# Q & A: detecting free-fall

Q:
A few days ago a friend of mine asked a very odd question. Say you are in an object in freefall,in her hypothetical she used a windowless room, you can't hear any wind to suggest that you're falling, there are no windows so you can't see that you're falling, would you be able to tell that you're falling? Obviously you would rise up and possibly hit the ceiling,but would you be able to distinguish that as falling or would you mistake that for a decrease in gravity's pull? What exactly would happen to you and how would you perceive it?
- Bailey (age 18)
Florida
A:

To be in free-fall, the room must not have any other forces acting on it, such as air friction. So let's say that is what she means.

There would be no effects of gravity in the room. That's another way of saying that it's in free-fall. You definitely can notice whether there are effects of gravity, so you can tell that you're in free fall. You won't, by the way, head for the ceiling or anywhere else, because you're in free-fall along with the room.

What you can't tell is what sort of gravitational field the room is in. It might be far away from any stars or planets, or it might be just about to crash into something pulling it in. The free-faller can't tell.

If you follow the logic of that fact (called "the equivalence principle") far enough, it says that the gravitational field is not really well-defined until you've chosen some coordinate system. The choices of coordinate system are described by General Relativity. I hope your friend follows this line of thought far enough to learn about that, and maybe become a physicist. It was worrying about just this problem that led Einstein to General Relativity.

Mike W.

(published on 09/04/2016)