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Message: <blockquote data-quote="Umbran" data-source="post: 6704988" data-attributes="member: 177">That example is frequently used to describe some of how relativity impacts the idea of simultaneity, yes.  But nobody actually physically performs that demonstration. It is done in the imagination, a "gedankenexperiment" (thought experiment - a term first coined by Einstein, precisely because he couldn't actually do physical experiments at the appropriate speeds)Overall, the universe *is* extremely flat.  Yes, there's some minor curvature around massive bodies, but we can correct for that.We don't usually go deeply into the assumptions in discussions with laymen, as they tend to clutter up the conversation.  Let us remember that Einstein developed his theories in the first part of the 20th century (the first publication of Special Relativity was in 1905).  Actually doing the experiment described above was out of the question.But, the assumptions of special relativity are quite simple.  And they have their own wikipedia article!<a href="https://en.wikipedia.org/wiki/Postulates_of_special_relativity" target="_blank">https://en.wikipedia.org/wiki/Postulates_of_special_relativity</a>Einstein assumed very few things:1) The speed of light in vacuum is a constant in all inertial frames of reference.2) The laws of physics are the same in all inertial frames of referenceSpecial Relativity then goes on to consider motion in those inertial frames of reference.  An "inertial frame" is one in which space is described homogeneously (it has the same properties at every point), isotropically (it is the same in every direction we look), and as time-independent.  It turns out that all inertial reference frames are in uniform linear motion with respect to each other.You can hack out some interesting things from special relativity when considering uniformly accelerating frames, which are not inertial, but the base assumptions are frames that are moving in straight lines at constant velocities with respect to one another:  So, for example, I am sitting on the ground, and a train goes by on a straight track at a constant speed.Which all boils down to:  Einstein assumed the outright simplest case he could for special relativity.For general relativity, we consider spaces that are not necessarily outright inertial frames, but in which the curvature of spacetime isn't too big, so that we can think of it as being made of an infinite number of frames in which space is locally flat*.  This is like finding the area under a curve by cutting it into an infinite number of rectangles and adding together their areas.  In other words, special relativity is the arithmetic of spacetime, while general relativity is the calculus.*This is why black hoes are such a big pain in the relativistic neck, because they are the cases where, at the singularity, the curvature of spacetime finally gets too big, and GR falls apart.</blockquote>

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