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Author Topic: How does physics explain the graviton?  (Read 11010 times)

Offline Vern

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How does physics explain the graviton?
« Reply #25 on: 14/03/2009 23:14:05 »
Oh yeah; that one too. I particularly don't like SUGRA.
 

Offline DoctorBeaver

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How does physics explain the graviton?
« Reply #26 on: 15/03/2009 00:43:27 »
I have half a spoon of it in coffee. Preferably Demerera.
 

Offline Chemistry4me

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How does physics explain the graviton?
« Reply #27 on: 15/03/2009 02:34:42 »
What the heck are you boys talking about?
 

Offline DoctorBeaver

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How does physics explain the graviton?
« Reply #28 on: 15/03/2009 04:51:01 »
SUGRA - SUperGRAvity
 

Offline Chemistry4me

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How does physics explain the graviton?
« Reply #29 on: 15/03/2009 04:56:28 »
What's Super Gravity? What is that all about?
 

Offline DoctorBeaver

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How does physics explain the graviton?
« Reply #30 on: 15/03/2009 09:14:41 »
erm... I'll let Vern explain

I can't explain it because I know nothing about its equivalence to strongly coupled 10d string theory or why M-theory says there is such an equivalence. All I know is that it's weakly coupled so it can be perturbated whereas strongly coupled string theory can't.

Of course, I was wrong when I said the graviton only exists in QFT. I forgot that it exists in SUGRA too as it's a field theory combining supersymmetry with general relativiy. Naturally, being supersymmetric, there is an anti-graviton too - the gravitino which has spin 3/2. But you don't want to hear about that.

And you wouldn't want to know about the long wavelength limit of M-theory being described by 11-dimensional supergravity.

No, you wouldn't want to hear about any of that.
« Last Edit: 15/03/2009 09:18:24 by DoctorBeaver »
 

Offline Chemistry4me

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How does physics explain the graviton?
« Reply #31 on: 15/03/2009 09:54:37 »
Who the heck comes up with this kind of stuff?!? ??????
Why is physics so confusing and complicated? :)???
 

Offline DoctorBeaver

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How does physics explain the graviton?
« Reply #32 on: 15/03/2009 09:57:43 »
Who the heck comes up with this kind of stuff?!? ??????

Daniel Z. Freedman, Peter van Nieuwenhuizen and Sergio Ferrara. That nice Ed Witten fiddled around with it somewhere along the line too.
 

Offline Chemistry4me

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How does physics explain the graviton?
« Reply #33 on: 15/03/2009 09:58:50 »
That was a rhetoric, but thank you for answering. :)

Now to the second question. :)
I think it's just me. :D
 

Offline Chemistry4me

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How does physics explain the graviton?
« Reply #34 on: 15/03/2009 10:00:04 »
Alright, I shall wait for Mr. Vern to enlighten me on this matter. :)
 

Offline DoctorBeaver

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How does physics explain the graviton?
« Reply #35 on: 15/03/2009 10:03:15 »
Now to the second question. :)

Stick to sheep. It's less confusing.
 

Offline Chemistry4me

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How does physics explain the graviton?
« Reply #36 on: 15/03/2009 10:05:45 »
But I'm confused already ??????

 

Offline Chemistry4me

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How does physics explain the graviton?
« Reply #37 on: 15/03/2009 10:09:14 »
I like the: 'Sheeplifters will be prosecuted' :)

Take note also of the: 'Physics graduate sheep'. :)
 

Offline Vern

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How does physics explain the graviton?
« Reply #38 on: 15/03/2009 12:40:34 »
This Wiki article is a pretty good treatment of Super Gravity. The quote below are some of the problems with SUGRA. I think SUGRA began as an attempt to explain gravity within the Super Symmetry theory. I can't add much. I really do hope nature does not behave super symmetrically.

Quote from: the article
The end of the SUGRA era

The initial excitement over 11-dimensional supergravity soon waned, as various failings were discovered, and attempts to repair the model failed as well. Problems included:

    * The compact manifolds which were known at the time and which contained the standard model were not compatible with super-symmetry, and could not hold quarks or leptons. One suggestion was to replace the compact dimensions with the 7-sphere, with the symmetry group SO(8), or the squashed 7-sphere, with symmetry group SO(5) times SU(2).

    * Until recently, the physical neutrinos seen in the real world were believed to be massless, and appeared to be left-handed, a phenomenon referred to as the chirality of the Standard Model. It was very difficult to construct a chiral fermion from a compactification the compactified manifold needed to have singularities, but physics near singularities did not begin to be understood until the advent of orbifold conformal field theories in the late 1980s.

    * Supergravity models generically result in an unrealistically large cosmological constant in four dimensions, and that constant is difficult to remove, and so require fine-tuning. This is still a problem today.

    * Quantization of the theory led to quantum field theory gauge anomalies rendering the theory inconsistent. In the intervening years physicists have learned how to cancel these anomalies.

Some of these difficulties could be avoided by moving to a 10-dimensional theory involving superstrings. However, by moving to 10 dimensions one loses the sense of uniqueness of the 11-dimensional theory.

The core breakthrough for the 10-dimensional theory, known as the first superstring revolution, was a demonstration by Michael B. Green, John H. Schwarz and David Gross that there are only three supergravity models in 10 dimensions which have gauge symmetries and in which all of the gauge and gravitational anomalies cancel. These were theories built on the groups SO(32) and E_8 \times E_8, the direct product of two copies of E8. Today we know that, using D-branes for example, gauge symmetries can be introduced in other 10-dimensional theories as well.
 

