The Story of Dark Matter | Crash Course Pods:
Unveiling the Universe's Mysteries: The Story of Dark Matter | Crash Course Pods
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so Katie I'm just going to start out by
telling you the truth which is that you
have almost a 0% chance of helping me
understand this like when you were
describing black holes I felt for the
first time that wondrous glimmer of
understanding that we seek in this world
where I believed you you know and I I
understood that there are these strange
light sucking
information destroying maybe
things in our universe and that they're
common and that's wild it is absolutely
but now you're about to tell
me look it's hard enough for me to
believe and understand that I'm on a
rock that's orbiting a star that's one
of hundreds of billions of stars many of
which are orbited by rocks similar to
mine that's in one Galaxy which is one
Galaxy out of hundreds of billions of
galaxies and that there's more stars in
the universe than there are grains of
sand on all the beaches on all of the
earth that is hard
enough that's a lot of stuff to try to
get my head around and you're about to
tell me that all of that stuff
combined is what like 15% of stuff 15%
of matter yeah yeah and the other 85% of
matter is
stuff that we can't touch or smell or
interact with or know anything about
except through inference you're you're
already there you you're telling me that
that I can't convince you but you
already you already know the whole story
of dark matter I definitely don't
because because I don't I that seems
utterly impossible man it is hard it is
hard enough to Grapple with how much
stuff I can see
[Music]
so as you just heard today's episode is
about dark matter and I'm going to be
honest with you there were moments
during this conversation when I started
to question my understanding of
extremely basic concepts for example in
about 90 seconds you'll hear me learn
what's actually happening when I pick up
an apple but I left this conversation
with a great appreciation for folks like
Dr Mack and the enormous amount of
effort it takes to learn pretty much
anything about our universe for the
complex Journey that scientists embark
on that we often don't learn about until
it's over and there's a shiny new
discovery to show for it that process
can be frustrating and fascinating and
to my surprise can sometimes involve
digging around old shipwrecks for
experimental materials here's our
conversation
[Music]
the way that we see regular matter is
through interactions with
electromagnetism so you have an atom
right and you know the atoms are
combined into some kind of solid thing
let's say you're looking at like an
apple or something right like you're
seeing that because there is light
that's reflected off of that into your
eye so it's it's able to reflect light
if you held it in front of a light you
would see that it would block out some
of that light so it's it's able to
absorb light and if you put it in a
totally dark room and then like looked
at it with an infrared camera as long as
it was a little bit warm it would emit a
little a little bit of light right so
it's it's it's able to interact with
light in a number of different ways
because the it's made of atoms and atoms
interact with electromagnetism because
the light can interact with the atoms in
that object the light can kind of bounce
off of those atoms could be absorbed by
those atoms can be emitted by those
atoms because those atoms have electrons
on them and protons and so there are all
these electromagnetic interactions that
are occurring so it it interacts with
the electromagnetic force okay fine I
get that yes I I realized the fine
sounded a little judgmental but I'm
getting I'm already getting anxious okay
but there are other ways I can interact
with an apple I can touch the Apple I
can smell the Apple I can step on the
Apple all of that is still
electromagnetism though no is that true
yeah cuz when you touch when you touch
anything what you're doing is you're
you're pushing the electrons in your
hand against the electrons in that thing
when you pick up the Apple it's because
your electrons can repel the electrons
in the Apple such that it your hand
won't pass through
M okay and and so it's it's
electromagnetism that's that's allowing
you to touch that thing taste and smell
are other interactions of the chemicals
in the object you know sort of
interacting with the receptor in your
body and that's also some kind of
electromagnetic thing happening because
those those are atoms interacting that's
chemistry that's that's you know
molecular bonds and things like that
right so it's it's all electromagnetism
that allows us to interact with things
and see things okay so if you can
imagine let's say there's a kind of
matter that doesn't interact with light
it doesn't have an electric charge it
doesn't absorb light it doesn't do
electromagnet system it doesn't interact
with the electromagnetic force that's so
possible we actually have already like
in a very detailed way studi a kind of
particle that has that property so
there's the neutrino which is this kind
of very sort of ghostly particle it's
produced in uh nuclear interactions in
the Sun and other stars and neutrinos
don't do electromagnetism they they
don't have an electric charge they're
called neutrinos cuz they're neutral and
they they just don't have any any
interaction with electromagnetism they
do interact via the weak nuclear force
that's why they can be produced in
radioactivity and in nuclear reactions
and they have some Mass so neutrino has
a very little bit of mass but it has
some mass and we've studied that
extensively we don't know exactly what
the masses are there are three different
kinds of neutrinos it's possible that
one is massless but at least two of them
have have some kind of mass you know
it's small but it's there so they
interact with gravity they interact with
the weak nuclear force but they don't do
electromagnetism and that means that
they can pass through the Earth without
interacting they can pass through us
without interacting there are trillions
of neutrinos passing through your body
every
second oh
no like uh like in the movie Ghost
almost yeah yeah just just like a just a
shower of nutrino coming from the Sun
coming from other stars and it's just
going through my body and I I don't
notice it and not only do I not notice
it I can't notice you can't notice it so
because a neutrino only interacts with
via the weak nuclear force it has to be
a really really Direct Hit with another
particle to make that happen the weak
nuclear force is a very short range
force and it's very weak and so like I
don't remember the numbers exactly but
it's something like you could have like
a lightyear of solid lead and in trino
passing through that has like a 5%
chance of interacting with anything wow
so neutrinos pass through your body all
the time I I was at a talk once about
neutrinos where they talking about they
were talking about these interaction
rates about the the chance of an of a
nutrino interacting with anything in
your body in your lifetime and if it
does interact with something in your
body it's like it's going to bump an
electron like it's going to ionize an
atom or something it's not going to do
anything complicated right but anyway
you get these uh these interactions very
very rarely and the the statistic they
said was you know over your lifetime
like maybe you'll have one interaction
with a nutrino wow even though there are
trillions going through me every second
yeah yeah I don't yeah and I don't
remember the exact number of neutrinos
going through you every second somebody
will correct me but it's but it's a lot
it's more than four yeah yeah and the
thing that stuck with me about that talk
was they said and the second one kills
you but that's a joke right to be clear
okay great that's an astrophysicist joke
that's an astrophysicist joke cuz I
laughed and I was like that's a funny
joke and then I was like uhoh no what if
the second one kills me you say that you
can't feel those of neutrinos going
through you every minute but I feel them
right now I have ever since I found out
about them I've been feeling them I feel
them very intensely that little like
tingle
[Music]
right so we interact with things with
regular matter through
electromagnetism that force that occurs
