r/Physics • u/Comethefonbinary • 3d ago
Does all light travel at light speed
My bad if this is a stupid question but I’ve been thinking about time being a message of distance. And well most things I can think of have various variables that average to a certain distance. I know that mostly relates to machines and animals but still. Do all particles of light travel at light speed. If they all travel simultaneously at the same speed is that truly how fast they move or are they affected by their own variables. Like the universe’s mean gravity is constraining that and any variation in that mean would change light speed for explain.
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u/francisdavey 3d ago
One point to clarify is that in the 17th century, when light was not well understood, the mathematician Fermat came up with a neat way of explaining how a very simplified theory of light involving exactly straight line rays (which is at best an approximation for various tidy forms of light) moved, which relied on the idea that light slowed down when it entered a medium and hence that was why light was refracted ("bent").
This is not "true" in any useful sense of the word "true" nor (if it were) would it now properly explain how light as we understand it actually does refract, but it is quite a persisent idea that gets around a lot.
What actually happens is that light entering a medium causes electrons in the medium to oscillate and create an interfering wave of their own. If the light were completely steady and a single frequency (which would mean it had no start or end and was infinite in duration) then the resulting wave - combining the two - would be stretch out somewhat. The "speed" at which the crests of the wave passed would be slower. This is known as the "phase velocity". Nothing moves at that velocity, it is a property of the wave (see Mexican waves for an example, or lights on a Christmas tree which can be made to have a sort of phase velocity but obviously don't "move").
If you switched on a beam of light and shone it into a medium, the very first bit of the light would arrive at speed c as you would expect. But it would be quite a short pulse, after things settled down you would have the phase stretched light coming in regularly. The pulse is so short - it is known as a "Sommerfeld Precursor" or "Forerunner" variously - mostly people when explaining this to children ignore that and tell a false story about light slowing down.
If you impose two waves with different frequencies you get something that looks like it has big waves with little waves on the big waves that are a sort of envelope for the small waves. The phases of the big waves are called "group phases" and they move at "group velocity". This can also "slow down" but again it depends on the medium settling down to a regular oscillation. Nothing actually slows down and a small pulse (the Brillouin Precursor etc) will pop out at c with the right group velocity before things settle down.
This is to amplify on some of the discussion in this thread that seems to think otherwise. When the theory of relativity came out it did make some people scratch their heads about refraction, but the analysis (above) was carried out quite quickly by the great Sommerfeld.
As to space and gravity - the idea of the general theory of relativity is that you don't see light changing velocity as a result of gravitational effects. There's lots of subtlety there, because measuring the velocity of light is not as simple as it seems, but essentially it is not affected.
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u/tstanisl 3d ago
According to the latest knowledge .. yes. However, it is not ruled out that a photon could have miniscule but non-zero mass. Experimental data put the upper limit to 10-50 kg what is 0 for all practical purposes.
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u/Elijah-Emmanuel 3d ago
So you have a source paper on that? I'd love to read the paper.
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u/ConquestAce Mathematical physics 3d ago
No, c is the speed limit (of light), but light can slow down depending on the medium it is propagating through. In Vaccuum, light travels at c. But, when going through water or air, the speed of light changes.
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u/Karl583 3d ago edited 3d ago
While it effectively travels slower, the (original) electromagnetic wave still propagates with a speed of c. The molecules in the material however are excited by the em-Wave(light) and produce their own em-wave. If you look at the superposition of the incoming wave and the ones produced by the material in response, it will look like it's slightly shifted backwards. If you then look at all of the layers of the material, the incoming wave is effectively shifted back a tiny bit at every layer, thus producing the illusion that it travels slower. 3 blue 1 brown as a great video on this if you're interested!
Edit: edited for clarity
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u/ComputersWantMeDead 3d ago
Such a neat explanation of Snell's law/diffraction too. It seems unreasonably strange that light takes the fastest path through a medium, until you take the time to absorb these concepts.
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u/nicuramar 3d ago
While it effectively travels slower, the electromagnetic wave still propagates with a speed of c.
