March 3, 2022 • 49 mins
Daniel and Jorge talk about whether dark matter feels dark forces.
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Speaker 1 (00:08):
Hey, Daniel, how do you like your coffee? Oh? I
like it very dark, basically black. What about your chocolate?
Also pretty dark? Maybe? Like? What about your wine? How
do you drink your wine? Well? I like a pretty
dark cabernet. Actually, now, are you optimistic about the fate
of humanity? You know, I'll be honest until we start
dismantling more nuclear weapons. I have a pretty dark view
(00:31):
of our future. You kind of a dark physicist. Maybe
you should lighten up a little, you know, try some dessert, wine,
some rose, maybe maybe even some white chocolate. No, no, no, man,
we live in a dark universe where dominated by dark matter,
dark energy, and dark chocolate. You know what Yoda says,
once you go into the dark side, forever your destiny
(00:54):
well dominate. Do you think Yoda like dark chocolate? I
always knew you were a dark fisicist. Daniel. Hi am
(01:18):
poor handmade cartoonists and the creator of PhD comics. Hi,
I'm Daniel. I'm a particle physicist and a professor UC Irvine.
And white chocolate is not chocolate. It is chocolate. Then
it's made from chocolate, made from chocolate, not the same
as chocolate. You just don't like it. You still want
to categorize this chocolate, But scientifically, I think if you
(01:38):
asked a you know, chemist, they would say it's a
it's chocolate. I don't know. I think it doesn't have
any cocoa in it. It can't be chocolate. Doesn't taste
like chocolate. You know, white chocolate is chocolate the same
way deep dish pizza is pizza. But you do know
it's made from chocolate, right, Like I think it uses
like the cocoa butter from chocolate beans. Yeah, it's sort
of like hangs around when chocolate is made, sort calls
(02:00):
itself chocolate nearby it's chocolate and I'll give it that much.
Oh man, how can it be a jacent if it
comes from chocolated Daniel, I think you're stretching things a
little bit here. My kids are coming from chocolate, but
they're not chocolate, right. You should be more candy inclusive, Tenny,
white chocolate is delicious. It's just not chocolate. But welcome
(02:20):
to our podcast. Daniel and jorhead the Bait Chocolate apparently
for an hour instead of talking about physics and explain
the universe, in which we explore all the questions out there,
the dark ones, the white ones, the milky ones, all
the questions of the universe that are rattling around in
your brain and in hours, the questions that make us human,
the questions that make us wonder why the universe is
(02:42):
this way and not some other way? Could it have
been this way? Did it have to turn out in
this particular manner? What are the rules of this universe?
And is it possible for them to make sense to us?
And is it possible for Daniel and Jorge to explain
them to you in less than an hour while eating chocolates? Apparently,
but only dark or milk? What about chocolate? Are you
a fan of milk chocolate? Milk chocolate is chocolate. But
(03:03):
I'm not a fan, I see, And there are different
categories here of exclusion for you. Oh yeah, I got
a whole matrix going here. But it is an amazing
and beautiful and sometimes even delicious universe, full of amazing
mysteries and tantalizing questions to answer and discover out there,
and fortunately humans are here for all of it, to
answer these questions and to wonder about the amazing things
(03:25):
that are out there. That's right, and one of the
most basic questions we ask about the universe is just
what's in it? What's out there in the universe sharing
this cosmos with us. There's this sense that if you
could like make an accounting of all the stuff, all
the forces in the matter, and all the particles in
the universe, you'd get a sense for why the universe
is the way that it is the way. For example,
(03:47):
if you'd like looked at the ingredients of a pizza,
you can get a sense for, like what makes a
pizza a pizza. Yeah, because for thousands of years, we
thought that, you know, the stuff that we were made
out of was all there was too stuff, you know,
the electron and corks and protons and neutrons that we're
made out of. We thought that was the whole thing
that was in the universe. But actually it turns out
that most of the universe is made out of something else,
(04:09):
something dark and mysterious. Yes, And the history of physics
is these waves of realization, discovering that we are not
at the center of the universe, we are not at
the center of the Solar system, we are not really
at the center of anything, and then discovering that our
kind of matter is not even that special that we
make up a tiny little fraction of this incredible universe pizza.
(04:29):
Oh boy, Daniel, are you saying we are universe at Jason.
We're not really part of the universe. We're not really
universe material. We're sort of dark matter adjacent, right, We're
around when the dark matter formed all this structure. We
sort of like we're followers, but we're not dark matter. Yeah.
Do you think there's some snob courmet I don't know,
universe eater out there judging us talking down about the
(04:52):
humanity and all the stars and planets and galaxies in
the universe, saying that's not really universe. Yeah, I hope. So,
I hope that there are some dark a adans out
there made of dark matter, with their incredibly dark dark
chocolate made of dark matter, and they're looking at our
dark chocolate. They're like, that's ridiculous. Well, I guess the
good news is they they're not gonna eat us, because
if they're there's nobby about it, like you are white chocolate.
(05:16):
That's good news for the white chocolate. Yeah. Here ago
you can turn anything into positive news. I appreciate that. Yeah,
but there is a lot of dark matter in the universe.
In fact, it's most of the stuff in the universe
is dark matter. Of the stuff in the universe. I mean,
that's right. Even though we can't see it directly, we
can tell that dark matter is there, that it's a
thing that is affecting the whole shape of the universe.
It affected their universe early on and its little wiggles
(05:38):
and jiggles. It affected the formation of structure in the universe.
It even shapes how galaxies spin today. So we know
that it's there, we know that it's out there, and
we know that it dominates the universe in terms of
like the budget of the universe is mostly dark matter,
not what we used to call normal matter. I e us. Yeah,
it's something like, what's the exact budget percentage of dark
(05:58):
matter in the universe compared to the regular stuff we're
made out of. It's about five to one dark matter
to normal matter, all right, So then that's like eight
percent of the universe is sort of dark matter. Yeah,
eighty percent of the universe. So you could almost neglect
our part of the universe and you still got to
be on your exam Befo, bueno, that's right, bueno. Barry
(06:20):
on is exactly. Yeah, you could ignore all the atoms
in the universe and that would only be five percent
of the energy in the universe or twenty percent of
the matter in the universe. So in some sense, the
question what is the universe made out of? What is
all this crazy stuff? Tells you that what we're made
out of is not a big part of it is
not a central element in the structure of the universe. Right,
(06:41):
And just to clarify, most of this stuff and energy
in the universe is dark energy, which is something totally different.
But if we're just talking about stuff like matter, things
that feel gravity, then most of it is dark matter.
That's right. The matter portion of the universe is about
and of that thirty percent, eight percent that is dark matter.
(07:03):
So if the matter portion of the universe it is
dark matter, well that's pretty amazing. It's almost like most
of the stuff in the universe is this other stuff
called dark matter, but we don't know actually what it is, right,
It's pretty mysterious. We've only know it's there, we haven't
actually seen it directly because it's dark. Yeah, but it's
a great story. You know. I got into physics for
exactly these kinds of realizations, discovering that the universe out
(07:26):
there is not the one that you thought it was,
or that scientists and deep thinkers for thousands of years
hadn't known the true picture of the universe. You know.
