Why is the speed of light constant?

[vish_al210]

Thanks Ash.. well, I get wht u r saying.. maybe I should also change my presentation of speed (atleast for a wave)..
But was the question in ur last statement directed to me or in general.. as I don't remember discussing that.. ?
Well in that question I hope u are referring to only the constituents of the light wave and not all other waves like sound, AC waveform, etc, as u may be battered by a flurry of q's and A's.. Just a safety clause thought... ;)

 

[vish_al210]

And I guess both of us saying the same thing..is not helping and also maybe we have pushed this thread off topic.. hope not...

 

[DevilsAvocado]

Gentlemen; Corvettes, wavelengths, frequencies and Pi is all interesting, but I think some of you are missing the point, discovered by this most distinguished man:

[PLAIN]http://upload.wikimedia.org/wikipedia/en/a/a7/Einstein_TIME_Person_of_the_Century.jpg

If you leave out Albert Einstein and his special theory of relativity (SR), you will never understand anything about the speed of light. Special relativity incorporates the principle that the speed of light is the same for all inertial observers regardless of the state of motion of the source.

We measure speed as the length 'something' has moved, divided by the time it took for it to move, right?

So how is this achieved!?

Well, if the speed of light is invariant (constant) for all observers in all inertial frame of reference, something else must change, right?

And this is exactly what happens according to SR; length contraction, time dilation and relativity of simultaneity, contradicting the classical notion that the duration of the time interval between two events is equal for all observers.

All of the SR predictions have been experimentally verified; if you are using GPS, it wouldn’t work without correction for the time dilation effect, resulting in a delay about 7 μs/day. Without corrections for SR & GR, errors of roughly 10 km/day would accumulate. This is a fact.

If SR states that that all uniform motion is relative, and that there is no absolute and well-defined state of rest (no privileged reference frames) – from mechanics to all the laws of physics, including the laws of electrodynamics, why bother about "the speed of light"??

We can’t even agree on what the "Universal Time" is, and what "Now" means, for God’s sake!

The 4D spacetime is a "stretchy rubber" and the only constant that’s the same in all inertial frame of reference is the speed of light = 1, period.

[URL]http://upload.wikimedia.org/wikipedia/commons/e/e4/Lorentz_transform_of_world_line.gif[/URL]
Spacetime along the world line of a
rapidly accelerating observer

Am I talking bull?

Well, let’s take this classical example of a speeding railroad car, demonstrating the relativity of simultaneity. Observer A is onboard and observer B is standing on the platform, and a flash of light is given off at the center of the railroad car just as the two observers pass each other:

[URL]http://upload.wikimedia.org/wikipedia/commons/thumb/c/ce/Traincar_Relativity1.svg/250px-Traincar_Relativity1.svg.png[/URL]
From the frame of reference of A, the light will reach the front and back of the railroad car at the same time.

[URL]http://upload.wikimedia.org/wikipedia/commons/thumb/7/72/Traincar_Relativity2.svg/294px-Traincar_Relativity2.svg.png[/URL]
From the frame of reference of B, the light will strike the back of the railroad car before it reaches the front.​

This is the only possible solution, without variant (faster than) speed of light, from the frame of reference of B.

But what happens if we put two 1967 Corvette Sting Ray in the middle of the railroad car, going in opposite directions at the exactly the same speed??

Well, they will of course strike the back and the front at the exactly the same time, for both observers A & B.

That’s the only "real" difference between a Corvette Sting Ray and speeding light.

Happy holidays!

 

[russ_watters]

I'd argue with this, I've tried to answer this question before and got into trouble with a moderator, but the answer I'd give this time is that it is not, as several people have said, a ratio of distance and time (or similar).

The answer is wavelength times frequency. (wavelength in metres, maybe, and frequency per second, maybe.)

Whenever you multiply wavelength by frequency you get the speed of light, in whatever units you choose to use, much the same as circumferance divided by diameter gives Pi.

It is the relationship between wavelength and frequency that we should be discussing.

(Maybe I'll get into trouble again?)

\lambda = \fract{v}{f}

That's a pretty basic relationship that offers nothing profound to say about the speed of light.

...different wavelengths have different periods, but wavelength/period is always a constant.

Um, well, yes - they are inverses of each other. That's also nothing profound. It's basically just a definition. The inverse of meters per cycle is cycles per meter.

 

[DevilsAvocado]

The inverse of meters per cycle is cycles per meter.

Good point!

 

[Rap]

Very interested, please osp details.

Sorry, I cannot give the reference either. I read the same thing on Wikipedia, on some relativity-related article, and it sounded valid.

 

[Buckleymanor]

Very interested, please osp details.

