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What is the speed of light? Here’s the history, discovery of the cosmic speed limit

On one hand, the speed of light is just a number: 299,792,458 meters per second. And on the other, it’s one of the most important constants that appears in nature and defines the relationship of causality itself.

As far as we can measure, it is a constant. It is the same speed for every observer in the entire universe. This constancy was first established in the late 1800’s with the experiments of Albert Michelson and Edward Morley at Case Western Reserve University . They attempted to measure changes in the speed of light as the Earth orbited around the Sun. They found no such variation, and no experiment ever since then has either.

Observations of the cosmic microwave background, the light released when the universe was 380,000 years old, show that the speed of light hasn’t measurably changed in over 13.8 billion years.

In fact, we now define the speed of light to be a constant, with a precise speed of 299,792,458 meters per second. While it remains a remote possibility in deeply theoretical physics that light may not be a constant, for all known purposes it is a constant, so it’s better to just define it and move on with life.

How was the speed of light first measured?

In 1676 the Danish astronomer Ole Christensen Romer made the first quantitative measurement of how fast light travels. He carefully observed the orbit of Io, the innermost moon of Jupiter. As the Earth circles the Sun in its own orbit, sometimes it approaches Jupiter and sometimes it recedes away from it. When the Earth is approaching Jupiter, the path that light has to travel from Io is shorter than when the Earth is receding away from Jupiter. By carefully measuring the changes to Io’s orbital period, Romer calculated a speed of light of around 220,000 kilometers per second.

Observations continued to improve until by the 19 th century astronomers and physicists had developed the sophistication to get very close to the modern value. In 1865, James Clerk Maxwell made a remarkable discovery. He was investigating the properties of electricity and magnetism, which for decades had remained mysterious in unconnected laboratory experiments around the world. Maxwell found that electricity and magnetism were really two sides of the same coin, both manifestations of a single electromagnetic force.

As Maxwell explored the consequences of his new theory, he found that changing magnetic fields can lead to changing electric fields, which then lead to a new round of changing magnetic fields. The fields leapfrog over each other and can even travel through empty space. When Maxwell went to calculate the speed of these electromagnetic waves, he was surprised to see the speed of light pop out – the first theoretical calculation of this important number.

What is the most precise measurement of the speed of light?

Because it is defined to be a constant, there’s no need to measure it further. The number we’ve defined is it, with no uncertainty, no error bars. It’s done. But the speed of light is just that – a speed. The number we choose to represent it depends on the units we use: kilometers versus miles, seconds versus hours, and so on. In fact, physicists commonly just set the speed of light to be 1 to make their calculations easier. So instead of trying to measure the speed light travels, physicists turn to more precisely measuring other units, like the length of the meter or the duration of the second. In other words, the defined value of the speed of light is used to establish the length of other units like the meter.

How does light slow down?

Yes, the speed of light is always a constant. But it slows down whenever it travels through a medium like air or water. How does this work? There are a few different ways to present an answer to this question, depending on whether you prefer a particle-like picture or a wave-like picture.

In a particle-like picture, light is made of tiny little bullets called photons. All those photons always travel at the speed of light, but as light passes through a medium those photons get all tangled up, bouncing around among all the molecules of the medium. This slows down the overall propagation of light, because it takes more time for the group of photons to make it through.

In a wave-like picture, light is made of electromagnetic waves. When these waves pass through a medium, they get all the charged particles in motion, which in turn generate new electromagnetic waves of their own. These interfere with the original light, forcing it to slow down as it passes through.

Either way, light always travels at the same speed, but matter can interfere with its travel, making it slow down.

Why is the speed of light important?

The speed of light is important because it’s about way more than, well, the speed of light. In the early 1900’s Einstein realized just how special this speed is. The old physics, dominated by the work of Isaac Newton, said that the universe had a fixed reference frame from which we could measure all motion. This is why Michelson and Morley went looking for changes in the speed, because it should change depending on our point of view. But their experiments showed that the speed was always constant, so what gives?

Einstein decided to take this experiment at face value. He assumed that the speed of light is a true, fundamental constant. No matter where you are, no matter how fast you’re moving, you’ll always see the same speed.

This is wild to think about. If you’re traveling at 99% the speed of light and turn on a flashlight, the beam will race ahead of you at…exactly the speed of light, no more, no less. If you’re coming from the opposite direction, you’ll still also measure the exact same speed.

This constancy forms the basis of Einstein’s special theory of relativity, which tells us that while all motion is relative – different observers won’t always agree on the length of measurements or the duration of events – some things are truly universal, like the speed of light.

Can you go faster than light speed?

Nope. Nothing can. Any particle with zero mass must travel at light speed. But anything with mass (which is most of the universe) cannot. The problem is relativity. The faster you go, the more energy you have. But we know from Einstein’s relativity that energy and mass are the same thing. So the more energy you have, the more mass you have, which makes it harder for you to go even faster. You can get as close as you want to the speed of light, but to actually crack that barrier takes an infinite amount of energy. So don’t even try.

How is the speed at which light travels related to causality?

If you think you can find a cheat to get around the limitations of light speed, then I need to tell you about its role in special relativity. You see, it’s not just about light. It just so happens that light travels at this special speed, and it was the first thing we discovered to travel at this speed. So it could have had another name. Indeed, a better name for this speed might be “the speed of time.”

Related: Is time travel possible? An astrophysicist explains

We live in a universe of causes and effects. All effects are preceded by a cause, and all causes lead to effects. The speed of light limits how quickly causes can lead to effects. Because it’s a maximum speed limit for any motion or interaction, in a given amount of time there’s a limit to what I can influence. If I want to tap you on the shoulder and you’re right next to me, I can do it right away. But if you’re on the other side of the planet, I have to travel there first. The motion of me traveling to you is limited by the speed of light, so that sets how quickly I can tap you on the shoulder – the speed light travels dictates how quickly a single cause can create an effect.

The ability to go faster than light would allow effects to happen before their causes. In essence, time travel into the past would be possible with faster-than-light travel. Since we view time as the unbroken chain of causes and effects going from the past to the future, breaking the speed of light would break causality, which would seriously undermine our sense of the forward motion of time.

Why does light travel at this speed?

No clue. It appears to us as a fundamental constant of nature. We have no theory of physics that explains its existence or why it has the value that it does. We hope that a future understanding of nature will provide this explanation, but right now all investigations are purely theoretical. For now, we just have to take it as a given.

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How Does Light Travel?

Ever since Democritus – a Greek philosopher who lived between the 5th and 4th century’s BCE – argued that all of existence was made up of tiny indivisible atoms, scientists have been speculating as to the true nature of light. Whereas scientists ventured back and forth between the notion that light was a particle or a wave until the modern era, the 20th century led to breakthroughs that showed us that it behaves as both.

These included the discovery of the electron, the development of quantum theory, and Einstein’s Theory of Relativity . However, there remains many unanswered questions about light, many of which arise from its dual nature. For instance, how is it that light can be apparently without mass, but still behave as a particle? And how can it behave like a wave and pass through a vacuum, when all other waves require a medium to propagate?

Theory of Light to the 19th Century:

During the Scientific Revolution, scientists began moving away from Aristotelian scientific theories that had been seen as accepted canon for centuries. This included rejecting Aristotle’s theory of light, which viewed it as being a disturbance in the air (one of his four “elements” that composed matter), and embracing the more mechanistic view that light was composed of indivisible atoms.

In many ways, this theory had been previewed by atomists of Classical Antiquity – such as Democritus and Lucretius – both of whom viewed light as a unit of matter given off by the sun. By the 17th century, several scientists emerged who accepted this view, stating that light was made up of discrete particles (or “corpuscles”). This included Pierre Gassendi, a contemporary of René Descartes, Thomas Hobbes, Robert Boyle, and most famously, Sir Isaac Newton .

Newton’s corpuscular theory was an elaboration of his view of reality as an interaction of material points through forces. This theory would remain the accepted scientific view for more than 100 years, the principles of which were explained in his 1704 treatise “ Opticks, or, a Treatise of the Reflections, Refractions, Inflections, and Colours of Light “. According to Newton, the principles of light could be summed as follows:

• Every source of light emits large numbers of tiny particles known as corpuscles in a medium surrounding the source.
• These corpuscles are perfectly elastic, rigid, and weightless.

This represented a challenge to “wave theory”, which had been advocated by 17th century Dutch astronomer Christiaan Huygens . . These theories were first communicated in 1678 to the Paris Academy of Sciences and were published in 1690 in his “ Traité de la lumière “ (“ Treatise on Light “). In it, he argued a revised version of Descartes views, in which the speed of light is infinite and propagated by means of spherical waves emitted along the wave front.

Double-Slit Experiment:

By the early 19th century, scientists began to break with corpuscular theory. This was due in part to the fact that corpuscular theory failed to adequately explain the diffraction, interference and polarization of light, but was also because of various experiments that seemed to confirm the still-competing view that light behaved as a wave.

The most famous of these was arguably the Double-Slit Experiment , which was originally conducted by English polymath Thomas Young in 1801 (though Sir Isaac Newton is believed to have conducted something similar in his own time). In Young’s version of the experiment, he used a slip of paper with slits cut into it, and then pointed a light source at them to measure how light passed through it.

According to classical (i.e. Newtonian) particle theory, the results of the experiment should have corresponded to the slits, the impacts on the screen appearing in two vertical lines. Instead, the results showed that the coherent beams of light were interfering, creating a pattern of bright and dark bands on the screen. This contradicted classical particle theory, in which particles do not interfere with each other, but merely collide.

The only possible explanation for this pattern of interference was that the light beams were in fact behaving as waves. Thus, this experiment dispelled the notion that light consisted of corpuscles and played a vital part in the acceptance of the wave theory of light. However subsequent research, involving the discovery of the electron and electromagnetic radiation, would lead to scientists considering yet again that light behaved as a particle too, thus giving rise to wave-particle duality theory.

Electromagnetism and Special Relativity:

Prior to the 19th and 20th centuries, the speed of light had already been determined. The first recorded measurements were performed by Danish astronomer Ole Rømer, who demonstrated in 1676 using light measurements from Jupiter’s moon Io to show that light travels at a finite speed (rather than instantaneously).

By the late 19th century, James Clerk Maxwell proposed that light was an electromagnetic wave, and devised several equations (known as Maxwell’s equations ) to describe how electric and magnetic fields are generated and altered by each other and by charges and currents. By conducting measurements of different types of radiation (magnetic fields, ultraviolet and infrared radiation), he was able to calculate the speed of light in a vacuum (represented as c ).

In 1905, Albert Einstein published “ On the Electrodynamics of Moving Bodies ”, in which he advanced one of his most famous theories and overturned centuries of accepted notions and orthodoxies. In his paper, he postulated that the speed of light was the same in all inertial reference frames, regardless of the motion of the light source or the position of the observer.

Exploring the consequences of this theory is what led him to propose his theory of Special Relativity , which reconciled Maxwell’s equations for electricity and magnetism with the laws of mechanics, simplified the mathematical calculations, and accorded with the directly observed speed of light and accounted for the observed aberrations. It also demonstrated that the speed of light had relevance outside the context of light and electromagnetism.

For one, it introduced the idea that major changes occur when things move close the speed of light, including the time-space frame of a moving body appearing to slow down and contract in the direction of motion when measured in the frame of the observer. After centuries of increasingly precise measurements, the speed of light was determined to be 299,792,458 m/s in 1975.

Einstein and the Photon:

In 1905, Einstein also helped to resolve a great deal of confusion surrounding the behavior of electromagnetic radiation when he proposed that electrons are emitted from atoms when they absorb energy from light. Known as the photoelectric effect , Einstein based his idea on Planck’s earlier work with “black bodies” – materials that absorb electromagnetic energy instead of reflecting it (i.e. white bodies).

At the time, Einstein’s photoelectric effect was attempt to explain the “black body problem”, in which a black body emits electromagnetic radiation due to the object’s heat. This was a persistent problem in the world of physics, arising from the discovery of the electron, which had only happened eight years previous (thanks to British physicists led by J.J. Thompson and experiments using cathode ray tubes ).

