Today I want to talk about neutrinos. They’re fundamental particles, here they are on the
standard model of particle physics, and they are created in the fusion reactions in the Sun
which gives out a colossal 10^38 neutrinos every second, then a massive 10^29 pass right through
the Earth every second as if it is not there and 300 trillion pass right through your body
every second, which is why they are often referred to in mainstream media as ghost particles.
Because so many pass through our bodies without us noticing, we definitely shouldn’t be able to
see them with our eyes, but, I’ve figured out that there is a very slight chance
that one of them could hit an atom in your eyeball and cause a visible flash of light.
The chance of this happening to you is very small, but because there are so many human eyeballs on
the planet, roughly 10 people per hour on earth will have a neutrino collide with one of their
eyes. Now nothing bad would happen, you’d just see a very faint flash of blue light, but I’ve
never heard anyone talk about this before even though according to my calculations, it’s true!
Neutrinos are the weirdest particles in the standard model,
and the ones we understand the least. There are four main reasons why they are weird.
First of all they are the only particles that only interact with other particles via the weak force.
There are four forces in nature: gravity, electromagnetism,
and the nuclear strong and weak forces,
and the weak force is called weak because it’s very short ranged, which means that it’s only
felt by particles that are really really close together, like 10^-18m apart - that’s a thousand
times smaller than the nucleus of an atom which is already incredibly small. This is why neutrinos
go through your body as if it’s not there because to them we just look like empty space.
Technically neutrinos also feel gravity, but they have such tiny masses and gravity is so weak we
can basically ignore it. In fact that’s the second weird thing about neutrinos, we have no idea where
they get their mass from. All the other particles with mass get their mass from the Higgs field,
but neutrinos get theirs from somewhere else, and we’ve got no idea where they get it from.
Also, when neutrinos are flying through space they morph from one kind to another. This is
the only particle in the standard model that spontaneously changes into another particle.
And finally we’ve only ever seen left handed neutrinos and right handed anti-neutrinos.
All other particles come in both left handed or right handed kinds of particle and antiparticle,
but not neutrinos. For more information about this check out my video and poster on the standard
model of particle physics. In fact, in case you were on the lookout for presents around now,
I have a load of educational posters available in my store, dosmaps.com, and I also have a
range of kids books for the youngsters covering space, physics, the human body and the oceans.
Dosmaps.com link in the description below. Anyway, like I said, neutrinos are weird and we
don’t understand them well because they rarely interact with anything. Despite them being the
most abundant particle in the universe: we think there’s even more neutrinos than there
are photons of light. And, even though neutrinos have incredibly small masses, less than 10^-37 kg,
the combined mass of all the neutrinos is about the same as all the stars in the Universe.
So, to try and understand neutrinos we’ve built a load of giant detectors underground
to try and detect the very rare occasions when they do get close enough to collide with matter.
This is what a neutrino detector looks like: this one is called Super-Kamiokande and it’s
deep underground to stop any other particles getting through to it. It’s a giant vat
50,000 m^3 of volume, filled with ultrapure water surrounded by photomultipliers,
which are basically very sensitive cameras. Other neutrino detectors follow similar designs
but are filled with different kinds of liquid which absorb different energies of neutrinos.
When a neutrino collides with one of the electrons in the liquid, the electron absorbs their energy
and wizzes off through the liquid giving out a special kind of light called cherenkov radiation.
Cherenkov radiation is kind of like a sonic boom but for light, because the electrons travel faster
than light does in the liquid. Which sounds weird because it sounds like I’m saying electrons travel
faster than light. Well yes they do, but only because light slows down in a liquid,
the electrons don’t go faster than light in a vacuum, that’s the thing that’s impossible.
But you can go faster than light if you slow down light enough,
like in a liquid, and in that case, you’ll emit this cherenkov radiation.
So with these massive vats of liquid people are seeing the sporadic neutrino impacts, collecting
data and trying to figure out how neutrinos really work, which is all good science and all that,
but I was thinking, these detectors are kind of like big eyes. If you think about it, our eyes are
vats of liquid, and your retinas are sensitive to light like the photomultipliers. So if a neutrino
hit an electron in your eye, would it also create cherenkov radiation? And could you see it?
Of course our eyes are way smaller than these giant detectors, but we have a lot of eyes.
So I tried to figure it out with maths, and I also ran it past a professor of these things,
and yeah, there is a very small chance a neutrino will collide
with your eye gloop and create a flash of light!
But, this will only happen to you on average once every 89 thousand years. So you’ll probably never
see one, but there are a lot of human eyes on earth. In fact if you collected all the human
eyeballs on Earth right now, you could fill up Super-Kamiokande, twice over. And that’s why
I love physics. Anyway, averaging all this out over the 7.7 billion people on earth we
can expect around ten people per hour will get a neutrino colliding with one of their eyeballs.
But, I hear you shout, how visible would this be? Would we even be able to see it? Well, our eyes
are pretty sensitive to light. If you are in a dark room, you can register flashes of as little
as 10 photons, and Cherenkov radiation from the neutrino impact creates around 200 visible
photons per cm through the eye. 200 photons is way bigger than 10 so a neutrino impact
would be visible, but only if you are in the dark because it would get swamped by daylight.
Also, there are some subtleties with what angle the neutrino hits your eye and whether
the cone of the cherenkov light overlaps with your retina. I’m not an eyeball expert,
so I don't really know how that would affect the statistics. But even if it’s like 10 times lower,
that’s still one person an hour on earth! We know astronauts have seen light flashes
inside their eyes when they travel outside the Earth’s magnetosphere, these are thought to be
caused by cosmic rays, but we don’t get this on Earth because we are mostly protected by the
magnetosphere and the atmosphere, but still some get through so we might get flashes of light in
our eyes caused by cosmic rays, or even background radiation from the Earth. But I think it’s very
cool that if you see a random flash of blue light in your eye, you could be looking at the remnants
of the most mysterious particle in the Universe. Although I should mention that statistically it’s
much more likely to be a detached retina, so if it keeps happening best to go see an opthamologist.
Anyway, I’m genuinely interested if any of you see a faint blue flash in one eye. If we get 90
thousand people to watch this video there’s a good chance one of you has seen one in the last
year. So I’m interested to find out, let me know in the comments below if that’s happened to you.
And if you want to learn the skills to do this kind of physics based problem solving yourself,
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They have a wide selection of courses in physics going from quantum mechanics to relativity,
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Thanks again for watching and see you in the next video.