Physics is this incredible technique to like reveal the truth
about the universe, to methodically build up knowledge and tell
us what's actually out there. So for it to discover
something shocking, like an enormous plot twist, like wow, it
(07:48):
turns out most of the stuff in the universe out
there is something you never heard about or thought about
or felt before. That's fantastic and it's exactly the kind
of thing that I hope happens, you know, again and again,
overturning everything we thought we knew about the universe. Yeah,
we know you went into physics for dark reasons. Daniels
that it's pretty close to home, given that a group
(08:09):
in Los Almos where they invented nuclear weapons which are
now being used to threaten genocide against civilian populations. So yeah,
that one stings a little bit, but it might say
humanity even if the media ever comes towards Earth and
we you know, break it up with nuclear weapons, then
then your parents would be heroes. There you go. All right,
So in some scenario we are saving the Earth instead
of destroying it. You know, you apply the quantum physics principle.
(08:31):
You know, it's all true. It's just the different probabilities.
There's a little white chocolate lining to that dark chocolate cloud.
Oh no, that ruins it for you though. It doesn't
taste as good, but it goes down easier. Yeah. Yeah,
So most of the stuff in the universe is made
out of dark matter, and so it kind of makes
you wonder if that's all that's dark in the universe.
(08:51):
Could there be other things in the universe that we're
not seeing or that we can't see. And it's certainly
true that most of the universe are things that you
cannot see directly. If you look out in front of you,
the information that's coming to you is just from photons,
and photons can only interact with things that have electric charges.
For example, there are particles flying right in front of
(09:12):
you right now that you cannot see. Billions of neutrinos
coming from the Sun raining down constantly in every square
centimeter per second, but they are invisible to you. So
the true universe out there. The actual reality of the
universe outside your skull is vastly different from the tiny
slice of it that you can see. Yeah, because if
we can't see of the stuff in the universe, it
(09:35):
kind of makes me wonder what else it's doing or
what else it can do, like is it maybe radiating
dark forces or dark light? Exactly? Because our kind of
matter does all sorts of complicated things. It doesn't just
sit there right. It shoots off photons at each other.
It has electromagnetic forces and strong forces and weak forces,
and forms complicated things like ice cream and pizza and chocolate,
(09:56):
and so a natural question is what's going on with
the dark matter? What else can it do? Can it
interact with itself in some way? So today on the podcast,
we'll be asking the question what is dark radiation? Now, Daniel,
this sort of sounds like an oxy moron, you know,
you're almost asking like, what is not light light? Exactly?
(10:19):
But this is the kind of game we play in
physics all the time. We don't really know how to
grapple with the unknown, so we tend to explore it
in terms of the known. Like when we think about photons,
we don't really know how to deal with the quantum objects.
We say, is it a particle? Is it a wave?
It's kind of both. That's kind of the same. It's
a contradiction there, right, and so here that's what we're doing.
We're saying, well, we know about photons, is there like
(10:41):
another kind of like dark matter version of the photon.
Were extrapolating from what we know into what we don't know. Interesting, well,
it sounds like the plot device for a great science
fic to novel or the next Marvel movie. You know,
dark radiation. That's how you know Stephen Strange gets his
powers or something. That's right. And if they are like
what ten avengers made of normal matter, there should be like,
(11:05):
you know, forty avengers made of dark matter, the dark avengers,
Oh many, they can make another gazillion dark billions of dollars.
That's right, and my one percent cut of that is
going to be pretty sweet. Hopefully not you don't like
sweet things exactly, it'll be pretty dark. Pile up those
dark dollars in my dark bank account. Only pay Daniel
(11:25):
with bitter dollars. We should start some sort of dark
matter currency, that's right, dark chocolate coin. But it is
a real question in physics, what is going on with
the dark matter? Is it interacting with itself? Are dark
matter particles shooting dark photons at each other? We just
(11:46):
don't know. It is a dark question. And so as usual,
we were wondering how many people out there had thought
about this question or even know what these two words
put together could mean. So, as usual, Daniel went out
there into the Internet to ask people what is dark radiation?
And I'm very grateful to those of you who volunteered
to try to answer this question without having the opportunity
(12:06):
to look it up or Google or get a PhD
in physics first, And so thank you to all of
those of you who volunteered. And if you would like
to hear your voice speculating baselessly on the podcast, please
don't be shy right to me. Two questions at Daniel
and Jorge dot com. Has anyone tried that? They're like, oh,
that's a great question, give me a seven years, I'll
come back with a PhD and answer. Or is that
(12:27):
what you do to your grad students every day? Yeah?
I take the answers people give me. I'm like, oh,
that's a good idea for a project, and then I
go write a grand proposal based on it nice, and
then you get dark bitter money for it, and my
grad students spend dark bitter years working on it. Yeah.
So here's what people had to say. When I figure
of radiation, I think of waves being admitted from something,
(12:48):
so that could be heat radiation, or it could be
light radiation. And even when something is invisible light to
humans radiation, UM, it's still quote unquote light. So dark
radiation I imagine. I think it has something to do
with dark energy. Probably I have to give all credit
(13:09):
to no Degress Tyson for why I know this. But
dark radiation is the mediator of dark matter particle interaction.
Say that five times fast. The best analogy for it is, um,
it's the dark matter equivalent of a photon. I think
dark radiation is radiation coming from dark matter right away.
It makes me think of dark matter and dark energy,
(13:31):
and I know they both interact with regular matter only
through gravity. So I would guess that it has something
to do with how dark matter radiates away or evaporates
in the same manner regular mass does with like a
black hole. All right, some pretty good answers here, mostly questions.
(13:53):
I feel like people answered back with some questions too. Yeah,
and people get the sense that it has something to
do with dark matter or dark inner g right, because
people have this concept that radiation energy, and so maybe
it falls into the like energy category rather than the
dark matter category. So yeah, some great answers. Yeah, I
feel like you've really sort of branded that word dark,
(14:13):
Like people automatically anything you associated with the word dark,
people know it's oh dark matter or dark energy. Yeah.
The funny thing is that the word dark and physics
just means we don't know anything about it. It's like
hidden from us, something we can't see, and so like
the only relationship between dark matter and dark energy really
is that we're pretty clueless about both of them. Well,
my question is, what's gonna happen when you do discover
(14:35):
what it is and when you can see it. Do
you need to change the name. Oh, that's a great question.
It depends, I suppose, on whether it remains dark or not.
You know, if we find some way to communicate with
the dark sector with these dark particles, then they won't
any longer be totally dark. So yeah, because it depends
specifically on what we figure out about it. Well, you
just said that it's you call it dark when you
(14:56):
don't know anything about it. So if you do know
something about it and can't see it, maybe think about
changing the name to like, you know, Horhi matter or
cam matter, just saying, you know, some suggestions. We'll put
that on the list, right next to white chocolate matter. Yeah,
I just realized its kind of running for a guy
named white sun. It sounds like you have some father issues. Yeah,
self loathing matter, that's what it is. That's that's pretty dark.
