Neutrinos — the universe playing hard-to-get

Neutrinos are little subatomic particles, a bit more obscure than electrons & protons, but have a particular set of skills that has made ’em famous.  Neutrons have no electric charge, unlike protons and electrons, so they’re unaffected by the electromagnetic force.  They are also immune to the strong force (the fundamental force that holds atomic nuclei together, despite the tendency of like-charged protons to repel each other).  They do feel the weak force (the 3^rd fundamental force), but that’s a very short-range force, so you don’t feel it unless you’re really close to another particle.  And finally, they do feel gravity, but just barely – neutrinos have just about the smallest mass you can have without being massless.  Their mass is so tiny¹ that we can barely measure it — we’ve barely nailed it down to within an order of magnitude.  The upshot of all of this:  they’re barely affected by anything else in the universe.

Neutrinos were also responsible for the demise of Planet Earth, in the movie 2012. Not cool, neutrinos.

When a neutrino heads your way, it will pass right through you as if you didn’t exist.  A neutrino headed straight for your forehead will zip right through your head and out the other side, as if you’re a ghost.  A giant block of lead?  Piece of cake — our little neutrino will head right through.  The entire planet earth?  No problem – a neutrino could plunge into the ground in Kansas and head right on through without noticing it just passed through an entire planet, without even slowing down.  If you tried to catch a neutrino with a giant block of lead a light-year across, you’d still only have a 50% shot of stopping it.

Why is that?  Well, since neutrinos are impervious to the charms of electric charge, and are so light that they barely feel gravity, they just keep on going until they happen to collide head-on with another subatomic particle.  Remember, our atoms are mostly empty space – plenty of room amongst the tiny atomic nucleus and outer electron shells for a neutrino to pass through.  A neutrino heading for earth will simply pass through the empty space in all the atoms it meets along the way.  Every once in a great while, its path will happen to line up just right that it will collide head-on with some proton or somesuch, and only in this case will the neutrino actually change its path.  The fact that a neutrino can pass through the earth with only a negligible chance of hitting anything tells you just how empty a typical atom really is.

Where do they come from?  They pop into existence thanks to various kinds of nuclear reactions, which means they aren’t commonly created around us in our day to day life.  But they’re created by the buttload in that giant ball of nuclear reactions in the sky, the sun – the sun emits a constant stream of ‘em, that travel the 93 million miles of empty space to earth, then keep on traveling right through us every second of every day.   And so by far the vast majority of neutrinos that we come into “contact” with day to day are from the sun.  Think of it like sunlight, except that this light shines right on through the earth.

In the next few neutrinular vignettes below, we explore these fascinating properties of neutrinos.  In each of the sections below, I derive some interesting Factwad about neutrinos using my ever-diminishing mathematical skills.   I trust you will find the results fascinating…

How many neutrinos are there?

There’s kind of a lot of them.  Wait — ain’t no “kind of” about it — there is a SHITLOAD of them.  There’s actually way more of them than there are of us!  Just how many are there?

>>>   READ ON

Exactly how many neutrinos pass through our bodies at any given moment?

I mentioned earlier that neutrinos don’t interact with matter, and so they just pass right through our bodies all the time.  But how many?  Let’s use some mathematical expertise (and some half-assed approximations) to guesstimate just how many are streaming through you right now.

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What if we had “neutrino-vision”?

We can’t actually see neutrinos.  That’s probably obvious by the fact that tons of them are streaming through you right now, and you don’ t even notice.  But what if we could see them?  What would the world look like?  I’ll give you a hint — you might have trouble getting to sleep at night.

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How often do they actually interact with us?

Neutrinos don’t always pass right through you.  Every once in a while, they do hit something — after all, we’ve got neutrino detectors that pick up a couple of them now and then.  So it stands to reason that once in a while, a neutrino must collide with a part of our bodies.  How often does this happen?

>>>   FLAME ON

Do they really go faster than the speed of light?

For a while in 2011, it looked like someone found solid experimental evidence that neutrinos may move faster than the speed of light.  Was that true?

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Howdy

Footnotes:

1.  *audience yelling* “How tiny is it???”