# Questions Regarding Speed of Light

1. Jul 18, 2015

### Stephanus

[Mentor's note: This post has been moved to it's own thread.]

Dear PF Forum, can I ask a random question here?
1. Is the speed of light defined by the speed of photon?
2. How do we measure EM wave speed? Just by putting transmitter and receiver and divide the distance by time?
3. If EM wave has no photon, is EM wave speed the same as light speed? or at the universe maximum speed?
4. If photon has mass, is light speed different than the universal speed limit? Altough only some tiny tiny percent?

If different cars have different speed (although the same car can have different speed when it's in a garage compared to when it's in highway or traffic jam)

5. Does photon always travel at the speed of light? For example, today the same photon travels at c, and yesterday the same photon travels at 0.5c or even 100 kmh.
6. Does neutrino always travel at the speed of light?

7. I know I should have googled it, just in case someone can give me yes/no answer, then I google it
The quote suggests that there are different kinds of neutrino type, is that right? Altough we have many types of atom, we only have 1 type of proton, right? I know this question should belong to particle physics, perhaps just yes/no answer will suffice before it is deleted by the admins

Last edited by a moderator: Jul 18, 2015
2. Jul 18, 2015

### andrewkirk

1. Is the speed of light defined by the speed of photon?
It's not really defined that way, because we can't both observe an individual photon being omitted at one place and being received somewhere else. But a photon is a particle that, under the theory, has a speed equal to the speed of light.

2. How do we measure EM wave speed? Just by putting transmitter and receiver and divide the distance by time?
Yes. Although I imagine there may be other ways based on measuring frequencies and wavelengths and then using the wave equation.

3. If EM wave has no photon, is EM wave speed the same as light speed? or at the universe maximum speed?
That would have to be in a different universe, because in this one, all EM waves have photons.
Say we lived in a universe in which all EMR was pure waves and there were no photons, and light was a form of EMR. Then I imagine we'd define 'light speed' to mean the speed of EMR waves. But since it's a different universe, with different rules of physics, it's possible that not all EMR has the same speed. That is science fiction and the limit is only imagination and the need for logical coherence.

4. If photon has mass, is light speed different than the universal speed limit? Altough only some tiny tiny percent?
Photons don't have mass. I imagine a particle physicist could tell you to what proportion of c particles with very small (but nonzero) masses have been accelerated in the LHC.

5. Does photon always travel at the speed of light? For example, today the same photon travels at c, and yesterday the same photon travels at 0.5c or even 100 kmh.
Yes. I don't know what you mean by your 'for example' bit though.
6. Does neutrino always travel at the speed of light?
I don't know much about neutrinos.

3. Jul 18, 2015

### Staff: Mentor

Relativity was discovered before quantum mechanics, so while you're learning relativity you should try to forget that you ever heard the word "photon". You can learn how photons work after you've learned relativity and are ready to take on QM; but until then the very complicated relationship between a flash of light and a bunch of photons will just get in the way.

But with that said:
1) In modern metrology, the meter is defined to be the distance that light travels in a particular amount of time, so we know what the speed of light is even without measuring it: it is 299792458 meters per second, because we've defined the length of one meter so that the speed of light has to come out to 299792458 meters per second. We do this because we can measure time more precisely than distance and we know that whatever the speed of light is, it is constant - so it makes sense to define the meter in terms of a time interval and the constant speed of light.

2) But back in the days when used meter sticks and clocks to measure the speed of light and see of it really was constant, yes, we measured the speed of light just by dividing the distance between source and receiver by the travel time.

3,4,5) as I said above, try to forget that you ever heard the word "photon". Photons have nothing to do with the behavior of light in relativity; they only come into the picture when you study the details of how a flash of light interacts with the devices that emit and receive it.

6) There are several different types of neutrinos. The current understanding is that all types have rest mass so move at less than c. However, they are so light that it is really easy to get them moving at speeds very close to c. If we know the energy of a neutrino, we can calculate its speed, and if that calculation comes out to be .99999999c and we want to know how long it takes for it to get from point A to point B.... Well, we can save ourself a lot of effort by rounding the speed off to exactly c before we calculate, and the rounding error will only be one part in one hundred million which is OK in most problems.

4. Jul 20, 2015

### Stephanus

3. If EM wave has no photon, is EM wave speed the same as light speed? or at the universe maximum speed?
That would have to be in a different universe, because in this one, all EM waves have photons.
- Are they EM wave?
- Do they have photon?

