I&#x27;ve also had a long-time question around this point. I&#x27;ve asked various university physics professors, and they all stutter and disagree (which means either we really don&#x27;t know or I&#x27;m going to all the wrong universities).  The underlying question is:  When we say in general relativity that particles can&#x27;t travel faster than light, what is that speed measured relative to?<p>If it&#x27;s photons emitting from flashlights pointed in opposite directions relative to each other, then are they compressing space as they travel so as not to exceed the speed of light?  Or even massive particles accelerated to  over half the speed of light at different times around an ellipse so there is a moment when their opposite directions make one traveling at greater than the speed of light relative to the other?  (Or even particle accelerators on two different planets already expanding away from each other - does this just mean that space is really twisting and turning all the time to make sure no particle ever exceeds the speed of light relative to any other particle?)<p>It has been my understanding - and appreciation at Einstein&#x27;s unbelievable insight and brilliance - that relativity is truly relative because there really isn&#x27;t any such thing as an arbitrary particle or space that all other motions are measured from.  But if this is true, does that really mean that space is really so constantly twisting and turning that SOL can&#x27;t be exceeded?<p>I&#x27;m sure there are great answers to this, but I&#x27;ve always wanted a definitive one.  Anyone have any comments on this?