If they are real, what exactly do gravitons do?

[benk99nenm312]

I typed up an essay for the annual gravity essay contest that addresses the question of how gravitons interact with gravitons. The essay got an "honorable mention" and was invited for review and publishing in IJMPD. The essay, as submitted to IJMPD is here:
http://www.brannenworks.com/gravity2009.pdf

The whole idea of gravity as created by gravitons is why I want to see the Feynman paper on the subject.

This is very interesting. Nice job. I lack some of the math to understand parts of this but I get the just of it. So, is it a consencus that gravitons interact with others or is it still debated?

 

[tom.stoer]

Let me ask another question: is there consensus what gravitons are?

 

[Max™]

No, they're unobserved, there is just a graviton shaped hole in current quantum theories which suggests we should find one.

 

[Vanadium 50]

benk99nenm312, let me try again. Your view of what a graviton is has nothing to do with what physicists mean when they say the word "graviton".

I can have a static electric field, and charged objects feel a force in that field. I can also have excitations in that field, and they are called photons. That doesn't mean that charged objects must emit photons - a charged object doesn't have to glow.

I can have a static gravitational field, and massive objects feel a force in that field. I can also have excitations in that field, and they are called gravitons. That doesn't mean that massive objects must emit gravitons for exactly the same reason a charged object doesn't have to glow.

Your complaints about gravitons are based on a model that has nothing to do with what physicists mean when they say the word "graviton".

 

[benk99nenm312]

I can have a static electric field, and charged objects feel a force in that field. I can also have excitations in that field, and they are called photons. That doesn't mean that charged objects must emit photons - a charged object doesn't have to glow.

But if the charged object in the field wants to interact with another charged object, it mediates photons between them, right?

 

[Vanadium 50]

But if the charged object in the field wants to interact with another charged object, it mediates photons between them, right?

No. That would mean it glows.

You might have heard virtual photons are exchanged. Virtual photons are not real photons. You can't see them; you can't measure them; you can't count them; you can't detect them. You don't even need them to do the calculation. They are mathematical artifacts - on par with drawing an auxiliary line in geometry.

 

[benk99nenm312]

No. That would mean it glows.

You might have heard virtual photons are exchanged. Virtual photons are not real photons. You can't see them; you can't measure them; you can't count them; you can't detect them. You don't even need them to do the calculation. They are mathematical artifacts - on par with drawing an auxiliary line in geometry.

So you are saying that gravitons are mathematical artifacts?

A photon is real when it is not mediating the electromagnetic force. When is a graviton real?

 

[tom.stoer]

That is exactly my question: what are gravitons? physical particles? virtual obejcts derived from perturbation theory (which in inconsistent for GR)? a mathematical concept in Hilbert space? ...

My feeling is that the failure of naive perturbation theory for GR forces us to think about different concepts. If you look at some recent approaches for quantum gravity (loops, dynamical triangulations) you wan't find anything like a graviton in the fundamental concepts. That does not mean that these theories are wrong, it simply means that the concept of ordinary quantum field theory cannot be applied to quantum gravity.

 

[Max™]

That is well known, I thought. If it did work perfectly, we'd have a very good candidate for a unified field theory.

As for LQG, the actual links between nodes in spin foam models could be described as gravitons at least metaphorically.

Trying to fit things to actual physical particles isn't a helpful idea though, you can speak sensibly of wave-like or particle-like behavior, but assuming there is a neat little transverse wave diagram or point like particle down in there is misleading at best.

We know that whatever this stuff is, it behaves in certain ways which we can often find easily understandable metaphors for in our day to day experience.

We still aren't sure what this stuff actually IS exactly though.

Applying the methods which have successfully produced the Standard Model to Gravity is very difficult, but a few things happen when you do.

When you include the SSB/Higgs Mechanism, you get a Spin 0 Massive Boson shaped "hole" in the theory, and Spin 2 Massless Boson shaped "holes". The relationship between other symmetry groups and their bosons suggests the Spin 0 one should be the Higgs, which should then couple to other particles in certain ways to produce their observed masses.

