<p>I am not sure this is a problem of visual communication. My incling would be to think this is a problem of language communication. The equations of relativity tell us that anything with zero rest-mass can only ever <em>exist</em> at the speed of light.<br />
So photons don't really accelerate, it is more a fundamental property of their existence to travel at c.</p>

CC BY-SA 4.0
2021-08-18T09:17:22.657Z
This is my intuition as well. Everything in the universe moves at c because that is simply the **natural** speed of things. But once a thing has mass then it interacts with the Higgs field and thus move at speeds other than c (slower).
slebetman
CC BY-SA 4.0
2021-08-16T18:02:17.513Z
2021-08-16T18:02:17.513Z
AtmosphericPrisonEscape
<p>&quot;Accelerate instantly&quot; would imply that a photon takes many different velocities at the same point in time. In fact, it would imply that a photon takes on <em>every</em> velocity between <span class="math-container">$0$</span> and <span class="math-container">$c$</span> <em>simultaneously</em>, but that clearly makes no sense at all - a particle cannot have many instantaneous velocities simultaneously.</p>
<p>When a photon is created, it is traveling at <span class="math-container">$c$</span>. A photon is always traveling at <span class="math-container">$c$</span>, there is no such thing as a stationary photon which is then accelerated. As an imperfect analogy, consider a pressure wave like sound traveling through air - the pressure wave itself is the movement of energy, you can't have a sound wave that remains stationary and does not move. Similarly, a stationary photon does not exist - if it exists, the electromagnetic wave (which <em>is the photon</em>) must be moving at <span class="math-container">$c$</span>.</p>
<p>As another analogy, consider what happens when you throw a rock into a pond - ripples expand from the point of contact. How fast were the ripples moving before you threw the rock? That's unanswerable, since the ripples did not exist before you threw the rock. The ripples are simply the movement of water, so the ripples require movement - the ripples come into existence already moving.</p>

CC BY-SA 4.0
2021-08-16T16:48:28.263Z
The point is; how can we visualise a particle which simply starts at c?Say you were a lecturer at a high school,how would you describe it to students?(Somebody who was in this situation consulted me ,so I wish to post it here)
AnnexRemotelearn
CC BY-SA 4.0
2021-08-16T17:00:54.930Z
@AnnexRemotelearn I suppose you could consider the wave-like transfer of energy. It's not a perfect analogy, but consider a sound wave as it travels through a medium - the pressure wave exists because it's moving, you can't have a pressure wave that remains stationary. The phenomenon itself requires velocity. Either it doesn't exist at all, or it comes into existence with some velocity.
Nuclear Hoagie
CC BY-SA 4.0
2021-08-16T20:23:43.020Z
@NuclearHoagie This comment looks to me like the best answer so far. Would you care to write an answer based on it?
Prallax
CC BY-SA 4.0
2021-08-17T11:38:46.600Z
"you can't have a sound wave that remains stationary and does not move." Sure, you can. They're called "standing waves".
nick012000
CC BY-SA 4.0
2021-08-17T13:17:48.140Z
@nick012000 Good point, but a standing wave is actually two waves moving in opposite directions. A standing pressure wave still requires movement of molecules - the air inside a pipe resonating with a standing wave is not stationary (https://en.wikipedia.org/wiki/File:Molecule2.gif).
Nuclear Hoagie
CC BY-SA 4.0
2021-08-17T19:07:24.963Z
