How often does mass effect space

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Discussion Overview

The discussion revolves around the nature of gravitation and how mass affects the geometry of spacetime, particularly focusing on whether this effect is continuous or a one-time occurrence. Participants explore the implications of general relativity and quantum mechanics on this topic, raising questions about the propagation of gravitational effects and the relationship between mass and spacetime geometry.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant questions whether mass must continuously affect spacetime to account for long-term gravitational effects, suggesting that a one-time effect would not suffice.
  • Another participant explains that Einstein's equations imply a one-time change could be sufficient, but acknowledges that quantum mechanics complicates this understanding.
  • Several participants express confusion about how spacetime would respond if mass were abruptly removed, questioning the continuity of the gravitational influence.
  • Some participants assert that the influence of mass is continuous and challenge the framing of the question regarding how often this effect occurs.
  • There is a discussion about the stress-energy tensor and its role in describing changes in spacetime geometry, with references to gravitational waves as a potential consequence of mass movement.
  • One participant uses an analogy about standing on a scale to illustrate the perceived absurdity of quantifying how often mass affects spacetime.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether the effect of mass on spacetime is continuous or can be treated as a one-time change. There are competing views regarding the implications of general relativity and quantum mechanics on this issue.

Contextual Notes

Participants highlight the complexity introduced by quantum mechanics and the limitations of general relativity in addressing changes in mass and their effects on spacetime. The discussion includes unresolved questions about the nature of gravitational influence and the assumptions underlying the participants' claims.

Nacho
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I have a question about gravitation, that may be a "duh" question and apparent -- to everybody but me.

Say that a mass effecting the geometry of space/time is the cause of gravitation, per what I understand about what Einstein said. Wouldn't that mean that the mass has to do this on a continuing basis? It seems to me that a one time effect couldn't keep up the long term effects that we see of gravitation.

If that is the case, I have a related question. How often is the effect propogated, and what is the limiting factor of how often/fast it is performed? The simple answer might be "it is done at the speed of light (or hypothetical gravitons)", but I'm not so much asking how fast as how OFTEN. Bringing in quantum actions, I don't know if it could be continuous. Is how often the effect performed somehow related to the HUP?
 
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Einstein's equation has a mathematical expression representing the geometry of spacetime on the left-hand side and a mathematical expression representing all the energy and interactions in all of spacetime on the right-hand side. Note that it's spacetime, not just space. That's why "once" is enough, or at least would have been enough if it hadn't been for quantum mechanics. Any local change of the stress-energy tensor caused by quantum effects would in theory propagate at the speed of light. The key word here is "any", but of course if the changes are small enough, quantum gravity effects will be important.
 
Fredrik said:
That's why "once" is enough, or at least would have been enough if it hadn't been for quantum mechanics.

I don't understand what you are saying here -- sorry. If "once" is enough per Einstein (and neglecting QM), then take the mass away abruptly. If the presence of the mass is not continually communicated, how does the local space know it is gone (maybe movement of mass VS presence of mass)?
 
First, the word is "affect", not "effect". Second, the influence of mass is continuous. There is no question of "how often".
 
HallsofIvy said:
First, the word is "affect", not "effect". Second, the influence of mass is continuous. There is no question of "how often".

Isn't that something that has to be reconciled with QM?
 
Nacho said:
I don't understand what you are saying here -- sorry. If "once" is enough per Einstein (and neglecting QM), then take the mass away abruptly. If the presence of the mass is not continually communicated, how does the local space know it is gone (maybe movement of mass VS presence of mass)?
The right-hand side of the equation specifies everything about the matter in spacetime. When you talk about taking mass away, you're already contradicting GR. The only kind of "change" that's possible in GR is already present in the stress-energy tensor (the right-hand side of the equation).
 
Fredrik said:
The right-hand side of the equation specifies everything about the matter in spacetime. When you talk about taking mass away, you're already contradicting GR. The only kind of "change" that's possible in GR is already present in the stress-energy tensor (the right-hand side of the equation).

I just meant remove that mass from that location in space, and move it somewhere else. That area of the space was effected by the presense of the mass, and and if it was a one-time change how that space would know to return to not being effected. I'm not talking about all mass and all locations in space, just a location in space that has an accumulation of mass, such as our sun, and the local gravitational effects caused by it.
 
I never thought you meant anything else. I'm just saying that this "move" is either described by the stress-energy tensor, or it isn't. The first alternative corresponds to one spacetime geometry and the second alternative to another geometry. The first of those two geometries describes gravitational waves propagating away from the relevant event. The second doesn't.
 
"How often" do your feet apply a force to a bathroom scale when you stand on it? See - the question doesn't really make any sense.
 

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