Detecting a neutrino at edge of world


By Tom Burns - Stargazing



At the South Pole, one of the harshest places on Earth, is one of our planet’s strangest observatories. IceCube Neutrino Detector is the equivalent to a telescope 20 kilometers wide. It was created by using pressurized hot water to create 86 shafts downward into the ice. Many of the shafts are more than a mile deep. Before the water refroze, 60 optical detectors were lowered into each of the shafts. The detectors are spread out over a square kilometer, but they are sensitive enough to detect a neutrino interaction with the ice 10 kilometers away.

Given the cold, the high winds, and the storms at the South Pole, one can only imagine the difficulty in creating the IceCube. The South Pole sits at 10,000 feet above sea level. Temperatures during the winter average -100 degrees Fahrenheit, but it seems much colder because of the high winds.

Even wearing complex protective gear, workers, technicians, and astronomers alike can spend only short periods outside. Their only access to supplies, crew replacements, and relief in case of emergency is through a ski-equipped Lockheed LC-130 Hercules.

During the winter months, which span from February – November, the cold, high winds, and high altitude prevent the planes from taking off and landing. Thus, for long stretches, the astronomers are isolated from the world. Physicians are on staff, but during any significant medical emergency, there’s no chance of reaching a hospital.

So why do astronomers endure such dangers? The IceCube is designed to detect an elusive subatomic particle called a neutrino. On a cold September day in 2017, it at long last detected one — a single neutrino. That’s why.

Their joy will perhaps require a bit of explanation.

Deep in the center of our sun and all the stars in the heavens, hydrogen atoms are forced together under enormous temperature and pressure. Hydrogen fuses into helium in what we have come to call the thermonuclear reaction, the same reaction that powers stars and our hydrogen bombs.

The energy released is in the form of light, which exists as subatomic particles called photons. Photons travel at the speed of light as they move through space, but they move much more slowly as they pass through the star’s substance. A photon interacts with the atoms of mostly hydrogen that make up the star and are lost in the bubbling, heaving conflagration.

After millions of years, the energy finally works its way to the surface of the star, and photons of light, finally free of impediments, erupt at the speed of light into space.

When those photons finally reach another chunk of matter, like the surface of the Earth, they are lost again in the interaction. The light from the sun striking your skin is converted to heat. The light you see from a distant star is converted to electrical impulses that are stored in your brain as a memory.

Besides helium and photons, another subatomic particle is a byproduct of the thermonuclear conflagration. Tiny, massless particles called neutrinos stream outward from the sun’s center. But neutrinos don’t interact with other matter. They stream unimpeded through a star at the speed of light. Like photons, they travel at the speed of light as they move through space.

When neutrinos finally strike a solid surface, they pass through it as if the surface weren’t there at all. Every second, you have trillions of them passing through your body and affecting it not at all.

On the level of the large, where we live, it seems odd indeed that anything could pass through anything undetected, but it’s a simple certainty of subatomic life.

In fact, we experience such transparency, or the lack of it, all the time. Glass is transparent to the parts of the energy spectrum that we call visible light. Those photons pass right through the glass, so our energy-detecting eyeballs can see through it.

However, the glass is not as transparent to the higher-energy ultraviolet part of the energy spectrum. That’s why you’ll get sunburned on your arm if you hang it out the window while you’re driving, but you won’t get sunburned with the window closed.

The walls of your house stop the photons of light, but they let at least part of the radio energy through. Thus, if you sit in a darkened room, you can’t see anything, but you can listen to the radio.

To a neutrino, everything — from the wall to your body to a pane of glass — is transparent.

Because they interact only very occasionally with the rest of the universe, neutrinos are exceedingly difficult to detect, let alone understand. Yet understanding neutrinos is one of the keys to understanding the explosive forces that drive our universe and perhaps how those forces began to function in the first place.

And thus it was that on Sept. 22, 2017, a single neutrino, one of uncountable trillions every second, passed into the expanse of ice that constitutes the IceCube Observatory. It had traveled almost four billion light years to get there. (One light year is equivalent to about 6 trillion miles.)

For the past 3.9 billion years, the neutrino traveled from its point of origin, passing through any impediment along the way as if it were not there at all. But at some point deep under the surface of the ice, it collided with an ice molecule. The product of the collision was a muon, a type of subatomic particle similar to an electron but heavier.

Everything in the universe, neutrinos, muons, and photons alike are limited in their velocity by the speed of light (186,000 miles/ second). However, the speed of particles as they travel through any substance, like glass or ice, is much slower.

The newly created muon was traveling faster than the local speed of light and thus created a brief eerie blue glow called Cherenkov radiation. The detectors in the IceCube detected that blue glow.

The process is much like the one that creates a sonic boom. As a jet accelerates toward the speed of sound, it forms a cone of densely packed air particles in front of it. As the jet breaks through the cone, it creates a shock wave that produces what we call a sonic boom.

As the muon reached and exceeded the local speed of light, it created a shockwave of photons, which the IceCube detectors saw as the eerie blue glow of Cherenkov radiation.

By measuring the direction the photon was traveling, astronomers can tell from which distant point it came. By measuring the intensity of the blue light, they determine how much energy the initial neutrino possessed.

The neutrino in question was far from the ordinary type created in the center of a hydrogen-bomb star. It formed in an object called TXS 0506+0564, an enormously high-energy source almost four billion light years away.

That distance is far beyond the boundaries of our Milky Way galaxy. That fact makes the neutrino the first one ever detected outside our cosmic environs. It is also far more energetic than a neutrino formed in the relatively sedate thermonuclear forge found inside a single star.

TXS 0506+056 is, in fact, a blasar, one of the most powerfully energetic places in the universe. Blasars happen mostly in newborn galaxies with trillions of stars.

At the center of TXS 0506+056 is an enormous black hole. As material from the galaxy is gravitationally sucked toward the black hole, a super-hot soup of subatomic particles is created in the cosmic vortex.

Before they can be sucked into the black hole completely, some of those particles are blasted back into space as enormously powerful jets.

One of those jets happened to be pointed toward Earth in such a way that a single neutrino collided with an ice molecule in just the right way to produce a faint blue glow.

We live in an odd place in the universe, and we experience it in such a limited way. Our human lives seem so complex, but our lives pale in comparison to the inconceivable complexity on the subatomic level in a single speck of dust.

When we created hydrogen bombs, we harnessed the power of a star. However, even a star is a tiny spark compared to the conflagration in a blasar like TXS 0506+056.

And how do we know? A single neutrino tells us so.

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By Tom Burns

Stargazing

Tom Burns is the former director of the Perkins Observatory in Delaware.

Tom Burns is the former director of the Perkins Observatory in Delaware.