Question

What does a person feel during free fall?
Given there's no air resistance. I've heard that a person feels weightless. Like in Zero-G airplane. But then, isn't there acceleration due to gravity? And people always feel acceleration. So you feel both weightless and being accelerated?

Answer 1

This is in fact one of the axioms of General Relativity: the effects of gravity at a point can be negated by moving to an appropriately accelerated reference frame. Or more formally: For any point on a spacetime manifold, there exists a coordinate system such that the local metric is Minkowski.

This means that the effects of gravity are indistinguishable from acceleration. If you were somehow in a closed windowless room in space somewhere and it was being accelerated towards the ceiling at 9.8ms^-2, there would be no experiment you could perform in order to establish whether the force you experience is from acceleration or gravity.

So to answer your question, when you're in freefall you feel no force. To an observer on the ground you are being accelerated by gravity, but this is not detectable in your own accelerated frame (other than measuring the ground accelerating towards you).

When you see astronauts in low-Earth orbit, the strength of gravity is something like 90% of the strength at the surface. An orbit is a type of freefall, but with a large sideways speed.

Answer 2

I didn't quite get this. I thought acceleration was one thing that is absolute in TOR, in contrast with uniform motion that is always relative. And how come you feel no force when there is one - the force of gravity - acting upon you? So, you are saying, that people in free fall feel no different than people who float weightless in space without being accelerated? But isn't there, for instance, a limit of acceleration that an average person can survive? Like 1g, 2g etc. Acceleration might feel really bad when it's large.

Answer 3

This is the paradox: submitting entirely and passively to gravity is what gives you the sensation of weightlessness.

Q:"that people in free fall feel no different than people who float weightless in space without being accelerated?"

A: that's precisely the point, the people who float weightless in space ARE being accelerated around the Earth by its gravitational force.

Answer 4

You have to think in terms of reference frames (i.e. relativistically). If you are standing on the ground your acceleration is zero relative to the ground, despite feeling a force. There is an equal and opposite electromagnetic force from all of the atoms that make up you and the ground beneath you etc that is opposing gravity and stopping you from falling to the Earth's core.

If you ride an elevator, you are being accelerated upwards. You feel this as an increase in gravity, even though gravity acts downwards. Upwards acceleration adds to the feeling of gravity. So if the lift drops, you are accelerating downwards with gravity, and you feel no upwards acceleration.

Answer 5

I could have written that better, but I hope you get what I mean. You "feel" gravity when there is a force opposing it. When you're on the ground, the feeling of gravity is equivalent to feeling an upwards acceleration.

Answer 6

Okay, let me finally get this straight: If there is nothing opposing your motion, you never anything, neither in uniform motion nor during acceleration? No matter how fast you accelerate? Say, you are falling toward a blackhole in vacuum really really fast and you still don't feel it? What about spinning in space? Wouldn't your feel blood flush into your arms and legs?

Answer 7

Rotating frames have inertial forces. You can tell if you're spinning from the apparent centrifugal and coriolis forces.

So what you've said is true for linear acceleration, but not rotational (torque).

If you're a point-like particle falling towards a black hole you won't ever feel a force in your own reference frame. If you're a human being falling into a black hole, you have non-zero size so (if you're falling feet first) at some point the gravity at your feet will be noticeably stronger than the gravity at your head, and you will get stretched until you snap (a bit horrific; don't go near black holes).

The amount of acceleration depends entirely on which frame of reference the observation is being made from. There is a covariant 4-vector for acceleration, just as there is for momentum. Covariance means "takes the same form in equations every frame, but not necessarily the same value".

Answer 8

Alright, I think I'm begining to understand the answer to my original question. I'll put rotating frames and covariant 4-vectors for later study. Thank you very much for your help.