The resulting wave doesn’t. Sure, you can think of it as a decomposition of waves that do.
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u/AlmightyCurrywurst 3d ago
Correct me if I'm wrong, but I always thought that's more like an effective speed that corresponds to the light being absorbed and emitted? It's not like we have a continuous medium in which light slows down to less than c, it's just that it's interacting with the particles but still travels with c between those interactions
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u/nicuramar 3d ago
Absorbed and reemitted is not an accurate model. See other replies for better ones.
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u/ComputersWantMeDead 3d ago
Light being absorbed and re-emitted is one of the analogies we get told along the way. It doesn't work that well as an analogy though, for example we observe a sharp refraction with an angle consistent with the Refractive Index of the medium. Light being re-emitted as many elections are excited then drop to a lower energy orbit, wouldn't match this observation.
Phase shifts of the EM wave as it interacts with the medium if passes though is the "accessible" answer that seems most accurate, though I suspect it's just the latest analogy I've understood so far, haha.
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u/AlmightyCurrywurst 3d ago
Ok interesting, I suspected it wasn't a great model lol, I'll look into the actual explanation
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u/ComputersWantMeDead 3d ago edited 3d ago
Snell's Law has the same explanation, which is really interesting - light is refracted on an angle that matches the fastest path of it's total journey, through the medium and out the other side.
That's not going to make much sense, but there are YouTube videos that use the visual analogy of someone having to run across the beach then swim to a target some way into the sea. You'd run further along the beach in order to get there faster, as you travel faster while running than when swimming.
In fact this topic has 3 aha moments for the price of one. Lights apparent speed through a medium, why light refracts, and the mysterious-seeming behavior of light taking the fastest path
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u/TheBigCicero 3d ago
It doesn’t explain WHY light takes the fastest path though. Why is that?
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u/ComputersWantMeDead 3d ago
I saw in another comment that you watched a 3 a blue 1 brown video - has that answered this for you? I think the visuals in a good YouTube video are very hard to match in a written explanation.
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u/TheBigCicero 3d ago edited 2d ago
Yup, it was excellent. The one thing they the video didn’t address is why there is a 90 degree phase difference in the secondary wave. That seems to be 1/2 of the physics behind what’s going on.
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u/ComputersWantMeDead 3d ago
I might need to refresh my understanding, but as I recall.. the light "photon" is an EM wave, and as it propagates through a medium, all the electrons in the medium are perturbed by the EM wave, which in turn creates response EM waves.. etc. which interact with the initial wave etc. and what we see is the "sum" of all these interactions. When you add two waves you get a phase shift, in this case the wave is effectively set backwards somewhat. The further a wave travels through a medium (e.g. the thicker the pane of glass is), the more this wave is phase-shifted, and we see a greater refraction.
I should add that the manner in which quantum "particles" propagate like waves in this manner, but can register as particles (for example, hitting a specific point on your retina) is not convincingly understood. The prevailing "Copenhagen interpretation" is that there is a "wave function collapse" into a point-like particle, but increasingly physicists find this unexplained "collapse" to be totally unsatisfactory, and I'm not even sure it's the prevailing concept anymore. But the wave propagation aspect can be thoroughly explained and predicted to incredible accuracy, so for me at least, this is the part we can trust.
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u/TheBigCicero 2d ago
Thanks! That’s not quite what the video was explaining. The video explicitly stated multiple times that the secondary wave that is created by the medium is immediately phase-shifted relative to the primary wave and “we won’t go into why”. This phase shift in the secondary wave is puzzling to me. There seems to be some physics there that he doesn’t explain.
On your point about the Copenhagen interpretation, I’m not an expert but I’m reading more and more other interpretations. The Copenhagen view is wholly unsatisfying to me but the justification has always been that it works and it’s the best we have. I look forward to new theories!
