the principle of equivalence states that an observer
cannot tell whether their frame is accelerating or if they are experiencing a gravitational field.
Physical processes obey the same laws.
An object dropped in an accelerating
spaceship will fall to the bottom of the cabin in just the same way as it would if the spaceship
was at rest on a planet's surface and it was the gravitational pull of the planet that was
making the object accelerate downwards.
Light entering a glass space ship that is accelerating will appear to bend
downwards as it crosses the spaceship. In reality the space ship accelerates upwards to
meet it.
However we are led to the conclusion that light bends due to the accelerated
frame.
However if we believe in the principle of equivalence then if light bends in an
accelerated frame it must also bend in a gravitational field. The amount of bending is very
small even in quite high intensity fields.
Think of a distorted space near
a massive body. These effects were observed during the solar eclipse of 1913. A predicted
0.8" deflection – 1.75 " was observed.