Gravitons

The graviton is a real hypothetical particle. It is massless. Therefore, gravity should have no effect on it. Beyond that my head starts to hurt.

Hi bhankiii, you wrote: "It is massless. Therefore, gravity should have no effect on it."

Remember, photons are also massless and gravity does have an effect on them! They are bent and in some cases swallowed by black holes, never able to escape again.

However, photons generated very near black holes can escape and if gravitons exist (which I doubt), they can also escape from just outside the event horizon.

How a black hole, with all its mass inside the event horizon, can radiate gravitons to tell space how to curve, is part of the "graviton problem".

-J

My limited understanding is that since photons have energy, they have mass. (m=hf/c²)

Since photons hitting a surface transfer momentum, they act like they have mass.

They have no rest mass, but then they're never at rest - except maybe in those new experiments where they freeze light.

Hi bhankiii, you wrote: "My limited understanding is that since photons have energy, they have mass. (m=hf/c²) "

I prefer to rather use the terms "since photons have energy, they have momentum". The concept "relativistic mass" (m=hf/c²) has fallen into discredit lately, due to the fact that it is an observer dependant quantity (mass observer dependant? - not nice!) Frequency is the culprit, because it depends on the speed of the observer (Doppler effect).

Momentum (p=hf/c) is also observer dependant, but that seems to be more natural, whatever that may mean...

-J

Jorrie, it's also hard to imagine how a massless particle could have an attrative force on a substrate, material body, etc. All known particles (outside of the atomic nucleus) have repulsive or "pushing" properties, e.g., the "space radiation sail", neutrons inside an nuclear pile, and so on.

"...it's also hard to imagine how a massless particle could have an attractive force..."

I have been waiting for a year for someone to broach this topic. (I didn't want to bring it up myself.) I have a question:

Why couldn't gravity be considered as a repulsive force? Wait, now- before you roll your eyes and skip the rest of this post- think a minute. Consider the possibility that gravitons are everywhere, moving in every direction (like the microwave background.) Imagine they mostly pass through solid matter like neutrinos. A fraction of them, however, colide with matter and pass on momentum. A solid with a great propensity to interact with gravitons is said to have great mass. If a massive body (such as the earth) is nearby, it partially shields us from gravitons. The net effect is that there is a greater flux of gravitons coming from the side away from the massive body (up). This greater flux of gravitons pushes us toward the large body (down)- what we sense as gravity.

I know this is a heretical notion, but it has a certain elegance that I find intriging. I would love to hear an explanation of why gravity can not be a pushing force.

Bill Morrow

Is it possible that gravitons are omnipresent in the universe and have varying densities and velocities in space and near matter? Furthermore could massive objects be essentially a void in the graviton density of space and the gravitons behave as fluid that flows INTO these holes? If I wear a massive hat will I be less affected by gravity?

I like your idea. This would explain why black holes have gravity (if they do).

Hi Bill and Cardio. You speculated on why gravitons have an attractive force.

Firstly: gravitons? What gravitons? Oh, you mean the one's QED postulates!

Secondly: luckily general relativity (GR) needs no such things.

Thirdly: the long sought-after-still-to-be-found theory of quantum gravity may need gravitons.

GR simply views gravity as the result of (static) space-time curvature. If the configuration of the space-time curvature changes, gravitational waves propagate the changes at the speed of light. This is the only time anything "moves" and these waves may (perhaps) be coherent gravitons.

As you stated, a massive body absorbs only a tiny amount of the energy of the gravitational waves (a tiny number of the gravitons?) The funny thing is that theoretically, this energy will not make the object recoil, but rather make it oscillate in directions perpendicular to the direction of travel of the gravitational wave!

This is the principle that hopefully will make detectors like LIGO work. Once directly detected, gravitational waves may lead to the detection of gravitons, but I am not convinced that it will.

When a gravitating body does not change its shape, I do not believe that it emits gravitons - how would a black hole emit them? To postulate "virtual gravitons" being exchanged between two bodies, causing the effect of gravity, while cancelling themselves out (equal number emitted and received), makes no sense to me.

-J

This is one for 'Jorrie'.

Gravitron of the family of Bosons, is part of the gravitational interaction, is single-pole attractive, of mass in equal rest to zero, not detected directly until now, is part of the fundamental interactions: fields electromagnetic, electroweak, electro-strong and presumably the gravitational one (graviton).

Your question not yet has answer, if it is real particle, is presumed that it is a by-product of the photon when they are created and destroyed by particles with electrical charge.

Hi Abstract02, you asked about the graviton.

Personally (as an amateur relativist) I tend towards a view of gravity as the curvature of space-time and gravitational waves as ripples in this curvature, propagating at the speed of light. The graviton is postulated in quantum theory (qt), but it is presently incompatible with general relativity (gr).

Perhaps a new theory (like super-strings or quantum gravity) will reconcile qt and gr , but until then I will stick to the gr view, i.e., no graviton.

-J

Hi, Jorrie... The photon (the carrier of the electromagnetic field) has mass equal to zero (thus doesn't exist) when it is still but has a mass (=energy) when it is moving... Can we assume the same thing for the "hypothetical" graviton (the carrier of the gravitational field)???

Hi G.K., you asked: "Can we assume the same thing [zero rest mass] for the "hypothetical" graviton (the carrier of the gravitational field)???"

Yes. The hypothetical graviton always moves at the speed of light. When it is "stopped", meaning absorbed by some massive body, it ceases to exist and only the energy that it transferred to the mass is measurable.

You must remember that gravitons are presumably only emitted if the gravitational field changes, not to keep it "up".

-J

I didn't get your last sentence... When a gravitational field changes then gravitational waves are produced ( e.g. when two black holes whirl around each other )... But when two masses attract each other due to gravity (and in that case the fields don't change their values), it is supposed that they transfer, continuously, gravitons between them, so each mass "sense" the field of the other mass (as a proton and an electron trasnfer "virtual photons", continuously, between them, in order to "attract" each other, as QED theory claims)... Am I right, or I miss something???...

Hi George, you wrote: "... it is supposed that they transfer, continuously, gravitons between them, so each mass "sense" the field of the other mass (as a proton and an electron transfer "virtual photons", continuously, between them, in order to "attract" each other, as QED theory claims)... "

I believe that if the situation is static, no effective gravitons are transferred between masses, because there is no energy transfer (maybe gravitons are exchanged but they cancel out).

When the bodies are allowed to free-fall towards (or orbit around) each other, the fields are not static and there may be a net graviton exchange between them. However, only if the pair loses total energy is there a net emission of gravitons to outside the pair of bodies.

I may be slightly off here, but that is more or less what general relativity (gr) says, although it (gr) does not predict gravitons - they are part of a QED postulate. Gr solves the issue by means of gravitational waves, not gravitons.

-J

Thanks, Jorrie... Hence, the gravitons are supossed to play the same role for the gravitational force as "virtual photons" for the electromagetic force... Gravitons are the "carriers" of the gravitational force...

Hi George, I guess you're right... "Virtual gravitons" invoke about the right "picture" in my mind's eye!

-J

If they wish to see in Wiky something but on graviton.