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35c3 preroll music
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Herald: Our speaker is Jost Migenda and he[br]is PhD student in astroparticle physics
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from the University of Sheffield in the UK[br]and Jost is going to talk about going deep
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underground to watch the stars. Please[br]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[br]managed the first day of congress. Now
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physics rarely makes highlight news. And[br]if and when it does it is often treated a
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black box, where you pourd in money and[br]scientist on one end, you wait a while and
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knowledge drops out. So today in this talk[br]I want to do this a bit differently. I
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want to give you a glimpse behind the[br]scenes of an experiment, I have been
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working on for over 4 years now. First as[br]part of my master's thesis and then as a
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PhD student. Now earlier this year we[br]published a design report which is over
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300 pages long and contains much more[br]detail about the experiment than you
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probably want to know. So I'll focus on[br]just some of the highlights in this talk.
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But before we actually talk about the[br]detector I'll have to introduce you to the
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particles we're looking for. And that[br]story begins over 100 years ago with
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radioactive beta decay. Now in radioactive[br]beta decay, you have a nucleus of one
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chemical element that turns into a nucleus[br]of a different element and emits an
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electron or in modern language we would[br]say a neutron decays into a proton and an
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electron. Now after that was discovered[br]there were lots of experiments done to
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measure the energy of the outgoing[br]electron and experiment after experiment
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found that there was some variance in[br]energy but was always lower than expected.
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And physicists at the time came up with[br]all sorts of possible explanations for
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what might be going wrong with these[br]experiments but they excluded those
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explanations very quickly as well. So[br]after a while physicists became desperate
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and some pretty well-known physicists[br]actually thought: "Well, maybe we'll just
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have to give up on conservation of[br]energy". So in this desperate situation a
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guy called Wolfgang Pauli came up with[br]what he himself call "a desperate way
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out". So in this letter to a group of his[br]colleagues which he addressed as "Dear
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radioactive ladies and gentlemen", Pauli[br]suggested that maybe there's another
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particle created in this beta decay. And[br]Pauli originally called this particle
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neutron but of course two years later the[br]particle we nowadays know as Neutron was
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discovered. So Pauli's particle was re-[br]named neutrino. Now you might be wondering
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well why didn't they observe this particle[br]already. And the answer is very simple.
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Neutrinos are like ghosts. So what I mean[br]by there is they can quite literally, you
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know, go through walls or through your[br]body. And in effect we can do a little
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experiment right now to try and detect[br]neutrinos. So to help me with this
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experiment, please give me thumb's up.[br]Everyone? Okay so there's two things
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happening right now. First thing of all[br]you're giving me a massive confidence
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boost. But, you know, more importantly[br]somewhere out there the sun is shining and
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it's producing a lot of neutrinos and[br]nuclear fusion. Now these neutrinos are
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flying to Earth through the roof of this[br]building and then through your thumbnail.
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And right now as you're listening to me[br]around 60 billion neutrinos are flying
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through your thumbnail. 60 billion[br]neutrinos flying through your thumbnail
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every second. How does that feel? Hmm? You[br]don't feel any of them? Right, so that's
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how ghost-like neutrinos are. And of[br]course physicists are clever and shortly
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after Pauli had this idea some of them[br]estimated that how often neutrinos
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interact with normal matter and they found[br]that there is no practically possible way
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of observing the neutrino. And that[br]remained true for over 20 years
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afterwards. So now that I have introduced[br]you to neutrinos. Let's talk about
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building a detector to actually detect[br]them. And the original motivation for
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building this detector was something a bit[br]different. I talked about beta decay and
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over the next decades physicists slowly[br]discovered more particles. They discovered
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that protons and neutrons are made up out[br]of quarks and in the 1970s theoretical
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physicists came up was are some Grand[br]Unified Theories basically precursors to
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string theory. And these theories[br]predicted that the proton should decay as
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well. So of course you know we build[br]detectors to look for that and a group in
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Japan built a detector near the town of[br]Kamioka which they called the Kamioka
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Nucleon Decay Experiment or Kamiokande for[br]short. Now they didn't observe any proton
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decay but shortly after they built it[br]somebody had a suggestion that if we
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changed just a little bit above their[br]detector, if we modified just a little, we
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would also be able to detect neutrinos[br]with that. So they modified the detector
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switched it back on and just a couple of[br]weeks later they actually observe
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neutrinos from an exploding star just[br]outside our Milky Way. And that was the
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birth of neutrino astronomy. And for that[br]the then-leader of the experiment received
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the Nobel Prize in 2002. Now after over a[br]decade of running physicists were
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basically hitting the limits of what we[br]could do with a detector of their size. So
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we needed to build a bigger detector and[br]that one was very creatively named Super-
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Kamiokande. And it's about 20 times[br]bigger, started running in 1996 and still
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running to this day. Now Super-K did not[br]discover proton decay but did detect a lot
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of neutrinos and made very fascinating[br]discoveries. For example, they discovered
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that different types of neutrinos can[br]change into each other back and forth as
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they travel. That's like you buying a cone[br]of vanilla ice cream and then as you walk
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out it suddenly turns into chocolate ice[br]cream. That's really weird. And for their
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discovery just a few years ago they[br]received the Nobel Prize again. But today
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we are again hitting the limit of you know[br]what we can learn from a detector that
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size. So of course the next step is to[br]build an even bigger detector and we're
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calling it Hyper-Kamiokande. By the way[br]"super" and "hyper" mean exactly the same
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thing. Just one is Latin and one is Greek.[br]So we're currently getting ready the plans
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to build Hyper-Kamiokande and we will[br]start construction probably in the spring
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of 2020. Details of the Noble Prize are[br]still to be determined. Now I said that 60
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billion neutrinos go through your[br]thumbnail every second. Of course Super-
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Kamiokande which is running right now is[br]much larger than your thumbnail. So
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there's not just 60 billion but 10000[br]billion billion neutrinos passing through
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every day and only 10 or 15 of those get[br]detected. So let's look at what this
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detection process looks like. Now this is[br]the water inside Super-K. And there's a
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bunch of electrons in there but they'll[br]show just one. And there's neutrinos
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flying through not just one, not just a[br]few, but loads of them. And most of them
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go straight through without leaving a[br]trace. But every once in a while we're
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lucky and one of those neutrinos will[br]actually hit the electron and give it a
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little kick. And that little kick you know[br]like billiard balls basically and that
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little kick accelerates the electron to[br]faster than the speed of light in water.
