How sure gravitons exist?

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OK we know beyond doubt that electrons exist in reality, even if no-one really knows what they actually are.
Can we claim the same certainty that gravitons exist in reality or not?
Thanks for the info.
 

jcsd

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We do know what electrons are and we can describe them with an extreme degree of exactitude. Gravitons almost certainly exist, but of course we cannot be as sure of this as we can about the existance of the well-known and well-described electron.
 

Tom Mattson

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The existence of gravitons is purely hypothetical, and is not on the same ground as the existence of electrons. Gravitons are predictions of TOE's, but the energies needed to experimentally probe gravitation at the quantum level are far beyond the reach of our current technology.
 
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Originally posted by jcsd
We do know what electrons are and we can describe them with an extreme degree of exactitude...
Thanks for reply.
If I may disagree: we don't know what they are at all, but we do know their properties. When I say we don't know what they are, I mean in the sense that we can not describe them at all, only describe measurements of their properties, e.g. we don't know if they are particles or waves or other but we do know their mass or wavelengh.

"Gravitons almost certainly exist..."
Aha! You've answered my question with word almost, so thanks for that.
 
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Originally posted by Tom
The existence of gravitons is purely hypothetical, and is not on the same ground as the existence of electrons. Gravitons are predictions of TOE's, but the energies needed to experimentally probe gravitation at the quantum level are far beyond the reach of our current technology.
Thanks for the info.I shall file gravitons next to dark matter and exotic matter.
 

jcsd

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Originally posted by Tom D
Thanks for reply.
If I may disagree: we don't know what they are at all, but we do know their properties. When I say we don't know what they are, I mean in the sense that we can not describe them at all, only describe measurements of their properties, e.g. we don't know if they are particles or waves or other but we do know their mass or wavelengh.

"Gravitons almost certainly exist..."
Aha! You've answered my question with word almost, so thanks for that.
Well they're particles, but all particles have wave-like properties. I don't know what more you want to describe them than the measuremnt of they're properties as that is all we can use to describe anything.
 
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well, if you say 'door', you can picture what it is, or a blind person could imagine how it feels, but if you say 'electron', you can only think of abstract numerical desciptions of it ( like anything too small to see )
 
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I'm not sure I understand the question. We know that GR describes gravity as a field like an em field but more complicated since it interacts with itself because gravity interacts with energy and gravity itslef has energy. So what would be the right quantum description of gravity? If it can be treated as a quantum field than there must be some concept of gravity particle since this is automatic with the way quantum field theory works. If it is really not a field like maybe it is a string or loop or something, it would still have to look like a field for semiclassical conditions so there would still be a graviton concept and there would be a corresponding concept like a string or loop when quantum field theory no longer works. The string or loop would have properties that correspond to graviton so we should speak of strings or loop of gravity still as a graviton even though it is not just like a graviton of gravity as a field.
 

selfAdjoint

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Gravitons are predicted by stringy theories, and as such they would have definite properties, massless, spin 2 and so on. Whether stringy physics of some kind is the way to the final answer is uncertain, and by the same token, nobody knows if gravitons are for real or not.
 
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Originally posted by eigenguy
I'm not sure I understand the question. We know that GR describes gravity as a field like an em field but more complicated since it interacts with itself because gravity interacts with energy and gravity itslef has energy. So what would be the right quantum description of gravity? If it can be treated as a quantum field than there must be some concept of gravity particle since this is automatic with the way quantum field theory works. If it is really not a field like maybe it is a string or loop or something, it would still have to look like a field for semiclassical conditions so there would still be a graviton concept and there would be a corresponding concept like a string or loop when quantum field theory no longer works. The string or loop would have properties that correspond to graviton so we should speak of strings or loop of gravity still as a graviton even though it is not just like a graviton of gravity as a field.
(SelfAdjoint):"Gravitons are predicted by stringy theories, and as such they would have definite properties, massless, spin 2 and so on. Whether stringy physics of some kind is the way to the final answer is uncertain, and by the same token, nobody knows if gravitons are for real or not.

Thanks for the replies.Very interesting food for thought.
 