The nature of Gravity in that it has no "poles" plus seems to propagate at light speed suggests the Spin 2 one should be a Graviton. This is at odds with the smooth continuum of spacetime described by GR, and one of the big problems seeking resolution which drives String/LQC/etc Theories.

 

[tom.stoer]

Max!

I fully agree with you - but reading the other posts I don't think that the colleagues have the same opinion.

I think that the graviton in LQG (if this concept makes sense at all) is the fundamental excitation of a spin network.

But the majority seems to have something in mind that is derived from ordinary quantum field theory, something like quantized plane waves. This is INCONSISTENT in GR. A plane wave in LQG is a very complex spin network state. I do not understand Rovelli's arxiv papers completely, but I understand enough to see that it is not the "ordinary graviton" as the majority may expect.

The problem with ST is that it works fairly well perturbatively (and therefore ST is able to tell us what the perturbative graviton is), but I do not see results for fully dynamical spacetimes with propagating gravitational fields (not necessarily plane waves).

Tom

 

[Vanadium 50]

So you are saying that gravitons are mathematical artifacts?

No, I am saying that virtual gravitons are mathematical artifacts. Just like virtual photons.

A photon is real when it is not mediating the electromagnetic force. When is a graviton real?

Just like when the photon is.

 

[benk99nenm312]

No, I am saying that virtual gravitons are mathematical artifacts. Just like virtual photons.

Just like when the photon is.

I don't understand. If the graviton were real at any given time, what would it do? What would be its effects? What would we measure, detect, observe? Why haven't we observed it then?

 

[Max™]

Are you falling towards the center of the Earth currently?

There's your answer, I think.

 

[benk99nenm312]

Are you falling towards the center of the Earth currently?

There's your answer, I think.

So are you saying that, when real, gravitons have no effect what-so-ever on me or anything else.

This would indicate to me that they are not 'real'.. ever. That's why I'm confused.
(The only way something has absolutely no effect on anything else is if it doesn't exist.)

 

[Max™]

Again, it's a very misleading way to think of things.

Mathematically it is convenient to describe electromagnetic fields in terms of particles shooting photons at each other, and it turns out that light, real photons, actually has electromagnetic properties.

The difference being you don't see magnets "glowing" at each other literally.

So right now your butt is "glowing" at the Earth with virtual gravitons in a convenient mathematical description.

So a gravity wave (literally a ripple in spacetime) would be akin to a photon in that it would be a defined waveform with gravitational properties instead of electromagnetic properties.

There's more truth and understanding to be found in that comparison if you really consider the nature of a magnetic field and light.

 

[benk99nenm312]

Again, it's a very misleading way to think of things.

Mathematically it is convenient to describe electromagnetic fields in terms of particles shooting photons at each other, and it turns out that light, real photons, actually has electromagnetic properties.

The difference being you don't see magnets "glowing" at each other literally.

So right now your butt is "glowing" at the Earth with virtual gravitons in a convenient mathematical description.

So a gravity wave (literally a ripple in spacetime) would be akin to a photon in that it would be a defined waveform with gravitational properties instead of electromagnetic properties.

There's more truth and understanding to be found in that comparison if you really consider the nature of a magnetic field and light.

Yes, now I see. This helps define a graviton. Now the OP's question is somewhat answered. So is some of mine. Thanks.

So here's my question. If a satelite is positioned behind earth, as before, then how does a virtual graviton emitted from the sun interact with the satelite? Is the answer simply that it does not exist? That it is a mathematical artifact, and therefore is useless to apply in real nature (which is by the way, what we do, right?)? We try to answer what is happening in nature, so a mathematical description is great, but if a virtual graviton is not a viable conceptual explanation, GR seems much more appropriate. So again what I guess I really want answered is, how does a graviton reach a satelite behind earth?

