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35C3 - Going Deep Underground to Watch the Stars

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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
  • 25:11 - 25:17
    actually take a picture of the sun with
    neutrinos. Now this is a bit blurry and
  • 25:17 - 25:21
    pixelated, not as nice as what you'd get
    from a, you know, from the optical
  • 25:21 - 25:28
    telescope. But this is still a completely
    different way of looking at the sun. And
  • 25:28 - 25:34
    what this tells us is that this, you know,
    giant glowing orb in the sky. That's not
  • 25:34 - 25:46
    some optical illusion but that actually
    exists. Okay. So onto our next topic.
  • 25:46 - 25:54
    Exploding stars or supernovae which is
    what my own research is mostly about. So
  • 25:54 - 26:00
    supernovae are these giant explosions
    where one single star, like in this
  • 26:00 - 26:06
    example here, can shine about as bright as
    a whole galaxy consisting of billions of
  • 26:06 - 26:15
    stars. And the rule of thumb is this:
    However big you think supernovae are
  • 26:15 - 26:24
    you're wrong. They're bigger than that.
    Randall Munroe as an XKCD fan had this
  • 26:24 - 26:30
    excellent example of just how big
    supernovae are so he asks which of the
  • 26:30 - 26:34
    following would be brighter in terms of
    the amount of energy delivered to your
  • 26:34 - 26:42
    retina. Option 1: A supernova, seen from
    as far away as the sun is from earth or
  • 26:42 - 26:50
    Option 2: A hydrogen bomb pressed against
    your eyeball. So which of these would be
  • 26:50 - 27:00
    brighter. What do you think? You remember
    the the rule of thumb we had earlier this
  • 27:00 - 27:09
    supernova is bigger than that. In fact
    it's a billion times bigger than that. So
  • 27:09 - 27:15
    supernovae are some of the biggest bangs
    since the original big bang. They also
  • 27:15 - 27:20
    leave a neutron star or a black hole which
    are really interesting objects to study in
  • 27:20 - 27:26
    their own right and the outgoing shockwave
    also leads to the creation of lots of new
  • 27:26 - 27:34
    stars. But maybe most importantly
    supernovae are where many of the chemical
  • 27:34 - 27:41
    elements around you come from. So whether
    it's things like the oxygen in your lungs
  • 27:41 - 27:47
    or the calcium in your bones or the
    silicon in your favorite computer chip.
  • 27:47 - 27:52
    Life as we know it, and congress as we
    know it could not exist without
  • 27:52 - 28:02
    supernovae. And yet we don't actually
    understand how these explosions happen...
  • 28:02 - 28:19
    well I have no idea what's happening....
    life as we know it could not exist without
  • 28:19 - 28:26
    supernovae and yet we don't actually
    understand how these explosions happen.
  • 28:26 - 28:31
    And even observing them with telescopes
    doesn't really help us because telescopes
  • 28:31 - 28:37
    can only ever see the surface of a star.
    They can't look inside the core of the
  • 28:37 - 28:45
    star where the explosion actually takes
    place. So that's why we need neutrinos.
  • 28:45 - 28:51
    And we have tens of thousands of
    supernovae with optical telescopes, with
  • 28:51 - 28:59
    neutrinos we've observed just one. This
    one in February of 1987. And we've seen
  • 28:59 - 29:07
    two dozen neutrinos which you see here on
    the right. That's what we know. So we know
  • 29:07 - 29:12
    basically that there were many neutrinos
    emitted during the first one second or so
  • 29:12 - 29:19
    and then fewer and fewer for the next 10
    seconds. We know that neutrinos make up
  • 29:19 - 29:26
    most of the energy of the explosion of the
    supernova, with the actual energy of the
  • 29:26 - 29:32
    explosion and the visible light making
    just up a tiny fraction. We know that the
  • 29:32 - 29:42
    neutrinos arrive a few hours before the
    light. And that's all, that's all we know.
  • 29:42 - 29:47
    And still about these two dozen events,
    these two dozen neutrinos more than
  • 29:47 - 29:53
    sixteen hundred papers are written. That's
    more than one paper a week for over 30
  • 29:53 - 30:00
    years. So this gives you an idea of just
    how important this event was. And how
  • 30:00 - 30:04
    creative physicists are. Or I guess, you
    could call it desperate.
  • 30:04 - 30:09
    Laughter
    But you know, I prefer creative. In fact
  • 30:09 - 30:15
    this, you know, this one Supernova, we
    observed was such big deal, that 30 years
  • 30:15 - 30:23
    later February of last year we had a
    conference in Tokyo on supernova neutrinos
  • 30:23 - 30:31
    and we had a 30th anniversary celebration.
    So there we were about 40 or 50 physicists
  • 30:31 - 30:38
    looking over the skyline of Tokyo having
    dinner, there's the now leader of the
  • 30:38 - 30:44
    Super-Kamiokande experiment who was a PhD
    student back then when it happened, and in
  • 30:44 - 30:50
    his hand he's holding the original data
    tape with the events that he himself
  • 30:50 - 30:57
