What causes weightlessness in a free fall?

[chaszz]

Einstein said one his happiest realizations was that a person in free fall would feel no weight. Unless I am mistaken he said this. But didn't Newton realize this?

 

[Mentz114]

I don't think Newton did realize this. Einstein was given a huge hint by a guy fell 60ft and lived to tell the tale. He is reported to have said he felt 'weightless' whilst falling.

 

[Meir Achuz]

It depends on the definition of 'weight', 'weightless', and 'felt'.

 

[Viracocha]

You wouldn't actually be weightless in a freefall, but you would feel like it because the forces are canceled out.

Einstein did realize that an observer accelerating due to a rocket or other force is analogous to an observer accelerating due to gravity: http://www.astronomynotes.com/relativity/s3.htm he used this to formulate general relativity from special relativity

 

[D H]

You wouldn't actually be weightless in a freefall, but you would feel like it because the forces are canceled out.

What forces are canceled out? In Newtonian physics there is no cancelation of forces for an object in free fall. There is only one force acting on such an object, and that force is gravity.

There is no cancelation of forces in general relativity, either. There is no force acting on an object in free fall in general relativity. Gravitation is not a force in general relativity. There is no local experiment that can distinguish free fall conditions from force-free conditions far removed from any gravitational source. The two situations are identical as far as general relativity is concerned.

Bottom line: You actually are weightless in a free fall in general relativity.