@AnnexRemotelearn I think the trick is to get past the concept that "at rest" is somehow privileged. (That is, that when you create something, "of course" it would start at rest.) When you actually get into special relativity (which is where "light always travels at c" comes from), "rest" isn't anything special. What's at rest for one person/particle is 90% the speed of light for another. There's reasons massive particles tend to be created at rest with respect to certain _specific_ reference frames (Conservation of Energy & Momentum), but they don't apply to photons the same way.
R.M.
CC BY-SA 4.0
2021-08-17T19:13:09.230Z
You may want to address this:  Photons in matter travel at a different velocity than photons in vacuum.  Does that mean it gets accelerated when it changes from one medium to another?
gerrit
CC BY-SA 4.0
2021-08-18T05:12:21.060Z
What about describing it from the point of view of the photon? You come into existence with a universe speeding by you at the speed of light!
Joshua Kolden
CC BY-SA 4.0
2021-08-19T04:22:12.903Z
@AnnexRemotelearn - "The point is; how can we visualise a particle which simply starts at c?" - The problem is, while there is a particle aspect to it, a photon is *not* a particle in the same sense a bullet is; it's an excitation of the underlying quantum field. In other words, it's a kind of a wave - so the ripples in a pond analogy does address your question; that's the visualization. But also, beyond that, visualizing a particle that starts at some speed is actually not that hard, and if you play games, you have likely *seen* it already - a lot of game weapon projectiles work like that.
Filip Milovanović
CC BY-SA 4.0
2021-08-19T09:44:53.043Z
@gerrit no, photons in matter always travel at $c$, just as in vacuum. Granted, their phase speed does decrease proportionally to refractive index, but the leading edge of an abruptly starting wave (however distorted by dispersion) will arrive at the other end of a slab of matter of width $d$ after $t=c/d$.
Ruslan
CC BY-SA 4.0
2021-08-19T16:11:06.637Z
@Ruslan saying that photon always travel at *c* even in matter, but their speed decreases in matter must make sense to a physicist, but  is **completely contradictory** to laypeople.
RonJohn
CC BY-SA 4.0
2021-08-16T16:30:53.733Z
2021-08-16T20:31:29.587Z
2021-08-16T20:31:29.587Z
Nuclear Hoagie
<p>You are looking for a way to visualize the fact that a photon is created traveling at the speed of light.</p>
<p>Remember that a photon is actually a perturbation of the <a href="https://en.wikipedia.org/wiki/Electromagnetic_field" rel="nofollow noreferrer">electromagnetic field</a>.  That field is everywhere at once.  It is part of the fundamental construct of the Universe and it has, essentially, always been so.</p>
<p>An approximation of photon propagation might be to consider a mallet hitting a gong.  The force waves (sound waves in the case of the gong) induced by the mallet striking the gong travel with an instantaneous speed; they do not accelerate, they simply propagate.  The velocity at which they propagate is determined by the physical characteristics of the gong itself; it's the speed of sound.</p>
<p>The electromagnetic field has the property that all of its waves propagate at the speed of light, <em>c</em>, no matter what else is happening.  because of this the gong comparison has its limits.</p>
<p>The key thing to remember is that the photon is not a particle; it is a perturbation.  It is the field which determines the velocity of that perturbation and, in our Universe, that velocity is <em>c</em>.</p>

