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.