between electrically charged particles
we can see something because light
reflects off of it we are able to touch
something because our electrons repel
its electrons but not everything
interacts with the electromagnetic force
and because of that we can't see them we
can't touch them we can't smell them
they may interact with other forces like
the weak nuclear force which is involved
with nuclear Decay but we humans can't
easily perceive that to us they might as
well not be there at all neutrinos are
an example of this as is dark matter so
if we can't interact with them how do we
know these things
exist basically to start off there are
these things called neutrinos that are
like ghosts in the sense that they can
walk through walls and walk through
bodies they can go through the Earth
they can go through the Earth and
they're going through me right now but
I'm fine the way we sort of detect
neutros is we I mean there are a few
different ways but the big ways you take
a giant tank of water you put it under a
mountain and you set up a bunch of
detectors around the inside of that
water tank and there are neutros passing
through all the time they you put it
under a mountain so that no other
particles can get through only the
neutrinos can get through and then every
once in a while a nutrino will hit
something in in one of the um one of the
atoms of of the water and it'll
accelerate that particle it'll it'll
strike it really hard it'll accelerate
that particle that particle will be
blasting through the water now at a
speed that's faster than things usually
go through water they say it exceeds the
the speed of light in water so it's not
going faster than the speed of light as
like a limit but it goes faster than
anything can usually go in water because
it's just accelerating and that means
that it makes it sort of like the light
equivalent of a sonic boom makes this
flash of light it's called trov
radiation and then that light is
detected by these detectors surrounding
the inside of this this water tank we
can actually use that as like a
telescope because we can tell which
direction the nutrio came from because
of which direction of the flash happens
and it turns out you can make an image
of the Sun in the nutrino flux right so
as the neutrinos are coming through most
of them are coming from the Sun so you
can actually like make a an image of the
sun based on where the neutrinos are
coming from and that image of the sun is
built up of all of the neutrinos that
are coming through whether it's day or
night if they're going through the Earth
or through the mountain or whatever and
it's like a fuzzy picture but it's a
picture of the nuclear interactions
happening inside the sun it's kind of
cool wow that's some CSI stuff right
there like that's like that's like
Sherlock Holmes business it's really
neat yeah but anyway this is a bit of a
a digression but the the point is that
we do know of the existence of particles
that act in the way that we think Dark
Matter acts in the sense that you can't
touch it you can't see it doesn't
interact with light maybe once in a
while it'll do weak Force intera action
we we don't know but all of the evidence
we have for Dark Matter makes it look
like something that's just a lot like
the neutrinos the reason that we think
the neutrinos themselves are not the
dark matter is at least the three
neutrinos we know about there could be
other kinds of neutrinos the the so
so-called sterile nutrino which is a
like a fourth version of a nutrino that
would be heavier and would act a little
bit differently but would have similar
properties but the reason that we know
that the three that we know about or not
the dark matter is that like they're too
light they move too quickly there are
not enough of them to make up all this
this extra stuff that we know is in the
universe there's just not enough of them
and they move too fast so like they
wouldn't be in big clumps where galaxies
are they would kind of disperse too much
cuz they move too fast okay but anyway
we do know that it's possible for there
to be a particle that just doesn't do
light does have gravity it does have a
mass and maybe it has maybe it does
something with a weak Force we don't
know okay so dark matter seems to be
something like that dark matter seems to
be something that we can't see doesn't
interact with light we can't touch it
because it doesn't seem to do
electromagnetism but it has gravity and
it's probably some kind of particle that
has a mass and it doesn't do electromag
doesn't have a charge maybe it does the
weak Force we don't know why do we need
Dark Matter to make sense of the
universe how do we know know that dark
matter is there so this is a long story
with lots of little pieces to it there
is a lot of evidence for dark matter
when people talk about dark matter in
the news they're usually talking about
one particular piece of Dark Matter
evidence which is about how stars move
around in galaxies so this is the one
that's a little that's sort of the
easiest to explain so picture a spiral
galaxy okay okay the stars are moving
around the center of that Galaxy um you
know our sun is orbiting around the
center of our galaxy it takes millions
and millions of years but you know we we
go around in a circle all the stars in
the spiral arms they're they're sort of
orbiting the center of the Galaxy and
when you picture a spiral galaxy you
have to picture like the central part of
that Galaxy is a lot brighter there's
like a bulge of stars there's like a a a
sort of big clump of stars in the center
and then the spiral arms themselves are
kind of wispy and thin okay so the way
that works is like most of the visible
map
in the galaxy is really concentrated in
that bulge in the center and the stars
and the spiral arms make up a very small
fraction of the visible matter in the
Galaxy it's mostly in the center and so
you'd expect that the stars that are
closer in are going faster and the stars
that are farther out are going slower in
just the same way that in our solar
system you know Mercury goes around the
Sun a lot faster than Jupiter or Neptune
right because it's closer in so it's
feeling the gravity of the sun stronger
and so it has to be going FAS fter to
stay in that orbit and not get sucked in
yeah not fall in and then the more
distant like Neptune if Neptune were
going a lot faster it would just leave
right it goes kind of slowly and it goes
around the Sun but if if you gave it too
much energy it would just it just leave
because it's not held on very tightly
now in the 1970s astronomers were
looking at the rotation of stars around
the centers of spiral galaxies I think
it start of started in the 60s and 70s
and they were noticing that it didn't
seem like the ones at the outside were
going a lot slower when they plotted out
how quickly the stars were moving around
the center of the Galaxy it really
looked like the ones in the center were
going kind of fast and the ones in the
middle were going kind of fast and the
ones on the outside were going kind of
fast and they were kind of it was kind
of the same speed like all these stars
were kind of going around in about the
same speed the person most famous for
these observations is Vera Rubin she's
one of the people who kind of made this
this discovery really well known and and
really sort of made it very convincing
to the community a whole bunch of people
contributed in other ways or or did some
of these observations around the same
time but the reason there are there's
now a major telescope project named
after her is because she was one of the
people who was like a Pioneer in this
field anyway so there was this weird
thing where it seemed like the stars
toward the edges of the Galaxy were
going too fast and they should just be
flying off into space this was even to
the very edge of the visible part of the
Galaxy like the farthest out Stars you
could see going around these spiral
galaxies we just we're going the same
speed as the ones really close in and
that just doesn't work if just the
visible matter is all the matter there
is because it really should be that
things have to move more slowly as they
get farther out because there's just
less matter to hold them in right and so
the sort of natural inference there is
that there has to be more matter than we
can see holding these stars in and it
