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35c3 preroll music
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Herald: Our speaker is Jost Migenda and he
is PhD student in astroparticle physics
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from the University of Sheffield in the UK
and Jost is going to talk about going deep
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underground to watch the stars. Please
give a huge round of applause for Jost
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Migenda.
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applause
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J: Good morning everybody. I'm glad you
managed the first day of congress. Now
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physics rarely makes highlight news. And
if and when it does it is often treated a
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black box, where you pourd in money and
scientist on one end, you wait a while and
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knowledge drops out. So today in this talk
I want to do this a bit differently. I
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want to give you a glimpse behind the
scenes of an experiment, I have been
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working on for over 4 years now. First as
part of my master's thesis and then as a
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PhD student. Now earlier this year we
published a design report which is over
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300 pages long and contains much more
detail about the experiment than you
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probably want to know. So I'll focus on
just some of the highlights in this talk.
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But before we actually talk about the
detector I'll have to introduce you to the
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particles we're looking for. And that
story begins over 100 years ago with
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radioactive beta decay. Now in radioactive
beta decay, you have a nucleus of one
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chemical element that turns into a nucleus
of a different element and emits an
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electron or in modern language we would
say a neutron decays into a proton and an
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electron. Now after that was discovered
there were lots of experiments done to
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measure the energy of the outgoing
electron and experiment after experiment
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found that there was some variance in
energy but was always lower than expected.
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And physicists at the time came up with
all sorts of possible explanations for
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what might be going wrong with these
experiments but they excluded those
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explanations very quickly as well. So
after a while physicists became desperate
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and some pretty well-known physicists
actually thought: "Well, maybe we'll just
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have to give up on conservation of
energy". So in this desperate situation a
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guy called Wolfgang Pauli came up with
what he himself call "a desperate way
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out". So in this letter to a group of his
colleagues which he addressed as "Dear
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radioactive ladies and gentlemen", Pauli
suggested that maybe there's another
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particle created in this beta decay. And
Pauli originally called this particle
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neutron but of course two years later the
particle we nowadays know as Neutron was
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discovered. So Pauli's particle was re-
named neutrino. Now you might be wondering
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well why didn't they observe this particle
already. And the answer is very simple.
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Neutrinos are like ghosts. So what I mean
by there is they can quite literally, you
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know, go through walls or through your
body. And in effect we can do a little
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experiment right now to try and detect
neutrinos. So to help me with this
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experiment, please give me thumb's up.
Everyone? Okay so there's two things
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happening right now. First thing of all
you're giving me a massive confidence
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boost. But, you know, more importantly
somewhere out there the sun is shining and
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it's producing a lot of neutrinos and
nuclear fusion. Now these neutrinos are
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flying to Earth through the roof of this
building and then through your thumbnail.
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And right now as you're listening to me
around 60 billion neutrinos are flying
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through your thumbnail. 60 billion
neutrinos flying through your thumbnail
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every second. How does that feel? Hmm? You
don't feel any of them? Right, so that's
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how ghost-like neutrinos are. And of
course physicists are clever and shortly
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after Pauli had this idea some of them
estimated that how often neutrinos
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interact with normal matter and they found
that there is no practically possible way
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of observing the neutrino. And that
remained true for over 20 years
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afterwards. So now that I have introduced
you to neutrinos. Let's talk about
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building a detector to actually detect
them. And the original motivation for
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building this detector was something a bit
different. I talked about beta decay and
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over the next decades physicists slowly
discovered more particles. They discovered
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that protons and neutrons are made up out
of quarks and in the 1970s theoretical
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physicists came up was are some Grand
Unified Theories basically precursors to
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string theory. And these theories
predicted that the proton should decay as
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well. So of course you know we build
detectors to look for that and a group in
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Japan built a detector near the town of
Kamioka which they called the Kamioka
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Nucleon Decay Experiment or Kamiokande for
short. Now they didn't observe any proton
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decay but shortly after they built it
somebody had a suggestion that if we
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changed just a little bit above their
detector, if we modified just a little, we
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would also be able to detect neutrinos
with that. So they modified the detector
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switched it back on and just a couple of
weeks later they actually observe
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neutrinos from an exploding star just
outside our Milky Way. And that was the
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birth of neutrino astronomy. And for that
the then-leader of the experiment received
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the Nobel Prize in 2002. Now after over a
decade of running physicists were
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basically hitting the limits of what we
could do with a detector of their size. So
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we needed to build a bigger detector and
that one was very creatively named Super-
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Kamiokande. And it's about 20 times
bigger, started running in 1996 and still
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running to this day. Now Super-K did not
discover proton decay but did detect a lot
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of neutrinos and made very fascinating
discoveries. For example, they discovered
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that different types of neutrinos can
change into each other back and forth as
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they travel. That's like you buying a cone
of vanilla ice cream and then as you walk
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out it suddenly turns into chocolate ice
cream. That's really weird. And for their
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discovery just a few years ago they
received the Nobel Prize again. But today
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we are again hitting the limit of you know
what we can learn from a detector that
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size. So of course the next step is to
build an even bigger detector and we're
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calling it Hyper-Kamiokande. By the way
"super" and "hyper" mean exactly the same
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thing. Just one is Latin and one is Greek.
