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  • They're everywhere, but you will never see one.

  • Trillions of them are flying through you right this second,

  • but you can't feel them.

  • These ghost particles are called neutrinos and if we can catch them,

  • they can tell us about the furthest reaches

  • and most extreme environments of the universe.

  • Neutrinos are elementary particles,

  • meaning that they can't be subdivided into other particles the way atoms can.

  • Elementary particles are the smallest known building blocks

  • of everything in the universe,

  • and the neutrino is one of the smallest of the small.

  • A million times less massive than an electron,

  • neutrinos fly easily through matter, unaffected by magnetic fields.

  • In fact, they hardly ever interact with anything.

  • That means that they can travel through the universe in a straight line

  • for millions, or even billions, of years,

  • safely carrying information about where they came from.

  • So where do they come from?

  • Pretty much everywhere.

  • They're produced in your body from the radioactive decay of potassium.

  • Cosmic rays hitting atoms in the Earth's atmosphere

  • create showers of them.

  • They're produced by nuclear reactions inside the sun

  • and by radioactive decay inside the Earth.

  • And we can generate them in nuclear reactors

  • and particle accelerators.

  • But the highest energy neutrinos are born far out in space

  • in environments that we know very little about.

  • Something out there, maybe supermassive black holes,

  • or maybe some cosmic dynamo we've yet to discover,

  • accelerates cosmic rays to energies over a million times greater

  • than anything human-built accelerators have achieved.

  • These cosmic rays, most of which are protons,

  • interact violently with the matter and radiation around them,

  • producing high-energy neutrinos,

  • which propagate out like cosmic breadcrumbs

  • that can tell us about the locations

  • and interiors of the universe's most powerful cosmic engines.

  • That is, if we can catch them.

  • Neutrinos' limited interactions with other matter

  • might make them great messengers,

  • but it also makes them extremely hard to detect.

  • One way to do so is to put a huge volume of pure transparent material in their path

  • and wait for a neutrino to reveal itself

  • by colliding with the nucleus of an atom.

  • That's what's happening in Antarctica at IceCube,

  • the world's largest neutrino telescope.

  • It's set up within a cubic kilometer of ice

  • that has been purified by the pressure

  • of thousands of years of accumulated ice and snow,

  • to the point where it's one of the clearest solids on Earth.

  • And even though it's shot through with boreholes holding over 5,000 detectors,

  • most of the cosmic neutrinos racing through IceCube will never leave a trace.

  • But about ten times a year,

  • a single high-energy neutrino collides with a molecule of ice,

  • shooting off sparks of charged subatomic particles

  • that travel faster through the ice than light does.

  • In a similar way to how a jet that exceeds the speed of sound

  • produces a sonic boom,

  • these superluminal charged particles leave behind a cone of blue light,

  • kind of a photonic boom.

  • This light spreads through IceCube,

  • hitting some of its detectors located over a mile beneath the surface.

  • Photomultiplier tubes amplify the signal,

  • which contains information about the charged particles' paths and energies.

  • The data are beamed to astrophysicists around the world

  • who look at the patterns of light

  • for clues about the neutrinos that produced them.

  • These super energetic collisions are so rare

  • that IceCube's scientists give each neutrino nicknames,

  • like Big Bird and Dr. Strangepork.

  • IceCube has already observed

  • the highest energy cosmic neutrinos ever seen.

  • The neutrinos it detects should finally tell us where cosmic rays come from

  • and how they reached such extreme energies.

  • Light, from infrared, to x-rays, to gamma rays,

  • has given us increasingly energetic

  • and continuously surprising views of the universe.

  • We are now at the dawn of the age of neutrino astronomy,

  • and we have no idea what revelations IceCube

  • and other neutrino telescopes may bring us

  • about the universe's most violent, most energetic phenomena.

They're everywhere, but you will never see one.

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TED-ED】なぜニュートリノは重要なのか - シルビア・ブラボー・ガラート (【TED-Ed】Why neutrinos matter - Sílvia Bravo Gallart)

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    稲葉白兎 に公開 2021 年 01 月 14 日
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