How sure gravitons exist?

  • Thread starter Tom D
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  • #26
zare
hi people. im new to this forum and i want to greet all of you first.

now for some of my opinions. i think there are low chances for graviton to exist, even lower that human scientist are going to probe it in near future. however, zero-point energy theory (a nice standing theory that's based on uncertanty principle) can explain gravity and inertia. inertia was declared as immediate property of matter, but some 10 years ago scientists discovered that inertia is electromagnetic drag force in ZPE system. when particle is pushed forward inertia tends to send it backward, due to ZPE fluctuations in quantum vacuum. that way it applies on whole molecules and every object constructed of those molecules. if some of you dont have background on ZPE, you can visualize this particular system as when you throw a rock into the water. drag force will try to push it up backward, as result of waves that were originated when rock entered the water and now are pushing it up. rock is particle, water is quantum vacuum, and waves are ZPE fluctuations.

so, if we use the example of interactions of electromagnetic quantum vacuum on electromagneticly interacting particles like quarks and electrons, we can see that gravity and inertia both generate from ZPE . it's long discovered that both of these forces must be on same principle in origin.

all work on ZPE is theoretical, but altrough it isnt used in calculations, it looks like direct effect of quantum theory. i know the topic was on gravitons, not on gravity. but who knows, maybe graviton is particle representation of energy given by ZPE reaction...
 
  • #27
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Hi,Zare.
ZPE features in this week's New Scientist and is (apparently) seemingly non-zero.
According to the article, ZPE=cosmological constant and dictates the rate of cosmic expansion.
...i know the topic was on gravitons, not on gravity...
It is all very interesting, nonetheless.

What if...[?]what if ZPE dictates gravity as well as cosmic expansion and gravity/inertia is a result of 'pressure' caused by the rest of the universe 'kicking back' on each particle as it 'pushes away' from each particle as it expands? Only a 'what if', remember.
 
  • #28
zare
nobody knows. what is known is there is underlying sea of energy in quantum vacuum, and it manifests as many forces witch react with particles.

i've researched ZPE via papers released on california institute of physics and astropyhsics. so probably they "weight" something...
 
  • #29
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The following bothers me about gravitons:

If gravitons are the exchange particles which transfer the force, then gravity should work like other forces: electric force, strong force .... (1)

On the other hand, main stream GR tell us that the gravitational acceleration is caused by the phenomenon of curved space-time. (2)

What does this now mean? Does gravity have two independent causes, which act in parallel? And why is gravity assumed to be different from other forces?

My opinion as an answer to the original question (How sure ...): none of both (1) or (2) is the true. Gravity is most probably a side effect of all other forces which exist in elementary particles. This would have the consequence that the gravitational field is dependant on the composition of the gravitational source. And this in turn would explain why the experimental determination of the gravitational constant G yields conflicting results.

Another question about gravitons (should they exist): If it is true that a photon cannot leave a black hole, can a graviton leave a black hole? And if the answer is YES, why should a graviton behave differently to a photon?

If the answer is NO, how can it happen that a black hole has a gravitational field?
 
  • #30
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Originally posted by Albrecht
If gravitons are the exchange particles which transfer the force, then gravity should work like other forces: electric force, strong force .... (1)
The electric force doesn't work like the strong force; that's why they're different forces. Neither of them should act like gravity.


On the other hand, main stream GR tell us that the gravitational acceleration is caused by the phenomenon of curved space-time. (2)

What does this now mean? Does gravity have two independent causes, which act in parallel?
Interestingly, you can go the other way: the other forces (strong, weak, electromagnetic) also have a geometric interpretation: the field tensor is a curvature, not of spacetime, but of the "internal gauge space".

This similarity is exploited in Kaluza-Klein unified theories, which attempt (unsuccessfully) to describe all forces as spacetime curvature.

As to your specific question: general relativity is a classical theory. Gravitons are an idea from quantum theory. It is not inconsistent to speak of both, any more than it is inconsistent to speak of the electromagnetic field as photons at one time, and as a classical E and B vector fields at another. The classical physics is a limit of the quantum physics.

Also, even a theory of quantum gravity will probably not have gravitons as fundamental particles, though it may have states (e.g., string modes, or excitations of quantum geometry) that behave like gravitons in an appropriate low-energy limit.


My opinion as an answer to the original question (How sure ...): none of both (1) or (2) is the true. Gravity is most probably a side effect of all other forces which exist in elementary particles. This would have the consequence that the gravitational field is dependant on the composition of the gravitational source.
There isn't any experimental evidence of that.


And this in turn would explain why the experimental determination of the gravitational constant G yields conflicting results.
Three points:

1. Who said the experimental determination of G yields conflicting results?

2. Even if it did, conflicting results appear in physics all the time when the experiments are hard to perform. It isn't evidence that GR is wrong; to say that, you'd need clear-cut evidence that G depends on composition.

3. Your idea doesn't explain anything about "conflicting results for G" unless it can predict how G should depend on composition, to account for the experiments in a consistent way.