So we're currently getting ready the plans
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to build Hyper-Kamiokande and we will
start construction probably in the spring
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of 2020. Details of the Noble Prize are
still to be determined. Now I said that 60
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billion neutrinos go through your
thumbnail every second. Of course Super-
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Kamiokande which is running right now is
much larger than your thumbnail. So
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there's not just 60 billion but 10000
billion billion neutrinos passing through
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every day and only 10 or 15 of those get
detected. So let's look at what this
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detection process looks like. Now this is
the water inside Super-K. And there's a
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bunch of electrons in there but they'll
show just one. And there's neutrinos
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flying through not just one, not just a
few, but loads of them. And most of them
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go straight through without leaving a
trace. But every once in a while we're
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lucky and one of those neutrinos will
actually hit the electron and give it a
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little kick. And that little kick you know
like billiard balls basically and that
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little kick accelerates the electron to
faster than the speed of light in water.
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Still slower than the speed of light in
vacuum which is the absolute cosmic tempo
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limit. But faster than the speed of light
in water. And then you get basically a
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sonic boom, but with light, which is this
cone of light. And let's just show the
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animation again. So you've got this cone
of light that hits the wall of the
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detector you see a little ring, this ring
of light. Well we've got very sensitive
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photo sensors all over the inside walls to
detect this flash of light and from how
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bright it is we can tell the energy of the
neutrino. And we can also tell, you know
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just like was billiard balls, we can
approximately tell what directions a
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neutrino came from just based on in which
direction it pushed the electron. So
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that's the basic idea how we detect
neutrinos from the sun. Now let's talk
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about what it's actually like to build one
of these detectors. So this is a drawing
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of Hyper-Kamiokande and you can see it's
78 meters high, 74 meters in diameter and
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on the top left there is a truck for
comparison. But maybe a better size
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comparison is to compare this to buildings
which you're familiar with. Like the
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entrance hall which you just came in
through this morning. Or the Statue of
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Liberty and it doesn't quite fit in there.
The arm still looks out. But you could
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drown the Statue of Liberty in this
detector which nowadays is probably some
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sort of political metaphor. So this is the
giant detector. And what's more we're
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building it inside the mountain about 650
meters underground. So that all the rock
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on top will act as a kind of a natural
shield against all sorts of other
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particles, that are raining down on the
atmosphere from outer space so that all
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other particles get stuck and only
Neutrinos can make it through. Now of
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course to build such a huge cavern inside
the mountain - that's something that we
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physicists can't do on our own. So we need
to talk to geologists who look at the rock
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quality and tell us, you know, what's a
good place to build this cavern - where is
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the rock stable enough to do that. And to
figure out the rock quality, they drill
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bore holes in what's actually called a
boring survey. laughter Now, during my
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years working on this experiment, I had to
listen to several hours of talks on these
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geological surveys and I can tell you that
name is quite appropriate. laughter
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though of course, there's a reason I'm not
a geologist, so, you know, take this with
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a grain of salt. But okay, let's say, you
know, we talked to geologists, they told
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us where we can build our detector. The
next step is: We need to actually excavate
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the cavern. And something to keep in mind
is that we are building this somewhere in
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the mountains of Japan, you know, pretty
far away from any major city. So we have
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to think about stuff like lack of local
infrastructure like what's the electricity
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supply like. Do we need to add the power
line. Or what are the local roads like.
