Do Gravitons Exist?

[Rhondazvous]

According to Wikipedia, gravitons are hypothetical elementary particles, so when I wrote about them in my novel, I treated them as such. But now I'm reading Hawking's Brief History of Time where he speaks of gravitons as if they really exist. I'd give Hawking a lot more credibility than Wikipedia, but maybe you can provide the synthesis for this seeming thesis/antithesis polemic.

[Alex K]

The fact is that nobody knows. We know that gravitational waves exist, but there is no real experimental test which could tell us that these waves have quanta the same way electromagnetism has photons. It seems overwhelmingly likely though, for the simple fact that matter causes gravitation, but matter is fundamentally made of particles subject to quantum uncertainty, and it is difficult to imagine how this quantum uncertainty would not get transferred to the gravity. For example, if an electron is in a quantum superposition of being in place A and in place B, what is its gravity? Half here half there? Does the superposition vanish when you measure the gravity? It should! So, in order not to mess up the logic of quantum mechanics, the gravitational force needs to play by the same quantum rules, and that would usually entail the existence gravitons.

[LastPoet]

According to Wikipedia, gravitons are hypothetical elementary particles, so when I wrote about them in my novel, I treated them as such. But now I'm reading Hawking's Brief History of Time where he speaks of gravitons as if they really exist.   I'd give Hawking a lot more credibility than Wikipedia, but maybe you can provide the synthesis for this seeming thesis/antithesis polemic.

What Alex said. We need to find out more. Hawkins has hyphothesised gravitons. We do not have evidence to admit their existence.

[Rhondazvous]

The fact is that nobody knows. We know that gravitational waves exist, but there is no real experimental test which could tell us that these waves have quanta the same way electromagnetism has photons. It seems overwhelmingly likely though, for the simple fact that matter causes gravitation, but matter is fundamentally made of particles subject to quantum uncertainty, and it is difficult to imagine how this quantum uncertainty would not get transferred to the gravity. For example, if an electron is in a quantum superposition of being in place A and in place B, what is its gravity? Half here half there? Does the superposition vanish when you measure the gravity? It should! So, in order not to mess up the logic of quantum mechanics, the gravitational force needs to play by the same quantum rules, and that would usually entail the existence gravitons.

"Does the superposition vanish when you measure the gravity?"

I may be going off in the wrong direction. I'm trying to understand this in terms of it not being possible to measure position and velocity at the same time with equal accuracy. But that would require me to make a connection between velocity and gravity. Well, what is gravity if not movement. You're measuring one objects ability to cause other objects to move toward it based on their mass. Correct?

So we are looking for a physical cause for gravitation. Maybe it is too small to see with current technology, or maybe like dark matter it can't be detected. But we know it's there.

I love scientists. Not because you have all the answers, but because of the way you address the questions. There's a lot of credibility there.

[Alex K]

@Rhondazvous,

If you want to take the uncertainty principle as a starting point, that works as well: from the gravity of a particle, one should be able to tell position and momentum. In order to prevent us from violating Heisenberg's principle like that, gravity itself needs to have quantum uncertainty.

[vorlon13]

The fact is that nobody knows. We know that gravitational waves exist, but there is no real experimental test which could tell us that these waves have quanta the same way electromagnetism has photons. It seems overwhelmingly likely though, for the simple fact that matter causes gravitation, but matter is fundamentally made of particles subject to quantum uncertainty, and it is difficult to imagine how this quantum uncertainty would not get transferred to the gravity. For example, if an electron is in a quantum superposition of being in place A and in place B, what is its gravity? Half here half there? Does the superposition vanish when you measure the gravity? It should! So, in order not to mess up the logic of quantum mechanics, the gravitational force needs to play by the same quantum rules, and that would usually entail the existence gravitons.

Would there be a spectrum of energy analog in regards to gravitons as there is with photons ?