(15:18):
That's pretty dark. I'm gonna go in some more dark
chocolate and now feel better. Alright, So Daniel, we're going
to answer this question. What is dark radiation? Which is
sounds very tantalizing, it sounds cool. It sounds maybe dangerous
dark radiation, or maybe dark radiation is good for you
because it's the opposite of regular radiation. It's not like
entire radiation like that. What you need is a dose
(15:39):
of dark radiation that'll cure Peter Parker, right, he'll no
longer be Spider Man. The you know, damage right out
of you. No, dark radiation is an analog to radiation,
but in dark matter. So in our kind of matter,
we have you know, electrons and quarks, and they can
radiate things like photons because there are forces between these particles,
(15:59):
and so dark radiation would imagine that between dark matter
particles there might be some new dark force, and that
new dark force would be capable of generating dark radiation.
But you know, I think technically in physics you use
the word radiation for basically any particle, any quantum particle
moving in a quantum field, right, I think that's isn't
(16:21):
that the technical definition? Unfortunately, we're not totally consistent that
when we talk about radiation. When we talk about radiation
sort of chemically like what is producing radiation, that can
include things like photons, but also, as you say, high
energy particles. Here, when we talk about radiation and we
distinguish it from matter, we're talking about the particles that
are associated with forces rather than the particles that are
(16:43):
associated with matter. So for example, a photon would be radiation,
whereas a cork would be matter. Even if a cork
moving at high speed, a chemist would call it radiation.
Oh I see, So if we're going with the chemist definition,
then radiation is just any particle moving kind of what
if you're going, if you're going with the day annual interpretation. Here,
radiation just means a force particle or like a particle
(17:04):
associated with a force moving through space. The idea is
that maybe dark matter also produces dark radiation. That's the idea,
and it's a really interesting area to study because you know,
we don't know what's going on with dark matter. We
just know that there's a lot of it in the universe.
But it's tempting to extrapolate from the kind of matter
that we have, right, the kind of matter we have
(17:25):
does all sorts of interesting crazy stuff. It feels forces,
it forms complex objects like ice cream and chocolate and pizza.
So we don't know what's going on with the dark matter.
There's various possibilities all the way from like it's totally
inert it feels no forces. It just sort of sits
there and feels gravity too. It has like seventy five
new forces we haven't even imagined. They're capable of forming intricate,
(17:47):
complex things that would blow our minds. And in that scenario,
those forces would be mediated by dark particles, and so
those would be dark radiation, right, Because I think all
we know about dark matter is that it it sort
of looks like it's a big giant, fuzzy clump. But
that's just what we can see of it. You know,
it could be something really detailed and organized. We just
(18:08):
have a very unfocused view of it. That's right, And
that's because so far, our only way to learn things
about dark matter, to interact with dark matter so as
to figure out like where it is and what it's doing,
is through gravity, and gravity is the weakest force in
the universe we've discovered, and not by a little bit,
but by like ten to the thirty six. It's like
(18:28):
super duper crazy week. So it's like trying to look
at dark matter, but you're looking through very very dark glasses,
so you can just barely see it. You can only
see huge clumps of it, and that gives us a
very fuzzy view of what's going on. The glasses are
made out of dark matter. I'm a little confused here.
They filter dark radiation. I think you mean like fuzzy glasses. Yeah, yeah,
(18:50):
all right, Well maybe for the people who are not
familiar with dark matter, maybe give us a quick refresher
of what it is and how we know it's there.
So we don't know that much about dark matter, but
we do know that it makes up most of the
stuff in the universe, and it's some kind of matter
by which we mean that it provides gravity. And you know,
we invented the idea of dark matter to explain why
(19:11):
there was gravity in the universe that we couldn't otherwise explain. So,
for example, we see that galaxies are rotating really really fast,
and there's apparently enough gravity to hold them together to
keep the stars from going out into interstellar space being
tossed out. But we can't explain where that gravity is
coming from, so we say probably dark matter. And if
we want to understand the structure of the universe, how
(19:33):
it got to have these galaxies and these superclusters and
all of this stuff. If you run a simulation of
the universe without dark matter, then it just doesn't form
galaxies and stars this quickly. You need more gravity to
pull that stuff together. So we need dark matter not
just to explain how galaxies are spinning, but also the
whole structure of the universe. And if we look in
(19:54):
the very early ever beginnings of the universe and we
see like how wiggles in the first plasma of the
universe formed, and how those wiggles propagated, they don't really
make sense unless you add dark matter to our calculations.
So we have a lot of evidence that dark matter
is a thing, that it's out there, that it clumps
together in these big structures that shaped our whole universe.
(20:15):
We just don't really know what it's made out of
other than something that gives gravity. Yeah, we we know
it's there and it has to be there to explain
what we see. But I think that's the crazy things
that we don't know what it is, Like, we really
have no idea, Like it could be a totally new
kind of thing that's not even a particle, or even
something sort of like cohesive. It could be anything, right, Yeah,
And we've done a lot of podcast episodes diving into
(20:37):
some of those examples. Sort Of the mainstream idea is
a whim weakly interacting massive particle that's sort of like,
you know, a generic idea like what's the simplest possible
explanation for dark matter? But there's lots of other more
exotic and fascinating ideas, from things like axions, these weird
heavy photons, to things like primordial black holes formed very
(20:59):
very early in the universe to things like super duper
long wave photons generated by dark stars. So we just
don't really know what it is. And as you say,
it might not even be a particle. It might be
that this whole idea of particles and fields and quantum
forces only applies to the little bit of the universe
that we've been studying for the last few hundred years,
(21:20):
and that you can't extrapolate to the rest of the
universe the same way you can't, like look at an
elephant's tail and assume the rest of the elephant is
like the tail. So it might be that we have
dramatic mind blowing lessons to be learned about dark matter,
or it could be super boring and just be one
particle that doesn't do anything. Some pretty extreme possibilities there.
But but are you saying that maybe, like maybe even
(21:41):
quantum mechanics only applies to a small part of the universe.
I think that's true. Yeah, All of the tests that
we've done of quantum mechanics use our kind of matter
and our kind of forces, and so we have a
pretty good quantum description of quantum particles, but we don't
really have a quantum description of gravity and gravity is
our poor to dark matter. The only thing we know
(22:01):
about dark matter is that it does feel gravity, and
so it could be some other new, weird kind of
thing that generates gravity but doesn't otherwise follow the rules
of quantum mechanics. Well, all right, so that's a dark matter. Now,
what would you say is a dark force? Besides the
obvious Star Wars reference, a dark force would be anybody
(22:22):
who tries to force me to eat white chocolate, the
dark force in my life. Now, a dark force would
be in the assumption that dark matter is made of
particles that follow the rules of quantum mechanics like our particles,
then you might imagine that they also feel some dark
forces between them. You know that too. Dark matter particles
(22:43):
can exchange some like dark photon or dark z boson
or dark higgs boson that they can like push or
pull on each other. They can do more than just
feel each other's gravity. Maybe there's some additional dark forces
and they're passing particles back and forth as a way
to mediate those forces. Rights. Even the matter in our
universe is sort of selective about which forces it feels
(23:04):
or which forces they give out, right, Like some particles
in our type of matter only feel like the magnetic forces,
or they don't feel the strong force and things like that. Right,
that's exactly right. Neutrinos, for example, only feel the weak force.