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And do they have enough capacity for, you
know, dozens of trucks every day to
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transport away the excavated rock. And, by
the way, where do you store all that
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excavated rock? Because we will be moving
something like half a million cubic meters
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of rock. You can't just store that in your
backyard. You need to find a place where
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all that fits. And, of course, if you've
listened to or watch the Lord of the
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Rings, you'll know it's dangerous to dig
too deeply, to greedily. So we need a
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Balrog early warning system as well. But
okay let's say we've got all those and we
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managed to build a cavern, and now we need
to fill it. And as detector material we
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use water. Both because it's actually
pretty good for detecting neutrinos, but
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also because it's cheap and there's lots
of it. So you can afford to build a
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detector of this size. A detector so big
that that little dot there is a scuba
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diver. But even with water, you hit limits
of, you know, how much you can get. So to
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fill Hyper-Kamiokande you need about as
much water as 5000 people use in a year.
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And that's for drinking, for showering,
for washing their car and so on. Now
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that's easy if you're near a big city. But
we are not, we're somewhere in the
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mountains in Japan where the next biggest
town has far fewer than 5000 people. So
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how do we get enough water to actually
fill our detector? And... we could use
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rivers nearby, we could use springs. We
could wait for for the end of winter and
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for the snow in the mountains to melt and
use that to fill our detector. But if you
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use melting snow to fill the detector, you
can only fill it once a year. So, you
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know, even "where do you get the water" is
is a pretty... pretty important question
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that you need to solve. And then, we're
not just using any water but we actually
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have, we will build our own water
purification system. So that we don't have
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any, you know, traces of radioactivity in
there, any trace of dust and stuff in
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there. And let me let me just tell you
just how pure this water will be. So, this
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is my supervisor, who, when he was a PhD
student, worked in the detector on some
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maintenance work, so he was working on a
boat doing the work, and then at the end
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of his shift he leaned back on the boat,
and just the tip of his long hair fell
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into the water, which he didn't know just
didn't think about too much until at the
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end of his shift. He went home, you know,
went to bed, fell asleep, and then woke up
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in the middle of the night, with his whole
head itching like mad. Now, what had
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happened there? The ultra pure water had
sucked all of the nutrients out the tip of
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his hair and then through osmosis over
time those had sucked the nutrients out of
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the rest of his hair, and then his skin.
So that's how pure that water is. Now, I
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said "all over the inside walls". And here
is, kind of, a photo of the inside of the
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detector. And all these kind of golden
hemispheres, those are what we call
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Photomultiplier tubes, of PMTs for short.
And those are, basically, giant very
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sensitive pixels. And we will have 40,000
of those lining the inside wall of the
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detector. Now, in smaller detectors, you
could just have from each PMT one cable
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leading to the top of the detector and
then have your computers there to analyze
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the signal. With a detector of this size,
you just can't do that because you would
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need 40,000 cables some of which are over
100 meters long. That wouldn't work. So we
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have to put some electronics in the water
to digitize the signal and combine signal
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from multiple PMTs into one, and then use
just a single cable, to bring it up
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to the top where we analyze the signal.
But that means we have to put electronics
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into the water, so that creates a whole
bunch of new problems. For example, we
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need these electronics to be watertight.
And I'm not talking about the level of
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watertight as you'd expect from your
smartwatch where it survives, you know,
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you standing under the shower for five
minutes. I'm talking below 60 metres of
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water for 20+ years. This electronics also
need to be very low power, because we
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can't heet up the water too much.
Otherwise these pixels, the
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Photomultiplier Tubes, would become noisy
and this would kill our signal. And then
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also, because we don't want one defective
cable to kill a whole section of the
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detector. We need to implement some sort
of mesh networking to introduce some
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redundancy. Now each of that by itself is
not... not a heart problem. Each of these
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problems can be solved. It's just a lot of
additional work you suddenly have to do,
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because your detectors is that huge. And
it gets even worse: This is what one of
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these PMTs looks like. It's about 50
centimeters in diameter, and inside that
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glass bulb is a vacuum. So it's under a
lot of pressure. Plus we add 60 metres of
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water on top of it, which adds additional
pressure. So you need to make absolutely
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sure when you're manufacturing those that
you don't have any weak points in the
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glass. And they don't just have to
withstand that water pressure, but there
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will probably be some PMTs that have some
weak points, some air bubbles or something
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in the glass. Some structural weakness.
