# Why is the speed of light exactly exactly 299 792 458 meters per second ?

1. Dec 15, 2008

### Strangerone

Is there any published theory that explain and proves why the speed of light is exactly 299 792 458 merters per second ? I do not know of any ! Do you ?

Best regards
Me :-)

2. Dec 15, 2008

### Phrak

The length of a meter is defined in terms the distance light travels in one second.

3. Dec 15, 2008

### Staff: Mentor

We define the speed of light to have that value, thereby defining the meter as the distance light travels in 1/299792458 second.

Before we did that (in 1983), we had great confidence that the speed of light was always that value, based on the theory of relativity and experimental measurements (taking experimental uncertainties into account of course).

See section 3 of What is the experimental basis of Special Relativity?

4. Dec 15, 2008

### templedog

No there isn't, so far. I just read a book called Beyond Belief by A. K Dewdney. The speed of light is one of the 8 unreasonable things speaks of.

5. Dec 15, 2008

### Naty1

Maxwell's equations compute the speed of light as 1/(sq rt [(permittivity)(permeability)]

Wikipedia, http://en.wikipedia.org/wiki/Maxwell's_equations

under "In Vacuum".....and played a big part in causing Einstein to develop special relativity

6. Dec 15, 2008

### Staff: Mentor

As Phrak mentioned this is a result of the http://www.bipm.org/en/si/si_brochure/chapter2/2-1/metre.html" [Broken]. This definition was chosen because measurements of the speed of light immediately prior to this time had become so precise that the primary source of error was the uncertainty in previous standards for the meter.

Last edited by a moderator: May 3, 2017
7. Dec 15, 2008

### epenguin

I think it's the third time we got this question in a week: others were "reason for value of c"
and "Why does light travel so fast???"

Does this happen every week? (Or is it like London buses, which come rarely but then three at a time?)

8. Dec 15, 2008

### Staff: Mentor

The most popular FAQs seem to be the value of c, the reference frame of a photon, and the twins' paradox. I think that all of them are about once per week on average, but very "streaky" with the first in a streak quickly prompting two or three others and then it dies down for a month or so.

I guess we are ending a "value of c" streak, we just finished a "reference frame of a photon" streak, and I think we just had the first installment of the next "twins' paradox" streak earlier today.

9. Dec 15, 2008

### Fredrik

Staff Emeritus
That must be a great comment, because I was thinking almost exactly the same thing. But you forgot to mention the mass of the photon.

10. Dec 15, 2008

### Staff: Mentor

D'oh! You are absolutely right. And that one is actually even more frequent than it appears since it gets split between here and the QM forum.

11. Dec 15, 2008

### epenguin

You should recombine them and get a diffraction pattern.

12. Dec 16, 2008

### Strangerone

Thanks to you all for good arguments and links ! :-)

Regarding the equations that include the permittivity and permeability:

Ok, this constants, the permittivity and permeability, works fine in the equations. But if I understands this correctly such an equation does not reveal anything about why the speed of light is exactly what we observe it to be. The equation only moves the problem from one area to another area, from the area of quantummechanics to the area of the permittivity and permeability of the medium of space ! And therefore this theory does no answar the question, if I understand it correctly.

Best regards
Me :-)

13. Dec 16, 2008

### Strangerone

The permittivity and permeability of the medium of space is just another constant.

14. Dec 16, 2008

### Staff: Mentor

The value of the permeability of free space is a result of the http://www.bipm.org/en/si/si_brochure/chapter2/2-1/ampere.html" [Broken]. And then (together with the definition of the meter which defines the speed of light) the permittivity of free space is also defined exactly.

You seem to think that there is some reason for the value of these constants other than the choice of units. There is not. The values of all of the dimensionful fundamental constants are simply an artifact of our choice of units and are therefore completely arbitrary. For example, http://en.wikipedia.org/wiki/Planck_units" [Broken] makes the convenient choice that all of these dimensionful fundamental constants are equal to 1.

Last edited by a moderator: May 3, 2017
15. Dec 16, 2008

### Phrak

The standard meter was originally supposed to be a convenient length of 1/10,000,000th the distance from the Equator to the pole. (The calculated distance from equator to pole was not as precise as hoped, but the calculated meter survives today, in refined form.) It became defined as the distance between two scratched on one particular beam of material stored in some environmentally controlled vault somewhere. France, I would guess. There were copies of this beam distributed around the world in various national institutes of standards. As the desire for precision increased the distance between the centers of two scratches became limiting. Added to this was the ever present fear that the original could be destroyed, throwing everyone's data and standards into bias.