They ignore the strong force, they ignore electromagnetism. Electrons they
do feel electromagnetism and the weak force, but they ignore
(23:25):
the strong force. Only the corks feel all of the forces,
including the strong force. And so for an individual particle,
you can ask, like whether it has a charge for
that force. So electromagnetism only interact with particles that have
electric charges. For example, we know that dark matter doesn't
have electric charges, and it doesn't have weak charges, or
it doesn't have strong charges, but it might have charges
(23:47):
for other forces we haven't yet discovered. Wow, like there
could be a whole different category of forces that we
just don't happen to feel in our kind of matter,
but that maybe dark matter it does feel exactly So
there could be like one or forty new forces that
we haven't yet discovered because our particles don't feel those,
or maybe more, not just forty, maybe four thousand exactly.
(24:11):
All right, Well, let's dig into what this new kind
of force might be, this dark force and what it
could all mean and whether or not we can never
hope to see it or feel it. But first let's
take a quick break. All right, we're talking about dark radiation,
(24:36):
and in this case, the word radiation sort of is
associated with the word force. So really we're asking does
dark matter feel dark forces that maybe are out there,
but that we in our kind of matter can feel
or touch or detect. But then maybe it is maybe
organizing dark matter into structures or maybe you know, even
making dark matter particles and Adams exactly. And it's part
(24:57):
of this imagination game we play, you know, we wonder
what's going on out there in the dark universe. Is
it simple, is it fascinating, is it complex? What is
going on? What is they're out there to learn? And
usually when we go out and explore the universe, we
find that things are crazier and more fascinating and richer
than we ever imagined. And so we're at this point
(25:19):
where we really don't know about most of the universe.
But at some point in the future, I hope a
hundred years, five hundred years from now, we will know
what's going on in the dark sector and those people
will look back at our ideas as ridiculous. Probably you
might think of us as the Dark Ages physics. They
like the really dark ages, not that the trial run
in the Middle Ages, but more serious. Yes, back when
(25:40):
people ate white chocolate, that's right. Maybe we're looking forward
to an age of dark enlightenment, in the age of oxymorons.
But yeah, So in our kind of matter, we feel
the electromagnetic force, which is what basically holds our atoms together.
And what you know when you touch something, when you
touch the table or you hold the banana, that's the
(26:01):
force you're feeling. And that force is transmitted by a
particle called the photon. So the photon is not just
lighted Tosso what transmits the electromagnetic force? And so the
question daniel I guess is does dark matter have the
equivalent of a photon or at least an electromagnetic force field.
We just don't know, and so we're playing this game
of trying to extrapolate from what we do know. And
(26:22):
in the same way that we imagine maybe dark matter
is made of a particle, because we're made of particles,
we wonder if there are dark forces that look like
our forces. And so one of the simplest forces to
understand is electromagnetism because it just has like one particle
and it interacts with lots of stuff, and so a
simple proposals to say, well, what if dark matter has
(26:44):
a dark electromagnetism, meaning a dark photon. This would be
a particle that has a lot of properties similar to
our photon, but it would be different. You know, those
dark matter particles would have like a dark electric charge
and the dark photon, but only interact with particles with
a dark electric charge, not ones with our electric charge
(27:05):
and our normal vanilla photons wouldn't interact with dark matter
because they don't have an electric charge, only a dark
electric charge. But it wouldn't necessarily be the same or
the sort of the analog of light. Right you're just
saying like, maybe it's a force, and let's why not
let's qual it the dark electromagnetic force. It doesn't have
to be, and it could be the dark matter fields
(27:26):
forces that are more similar to like our weak force,
or forces that are more similar to our strong force,
or forces that are totally different from these forces and
have other new weird capacities. Right before we discovered the
strong force, who could have imagined this crazy force, you know,
where it gets stronger as things get further apart, and
it has like eight different kinds of gluons pulling it together.
(27:46):
It's bonkers. Nobody would have written that into the script.
So it's possible that there are other kinds of forces.
But if there is a dark force that's very similar
to electromagnetism, then that dark photon would have properties very
similar to our photon, because those properties come from the
structure of the force, all right. And also it makes
for a cool name, dark photon, because it kind of
(28:07):
it trips up your brain, right, Like, how can a
photon which transmits light be dark? That's right, because it
transmits dark light into dark eyeballs. Oh yeah, I feel
like you should maybe just call it something different. What
would you call it the dark on the invisible photon?
I don't know, dark on is good, But then what
do you find another force? Yeah, we'll have to be
(28:27):
even more creative with our names. We'll just have, you know,
a brainstorming session with lots of chocolate, and we'll come
up with something good and then and then who knows
what you'll get out of it? Maybe a heart attack,
all right? So dark matter might feel this new kind
of or strange new kind of force, a dark force.
So how would we ever detected Daniel, would we ever
hope to feel it or you know, know it's there
if we can't touch it or see it. So in
(28:49):
the end, this question is really asking what dark matter
does to itself? You know, can we speculate about what's
going on in the dark sector beyond these particles existing
can like touch each other and become sticky and do stuff,
And so we have sort of two ways to probe this.
The first category is sort of like indirect ways to
look at it, and broadly speaking, we can tell that
(29:10):
dark matter is doing things because of its gravity. And
even though it's not a great way to study dark matter,
we can use its gravitational effects to ask questions like
is it clumping? Is it interacting with itself? Because if
it interacts with itself, it forms sort of different structures
that have different gravity. So we can sort of ask
questions about whether dark matter is feeling itself by trying
(29:30):
to look to see how it organizes itself in the universe.
I see, we can't maybe see it directly or touch
dark matter, but if we can somehow kind of know
what it's doing, And if we see it doing stuff
to itself, then we know that it's there must be
some kind of force that it does interact with exactly.
For example, if dark matter felt a version of the
strong force, if it interacted with itself very very strongly
(29:53):
and could like clump together very very very sticky ways,
then it would form much denser objects than we see
currently in the universe. Currently, dark matter looks really big
and fluffy, like the dark matter in our galaxy isn't
nearly as compact as the normal matter in our galaxy.
It spreads out much further out. It's a big, fluffy
cloud hasn't collapsed nearly as much, and we think that's
(30:16):
because it's not as sticky as our matter. We think
that it doesn't feel like a super powerful force that
sticks it together, because that would help it like form
denser objects like dark planets and dark stars. So we
already know very broadly something about how dark matter can't
feel itself because it stays sort of fluffy and doesn't
seem to clump as much as normal matter. Right, But
(30:38):
I wonder you know it sort of looks fluffy to
us because we don't have a good way of seeing it.