Another question about gravitons (should they exist): If it is true that a photon cannot leave a black hole, can a graviton leave a black hole? And if the answer is YES, why should a graviton behave differently to a photon?

If the answer is NO, how can it happen that a black hole has a gravitational field?
Real gravitons cannot leave a black hole; virtual gravitons can. Real photons cannot leave a black hole; virtual photons can. This is a FAQ:

http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/black_gravity.html

Black holes can have a gravitational field for the same reason that they can have an electric field.
 
  • #31
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Originally posted by Ambitwistor

1. Who said the experimental determination of G yields conflicting results?

An article that I read today interestingly... New scientist October 2002 I think. There three latest results have a problem in that two agree with each other, but one taken in Paris disagrees slighty. The difference between the results is greater than the calculated errors.

The article was discussing whether big G actually varied depending where it was measured (a very odd thought!) or whether something was wrong wtih the results.

Incidentally, I measured big G with a group in the lab today and got a result of 4.7x10^-11 .... Maybe I should write to the new scientist?
 
  • #32
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Originally posted by Ambitwistor
Interestingly, you can go the other way: the other forces (strong, weak, electromagnetic) also have a geometric interpretation: the field tensor is a curvature, not of spacetime, but of the "internal gauge space".
Yes, it is interesting, this is part of the history of "geometrizing" physical phenomena and theories. We know (since 200 years) that this always can be done. Question is whether it helps us.

general relativity is a classical theory. Gravitons are an idea from quantum theory. It is not inconsistent to speak of both, any more than it is inconsistent to speak of the electromagnetic field as photons at one time, and as a classical E and B vector fields at another. The classical physics is a limit of the quantum physics.
I believe that we all can agree that this cannot be the final state of physics. The particles and the fields do not care that humans use different approaches for different phenomena. This is one physical world which has to be described by one consistent theory.

3. Your idea doesn't explain anything about "conflicting results for G" unless it can predict how G should depend on composition, to account for the experiments in a consistent way.
The experimental situation is in fact difficult.

On one hand the experiments which were performed have mixed the composition-dependency of the source mass with the dependency of the test mass. From the equivalence principle no dependency on the test mass should exist.

On the other hand the measurement of the dependency of the source mass is very difficult. If one takes a big source mass (like the earth) we do not know exactly it's composition, and we have no alternative object. If we use a source mass in the lab with known composition and known shape, it will to be small and the effects are consequently extremely small.

But this does not mean that this dependency does not exist. We will anyway have to do something as there are lots of open problems.
 
  • #33
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"...geometric interpretation..."
"...What does this now mean? Does gravity have two independent causes, which act in parallel?..."

[?] What if.....
Well, I see it like this.if space is dynamic as implied by the apparent expansion of universe where space stretches outwardly from all points and similarly but 'oppositely' gravity 'source' such as black hole has event horizon where space (must surely?) be stretched inwardly at a rate equal in speed to c so a photon is swimming against the tide as it were and can't escape.
So dynamic space interprets a geometric flow of space and, I speculate requires a particle to commuicate the local geometry, which might, I guess, require information on sub-planck lenghts i.e. a string perhaps which does not manifest on bigger than plank length scales which explains why we can't find them yet.If it it exists, I propose the name: geometricinformotron :D
 
  • #34
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Propagation speed of gravity. I had always assumed gravity travels at light speed and not above. Gravity is information.

Imagine there is an SMBH at the core of our galaxy and one day it decided to Suprahypernova and convert itself completely to cosmic ray high energy photons.

After a few million years we would be able to watch this as the photons arrived and cooked us.

But would we detect the change in gravity at the same instant we start cooking ? or would the gravity change arrive earlier (assuming we could detect it). Or would it arrive later ?

Its always been clear to me the gravity would arrive at the same time as light until I read this thread.
 
  • #35
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No, you're right. Changes in the gravitational field propagate at the speed of light; we have indirect evidence of this from the Taylor-Hulse binary pulsar system (1993 Nobel Prize), and if our gravitational wave detectors ever work, we will have direct evidence of it.
 
  • #36
marcus
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Originally posted by Beast

Its always been clear to me the gravity would arrive at the same time as light until I read this thread.
I just went back and re-read Patrick van Esch post
where he says among other things that news of changes in the gravity field propagates at c, just like news of changes in the electric field.

You might like a caltech animation of a the field around a moving pointcharge, if you havent seen it. There is a subtle point about linear motion---the electric and so presumably the gravitational field "anticipates" it. (It can be seen as static in some frame I guess.) So it's only when something accelerates that it counts as news. I'll look up the caltech link, circular motion makes a nice picture---they have various motions including where you drag the charge around with the mouse and see the effects ripple out at the speed of light.

http://www.cco.caltech.edu/~phys1/java/phys1/MovingCharge/MovingCharge.html

Never heard of macroscopic BH going "nova". Microscopic holes may be able to evaporate in a flash of Hawking radiation, but the big ones radiate by sucking in ordinary matter, which gets hot on the way in. So I cant picture a SMBH doing what you say.
But so-called "hypernovae" or powerful "gammaray bursts" are observed and sometimes attributed to the sudden collapse of a neutron star into a BH. In that case, if we had gravity wave detectors sensitive enough, it would be like what you picture. We would not get fried but we would detect the collapse-wave at the same time as we "see" the gamma flash. Nice thought, maybe will happen sometime.
 
