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Neutrinos are one of the most bizarre of known particles.
Black holes are probably the most bizarre of astrophysical objects.
So it makes sense we should use one to study the other, rig?
Well, today we’re doing just that.
There’s been a lot of hype about our shiny new observatories and the new windows to the
universe that they open.
There’s the James Webb Space Telescope and its infrared supervision and of course LIGO
with its ability to see gravitational waves.
And then there’s neutrino astronomy.
It gets far less attention in the media - in fact it’s almost as elusive as the particle
it depends on - and yet mapping the neutrino sky will surely unlock secrets no other method
can access.
Today we’re going to look at some of the first neutrino astrophysics in this new astronomical
era, as we discuss a new result from the IceCube collaboration.
They report seeing neutrinos produced in the colossal magnetic fields surrounding a black
hole with the mass of 10 million Suns.
No big deal.
First up, let’s review what neutrinos are.
These elementary particles are fermions - so particles of matter rather than force-carrying
bosons like the photons of regular astronomy, and neutrino's fermion type is lepton, so they're cousins
of the electron, muon and tau particle.
There’s one neutrino type corresponding to each of these three; with the most striking
differences being that neutrinos are electrically neutral and have much lower masses than the
charged counterparts.
The extremely low mass of neutrinos means that they tend to travel at very close to the speed
of light, just because it doesn’t take much energy to get them close to the cosmic speed limit.
But perhaps the most characteristic property of the neutrino is that they only interact
via the weak nuclear force and gravity.
That makes them very, very difficult to detect.
For reference, if you want to stop a low-energy neutrino with a wall of lead, the wall would
need to be a light year thick to have even a 50-50 chance that the neutrino gets close
enough to a lead nucleus to interact.
So you can see how doing neutrino astronomy might be challenging, given that each neutrino
has a minuscule interaction probability most of them pass straight through your detector.
But miniscule isn’t zero.
With enough neutrinos you will get interactions.
And space does generate a lot of neutrinos.
By far the brightest neutrino source on the sky is the Sun.
It creates 10^38 - a hundred trillion trillion trillion neutrinos every second in the fusion
reactions in its core.
These spread out in every direction, so a mere 100 trillion pass through your body every
second.
In fact, the vast majority pass straight through the entire planet.
To have a chance of catching a neutrino you need a big detector, and the biggest neutrino detector
is IceCube.
There are two famous musicians who are also astrophysicists - those are Brian May and
Brian Cox - so no, IceCube is not named after the rapper.
It’s named for the fact that it’s literally an ice cube.
It’s a full cubic kilometer of glacial ice at the South Pole.
Let’s see how ice can be used to see neutrinos.
When neutrinos pass through the ice of ice cube, one in a million interact with water
molecules via the weak force.
For high-energy neutrinos, interaction are with an atomic nucleus and that interaction can transmute
the neutrino into its high-mass lepton counterpart - an electron, muon or tau.
If the neutrino becomes a tau then it decays almost instantly, but an electron or muon
will continue through the ice, emitting light as it interacts with other charged particles.
This is seen as a cone of blue light that trails the particle.
It trails behind because the lepton is traveling faster than light.
See, the speed of light is reduced in any medium inside a vacuum.
In ice it’s around 25% slower.
But neutrinos are not slowed down, so the electron or muon that it creates also start
out with a speed faster than the reduced speed of light in the ice.
The result is the electromagnetic analog of a sonic boom.
The expanding EM waves created by the charged particle expand slower than the particle itself,
so their wavefronts overlap each other and you get constructive interference in this
cone shape that follows the particle.
This is Cherenkov radiation, and it’s ultimately how we detect the neutrinos.
The actual detectors are sensitive photomultipliers - basically extremely sensitive light detectors
- suspended in deep boreholes.
A grid of over 5000 photomultiplies spans a cubic kilometer of the glacier, starting
at a depth of 1.5km.
In this way, IceCube sees the Cherenkov radiation from neutrinos generating both electrons and
muons, but it’s the muons that are really useful.
Electrons interact very strongly with the water molecules and so begin to bounce around
immediately, leading to a bubble of Cherenkov near their creation point.
But muons can travel for kilometers without changing course, so their Cherenkov cones
trace out a straight line in the direction of the original neutrino path.
In this way it’s possible to figure out where on the sky the neutrino came from, and
with enough neutrinos even build up a rather blurry image.