At the time, scientists still believed that electromagnetic energy behaved as a wave, and were therefore hoping to be able to explain it in terms of classical physics. Einstein’s explanation represented a break with this, asserting that electromagnetic radiation behaved in ways that were consistent with a particle – a quantized form of light which he named “photons”. For this discovery, Einstein was awarded the Nobel Prize in 1921.

Wave-Particle Duality:

Subsequent theories on the behavior of light would further refine this idea, which included French physicist Louis-Victor de Broglie calculating the wavelength at which light functioned. This was followed by Heisenberg’s “uncertainty principle” (which stated that measuring the position of a photon accurately would disturb measurements of it momentum and vice versa), and Schrödinger’s paradox that claimed that all particles have a “wave function”.

In accordance with quantum mechanical explanation, Schrodinger proposed that all the information about a particle (in this case, a photon) is encoded in its wave function , a complex-valued function roughly analogous to the amplitude of a wave at each point in space. At some location, the measurement of the wave function will randomly “collapse”, or rather “decohere”, to a sharply peaked function. This was illustrated in Schrödinger famous paradox involving a closed box, a cat, and a vial of poison (known as the “ Schrödinger Cat” paradox).

According to his theory, wave function also evolves according to a differential equation (aka. the Schrödinger equation ). For particles with mass, this equation has solutions; but for particles with no mass, no solution existed. Further experiments involving the Double-Slit Experiment confirmed the dual nature of photons. where measuring devices were incorporated to observe the photons as they passed through the slits.

When this was done, the photons appeared in the form of particles and their impacts on the screen corresponded to the slits – tiny particle-sized spots distributed in straight vertical lines. By placing an observation device in place, the wave function of the photons collapsed and the light behaved as classical particles once more. As predicted by Schrödinger, this could only be resolved by claiming that light has a wave function, and that observing it causes the range of behavioral possibilities to collapse to the point where its behavior becomes predictable.

The development of Quantum Field Theory (QFT) was devised in the following decades to resolve much of the ambiguity around wave-particle duality. And in time, this theory was shown to apply to other particles and fundamental forces of interaction (such as weak and strong nuclear forces). Today, photons are part of the Standard Model of particle physics, where they are classified as boson – a class of subatomic particles that are force carriers and have no mass.

So how does light travel? Basically, traveling at incredible speeds (299 792 458 m/s) and at different wavelengths, depending on its energy. It also behaves as both a wave and a particle, able to propagate through mediums (like air and water) as well as space. It has no mass, but can still be absorbed, reflected, or refracted if it comes in contact with a medium. And in the end, the only thing that can truly divert it, or arrest it, is gravity (i.e. a black hole).

What we have learned about light and electromagnetism has been intrinsic to the revolution which took place in physics in the early 20th century, a revolution that we have been grappling with ever since. Thanks to the efforts of scientists like Maxwell, Planck, Einstein, Heisenberg and Schrodinger, we have learned much, but still have much to learn.

For instance, its interaction with gravity (along with weak and strong nuclear forces) remains a mystery. Unlocking this, and thus discovering a Theory of Everything (ToE) is something astronomers and physicists look forward to. Someday, we just might have it all figured out!

We have written many articles about light here at Universe Today. For example, here’s How Fast is the Speed of Light? , How Far is a Light Year? , What is Einstein’s Theory of Relativity?

If you’d like more info on light, check out these articles from The Physics Hypertextbook and NASA’s Mission Science page.

We’ve also recorded an entire episode of Astronomy Cast all about Interstellar Travel. Listen here, Episode 145: Interstellar Travel .

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56 Replies to “How Does Light Travel?”

“HOW DOES LIGHT TRAVEL?”

it travels lightly. 😀

Light doesn’t exist. This is an observation from light’s point of view and not ours. Traveling at the speed of (wait for it) light, absolutely no time passes between leaving it’s source and reaching it’s destination for the photon. This means, to the photon hitting your retina, it is also still on that star you are observing 10 light years away. How is this possible? Maybe John Wheeler was right when he told Richard Feynman that there is only one electron in the universe and it travels forward in time as an electron, then back in time as a positron and every electron we see is the same electron.

MY QUESTION IS: Whether light is a wave , particle or both.. where does it get the energy to move through space/time. In other words is the energy of light infinite? Does it continue on without lose of energy…..forever…….

I believe that Special Relativity says that the energy of light is infinite due to the very fact it has no mass. E=MC^2

In reverse, this is also why something with mass to begin with. If accelerated toward the speed of light, will see their mass and gravity increase to infinite points as they near relativistic speed (it actually starts around 95% with a steep upward curve from there), with a relative slowing to a stop of time.

Join the discussion

Light and the universe are only illusions that are formed in our minds via technology that sends information from the simulation program we’re living in. That information comes in the form of invisible wavelengths that includes wavelengths that we perceive as light. The visible retinas in our eyes are like tiny video screens where these particles are arranged into patterns that form into all the various objects we think are real objects. This information is also converted into thoughts within our minds which are like computer processors that process that information.

We are living in a computer simulation that is much more advanced than anything the characters in the program have built according to the information called the Beast.

Brad,…So You’re suggesting that “life” as we know and call it “is some kind of retro-virus” or “bio-intelligent format” heaped upon a perceived “set of accepted data sets” that are not in sync with each other in most cases with exception to Math 94% of the time….Even then it can vary which suggests Your idea would mean we all live in a fairy tale. That is what you suggest,…right?……

Brad has watched the Matrix too many times.

Correction: Even gravity doesn’t slow light down. Light (EM radiation of any wavelength) always travels at speed c, relative to any local inertial (Lorentz) frame. It could also be noted that the wavelength of an EM wave is not a characteristic of that wave alone; it also depends on the state of motion of the observer. You might even say, “One man’s radio wave is another man’s gamma ray.”

Light actually “slows down” every time it has to travel through anything but a vacuum. Look up Cherenkov radiation to see what happens when light initially travels faster than it can through a particular substance, like water. Light speed is not constant when traveling through any medium except pure vacuum. In fact that is why your pencil looks bent when you drop it in a glass of water. Light bends to find it’s fastest path through any medium, and it slows down in that medium.

if all you scientist could ever get it in your pie brain that there is no time, no light speed, no warping space, no black holes for the purpose of moving through space quickly, no smallest no biggest when it comes to space and that all of everything has always been in existence but not necessarily as it is now. you will never find the smallest because if it exist it has an inside, and you will never find the end of space because it is infinite.

What are you smoking?

The article started out nicely, but I lost interest as mistakes began to appear. First Einstein did not “propose” the photoelectric effect. The photoelectric effect was first observed by Heinrich Hertz in 1887. Einstein used the idea of photons to explain the photoelectric effect and derive the photoelectric equation. Also, Max Plank had already derived the blackbody distribution, by assuming that electromagnetic energy of frequency f could only be emitted in multiples of energy E=hf, by 1900. Einstein’s paper on the photoelectric effect was published in his “miracle” year of 1905. The photoelectric effect has nothing to do with black body radiation.

Einstein did not coin the name “photons” for light quanta, as stated in this article. This term was first used by Arthur Compton in 1928.

I have to say that I do not know what the author of the article means when he says ” calculating the wavelength at which light functioned” in reference to Louis-Victor de Broglie. Louis de Broglie used the dual nature of light to suggest that electrons, previously thought of as particles, also had wave characteristics and used this notion to explain the Bohr orbits in the hydrogen atom.

I gave up on the article after seeing these errors. I’m afraid I have a low tolerance for sloppy writing.

Oh, it’s BCE now, “Before the Common Era” BC has worked for 2000 years but now the PC police have stepped in so as not to offend who? Some Muslims?

mecheng1, you must be very young. BCE has been in used in academia for decades. It’s nothing “new”, just out of your circle of knowledge.

Decades??? Really?? How does that compare to 2000 years?

Only in Euro-centric texts have your assertions been true, McCowen. The rest of the world not influenced by Christianity have used their own calendars and a “0” year or a “year 1” from which to reckon the passage of time, largely based on their own religions or celestial observations.

Over the last century or so, through commerce, most of the world has generally accepted the use of a Western calendar (or use it along with their own for domestic purposes, like we here in the US still use Imperial units of measure that have to be converted to metric for international commerce). So, we are in a “common era” insofar as non-Christian societies are incorporating the Gregorian Calendar and the generally-accepted “year 1” established by that calendar (which is supposed to be the year of Jesus’s birth, but it probably isn’t according to current scholarship). Besides, the Gregorian calendar is an improved derivative of the Roman calendar – even the names of the months come from the Romans.

In short, it is more accurate, as well as respectful, to go with BCE in these global times.

Where is the information carried on a photon hitting my eye(s), or cluster/group/pack of photons hitting my eyes(s), that I see as other distant galaxies and planets going around stars?

That’s the mystery, isn’t it? Even in scattering, light remains coherent enough to convey an enormous amount of information.

Since the miniscule equal masses with opposite charges, that make up the photon structure, interact at 90 degrees, this induces a spin (a finding from the 80’s by the LANL plasma physics program) which creates a centrifugal force that counterbalances the charge attraction of the opposite charges. This establishes a stable structure for energies less than 1.0216 MeV, the pair-formation threshold, separating these “neutrino” sub-components by a specific distance providing wavelengths varying with photon energy. This composite photon propagates transversely at c/n, the speed of light divided by the index of refraction of the material traversed. In spite of the mass being defined as zero, for convenience in calculating atomic masses, there is actually an infinitesimal but non-zero mass for the photon that is required for calculations that describe its properties.

Tim, you poor guy! You have a discombobulated brain! Everything you wrote is just gibberish.

i would like to know the temperature in a black hole…maybe absolute zero? is absolute zero the moment that time stop?

I think the temp inside a black hole would be extremely high since temperature seems to increase with mass. Comparing absolute zero to time stopping is very interesting though. To the observer they would appear the same.

Theoretically there is no temperature in a black hole from any observer POV because time is stopped. Although JALNIN does bring up that point, and he also brings up the point of increasing mass corresponding to increasing energy. Everything in Hawking and Einstein’s equations though, suggest that any energy would be absorbed back by the singularity, so there wouldn’t be any heat. In fact it should be infinitely cold. But time is no more, so technically no heat or energy is emitted anyway from any observers POV. Yet recent images of black holes from Chandra show that they emit powerful Gamma Jets along their spin axis just like Neutron stars, and Pulsars. BTW edison. The accretion disk can reach temperatures of 20MN Kelvin on a feeding SM black hole (quasar). NASA just published an article on it through the Chandra feed a while back.

Light doesn’t travel, it just IS. It is we, the condensed matter, that travels, through time.

Oh really? Is this just your imagination/illusion or you have published a paper on it?

So you don’t believe you travel through time?

I wish I understood just a portion of I just read, love sicence so bad BUT, sighs

It would be easier to understand if it wasn’t pure gibberish written by someone with no science background.

I have two “mind-bending relativity side effects” to share. At least they are mind-bending to me.

1) Light travels the same speed relative to all particles of mass, regardless of how those particles move relative to each other:

I can conceptualize this if we are only talking about two mass-particles/observers and the examples I’ve seen always involve only two observers. But if you have many mass-particles/observers, how does the space-time seem to know to adjust differently for all of them. I am sure i am understanding this correctly as it is a basic concept of special relativity and nobody seems to bring this issue up. But it “bends my mind” when i try to include more than two observers. Maybe you can help.

2) General Relativity’s (“GR”) prediction that the big bang started with “Infinite” energy and now the universe appears to have finite mass energy and Regarding the first effect: How can something infinite turn into something finite? Is the answer that at that early in the universe, quantum takes over and GR’s prediction of infinite mass-energy at the start of the universe is just wrong?

I need to correct a typo in my previous comment. Where i say “i am sure am understanding this correctly” I meant to include the word NOT. so it should read “i am sure am NOT understanding this correctly” Mark L.

