I picked this up on the net:
Einstein came to realize the principle of equivalence, and it states that an accelerated system is completely physically equivalent to a system inside a gravitational field.
When I am being accelerated I will pick up speed over time. This will have a noticeable effect, e.g. there will be an increasing blue-shift in the light of stars which lie in the direction of acceleration. This will not happen if I am just in a gravitational field and I could well distinguish acceleration from gravity.
I suspect that being completely physically equivalent may be a bit of an overstatement and is only true as long as I don't look outside?
Or is the solution, that I cannot really know, whether those stars are being accelerated too? So when I feel the gravity and I see a constant blue shift, I might conclude that I am being accelerated, but all these stars are being accelerated the same as me?
Or is there a way to interpret gravity as acceleration, which also leads to an increasing red/blue shift?