You know, we talked earlier about having we kind of
have a fuzzy lens when we look at dark matter
because we can only see it through gravity. You know,
is it maybe even possible that there are dark matter
stars out there, which is don't have the resolution to
see them. You know, absolutely, there's a very big loophole
in this argument that you allude to, which is that
(31:00):
you can see sort of the bulk of dark matter,
but we can't tell what it's doing in detail. In
an episode recently about where is the dark matter, and
we talked about how we can tell sort of the
dark matters in these big fluffy clouds, but we also
can't really tell if it is forming clumps. So you
could imagine that dark matter maybe it's made out of
a few different kinds of things. So most of it,
(31:20):
maybe like you know, half of the dark matter or
three fourths of the dark matter, is some big, fluffy
stuff that hardly interacts with itself. But there could be
a component of the dark matter that does do interesting,
complicated things and has crazy interactions and forms dark stars
and dark planets and dark life, and that would not
mess up the distribution of dark matter, because remember, there's
like so much dark matter out there that even like
(31:43):
if a quarter of it does more interesting stuff, that's
still more than all of our kind of matter. So
there's plenty of room to have like a component that
has complex interactions without missing up these constraints about the
big picture of dark matter. So I feel like dark
matter could be in the shape of I don't know,
like rubber duckies. It's just that they're sort of distributed
out in space, these rubber duckies, and to us it
(32:06):
just seems like a big fluffy cloud. Yeah, most of
it has to be pretty big and fluffy to form
that big fluffy cloud. It can't stick to itself. But
it could be that inside that big fluffy cloud there
are like rubber duckies made out of like a special
kind of dark matter, you know, the way they like
our matter. There's lots of different kinds is electrons, is
different kinds of quarks. Right, we have twelve different matter particles.
(32:27):
Dark matter could have like fifty different kinds of particles,
and maybe most of them don't interact and make a
big fluffy cloud, but one or two of them form
dark rubber duckies that are floating out there in the universe,
and we couldn't tell the difference because the only way
we can see them is through their overall gravitational interaction.
I'm pretty sure that's what dark Vader uses in his bathtuff.
But I guess the question is, you know, isn't the
(32:48):
fact that dark matter does seem to be sort of
fluffy and doesn't seem to stick to itself. Is that
evidence that maybe it doesn't interact with itself other than
with gravity. It's evidence that it doesn't interact with itself
very very strongly. But again, the loophole is that some
component of it might be able to Now. The way
to probe that a little bit more deeply is to
do experiments. Is to take like two big clouds of
(33:11):
dark matter and throw them against each other and see
if they pass right through each other, or if maybe
they stick together and make them like dark explosions. That
kind of experiment is pretty tough to do, but we
were lucky and the universe did it for us. It
collided two huge clusters of galaxies, each of which have
their own dark matter associated with them, smashed them together
(33:32):
millions of years ago, and we got to see what
happened and what did happen, and so this is called
the bullet cluster. You can go ahead and look at
these pictures online if you're interested. It's really beautiful. What
happened is that the stars in these galaxy clusters mostly
passed through themselves because stars are pretty sparse, so it's
like two clouds of sand passing through each other. The
gas and the dust that were in these galaxies smashed
(33:53):
together and admitted a lot of light and radiation. But
the dark matter looks like it basically passed right through itself.
You might ask school like, how could we know where
the dark matter is? We can see the dark matter
because it distorts the stuff behind it. From the dark
matter is matter, it feels gravity, so it bends space
a little bit, which creates like gravitational distortions in the
light that passes through it. So from this bullet cluster,
(34:16):
it looks like these two clouds mostly passed right through
each other. So not it doesn't have any strong interactions
with itself, but it could still have some interactions with itself,
which would maybe give rise to something like a dark photon.
That's right, and so people often cite the bullet cluster
is evidence that dark matter can't feel anything with itself,
and that's not exactly true, because, as you say, there's
still room in there for some kind of interaction. Because
(34:38):
remember that the stars, which do definitely feel each other,
also passed right through themselves. Right. The star is hardly collided,
and that's just because it's pretty sparse, and so could
be that dark matter forms the structures, but they didn't
collide with each other because just like the stars, they're
pretty sparse and space is pretty big. And it could
also be that some components of the dark matter did
(34:58):
collide with itself, did get stuck sort of in the
middle there. But we don't really have the resolution to tell.
We can't very precisely measure how much dark matter passed
through and how much stuck. We can just tell that
a lot of it passed through, but some component could
have gotten stuck and could be doing like crazy stuff
there in the middle. I see, like maybe the Rubert
duckies didn't all crash into each other, but some just
(35:19):
kept going. I think the key point to understand is
that there's so much dark matter out there that even
if a tiny fraction of it is doing interesting things
we couldn't tell, and that tiny fraction would still be
more stuff than all of the stars and gas and
dust in the universe that we know. All right, well,
let's get into other ways that we might be able
to detect this dark radiation and maybe even see these
(35:39):
dark or not see these dark photons. I'm a little
confused about how he might see are not photon But
first let's take another quick break. All right, we're talking
abot dark gradiation and dark photons, which it might be
(36:03):
what dark matter sees and feels with itself when dark
matter looks in the mirror. That's what they would see
dark photons. Yeah, exactly. I hope they haven't eating too
much dark chocolate. But to us it's pretty elusive because
we can see or feel the dark matter or maybe
even these forces. So, Daniel, what are some of the
other ways that we might hope to detect whether or
not dark matter has these dark interactions. There are a
(36:26):
couple other indirect ways, basically again using gravity. One of
them is looking at neutron stars. Neutron stars are these
crazy dense objects that are the remnants of stars that
have blown up and left this very very compact core,
and sometimes neutron stars come in pairs and then those
pairs spin around each other in this sort of like
(36:47):
death spiral before they collide, and when they do that,
they create gravitational waves that we can see here from Earth.
This is like the shaking of space itself. If you're
interested in learning more about that, we have a couple
of episodes about gravitational waves. It's really cool because the
gravitational waves reveal in detail how these stars are spiraling
towards the center, how they're losing energy, and we can
(37:09):
calculate with great precision how fast they should be going
and how much energy they're losing. And so this is
a really nice test of sort of like what's going
on with this kind of matter, Because if these neutron
stars were capable, for example, of feeling this dark radiation
at some level, if the particles in there could feel
this force at all, then they would radiate a little
(37:29):
bit of those dark photons and it would change the
way this inspiral happens, and we would be able to
see that by looking at the gravitational waves. Wait, so
these neutron stars would be made out of regular matter,
the kind we're made out of. But you're saying that
even though the regular matter it might admit this dark radiation,
how is that possible. It might be that our matter
can feel these sort of like new forces, just at
(37:50):
a very very low level. We don't know, and so
this is like, you know, one way to look for
it under the assumption that our kind of matter could
feel these dark forces, which is not something we know.
It's just sort of like a guess. We do this
in physics a lot. We're like, we don't know if
it's possible to see it this way, but let's check,
because if it does feel this force, if neutron stars
can make these dark photons, we would be able to
(38:12):
see that. I see neutron stars are so it's such
an extreme event when they crash into each other that
that's one possible, you know, crazy scenario where it might
make dark gradiations exactly. And it's a place where we
can make very very precise calculations about the gravitational waves
and compare them to very very detailed data. So it's
an opportunity to look for like very small deviations, you know,
(38:33):
things that the neutron stars are doing that we can't
otherwise explain that might be explainable through dark photons. I see,
all right, what are some other ways we could maybe
feel this dark force? All the most generic ways you
just use gravity the kind of tests that we did before,
But there are some other approaches to look for these
dark photons, hoping that there's some connection between our kind
of matter and the dark matter, Like hoping that somehow
(38:56):
maybe these dark photons can turn into normal photon very
very rarely, and then we could detect that what how
can a dark photon turn into a regular photon. It
turns out that if you have another kind of force
that's very similar to the photon, has sort of like
a similar mathematical structure, that these forces like to talk
to each other, that there's like it's very easy to
(39:18):
build a physics model in which another kind of photon
can turn into our photon wha like spontaneously or only
in like high energy completions or you know, moments where
you have kind of this a state of pure energy.