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Still slower than the speed of light in[br]vacuum which is the absolute cosmic tempo
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limit. But faster than the speed of light[br]in water. And then you get basically a
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sonic boom, but with light, which is this[br]cone of light. And let's just show the
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animation again. So you've got this cone[br]of light that hits the wall of the
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detector you see a little ring, this ring[br]of light. Well we've got very sensitive
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photo sensors all over the inside walls to[br]detect this flash of light and from how
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bright it is we can tell the energy of the[br]neutrino. And we can also tell, you know
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just like was billiard balls, we can[br]approximately tell what directions a
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neutrino came from just based on in which[br]direction it pushed the electron. So
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that's the basic idea how we detect[br]neutrinos from the sun. Now let's talk
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about what it's actually like to build one[br]of these detectors. So this is a drawing
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of Hyper-Kamiokande and you can see it's[br]78 meters high, 74 meters in diameter and
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on the top left there is a truck for[br]comparison. But maybe a better size
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comparison is to compare this to buildings[br]which you're familiar with. Like the
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entrance hall which you just came in[br]through this morning. Or the Statue of
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Liberty and it doesn't quite fit in there.[br]The arm still looks out. But you could
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drown the Statue of Liberty in this[br]detector which nowadays is probably some
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sort of political metaphor. So this is the[br]giant detector. And what's more we're
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building it inside the mountain about 650[br]meters underground. So that all the rock
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on top will act as a kind of a natural[br]shield against all sorts of other
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particles, that are raining down on the[br]atmosphere from outer space so that all
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other particles get stuck and only[br]Neutrinos can make it through. Now of
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course to build such a huge cavern inside[br]the mountain - that's something that we
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physicists can't do on our own. So we need[br]to talk to geologists who look at the rock
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quality and tell us, you know, what's a[br]good place to build this cavern - where is
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the rock stable enough to do that. And to[br]figure out the rock quality, they drill
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bore holes in what's actually called a[br]boring survey. laughter Now, during my
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years working on this experiment, I had to[br]listen to several hours of talks on these
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geological surveys and I can tell you that[br]name is quite appropriate. laughter
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though of course, there's a reason I'm not[br]a geologist, so, you know, take this with
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a grain of salt. But okay, let's say, you[br]know, we talked to geologists, they told
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us where we can build our detector. The[br]next step is: We need to actually excavate
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the cavern. And something to keep in mind[br]is that we are building this somewhere in
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the mountains of Japan, you know, pretty[br]far away from any major city. So we have
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to think about stuff like lack of local[br]infrastructure like what's the electricity
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supply like. Do we need to add the power[br]line. Or what are the local roads like.
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And do they have enough capacity for, you[br]know, dozens of trucks every day to
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transport away the excavated rock. And, by[br]the way, where do you store all that
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excavated rock? Because we will be moving[br]something like half a million cubic meters
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of rock. You can't just store that in your[br]backyard. You need to find a place where
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all that fits. And, of course, if you've[br]listened to or watch the Lord of the
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Rings, you'll know it's dangerous to dig[br]too deeply, to greedily. So we need a
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Balrog early warning system as well. But[br]okay let's say we've got all those and we
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managed to build a cavern, and now we need[br]to fill it. And as detector material we
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use water. Both because it's actually[br]pretty good for detecting neutrinos, but
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also because it's cheap and there's lots[br]of it. So you can afford to build a
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detector of this size. A detector so big[br]that that little dot there is a scuba
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diver. But even with water, you hit limits[br]of, you know, how much you can get. So to
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fill Hyper-Kamiokande you need about as[br]much water as 5000 people use in a year.
