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The Problem with Gravitons

JesFine

Critical Thinker
J
Joined
Jun 5, 2002
Messages
449
Disclaimer: I am basing this entire thing on premises which may be faulty or incomplete. I will try to state my premises up front so you can show me if they are wrong and the incorrect premise propogates to my conclusion.

OK, photons and gravitons are "virtual particles". I understand this to mean that, while they may or may not technically exist, you can explain certain phenomena by acting as though they do. These particles also have zero mass and travel at the speed of light -- well, theoretically in the case of gravitons as we have not really observed anything that conclusively backs up this claim.

A photon is a virtual particle that "carries" electromagnetic force (ie light). A graviton is a virtual particle that "carries" gravitational force. We have observed photons (or at least directly observed side effects of photons), but we have not observed gravitons.

Lastly, in a black hole, gravity is so strong that nothing can escape, including light (ie photons). Somehow, the gravity is so intense, that even a massless particle traveling at the speed of light cannot break its grasp.

Now my problem: If photons can't escape, then why can gravitons? Obviously they escape, because they carry the gravitational force, and the gravitational pull of black holes is extreme.

This quickly becomes circular in my mind. If the gravitational pull of a black hole was really that huge, then shouldn't gravitons also get sucked back in? And if that were the case, wouldn't the gravitational pull of the black hole be zero? And in that case, wouldn't te gravitons be able to fly out? Repeat.

My amateur brain says it must be because gravitons either don't exist, have "negative" mass, or travel faster than the speed of light. Or one or more of my premises is flawed.

Thanks.
 
Amature going out on a limb. If I'm wrong, have fun.

Photons have mass I'm pretty sure.
 
JesFine,

Disclaimer: I am basing this entire thing on premises which may be faulty or incomplete. I will try to state my premises up front so you can show me if they are wrong and the incorrect premise propogates to my conclusion.

OK, photons and gravitons are "virtual particles". I understand this to mean that, while they may or may not technically exist, you can explain certain phenomena by acting as though they do.
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Problem number 1. That is not what the term "virtual particles" means. Furthermore, not all photons and gravitons are virtual particles.

Here is how it works. Electromagnetic waves are quantized. The quanta of those waves are called photons. When you have an actual electromagnetic wave, those photons are "real" photons. A virtual photon is a statistical electromagnetic fluctuation in the vacuum. These fluctuations are also quantized, and manifest as photons which appear for a short time, and then vanish.

In principle, the same should be true for gravity.

These particles also have zero mass and travel at the speed of light -- well, theoretically in the case of gravitons as we have not really observed anything that conclusively backs up this claim.
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We have indirectly observed gravitational waves. We know they exist and that they travel at the speed of light. This means that if the gravitational field is quantized, then its quanta must have zero rest mass, and travel at the speed of light. We can also determine other properties it must have, such as its spin, from the behavior of the macroscopic gravitational fields and waves.

A photon is a virtual particle that "carries" electromagnetic force (ie light). A graviton is a virtual particle that "carries" gravitational force. We have observed photons (or at least directly observed side effects of photons), but we have not observed gravitons.
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We have not yet confirmed that gravity is quantized. The gravitational force is extraordinarily weak, and an individual quanta would be several orders of magnitude smaller than anything we can currently measure.

Lastly, in a black hole, gravity is so strong that nothing can escape, including light (ie photons). Somehow, the gravity is so intense, that even a massless particle traveling at the speed of light cannot break its grasp.

Now my problem: If photons can't escape, then why can gravitons? Obviously they escape, because they carry the gravitational force, and the gravitational pull of black holes is extreme.
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Gravitons do not have to escape. It is a misconception that forces like electromagnetism and gravity are "transmitted" in the form of photons and gravitons. Like I said, they are the quanta of electromagnetic and gravitational waves. And guess what? Even under non-Quantum General Relativity, gravitational waves cannot escape from a black hole.

This quickly becomes circular in my mind. If the gravitational pull of a black hole was really that huge, then shouldn't gravitons also get sucked back in? And if that were the case, wouldn't the gravitational pull of the black hole be zero? And in that case, wouldn't te gravitons be able to fly out? Repeat.
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Nope. All it means is that the mass inside the black hole could be pulsating, which would normally create gravitational waves, but those waves would not escape. The gravitation "force" does not have to escape.

