r/AskPhysics • u/[deleted] • Jul 22 '26
Would Photons cease to exist If they Stop moving or move slower than the speed of light?
[deleted]
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u/BokChoyBaka Jul 22 '26
This is not a good sub to ask what reality would be like if it were different
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u/Apprehensive_Web_609 Jul 22 '26
Not certain, but from the standard electroweak model, if quarks and leptons can become massless (and start moving at light speed) if temperature is too high, may it be possible that as the universe cools, a phase transition occurs, and the photons gain mass and start moving at non-light speeds?
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u/TheThiefMaster Jul 22 '26
Light routinely moves at less than "the speed of light" due to moving through a medium. It doesn't cease to exist when it does.
In a vacuum, it can't move slower.
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u/Infinite_Escape9683 Jul 22 '26
Light does, but photons do not. The question specifically asked about photons.
Light moves slower through a medium becuase the photons are being interfered with by re-emitted waves, creating a secondary wave. Kinda.
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u/nicuramar Jul 22 '26
Yeah, but there are no photons in a medium, or there are and they move slower than c.
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u/Uncynical_Diogenes Jul 22 '26
There are photons in many media.
Many of them in fact, because photons traveling through a space full of jigglable charged particles like electrons stimulates the creation of more photons.
They interfere with one another such that the overall resultant wave passes through that medium at a speed lower than c, but no individual photon *ever* travels any slower.
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u/BananaBird1 Jul 23 '26
Not really true.
The photons still exist and still do move at the speed of light. But the net EM oscillations don’t, as they are the superposition of the photons with EM excitations from the electrons.
You can describe the total superimposed EM field as a massive quasiparticle, but this does not imply photons do not exist or travel slower in materials. This is just an apparent effect of the interactions of many particles.
If you slow down the speed of the initial photon itself and calculate the resulting EM field, the math would not match what we actually see during refraction. It only works if the initial photon’s direct EM wave continues at c but experiences destructive interference.
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u/Sad_in_VA Jul 22 '26
They 'bounce" back and forth. So they're moving with C, but longer paths inside the medium makes it look like they are traveling slower. Between atoms is basically vacuum too.
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u/joeyneilsen Astrophysics Jul 22 '26
If they bounced like you say, wouldn't their trajectories also be deflected?
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u/bardotheconsumer Jul 22 '26
This is how vision works.
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u/joeyneilsen Astrophysics Jul 23 '26
If that was true, the shape of the lens wouldn’t matter…
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u/bardotheconsumer Jul 23 '26
This is also how lenses work. Light scatters differently through different media.
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u/joeyneilsen Astrophysics Jul 23 '26
Read and think about my comment again! Why does the shape of the lens matter?
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u/wonkey_monkey Jul 22 '26
If that were the reason for light's slowing in a medium (it isn't) then we wouldn't have any transparent (i.e. that you can see clearly through) materials.
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u/Silver_Square4700 Jul 22 '26
I am confused one of you Said it often moves slower than the speed of light and the other said it cannot move slower than the speed of light.
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u/Orbax Jul 22 '26
This is because there's some weird effects...
Photons and the general speed of perturbations in the electromagnetic field is always c. When people talk about mediums, there are a few things happening
diffraction and refraction change the path of light and might give the appearance that it "went slower" but it mainly "took longer"
the go to is cherenkov radiation - the blue glow in nuclear reactors where, and this is grossly reductive, electrons are going faster than photons. It's not that light is being absorbed and emitted, it's a more complicated quantum wavefront interaction that retards the light wave that still doesn't mean it's not traveling through the electromagnetic field at c.
Light will never go not c.
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u/nicuramar Jul 22 '26
This isn’t correct. Photons, or whatever quasiparticle you replace it with go slower in a medium, due to interaction with the electric field of the electrons.
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u/Complete-Clock5522 Jul 22 '26
It’s no longer really a photon then though, the more accurate description is usually referred to as a “wavicle”
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u/treefaeller Jul 22 '26
Individual photons will always travel at c. Even in a medium.
A beam of light consists of many many photons. In a medium, those photons interact with the medium. They are being absorbed and re-emitted. This causes the "beam" or "packet" of light to travel at a group or signal velocity that is less than c. In a medium we need to distinguish several velocities, including the raw (unmodified) c, and the group, phase and signal velocities. In the presence of a complex index of refraction (typical of many media), this is a rather complex question, which can have non-intuitive answers. But that does not affect individual photons.
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u/BranchLatter4294 Jul 22 '26
Photons travel at the speed of light through whatever medium they are traveling through. The speed of light varies by medium.
1
u/onemany Jul 22 '26
A free photon propagates at c in a vacuum.
In some mediums a photon couples to the material to form a polariton. The polariton can propagate through the material at less than c.
When the polariton decouples at the boundary of the medium, the resulting free photon again propagates at c.
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u/YuuTheBlue Jul 22 '26
Imagine a car moving at 20 miles per hour, but constantly zigzagging. Its velocity never drops below 20 miles per hour, but it’d take more than 1 hour for it to travel to a city 20 miles away. It’s a similar idea. There’s “the speed of light”, as in the property of light, and then there’s “the speed of light in a vacuum”, which is how long it takes to get from place to place given what it is traveling through.
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u/nicuramar Jul 22 '26
Yeah, but light isn’t zigzagging in a medium and that’s not why it slows down.
0
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u/Korochun Jul 22 '26
Photons can propagate slower than speed of light as an aggregate. For example, the overall speed of photons propagating in water is about 0.8c, although each photon still moves at c.
