A question about gravitons&photons

[Loren Booda]

Speculation:

Bosons such as photons or gravitons are indistinguishable among their own species. Photons contribute to energy density though, as would gravitons, and thus interact through spacetime curvature. Gravitons, being quadrupole, also warp spacetime by inducing complementary gravity waves.

 

[Stingray]

Originally posted by startic84
Okay, to the crux of this post now, i seem to get the impression that photons and gravitons wouldn't interact, right? Does this mean they will whizz pass each other like ghosts? Then again, gravity bends light. Doesn't this mean there exists an interaction between those two class of particles when brought together? I am confused...anybody care to enlighten?

Once again, your question is ill-defined because gravitons do not even exist in any accepted physical theory. They are hypothesized to exist "at some point" with properties unknown until then. Anyone who says they know what gravitons do is either pushing their pet theory, or doesn't know what they are talking about.

In fact, the usual concept of a quantum particle makes very little sense even in quantum field theory formulated in a classical curved spacetime. This is a little-appreciated fact by the particle physicists who've only bothered to learn how to calculate cross-sections for their particle accelerators.

 

[Nereid]

startic84 wrote: I feel that this phenomenon is understandable since topics like light/gravity still have their certain areas left un-discovered and un-explained.

There are surely many, many things about light and gravity which remain un-discovered ... however, until we 'discover' them - through an experiment here on Earth, or observation of something in a galaxy far, far away (and a long, long time ago) - we can't even begin to 'explain' them!

AFAIK, there are no experiments involving light and gravity (separately or together) which are 'un-explained' in terms of general relativity (GR, the best theory of gravity that we have) or quantum electro-dynamics (QED, the best theory involving light that we have).

To drive home Stingray's point: photons have a home in QED; there are no gravitons in GR.

Now for the caveat (there's always a caveat in science; if there weren't, it wouldn't be science!): When it comes to observational cosmology, there is some data which doesn't seem to fit the best cosmological theories we have today; however QED and GR have to be reconciled for any decent cosmological theory, but no one can claim much success with that yet.

 

[TeV]

Originally posted by Stingray
Once again, your question is ill-defined because gravitons do not even exist in any accepted physical theory. They are hypothesized to exist "at some point" with properties unknown until then. Anyone who says they know what gravitons do is either pushing their pet theory, or doesn't know what they are talking about.

In fact, the usual concept of a quantum particle makes very little sense even in quantum field theory formulated in a classical curved spacetime. This is a little-appreciated fact by the particle physicists who've only bothered to learn how to calculate cross-sections for their particle accelerators.

Properties of free graviton should not differ than that of free traveling gravity wave if quantuum field of gravity and GR are on the course to unite one day.We don't know yet if this is possible.
In the same fashion It is not usual to talk about radio wave as photon and vise versa.But this is basically the same thing-sometimes show particle and sometimes wave characteristics depending on the energy.One way or another I would propose the guy who asked orginal question to study "interaction" of localised traveling gravity wave (as described in GR) and TEM in Minkowski STR reference frame.Frontal collision in one line is without exchange of impulse ,and energy and direction of propagation if observed from spacetime points just before and after " interaction".This is what says GR about it.That was startlic original question wasn't it?Other cases of collision "angle" always result in both changed clock times and direction of propagation paths.
Althought "interaction" cannot be treated in usual sense since spacetime characteristics are altered by the presence of gravity wave itself.

 

[TeV]

I forgot to stress that free graviton due to the all the mentioned and reasons is NOT observable or detectable as a PARTICLE at all.

 

[p-brane]

Originally posted by turin
I think that gravity is physical. How else would you account for the effect on the physical universe?

I can account for gravity's effect on the physical by way of this analogy:

The lack of food causes starvation. The food is not present (ie: not physically present) but this non-presenting condition does present an effect on a very physical condition.

 

[Nereid]

Is gravity physical?

What does a question "Is {X} physical?" mean?

So far as gravity is concerned, we have a theory of gravity - Einstein's General Relativity ("GR") - which is pretty good. In fact, AFAIK, there are no observations which are inconsistent with GR; in fact some aspects have recently been shown to match observations to 1 part in ~20,000.

However, GR is NOT consistent with quantum mechanics. So, let's do some experiments to test whether QM or GR (or neither) is correct! Problem is that the conditions necessary for such experiments cannot be created here on Earth, nor are we close enough to any black holes to try looking there, and our observational (and theoretical) cosmology has a long way to go before they'll help answer the questions.

