0:00:06.638,0:00:10.134
They're everywhere, [br]but you will never see one.

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Trillions of them are flying [br]through you right this second,

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but you can't feel them.

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These ghost particles are called neutrinos[br]and if we can catch them,

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they can tell us about [br]the furthest reaches

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and most extreme environments [br]of the universe.

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Neutrinos are elementary particles,

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meaning that they can't be subdivided[br]into other particles the way atoms can.

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Elementary particles are the smallest[br]known building blocks

0:00:35.659,0:00:37.712
of everything in the universe,

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and the neutrino is one [br]of the smallest of the small.

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A million times less massive [br]than an electron,

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neutrinos fly easily through matter,[br]unaffected by magnetic fields.

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In fact, they hardly ever [br]interact with anything.

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That means that they can travel[br]through the universe in a straight line

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for millions, or even billions, of years,

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safely carrying information [br]about where they came from.

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So where do they come from?

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Pretty much everywhere.

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They're produced in your body[br]from the radioactive decay of potassium.

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Cosmic rays hitting atoms [br]in the Earth's atmosphere

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create showers of them.

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They're produced by nuclear [br]reactions inside the sun

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and by radioactive [br]decay inside the Earth.

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And we can generate them [br]in nuclear reactors

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and particle accelerators.

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But the highest energy neutrinos[br]are born far out in space

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in environments that [br]we know very little about.

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Something out there,[br]maybe supermassive black holes,

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or maybe some cosmic dynamo[br]we've yet to discover,

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accelerates cosmic rays to energies[br]over a million times greater

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than anything human-built[br]accelerators have achieved.

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These cosmic rays, [br]most of which are protons,

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interact violently with the matter[br]and radiation around them,

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producing high-energy neutrinos,

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which propagate out [br]like cosmic breadcrumbs

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that can tell us about the locations

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and interiors of the universe's most[br]powerful cosmic engines.

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That is, if we can catch them.

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Neutrinos' limited interactions[br]with other matter

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might make them great messengers,

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but it also makes them [br]extremely hard to detect.

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

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and wait for a neutrino to reveal itself

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by colliding with the nucleus of an atom.

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That's what's happening [br]in Antarctica at IceCube,

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the world's largest neutrino telescope.

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It's set up within [br]a cubic kilometer of ice

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that has been purified by the pressure

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of thousands of years [br]of accumulated ice and snow,

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to the point where it's one [br]of the clearest solids on Earth.

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

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most of the cosmic neutrinos racing[br]through IceCube will never leave a trace.

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But about ten times a year,

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a single high-energy neutrino[br]collides with a molecule of ice,

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shooting off sparks of charged[br]subatomic particles

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that travel faster through the ice[br]than light does.

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In a similar way to how a jet[br]that exceeds the speed of sound

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produces a sonic boom,

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these superluminal charged particles[br]leave behind a cone of blue light,

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kind of a photonic boom.

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This light spreads through IceCube,

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hitting some of its detectors[br]located over a mile beneath the surface.

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Photomultiplier tubes amplify the signal,

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which contains information about [br]the charged particles' paths and energies.

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The data are beamed [br]to astrophysicists around the world

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who look at the patterns of light

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for clues about the neutrinos [br]that produced them.

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These super energetic collisions[br]are so rare

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that IceCube's scientists give each[br]neutrino nicknames,

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like Big Bird and Dr. Strangepork.

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IceCube has already observed

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the highest energy [br]cosmic neutrinos ever seen.

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The neutrinos it detects should finally[br]tell us where cosmic rays come from

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and how they reached [br]such extreme energies.

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Light, from infrared,[br]to x-rays, to gamma rays,

0:04:06.205,0:04:08.365
has given us increasingly energetic

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and continuously surprising [br]views of the universe.

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We are now at the dawn[br]of the age of neutrino astronomy,

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and we have no idea [br]what revelations IceCube

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and other neutrino telescopes may bring us

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about the universe's most violent,[br]most energetic phenomena.