Question about Basics of Gravity

[Genady]

Just now, mistermack said:

Which is every particle and every bit of energy in the universe, isn't it? 

In the observable universe. Diluted by the distances and by the redshift. So, the emission and absorption might be in balance.

[geordief]

If we have an object like a black hole standing in more or less isolation how does it work that the spacetime is curved by it at a distance?

 

I understand that it curves spacetime in its immediate vicinity but the effect continues at a distance from the object. 

Is  this perhaps an example of gravitation being itself a source  of gravity?

[Genady]

14 minutes ago, Mordred said:

Try this for a thought experiment  take a uniform distribution of mass where every point has the same mass. Then apply Newtons Shell theorem using any random point as the designated centre of mass. In this case you would experience no gravity at any location. You need regions of non uniform mass distribution.

This is incorrect. I remember us discussing it about a year ago. I gave you then a link to Alan Guth's MIT lecture where he shows that this is incorrect.

[Mordred]

Try it, which side would you get a net force if the mass is evenly distributed surrounding any point you choose ? Aside from a net force of zero.

20 minutes ago, Genady said:

This is incorrect. I remember us discussing it about a year ago. I gave you then a link to Alan Guth's MIT lecture where he shows that this is incorrect.

I also don't believe Allen Guth claimed Newtons Shell theorem as being incorrect. More likely he added some detail or scenario. If you can find the link we can examine it.

36 minutes ago, mistermack said:

Which is every particle and every bit of energy in the universe, isn't it? 

But then there's a mechanism in which the path of least action is shaped, that needs explaining. 

Are you familiar with the Principle of least action that equates potential and kinetic energy relations ? You seem to keep wanting particle to interaction for your mechanism however you don't require this. 

Potential energy being the energy due to location aka field or collection of fields energy.

Kinetic energy being the particles momentum.

Edited May 8, 2023 by Mordred

[mistermack]

Another question that bothers me, is how, if gravity is mediated by a particle, how does a black hole cause gravity? We know that nothing can exceed the speed of light, and that light cannot escape a black hole, so how does a particle like a graviton exit a black hole? Gravity travels at the speed of light, and the speed of light is not enough to escape the event horizon. 

[Genady]

20 minutes ago, Mordred said:

Try it, which side would you get a net force if the mass is evenly distributed surrounding any point you choose ? Aside from a net force of zero.

35 minutes ago, Genady said:

This is incorrect. I remember us discussing it about a year ago. I gave you then a link to Alan Guth's MIT lecture where he shows that this is incorrect.

I also don't believe Allen Guth claimed Newtons Shell theorem as being incorrect. More likely he added some detail or scenario. If you can find the link we can examine it.

It is a bit more subtle than this.

First I saw this explanation in one of the Susskind lectures, but it will be easier for me to find the Guth's one. I will be happy to discuss it.

[Mordred]

I think you may be referring to negative and positive pressure influences in his inflationary lecture. Where negative pressure can induce repulsive gravity.

[mistermack]

24 minutes ago, Mordred said:

You seem to keep wanting particle to interaction for your mechanism however you don't require this. 

I'm actually explaining the problem I have with the concept of a particle mediating gravity. I realise that there is no accepted theory of gravity as yet, but the existence or otherwise of a graviton particle hasn't been determined, as far as I can tell. 

[Mordred]

No it hasn't we count is as a possibility but physicists also realize a graviton as a mediator is not required when spacetime itself is sufficient.

[Genady]

6 minutes ago, Mordred said:

I think you may be referring to negative and positive pressure influences in his inflationary lecture. Where negative pressure can induce repulsive gravity.

No, it's not it. It just plain old Newtonian gravity.

[Mordred]

Ok I was thinking he may have incorporated the stress energy momentum tensor which he does without referring to it directly in his inflationary lecture.

[mistermack]

25 minutes ago, Mordred said:

spacetime itself is sufficient.

But as I said earlier, spacetime describes the effect but doesn't explain the mechanism. What is spacetime, and what mechanism causes mass and energy to curve it?

[Genady]

26 minutes ago, Mordred said:

Ok I was thinking he may have incorporated the stress energy momentum tensor which he does without referring to it directly in his inflationary lecture.

I think this is it. The first 12 minutes is all we need to discuss.

Lecture 6: The Dynamics of Homogeneous Expansion, Part II | The Early Universe | Physics | MIT OpenCourseWare

[Mordred]

Ok I had to go back one lecture towards the end of lecture one to get the breadth of his statement. If you sum the masses symmetrically then Newtons Shell theorem is accurate. However if you choose a different point q off centre of point P you can get any arbitrary answer. Which highlights to ordering of how you add the mass. In the first case. The order doesn't matter as it's commutative in the second case it's non commutative 

 I wouldn't necessarily consider Newtons Shell theorem incorrect if the mass is added symmetrically it works in that case. However I can see his argument that the Poisson equations are more accurate in all cases.

I'm not sure I fully buy his argument. If I rotate q at the same radius from p and continue to sum the concentric circles I should return to sum zero

Edited May 9, 2023 by Mordred

[Genady]

19 minutes ago, Mordred said:

If I rotate q at the same radius from p and continue to sum the concentric circles I should return to sum zero

But you cannot do this, because you would be counting the same regions multiple times.

Newton Shell Theorem is correct and valid in all cases. It is just that when we apply it to an infinite distribution of masses it gives a conditionally converging integral, and this leads to an undefined answer.

The outcome of the homogeneously collapsing universe is fully symmetrical: each point is a center of the collapse in the same way as in expanding universe each point is a center of the expansion.

