Is the speed of light a constant

[smurfcatcher]

Is the speed of light a constant or can it be slowed down or sped up?

 

[madmike159]

It can't be faster than it is in a vacuum (about 3x10^8 ms^-1) but it is slower in different mediums. I think there is a FAQ section on it.

 

[BryanP]

It can't be faster than it is in a vacuum (about 3x10^8 ms^-1) but it is slower in different mediums. I think there is a FAQ section on it.

Such as water.

Anyways, this been talked about before already. In fact you can just scroll down and see the related threads to find your answer: https://www.physicsforums.com/showthread.php?t=183408

 

[peter0302]

Rhg. The speed of light is not slower in different mediums. The photons just propagate slower.

There was a purported physicist, I think in Maryland, who at one time believed that he could make a time machine by slowing down the speed of light in the medium he was using, believing that this slower value of the speed of light would reduce the energy requirements given by the Einstein field equations. Thankfully he realized he was out of his mind.

'c' is always a constant, and if ever shows up in an equation, it cannot ever change as far as we know. Don't confuse that with photons propagating more slowly in a medium.

 

[Nick89]

Rhg. The speed of light is not slower in different mediums. The photons just propagate slower.

There was a purported physicist, I think in Maryland, who at one time believed that he could make a time machine by slowing down the speed of light in the medium he was using, believing that this slower value of the speed of light would reduce the energy requirements given by the Einstein field equations. Thankfully he realized he was out of his mind.

'c' is always a constant, and if ever shows up in an equation, it cannot ever change as far as we know. Don't confuse that with photons propagating more slowly in a medium.

Are you sure? I have always thought individual photons always travel at the speed of light in vaccuum, no more, no less. In a medium, the group of photons that form a lightpulse interact in such a way that the lightpulse itself moves slower (not the individual photons). Am I wrong?

 

[matheinste]

Hello smufcatcher.

c is the speed of light in a vacuum. It is constant. Light cannot exceed this speed in any medium but can be slowed down in denser media. When it is slowed down its speed is not c as c is the speed of light in a vacuum.

Matheinste.

 

[granpa]

the phase velocity of light can be less that c. the group velocity can be greater than or less that c.

 

[Littlepig]

Are you sure? I have always thought individual photons always travel at the speed of light in vaccuum, no more, no less. In a medium, the group of photons that form a lightpulse interact in such a way that the lightpulse itself moves slower (not the individual photons). Am I wrong?

What you mean with lightpulse?

Well, Photons move in vacuum at c, in some medium the photons just interact with electrons, exciting then. When they go back to ground state, emits another photon. That's why, according to QED, speed light is slower than c in a medium.(Now, between interactions, yes, they move at c, it comes from invariance of electromagnetic waves and particle-wave duality i suppose).

 

[peter0302]

Yeah I don't know what is meant by "lightpulse" either.

There is a certain amount of time when the photon in a medium is exciting the electrons and they bounce around a little bit and emit new photons. The group velocity winds up being less than c but if you isolate anyone photon, it will be moving at c.

 

[Nick89]

But you said 'photons propagate slower in a medium'... They don't? The individual photons still move at c... Right?

 

[BryanP]

But you said 'photons propagate slower in a medium'... They don't? The individual photons still move at c... Right?

As said, it depends on the medium.

However, /c/ in a vacuum will always be the same.

 

[madmike159]

But you said 'photons propagate slower in a medium'... They don't? The individual photons still move at c... Right?

They do propagate slower in different mediums. The 'speed of a photon' in dimond is about 0.5c. The speed of light in a vacuum is always c. Saying can the speed of light be changed is wrong because the speed of light normaly means 'the speed of light in a vacume'.

 

[Nick89]

But I thought a massless particle like a photon cannot move at any speed other than c, the speed of light in a vaccuum? I thought it was the interaction with the medium that made the speed of light in a medium slower, but it does not make the individual photons slower. Note that with 'the speed of light in a medium', I mean the speed at which a pulse of light travels through the medium, NOT the speed of the photons.

Am I really wrong? I'm so sure that massless particles always move at c... Do you have any source for your statement?

 

[madmike159]

The speed of light through a medium depends on its refractive index, which depends on the frequency of thel light.
http://en.wikipedia.org/wiki/Speed_of_light

 

[Nick89]

The speed of light through a medium depends on its refractive index, which depends on the frequency of thel light.
http://en.wikipedia.org/wiki/Speed_of_light

Yes, I know... I'm not talking about the speed of light, I'm talking about the speed of the individual photons. They are not the same...! (they are only the same in a vaccuum)

I know the speed of light is lower in a medium, but the speed of the individual photons take make up the light is still c, the speed of light in a vaccuum.

 

[madmike159]

The photons must go slower because that is what light is. If you fired one photon under water it would have to move at the speed of light in water.

 

[Nick89]

The photons must go slower because that is what light is. If you fired one photon under water it would have to move at the speed of light in water.

As far as I know, no!

There was a thread about this some time ago... The individual photons travel at the speed of light in a vaccuum, ALWAYS. However, in a medium, they interact with the atoms which makes the (average?) speed of a pulse of light slower.

It's not exactly what happens, but you can imagine it as if you are driving your car through a city. Your car resembles the photon, and you can only move at say 50 mph. However, you must sometimes stop for traffic lights or pedestrians or things like that (resembling the atoms).
Now imagine the exact same car driving in a straight road outside of the city (resembling a vacuum). While the distance traveled through the city and over the straight road is equal, and obviously the speed of the car (50 mph) is equal, the car driving through the city will arrive at the destination later, making it's average speed less then 50 mph (or c).