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And that's for drinking, for showering,[br]for washing their car and so on. Now
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that's easy if you're near a big city. But[br]we are not, we're somewhere in the
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mountains in Japan where the next biggest[br]town has far fewer than 5000 people. So
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how do we get enough water to actually[br]fill our detector? And... we could use
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rivers nearby, we could use springs. We[br]could wait for for the end of winter and
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for the snow in the mountains to melt and[br]use that to fill our detector. But if you
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use melting snow to fill the detector, you[br]can only fill it once a year. So, you
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know, even "where do you get the water" is[br]is a pretty... pretty important question
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that you need to solve. And then, we're[br]not just using any water but we actually
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have, we will build our own water[br]purification system. So that we don't have
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any, you know, traces of radioactivity in[br]there, any trace of dust and stuff in
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there. And let me let me just tell you[br]just how pure this water will be. So, this
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is my supervisor, who, when he was a PhD[br]student, worked in the detector on some
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maintenance work, so he was working on a[br]boat doing the work, and then at the end
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of his shift he leaned back on the boat,[br]and just the tip of his long hair fell
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into the water, which he didn't know just[br]didn't think about too much until at the
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end of his shift. He went home, you know,[br]went to bed, fell asleep, and then woke up
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in the middle of the night, with his whole[br]head itching like mad. Now, what had
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happened there? The ultra pure water had[br]sucked all of the nutrients out the tip of
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his hair and then through osmosis over[br]time those had sucked the nutrients out of
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the rest of his hair, and then his skin.[br]So that's how pure that water is. Now, I
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said "all over the inside walls". And here[br]is, kind of, a photo of the inside of the
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detector. And all these kind of golden[br]hemispheres, those are what we call
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Photomultiplier tubes, of PMTs for short.[br]And those are, basically, giant very
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sensitive pixels. And we will have 40,000[br]of those lining the inside wall of the
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detector. Now, in smaller detectors, you[br]could just have from each PMT one cable
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leading to the top of the detector and[br]then have your computers there to analyze
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the signal. With a detector of this size,[br]you just can't do that because you would
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need 40,000 cables some of which are over[br]100 meters long. That wouldn't work. So we
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have to put some electronics in the water[br]to digitize the signal and combine signal
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from multiple PMTs into one, and then use[br]just a single cable, to bring it up
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to the top where we analyze the signal.[br]But that means we have to put electronics
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into the water, so that creates a whole[br]bunch of new problems. For example, we
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need these electronics to be watertight.[br]And I'm not talking about the level of
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watertight as you'd expect from your[br]smartwatch where it survives, you know,
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you standing under the shower for five[br]minutes. I'm talking below 60 metres of
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water for 20+ years. This electronics also[br]need to be very low power, because we
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can't heet up the water too much.[br]Otherwise these pixels, the
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Photomultiplier Tubes, would become noisy[br]and this would kill our signal. And then
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also, because we don't want one defective[br]cable to kill a whole section of the
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detector. We need to implement some sort[br]of mesh networking to introduce some
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redundancy. Now each of that by itself is[br]not... not a heart problem. Each of these
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problems can be solved. It's just a lot of[br]additional work you suddenly have to do,
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because your detectors is that huge. And[br]it gets even worse: This is what one of
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these PMTs looks like. It's about 50[br]centimeters in diameter, and inside that
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glass bulb is a vacuum. So it's under a[br]lot of pressure. Plus we add 60 metres of
0:20:06.760,0:20:12.370
water on top of it, which adds additional[br]pressure. So you need to make absolutely
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sure when you're manufacturing those that[br]you don't have any weak points in the
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glass. And they don't just have to[br]withstand that water pressure, but there
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will probably be some PMTs that have some[br]weak points, some air bubbles or something
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in the glass. Some structural weakness.[br]And the neighboring PMT don't only have to
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survive the normal water pressure. They[br]also need to survive whether their
0:20:44.950,0:20:51.100
neighboring PMT is imploding and sending[br]out a pressure wave. And that's not just a
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hypothetical - that actually happened, you[br]know, 18 years ago. It's 17 years ago in
0:20:57.950,0:21:05.120
Superkamiokande. And that, you know,[br]within seconds killed more than half of
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the PMTs we had in there and it took years[br]to restore the detector to full capacity.
