35C3 - Going Deep Underground to Watch the Stars
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0:00 - 0:1835c3 preroll music
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0:18 - 0:27Herald: Our speaker is Jost Migenda and he
is PhD student in astroparticle physics -
0:27 - 0:35from the University of Sheffield in the UK
and Jost is going to talk about going deep -
0:35 - 0:42underground to watch the stars. Please
give a huge round of applause for Jost -
0:42 - 0:43Migenda.
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0:43 - 0:51applause
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0:51 - 0:56J: Good morning everybody. I'm glad you
managed the first day of congress. Now -
0:56 - 1:02physics rarely makes highlight news. And
if and when it does it is often treated a -
1:02 - 1:10black box, where you pourd in money and
scientist on one end, you wait a while and -
1:10 - 1:16knowledge drops out. So today in this talk
I want to do this a bit differently. I -
1:16 - 1:20want to give you a glimpse behind the
scenes of an experiment, I have been -
1:20 - 1:26working on for over 4 years now. First as
part of my master's thesis and then as a -
1:26 - 1:34PhD student. Now earlier this year we
published a design report which is over -
1:34 - 1:38300 pages long and contains much more
detail about the experiment than you -
1:38 - 1:45probably want to know. So I'll focus on
just some of the highlights in this talk. -
1:45 - 1:49But before we actually talk about the
detector I'll have to introduce you to the -
1:49 - 1:55particles we're looking for. And that
story begins over 100 years ago with -
1:55 - 2:02radioactive beta decay. Now in radioactive
beta decay, you have a nucleus of one -
2:02 - 2:08chemical element that turns into a nucleus
of a different element and emits an -
2:08 - 2:16electron or in modern language we would
say a neutron decays into a proton and an -
2:16 - 2:22electron. Now after that was discovered
there were lots of experiments done to -
2:22 - 2:27measure the energy of the outgoing
electron and experiment after experiment -
2:27 - 2:35found that there was some variance in
energy but was always lower than expected. -
2:35 - 2:40And physicists at the time came up with
all sorts of possible explanations for -
2:40 - 2:46what might be going wrong with these
experiments but they excluded those -
2:46 - 2:52explanations very quickly as well. So
after a while physicists became desperate -
2:52 - 2:56and some pretty well-known physicists
actually thought: "Well, maybe we'll just -
2:56 - 3:04have to give up on conservation of
energy". So in this desperate situation a -
3:04 - 3:09guy called Wolfgang Pauli came up with
what he himself call "a desperate way -
3:09 - 3:15out". So in this letter to a group of his
colleagues which he addressed as "Dear -
3:15 - 3:21radioactive ladies and gentlemen", Pauli
suggested that maybe there's another -
3:21 - 3:29particle created in this beta decay. And
Pauli originally called this particle -
3:29 - 3:34neutron but of course two years later the
particle we nowadays know as Neutron was -
3:34 - 3:40discovered. So Pauli's particle was re-
named neutrino. Now you might be wondering -
3:40 - 3:47well why didn't they observe this particle
already. And the answer is very simple. -
3:47 - 3:52Neutrinos are like ghosts. So what I mean
by there is they can quite literally, you -
3:52 - 3:59know, go through walls or through your
body. And in effect we can do a little -
3:59 - 4:04experiment right now to try and detect
neutrinos. So to help me with this -
4:04 - 4:13experiment, please give me thumb's up.
Everyone? Okay so there's two things -
4:13 - 4:17happening right now. First thing of all
you're giving me a massive confidence -
4:17 - 4:24boost. But, you know, more importantly
somewhere out there the sun is shining and -
4:24 - 4:28it's producing a lot of neutrinos and
nuclear fusion. Now these neutrinos are -
4:28 - 4:35flying to Earth through the roof of this
building and then through your thumbnail. -
4:35 - 4:43And right now as you're listening to me
around 60 billion neutrinos are flying -
4:43 - 4:49through your thumbnail. 60 billion
neutrinos flying through your thumbnail -
4:49 - 4:58every second. How does that feel? Hmm? You
don't feel any of them? Right, so that's -
4:58 - 5:03how ghost-like neutrinos are. And of
course physicists are clever and shortly -
5:03 - 5:08after Pauli had this idea some of them
estimated that how often neutrinos -
5:08 - 5:14interact with normal matter and they found
that there is no practically possible way -
5:14 - 5:18of observing the neutrino. And that
remained true for over 20 years -
5:18 - 5:28afterwards. So now that I have introduced
you to neutrinos. Let's talk about -
5:28 - 5:36building a detector to actually detect
them. And the original motivation for -
5:36 - 5:44building this detector was something a bit
different. I talked about beta decay and -
5:44 - 5:49over the next decades physicists slowly
discovered more particles. They discovered -
5:49 - 5:56that protons and neutrons are made up out