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  • #37
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Originally posted by marcus
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.
yes, of course gravitons are an artifact of flat spacetime! and whats more, so are all particles!

what i mean by that is, well you noticed how the original poster said we were sure about the existence of electrons. but in nonflat spacetime, what is an electron in one frame might not be an electron in another!

a particle has quantum numbers that are invariants of its symmetry group, which is the Poincaré group, in flat spacetime. so mass and spin are how we characterize the spacetime properties of particles, in flat spacetime.

all that goes out the window in nonflat backgrounds. not only do you not have a graviton, you do not have an electron or a photon! it is possible to to perturbation around a nonflat background of course, but depending on what the symmetries of the background are, it may be very hard to decide on what you want to call a particle.

also, the situation is analogous for the electromagnetic field. if you are in a vacuum, then perturbation theory makes a photon a useful approximation. but if there is some strong field electromagnetic background, then this notion becomes more troublesome. you can treat the background as classical, and photons as perturbations around that classical background, but it is difficult to construct a strong classical background out of particles.

(i guess this has something to do with coherent states. i don t know much about that)

the point of the story is: a particle interpretation is most useful in perturbation theory around a flat background, and it becomes strained, if not completely useless, in other situations.

nevertheless, we expect to be able to do physics in these weak field regimes, where the particle interpretation is intact, so we expect there to be a graviton in any quantum theory of gravity.


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.
this is not correct! an electron is a relative term as well!


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
of course it is perturbative.
 
  • #38
NateTG
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Original Topic Questions

Let's say there are gravitons as wave/particles.

If they actually travel at c, then we can expect them to have zero rest mass.

Would gravitons be red/blue shifted by relative motion, and if so, how? What about red/blue shifting by gravity?

How would graviton radiation interact with mater? Do graviton theories provide for e.g. the analog of Cherenkov radiation? Would it be possible to use some kind of lens to bend gravitons or mirror to reflect them?

Would the graviton have the same kind of quantum restrictions as a photon regarding energy states?
 
  • #39
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The dynamics of the string theory graviton are governed by Einstein's field equation, so it behaves in a GR-ish way. It does have zero mass, and coherent gravitons can create gravitational waves. In order to create a Cerenkov effect you would have to find a substance in which gravity travels more slowly than it does in a vacuum.
 
  • #40
Nereid
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A long time ago, in a post far far away

vanesch/Patrick wrote: 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.
IIRC, VIRGO (etc), and LISA (no etcs) look for signals predicted by theory - partly a result of weakness of the expected signals. How much work is being done to develop 'unbiased' analyses of the raw data streams?

What tests - other than gravitational wave detection - are being contemplated for 'strong field' and 'dynamic' GR regimes?
 
  • #41
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IIRC, VIRGO (etc), and LISA (no etcs) look for signals predicted by theory - partly a result of weakness of the expected signals. How much work is being done to develop 'unbiased' analyses of the raw data streams?
I wouldn't say that the analyses are "biased": methods like matched filtering will pull out signals from the noise if you know what the signals look like, but they don't really introduce false signals. (It will pull out a false signal if the noise looks like the signal it's looking for, but that's what it should do: find things that look like signals.) The disadvantage of matched filtering is just that you need to know what the waveforms look like ahead of time to use it.

But anyway, there is some work being done on signal detection that doesn't rely on precalculated waveforms, namely detection of bursts and other anomalies ("things that go bump in the night"). They just look for sudden changes in the general statistics of the signal (such as its average power or variance).


What tests - other than gravitational wave detection - are being contemplated for 'strong field' and 'dynamic' GR regimes?
Not a whole lot, other than the ones already being performed to provide evidence of black holes (attempts to astronomically observe innermost circular orbits, Lense-Thirring effect, sudden vanishing of luminosity near a purported horizon, etc.) Although it depends on exactly what you mean by "strong field" and "dynamic GR" regimes.
 
  • #42
Nereid
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Ambitwistor wrote, re tests of 'strong field' and 'dynamic' GR regimes: Not a whole lot, other than the ones already being performed to provide evidence of black holes (attempts to astronomically observe innermost circular orbits, Lense-Thirring effect, sudden vanishing of luminosity near a purported horizon, etc.) Although it depends on exactly what you mean by "strong field" and "dynamic GR" regimes.
Taking vanesch/Patrick's comment about experimental/observational verification of GR as a starting point, the current status is:
- weak, quasi-static regimes: verified to at least 1:1,000, some aspects to 1:20,000 or better
- strong regimes: BH existence etc verifies GR to ~1:10 (or is that just a wish?)
- dynamic regimes: awaiting clear signals from colliding BHs or neutron stars (etc).

So, in the sense that GR is experimentally verified, the "classical [gravitational] waves" must exist, but 'merely' await direct observational verification.

Gravitons? Stay tuned.
 

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