4. If photon has mass, is light speed different than the universal speed limit? Altough only some tiny tiny percent?
Photons don't have mass.
Just curious though if the scientist have settled if proton has mass or not.

5. Does photon always travel at the speed of light? For example, today the same photon travels at c, and yesterday the same photon travels at 0.5c or even 100 kmh.
I mean this...
Take a car for example. This hour your car parks in a garage. Speed: 0
Later in the morning, your car comes out of garage to city traffic, speed: perhaps 10 mph
Then, entering highway, you could drive as fast as 100 mph.
My question is
Does proton always travel, have to travel, no chance or possibilty to stay at rest?
And if it travels, does it always travel at c, no less?

6. Does neutrino always travel at the speed of light?
I don't know much about neutrinos
okay...

Thanks anyway

5. Jul 20, 2015

### Stephanus

Obviously I didn't read Nugatory respond before I post those question

6. Jul 21, 2015

### Janus

Staff Emeritus
A photon, simply put, is a quantum of electromagnetic radiation. By quantum I mean that it is the smallest unit of electromagnetic radiation energy you can have for a given frequency. EM waves and photons are not separate things, they are different ways of treating the same thing. In Relativity, it doesn't matter whether we treat light as continuous waves or a stream of particle-like photons. Thus photons always travel at the speed of light, and because of this they are massless.

7. Jul 21, 2015

### Primordial

To my understanding the photon's reality is dependent on the observers energy state as observed from a quantized superposition. Is this an appropriate view? Thank you.

8. Jul 21, 2015

### vanhees71

I cannot object more to that statements. We can never describe electromagnetic phenomena as a stream of particle-like somethings but very well as photons. In fact Quantum Electrodynamics is one of the best-established theoretical frameworks ever discovered by men, but it doesn't say that electromagnetic phenomena are described by particle-like entities. Photons are "single-particle" Fock states of the electromagnetic field (unfortunately there's no other word than "single-particle" here, which can give rise to misunderstandings when communicated to lay men who are not familiar with the proper mathematics underlying relativistic QFT). You cannot say anything else about them. It's a fundamental notion. A photon is far from being in any way like a "particle" in everyday life, because you cannot even define a position for it, i.e., it's not a localizable object like a little macroscopic body, which we call a "particle".

9. Jul 21, 2015

### Stephanus

Okay, thanks everybody.
One last question.
Is the speed of light = the maximum speed in the universe?
Is the speed of em wave = the maximum speed in the universe?
Because in Newtown experiment with prism, light bends/bent.
Perhaps this is the more appropriate question?
Is the speed of light in vacuum = the maximum speed of the universe?

10. Jul 21, 2015

### vanhees71

As far as we know the relativistic space-time description works with utmost accuracy. Thus there's a "cosmic speed limit", which is valid for all physical processes, inclusively electromagnetic waves.

To a very high accuracy, the phase velocity of electromagnetic waves in a vacuum is this limiting speed.

The "bending of light" due to refraction in a prism is not very surprising and can be understood also with the non-relativistic treatment of matter. It's simply the scattering of light on the charged particles making up the material the light wave propagates through.

What's more related to General Relativity is the "bending of light" on heavy bodies like the Sun due to the gravitational interaction. Besides the perihelion shift of Mercury the correct prediction of the "bending of light" on the Sun lead to the breakthrough of Einstein GR in 1919.

11. Jul 21, 2015

### Staff: Mentor

Yes, the speed of light in a vacuum is the maximum speed in the universe (although there is a subtlety about how we define relative speeds of objects that are widely separated in curved spacetime).

Light in a medium will move at a slower (and, in general, frequency-dependent) speed, and that's how a prism does its light-bending thing.

12. Jul 21, 2015

### vanhees71

The speed of light in GR, as I understand it in my previous posting #10 should be considered a local speed, indeed.

When describing the speed of a wave like light in a medium, you must, however, be careful. First you have to tell, which speed you are referring to, and the only thing that is really subject to the "general speed limit" is the front velocity of wave packets with compact spacial support. All other notions of speed (particularly phase velocity and group velocity) are not subject to the "general speed limit", but this doesn't invalidate the relativistic space-time structure and causality. This is well-understood (at least in principle) already for more than 100 years (Sommerfeld 1907, Sommerfeld and Brillouin 1912).