Scientists used "ghost particles" to measure Earth's mass



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The Earth is too large to fit on an X-ray or MRI scanner, so a group of physicists in Spain found a fascinating new way to look inside – they used the subatomic particles that were constantly flowing through the planet.

The team examined the Earth's interior using neutrinos – and even used them to conduct an entirely new measure of our planet's mass.

Neutrinos are weird little things. They are among the most abundant particles in the Universe, and are still hard to detect. They are similar to electrons, but they have no electrical charge and their mass is almost zero, so they interact very little with normal matter as they flow through the Universe at speeds close to that of light.

Billions of neutrinos are now going through your body. You can see why they are called "ghost particles".

Although historically our detection efforts have been weak, the IceCube neutrino detector in Antarctica has opened up a brave new world of neutrino science.

Using a year's data from the IceCube, physicists at the Institute of Corpuscular Physics (IFIC) in Valencia, Spain, were able to study the bowels of the Earth.

It worked almost exactly like an x-ray. When you are being x-rayed, the rays are sent through your body. They pass primarily through soft tissues such as muscles and organs. However, denser material, such as bone, absorbs the beams at a higher rate, so fewer X-rays hit the detector on the other side, producing an image of your skeleton.

Instead of X-rays, however, the team used atmospheric neutrinos, whose showers are created when energetic particles from space collide with Earth's atmosphere. These then extend through the Earth – but can be absorbed by the atomic nuclei of the material through which they pass.

The denser the material, the higher the absorption rate; which, in turn, allowed the research team to assemble a study of the density of the planet.

"The use of atmospheric neutrinos allows us to have neutrinos coming from all directions, with a wide range of energy and a known flow with sufficient accuracy," said IFIC physicist Sergio Palomares-Ruiz.

"The amount of absorption of the atmospheric neutrino flux depends on the amount of material traversed, as well as the energy of the neutrinos, so by studying the variation of the amount of absorption in different directions for neutrinos of different energy, we can determine the density distribution of Earth" .

neutrinos through the earth(Donini et al. / Physics of Natura)

The different angles are a big factor here. Some neutrinos traveled throughout the Earth's core, while others traveled at an oblique angle that completely ignored the nucleus.

Concentration and angle analysis gave the team the tools to calculate the density of the planet at various depths.

Traditionally, the density of the Earth is extrapolated from the way the seismic waves produced by earthquakes propagate. However, seismic waves can not penetrate the inner core.

"Neutrinos, on the other hand, go through all this by providing valuable information about the Earth's unknown core where the planet's magnetism is generated," said IFIC physicist Andrea Donini.

The team has learned nothing new about our planet. The density map of the Earth's interior, the mass of the planet and its moment of inertia were calculated using neutrino data – and all were consistent with previous measurements made by other means and less detailed.

But the aim of the research was not to learn something new about Earth – it was to learn something new about what we can learn from neutrinos. And in that score the results are really incredible.

"Our results demonstrate the feasibility of this approach to study the internal structure of the Earth, which is complementary to traditional methods of geophysics," the researchers wrote in their article.

At the moment, however, neutrino data are still relatively scarce; Researchers hope their research will encourage the release of more recent IceCube data as well as more neutrino science in the coming years.

The article was published in Physics of Nature.

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