The Story of Dark Matter | Crash Course Pods:

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

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