Question about Basics of Gravity

[Genady]

17 minutes ago, geordief said:

So is the speed of em radiation a vacuum   just a function of the ratio of space to time ?

Yes.

 

17 minutes ago, geordief said:

could it be the other way round?

No, because this ratio affects phenomena other than EM radiation.

[Eise]

1 hour ago, Genady said:

And, at the end of the day, c is just a unit conversion constant as well, the conversion between units of distance and time.

This also seems to me the most fundamental description of what c is. However, I think there is another description, and it would be great, if people agree, can shed some more light on this: c is the maximum speed of causality.

47 minutes ago, geordief said:

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?)

So, no, as Genady already said. Only particles without rest mass, can travel at this speed, or better, can only exist as particles with this speed. Light and gravity are as far as we know the only phenomena that have this speed. (Neutrinos were also suspected to travel at light speed, until other experiments showed that they must have a tiny rest mass.)

[Genady]

6 minutes ago, Eise said:

Light and gravity are as far as we know the only phenomena that have this speed.

Gluons?

[geordief]

55 minutes ago, Genady said:

 

 

No, because this ratio affects phenomena other than EM radiation.

Can I be nosey and ask for an example or two?

[joigus]

17 minutes ago, Genady said:

Gluons?

Gluons are dressed with effective mass. They appear as massless in the Lagrangian (if I remember correctly, some people call them 'Lagrangian gluons'), but as soon as they 'get real' (  ) they acquire a mass. The reason is the non-Abelian character of QCD. Because gluons have QCD charge, they polarise the QCD vacuum. This doesn't happen for photons. Maybe it doesn't for gravitons either, but I don't know that anybody has calculated vacuum polarisation with quantum gravity, really. The way things normally work in QFT, an interaction with infinite range is effectively mediated by massless particles.

Similar comments apply to W and Z for the weak interaction. But in that case masses appear also because of the Higgs mechanism.

Hardly ever is Eise wrong, if at all.  

[Genady]

11 minutes ago, geordief said:

Can I be nosey and ask for an example or two?

As @Eise said above, the maximum speed of causality. 

Transformation of spacetime coordinates between relatively moving reference frames.

Speed of gravitational waves.

Relation between energy and mass.

Schwarzschild radius.

.....

4 minutes ago, joigus said:

Gluons are dressed with effective mass. They appear as massless in the Lagrangian (if I remember correctly, some people call them 'Lagrangian gluons'), but as soon as they 'get real' (  ) they acquire a mass. The reason is the non-Abelian character of QCD. Because gluons have QCD charge, they polarise the QCD vacuum. This doesn't happen for photons. Maybe it doesn't for gravitons either, but I don't know that anybody has calculated vacuum polarisation with quantum gravity, really. The way things normally work in QFT, an interaction with infinite range is effectively mediated by massless particles.

Similar comments apply to W and Z for the weak interaction. But in that case masses appear also because of the Higgs mechanism.

Thank you!

[Eise]

1 hour ago, joigus said:

Hardly ever is Eise wrong, if at all.

I doubt that, but in this case... I just wanted to apologise to Genady for forgetting about gluons. So this time I was right by being wrong.

Maybe I show my stupidity with the next question: are gluons in a nucleon off-shell, i.e. virtual particles? Or is that a different topic?

[Mordred]

As a vector gauge boson under QFT gluond are required to be massless. Vector gauge bosons are typically offshell. Experiments show less than a few Mev if they have any mass at all.

 One way to think of it is vector bosons on Feymann diagram are internal wavy lines. Real particles are on the external lines. All vector bosons are off shell internal lines on the Feymann diagram as the field mediator.

Edited May 9, 2023 by Mordred

[geordief]

8 hours ago, Genady said:

As @Eise said above, the maximum speed of causality. 

 

 

Am I nitpicking to ask what is the difference between the maximum speed of causality and the maximum speed in a vacuum?

Does the latter not imply the former?

[Genady]

11 minutes ago, geordief said:

Am I nitpicking to ask what is the difference between the maximum speed of causality and the maximum speed in a vacuum?

Does the latter not imply the former?

What do you mean by "the maximum speed in a vacuum?" Does such thing exist?

Edited May 9, 2023 by Genady

[Mordred]

2 minutes ago, Genady said:

What do you mean by "the maximum speed in a vacuum?" Does such thing exist?