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I think the wave form of gravity is a scalar electromagnetic wave. Ditto if one replaces 'wave' with 'particle' the first time and 'photon' the second.
 

jcsd

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I disagree self-adjoint, gravitons are firmly predicted by theory and any theory that quantizes gravity will almost certainly include them, it's just that at the moment we have absolutely no experimental evidence for them, though this is not suprising given their predicted nature.
 

vanesch

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It is fair to say that we are much more sure about the existance of electrons than of gravitons, in nature. Of course, if you talk about the graviton as a theoretical-mathematical concept (such as "real number" or "complex polynomial"), then gravitons are quite firmly established in the linearised version of gravity.
I suppose it is ok to say that we can confirm that an entity (which starts out as a theoretical entity in a theory) "exists" for a physicist, if several crucial experiments all give a consistent picture of what one expects to see in the frame of the theory predicting the entity under consideration. As such, for the electron, there is no doubt.
Let us look at the situation a few years back, when the top quark wasn't yet discovered. People were pretty sure it existed as the sixth flavour in the standard model ; the top quark was really a part of the standard model that had made a lot of verified predictions, and it would have been a very serious puzzle if the top quark didn't exist. The standard model predicted several "signatures" of the top quark (in fact t-t-bar states), the only thing missing was the precise mass. And then, I think it was in 1996, finally these signatures were measured by two independent groups, so people concluded that the "top quark was discovered". But there was not much doubt before that this top quark somehow had to exist.

We are now in a similar period for two other things: the Higgs and superpartners. The standard model needs the Higgs particle somehow to be coherent. Only, the Higgs is a very different particle than the quarks, so I think people strongly suspect that the Higgs will be discovered (through signatures predicted by the standard model). But they are probably less sure than they were for the top quark. If you have already discovered 5 quarks, and you should have 6, you feel a lot more certain than if it is the first of a kind. The Higgs would be the first scalar particle that is discovered. Maybe the Higgs mechanism (a theoretical construction at the heart of the standard model) is not correct. So the Higgs is expected, but it is already less sure than was the top quark.

Next come the superpartners. Now supersymmetry is nice allright, but there is, except for mathematical beauty, not really a need for it. The existance of superpartners is a lot less certain. If they don't appear, mmm, a lot of current theory devellopment (SUSY, superstrings, supergravity, etc...) goes down the drain ! So let us say that some people HOPE for these superpartners.

The graviton is much less certain. First of all, as a theoretical concept, the graviton is supposed to be the boson associated with gravitational waves, which are themselves a prediction of General Relativity. Although general relativity is a nice theory, its experimental verification is much less advanced than is the standard model for example. Weak effects of curvature and time dilatation, and the equivalence principle using normal matter, has been checked, so the "low field quasi-static" limit is ok. Indirect measurement of the energy loss of a pulsar is the only indication we have that classical gravity waves exist. We should first detect classical waves (Virgo experiment for example) to really know that gravity waves exist. The quantum version is then even more elusive.
In fact, GR predicts gravity waves that have a tensorial nature (tide waves). From this follows that the quantum equivalent, if treated as a linearised wave equation, must be a spin-2 system. It turns out that the standard techniques to quantize this correctly all fail. The main interest stringy theories have, is that spin-2 particles seem to be always required in them, so they always reduce to general relativity or one of its cousins.
So you see that there are some theoretical reasons to expect that something like a graviton must exist, but that these reasons are much, much less compelling than was the case for other particles. We haven't even measured directly the classical phenomenon (gravity waves) !

Now, don't get me wrong. Don't think I've been saying that all these theories are somehow BS. But in science (and sometimes modern theorists seem to forget this !) one must always make a distinction between what is established and what is "speculative". Of course in speculative, there is the well-motivated speculation, the wild dreams (and then of course the uninformed crackpottery). I don't know where to put the graviton, it is somewhere between motivated speculation, and not-so-wild dreams...

cheers,
Patrick.
 
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"Now, don't get me wrong. Don't think I've been saying that all these theories are somehow BS. But in science (and sometimes modern theorists seem to forget this !) one must always make a distinction between what is established and what is "speculative". Of course in speculative, there is the well-motivated speculation, the wild dreams (and then of course the uninformed crackpottery). I don't know where to put the graviton, it is somewhere between motivated speculation, and not-so-wild dreams..."{ vanesch aka Patrick }

Very wise words and thasnk you for the reply--- it was just the type of information I was looking for.
Since school, I have had to unlearn concepts and 'facts' that turned out to be ( seemingly ) incorrect speculations. If only the degree of certainty were more openly and commonly stated then I, and I believe many others, would probably not have to unlearn, which is possibly very destructive to the learning process ( but maybe not ).
I have read in a certain 'reputable' magazine many times of speculations dressed as facts.
 

Chi Meson

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Am I correct in thinking that gravitons are not compatible with the General Theory? And is it still believed that IF there are gravitons, then they would have to travel at 20 billion times c ?
 
