Special relativity and flash bulbs

[matness]

Homework Statement

this is a problem from rindler

Suppose there are flush bulbs fixed at all lattice points of some inertial frame and suppose they all flash at once .what actually seen by an observer sitting at the origin?

Homework Equations

The Attempt at a Solution

i don' t understand the question itself ."they all flash at once": relative to whom

 

[Doc Al]

"they all flash at once": relative to whom

Relative to the given inertial frame.

 

[matness]

ok. But what changes then relative to the observer? The observer should see everything same. If it was so ,then i think rindler would not use this as a problem.there must be some poinnt that i missed

 

[Doc Al]

You are confusing (1) the bulbs flashing simultaneously, with (2) a person seeing (receiving the light from) the flashes simultaneously.

Example: You are at point x = 0. One bulb is at point x =0, another is at point x = 1 light year. If the two bulbs flash simultaneously (at the same time according to their synchronized clocks), when do you see the light from each bulb?

 

[matness]

So i have to think two inertial frames: one of observer's and the other one synchronized stars'. And therefore only the bulbs which are at the same distance to the observer will flash simultaneously.and the further ones will flash later
Is it correct?

 

[Doc Al]

It's not a matter of multiple inertial frames--there is only one frame in this problem. Instead it's a matter of realizing that in order to interpret raw observations, one must account for light travel time.

Generally, in relativity thought experiments, one imagines an inertial frame as having zillions of observers (each with his own synchronized clock) located at every possible position. Thus when the lights flash, there will be a co-located observer able to confirm that the lights all flashed at the same time. (The various observers just compare their clock readings later.) But if an observer sees some event happen that is remote from him, he must subtract the light travel time from the time when he saw the light to find the actual time (according to clocks in his frame) that the event happened.

Of course, observers in a different frame (moving with respect to the bulbs) will disagree that the bulbs flashed simultaneously.