CC BY-SA 4.0
2021-08-16T17:22:13.880Z
I'm not downvoting, but I don't think an explanation on the level of quantum field theory and quantum electrodynamics  is appropriate for someone who wants a high school level explanation.
David Hammen
CC BY-SA 4.0
2021-08-16T17:28:17.567Z
Yes I can understand waves are disturbances/pertubations,but what is difficult is ;if you were watching the disturbance start,why do you see it instantaenously start at c?Say I had a photon detector,I would have to see some physical effect to measure c,and no physical effect's rate of occurrence can equal c.,as the pyhsics textbooks say?
AnnexRemotelearn
CC BY-SA 4.0
2021-08-18T06:51:27.970Z
P@David Hammen; Popular science programs on television cover this much, and I didn't think it would be too deep.  I thought this about the level given to "Why can't objects travel at the speed of light?" type questions.
JohnHunt
CC BY-SA 4.0
2021-08-18T07:05:05.477Z
@AnnexRemotelearn; If you strike a stretched bit of wire, say a piano string, with a mallet, the wave that is produced does not "speed up".  The energy from the mallet is simply transferred to the string and it continues on its merry way.  Light works the same way.  The photon is a packet of energy which is transferred to the EM field in the same sense that the energy from the mallet transferred to the string.  In neither case does that energy wave accelerate. It moves through the medium (String/EM) at the medium's speed of sound.  The "speed of sound" for the EM field is C.
JohnHunt
CC BY-SA 4.0
2021-08-18T09:19:49.943Z
@AnnexRemotelearn Yes, the same as when you create waves on water you don't see the waves accelerate but moves at wave-speed from the moment you created it to when it reaches the other end of the pool
slebetman
CC BY-SA 4.0
2021-08-18T18:10:45.790Z
> The Electro-Magnetic field has the property that all of its waves propagate at the speed of light, C, no matter what else is happening. < I don't think this is the case. _c_ is the same everywhere, no matter what, but _c_ represents the speed of light _in a vacuum_. Light not in a vacuum is free to move slower, and does--sometimes very slowly indeed. https://en.wikipedia.org/wiki/Slow_light
user3067860
CC BY-SA 4.0
2021-08-16T17:09:04.413Z
2021-08-19T13:27:02.863Z
2021-08-19T13:27:02.863Z
JohnHunt
<p>Take a look at this diagram from <a href="https://www.feynmanlectures.caltech.edu/III_18.html" rel="noreferrer">Feynman's lectures at Caltech</a> on angular momentum.  Here, an atom with angular momentum <span class="math-container">$m=1$</span> starts out in an excited state on the left hand side of the diagram.  Then it moves to a ground state as it emits a photon, traveling at <span class="math-container">$c$</span>.  Angular momentum is conserved, so the photon now has an angular momentum of 1.</p>
<p><a href="https://i.sstatic.net/g45k7.png" rel="noreferrer"><img src="https://i.sstatic.net/g45k7.png" alt="enter image description here" /></a></p>
<p>As an analogy, suppose you are on the edge of a merry-go-around, which is spinning very fast.  If you let go, you won't speed up, instead you will continue moving in a straight line with linear momentum.  The merry-go-around system will lose a corresponding amount of angular momentum so that momentum is conserved. No acceleration is necessary around the time you let go! This analogy, of course has its limitations, since a photon may be emitted off the rotational plane.</p>

