Science Forums 2

[TeleMad]

Someone above mentioned that gravity is a pseudo-force in general relativity...how can that be?

Einstein introduced the equivalence principle which states that gravity and acceleration are indistinguishable - they are equivalent. Mass has two interpretations. Gravitational mass determines how strongly an object attracts others gravitationally and inertial mass determines how much an object resists attempts to change its state of motion: i.e., to accelerate it. Yet these two seemingly unrelated masses turn out to be not only proportional to each other, but to be exactly equal (that's why heavy and light objects fall at the same rate: the gravitational attraction for the more massive object is greater, but it also takes a greater force to accelerate the more massive object - since the two opposing factors are exactly equal, they cancel out). Many people had pondered on why this was but Einstein explained it.

Here's the idea in a nutshell. Picture yourself in a regular elevator. Naturally, you are standing on the floor because gravity pulls you down and holds you to it. Now, imagine that the elevator is not here on Earth but is instead out in deep intergalactic space, where there is no gravity. You are floating around inside the elevator. But if the elevator were to be accelerated at 9.80 m/s^2 in a direction towards the top of the elevator, you would find yourself standing on the floor and being held there by the accelerating elevator. You'd feel a force on your feet equal to the force of gravity if you were on Earth and the elevator motionless. The two situations are indistinguishable. Even if you perform an experiment, you can't tell the difference. For example, if you had a ball and dropped it, in either situation it woud fall towards to the floor with an acceleration of 9.80 m/s^2. Einstein concluded that since there is no experiment you could perform that would tell which is the case - on Earth and motionless or out in space and accelerating - that gravity and acceleration can be considered equivalent and gravity can be transformed away by replacing it with a suitable acceleration (or vice versa).


PS: Technically, tidal forces can be used to determine which is the case if the reference frames are large. For example, if you had 2 balls and dropped them on opposite sides of a very wide and tall elevator, you would see the two balls approach each other horizontally as they fell because of their non parallel paths directed towards Earth's center of mass. However, in space, this would not occur: the balls would fall perfectly parallel to each other. So the equivalence principle holds truly for only very small reference frames.