Astronomy Lecture 2 Questions

Lots of really good questions from the students this week! Frontiers‘ second astronomy lecture was mainly about the stellar life-cycle and how we learn about it.

Questions mainly were about two things: how do light and temperature relate and what the heck are these neutrino thingies? I wanted to try to address questions about neutrinos here since they are very interesting and cutting edge, but I don’t know if I’ll be able to talk about them much in seminar.

A) What are neutrinos? Where do they come from?

Neutrinos are neutral (hence the “neutri” part of the name) particles with very very tiny masses. In fact, for a while it was thought that neutrinos had no mass (more on this below).

Neutrinos come from a number of sources, but the simplest to understand is the decay of the neutron. Just from the words alone you might have expected that neutrons and neutrinos are related. They are both neutral particles (not charged).

Now, a neutron is actually slightly unstable. If you have a neutron in your lab and you wait some time, it may randomly decay into an electron and a proton. Notice that the two new particles are charged but they have opposite charges, so the net charge is zero.

Suppose your neutron is just sitting there. Even when a particle isn’t moving, it has an intrinsic energy–the energy of its own mass. The amount of energy stored in a particle of mass m is

E = mc^2

where c is the speed of light. Notice that a 1 kg mass stores almost 10^17 Joules or about 10^14 Calories worth of energy! That’s a lot.

A neutron that is just sitting around has energy stored in its mass. When the neutron decays, the total energy of the two particles that emerge from the decay should equal the energy that the neutron had in its mass. Well, people measured this, and they found out that things didn’t match up. Rather than scrap conservation of energy (which is one of the most fundamental principles of physics) people suggested that a new particle must exist. The particle had to be neutral since the electron and proton already had charges that cancelled. Thus, the idea for the neutrino was born.

So to reiterate, a neutrino is a neutral particle with a very small mass and most commonly arises from “beta” decay of neutrons–neutrons decay by spitting out an electron, a neutrino, and turning into a proton.

B) How do we detect them?

Aside from measuring the absence of energy in some process, how do we try to detect a neutrino?

Well, in order to detect anything, the thing has to interact with other matter and we need to be able to observe those interactions. Now, there are four possible ways for matter to interact:

* Gravity: all mass and energy tugs on other mass and energy. However, this is very weak. For particles with very small masses, this is pretty much useless as a method of detection.

* Electromagnetism: charged objects cause other charged objects to move due to electric and magnetic forces. However, neutrinos are neutral, so you can’t observe them this way.

* Strong nuclear force: this is a force that holds atomic nuclei together–remember protons in the nucleus want to push apart, and that electric repulsion is quite powerful. You need a REALLY strong force to hold them together. Thus, the strong nuclear force is orders of magnitude stronger than electromagnetism. BUT, neutrinos are not affected by it since the strong nuclear force actually only acts on quarks and things that are made of them (neutrons, protons). Neutrinos are not made of quarks, so they don’t have “strong force” charge (just like the fact that they have no electric or magnetic charge).

* Weak nuclear force: we’re left with this force. In fact, this force was first posited to explain things like beta decay. Indeed, this is how neutrinos interact. However, the weak force is very weak: while it is orders of magnitude stronger than gravity, it is minuscule compared to electromagnetism and the strong force. This is why neutrinos are really really hard to detect: they pass through everything with a very very low probability of interacting weakly with other particles.

So how do you detect something that interacts only weakly, with a very low frequency of interaction? Well, you build a giant vat of stuff–water, and you tuck it way underground so that other particles that originate in the atmosphere don’t interfere with your observations. You also look for a source of neutrinos–many of them. As luck would have it, the sun is a huge source of neutrinos since they are produced in large quantities by nuclear reactions. There are lots of atoms of water in this giant vat. There are lots of neutrinos passing through this vat of water. So, even though the probability of any interaction occurring is very low, the numbers work out so that you will detect some smallish number of neutrinos through their interactions with the water nuclei or electrons.

But how do you know an interaction occurred? When the neutrino hits an electron in the water molecule, it can cause a charged particle to momentarily move faster than the speed of light IN WATER (note: things can move faster than light in some medium other than a vacuum. Nothing can move faster than light in a vacuum). This produces a sonic boom of light–something called Cherenkov radiation. It looks like a cone of blue light. Now you cleverly lined your giant vat with 10,000 photo-detectors. All of the sudden, a small ring of these detectors go off, indicating that a neutrino has collided with one of your water molecules. That’s how you detect the neutrinos.

There are other methods as well, but they all involve painstakingly careful and patient observation.

C) Three kinds of neutrinos? And why does this have to  do with their mass?

So now there’s a catch. When people started observing neutrinos using the methods mentioned above, they actually found that the number of neutrinos observed was a third of what was predicted! This was weird since the physics of the sun is actually pretty straightforward. So either something about neutrinos was wrong, OR something else about physics was really wrong. We try to modify as little as possible when we have a very successful theory that correctly predicts lots of other things, so people suggested that the issue was with our understanding of neutrinos.

It turns out that there are three “flavors” of neutrino: the electron-neutrino, muon-neutrino, and tau-neutrino. How did theorists come up with that? Because we know that there are electrons, muons, and taus, and that they are all analogous to each other, except that their masses are different. So since we knew that electrons were partnered with a neutrino, the other analog particles needed their own partners (this is related to the deep principle of symmetry in physics, something I won’t talk about here unless somebody wants me to).

Well, there are three kinds of neutrino and we were detecting only 1/3 what we expected…coincidence? NO! In fact, we were only detecting one type of neutrino out of the three because the molecules of water in our giant vats only will interact with the electron-neutrino, not the other types. So you might think that the story is over, the sun produces three kinds of neutrinos in equal proportion and we only detected one kind.

But it’s a bit more complicated. The sun’s nuclear reactions will actually only produce electron-neutrinos. So at first blush the above explanation doesn’t work. If the sun only produces electron-neutrinos, why do we only detect a third of them? The answer, as it happens, is that something funny happened on the way to the detector!

As the electron-neutrinos travel through space they can change their flavor. They have some probability of turning into a muon or tau neutrino. The connection with mass is this: the probability depends on the difference between the masses of different types of neutrinos. If the masses were the same, then the neutrinos wouldn’t change into one another


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