ROBERT SIEGEL, Host:

Earlier this month scientists in the Midwest reported their first results from an experiment that sent subatomic particles through the Earth. The particles are called neutrinos, and once they are made they pass through just about everything without leaving a mark. That feat got NPR's David Kestenbaum wondering where they go.

DAVID KESTENBAUM: Neutrinos have always been seen as odd little beasts. The physicist who proposed them back in 1931 was almost sheepish about it. No one had ever observed one and at the time it was unclear if anyone ever would. Marvin Marshak is a physicist at the University of Minnesota.

MARVIN MARSHAK: We physicists, we're tough on the outside but we're sensitive on the inside. We don't like to be laughed at.

KESTENBAUM: If you want to try to understand the weird nature of neutrinos, do this experiment at home. Try to walk through the wall.

(SOUNDBITE OF SOMEONE HITTING A WALL)

KESTENBAUM: Well maybe it's not such a good experiment. The reason you can't walk through a wall is not that you're too big. You and I and the wall are mostly empty space. We're made of atoms and atoms are mostly nothing. There's a tiny nucleus and tiny electrons, so you might think you could just pass through. The reason you can't walk through a wall is that your atoms and the atoms in the wall interact with each other. They speak the same language. They push on each other because they have electric charge. Neutrinos have almost none of the usual atomic qualities. They don't carry an electric charge, they weigh almost nothing.

MARSHAK: Really, really light yeah.

KESTENBAUM: So neutrinos have kind of a sad life. Once a neutrino has popped into existence it has almost no way to touch the physical world again.

MARSHAK: They're like ghosts slipping through the night.

KESTENBAUM: The only chance neutrinos have to mingle is through this force called the weak interaction, which most of the time is too feeble to do anything. So the number of these lonely neutrinos is probably increasing. The Big Bang made a bazillion, that's not the real number. Nuclear reactors make them, the sun pumps out neutrinos. We are soaking in them. Marshak and his colleagues are making them, too. =They use a big machine called a particle accelerator. It's underground in Illinois and it pumps out bursts of neutrinos moving at the speed of light. That's not the real sound. They fly down a long tunnel like tiny bullets, and at the end there's a wall. Someone put up a funny sign saying Minnesota with an arrow because that's the direction the neutrinos are headed, right through the wall.

MARSHAK: The neutrinos just keep going. They go under Wisconsin, a little bit of east of Madison, under Lake Superior and into Minnesota.

KESTENBAUM: Where there is an old iron mine. About a half-mile down scientists have built a big iron detector. But even though a lot of neutrinos hit the detector...

MARSHAK: A million, billion a day.

KESTENBAUM: They only detect a few.

MARSHAK: Two, three.

KESTENBAUM: And on a bad day?

MARSHAK: None.

KESTENBAUM: They're pretty hard to catch.

MARSHAK: Pretty hard to catch, yeah.

KESTENBAUM: So a handful of lucky neutrinos make their mark at the bottom of the mine, but most do not.

MARSHAK: They all just keep going. They come out of the earth somewhere just south of the U.S. Canadian border and then they just keep going out into space.

KESTENBAUM: The experiment has confirmed that as neutrinos travel they actually change from one form into another. They don't even seem to know what they are. And what happens to them in the end? Are the cursed to wander the universe in solitude forever?

GIORGIO GRATTA: Good question actually.

KESTENBAUM: GORGIO Gratta is a physicist at Stanford University. It all depends on how the universe ends.

GRATTA: If the universe will collapse on itself at the end, you know, we die by big crunch. But then it would become dense enough that all the neutrinos that were around will be absorbed or would be starting interacting with other things.

KESTENBAUM: There you have it, for neutrinos an upside to the end of the universe. David Kestenbaum, NPR News.

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