Is Gravity Affected by All Types of Energy?

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Discussion Overview

The discussion revolves around the nature of gravity and its relationship with various types of energy, particularly in comparison to other fundamental forces. Participants explore theoretical implications, the role of energy-momentum density, and the philosophical aspects of understanding why gravity behaves differently from other forces.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant questions why gravity is "special" compared to other forces, suggesting that all forms of energy create disturbances in the gravitational field.
  • Another participant asserts that gravity couples to the energy-momentum density of all fields, while other forces couple to specific current densities.
  • A participant expresses confusion but acknowledges the idea that everything affects the gravitational field, while only certain entities affect other fields.
  • Some participants note that the reasons behind these differences in coupling are not well understood, with one stating that nobody knows why this is the case.
  • There is a mention of mathematical models that describe these behaviors in certain regimes, but these do not explain why such models are realized in nature.
  • One participant emphasizes the distinction between the "how" of physics and the "why," suggesting that many constants in the Standard Model lack known reasons for their values.
  • Another participant references ongoing research in string theory and loop quantum gravity as potential avenues for answering these questions in the future.
  • A later reply discusses a specific argument from Weinberg regarding the universal coupling of gravity to all energy, referencing a paper for further details.

Areas of Agreement / Disagreement

Participants express a range of views on the nature of gravity and its relationship with energy, with no consensus reached on the underlying reasons for these phenomena. The discussion remains unresolved regarding the philosophical implications and the existence of a comprehensive theoretical framework.

Contextual Notes

Participants acknowledge limitations in current understanding, particularly regarding the lack of a universally accepted theory that explains the differing behaviors of gravity compared to other forces. There is also mention of unresolved mathematical steps in existing models.

Runner 1
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Why is gravity "special"?

(You may think this belongs in the General Relativity section, but the question really relates more to particle physics).

As I understand it, anything with any energy whatsoever creates a disturbance in the gravitational field. So this means that the potential energy from interactions via W bosons, Z bosons, gluons, or photons all have effects on gravity (even if those effects are so small as to be undetectable).

On the other hand, the fields due to other forces (strong, weak, and electromagnetic) are -- as far as I know -- affected ONLY by the particles that correspond to those fields. In other words, no amount of charge is going to affect the strong force.

I'm not great at lucidly describing what I mean so I hope someone sees what I'm getting at. Can someone help explain this?

Thanks
 
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Gravity is a universal force coupling to energy-momentum density of all fields, whereas other forces couple to special current densities of a subset of fields.
 


Huh, okay. Thanks. A little over my head but I kind of think I know what you mean.

I also thought of a better way to posit my question:

Everything affects the gravitational field, but only certain things affect the electromagnetic field, the Higgs field, etc. Why?
 


Runner 1 said:
Everything affects the gravitational field, but only certain things affect the electromagnetic field, the Higgs field, etc.
Yes

Runner 1 said:
Why?
Nobody knows.

One can write down a rather sophisticated mathematical model describing this behaviour in certain regimes (not yet in the quantum gravity regime); but that does not explain why this models is realized in nature, whereas others aren't.
 


In this, as in many thing, the point of physics is to determine HOW things work. WHY is philosophy. To extend Tom's excellent point, we know all KINDS of things about the universe and the interactions that take place. A lot of the WHY is a total mystery. For example, there are 23 (I think it is) constants in the Standard Model. Things like the mass of the electron. NONE of them are what they are for any known reason, they just ARE. Maybe someday we'll know more of the why.
 


phinds said:
In this, as in many thing, the point of physics is to determine HOW things work. WHY is philosophy. To extend Tom's excellent point, we know all KINDS of things about the universe and the interactions that take place. A lot of the WHY is a total mystery. For example, there are 23 (I think it is) constants in the Standard Model. Things like the mass of the electron. NONE of them are what they are for any known reason, they just ARE. Maybe someday we'll know more of the why.

That isn't really where I was going with this.

If someone asks why two electrons repel, we can describe that using a model we've developed.

If someone asks why there is a tide, we can give a reason involving the gravitational pull of the moon.

And if someone asks why everything affects the gravitational field but not everything affects the other fields, and if there is a mathematical model that describes these cases, then that would be absolutely great to know. ;)

(If you can't tell, I'm not fond of interpreting semantics in the strictest possible sense -- were this the case, communication among humans would be all but impossible. It's like the "Can I... ?" question where someone snidely responds "Of course you can...". Not to mention the fact that I even included a disclaimer in my original post to try and avoid these situations).
 
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Runner 1 said:
And if someone asks why everything affects the gravitational field but not everything affects the other fields, and if there is a mathematical model that describes these cases, then that would be absolutely great to know. ;)

I think the best that can be said is that some people are working along paths such as string theory and loop quantum gravity, that might eventually answer questions like this. Right now there are no generally-accepted and experimentally-tested theories that do this.
 


In the view of gravity as a [url="http://arxiv.org/abs/astro-ph/0006423]massless spin 2[/url] particle on flat spacetime, Weinberg has an argument I'm trying to understand for why gravity is universally and identically affected by all energy. It's described in http://arxiv.org/abs/1007.0435v2 section 2.2.2:

"Lorentz invariance for the emission of a soft massless spin-two particle leads to the conservation law ... On the other side, translation invariance implies momentum conservation ... Therefore, for generic momenta, Poincare invariance requires all coupling constants to be equal ... In other words, massless particles of spin-two must couple in the same way to all particles at low energies."
 


Runner 1 said:
That isn't really where I was going with this.

If someone asks why two electrons repel, we can describe that using a model we've developed. ...

OK, gotcha ... I was going off in the wrong direction.
 

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