 

[Loren Booda]

I recall that GR is accurate (theory vs measurement) to one part in ~10¹⁰ for pulsar decelleration, as is QED for some magnetic moments.

 

[Creator]

Originally posted by Loren Booda
I recall that GR is accurate (theory vs measurement) to one part in ~10¹⁰ for pulsar decelleration, ...

I don't think so, Loren. If you are referring to Taylor-Hulse binary gravitational/orbital rate decay, its accuracy is only about .5%; and at last check I have seen no other pulsar decay rates showing agreement with GR better than 0.1%

Creator

 

[p-brane]

Originally posted by Loren Booda
I recall that GR is accurate (theory vs measurement) to one part in ~10¹⁰ for pulsar decelleration, as is QED for some magnetic moments.

Although I've never starved to death (hence I am typing this) I tend to understand what I said better than this quote (probably because I am not a physicist or a patent office clerk)! I know that gravity by the definitions offered in the theory of General Relativity show that relative to a gravitational field time and space will change accordingly. What GR does not state is the actual composite nature of gravity. If I've got it right - GR only explains that gravity will effect its surroundings, in a relative manner.

I get the feeling that the ultimate example of gravity, a "black hole" is just that - a lack of fabric or a lack of space and time. It is a dense mass, so they say, but it is also analogized as a "hole" in space and time. This is where I get the idea that it is more of a subtraction of the fabric of space and time than an addition to the fabric.

When I look at gravity this way, it begins to look like it interupts the medium that carries electromagnetic waves by presenting the lack of a carrier. This, relative to the mass creating the "hole" will either swallow light or bend it, respectively. Any contary explanation will be considered!

 

[Loren Booda]

Thanks, Creator. Can anyone come up with the most accurately verified predictions of QM or GR?

 

[TeV]

Originally posted by Creator
I don't think so, Loren. If you are referring to Taylor-Hulse binary gravitational/orbital rate decay, its accuracy is only about .5%; and at last check I have seen no other pulsar decay rates showing agreement with GR better than 0.1%

Creator

Yes.The accuracy of the observation of Taylor-Hulse binary system orbital time changes is only about 0.5%.Due to the lack of preceise data of the system though.Still the prediction is in the width of error limit and is find to be the first experimental proof of the existence of gravity waves.
Supernova explosions are expected (and awaited for) to give us more info about these phenomena.

 

[Nereid]

http://relativity.livingreviews.org/Articles/lrr-2001-4/ gives an overview of where GR stood in 2000. As to how accurately it's been tested, well, you need to ask 'which part'? There are some tables at the end of the later sections in Chapter 3, and other numerical figures in later chapters. In general, GR has been shown to be consistent with observation and experiment to at least 0.1%, and in some respects to ~10^-5.

Last year there were some press releases about results from the Cassini spacecraft (on its way to Saturn) and a light deflection (gravitational bending) test of GR, which gave a limit of ~1:20,000 for any inconsistency (I do not know whether the paper is in print yet).

http://www.phys.washington.edu/Department/Gradweb/Res_ExpAtomicAndMolcPhys.html gives the accuracy of at least one prediction of QED at 1:10¹²:

"g(electron)/2 = 1.001 159 652 188(4)
g(positron)/2 = 1.001 159 652 188(4)
g(positron)/g(electron) = 1.000 000 000 001(2)
The value of the electron/postiron g factor is the most precisely determined parameter of an elementary particle and so far it has stood up to all our tests. The close agreement of the theoretical value with the experimental one may be interpreted as the most stringent test of QED theory. "

 

[turin]

Originally posted by p-brane
The lack of food causes starvation.

The lack of food does not cause starvation any more than the lack of a driving force causes signal decay. The body eating itself causes starvation. It isn't food's fault.

 

[p-brane]

Originally posted by turin
The lack of food does not cause starvation any more than the lack of a driving force causes signal decay. The body eating itself causes starvation. It isn't food's fault.

Fault has nothing to do with what I'm saying. The body eats itself because there is no other source of food.

Light is trapped and its energy either entropied or transformed because it encounters an area in space/time where background radiation has condensed into a mass, leaving a hole (or "spongy medium) proportinate to the density of the mass observed.

 

[Loren Booda]

What were the results of the Einstein probe in regards to GR?

 

[turin]

Originally posted by p-brane
I get the feeling that the ultimate example of gravity, a "black hole" is just that - a lack of fabric or a lack of space and time.