Mark,….I think you’re understanding it just fine from the standpoint of multiple observers, The point might be that in space, the density of “emptiness” or “lack of emptiness” might be impacted from one area of observation to another by an observer who’s perceptions are not equal but not being taken into consideration by each observer. ( an example if I may?) If you were to use a Clear medium which is oil based beginning with 5 gallons of mineral spirits in a large barrel and keep adding 5 gallons of thicker clear oil and then heavy grease and stop with using a clear heavy wax,…what happens is you end up with a barrel of clear fluid that begins with a floating substrate but the liquid begins to keep floating and the heaviest stuff goes to the bottom,…You end up with a sort of solid tube of clear fluids which if you could keep them in shape here on the earth, “you could observe them” from several positions, #1. the fluid end #2, the less fluid part, #3, the semi solid part #4. the seemingly solid part #5. the almost solid part & #6. the solid part……all of which would be transparent….You could then shine a laser through all of it and perhaps do that again from different places and see what happens at different angles…..I think what happens as a result would be, an observer would end up be influenced as per his or her ideas thusly because of the quasi-nature of what the density of space is at the point of space is where the observation is made. just a guess.

All Special Relativity really says about light is that it appears to move at the same rate from any observer POV. There are other more advanced rules relating to light speeds. One of them is the implication of infinite energy in a photon because of the fact it’s mass-less, therefore it can move at the maximum rate a mass-less particle or wave can (not necessarily that it does) Later when the electron was discovered (also mass-less particle or wave), it was also found to conform to the rules of special relativity.

As far as the big bang, there are a lot of cracks in that theory, and many different ones are beginning to dispute some of the common ideas behind the “Big Bang” as well as “Inflationary Cosmology”. Honestly though, both standard and quantum physics applied, and yet both went out the window at the same time at some point. That’s what all the theories really say. At some point, everything we know or think we know was bunk, because the math just breaks down, and doesn’t work right anymore.

i think until there is an understanding of the actual “fabric” of space itself, the wave vs particle confusion will continue. another interesting article recently was the half integer values of rotating light. planck’s constant was broken? gravity? a bump in the data? lol these are interesting times.

There’s no fabric.

Tesla insists there is an aether, Einstein says not. Tesla enjoyed far less trial and error than Einstein. The vast majority of Tesla’s projects worked the first time around and required no development or experimentation. I’ll go with Tesla; there is an aether as a fabric of space.

http://weinsteinsletter.weebly.com/aether.html

Maybe Special Relativity is not correct? 🙂

Feynman said unequivocally that QED is NOT a wave theory. In fact, the math only looks like Maxwell’s wave function when you are looking at a single particle at a time, but the analogy breaks down as soon as you start looking at the interactions of more than one, which is the real case. There’s no light acting alone, but always an interaction between a photon and some other particle, an electron, another photon, or whatever. He said “light is particles.” So the question re: how can light travel through a vacuum if it’s waves is a nonsensical question. There are no collapsing wave functions in light. There’s only probabilities of position that look like waves on a freaking piece of paper. Even calling light properties as “wavelengths” is nonsensical. Light comes in frequencies, i.e., the number of particles traveling tightly together. Higher frequency is more energy because it’s more particles (E=MC[squared]). “Wavicles” is pure bullshit.

I don’t agree with the John Wheeler theory that there is only one electron since the computer I am using was built by ion implantation and uses a very large number of them simultaneously to function.

Black holes don’t stop or slow light, if they even exist. A black hole could phase shift light, which is why we see things emitting xrays and call them black holes….but they could be something else too.

Photons have no mass but they do have energy. Energy and mass are transformable into each other. Gravity works on energy as well as mass. As massive particles approach the speed of light their measurable mass increases to infinity. But since energy is equivalent to mass, why doesn’t the photon, which has energy, not seem to have infinite mass?

NO other wave travels thru a vacuum? what about radio?

Radio waves are a specific frequency range of light.

Technically speaking, radio waves are emitted at various frequencies that share the same space time as light. They are not however light. They’re modulated electrons. Modulated photons certainly can be used to carry a vast amount of information a great distance. It cannot do it any faster or better than a radio wave though. Both electrons and photons are mass-less, therefore they both conform to the rules of Special Relativity in the same way. Both travel at the speed of light.

I just don’t understand is it a particle of a wave? It seems like it behaves like wave and sometimes like particle and in some situations is like a what ever you are going to call it.

So, the logical idea would to have formula Photon_influence * weight_for_particle + Wave_influence * weight_for_wave

Make it more compact.

This article is good but the title is bad as by the end we still weren’t told how light travels through space. Also, there are some historical mistakes as already pointed out. Now for my contribution: I think that light and Gravity have a lot in common; for one – an atom’s electrons transmit light and an atom contains the tiny heavy place that knows everything there is to know about gravity, that is, the nucleus. Light and Gravity are both related to the same entity, the atom. Unfortunately, we, still cannot grasp how what’s heavy brings about gravitation. For those of you with a creed for new ideas go to: https://www.academia.edu/10785615/Gravity_is_emergent It’s a hypothesis…

Gravity and light are infinite, like space and time… Mind the concept that there are waves within waves, motions within motion, vibrations within vibration, endless overtones and universal harmony…

From this article, I have “And in the end, the only thing that can truly slow down or arrest the speed of light is gravity”

Doesn’t light slow down in water and glass and other mediums. I was only a Physics minor, but I do remember coivering this though way back in the early 80’s. And in my quick checking online, I found the following.

“Light travels at approximately 300,000 kilometers per second in a vacuum, which has a refractive index of 1.0, but it slows down to 225,000 kilometers per second in water (refractive index = 1.3; see Figure 1) and 200,000 kilometers per second in glass (refractive index of 1.5).”

Were they saying something else here. I did like the article.

Photons are not massless, but their mass is incredibly small even compared to a proton or neutron. So, by Einstein’s E=MC^2, the energy required for a photon to move is greatly reduced, but photons do have mass and are affected by gravity. If photons had no mass at all, then gravity would have no affect on them, but gravity does. Gravity bends light and can change it’s course through space. We see that in the actual test first performed to prove Einstein’s theory buy observing the distorted placement of stars as their light passes near the sun observed during an eclipse. We can also see it through gravitational lensing when viewing deeps space objects. And the fact that there are black holes that are black because light cannot escape it’s gravity. So photons do have mass, be it miniscule, and with that their propagation with light waves through space will eventually run out of energy and stop. but this would probably require distances greater to several widths of our universe to accomplish. Light from the furthest reaches of the universe are not as bright, or as energetic, as they are at anyplace between here and their origins. That reduction in their energy is also attributed to Einstein’s equation and the inverse square law, where the intensity of light is in relation to the inverse square of the distance. That proves that light looses energy the further it travels, but it still moves at the speed of light. As light looses energy, it doesn’t slow the light wave.

It has been proven that more energetic light does in fact travel slightly faster. You can find the experiments done with light that has traveled billions of light years, the more energetic is in fact faster over a number of seconds, around 10 -15 or so. As people encounter this information, they see that many accepted theories can now be debunked.

The point of the article is nothing new; light acts like a particle AND a beam. So when you sit behind a closed door and someone shines a light on the door, the light will engulf the door and wave through and around the edges, the particle does not just bounce straight back. You can focus a beam of light on an object, but it will sneak though the corners and underneath the door, through any opening,. And yes, light travels forever. It is a constant, that cannot be sped up. We can slow it down by focusing it through prisims or crystals. But it still is traveling at 186,000/MPS.and that speed does not change. So, that is why we can see the outer edge of the universe: 13,8B light years away *the time that it takes for light to travel in one year, is one light year. So, it has taken 13,8B light years for the light of other galaxies to get here, so those galaxies could be gone by now, since it took so long to reach us, We are truly looking back in time as we see the light emitted from those galaxies and stars.

It propagates through the quantum mish-mash know as the aether . . .

If light is a particle and particles have mass why does not the mas increase with it speed?

Wow…there are errors in the article, yes…the enthusiasm demonstrated by all the comments is encouraging…but when I read these comments, I am a bit dismayed at the lack of understanding that is evident in most of them…confusing energy and intensity and wavelength…confusing rest mass and inertial mass…not to mention some off-the-wall hypotheses with no experimental evidence to support them. There are some great primers out there…books, documentaries, podcasts (like Astronomy Cast). Good luck!

Precisely correct. Sci-fi rules basic physics, which reflects on the poor education system. Pity.

First time I heard about A. A. and his theory about light I really didn’t like him. Why? Because light was the the fastest thing in the universe and there is no other thing faster than the light. Later, when I have red about angular speed I have asked my self if you have linear and angular speed and both of them are speeds how that will result in the maximum speed. Since then, I have not had a chance to get right answer.

Comments are closed.

Why is the speed of light the way it is?

It's just plain weird.

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of Ask a Spaceman and Space Radio , and author of " How to Die in Space ." He contributed this article to Space.com's Expert Voices: Op-Ed & Insights . 

We all know and love the speed of light — 299,792,458 meters per second — but why does it have the value that it does? Why isn't it some other number? And why do we care so much about some random speed of electromagnetic waves? Why did it become such a cornerstone of physics? 

Well, it's because the speed of light is just plain weird.

Related: Constant speed of light: Einstein's special relativity survives a high-energy test

Putting light to the test

The first person to realize that light does indeed have a speed at all was an astronomer by the name of Ole Romer. In the late 1600s, he was obsessed with some strange motions of the moon Io around Jupiter. Every once in a while, the great planet would block our view of its little moon, causing an eclipse, but the timing between eclipses seemed to change over the course of the year. Either something funky was happening with the orbit of Io — which seemed suspicious — or something else was afoot.

After a couple years of observations, Romer made the connection. When we see Io get eclipsed, we're in a certain position in our own orbit around the sun. But by the next time we see another eclipse, a few days later, we're in a slightly different position, maybe closer or farther away from Jupiter than the last time. If we are farther away than the last time we saw an eclipse, then that means we have to wait a little bit of extra time to see the next one because it takes that much longer for the light to reach us, and the reverse is true if we happen to be a little bit closer to Jupiter.

The only way to explain the variations in the timing of eclipses of Io is if light has a finite speed.

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Making it mean something

Continued measurements over the course of the next few centuries solidified the measurement of the speed of light, but it wasn't until the mid-1800s when things really started to come together. That's when the physicist James Clerk Maxwell accidentally invented light.

Maxwell had been playing around with the then-poorly-understood phenomena of electricity and magnetism when he discovered a single unified picture that could explain all the disparate observations. Laying the groundwork for what we now understand to be the electromagnetic force , in those equations he discovered that changing electric fields can create magnetic fields, and vice versa. This allows waves of electricity to create waves of magnetism, which go on to make waves of electricity and back and forth and back and forth, leapfrogging over each other, capable of traveling through space.

And when he went to calculate the speed of these so-called electromagnetic waves, Maxwell got the same number that scientists had been measuring as the speed of light for centuries. Ergo, light is made of electromagnetic waves and it travels at that speed, because that is exactly how quickly waves of electricity and magnetism travel through space.

And this was all well and good until Einstein came along a few decades later and realized that the speed of light had nothing to do with light at all. With his special theory of relativity , Einstein realized the true connection between time and space, a unified fabric known as space-time. But as we all know, space is very different than time. A meter or a foot is very different than a second or a year. They appear to be two completely different things.

So how could they possibly be on the same footing?

There needed to be some sort of glue, some connection that allowed us to translate between movement in space and movement in time. In other words, we need to know how much one meter of space, for example, is worth in time. What's the exchange rate? Einstein found that there was a single constant, a certain speed, that could tell us how much space was equivalent to how much time, and vice versa.

Einstein's theories didn't say what that number was, but then he applied special relativity to the old equations of Maxwell and found that this conversion rate is exactly the speed of light.

Of course, this conversion rate, this fundamental constant that unifies space and time, doesn't know what an electromagnetic wave is, and it doesn't even really care. It's just some number, but it turns out that Maxwell had already calculated this number and discovered it without even knowing it. That's because all massless particles are able to travel at this speed, and since light is massless, it can travel at that speed. And so, the speed of light became an important cornerstone of modern physics.

