Do you know how big one square centimeter is? Point your index finger at your eye. The end you see is about a square centimeter. Do you know how big 65,000,000,000 is? It’s 6.5E10 in scientific notation. It looks big. Considering that there are about 1E19 atoms of air in a cubic centimeter of air at sea level, 65,000,000,000 starts to look small. But what if I said that every second, about 65 billion solar neutrinos pass through every square centimeter of the Earth that is aimed at the sun? Seems like a pretty big number now, doesn’t it?
Solar neutrinos are particles emitted by the sun. Neutrinos have no mass, they have no electrical charge, and they are unaffected by magnetic fields. They zip through most things because most things are made of empty space and the only thing that changes the course of a neutrino is the weak nuclear force. To get that to change your course you have to fly through the nucleus of an atom. For comparison, the likelihood of that happening is about the same as a random rock floating through our solar system to crash into the sun. Not very likely.
Neutrinos are cool for many reasons, but the best one is that they react gravitationally to other particles. Which is really cool. A massless particle (the neutrino) will react to another massful particle’s gravitational field. Wow!
Why is this cool? Because in order to solve a particular physics problem, neutrinos have to change flavors. (the flavor of a neutrino is like the color of light – it’s an attribute.) In order to do this, they need to have mass. So here we have this massless particle that has to have mass. Okay, so let’s say they have mass. And they have spin. Spinning massful particles have magnetic moments, which mean that they will interact with electromagnetic forces.
Ah, the lonely little neutrino becomes even more complex the more we look at it. But so what? What’s a neutrino have to do with time travel? Enter Supernova 1987a.
SN1987A was a supernova in the nearby Large Magellanic Cloud – a dwarf galaxy that orbits our Milky Way. About 150,000 light years away, the light from its explosion reached Earth on February 23, 1987. Unfortunately for this Astropotamus, you had to be in the southern hemisphere to see it. It was the first chance for modern astronomers to see a supernova since the Crab Nebula light reached our planet in 1604.
So about the time humans started living in the first metropolis, some star in the LMC blew up and we got to see it 168,000 years later. Luckily, a very sensitive Time Machine, buried in an old mine in Japan, was watching. It recorded 24 anti-neutrinos hitting its detectors about three hours before the main light from the supernova hit the Earth. This is consistent with a neutrino burst emitting from the core of the sun during explosion, which precedes the visible light, since visible light is caused by the shockwave hitting the stellar surface (and therefore, slightly delayed).
24 anti-neutrinos from the depths of a star 168,000 light years away reaching our planet in February of 1987 was the birth of neutrino astronomy. The results were consistent with models that predicted that 99% of the energy of the collapse is radiated away by neutrinos and is consistent with proposed values of the energy (and mass) levels of a neutrino. Since any light produced in the core of our Sun will interact with the gas in the outer layers, neutrinos are the only way to study what happens deep inside our stellar companion without interference. When we detect neutrinos from the Sun, they can tell us the story of something that happened more than 8 minutes and 13 seconds ago, since that neutrino may have been wrapped around a gravitational force line for quite sometime before it finally was ejected.
So let’s hear it for the tiny particle that traveled more than 160,000 years to drop on into a mineshaft and bounce off a particle of water to make a detector go ding and introduce all of humanity to the science of what happens deep inside a star!