of quarks and in the 1970s theoretical -
5:56 - 6:01physicists came up was are some Grand
Unified Theories basically precursors to -
6:01 - 6:07string theory. And these theories
predicted that the proton should decay as -
6:07 - 6:15well. So of course you know we build
detectors to look for that and a group in -
6:15 - 6:21Japan built a detector near the town of
Kamioka which they called the Kamioka -
6:21 - 6:27Nucleon Decay Experiment or Kamiokande for
short. Now they didn't observe any proton -
6:27 - 6:33decay but shortly after they built it
somebody had a suggestion that if we -
6:33 - 6:37changed just a little bit above their
detector, if we modified just a little, we -
6:37 - 6:45would also be able to detect neutrinos
with that. So they modified the detector -
6:45 - 6:49switched it back on and just a couple of
weeks later they actually observe -
6:49 - 6:58neutrinos from an exploding star just
outside our Milky Way. And that was the -
6:58 - 7:04birth of neutrino astronomy. And for that
the then-leader of the experiment received -
7:04 - 7:12the Nobel Prize in 2002. Now after over a
decade of running physicists were -
7:12 - 7:16basically hitting the limits of what we
could do with a detector of their size. So -
7:16 - 7:22we needed to build a bigger detector and
that one was very creatively named Super- -
7:22 - 7:29Kamiokande. And it's about 20 times
bigger, started running in 1996 and still -
7:29 - 7:40running to this day. Now Super-K did not
discover proton decay but did detect a lot -
7:40 - 7:44of neutrinos and made very fascinating
discoveries. For example, they discovered -
7:44 - 7:49that different types of neutrinos can
change into each other back and forth as -
7:49 - 7:55they travel. That's like you buying a cone
of vanilla ice cream and then as you walk -
7:55 - 8:02out it suddenly turns into chocolate ice
cream. That's really weird. And for their -
8:02 - 8:09discovery just a few years ago they
received the Nobel Prize again. But today -
8:09 - 8:13we are again hitting the limit of you know
what we can learn from a detector that -
8:13 - 8:19size. So of course the next step is to
build an even bigger detector and we're -
8:19 - 8:26calling it Hyper-Kamiokande. By the way
"super" and "hyper" mean exactly the same -
8:26 - 8:34thing. Just one is Latin and one is Greek.
So we're currently getting ready the plans -
8:34 - 8:38to build Hyper-Kamiokande and we will
start construction probably in the spring -
8:38 - 8:47of 2020. Details of the Noble Prize are
still to be determined. Now I said that 60 -
8:47 - 8:53billion neutrinos go through your
thumbnail every second. Of course Super- -
8:53 - 8:58Kamiokande which is running right now is
much larger than your thumbnail. So -
8:58 - 9:04there's not just 60 billion but 10000
billion billion neutrinos passing through -
9:04 - 9:14every day and only 10 or 15 of those get
detected. So let's look at what this -
9:14 - 9:20detection process looks like. Now this is
the water inside Super-K. And there's a -
9:20 - 9:24bunch of electrons in there but they'll
show just one. And there's neutrinos -
9:24 - 9:32flying through not just one, not just a
few, but loads of them. And most of them -
9:32 - 9:37go straight through without leaving a
trace. But every once in a while we're -
9:37 - 9:41lucky and one of those neutrinos will
actually hit the electron and give it a -
9:41 - 9:47little kick. And that little kick you know
like billiard balls basically and that -
9:47 - 9:55little kick accelerates the electron to
faster than the speed of light in water. -
9:55 - 10:00Still slower than the speed of light in
vacuum which is the absolute cosmic tempo -
10:00 - 10:07limit. But faster than the speed of light
in water. And then you get basically a -
10:07 - 10:13sonic boom, but with light, which is this
cone of light. And let's just show the -
10:13 - 10:20animation again. So you've got this cone
of light that hits the wall of the -
10:20 - 10:26detector you see a little ring, this ring
of light. Well we've got very sensitive -
10:26 - 10:33photo sensors all over the inside walls to
detect this flash of light and from how -
10:33 - 10:40bright it is we can tell the energy of the
neutrino. And we can also tell, you know -
10:40 - 10:44just like was billiard balls, we can
approximately tell what directions a -
10:44 - 10:51neutrino came from just based on in which
direction it pushed the electron. So -
10:51 - 10:57that's the basic idea how we detect
neutrinos from the sun. Now let's talk -
10:57 - 11:03about what it's actually like to build one
of these detectors. So this is a drawing -
11:03 - 11:11of Hyper-Kamiokande and you can see it's
78 meters high, 74 meters in diameter and -
11:11 - 11:16on the top left there is a truck for
comparison. But maybe a better size -
11:16 - 11:22comparison is to compare this to buildings
which you're familiar with. Like the -
11:22 - 11:29entrance hall which you just came in
through this morning. Or the Statue of -
11:29 - 11:37Liberty and it doesn't quite fit in there.