But still, why that number, with that value, and not some other random number? Why did nature pick that one and no other? What's going on?

Related: The genius of Albert Einstein: his life, theories and impact on science

Making it meaningless

Well, the number doesn't really matter. It has units after all: meters per second. And in physics any number that has units attached to it can have any old value it wants, because it means you have to define what the units are. For example, in order to express the speed of light in meters per second, first you need to decide what the heck a meter is and what the heck a second is. And so the definition of the speed of light is tied up with the definitions of length and time.

In physics, we're more concerned with constants that have no units or dimensions — in other words, constants that appear in our physical theories that are just plain numbers. These appear much more fundamental, because they don't depend on any other definition. Another way of saying it is that, if we were to meet some alien civilization , we would have no way of understanding their measurement of the speed of light, but when it comes to dimensionless constants, we can all agree. They're just numbers.

One such number is known as the fine structure constant, which is a combination of the speed of light, Planck's constant , and something known as the permittivity of free space. Its value is approximately 0.007. 0.007 what? Just 0.007. Like I said, it's just a number.

So on one hand, the speed of light can be whatever it wants to be, because it has units and we need to define the units. But on the other hand, the speed of light can't be anything other than exactly what it is, because if you were to change the speed of light, you would change the fine structure constant. But our universe has chosen the fine structure constant to be approximately 0.007, and nothing else. That is simply the universe we live in, and we get no choice about it at all. And since this is fixed and universal, the speed of light has to be exactly what it is.

So why is the fine structure constant exactly the number that it is, and not something else? Good question. We don't know.

Learn more by listening to the episode "Why is the speed of light the way it is?" on the Ask A Spaceman podcast, available on iTunes and on the Web at http://www.askaspaceman.com. Thanks to Robert H, Michael E., @DesRon94, Evan W., Harry A., @twdixon, Hein P., Colin E., and Lothian53 for the questions that led to this piece! Ask your own question on Twitter using #AskASpaceman or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter.

Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: [email protected].

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute in New York City. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy, His research focuses on many diverse topics, from the emptiest regions of the universe to the earliest moments of the Big Bang to the hunt for the first stars. As an "Agent to the Stars," Paul has passionately engaged the public in science outreach for several years. He is the host of the popular "Ask a Spaceman!" podcast, author of "Your Place in the Universe" and "How to Die in Space" and he frequently appears on TV — including on The Weather Channel, for which he serves as Official Space Specialist.

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• voidpotentialenergy This is just my opinion but i think L speed is it's speed because the particle part of it is the fastest it can interact with the quanta distance in quantum fluctuation. Light is particle and wave so the wave happens in the void between quanta. Gravity probably travels in that void and why gravity seems instant. Reply
• rod The space.com article wraps up the discussion with, "So on one hand, the speed of light can be whatever it wants to be, because it has units and we need to define the units. But on the other hand, the speed of light can't be anything other than exactly what it is, because if you were to change the speed of light, you would change the fine structure constant. But our universe has chosen the fine structure constant to be approximately 0.007, and nothing else. That is simply the universe we live in, and we get no choice about it at all. And since this is fixed and universal, the speed of light has to be exactly what it is. So why is the fine structure constant exactly the number that it is, and not something else? Good question. We don't know." It seems that the *universe* made this decision, *But our universe has chosen the fine structure constant to be...* I did not know that the universe was capable of making decisions concerning constants used in physics. E=mc^2 is a serious constant. Look at nuclear weapons development, explosive yields, and stellar evolution burn rates for p-p chain and CNO fusion rates. The report indicates why alpha (fine structure constant) is what it is and c is what it is, *We don't know*. Reply

Admin said: We all know and love the speed of light, but why does it have the value that it does? Why isn't it some other number? And why did it become such a cornerstone of physics? Why is the speed of light the way it is? : Read more

rod said: The space.com article wraps up the discussion with, "So on one hand, the speed of light can be whatever it wants to be, because it has units and we need to define the units. But on the other hand, the speed of light can't be anything other than exactly what it is, because if you were to change the speed of light, you would change the fine structure constant. But our universe has chosen the fine structure constant to be approximately 0.007, and nothing else. That is simply the universe we live in, and we get no choice about it at all. And since this is fixed and universal, the speed of light has to be exactly what it is. So why is the fine structure constant exactly the number that it is, and not something else? Good question. We don't know." It seems that the *universe* made this decision, *But our universe has chosen the fine structure constant to be...* I did not know that the universe was capable of making decisions concerning constants used in physics. E=mc^2 is a serious constant. Look at nuclear weapons development, explosive yields, and stellar evolution burn rates for p-p chain and CNO fusion rates. The report indicates why alpha (fine structure constant) is what it is and c is what it is, *We don't know*.

• rod FYI. When someone says *the universe has chosen*, I am reminded of these five lessons from a 1982 Fed. court trial. The essential characteristics of science are: It is guided by natural law; It has to be explanatory by reference to natural law; It is testable against the empirical world; Its conclusions are tentative, i.e., are not necessarily the final word; and It is falsifiable. Five important points about science. Reply
• Gary If the universe is expanding , how can the speed of light be constant ( miles per second , if each mile is getting longer ) ? Can light's velocity be constant while the universe expands ? So, with the expansion of the universe , doesn't the speed of light need to increase in order to stay at a constant velocity in miles per second ? Or, do the miles in the universe remain the same length as the universe 'adds' miles to its diameter ? Are the miles lengthening or are they simply being added / compounded ? Reply
• Gary Lets say we're in outer space and we shoot a laser through a block of glass. What causes the speed of the laser light to return to the speed it held prior to entering the block of glass ? Is there some medium in the vacuum of space that governs the speed of light ? Do the atoms in the glass push it back up to its original speed. If so, why don't those same atoms constantly push the light while it travels through the block of glass ? Reply

Gary said: Lets say we're in outer space and we shoot a laser through a block of glass. What causes the speed of the laser light to return to the speed it held prior to entering the block of glass ? Is there some medium in the vacuum of space that governs the speed of light ? Do the atoms in the glass push it back up to its original speed. If so, why don't those same atoms constantly push the light while it travels through the block of glass ?

Gary said: If the universe is expanding , how can the speed of light be constant ( miles per second , if each mile is getting longer ) ? Can light's velocity be constant while the universe expands ? So, with the expansion of the universe , doesn't the speed of light need to increase in order to stay at a constant velocity in miles per second ? Or, do the miles in the universe remain the same length as the universe 'adds' miles to its diameter ? Are the miles lengthening or are they simply being added / compounded ?

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What is the speed of light?

Light is faster than anything else in the known universe, though its speed can change depending on what it's passing through.

The universe has a speed limit, and it's the speed of light. Nothing can travel faster than light — not even our best spacecraft — according to the laws of physics.

So, what is the speed of light? 

Light moves at an incredible 186,000 miles per second (300,000 kilometers per second), equivalent to almost 700 million mph (more than 1 billion km/h). That's fast enough to circumnavigate the globe 7.5 times in one second, while a typical passenger jet would take more than two days to go around once (and that doesn't include stops for fuel or layovers!). 

Light moves so fast that, for much of human history, we thought it traveled instantaneously. As early as the late 1600s, though, scientist Ole Roemer was able to measure the speed of light (usually referred to as c ) by using observations of Jupiter's moons, according to Britannica . 

Around the turn of the 19th century, physicist James Clerk Maxwell created his theories of electromagnetism . Light is itself made up of electric and magnetic fields, so electromagnetism could describe the behavior and motion of light — including its theoretical speed. That value was 299,788 kilometers per second, with a margin of error of plus or minus 30. In the 1970s, physicists used lasers to measure the speed of light with much greater precision, leaving an error of only 0.001. Nowadays, the speed of light is used to define units of length, so its value is fixed; humans have essentially agreed the speed of light is 299,792.458 kilometers per second, exactly.

Light doesn't always have to go so fast, though. Depending on what it's traveling through — air, water, diamonds, etc. — it can slow down. The official speed of light is measured as if it's traveling in a vacuum, a space with no air or anything to get in the way. You can most clearly see differences in the speed of light in something like a prism, where certain energies of light bend more than others, creating a rainbow.

— How many moons does Earth have ?

— What would happen if the moon were twice as close to Earth?

— If you're on the moon, does the Earth appear to go through phases?

Interestingly, the speed of light is no match for the vast distances of space, which is itself a vacuum. It takes 8 minutes for light from the sun to reach Earth, and a couple years for light from the other closest stars (like Proxima Centauri) to get to our planet. This is why astronomers use the unit light-years — the distance light can travel in one year — to measure vast distances in space.

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Because of this universal speed limit, telescopes are essentially time machines . When astronomers look at a star 500 light-years away, they're looking at light from 500 years ago. Light from around 13 billion light-years away (equivalently, 13 billion years ago) shows up as the cosmic microwave background, remnant radiation from the Big Bang in the universe's infancy. The speed of light isn't just a quirk of physics; it has enabled modern astronomy as we know it, and it shapes the way we see the world — literally.

Briley Lewis (she/her) is a freelance science writer and Ph.D. Candidate/NSF Fellow at the University of California, Los Angeles studying Astronomy & Astrophysics. Follow her on Twitter  @briles_34 or visit her website  www.briley-lewis.com .

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Why Does Light Travel in a Straight Line?

Most recent answer: 11/20/2010

(published on 11/20/2010)

Follow-Up #1: gravity bending light

Yes. Light from a torch is just like any other light. Astronomers routinely see evidence of light being bent by gravity.

(published on 03/08/2018)

Follow-up on this answer

Related Questions

• magnetism and gravity in the cosmos
• Questions about nebulae
• How do we know that half of the Galaxies are not Anti-matter?
• Bending light with gravity
• How do we know a galaxy or star is 13 billion light years away?
• Is cosmic microwave background from old galaxies?
• Is there a danger of a GRB striking the earth or sun?
• Rotation of galaxies
• Do we feel acceleration in an accelerating universe?
• Can stars exist independently of galaxies ?

Still Curious?

Expore Q&As in related categories

• The Rest of the Universe

• Light Travels In a Straight Line

Light travels in a straight line can be observed by keeping an object in the path of light. In an atmosphere which is bit dusty, we can see light traveling in a straight line. Light emerging from the torch, train and lamps always travel in a straight line. Let us study in detail how does light travel in a straight line.

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Light travels along a straight line.

Life without light would have been pretty dull. Light travels at a speed of 186,000 miles per second. You must have observed that in your house that whenever a beam of light enters a dark room through a tiny hole in the window, the lightwave always travels in a straight line.

Let us carry out a small activity to show that lightwave travels along a straight line. Take three CD’s and align them together. Align them in such a way that all the CD’s line in a straight line. Now take a candle and place it at the other end. Do make sure that the tip of the candle and the holes of the CD’s all lie in the straight line. Ensure that the height of the CD’s and the tip of the candle are same. Observe the flame of the candle. We are able to see the flame of the candle because the light wave travels through the holes and reaches our eye.

Browse more Topics under Light

• Reflection of Light
• Sunlight – White or Coloured
• Images Formed By Lenses

Now if suppose we displace the center of the CD’s we observe that we are not able to see the flame of the candle. Why does that happen? This is because the light gets blocked. If the light could have the ability to take a curve and travel, we could have seen the lightwave. But since light travels in a straight line, we were unable to see the flame of the candle when the CD is displaced. This proves that light travels along a straight line.

(Source: Wikipedia)

In the above picture, we can clearly see that light coming through the holes in the window travel along a straight line.

Questions For You

Q1. The phenomenon in which the moon’s shadow falls on earth,  or the earth casts its shadow on the moon, is known as

• Lateral deviation

Answer: C. The phenomenon in which the moon’s shadow falls on earth or the earth casts its shadow on the moon is known as an eclipse. During a solar eclipse, moon’s shadow falls on the earth. During a lunar eclipse, earth’s shadow falls on the moon.