If I remember the main premis of the M.M. experiment it was that if there was any difference in the speed of light in one direction to another direction it would show up in a decoherent interferance pattern when the two beams of light from the interferometer were recombined.
There wavelengths would be out of sink.
The two beams of light were configuered at right angles to each other and the whole apparatus could be rotated.
It was thought at the time that if there was an aether wind light traveling across the wind would take a longer period to come back to the observer than light traveling with the wind and back.
A popular analogy was that a swimmer traveling up stream and back would take less time than the same swimmer traveling the same distance across the stream and back.
Or was it the other way round.
Anyway it was decided that there would be a difference.
The problem with this is that if there is an increase in speed in one direction after the light hits the mirror it will loose any gain in speed in the return journey to the observer.
The amount of speed lost will be exactly the same amount as any gained it will be exactly canceled out.
There would be no difference if the interferometer was at right angles at 180degrees or any specific angle.
The light traveling at right angles to the horizontal would not gain or loose any speed.
Where as the light traveling along the horizontal would gain or loose speed in one direction then gain or loose speed in the other.
The net result being that when the light from the horizontal and that at right angles was recombined both beams would be traveling at exactly the same speed and moveing along the same plane.
So result would show no increase in speed as far as the observer was concerned allthough the light had been traveling at different speeds throughout it's journey.

 

[DevilsAvocado]

And you did take the distance between the Sun and the Earth into your calculations?

 

[jhmar]

If I remember the main premis of the M.M. experiment it was that if there was any difference in the speed of light in one direction to another direction it would show up in a decoherent interferance pattern when the two beams of light from the interferometer were recombined.
There wavelengths would be out of sink.
The two beams of light were configuered at right angles to each other and the whole apparatus could be rotated.
It was thought at the time that if there was an aether wind light traveling across the wind would take a longer period to come back to the observer than light traveling with the wind and back.
A popular analogy was that a swimmer traveling up stream and back would take less time than the same swimmer traveling the same distance across the stream and back.
Or was it the other way round.
Anyway it was decided that there would be a difference.
The problem with this is that if there is an increase in speed in one direction after the light hits the mirror it will loose any gain in speed in the return journey to the observer.
The amount of speed lost will be exactly the same amount as any gained it will be exactly canceled out.
There would be no difference if the interferometer was at right angles at 180degrees or any specific angle.
The light traveling at right angles to the horizontal would not gain or loose any speed.
Where as the light traveling along the horizontal would gain or loose speed in one direction then gain or loose speed in the other.
The net result being that when the light from the horizontal and that at right angles was recombined both beams would be traveling at exactly the same speed and moveing along the same plane.
So result would show no increase in speed as far as the observer was concerned allthough the light had been traveling at different speeds throughout it's journey.

Perhaps the following is what you are referring to:

http://en.wikipedia.org/wiki/Michelson–Morley_experiment

It is very interesting, but it does not proove Einstein's theory right or wrong, it simply negates the experiment; my sugestion is a none mathematical hypothesis of the action that causes Einstein theory to work.

 

[PhanthomJay]

I got the feeling from some of the posts here that the speed of light is just what it is, sort of a philosophical speed, and was wondering if it might be possible for humans, after a million years of math/phiysics, to change that outlook, to prove that c must be what it is, rather than just accept is as a sort of "god number." ... is such a feat fundamentally impossible?

The finite speed of light is not a philosophical speed. In our Universe, it is a function of space permeability, permissivity, Planck's constant, and all that other good stuff, which, in turn , are functions of the Fine Structure Constant of OUR universe, which happens to be , as was pointed out earlier, approximately 1/137. So it is best to ask why this constant is what it is, rather than ask why the speed of light is what it is, because now you avoid having to include the the standard definitions of meters and seconds, which puts you in an endless cyclic loop of defining one in terms of the other.

So why is the Fine Structure Constant of our Universe 1/137? I think Hawking/Mlodinow in their latest book hint at this answer (well, its not yet an answer and won't be until further advances in theories of quantum gravity, M-Theory, TOE, etc., or maybe there will never be an answer), but until I finish the final chapter, I'll provide comment later on my understanding, for what it may be worth.

 

[Oldfart]

The finite speed of light is not a philosophical speed. ...or maybe there will never be an answer),

Well, if the answer may never be known, it sounds philosophical to me -- though I must admit that the speed of philosophy is not well defined...

Thanks for the reply, happy new year!

 

[PhanthomJay]

The finite speed of light is not a philosophical speed. ...or maybe there will never be an answer),

Well, if the answer may never be known, it sounds philosophical to me --

you may be right

though I must admit that the speed of philosophy is not well defined...

I thought it was the same as the speed of time

Thanks for the reply, happy new year!

Same to you, thanks. I Read Hawking's last chapter just after Dick Clark's countdown. I'm told the million dollar ball dropped, but i guess i was not watching carefully, because I never saw the damn thing drop. Maybe it was one of those Vacuum Fluctuations...

Anyway, Hawking/Mladinow's book fell short of a masterpiece, but nevertheless, it is a MUST read that reflects the latest thinking on the the Universe on both the Quantum and Cosmological scales that one day will be merged, perhaps. It didn't really answer anything that I haven't already learned from the expert responders on the Cosmology forum ..they are second to none in my estimatiion...especially, but not limited to, Chalnoth, who must come from the Great Beyond somewhere.