    analyzed back then. So there we were, and
    at one point that evening we actually
  • 30:57 - 31:04
    started to sing Happy Birthday. So you
    know how it goes - happy birthday to you,
  • 31:04 - 31:12
    happy birthday to you, happy birthday dear
    supernova 1987-A. You know it absolutely
  • 31:12 - 31:21
    doesn't work, but it was still amazing. So
    that's all we know, and then there's what
  • 31:21 - 31:27
    we think we know, and most of that comes
    from computer simulations of supernovae.
  • 31:27 - 31:33
    But the problem is, those are really
    really hard. You know it's one of these
  • 31:33 - 31:39
    extremely rare situations where all four
    fundamental forces: gravity,
  • 31:39 - 31:45
    electromagnetism, and the weak, and the
    strong nuclear force, all play a role. You
  • 31:45 - 31:49
    know normally, in particle physics you
    don't have to worry about gravity, and in
  • 31:49 - 31:54
    pretty much all other areas of physics you
    only have to worry about gravity and
  • 31:54 - 32:00
    electromagnetism. Here all four play a
    role. You've also got nonlinear
  • 32:00 - 32:05
    hydrodynamics of the gas and plasma inside
    the star. You've got the matter moving
  • 32:05 - 32:10
    relativistically at 10 or 20 percent of
    the speed of light and you've got extreme
  • 32:10 - 32:14
    pressures and extreme temperatures that
    are sometimes beyond what we can produce
  • 32:14 - 32:23
    in the laboratory on earth. So that's why
    these simulations even in 2018 are still
  • 32:23 - 32:28
    limited by the available computing power.
    So we need to do a lot of approximations
  • 32:28 - 32:35
    to actually get our code to run in a
    reasonable time, but that gives you, some,
  • 32:35 - 32:41
    that produces some problems, and in fact
    the week I started my PhD one of those
  • 32:41 - 32:46
    groups doing the supernova simulations
    published a paper saying that there is a
  • 32:46 - 32:52
    long list of numerical challenges and code
    verification issues. Basically we're using
  • 32:52 - 32:58
    this approximations and we don't know
    exactly how much error they introduce, and
  • 32:58 - 33:03
    the results of different groups are still
    too far apart, and that's not because
  • 33:03 - 33:07
    those people are dummies. Quite the
    opposite they're some of the smartest
  • 33:07 - 33:15
    people in the world. It's just that the
    problem is so damn hard. In fact, in many
  • 33:15 - 33:20
    of these simulations the stars don't even
    explode on their own, and we don't know
  • 33:20 - 33:25
    whether that means that some of these
    approximations just introduce numerical
  • 33:25 - 33:31
    errors which change the result, or whether
    it means that there are some completely
  • 33:31 - 33:36
    new physics happening in there which we
    don't know about, or whether that is
  • 33:36 - 33:44
    actually realistic and some stars you know
    in the universe don't explode but just
  • 33:44 - 33:53
    implode silently into a blackhole. We
    don't know. We just don't know. So take
  • 33:53 - 34:02
    any, you know, any results of the
    simulations with a grain of salt. That said
  • 34:02 - 34:09
    here's our best guess for what happens. So
    we start out was a massive star that's at
  • 34:09 - 34:15
    least eight times the mass of our sun, and
    it starts fusing hydrogen to helium, and
  • 34:15 - 34:23
    then on and on into heavier elements until
    finally it reaches iron, and at that point
  • 34:23 - 34:31
    fusion stops because you can't gain energy
    from fusing two iron nuclei. So there the
  • 34:31 - 34:37
    iron just accretes in the core of the star
    while in the outer layers - here in orange
  • 34:37 - 34:44
    - nuclear fusion is still going on, but as
    more and more iron accretes and that core
  • 34:44 - 34:51
    reaches about one-and-a-half solar masses
    it can't hold its own weight anymore and
  • 34:51 - 34:56
    it starts to collapse, and inside the core
    in nuclear reactions you're starting to
  • 34:56 - 35:05
    form neutrinos which I'm showing us ghost
    emoji here. Now let's zoom in a bit. The
  • 35:05 - 35:13
    core continues to collapse until at the
    center it surpasses nuclear density, and
  • 35:13 - 35:22
    at that point it's so dense that the
    neutrinos are actually trapped in there.
  • 35:22 - 35:27
    So even neutrinos which literally can go
    through walls can not escape from there
  • 35:27 - 35:36
    because matter is so dense, and the
    incoming matter basically hits a wall
  • 35:36 - 35:40
    because the matter in the center can't be
    compressed any further, so just hits the
  • 35:40 - 35:46
    wall and bounces back. So from that
    collapse you suddenly get an outgoing
  • 35:46 - 35:51
    shock wave, and in the wake of that
    shockwave suddenly you get a whole burst
  • 35:51 - 35:59
    of neutrinos which escape the star
    quickly. Now that shockwave moves on and
  • 35:59 - 36:06
    slows down and as it slows down the matter
    from outer layer still falls in, and in
  • 36:06 - 36:13
    this kind of collision region where
    neutrinos in the center are still trapped