We need to sort out one more thing to understand how IceCube actually takes neutrino pics of
the sky.
It turns out that our environment is very noisy with neutrinos.
The sun produces an enormous number, but those are easy to distinguish because they come
from the direction of the Sun.
More challenging are confounding particles from our own atmosphere.
When cosmic rays hit molecules in our atmosphere, many different particles can be produced,
but the most annoying are muons and neutrinos.
Those muons can cause Cherenkov cones, but they’re easy to sort out because almost
all of them come from above.
If we only pay attention to signals from below we eliminate atmospheric muons.
This makes a neutrino observatory the only type of observatory where you need to wait
until your object sets below the horizon before you can observe it.
Atmospheric neutrinos are still a problem, because if they’re produced on the other
side of the planet they can still reach IceCube.
In order to confidently say you detected the neutrinos of a cosmic object, you need to
collect significantly more neutrinos from that particular direction than the smooth
background neutrino rate would give you.
Which is why it’s taken a decade of collecting neutrinos to confidently detect the amazing
space object that we’re finally ready to talk about.
This is the neutrino map of the sky produced by IceCube since it began operations in 2011.
And this spot here is the only place where we see a significant over-abundance of neutrinos.
There’s only about 50 neutrinos more than you would expected from cosmic rays in the atmosphere, but it’s
pretty clearly something out there in space.
There’s only a 1 in 10,000 chance of getting such an over abundance anywhere on the map
by pure chance, which in science-speak means a 4.2 sigma detection.
So what could it be?
Well, this blurry patch of sky in the constellation of cetus includes a lot of Milky Way stars,
a lot of very distant galaxies, but there’s only one thing that is a plausible source
of neutrinos.
And that’s this.
It's M77, also known as NGC 1068.
It’s a beautiful spiral galaxy 47 million light years away.
Now a regular galaxy should not produce so many neutrinos.
But M77 is not regular.
At its heart it holds a terrible secret.
As with most galaxies, including our own, there’s a gigantic black hole at its core.
In this case with a mass of around 10 million suns.
But unlike most galaxies, M77's supermassive black hole is not quiet.
It’s currently in a feeding phase - gas from the surrounding galaxy has been driven
to the center, forming a whirlpool of searing plasma - an accretion disk - as it falls into
the black hole.
M77 has what we call an active galactic nucleus.
It’s an AGN.
The most powerful AGNs are called quasars, and shine out from across the universe.
M77 is not that powerful, it's known as a Seyfert galaxy, and the light of its accretion disk is hidden from
us by a wreath of dust and gas surrounding the nucleus.
Being one of the closest AGNs to our galaxy makes M77 a prime candidate as the source
of neutrinos.
We know that AGNs have powerful magnetic fields because in many of them we see jets of high energy
particles blasted out from the vicinity of the black hole, and these have all the signatures
of having been accelerated and focused by magnetic fields.
They are natural particle accelerators, far more powerful than the ones we can build on
earth.
And collisions of magnetic-field-accelerated particles is exactly how we make neutrinos
in our experiments.
On that note, I should mention that M77 may not be the first AGN to be detected by IceCube.
In 2017 a single very high energy neutrino was detected by IceCube coming from the direction of
a known blazar.
Now a blazar is an AGN where a magnetically-channeled jet happens to be pointing more or less directly
at us.
This leads to massive amplification of the jet’s light due to Einstein stuff.
Relativistic boosting due to the jet particles racing towards us at near the speed of light.
Many telescopes swiveled to the blazar after the neutrino detection and it was revealed
to be in a particularly active phase, so there’s a good chance it was spitting out high energy
neutrinos at an enhanced rate.
But there’s only a 3-sigma confidence that the neutrino is associated with the blazar
in this case - not enough to claim a sure detection.
That makes the neutrino signal from M77 far more exciting, because we’re approaching
certainty in this case.
The only other sure neutrino detections from space are the Sun and the remnant of supernova 1987A
in the large magellanic cloud.
So that means we can finally start doing real neutrino astrophysics.
In fact, that work is already well underway.
A number of studies have come out describing the possible mechanisms and environment of the
neutrino production.
We stand to learn a ton about the regions around these gigantic black holes, and eventually
from the many other energetic environments that probably produce neutrinos.