Offline DoctorBeaver

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How does physics explain the graviton?
« Reply #39 on: 15/03/2009 13:02:11 »
If you knew SUSY like I know SUSY!

I too have doubts about it. It seems like a kludge to me.
 

Offline Chemistry4me

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How does physics explain the graviton?
« Reply #40 on: 16/03/2009 05:16:25 »
Jeez, that stuff is confusing. ??????
 

Offline Chemistry4me

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How does physics explain the graviton?
« Reply #41 on: 16/03/2009 10:18:12 »
I'm still here you know? :)
 

Offline Chemistry4me

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How does physics explain the graviton?
« Reply #42 on: 16/03/2009 10:26:11 »
Why are you talking about me behind my back?

 

Offline DoctorBeaver

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How does physics explain the graviton?
« Reply #43 on: 16/03/2009 14:31:06 »
Why are you talking about me behind my back?



Coz it's easier on the eye than your front  :P
 

Offline yor_on

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How does physics explain the graviton?
« Reply #44 on: 17/03/2009 00:08:32 »
Mr Chem, that was a sheep trick, introducing Lambert to gravity.


No disrespect meant here Neil

As for "I think it's just me." :)
Count me in.
 

Offline Chemistry4me

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How does physics explain the graviton?
« Reply #45 on: 17/03/2009 07:48:11 »
Why are you talking about me behind my back?



Coz it's easier on the eye than your front  :P

Oi...! Com' ova 'ere you!

--------

Mr Chem, that was a sheep trick, introducing Lambert to gravity.

I liked it. :)
 

Offline DoctorBeaver

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How does physics explain the graviton?
« Reply #46 on: 17/03/2009 11:06:25 »
AHEM... going back to the question (sort of; and even then via the scenic route):

I have serious doubts about whether gravity would unify with the other forces at high energy. I am very taken by the idea of braneworlds in a higher dimensional bulk. The other forces we experience (EM, weak & strong) are sequestered on our 3d brane (3 spatial dimensions; leave time out of this for now) but gravity is everywhere.

If our universe is a brane that was brought into existence at the moment of the Big Bang then gravity would already have been around in the higher dimension whereas the other forces came into existence with the Big Bang. There is no reason to suggest they exist anywhere else.

To my mind, attempts at unifying gravity with the other forces are pointless because no such unity exists. TOEs are full of fudges & kludges that seem to be no more than mathematicians' playthings. Layer upon layer of complexity is added to patch holes that exist in the theories. Those layers, in turn, have holes in them that require more patches and so on. It's worse than a Microsoft OS (OK, maybe not). By accepting that gravity is totally separate from the other forces we can do away with all that complexity & desperate fine-tuning.

That doesn't, though, preclude the possibility of gravitons (See? I did get back to them). If they exist they would be the most fundamental of particles as they were the only ones that existed before the Big Bang. Although it hasn't been proven, it's possible that gravity affects every type of particle so maybe they can all produce gravitons in the same way that electrons can produce photons. Or maybe every other type of particle is made of gravitons; even electrons & quarks. It's beyond my little brain to contemplate that possibility too much so I'll leave it to those with more understanding of the subject.
 

Offline Vern

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How does physics explain the graviton?
« Reply #47 on: 17/03/2009 13:55:20 »
Quote from: DoctorBeaver
To my mind, attempts at unifying gravity with the other forces are pointless because no such unity exists. TOEs are full of fudges & kludges that seem to be no more than mathematicians' playthings. Layer upon layer of complexity is added to patch holes that exist in the theories. Those layers, in turn, have holes in them that require more patches and so on. It's worse than a Microsoft OS (OK, maybe not). By accepting that gravity is totally separate from the other forces we can do away with all that complexity & desperate fine-tuning.
I see no problem at all with unifying gravity with the other forces. Photons attract each other. Everything is made of photons. Bingo; we have gravity :)

That is a simplistic way of putting it, but it is essentially the way to get gravity and electromagnetism united. The exact mechanism is only slightly more complicated. I don't feel a real need to plunge into multi-brane universes :)
 

Offline JukriS

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How does physics explain the graviton?
« Reply #48 on: 17/03/2009 14:12:48 »
When particle is new, is hot/density and that particle time is slowly!

Thats why new particle dont emit energy fast and that why is not giving kinetic energy for expanding atoms nucleus!

When particle is old, is not so hot/density and time is not so slowly and thats why particle emit energy fast. Thats why particle giving kinetic energy for expanding atoms nucleus!

thats why we stay earth skin with out gravity, you know!

thats why you can see old light who is redshifting!

Not, because space expanding!

space dont expanding or curving/bending at all!


.
 

Offline Vern

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How does physics explain the graviton?
« Reply #49 on: 17/03/2009 15:04:24 »
When particle is new, is hot/density and that particle time is slowly!

Thats why new particle dont emit energy fast and that why is not giving kinetic energy for expanding atoms nucleus!

When particle is old, is not so hot/density and time is not so slowly and thats why particle emit energy fast. Thats why particle giving kinetic energy for expanding atoms nucleus!

thats why we stay earth skin with out gravity, you know!

thats why you can see old light who is redshifting!

Not, because space expanding!

space dont expanding or curving/bending at all!
.

I like your videos. My suspicion is that you are better at making videos than you are at making GUTs. :) 
 

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How does physics explain the graviton?
« Reply #49 on: 17/03/2009 15:04:24 »

 

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