CC BY-SA 4.0
2021-08-16T20:05:40.990Z
2021-08-16T20:05:40.990Z
Connor Garcia
<p>I think your core misunderstanding is this :</p>
<blockquote>
<p>In physics, it always takes some time for a particle to move from rest to some speed.</p>
</blockquote>
<p>If a particle breaks up into two other particles, then the moment those two particles are created they have whatever velocities are required to conserve momentum (i.e. momentum before the break up must equal total momentum after).  There's no acceleration required - they get that velocity the instant they are created.</p>
<p>Now photons have another property - they absolutely always travel at the speed of light.  For photons their momentum and energy are dependent on their frequency (or wavelenght if you prefer), so momentum/energy is not dependent on velocity for a photon.  This means there is no problem at all with the photon starting out at the speed of light.</p>

CC BY-SA 4.0
2021-08-17T06:38:28.187Z
"Photons... always travel at the speed of light" in a vacuum.  But when they travel through matter - the glass of a lens, for instance - they slow down, and speed up when they leave the glass.  But AFAIK (I'm no expert) there's no force applied to slow them down, or speed them up again, and the photons always have the same energy coming out as they did going in.  Seemingly another case where photons don't act like ordinary matter.
jamesqf
CC BY-SA 4.0
2021-08-17T07:50:41.157Z
@jamesqf Photons don't act like ordinary matter because they are not matter. You are looking for a simple, classical explanation for a complex, quantum phenomenon. If it helps, consider that photons are massless - so can undergo instantaneous acceleration.
Oscar Bravo
CC BY-SA 4.0
2021-08-17T13:05:52.027Z
@OscarBravo The point I'm making is that they *never* travel at any other velocity than the speed of light (traveling in a medium is a complex case).  They *never* accelerate because they always *start* at the speed of light.
StephenG - Help Ukraine
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2021-08-17T14:25:34.300Z
@StephenG I know. I was replying to jamesqf, who thought photons are matter...
Oscar Bravo
CC BY-SA 4.0
2021-08-17T16:11:36.637Z
@Oscar Bravo: Depends on your definition of matter, doesn't it?  But linguistic problems aside, it's that "complex case" that's interesting.  The photons change speed because the speed of light changes, or vice versa. but there's no acceleration.
jamesqf
CC BY-SA 4.0
2021-08-17T16:41:35.983Z
@jamesqf If you are interested in the reason why light in a medium apparently travels slower than the speed of light in a vacuum then I'd suggest starting with [this Q&A on Physics SE](https://physics.stackexchange.com/questions/1898/do-photons-gain-mass-when-they-travel-through-glass).
StephenG - Help Ukraine
CC BY-SA 4.0
2021-08-18T12:52:47.913Z
@jamesqf There is no "your" definition of matter. It's not a question of linguistics; it's physics, which is not a matter of opinion. In the Standard Model, photons, along with all other gauge bosons, are not matter particles.
Oscar Bravo
CC BY-SA 4.0
2021-08-18T16:57:30.707Z
@Oscar Bravo: But in Einstein's theory, E=mc^2.
jamesqf
CC BY-SA 4.0
2021-08-19T09:48:40.527Z
@jamesqf You're mixing up mass with matter. Matter is fermions - quarks and leptons. Bosons are the force carriers - photons, gluons, W, Z. Bosons can have a mass, if they interact with the Higgs field (W, Z), but they're not matter particles.
Oscar Bravo
CC BY-SA 4.0
2021-08-20T16:08:10.940Z
@Oscar Bravo: I don't think I'm mixing up anything.  I think I'm just unable to write clearly enough for you to understand.
jamesqf
CC BY-SA 4.0
2021-08-17T02:11:06.633Z
2021-08-17T13:38:37.140Z
2021-08-17T13:38:37.140Z
StephenG - Help Ukraine
<blockquote>
<p>In physics,some time is always need for a particle to move from rest to some speed.</p>
</blockquote>
<p>This is not true in quantum mechanics. Particles created during a quantum interaction can be born with non-zero linear momentum with respect to the rest frame of the particles that generate the new particle. There is no acceleration. For example, there is no acceleration when a pair of photons are created via a electron-positron collision. The two photons are moving at the speed of light from the instant the collision created them.</p>
<p>Regarding a visualization,</p>
<p><a href="https://i.sstatic.net/5WMS8.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/5WMS8.png" alt="Feynman diagram depicting an electron-positron annihilation" /></a></p>
<p>This is a <a href="https://en.wikipedia.org/wiki/Feynman_diagram" rel="nofollow noreferrer">Feynman diagram</a> that depicts an electron-positron annihilation. Starting at the bottom of the diagram, the diagram shows an electron and a positron moving toward one another. At some point in time, the two particles get close enough together that their fields interact. This is the horizontal line in the middle of the diagram. At that point in time, a pair of photons are created that conserve the energy and momentum of the incoming particles. The created photons are born moving at the speed of light.</p>

CC BY-SA 4.0
2021-08-16T16:28:01.120Z
How is such a thin visualisable?
AnnexRemotelearn
CC BY-SA 4.0
2021-08-16T17:30:11.160Z
Thanks for the visual and URL;shall read through it
AnnexRemotelearn
CC BY-SA 4.0
2021-08-16T18:15:30.990Z
I don't think that Feynman diagrams are appropriate to educate somebody with little to no background in physics,
Thomas
CC BY-SA 4.0
2021-08-19T06:56:04.580Z
@Thomas I respectfully disagree. Feynman diagrams are extremely easy to understand, even for children. **"This straight line is a particle, which is something you can hold. This wiggly line is a wave, just like the waves in the ocean. We can't hold them, but we can feel them push us."** It doesn't have to be 100% accurate, it just has to be accurate enough for the layman to grasp.
dotancohen
CC BY-SA 4.0
2021-08-16T16:19:55.790Z
2021-08-16T17:13:20.910Z
2021-08-16T17:13:20.910Z
David Hammen
<p>In physics, it always takes some time for a particle to move from rest to some speed.</p>
<p>However, photons (light particles) accelerate instantly from zero to c. How? (A visualization would be helpful.)</p>

