Gravity, behaves a certain way?

[Syntho-sis]

Does anyone happen to know why matter when acted on by gravity will always fall towards the source of gravity?

 

You all know the pool full of jello and the bowling ball analogy. I understand that somewhat, but why doesn't the matter acted on move away from the source? What makes this work the way it does?

 

And how exactly do gravitons fit in?

 

EDIT: Does it always fall towards the source? That was an assumption btw, correct me if I'm wrong.

[Mr Skeptic]

Because gravity is an attractive force, of course. In other words, it just does.

 

To say why this is so you would need to deduce gravity from first principles.

[Syntho-sis]

Because gravity is an attractive force, of course. In other words, it just does.

 

To say why this is so you would need to deduce gravity from first principles.

 

Question one: What causes it to be attractive? Why not repelling, or even sometimes both?

 

Question two: What do you mean by first principles?

[ajb]

The positive mass theorem and the properties of spin-2 fields?

 

(If I remember correctly)

[swansont]

Question two: What do you mean by first principles?

 

Come up with how gravity should behave, without first observing how gravity behaves. At best, basing it on a few axioms and/or other established laws, but not on data.

[elas]

The positive mass theorem and the properties of spin-2 fields?

 

(If I remember correctly)

 

Observation of Black Holes shows that spin is independent of mass and presumably, therefore independent of gravity. As Swansont implies observation takes priority over 'a few axioms and/or other established laws',.

 

But neither 'observation' nor 'a few axioms and/or other established laws', explain the 'cause' of gravity or 'how' or 'why' gravity works as observed.

 

Max Jammar and Lee Smolin have written speculative books on mass and gravity respectively; neither reach a definitive explantion. The speculation made by Newton is (in my opinion) still the nearest anyone came to to reaching a working model. Newton's speculation came in his statement that "'perhaps the universe is corpuscular in nature'". Neither 'Jammar' nor 'Smolin' developed Newton's speculation prefering instead to confine their speculations to 'Relativity' and 'Quantum Theory'. Of the three (Newton, Relativity, and QT) only Newton's speculation can be developed to explain 'how' and 'why'.

[ajb]

Observation of Black Holes shows that spin is independent of mass and presumably, therefore independent of gravity.

 

The gravitational field is spin-2.

 

You can show that the nature of interactions does depend on the spin of the field. I forget the details, but if I recall correctly spin-2 with a positive mass can only be attractive. (A theorem is only as good as what you insist on to establish it, so there is probably some leeway here.)

 

Of course, nature always has the final say.

---

Merged post follows:

Consecutive posts merged

But neither 'observation' nor 'a few axioms and/or other established laws', explain the 'cause' of gravity or 'how' or 'why' gravity works as observed.

 

The best physics can ever really do is produce models of phenomena. You can then to some extent answer the "cause" (the sources) and "how" (curvature of space-time) buy "why" is outside physics.

[elas]

The best physics can ever really do is produce models of phenomena. You can then to some extent answer the "cause" (the sources) and "how" (curvature of space-time) buy "why" is outside physics.

 

Many times I have quoted experts who disagree with your statement, to do so again would be repetitive and straying from the subject. There also seems to be a defeatist attitude in your reply which I am sure is unintended. Newton gave the possible cause, 'infinity' can be used to explain why the cause exists. Both are speculative; I can get away with quoting speculations by Newton, Jammer and Smolin; I cannot get away with quoting speculations by 'elas' so no further comment is possible, but I have always thought that science is the pursuit of knowledge which surely must embrace all three ‘how, why’ and ‘cause’.

[ajb]

Many times I have quoted experts who disagree with your statement, to do so again would be repetitive and straying from the subject.

 

I am no expert, but I feel qualified to make such statements. There will of course be academics out there who disagree, but that is fine. These issues are more metaphysical.

 

There also seems to be a defeatist attitude in your reply which I am sure is unintended.

 

My philosophy is that one must be careful to make the distinction between the mathematical world and the physical world. Theoretical physics (phenomenology in particular) tries to make a link between these worlds.

 

I do not mean to be defeatist, but this distinction is largely the reason I say that "why" is outside physics. This is not to be confused with describing mechanisms and interpretations.

 

Physics can and often does proved clear mechanisms, if this is what you mean by "why". Which is really the same as "how".

