What is a ghost particle?

China is building the world’s largest “ghost particle” detector below the South China Sea. But what is it exactly?

Understanding neutrinos could help solve the mystery of the origins of cosmic rays. | Photo Credit: Image: Mark Garlick/Science Photo Library/IMAGO

China is building an enormous telescope in the western Pacific Ocean. Its job will be to detect “ghost particles”, also known as neutrinos. The telescope will be the largest of its kind, scientists say in an article published in Nature earlier this month. Here’s everything you need to know about neutrinos.

What is a ghost particle?

In order to understand what a ghost particle or neutrino is, you need to understand the importance of atoms. Atoms make up our universe. Anything that has mass—you, me, the book on the shelf next to you and the cup of coffee you’re drinking from — is made up of atoms.

For a long time, scientists thought atoms were the smallest particle in existence—before discovering that they are themselves comprised of even tinier “subatomic” particles: protons (which have a positive charge), electrons (negative charge), and neutrons (no charge).

Neutrinos are a type of electron but, like neutrons, they do not have any charge. They are among the most abundant particles in our universe — with trillions of neutrinos passing through you at any given second—and also among the tiniest. Neutrinos were long believed to be massless, until scientists found evidence that they do have a very small mass.

Neutrinos’ weak charge and almost nonexistent mass have made them notoriously difficult for scientists to observe. They can only been “seen” when they interact with other particles. The rarity of interactions with other particles makes them almost impossible to track. That’s why they’re called ghost particles—the vast majority skirt around undetected.

How do scientists detect ghost particles?

Ghost particles rarely interact with other particles. But rarely doesn’t mean “never”. Sometimes they interact with water molecules, which is why China is building its ghost molecule telescope underwater.

Scientists have observed ghost particles in fleeting instances when the particles create byproducts after traveling through water or ice. These “muons” create flashes of light that can be detected by sophisticated underwater telescopes and offer one of the fews ways to study the energy and source of neutrinos.

Right now, the largest neutrino-detecting telescope is the University of Madison-Wisconson’s “IceCube” telescope. Situated deep in the Antarctic, the telescope’s sensors span around 1 cubic kilometer. China says its new telescope, called “Trident”, will span 7.5 cubic kilometers in the South China Sea.

Scientists say that its size will allow it to detect more neutrinos and make it “10,000 times more sensitive” than existing underwater telescopes. Construction has already begun and is slated to finish within this decade, scientists say.

Why does the detection of ghost particles matter?

Okay, you might be thinking, that all sounds fine, but what’s so special about neutrinos? Why should I care? Here’s the thing—scientists don’t really know why the massively abundant neutrinos act the way they do. They defy established rules of physics.

And it’s not clear where the particles come from. Scientists think they might have played a role in the early universe, right after the big bang. But this is just a hypothesis, nothing they’ve been capable of proving yet.

It’s thought that a sound understanding of neutrinos will help solve a number of scientific mysteries—like the origin of the mysterious cosmic rays, which are known to contain neutrinos. Researchers believe that understanding the source of neutrinos will enable them to explain the origins of cosmic rays—something scientists have been trying to do for centuries.

There’s evidence that neutrinos are essential for understanding the origins of our universe. The construction of Trident is set to bring us one step closer to that knowledge.