Unfortunately the common descriptive of c is the speed of light in a vacuum. That likely is what Geordief is referencing. 

[Genady]

1 minute ago, Mordred said:

Unfortunately the common descriptive of c is the speed of light in a vacuum. That likely is what Geordief is referencing. 

Yes, but I'm not sure he is talking about light now. I suspect he means any speed. Also, I don't know why he specifies vacuum...

[geordief]

50 minutes ago, Genady said:

Yes, but I'm not sure he is talking about light now. I suspect he means any speed. Also, I don't know why he specifies vacuum...

Yes ,I meant any speed (but the fastest  speeds occur in a vacuum and with massless objects)

I have also seen it described as the cosmic speed limit and again it seemed to me helpful to also say "in a vacuum" since any other medium  would only permit lower speeds.

 

So I was just asking whether the maximum possible speed  (neglecting the speed of expansion) also implies a maximum "speed  of causality" since causality depends on the transmission of a signal or the transit of a particle  or radiation. 

 

Hope I am clear?

Edited May 9, 2023 by geordief

[Mordred]

Yes the speed limit also results in a maximum speed limit to causality.

[Genady]

4 minutes ago, geordief said:

Yes ,I meant any speed (but the fastest  speeds occur in a vacuum and with massless objects)

I have also seen it described as the cosmic speed limit and again it seemed to me helpful to also say "in a vacuum" since any other medium  would only permit lower speeds.

 

So I was just asking whether the maximum possible speed  (neglecting the speed of expansion) also implies a maximum "speed  of causality" since causality depends on the transmission of a signal or the transit of a particle  or radiation. 

 

Hope I am clear?

Firstly, as you said, any other medium would only permit lower speeds, saying "maximum speed" says it all.

Secondly, there is no "maximum speed". For example, a shadow of moving object can move with any speed. Or a spotlight of projector. Or a dot on computer screen. Etc.
 

[geordief]

14 minutes ago, Genady said:

Firstly, as you said, any other medium would only permit lower speeds, saying "maximum speed" says it all.

Secondly, there is no "maximum speed". For example, a shadow of moving object can move with any speed. Or a spotlight of projector. Or a dot on computer screen. Etc.
 

When I said I was nitpicking I didn't say you could  nitpick my nit

[Genady]

11 minutes ago, geordief said:

When I said I was nitpicking I didn't say you could  nitpick my nit

LOL

But seriously, "maximum speed of causality" is rigorous. It means that two events can be causally related if and only if their distance d and time difference t are such that d <= ct.

Edited May 10, 2023 by Genady

[geordief]

7 minutes ago, Genady said:

LOL

But seriously, "maximum speed of causality" is rigorous. It means that two events can be causally related if and only if their distance d and time difference t are such that d <= ct.

Agreed,but I  have probably gone well off topic now.

[MigL]

Causality necessitates the transfer of information.
A moving shadow, or dot of light, does not transfer any information.
The maximum speed of information transfer is c .

[Markus Hanke]

17 hours ago, geordief said:

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

No. It only places a constraint on how - given a gravitational source - the geometry external to that source can look like, ie it reduces the possibilities of what that geometry can be. In order to uniquely determine that geometry (ie reduce the remaining possibilities to just one specific one), you also need a field equation (which are the Einstein equations, which follow from the stationary action principle that Genady has mentioned), as well as some more boundary conditions. 

Ultimately though, the geometry at a distance is what it is because the possibilities are so heavily constrained by boundary conditions and the Einstein equation, that all but one possible metric is eliminated. So it is largely a question of internal consistency.

[MigL]

On 5/9/2023 at 1:01 AM, Markus Hanke said:

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

Yet we find that at Planck scale energies, all interactions, including gravity, are pretty well equal.
This would seem to indicate that a Grand Unified 'Force' may have been present doring the Planck epoch, and gravity dissociated from it shortly thereafter when energies dropped.

I have also always considered renormalization a mathematical 'trick', and maybe we simply haven't found the right rick to handle the infinities of quantum gravity field theory.

But whatever method is finally devised  ( I like LQG also ), any quantized field theory, even if the quantized field is geometric, will have a mediator particle.

So don't throw the graviton out yet.

[Mordred]

One of the QFT tricks to handle renormalization to handle divergences in a field theory is to employ a regulator operator. The main divergences being IR and UV divergence. Using Planck units provide a lower and upper bound (upper being Planch temp as one example) were all familiar with the lower lol.