  • 36:13 - 36:19
    in that collision region, neutrinos are,
    you know, being produced at a relatively
  • 36:19 - 36:25
    steady rate and the shock wave has pretty
    much stopped and just wavers back and
  • 36:25 - 36:33
    forth, and we see some neutrino emission.
    Now after about half a second, maybe a
  • 36:33 - 36:38
    second, neutrinos from the center are
    slowly starting to escape. You know most
  • 36:38 - 36:44
    of them are still trapped but some are
    making their way outside and some of those
  • 36:44 - 36:49
    actually manage to leave the star while
    others interact with matter in this
  • 36:49 - 36:54
    shockwave layer and give that matter a
    little energy transfer a little push and
  • 36:54 - 37:03
    heat it back up. So the shock wave gets
    revived and the star actually explodes,
  • 37:03 - 37:10
    and all of that took just one second, and
    then over the next 10 seconds or so the
  • 37:10 - 37:16
    neutrinos remaining at the core slowly
    make their way outwards and then travel
  • 37:16 - 37:23
    away at the speed of light hopefully to
    Earth to our detector. While that shock wave
  • 37:23 - 37:27
    moves much slower than the speed of light,
    you know, slowly makes its way outwards
  • 37:27 - 37:33
    and only a few hours later when that shock
    wave reaches the surface of the star do we
  • 37:33 - 37:41
    actually see something with telescopes.
    So, remember earlier the neutrinos signal
  • 37:41 - 37:46
    we saw was something like a bunch of
    neutrinos in the first second and then
  • 37:46 - 37:54
    fewer and fewer neutrinos for 10 seconds.
    Not a lot of detail but what we might see
  • 37:54 - 37:59
    is something like this: a brief and
    intense burst when the matter hits the
  • 37:59 - 38:04
    wall and is thrown back in this first
    shock wave, then as the shock wave
  • 38:04 - 38:09
    stagnates we might see some wiggles
    corresponding to the shock wave, you know,
  • 38:09 - 38:17
    sloshing around aimlessly until the shock
    wave is revived. The explosion starts and
  • 38:17 - 38:22
    then over the next 10 or so seconds we
    would see fewer and fewer neutrinos as a
  • 38:22 - 38:29
    star cools down and as neutrinos escape.
    So if we have good neutrino detectors we
  • 38:29 - 38:34
    should be able to watch, you know,
    millisecond by millisecond what exactly
  • 38:34 - 38:43
    happens inside the star. Now luckily we've
    got many more neutrino detectors by now.
  • 38:43 - 38:47
    Probably the biggest one is the Super-
    Kamiokande in the Mozumi Mine which would
  • 38:47 - 38:52
    see about 4000 events from an average
    supernova in our Milky Way, and then we've
  • 38:52 - 38:58
    got a bunch of other detectors which was
    typically you know hundreds of events, and
  • 38:58 - 39:02
    some of these detectors are part of
    something called the supernova early
  • 39:02 - 39:08
    warning system or SNEWS, and snooze is
    meant to act as a wake up call to
  • 39:08 - 39:14
    astronomers. So when when these detectors
    observe neutrinos which are probably from
  • 39:14 - 39:20
    a supernova they will send out an alert to
    astronomers to get their telescopes ready
  • 39:20 - 39:27
    to be able to see that supernova from the
    very beginning and then of course just in
  • 39:27 - 39:32
    the past few years we've also had
    gravitation wave detectors like LIGO in
  • 39:32 - 39:40
    the U.S, Virgo in Italy, and in just a few
    years we will get another one called KAGRA
  • 39:40 - 39:46
    which is located in Japan actually inside
    the same mountain as Super-Kamiokande. So
  • 39:46 - 39:50
    they're literally next door neighbors, and
    then we might get another detector in
  • 39:50 - 39:56
    India, maybe one China in the future. So
    that's three completely different ways of
  • 39:56 - 40:05
    looking at supernovae. So when we observe
    a supernova it will be headline news, and
  • 40:05 - 40:10
    now you know what's behind those
    headlines. So I've introduced you to
  • 40:10 - 40:17
    neutrinos, I've told you a bit about what
    it's like to work on on such a detector
  • 40:17 - 40:22
    and, you know the challenges of building a
    detector of this scale and I've showed you
  • 40:22 - 40:28
    how with neutrinos we can observe things
    but we can't directly observe otherwise
  • 40:28 - 40:34
    like the interior of exploding stars, and
    with that I want to thank you for your
  • 40:34 - 40:37
    attention and please let me know if you
    have any questions.
  • 40:37 - 40:51
    applause
  • 40:51 - 40:53
    Herald: Thank you Jost, it was an amazing
  • 40:53 - 41:00
    talk. We have plenty of time for questions
    and there are two microphones. Microphone
  • 41:00 - 41:04
    one is on the left side of the stage,
    microphone two is in the middle so queue
  • 41:04 - 41:11
    up and we're going to take some questions.
    First question from microphone two.
  • 41:11 - 41:17
    Q: Yeah, thank you, I do have a question -
    I come from a mining area and I just
  • 41:17 - 41:24
    looked up how deep other mines go and I'm