The arm still looks out. But you could -
11:37 - 11:42drown the Statue of Liberty in this
detector which nowadays is probably some -
11:42 - 11:51sort of political metaphor. So this is the
giant detector. And what's more we're -
11:51 - 11:57building it inside the mountain about 650
meters underground. So that all the rock -
11:57 - 12:02on top will act as a kind of a natural
shield against all sorts of other -
12:02 - 12:07particles, that are raining down on the
atmosphere from outer space so that all -
12:07 - 12:13other particles get stuck and only
Neutrinos can make it through. Now of -
12:13 - 12:18course to build such a huge cavern inside
the mountain - that's something that we -
12:18 - 12:26physicists can't do on our own. So we need
to talk to geologists who look at the rock -
12:26 - 12:32quality and tell us, you know, what's a
good place to build this cavern - where is -
12:32 - 12:38the rock stable enough to do that. And to
figure out the rock quality, they drill -
12:38 - 12:47bore holes in what's actually called a
boring survey. laughter Now, during my -
12:47 - 12:52years working on this experiment, I had to
listen to several hours of talks on these -
12:52 - 12:58geological surveys and I can tell you that
name is quite appropriate. laughter -
12:58 - 13:02though of course, there's a reason I'm not
a geologist, so, you know, take this with -
13:02 - 13:09a grain of salt. But okay, let's say, you
know, we talked to geologists, they told -
13:09 - 13:15us where we can build our detector. The
next step is: We need to actually excavate -
13:15 - 13:21the cavern. And something to keep in mind
is that we are building this somewhere in -
13:21 - 13:26the mountains of Japan, you know, pretty
far away from any major city. So we have -
13:26 - 13:33to think about stuff like lack of local
infrastructure like what's the electricity -
13:33 - 13:41supply like. Do we need to add the power
line. Or what are the local roads like. -
13:41 - 13:44And do they have enough capacity for, you
know, dozens of trucks every day to -
13:44 - 13:51transport away the excavated rock. And, by
the way, where do you store all that -
13:51 - 13:57excavated rock? Because we will be moving
something like half a million cubic meters -
13:57 - 14:01of rock. You can't just store that in your
backyard. You need to find a place where -
14:01 - 14:08all that fits. And, of course, if you've
listened to or watch the Lord of the -
14:08 - 14:15Rings, you'll know it's dangerous to dig
too deeply, to greedily. So we need a -
14:15 - 14:23Balrog early warning system as well. But
okay let's say we've got all those and we -
14:23 - 14:31managed to build a cavern, and now we need
to fill it. And as detector material we -
14:31 - 14:37use water. Both because it's actually
pretty good for detecting neutrinos, but -
14:37 - 14:42also because it's cheap and there's lots
of it. So you can afford to build a -
14:42 - 14:50detector of this size. A detector so big
that that little dot there is a scuba -
14:50 - 14:59diver. But even with water, you hit limits
of, you know, how much you can get. So to -
14:59 - 15:05fill Hyper-Kamiokande you need about as
much water as 5000 people use in a year. -
15:05 - 15:14And that's for drinking, for showering,
for washing their car and so on. Now -
15:14 - 15:18that's easy if you're near a big city. But
we are not, we're somewhere in the -
15:18 - 15:24mountains in Japan where the next biggest
town has far fewer than 5000 people. So -
15:24 - 15:31how do we get enough water to actually
fill our detector? And... we could use -
15:31 - 15:38rivers nearby, we could use springs. We
could wait for for the end of winter and -
15:38 - 15:43for the snow in the mountains to melt and
use that to fill our detector. But if you -
15:43 - 15:49use melting snow to fill the detector, you
can only fill it once a year. So, you -
15:49 - 15:55know, even "where do you get the water" is
is a pretty... pretty important question -
15:55 - 16:04that you need to solve. And then, we're
not just using any water but we actually -
16:04 - 16:09have, we will build our own water
purification system. So that we don't have -
16:09 - 16:16any, you know, traces of radioactivity in
there, any trace of dust and stuff in -
16:16 - 16:23there. And let me let me just tell you
just how pure this water will be. So, this -
16:23 - 16:29is my supervisor, who, when he was a PhD
student, worked in the detector on some -
16:29 - 16:36maintenance work, so he was working on a
boat doing the work, and then at the end -
16:36 - 16:41of his shift he leaned back on the boat,
and just the tip of his long hair fell -
16:41 - 16:48into the water, which he didn't know just
didn't think about too much until at the -
16:48 - 16:55end of his shift. He went home, you know,
went to bed, fell asleep, and then woke up -
16:55 - 17:01in the middle of the night, with his whole
head itching like mad. Now, what had -
17:01 - 17:07happened there? The ultra pure water had
sucked all of the nutrients out the tip of -
17:07 - 17:12his hair and then through osmosis over
time those had sucked the nutrients out of -
17:12 - 17:31the rest of his hair, and then his skin.
So that's how pure that water is. Now, I -
17:31 - 17:35said "all over the inside walls". And here
is, kind of, a photo of the inside of the -
17:35 - 17:42detector. And all these kind of golden
hemispheres, those are what we call -
17:42 - 17:49Photomultiplier tubes, of PMTs for short.