Q2. Two examples of non-luminous objects are

• Stars and Moon
• Burning candle, glowing bulb
• The moon, a spoon
• Stars, a spoon

Answer: C. Non-luminous objects are those that do not emit light. The moon and the spoon do not emit light. So these two are good examples of non-luminous objects.

Q3. We can see the objects only when

• Reflected light from the object reaches our eye.
• The objects absorb all the light.
• When the objects allow all the light to pass through them.
• None of these.

Answer: A. Objects can only be seen when light falls on the object and are reflected back to our eyes.

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Does Light Travel in a Straight Line? Can It Be Bent?

Last Updated on Jan 27 2023

A basic principle of physics states that light travels in a straight line. It’s easy to prove, too. Simply shine a light through a parallel series of openings and it will pass through each one successively. You can also see it in real-time when looking at shadows. The division between the lighted area in the background and the object obstructing the light follows the perimeter. 

Keep reading to learn more about how light travels and more pertinent information regarding the subject.

• Light vs. Sound

We can learn more about the properties of light by comparing it to another intangible force: sound. Sound is variable, depending on what it’s traveling through it and its temperature. At 59℉ at sea level, it will go 761.2 miles per h our (mph) . That may sound fast until you start delving into Albert Einstein’s Theory of Special Relativity . Essentially, nothing can exceed the speed of light.

While sound is moving around at 761.2 mph, light is zipping along at a blazing 983,571,056 feet or 186,282 miles per second. It’s worth noting that this measurement is in a vacuum. Light sound, the medium—in this case, air—can affect its speed.

We can put the two properties in perspective with lightning and thunder. Remember that that bolt is moving 186,282 miles per second. The thunder is lagging behind at 1,100 feet per second . Both are moving at a constant speed, making it easy to calculate the distance between the lightning and the clap of thunder. Count the seconds between the two and divide by five to get the number of miles away.

If you’ve seen lightning strike, you’ll notice it’s following a straight path, although it may come at an angle. However, does that mean that light never deviates from this course? The answer is no.

• Scattering the Light

If you’ve seen light shine through a cloud of dust, you may notice that it’s traveling in different directions. That’s the variations in the air medium changing with the suspended particles. Another classic example involves putting an object like a spoon in a glass of water. It will look like it’s bent. What you’re seeing is the difference between traveling through air and water.

Air is composed primarily of nitrogen, oxygen, water, and carbon dioxide. Of course, water is hydrogen and oxygen. However, there’s also the glass, which adds another factor to the mix. A prism will have a similar effect by refracting light into its various colors. However, we still have to dig a bit deeper. All things being equal, can light ever bend on its own?

• Bending Light

Scientists thought that they had solved these riddles until they discovered the Airy waveform in the late 1970s. Researchers found that light could bend ever so slightly. Next, fast forward to 2012. The reason behind the discovery is based on heavy-duty mathematics and physics . Suffice to say that self-bending light is possible, opening up opportunities to use it for various purposes, such as redirecting lasers.

It might not be something that you would ever need to do. However, it does answer some questions. So, yes, light can travel in a straight line and also bend.

• Final Thoughts

Understanding how light and sound travel tells us a lot about physics, mathematics, and science. It also shows us how much we have yet to learn about our planet and its place in the Solar System . While light does travel in a straight line, there are also times when it can bend.

• https://www.livescience.com/37022-speed-of-sound-mach-1.html
• https://www.space.com/36273-theory-special-relativity.html
• https://www.space.com/15830-light-speed.html
• https://www.grc.nasa.gov/www/k-12/airplane/sound2.html
• http://tornado.sfsu.edu/geosciences/classes/m201/Atmosphere/AtmosphericComposition.html
• https://www.britannica.com/technology/prism-optics
• https://www.science.org/content/article/light-bends-itself
• https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.108.163901

Featured Image Credit: donatas1205, Shutterstock

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About the Author Chris Dinesen Rogers

Chris has been writing since 2009 on a variety of topics. Her motto with all of her writing is “science-based writing nurtured by education and critical thinking.” Chris specializes in science topics and has a special love for health and environmental topics, and animals of all shapes and sizes.

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Speed of light not so constant after all.

Pulse structure can slow photons, even in a vacuum

SHIFTING SPEEDS  Even in vacuum conditions, light can move slower than its maximum speed depending on the structure of its pulses. The finding could be important for physicists studying extremely short light pulses.

Jeff Keyzer/Flickr ( CC BY-SA 2.0 )

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By Andrew Grant

January 17, 2015 at 6:00 pm

Light doesn’t always travel at the speed of light. A new experiment reveals that focusing or manipulating the structure of light pulses reduces their speed, even in vacuum conditions.

A paper reporting the research, posted online at arXiv.org and accepted for publication, describes hard experimental evidence that the speed of light, one of the most important constants in physics, should be thought of as a limit rather than an invariable rate for light zipping through a vacuum.

“It’s very impressive work,” says Robert Boyd, an optical physicist at the University of Rochester in New York. “It’s the sort of thing that’s so obvious, you wonder why you didn’t think of it first.”

Researchers led by optical physicist Miles Padgett at the University of Glasgow demonstrated the effect by racing photons that were identical except for their structure. The structured light consistently arrived a tad late. Though the effect is not recognizable in everyday life and in most technological applications, the new research highlights a fundamental and previously unappreciated subtlety in the behavior of light.

The speed of light in a vacuum, usually denoted c, is a fundamental constant central to much of physics, particularly Einstein’s theory of relativity. While measuring c was once considered an important experimental problem, it is now simply specified to be 299,792,458 meters per second, as the meter itself is defined in terms of light’s vacuum speed. Generally if light is not traveling at c it is because it is moving through a material. For example, light slows down when passing through glass or water.

Padgett and his team wondered if there were fundamental factors that could change the speed of light in a vacuum. Previous studies had hinted that the structure of light could play a role. Physics textbooks idealize light as plane waves, in which the fronts of each wave move in parallel, much like ocean waves approaching a straight shoreline. But while light can usually be approximated as plane waves, its structure is actually more complicated. For instance, light can converge upon a point after passing through a lens. Lasers can shape light into concentrated or even bull’s-eye–shaped beams.

The researchers produced pairs of photons and sent them on different paths toward a detector. One photon zipped straight through a fiber. The other photon went through a pair of devices that manipulated the structure of the light and then switched it back. Had structure not mattered, the two photons would have arrived at the same time. But that didn’t happen. Measurements revealed that the structured light consistently arrived several micrometers late per meter of distance traveled.

“I’m not surprised the effect exists,” Boyd says. “But it’s surprising that the effect is so large and robust.”

Greg Gbur, an optical physicist at the University of North Carolina at Charlotte, says the findings won’t change the way physicists look at the aura emanating from a lamp or flashlight. But he says the speed corrections could be important for physicists studying extremely short light pulses.

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Speed of Light Calculator

Table of contents

With this speed of light calculator, we aim to help you calculate the distance light can travel in a fixed time . As the speed of light is the fastest speed in the universe, it would be fascinating to know just how far it can travel in a short amount of time.

We have written this article to help you understand what the speed of light is , how fast the speed of light is , and how to calculate the speed of light . We will also demonstrate some examples to help you understand the computation of the speed of light.

What is the speed of light? How fast is the speed of light?

The speed of light is scientifically proven to be the universe's maximum speed. This means no matter how hard you try, you can never exceed this speed in this universe. Hence, there are also some theories on getting into another universe by breaking this limit. You can understand this more using our speed calculator and distance calculator .

So, how fast is the speed of light? The speed of light is 299,792,458 m/s in a vacuum. The speed of light in mph is 670,616,629 mph . With this speed, one can go around the globe more than 400,000 times in a minute!

One thing to note is that the speed of light slows down when it goes through different mediums. Light travels faster in air than in water, for instance. This phenomenon causes the refraction of light.

Now, let's look at how to calculate the speed of light.

How to calculate the speed of light?

As the speed of light is constant, calculating the speed of light usually falls on calculating the distance that light can travel in a certain time period. Hence, let's have a look at the following example:

• Source: Light
• Speed of light: 299,792,458 m/s
• Time traveled: 100 seconds

You can perform the calculation in three steps:

Determine the speed of light.

As mentioned, the speed of light is the fastest speed in the universe, and it is always a constant in a vacuum. Hence, the speed of light is 299,792,458 m/s .

Determine the time that the light has traveled.

The next step is to know how much time the light has traveled. Unlike looking at the speed of a sports car or a train, the speed of light is extremely fast, so the time interval that we look at is usually measured in seconds instead of minutes and hours. You can use our time lapse calculator to help you with this calculation.

For this example, the time that the light has traveled is 100 seconds .

Calculate the distance that the light has traveled.

The final step is to calculate the total distance that the light has traveled within the time . You can calculate this answer using the speed of light formula:

distance = speed of light × time

Thus, the distance that the light can travel in 100 seconds is 299,792,458 m/s × 100 seconds = 29,979,245,800 m

What is the speed of light in mph when it is in a vacuum?

The speed of light in a vacuum is 670,616,629 mph . This is equivalent to 299,792,458 m/s or 1,079,252,849 km/h. This is the fastest speed in the universe.

Is the speed of light always constant?

Yes , the speed of light is always constant for a given medium. The speed of light changes when going through different mediums. For example, light travels slower in water than in air.

How can I calculate the speed of light?

You can calculate the speed of light in three steps:

Determine the distance the light has traveled.

Apply the speed of light formula :

speed of light = distance / time

How far can the speed of light travel in 1 minute?

Light can travel 17,987,547,480 m in 1 minute . This means that light can travel around the earth more than 448 times in a minute.

Speed of light

The speed of light in the medium. In a vacuum, the speed of light is 299,792,458 m/s.

It’s a wonderful world — and universe — out there.

Come explore with us!  

Science News Explores

Understanding light and other forms of energy on the move.

This radiation includes visible light, radio signals — even medical X-rays

Light is a form of energy created by the movement of electrons. Different wavelengths appear as different colors, although most wavelengths are not visible to the human eye.

Natasha Hartano/Flickr ( CC BY-NC 2.0 ); adapted by L. Steenblik Hwang

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By Jennifer Look

July 16, 2020 at 6:30 am

Light is a form of energy that travels as waves. Their length — or wavelength — determines many of light’s properties. For instance, wavelength accounts for light’s color and how it will interact with matter. The range of wavelengths, from super short to very, very long, is known as the light spectrum. Whatever its wavelength, light will radiate out infinitely unless or until it is stopped. As such, light is known as radiation.

Light’s formal name is electromagnetic radiation. All light shares three properties. It can travel through a vacuum. It always moves at a constant speed, known as the speed of light, which is 300,000,000 meters (186,000 miles) per second in a vacuum. And the wavelength defines the type or color of light.

Just to make things interesting, light also can behave as photons , or particles. When looked at this way, quantities of light can be counted, like beads on a string.

Humans have evolved to sense a small part of the light spectrum. We know these wavelengths as “visible” light. Our eyes contain cells known as rods and cones. Pigments in those cells can interact with certain wavelengths (or photons) of light. When this happens, they create signals that travel to the brain. The brain interprets the signals from different wavelengths (or photons) as different colors.

The longest visible wavelengths are around 700 nanometers and appear red. The range of visible light ends around 400 nanometers. Those wavelengths appear violet. The whole rainbow of colors falls in between.

Most of the light spectrum, however, falls outside that range. Bees, dogs and even a few people can see ultraviolet (UV) light . These are wavelengths a bit shorter than violet ones. Even those of us without UV vision can still respond to UV light, however. Our skin will redden or even burn when it encounters too much.

Many things emit heat in the form of infrared light. As that name suggests, infrared wavelengths are somewhat longer than red’s. Mosquitoes and pythons can see in this range. Night-vision goggles work by detecting infrared light.

Light also comes in many other types. Light with really short, high-energy waves can be gamma rays and X-rays (used in medicine). Long, low-energy waves of light fall in the radio and microwave part of the spectrum.