 

Newton gave the possible cause, 'infinity' can be used to explain why the cause exists. Both are speculative; I can get away with quoting speculations by Newton, Jammer and Smolin; I cannot get away with quoting speculations by 'elas' so no further comment is possible, but I have always thought that science is the pursuit of knowledge which surely must embrace all three ‘how, why’ and ‘cause’.

 

You probably should try to define carefully what you mean by how, why and cause. I would not be surprised if in the context of a physical theory they are one and the same. Thus, the confusion on both our parts.

[swansont]

Observation of Black Holes shows that spin is independent of mass and presumably, therefore independent of gravity. As Swansont implies observation takes priority over 'a few axioms and/or other established laws',.

 

Did you understand what "spin-2" means? Your statement implies that these observations of spin are at the level of 2*hbar, which is, frankly, outlandish.

[Syntho-sis]

Did you understand what "spin-2" means? Your statement implies that these observations of spin are at the level of 2*hbar, which is, frankly, outlandish.

 

What exactly does "spin-2" mean?

[pioneer]

The other three forces of nature give off energy when the force potential is lowered. If gravity is a force, what type of exothermic output does gravity give off when potential lowers? There is no output that is very obvious. As such, this is one way that gravity behaves differently than other forces. It does not output obvious energy. However, lowering of potential and the conservation of energy, implies something, or we are in violation.

 

Say gravity did have some type of output, that is not obvious to the naked eye, its impact would be loosely analogous to that of the other forces. For example, the EM force gives off photons, which can act on other matter held together with EM force. Its impact is to reverse the effect of the force somewhere else. But the second law states entropy must increase such that the impact goes into entropy to lower the reverse impact, as it propagates.

 

If gravity gave off an unknown output, like other forces, the impact of this output would reverse gravity, elsewhere, but in a way that increases net entropy for the second law. The expansion of the universe seems to satisfy the criteria.

 

Let us so this another way. Gravity is defined by GR, with lowering gravitational potential compacting the matter in space-time increasing the local space-time contraction. The exothermic output should cause space-time to expand, elsewhere, if gravity was behaving like a force and giving off potential as it lowers potential. With entropy having to increase, it will not show up as a parallel galaxy expansion but something that is putting potential into space-time expansion effect with increasing entropy.

 

If we look at the earth, condensing core iron from liquid to solid, increases the density and lowers entropy. Does this increase the space-time contraction of the iron core over time due to higher mass density over more space-time. Does the output from this have an anti-gravity/entropy effect in the liquid part of the core? Does these cancel so the surface stays the same or does the surface increase or decrease gravity? Does it show up in anti-gravity effects such as volcanoes, where matter moves toward lowered space-time contraction by increasing gravity potential?

[ajb]

The other three forces of nature give off energy when the force potential is lowered. If gravity is a force, what type of exothermic output does gravity give off when potential lowers?

 

What about gravitational waves?

[swansont]

What exactly does "spin-2" mean?

 

A spin-2 particle has an intrinsic spin angular momentum of 2*hbar, where hbar is planck's constant divided by 2pi. In QM, spin is quantized, and can only vary in increments of hbar. Electrons, for example, have a spin of 1/2, so they will have either +1/2 or -1/2 units; the vector is often describes as pointing up or down. No other value can be measured. One should note that spin is not due to a physical spinning of some particle; this is quantum mechanics, and such classical notions simply do not apply.

 

Fermions have half-integral spin. Bosons have integral spin. Photons, the exchange particle for the electromagnetic force, are spin-1, and this has some implications on how interactions occur. Because of the properties gravity needs to have, stemming from the math that describes it, gravitons have to be spin-2. Gravitons have not yet been detected, though, so any claim about measurement of the spin of a black hole is not related to the quantum-mechanical properties.

 

Your angular momentum is quantized, too, but nobody is going to be able to measure an hbar change in that value as you spin around in your chair. It's absurd; the values differ by around 34 orders of magnitude. Anyone making a claim that such a measurement disproves the QM prediction is demonstrating a lack of grasp of the material in question.

[elas]

Did you understand what "spin-2" means? Your statement implies that these observations of spin are at the level of 2*hbar, which is, frankly, outlandish.

 

The statement does not originate from me, I am simply restating what is on the Wiki Black Hole page.

[swansont]

The statement does not originate from me, I am simply restating what is on the Wiki Black Hole page.

 

You quoted it in a context where it is not relevant. It was a non-sequitur.

 

And you did not state in that post that you were just cribbing from Wikipedia.