It would be spontaneous, and in order for that to happen,
they would have to be some kind of particle in
the universe that has both kinds of charges. So imagine
(39:41):
some new kind, very very rare dark matter that does
have electric charges and also has some new kind of
dark electric charge. That particle could effectively be like a
portal that connects our photons to these other photons, and
it might be that these particles never really exist in
the universe, the way like top corks almost never exist
(40:02):
in the universe on their own, but they're sort of
like on the list of possibilities. As long as it's
on the list of possibilities, then dark photons can use
that as a portal to become normal photons and vice versa.
This allows them to spontaneously turn from one into the other. Whoa,
So that wouldn't make dark matter not dark, right, It
would mean that you can sort of see them. It would,
(40:23):
but these particles again wouldn't actually have to exist in
the universe. So imagine now the dark sector is some
kind of particle that's actually out there, that most of
the dark matter is made out of this particle. And
then there's the possibility for this other particle that has
electric charges and dark charges, even though it's never really
out there in the universe, as long as the possibility
for it to exist is part of sort of nature's menu.
(40:47):
That possibility allows dark photons to turn into photons. It's
called kinetic mixing because we see it happening with regular matter, right,
Like our photons turned into other kinds of stuff all
the time, yes, exactly, Like photons turned into pairs of
particles electrons and positrons, and then those can turn into
z bosons, right because z's also interact with electrons impositron.
(41:08):
So even if there weren't a single electron or positron
in the universe, a photon could still turn into a
z boson. And in the same way, as long as
there's the possibility for some virtual particle that connects these
two forces to exist, then photons could turn into dark
photons and vice versa. And that's kind of our only
hope of seeing this dark force directly. That's the best
(41:29):
way to see these dark forces directly, exactly, And so
people have built really cool experiments to try to like
make this happen more often, try to like create the
scenarios that would help induce dark photons to spontaneously turn
into photons. They have these resonant cavities at Fermilab, for example,
that should enhance the rate of this happening. If you
(41:50):
like build something with the right shape and size, it
creates a situation where this kind of spontaneous transformation is
more likely to happen. And so they look for this
kind of signature. Wait, that's being built right now, Like
there are people building basically dark forest boxes. These things
exist already and they are being run and they're just
building them bigger and bigger. It's sort of like the
(42:11):
way people are looking for a dark matter. They look
in these containers underground, and they started with small ones,
and then they're making bigger and bigger and bigger ones.
Nobody's seeing a dark photon yet, but they're hoping as
they make these cavities more precise and larger to induce
a dark photon to turn into a normal photon. Cool,
and how else can you look for these? Another way
to look for these is to look for particles appearing
(42:32):
where they shouldn't be. So one of my favorite kind
of experiments, it's called a beam dump experiment, where you
take a particle beam you were using for something else
and you dump it, meaning you just like shoot it
into a huge block of concrete or into a mountain side,
just because you know, it's got to go somewhere. You
don't want to like sprayed over the neighborhood because it's dangerous, right,
just shoot it through the earth at people on the
other side of the earth. Is that what you're saying, Yeah, exactly.
(42:54):
And so Sho didn't do a mountain. It was on
the other side of Daniel. It's not California, is I'm
to have to look that up. But if you shoot
this particle beam into a mountain, for example, and then
you put a particle detector on the other side of
the mountain, none of the particles should make it through
the mountain, but you might see particles appearing in your
detector on the other side of the mountain, because dark
(43:17):
photons might make it through the mountain because they basically
see the mountain is invisible as transparent. The ideas if
your beam sometimes occasionally produces dark photons, those dark photons
would make it through the mountain. And so essentially it's
like looking for light shining through walls or I see
so many the ideas that you send this beam out
into the mountain, some of it turns into dark photons,
(43:38):
and then somehow it turns back into regular photons before
it hits your detector. Yeah, that's exactly right. And so
you're hoping that sometimes those dark photons turned back into
normal photons. So you're assuming a lot of things here.
You're assuming that maybe sometimes dark photons are created in
your beam, and that sometimes they turn back into normal photons. Right,
But if they turn back into regular photons, how would
(43:59):
you know they turned into dark photons in the middle. Yeah,
you wouldn't know for sure. You would just know that
something passed through the mountain that typically shouldn't be able to.
So you calculate, like how often should particles be able
to pass through the mountain and turn into photons? And
if that happens more often than you expect, then you
know there's something else and new there, maybe a dark photon,
(44:20):
maybe something else. But you know, that's often the case
with particle physics. We find something new, we're not exactly
sure what it is, and then we study it in detail,
we try to characterize exactly what it is, just like
with the Higgs boson. First thing, we saw some new
particle decaying into two photons, and we weren't sure is
this the Higgs boson or is it something else? We
didn't expect, and over decades of study we sort of
(44:41):
pinned down what it must be because of all of
its behaviors. I see, it's like, the only way it
could have made it through the mountain to and out
into California to hit us it was if it turned
into something invisible like dark photons somewhere in the middle.
Otherwise it wouldn't have made it through. Exactly, I have
to phase through the mountain by turning into something that
doesn't interact with mountains. All right, cool, Well, what are
(45:04):
some other ways we might be able to see these
dark forces? A lot of the other experiments are very similar.
There's one at CERN called Phaser Forward Search Experiment. It's
a pretty tortured acronym, but a really cool idea for
I don't even know what is there an A actually
in the name, there's an A and forward and an
R experiment. Oh man, it's a really cool idea because
(45:27):
they're taking an already existing experiment Atlas where we collide
protons together and they're wondering like maybe dark photons are
created and shoot down the beam. So they built a
detector like really far down along the beam past where
the collisions happen to see if maybe dark photons are
created in those collisions and then fly along the beam
and then turn into something that they can see sort
(45:49):
of downstream. So they added this little bit to the
detector that can do something totally novel and new. It's
actually led by a team here at you see Irvine,
m M, I see. But we don't actually know if
dark matter can switch back and forth that easily, right,
We don't know. And again, it could be that dark
matter is only some inert particle that feels gravity and
nothing else and there are no dark forces and no
(46:11):
way to interact. It could also be that dark matter
does feel some new dark force, but can only interact
with itself, and none of those dark forces can ever
turn into normal matter or normal photons. Here, we're just guessing.
