How do you explain Einstein"s Equivalency principle In his Lift thought experiment?

Question

Ball and Lift collision ?

the motion of the ball is imaginary and the motion of the Lift is the real motion?

Answer 1

Einstien used the thought experiment to show that the effects of gravity and acceleration are exactly the same. This is the equivalency theory. In the experiment, you are in a lift. Since you cannot see outside of it, you do not know if you are being pulled down by gravity, or you are in space being accelerated by rockets. Just the same, if the lift snapped, you would feel weightless because the lift is falling at the same speed as gravity. This thought experiment led Einstien to believe that acceleration and gravity are the same. This is the basis for Einstein's theory of general relativity. One of the implications of this is that light is bent by gravity, something that was proven during Einstein's lifetime.

Answer 2

The equivalence principle has to do with curved paths through spacetime. The effect of curved paths, regardless of what are causing them, are all the same. If I travel on a curved path due to an acceleration or I travel on a curved path due to gravity, it will "feel" the same and actually have exactly the same physical effects. It's the curved path through spacetime that matters, not the source of the curvature.

Let me explain in more detail.

Relativity posits that every entity in the universe moves through a 4 dimensional space often called spacetime. Because we always experience time moving forward, we are always moving through this spacetime, even when we're sitting in one place.

Imagine that the universe only had one spatial direction. That is, imagine that everything in the universe existed on a line. In that case, movement could only be "left" or "right" or sitting still. "Spacetime" in this universe would be two dimensional, so you can draw it on a piece of paper. Label one edge of the paper "space" and the other edge of the paper "time." As you can see, because time is always moving forward, if you sit still, you move in a "straight line" on the paper advancing along the "time" edge but staying the same along the "space" edge.

Now imagine that you accelerate "right" along the space direction. Time is still moving forward, so now you move in a "curved" trajectory. That is, instead of tracing a straight line as time goes forwad, you trace a curved line because you keep getting faster and faster in the "right" direction.

Now, imagine that the paper was floating while it was being held up by a spring in each corner. The springs hold the paper up; however, if you drop a marble in the middle of the paper it causes the whole thing to deform and sort of "curve" around the marble. Now, pretend like you were "sitting still" and draw your trajectory on this paper. You're going to again move parallel to the "time" edge and perpendicular to the "space" edge, but because the paper is deformed, you actually are moving in a "curved" trajectory with respect to the original undeformed paper.

This is what Einstein meant by equivalence. Gravity forces you to travel in curved trajectories through SPACETIME. Any curved trajectory can be thought of as EITHER a gravitational effect or as an acceleration effect. We "feel" acceleration "forces" due to the curved trajectories they force us to follow in spacetime. As long as it's curved, it doesn't matter what is causing the curve; it will feel the same regardless of the reason for the curvature.

(an example: the next time you are in a car, bring a helium filled balloon inside the car. The balloon will stick to the ceiling of the car because its density is less than the density of air. Now accelerate forward (or turn left or right) so that you can feel the acceleration push you against the back of your seat. You might expect that the balloon will move backwards in the car, just as you were pushed backwards. However, as long as the friction between the balloon and the ceiling is not too great, the balloon will move to the FRONT of the car. This is because the density of the air is greater than it, so air moves to the back of the car and the balloon is pushed to the front. However, you can also think of the acceleration as putting a gravitational field pulling things toward the back of the car. Just as the balloon "resists" gravity when sitting still and floats up, it will "resist" gravity in the car and move to the front)