Desiré Whitmore is a physics educator at the Exploratorium in San Francisco, Calif. Teaching people about light as radiation can be difficult, she says. “People are afraid of the word ‘radiation.’ But all it means is that something is moving outward.”

The sun emits lots of radiation in wavelengths that span from X-rays to infrared. Sunlight provides almost all of the energy required for life on Earth. Small, cool objects release much less radiation. But every object emits some. That includes people. We give off small amounts of infrared light generally referred to as heat.

Whitmore points to her cell phone as a common source of many types of light. Smartphones use visible wavelengths to light up the screen display. Your phone talks to other phones via radio waves. And the camera has the ability to detect infrared light that human eyes cannot see. With the right app, the phone transforms this infrared light into visible light that we can see on the phone’s screen.

“This is fun to try out with your cell phone’s front-facing camera,” Whitmore says. Use a remote control for a television or other device. Its light is infrared, she notes, “so we cannot see it. But when you point the controller at your phone’s camera and press a button, “you can see a bright pink light appear on the screen!”   

“All these different types of radiation help improve our lives,” Whitmore says. They “have been shown to be safe when used in reasonable amounts,” she notes — but can be “dangerous when you use too much of it.”

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How Light Travels: The Reason Why Telescopes Can See the Invisible Parts of Our Universe

Due to how light travels, we can only see the most eye-popping details of space—like nebulas, supernovas, and black holes—with specialized telescopes.

• Our eyes can see only a tiny fraction of these wavelengths , but our instruments enable us to learn far more.
• Here, we outline how various telescopes detect different wavelengths of light from space.

Light travels only one way: in a straight line. But the path it takes from Point A to Point B is always a waveform, with higher-energy light traveling in shorter wavelengths. Photons , which are tiny parcels of energy, have been traveling across the universe since they first exploded from the Big Bang . They always travel through the vacuum of space at 186,400 miles per second—the speed of light—which is faster than anything else.

Too bad we can glimpse only about 0.0035 percent of the light in the universe with our naked eyes. Humans can perceive just a tiny sliver of the electromagnetic spectrum: wavelengths from about 380–750 nanometers. This is what we call the visible part of the electromagnetic spectrum. The universe may be lovely to look at in this band, but our vision skips right over vast ranges of wavelengths that are either shorter or longer than this limited range. On either side of the visible band lies evidence of interstellar gas clouds, the hottest stars in the universe, gas clouds between galaxies , the gas that rushes into black holes, and much more.

Fortunately, telescopes allow us to see what would otherwise remain hidden. To perceive gas clouds between stars and galaxies, we use detectors that can capture infrared wavelengths. Super-hot stars require instruments that see short, ultraviolet wavelengths. To see the gas clouds between galaxies, we need X-ray detectors.

We’ve been using telescopes designed to reveal the invisible parts of the cosmos for more than 60 years. Because Earth’s atmosphere absorbs most wavelengths of light, many of our telescopes must observe the cosmos from orbit or outer space.

Here’s a snapshot of how we use specialized detectors to explore how light travels across the universe.

Infrared Waves

We can’t see infrared waves, but we can feel them as heat . A sensitive detector like the James Webb Space Telescope can discern this thermal energy from far across the universe. But we use infrared in more down-to-Earth ways as well. For example, remote-control devices work by sending infrared signals at about 940 nanometers to your television or stereo. These heat waves also emanate from incubators to help hatch a chick or keep a pet reptile warm. As a warm being, you radiate infrared waves too; a person using night vision goggles can see you, because the goggles turn infrared energy into false-color optical energy that your eyes can perceive. Infrared telescopes let us see outer space in a similar way.

Astronomers began the first sky surveys with infrared telescopes in the 1960s and 1970s. Webb , launched in 2021, takes advantage of the infrared spectrum to probe the deepest regions of the universe. Orbiting the sun at a truly cold expanse—about one million miles from Earth—Webb has three infrared detectors with the ability to peer farther back in time than any other telescope has so far.

Its primary imaging device, the Near Infrared Camera (NIRCam), observes the universe through detectors tuned to incoming wavelengths ranging from 0.6 to 5 microns, ideal for seeing light from the universe’s earliest stars and galaxies. Webb’s Mid-Infrared Instrument (MIRI) covers the wavelength range from 5 to 28 microns, its sensitive detectors collecting the redshifted light of distant galaxies. Conveniently for us, infrared passes more cleanly through deep space gas and dust clouds, revealing the objects behind them; for this and many other reasons, the infrared spectrum has gained a crucial foothold in our cosmic investigations. Earth-orbiting satellites like NASA’s Wide Field Infrared Survey Telescope ( WFIRST ) observe deep space via longer infrared wavelengths, too.

Yet, when stars first form, they mostly issue ultraviolet light . So why don’t we use ultraviolet detectors to find distant galaxies? It’s because the universe has been stretching since its beginning, and the light that travels through it has been stretching, too; every planet, star, and galaxy continually moves away from everything else. By the time light from GLASS-z13—formed 300 million years after the Big Bang—reaches our telescopes, it has been traveling for more than 13 billion years , a vast distance all the way from a younger universe. The light may have started as ultraviolet waves, but over vast scales of time and space, it ended up as infrared. So, this fledgling galaxy appears as a red dot to NIRCam. We are gazing back in time at a galaxy that is rushing away from us.

Radio Waves

If we could see the night sky only through radio waves, we would notice swaths of supernovae , pulsars, quasars, and gassy star-forming regions instead of the usual pinprick fairy lights of stars and planets.

Tools like the Arecibo Observatory in Puerto Rico can do the job our eyes can’t: detect some of the longest electromagnetic waves in the universe. Radio waves are typically the length of a football field, but they can be even longer than our planet’s diameter. Though the 1,000-foot-wide dish at Arecibo collapsed in 2020 due to structural problems, other large telescopes carry on the work of looking at radio waves from space. Large radio telescopes are special because they actually employ many smaller dishes, integrating their data to produce a really sharp image.

Unlike optical astronomy, ground-based radio telescopes don’t need to contend with clouds and rain. They can make out the composition, structure, and motion of planets and stars no matter the weather. However, the dishes of radio telescopes need to be much larger than optical ones to generate a comparable image, since radio waves are so long. The Parkes Observatory’s dish is 64 meters wide, but its imaging is comparable to a small backyard optical telescope, according to NASA .

Eight different radio telescopes all over the world coordinated their observations for the Event Horizon Telescope in 2019 to put together the eye-opening image of a black hole in the heart of the M87 galaxy (above).

Ultraviolet Waves

You may be most familiar with ultraviolet, or UV rays, in warnings to use sunscreen . The sun is our greatest local emitter of these higher-frequency, shorter wavelengths just beyond the human visible spectrum, ranging from 100 to 400 nanometers. The Hubble Space Telescope has been our main instrument for observing UV light from space, including young stars forming in Spiral Galaxy NGC 3627, the auroras of Jupiter, and a giant cloud of hydrogen evaporating from an exoplanet that is reacting to its star’s extreme radiation.

Our sun and other stars emit a full range of UV light, telling astronomers how relatively hot or cool they are according to the subdivisions of ultraviolet radiation: near ultraviolet, middle ultraviolet, far ultraviolet, and extreme ultraviolet. Applying a false-color visible light composite lets us see with our own eyes the differences in a star’s gas temperatures.

Hubble’s Wide Field Camera 3 (WFC3) breaks down ultraviolet light into specific present colors with filters. “Science visuals developers assign primary colors and reconstruct the data into a picture our eyes can clearly identify,” according to the Hubble website . Using image-processing software, astronomers and even amateur enthusiasts can turn the UV data into images that are not only beautiful, but also informative.

X-Ray Light

Since 1999, the orbiting Chandra X-Ray Observatory is the most sensitive radio telescope ever built. During one observation that lasted a few hours, its X-ray vision saw only four photons from a galaxy 240 million light-years away, but it was enough to ascertain a novel type of exploding star . The observatory, located 86,500 miles above Earth, can produce detailed, full-color images of hot X-ray-emitting objects, like supernovas, clusters of galaxies and gases, and jets of energy surrounding black holes that are millions of degrees Celsius. It can also measure the intensity of an individual X-ray wavelength, which ranges from just 0.01 to 10 nanometers. Its four sensitive mirrors pick up energetic photons and then electronic detectors at the end of a 30-foot optical apparatus focus the beams of X-rays.

Closer to home, the Aurora Borealis at the poles emits X-rays too. And down on Earth, this high-frequency, low-wavelength light passes easily through the soft tissue of our bodies, but not our bones, yielding stellar X-ray images of our skeletons and teeth.

Visible Light

Visible color gives astronomers essential clues to a whole world of information about a star, including temperature, distance, mass, and chemical composition. The Hubble Telescope, perched 340 miles above our planet, has been a major source of visible light images of the cosmos since 1990.

Hotter objects, like young stars, radiate energy at shorter wavelengths of light; that’s why younger stars at temperatures up to 12,000 degrees Celsius, like the star Rigel, look blue to us. Astronomers can also tell the mass of a star from its color. Because mass corresponds to temperature, observers know that hot blue stars are at least three times the mass of the sun. For instance, the extremely hot, luminous blue variable star Eta Carina’s bulk is 150 times the mass of our sun, and it radiates 1,000,000 times our sun’s energy.

Our comparatively older, dimmer sun is about 5,500 degrees Celsius, so it appears yellow. At the other end of the scale, the old star Betelgeuse has been blowing off its outer layer for the past few years, and it looks red because it’s only about 3,000 degrees Celsius.

A View of Earth

Scientists use different wavelengths of light to study phenomena closer to home, too.

Detectors in orbit can distinguish between geophysical and environmental features on Earth’s changing surface, such as volcanic action. For example, infrared light used alongside visible light detection reveals areas covered in snow, volcanic ash, and vegetation. The Moderate Resolution Imaging Spectroradiometer ( MODIS ) infrared instrument onboard the Aqua and Terra satellites monitors forest fire smoke and locates the source of a fire so humans don’t have to fly through smoke to evaluate the situation.

Next year, a satellite will be launched to gauge forest biomass using a special radar wavelength of about 70 centimeters that can penetrate the leafy canopy.

💡 Why is the sky blue? During the day, oxygen and nitrogen in Earth’s atmosphere scatters electromagnetic energy at the wavelengths of blue light (450–485 nanometers). At sunset, the sun’s light makes a longer journey through the atmosphere before greeting your eyes. Along the way, more of the sun’s light is scattered out of the blue spectrum and deeper into yellow and red.

Before joining Popular Mechanics , Manasee Wagh worked as a newspaper reporter, a science journalist, a tech writer, and a computer engineer. She’s always looking for ways to combine the three greatest joys in her life: science, travel, and food.

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How to Prove That Light Travels in a Straight Path

Last Updated: April 24, 2024 Fact Checked

This article was co-authored by Chris Hasegawa, PhD . Dr. Chris Hasegawa was a Science Professor and the Dean at California State University Monterey Bay. Dr. Hasegawa specializes in teaching complex scientific concepts to students. He holds a BS in Biochemistry, a Master’s in Education, and his teaching credential from The University of California, Davis. He earned his PhD in Curriculum and Instruction from The University of Oregon. Before becoming a professor, Dr. Hasegawa conducted biochemical research in Neuropharmacology at the National Institute of Health. He also taught physical and life sciences and served as a teacher and administrator at public schools in California, Oregon, and Arizona. This article has been fact-checked, ensuring the accuracy of any cited facts and confirming the authority of its sources. This article has been viewed 213,427 times.

Light is an essential part of your day. It allows you to see objects, shapes, and colors. In fact, the pupils in your eyes filter in light to help you see everything around you. As part of a school assignment, you may be asked to prove that light travels in a straight line. You can do this using basic household items in three easy experiments.

Making a Light Pinhole

• Three index cards.
• A piece of modeling clay or sticky tack. You can also use double sided tape.

• A hole puncher.

• Take the hole puncher and punch a hole at the center of the card where the two lines intersect. Do this for the other two cards.

