- #106
PeterDonis
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rede96 said:how massive are the two objects, how far apart they are
Since it is specified that there is no other matter between them, just empty space, this doesn't make any difference qualitiatively; all it affects is how fast the objects will fall together. Of course that specification might not be realistic for large enough separations.
rede96 said:If there is dark energy present then that is an additional interaction.
I thought we had ruled out dark energy for this particular scenario. You said there is "nothing inside the boundary except those two objects". That means no dark energy. Obviously the presence of dark energy will change things--but the separation would have to be very, very large (tens to hundreds of millions of light years) for it to change things significantly.
Also, if you are trying to understand what effect "expansion" has in itself, it would seem that you would want to rule out dark energy.
rede96 said:Space (which includes dark energy)
"Space" in itself does not have to include dark energy. It happens to in our actual universe, but we are considering thought experiments in order to understand the underlying physical principles involved. In such thought experiments it's perfectly reasonable to say there is no dark energy. "Space" without dark energy is perfectly consistent physically.
rede96 said:I agree that there will always be an attraction between the two objects in proportion to the inverse square law. But that attractive force does not tell me anything about the relative motion between the two bodies.
It does if it's the only force acting, which is what I thought the specification of the scenario was.
rede96 said:I am now thinking that we shouldn't talk about expansion without dark energy, as it is a real part of 'space'.
No, it isn't. Dark energy is something separate from "space". It happens to be present everywhere in our universe, but that is not required by the laws of physics. It's just a contingent fact about our universe. If you are trying to understand expansion in and of itself, it is perfectly reasonable to assume, for purposes of a thought experiment, that there is no dark energy; the physical model you get is perfectly consistent. See my comments above.
rede96 said:One of the other big confusion factors is when people talk about recession as distant bodies being at rest wrt each other and the space growing between them.
Yes, and I am saying that a good way to avoid this confusion is to understand that there is no force associated with expansion. There is a force associated with dark energy, but that is a force associated with accelerating expansion. You can have expansion without having accelerated expansion.
To put this another way, there is no force associated with "space" in and of itself. "Space" cannot expand in the sense of pushing or pulling things apart. Dark energy can push things apart, but that's because dark energy is not "space"; it's something separate from space. It can be thought of as a kind of "exotic" substance that causes repulsive instead of attractive gravity. It's not the same as empty space.