has to be such that you know as you go
farther out there's the same amount of
like gravity because there's just way
more matter and one of the ways you can
do that is if if you have a spherical
distribution of matter so their spiral
galaxy is is a dis right but if it's if
it's embedded in this giant blob of a
spherical distribution of matter where
the matter is more concentrated at the
center less concentrated as you go out
turns out the math will work out such
that the amount of gravitational force
felt by the really distant stars is
going to be about the same as the ones
as the mount felt by the interior stars
and and that's because you know in this
spherical distribution If you're sort of
partway in you're only feeling the
gravity of the stuff interior to you if
you're toward the edge you're feeling
the gravity of all of the stuff all the
stuff yes yes got it got it and we we
talked about this a little bit U
previously right I remember and so you
can work out the math and it it works
out that if the disc Galaxy is embedded
in this giant spherical clump of
invisible stuff then that naturally
explains why those distant stars are
moving so fast now one of the things
that always comes up when people talk
about this explanation is well what if
we just got gravity wrong right because
because this this explanation assumes
that we know how gravity works and so
there has to be extra matter but what if
there's something about when you get to
really weak gravity toward the edges of
the Galaxy like it's just you know
you're no long you don't have the same
weakening of gravity as you go out
farther you know maybe that Law changes
I mean what we usually have with
Newton's gravity we have what's called
an inverse Square law so if you get
twice as far away from the gravitational
object the force of gravity is a quarter
as strong okay so it it weakens as the
square of the distance right and so if
you're four times further it's 16 times
weaker yeah yeah okay I got it but the
argument is maybe that doesn't apply at
a Galaxy scale maybe there's something
that we're missing yeah there could be
some weird acceleration scale where like
things change as you get to a certain
kind of strength of gravity and this
this is an idea that's been around for
many many years the most famous version
is called M uh stands for modified
Newtonian Dynamics and it was written to
just explain this rotation curve thing
to say well what if there's just some
weird scale that you add to you know
change the the law of gravity and then
this all works out with these rotation
curves and and you can do that and you
can get the rotation curves to work just
as well by modifying gravity so so if
you really want to tell the difference
between dark matter as like an extra
stuff or Gravity the law of gravity
changing you need different kinds of
evidence because those two pieces look
the same it turns out that within the
solar system in the history of
understanding the solar system there
were two situations where it went in
opposite directions around is there is
there extra mass or is there extra
gravity okay are you changing Mass or
are you changing gravity and those
examples are Mercury and Neptune you
know the way that the way that we
discovered that Neptune exists was that
astronomers saw that the orbit of Uranus
was weird it was like being perturbed by
something and they inferred that that
could be explained by the existence of
an extra Planet farther out and so they
did those calculations and they worked
out maybe there's some extra Planet out
there that's messing with Uranus and
they went and found Neptune there
probably more to the story but that's
that's the kind of version I know with
Mercury it had been known for a long
time that the orbit of mercury was a
little bit wonky it was kind of
processing in this weird way so there
were ideas that maybe there was some
extra Planet near the Sun that was
messing with the orbit of mercury turns
out no Mercury's orbit is weird because
of general relativity because Mercury is
so close to the Sun that the Space is
really really curved and that changes
the way that Mercury orbits so we have
these two examples of you know changing
the matter content in one side or
changing gravity on the other side and
so in our solar system you can you know
people can argue in either direction
about like oh well we have this example
like yeah we have both examples so with
dark matter you know the way that things
sit now there are definitely people who
still argue you know we just need to
change gravity if we if we find the
right way to change gravity we can
explain all this stuff we don't need
dark matter but the abundance of
evidence is very much pointing toward
dark matter just being real um because
it's not just rotation curves there are
a huge number of pieces of evidence for
dark matter I'll talk about a few of
them so the first piece of evidence for
Dark Matter actually came even before
this rotation curve stuff when
astronomers were looking at galaxies
moving around in clusters of galaxies
and seeing that basically the galaxies
and the cluster were moving too fast and
it was a Sim similar argument to the
rotation curve thing um but uh at a sort
of very different scale and that gave
evidence for dark matter because there
you're seeing that they're moving around
each other as if they had much more mass
than they are observed to have as if
there was more mass holding all of the
galaxies into the cluster right okay so
like the cluster kind of it's kind of
like a hive of bees right all these
galaxies are sort of orbiting around the
central region of the cluster and
they're all kind of gravitationally
bound in a clump but if you just count
the galaxies there's not enough matter
to account for all of them in fact
though in a cluster of galaxies like
most of the visible matter isn't even
the galaxies most of the visible matter
is a bunch of to hot gas that's kind of
also bound into that cluster from the
gravity that hot gas is just like hot
ionized gas that if you look at the
Galaxy cluster with x-ray telescopes
it's glowing in x-ray light so most of
the visible matter is that cluster gas
intercluster medium is what it's called
but even with that gas there isn't
nearly enough of it to explain why
they're moving the way they are yeah and
and the gas itself is another piece of
evidence for extra matter because it's
so hot that it should like disperse it's
the gas is so hot there's a lot of
pressure and it should be dispersing and
it doesn't it's bound into that cluster
in such a way that there has to be extra
matter holding that gas in so that's
that's another piece of evidence that
there's there's more stuff but then
there's also gravitational lensing so
the way that that light can bend around
massive objects in in the universe we we
see that I mean that's very dramatic
around like black holes where the light
can bend around the black hole and kind
of fall into the black hole but even
with just regular galaxies or clusters
of galaxies they can bend the light
coming from things behind them and we
can see these like we see these really
amazing structures in the universe where
there'll be a picture of a you can find
these on online where there's a picture
of a cluster of galaxies and it's like
you know a bunch of little bright
objects in a sort of Clump and then
there will the there'll be these weird
like arcs around it um these little like
usually there like blue or red arcs
around that cluster sort of like framing
it like you know going around sort of in
circular patterns and those arcs are
actually the light from galaxies behind
the cluster that's bent and distorted by
the gravity of the cluster itself and
The more mass in the Galaxy cluster the
more that light gets bent and the more
of these arked images of the background
galaxies you get and so that gives you a
way of measuring the amount of mass in
the cluster because that cluster of
galaxies bends space according to its
mass not according to how much stuff you
can see and so that gives you a very
objective measurement of the mass of the
cluster and that allows you to tell
where the matter is even if you can't
see the matter itself you can see how
much effect it's having and you can see
that there's a way bigger dent in space
basically than could happen with just
the stuff you can see and so that gives
you a way of of also you know
