If force is what causes gravity (motion), what is causing force?

  • #1
I'm tying this into mass manipulating space and these curvatures being what matter (mass) follow.
 
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  • #2
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A very good question indeed.
 
  • #3
Yeah, something isn't right. This isn't making any sense. The scientific community is overlooking something.
 
  • #4
Doc Al
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Yeah, something isn't right. This isn't making any sense. The scientific community is overlooking something.
Please restate your question. What do you think is being "overlooked"?

Did you mean to post this in the Relativity forum? Do you realize that General Relativity is a detailed theory of gravity?
 
  • #5
Doc Al:

Yes, I'm aware of that. I'm tying this into mass manipulating space and these curvatures being what matter (mass) follow.
 
  • #6
Doc Al
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I'm tying this into mass manipulating space and these curvatures being what matter (mass) follow.
And your question is what?
 
  • #7
I think the science community is missing language. That's the problem. Much of its language seems to cancel out (other language). For instance, we have motion, but we find that it is relative. We have time, but it's relative. Motion and force, as words, aren't cutting it. There is something else going on.
 
  • #8
PeterDonis
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Why do you think "force is what causes gravity" in the first place? In General Relativity, gravity is not a "force", so your whole OP is based on an incorrect premise.
 
  • #9
Doc Al
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I think the science community is missing language. That's the problem. Much of its language seems to cancel out (other language).
Why do you think that?

For instance, we have motion, but we find that it is relative.
Yes it is. So?

We have time, but it's relative.
Yes it is. So?

Motion and force, as words, aren't cutting it.
Why not?

There is something else going on.
How mysterious!
 
  • #10
Gravity is not one of the forces of nature?
 
  • #11
PeterDonis
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I think the science community is missing language.

No, you are missing language: the language of relativistic invariants. All of these "words" that you say "aren't cutting it" are *not* the words that GR actually uses to describe the underlying physics; they are words that people overlay on the underlying physics to try to make it match up with their intuitions.

The language you should be using, if you want to describe the underlying physics the way GR does, is the language of covariant and invariant quantities: things like the metric tensor, the curvature tensor, proper time (as opposed to just "time" without qualification, which is ambiguous), etc. So the real problem is not that the words of physics don't cut it; the problem is that you are not using the words of physics, you're using the words of lay people's intuitive picture of physics.
 
  • #12
PeterDonis
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Gravity is not one of the forces of nature?

It depends on what you mean by "force". That's why "force" is one of those words that doesn't appear in the underlying physics, but only in lay people's intuitive picture of physics; it's ambiguous and doesn't really match up well with the underlying physics.

According to GR, "gravity" is a manifestation of spacetime curvature (some physicists would say that gravity, or at least tidal gravity, *is* spacetime curvature; they are just two different words for the same thing). That makes it fundamentally different from things like electromagnetism which are often viewed as "forces": an object moving solely under the influence of "gravity" feels no weight and has zero proper acceleration. Objects moving under the influence of other "forces" do feel weight and have nonzero proper acceleration.

According to quantum field theory, "gravity" is a manifestation of a quantum field: a massless, spin-two field, to be exact. Other things that are often viewed as "forces" are manifestations of other quantum fields; electromagnetism is a manifestation of a massless spin-one field, for example. On this view, "forces" are just particular manifestations of quantum fields; but not all such manifestations are usefully viewed as "forces". "Gravity" is just another quantum field on this view, but it still does have some unique features--the ones that lead, in the classical limit, to the GR view of "gravity" as spacetime curvature rather than as a "force".
 
  • #13
Peter, I actually don't like the word either. I wish I could delete this post and start over with words like motion. Someone just woodshedded me on my not knowing enough about "force" (which I'm sure I don't!) and so that's why I threw the word out there. I regret doing so.
 
  • #14
Questions:

What is "underlying physics"?

What do you mean by "manifestation of spacetime curvature" as opposed to gravity being "spacetime curvature"?

Why does light, weak and strong, when in conjunction with gravity, give "weight" and nonzero proper acceleration (whatever that is) to objects?
 
  • #15
PeterDonis
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I wish I could delete this post and start over with words like motion.

"Motion" wouldn't work any better, since it's also relative. :wink: In SR and GR, you don't actually need the concept of "motion" at all; you can analyze everything using spacetime as a 4-dimensional geometric object, with the worldlines of objects being curves within this geometry, and all of the physics being contained in geometric quantities like the lengths of curves, the angles between them at intersections, etc. (Similarly, in quantum field theory, you don't need the concept of "motion" either: you can analyze everything using path integrals, which is basically the quantum version of the "spacetime" viewpoint in SR and GR.)
 
  • #16
Dale
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Closed pending moderation.
 

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