I hope I haven't been too inventive in the above. I'm recalling this from memory.

Last edited: Dec 17, 2008
16. Dec 16, 2008

### Strangerone

There seem to be a manmade constant at the bottom of this bottle wherever I look ! First, the constant of C, then the constant "permittivity and permeability" related to the medium of space, and now the value of "ampere" which is a consequense of the "magnetic constant", also known as the permeability of free space, measured at exactly 4 x 10–7 henries per metre, 0 = 4 x 10–7 H/m.

17. Dec 16, 2008

### Naty1

If I understand this comment correctly, I could not disagree more. The purpose of physics is to both explain how things work (say via math) and also why they work. We are not so good at the latter as the former, I think.

There has to be an underlying reason why light speed is what it is, regardless of units chosen...Lights moves at "it's own pace" regardless of the units we choose to use to describe it's speed. It's a contsant for a reason and has a value for a reason. Electromagnetic waves propagate at "c" for a fundamental reason(s) that is as yet unknown. (I can guess why c is constant because if it varied a lot, nothing would be here...electromagnetic waves would take on a life of their own and atoms would likely never form or if they did would likely break apart immediately....)

And there is a mass energy equivalent involving lightspeed for reasons I have not seen...........why should mass and energy be related by lightspeed? Exactly what causes mass? all of these are unanswered "why" questions.

18. Dec 16, 2008

### Alfi

lol @ Strangerone
I'm one of the 'zero' haters.
It does go a bit round and round to prove itself.

I believe the Kilogram is the last artifact yet to be mathematically derived.

hehehe
Mass must weigh heavily upon physicists.

19. Dec 16, 2008

### Strangerone

What is the exact physical reality behind these observed constants. Theres is, as far as I know, no published theory that can tell this. There is only math related to these observed constants. But why does scientists accept this?

20. Dec 16, 2008

### Naty1

while reading Wikipedia just now on Lorentz Transformations,
under DERIVATIONS
http://en.wikipedia.org/wiki/Lorentz_transformation

I (accidentally) came across the following....

(my boldface.)

I don't doubt what is said, but neither do I understand it....this is kind of the point I was trying to make in my last post above....I've never seen a statement quite like this...Anybody know what the above text says about WHY the speed of information transmission must be invariant???

21. Dec 16, 2008

### Naty1

I don't think we should assume this a "acceptable", just that it reflects our current lack of understanding. Maybe when we eventually unify quantum theory and relativity we may gain insights as to how all the constants emerged from "nothing" at the start of the universe...maybe there are an infinite number of such combinations and only a few lead to viable universes, maybe there are only a fixed number of possibilities (analogous to the only three possible shapes of our universe, flat, spherical, saddle shaped) or maybe quantum foam has a group of rigidly fixed "constants" hiding....just waiting to pop out....

It's like asking "Why do scientists accept that we can't cure the common cold". It's because so far "virus's" are smarter than we are!!!

22. Dec 16, 2008

### Strangerone

Extreemly well observed Naty1 :-)

Have a nice evening .

23. Dec 16, 2008

### MeJennifer

I do not think you understand it.

The question is only whether the speed of light is limited or unlimited. Clearly it is limited but the numerical value is completely arbitrary since it depends solely on the chosen units of measurement.

24. Dec 16, 2008

### lightarrow

*IF* you can prove what Landau says, that is that:
"there exists a theoretical maximal speed of information transmission"
then it's quite easy to prove it's invariant: if it wouldn't be, you could always find a frame of reference in which it has a greater value, so it wouldn't have been the maximal...
The problem is to prove that statement...

25. Dec 16, 2008

### lightarrow

If with "why light speed is what it is" you mean the *numerical value*, then, as others have explained, there is no deep reason, just choice of units. If you mean "It's a constant for a reason" and you are talking about the experimental fact (and not the theory, because in the theory it's constant as postulate), then we can discuss about it...
About mass and energy, if you chose the units of time and space to have a completely different value of c, then the energy value changes too and you still have the same value for your mass. The fact that mass and energy are related by that equation it's another story, it doesn't have to do with the numerical value of c.