determining that the extra matter sort
of makes sense in this picture you can
see little distortions in background G
that kind of give you an idea of the
broader distribution of of matter in the
universe and that's a subtler Effect
called weak lensing but that allows you
to really like map out where all the
dark matter is and so we've been able to
see like if you have two clusters of
galaxies there's like a filament of Dark
Matter stretching between them um and
we've been able to detect that even
though there aren't a lot of like
galaxies on that filament because of the
lensing of the distant stuff behind it
you can can trace out where that is so
there's all sorts of things like that
where we have these these indirect
measurements but then there are a couple
that are a little bit more subtle but
really important so I mean we talked
before about the like large scale
structure of the universe right so how
the universe went from being this kind
of blotchy plasma in the early times to
then that those little blotches the bits
of higher density plasma s kind of
growing up into clusters of galaxies and
I mentioned that you can do simulations
where you give the simulation the
distribution of matter from the cosmic
microwave background from this sort of
blotchy early plasma you give the Sim
simulation that distribution of matter
you turn on gravity you let it evolve
over time and it creates this Cosmic web
this distribution of galaxies on the
largest scales in the universe and I
don't know if I mentioned it at the time
but those simulations are done with only
Dark Matter oh because the visible
matter is such a small percentage that
it doesn't really matter very much
yeah exactly so when you do those
simulations when you put in the
distribution of matter in those
simulations you just make it dark matter
because it makes the simulation easier
because then the only thing that's
happening is gravity like if you tried
to do that simulation with gas that gas
would have a bunch of pressure and it
would sort of resists collapsing it
would bounce off it would it would heat
up it would get really complicated and
and you can you can try a simulation
where instead of dark matter that you
know that only does gravity you put all
the matter in with gas with pressure and
all this stuff you just just can't
really form galaxies like it's so much
harder to get the matter to come
together and create galaxies if you
don't have that collisionless stuff
because when you try and push all the
gas together it heats up and then it
pushes out and then so you know you have
to find a way for it to cool effectively
to to fall together and and it just
takes much much longer if you want
galaxies to exist today the way that we
see them you need the dark matter to
bring all the all the matter together
and to create this Cosmic web and so
these simulation of the large scale
structure of the universe only work if
you do have that dark matter component
acting as like extra sort of glue to
hold together all of the regular matter
so if we do these
simulations with the stuff that we see
the world that we see the world we or
the universe we encounter with our eyes
and our telescopes and our other forms
of sensing the universe it just does not
work but if you make it 85 % Dark Matter
suddenly the universe looks like we
would expect the universe to look like
looks like our universe yeah or even if
you just do the simulation with 100%
dark matter and then when it's done you
say okay the places with higher density
that's where the galaxies live that also
works M
[Music]
wow so we can't see dark matter we can't
touch it but it does have mass and it
does have gravity and even though we
can't see it we can be reasonably
confident Dark Matter exists there's a
variety of evidence like the way some
Stars within spiral galaxies move and
even the way some galaxies within Galaxy
clusters move based on the amount of
visible matter around them there's a
missing piece there which could very
likely be dark matter and not only does
Dark Matter explain the movements of
some galaxies and stars it also allows
us to explain the for
of galaxies to begin
[Music]
with we talked about how in the very
very early Universe the whole universe
is as hot as like the center of the sun
right and so you have these nuclear
reactions you have hydrogen turning into
helium and a few other elements and we
can look at the abundance of of elements
in the universe the abundance of
hydrogen helium and lithium and a little
bit of burum and and and we can we can
see the abundances of these elements
and calculate like how that big bang
nucleosynthesis must have happened and
and then we can calculate from this from
this stuff like how much regular matter
there had to have been versus like how
much total matter there had to have been
for these interactions to occur and we
find that there had to have been mostly
dark matter like the the amount of
regular matter is only about 15% of what
you would need to get all these
interactions to happen and to get the
nucleosynthesis uh to work out out the
way that we see it and that's one of the
pieces of evidence that is really very
hard to argue against because that's
just saying like the regular matter
can't be more than a small fraction of
the total matter in the universe so we
have all of these reasonably independent
ways maybe not wholly independent but
reasonably independent ways of looking
obviously not at dark matter directly
but at the universe as we find it and
all of them seem to indicate the same
thing which is about 85% of all matter
in the universe is dark matter exactly
yeah not so not only do they indicate
that there is more matter than we can
see they all point to about the same
abundance of that extra matter and the
same behavior of that extra matter and
so you know it's one of these things
where you're you're putting together all
these different pieces of evidence and
they all match in a way that becomes
really compelling when you start to add
them up there's there's another one I
want to mention just just because
sometimes people talk about it as The
Smoking Gun for for Dark Matter oh and
that's that's even even more sort of
sort of appropriate because it's it's a
a Galaxy cluster called the bullet
cluster oh that's a good name yeah
that's much better than Sagittarius A
star which isn't even a star it's not a
star no it's it's terrible it's terrible
but yeah so the Bullet cluster the
reason it's called the bullet cluster is
because you have these two clusters of
galaxies one is little and the one is
big and the little one like shot through
the big one like a bullet oh sometime
you know millions and millions of years
ago um don't want to stop you but just
real quick that's not going to happen to
us right no no I mean I mean we're going
to collide with the Andromeda galaxy in
about 4 billion years but nothing's
going to shoot through us as far as we
know no great okay go on okay so this
little cluster of galaxies like shot
through this bigger cluster of galaxies
and the way that we know that is that
when we look at these clust with x-ray
light what we see is that there's a
bunch of gas uh with like a shock wave
going through it of like a little bit
like so there's a clump of gas in one
place and then right next to it there's
this little like triangular shock wave
where this little clump of gas like you
know pounded through it and uh heated up
and so this gas from the Clusters got
sort of stuck in the middle of this this
Collision whereas the galaxies
themselves pass through because even in
a cluster of G the galaxies are not so
close together that if you Collide the
Clusters they're going to directly hit
they kind of fly past each other you
know like two flocks of birds if the
birds are spread out far enough they
don't hit in the sky so the galaxies
themselves went through most of this
cluster gas got stuck in the middle and
made this little shock wave that gives
the cluster its name so now it's like a
a big cluster formed of these two
subclusters where they collided in the
past but the the galaxies went through
the gas got stuck in the middle and the
reason that this is an interesting piece
of evidence for dark matter is because
in a cluster Collision what you expect
to happen is you expect that the gas
will get stuck in the middle because
it's you know it's this sort of dense
