In describing Einstein's theory of Special Relativity, Isaacson uses the following example (let's see if I get this right...if you're interested in the subject you should definitely browse around and make sure I didn't screw up this explanation). Let's say Joe is standing at point X when two lightning bolts strike at A and B:
The light from both strikes reaches Joe at precisely the same time, so he perceives the events as occurring simultaneously. So we can safely say that lightning struck the two places simultaneously, right?
Wrong.
Bob is traveling at a very high, constant velocity toward point X. He is directly on X when the lightning strikes occur. However, the light from the lightning strike at A will reach him sooner than the light from lightning strike B because of his motion toward A and away from B. So from his frame of reference, the lightning will strike A slightly before it will strike B.
Thus, there is no such thing as absolute simultaneity. You can only describe two things happening at once in relative terms. What determines whether two things happen at the same time depends on how fast an observer (or measurement device) is traveling relative to them.
I think I got that right.
And I think this is part of the reason why Einstein had such a problem with "spooky action at a distance", the idea that two particles that are entangled could affect one another simultaneously regardless of how far apart they were. As far as I understand, this phenomenon has been tested in the lab, at very short distances, but there are criticisms. However, if two particles were entangled, and one stayed on earth while the other was transported to the moon, and then the spin of one was modified and it affected the spin of the other...that would certainly be strong evidence.
I'm looking forward to Isaacson's account of Einstein and quantum mechanics.
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