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So, lots of problems to solve, and, you[br]know, one group, one university can't
0:21:19.830,0:21:24.539
solve all these on their own. So we've got[br]this multinational collaboration with over
0:21:24.539,0:21:29.919
300 people from 17 different countries[br]marked in green here and across many
0:21:29.919,0:21:36.850
different time zones. So right here right[br]now, here it's about you know just before
0:21:36.850,0:21:42.430
noon. In Japan people have already had[br]dinner and they're going to bed soon. In
0:21:42.430,0:21:47.410
the U.S., people haven't even got up yet[br]in the morning. So good luck finding a
0:21:47.410,0:21:57.540
time for phone meetings which works for[br]all of these people. So, that's kind of a
0:21:57.540,0:22:01.720
glimpse behind the scenes of what it's[br]like to work on this detector and to
0:22:01.720,0:22:07.780
actually build it. But now I want to talk[br]about what we use the detector for. And
0:22:07.780,0:22:13.330
I've got two examples. But of course[br]there's a whole bunch more that we do, I
0:22:13.330,0:22:23.940
just don't have time to talk about it[br]today. So, first example: Why does the sun
0:22:23.940,0:22:30.060
shine? That seems like such a simple[br]question, right? And yet it turns out it's
0:22:30.060,0:22:36.330
really difficult to answer. So, in the[br]days of the Industrial Revolution, when,
0:22:36.330,0:22:41.480
you know, burning coal and steam power was[br]all the rage, people thought that, you
0:22:41.480,0:22:48.120
know, maybe it's, you know, a giant ball[br]of burning coal. But when you when you do
0:22:48.120,0:22:54.780
the math, it turns out that would burn for[br]a few thousand years maybe. So that
0:22:54.780,0:23:01.000
definitely doesn't work. A bit later,[br]physicists suggested that maybe the sun is
0:23:01.000,0:23:06.530
just slowly shrinking and shrinking and[br]it's that gravitational energy which is
0:23:06.530,0:23:13.050
released as light. And that would give you[br]a life time of a few million years. But
0:23:13.050,0:23:17.070
then you've got, you know pesky geologists[br]coming along and saying "no no, we've got
0:23:17.070,0:23:21.830
these rock formations or, I don't know,[br]fossils maybe, for more than a few million
0:23:21.830,0:23:28.310
years old on Earth. So the sun has to live[br]longer than a few million years. And the
0:23:28.310,0:23:33.750
arguments from the 19th century between[br]Lord Kelvin and the geologists back then
0:23:33.750,0:23:41.400
are just amazing to read. If you find[br]those somewhere. But, of course, nowadays
0:23:41.400,0:23:49.250
we know that the answer is nuclear fusion.[br]And here's, you know, a bunch of reactions
0:23:49.250,0:23:54.790
which lead you the energy generation of[br]the sun. But now the question is "how can
0:23:54.790,0:24:02.720
we check that? How can we check that this[br]is actually what's going on?" And the
0:24:02.720,0:24:08.570
answer is neutrinos, because many of these[br]reactions produce neutrinos, and we can
0:24:08.570,0:24:16.309
detect them. So the one we typically[br]detect in Super- and later Hyperkamiokande
0:24:16.309,0:24:21.220
is the one on the bottom right here,[br]called "Bore and eight neutrinos" because
0:24:21.220,0:24:28.929
those have the highest energy. So they are[br]easiest for us to detect. And the rate of
0:24:28.929,0:24:35.559
various of these processes depends very[br]much on the temperature. So by measuring
0:24:35.559,0:24:40.320
how many of these neutrinos we see, we can[br]measure the temperature inside the core of
0:24:40.320,0:24:48.650
the sun. And we have done that, and we[br]know that it's about 15.5 million Kelvin, plus or
0:24:48.650,0:24:54.740
minus 1 percent. So we know the[br]temperature in the core of the sun to less
0:24:54.740,0:25:05.640
than 1 percent uncertainty. That's pretty[br]amazing if you ask me. And, you know, I
0:25:05.640,0:25:10.560
said that we could detect the directions[br]the neutrinos were coming from. So we can
0:25:10.560,0:25:16.700
actually take a picture of the sun with[br]neutrinos. Now this is a bit blurry and
0:25:16.700,0:25:21.400
pixelated, not as nice as what you'd get[br]from a, you know, from the optical
0:25:21.400,0:25:27.810
telescope. But this is still a completely[br]different way of looking at the sun. And
0:25:27.810,0:25:33.600
what this tells us is that this, you know,[br]giant glowing orb in the sky. That's not
0:25:33.600,0:25:45.900
some optical illusion but that actually[br]exists. Okay. So onto our next topic.
0:25:45.900,0:25:54.279
Exploding stars or supernovae which is[br]what my own research is mostly about. So
0:25:54.279,0:25:59.669
supernovae are these giant explosions[br]where one single star, like in this
0:25:59.669,0:26:05.760
example here, can shine about as bright as[br]a whole galaxy consisting of billions of
0:26:05.760,0:26:15.060
stars. And the rule of thumb is this:[br]However big you think supernovae are
0:26:15.060,0:26:23.830
you're wrong. They're bigger than that.[br]Randall Munroe as an XKCD fan had this
0:26:23.830,0:26:30.290
excellent example of just how big[br]supernovae are so he asks which of the
0:26:30.290,0:26:33.980
following would be brighter in terms of[br]the amount of energy delivered to your
0:26:33.980,0:26:42.441
retina. Option 1: A supernova, seen from[br]as far away as the sun is from earth or
0:26:42.441,0:26:49.920
Option 2: A hydrogen bomb pressed against[br]your eyeball. So which of these would be
0:26:49.920,0:27:00.280
brighter. What do you think? You remember[br]the the rule of thumb we had earlier this
0:27:00.280,0:27:08.769
supernova is bigger than that. In fact[br]it's a billion times bigger than that. So
0:27:08.769,0:27:15.340
supernovae are some of the biggest bangs[br]since the original big bang. They also
0:27:15.340,0:27:19.990
leave a neutron star or a black hole which[br]are really interesting objects to study in
0:27:19.990,0:27:26.240
their own right and the outgoing shockwave[br]also leads to the creation of lots of new
0:27:26.240,0:27:33.770
stars. But maybe most importantly[br]supernovae are where many of the chemical
0:27:33.770,0:27:40.779
elements around you come from. So whether[br]it's things like the oxygen in your lungs
0:27:40.779,0:27:46.860
or the calcium in your bones or the[br]silicon in your favorite computer chip.
