Are gravitons a certainty like electrons in reality?

[Tom D]

"...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

 

[Beast]

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.

 

[marcus]

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 haven't 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 can't 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.

 

[lethe]

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 what's 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.

 

[NateTG]

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?

 

[selfAdjoint]

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.

 

[Nereid]

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?

 

[Nereid]

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.