# Proof that the speed of light is constant?

1. Dec 9, 2005

### Phalanx

hey, im looking for an experiment that prooves that the speed of light is constant as a preamble for a talk on the phenomena related to special relativity such as time dilation and length contraction.

As far as I can see many websites are claiming that the Michelson-Morley interferometer prooves that the speed of light is constant but as far as I can see this experiment doesnt work because the light traveling back on itself would cancel out any effect the rotation of the earth would have. Even if this experiment does work, i do not understand how the speed of light must be constant just because there is no stationary ether.

2. Dec 9, 2005

### ZapperZ

Staff Emeritus
Er.. be careful with the use of the word "proof". In mathematics, these are logical derivations at which you arrive. In physics, many of these things are postulates which are VERIFIED to be either valid or not. The constant speed of light is a postulate of Special Relativity, which are then verified either via measurements, or through the consequences of such a postulate. You don't "prove" it, you verify its validity.

Zz.

3. Dec 9, 2005

### ComputerGeek

but.. it is not, Scientists have stopped light and even sped it up past c.

it might only be on a quantum level that those effects can be had, but it does show that c is not immutable... but then I guess you would have to ask if in those instances if it was just the frame of reference that the experiment had.

4. Dec 9, 2005

### Staff: Mentor

Substitute the word "supports" for "proves" and you'll make people here a lot happier.
As for experiments, take your pick:
List of experiments on speed of light

5. Dec 9, 2005

### Staff: Mentor

In a medium, yes, but not in a vacuum. The fact that light travels in different speeds in different media does not contradict relativity.

6. Dec 9, 2005

### nnxion

c is just saying it's constant, it doesn't say anything about the speed of the light.

7. Dec 9, 2005

### ComputerGeek

but, does the fact that they were able to increase the speed of a photon past c show that the speed that light moves in a vacuum is not the maximum speed of particle sin the universe?

8. Dec 9, 2005

### gta-maloy]

c simply states that light will remain at a constant speed unless slowed by a medium.

If they managed to make light faster than the "normal" speed at which it travels, what is used to fill "c" in e=mc^2, simply means that they either removed or replaced a medium. This doesn't contradict the fact that the speed of light is constant, it mearely backs up the fact that it will slow down in a median that is more dense than the previous....and vice-versa.

9. Dec 9, 2005

### ZapperZ

Staff Emeritus
No such thing occurs. The NEC experiment of apparent superluminal signal does NOT violate the c limit. In fact, if you read the paper, no part of the pulse exceeded c. It is due to the pulse shapping due to the medium and thus, changing the group velocity of the pulse, NOT the photon.

Zz.

10. Dec 9, 2005

### Integral

Staff Emeritus
Stopping light is no big deal it happens all the time, OTOH you will need to post a link to experimental evidence of LIGHT traveling faster then c.

11. Dec 9, 2005

### Phalanx

okay... well then what experiments support that the speed of light is constant? and can someone please explain to me how the michelen-morley interferometer works or direct me somewhere?

12. Dec 9, 2005

### ukmicky

The people who ran the experiment are confusing things because when they say that they managed to get light to exceed c they don’t explain themselves properly.
Firstly when Relativity say light cannot travel faster than c it is really taking about information in light and not light.
In the experiments light manages to exceed c because their measurements are group velocity rather than front velocity. it is the front part of the wave which contains information, this part cannot exceed c ,the central fat part which dosent contain information and is basically pure light can exceed c if travelling through certain meduims like cesium , which is specially prepared for the experiment.

13. Dec 9, 2005

### Integral

Staff Emeritus
Not quite. Specially prepared Cesium can capture, then be triggered to emit a photon some time later. The speed of light is not, and has not been execeded.

14. Dec 9, 2005

### ukmicky

Not in a vacuum no .
what i was trying to say just not very well, was that even though light did travel through the medium (cesium)faster than c,as it was travelling through cesium and not a vacuum and as the only important part in regards to the rules being violated is the part of the wave which contains the information the front of the wave and as that part didnt exceed the speed limit no rules were violated.

Last edited: Dec 9, 2005
15. Dec 9, 2005

### rbj

i think we can have a feel for why the speed of E&M propagation should be the same for all inertial reference frames. it really just comes from Maxwell's Eqs. and the knowledge that there is no ether medium that E&M is propagated in. i mean, how do we tell the difference between a moving vacuum and a stationary vacuum? if we can't, if there really is no difference between a moving vacuum and a stationary vacuum, that such a concept is really meaningless, then whether the light that you are measuring originated from a flashlight mounted on a rocket moving past you at $c/2$ or from a stationary flashlight, how does that change the fact that a changing E field is causing a changing B field which is causing a changing E field, etc.? that propagation of an E field and B field disturbance, which has velocity $1/ \sqrt{ \epsilon_0 \mu_0 }$? how is it different?
whether you are holding the flashlight or moving past it at high velocity, Maxwell's Eqs. say the same thing regarding the nature of E&M in the vacuum. that is qualitatively different for waves that require a substantive medium to travel in, e.g. sound. if the wind is moving past you from left to right at 10 m/s, a measurement of the speed of sound coming from a source on your left will be 20 m/s faster than a sound emanating from a source on your right. but there is no (ether) wind for electromagneting radiation. so once the E&M wave has left its source, does it have any contact with it anymore? can it "remember" that it came from a high speed moving flashlight (relative to some observer) or a stationary flashlight? it can't. and if there is no "wind" to carry E&M, what would be left for it to know it should move any faster, just because its source was moving?

16. Dec 9, 2005

### Staff: Mentor

See the link that I posted in posting #4 in this thread.

17. Sep 22, 2009

### mauroz

Lets give this a go.
Do you accept e = mc2 to be true?
then c = $$^{+}_{-}$$$$\sqrt{e/m}$$ lets consider the + only.
c = $$\sqrt{e/m}$$
Do you accept that the amount of energy associated with a specific mass is constant?
If so then e/m is constant, implying that c is constant.

Mauro.

18. Sep 22, 2009

### Staff: Mentor

Yes, C is constant. I'm not sure I see your point...

19. Sep 22, 2009

### k_squared

I think I see what he is saying, not that I'm well versed in this at all, please allow me to clarify:

c=+- e/m. (I'm going to guess we can throw out the negative...)
c=e/m?

So he is saying is there a specific amount of energy for any given amount of mass, regardless of whatever composes this mass? (ie, does a ton of feathers have the same energy as a ton of Uranium.) Because, the mass going into the energy, according to this, should be constant. I think. Mabye.

20. Sep 23, 2009