Thread: Well, he did say it was only a theory....

1. Originally Posted by EVOEx
Sorry for snipping your quote, I didn't want to fill an entire page with it again. Thanks for your help, guys. It makes sense, except that I don't understand one single part of it: if we would make a photon (or multiple, but let's stay with one for simplicity) bounce back and forth between mirrors, it would make sense, but when the mirrors would start moving (when they're inside a spaceship for instance), the photon wouldn't start moving along with it, would it? So the photon would simply be lost, from what I understand. To make it bounce back and forth indefinitely, would it not have to be sent out diagonally, matched up with the speed of the mirrors, which is exactly what an external observer would see?
What am I misunderstanding here?
With respect to non-inertial (accelerating/decelerating) reference frames, your assumption is correct; the photon would indeed be deflected. However, once the ship has attained it's target velocity, any emitted photon would obey a straight-line path (within the ship, that is).

For example, suppose our mirrors are one atom wide and distanced one light-second apart. Initially, our ship is "stationary" (purely inertial) and a photon is emitted from one of the (half-silvered or what have you) mirrors. Two seconds elapse, and the photon returns to strike the emitter, just as expected. Now a second photon is emitted, but immediately thereafter the engines are throttled up and the ship lurches forward some distance. Clearly, the returning photon must necessarily miss it's mark. Shut off the engines (whatever the speed) and the results of the experiment once again demonstrate straight-line propagation; "being stationary" and "moving at a constant speed" are equivalent (and thus completely indistinguishable) states.

Ergo, a photon emitted from an inertial reference frame is somehow "resonant" to that frame, in the sense that it acts much like an ordinary particle would by obeying straight-line paths with respect to that frame. Conversely, non-inertial frames (including gravitational fields) bend or deflect the path of light.

2. Originally Posted by gardhr
Shut off the engines (whatever the speed) and the results of the experiment once again demonstrate straight-line propagation; "being stationary" and "moving at a constant speed" are equivalent (and thus completely indistinguishable) states.
Thanks everyone for your answers, I understand it now how it would seem that light slows down. And sorry for hijacking this thread. But if we have no way of finding out how fast you're moving (other than measuring the speed of light), how could we ever have measured the true speed of light in the first place? Because we are moving extremely quickly, aren't we: around earth's axis, around the sun and away from "the place" the big bang occurred.
If we can't even really tell how quickly we're moving, how could we ever know the speed of light? If we would send a rocket in the direction of the big bang at the speed we're moving away from it, wouldn't that speed up light for the observer?

3. Originally Posted by EVOEx
If we can't even really tell how quickly we're moving, how could we ever know the speed of light? If we would send a rocket in the direction of the big bang at the speed we're moving away from it, wouldn't that speed up light for the observer?
No, the frequency of the light would increase, see post #16. Or rather, if the observer were us and the ship moving away, the frequency of the light from the ship would decrease (red shift).

But that is still an interesting thought, since AFAICT, we are only assuming that what is true here is true everywhere. So why wouldn't light speed up somewhere else (for unknown reasons)? E=MC2 is totally dependent on properties of matter as they are observed here on Earth; we simply assume these properties are universal because of Occam's razor.

For anything beyond I would guess the orbit of Mars, the only empirical information we have is from light itself, which when it gets here of course behaves as the Romans do in Rome, providing evidence of ???

4. Originally Posted by MK27
No, the frequency of the light would increase, see post #16. Or rather, if the observer were us and the ship moving away, the frequency of the light from the ship would decrease (red shift).

But that is still an interesting thought, since AFAICT, we are only assuming that what is true here is true everywhere. So why wouldn't light speed up somewhere else (for unknown reasons)? E=MC2 is totally dependent on properties of matter as they are observed here on Earth; we simply assume these properties are universal because of Occam's razor.

For anything beyond I would guess the orbit of Mars, the only empirical information we have is from light itself, which when it gets here of course behaves as the Romans do in Rome, providing evidence of ???
Yeah, I realized my mistake before.. I'm having a bit of a hard time understanding why this observed slowing down of light slows down time itself... But I'll read up on the rest online, you guys made me understand quite a lot of new stuff and I find it very interesting...

Again, thanks everyone!

5. Originally Posted by EVOEx
Yeah, I realized my mistake before.. I'm having a bit of a hard time understanding why this observed slowing down of light slows down time itself... But I'll read up on the rest online, you guys made me understand quite a lot of new stuff and I find it very interesting...

Again, thanks everyone!
"Special relativity is not hard to understand. It is hard to believe".
from "The Six Core Theories of Modern Physics" - Charles F. Stevens

There are a lot of good sites. This one has some good animations:

Time Dilation, Length Contraction and Simultaneity (from Einstein Light)

The play, stop, etc buttons are at the top of each animation image. You might not see the animations
if your security settings are on "high".

I don't think you will find a good explanation for why light speed is constant relative to everyone,
regardless of their own speed. The answer lies in Maxwell's field equations. It has to do with the
fact that the electrical and magnetic fields themselves are relative to the observer.

I simply accept the constancy of light speed, although I do have a way of explaning it to myself.
If you look up Maxwell's equations, I think you will see why I decided to simply accept it, and not
try to understand it.