0:27:46.860,0:27:51.539
Life as we know it, and congress as we[br]know it could not exist without
0:27:51.539,0:28:01.569
supernovae. And yet we don't actually[br]understand how these explosions happen...
0:28:01.569,0:28:18.890
well I have no idea what's happening....[br]life as we know it could not exist without
0:28:18.890,0:28:25.560
supernovae and yet we don't actually[br]understand how these explosions happen.
0:28:25.560,0:28:31.380
And even observing them with telescopes[br]doesn't really help us because telescopes
0:28:31.380,0:28:37.059
can only ever see the surface of a star.[br]They can't look inside the core of the
0:28:37.059,0:28:44.529
star where the explosion actually takes[br]place. So that's why we need neutrinos.
0:28:44.529,0:28:51.020
And we have tens of thousands of[br]supernovae with optical telescopes, with
0:28:51.020,0:28:58.710
neutrinos we've observed just one. This[br]one in February of 1987. And we've seen
0:28:58.710,0:29:06.889
two dozen neutrinos which you see here on[br]the right. That's what we know. So we know
0:29:06.889,0:29:11.770
basically that there were many neutrinos[br]emitted during the first one second or so
0:29:11.770,0:29:18.980
and then fewer and fewer for the next 10[br]seconds. We know that neutrinos make up
0:29:18.980,0:29:25.990
most of the energy of the explosion of the[br]supernova, with the actual energy of the
0:29:25.990,0:29:32.390
explosion and the visible light making[br]just up a tiny fraction. We know that the
0:29:32.390,0:29:42.450
neutrinos arrive a few hours before the[br]light. And that's all, that's all we know.
0:29:42.450,0:29:46.830
And still about these two dozen events,[br]these two dozen neutrinos more than
0:29:46.830,0:29:52.710
sixteen hundred papers are written. That's[br]more than one paper a week for over 30
0:29:52.710,0:29:59.730
years. So this gives you an idea of just[br]how important this event was. And how
0:29:59.730,0:30:03.770
creative physicists are. Or I guess, you[br]could call it desperate.
0:30:03.770,0:30:09.079
Laughter[br]But you know, I prefer creative. In fact
0:30:09.079,0:30:14.740
this, you know, this one Supernova, we[br]observed was such big deal, that 30 years
0:30:14.740,0:30:22.690
later February of last year we had a[br]conference in Tokyo on supernova neutrinos
0:30:22.690,0:30:30.700
and we had a 30th anniversary celebration.[br]So there we were about 40 or 50 physicists
0:30:30.700,0:30:38.270
looking over the skyline of Tokyo having[br]dinner, there's the now leader of the
0:30:38.270,0:30:43.680
Super-Kamiokande experiment who was a PhD[br]student back then when it happened, and in
0:30:43.680,0:30:49.650
his hand he's holding the original data[br]tape with the events that he himself
0:30:49.650,0:30:56.830
analyzed back then. So there we were, and[br]at one point that evening we actually
0:30:56.830,0:31:04.419
started to sing Happy Birthday. So you[br]know how it goes - happy birthday to you,
0:31:04.419,0:31:11.779
happy birthday to you, happy birthday dear[br]supernova 1987-A. You know it absolutely
0:31:11.779,0:31:21.029
doesn't work, but it was still amazing. So[br]that's all we know, and then there's what
0:31:21.029,0:31:26.689
we think we know, and most of that comes[br]from computer simulations of supernovae.