And those are, basically, giant very -
17:49 - 17:55sensitive pixels. And we will have 40,000
of those lining the inside wall of the -
17:55 - 18:02detector. Now, in smaller detectors, you
could just have from each PMT one cable -
18:02 - 18:06leading to the top of the detector and
then have your computers there to analyze -
18:06 - 18:13the signal. With a detector of this size,
you just can't do that because you would -
18:13 - 18:22need 40,000 cables some of which are over
100 meters long. That wouldn't work. So we -
18:22 - 18:26have to put some electronics in the water
to digitize the signal and combine signal -
18:26 - 18:34from multiple PMTs into one, and then use
just a single cable, to bring it up -
18:34 - 18:41to the top where we analyze the signal.
But that means we have to put electronics -
18:41 - 18:47into the water, so that creates a whole
bunch of new problems. For example, we -
18:47 - 18:51need these electronics to be watertight.
And I'm not talking about the level of -
18:51 - 18:55watertight as you'd expect from your
smartwatch where it survives, you know, -
18:55 - 19:00you standing under the shower for five
minutes. I'm talking below 60 metres of -
19:00 - 19:08water for 20+ years. This electronics also
need to be very low power, because we -
19:08 - 19:13can't heet up the water too much.
Otherwise these pixels, the -
19:13 - 19:22Photomultiplier Tubes, would become noisy
and this would kill our signal. And then -
19:22 - 19:26also, because we don't want one defective
cable to kill a whole section of the -
19:26 - 19:31detector. We need to implement some sort
of mesh networking to introduce some -
19:31 - 19:38redundancy. Now each of that by itself is
not... not a heart problem. Each of these -
19:38 - 19:43problems can be solved. It's just a lot of
additional work you suddenly have to do, -
19:43 - 19:52because your detectors is that huge. And
it gets even worse: This is what one of -
19:52 - 19:58these PMTs looks like. It's about 50
centimeters in diameter, and inside that -
19:58 - 20:07glass bulb is a vacuum. So it's under a
lot of pressure. Plus we add 60 metres of -
20:07 - 20:12water on top of it, which adds additional
pressure. So you need to make absolutely -
20:12 - 20:17sure when you're manufacturing those that
you don't have any weak points in the -
20:17 - 20:26glass. And they don't just have to
withstand that water pressure, but there -
20:26 - 20:33will probably be some PMTs that have some
weak points, some air bubbles or something -
20:33 - 20:40in the glass. Some structural weakness.
And the neighboring PMT don't only have to -
20:40 - 20:45survive the normal water pressure. They
also need to survive whether their -
20:45 - 20:51neighboring PMT is imploding and sending
out a pressure wave. And that's not just a -
20:51 - 20:58hypothetical - that actually happened, you
know, 18 years ago. It's 17 years ago in -
20:58 - 21:05Superkamiokande. And that, you know,
within seconds killed more than half of -
21:05 - 21:14the PMTs we had in there and it took years
to restore the detector to full capacity. -
21:14 - 21:20So, lots of problems to solve, and, you
know, one group, one university can't -
21:20 - 21:25solve all these on their own. So we've got
this multinational collaboration with over -
21:25 - 21:30300 people from 17 different countries
marked in green here and across many -
21:30 - 21:37different time zones. So right here right
now, here it's about you know just before -
21:37 - 21:42noon. In Japan people have already had
dinner and they're going to bed soon. In -
21:42 - 21:47the U.S., people haven't even got up yet
in the morning. So good luck finding a -
21:47 - 21:58time for phone meetings which works for
all of these people. So, that's kind of a -
21:58 - 22:02glimpse behind the scenes of what it's
like to work on this detector and to -
22:02 - 22:08actually build it. But now I want to talk
about what we use the detector for. And -
22:08 - 22:13I've got two examples. But of course
there's a whole bunch more that we do, I -
22:13 - 22:24just don't have time to talk about it
today. So, first example: Why does the sun -
22:24 - 22:30shine? That seems like such a simple
question, right? And yet it turns out it's -
22:30 - 22:36really difficult to answer. So, in the
days of the Industrial Revolution, when, -
22:36 - 22:41you know, burning coal and steam power was
all the rage, people thought that, you -
22:41 - 22:48know, maybe it's, you know, a giant ball
of burning coal. But when you when you do -
22:48 - 22:55the math, it turns out that would burn for
a few thousand years maybe. So that -
22:55 - 23:01definitely doesn't work. A bit later,
physicists suggested that maybe the sun is -
23:01 - 23:07just slowly shrinking and shrinking and
it's that gravitational energy which is -
23:07 - 23:13released as light. And that would give you
a life time of a few million years. But -
23:13 - 23:17then you've got, you know pesky geologists
coming along and saying "no no, we've got -
23:17 - 23:22these rock formations or, I don't know,
fossils maybe, for more than a few million -
23:22 - 23:28years old on Earth. So the sun has to live
longer than a few million years. And the -
23:28 - 23:34arguments from the 19th century between
Lord Kelvin and the geologists back then -
23:34 - 23:41are just amazing to read. If you find
those somewhere. But, of course, nowadays -
23:41 - 23:49we know that the answer is nuclear fusion.
