Principle of equivalence?

Question

In demonstrating the principle of equivalence, it is shown that for an observer inside an accelerating spaceship moving upwards, a horizontal beam of light is seen to bend downwards...

What would the observer inside the spaceship see if the spaceship is just moving upward at a constant velocity? The light beam will remain unbent?

Answer 1

That's correct. The PoE says that acceleration has exactly the same effects as a gravitational field.

Answer 2

Right. If the beam comes straight in thru a hole in the wall as viewed by an outside observer, the translation of the elevator during the beam's trip across will just make it look like it came in at a downwards angle to a passenger. It would still look straight in a smoky elevator.

It the light source was hanging on the elevator wall, the beam would appear to go straight across to a passenger, but to an outside observer, it would appear to be angled up.

Answer 3

In space, there is really no "up". But if your spaceship moves in any direction at a constant velocity (zero tangential and radial acceleration), without changing speed or direction, you are in an inertial frame of reference, and therefore a light beam will behave in accordance with Newtonian physics, that is, it will go in a perfectly straight line.