 

[apeiron]

Isn't there an equivalent question now in how does the sun's magnetic field reach through the Earth to affect the satellite.

So in everyday physics, what is the effect of an insulator placed in the path of a magnetic field?

 

[Max™]

What would it take to insulate something from gravity?

 

[glueball8]

but some also are dragged by gravity and bend aound the earth...

Photons can still bend around earth, because they are now interacting with gravitons, but gravitons can't do that now (unless they somehow interact with themselves).

Is it possible that "gravitons can create gravitons"? Since Graviton is something therefore it has energy. All energy can curve space-time. Therefore gravitons can create gravitons?

This is much like in Q.C.D where Gluons can make Gluons(called "Gluon Ball"?). Maybe this property can unify gravity and the strong force?

By this wouldn't gravitons be able to bend around earth? Since it would create a gluey, stretchy and bendy property so it would cover Earth and everywhere.

 

[benk99nenm312]

Is it possible that "gravitons can create gravitons"? Since Graviton is something therefore it has energy. All energy can curve space-time. Therefore gravitons can create gravitons?

This is much like in Q.C.D where Gluons can make Gluons(called "Gluon Ball"?). Maybe this property can unify gravity and the strong force?

By this wouldn't gravitons be able to bend around earth? Since it would create a gluey, stretchy and bendy property so it would cover Earth and everywhere.

"All energy can curve space-time" according to relativity. Does relativity still hold after one throws out the notion of a graviton? That is the key. To me, that's like using gravity to explain gravity. If gravitons do curve space-time, someone please let me know, because then, everything will make sense.

 

[benk99nenm312]

What would it take to insulate something from gravity?

Well that's my point. in reality, I don't know of anything that can. I mean, when electric force repels, it over-powers and counters gravity, but nothing just simply sitting in the way, like the earth, is going to counter gravity.

 

[benk99nenm312]

Isn't there an equivalent question now in how does the sun's magnetic field reach through the Earth to affect the satellite.

So in everyday physics, what is the effect of an insulator placed in the path of a magnetic field?

I think I already gave answer to this a few posts back.

 

[apeiron]

What would it take to insulate something from gravity?

That was my point. Doesn't the same issue apply to virtual force particles of both kinds of field?

So citing wiki: even a Faraday cage will not shield its contents from static magnetic fields

http://en.wikipedia.org/wiki/Faraday_cage

 

[benk99nenm312]

Now I might be piecing this together. I thought that the Earth being in the way would dicipate the electromagnetic force, because less photons are recieved. And the only reason some are received is because they are bent around the Earth due to gravity. But the electromagnetic force runs on virtual photons. They are evidently so unreal that they can jump right through the earth, I think. I don't know for sure, or how. But the electromagnetic force must be able to get to the other side somehow.

So, now I ask, is the electromagnetic force dicipated by the Earth's presence? If so, I still don't have an answer to what's bothering me. If not, then we're good.

 

[glueball8]

No. That would mean it glows.

You might have heard virtual photons are exchanged. Virtual photons are not real photons. You can't see them; you can't measure them; you can't count them; you can't detect them. You don't even need them to do the calculation. They are mathematical artifacts - on par with drawing an auxiliary line in geometry.

Are you saying that you can not detect all of virtual particles? What about Casimir effect?

 

[glueball8]

Now I might be piecing this together. I thought that the Earth being in the way would dicipate the electromagnetic force, because less photons are recieved. And the only reason some are received is because they are bent around the Earth due to gravity. But the electromagnetic force runs on virtual photons. They are evidently so unreal that they can jump right through the earth, I think. I don't know for sure, or how. But the electromagnetic force must be able to get to the other side somehow.

So, now I ask, is the electromagnetic force dicipated by the Earth's presence? If so, I still don't have an answer to what's bothering me. If not, then we're good.