We're trying to like study the various possibilities in the
various ways that those possibilities might manifesto well, And so
(46:32):
what does it all mean? Do you think we will
find it has interactions with itself or do you think
it just represents this whole part of the universe that
we will never be able to see or touch or
even confirm that it's there. You know, like we could
be swimming in dark rubber duckies and never ever ever, Yeah,
part of it comes down to what you think is
more natural. Does it make sense to you to have
(46:54):
a huge component of the universe be sort of simple
and inert and not really doing any being interesting, or
does this seem more natural for it to be complex
and interactive the way that our matter is. Now, what
I know is that the universe doesn't follow what we
think is natural. Our conceptions of how the universe should
work don't seem to be very well aligned with how
(47:14):
it usually does work, and so I suspect there are
some surprises out there. The other side of that question
is what you asked, is might we ever be able
to tell? And it could be that dark matter doesn't
feel any of our forces, and none of these forces
can talk to our forces, and we can never find
these dark rubber duckies. But I have faith in physicists
and engineers to come up with clever ways to probe
these things, ways that we can't even today. Imagine kinds
(47:37):
of experiments that people might think of in ten years
or twenty years, experiments thought up by you know, clever
Listeners to this podcast who aren't constrained by the kind
of thinking that academic physicists have been trained to do.
I think I know how you can do it, Daniel,
Dark Chocolate collisions. You just gotta want it enough. You know,
you have to really want it, because, as Yoda says,
(47:58):
you know, wanting these to pain and pain needs to suffering,
and suffering leads to the dark side, and so that's
that's how you can get there faster. Maybe, Yeah, there
is no try. I should just do it, that's right,
Just I mean, do it? You should, yes, all right.
I don't know why I didn't think of that before.
I'm just gonna go do it right after we're done
with this podcast, or not do it. I guess we're
(48:20):
doing in the dark. Gosh, I'm so confused. But to me,
it really touches on these deep mysteries of physics that
we know the universe out there is telling stories that
we haven't heard yet. We're desperate to hear those stories.
We have hints that we know the shape of those stories,
but we don't know any of the details yet, and
I just hope that one day we'll be able to
fill those in. Yeah, it's almost like there's a whole
universe out there, sitting right in front of us that
(48:42):
we have yet to discover, and that anyone listening to
this could become part of that search. That's right. So
there's plenty more of the universe to find out, and
we desperately need new ideas. And whether the universe counts
as dark chocolate or not, that's for other people to debate.
That's right. And if you love white chocolate, take my apologies.
I love all of them. You love listeners more than
(49:03):
you love your chocolate, exactly, And if you eat white
chocolate while listen to this podcast, I forgive you because
he can't see your or feel you, so it doesn't
matter to exactly. Please accept my dark apologies. All right, Well,
we hope you enjoyed that. Thanks for joining us, See
you next time. Thanks for listening, and remember that Daniel
(49:30):
and Jorge Explain the Universe is a production of I
Heart Radio. For more podcast for my heart Radio, visit
the I heart Radio app, Apple Podcasts, or wherever you
listen to your favorite shows. Yeah,
Hey, Daniel, how do you like your coffee? Oh? I
like it very dark, basically black. What about your chocolate?
Also pretty dark? Maybe? Like? What about your wine? How
do you drink your wine? Well? I like a pretty
dark cabernet. Actually, now, are you optimistic about the fate
of humanity? You know, I'll be honest until we start
dismantling more nuclear weapons. I have a pretty dark view
(00:31):
of our future. You kind of a dark physicist. Maybe
you should lighten up a little, you know, try some dessert, wine,
some rose, maybe maybe even some white chocolate. No, no, no, man,
we live in a dark universe where dominated by dark matter,
dark energy, and dark chocolate. You know what Yoda says,
once you go into the dark side, forever your destiny
(00:54):
well dominate. Do you think Yoda like dark chocolate? I
always knew you were a dark fisicist. Daniel. Hi am
(01:18):
poor handmade cartoonists and the creator of PhD comics. Hi,
I'm Daniel. I'm a particle physicist and a professor UC Irvine.
And white chocolate is not chocolate. It is chocolate. Then
it's made from chocolate, made from chocolate, not the same
as chocolate. You just don't like it. You still want
to categorize this chocolate, But scientifically, I think if you
(01:38):
asked a you know, chemist, they would say it's a
it's chocolate. I don't know. I think it doesn't have
any cocoa in it. It can't be chocolate. Doesn't taste
like chocolate. You know, white chocolate is chocolate the same
way deep dish pizza is pizza. But you do know
it's made from chocolate, right, Like I think it uses
like the cocoa butter from chocolate beans. Yeah, it's sort
of like hangs around when chocolate is made, sort calls
(02:00):
itself chocolate nearby it's chocolate and I'll give it that much.
Oh man, how can it be a jacent if it
comes from chocolated Daniel, I think you're stretching things a
little bit here. My kids are coming from chocolate, but
they're not chocolate, right. You should be more candy inclusive, Tenny,
white chocolate is delicious. It's just not chocolate. But welcome
(02:20):
to our podcast. Daniel and jorhead the Bait Chocolate apparently
for an hour instead of talking about physics and explain
the universe, in which we explore all the questions out there,
the dark ones, the white ones, the milky ones, all
the questions of the universe that are rattling around in
your brain and in hours, the questions that make us human,
the questions that make us wonder why the universe is
(02:42):
this way and not some other way? Could it have
been this way? Did it have to turn out in
this particular manner? What are the rules of this universe?
And is it possible for them to make sense to us?
And is it possible for Daniel and Jorge to explain
them to you in less than an hour while eating chocolates? Apparently,
but only dark or milk? What about chocolate? Are you
a fan of milk chocolate? Milk chocolate is chocolate. But
(03:03):
I'm not a fan, I see, And there are different
categories here of exclusion for you. Oh yeah, I got
a whole matrix going here. But it is an amazing
and beautiful and sometimes even delicious universe, full of amazing
mysteries and tantalizing questions to answer and discover out there,
and fortunately humans are here for all of it, to
answer these questions and to wonder about the amazing things
(03:25):
that are out there. That's right, and one of the
most basic questions we ask about the universe is just
what's in it? What's out there in the universe sharing
this cosmos with us. There's this sense that if you
could like make an accounting of all the stuff, all
the forces in the matter, and all the particles in
the universe, you'd get a sense for why the universe
is the way that it is the way. For example,
(03:47):
if you'd like looked at the ingredients of a pizza,
you can get a sense for, like what makes a
pizza a pizza. Yeah, because for thousands of years, we
thought that, you know, the stuff that we were made
out of was all there was too stuff, you know,
the electron and corks and protons and neutrons that we're
made out of. We thought that was the whole thing
that was in the universe. But actually it turns out
that most of the universe is made out of something else,
(04:09):
something dark and mysterious. Yes, And the history of physics
is these waves of realization, discovering that we are not
at the center of the universe, we are not at
the center of the Solar system, we are not really
at the center of anything, and then discovering that our
kind of matter is not even that special that we
make up a tiny little fraction of this incredible universe pizza.
(04:29):
Oh boy, Daniel, are you saying we are universe at Jason.
We're not really part of the universe. We're not really
universe material. We're sort of dark matter adjacent, right, We're
around when the dark matter formed all this structure. We
sort of like we're followers, but we're not dark matter. Yeah.
Do you think there's some snob courmet I don't know,
universe eater out there judging us talking down about the
(04:52):
humanity and all the stars and planets and galaxies in
the universe, saying that's not really universe. Yeah, I hope. So,
I hope that there are some dark a adans out
there made of dark matter, with their incredibly dark dark
chocolate made of dark matter, and they're looking at our
dark chocolate. They're like, that's ridiculous. Well, I guess the
good news is they they're not gonna eat us, because
if they're there's nobby about it, like you are white chocolate.
