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At the South Pole, scientists are using the universe's tiniest and most fleeting particles to take a glimpse at our planet's interior. Ivan Semeniuk explains how they did it

What can carry the energy of a flying mosquito and pass through the entire Earth in a fraction of a second? Only a neutrino, the most ephemeral subatomic particle we know of in the universe.

On Wednesday, scientists behind a unique experiment embedded in the ice beneath this facility at the South Pole reported that for the first time they have used some of the highest energy neutrinos yet detected to sense the Earth's interior. The result, published in the journal Nature, suggests the method will offer new avenues for testing the laws of physics in a domain that researchers have not been able to access before. It may also reveal new information about our planet's structure and composition.

"It's sort of like taking a neutrino X-ray of the Earth," says Darren Grant, a physicist at the University of Alberta and spokesperson for the experiment, which is called IceCube.

Image: Ian Rees, IceCube/NSF

The experiment uses thousands of light-sensitive detectors that have been sunk up to 2.8 kilometres into the Antarctic ice sheet. The detectors hang on long strings that convey information back up to the surface. At great depth, the ice is essentially as clear as glass so the detectors can sense faint bursts of light produced when neutrinos collide with atoms anywhere within the volume of the vast, one-cubic-kilometre experiment.

There are plenty of flashes for IceCube to sense. The ones scientists pay attention to are the ones caused by particles coming up through the Earth's interior. Those are the neutrinos. They are generated on the other side of the planet when high-energy cosmic rays come tearing into the Earth's atmosphere. The cosmic rays produce cascades of particles that are all stopped when they plow into the ground, except neutrinos, which can easily pass through the Earth. Essentially, IceCube uses the Earth as a filter so that other kinds of particles don't drown out the neutrino signal.

Image: Jamie Yang, IceCube Collaboration

After the neutrinos speed through the Earth and come out the other side, a few will occasionally smash into an atom in the ice near the IceCube experiment. The collision generates a telltale cone of light that indicates the direction the neutrino was originally coming from.

Scientists found that fewer neutrinos were coming from directly beneath the experiment compared with various angles from the side. This allowed them to calculate how likely it is that the solid rock and metal that make up the Earth can stop a neutrino – an extremely difficult measurement to make since neutrinos mostly pass through everything they encounter. The experiment was done using the highest energy neutrinos, which are more likely to interact with other forms of matter. Because high-energy neutrinos are rare, the measurement could only be made with an experiment as large as IceCube.

So why does all of this mean? Particle physicists are on the hunt for new laws of nature that can explain our universe. One way to do that is to watch how particles behave under extreme circumstances and see whether they diverge from existing theories that predict how they ought to behave. Because they are carrying some of the energy of the cosmic rays that generated them, the neutrinos measured in this experiment are far more powerful than any particle accelerator on Earth could hope to generate. In time, Dr. Grant and his colleagues hope to push the experiment to even higher energies.

Meanwhile, the accumulating data from IceCube, which has been operating since 2010, may eventually provide a more detailed and direct picture of the Earth's interior.

"The ability to some day use this technique to refine our understanding of the planet we live on – that's just incredible," Dr. Grant said.

Image: NSERC

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