CC BY-SA 4.0
2021-08-17T05:19:59.330Z
A photon is never at any other speed than c. If you look at it from the photons perspective, in **it's frame of reference** it is a continuous spear going from creation to annihilation. It is merely a bridge between two events that are separated by spacetime.
Stian
CC BY-SA 4.0
2021-08-17T06:14:49.270Z
[Does a photon instantaneously gain  c  speed when emitted from an electron?](https://physics.stackexchange.com/questions/94049/does-a-photon-instantaneously-gain-c-speed-when-emitted-from-an-electron)
John Rennie
CC BY-SA 4.0
2021-08-17T11:21:28.500Z
There is nothing that accelerates to c non-instantaneously.
fraxinus
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2021-08-17T13:21:32.833Z
A change in speed would imply that the photon is subject to a force, but photons are not subject to forces.
Buck Thorn
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2021-08-17T14:05:39.220Z
@StianYttervik Correct me if I'm wrong, but I thought photons didn't have a frame of reference?
gardenhead
CC BY-SA 4.0
2021-08-17T14:49:40.050Z
@fraxinus,Hi I also wanted initially to include neutrinos in this question,but refrained given it was unclear whether they travel at c or slightly below it.So to 'preview' a question on them,if you have the expertise to anwser ;why do neutrinos accelerate instantly to the 'neutrino-speed'? And why is it said they have a rest-mass yet are never observed at rest?
AnnexRemotelearn
CC BY-SA 4.0
2021-08-17T15:10:04.560Z
I’m voting to close this question because there doesn't seem to be an astronomy angle to this question.  It is about the properties of light, not the properties of astronomical objects.  It's not a bad question for [physics.se], so I've upvoted.
James K
CC BY-SA 4.0
2021-08-17T21:08:18.250Z
Who told you no other phenomenon do it? Gravitational waves and gluons also travel at c. As would any other particle or wave we might discover that doesn't have a rest mass.
Shufflepants
CC BY-SA 4.0
2021-08-17T21:23:51.753Z
@AnnexRemotelearn We have a hard enough time observing neutrinos that are moving very fast...
Michael
CC BY-SA 4.0
2021-08-18T05:08:32.513Z
@gardenhead not a useful one, you are right. Don't try transforming any laws to this frame. As a thought experiment.
Stian
CC BY-SA 4.0
2021-08-19T00:21:55.257Z
@gardenhead My understanding is that you can't construct a frame of reference moving at c (relative to anything else), but you can think about the frame you get as the limit of its speed goes to c. As the frame's speed gets arbitrarily close to c, length contraction makes distance along the direction of travel arbitrarily close to 0. So sometimes people "complete the limit" and talk about "a phonton's frame of reference" as a frame where the distance between all points on its path is actually 0. Not formally workable, but not really intended to be either; it's an analogy.
Ben
CC BY-SA 4.0
2021-08-19T16:06:24.810Z
@JamesK should it be *migrated* to Physics.SE?
RonJohn
CC BY-SA 4.0
2021-08-19T20:08:49.293Z
It would fit better there than here, but that is a mod decision, it can't be done by the community.
James K
<light><time><velocity>
Why does light accelerate instantaneously to c, while no other phenomena do it?
AnnexRemotelearn
# Why does light accelerate instantaneously to c, while no other phenomena do it?

(Asked by: username_0 on 2021-08-16)

In physics, it always takes some time for a particle to move from rest to some speed.

However, photons (light particles) accelerate instantly from zero to c. How? (A visualization would be helpful.)

**Tags:** light, time, velocity
> **Comments:**
> **username_1**: A photon is never at any other speed than c. If you look at it from the photons perspective, in **it's frame of reference** it is a continuous spear going from creation to annihilation. It is merely a bridge between two events that are separated by spacetime.  
> **username_2**: [Does a photon instantaneously gain  c  speed when emitted from an electron?](https://physics.stackexchange.com/questions/94049/does-a-photon-instantaneously-gain-c-speed-when-emitted-from-an-electron)  
> **username_3**: There is nothing that accelerates to c non-instantaneously.  
> **username_4**: A change in speed would imply that the photon is subject to a force, but photons are not subject to forces.  
> **username_5**: @StianYttervik Correct me if I'm wrong, but I thought photons didn't have a frame of reference?  
> **username_0**: @fraxinus,Hi I also wanted initially to include neutrinos in this question,but refrained given it was unclear whether they travel at c or slightly below it.So to 'preview' a question on them,if you have the expertise to anwser ;why do neutrinos accelerate instantly to the 'neutrino-speed'? And why is it said they have a rest-mass yet are never observed at rest?  
> **username_6**: I’m voting to close this question because there doesn't seem to be an astronomy angle to this question.  It is about the properties of light, not the properties of astronomical objects.  It's not a bad question for [physics.se], so I've upvoted.  
> **username_7**: Who told you no other phenomenon do it? Gravitational waves and gluons also travel at c. As would any other particle or wave we might discover that doesn't have a rest mass.  
> **username_8**: @AnnexRemotelearn We have a hard enough time observing neutrinos that are moving very fast...  
> **username_1**: @gardenhead not a useful one, you are right. Don't try transforming any laws to this frame. As a thought experiment.  
> **username_9**: @gardenhead My understanding is that you can't construct a frame of reference moving at c (relative to anything else), but you can think about the frame you get as the limit of its speed goes to c. As the frame's speed gets arbitrarily close to c, length contraction makes distance along the direction of travel arbitrarily close to 0. So sometimes people "complete the limit" and talk about "a phonton's frame of reference" as a frame where the distance between all points on its path is actually 0. Not formally workable, but not really intended to be either; it's an analogy.  
> **username_10**: @JamesK should it be *migrated* to Physics.SE?  
> **username_6**: It would fit better there than here, but that is a mod decision, it can't be done by the community.  