And here's, you know, a bunch of reactions -
23:49 - 23:55which lead you the energy generation of
the sun. But now the question is "how can -
23:55 - 24:03we check that? How can we check that this
is actually what's going on?" And the -
24:03 - 24:09answer is neutrinos, because many of these
reactions produce neutrinos, and we can -
24:09 - 24:16detect them. So the one we typically
detect in Super- and later Hyperkamiokande -
24:16 - 24:21is the one on the bottom right here,
called "Bore and eight neutrinos" because -
24:21 - 24:29those have the highest energy. So they are
easiest for us to detect. And the rate of -
24:29 - 24:36various of these processes depends very
much on the temperature. So by measuring -
24:36 - 24:40how many of these neutrinos we see, we can
measure the temperature inside the core of -
24:40 - 24:49the sun. And we have done that, and we
know that it's about 15.5 million Kelvin, plus or -
24:49 - 24:55minus 1 percent. So we know the
temperature in the core of the sun to less -
24:55 - 25:06than 1 percent uncertainty. That's pretty
amazing if you ask me. And, you know, I -
25:06 - 25:11said that we could detect the directions
the neutrinos were coming from. So we can -
25:11 - 25:17actually take a picture of the sun with
neutrinos. Now this is a bit blurry and -
25:17 - 25:21pixelated, not as nice as what you'd get
from a, you know, from the optical -
25:21 - 25:28telescope. But this is still a completely
different way of looking at the sun. And -
25:28 - 25:34what this tells us is that this, you know,
giant glowing orb in the sky. That's not -
25:34 - 25:46some optical illusion but that actually
exists. Okay. So onto our next topic. -
25:46 - 25:54Exploding stars or supernovae which is
what my own research is mostly about. So -
25:54 - 26:00supernovae are these giant explosions
where one single star, like in this -
26:00 - 26:06example here, can shine about as bright as
a whole galaxy consisting of billions of -
26:06 - 26:15stars. And the rule of thumb is this:
However big you think supernovae are -
26:15 - 26:24you're wrong. They're bigger than that.
Randall Munroe as an XKCD fan had this -
26:24 - 26:30excellent example of just how big
supernovae are so he asks which of the -
26:30 - 26:34following would be brighter in terms of
the amount of energy delivered to your -
26:34 - 26:42retina. Option 1: A supernova, seen from
as far away as the sun is from earth or -
26:42 - 26:50Option 2: A hydrogen bomb pressed against
your eyeball. So which of these would be -
26:50 - 27:00brighter. What do you think? You remember
the the rule of thumb we had earlier this -
27:00 - 27:09supernova is bigger than that. In fact
it's a billion times bigger than that. So -
27:09 - 27:15supernovae are some of the biggest bangs
since the original big bang. They also -
27:15 - 27:20leave a neutron star or a black hole which
are really interesting objects to study in -
27:20 - 27:26their own right and the outgoing shockwave
also leads to the creation of lots of new -
27:26 - 27:34stars. But maybe most importantly
supernovae are where many of the chemical -
27:34 - 27:41elements around you come from. So whether
it's things like the oxygen in your lungs -
27:41 - 27:47or the calcium in your bones or the
silicon in your favorite computer chip. -
27:47 - 27:52Life as we know it, and congress as we
know it could not exist without -
27:52 - 28:02supernovae. And yet we don't actually
understand how these explosions happen... -
28:02 - 28:19well I have no idea what's happening....
life as we know it could not exist without -
28:19 - 28:26supernovae and yet we don't actually
understand how these explosions happen. -
28:26 - 28:31And even observing them with telescopes
doesn't really help us because telescopes -
28:31 - 28:37can only ever see the surface of a star.
They can't look inside the core of the -
28:37 - 28:45star where the explosion actually takes
place. So that's why we need neutrinos. -
28:45 - 28:51And we have tens of thousands of
supernovae with optical telescopes, with -
28:51 - 28:59neutrinos we've observed just one. This
one in February of 1987. And we've seen -
28:59 - 29:07two dozen neutrinos which you see here on
the right. That's what we know. So we know -
29:07 - 29:12basically that there were many neutrinos
emitted during the first one second or so -
29:12 - 29:19and then fewer and fewer for the next 10
seconds. We know that neutrinos make up -
29:19 - 29:26most of the energy of the explosion of the
supernova, with the actual energy of the -
29:26 - 29:32explosion and the visible light making
just up a tiny fraction. We know that the -
29:32 - 29:42neutrinos arrive a few hours before the
light. And that's all, that's all we know. -
29:42 - 29:47And still about these two dozen events,
these two dozen neutrinos more than -
29:47 - 29:53sixteen hundred papers are written. That's
more than one paper a week for over 30 -
29:53 - 30:00years. So this gives you an idea of just
how important this event was. And how -
30:00 - 30:04creative physicists are. Or I guess, you
could call it desperate. -
30:04 - 30:09Laughter
But you know, I prefer creative. In fact -
30:09 - 30:15this, you know, this one Supernova, we
observed was such big deal, that 30 years -
30:15 - 30:23later February of last year we had a
conference in Tokyo on supernova neutrinos -
30:23 - 30:31and we had a 30th anniversary celebration.
