So that's a very nice story. I gave you a whole rigmarole and I claimed that this
is how the sun produces it's energy. How would you go about verifying such a
thing? You can't get inside the suns, there's
hundred of thousands of kilometers of sun between you and the core, where these
reactions are going on. Which is very good because it prevents
all the gamma rays from escaping. And the basic reason we knew how stars
worked in the 30's was because Eddington had the theoretical idea and later
figured out how to get through the weak decay process.
But it turns out that we can in fact look right inside the sun. With a beautiful
experiment, the idea is due to an experimentalist called Ray Davis and a
theorist called John Bahcall who in the 60's, come up with the idea that, wait a
minute. One of the properties one of the products
of this PP process that we discussed, is electron neutrinos.
Two electron neutrinos for every alpha particle, for every helium nucleus form.
And we know how many helium nuclei are formed per second, because we know the
total luminosity. So we know how many neutrinos the Sun
produces every second. These neutrinos are produced in the core,
but, being neutrinos, they sail right through hundreds of thousands of
kilometers of sun, with essentially no attenuation, maybe three interact, it
doesn't matter. This is great because this gives us a
telescope that essentially eliminates all of the distraction of the sun and lets us
look right into the core. The disadvantage, of course, is [LAUGH]
that they also sail through the earth and through whatever your detector is.
And most of the neutrinos will cause absolutely no interaction to give you a
sense, as I said, ten to the eleven solar neutrinos hit every meter squared of
earth every second. So you are being bombarded with millions
of electrons, of neutrinos as we speak. Obviously nothing happens.
So, how do you build a detector such that something will happen.
Well in the 60's these two guys put together this brilliant experiment, where
essentially they take a huge tank of cleaning solvent, bleach, which
containing's chlorine. They stick it underground in a gold mine and the other
reason they stick underground is to prevent cosmic ray and to insulate it
from all kinds of noise. and once in a very long while an electron
neutrino, penetrating this tank of chlorine will interact with a neutron and
a chlorine atom and convert that neutron, through the reaction that we talked
about, where an electron neutrino plus a neutron can become a proton plus an
electron. through that reaction this occurs inside
the nucleus that converts one of the neutrons here to a proton that converts
chlorine to an radioactive, unstable isotope of argon, a noble gas.
Unfortunately, the actual neutrinos from the pp process that I showed, the way it
works would not have sufficient energy to lead to this reaction.
But there are some other modes of interaction that do produce high-energy
neutrinos. And so one could imagine detecting the
neutrinos from those. And if you understand your model
correctly, then you can relate how many pp processes go on to how many of these
other processes are going on because you understand the rates.
You do the calculation, you figure out how many high energy neutrinos the sun
produces, it's a copious number. How many of them hit your detector?
It's a huge number. How many of them interact?
One argon atom should be formed in this big tank of bleach every six days.
So Mr. Davis' job is to detect one atom of argon
every six days, he flushes the detector and looks for the radioactive signature
of the argon, which would be in the noble gas bubbles out.
And he's trying to find an atom every six days, he empties the tank every couple of
months and looks for fifteen atoms. And he finds some argon in the tank but
less than predicted. And this is a result that comes out in
the late 60s and over 30 or 40 years 30 years I guess, Davis is making these
measurements and continually obtaining results that are too small that do not
match predictions. Remember we know how many neutrinos the
sun forms, because we form precisely two neutrinos every time we form an alpha
particle. And that's how we make our energy.
So maybe people say the solar model is wrong.
maybe our understanding of course, we know how many alpha particles are
produced. But maybe the details of the rate, ratios
of the rates of the high energy and low energy neutrinos are somehow wrong.
people, this, this is a great incentive for people like Bacall to refine the
solar model to exquisitely high precision.
maybe our understanding of the processes in the detector are wrong.
in fact it turns out in that, that, that the answers given in 2001, by
measurements using the Sudbury Neutrino Observatory this is a completely
different technology. It's sensitive to all different kinds of
neutrinos, including the low-energy neutrinos.
And if you detect the problem with the flux in those then you'd discover that
the Sun doesn't shine. And so the Sudbury Observatory in 2001
realizes, resolves the issue, and what we realize is that what was wrong was our
understanding of particle physics. Particle physics again?
Yeah, I'm sorry. Here it comes.
This is a going to be a fun story, though.
So, we need to extend our table of particles.
Here I'm not bothering with the antiparticles because we understand that
everything comes in doubles. But here is the list excluding the photon
that I didn't bother to list, of all the particles we've seen so far.
I've added their masses in these units we like to use, that we'll discuss later.
And you'll see a neutron is a little bit heavier than a proton as promised.
And the mass of the neutrino is effectively zero.
It turns out that along with electrons, there are other negatively charged
elementary particles much more massive than the electron.
First 200 and then about 3,500 times the mass of the electron and these are the
Mu-Meson and the Tau-Meson. otherwise they're for all intents and
purposes electrons except heavy electrons.
and each of them it turns out, is accompanied by a neutrino.
So just as an electron has an electron neutrino, there's a muon and a muon
neutrino, a tau meson and a tau neutrino. All of the mesons are negatively charged.
All the neutrinos are neutral and don't interact with anything.
except for the fact that the electron neutrino carriers electron number and we
have two analogous conserved numbers for new number carried only by the muon and
his electron and tau number carried only by the tau meson and his electron.
And, so, for example the favorite decay mode, of one or the decay
modes, of a mu meson is miu which is negatively charged decays to an electron
which is negatively charged. plus, well electron number has been
violated. But not if I produce an empty electron
neutrino, and now muon number has disappeared but not if I have a new mu,
and this is indeed one of the allowed and existing decay modes of the negative
muon.. So we have all of these particles and
they're interactions of electrons, the weak interactions of electrons and muons
and so on conserve not only electron number but also mu number and tau number.
And so, for example, the decay in the sun that converts a proton to a neutron
produces, not a muon neutrino, but an electron neutrino.
Similarly, the process by which chlorine changes to argon, can only occur in the
presence of an electron neutrino. And it turns out that neutrinos oscillate
from one kind of neutrino into the other. In other words, electron muon and tau
number are not in fact conserved. Not by the interactions but by the
neutrino propagation itself, neutrinos spontaneously change from one flavor to
the other, other en route, this is called neutrino oscillation it's the discovery
at 2001, I think it was the 2002 Nobel prize including Ray Davis for his
prediction. the PP process as I said produces
electron neutrinos, by the time they get here they've all mixed up and only a
third of them are electron neutrinos, the other two thirds are mu or tau neutrinos
which do not interact in the way that Davis was looking for in his detector.
And in particular this implies that despite the fact that we thought
neutrinos are massless, the question marks on their masses in the previous
slide are because neutrino oscillations and there have been other discoveries on
neutrino oscillations since show us that neutrinos cannot possibly be massless,
and massless particle could not oscillate.
And so we made by us trying to understand the structure of the sun.
Well we got very well refined solar models we also discovered a whole new
property of fundamental particle physics, and I find that really exciting.
We'll meet these neutrinos again, they turn out to be more useful than you'd
imagine. But I thought you would enjoy this great
story where after decades, and I remember sitting in conferences where the particle
physicists would argue with the astro physicists over who was wrong.
And the net result at the end of the day was that we were wrong and they were
right.