Someone said above in this thread that the photons are absorbed and re-emited by the atoms, but I believe it was established in the other thread that this was not what happened. I don't know what exactly happens (probably some advanced quantum mechanical stuff) but I am pretty sure about my point that the individual photons can NOT move slower (or faster) than c, the speed of light in a vacuum.

 

[madmike159]

As far as I know, no!

There was a thread about this some time ago... but I am pretty sure about my point that the individual photons can NOT move slower (or faster) than c, the speed of light in a vacuum.

Yea there was and there is also a FAQ explaining it. They would still move at c sometimes because inside a atom between the electron orbits and nucleus is a vacume.

 

[Nick89]

No, not sometimes, always... If you say that between the electron orbits and the nucleus is a vacuum, then everything is a vacuum for the photon. (As far as I know it does not make sense to say that a photon is inside an electron for example)...

Where is the FAQ you mention?

 

[George Jones]

Where is the FAQ you mention?

https://www.physicsforums.com/showpost.php?p=899393&postcount=4

 

[Nick89]

Well, that proves my point doesn't it?

Photons interact with the medium, but not by being absorbed and re-emited (I said this before), but by another method. This means that the individual photons move at the speed of light in a vaccuum, and it's only the interactions with the medium that cause the speed of light (not the speed of the photons) in that medium to be slower. Correct or not?

 

[DaveC426913]

The photons must go slower because that is what light is. If you fired one photon under water it would have to move at the speed of light in water.

Does a photon romping freely in the empty spaces between a bunch of H's bonded to a bunch of Os know that it's in water?

 

[madmike159]

No, not sometimes, always... If you say that between the electron orbits and the nucleus is a vacuum, then everything is a vacuum for the photon. (As far as I know it does not make sense to say that a photon is inside an electron for example)...

Where is the FAQ you mention?

Yea that's a good point. I know photons go slower in different mediums but I can't explain why very well.

 

[Nick89]

If you can't explain why you know photons go slower than c in a medium, how do you know it's true? I don't believe it, I have always heard photons cannot move slower or faster than c, and in fact, every massless particle must travel at c, and only c.

 

[madmike159]

No I know they do or else refraction wouldn't work. When light moves from a fast to slower medium at any angle other than 90 degrees it gets refeacted. This is because the light slows down, so wave fronts bend because one part slows down before the other.
There are lots of posts explaining why and also a FAQ, I don't think any of them are written that well though.

*Edit* In fact most sources I can find on the internet seem to think that photons travle at c, but get asorbed but atoms then re-emmited lowering the avrage speed. Alot of people on these forums seem to think that this is wrong though.

 

[Nick89]

You keep confusing the speed of light with the speed of an individual photon. Yes, light refracts because the light slows down in a medium, but it's NOT the individual photons that are slowing down.

About the photons being absorbed and re-emitted, that is probably a simplistic way to explain what happens. If atoms would really absorb the photons (exciting an electron) and a time later re-emit a photon (electron falls back), then the light that came through a medium should have a distinct line spectrum because not all the light frequencies can excite electrons. This is not what happens.
But, there is something happening that causes the photons to interact with the matter, and that something causes the speed of light (again, NOT the speed of the photons) to go down.

 

[BryanP]

No I know they do or else refraction wouldn't work. When light moves from a fast to slower medium at any angle other than 90 degrees it gets refeacted. This is because the light slows down, so wave fronts bend because one part slows down before the other.
There are lots of posts explaining why and also a FAQ, I don't think any of them are written that well though.

*Edit* In fact most sources I can find on the internet seem to think that photons travle at c, but get asorbed but atoms then re-emmited lowering the avrage speed. Alot of people on these forums seem to think that this is wrong though.

The SPEED of LIGHT slows down in a medium, but the speed of the individual photons do not. Again, it is the interaction between the properties of the medium and the light that causes it to move slower than it would in a vacuum.

edit: Pretty much what Nick said (for some reason I didn't see that reply on my screen initially!)

 

[Hootenanny]

There are lots of posts explaining why and also a FAQ, I don't think any of them are written that well though. I don't think any of them are written that well though.

What's the problem with the writing? Is it that the posts/FAQ's don't agree with your point? Or is it that you can't understand them?

 

[madmike159]

No its just there is a lot of different views. Some one said (in a old post) what Nick89 and BryanP just said, and then some one else told them it was wrong and their explination was exactly the same.

 

[Hootenanny]

No its just there is a lot of different views. Some one said (in a old post) what Nick89 and BryanP just said, and then some one else told them it was wrong and their explination was exactly the same.

Could you try and find that old post and link to it here? For the record what BryanP and Nick86 have said in their last posts and what is written in the FAQ is correct. To emphasise the point once more, the speed of light in a particular medium is not the same as the speed of individual photons in that same media. Whilst photons always travel at c, irrespective of the media, the group and phase velocities of the light may be less (or in some cases greater) than c. The velocity that is shown in the refraction relation ($n = c/v$) is actually the phase velocity of the EM wave in a medium with refractive index $n$.

 

[George Jones]

Let me cloud the picture further; the group velocity of a pulse of light in an anomalously dispersive medium can be greater than c.

http://gregegan.customer.netspace.net.au/APPLETS/20/20.html

 

[lightarrow]

Infact what count is "signal velocity" which however is not always very easy to define.

 

[George Jones]

Infact what count is "signal velocity" which however is not always very easy to define.

Yes.

In a medium with normal dispersion and n > 1, most of the energy propagates at a speed equal to the group velocity (less than c), but there is an initial small signal, called a precursor, that propagates almost at c. So, if one has sensitive detectors, signal velocity in any medium is very close to c. Sommerfeld was one of the first people to work this out, and I first learned about this in Jackson.