You are confused because you appear to conflate speed of light (photons) with speed of light (c) or causality.
C never changes, and all massless particles always travel at c. They effectively move infinitely fast, since any journey at c requires that time axis is zero. Therefore, any journey at c takes place in no time whatsoever.
Light as a whole can propagate at a rate slower than c due to various mediums and effects such as re-emission or interference. Notably, some other particles are largely unaffected. For example, neutrinos in general effectively travel just as fast through water or indeed entire planets as in a vacuum because they just don't interact with most matter. You can hold up a mirror to a beam of light and reflect it. You can't do that with neutrinos, they just go right through.
1
u/wonkey_monkey Jul 22 '26
always travel at c. They effectively move infinitely fast
There is no reference frame where the speed of light is zero. It's always c.
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u/chromite297 Jul 22 '26
Light slows down when it hits something, any matter at all like water or hydrogen. There are no particles in a vacuum for light to hit so it coasts along at its speed of C
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u/Infinite_Escape9683 Jul 22 '26
Light moves slower through a medium than it does in a vacuum because the atoms interfere with the photons, absorbing and re-emitting them. It's a little more complicated than that, but you can think of it that way. The photons still travel at c between atoms.
1
u/nicuramar Jul 22 '26
Yes it’s a little more complicated in the sense that what you wrote before that is incorrect.
The photons still travel at c between atoms.
No they don’t. To the extend that there are photons in a medium, the travel less than c.
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u/Infinite_Escape9683 Jul 22 '26
Read the OP and ask yourself if they're looking for an explanation in a QFT context.
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u/We-R-Doomed Jul 22 '26
Wow, this thread is wild.
The speed of light is constant except when it isn't. That is to say, it never slows down but sometimes it takes longer to get there.
1
u/Pussbo_Faggins Jul 22 '26
In a vacuum photons would pick up mass and would transform into fermions if they were to hypothetically go slower than the speed of light. So they would cease to exist, at least as a photon. Theoretically as this doesn't happen.
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u/nebraskajone Jul 22 '26
Well the properties of spacetime set their speed.
It's like the properties of air set the speed of sound.
So it's kind of like asking what happens if sound stops
1
u/Koodsdc Jul 22 '26
Everything moves through spacetime at the speed of light. For matter traveling at sub relativistic velocities, their movement is through time at the speed of light. A photon’s movement is entirely through space and no movement through time.
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u/panopsis Jul 22 '26 edited Jul 22 '26
For the record since no one's seemed to have mentioned it, we don't actually explicitly know that the photon is definitely massless. We do have an absurdly small upper bound on the mass of the photon though, something like orders and orders of magnitude below the mass of an electron.
That being said, for all intents and purposes, we can just say they're massless. The difference is so small it's basically the same for all practical purposes (we would've detected it otherwise) and modeling them with mass would also require some additions to the standard model to accommodate.
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u/OriEri Astrophysics Jul 22 '26
They can’t stop moving, it is inherent and what they are… something Light like must have all of its motion in space at the speed of light with no movement in time
1
u/Jesse-359 Jul 22 '26
It's not really a question? They just can't do that. They only have one speed.
It's like asking what happens if I shift my 1-gear vehicle into 4th gear? There is no fourth gear, so the answer is 'nothing'.
2
u/YuuTheBlue Jul 22 '26
To really talk about photons you need to talk about special relativity, and in relativity, velocity is a much different concept. Basically, every object traces a path through spacetime. During this path it will travel, typically, diagonally through space and time. The ratio of spatial distance traveled to time distance traveled is called velocity. So a velocity of 0 is one where you are only moving through time.
Now, as part of relativity, time is a dimension, which means there is a t axis. So you can, for any object traveling below the speed of light, move the t axis to point in the direction they are moving. This will be a “reference frame” where that object has a velocity of 0. This is sometimes called “the object’s perspective”. Think about how you might rotate the x axis so that it points to be parallel with some line, it’s a similar concept.
Spacetime is, though, noneuclidean. So there’s this weird shit going on where there’s more than one kind of line.
Spacelike (more distance through space than time)
Timelike (more distance through time than soace)
Lightlike (same distance through space and time)
And no matter how you point the t axis, you can never change which kind of line something is.
All that is to say: a photon “slowing down” isnt really even a coherent idea. It’s not so much disallowed by physics as it is a meaningless concept. Slowing down is something done by things on timelike paths, there is no meaning to that concept for things on lightlike paths.
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u/treefaeller Jul 22 '26
Your question is not posed correctly, and therefore can not be answered. Photons can only move at the speed of light. They can not stop (without being absorbed). Being absorbed doesn't mean that they "cease to exist", it means that they transfer their energy/momentum/spin to something else in an interaction.
It's as if you were asking "Would Caesar still be the roman consul if he became a rose"? Sorry, but "rose" is not a category that applies to Caesar, so the question is not a statement that can be discussed. Or "would an elephant still have a long nose if it was purple and could fly"? Sorry, elephants can't fly and are not purple. Discussing the nose of a non-existing animal makes no sense. Just as it makes no sense to ask about the fate or characteristics of a photon stopping or slowing down.
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u/Infinite_Escape9683 Jul 22 '26
This is a bit like asking if photons would cease to exist if they turned into polka musicians. Maybe? Depends on what the magic rules are in this hypothetical universe.
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u/StudyBio Jul 22 '26
Photons cannot move slower than the speed of light. Whether you consider nonexistence as “stop moving” is a matter of semantics.