Hi, Dr. Z – I read something about neutrino astronomy the other day. But I thought that neutrinos can pass through our whole planet – how do astronomers even see them? And what do they learn from them?
You know, you’re right – every now and again there’s a story in the news about some wildly improbable type of telescope and it always seems to be a way to look for neutrinos. At the South Pole astronomers have drilled holes more than a mile deep into the ice cap and installed sensitive light detectors; in the Mediterranean Sea astronomers have put detectors deep underwater; in Japan and Canada and the US scientists have put neutrino detectors deep underground. But saying that neutrinos can pass through our entire planet is actually a bit of an understatement – neutrinos can pass through as much as five trillion miles of lead and still have a good chance of emerging out the far side. Neutrinos are hard to detect and, because of this, the instruments used to “see” them are among the largest scientific instruments ever built. But what we can learn from them…it’s worth it!
To start, neutrinos are subatomic particles that barely interact with matter at all. And – yes – most neutrinos zip right through the Earth without ever interacting with anything. What that means is that any single neutrino has a vanishingly small chance of interacting with any of the atoms that make up our planet. But if we have a LOT of neutrinos that are passing by a LOT of atoms, we’ll get a few interactions – the occasional neutrino will strike the occasional atom, giving off a little flash of light. And if we can catch that tiny flash of light, we can start to do some work! We can figure out where the neutrino came from, how much energy it has, how it came to be produced, and more. In other words – we can use neutrinos to learn more about the universe, and that’s what astronomy is all about.
Here are some of the basics – in addition to being tremendously difficult to detect, neutrinos are among the most common particles in the universe. Every time two atoms fuse together in our Sun a neutrino is emitted; for that matter, every time a radioactive atom emits a beta particle a neutrino is emitted as well. When a star explodes, about 1058 (that’s a 1 followed by 58 zeros (ten billion trillion trillion trillion trillion) neutrinos will be given off. But, because neutrinos are so unlikely to strike an atom in one of our cells, neutrinos from even a star that explodes near to us are unlikely to cause us any harm at all.
When a star explodes, most of the hydrogen atoms (which are simply protons) in the star will absorb an electron and turn into neutrons, forming a neutron star. Each one of them emits a neutrino – that’s where all the trillions of trillions of trillions of trillions of neutrinos come from. Those neutrinos pass through the outer layers of the dying star and through the material blasted off by the explosion; and they pass through all the gas and dust and everything else between the star and the Earth, reaching us virtually undiminished in number. And when they reach Earth, they will encounter the atoms in the ocean and the ice and a vanishingly small fraction will actually interact with an atom…and when that happens, a tiny flash of light will be given off. It’s that tiny flash of light that the astronomers are waiting for – and it’s detected by highly sensitive light sensors that are strung through the ice – or in the ocean’s depths or in salt mines deep underground.
Detecting neutrinos is sort of like playing the lottery – the odds of seeing any individual neutrino are horrible. But the more tickets you buy (or the more atoms you put out there) the better the chance that you’ll get a “hit.” With the lottery, that means you’ve got a better chance of hitting the lottery if you buy a LOT of tickets; with neutrino astronomy that means you need to put a LOT of atoms out there for neutrinos to interact with. There’s a neutrino telescope in the Mediterranean Sea that’s a cubic km in size – it contains about 1015 (about 1 trillion liters) of water; enough so that a few of the torrent of neutrinos from an exploding star might strike an atom, giving off a few photons that will be detected, marking the transit of a ghostly particle produced hundreds or thousands or hundreds of thousands of light years away.
Here’s what’s cool about neutrinos – because they barely interact with matter, they zip through things like they’re not there. Believe it or not, it can take a photon up to 10,000 years to make its way from the Sun’s core to the surface – the sunlight we see today…most of those photons were created around the time humans were learning to be farmers, to make shoes, to use copper, and to start living in villages. The neutrinos we detect from the Sun though? They left the Sun’s core about eight minutes ago, giving us information on what’s happening in the core of the Sun in near-real-time. So that’s how neutrinos can give us information about a nearby well-behaved star…what about more distant stars when they get a bit testy?
Remember all those neutrinos that are given off by supernovae? Every proton that merges with an electron produces a neutrino, which flies into space at very nearly the speed of light. The visible light, ultraviolet, and even gamma radiation from the exploding star are all blocked by the star’s outer layers as they’re blasted into space – the neutrinos, though, cut right through all of that, reaching us before the flash of light from the detonating star. It’s not that neutrinos are breaking the cosmic speed limit – it’s just that they’re not delayed by having to filter through all of the star’s outer layers. In the case of the 1987 supernova that went off in the Large Magellanic Cloud, the neutrinos arrived about three hours before we first saw the star begin to brighten. And after travelling for just under 170,000 years, a few dozen of those neutrinos ran into a handful of atoms in neutrino detectors in Japan, the US, and in the Caucasus Mountains where the brief pulses of light lit up some photomultiplier tubes in a pattern that told astronomers they’d caught neutrinos.
January 1987 was the birth of neutrino astronomy; since then the detectors have become larger, more complex, and more sensitive. The IceCube telescope, for example, laces the deep Antarctic ice with more than 5000 sensors sunk more than a mile deep in the ice. With so many detectors, the astronomers not only detect the neutrinos, but can also tell which direction it was traveling and how much energy it carries – a surprising number have already passed through the Earth, exiting our planet through the IceCube array. Similarly huge arrays of light detectors have identified neutrinos as originating around the massive black hole at the center of our galaxy, from star-forming regions of other galaxies, and other high-energy events in our part of the universe. But even so, more than three quarters of the neutrinos detected come from unknown sources. With luck, as the new telescopes come on line this fraction will go down.
References:
- Header Image Reference: IceCube Neutrino Observatory in 2023 by Christopher Michel. Taken on 6 January 2023, 07:21:57. Source – Cmichel67 – https://commons.wikimedia.org/wiki/User:Cmichel67, https://commons.wikimedia.org/wiki/File:IceCube_Neutrino_Observatory_in_2023_02.jpg