I did learn , per my understanding, a few new things new, however, such as

-The nature of the Time Dimension in the very very very early stages of the Universe;
- The Sum of Histories leading to the multiverses (I've got a lot of work to due on this, however);
- the nature of Inelligent Life, and,
here's the biggy:
- The Theist may be Gravity...unless i misunderstood, which i probably did.

I'll be posting more questions to the Cosmologists here, on the Cosmology forum.

 

[DevilsAvocado]

So it is best to ask why this constant is what it is, rather than ask why the speed of light is what it is,

Excellent PhanthomJay! Just Excellent!

I’ll order that book NOW! I have http://en.wikipedia.org/wiki/The_Universe_in_a_Nutshell" and it’s just wonderful + fantastic graphics!

P.S. I also believe in Gravity.

 

[DevilsAvocado]

P.S.2. It’s this one you’re reading, right?

http://en.wikipedia.org/wiki/The_Grand_Design_(book)"

 

[PhanthomJay]

P.S.2. It’s this one you’re reading, right?

http://en.wikipedia.org/wiki/The_Grand_Design_(book)"

Yeah, that's the One!

I believe in Gravity, ---World without End...Amen!

 

[DevilsAvocado]

Thanks!

Amen

 

[Rap]

Well, I've been thinking about this. There are a bunch of dimensionless constants of the universe, like, for example, the fine structure constant α which has to do with the electromagnetic force, and involves the charge and mass of an electron. Another one is the gravitational coupling constant α_G which has to do with the gravitational force, and involves the mass of an electron and the gravitational constant. I guess there are others, having to do with the other forces, weak, strong, etc. I always thought that if you had another universe where all the dimensionless constants were the same, you would not be able to detect that you were in a "different" universe. In other words, all the lengths, times, masses, etc. might be "different", but if you used the same procedures to define units of length, time, mass, etc., the numerical value of the fundamental constants (speed of light, Planck constant, mass of electron, electron charge, vacuum permittivity, gravitational constant, etc.) would all be the same. If they were not, you would be detecting that you were in a "different" universe.

But I can't get it to work. If you take three fundamental constants, electron mass (m), Planck constant (h) and speed of light (c), you can form a fundamental mass (m), a fundamental length (L=h/mc) which is the Compton wavelength, and a fundamental time (T=h/mc^2) which is 1 over the Compton frequency. If you go to a new universe, where the speed of light is c' (as measured by our present universe meter sticks and clocks) but all the dimensionless constants remain the same, then some or all of the fundamental constants will have to change their values in order to keep the dimensionless constants the same. That means that the Compton wavelength may change by some unknown factor due to the change in the speed of light, and the possible corresponding changes in the Planck constant and the mass of the electron required to keep the dimensionless constants the same. Suppose the values of h and m in the new universe are h' and m' as measured by our clocks and meter sticks. Then the new Compton wavelength will be L'=h'/m'c' and the new Compton time will be T'=h'/m'c'^2. The new speed of light will be measured as L'/T'=c', not c!. For example, if, in the new universe, the speed of light were cut in half, and h and m remained the same, and the dimensionless constants were kept constant by changing the vacuum permittivity and the gravitational constants, then all lengths would double in the new universe, but all times would quadruple. A meter stick would be twice as long, but a clock would click 4 times more slowly, and the numerical value of c would be half of what it is in our universe. It appears that the speed of light is something that must have its particular value in order for the universe to be what it is, and it is a valid question to ask why it has the value it does. Either that or I missed something...

 

[PhanthomJay]

It appears that the speed of light is something that must have its particular value in order for the universe to be what it is, and it is a valid question to ask why it has the value it does. Either that or I missed something...

Thanks for the response! The most popular responses to the question regarding why the speed of light is what it is are:
1.) "It just is", and
2.) "Its the way we define the meter and second"

Neither one of these satisfies me...I think it's got something to do with Feynman's sum of histories...but what do I know about the quantum world...and If I read Hawking correctly, he's on his knees worshipping Gravity...

Thanks again.

 

[Rap]

Number 1 is not very satisfying and 2 is wrong. My wrong (I think) idea was that if the speed of light changed, but every dimensionless constant stayed the same, you would not be able to tell that anything had changed, because other things would have to change to keep the dimensionless constants the same, and the dimensionless constants are what determines the physics of the universe. The meter stick would change, the second would change, and the numerical value of the speed of light in meters/sec would come out the same.

 

[Rap]

I think I figured out where my argument went wrong: c' is the speed of light in the other universe as measured by us, not by them. So cancel that whole argument. I go back to my original argument. It makes no sense to ask how fast is light any more than it makes sense to ask how long is a meter, or how long is a second. The minute you answer that question, you have made reference to some other velocity, length, or time. So what you are really asking is why is that ratio what it is.