A few questions:

-Does photons interact with itself?
-Does gluons interact with itself? If two gluons can create a gluon does that mean it interact with itself?
-How can gravitons interact with photons?
-What would the properties of gluon be like if its power is decreased significantly, to about gravity. That means it can travel farther?

 

[tom.stoer]

I'll try to answer that

Photons can only interact with photons indirectly via virtual electron pairs. Two photons cannot exchange a virtual photon, but the two photons can decay into two virtual electron-positron pairs. The particles in these pairs can exchange virtual photons. Because of the many virtual particles involved this process is highly suppressed and as far as I know not measurable directly.

For gluons you already gave the answer: yes, by exchange of a virtual gluon.

Regarding gravitons I can only repeat what I have mentioned a couple of times here. The electromagnetic field interacts with the gravitational field (light deflection near the sun, measured during eclipses). But if you try to quantize gravity like an ordinary QFT (which may provide some notation of gravitons and describe this scenario including quantum corrections) the theory becomes inconsistent (non-renormalizable).
0) In perturbatively quantized QG you have a notation of gravitons, you have a coupling of gravitons to other gravitons and to photons, but if you try to calculate a cross sections or other physical quantities, the theory gives you infinite answers; this cannot be cured and therefore it does not make sense to talk about vitual gravitons (at least not in this simple picture).
So you have to try other approaches in quantizing gravity:
1) supergravity may eventually allow to describe photons and gravitons; for gravitons it's fine, they are there. But I don't know if there's a realistic SUGRA theory containing all particles and forces of the standard model; I haven't seen any! And SUGRA may cure the problem of non-renormalizibility; there are steps into that direction but I do not know if there is a solid proof; I haven't seen any!
2) the same for string theory; up to know there's no low-energy limit which reproduces the standard model exactly; and there is no solid proof of renormalizibility and/or perturbative finiteness.
3) in LQG the situation is such that photons couple to a "quantized gravitational field", but in all approaches I know you somehow loose the notation of gravitons: the elementary quanta of the gravitational field are fundamentally different from something like quantized plane waves. This is not a drawback but a feature: the theory need not produce gravitons (nobody has ever detected a graviton) butmust instead provide a consistent quantization scheme (a replacement of the perturbative one).

Good question. What do you mean by reducing the "power of the gluon"? do you mean the strength of the strong coupling? I would say that glue balls would still exist, but they would have less mass and would be much larger.

 

[Ultimaton]

I’m having a hard time believing gravitons exist. This is how I’m thinking:

We know that Mass distorts time-space like the Einstein example of the bowling ball on the trampoline warping the fabric. Another ball will travel around the bowling ball due to the warped space [warped fabric]. But that doesn’t explain everything.

Since mass is defined as an objects inertia, or resistance to force casing a change in it’s velocity through the medium of Space-Time. Therefore the mass of the object could increase, as it is accelerated faster.

I like to think of Space-Time like the propagation of sound in air. As an airplane travels through the air, it creates a sound pressure wave. That is the warping of the atoms, forcing atoms into each other, transferring that wave from atom to atom through the medium. You reach a maximum speed, which is the speed this wave can be propagated through the medium: The speed of sound.

Now Space-Time works in a similar fashion. When a mass is accelerated faster, it reaches a point where this shock wave interferes with the velocity, requiring more and more force to obtain higher speeds. Since mass is the resistance to force which causes velocity changes, that objects mass is increased, thus will never obtain that maximum speed, due to the increased resistance.

When the Earth travels through the warped Space-Time caused by the sun, it encounters this warped Space-Time. Adding it’s own mass related warp to the Space-Time itself, it encounters more resistance to velocity change, and realizes a time warp towards the sun, which translates into a force which we have called gravity.

Therefore, Gravity does not exist. We realize the effects of warped Space-Time, and thus ‘feel’ a force towards this warping.

The only graviton I can comprehend, is the speed at which mass related warping can propagate through the medium of Space-Time. That is C.

Does this make sense?