[Mordred]

Well that I agree with though our universe isn't static so we will always have fluxes of mass distribution as every body is is motion and tends to clump into LSS. Anyways we both agree the Shell theorem is accurate.

 Which is good in so far as the FLRW metric critical density formula applies the Shell theorem as its basis.

Edit

Granted the scenario in the infinite case amounts to an impossibility once you consider causality. The influence of gravity also being limited to c

Edited May 9, 2023 by Mordred

[MigL]

2 hours ago, Mordred said:

You seem to keep wanting particle to interaction for your mechanism however you don't require this. 

You uys seem to be talking past each other.
A geometric field theory of gravity does not require a mediator particle, as Mordred states; it is 'sufficient' on its own.
Yet we know that it isn't exactly 'sufficient' at small separations/hi energies, and a quantum field theory of gravity is required to cover those areas of applicability.
We also know that a quantum field theory of gravity does require a spin2, massless mediator particle which we have termed the graviton, as MisterMack states.
It could be quite a while before quantum gravitytheory and gravitons are an established fact, though.

[Genady]

2 hours ago, mistermack said:

What is spacetime, and what mechanism causes mass and energy to curve it?

Spacetime is an arrangement of events. The structure and dynamics of the arrangement are determined by the events.

The arrangement is quantitatively described as geometry, and the events are quantitatively described as distribution of energy, momenta, and their various derivatives.

[Mordred]

@Genady by the way thanks I did enjoy watching the lecture. It's nice to get a good mental challenge with a physics subject once in a while. 

[Genady]

Just now, Mordred said:

@Genady by the way thanks I did enjoy watching the lecture. It's nice to get a good mental challenge with a physics subject once in a while. 

You're welcome. I agree, and I also agree with your comments above regarding it.

[Markus Hanke]

5 hours ago, geordief said:

Is  this perhaps an example of gravitation being itself a source  of gravity?

Yes, at least partly. Even if there is no conventional source in the immediate vicinity, the field itself has a gravitational effect, so it extends outwards from the central source. Another important reason is internal consistency - spacetime is required (by definition) to be smooth and continuous everywhere, so the geometry in the interior of a body must connect to an external geometry in a way that guarantees this requirement to hold. It turns out that this provides a strong constraint on what an external geometry can look like - in 4D you simply can't have a flat external geometry connect to a non-flat internal one in a way that preserves smoothness and continuity at the boundary. 

4 hours ago, mistermack said:

I'm actually explaining the problem I have with the concept of a particle mediating gravity.

The entire concept of a 'graviton' stems from an attempt to apply the framework of quantum field theory to gravitational interactions, just like it was done for the other fundamental interactions. However, while this works just fine for EM, weak, and strong interactions, we already know that this approach fails when applied to gravity - you can write down a QFT for a massless spin-2 field, but you will find that this must necessarily contain infinities that cannot be removed from the model by any means. So it cannot make any physical predictions, and is thus entirely useless. The approach simply doesn't work, which is why I think the concept of a 'graviton' is fundamentally meaningless. Gravity simply doesn't quantise in the same way as the other interactions, which is why much active research is conducted on alternative approaches, such as for example Loop Quantum Gravity.

[Genady]

11 hours ago, geordief said:

If we have an object like a black hole standing in more or less isolation how does it work that the spacetime is curved by it at a distance?

The gravitational field, like other fields, assumes a configuration that makes a function called action, stationary: the principle of stationary, or least, action. Given the source of the field located somewhere and vacuum everywhere else, it assumes a specific curved shape whose action is stationary.

[geordief]

34 minutes ago, Genady said:

The gravitational field, like other fields, assumes a configuration that makes a function called action, stationary: the principle of stationary, or least, action. Given the source of the field located somewhere and vacuum everywhere else, it assumes a specific curved shape whose action is stationary.

Is the configuration of the gravitational field  dependent on  anything comparable to the electric and magnetic permittivity of the vacuum?

 

Changes in gravitational  fields  propagate at c ,which is a function of those values so is there any connection there?

6 hours ago, Markus Hanke said:

Yes, at least partly. Even if there is no conventional source in the immediate vicinity, the field itself has a gravitational effect, so it extends outwards from the central source. Another important reason is internal consistency - spacetime is required (by definition) to be smooth and continuous everywhere, so the geometry in the interior of a body must connect to an external geometry in a way that guarantees this requirement to hold. It turns out that this provides a strong constraint on what an external geometry can look like - in 4D you simply can't have a flat external geometry connect to a non-flat internal one in a way that preserves smoothness and continuity at the boundary. 

Does the second of those two phenomena explain the first?

 

Ie does the simple requirement that the field be continuous  mean that the field can be modeled as acting on itself?

[Genady]

15 minutes ago, geordief said:

Is the configuration of the gravitational field  dependent on  anything comparable to the electric and magnetic permittivity of the vacuum?

Aren't they just unit conversion constants?

 

16 minutes ago, geordief said:

Changes in gravitational  fields  propagate at c ,which is a function of those values so is there any connection there?

c is not a function of those values, but it can be calculated knowing those values. And, at the end of the day, c is just a unit conversion constant as well, the conversion between units of distance and time.

All of these constants can, and are, set to 1 in appropriate units.

[geordief]

23 minutes ago, Genady said:

Aren't they just unit conversion constants?

 

 

I don't know (a bit out of my depth)

25 minutes ago, Genady said:

Aren't they just unit conversion constants?

 

c is not a function of those values, but it can be calculated knowing those values. And, at the end of the day, c is just a unit conversion constant as well, the conversion between units of distance and time.

 

So is the speed of em radiation a vacuum   just a function of the ratio of space to time ? (could it be the other way round?)