High-energy subatomic particles nicknamed "ghost particles" for their ability to pass through just about anything can be stopped, scientists have confirmed.
That doesn't require kryptonite or any other special substance — scientists have observed some high-energy neutrinos being blocked and absorbed by the Earth itself as they zip through the planet from the atmosphere or from deep space, reports the international "IceCube" research collaboration in a new paper published today in the journal Nature.
Currently, the IceCube Collaboration includes 300 people from 49 institutions in 12 countries, including Canada.
The team's observations came from a massive one-cubic-kilometre block of ice filled with sensors buried more than a kilometre in the ice of Antarctica. It was completed in 2010.
"This achievement is important because it shows, for the first time, that very-high-energy neutrinos can be absorbed by something," said Penn State University physics and astronomy professor Doug Cowen, a member of the collaboration, in a statement.
He added that lower energy neutrinos "pass through just about anything," and although theory suggested that higher energy neutrinos might not be quite as ghostly, "no previous experiments had been able to demonstrate convincingly that higher-energy neutrinos could be stopped by anything."
Passing through you
Neutrinos are neutral subatomic particles with a very small mass that generally pass through matter — including objects, people and even planets — without leaving a trace. That's how they came to be known as "ghost particles," says Darren Grant, a University of Alberta physicist and spokesman for the IceCube collaboration.
Huge quantities of low-energy neutrinos are spewed by the sun, nuclear reactors and particle accelerators each day (and trillions pass through your body each second at the speed of light without you noticing).
Neutrinos with a million times more energy are generated by the interaction of cosmic rays with atoms in the Earth's atmosphere and by cataclysmic events in space like supernovas and black holes. Those high-energy neutrinos are much rarer and therefore harder to detect and study.
In fact, the very first detection of neutrinos from deep space was made by IceCube just five years ago. Those came at the detector from the sky above.
Now, researchers have measured neutrinos coming at the detector from below after passing through the Earth.
"There's no other particle that can travel all the way through the Earth and then enter from below," Grant said.
By calculating the neutrinos' energies and trajectories and comparing them with predictions, researchers found that they detected fewer high-energy neutrinos coming from straight through the Earth, and more coming from shallower angles (where they would not have passed through as much of the Earth), suggesting that the Earth was absorbing a certain amount.
As expected, higher energy neutrinos were more likely to interact with the Earth. That's because the interaction requires an intermediary particle that needs a lot of energy to generate, and low-energy neutrinos seldom have enough.
Based on the data, the researchers calculated the probability that neutrinos with different energies will interact with matter in the Earth as they pass through.
'X-raying' the Earth
In the future, that information could be used to make a detailed picture of the inner Earth, using neutrinos instead of X-rays, Grant said. When you take an X-ray image, your bones show up and your skin and flesh are transparent because the X-rays are absorbed by the bones, but pass right through the flesh. Similarly, denser areas in the Earth, such as the solid core that's thought to exist, will absorb more neutrinos than the surrounding layers of molten liquid and should appear less transparent.
IceCube is a block of extremely clear ice one cubic kilometre in size and 1.5 to two kilometres below the surface, where it is very dark and high pressures keep the ice clear and bubble-free. The ice is embedded with 5,160 sensors that detect very faint amounts of light.
Neutrinos are detected when they interact with the ice itself as they pass through the detectors. When that happens, Grant said, they produce a charged particle such as an electron. Typically, light travels fastest in a vacuum and more slowly in ice. But a particle produced by a neutrino interacting with the ice travels faster than the typical speed of light in ice.
When it does that, a burst of blue light, known as Cherenkov radiation, is produced. Grant described it as "almost the optical equivalent of sonic boom."
The higher energy the neutrino, the more ice it lights up. The researchers use that to detect each neutrino and calculate its energy.