The special theory of relativity has been explained above, and acceleration and gravitation shall be taken up in the following topics.
Let us assume that we are in an opaque elevator. As the elevator begins to go up, or when upward acceleration occurs, we would experience a feeling of being pulled downward or of being heavy.
Let us assume that the earth below the elevator grows larger momentarily (although this is impossible). In such case, gravitation would increase and we would be pulled downward with a greater force.
Therefore, acceleration and gravitation are equivalent.