My amateur brain says it must be because gravitons either don't exist, have "negative" mass, or travel faster than the speed of light. Or one or more of my premises is flawed.
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Your premises are flawed. Unfortunately, I do not think I can explain the actual role that virtual particles play in forces like EM and gravity here. I am not enough of an expert in it myself to try to explain it to a layman. Perhaps Tez can help out? He knows a lot more about this stuff than I do.


Dancing David,

Uh, my guess is that while photons do have a rest mass, gravitons don't.
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Photons do not have rest mass. Nothing with nonzero rest mass can move at the speed of light.


Dr. Stupid
 
Two more points I'd like to make.

Virtual particles can have mass. The particles that mediate the weak force (the W and Z bosons) have mass and charge, but when they act as force mediators they are still virtual particles.

Secondly, one of the few pieces of information about infalling matter that remains once it has entered a black hole is its charge. In other words, although light (electromagnetic waves) cannot escape a black hole, electromagnetic force can.
 
Thanks Dr. Stupid, I was out on a limb, thanks for bring me in.

So do photon only have mass becaise they do move? Stupid question for Dr. Stupid. I know that photons have mass because they are effected by a gravitational field, correct. So if the photon were to cease it's motion it's rest mass would be zero. Wow, not just so miniscule that it could travel at the speed of light?
 
Dancing David,

So do photon only have mass becaise they do move? Stupid question for Dr. Stupid. I know that photons have mass because they are effected by a gravitational field, correct. So if the photon were to cease it's motion it's rest mass would be zero. Wow, not just so miniscule that it could travel at the speed of light?
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Look at it this way. A particle with non-zero rest mass can never reach the speed of light. Likewise, a particle moving at the speed of light can not slow down.

Apply a force to a photon, and depending on the relative directions of the force and the photon's motion, the following will happen.

1) The force is perpendicular to the photon's motion. The photon changes direction, but not speed.

2) The force is with the motion of the photon. The photon is blue shifted. Its energy increases, but its velocity does not change.

3) The force is against the motion of the photon. The photon is red shifted. Its energy decreases, but its velocity does not change.

You can red-shift a photon down to nothing, but you can never have a photon which is at rest.

Dr. Stupid
 
Dr. Stupid,
so why do photons respond to a gravitational field, is it because the field bends thier path or because the field bends space. Do photons really have mass, or is it a product of the virtual particels they craete, thank you.
 
Dancing David,

so why do photons respond to a gravitational field, is it because the field bends thier path or because the field bends space. Do photons really have mass, or is it a product of the virtual particels they craete, thank you.
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Remember that mass and energy are equivalent. E=Mc^2.

All forms of energy gravitate. A moving electron weighs more than a stationary one. Even potential energy gravitates. The binding energy between the planets and the sun actually mean that, to a distant observer, the solar system weighs slightly less than it would if it were more spread out.

That said, it is kind of misleading to say that a photon has no rest mass. A more accurate statement would be that rest mass is meaningless for a photon, since a photon can never be at rest.

The total energy of a particle is equal to its rest energy (rest mass / c^2), plus its kinetic energy. Since an electron's total energy is equal to its kinetic energy, it is convenient to say that its rest mass is zero. But its kinetic energy is still gravitationally active.

The curved space-time thing doesn't really apply at the quantum level. Attempts to unify gravity with quantum mechanics treat space-time itself as an emergent property of QM. In other words, instead of having a curved space-time, in which QM takes place, the idea is that what we think of as space-time is a macroscopic limit of something far more complicated. The geometry of space-time at the macroscopic level is thus determined by the quantum interactions that it is a manifestation of.

I'm sorry if that sounds vague or confusing. Like I said, I am not an expert on General Relativity, Cosmology, or Quantum Mechanics.


Dr. Stupid
 
No , is is an answer and gets to the points I asked, the mass of a photon at rest is meaningless, so while it's rest mass may be zero that is a moot point. Thanks Dr. Stupid.
 
Stimpson J. Cat said:

A moving electron weighs more than a stationary one.
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I don't think I agree with this statement, though most of what you said is certainly spot on. Here's a slightly different explanation of the situation:

Everything (photons, electrons, whatever) has a relativistic 4-component momentum. In an object's rest frame, the spatial components are zero (it's not moving) and the time component (the energy) is given my mc^2. Photons and the like are considered massless because you cannot transform to a rest frame for an object moving at c. When massive objects are viewed in a frame where they are moving, the time component of their relativistic momentum is larger than at rest, which is what leads some to describe this as an increase in mass. And it becomes larger without limit as c is approached.