Fabric? So you're a creationist then? (sorry, I'm just kidding, I don't care whether or not you are.)

But a black hole is not a lack of anything: there is a metric inside the horizon (there is a "space-time fabric"), light and matter can have fallen in (on their way to inevitable destruction). It is not necessarily empty at all. It is called a "black hole" simply because, if you could get close enough to look at one (visually, optically), then it would literally look like there was a big black hole in the star field (and the optical lensing makes this appearance even more pronounced than just some missing stars). The only place where the physical theory does not go is the singularity. But even there, I'm pretty sure it is believed that something exists, and even if it is not so believed, the singularity does not pull on things, gravity does. The singularity is just there. Space-time is curved. A black hole is not an example of gravity, it is an example of an object that gravitates.

Originally posted by p-brane
The body eats itself because there is no other source of food.

The body eats itself because that is precisely the mechanism of metabolism. When, as you say, "other food" is ingested, it is obsorbed into the body and eaten. The body is always eating (well, I don't have a reference for this, so, if you disagree, then I guess I'll have to shut my mouth until I find one). The presense or absense of "other food" does not change this (see previous parenthetical).

There is no food on my desk right now. But my desk is not starving. A lack of food does not cause starvation.

Originally posted by p-brane
Light is trapped and its energy either entropied or transformed because it encounters an area in space/time where background radiation has condensed into a mass, leaving a hole (or "spongy medium) proportinate to the density of the mass observed.

What does it mean for energy to be "entropied?" A hole or spongy medium? I don't follow. But, if there is some spongy medium, then I would say that sounds pretty physical (i.e. it has the property of being spongy, and it is a medium).

 

[bettysfetish]

Good to be back. House fire fried my computer. It's nice to see some of the same names; Nereid,"Is X physical', that's excellent as usuall. I like the way you can simplfy, or "dumbdify", a question. How "Could" one know if "X" is physical before "X" has been quantified? Turin, your descriptions are very clear as usuall.
Question: Would a beam of photons in vacuum eventually difuse? It seems the photon has no charge but does have an EM field. Now if we can someday observe a "graviton", what properties might we expect?
Gravitons, if they exsist, would be more defuse as distance from mass increases. However they would permiate the universe. As the photon passes through varying "clouds of gravitons" coming and going to and from all mass it should be pulled to and fro until it strikes an object.
Is it possible that mass effects light but light has no effect on mass? Can this be a violation of CPT? I don't think I've read of anything other than the "K" meson that shows that property. Perhaps the photon does have some minute effect on the graviton, if it exsists, but it would be well below our capabilitys to measure it, even in quanta, right now. That would keep things conserved.
L8R - - - p.s. for anyone new, I'm a Male. The name comes from a band I'm in; just to clear things up.

 

[p-brane]

Originally posted by turin
Fabric? So you're a creationist then? (sorry, I'm just kidding, I don't care whether or not you are.)

But a black hole is not a lack of anything: there is a metric inside the horizon (there is a "space-time fabric"), light and matter can have fallen in (on their way to inevitable destruction). It is not necessarily empty at all. It is called a "black hole" simply because, if you could get close enough to look at one (visually, optically), then it would literally look like there was a big black hole in the star field (and the optical lensing makes this appearance even more pronounced than just some missing stars). The only place where the physical theory does not go is the singularity. But even there, I'm pretty sure it is believed that something exists, and even if it is not so believed, the singularity does not pull on things, gravity does. The singularity is just there. Space-time is curved. A black hole is not an example of gravity, it is an example of an object that gravitates.




The body eats itself because that is precisely the mechanism of metabolism. When, as you say, "other food" is ingested, it is obsorbed into the body and eaten. The body is always eating (well, I don't have a reference for this, so, if you disagree, then I guess I'll have to shut my mouth until I find one). The presense or absense of "other food" does not change this (see previous parenthetical).

There is no food on my desk right now. But my desk is not starving. A lack of food does not cause starvation.




What does it mean for energy to be "entropied?" A hole or spongy medium? I don't follow. But, if there is some spongy medium, then I would say that sounds pretty physical (i.e. it has the property of being spongy, and it is a medium).

So far I'd say I'm striking out with you turin, so I won't waste much more time with your questions. I do agree with you that a desk won't starve, food or no food, that's because its either made of metal or the remains of a dead organism. Starvation usually refers to a living organism and its reaction to the absence of of food. Thank you.