# Is it experimentally verified that the neutrinos are affected by gravity?

If neutrinos (or any other particles) wasn't affected by gravity that would contradict the general theory of relativity. I'm convinced that the postulate of the equivalence between inertial mass and gravitational mass is adequate, but not totally convinced that it is the truth.

From my dialectical point of view there are no total unity. In every distinction there is a gap to explore. What is supposed to be identical will be found split complex when examined more closely.

And therefor I would like to know if it's experimentally verified that the neutrinos are affected by gravity?

• Related (but not a duplicate, since it does not ask for the evidence) physics.stackexchange.com/q/23899 – Rob Jeffries Jul 18 '15 at 17:18
• At present there is neither the energy nor the angular resolution to attempt such a measurement in a terrestrial setting. No idea if anyone can work out a way forward using astrophysical sources. – dmckee Jul 18 '15 at 17:20
• why would there have to be a difference, as far as we know the definitions are equivalent. the big difference between physics and mathematics is that we can never know if something is true, only if something is false. – john Jul 18 '15 at 20:12
• I thought Rob's answer was good. GR is a well-tested theory, and gravity is all about energy. By the by, if you're not convinced about the equivalence between inertial mass and gravitational mass, you could always ask a question about it. – John Duffield Jul 18 '15 at 20:42
• @john, there don't have to be a difference, it's just that I'm not totally convinced. – Lehs Jul 18 '15 at 22:15

It would help if you gave some context. Is there any evidence, or even theoretical work, that suggests neutrinos are not affected by gravity?

I suppose you could argue that the similar arrival times of photons and neutrinos from SN 1987A was evidence that neutrinos and photons are following the same path through spacetime and both being "gravitationally delayed" by the same amount as they travel from the Large Magellanic Cloud (see Shapiro delay). However, I am unsure to what extent this is degenerate/confused with assumptions about the neutrino masses.

There must also be indirect evidence in the sense that if neutrinos had mass but were unaffected by gravity, then the large scale structure in the universe could look very different. However, I feel that given neutrinos are already an example of hot dark matter, such a signature could be extremely elusive.

Firm evidence may need new neutrino telescopes. One test would be to search for neutrinos from the centres of other stars using the gravitational focusing effect of the Sun. There are predictions that, for instance, the neutrinos from Sirius would be focused at around 25 au from the Sun and would have an intensity about one hundredth of the neutrino flux from the Sun at the Earth. Such a detection would be very clear evidence that neutrinos are being affected by gravity as expected (Demkov & Puchkov 2000).

In a similar vein, any positive detection of the cosmic neutrino background should be modulated by gravitational focusing by the Sun at the level of about 1 per cent (Safdi et al. 2014). This is because an isotropic neutrino background will form a "wind" that the Sun passes through. When the Earth is leeward of the Sun, neutrinos would be gravitationally focused and there should be a larger flux.

• Ah. So all we need to do is place a Super-K sized detector (and it's shielding) in orbit between Uranus and Neptune. Now where is that grant proposal template, anyway? – dmckee Jul 18 '15 at 18:24
• @dmckee I haven't worked out the details, but presumably it has to be there at a particular time too, so the Sun, Sirius and the detector are in line. – Rob Jeffries Jul 18 '15 at 18:26
• Perhaps indirect evidence for (or against) is worth while thinking about? And no, I have no indication of that kind, it's only a perhaps faulty intuition. – Lehs Jul 18 '15 at 22:43

Even if neutrinos were massless, which they aren't, they would still be affected by gravity because of their energy content. For example light is composed of massless photons but these can and are still affected by gravity because of $E=mc^2$

But that's null because we know neutrinos are massive because they oscillate.

• I agree with your observation about photons. But I would be careful with saying something like this about neutrinos. Experimentally there is only an upper bound for the neutrino masses, and the fact that neutrinos oscillate might imply that they have mass, but there is no consistent frame work for describing neutrinos, so the implication might also be false. If Neutrinos had mass we would also expect to see neutrinoless double beta decay, which we haven't seen (yet) – john Jul 18 '15 at 20:00
• @john Neutrino mass only implies neutrinoless double beta decay if the neutrinos have Majorana rather then Fermi nature. That is what the $0\nu2\beta$ experiments are working on. – dmckee Jul 18 '15 at 20:39
• ok your right, I mixed something up there. but still, saying that neutrinos have mass, because... is a bit daring – john Jul 18 '15 at 20:51
• How would you explain neutrino oscillations otherwise? – Nontriviality Jul 18 '15 at 22:20
• In the current SM the neutrinos have zero mass. I think all attempts to incorporate a finite neutrino mass into the SM have failed so far. So there could be a different reason why neutrinos oscillate, Physics beyond the SM, or does this make no sense? – john Jul 19 '15 at 17:10