Assume that a beam of light horizontally enters a rocket with a horizontal width of 300,000 km for example.
If the rocket moves upward with a uniform velocity, the light beam advances as illustrated in the following picture.
To observers inside the rocket, the light beam appears to advance at an angle.
If the rocket moves at an accelerated pace, the light beam advances as illustrated in the following picture.
The light beam appears to form a parabola to observers inside the rocket.
If I were a light beam, I would say that I advance in a straight line. However, the observers in the rocket would say that the light beam forms a parabola. Although this occurs in a three-dimensional space, it is in fact bent due to the presence of a gravitational field.
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