So there we were about 40 or 50 physicists -
30:31 - 30:38looking over the skyline of Tokyo having
dinner, there's the now leader of the -
30:38 - 30:44Super-Kamiokande experiment who was a PhD
student back then when it happened, and in -
30:44 - 30:50his hand he's holding the original data
tape with the events that he himself -
30:50 - 30:57analyzed back then. So there we were, and
at one point that evening we actually -
30:57 - 31:04started to sing Happy Birthday. So you
know how it goes - happy birthday to you, -
31:04 - 31:12happy birthday to you, happy birthday dear
supernova 1987-A. You know it absolutely -
31:12 - 31:21doesn't work, but it was still amazing. So
that's all we know, and then there's what -
31:21 - 31:27we think we know, and most of that comes
from computer simulations of supernovae. -
31:27 - 31:33But the problem is, those are really
really hard. You know it's one of these -
31:33 - 31:39extremely rare situations where all four
fundamental forces: gravity, -
31:39 - 31:45electromagnetism, and the weak, and the
strong nuclear force, all play a role. You -
31:45 - 31:49know normally, in particle physics you
don't have to worry about gravity, and in -
31:49 - 31:54pretty much all other areas of physics you
only have to worry about gravity and -
31:54 - 32:00electromagnetism. Here all four play a
role. You've also got nonlinear -
32:00 - 32:05hydrodynamics of the gas and plasma inside
the star. You've got the matter moving -
32:05 - 32:10relativistically at 10 or 20 percent of
the speed of light and you've got extreme -
32:10 - 32:14pressures and extreme temperatures that
are sometimes beyond what we can produce -
32:14 - 32:23in the laboratory on earth. So that's why
these simulations even in 2018 are still -
32:23 - 32:28limited by the available computing power.
So we need to do a lot of approximations -
32:28 - 32:35to actually get our code to run in a
reasonable time, but that gives you, some, -
32:35 - 32:41that produces some problems, and in fact
the week I started my PhD one of those -
32:41 - 32:46groups doing the supernova simulations
published a paper saying that there is a -
32:46 - 32:52long list of numerical challenges and code
verification issues. Basically we're using -
32:52 - 32:58this approximations and we don't know
exactly how much error they introduce, and -
32:58 - 33:03the results of different groups are still
too far apart, and that's not because -
33:03 - 33:07those people are dummies. Quite the
opposite they're some of the smartest -
33:07 - 33:15people in the world. It's just that the
problem is so damn hard. In fact, in many -
33:15 - 33:20of these simulations the stars don't even
explode on their own, and we don't know -
33:20 - 33:25whether that means that some of these
approximations just introduce numerical -
33:25 - 33:31errors which change the result, or whether
it means that there are some completely -
33:31 - 33:36new physics happening in there which we
don't know about, or whether that is -
33:36 - 33:44actually realistic and some stars you know
in the universe don't explode but just -
33:44 - 33:53implode silently into a blackhole. We
don't know. We just don't know. So take -
33:53 - 34:02any, you know, any results of the
simulations with a grain of salt. That said -
34:02 - 34:09here's our best guess for what happens. So
we start out was a massive star that's at -
34:09 - 34:15least eight times the mass of our sun, and
it starts fusing hydrogen to helium, and -
34:15 - 34:23then on and on into heavier elements until
finally it reaches iron, and at that point -
34:23 - 34:31fusion stops because you can't gain energy
from fusing two iron nuclei. So there the -
34:31 - 34:37iron 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 - 34:51reaches about one-and-a-half solar masses
it can't hold its own weight anymore and -
34:51 - 34:56it starts to collapse, and inside the core
in nuclear reactions you're starting to -
34:56 - 35:05form neutrinos which I'm showing us ghost
emoji here. Now let's zoom in a bit. The -
35:05 - 35:13core continues to collapse until at the
center it surpasses nuclear density, and -
35:13 - 35:22at that point it's so dense that the
neutrinos are actually trapped in there. -
35:22 - 35:27So even neutrinos which literally can go
through walls can not escape from there -
35:27 - 35:36because matter is so dense, and the
incoming matter basically hits a wall -
35:36 - 35:40because the matter in the center can't be
compressed any further, so just hits the -
35:40 - 35:46wall and bounces back. So from that
collapse you suddenly get an outgoing -
35:46 - 35:51shock wave, and in the wake of that
shockwave suddenly you get a whole burst -
35:51 - 35:59of neutrinos which escape the star
quickly. Now that shockwave moves on and -
35:59 - 36:06slows down and as it slows down the matter
from outer layer still falls in, and in -
36:06 - 36:13this kind of collision region where
neutrinos in the center are still trapped -
36:13 - 36:19in that collision region, neutrinos are,
you know, being produced at a relatively -
36:19 - 36:25steady rate and the shock wave has pretty
much stopped and just wavers back and -
36:25 - 36:33forth, and we see some neutrino emission.