And the neighboring PMT don't only have to
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survive the normal water pressure. They
also need to survive whether their
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neighboring PMT is imploding and sending
out a pressure wave. And that's not just a
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hypothetical - that actually happened, you
know, 18 years ago. It's 17 years ago in
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Superkamiokande. And that, you know,
within seconds killed more than half of
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the PMTs we had in there and it took years
to restore the detector to full capacity.
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So, lots of problems to solve, and, you
know, one group, one university can't
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solve all these on their own. So we've got
this multinational collaboration with over
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300 people from 17 different countries
marked in green here and across many
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different time zones. So right here right
now, here it's about you know just before
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noon. In Japan people have already had
dinner and they're going to bed soon. In
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the U.S., people haven't even got up yet
in the morning. So good luck finding a
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time for phone meetings which works for
all of these people. So, that's kind of a
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glimpse behind the scenes of what it's
like to work on this detector and to
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actually build it. But now I want to talk
about what we use the detector for. And
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I've got two examples. But of course
there's a whole bunch more that we do, I
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just don't have time to talk about it
today. So, first example: Why does the sun
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shine? That seems like such a simple
question, right? And yet it turns out it's
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really difficult to answer. So, in the
days of the Industrial Revolution, when,
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you know, burning coal and steam power was
all the rage, people thought that, you
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know, maybe it's, you know, a giant ball
of burning coal. But when you when you do
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the math, it turns out that would burn for
a few thousand years maybe. So that
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definitely doesn't work. A bit later,
physicists suggested that maybe the sun is
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just slowly shrinking and shrinking and
it's that gravitational energy which is
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released as light. And that would give you
a life time of a few million years. But
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then you've got, you know pesky geologists
coming along and saying "no no, we've got
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these rock formations or, I don't know,
fossils maybe, for more than a few million
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years old on Earth. So the sun has to live
longer than a few million years. And the
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arguments from the 19th century between
Lord Kelvin and the geologists back then
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are just amazing to read. If you find
those somewhere. But, of course, nowadays
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we know that the answer is nuclear fusion.
And here's, you know, a bunch of reactions
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which lead you the energy generation of
the sun. But now the question is "how can
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we check that? How can we check that this
is actually what's going on?" And the
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answer is neutrinos, because many of these
reactions produce neutrinos, and we can
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detect them. So the one we typically
detect in Super- and later Hyperkamiokande
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is the one on the bottom right here,
called "Bore and eight neutrinos" because
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those have the highest energy. So they are
easiest for us to detect. And the rate of
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various of these processes depends very
much on the temperature. So by measuring
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how many of these neutrinos we see, we can
measure the temperature inside the core of
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the sun. And we have done that, and we
know that it's about 15.5 million Kelvin, plus or
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minus 1 percent. So we know the
temperature in the core of the sun to less
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than 1 percent uncertainty. That's pretty
amazing if you ask me. And, you know, I
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said that we could detect the directions
the neutrinos were coming from. So we can
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actually take a picture of the sun with
neutrinos. Now this is a bit blurry and
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pixelated, not as nice as what you'd get
from a, you know, from the optical
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telescope. But this is still a completely
different way of looking at the sun. And
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what this tells us is that this, you know,
giant glowing orb in the sky. That's not
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some optical illusion but that actually
exists. Okay. So onto our next topic.
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Exploding stars or supernovae which is
what my own research is mostly about. So
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supernovae are these giant explosions
where one single star, like in this
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example here, can shine about as bright as
a whole galaxy consisting of billions of
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stars. And the rule of thumb is this:
However big you think supernovae are
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you're wrong. They're bigger than that.