• Form a stand for the cards using the clay so the cards are straight and upright. Use the ruler to ensure the cards are two to five inches from each other.
• You can also use double sided tape to attach the cards to a surface in a vertical position. Do not cover or obstruct the hole in the center of the cards with modeling clay or tape.

• Note that the light can be seen through all the holes. You should be able to see the light go through all the holes and land on a wall or surface beyond the last index card.

Using a Mirror and a Flashlight

• Two to three sheets of black paper.
• Small objects like buttons, bottle caps, or dimes.

• The other person will use the small mirror to reflect the flashlight so it hits the objects. Move close to the light, at an angle, to catch the light so it hits the objects.
• You may need to position more than one mirror to create a light path that shines on the objects. Play around with reflecting the light on the mirrors until the light hits the objects. You can also move the objects around the room to create a more complicated light path, using the flashlight as the light source.
• This experiment shows that light travels in a straight line in the air. But it also bounces off of a reflective surface, like a mirror. The angle of the light as it bounces off the mirror will be the same as the angle of the light as it hits the mirror. The mirror reflects the light and changes its path from a straight line to an angled straight line.

Using Water and Oil

• A large glass jar.
• Access to water.
• One cup of oil.

• Make sure the jar is large enough to fit the ruler.

• Note that the numbers appear stretched or magnified as the light rays bend in the oil and the water. Move the ruler from side to side to note the different appearances of the ruler numbers in the oil and in the water.
• This will show that light travels at different speeds in different mediums, such as air, oil, and water. It will travel in a straight line in the air, but it will bend when it changes speed due to contact with a certain medium, like oil or water.

Expert Q&A

Things You'll Need

• A piece of modeling clay or sticky tack. You can also use tape.
• A flashlight or a laser pointer.
• A flashlight.
• A small mirror.

You Might Also Like

• ↑ http://www.ducksters.com/science/experiment_light_travel.php
• ↑ Chris Hasegawa, PhD. Retired Science Professor & Dean. Expert Interview. 29 July 2021.
• ↑ https://www.science-sparks.com/science-fair-projects-light-maze/
• ↑ https://www.scientificamerican.com/article/now-you-see-it-testing-out-light-refraction/

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Checked Bag Fees: Airline-by-Airline Guide for US Travel in 2024

C hecking a bag is something frequent flyers try mightily to avoid. It adds time, extra logistics, and cost to air travel. When little ones come along, however, packing light just isn’t always possible. Flying with a baby or toddler requires gear! The time comes for traveling parents to face the reality of needing to check a bag…or two.

So just how much will checked bags set you back? Checked bag fees vary from airline to airline. It is important to know what the airlines charge before booking your flight. You need to consider the costs as you are comparing ticket prices between different carriers to get the best overall deal. This is especially true for families, because bag fees can add up quickly with multiple people. And with many major airlines increasing checked bag fees again in early 2024, the costs grow ever higher.

Last updated : March 6, 2024. (Updated to include increased bag fees on Delta Airlines. Note that American, United, and Alaska all increased their fees in 2024 a few weeks prior. Those increased fees are reflected below.)

Checked Bag Fees on Every U.S. Airline

So just how much do airlines within the United States charge for checked bag fees?

The only major US airline that doesn’t charge checked baggage fees at all is Southwest Airlines . All of the legacy carriers ( United , American, Delta, Alaska, etc.) tend to charge right about the same for their checked bags – approximately $35 for the first bag as of March 2024.

The ultra-low cost carriers (Frontier, Allegiant, Spirit, etc.) have pricier bag fees that are more opaque. Most of these airlines charge based on variables like flight distance or how early you pre-pay for a bag. Some of these airlines have bag fee check tools on their website, so I highly recommend going there first for an estimate before you waste time going through multiple steps in the booking process.

Related: What are the Best Airlines to Fly with Kids in the USA?

Here’s a chart showing the current bag check fees for domestic travel on all the major US carriers.

Note that most airlines classify a standard checked bag as one weighing up to 50 lbs. The standard dimensions for checked bags on most airlines are normally 62 inches maximum, which is total length + width + depth, allowing for multiple bag shapes. A few airlines, however, have even more restrictive weights and dimensions, so I’ve noted those rules below.

Special Checked Bag Fee Rules To Watch Out For

As the chart makes clear, there are lots of asterisks and fine print in the world of checked bag fees. Here are some special bag check situations and rules to look out for so you aren’t surprised and what may cost you more (or conversely, what you might not expect to be free!).

• Pre-Purchased Bags : Quite a few airlines (in particular low cost carriers) charge you more based on when you pay for your checked bag. If you book a checked bag when you purchase a ticket, you’ll often pay quite a bit less. Sometimes there is an intermediate price if you add a bag after you’ve book, such as at online check-in. Whatever you do, never ever wait to pay until you get to the airport ticket counter. That is when prices are highest – sometimes several times more!
• Car Seats and Strollers : Believe it or not, US domestic carriers do not charge anything for checking a stroller and/or car seat . That’s a nice and rare break for traveling parents. Note though that at least one airline doesn’t allow you to gate check strollers that are either non-folding or that exceed 20 pounds (see tips for flying American Airlines with a stroller ).
• Overweight/oversize bags : Oversized or overweight bags or special equipment like skis or golf clubs can really cost you. Most airlines consider any bag over 50 pounds to be overweight (and >62 inches in total dimension to be oversized). But there are a couple of tricky airlines that set the maximum at 40 pounds (ahem, Frontier & Spirit). This maximum is easy to exceed packing a standard sized suitcase with a reasonable amount in it. What airlines charge for oversize bags varies wildly – even more than regular checked bag fees. So check the fine print closely. The chart below summarizes the major overweight/oversize fee rules for each airline.

Ways to Avoid Checked Bag Fees

Although most airlines charge checked bag fees these days, there are increasingly a number of ways to avoid them or at least minimize them. Here are all the strategies my family and I have used to control our bag check costs over the last decade and a half of flying with kids.

Related: 7 Ways to Avoid Airline Fees

Elite Status

If you have elite status on an airline, you probably get at least one free bag (or more), usually for each person traveling on the same reservation with the elite member. Double check the program benefits for the airline you are flying on, however, because airlines offer different benefits at each elite status level and are constantly tweaking these rules in small ways.

Airline Co-Branded Credit Card

If you have one of the airlines’ credit cards, you (and others booked under the same reservation) usually get a free bag if you have booked your tickets with that credit card. Many of these airline credit cards have annual fees of about $100 a year, so if your family checks two bags on a roundtrip vacation on that airline once a year, you’ll at least break even on the fees.

Credit Card Airline Fee Credit

Some credit cards (especially those with higher annual fees targeted at a traveling demographic) come with perks like an airline fee credit. You may be able to charge bag fees to that card and have some or all of the fees refunded. My husband and I have had several cards the past few years that have this benefit that we’ve used to cover fees on airlines we don’t fly as often. These cards aren’t affiliated with a particular airline program but you may have to select your preferred airline (often once a year in January) in order to access the credit.

Fare Types that Include Free Checked Bags

A number of airlines have fare types or bundles that come with one or more extras. One of the included perks may be a checked bag or two. You’ll of course pay more for these fare types than the cheapest economy ticket, but there may be savings in the overall bundling, especially if you need some of the other perks too like seat assignments or a larger carry on bag.

Premium Cabins

If you are lucky enough to be traveling on a business class or first class ticket, you usually will not have to worry with bag fees. If you are booking these kinds of fares with cash though, you’ll nearly always be paying much more for these cabins, so the “savings” aren’t necessarily savings. Often first and business class passengers are able to check bags that weigh more than the standard sizes (sometimes 70 pounds instead of a 50 pound maximum).

Last but certainly not least, families can often save on bag fees just by packing strategically. Don’t underestimate the savings to be had just by sharing bags or smartly distributing your packed items between and among checked vs. carry on bags, based upon what the airline charges for each.

For example, some legacy carriers like American or Delta don’t charge for carry ons, even for passengers purchasing basic economy tickets. Families may want to carry on more bags when flying those airlines to save on checked bag fees.

Conversely, some ultra low cost carriers charge more for a roll-aboard carry on than a larger checked bag. Families may save more packing clothes for everyone in one or two checked bags instead of bringing larger carry ons at all.

The post Checked Bag Fees: Airline-by-Airline Guide for US Travel in 2024 appeared first on Trips With Tykes .

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Google's top 10 summer travel destinations for 2024

Precious Rongmei , TIMESOFINDIA.COM , TRAVEL TRENDS , WORLD Created : Jun 25, 2024, 10:00 IST

Summer is already here, and most of us are busy making summer travel plans. While we are forever in love with domestic travel, it’s always good to add international travel every now and then. Whether you're looking to immerse yourself in bustling cityscapes, lounge on pristine beaches, or delve into rich cultural experiences, there's a perfect destination awaiting you. Google’s top picks for summer 2024 encompass a diverse range of experiences, from vibrant urban hubs to serene natural escapes, ensuring every traveller finds their ideal getaway. Here are the top 10 summer destinations for 2024:

London, United Kingdom

London, a vibrant city blending history and modernity, offers iconic landmarks like the Tower of London, Buckingham Palace, and the British Museum. Summer in London is perfect for picnics in Hyde Park, outdoor markets, and world-class theatre in the West End. The city's diverse culinary scene and bustling nightlife make it a dynamic summer destination.

Paris, France

The City of Light dazzles visitors with its romantic ambiance, stunning architecture, and rich culture. Summer in Paris means strolls along the Seine, picnics by the Eiffel Tower, and exploring charming neighbourhoods like Montmartre. Don’t miss the Louvre, Notre-Dame, and the chic cafes that line the streets.

Tokyo, Japan

Explore historic temples like Senso-ji, cutting-edge districts such as Shibuya and Akihabara, and serene gardens like Shinjuku Gyoen. The city's culinary scene, from sushi to street food, is a treat for all food lovers.

Rome, Italy

Rome, a city steeped in history and culture, offers awe-inspiring sights like the Colosseum, Vatican City, and the Pantheon. Summer is ideal for exploring its ancient ruins, indulging in gelato, and enjoying outdoor cafes. The city's charming piazzas, beautiful fountains, and lively street life create an unforgettable summer experience.

New York, USA

The Big Apple buzzes with energy and excitement, especially in summer. Iconic landmarks such as the Statue of Liberty, Central Park, and Times Square draw visitors from around the world. Summer events like outdoor concerts, rooftop bars, and festivals add to the city's dynamic atmosphere. Don’t forget to explore diverse neighbourhoods and savour the eclectic food scene.

Cancun, Mexico

Cancun is synonymous with pristine beaches, turquoise waters, and all-inclusive resorts. Summer in Cancun means enjoying water sports, exploring Mayan ruins like Chichen Itza, and indulging in vibrant nightlife. The Riviera Maya offers stunning natural beauty, from cenotes to coral reefs, making it a paradise for adventure and relaxation.

Orlando, USA

Orlando is a dream destination for families and thrill-seekers. Home to world-famous theme parks like Walt Disney World, Universal Studios, and SeaWorld, it offers endless entertainment. Summer is perfect for water parks, outdoor adventures, and exploring the nearby natural springs.

Las Vegas, USA

Las Vegas, the Entertainment Capital of the World, offers more than just casinos. Summer in Vegas is about pool parties, spectacular shows, and fine dining. The Strip dazzles with its bright lights and extravagant resorts, while nearby attractions like the Grand Canyon and Red Rock Canyon provide stunning natural escapes.

Seattle, USA

Seattle combines urban sophistication with natural beauty. Summer brings mild weather perfect for exploring Pike Place Market, the Space Needle, and the Museum of Pop Culture. The city's waterfront, vibrant neighbourhoods, and outdoor activities like hiking and kayaking in the nearby mountains and lakes make it a delightful summer destination.

Athens, Greece

Athens, the cradle of Western civilisation, boasts a blend of ancient history and modern culture. Summer is ideal for exploring iconic sites like the Acropolis, Parthenon, and ancient Agora. The city's lively street markets, delicious Mediterranean cuisine, and vibrant nightlife add to its charm. Nearby beaches and islands provide perfect spots for relaxation.

