The Science Thread

[Corribus]

/delurk

First of all, photons do have momentum.

Second of all, to address your question, think of it this way: time is (in relativity) a function of velocity. As velocity increases, time slows down, until, at the speed of light (speed of massless particles), time stops completely. You can't go faster than light because at the point your velocity reaches the velocity of light, there is no more time. I.e., time cannot slow down any more. Thus, the limitation of velocity is the extent to which time can still flow.

Well, that's the way I look at it anyway. Probably a poor explanation. Relativistic physics isn't my specialty.

/lurk

[Pol]

/delurk

Thus, the limitation of velocity is the extent to which time can still flow.

/lurk

So, if we would be doing our work really fast, we could be doing it infinitively long?

[Kristo]

As velocity increases, time slows down, until, at the speed of light (speed of massless particles), time stops completely.

But isn't that a result of the premise that c is the same in all reference frames? I agree that's how the math works out, but it just doesn't make physical sense to me.

[Banedon]

Let me try ...

The principle of relativity basically says: all laws of physics are the same in all reference frames. There is nothing mystical about this and nothing hard to comprehend, just what is a very reasonable statement that seems to correspond to reality.

Now the laws of electricity and magnetism (Maxwell's laws) take a very specific form in a vacuum. There are equations governing how the electric field will evolve, how the magnetic field will evolve, and how changing one necessarily changes the other. From these equations you can construct a wave equation made up of electric and magnetic fields. Wave equations describe waves, e.g. water waves and sound waves. From the standard form of the wave equation you can read off the speed of the wave. It turns out that the speed of this wave created from the electric and magnetic fields is the speed of light - you can see this yourself: c = 1 / sqrt(e0 * u0), where e0 and u0 are the permittivity and permeability of free space respectively.

Now the permittivity and permeability of free space are (obviously) constants. They don't move and they don't change as you move from one reference frame to another. But if they don't change the speed of light doesn't change either!

... therefore either the principle of relativity fails to hold, or the laws of electromagnetism are incomplete, or the speed of light is truly constant. And experimentally, it works out to be the last option.

[Kalah]

permitvi..tvy.... um..

Yeah. That's probably right, what B said.

[ThunderTitan]

You can't go faster than light because at the point your velocity reaches the velocity of light, there is no more time. I.e., time cannot slow down any more. Thus, the limitation of velocity is the extent to which time can still flow.

So when they made light go faster the c (which is the speed of light in vacuum) what happened?

[Kristo]

Again, I can't argue with the math. I guess my brain isn't wired up to understand this. I can do the math for a wave of water and figure out how fast it's moving. If I'm floating along at the same rate as the wave of water, the wave will appear to be standing still. But I can't do that with light apparently. No matter how fast I'm moving, light will always appear to be moving at c relative to me. Why does light have to be special? Why can't I move relative to it?

FWIW, I realize that my intuition is in conflict with the last 100 years of science. That's why I'm looking to understand it.

[Banedon]

Why does light have to be special? Why can't I move relative to it?

Well if you could, the principle of relativity would be violated.

You see? The speed of light depends on two things: the permittivity and permeability of free space, i.e. a vacuum. If you are moving relative to the vacuum, the vacuum is still a vacuum (you don't create particles by moving). The permittivity and permeability therefore don't change. If they don't change then the speed of light doesn't change either. Light is special in this sense because its speed depends on constants, not on anything else.

[Gaidal Cain]

That's not an explanation. You're using circular reasoning.

[ThunderTitan]

That's not an explanation. You're using circular reasoning.

Well it is physics... as long as it works who cares it makes no sense...



But from the explanations here i'm guessing that it's a constant coz you can't go faster...

[Banedon]

That's not an explanation. You're using circular reasoning.

Lol

Why don't you try then?

[Kristo]

That's exactly my problem with understanding this. Every explanation I've ever seen basically reduces to, "it has to be that way or else Einstein would be wrong." I have no problem with your flashlight not working on a train traveling near c. I don't see how traveling faster than c violates causality. It's your fault if your measurement device (e.g., photons) is too slow to accurately measure what's going on.

[Banedon]

Do you find the principle of relativity hard to accept?

By the way why traveling faster than the speed of light violates causality is a rather complex question. Let's just say you have a problem with it because you're grounded in the Newtonian view of space and time - a view where there is no upper speed limit. Einstein's great insight (and supported by experiment, too) is that space and time ARE closely interwined, and that you can cause clocks to slow down by moving, and Newton's idea of absolute time wrong.

Also by the way the explanations don't reduce to "it has to be that way or else Einstein would be wrong". Einstein is a scientist, and he can be wrong. He has been, elsewhere. But relativity passes the litmus test of experiment. If something is verified by experiment but theory can't explain it, then theory is wrong, pretty much.

[Gaidal Cain]

Why don't you try then?

Well, I don't think there is any good explanation. Modern physics is pretty counterintuitive. You could appeal to the Maxwell equations and show how they are invariant under Lorentz transformation or something, but that would just be to hide behind mathematics.

[ThunderTitan]

Wait, isn't it because after it mass doesn't exist at all anymore...

[Kristo]

Can we even set up an experiment to test whether the speed of light is still c when moving sufficiently fast? I don't know if we can move something fast enough where you couldn't explain away any differences to experimental errors. We're fixed relative to the Earth, but the Earth is moving at 20-30 km/sec around the sun, the sun is moving at 200 km/sec through the galaxy, and the galaxy is moving at who knows how fast across the universe. And I'll bet even that last number isn't large enough relative to c to matter.

I'm comfortable with the laws of physics being the same in all reference frames. I can't wrap my head around "Speed of light = c" being one of those laws. To my Newtonian mind, light is always in the absolute reference frame. I can't comprehend a reason why you can't move relative to it.

[Pol]

@Kristo
But light (speed of light) isn't a constant. Can't be - there is no "ideal" light not in theory and not in the real world.

Ponder:
Gravity affects light.
It's a wave.
There's not exactly defined what is "the light".
Eyes are most deceiving human sense.
=not a constant although you may set one for a future reference, with more exact definition. (As always.)

[Gaidal Cain]

Can we even set up an experiment to test whether the speed of light is still c when moving sufficiently fast?

Nope, we basically lost that chance when we decided to use the speed of light to define the meter. Under the current system, it's impossible to measure the speed of light: we would be measuring how accurate our instruments are instead.

What we do have is ample evidence that the theory of relativity is correct. For example, GPS wouldn't work as well if we didn't add relativistic corrections.

[Kristo]

Yes, and we've defined time in terms of the decay rate of a cesium atom. Do we really know enough about the effects of gravity and high angular velocity on cesium atoms to be sure we're really observing time dilation?

[Kalah]

You guys are weirding me out...