Now after about half a second, maybe a -
36:33 - 36:38second, neutrinos from the center are
slowly starting to escape. You know most -
36:38 - 36:44of them are still trapped but some are
making their way outside and some of those -
36:44 - 36:49actually manage to leave the star while
others interact with matter in this -
36:49 - 36:54shockwave layer and give that matter a
little energy transfer a little push and -
36:54 - 37:03heat it back up. So the shock wave gets
revived and the star actually explodes, -
37:03 - 37:10and all of that took just one second, and
then over the next 10 seconds or so the -
37:10 - 37:16neutrinos remaining at the core slowly
make their way outwards and then travel -
37:16 - 37:23away at the speed of light hopefully to
Earth to our detector. While that shock wave -
37:23 - 37:27moves much slower than the speed of light,
you know, slowly makes its way outwards -
37:27 - 37:33and only a few hours later when that shock
wave reaches the surface of the star do we -
37:33 - 37:41actually see something with telescopes.
So, remember earlier the neutrinos signal -
37:41 - 37:46we saw was something like a bunch of
neutrinos in the first second and then -
37:46 - 37:54fewer and fewer neutrinos for 10 seconds.
Not a lot of detail but what we might see -
37:54 - 37:59is something like this: a brief and
intense burst when the matter hits the -
37:59 - 38:04wall and is thrown back in this first
shock wave, then as the shock wave -
38:04 - 38:09stagnates we might see some wiggles
corresponding to the shock wave, you know, -
38:09 - 38:17sloshing around aimlessly until the shock
wave is revived. The explosion starts and -
38:17 - 38:22then over the next 10 or so seconds we
would see fewer and fewer neutrinos as a -
38:22 - 38:29star cools down and as neutrinos escape.
So if we have good neutrino detectors we -
38:29 - 38:34should be able to watch, you know,
millisecond by millisecond what exactly -
38:34 - 38:43happens inside the star. Now luckily we've
got many more neutrino detectors by now. -
38:43 - 38:47Probably the biggest one is the Super-
Kamiokande in the Mozumi Mine which would -
38:47 - 38:52see about 4000 events from an average
supernova in our Milky Way, and then we've -
38:52 - 38:58got a bunch of other detectors which was
typically you know hundreds of events, and -
38:58 - 39:02some of these detectors are part of
something called the supernova early -
39:02 - 39:08warning system or SNEWS, and snooze is
meant to act as a wake up call to -
39:08 - 39:14astronomers. So when when these detectors
observe neutrinos which are probably from -
39:14 - 39:20a supernova they will send out an alert to
astronomers to get their telescopes ready -
39:20 - 39:27to be able to see that supernova from the
very beginning and then of course just in -
39:27 - 39:32the past few years we've also had
gravitation wave detectors like LIGO in -
39:32 - 39:40the U.S, Virgo in Italy, and in just a few
years we will get another one called KAGRA -
39:40 - 39:46which is located in Japan actually inside
the same mountain as Super-Kamiokande. So -
39:46 - 39:50they're literally next door neighbors, and
then we might get another detector in -
39:50 - 39:56India, maybe one China in the future. So
that's three completely different ways of -
39:56 - 40:05looking at supernovae. So when we observe
a supernova it will be headline news, and -
40:05 - 40:10now you know what's behind those
headlines. So I've introduced you to -
40:10 - 40:17neutrinos, I've told you a bit about what
it's like to work on on such a detector -
40:17 - 40:22and, you know the challenges of building a
detector of this scale and I've showed you -
40:22 - 40:28how with neutrinos we can observe things
but we can't directly observe otherwise -
40:28 - 40:34like the interior of exploding stars, and
with that I want to thank you for your -
40:34 - 40:37attention and please let me know if you
have any questions. -
40:37 - 40:51applause
-
40:51 - 40:53Herald: Thank you Jost, it was an amazing
-
40:53 - 41:00talk. We have plenty of time for questions
and there are two microphones. Microphone -
41:00 - 41:04one is on the left side of the stage,
microphone two is in the middle so queue -
41:04 - 41:11up and we're going to take some questions.
First question from microphone two. -
41:11 - 41:17Q: Yeah, thank you, I do have a question -
I come from a mining area and I just -
41:17 - 41:24looked up how deep other mines go and I'm
wondering why do you dig into useless rock -
41:24 - 41:31if you can just go to some area where
there are mines that are no longer used, -
41:31 - 41:37my area they go as deep as 1,200 metres I
think. I just looked up and was surprised -
41:37 - 41:41that the deepest mine on Earth is almost
four kilometres in South Africa - an -
41:41 - 41:47active goldmine - so why don't you use
those? -
41:47 - 41:52A: So, part of why we're using that
particular location is because we used it -
41:52 - 42:00for Super-K and Kamiokande before, and the
mountain that Kamiokande was in actually -
42:00 - 42:08is a mine. So we had some previous
infrastructure there, and then there's I -
42:08 - 42:12guess some tradeoff between the benefits
you get from going deeper and deeper and -
42:12 - 42:20the additional cost I think.