plasma gas and and it collides and it
you know it loses the energy through
that Collision it gets stuck like right
it's it's it sticks the Galaxies have to
pass through because you know they
they're not likely to actually hit and
they can go through that cluster gas
fine but the Dark Matter should go
through through as well because it
doesn't do collisions right so if each
cluster has galaxies gas and dark matter
then what you would expect to see if if
dark matter is really a thing is that
the Dark Matter should stick with the
galaxies the cluster gas should stay in
the middle right and we can figure out
where the dark matter is through
gravitational lensing through looking at
how the space is being bent by these
objects and what we see is that most of
the lensing most of the bending of space
is where the galaxies are not where the
gas is in the middle
M so if it were just that regular matter
had like extra gravity then the gensing
should be in the middle cuz that's where
most of the regular matter is right
that's where that gas is because that
gas outweighs the galaxies right but the
the lensing is where the galaxies are
which means it passed through the
Collision which means it really is this
extra dark matter collisionless stuff
right and that's that's something that's
quite hard to explain by just changing
the law of gravity because you have to
move the gravity somewhere else to do
that you have to to move it away from
where the regular matter is people come
up with complicated explanations that
involve sort of a delay of the
gravitational force moving or I don't
know um people people try but it's it's
a difficult thing to explain without
just there's a another kind of stuff and
it doesn't Collide so the current
consensus among most
cosmologists is that there is this dark
matter and there may down the road of
course be evidence that causes us to
change our understandings of the
universe that's the that's how science
works but but that's that's where we are
right now yeah and the way that I the
way that I sometimes talk about it is
like it's like if you're walking down
the street and as you're walking down
the street you hear the leaves rustling
in the trees and you look over and the
the trees are kind of bending over and
the stop sign is kind of waving back and
forth and a plastic bag moves around
moves past in front of you and and you
feel really cold on one side
you you you figure there's wind right
there's there's all these different
phenomena but they're all consistent
with something invisible moving the
stuff you can see or affecting the stuff
that you can see and and dark matter is
kind of like that we can't see it just
like you can't see the wind but we can
see all these different pieces of
evidence that are all pointing to the
same explanation right and we can see
things moved by the wind we can see
things affected by the wind even if we
can't see the wind itself and so when we
when we talk talk about feeling or
seeing the wind what we're really
talking about is feeling or seeing the
things that the wind
does I know Dr Mack dislikes it when I
metaphorize science into Human
Experience but who Among Us has not been
buffeted by forces that despite being
unseen and unobserved still shape our
experience of the world like isn't that
the basic human condition and to over
metaphorize the future is a kind of dark
matter right we know it's out there we
know it's most of what's out there and
yet we cannot see it anyway that's why
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just
[Music]
saying okay so to recap we can assume
there's dark matter in the universe from
different kinds of evidence one way is
by looking at the abundance of elements
in the universe and sort of reverse
engineering how much ordinary matter
needed to be there at the beginning to
end up with all that stuff another is by
looking at the bullet cluster where two
Galaxy clusters collided leaving a blob
of gas in the middle but instead of the
space around it being bent mostly by gas
it's bent more by the less massive
galaxies indicating that dark matter
passed through the collision with them
by observing the effects we find a
cause we're trying to find ways to
detect dark matter where we would get
other evidence of it existing in the
same way that like with neutrinos we
found a way to detect neutrinos even
though they don't do electromagnetism
you know by making these giant tanks and
let letting them interact with a weak
nuclear force and like we have a way to
detect neutrinos we would love a way to
detect Dark Matter similarly but it
might not work out so so far it hasn't
worked out so far all of the experiments
to detect dark matter have been either
inconclusive or have found nothing
nothing so um there are there are a lot
of different ways that people have tried
to detect Dark Matter there are kind of
three different branches of experiments
around dark matter characterization I
would call it trying to figure out what
the dark matter is made of what is this
new particle if it even is a particle
there's direct detection which is where
you build a detector and you hope that a
dark matter particle will come in and
bounce off something in a particular way
that would indicate that the Dark Matter
came through so very similar to the nutr
detector idea but use different detector
technology and you know sort of set up
differently but this the same idea you
wait for a dark matter particle to come
in and interact via the weak force with
something in your detector and then you
look for that bump right it's called a
nuclear recoil experiment you look for
for the particle to to bump into one of
the nuclei of the atoms in your detector
and you it's hard because you have to
Shield against everything else that
might bump into your detector and you
can't Shield against neutrinos which
should be fine because neutrinos don't
add that much energy and so as long as
you you know set it up properly it
shouldn't be a problem but if the Dark
Matter turns out to be you know light
enough or the coupling strength to
regular matter is small enough for
whatever you could get to a point where
the amount of energy particle gives your
stuff is going to be the same as a
nutrino and then you're you're screwed
right then you have no way of knowing
yeah and and there are there are kind of
subtle ways to try and get around that
by figuring out where the dark matter is
coming from or how it changes over the
course of the year but it gets really
hard at that point and we're pushing
toward that limit now with these
detectors they're getting really really
sensitive and you know haven't seen
those interactions so direct detection
is complicated I should mention there's
one direct detection experiment that has
claimed a detection but in a complicated
way that most of the rest of the
community thinks is probably not valid
or like
um just because all of the other
experiments disagree uh but it has to do
with the way that the Dark Matter signal
should change over the course of the
year and they saw basically in certain
times of year because of the way the
Earth is orbiting the Sun it's going
sort of more into the dark matter and
sometimes it's going more out of the
dark matter because of on its orbit no
way seriously okay so the sun is moving
around the Galaxy yeah and the Galaxy is
in a cloud of dark matter just picture a
big spherical cloud of dark matter and
the Sun is moving around the Galaxy yep
inside that big spherical yeah inside
the big spherical Cloud okay so the
direction in which the sun is moving is
the direction where it's going to hit
more Dark Matter the Dark Matter wind is
going to be coming from that direction
for the most part so like you know like
if you're driving a car you know the
wind is coming from in front of you
because you're moving through the air
right right right so the the sun is
moving through the Dark Matter cloud in
a particular Direction and at certain
times of year the way the Earth is
orbiting the Sun it's going more in that
direction and another times of year it's
going kind of in away so it's that it's
tilted but in in June it's going more
toward the direction the Sun is going
and December it's going more in the
other direction okay so in June it's
headed into the dark matter cloud in
December it's sort of like the wind is
at our back almost yeah yeah exactly
