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Massive Gravity, or it's OK if current models fray at extreme scales

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#1 Jay_Bird

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Posted 25 January 2020 - 05:41 PM

Saw this to add to new physics talk...as others already mentioned, at largest or smallest scales we may learn new things that don't make sense in, or really affect our meso-scale experiences or models*, but the new understanding finally explains phenomena at the extreme scales.

 

https://www.theguard...k-energy-riddle

 

Never read of this idea before.  Gravitational wave astronomy in coming decades may help understand.

 

 

 

* which reminds of the Eames' classic film "powers of ten"


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#2 sg6

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Posted 25 January 2020 - 06:09 PM

Have seen topics that indicate gravity may be "repulsive" at distances also.

 

The one problem is "distance".

If gravitons have a mass, then gravity is expected to have a weaker influence on very large distance scales, which could explain why the expansion of the universe has not been reined in.

 

Where is "distance" measured from. I do not believe it is correct to say "from us". "from us" has no real basis. It is like saying that as distance from us increases then the laws of physics (ones we are aware of) change also.

 

Also would the weaker influence mean the universe expands "not as slowly" as opposed to accelerates. One would seem to be a negative value the other a positive one.

 

Agreed I know little and the artical is likely very simplified, just some of it seems questionable or ill worded and so comes across as questionable.

 

But as said in another post I hear too much of what is "Newtonian gravity" talked of in many (all) cosmology talks I get the chance to attend.



#3 Keith Rivich

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Posted 26 January 2020 - 10:10 PM

Isn't this MOND? I thought that got shot down years ago...



#4 llanitedave

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Posted 27 January 2020 - 10:35 AM

Isn't this MOND? I thought that got shot down years ago...

Not sure, but MOND as I used to understand it operated on galactic scales, while this seems to be larger than the scale of galaxy clusters.  So not quite the same thing.



#5 Jim_V

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Posted 27 January 2020 - 11:05 AM

It will be interesting if they can find observable data to go with the theoretical.



#6 DaveC2042

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Posted 27 January 2020 - 03:32 PM

Not sure, but MOND as I used to understand it operated on galactic scales, while this seems to be larger than the scale of galaxy clusters. So not quite the same thing.


I don't think this is MOND.

As I understand it, MOND is a tweak to how gravity interacts with acceleration. This is suggesting gravitons have mass, which is very different.

And MOND is very much a minority view, but I'm not sure it's fair to describe it as 'shot down'. It's extremely difficult to disprove this kind of thing.
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#7 EJN

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Posted 27 January 2020 - 03:50 PM

I don't think this is MOND.

As I understand it, MOND is a tweak to how gravity interacts with acceleration. This is suggesting gravitons have mass, which is very different.

And MOND is very much a minority view, but I'm not sure it's fair to describe it as 'shot down'. It's extremely difficult to disprove this kind of thing.


Yes, MOND stands for MOdified Newtonian Dynamics, and is a classical theory which ignores QM.

This new theory of gravitons would be a quantum theory, in that gravitons are a hypothetical boson which are the carriers of gravitational force.

Due to theoretical arguments, they are believed to be spin 2, mass 0 bosons. However, this new theory has gravitons with a slight mass, so at long distances gravity would be weaker than in GR.

One thing that has baffled me is that if gravity is really curved spacetime, why are gravitons even needed?

Edited by EJN, 27 January 2020 - 04:06 PM.

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#8 DaveC2042

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Posted 27 January 2020 - 06:02 PM

Yes, MOND stands for MOdified Newtonian Dynamics, and is a classical theory which ignores QM.

This new theory of gravitons would be a quantum theory, in that gravitons are a hypothetical boson which are the carriers of gravitational force.

Due to theoretical arguments, they are believed to be spin 2, mass 0 bosons. However, this new theory has gravitons with a slight mass, so at long distances gravity would be weaker than in GR.

One thing that has baffled me is that if gravity is really curved spacetime, why are gravitons even needed?

Possibly getting above my paygrade here, but...

 

My sense is that physics sees QM as 'more fundamental' than GR, in the sense that inconsistency between the two is to be resolved by absorbing GR into QM.  This makes intuitive sense to me, as the glaring problem with GR is that it produces singularities.  If you introduced Heisenberg-style uncertainty to spacetime, you'd expect this problem to naturally disappear, as there would no longer really be such a thing as a well-defined point in spacetime, so a singularity simply becomes impossible.