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Planning on traveling for the Fourth of July holiday? Here’s how to avoid the rush

FILE - Motorists head southbound in the local and express lanes on Interstates 90-94 in slow and thickening traffic as a CTA train enters a station on the first day of the Fourth of July holiday weekend, July 1, 2022, in Chicago. Millions of Americans are preparing to get out of town sometime in the coming Fourth of July holiday week, which will likely mean busy roads as well as packed airports and train stations. (AP Photo/Charles Rex Arbogast, File)

FILE - Travelers walk with their luggage through Union Station in Washington ahead of the Fourth of July holiday, July 1, 2023. Millions of Americans are preparing to get out of town sometime in the coming Fourth of July holiday week, which will likely mean busy roads as well as packed airports and train stations. (AP Photo/Stephanie Scarbrough, File)

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NEW YORK (AP) — The Fourth of July is right around the corner, and the travel rush is already heating up.

Millions of Americans are preparing to get out of town sometime in the coming holiday week. That will likely mean busy roads, as well as packed airports and train stations.

Motor club AAA projects that some 70.9 million travelers will head 50 miles (80 kilometers) or more from their homes over a nine-day Independence Day travel period — surpassing pre-pandemic numbers for the U.S. holiday. And the Transportation Security Administration expects to screen over 32 million individuals in airports from this Thursday through July 8, up 5.4% from last year’s numbers.

Are you traveling for the Fourth? Here’s a rundown of what you need to know.

When is the best time to hit the road for July Fourth?

Smooth sailing for travel around any holiday is never a given. But avoiding the most hectic times, when others are rushing out of town, is a good way to start.

If you’re traveling by car for the Fourth of July, it’s best to hit the road in the morning, according to transportation data and insights provider INRIX. Peak traffic congestion varies by location, INRIX data published by AAA shows, but the worst times to drive on, or leading up to, the holiday are generally between 2 p.m. and 7 p.m. Either way, be prepared for the roads to be jammed.

“Road trips over the holiday week could take up to 67% longer than normal,” Bob Pishue, transportation analyst at INRIX, said in a prepared statement.

July Fourth falls on a Thursday this year, and many travelers will likely take Friday July 5th off to extend their trip into a four-day weekend. Drivers in large metro areas can expect the biggest delays on Wednesday July 3 and Sunday July 7 — as travelers leave and return to town, Pishue added.

And if you’re renting a car ahead of July Fourth, the busiest pickup days will be Friday, Saturday and Wednesday before the holiday, AAA notes.

When will airports be busiest?

Airports will also likely be packed all week long — but the TSA expects most people will take to the skies on Friday.

It anticipates that it will screen more than 3 million individuals Friday. That would surpass the agency’s current record for most people screened on a single day, which reached just under 3 million last Sunday.

“We expect this summer to be our busiest ever,” TSA Administrator David Pekoske said, adding that travel typical peaks around Independence Day.

Last year, the busiest day for Fourth of July air travel was also the Friday ahead of the holiday, TSA data shows. If past trends hold, travel will likely be higher on the days before and after the Fourth — particularly closer to the weekend. In 2023, for example, more than 2 million people were screened on the Fourth, which landed on a Tuesday last year, down from 2.88 million the Friday before.

What should I do if my flight is delayed or canceled?

Flights can be delayed or canceled for an array of reasons — from plane-specific mechanical problems to major storms impacting popular travel paths.

If your flight is canceled, airlines are required to provide refunds for customers, even if the cancellation is due to weather . Delays are trickier, because they typically have to meet certain criteria for relief, such as refunds or compensation — but carriers will often give customers to chance to switch to alternative flights, if available, at no cost.

In April, the Biden administration issued final rules that include requiring airlines to provide automatic cash refunds within a few days for canceled flights and “significant” delays. Those rules are set to take effect over the next two years, but the Department of Transportation has a site that lets consumers see the commitments each airline has made for refunds and covering other expenses when flights are canceled or significantly delayed.

Always check your itinerary before leaving home

It’s better to be stuck at home than locked in hourslong traffic or stranded in an airport terminal. Before heading out the door this holiday week, do yourself a favor and check the status of your travel plans.

Was your flight, train or bus ride delayed? Are there are traffic incidents set to disrupt your drive? And what about the weather? A quick look through your itinerary — such as trip updates on a carrier’s website — checking weather forecasts and monitoring traffic safety through services like the 511 hotline or your phone’s navigation apps can go a long way toward avoiding travel misery.

Here are a few more tips to keep in mind:

— Leave early: There are more people everywhere during a holiday week, so lines will be longer and roads will be busier. Give yourself more time to get to your destination or to make your way through airport security.

— Keep an eye on the weather — and not just for your destination: Look at the weather for your entire travel path. Even if it’s sunny skies both at home and the place you’re headed, it’s important to keep an eye out for any storms in between. You may need to do some rerouting.

— Be kind: A trip delay or cancellation can be really frustrating — but if you’re running into disruptions, chances are others are too. Customer service agents have a lot on their plate at this time of year, and it’s important to be patient and respectful as they try to help you.

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NBC Select Travel Awards 2024: The best hardshell and softshell suitcases

At NBC Select, we spend hours researching, testing and reviewing products so that you can be more selective and informed about the things you buy. For our first-ever NBC Select Travel Awards, we spent months thoroughly testing travel gear covering various categories.

We traveled across the globe to Tokyo with hardshell suitcases and packing cubes , commuting to and from our New York City office with weekenders and duffel bags and tried toiletry bags , noise-canceling headphones and more.

After all of our testing, we collected thoughtful feedback for every product. After sifting through all editors’ reviews, we found the products deserving of the Travel Award winner’s stamp of approval.

Below, we rounded up the best hardshell and softshell suitcases, plus information about how we evaluated each product.

SKIP AHEAD How we picked the best hardshell and softshell suitcases | Best hardshell suitcases | Best softshell suitcases

select We took over 30 duffels, backpacks and weekenders on planes, trains and more for our Travel Awards. These are the best

How we picked the best hardshell and softshell suitcases.

Arguably one of the most important types of bags you’ll take with you on a long-haul trip, a great hardshell and softshell suitcase should be durable, spacious, easy to maneuver and able to hold up well against scratches, scuffs and potential damage (especially if you regularly check in your bags).

• Durability: Overall, we wanted to see that once the bag was fully packed, it was easy to pull and roll around on various surfaces, including carpet, tile and pavement. We also wanted to see if the fully packed suitcases held their shape well with items like clothes and shoes inside. Plus, it was important to ensure that the suitcases held up well against elements such as water or accidental spills.
• Maneuverability: Similar to durability, we prioritized maneuverability in the hardshell softshell suitcases we tested. We wanted to ensure the handles weren’t flimsy or prone to breaking, especially after pulling and handling the bag for hours. Additionally, we wanted to make sure the suitcases were relatively easy for the average person to lift and carry if they plan on using them as carry-ons.
• Spaciousness: We wanted to make sure the suitcases we awarded had sufficient room to hold common articles of clothing such as t-shirts, pants, shorts, underwear, and certain types of shoes. Ideally, the bag would hold at least a few days' worth of outfits. We also wanted to make sure they were compatible with packing cubes, which help to keep belongings more organized. 

Best hardshell suitcases 

Best overall hardshell suitcase: away the bigger carry-on.

Away The Bigger Carry-On

Away’s The Bigger Carry-On is the best overall hardshell suitcase because it delivered when it came to all of our testing criteria. This Away suitcase, as the name implies, is a subtly larger version of the brand’s standard carry-on suitcase. The bag can fit multiple packing cubes and has ample room for clothing, toiletries and shoes. It also comes with a small bag for storing dirty laundry.

Additionally, it comes with a TSA-approved lock, which NBC Select associate reporter Bianca Alvarez found useful during her travel testing.  “I loved using this carry-on and was able to pack roughly around 10 days worth of clothes with the help of packing cubes,” she says. “I also thought that the inside features were convenient.” Aside from space for clothing, it also has straps and buckles, which help flatten and secure the items in the bag, which makes it easier to close. The handle is easy to adjust and the wheels roll smoothly and handle various surfaces well, whether upright or at an angle.

Best lightweight hardshell suitcase: Quince 21” Carry-On

Quince 21” Carry-On

During our testing, we found that the Quince 21” Carry-On balances durability and maneuverability well. While the suitcase keeps belongings safe and secure, it’s also easy to lift and place in an overhead compartment or into the trunk of a car, especially when it is fully packed. NBC Select associate updates editor Zoe Malin tested the bag and loved how well it handled different terrain. “I dragged this bag through the streets of New York, on multiple flights, checked and not checked, and dragged it up flights of stairs and it looks great,” she says. While the bag is prone to some scrapes (the case with many hardshell bags), it comes with a small sponge to help wipe scuffs away.

Best budget hardshell suitcase: Bagsmart Carry-On  

Bagsmart Carry-On

The Bagsmart Carry-On can hold a surprisingly large amount given its size, according to NBC Select managing editor Leah Ginsberg, who tested the bag. “I was really impressed by this carry-on. Its dimensions are smaller than the average carry-on,” she says. “But it held a lot of things and the quality is good for the price.” The suitcase is slightly smaller than the standard carry-on size for most major airlines but that doesn’t mean you have to sacrifice space for storing clothing or other travel items. It also has 360-degree wheels, an adjustable handle with four heights, TSA-approved locks on the zippers, multiple compartments for organizing clothes and an expandable zipper on the side.

Best softshell suitcases 

Best overall: thule subterra 2 carry-on spinner.

Thule Subterra 2 Carry-On Spinner

Thule’s Subterra 2 suitcase is the perfect softshell carry-on for your travel needs, whether you’re using it for a quick trip or taking it on a long-haul vacation. Former NBC Select editorial operations manager Shari Uyehara says that while testing the bag, it “helped make traveling a lot easier” — she loves its sleek, stylish design, and how well it’s held up after multiple trips. It’s made from a polycarbonate shell and a durable nylon/polyester blend that’s designed to be water-resistant and to withstand the impact of travel wear and tear, according to the brand.

The suitcase also includes an easily-accessible removable panel that compresses your clothes to maximize packing space, which Uyehara says she loves most of all. “I love how spacious it is and that it can easily hold all of my items,” she says. She also says that, compared to other carry-on suitcases she’s tried, this one is lightweight and very easy to wheel around.

Best lightweight: Calpak Luka Soft Sided Carry-On

Calpak Luka Soft Sided Carry-On

Malin, who’s never used soft-shell luggage before, says that this suitcase is “ideal to use for a short trip”, because of its spacious yet lightweight design. When testing the suitcase for a weekend trip, Malin says she fit three full day outfits, three pairs of pajamas and three workout outfits inside of the suitcase’s main compartment, plus underwear, bras and socks.

It’s made from Calpak’s puffy, polyester Luka material, which the brand says is designed to withstand travel — the flexible material also makes it easier to close (and overstuff, according to Malin). The carry-on also has 360-degree spinner wheels and two front pockets with a padded laptop compartment, which Malin says is her favorite part of the bag. “The sleeve makes it really easy to take my laptop out, which would be super helpful if I was going through security at the airport,” she says. Malin says she can store extras like her resistance bands and Bala bangles in the larger part of the pocket, and it’s an easy bag to wipe clean.

Why trust NBC Select?

Products underwent an eight-week trial before being selected as our favorite in their respective categories. We utilized our editors as shopping and trying experts. Each editor was given the same criteria during the testing period to ensure each item was held to the same standards and procedures. We looked at spaciousness, durability, comfort, zippers and much more. You can read more about our process here .

What our badge means

When a product carries the NBC Select badge, you can trust that our team of editors as shopping experts and professionals vetted the item thoroughly. First and foremost, we are journalists, so we will always do our research and reporting.

Catch up on NBC Select’s in-depth coverage of personal finance , tech and tools , wellness and more, and follow us on Facebook , Instagram , Twitter and TikTok to stay up to date.

Select Staff on NBC News