# Einstein equivalence principle

1. Nov 15, 2015

### Sobi

I can not understand what exactly this principle says.while reading a book about it ,it was written that einstein abolished gravity by saying the lift is free falling can some explain clearly what does that mean?

2. Nov 15, 2015

### Smattering

It means that (apart from tidal forces) experiments that you perform in a free falling lift will have the same results as experiments that you perform in a lab in empty space.

3. Nov 15, 2015

### Chalnoth

A lab in empty space would be a lab in free fall.

4. Nov 15, 2015

### Smattering

Yes, but as far as I know this view had not been established when Einstein began working in GR.

5. Nov 15, 2015

### Chalnoth

Yes, it had. This realization is based upon Newtonian gravity. Newtonian gravity is an infinite-range force, which means that a lab in orbit still experiences gravity. People in that lab only feel weightless because the lab is in free fall.

Einstein's philosophical contribution here was to state that this equivalence between acceleration and gravity is a real thing, not just a trick of the math.

6. Nov 15, 2015

### Aniruddha@94

It means that if you're in a closed box, you cannot say whether you're in a gravitational field or the box is accelerating. Both have the same effects.

7. Nov 15, 2015

### Smattering

Hm ... I thought the version that was established before Einstein was only the so-called weak equivalence principle (equivalence of inertial and gravitational mass), and it was Einstein who extended this to the strong equivalence principle (invariance of physical laws in nonrotating laboratories freely falling in a uniform gravitational field).

8. Nov 15, 2015

### Sobi

it has also written that the free fall is a local inertial frame.well I will accept that any thing inside the lab will feel that it is moving with a constant speed compared to the other things inside the lab,but inside the book it stated that this was a problem for Einstein because then special relativity had to hold true there.i also need an explanation wether special relativity will be true there or not?