okay okay and so if you have an
experiment that detects Dark
Matter like you'd expect to see more of
it in June than than in December and so
there's this experiment called dhamma
Libra where they don't have a good way
of determining what they're seeing
they're detecting some kind of
interactions with their experiment and
they're they're detecting more of them
in June than in
December and so their explanation is
well that that could be the dark matter
right the Dark Matter could be happening
more in June than December and so maybe
that's why we get more flux of something
in June in December the problem is there
are a lot of things that can change with
the seasons right and so it could be
some kind of weird systematic effect due
to like changes in the atmosphere that
affected neutrino flux or like who knows
right like there's stuff that could
happen things that could change the
detector capabilities in some ways and
it's just very hard to know for sure and
so there are now some experiments going
on that are trying to test it by doing
the same experiment but in the southern
hemisphere oh so you're swapping the
seasons but keeping the Dark Matter wind
the same right so there's an experiment
that I've been involved with called
saber they're doing two experiments they
have one in Australia and one in Italy
and they're going to try and do the same
measurement with the same kind of
detector on opposite sides of the world
to see if they have high flux at the
same time of year or the opposite time
of year or if they just don't see
anything at all if they both have high
flux in June that would maybe indicate
that it could be more likely to be dark
matter exactly exactly whereas if the if
the one in in Italy sees it in June but
the one in Australia sees it in December
it's probably seasonal yeah it's
probably Seasons right wow that's so
cool yeah yeah it's really neat there's
even weirder stuff around dark matter
detectors so one of the problems with
dark matter detectors is that you really
have to reduce the backgrounds which
means that you you just have to make
sure that there is nothing going to come
in and bump into something in your
detector so one way is you put it deep
under ground you put a bunch of
shielding around it but like even the
the shielding if it has any any
radioactivity in it it can be a really
big problem right because that
radioactivity creates neutrons that bump
into your detector and those neutrons
look just like dark matter to the
detector CU they're electrically neutral
and they just bump into something they
have some mass and so like radioactivity
is a really big problem so you know you
have to go around and measure all the
radon and all this kind of stuff in your
detector and it turns out that uh iron
that is made made or like smelted or
whatever
after the atomic bombs happened has more
radioactivity than the stuff that's made
that was smelted before it
wow wow so wait do you have to try to
find like old steel yeah so people go
out and look for shipwreck
steel wow like so there's there's this
like cottage industry of going out and
like salvaging shipwrecks to do dark
matter experiments man because you need
the uh you need the low background
that's wild that's wild yeah and like
there's there are sort of rumors about
certain experiments where maybe they got
that steel like not fully
honestly but but it's like problematic
steel yeah okay cuz you had to you had
to get yeah so so anyways shipwreck seal
is is a thing it's like you try and you
try to get really old steel so that it
doesn't have as much background and and
there's there's like a limited amount of
it and and you know people have to
excavate to try to to do that anyway
it's it's it's wild I can't believe that
you waited 45 minutes to tell me that
there's special shipwreck steel that
gets used in these experiments yeah what
other amazing secrets are you hiding
from me I mean I mean physics is full of
these little things where like you you
start to dig into something and and you
find just you find something just
utterly wild went on yeah you know all
of these experiments you know they're
happening in the real world and there
are there are practical considerations
that you have to deal with like the fact
that ligo the gravitational wave
detectors there are two ligo sites and
they're kind of out in the middle of
nowhere you know in their respective
places because they need to be like you
know seismically isolated and and all of
this stuff and um I mean for one thing
they can detect they can detect the
waves lapping at the at the ocean
uh their the seismic detections are so
so careful but they had to put a
concrete barrier around at least one of
them and maybe both but they had to sort
of cover the tube with concrete cuz
people were like shooting at the vacuum
tube like shooting guns at it yeah yeah
oh that's a bummer so they so they had
to encase it in concrete so that the
experiment wouldn't like implode from
people shooting at it wow yeah all of
this is a reminder to me that where on a
planet and we're humans and we're doing
our
best but sometimes we're also doing our
worst as in the case of shooting up a
gravitational wave detector
gravitational wave detector yeah it just
reminds me of the essential Humanity of
science which it's easy to forget about
it's easy to see it as something that
like isn't done by or for
humans but but we're we're doing this
not me as much but but we in the larger
sense
[Music]
yeah I was going to tell you about the
two other ways that we look for try to
figure out what dark matter is great
tell me about the two other ways we try
to figure out what dark matter is aside
from this sort of direct observation
strategy the way that comes up the most
in astronomy is called indirect
detection and I should say that like
direct indirect these are all kind of
relative terms right because you're
never going to actually see it because
it doesn't interact with light so even
in a detector what you're detecting is
like the motion of something else due to
the interaction yeah you're detecting
the leaf shaking in the wind not the
wind exactly and is is that more direct
than detecting the motion of stars
around a Galaxy or the bending of space
through gravitational lensing I don't
know right I don't know but anyway these
are the terms that we use so direct
detection is where you use a detector
indirect detection is what happens when
in
astronomy if if the dark matter is doing
something interesting other than just
gravity something in interesting in a
particle physics sense other than just
gravity then we might be able to see the
effects of that in the sky so one
example and the most common example for
for indirect
detection is it's possible that dark
matter could be its own antiparticle so
so
antimatter is oh no wait whoa whoa whoa
whoa whoa Dark Matter isn't antimatter
dark matter is not antimatter oh in the
way that we think about antimatter in
general okay great I was going to ask
that question so I'm glad got to it all
right okay there's something else called
antimatter yes yeah got it so antimatter
is it's a kind of matter that is in some
way like a mirror image of regular
matter okay so for example there's the
electron the electron has a negative
charge there's also a particle called a
positron which is the antimatter version
of the electron and it's just like the
electron except that it's got a positive
charge and in some sense like spins in
the other direction something like that
but but but does is it
matter it's matter in the sense that it
has mass but what is anti about it so
the anti is that the charge flips so
it's anti because the charge is
different yeah and and there's there's a
sense of the like spin will be the
opposite or something the spin will be
different yeah but it still has mass and
we can observe it yes okay yeah and that
idea that it still has mass this is
something that was actually only
recently like really strongly
proven um so there's an experiment at
CERN where they make anti-hydrogen and
they drop it and they see if it falls in
the same way as regular hydrogen and it
does okay but that was not 100% certain
that was that was assumed but it was not
100% certain okay so a positron has the
same mass as an electron yeah an
anti-hydrogen has the same mass as
hydrogen it just has opposite charges
and Spins yeah yeah so dark matter is