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I would be interested to know where does the 20 billion come from?
Does it have anything to do with the esitmated size of a photon horizon or am I barking up the wrong tree?
 

Chi Meson

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RE: 20 billion times c

THe problem that Einstein saw immediately once Special relativity was figured out, is that if no information can travel faster than light, then how do we out here in the solar system feel a gravitational pull that is directed toward the actual center of the galaxy. THe center of the galaxy is not in the place where is appears to be since that light left a long time ago.

In astrophysics we had to treat the gravitational force as being instantaneous (when using Newtonian law), otherwise there would be no conservation of angular momentum in the solar system, and all orbits would collapse.

If Einstein is right, then our gravitaional effect is due to a condition of the space that we are in, and not of gravitational interaction between ouselves and the thing we are orbiting.

Gravitons would be the force carrier of a quantum gravitational law (and Newtonian Physics approximates Quantum physics when dealing with billions of particles). If there is the graviton, and not General relativity, then this force carrier would have to travel much much faster than light in order for the effects to appear instantaneous.

Here's a good website: www.ldolphin.org/vanFlandern/gravityspeed.html
Here it suggests gravity travels only 20 time the speed of light. I remember once that some were saying it would have to be billions of c. I might have conflated the "20" with the "billions," but hey, I don't know, I asking!

String theory provides another way out of this conundrum, but I never studied it.
 
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jcsd

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Chi meson gravity is thought to propagate at c, your point about how gravity 'knows' were the centre of the galaxy is s'no problem as it is a static gravitational field which just like a static electric field is propgated by virtual particles. This is one of the problems in detecting gravitons (and more genrally gravity waves), i.e. most of them are virtual. It is only an event such as the coalescense of two black holes in which we should see real gravitons.
 

marcus

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Originally posted by vanesch
It is fair to say that we are much more sure about the existance of electrons than of gravitons, in nature. Of course, if you talk about the graviton as a theoretical-mathematical concept (such as "real number" or "complex polynomial"), then gravitons are quite firmly established in the linearised version of gravity.

I suppose it is ok to say that we can confirm that an entity (which starts out as a theoretical entity in a theory) "exists" for a physicist, if ...

...So you see that there are some theoretical reasons to expect that something like a graviton must exist, but that these reasons are much, much less compelling than was the case for other particles. We haven't even measured directly the classical phenomenon (gravity waves)!...
...
cheers,
Patrick.
Carefully chosen words!

Patrick perhaps you have a reference for this: I have read several places that "graviton" may be an artifact of using a flat space----but now I cannot lay my hand on the reference.

that is, some writers give the impression that on a curved background space the mathematics does not show anything you can clearly and obviously label as a "graviton", so perhaps it is not so good that some theories "predict" their existence?

I may not be remembering correctly but I got the impression that YES there is the field, it exists and it is dynamically changing and it can undulate. But only in certain limited circumstances can the undulations of the gravitational field be identified as "gravitons"

Maybe you can confirm this, or perhaps it is a misconception which you can correct. I have concluded, then, that the idea of an
electron is very useful because it is not just an artifact of mathematical circumstances and it does NOT go away when you change the problem. You can accelerate it, make atoms with it, run it through wires, charge a battery, and you can make it live in curved space, like around a black hole.

But what I have read seems to suggest that by contrast the "graviton" is not such a useful idea because it goes away if you put it in different mathematical circumstances like a space which is the wrong shape.

Or as you say (and there may be a difference) "gravitons" appear in the math when one uses a "linearized" model. This makes me think that we are talking about an approximation out of, say, a pertubative analysis. Please be more specific, if it would not be too much trouble for you. Thanks,

marcus
 
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Chi Meson

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Originally posted by jcsd
Chi meson gravity is thought to propagate at c, your point about how gravity 'knows' were the centre of the galaxy is s'no problem as it is a static gravitational field which just like a static electric field is propgated by virtual particles. This is one of the problems in detecting gravitons (and more genrally gravity waves), i.e. most of them are virtual. It is only an event such as the coalescense of two black holes in which we should see real gravitons.
The recent "speed of gravity" experiment appears to confirm this, although there are still many sceptics. I'm sort of neutral, but I'm rooting for General Relativity because I spent so much time trying to understand it.

Can the following question be answered with a "yes" or "no" :

Does the gravity wave/graviton model exclude GR?
Or can they both be correct simultaneously?
 

vanesch

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To Marcus:

I'm not such an expert on quantum gravity, to say the least. The references I can give you is first of all, Wald, p 411, somewhere in the middle:
" the quantum back-reaction effects caused by gravitons (the quantized degrees of freedom of the linearized gravitational field).