0:31:26.689,0:31:33.440
But the problem is, those are really[br]really hard. You know it's one of these
0:31:33.440,0:31:38.880
extremely rare situations where all four[br]fundamental forces: gravity,
0:31:38.880,0:31:44.910
electromagnetism, and the weak, and the[br]strong nuclear force, all play a role. You
0:31:44.910,0:31:49.399
know normally, in particle physics you[br]don't have to worry about gravity, and in
0:31:49.399,0:31:53.899
pretty much all other areas of physics you[br]only have to worry about gravity and
0:31:53.899,0:31:59.620
electromagnetism. Here all four play a[br]role. You've also got nonlinear
0:31:59.620,0:32:05.170
hydrodynamics of the gas and plasma inside[br]the star. You've got the matter moving
0:32:05.170,0:32:09.620
relativistically at 10 or 20 percent of[br]the speed of light and you've got extreme
0:32:09.620,0:32:14.320
pressures and extreme temperatures that[br]are sometimes beyond what we can produce
0:32:14.320,0:32:23.470
in the laboratory on earth. So that's why[br]these simulations even in 2018 are still
0:32:23.470,0:32:28.260
limited by the available computing power.[br]So we need to do a lot of approximations
0:32:28.260,0:32:34.929
to actually get our code to run in a[br]reasonable time, but that gives you, some,
0:32:34.929,0:32:41.429
that produces some problems, and in fact[br]the week I started my PhD one of those
0:32:41.429,0:32:46.210
groups doing the supernova simulations[br]published a paper saying that there is a
0:32:46.210,0:32:52.080
long list of numerical challenges and code[br]verification issues. Basically we're using
0:32:52.080,0:32:57.890
this approximations and we don't know[br]exactly how much error they introduce, and
0:32:57.890,0:33:03.390
the results of different groups are still[br]too far apart, and that's not because
0:33:03.390,0:33:06.670
those people are dummies. Quite the[br]opposite they're some of the smartest
0:33:06.670,0:33:14.799
people in the world. It's just that the[br]problem is so damn hard. In fact, in many
0:33:14.799,0:33:20.090
of these simulations the stars don't even[br]explode on their own, and we don't know
0:33:20.090,0:33:25.070
whether that means that some of these[br]approximations just introduce numerical
0:33:25.070,0:33:31.390
errors which change the result, or whether[br]it means that there are some completely
0:33:31.390,0:33:36.340
new physics happening in there which we[br]don't know about, or whether that is
0:33:36.340,0:33:44.149
actually realistic and some stars you know[br]in the universe don't explode but just
0:33:44.149,0:33:52.730
implode silently into a blackhole. We[br]don't know. We just don't know. So take
0:33:52.730,0:34:02.280
any, you know, any results of the[br]simulations with a grain of salt. That said
0:34:02.280,0:34:09.029
here's our best guess for what happens. So[br]we start out was a massive star that's at
0:34:09.029,0:34:15.179
least eight times the mass of our sun, and[br]it starts fusing hydrogen to helium, and
0:34:15.179,0:34:22.659
then on and on into heavier elements until[br]finally it reaches iron, and at that point
0:34:22.659,0:34:31.440
fusion stops because you can't gain energy[br]from fusing two iron nuclei. So there the
0:34:31.440,0:34:37.378
iron just accretes in the core of the star[br]while in the outer layers - here in orange
0:34:37.378,0:34:43.668
- nuclear fusion is still going on, but as[br]more and more iron accretes and that core
0:34:43.668,0:34:50.839
reaches about one-and-a-half solar masses[br]it can't hold its own weight anymore and
0:34:50.839,0:34:55.819
it starts to collapse, and inside the core[br]in nuclear reactions you're starting to
0:34:55.819,0:35:04.770
form neutrinos which I'm showing us ghost[br]emoji here. Now let's zoom in a bit. The
0:35:04.770,0:35:13.279
core continues to collapse until at the[br]center it surpasses nuclear density, and
0:35:13.279,0:35:21.669
at that point it's so dense that the[br]neutrinos are actually trapped in there.
0:35:21.669,0:35:27.469
So even neutrinos which literally can go[br]through walls can not escape from there
0:35:27.469,0:35:35.779
because matter is so dense, and the[br]incoming matter basically hits a wall
0:35:35.779,0:35:39.690
because the matter in the center can't be[br]compressed any further, so just hits the
0:35:39.690,0:35:46.209
wall and bounces back. So from that[br]collapse you suddenly get an outgoing
0:35:46.209,0:35:51.299
shock wave, and in the wake of that[br]shockwave suddenly you get a whole burst
0:35:51.299,0:35:58.650
of neutrinos which escape the star[br]quickly. Now that shockwave moves on and
0:35:58.650,0:36:05.920
slows down and as it slows down the matter[br]from outer layer still falls in, and in
0:36:05.920,0:36:13.390
this kind of collision region where[br]neutrinos in the center are still trapped
0:36:13.390,0:36:18.660
in that collision region, neutrinos are,[br]you know, being produced at a relatively
0:36:18.660,0:36:25.140
steady rate and the shock wave has pretty[br]much stopped and just wavers back and
0:36:25.140,0:36:32.529
forth, and we see some neutrino emission.[br]Now after about half a second, maybe a
0:36:32.529,0:36:37.589
second, neutrinos from the center are[br]slowly starting to escape. You know most
0:36:37.589,0:36:44.079
