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*all*inertial observers. This postulate implies that no two inertial frames can differ by a relative speed equal to that of light.

But the issue of whether two such inertial frames exist is experimentally unfalsifiable, for the following reason: Suppose we modify the constancy-of-speed-of-light postulate of special relativity to the following instead:

If an object travels at the speed of light with respect to one inertial observer (call him Observer 1), then the object must travel at the speed of light with respect to

*all*inertial observers

*. Moreover, for an observer (call him Observer 2) and a massless object that both travel at the speed of light with respect to Observer 1, either the object travels at the speed of light also with respect to Observer 2, or the object is nonobservable with respect to Observer 2 (suggestive of the correct fact that a massless object only gains relativistic energy and becomes physically observable through traveling at the speed of light).*

**that move at a speed lesser than that of light with respect to Observer 1**It is easy to see that the new version of the special theory of relativity that would form under this new postulate would predict

*identical*experimental results to the original version, and yet the "new" version does

*not*ban two inertial frames from differing by the speed of light, unlike the original version. This shows that the issue of whether two different inertial frames can differ by the speed of light is itself experimentally meaningless.

Yet, I am confused because special relativity textbooks seem to emphasize that no inertial frames can travel at the speed of light with respect to another inertial frame, even though this is something that has not been (and cannot be) experimentally verified or proven (unlike time dilation and E=mc^2, which are very well tested by experiment). What scientific purpose is there to even deal with issues concerning whether an inertial observer can actually travel at the speed of light or not if there is no way to experimentally test any claims made on these issues? Physics is an experimental science and I don't see how physicists can seriously make "scientific" claims about the natural world that cannot be tested experimentally?