>if I fire a photon from the floor to the ceiling, it should take longer to complete its journey than one fired in the opposite direction.

Not relative to you who's traveling with it. Both beams of light are objectively traveling at the same speed. Possibly in extreme relativistic scenarios (such near an event horizon) you may see one beam slow down or speed up as it traveled away, however this is just a perception from your view and if you were to adjust your frame of reference to its, it would appear at it's proper speed.

Pretty much you have a viewpoint of the universe that's your own, only those traveling similarly through space (like the rest of Earth) will generally agree with you on how things look. Those traveling slower, faster, or oppositely will have drastically different accounts. Lightspeed to all of them is 299,792,458 m/s however, and each is as right as any other.

This is general relativity (at least insofar as I understand/remember it; which isn't too well. If something conflicts with what I said, take it over me).


>Does that mean that if I do that in a gravitational field I will see the same thing or is that just completely wrong?

As in the above black hole example, yes, but only via perception. Acceleration and gravity are of the same fundamental nature according to general relativity (again insofar as I understood and remember) so it's all the same talking point.


If you'd like something to digest about all this sorta stuff, check out the NOVA series 'The Fabric of the Cosmos', or the identically titled book it's made after. It covers exactly this subject and is great brain food.

>If I fire a photon from the floor to the ceiling, it should take longer to complete its journey than one fired in the opposite direction.

No, that is not the way it works. A photon emitted from the floor in the direction of acceleration would redshift - by a factor inverse to that associated with the blueshift of an oppositely traversing photon. The two photons would complete their journeys in the same time. An observer in a gravitational field with local gravitational acceleration matching that of the acceleration of your hypothetical room experiences the same situation. These links should help:

http://en.wikipedia.org/wiki/Gravitational_redshift

http://en.wikipedia.org/wiki/Equivalence_principle

>>209
>>208
Thanks! I'll look into those.

>>208
>>209
You guys are overthinking this. Yes the photon would take longer to reach the ceiling because it's traveling a longer distance. Gravity shouldn't effect it unless it's really strong, like a black hole.

Because OP's question interests me, I would like to propose an addendum to this:

Suppose you were in sed room travelling upwards at 0.999c and you had a photon emitter in the room traveling 0.999c with you that emitted one photon up and one down and you also had a stationary emitter that passed through the room, again firing one photon up and one down for the instant it was in the middle of the room? What would be the difference between the photons fired from a stationary emitter vs those from the moving one and what would would that look like to the observer within the room?

Excuse me if my question is retarded, I'm new at physics.

>>222
That's not even a little bit right.

SAGE has been used.

>>240
(relative speeds)

relativity is pretty much based on these two concepts:
a) inertial frames of reference, meaning if you're running past me at speed x, i'm... um... standing past you at speed x
b) the speed of light is constant in any/every frame of reference

the universe will literally bend over backwards to keep both of these facts true.

>>251
err, not accelerating... that's kinda different.

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