of them are still trapped but some are[br]making their way outside and some of those
0:36:44.079,0:36:48.809
actually manage to leave the star while[br]others interact with matter in this
0:36:48.809,0:36:54.410
shockwave layer and give that matter a[br]little energy transfer a little push and
0:36:54.410,0:37:02.670
heat it back up. So the shock wave gets[br]revived and the star actually explodes,
0:37:02.670,0:37:09.520
and all of that took just one second, and[br]then over the next 10 seconds or so the
0:37:09.520,0:37:16.410
neutrinos remaining at the core slowly[br]make their way outwards and then travel
0:37:16.410,0:37:22.719
away at the speed of light hopefully to[br]Earth to our detector. While that shock wave
0:37:22.719,0:37:27.160
moves much slower than the speed of light,[br]you know, slowly makes its way outwards
0:37:27.160,0:37:32.999
and only a few hours later when that shock[br]wave reaches the surface of the star do we
0:37:32.999,0:37:41.049
actually see something with telescopes.[br]So, remember earlier the neutrinos signal
0:37:41.049,0:37:45.759
we saw was something like a bunch of[br]neutrinos in the first second and then
0:37:45.759,0:37:53.760
fewer and fewer neutrinos for 10 seconds.[br]Not a lot of detail but what we might see
0:37:53.760,0:37:58.730
is something like this: a brief and[br]intense burst when the matter hits the
0:37:58.730,0:38:03.680
wall and is thrown back in this first[br]shock wave, then as the shock wave
0:38:03.680,0:38:08.690
stagnates we might see some wiggles[br]corresponding to the shock wave, you know,
0:38:08.690,0:38:17.269
sloshing around aimlessly until the shock[br]wave is revived. The explosion starts and
0:38:17.269,0:38:22.079
then over the next 10 or so seconds we[br]would see fewer and fewer neutrinos as a
0:38:22.079,0:38:29.010
star cools down and as neutrinos escape.[br]So if we have good neutrino detectors we
0:38:29.010,0:38:34.099
should be able to watch, you know,[br]millisecond by millisecond what exactly
0:38:34.099,0:38:43.430
happens inside the star. Now luckily we've[br]got many more neutrino detectors by now.
0:38:43.430,0:38:46.690
Probably the biggest one is the Super-[br]Kamiokande in the Mozumi Mine which would
0:38:46.690,0:38:52.359
see about 4000 events from an average[br]supernova in our Milky Way, and then we've
0:38:52.359,0:38:58.069
got a bunch of other detectors which was[br]typically you know hundreds of events, and
0:38:58.069,0:39:01.579
some of these detectors are part of[br]something called the supernova early
0:39:01.579,0:39:07.789
warning system or SNEWS, and snooze is[br]meant to act as a wake up call to
0:39:07.789,0:39:14.130
astronomers. So when when these detectors[br]observe neutrinos which are probably from
0:39:14.130,0:39:20.309
a supernova they will send out an alert to[br]astronomers to get their telescopes ready
0:39:20.309,0:39:27.150
to be able to see that supernova from the[br]very beginning and then of course just in
0:39:27.150,0:39:31.769
the past few years we've also had[br]gravitation wave detectors like LIGO in
0:39:31.769,0:39:40.009
the U.S, Virgo in Italy, and in just a few[br]years we will get another one called KAGRA
0:39:40.009,0:39:45.709
which is located in Japan actually inside[br]the same mountain as Super-Kamiokande. So
0:39:45.709,0:39:50.260
they're literally next door neighbors, and[br]then we might get another detector in
0:39:50.260,0:39:56.300
India, maybe one China in the future. So[br]that's three completely different ways of
0:39:56.300,0:40:05.200
looking at supernovae. So when we observe[br]a supernova it will be headline news, and
0:40:05.200,0:40:10.209
now you know what's behind those[br]headlines. So I've introduced you to
0:40:10.209,0:40:16.631
neutrinos, I've told you a bit about what[br]it's like to work on on such a detector
0:40:16.631,0:40:22.479
and, you know the challenges of building a[br]detector of this scale and I've showed you
0:40:22.479,0:40:27.519
how with neutrinos we can observe things[br]but we can't directly observe otherwise
0:40:27.519,0:40:33.619
like the interior of exploding stars, and[br]with that I want to thank you for your
0:40:33.619,0:40:36.949
attention and please let me know if you[br]have any questions.
0:40:36.949,0:40:51.044
applause
0:40:51.044,0:40:53.019
Herald: Thank you Jost, it was an amazing
0:40:53.019,0:40:59.530
talk. We have plenty of time for questions[br]and there are two microphones. Microphone
0:40:59.530,0:41:03.630
one is on the left side of the stage,[br]microphone two is in the middle so queue
0:41:03.630,0:41:10.839
up and we're going to take some questions.[br]First question from microphone two.
0:41:10.839,0:41:17.410
Q: Yeah, thank you, I do have a question -[br]I come from a mining area and I just
0:41:17.410,0:41:24.450
looked up how deep other mines go and I'm[br]wondering why do you dig into useless rock
0:41:24.450,0:41:30.729
if you can just go to some area where[br]there are mines that are no longer used,
0:41:30.729,0:41:37.059
my area they go as deep as 1,200 metres I[br]think. I just looked up and was surprised
0:41:37.059,0:41:41.289
that the deepest mine on Earth is almost[br]four kilometres in South Africa - an
0:41:41.289,0:41:46.839
active goldmine - so why don't you use[br]those?