I guess I was assuming the LIGO device was measuring gravitational waves 'composed' of gravitons of identical energy equivalence.  If individual gravitons can have varying levels of energy, well, there outta be some really biguns leftover from the big bang flying around still and scattered sufficiently they are no longer organized into detectable waves but are more like the 'sea of neutrinos' we are saturated in.

And if there is enough of 'em is that the dark matter, or dark energy or quintessence, or ylem they are looking for and not finding ?

[vorlon13]

Don't just kudos that !


I was hoping you KNEW !!

[Alex K]

The fact is that nobody knows. We know that gravitational waves exist, but there is no real experimental test which could tell us that these waves have quanta the same way electromagnetism has photons. It seems overwhelmingly likely though, for the simple fact that matter causes gravitation, but matter is fundamentally made of particles subject to quantum uncertainty, and it is difficult to imagine how this quantum uncertainty would not get transferred to the gravity. For example, if an electron is in a quantum superposition of being in place A and in place B, what is its gravity? Half here half there? Does the superposition vanish when you measure the gravity? It should! So, in order not to mess up the logic of quantum mechanics, the gravitational force needs to play by the same quantum rules, and that would usually entail the existence gravitons.

Would there be a spectrum of energy analog in regards to gravitons as there is with photons ?

I guess I was assuming the LIGO device was measuring gravitational waves 'composed' of gravitons of identical energy equivalence.  If individual gravitons can have varying levels of energy, well, there outta be some really biguns leftover from the big bang flying around still and scattered sufficiently they are no longer organized into detectable waves but are more like the 'sea of neutrinos' we are saturated in.

And if there is enough of 'em is that the dark matter, or dark energy or quintessence, or ylem they are looking for and not finding ?

In the simplest case, I'd even say in the most likely case by far, gravitons behave like massless particles, and grav. waves are a collective effect of a superposition of many gravitons just as a coherent light wave or radio wave is a superposition of many photons. The energy of an individual graviton would also be given by planck's constant h times the frequency, like with photons.

When you measure the electric field of an em wave, you measure the collective effect of many photon particles, and likewise, when you measure the displacement of LIGO, of gravitons.

Since gravity does not get exponentially suppressed at the distance, gravitons need to be massless to very high precision. This would mean that they get diluted in cosmic expansion with the expansion factor to the fourth power like radiation, and would not form structure. Thus, normal individual gravitons are not a candidate for dark matter. Balls of tightly bound gravitons, i.e. black holes, could serve as DM. Before you cross this threshold of black hole formation, the gravitons would move at light speed.

[Alex K]

With the dark energy, the problem isn't so much that they can't find it - there is a free parameter in Einstein's field equation, the cosmo. constant, which regulates dark energy. So from a theoretician's perspective, the mere existence of DE isn't so surprising, nor necessarily demanding an explanation. The trouble is rather that all the virtual particles and the higgs field on paper contribute such a ridiculous amout of dark energy that the actual puzzle is - why is there so little of it! On paper, you need to set the cosmo constant to 120 digits precision to cancel the dark energy coming from all the virtual particles down to the observed value. That seems strange, and that's the open question - is that an artifact of the way we formulate our theory, is it a sign that we don't understand quantum gravity, is it an anthropic tuning, or is it just that way and there's no explanation?

[Rhondazvous]

Would there be a spectrum of energy analog in regards to gravitons as there is with photons ?

I guess I was assuming the LIGO device was measuring gravitational waves 'composed' of gravitons of identical energy equivalence.  If individual gravitons can have varying levels of energy, well, there outta be some really biguns leftover from the big bang flying around still and scattered sufficiently they are no longer organized into detectable waves but are more like the 'sea of neutrinos' we are saturated in.

And if there is enough of 'em is that the dark matter, or dark energy or quintessence, or ylem they are looking for and not finding ?

That's interesting. When you say gravity might be more like a sea rather than a wave, are you suggesting that gravity causes other things to move but does not itself move? The universe is floating in a sea. According to Hawking, Einstein said gravity is not like other forms of energy.