As superpowers go, invisibility is an anomaly. Super-strength, X-ray vision, walking through walls: These are all fun. But invisibility is both ability and disability, blessing and curse, sport and misery. Turning invisible might be fun, but feeling invisible is definitely not. And for every fly-on-the-wall, catching-the-bad-guy-unawares romp you can imagine, there is an equal and opposite wretchedness: the seventh-grade dance you slunk out of, the staff meeting where the boss called you by the intern's name, the photo of your new haircut that didn't get even one measly "like."

Maybe these mixed emotions around invisibility are part of why we tend to get all lyrical about neutrinos. Neutrinos are as close to invisible as ordinary matter can get. They are subatomic particles so small that it took physicists almost 70 years to figure out that they had any mass at all; the precise measurement is still up for grabs. Neutrinos tear across the universe at almost the speed of light, with no need to slow down or change course, heedless of the magnetic fields that divert charged particles like electrons and protons. They cut through galaxies, planets and even people like you and me, unnoticed and unseen.


All of these qualities make neutrinos excellent long-distance messengers, but they also make them very hard to study, especially if you're interested in high-energy neutrinos coming from beyond the Milky Way. These neutrinos are rarer and more energetic than the ones made locally, inside the sun or in nearby supernovae.

That's where the IceCube Neutrino Observatory comes in.

A neutrino, having interacted with a molecule of ice, produces a secondary particle — a muon — that moves at relativistic speed in the ice,
A neutrino, having interacted with a molecule of ice, produces a secondary particle — a muon — that moves at relativistic speed in the ice, leaving a trace of blue light behind it. (Nicolle R. Fuller / National Science Foundation)

IceCube is actually an array of some 5,000 light detectors, strung like pearls and sunk into boreholes cut in the extraordinary clear ice near the Amundsen-Scott South Pole Station. Though many millions of neutrinos have passed uneventfully through the ice, every once in a while, a neutrino runs straight into an atomic nucleus somewhere in the observatory, creating a charged particle that makes a streak of light as it pushes through the ice.

I mentioned earlier that because they are electrically neutral, neutrinos don't change course when they encounter magnetic fields. That is one reason astrophysicists like them so much: In principle, researchers can follow neutrinos back to their origin and learn something about the cosmic processes that generate them.

But actually following that trail isn't so easy. Even though astronomers had tried to match IceCube's neutrino finds with known cosmic objects, chasing after them with a fleet of telescopes working across the electromagnetic spectrum, they had not been able to make a definite link.

Then, on Sept. 22, 2017, a neutrino came hurtling through IceCube, generating a subatomic particle called a muon, which in turn triggered the detectors. X-ray and gamma-ray telescopes quickly followed up and zeroed in on a likely source: a jet of particles shooting out of a supermassive black hole in a galaxy about 4 billion light-years from Earth. Because the jet is pointed in our general direction, the object is classified as a blazar (blazar TXS 0506+056, to be specific), and it happened to be "blazing" up a gamma-ray storm at the same time that the neutrino arrived.

That alone wasn't enough to convince the researchers that their neutrino came from TXS 0506+056. To build their case, they looked back over 10 years of IceCube data and found that, for a five-month period starting in 2014, they'd picked up extra neutrinos coming from the same spot in the sky. Combined, these two facts give astrophysicists more confidence that the neutrinos really are coming from the blazar.

That's pretty neat, but the bigger news is that the discovery demonstrates how astrophysicists can combine information from completely different cosmic "messengers" — in this case, neutrinos and electromagnetic waves — to create a deeper understanding of our universe.

So, is invisibility a blessing or a curse? As every superhero knows, disappearing is just a trick. Invisibility's real super power is its ability to reveal the visible world, unmask it and capture it like never before.

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