its own own antimatter maybe in the
sense that it well it doesn't have a
charge it doesn't have a charge right so
does it have a spin we don't know yeah
we don't know um is it a little more
complicated than the way I stated it
it's more complicated than saying it has
the opposite spin of course it has the
opposite parody it's kind of like a
mirror image
version
um where I'm going to I'm going to I'm
going to stick with my thing okay that's
fine yeah no I mean I said spin also it
but it it like parody has to do with
like sort of which way it goes in a way
that's more subtle than just spin okay
but it it doesn't really matter well
certainly not to me yeah but it matters
to astrophysicists it's just some level
yeah okay so the sort of defining
feature the most interesting feature of
antimatter is that if you take a
particle and it's antiparticle and you
put them together they will annihilate
like they cease to exist like they turn
into an
energy oh okay great so if you take
anti-hydrogen and hydrogen and you put
them together they are not anything
other than energy they explode basically
they explode they they would turn into
Gamay can you make bombs this way
because I'm surprised we haven't um so
in Star Trek the uh the engines are are
run through antimatter matter
combinations right I mean this is a
common feature in science fic that you
have matter antimatter interactions that
create your energy because it's the most
efficient thing you can possibly do
because if you take an electron and a
positron the antimatter particle of the
electron if you combine an electron and
a positron they both weigh 511 kilo
electron volts that's the measure of
energy of the mass of these particles
then you will get exactly that amount of
energy out so it's basically
lossless energy creation it's 100%
efficient
energy creation yeah all right cool so
it's the it's the biggest kind of
explosion you could possibly do the
reason that we don't use it for energy
is is because it's quite hard to contain
and so first of all it's quite hard to
create antimatter you have to have some
kind of some kind of high energy
experiment that'll create the antimatter
because that's not an easy thing to do
it say it doesn't just kind of exist
everywhere but then also you have to
contain it and that's hard to do because
if it touches the wall of your container
it it it annihilates right right so
that's not straightforward um and and in
in in you know science fiction you know
losing the antimatter containment is is
the big uh the big disaster that blows
up the spaceship um wait because then
could it get out of control well because
then it just it annihilates with
everything all the regular matter so it
could have a kind of chain reaction if
you created okay I mean it'll it's not a
chain reaction it's just that any
regular matter it touches however much
antimatter you have if it touches
regular matter that much regular matter
will blow up which could be a problem
for a spaceship exactly exactly got it
got it got it yeah and I think we talked
very briefly in the in the one of the
earlier episodes about the question of
why there is anything and the fact that
like we think that in the very early
Universe the same amount of matter and
antimatter should have been created and
somehow there was more more regular
matter created because regular matter is
is what's left because if there were
truly equal amounts then we would just
be Pure Energy Right CU everything would
have annihilated and somehow there was
an imbalance this this asymmetry that
allowed regular matter to persist and
now antimatter just kind of happens when
there's like a really high energy event
and some's created okay but it's not
like out there just hanging around right
but there was this really high energy
event at the very beginning mhm that
should have as far as our current models
tell us created the same amount of
matter and antimatter exactly which
should mean that we're not here yeah
yeah but we are we don't know why so
that's that's a mystery we are here we
don't know why and we're trying to
indirectly observe Dark Matter yes yes
so yeah so back to that so we know that
if you take an electron and a positron
put them together they annihilate it's
possible that some particles are their
own antiparticles so if this is true for
Dark Matter then a dark matter particle
if it collides with another Dark Matter
particle in just the right way then
those two would annihilate okay into
energy into well into in this case
they'd annihilate into other kinds of
particles high energy particles um so
they might they might annihilate into
quirks or leptons okay they wouldn't go
straight into gamma rays because that
would require an interaction with
electromagnetism that they don't have
but which they don't have yeah but they
could they could turn into other
particles okay and then those particles
would then turn into eventually they
they turn into Gamay because they would
decay in some in some way and so there
are a bunch of efforts to look for just
like a whole lot of extra energy coming
from places where there's a lot of dark
matter so for example in the center of
our galaxy we know there's got to be a
lot of dark matter in the center of our
galaxy if Dark Matter annihilates then
there should be a lot of annihilation
energy coming from the center of our
galaxy and you know whatever it
annihilates into it should eventually
turn into gamma rays just or just like
high energy particles positrons
something like that and so there's
efforts to look for that and there's
efforts to look for a whole lot of
annihilation energy coming from the
centers of like nearby small galaxies
dwarf galaxies like a dwarf Galaxy would
be a nice place to look because a dwarf
Galaxy is a Galaxy where there's it's so
low mass that that it doesn't have a lot
of gas in it doesn't have a lot of stars
in it because basically they're just not
bound very tightly and so when the star
goes off like goes Supernova it throws a
lot of gas out so they're mostly dark
matter these little dwarf galaxies and
so if there is dark matter annihilating
they're then it should be noticeable
compared to the other stuff that's going
on in those galaxies it's hard in the
galactic center because the galactic
center has a whole big clump of stars a
whole lot of gas magnetic fields a super
massive black hole like there's a lot
going on in the galactic center and it's
quite hard to see because we have to
look through the whole disc of the
Galaxy you know 26,000 Lighty years of
stuff to see to see the center of the
Galaxy so if there's Dark Matter
annihilating there we have to figure out
like what else is happening to know that
that it's dark matter that's doing that
but that said there is a weird gamma ray
access in the center of the Galaxy and
we don't know how to explain it at the
moment and it could be dark matter
Annihilation there's models that suggest
that that would work out the numbers
would be okay for that but it could also
be a population of pulsars that we
didn't know about before that are
creating a bunch of positrons that then
annihilate with their surroundings and
create gamma rays M okay you know could
be some other thing that we haven't
taken account of so it's kind of
inconclusive some people are pretty well
convinced by this the galactic center
exess some are not there there have been
other weird excesses of energy in
various places uh extra x-rays coming
from Galaxy clusters that have been
suggested as a possible sign of Dark
Matter decaying in in a different way um
there's uh there's been an excess of
high energy positrons in sort of the
solar neighborhood that have been
pointed at as a possibility of Dark
Matter Annihilation creating a bunch of
positrons that one's less likely now
based on our understanding but like a
few things like that where it's like
yeah maybe but the problem with indirect
detection is that you have to really
know about everything else that's going
on in the universe to be sure that this
extra bit of energy or you know extra
bit of sort of high energy particles or
radiation is really something new and
that's tough because there are a lot of
things that we just might not be sort of
counting correct
so at the moment it's inconclusive but
that's one of the the things that uh
that people look for and and it's sort
of related to my research too because in