 

Once you go down that path, then, gravitons are more or less inevitable.

 

Now my extremely limited understanding is that mathematically, gravitons in quantum gravity act like quantizations of spacetime in the same way that photons act like quantizations of the EM field.  As a result, they produce the apparent curvature effects, in the same way photons produce apparent forces between charged particles.

 

The difficulty of course, is that word mathematically.  My mathematical learning (and possibly my innate ability to understand) gets me to (Feynmann-style) relativistic quantum mechanics, and standard general relativity - neither of which I found easy at all.  I don't think I have any real hope of properly understanding the maths involved in quantum gravity, particularly not while holding down a full-time finance job.



#9 EJN

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Posted 27 January 2020 - 09:37 PM

Possibly getting above my paygrade here, but...

 

My sense is that physics sees QM as 'more fundamental' than GR, in the sense that inconsistency between the two is to be resolved by absorbing GR into QM.  This makes intuitive sense to me, as the glaring problem with GR is that it produces singularities.  If you introduced Heisenberg-style uncertainty to spacetime, you'd expect this problem to naturally disappear, as there would no longer really be such a thing as a well-defined point in spacetime, so a singularity simply becomes impossible.

 

Once you go down that path, then, gravitons are more or less inevitable.

 

Now my extremely limited understanding is that mathematically, gravitons in quantum gravity act like quantizations of spacetime in the same way that photons act like quantizations of the EM field.  As a result, they produce the apparent curvature effects, in the same way photons produce apparent forces between charged particles.

 

The difficulty of course, is that word mathematically.  My mathematical learning (and possibly my innate ability to understand) gets me to (Feynmann-style) relativistic quantum mechanics, and standard general relativity - neither of which I found easy at all.  I don't think I have any real hope of properly understanding the maths involved in quantum gravity, particularly not while holding down a full-time finance job.

 

That's my understanding too, although to understand the mathematics better I am looking at

getting a strap-on auxiliary brain pack...

 

In one of his books, Roger Penrose gave 3 possibilities:

 

1) The majority view: gravity is ultimately quantized approaching the Planck scale.

 

2) The minority view: gravity & curved spacetime is fundamental, and particle physics

can ultimately be described by spacetime geometry.

 

3) An outside possibility: gravity and quantum mechanics are fundamentally different

phenomena and although it might be possible to find some mathematical way of unifying them, it won't

make physical sense.

 

 

As for #3, I am reminded of string theory, which appears to incorporate gravity, but at the cost

of requiring at least 10 dimensions and having a "landscape" of 10500 vacuum solutions.



#10 DaveC2042

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Posted 27 January 2020 - 09:50 PM

That's my understanding too, although to understand the mathematics better I am looking at

getting a strap-on auxiliary brain pack...

 

In one of his books, Roger Penrose gave 3 possibilities:

 

1) The majority view: gravity is ultimately quantized approaching the Planck scale.

 

2) The minority view: gravity & curved spacetime is fundamental, and particle physics

can ultimately be described by spacetime geometry.

 

3) An outside possibility: gravity and quantum mechanics are fundamentally different

phenomena and although it might be possible to find some mathematical way of unifying them, it won't

make physical sense.

 

 

As for #3, I am reminded of string theory, which appears to incorporate gravity, but at the cost

of requiring at least 10 dimensions and having a "landscape" of 10500 vacuum solutions.

I thought Penrose himself (with Hawking) had really knocked number 2 on the head when they showed that singularities are an intrinsic property of GR.



#11 Keith Rivich

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Posted 27 January 2020 - 09:51 PM

That's my understanding too, although to understand the mathematics better I am looking at

getting a strap-on auxiliary brain pack...

 

In one of his books, Roger Penrose gave 3 possibilities:

 

1) The majority view: gravity is ultimately quantized approaching the Planck scale.

 

2) The minority view: gravity & curved spacetime is fundamental, and particle physics

can ultimately be described by spacetime geometry.

 

3) An outside possibility: gravity and quantum mechanics are fundamentally different

phenomena and although it might be possible to find some mathematical way of unifying them, it won't

make physical sense.

 

 

As for #3, I am reminded of string theory, which appears to incorporate gravity, but at the cost

of requiring at least 10 dimensions and having a "landscape" of 10500 vacuum solutions.

My thoughts have always been a combination of your 2 and 3. Gravity and QM are elementary forces...much like an electron and a photon. You can't marry the two, I don't think. 




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