Supernovae, colliding neutron stars and black holes, tidal disruption events when black
holes rip apart stars, you name it.
To achieve all of this we need bigger and more sensitive neutrino observatories.
There are already plans to improve the sensitivity of IceCubes detectors, and also to expand
it to a full 10 cubic kilometers, which should approximately increase the detection rate
by a factor of 10.
We can also look for neutrino Cherenkov radiation at radio wavelengths, which allows us to scan
vast tracks of the Antarctic glacier with detectors placed above the ice.
In fact we already do so, for example with the ANITA balloon experiment.
These experiments can also be expanded.
One possibility is looking for radio-Cherenkov from the Moon.
So yeah, using the entire moon as a neutrino telescope.
The future of neutrino astronomy is bright, and it’s hard to even guess what we’ll
learn as we slowly build our neutrino map of space time.
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Patreon is an incredibly important source of support for this show, so thanks to everyone
who helps us out that way.
Today I want to give a very special thank you to Steffen Bendel who’s supporting us
at the Big Bang level.
Steffen, you clearly understand the great importance of science communication, but we
thought it would also be nice if your contribution directly supported actual scientific research.
So I went ahead and launched a research program in your name to address a critical question.
That question is the following: Is it actually possible to see a neutrino?
And after nearly an hour of intensive investigation we have a result.
The answer is yes!
There’s a tiny chance that a neutrino passing through the vitreous fluid of your eye will
interact with a nucleus and produce Cherenkov radiation.
We know that this Cherenkov is potentially visible based on reports of flashes by radiation
therapy patients.
So, how likely is this for an individual human?
100 trillion neutrinos pass through your body every second, and on average one single neutrino
will be stopped by your body if you live 100 years.
The human eye has a volume of around 6 cubic centermeters, which is 5,000 times smaller
than your whole body, counting both eyes - so it’ll take 100 years times 5000 to stop
a neutrino with your eyes - around a half million years.
Round to a million because you have to be awake when it happens.
In other words, fewer than 1 in a million people see a neutrino once in their life.
But with 8 billion people on the planet, that’s a few every day.
So there you go, Steffen, you’ve sponsored some cutting edge scientific research.
Thank you for your contribution to the growth of human knowledge.
And by the way kids - if you start seeing mysterious flashes, you might be one of those
lucky few, but you should talk to your doctor anyway.
Today’s comment responses are for our recent episode on the island of stability - the hypothetical
collection of stable-ish elements beyond the end of the current periodic table.
Feynstein 100 asks if stable heavy elements like the island of stability could exist,
then shouldn't they be present on earth and be found already?
Let me clarify something about the island of stability.
Those elements are unlikely to be perfectly stable.
They’re likely to decay, but on a longer timescale than the other elements between
the island and the current top of the periodic table.
So any that formed naturally, say in a supernova, may have decayed by now.
Also, because they have such extreme masses they’d be hard to produce naturally.
Extremely heavy elements are produced in supernovae or in colliding neutron stars by the R-process
- basically bombarding lighter elements with neutrons.
These then decay into the first stable isotope, with some neutrons converting into protons
on the way.
There’s a limit to how large a nucleus you can make this way - and that’s when the
decay timescale is shorter than the rate of neutron bombardment.
To get to the island of stability, you have to get through a region of extremely fast
decay nuclei, and my guess is that known astrophysical processes can’t generate the neutron flux
needed - at least, to produce enough of the stuff to last very long.
Brian Diehl mentioned the prospect of a 'continent of stability’ - beyond the periodic table.
Well, I had to look this one up, but you’re right.
The idea is that there may be a new way for quarks to form stable clumps.
Normally quarks bid together in groups of 3 - protons or neutrons.
But at nuclear masses much higher than the current periodic it may be possible for quarks
to exist in a free-flowing state without the standard nucleons.
Originally it was thought that strange quarks were required, and we’ve been searching
for strange quark matter since the ’80’s.
But very new work shows that this may also be possible with the regular up and down quarks of normal
matter, and that this up-down-quark matter may lead to perfectly stable “nuclei”
with atomic masses as low as 300.
And finally, apparently I pronounced technetium incorrectly.
It’s technetium, not technetium.
While you can trust much of what I say, there are a few areas where you should not entirely
rely on some random astrophysicist on YouTube.
In fact there are a great many such areas, but most importantly don’t copy their
pronunciation of obscure chemical elements.