Randall Munroe as an XKCD fan had this
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excellent example of just how big
supernovae are so he asks which of the
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following would be brighter in terms of
the amount of energy delivered to your
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retina. Option 1: A supernova, seen from
as far away as the sun is from earth or
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Option 2: A hydrogen bomb pressed against
your eyeball. So which of these would be
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brighter. What do you think? You remember
the the rule of thumb we had earlier this
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supernova is bigger than that. In fact
it's a billion times bigger than that. So
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supernovae are some of the biggest bangs
since the original big bang. They also
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leave a neutron star or a black hole which
are really interesting objects to study in
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their own right and the outgoing shockwave
also leads to the creation of lots of new
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stars. But maybe most importantly
supernovae are where many of the chemical
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elements around you come from. So whether
it's things like the oxygen in your lungs
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or the calcium in your bones or the
silicon in your favorite computer chip.
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Life as we know it, and congress as we
know it could not exist without
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supernovae. And yet we don't actually
understand how these explosions happen...
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well I have no idea what's happening....
life as we know it could not exist without
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supernovae and yet we don't actually
understand how these explosions happen.
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And even observing them with telescopes
doesn't really help us because telescopes
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can only ever see the surface of a star.
They can't look inside the core of the
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star where the explosion actually takes
place. So that's why we need neutrinos.
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And we have tens of thousands of
supernovae with optical telescopes, with
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neutrinos we've observed just one. This
one in February of 1987. And we've seen
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two dozen neutrinos which you see here on
the right. That's what we know. So we know
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basically that there were many neutrinos
emitted during the first one second or so
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and then fewer and fewer for the next 10
seconds. We know that neutrinos make up
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most of the energy of the explosion of the
supernova, with the actual energy of the
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explosion and the visible light making
just up a tiny fraction. We know that the
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neutrinos arrive a few hours before the
light. And that's all, that's all we know.
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And still about these two dozen events,
these two dozen neutrinos more than
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sixteen hundred papers are written. That's
more than one paper a week for over 30
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years. So this gives you an idea of just
how important this event was. And how
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creative physicists are. Or I guess, you
could call it desperate.
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Laughter
But you know, I prefer creative. In fact
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this, you know, this one Supernova, we
observed was such big deal, that 30 years
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later February of last year we had a
conference in Tokyo on supernova neutrinos
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and we had a 30th anniversary celebration.
So there we were about 40 or 50 physicists
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looking over the skyline of Tokyo having
dinner, there's the now leader of the
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Super-Kamiokande experiment who was a PhD
student back then when it happened, and in
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his hand he's holding the original data
tape with the events that he himself
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analyzed back then. So there we were, and
at one point that evening we actually
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started to sing Happy Birthday. So you
know how it goes - happy birthday to you,
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happy birthday to you, happy birthday dear
supernova 1987-A. You know it absolutely
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doesn't work, but it was still amazing. So
that's all we know, and then there's what
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we think we know, and most of that comes
from computer simulations of supernovae.
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But the problem is, those are really
really hard. You know it's one of these
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extremely rare situations where all four
fundamental forces: gravity,
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electromagnetism, and the weak, and the
strong nuclear force, all play a role. You
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know normally, in particle physics you
don't have to worry about gravity, and in
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pretty much all other areas of physics you
only have to worry about gravity and
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electromagnetism. Here all four play a
role. You've also got nonlinear
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hydrodynamics of the gas and plasma inside
the star. You've got the matter moving
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relativistically at 10 or 20 percent of
the speed of light and you've got extreme
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pressures and extreme temperatures that
are sometimes beyond what we can produce
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in the laboratory on earth. So that's why
these simulations even in 2018 are still
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limited by the available computing power.
So we need to do a lot of approximations
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to actually get our code to run in a
reasonable time, but that gives you, some,
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that produces some problems, and in fact
the week I started my PhD one of those
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groups doing the supernova simulations
published a paper saying that there is a
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long list of numerical challenges and code
verification issues. Basically we're using
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this approximations and we don't know
exactly how much error they introduce, and
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the results of different groups are still
too far apart, and that's not because
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those people are dummies. Quite the
opposite they're some of the smartest
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people in the world. It's just that the
problem is so damn hard. In fact, in many
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of these simulations the stars don't even
explode on their own, and we don't know
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whether that means that some of these
approximations just introduce numerical
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errors which change the result, or whether
it means that there are some completely
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new physics happening in there which we
don't know about, or whether that is
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actually realistic and some stars you know
in the universe don't explode but just
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implode silently into a blackhole. We
don't know. We just don't know. So take
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any, you know, any results of the
simulations with a grain of salt. That said
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here's our best guess for what happens. So
we start out was a massive star that's at
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least eight times the mass of our sun, and
it starts fusing hydrogen to helium, and
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then on and on into heavier elements until
finally it reaches iron, and at that point
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fusion stops because you can't gain energy
from fusing two iron nuclei. So there the
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iron just accretes in the core of the star
while in the outer layers - here in orange
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- nuclear fusion is still going on, but as
more and more iron accretes and that core
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reaches about one-and-a-half solar masses
it can't hold its own weight anymore and
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it starts to collapse, and inside the core
in nuclear reactions you're starting to
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form neutrinos which I'm showing us ghost
emoji here. Now let's zoom in a bit. The
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core continues to collapse until at the
center it surpasses nuclear density, and
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at that point it's so dense that the
neutrinos are actually trapped in there.
