# Why of all things LIGHT?

1. Nov 16, 2005

### cd27

why on Earth did we choose light to say it has a constant velocity? why did we not choose something else? if i'm not mistaken, it has been shown that light can in fact be slowed down or even stoppen in some cases in some special labrotories.

but my question is, why do we use light? isn't there something else? i mean, the entire reason light was declared to be absolute in the first place was because we needed an absolute speed to bounce everything else off with...wouldn't you think the reason light was chosen was b/c it was the fastest thing we KNEW of...not what is really out there.

cd

2. Nov 16, 2005

### Staff: Mentor

Einstein arrived at the theory of relativity via thinking about light, basically as a historical accident. It just happened that the first phenomena that needed explaining via relativity theory, were phenomena involving light.

A more fundamental view of relativity says only that there is a constant, invariant speed, c, that represents the maximum speed of any object or influence. It also says that a particle that has zero rest mass (a.k.a "invariant mass") must travel at that speed and no other. Light happens to fill the bill: photons have zero rest mass.

Until recently, it was thought that neutrinos might also have exactly zero rest mass, in which case they would also travel at speed c, just like photons do.

c is the speed of light in vacuum. Those experiments have light traveling through a non-vacuum.

Last edited: Nov 16, 2005
3. Nov 16, 2005

### Integral

Staff Emeritus
Where did you get this idea? We did NOT choose light. First the speed of light was measured then Maxwell PREDICTED that the speed of electromagnetic waves was a constant
$$c = \sqrt { \frac 1 {\mu_0 \epsilon_0}$$. When Maxwell computed the numerical value of this constant, he found it to be equivalent to the then known measured value of the speed of light.

The speed of light was not chosen to be constant it was FOUND to be constant. Do you see the difference? We do not choose the laws of the universe. Physics is the science of DISCOVERING the properties of the universe.

4. Nov 16, 2005

### HallsofIvy

Staff Emeritus
Relativity, in the classical sense, goes back to Galileo. He argued that if you are in a closed "carriage" moving in a straight line at constant speed, there is no experiment you could do that would determine how fast you were moving or if you were moving at all. That is, of course, assuming perfectly smooth motion- no bumps, etc. In a sense this is because "F= ma": the force we feel depends on acceleration, not velocity.
In particular, that means that there is no such thing as an "absolute" speed. Every speed must be "relative" to something else.

Of course, Galileo did not know about electricity or magnetism! It was shown, centuries later, that the force a magnetic field exerts on an electron depends upon the speed of the electron, not acceleration! That seemed to imply that some sort of "electro-magnetic" experiment could determine an "absolute" speed rather than just "relative" speed. The problem was how to do such an experiment accurately enough. It was when Maxwell showed, as Integral mentioned, that light itself was waves in an electromagnetic field, that the idea of using light in such an experiment developed. It was the "Michaelson-Morley" experiment that tried to do that- and gave a "null" result. It found no "absolute" speed. The "Michaelson-Morley" experiment is one of the most repeated experiments in history- always with better accuracy. It is the experiment that implies that the speed of light, striking receptor does NOT depend on the speed of the receptor.

No, that's not true. No one "declared" light to be absolute- the constancy of the speed of light is the result of experimental evidence.
It was not "declared" to be constant- it was found, experimentally, to be constant.

5. Nov 16, 2005

### cd27

interesting.....thanks for telling me this...i would've been living with a lie wouldn't i? a question would like to ask though...is there a possibility that there is something faster than light...or perhaps, speed isn't the answer...perhaps it's in another place where we haven't looked yet, but when we do, it'll blow our minds away...is that possible?

cd

6. Nov 16, 2005

### Staff: Mentor

There are ways around this "speed limit". With time dilation and length contraction, you can travel to distant places in less time than you might otherwise think it should take. But there is always a catch: If you want to come back to earth after making such a trip, you may find that everyone you knew died years ago!

7. Nov 16, 2005

### Hooloovoo

A particle that travels faster than c is called a "tachyon." A theoretical particle, but there's little reason to think such a particle doesn't exist.

Any such tachyon would experience spacetime in the opposite time direction as we do -- it would appear to be traveling from the "end" of the universe towards its "beginning."

But light speed -- c -- is still the limit. The amount of energy you'd need to pour into a tachyon to "slow down" to light speed approaches infinity, just as the amount of energy you'd need to speed up to light speed would approach infinity. (Just like a limit function in math. If the x axis on the graph is velocity, and the y axis is the energy required to push a particle to that velocity, then the graph would curve sharply up to infinity on either side of c. Tachyons would be traveling at speeds to the right of c.)

8. Nov 16, 2005

### cd27

uh..why?

cd

9. Nov 16, 2005

### Allday

allday

i believe there are reasons that tachyons are considered pretty improbable.

There is always an inertial frame where the energy of a tachyon is negative. Not a huge problem in and of itself but If they interact with normal particles moving at less than c, those normal particles can emit a tachyon with total energy and three momentum being conserved. This suggests that a universe containing tachyons would be unstable.

PS
anyone who has Hartles GR book, this is problem 23 in chapter 9.

gabe

Last edited: Nov 16, 2005
10. Nov 16, 2005

### JesseM

One of the big reasons tachyons are considered unlikely is that if relativity is true, then if tachyons existed you could use them to send information backwards in time! This was discussed a bit on this thread, here was my explanation:

11. Nov 17, 2005

### cd27

interesting...

cd

12. Nov 17, 2005

### Hooloovoo

Not really a problem conceptually. All this requires at the quantum/string level is an imaginary-number mass. As imaginary numbers are all over the math for slower-than-light stuff, it's not much of a leap to say why not here. Tachyons are acceptable string states, for example.

Causality could be a problem, if a tachyon contacting a normal particle were to affect their world-lines so that past and future were indistinguishable, so that information could be sent to either particle's past. But on the other hand, if each particle's world-line was its and its alone, then there's no causality problem because (to flirt with tautology) they are what they are.

(Okay, I wasn't just flirting with tautololgy; I got it drunk and took it to a cheap motel. But you get my point.)