---

## 6 answers

### Answer by username_13 on 2021-08-16. Score: 72

"Accelerate instantly" would imply that a photon takes many different velocities at the same point in time. In fact, it would imply that a photon takes on *every* velocity between $0$ and $c$ *simultaneously*, but that clearly makes no sense at all - a particle cannot have many instantaneous velocities simultaneously.

When a photon is created, it is traveling at $c$. A photon is always traveling at $c$, there is no such thing as a stationary photon which is then accelerated. As an imperfect analogy, consider a pressure wave like sound traveling through air - the pressure wave itself is the movement of energy, you can't have a sound wave that remains stationary and does not move. Similarly, a stationary photon does not exist - if it exists, the electromagnetic wave (which *is the photon*) must be moving at $c$.

As another analogy, consider what happens when you throw a rock into a pond - ripples expand from the point of contact. How fast were the ripples moving before you threw the rock? That's unanswerable, since the ripples did not exist before you threw the rock. The ripples are simply the movement of water, so the ripples require movement - the ripples come into existence already moving.

> **Comments:**
> **username_0**: The point is; how can we visualise a particle which simply starts at c?Say you were a lecturer at a high school,how would you describe it to students?(Somebody who was in this situation consulted me ,so I wish to post it here)  
> **username_13**: @AnnexRemotelearn I suppose you could consider the wave-like transfer of energy. It's not a perfect analogy, but consider a sound wave as it travels through a medium - the pressure wave exists because it's moving, you can't have a pressure wave that remains stationary. The phenomenon itself requires velocity. Either it doesn't exist at all, or it comes into existence with some velocity.  
> **username_14**: @NuclearHoagie This comment looks to me like the best answer so far. Would you care to write an answer based on it?  
> **username_15**: "you can't have a sound wave that remains stationary and does not move." Sure, you can. They're called "standing waves".  
> **username_13**: @nick012000 Good point, but a standing wave is actually two waves moving in opposite directions. A standing pressure wave still requires movement of molecules - the air inside a pipe resonating with a standing wave is not stationary (https://en.wikipedia.org/wiki/File:Molecule2.gif).  
> **username_16**: @AnnexRemotelearn I think the trick is to get past the concept that "at rest" is somehow privileged. (That is, that when you create something, "of course" it would start at rest.) When you actually get into special relativity (which is where "light always travels at c" comes from), "rest" isn't anything special. What's at rest for one person/particle is 90% the speed of light for another. There's reasons massive particles tend to be created at rest with respect to certain _specific_ reference frames (Conservation of Energy & Momentum), but they don't apply to photons the same way.  
> **username_17**: You may want to address this:  Photons in matter travel at a different velocity than photons in vacuum.  Does that mean it gets accelerated when it changes from one medium to another?  
> **username_18**: What about describing it from the point of view of the photon? You come into existence with a universe speeding by you at the speed of light!  
> **username_19**: @AnnexRemotelearn - "The point is; how can we visualise a particle which simply starts at c?" - The problem is, while there is a particle aspect to it, a photon is *not* a particle in the same sense a bullet is; it's an excitation of the underlying quantum field. In other words, it's a kind of a wave - so the ripples in a pond analogy does address your question; that's the visualization. But also, beyond that, visualizing a particle that starts at some speed is actually not that hard, and if you play games, you have likely *seen* it already - a lot of game weapon projectiles work like that.  
> **username_20**: @gerrit no, photons in matter always travel at $c$, just as in vacuum. Granted, their phase speed does decrease proportionally to refractive index, but the leading edge of an abruptly starting wave (however distorted by dispersion) will arrive at the other end of a slab of matter of width $d$ after $t=c/d$.  
> **username_10**: @Ruslan saying that photon always travel at *c* even in matter, but their speed decreases in matter must make sense to a physicist, but  is **completely contradictory** to laypeople.  

