# Speed of light and light

1. Sep 10, 2006

let me start off saying that i dont know if this is in the right section.

then let me second that by saying this may be a stupid question.

now let me begin the actual question phase.

1. i watched a video online, i forgot where it was and what it was about, the only thing i picked up from it was that it said a photons move at the speed of light, so therefore doesnt have mass. but then it said that anything that moves the speed of light has infinite mass, is that correct? i donut understand that.

2. is it possible to slow the speed of a photon down? what would u see if this would be possible? if you could, THEN would it gain mass?

2. Sep 10, 2006

### Tomsk

If something has a rest mass (that is, mass when it is not moving), it would require infinite energy to get to the speed of light, which would give it infinite mass (as energy has mass). This is why nothing (with rest mass!) can travel at c, as nothing can have infinite energy or mass. Photons don't have rest mass, so can travel at c. In fact, because they have no rest mass, they must travel at c. To confuse matters further, photons do have some energy, and therefore mass, but it is not rest mass, it comes from the photon's motion.

They can't be slowed down, although in various media, they can appear to travel slower, but between the atoms of the medium they are still travelling at c.

I hope that helps. It's pretty confusing, and quite hard to explain.

3. Sep 10, 2006

### HallsofIvy

No, it didn't say that. I suspect it said that if an object with non-zero (rest) mass could move at the speed of light then it would have infinite mass. But it had just said that anything moving at the speed of light doesn't have mass hadn't it?

No, it is not possible. As long as a photon exists, it moves at the speed of light relative to any coordinate system.

4. Sep 10, 2006

### JustinLevy

Well, in any inertial coordinate system.

I remember there was a big arguement on this forum once about someone claiming to have measured the speed of light in a coordinate system independant manner (which of course is not possible). So it is probably best to be careful about the wording of such statements.

Although if one reads your post literally, a photon does move at the speed of light in any frame because light is composed of photons :). Or is that what you meant in the first place?

Anyway, to answer Quadruple Bypass's second question, it has been suggested that due to QED effects it may be possible to very slightly increase the speed of a photon ( http://en.wikipedia.org/wiki/Scharnhorst_effect ). A photon leaving such a situation would also then "slow down" I guess. This doesn't cause it to "gain mass" though.

5. Sep 10, 2006

### HallsofIvy

Well, yes, any inertial coordinate system. Unfortunately, I tend to think of that as "given" in any SR question!

No, that would be circular! I am tempted, however, to ask how you define "photon". Any object, having 0 rest mass (as is true of photons) can only exist at the speed of light.

6. Sep 10, 2006

### JustinLevy

Is this comment about the Scharnhorst reference? I'm not really sure what you are objecting to.

One person's response to the Scharnhorst effect was along the lines of: there is a maximum velocity in an inertial frame, unfortunately this was called the speed of light for historical reasons, but a light interacts with the virtual particles in vacuum and thus macroscopically travels very slightly slower than the maximum velocity.

It is an interesting view of the Scharnhorst effect, and I kind of prefer that interpretation. Although others have written a paper explaining why the Scharnhorst effect doesn't destroy causality.

7. Sep 10, 2006

ah thanks for the replies. it helped :D

light is so cool lol :S

8. Sep 10, 2006

### danriley

Variable c, neutrinos, 1987a

Hi,
I'm not sure the proper way to start new questions, but if some people could answer this one, and tell me how to, I hope that's OK.

If (if!) c were variable over time because of changing permitivity/ permeability, or because of changing free-space energy, changing density of virtual EM-interacting particles...that sort of thing, then:

would it be fair to have expected a severe differential in arrival times of light and neutrinos from 1987a?

Since, even given an analogous impediment of neutrinos from interaction with their respective virtual particles, it would hardly be likely to have given so close a match in arrival time with that of light, in which the expected difference in arrival (from differing production proccesses of a supernova of light and neutrinos) matches the difference in actual observation.

The observations suggest strong and elegant confirmation both that c is truly invariant, and that the speed of all particles is ultimately linked to it.

Thanks for any answers. Please keep it simple, as I'm a dumb layman.

