I've already read the below questions (and their answers) regarding neutrinos vs. electromagnetic waves propagating through space, but I'm still not clear on something.
- Neutrinos arrived before the photons (supernova)
- The delay between neutrinos and gammas in a supernova, and the absolute mass scale of neutrinos
- Neutrino Speed in Supernova
- Speed of neutrinos (Especially dmckee's answer)
Given that
- Light from SN 1987A arrived 2 or 3 hours after its neutrinos, implying that it was "slowed down" relative to the neutrinos
- Light from SN Refsdal has been "lensed" multiple times to re-appear on time scales of several decades, implying that light interacts with matter (mass)
- Neutrinos interact extremely little with matter but are known to have mass and energy
Question
Why did neutrinos (with their mass and momentum) arrive before the light (considered to be massless) from SN 1987A? Considering SR and GR, this seems to be a contradiction. What am I missing?
Postscript
I've tried desperately to avoid using the word "photon" above (in reference to light) after learning of the Lamb Controversy™ (via related discussions here and here on Phys SE).
Best Answer
Both neutrinos and photons were produced in the core of the star but photons have a much stronger probability of interacting with the outer layer of the star than the neutrinos. Thus the photons were trapped whereas the neutrinos easily escaped. This has nothing to do with mass and all to do with the cross-section of interaction with protons/electrons for photons on one hand and for neutrinos on the other.
Reading @dmckee's answer made me realise that the phrasing of the previous paragraph makes it sound like the light flash we observe might be due to those photons eventually escaping. This is not what I meant: it would take millions of years for those photons to escape, as is well known for our own Sun. It is only because the outer layers of the star are eventually blown off that we see a light flash.
I should also have pointed out that electron neutrinos can escape only in the early stages of the collapse of type II supernovae. As the density increases beyond a few times $10^{11} \text{g}\ \text{cm}^{-3}$, the scattering of neutrinos with stellar matter is sufficient to make the timescale of the diffusion of neutrinos out of the star shorter than the collapse timescale. This is a combination of increasing density (and therefore increasing interactions) and accelerating collapse. So the neutrino flash measured on Earth came from the very beginning of the evolution into a supernova.
Let me add some orders of magnitude. The cross-section of photon-electron scattering is of the order of $10^{-24} \text{cm}^2$. Compare this with the neutrino-nucleon scattering. It varies as the square of the neutrino energy:
$$\sigma_\nu \approx 10^{-44} E_\nu^2\ \text{cm}^2$$
with the energy in MeV. So that's 20 orders of magnitude, give or take.
Where does this huge difference come from? Neutrinos interact solely through the weak interaction whereas photons interact through the electromagnetic interaction with charged nuclei and electrons in the star plasma. So this is just a reflection of the relative strength of both interactions. There is no reason it should be like that: it is just the way our universe is! We would not be here to discuss these matters if it were not, actually…