One of the problems is that we do not know of any upper boundary to mass density.

[Markus Hanke]

13 hours ago, MigL said:

Yet we find that at Planck scale energies, all interactions, including gravity, are pretty well equal.

That's really just conjecture, since we haven't got any working model of physics at the Planck scale. The best we can currently do is extrapolate from already known physics, but I'd by really careful with this.

13 hours ago, MigL said:

and maybe we simply haven't found the right rick to handle the infinities of quantum gravity field theory.

Actually, it can be formally proven that any QFT equivalent of Einsteinian GR is necessarily non-renormalisable. I highlighted the 'able' part, because this isn't a case of us not knowing how to do it, but of it not being mathematically possible by any means. There are several technical reasons for this, but the main stumbling block is the nature of the coupling constant in GR - expressed in Planck units it is of negative dimension, unlike is the case for the couplings of the other fundamental interactions. As a result of this the renormalisation procedure must always fail, since each power term in the series requires a new term of higher power, and it can never be cut off anywhere - in fact, the series 'fluctuates' more and more wildly the more terms you add to it.

On a perhaps more intuitive level, GR is background-independent, whereas the other interactions are not. So whatever way you look at it, gravity is of fundamentally different nature than the other interactions, so it isn't surprising that quantisation via QFTs doesn't work here.

14 hours ago, MigL said:

But whatever method is finally devised  ( I like LQG also ), any quantized field theory, even if the quantized field is geometric, will have a mediator particle.

The entire notion of 'particle' and 'mediation' already presupposes that your field theory lives on a smooth and continuous spacetime with a pre-set causality structure. That's background-dependence. That's what we presuppose for the other interactions, but it is also precisely what gravity is not. In GR, spacetime itself provides the only dynamics there are - there's no background of any kind, so it is difficult to even make self-consistent sense of the notion of a 'graviton'. If you mean by this just some spin-2 massless particle that lives in spacetime, like any other particle, then clearly you haven't quantised GR at all - you've just added another field into the Standard Model, which already requires a fixed geometry as background. At best you wouldn't have gained anything, and at worst you end up with a logical contradiction. So again, I'm not surprised at all that this approach doesn't work out, mathematically speaking.

In contrast, approaches such as covariant LQG remain true to GR in that they don't require any background, but instead describe spacetime as the classical limit of more fundamental dynamics. To do this it does away with fields and particles altogether, and introduces more general objects, namely Wilson loops and spin foams. As a result, spacetime itself (rather than any fields living on it) becomes 'quantised' in the sense that it is meaningless to speak of separation between events below a certain order of magnitude. So rather than QFTs in spacetime, we have a quantisation of spacetime.

14 hours ago, MigL said:

So don't throw the graviton out yet.

That wasn't my intention - for the simple reason that for the time being, all of our candidate models are merely hypotheses, so we really don't know yet what the answer will turn out to be in the end. However, I personally won't be betting any money on gravitons.

[MigL]

Maybe it's too subtle for me to grasp, but I fail to see a difference.

All other interactions are fields acting on a fixed background stage.
Gravity, as mdelled by GR ( and LQG ) goes a step further, and the formerly fixed background stage, space-time ( more specifically, its geometry ) becomes an active participant in the interaction and can be modified by it.
The difference is not that large, and maybe it is renormalization ( which I find somewhat ad-hoc ) that needs to be scrapped, and a new, more robust method, devised to handle the divergences.

You could be right, and it may be just wishful thinking on my part, but I like the idea of all interactions descending from an original 'superforce' at the beginning.
It would make things a lot neater.

[Markus Hanke]

13 hours ago, MigL said:

Gravity, as mdelled by GR ( and LQG ) goes a step further, and the formerly fixed background stage, space-time ( more specifically, its geometry ) becomes an active participant in the interaction and can be modified by it.

It's not that GR modifies the background, it's that it does not have any background. It's a fully background-independent theory - in contrast to the other interactions.

13 hours ago, MigL said:

but I like the idea of all interactions descending from an original 'superforce' at the beginning.

This may well be the case still, it's just that in all likelihood this would not be described by a quantum field theory. I think such a unification of forces will require us to let go of the notion of a smooth and continuous spacetime, which renders the concept of a QFT meaningless.