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My PhD topic (repost)

November 18, 2013

(this was originally a guest post on another blog in mid-2010, but that blog seems to have disappeared so I’m putting it here so it can live forever, complete with hand-crafted pictures)

Learning about ourselves and others is all the rage these days. But since the self is no-self and hell is other people, how about finding out some true facts about the real Universe?

If you want to know facts about about anything other than stamps, you have to enroll in a PhD in physics. This follows from Rutherford’s classification system, viz. “All science is either physics or stamp collecting”. Or else you can read on and I will tell you some facts about the Universe that I have learned.

Let us begin with stars. Stars are atom crushers. They are giant balls of gas that run on gravity and pressure. Gravity pulls them together and forces atoms together in the centre, which releases energy in the form of light and heat. This energy causes pressure which keeps the star from collapsing forever. Here’s a picture of the fundamental process that makes stars work:

fig1

There are an awful lot of types of subatomic particles, but the only ones that you need to know about to understand my research are protons, neutrons, electrons, and neutrinos. Protons and neutrons are very similar, except that protons have a positive electric charge and neutrons have no charge. Neutrons also weigh a tiny bit more. They are quite big particles, in that they are heavy. (Also, they are actually made up of smaller, more fundamental particles (quarks), so they do have a definable ‘size’. But that is not too important here.) Electrons are much lighter, have a negative charge, and tend to orbit protons in atoms. An atom is some number of neutrons and protons stuck together in a tight ball, with electrons orbiting it.

Finally neutrinos, the little babies of the particle world. They have almost no mass, no charge, and barely interact with anything at all. Effectively they can travel through an entire star without hitting anything. Here is a little table to demonstrate how small they are in terms of mass (which is kind of a rough proxy for their ‘size’ but such concepts are tricky to explain for point particles…but I don’t want to get too sidetracked).

tab1

If neutrinos are so tiny and hard to see, why are they interesting? Well, when a star reaches the end of its life, it emits neutrinos. A lot of neutrinos. Looking for them can tell us about how the star exploded and what the structure of it is at the deepest layers, which normal light and hence normal telescopes cannot penetrate.

As I told you before, stars are atom crushers. But not all atoms can be usefully crushed. For some reasons (which I will spare you here), you can’t crush atoms of iron together without putting in energy. This means that once the core of the star is all crushed down to iron there is no more energy being released at the centre. Remember the pressure/gravity balance? No pressure means gravity wins. But instead of crushing iron into heavier atoms, gravity crushes entire atoms into neutrons. Here’s a cartoon of that:

fig2

Now you’ll notice that this process releases neutrinos. Practically the entire core of the star, about 20 km across, gets turned into neutrons, forming a so-called neutron star. During this process, for several seconds the supernova releases about ten to the power of 55 neutrinos per second. To give some idea of how big a number that is, think about how many atoms there must be in the entire Earth. Now imagine every single one of those atoms emits a million neutrinos. That’s a decent approximation to what this neutron star is doing every second. Now you might wonder, since these neutrinos are so tiny, maybe that’s just a bit like having a beach lose a few thousand grains of sand. But in fact these neutrinos carry off about 90% of the energy of the explosion as they fly out of the supernova. Keep in mind that the supernova core is essentially acting like an atomic bomb that weighs as much as the Earth.

Here’s a rough sketch of what the exploding star looks like in profile:

fig3

There’s one more thing you need to know about neutrinos before you can really understand my research. That is that neutrinos have flavours. There are three different flavours, electron, muon, and tau. The electron neutrino is a tiny partner to the electron, and the other two are partners of heavier versions of the electron that also happen to exist, the muon and tauon. The strange thing with neutrinos is, they can change which flavour they are. Electrons are always electrons, they never change into muons, but electron neutrinos can change into mu or tau neutrinos with no problem at all.

So let’s briefly review: a star explodes, releasing ridiculous numbers of neutrinos, which have certain a flavour and energy distribution which can be figured out based on the theory of neutron stars. And these flavour can all change around in some way. There are even more complications, however, because the way that neutrinos change their flavour depends on their surroundings. If there is dense matter (like the outer layers of the exploding star) then this has certain effects. If there are enough neutrinos, the neutrinos all bounce off each other and their flavours are all connected together in a complicated way.

The way the theory works out (you can read the first fifty-odd pages of my thesis if you want proof), the massive cloud of neutrinos acts a bit like a pendulum. Imagine a grandfather-clock type pendulum that can spin all the way around from vertically upwards to vertically down. With some magic maths ‘up’ represents electron neutrinos, and ‘down’ represents the other two flavours. As the cloud of neutrinos moves out from the core, it starts off at the top position, eventually falls over, then comes back up again but not quite so high, and so on. The end state of the pendulum is pointing down with gravity, which means that the flavour that the neutrinos end up in is completely flipped from where they started.

But there are even more complications! Really the pendulum has a spinning top on the end! And that makes it do all sorts of weird things. But the major complication is, there is an unknown parameter in physics, called the neutrino mass hierarchy, which corresponds to the initial direction of this crazy neutrino flavour pendulum, up or down. If it starts off in a downwards position, as you’d guess, gravity just keeps it there and there’s no change in flavour.

So the aim of all this is to measure neutrinos from a supernova, see whether their flavours have all flipped, and from that figure out the neutrino mass hierarchy. And it turns out supernovae are the only places where this pendulum thing can happen because there just aren’t enough neutrinos anywhere else in the Universe.

My contribution to all of this is to test how stable this pendulum picture is. In theoretical models we usually assume everything is nice and smooth around the supernova, but in reality it probably looks more like this:

messy

So I took a nice well-behaved theoretical model and added a bunch of turbulence to it, and then calculated what effect this has on the neutrino flavours. Summary: the pendulum is a bit more crazy, but it still acts like a pendulum in essence, and falls down or not depending on the neutrino mass hierarchy.

And that is what the first two chapters of my PhD thesis are about.

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One Comment leave one →
  1. November 18, 2013 11:50 pm

    I love this post Giles, when I read it, I understand what you did for all those months and months… at least until the fluffy dog thoughts and song lyrics invade again! You are indeed a brilliant communicator of tricky stuff to the masses!

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