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So even neutrinos which literally can go
through walls can not escape from there
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because matter is so dense, and the
incoming matter basically hits a wall
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because the matter in the center can't be
compressed any further, so just hits the
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wall and bounces back. So from that
collapse you suddenly get an outgoing
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shock wave, and in the wake of that
shockwave suddenly you get a whole burst
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of neutrinos which escape the star
quickly. Now that shockwave moves on and
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slows down and as it slows down the matter
from outer layer still falls in, and in
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this kind of collision region where
neutrinos in the center are still trapped
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in that collision region, neutrinos are,
you know, being produced at a relatively
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steady rate and the shock wave has pretty
much stopped and just wavers back and
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forth, and we see some neutrino emission.
Now after about half a second, maybe a
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second, neutrinos from the center are
slowly starting to escape. You know most
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of them are still trapped but some are
making their way outside and some of those
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actually manage to leave the star while
others interact with matter in this
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shockwave layer and give that matter a
little energy transfer a little push and
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heat it back up. So the shock wave gets
revived and the star actually explodes,
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and all of that took just one second, and
then over the next 10 seconds or so the
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neutrinos remaining at the core slowly
make their way outwards and then travel
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away at the speed of light hopefully to
Earth to our detector. While that shock wave
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moves much slower than the speed of light,
you know, slowly makes its way outwards
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and only a few hours later when that shock
wave reaches the surface of the star do we
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actually see something with telescopes.
So, remember earlier the neutrinos signal
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we saw was something like a bunch of
neutrinos in the first second and then
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fewer and fewer neutrinos for 10 seconds.
Not a lot of detail but what we might see
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is something like this: a brief and
intense burst when the matter hits the
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wall and is thrown back in this first
shock wave, then as the shock wave
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stagnates we might see some wiggles
corresponding to the shock wave, you know,
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sloshing around aimlessly until the shock
wave is revived. The explosion starts and
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then over the next 10 or so seconds we
would see fewer and fewer neutrinos as a
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star cools down and as neutrinos escape.
So if we have good neutrino detectors we
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should be able to watch, you know,
millisecond by millisecond what exactly
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happens inside the star. Now luckily we've
got many more neutrino detectors by now.
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Probably the biggest one is the Super-
Kamiokande in the Mozumi Mine which would
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see about 4000 events from an average
supernova in our Milky Way, and then we've
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got a bunch of other detectors which was
typically you know hundreds of events, and
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some of these detectors are part of
something called the supernova early
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warning system or SNEWS, and snooze is
meant to act as a wake up call to
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astronomers. So when when these detectors
observe neutrinos which are probably from
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a supernova they will send out an alert to
astronomers to get their telescopes ready
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to be able to see that supernova from the
very beginning and then of course just in
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the past few years we've also had
gravitation wave detectors like LIGO in
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the U.S, Virgo in Italy, and in just a few
years we will get another one called KAGRA
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which is located in Japan actually inside
the same mountain as Super-Kamiokande. So
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they're literally next door neighbors, and
then we might get another detector in
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India, maybe one China in the future. So
that's three completely different ways of
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looking at supernovae. So when we observe
a supernova it will be headline news, and
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now you know what's behind those
headlines. So I've introduced you to
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neutrinos, I've told you a bit about what
it's like to work on on such a detector
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and, you know the challenges of building a
detector of this scale and I've showed you
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how with neutrinos we can observe things
but we can't directly observe otherwise
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like the interior of exploding stars, and
with that I want to thank you for your
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attention and please let me know if you
have any questions.