I vaguely also remember that Scardon (Advanced quantum theory) has some part on this.

It might very well be that people do have "gravitons" in some fuller, non-linearised theory. But as my meager understanding of superstrings is correct, also there the spin-2 particle appears in perturbative expansions on a background metric (which can be flat or curved). But I'm out of my depth here.

To Chi meson: let us say that Tom van Flandern is, ahum, known for his rather unorthodox views, if you see what I mean. So I wouldn't take him as a reference to learn about gravity.
There is no contradiction, and it is even a typical exercise in GR, to work out that in the 2-body problem, gravity "appears to propagate instantaneously", which explains the stability of the orbits in the solar system. However, a sudden change (imagine a titanic explosion suddenly shooting the sun out of the solar system) propagates at the speed of light. All this is included in GR. Even more: classical electromagnetism has a similar effect in it ! You find this when you work out synchrotron radiation. It is rather heavy stuff, ,worked out in Jackson for example (I did it once, and I'm not ready to redo it). An accelerated charge has a field that behaves as if the charge wasn't accelerated, and there is a Coulomb type contribution to the E-field that points not to the particle with retardation, but to where the particle would be right now if it hadn't accelerated. This is exactly the same as what you have in GR.
But nobody makes a deal out of it...

cheers,
Patrick.
 

Chi Meson

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Thanks Patrick.

I was aware that this site was that of the "outside view point." I thought it was good in that it explained the unorthodox view rather well. I was not sure to what extent this guy was appreciated or reviled.

And thanks for answering the question. I was under the impression that the two couldn't coexist (GR and gravity waves), but through the years I was wondering if they, like our wave-particle duality, were two aspects of the same thing. I'm guessing now that it isn't even so "compementary" as that. As I said before, I spent so much time on GR just to pass the class, I'd hate to find that it was all for nothing.
 
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Do gravitons exist?

According to the Austrian physicist Roman Sexl (and with some necessary additions) gravity and all other phenomena of General Relativity can be understood quite easily from the fact that the velocity of light c is reduced in a gravitational field. Gravitational acceleration is nothing different than the refraction of light-like particles at this field.

If this is taken as a fact then the existence of gravitons should be able to explain why the velocity of light is reduced.

I believe that it is more likely that the reduction of "c" is caused by the other fields we know and which are related to a massive body: The electric and the strong force, or better: the exchange particles of those fields.

For the explanation of gravity (and other points of GR) I would like to refer to my site about this:
http://www.ag-physics.org/gravity
 
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Originally posted by ohlookitsme
-we know that there is a force known as gravity. this is the force that causes attraction between 2 objects with a mass >0.
Gravitation exists even between massless objects, such as photons. (Energy gravitates.)


-since this force does not just appear out of thin air, there must be a particle that causes it.
The chain of logic breaks here: we cannot simply assume that a particle causes it. In fact, in the leading theories of quantum gravity (string theory, loop quantum gravity), there is no graviton particle.

(There are no particles in string theory, though there are string vibrational modes which act like particles and are named after them. Loop quantum gravity drifts away from the whole "force mediated by some kind of object" idea, and considers quantum spacetime geometry.)

Gravitons appear as fundamental particles only in perturbative quantum gravity, which fails (is non-renormalizable).

However, any theory of quantum gravity must have solutions that behave like gravitons, even if gravitons aren't really fundamental objects in the theory. So it does make sense to speak of gravitons and their properties, at least as an approximation.


-this particle itself does not have any mass, because if it did then it would be attracted to other particles of its type, thus sending the universe into chaos.
Gravitons are massless, because otherwise the gravitational interaction would be short-ranged. But gravitons do attract each other: gravity is a universal interaction, which couples to all particles/fields, including gravitons themselves.


-since we have established that there is a force that causes gravity that has no mass itself, it must be the graviton.
That sounds kind of circular. A particle that is responsible for gravity is defined to be a graviton. If we have assumed that gravitons exist, we can then derive their properties from the known properties of classical gravity. We find that they are massless and spin 2. There is a good discussion of this in the Feynman Lectures on Gravitation.
 
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Originally posted by Chi Meson
RE: 20 billion times c

...Here's a good website: www.ldolphin.org/vanFlandern/gravityspeed.html
Here it suggests gravity travels only 20 time the speed of light. I remember once that some were saying it would have to be billions of c. I might have conflated the "20" with the "billions," but hey, I don't know, I asking!

String theory provides another way out of this conundrum, but I never studied it.
Yes, it does say 20 billion although I haven't yet checked it all out
 

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