Detecting Gravitons: Challenging the Foundations of General Relativity

[KallaNikhil]

If we were to able to detect gravitons then is it not that the basic assumption over which the general theory of relativity is flawed ?

 

[mfb]

General relativity doesn't have gravitons. Yes.
It is expected that GR is only an approximation, the classical limit of some yet unknown theory of quantum gravity. This is similar to Newtonian mechanics which is an approximation of nonrelativistic quantum mechanics.

 

[PeterDonis]

If we were to able to detect gravitons

Note that we have not detected gravitons; we have only detected gravitational waves, which classical GR can and does predict and model.

 

[mfb]

Note that we have not detected gravitons

And we have no idea how to build a detector sensitive enough to see individual gravitons (unless there are extra dimensions or similar things).

 

[PAllen]

And we have no idea how to build a detector sensitive enough to see individual gravitons (unless there are extra dimensions or similar things).

Yes, I liked this paper on graviton detection: https://arxiv.org/abs/gr-qc/0601043

 

[mfb]

An interesting paper. I'm a bit puzzled by the assumptions made there (e. g. to get equation 6.2). A Jupiter-sized detector is perfectly fine, but we can't get it closer than 100,000 light years to a black hole? Build it around the primordial black hole and you get R=100,000 km, a factor 10¹³ closer, or 10²⁶ higher detection rate. Alternatively, you get the same detection rate with just 20 kg of detector material. The neutrino background stays a problem, of course.
The assumption that the detector readout happens at the surface of the Jupiter-sized detector only (p16) is odd as well.

 

[PAllen]

An interesting paper. I'm a bit puzzled by the assumptions made there (e. g. to get equation 6.2). A Jupiter-sized detector is perfectly fine, but we can't get it closer than 100,000 light years to a black hole? Build it around the primordial black hole and you get R=100,000 km, a factor 10¹³ closer, or 10²⁶ higher detection rate. Alternatively, you get the same detection rate with just 20 kg of detector material. The neutrino background stays a problem, of course.
The assumption that the detector readout happens at the surface of the Jupiter-sized detector only (p16) is odd as well.

Yeah, he is assuming we stay in the solar system, and that there do not happen to be any unusually close PBH.

As for the detector, through most of the paper, they do not assume only surface detectors. However, they do justify adding this constraint for more realism based on the mechanism of the proposed detector, and they then discuss ways to distribute detectors through a volume. Do you disagree with their electron mean free path discussion?

 

[mfb]

I don't disagree with the short mean free path, but if we have the methods to make such a large detector, we would instrument it in 3D like we do with current detectors. If we don't go closer to the source then there is no need to make a single large detector, multiple smaller ones would be fine. Closer to a source a ring around the source could work nicely, or some Dyson swarm like structure.

 

[vanhees71]

Another question I have is, how to create single-graviton states to begin with. Given the fact that not long ago it was pretty difficult to prepare single-photon states, and that "macroscopic electromagnetic waves" have been created and detected for more than 100 years now and we just are beginning to be able to detect macroscopic gravitational waves, I'd expect it to take some more decades to construct single-graviton sources and single-graviton detectors, provided either of them exist at all. Don't forget that we don't have a satisfying theory of quantum gravity yet!

 

[mfb]

You don't need a single-graviton source to demonstrate that the energy is transferred only in discrete steps. As discussed in the paper, this is not the best possible demonstration of quantization of the radiation, but it would be a significant step beyond current detectors.

 

[Haelfix]

It has been argued that there is a sense in which inflation also probes the quanta of the gravitational field, but you wouldn't necessarily be able to show uniqueness (you could imagine different physics also contributing the same signal).

https://arxiv.org/abs/1309.5343