As described in Wikipedia as well as this entry http://physics.stackexchange.com/qu...xplanation-of-the-equivalence-principle-wrong, "being at rest on the surface of the Earth is equivalent to being inside a spaceship (far from any sources of gravity) that is being accelerated by its engines." Even more fundamentally, in an elevator in space, how do you know you are moving at a constant speed or are stationary? I am confused about something. Consider the following situation: An alien with spectacular vision (that can resolve time differences on the order of 10^-12 seconds) is in a train that is stationary and is looking at a light that is very briefly pulsed on-off about a meter away from his eye. The light is pulsed at time t1 and he perceives it at t2.( even though it has been off for a long time by then). Now consider the situation when the train starts to move. The alien sees the same event. There are no windows and there is no way the alien can get information from outside the cabin. According to the fact that the train is not accelerating but moving at a constant velocity, nothing the alien can do should tell him that he is moving. But consider which photons the alien is seeing. When the light is pulsed, the photons go in all directions. In a stationary train, the alien sees the photon going straight up, but in the moving train, the photon that reaches his eye, is the one that takes off from the light at an angle. Now make a stencil with numbers cut out on it like so: and put it right over the light like so: Please forgive the scale. I wanted to make the numbers readable so I made the mask very large. You might be tempted to say that by the time the light pulse traveling obliquely reaches the mask, the mask will have moved so the alien will really see a different number (2 maybe?). But make the mask really small so it is really close to the source. And make the stenciling really small so you can resolve small differences. When the train is stationary, the alien sees the number '1' but when the train is moving he sees the number '5'. Again, remember, the mask is really, really close to the light source. If the mask were really big the way I drew it (for clarity) then the alien would always see the number '1'. So it seems to me that such a 'light speedometer' can tell us how fast our frame of reference is moving even without any information from outside our frame. It relies on the fact that we know the speed of light is constant in all inertial reference frames. This seems to suggest that we can distinguish inertial frames of reference. It goes all the back to the idea of Newton who spoke about absolute reference frames here http://plato.stanford.edu/entries/spacetime-iframes/ Please note, I have not made any claims about time or the concept of simultaneity. I am not discussing special relativity. Yet the concept of the space elevator introduced by Einstein now bothers me. With such a 'light speedometer' shouldn't I be able to know how fast I am going in an absolute sense?