Weak and strong equivalence- what is the difference really?

[trelek2]

Hi!

My lecture notes make me really confused as to what is the difference between weak and strong equivalence. I also read about it on wikipedia, but I'm still not sure. Can anyone give an example how in real life (thought experiment?) of what weak and strong equivalence is.

 

[bcrowell]

The Wikipedia article http://en.wikipedia.org/wiki/Equivalence_principle seems messed up. Its statement of the strong and weak equivalence principles are basically identical. I think what they call the Einstein equivalence principle is what most people call the strong equivalence principle.

 

[atyy]

http://relativity.livingreviews.org/Articles/lrr-2006-3/

"One elementary equivalence principle is the kind Newton had in mind when he stated that the property of a body called “mass” is proportional to the “weight”, and is known as the weak equivalence principle (WEP). An alternative statement of WEP is that the trajectory of a freely falling “test” body (one not acted upon by such forces as electromagnetism and too small to be affected by tidal gravitational forces) is independent of its internal structure and composition. In the simplest case of dropping two different bodies in a gravitational field, WEP states that the bodies fall with the same acceleration (this is often termed the Universality of Free Fall, or UFF)."

"The Einstein equivalence principle (EEP) is a more powerful and far-reaching concept; it states that:
1. WEP is valid.
2. The outcome of any local non-gravitational experiment is independent of the velocity of the freely-falling reference frame in which it is performed.
3. The outcome of any local non-gravitational experiment is independent of where and when in the universe it is performed."

"These ideas can be summarized in the strong equivalence principle (SEP), which states that:
1. WEP is valid for self-gravitating bodies as well as for test bodies.
2. The outcome of any local test experiment is independent of the velocity of the (freely falling) apparatus.
3. The outcome of any local test experiment is independent of where and when in the universe it is performed."