Herald: Thank you, we have a question from -
42:20 - 42:25the Internet, that's going to be narrated
by our wonderful Signal Angel. -
42:25 - 42:38Jost: Hello Internet. So the question, I
didn't understand then whole question, but -
42:38 - 42:43something about earthquake. Okay.
Q: Does the earthquake affect the -
42:43 - 42:50detector.
A: Well there's two parts of the answer: -
42:50 - 42:59Part one I'm not a geologist. Part two I
think the earthquakes are mostly centered -
42:59 - 43:06in the cavern on the east coast of Japan
and we're about 200-300 kilometers away -
43:06 - 43:15from there, so the region we're in is
relatively stable, and in fact we've been -
43:15 - 43:23running, since 1983 and we haven't had
problems with earthquakes, and during the -
43:23 - 43:31Fukushima earthquake our detector was
mostly fine but we've actually had, so in -
43:31 - 43:38addition to what I was talking about we're
also producing a beam of neutrinos at an -
43:38 - 43:43accelerator which we shoot at the
detector, and that accelerator is right at -
43:43 - 43:49the east coast. So, the only damage from
the Fukushima earthquake was to that -
43:49 - 43:57accelerator, not to the detector itself. I
hope that answers your question. -
43:57 - 44:03Herald: Next is from microphone two.
Q: Hello, thanks for an interesting talk. -
44:03 - 44:10Do you, or does science, have any theory
if the neutrinos who hit the electron are -
44:10 - 44:17affected themselves from this hit, are
they like directed in another direction or -
44:17 - 44:23lose some sort of energy themselves or
just hit the electron and pass through. -
44:23 - 44:28A: So, conservation of energy and of
momentum still holds, so they would lose -
44:28 - 44:31some energy as they give the electron a
little kick. -
44:31 - 44:35OK. Thank you.
Herald: Thank you, one more question from -
44:35 - 44:38mic two.
Q: Hello, thanks for your talk. My -
44:38 - 44:44question is you said that the only
supernova where we detected some neutrinos -
44:44 - 44:51from is from the 80's. So what is so
special about that supernova that with all -
44:51 - 44:55the new detectors built there was never
another detection? -
44:55 - 45:00A: So, the special thing about that one is
that it was relatively close. So it was in -
45:00 - 45:06the Large Magellanic Cloud about 150,000
light years away which is, you know, on -
45:06 - 45:13cosmic scales our next door neighbor,
whereas other supernovae which we observe -
45:13 - 45:17can be you know millions of light years
away. We can easily see them at that -
45:17 - 45:23distance but we can't detect any
neutrinos, and we expect about between 1 -
45:23 - 45:29and 3 supernovae in our Milky Way per
century, so we're in this for the long -
45:29 - 45:32term. Okay.
-
45:32 - 45:36Herald: Thank you, microphone two again
please. -
45:36 - 45:44Q: Hi, thanks for your talk. My question
is you said that changing the water once a -
45:44 - 45:50year is not often enough, how often do you
change the water? -
45:50 - 45:57A: How often do we change the water in the
detector? - Yes - So we completely drain -
45:57 - 46:06and refill the detector only for repair
work which, you know, happens every -
46:06 - 46:11depending on what we want to do but
typically every couple of years to, you -
46:11 - 46:18know, 10 plus years and apart from that we
recirculate the water all the time to -
46:18 - 46:22purify it because there will always be
some traces of radioactivity from the -
46:22 - 46:28surrounding rock which make the way in the
water over time. -
46:28 - 46:34Mic 2: Thank you.
Herald: Thank you, and that would be all, -
46:34 - 46:39that was a wonderful start to the
Congress, thank you Jost. -
46:39 - 46:41applause
-
46:41 - 46:4735c3 postroll music
-
46:47 - 47:03subtitles 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
- Video Language:
- English
- Duration:
- 47:03
Bar Sch edited English subtitles for 35C3 - Going Deep Underground to Watch the Stars | ||
Bar Sch edited English subtitles for 35C3 - Going Deep Underground to Watch the Stars | ||
Bar Sch edited English subtitles for 35C3 - Going Deep Underground to Watch the Stars | ||
C3Subtitles edited English subtitles for 35C3 - Going Deep Underground to Watch the Stars | ||
C3Subtitles edited English subtitles for 35C3 - Going Deep Underground to Watch the Stars | ||
Bar Sch edited English subtitles for 35C3 - Going Deep Underground to Watch the Stars | ||
Bar Sch edited English subtitles for 35C3 - Going Deep Underground to Watch the Stars | ||
Bar Sch edited English subtitles for 35C3 - Going Deep Underground to Watch the Stars |