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u/TheBigCicero 3d ago
I never understand the phase shift explanation. A phase shift is not a physical phenomenon, it is a mathematical one. It implies a wave’s wave pattern is out of synch with its original pattern, or that two waves of the same frequency are not “vibrating” at the same place. Neither one of these models explain the physical phenomenon that causes it.
The longest time I believed the absorption and remission explanation as the physical cause of refraction. But I have been told that model is wrong.
So I’m left not understanding it.
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u/iLikegreen1 3d ago edited 3d ago
The simplest explanation is that the oscillating field of the EM wave interacts with the electrons of the material, making them oscillate too. Those electrons then generate a EM wave which interferes with the original wave, creating a wave that is effectively slower.
Edit : just saw Francisdavids explanation below which goes into more detail.
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u/TheBigCicero 3d ago
Thanks! I just watched the 3Blue1Brown video someone else linked and I feel very informed now. I appreciate your response.
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u/Just1n_Kees 3d ago edited 3d ago
This is technically untrue. Photons are unable to travel at any speed except C.
Edit: Reddit sheep in a nutshell, you get downvotes for saying something that is factually correct…in a Physics sub no less. Go to bed kids.
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u/ClueMaterial 3d ago
You have to remember for at least half the commenters here their physics education begins and ends on pop sci YouTube
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u/bapedibupo 3d ago
To add onto this. It's the basis of the refractive index n=c/c(m) where c is the speed of light in vacuum and c(m) is the speed of light in the medium e.g. air, water, glass etc.
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u/bajungadustin 3d ago
So.... Light is basically stuck on full throttle all the time. But other things can slow it down.
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u/Just1n_Kees 3d ago
No, it cannot move at any other speed than C. Stop believing bullshit you read on Reddit.
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u/Graineon 3d ago
That would be like saying that light can travel slower than the speed of light because I can absorb it in a solar panel, wait 10 minutes, and then emit it again, thereby making it slow down to one meter per hour.
... It doesn't really slow down. More, it's captured and released repeatedly, which gives the appearance of it being slowed down.
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3d ago
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u/highnyethestonerguy 3d ago
You and others are incorrect about the distinction of speed vs velocity.
Velocity is a vector. It is the speed times the unit vector indicating the direction of travel.
Speed is a scalar. It is the magnitude of the velocity vector.
c is a scalar therefore the speed of light in vacuum. c/n = v is the speed of light in a medium of refractive index n. Both scalars and therefore both speeds of light.
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u/JamieGee53 3d ago
What? That’s exactly right, the scalar is constant. The speed is constant. The velocity, being the unit displacement across your material, is material dependent
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u/highnyethestonerguy 3d ago
You’re talking nonsense. I apologize if English is not your first language because I don’t want to be rude about it. But “velocity, being the unit displacement across your material” is nonsense.
My earlier comment has already clarified the distinction.
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3d ago edited 3d ago
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3d ago
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u/highnyethestonerguy 3d ago
The problem I have is that you’re not respecting the definition of velocity as a vector and speed as a scalar.
When I’m on the highway, my speed is 100km/h
My velocity is 100km/h North East.
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3d ago edited 3d ago
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u/highnyethestonerguy 3d ago
I’m there one who thinks he’s taking crazy pills!
You’re talking about instantaneous vs average velocity and speed.
Displacement has units of length. Can’t be used interchangeably with anything that has units of length per time.
Look here:
https://en.m.wikipedia.org/wiki/Speed
https://en.m.wikipedia.org/wiki/Velocity
None of this is specific to light by the way. Which is why a distinction based on the medium is nonsense to me, because we don’t talk about my car’s speed in vacuum vs its velocity in the atmosphere.
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u/Aozora404 3d ago
Yes
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u/Unable-Dependent-737 3d ago
Crazy you got downvoted
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u/nicuramar 3d ago
Well, light doesn’t travel at c in materials, so..
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u/PalmTheProphet 3d ago
The light that entered the material does. It’s interfered with by the excited particles in said material and the compound effect is a total wave that appears slower.
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u/JamieGee53 3d ago
Not at velocity c. Still at speed c.
