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I have understood the following: in SR, time intervals and space intervals (distances, lengths) are relative and inertial reference frame dependent. Space and time is not absolute anymore. However, acceleration is still absolute: different inertial frames see the same acceleration.

When introduced to GR, we run into the equivalence principle:

**an accelerated frame of reference in the absence of a gravitational field is indistinguishable from an inertial (unaccelerated) frame in the presence of a homogeneous gravitational field: the local effects of gravity are the same as those of being in an accelerating reference frame.**

I understand this, as well as the example of the light beam curved downward trajectory from an accelerated spaceship frame while an inertial observer on earth concludes that the light path is curved by gravity (but doesn't the earth observer see the path as straight and notices the upward accelerating spaceship?)

Why is the equivalence principle so useful? Why is it so powerful and novel to state that the local effects of gravity are the same as those of being in an accelerating reference frame?

In classical physics, gravity is a force acting on massive objects. Light, which is radiation, should not be affected and curved by mass, i.e. gravity...What does the equivalence principle add to the discussion? Is the mentioned equivalence the equivalence between inertial mass and gravitational mass? How does that equality bring us to the idea that mass-energy curves spacetime and light truly follows this geodesic paths?

Thank you!!!