---

### Answer by username_11 on 2021-08-16. Score: 22

I am not sure this is a problem of visual communication. My incling would be to think this is a problem of language communication. The equations of relativity tell us that anything with zero rest-mass can only ever *exist* at the speed of light.  
So photons don't really accelerate, it is more a fundamental property of their existence to travel at c.

> **Comments:**
> **username_12**: This is my intuition as well. Everything in the universe moves at c because that is simply the **natural** speed of things. But once a thing has mass then it interacts with the Higgs field and thus move at speeds other than c (slower).  

---

### Answer by username_21 on 2021-08-16. Score: 14

You are looking for a way to visualize the fact that a photon is created traveling at the speed of light.

Remember that a photon is actually a perturbation of the [electromagnetic field](https://en.wikipedia.org/wiki/Electromagnetic_field). That field is everywhere at once. It is part of the fundamental construct of the Universe and it has, essentially, always been so.

An approximation of photon propagation might be to consider a mallet hitting a gong. The force waves (sound waves in the case of the gong) induced by the mallet striking the gong travel with an instantaneous speed; they do not accelerate, they simply propagate. The velocity at which they propagate is determined by the physical characteristics of the gong itself; it's the speed of sound.

The electromagnetic field has the property that all of its waves propagate at the speed of light, *c*, no matter what else is happening. because of this the gong comparison has its limits.

The key thing to remember is that the photon is not a particle; it is a perturbation. It is the field which determines the velocity of that perturbation and, in our Universe, that velocity is *c*.

> **Comments:**
> **username_22**: I'm not downvoting, but I don't think an explanation on the level of quantum field theory and quantum electrodynamics  is appropriate for someone who wants a high school level explanation.  
> **username_0**: Yes I can understand waves are disturbances/pertubations,but what is difficult is ;if you were watching the disturbance start,why do you see it instantaenously start at c?Say I had a photon detector,I would have to see some physical effect to measure c,and no physical effect's rate of occurrence can equal c.,as the pyhsics textbooks say?  
> **username_21**: P@David Hammen; Popular science programs on television cover this much, and I didn't think it would be too deep.  I thought this about the level given to "Why can't objects travel at the speed of light?" type questions.  
> **username_21**: @AnnexRemotelearn; If you strike a stretched bit of wire, say a piano string, with a mallet, the wave that is produced does not "speed up".  The energy from the mallet is simply transferred to the string and it continues on its merry way.  Light works the same way.  The photon is a packet of energy which is transferred to the EM field in the same sense that the energy from the mallet transferred to the string.  In neither case does that energy wave accelerate. It moves through the medium (String/EM) at the medium's speed of sound.  The "speed of sound" for the EM field is C.  
> **username_12**: @AnnexRemotelearn Yes, the same as when you create waves on water you don't see the waves accelerate but moves at wave-speed from the moment you created it to when it reaches the other end of the pool  
> **username_23**: > The Electro-Magnetic field has the property that all of its waves propagate at the speed of light, C, no matter what else is happening. < I don't think this is the case. _c_ is the same everywhere, no matter what, but _c_ represents the speed of light _in a vacuum_. Light not in a vacuum is free to move slower, and does--sometimes very slowly indeed. https://en.wikipedia.org/wiki/Slow_light  

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### Answer by username_24 on 2021-08-16. Score: 11

Take a look at this diagram from [Feynman's lectures at Caltech](https://www.feynmanlectures.caltech.edu/III_18.html) on angular momentum. Here, an atom with angular momentum $m=1$ starts out in an excited state on the left hand side of the diagram. Then it moves to a ground state as it emits a photon, traveling at $c$. Angular momentum is conserved, so the photon now has an angular momentum of 1.

[![enter image description here](https://i.sstatic.net/g45k7.png)](https://i.sstatic.net/g45k7.png)

As an analogy, suppose you are on the edge of a merry-go-around, which is spinning very fast. If you let go, you won't speed up, instead you will continue moving in a straight line with linear momentum. The merry-go-around system will lose a corresponding amount of angular momentum so that momentum is conserved. No acceleration is necessary around the time you let go! This analogy, of course has its limitations, since a photon may be emitted off the rotational plane.