9. Sep 10, 2006

### rbj

it is operationally meaningless to consider a changing $c$ or a changing $\epsilon_0$ or $\mu_0$ or any other dimensionful constant since their value is purely a human construct. now a chainging dimensionless constant like $\alpha$, that's a different story.

there's a very fundamental reason for that invariance of $c$ (like it's a defined constant). if $c$ were to change, you have to tell us exactly what measurement would change? that measurement was originally a dimensionless number which means that $c$ was measured against a like-dimensioned standard. if that dimensionless quantity was observed to change, there are other quantities in that dimensionless quantity that could have changed instead of $c$. all we know is this dimensionless quantity involving $c$ and other physical quantities has changed. we cannot ascribe that change to just $c$ since there are other quantities that went into it.

check out the wikipedia articles on VSL, Planck units, Natural units, etc.

10. Sep 10, 2006

### MeJennifer

That is correct c is exactly 299,792,458 metres per second, by definition!

11. Sep 12, 2006

### danriley

I think you mean exactly 299,792,458 metres per second, by definition, in vacuo.

I appreciate constancy in Einsteinian relativity. But there are variant relativities, and Majuiro (sp?), et al have challenged this fundamental idea of constancy.

I'd like to examine the experimental side for a moment, not the theoretical. Would the non-differential (after very long distances and time) in the arrival of light and neutrinos constitute strong experimental evidence of the de facto constancy of c, and that Dirac particle pairs offer no impediment whatsoever to the propagation of light?

12. Sep 12, 2006

### Staff: Mentor

At the top of the list of discussion topics (or "threads") in this forum, there is a button labeled "New Topic."

13. Sep 12, 2006

### MeJennifer

It seems you do not understand, c is a constant not a measurement.

Furthermore the meter is defined by the speed of light. One meter is exactly the distance traveled by light in a vacuum in 1/299,792,458 of a second.

14. Sep 12, 2006

### danriley

I don't see where I said or implied 'measurement'. I used the term 'constancy' three times. Using light to define the measure of a metre is a convention. If I recall, it was formerly defined as 1/10^6 of the distance from...

...but enough of that; let's salvage the rest of my query....

[I think you mean exactly 299,792,458 metres per second, by definition, in vacuo.]

I appreciate constancy in Einsteinian relativity. But there are variant relativities, and Majuiro (sp?), et al have challenged this fundamental idea of constancy, i.e., VSL.

I'd like to examine the experimental side for a moment, not the theoretical. Would the non-differential (after very long distances and time) in the arrival of light and neutrinos from supernova 1987a constitute strong experimental evidence of the de facto constancy of c, and that Dirac particle pairs offer no impediment whatsoever to the propagation of light?

Last edited: Sep 12, 2006
15. Sep 12, 2006

### MeJennifer

You do not seem to get the subtle point here.

Since the meter is defined as:

the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second,

c is constant by definition! Afteral we measure speed as meters per second right?

Of course you can question the constancy of the speed of light but don't mix it up with questioning the constancy of c.

16. Sep 12, 2006

### danriley

I hope I don't understand the subtle point: that you guys send newbies to another forum to look for a left-handed tensor.

Google "c speed of light constant definition" and examine the variety of loose and formalist treatments of the 'meaning' of 'c', and that will have to suffice for my response to this pendantic exchange. :)

Now let's get on with it:

I'd like to examine the experimental side for a moment, not the theoretical. Would the non-differential (after very long distances and time) in the arrival of light and neutrinos from supernova 1987a constitute strong experimental evidence of the de facto constancy of c, and that Dirac particle pairs offer no impediment whatsoever to the propagation of light?

17. Sep 12, 2006

### Aether

This sounds like it could be good experimental confirmation that the one-way speed of light is independent of its wavelength.

18. Sep 12, 2006

### rbj

i think Michael Duff, John Baez, Steve Carlip (and some other author i can't remember) has blown away the "challenges" from Magueijo, Davies, Moffat and the other VSLers.

it's the dimensionless "constants" that matter. if any of them change, we would know the difference. changing c or G is like changing how many Newtons of force it takes to accelerate 1 kilogram at a rate of 1 m/s2.

19. Sep 13, 2006

### Aether

Good point. Here is a thorough treatment of this important subject: J.P. Uzan, The fundamental constants and their variation: observational and theoretical status, Rev. Mod. Phys. 75, 403, (2003).

20. Sep 18, 2006

### danriley

hang the formalists; let's get on with the real topic

I guess I just have to keep repeating this.