my research I'm looking at the
possibility that dark matter
Annihilation was happening when the
first galaxies were forming in these
little clumps of of dark matter that
house the first galaxies and that that
extra energy deposition would affect the
gas in those galaxies and change the way
those galaxies would evolve but that's a
hard calculation too because you know
you know we don't know a whole lot about
how those first galaxies behaved and
what was going on in them and so you
have to be pretty confident that you
understand all of those processes to be
able to say oh the Dark Matter would
have had this effect and would be
noticeable you know right yeah it sounds
like it's really challenging to kind of
sort the noise yes and understand you
know where where you can sense a strong
signal versus where you're interpreting
noise as a signal exactly so that's kind
of where things are at with indirect
detection the final Vue is collider
experiments so if it's true that dark
matter particles can Collide and
annihilate into regular matter then it
should also be true that regular matter
particles can Collide and annihilate
into Dark Matter oh those should be
reversible processes and and and we have
some reason to believe that that might
be that might have happened in the very
early Universe where everything was
super super dense and there were these
back and forth conversions happening all
the time and then when the universe got
less dense Dark Matter was kind of left
alone and stopped annihilating so much
and then just kind of became an extra
component of of matter but uh in any
case this process should go both ways if
you can annihilate into regular matter
from dark matter you should be able to
go the other direction and so there are
experiments looking for the possibility
that proton collisions in the large hon
collider could be making some dark
matter too and that's a difficult
experiment but basically the explanation
is just like you Collide these protons
together the detectors at the Collision
site count count up all of the stuff
that comes out of that Collision so you
create a whole shower of different kinds
of particles and Gamay and you know
whatever so if you count up all the
energy of all the stuff that comes out
of the Collision it should equal the
amount of energy you put into the
Collision right but if there's something
missing then maybe you created some dark
matter and it just escaped because it
didn't interact with your detector at
all right and we we don't we don't have
a signal so oh so there hasn't there
hasn't been any compelling evidence in
that direction of like missing energy uh
signatures okay well that bummed me out
you said you you set it up like I know I
know it would be nice but so most of the
stuff that comes out of large haton
collider studies around dark matter now
are like looking for the possibility of
of new exotic particles that could be
somehow connected to the dark matter
like have their own interactions with
the dark matter but at the moment
there's there's nothing conclusive there
which is kind of a bummer because one of
the most comping sort of theories of
what the Dark Matter particle could be
was something that should have had a
bunch of other particles that would have
been detected by the large H collider
and uh and they haven't been seen so
just another reminder that that we're in
the middle of this you know like we're
not in the part of science or the part
of astrophysics where we tell you
everything that is we're in the part
where we tell you everything that we're
asking questions about trying to figure
out yeah yeah yeah exactly exactly and
and you know and and we don't know where
the next step in in this process is
going to take us this is something I
think about so much because we always
credit the person like imagine that
there's a a circular staircase that goes
in every direction and there's a bunch
of people just kind of standing in the
middle and everybody has to go to a
different part of the staircase and walk
up the stairs and be like no this isn't
the stair staircase that leads to the
next floor I'm sorry yeah uh and then
and then one person or a few people
happen to be in the place and I know
it's not just luck but it's partly luck
they they happen to be in the place
where they walk up the stairs and
they're like oh there's something up
here guys yeah yeah and then we give
them Nobel prizes right right but but
it's not entirely because
they and I think I think Nobel laurates
would say this that it's not entirely
because they were brilliant it it's
mostly because there was this circle
staircase and everybody went up a
different part of it and then it turned
out that there was only one path forward
yeah yeah yeah no 100% and those people
who went up that the the correct
staircase like they were watching
everybody go up the other staircases
yeah and it only because all those other
people were reporting back what they did
or didn't see that that led the person
to go up the right way right that's how
how this always works you know you see
which which things are are working which
things are not working and you kind of
adjust BAS based on that it's like a a
very very long relay race and and you
only give the medal to the person who's
who runs the final leg right right right
it's a tricky business I mean yeah and
and I don't know which way it's going to
go like I think that you know what's
really exciting about about Dark Matter
research is that there are so many
different ways to attack this problem
right there's tons of different ways to
look for it both in indirect detection
and direct detection uh different kinds
of Technologies and then with colliders
you know trying do clever things uh
looking for particles that might be
connected to the dark matter there's
lots of room to be creative there's lots
of room to take a lot of different paths
to do different kinds of science looking
for the same goal it's it's a neat field
to be in I I I really enjoy how many
different kinds of science I get to
learn about in the effort to understand
Dark Matter well I for one am grateful
that you're doing that work not least
because I couldn't do it I will
say after this
I'm surprised to be able to tell you
that
I I think I get something about it I
don't think I get it but I think I get
something about it that I didn't get
before and when I started off in a
really negative head space and told you
that you weren't going to convince me
because it didn't make any dang
sense it now makes sense good excellent
I I feel good about that I'm very happy
to hear that so A reluctant thank you I
thank you very much I appreciate that
I'm happy to talk about Dark Matter
anytime for any length of
time yeah I have a feeling we could have
made this like a 5 hour episode yeah
[Music]
yeah so holding smelling and tasting an
apple are all electromagnetic
interactions and there's a type of
matter that we cannot see or touch and
that invisible matter makes up about
about 85% of all matter in the universe
and is key to the creation of galaxies
that was incredible new information to
me today but there was a time when it
was also new information to everyone and
those discoveries require people doing a
lot of work over a long period of time
and as the high demand of Steel from old
shipwrecks indicates that work is still
ongoing as I said earlier we're still
very much in the middle of the
process what an exciting place to be
next episode we'll talk about the
creation of a galaxy that I'm a big fan
of the one we know as The Milky
[Music]
Way this show is hosted by me John Green
and Dr Katie Mack this episode was
produced by Hannah West edited by lonus
openhouse with music and mix by Joseph
tuna medish special thanks to the
perimeter Institute for theoretical
physics our associate script editor is
Annie fillenworth our editorial
directors are Dr Darcy Shapiro and Megan
moery and our executive producers are
Heather D Diego and Seth Radley this
show is a production of complexly if you
want to help keep crash course free for
everyone forever you can join our
community on patreon at
patreon.com/crashcourse
About the Creator
Ярема
Yarema is an inquisitive and active world explorer who has dedicated his life to studying news and adventures happening in different parts of the world.
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