The problem I have with saying that an electron or any other massive particle has an increasing mass (or weight) as it moves faster is that it suggests that the gravitational field from a single particle should change as you change reference frames. Consider an object a little shy of being massive enough to be a black hole. Can you find a coordinate system in which this object is a black hole? The answer should be no, if it's a black hole in one frame it should be a black hole in all frames, and vice versa. So moving to a frame where this object has huge kinetic energy many times its rest mass shouldn't turn it into a black hole. So I don't think it's really useful to talk about the mass or weight actually increasing as you increase kinetic energy (or equivalently change reference frames).
 
Some interesting questions. Unfortunately I head to a conference tomorrow, and am madly preparing a poster and a talk in my least favourite program of all time - powerpoint.

Some of these questions were discussed in the threads:

http://www.randi.org/vbulletin/showthread.php?s=&threadid=20661

http://www.randi.org/vbulletin/showthread.php?s=&threadid=7973

I have a slightly better understanding now of quantum black holes, coz we had a visitor here recently whom I pestered. I'll try and post something more coherent when I get back...
 
Interesting stuff in this thread. I'm going to read it a few times until I understand it. Ok, maybe more than a few times.

In the meantime, I googled on this -- and the strange part is, I googled before I posted the original question and found nothing, this time, I found something. No idea. Anyway, I found this site:

http://curious.astro.cornell.edu/question.php?number=264

Which basically says that the gravitational effects of black holes is due to the curvature of space time, and not the gravitons themselves. So forget about gravitons when talking about black holes.

And this site:

http://www.fnal.gov/pub/inquiring/questions/blackholes.html

Which basically says it is tough to combine quantum field theory with general relativity, but gives an photon analogy to describe the behavior of gravitons. Kind of a cop out.

Also, I'm not sure if the answers given in those sites agree with what has been posted here. Is there some debate on this matter?

Stimpy/Dr. Stupid,

I didn't understand this:
All it means is that the mass inside the black hole could be pulsating, which would normally create gravitational waves, but those waves would not escape. The gravitation "force" does not have to escape.
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Gravitational waves are why we feel gravity, correct? So, why then do we feel gravity from a black hole if the gravitational waves do not escape? I am starting to think I am confusing the terms "gravitational wave" and "gravitational force" -- are these terms interchangeable?

Thanks again! You guys are too smart.
 
JesFine,

I didn't understand this:

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All it means is that the mass inside the black hole could be pulsating, which would normally create gravitational waves, but those waves would not escape. The gravitation "force" does not have to escape.
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Gravitational waves are why we feel gravity, correct? So, why then do we feel gravity from a black hole if the gravitational waves do not escape? I am starting to think I am confusing the terms "gravitational wave" and "gravitational force" -- are these terms interchangeable?
Click to expand...

No. Gravitational waves are propagating changes in the gravitational field which occur when masses accelerate. Just as electromagnetic waves are propagating changes in the electromagnetic field which occur when electric charges accelerate.

A stationary mass does not emit gravitational waves. The force it exerts is due to the static gravitational field.


It is helpful to consider the classical view of electromagnetism first, then the classical view of gravity (General Relativity) and the quantum view of electromagnetism. Only once you understand all of those, is idea of quantum gravity likely to make any sense.

Classically, a static configuration of electrical charges has associated with it an electric field. That field determines the force exerted by those charges on each other. When those charges begin moving, the field does not just instantaneously change. Instead the change in the field propagates at the speed of light in the form of electromagnetic waves. Those waves carry energy and momentum, and photons are the quanta of those waves.

The situation is analogous for gravity. Gravitational waves cannot escape from a black hole, but that simply means that we can never have any information about changes in the configuration of mass within the event horizon. That is fine, though, because according to General Relativity, it is meaningless to talk about the configuration of mass within the event horizon of a black hole.

Instead, we must view a black hole as a fundamental particle. It has mass, momentum, angular momentum, electric charge, and magnetic moment, but that is all. These quantities completely describe the black hole.


Dr. Stupid
 
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