60 points by extraterra 4 months ago
Lengthy discussion: https://news.ycombinator.com/item?id=18081920
I don't know enough on this topic, so kindly pardon my ignorance, but while I'm aware that neutrino detectors can pick up particles punching through the ground, that's a few dozen meters of ground tops, is it not?
For a particle to survive tens of thousands of miles through dense rock and come out the other side and register with the detector makes me ask:
1. Just how many particles are discharged by the event and/or passing through the earth at the time of the reading?
2. What's so special about the composition of the detector that it's able to pick up particles that have not been stopped by the entirety of the Earth?
I'm not asking these questions dismissively. I'm keen to learn. I'm also aware I'm making many assumptions here. Please question or contradict all of my assumptions if possible.
I am not sure about what particles you are specifically asking, but the neutrino flux on Earth for example is on the order of billions per square centimeter and second. And there is nothing really special about the detector material, they just detect the tiny fraction of particles that happens to interact by pure chance within the detector while most particles just pass through the entire detector unnoticed.
Tens of thousands of miles? The planet's a little less than 8000 miles in diameter. As for "a few dozen meters", the Sudbury Neutrino Observatory was 2.1 kilometres down (at the nominal 6800 foot level in the old Creighton Mine).
Don't know why I thought it was 12. Good catch.
Neutrinos are weird little things, and pass clean through any sort of matter almost every time. The famous statistic is that you'd need a light-year of solid lead just to block half of all neutrinos.
What surprises me about this article is that catching neutrinos tunneling up through the earth is considered surprising. I had thought that neutrino detectors generally caught just as many neutrinos from below as from above; the earth is essentially transparent to them.
To answer your question #2: the detector is no better than anything else at stopping neutrinos (just as detecting their stoppage), but--even though the chance of any given neutrino stopping anywhere in the earth is almost nil--there are so very very many of them that the probabilities add up to a decent number impacting within the detector.
Neutrino detectors are mainly looking for (neutrino) events coming from below in order to avoid noise from cosmic rays coming from above.
The surprising part is if other particles are observed coming in from below.
These detectors are underground presumably to remove "noise" from other sources - sorry, I have no source for this, it's just a memory from a physics professor in high school describing such a facility in Italy (Gran Sasso).
> What surprises me about this article is that catching neutrinos tunneling up through the earth is considered surprising. I had thought that neutrino detectors generally caught just as many neutrinos from below as from above; the earth is essentially transparent to them.
This is what I thought too, but apparently, the Earth is opaque to neutrinos at energies above 1 PeV. (That is, the Standard Model predicts so.)
The article states "The particles had energies that were high enough for the Standard Model to prohibit that kind of careless disregard for matter". Although I think it could have been explained better, explanation is there.
The ANITA detector itself does not detect neutrinos. It detects showers of secondary radiation  that result from a diminutive portion of the total neutrino flux interacting with the large test mass that is a million cubic kilometers of Antarctic ice.
> What's so special about the composition of the detector that it's able to pick up particles that have not been stopped by the entirety of the Earth?
With enough particles, and enough time, it'll happen eventually at every point.
This seems plausible to me. It’s something that has been hypothesized for a while, it’s not proposing anything implausible like ftl neutrinos, and the effect seems small enough to explain why we haven’t seen them before.
It’s very exciting.