Hacker News Reader

" - When a neutrino, rarely, collides with an atom, it produces charged particles. In ice - not vacuum! - certain particles can travel faster than light."

That confused me for a moment so it seems worth clarifying: in ice, certain particles can travel faster than light does in ice.

This is also the source of the classic blue glow in water cooled nuclear reactors. Electrons move through the water faster than light and leave behind a kind of 'sonic boom' of photons in its wake.

Thank you, I lost my mind, briefly, reading that part.

Light slows down in dielectric materials because as an em wave it interacts with the polarization of molecules. Neutrino has very little interaction with matter, OTOH.

https://news.ycombinator.com/item?id=49977885

>Neutrinos give us a pristine snapshot into the origin of the universe. They have travelled billions of years and trillions of miles without interacting with anything. Unless...we catch them!

I assume this is not a hard, totalizing law since it seems we're able to get samples of neutrino collisions in just a kilometer sized chunk of ice on Earth (meaning that the probability of collision is not absolutely zero and there's no way to know the full neutrino travelogue through the universe)

Correct, they do have a probability of interacting, but it's extremely low, depends of the energy of the neutrino (its momentum) but basically it can go through 1 light year of lead and still have only a 50/50 chance of interacting.

to put things to scale 1 ly is 9,460,730,472,580,800 meters, or 63,241.077 astronomical units, the distance from the Earth to the Sun.

Needless to say, it's pretty weakly interacting as far as interaction goes...

> a cubic kilometer of ice into a neutrino detector. How? Scientists drilled boreholes 2.5km deep into the ice

Seems odd that boring 2.5km into ice would protrude 1.5km out the other side of a cubic km.

A cubic kilometer is a volume, it does not dictate the shape.

The upper layers of detectors are buried over a kilometer below the surface in order to shield the array from most cosmic rays.

If they drilled down any further they'd reach the interior of the hollow earth, obviously not permitted

We wouldn't want to cause a leak!

> neutrinos coming from outside the solar system How do we know this and why do we care where they came from?

Since neutrinos don’t interact with much of anything, its path doesn’t deviate from its origin. Different origins emit neutrinos with different levels of energy. When a neutrino hits a nucleus, the blue light emitting particle in ice follows closely the same direction. Measuring the direction and the magnitude (lighting energy) of the vector we can classify its origin. Collecting a bunch of the same signature we know there's an object at that direction emitting at that energy level. From the vector and energy we kind of know whether it’s from the solar system or not.

Identifying the origin let us see objects behind dust clouds or other obstacles.

> When a neutrino hits a nucleus, the blue light emitting particle in ice follows the same direction.

How does this not violate conservation of momentum? Are you saying every single collision is head-on?

Most paths of the high energy observable collisions follow a very close alignment to the original neutrino's path. The forward momentum of a high energy neutrino completely dominates the collision. Imagine an one-ton ball hitting a billiard ball. Doesn't matter what direction the billiard ball was going; the resulting path is the one-ton ball's path.

In other cases, the particle being collided is scattered and won't show up in the detector. Low energy neutrino collisions also have very faint light and difficult to detect. Solar system produced neutrinos are in this category.

We know this because

- In the beginning, for the first couple of events, only because they have way higher energies than anything in the solar system could produce - By now, with enough data collected, their origin correlates very well with the milky way - we are close to identifying several far away galaxies as well

This is the significance. Contrary to some statements published today, IceCube was not primarily built to study Neutrinos, it was built to study the universe using Neutrinos.

That's why we care where they are coming from, we want to learn about the astrophysical objects that produce them.

" in February 2023, [KM3NeT] detected the most energetic neutrino ever observed. It's called KM3-230213A, and its estimated energy was 220 PeV (220 x 10^15 electron volts or 220 million billion electron volts) "

- https://phys.org/news/2026-04-energetic-neutrino-primordial....

That's an insane amount of energy. If your are knowledgeable about this do you know why some neutrinos have more energy than others? surely it is just a relationship with it's speed. I thought all neutrinos moved close to the speed of light.

If my quick math is correct that is equivalent to dropping 3.59g one meter in a vacuum in earths gravity. insane that a subatomic particle can have that much energy.

Neutrinos interact only to the weak interaction and (IF they have rest mass) to gravity. (They have no charge.)

This article has some ideas on such an energy:

https://phys.org/news/2025-05-physicists-route-neutrinos.htm...

It says "typically, energetic neutrinos from active galactic centers are thought to originate from interactions between protons and photons, producing gamma rays of comparable intensity." and suggests other theorized sources.

Another good source on this new branch of astronomy:

https://arxiv.org/html/1501.07798v1

Newer idea (May 2026): blazars

https://www.sciencedaily.com/releases/2026/05/260523103912.h...

Particle physics processes in the sources we suspect accelerate particles in a way that results in a power law distribution of energies. So you can get extremely high energy particles, but the higher the energy the rarer.

And sources have a maximum energy they can accelerate particles to.

https://en.wikipedia.org/wiki/Fermi_acceleration

Didn’t the big bang theory show have an episode about this?

If it did they should get the Novel prize.

what is so special about ice?

Neutrinos are very weakly interacting and hence you need a massive amount of some matter through which they can pass so as to have a decent probability of getting an interaction.

Ice at kilometer level depths is exceptionally clear and transparent since the pressure (from layers above) removes any air bubbles and hence any Cherenkov radiation due to neutrino interaction does not get scattered. This means observations can be done clearly and precisely.

Finally, the rigidity of Ice can hold the sensors precisely and the depth guarantees that no unnecessary external noise from the surface can affect the measurements.

All the above are ideally met by the Ice at the south pole and hence the IceCube Neutrino Observatory was established there.

H2O is the key, not ice. Ice is nice because it's rigid, so you don't need a boat [1]. They're also working on making detector under the sea [2].

[1] https://www.amusingplanet.com/2015/07/the-surreal-world-of-n...

[2] https://news.cnrs.fr/videos/detecting-neutrinos-at-the-botto...

Among other things this ice is super transparent.

interesting