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### Answer by username_25 on 2021-08-17. Score: 7

I think your core misunderstanding is this :

> In physics, it always takes some time for a particle to move from rest to some speed.

If a particle breaks up into two other particles, then the moment those two particles are created they have whatever velocities are required to conserve momentum (i.e. momentum before the break up must equal total momentum after). There's no acceleration required - they get that velocity the instant they are created.

Now photons have another property - they absolutely always travel at the speed of light. For photons their momentum and energy are dependent on their frequency (or wavelenght if you prefer), so momentum/energy is not dependent on velocity for a photon. This means there is no problem at all with the photon starting out at the speed of light.

> **Comments:**
> **username_26**: "Photons... always travel at the speed of light" in a vacuum.  But when they travel through matter - the glass of a lens, for instance - they slow down, and speed up when they leave the glass.  But AFAIK (I'm no expert) there's no force applied to slow them down, or speed them up again, and the photons always have the same energy coming out as they did going in.  Seemingly another case where photons don't act like ordinary matter.  
> **username_27**: @jamesqf Photons don't act like ordinary matter because they are not matter. You are looking for a simple, classical explanation for a complex, quantum phenomenon. If it helps, consider that photons are massless - so can undergo instantaneous acceleration.  
> **username_25**: @OscarBravo The point I'm making is that they *never* travel at any other velocity than the speed of light (traveling in a medium is a complex case).  They *never* accelerate because they always *start* at the speed of light.  
> **username_27**: @StephenG I know. I was replying to jamesqf, who thought photons are matter...  
> **username_26**: @Oscar Bravo: Depends on your definition of matter, doesn't it?  But linguistic problems aside, it's that "complex case" that's interesting.  The photons change speed because the speed of light changes, or vice versa. but there's no acceleration.  
> **username_25**: @jamesqf If you are interested in the reason why light in a medium apparently travels slower than the speed of light in a vacuum then I'd suggest starting with [this Q&A on Physics SE](https://physics.stackexchange.com/questions/1898/do-photons-gain-mass-when-they-travel-through-glass).  
> **username_27**: @jamesqf There is no "your" definition of matter. It's not a question of linguistics; it's physics, which is not a matter of opinion. In the Standard Model, photons, along with all other gauge bosons, are not matter particles.  
> **username_26**: @Oscar Bravo: But in Einstein's theory, E=mc^2.  
> **username_27**: @jamesqf You're mixing up mass with matter. Matter is fermions - quarks and leptons. Bosons are the force carriers - photons, gluons, W, Z. Bosons can have a mass, if they interact with the Higgs field (W, Z), but they're not matter particles.  
> **username_26**: @Oscar Bravo: I don't think I'm mixing up anything.  I think I'm just unable to write clearly enough for you to understand.  

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### Answer by username_22 on 2021-08-16. Score: 3

> In physics,some time is always need for a particle to move from rest to some speed.

This is not true in quantum mechanics. Particles created during a quantum interaction can be born with non-zero linear momentum with respect to the rest frame of the particles that generate the new particle. There is no acceleration. For example, there is no acceleration when a pair of photons are created via a electron-positron collision. The two photons are moving at the speed of light from the instant the collision created them.

Regarding a visualization,

[![Feynman diagram depicting an electron-positron annihilation](https://i.sstatic.net/5WMS8.png)](https://i.sstatic.net/5WMS8.png)

This is a [Feynman diagram](https://en.wikipedia.org/wiki/Feynman_diagram) that depicts an electron-positron annihilation. Starting at the bottom of the diagram, the diagram shows an electron and a positron moving toward one another. At some point in time, the two particles get close enough together that their fields interact. This is the horizontal line in the middle of the diagram. At that point in time, a pair of photons are created that conserve the energy and momentum of the incoming particles. The created photons are born moving at the speed of light.

> **Comments:**
> **username_0**: How is such a thin visualisable?  
> **username_0**: Thanks for the visual and URL;shall read through it  
> **username_28**: I don't think that Feynman diagrams are appropriate to educate somebody with little to no background in physics,  
> **username_29**: @Thomas I respectfully disagree. Feynman diagrams are extremely easy to understand, even for children. **"This straight line is a particle, which is something you can hold. This wiggly line is a wave, just like the waves in the ocean. We can't hold them, but we can feel them push us."** It doesn't have to be 100% accurate, it just has to be accurate enough for the layman to grasp.  

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