0:41:46.839,0:41:52.059
A: So, part of why we're using that[br]particular location is because we used it
0:41:52.059,0:42:00.390
for Super-K and Kamiokande before, and the[br]mountain that Kamiokande was in actually
0:42:00.390,0:42:07.759
is a mine. So we had some previous[br]infrastructure there, and then there's I
0:42:07.759,0:42:12.470
guess some tradeoff between the benefits[br]you get from going deeper and deeper and
0:42:12.470,0:42:19.670
the additional cost I think.[br]Herald: Thank you, we have a question from
0:42:19.670,0:42:24.650
the Internet, that's going to be narrated[br]by our wonderful Signal Angel.
0:42:24.650,0:42:37.839
Jost: Hello Internet. So the question, I[br]didn't understand then whole question, but
0:42:37.839,0:42:42.520
something about earthquake. Okay.[br]Q: Does the earthquake affect the
0:42:42.520,0:42:49.969
detector.[br]A: Well there's two parts of the answer:
0:42:49.969,0:42:58.660
Part one I'm not a geologist. Part two I[br]think the earthquakes are mostly centered
0:42:58.660,0:43:06.440
in the cavern on the east coast of Japan[br]and we're about 200-300 kilometers away
0:43:06.440,0:43:15.009
from there, so the region we're in is[br]relatively stable, and in fact we've been
0:43:15.009,0:43:22.949
running, since 1983 and we haven't had[br]problems with earthquakes, and during the
0:43:22.949,0:43:31.380
Fukushima earthquake our detector was[br]mostly fine but we've actually had, so in
0:43:31.380,0:43:37.599
addition to what I was talking about we're[br]also producing a beam of neutrinos at an
0:43:37.599,0:43:43.180
accelerator which we shoot at the[br]detector, and that accelerator is right at
0:43:43.180,0:43:48.510
the east coast. So, the only damage from[br]the Fukushima earthquake was to that
0:43:48.510,0:43:57.379
accelerator, not to the detector itself. I[br]hope that answers your question.
0:43:57.379,0:44:03.329
Herald: Next is from microphone two.[br]Q: Hello, thanks for an interesting talk.
0:44:03.329,0:44:10.090
Do you, or does science, have any theory[br]if the neutrinos who hit the electron are
0:44:10.090,0:44:16.680
affected themselves from this hit, are[br]they like directed in another direction or
0:44:16.680,0:44:22.979
lose some sort of energy themselves or[br]just hit the electron and pass through.
0:44:22.979,0:44:27.999
A: So, conservation of energy and of[br]momentum still holds, so they would lose
0:44:27.999,0:44:31.289
some energy as they give the electron a[br]little kick.
0:44:31.289,0:44:34.849
OK. Thank you.[br]Herald: Thank you, one more question from
0:44:34.849,0:44:38.279
mic two.[br]Q: Hello, thanks for your talk. My
0:44:38.279,0:44:44.359
question is you said that the only[br]supernova where we detected some neutrinos
0:44:44.359,0:44:50.509
from is from the 80's. So what is so[br]special about that supernova that with all
0:44:50.509,0:44:54.670
the new detectors built there was never[br]another detection?
0:44:54.670,0:45:00.149
A: So, the special thing about that one is[br]that it was relatively close. So it was in
0:45:00.149,0:45:05.940
the Large Magellanic Cloud about 150,000[br]light years away which is, you know, on
0:45:05.940,0:45:13.279
cosmic scales our next door neighbor,[br]whereas other supernovae which we observe
0:45:13.279,0:45:17.059
can be you know millions of light years[br]away. We can easily see them at that
0:45:17.059,0:45:23.249
distance but we can't detect any[br]neutrinos, and we expect about between 1
0:45:23.249,0:45:28.999
and 3 supernovae in our Milky Way per[br]century, so we're in this for the long
0:45:28.999,0:45:32.249
term. Okay.
0:45:32.249,0:45:36.039
Herald: Thank you, microphone two again[br]please.
0:45:36.039,0:45:43.630
Q: Hi, thanks for your talk. My question[br]is you said that changing the water once a
0:45:43.630,0:45:50.169
year is not often enough, how often do you[br]change the water?
0:45:50.169,0:45:56.709
A: How often do we change the water in the[br]detector? - Yes - So we completely drain
0:45:56.709,0:46:05.789
and refill the detector only for repair[br]work which, you know, happens every
0:46:05.789,0:46:10.680
depending on what we want to do but[br]typically every couple of years to, you
0:46:10.680,0:46:17.519
know, 10 plus years and apart from that we[br]recirculate the water all the time to
0:46:17.519,0:46:22.479
purify it because there will always be[br]some traces of radioactivity from the
0:46:22.479,0:46:27.599
surrounding rock which make the way in the[br]water over time.
0:46:27.599,0:46:33.640
Mic 2: Thank you.[br]Herald: Thank you, and that would be all,
0:46:33.640,0:46:38.635
that was a wonderful start to the[br]Congress, thank you Jost.
0:46:38.635,0:46:41.405
applause
0:46:41.405,0:46:46.598
35c3 postroll music
0:46:46.598,0:47:03.000
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