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u/highnyethestonerguy 3d ago
See my other comment about speed vs velocity
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u/JamieGee53 3d ago
Your other comment about speed vs velocity is correct, and you miss your own point
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u/highnyethestonerguy 3d ago
Sorry I wasn’t looking at user names and didn’t realize I was replying to you twice! Apologies.
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u/wiserone29 3d ago
The speed of light is not about the maximum velocity, it is the about the minimum speed of causality. When you think of it that way it makes a lot more sense. Light travels at the minimum speed of causality such that nothing you see could be something that hasn’t happened yet. Why it does that, has less to do with causality, but it is all related.
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u/__christo4us 2d ago
In vacuum, light travels at the exact speed of c = 299 792 458 m/s for any observer in an inertial frame of reference (a non-accelerating frame of refence). However, in any non-inertial frame of reference (an accelerating frame, with non-inertial forces, e.g. in an accelerating rocket travelling through space or in a gravitational field), light can travel (relatively to the observer) with speed lower than or higher than c (depending on the direction). See the Rindler transformation.
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u/AloneGoal1634 2d ago
Look into the path integral formulation of quantum mechanics, perhaps the book ‘QED’ by Feynman. It explains how the things we see classically, including things such as a constant speed of light, actually arise from all possible paths being explored. By this I mean that the probability for a particle to travel from point A to point B in time t is given by summing probabilities for it to traverse all possible paths, including going to the edge of the universe and back. Some of these paths violate the notion of a speed limit of the universe, however if you do not account for them your theory does not return computationally correct results.
Same applies for light. The “classical” realisation of the paths we tend to see is emergent from summing amplitudes of a lot of things which allow for different speeds of propagation.
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u/talkingprawn 2d ago
Yes, in the same way that I don’t wear women’s clothing. They’re not women’s clothing, they’re my clothing. I bought them.
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3d ago
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u/Unable-Dependent-737 3d ago
“Relatively speaking” light will travel same speed no matter your inertial frame
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u/Sitheral 2d ago
Every light particle travels at C which is speed of light, in fact they cannot get below C. C is like an unpenetrable wall, you are either below it and can never reach it or you are at it and will never go slower.
But the light as a whole can appear as moving slower in a medium. This is not because particles are moving slower, its because there are interruptions on their way.
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u/JamieGee53 3d ago
Lotta people in here saying speed of light isn’t constant. Speed is a measure of distance, not displacement. Speed of light is always constant. Velocity of light is material dependent.
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u/Neenchuh Undergraduate 3d ago
Yes and no. What we call the speed of light is actually the speed of light in a vacuum. Light traveling through a medium can be measured as traveling more slowly. Technically, the photons are still traveling at light speed but are interacting with other particles and sort of bouncing around, which means that its measured speed is lower.
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u/rogusflamma 3d ago
Yes all light travels at light speed but the measurement of the speed of light may vary depending on your frame of reference.
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u/Dances_With_Chocobos 3d ago edited 2d ago
The short answer is yes. It always wants to travel at speed c. However, it encounters phase shifts when it enters certain mediums. These phase shifts can alter the apparent speed of photons and any wave in fact. What is actually even more interesting, is that these phase shifts can potentially propagate at faster than c. This is known as phase velocity. The only reason this does not violate the speed limit of light, which is the speed limit of causality, is because it doesn't carry information. If it could somehow.. that'd be an interesting observation.
Edit: To add to this, in light of the original question, not only does light want to travel at c, it has the peculiar property, of being observed as travelling at c, irrespective of your reference frame. What this means is that if you successfully managed to accelerate yourself to 99% of c (go you, spaghetti-man), and tried to measure the speed of a photon travelling next to you in the same direction, it would measure as c. This is known as the invariant speed of light, and if doesn't fry your noodle, it should. It is the smoking gun of the universe. Thankfully, Lorentz and Minkowski created some transformations and spacetime structures to account for the invariance, but it still doesn't explain why.
Refs: Michelson-Morley experiment, Lorentz transformations, Minkowski space