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applause
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Herald: Thank you Jost, it was an amazing
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talk. We have plenty of time for questions
and there are two microphones. Microphone
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one is on the left side of the stage,
microphone two is in the middle so queue
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up and we're going to take some questions.
First question from microphone two.
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Q: Yeah, thank you, I do have a question -
I come from a mining area and I just
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looked up how deep other mines go and I'm
wondering why do you dig into useless rock
385
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if you can just go to some area where
there are mines that are no longer used,
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my area they go as deep as 1,200 metres I
think. I just looked up and was surprised
387
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that the deepest mine on Earth is almost
four kilometres in South Africa - an
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active goldmine - so why don't you use
those?
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A: So, part of why we're using that
particular location is because we used it
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for Super-K and Kamiokande before, and the
mountain that Kamiokande was in actually
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is a mine. So we had some previous
infrastructure there, and then there's I
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guess some tradeoff between the benefits
you get from going deeper and deeper and
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the additional cost I think.
Herald: Thank you, we have a question from
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the Internet, that's going to be narrated
by our wonderful Signal Angel.
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Jost: Hello Internet. So the question, I
didn't understand then whole question, but
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something about earthquake. Okay.
Q: Does the earthquake affect the
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detector.
A: Well there's two parts of the answer:
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Part one I'm not a geologist. Part two I
think the earthquakes are mostly centered
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in the cavern on the east coast of Japan
and we're about 200-300 kilometers away
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00:43:06,440 --> 00:43:15,009
from there, so the region we're in is
relatively stable, and in fact we've been
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running, since 1983 and we haven't had
problems with earthquakes, and during the
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Fukushima earthquake our detector was
mostly fine but we've actually had, so in
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addition to what I was talking about we're
also producing a beam of neutrinos at an
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accelerator which we shoot at the
detector, and that accelerator is right at
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the east coast. So, the only damage from
the Fukushima earthquake was to that
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00:43:48,510 --> 00:43:57,379
accelerator, not to the detector itself. I
hope that answers your question.
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Herald: Next is from microphone two.
Q: Hello, thanks for an interesting talk.
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Do you, or does science, have any theory
if the neutrinos who hit the electron are
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affected themselves from this hit, are
they like directed in another direction or
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lose some sort of energy themselves or
just hit the electron and pass through.
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00:44:22,979 --> 00:44:27,999
A: So, conservation of energy and of
momentum still holds, so they would lose
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some energy as they give the electron a
little kick.
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00:44:31,289 --> 00:44:34,849
OK. Thank you.
Herald: Thank you, one more question from
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mic two.
Q: Hello, thanks for your talk. My
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00:44:38,279 --> 00:44:44,359
question is you said that the only
supernova where we detected some neutrinos
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from is from the 80's. So what is so
special about that supernova that with all
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00:44:50,509 --> 00:44:54,670
the new detectors built there was never
another detection?
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00:44:54,670 --> 00:45:00,149
A: So, the special thing about that one is
that it was relatively close. So it was in
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00:45:00,149 --> 00:45:05,940
the Large Magellanic Cloud about 150,000
light years away which is, you know, on
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00:45:05,940 --> 00:45:13,279
cosmic scales our next door neighbor,
whereas other supernovae which we observe
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can be you know millions of light years
away. We can easily see them at that
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00:45:17,059 --> 00:45:23,249
distance but we can't detect any
neutrinos, and we expect about between 1
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and 3 supernovae in our Milky Way per
century, so we're in this for the long
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term. Okay.
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Herald: Thank you, microphone two again
please.
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Q: Hi, thanks for your talk. My question
is you said that changing the water once a
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year is not often enough, how often do you
change the water?
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A: How often do we change the water in the
detector? - Yes - So we completely drain
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and refill the detector only for repair
work which, you know, happens every
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depending on what we want to do but
typically every couple of years to, you
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know, 10 plus years and apart from that we
recirculate the water all the time to
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purify it because there will always be
some traces of radioactivity from the
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surrounding rock which make the way in the
water over time.
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Mic 2: Thank you.
Herald: Thank you, and that would be all,
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that was a wonderful start to the
Congress, thank you Jost.
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applause
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