Is it proven that Brian Greene is correct about if you jump out of a window it is the Earth that rushes up to hit you?

Question

https://m.youtube.com/watch?v=HneFM-BvZj4 (excerpt from the 2014 World Science Festival Program Dear Albert. Actor Alan Alda and physicist Brian Greene discuss Einstein's relationship with the "unruly child" of quantum mechanics, and how the famed physicist came up with the Special Theory of Relativity).

From about 9:30 mins to 10:30 mins (then he purports to demonstrate the principle by dropping a leaking bottle of water).

And what does he mean by 'execute a certain type of motion', just like moving off something you were on?

I note a question has been asked before about which moves towards which, but one answer seems to be Newtonian while one seems to be Einsteinian.

There is a discussion of related issues in If gravity isn't a force, then how are forces balanced in the real world? — John Rennie, May 17 '20 at 19:14
I've removed a number of comments that were attempting to answer the question and/or responses to them. Commenters, please keep in mind that comments should be used for suggesting improvements and requesting clarification on the question, not for answering. — David Z, May 18 '20 at 05:45
Does this answer your question? How do frames of reference work in general relativity, and are they described by coordinate systems? — Charles Francis, May 18 '20 at 06:18
@CharlesFrancis It may do but I can't follow. Basically I know Greene can't mean that the earth is actually (in our brains' default perception simulation) rushing out to hit every object dropped into free fall. So I guess it's to do with the earth bending spacetime but I can't imagine why he calls that the earth moving. — Gimloyp, May 18 '20 at 08:22
@JohnRennie So, the answer there seems to contradict Greene's statement about Earth moving, by saying 'The answer is that both are correct. Whether it's the apple or the table that is stationary is just a choice of rest frame'? — Gimloyp, May 18 '20 at 13:01

score 1 · Answer 1 · answered May 18 '20 at 11:03

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In relativity we regard motion as relative. Actually, even position is relative. You cannot say where something is unless you say where it is relative to something else. When we say we fall to the ground, we are thinking in terms of our motion, relative to the ground. But suppose we were in an elevator in which we cannot see out. We would not be able to see the ground and we could only describe motions relative to the elevator.

Relativity describes a special class of reference frames, inertial reference frames, in which Newton's first law holds. Typically the elevator is not an inertial frame. If you drop an object it will accelerate downwards. Now imagine that the cord is cut, and the elevator is in free fall. This is now an inertial frame. If you drop an object, it will not fall in this frame, but will remain stationary or move uniformly, as described in Newton's first law.

So, when Brian Greene says the Earth is rushing up to hit you, he is talking of the motion of the surface of the Earth relative to an inertial frame.

answered May 18 '20 at 11:03

Charles Francis

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If two people on opposite sides of the Earth jump out of windows, does the Earth rush upwards toward both of them? Seems unlikely! – S. McGrew May 18 '20 at 13:23
@S.McGrew, Inertial frames are necessarily local. This comparison is not meaningful in the context in which it is made. – Charles Francis May 18 '20 at 13:29
@S.McGrew based on https://www.physicsforums.com/threads/einstein-says-objects-do-not-fall-to-the-earth.781200/ i think the answer might be that the inside of the earth stops the surface following the curved geodesic into the centre of the earth? – Gimloyp May 20 '20 at 13:03
@gimloyp, inertial frames are much smaller. Tidal forces (I.e. differences in gravity) are not apparent in inertial frames. – Charles Francis May 20 '20 at 15:03
@CharlesFrancis i thought i was talking about the accelerating frame, like someone on the surface who feels their weight. – Gimloyp May 20 '20 at 21:30

score 0 · Answer 2 · answered May 19 '20 at 02:03

In the following I'm going to talk about the relation between information and science

When the information that is available to you is purely local information your assessment of your situation is inherently limited to that local information.

Let's say you find yourself in a small room, and as far as you can tell there is zero G. Your own physical sense is that there is zero G, accelerometers attached to the wall of the room read zero G.

You could be in a spacecraft in interstellar space, or you could be in a spacecraft in orbit around a planet in a solar system. Maybe very, very sensitive can be developed that can tell the difference, but for the purpose of this thought experiment: local measurement cannot tell the difference.

Next you widen your perspective, and the information available to you now is that the spacecraft is orbiting an earth sized celestial body. Newtonian theory and Einsteinian theory have the following in common: size matters. Given that the mass of the celestial body is many many times the mass of the spacecraft we say unequivocally: the spacecraft is orbiting the celestial body.

Next question:
Can we determine with purely local measurement whether the celestial body is orbiting a star or in interstallar space? That is, in this thought experiment you are only allowed to look inward; no observation of anything that happens away from the surface of the celestial body. The confinement to local measurement is very restrictive. Rotation of the celestial body: no problem, our technology has rotation sensors that can measure very slow rotation rates. But if the information available to you is local measurement only I think you'd be hard pressed to tell whether the celestial body is orbiting a star.

You widen the perspective again: the solar system as a whole. Now again there is that common ground of newtonian and einsteinian theory: the planets are orbiting the common center of mass of the solar system as a whole.

Widen your perspective again: the stars of the Galaxy are orbiting the center of mass of the Galaxy.

Each of these levels of perspective take a group of gravitationally bound objects. Such a grouping is natural level of perspective.

Going back to the example of jumping of and starting to fall:
If you confine yourself to local information only then you cannot tell the difference. prior to jumping: an accelerometer on your person reads that you are experiencing an acceleration of 1 G, upwards. During the fall the accelerometer on your person reads that you are experiencing zero G.

What Brian Greene is referring to is this inability to tell the difference if you are confined to local measurement only.

From a scientific point of view: confining yourself to limited information is the last thing you want to do.

The whole point of doing science is that you bring all available information to bear.

Let me make a comparison:
In the history of physics there was a time that the caloric theory of heat was a prominent theory. For example, when Sadi Carnot developed the work that is the foundation of thermodynamics he was thinking in terms of caloric theory. That is, in its day caloric theory was scienfifically fruitful, it has been conducive to science that has stood the test of time. Over time caloric theory was replaced with the molecular theory of heat.

Thought experiment: if you deprive yourself of the information that caloric theory is obsolete (and why it is obsolete) then caloric theory is valid and relevant.

Obviously that's a pointless exercise. Depriving yourself of relevant information is the opposite of what science is about.

Is it proven that Brian Greene is correct about if you jump out of a window it is the Earth that rushes up to hit you?

2 Answers2