    wondering why do you dig into useless rock
  • 41:24 - 41:31
    if you can just go to some area where
    there are mines that are no longer used,
  • 41:31 - 41:37
    my area they go as deep as 1,200 metres I
    think. I just looked up and was surprised
  • 41:37 - 41:41
    that the deepest mine on Earth is almost
    four kilometres in South Africa - an
  • 41:41 - 41:47
    active goldmine - so why don't you use
    those?
  • 41:47 - 41:52
    A: So, part of why we're using that
    particular location is because we used it
  • 41:52 - 42:00
    for Super-K and Kamiokande before, and the
    mountain that Kamiokande was in actually
  • 42:00 - 42:08
    is a mine. So we had some previous
    infrastructure there, and then there's I
  • 42:08 - 42:12
    guess some tradeoff between the benefits
    you get from going deeper and deeper and
  • 42:12 - 42:20
    the additional cost I think.
    Herald: Thank you, we have a question from
  • 42:20 - 42:25
    the Internet, that's going to be narrated
    by our wonderful Signal Angel.
  • 42:25 - 42:38
    Jost: Hello Internet. So the question, I
    didn't understand then whole question, but
  • 42:38 - 42:43
    something about earthquake. Okay.
    Q: Does the earthquake affect the
  • 42:43 - 42:50
    detector.
    A: Well there's two parts of the answer:
  • 42:50 - 42:59
    Part one I'm not a geologist. Part two I
    think the earthquakes are mostly centered
  • 42:59 - 43:06
    in the cavern on the east coast of Japan
    and we're about 200-300 kilometers away
  • 43:06 - 43:15
    from there, so the region we're in is
    relatively stable, and in fact we've been
  • 43:15 - 43:23
    running, since 1983 and we haven't had
    problems with earthquakes, and during the
  • 43:23 - 43:31
    Fukushima earthquake our detector was
    mostly fine but we've actually had, so in
  • 43:31 - 43:38
    addition to what I was talking about we're
    also producing a beam of neutrinos at an
  • 43:38 - 43:43
    accelerator which we shoot at the
    detector, and that accelerator is right at
  • 43:43 - 43:49
    the east coast. So, the only damage from
    the Fukushima earthquake was to that
  • 43:49 - 43:57
    accelerator, not to the detector itself. I
    hope that answers your question.
  • 43:57 - 44:03
    Herald: Next is from microphone two.
    Q: Hello, thanks for an interesting talk.
  • 44:03 - 44:10
    Do you, or does science, have any theory
    if the neutrinos who hit the electron are
  • 44:10 - 44:17
    affected themselves from this hit, are
    they like directed in another direction or
  • 44:17 - 44:23
    lose some sort of energy themselves or
    just hit the electron and pass through.
  • 44:23 - 44:28
    A: So, conservation of energy and of
    momentum still holds, so they would lose
  • 44:28 - 44:31
    some energy as they give the electron a
    little kick.
  • 44:31 - 44:35
    OK. Thank you.
    Herald: Thank you, one more question from
  • 44:35 - 44:38
    mic two.
    Q: Hello, thanks for your talk. My
  • 44:38 - 44:44
    question is you said that the only
    supernova where we detected some neutrinos
  • 44:44 - 44:51
    from is from the 80's. So what is so
    special about that supernova that with all
  • 44:51 - 44:55
    the new detectors built there was never
    another detection?
  • 44:55 - 45:00
    A: So, the special thing about that one is
    that it was relatively close. So it was in
  • 45:00 - 45:06
    the Large Magellanic Cloud about 150,000
    light years away which is, you know, on
  • 45:06 - 45:13
    cosmic scales our next door neighbor,
    whereas other supernovae which we observe
  • 45:13 - 45:17
    can be you know millions of light years
    away. We can easily see them at that
  • 45:17 - 45:23
    distance but we can't detect any
    neutrinos, and we expect about between 1
  • 45:23 - 45:29
    and 3 supernovae in our Milky Way per
    century, so we're in this for the long
  • 45:29 - 45:32
    term. Okay.
  • 45:32 - 45:36
    Herald: Thank you, microphone two again
    please.
  • 45:36 - 45:44
    Q: Hi, thanks for your talk. My question
    is you said that changing the water once a
  • 45:44 - 45:50
    year is not often enough, how often do you
    change the water?
  • 45:50 - 45:57
    A: How often do we change the water in the
    detector? - Yes - So we completely drain
  • 45:57 - 46:06
    and refill the detector only for repair
    work which, you know, happens every
  • 46:06 - 46:11
    depending on what we want to do but
    typically every couple of years to, you
  • 46:11 - 46:18
    know, 10 plus years and apart from that we
    recirculate the water all the time to
  • 46:18 - 46:22
    purify it because there will always be
    some traces of radioactivity from the
  • 46:22 - 46:28
    surrounding rock which make the way in the
    water over time.
  • 46:28 - 46:34
    Mic 2: Thank you.
    Herald: Thank you, and that would be all,
  • 46:34 - 46:39
    that was a wonderful start to the
    Congress, thank you Jost.
  • 46:39 - 46:41
    applause
  • 46:41 - 46:47
    35c3 postroll music
  • 46:47 - 47:03
    subtitles created by c3subtitles.de
    in the year 2019. Join, and help us!
Title:
35C3 - Going Deep Underground to Watch the Stars
Description:

https://media.ccc.de/v/35c3-9913-going_deep_underground_to_watch_the_stars

Neutrino Astronomy with Hyper-Kamiokande

Neutrinos are “ghost-like” elementary particles that can literally go through walls. They can bring information from places that are impossible to observe through other means.
This talk provides a glimpse behind the scenes of a next-generation neutrino detector called Hyper-Kamiokande – a cylindrical water tank the size of a high-rise building. I will describe some of the problems you encounter when planning a subterranean detector of this size, and explain how this detector helps us figure out why the sun shines and how giant stars explode.

Neutrinos are tiny elementary particles that do not interact through the electromagnetic force. Almost like ghosts, they can literally go through walls and escape places that are inaccessible by other means, giving us a unique way of observing the interior of stars or nuclear reactors.

Hyper-Kamiokande – a cylindrical water tank that is 62 m high and 76 m in diameter – is a next-generation neutrino detector, which will be built inside a mountain 250 km northwest of Tokyo starting in 2020. The talk will give an overview on the process of designing and building a subterranean detector of this size, starting from preparations for cavern construction and ending with the design of photodetectors, electronics and data analysis.

In addition, the talk will cover selected areas of the physics programme of this detector. It will be explained how detecting neutrinos from our sun lets us figure out why the sun shines and how we can measure the temperature at its core to a precision of about 1%. Finally, I will explain how such a neutrino detector can help us watch, millisecond by millisecond, how giant stars explode in a supernova, creating many of the chemical elements that are necessary for life and computers to exist.

Jost Migenda

https://fahrplan.events.ccc.de/congress/2018/Fahrplan/events/9913.html

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Video Language:
English
Duration:
47:03

English subtitles

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