(05:16):
That's good news for the white chocolate. Yeah. Here ago
you can turn anything into positive news. I appreciate that. Yeah,
but there is a lot of dark matter in the universe.
In fact, it's most of the stuff in the universe
is dark matter. Of the stuff in the universe. I mean,
that's right. Even though we can't see it directly, we
can tell that dark matter is there, that it's a
thing that is affecting the whole shape of the universe.
It affected their universe early on and its little wiggles
(05:38):
and jiggles. It affected the formation of structure in the universe.
It even shapes how galaxies spin today. So we know
that it's there, we know that it's out there, and
we know that it dominates the universe in terms of
like the budget of the universe is mostly dark matter,
not what we used to call normal matter. I e us. Yeah,
it's something like, what's the exact budget percentage of dark
(05:58):
matter in the universe compared to the regular stuff we're
made out of. It's about five to one dark matter
to normal matter, all right, So then that's like eight
percent of the universe is sort of dark matter. Yeah,
eighty percent of the universe. So you could almost neglect
our part of the universe and you still got to
be on your exam Befo, bueno, that's right, bueno. Barry
(06:20):
on is exactly. Yeah, you could ignore all the atoms
in the universe and that would only be five percent
of the energy in the universe or twenty percent of
the matter in the universe. So in some sense, the
question what is the universe made out of? What is
all this crazy stuff? Tells you that what we're made
out of is not a big part of it is
not a central element in the structure of the universe. Right,
(06:41):
And just to clarify, most of this stuff and energy
in the universe is dark energy, which is something totally different.
But if we're just talking about stuff like matter, things
that feel gravity, then most of it is dark matter.
That's right. The matter portion of the universe is about
and of that thirty percent, eight percent that is dark matter.
(07:03):
So if the matter portion of the universe it is
dark matter, well that's pretty amazing. It's almost like most
of the stuff in the universe is this other stuff
called dark matter, but we don't know actually what it is, right,
It's pretty mysterious. We've only know it's there, we haven't
actually seen it directly because it's dark. Yeah, but it's
a great story. You know. I got into physics for
exactly these kinds of realizations, discovering that the universe out
(07:26):
there is not the one that you thought it was,
or that scientists and deep thinkers for thousands of years
hadn't known the true picture of the universe. You know.
Physics is this incredible technique to like reveal the truth
about the universe, to methodically build up knowledge and tell
us what's actually out there. So for it to discover
something shocking, like an enormous plot twist, like wow, it
(07:48):
turns out most of the stuff in the universe out
there is something you never heard about or thought about
or felt before. That's fantastic and it's exactly the kind
of thing that I hope happens, you know, again and again,
overturning everything we thought we knew about the universe. Yeah,
we know you went into physics for dark reasons. Daniels
that it's pretty close to home, given that a group
(08:09):
in Los Almos where they invented nuclear weapons which are
now being used to threaten genocide against civilian populations. So yeah,
that one stings a little bit, but it might say
humanity even if the media ever comes towards Earth and
we you know, break it up with nuclear weapons, then
then your parents would be heroes. There you go. All right,
So in some scenario we are saving the Earth instead
of destroying it. You know, you apply the quantum physics principle.
(08:31):
You know, it's all true. It's just the different probabilities.
There's a little white chocolate lining to that dark chocolate cloud.
Oh no, that ruins it for you though. It doesn't
taste as good, but it goes down easier. Yeah. Yeah,
So most of the stuff in the universe is made
out of dark matter, and so it kind of makes
you wonder if that's all that's dark in the universe.
(08:51):
Could there be other things in the universe that we're
not seeing or that we can't see. And it's certainly
true that most of the universe are things that you
cannot see directly. If you look out in front of you,
the information that's coming to you is just from photons,
and photons can only interact with things that have electric charges.
For example, there are particles flying right in front of
(09:12):
you right now that you cannot see. Billions of neutrinos
coming from the Sun raining down constantly in every square
centimeter per second, but they are invisible to you. So
the true universe out there. The actual reality of the
universe outside your skull is vastly different from the tiny
slice of it that you can see. Yeah, because if
we can't see of the stuff in the universe, it
(09:35):
kind of makes me wonder what else it's doing or
what else it can do, like is it maybe radiating
dark forces or dark light? Exactly? Because our kind of
matter does all sorts of complicated things. It doesn't just
sit there right. It shoots off photons at each other.
It has electromagnetic forces and strong forces and weak forces,
and forms complicated things like ice cream and pizza and chocolate,
(09:56):
and so a natural question is what's going on with
the dark matter? What else can it do? Can it
interact with itself in some way? So today on the podcast,
we'll be asking the question what is dark radiation? Now, Daniel,
this sort of sounds like an oxy moron, you know,
you're almost asking like, what is not light light? Exactly?
(10:19):
But this is the kind of game we play in
physics all the time. We don't really know how to
grapple with the unknown, so we tend to explore it
in terms of the known. Like when we think about photons,
we don't really know how to deal with the quantum objects.
We say, is it a particle? Is it a wave?
It's kind of both. That's kind of the same. It's
a contradiction there, right, and so here that's what we're doing.
We're saying, well, we know about photons, is there like
(10:41):
another kind of like dark matter version of the photon.
Were extrapolating from what we know into what we don't know. Interesting, well,
it sounds like the plot device for a great science
fic to novel or the next Marvel movie. You know,
dark radiation. That's how you know Stephen Strange gets his
powers or something. That's right. And if they are like
what ten avengers made of normal matter, there should be like,
(11:05):
you know, forty avengers made of dark matter, the dark avengers,
Oh many, they can make another gazillion dark billions of dollars.
That's right, and my one percent cut of that is
going to be pretty sweet. Hopefully not you don't like
sweet things exactly, it'll be pretty dark. Pile up those
dark dollars in my dark bank account. Only pay Daniel
(11:25):
with bitter dollars. We should start some sort of dark
matter currency, that's right, dark chocolate coin. But it is
a real question in physics, what is going on with
the dark matter? Is it interacting with itself? Are dark
matter particles shooting dark photons at each other? We just
(11:46):
don't know. It is a dark question. And so as usual,
we were wondering how many people out there had thought
about this question or even know what these two words
put together could mean. So, as usual, Daniel went out
there into the Internet to ask people what is dark radiation?
And I'm very grateful to those of you who volunteered
to try to answer this question without having the opportunity
(12:06):
to look it up or Google or get a PhD
in physics first, And so thank you to all of
those of you who volunteered. And if you would like
to hear your voice speculating baselessly on the podcast, please
don't be shy right to me. Two questions at Daniel
and Jorge dot com. Has anyone tried that? They're like, oh,
that's a great question, give me a seven years, I'll
come back with a PhD and answer. Or is that
(12:27):
what you do to your grad students every day? Yeah?
I take the answers people give me. I'm like, oh,
that's a good idea for a project, and then I
go write a grand proposal based on it nice, and
then you get dark bitter money for it, and my
grad students spend dark bitter years working on it. Yeah.
So here's what people had to say. When I figure
of radiation, I think of waves being admitted from something,
(12:48):
so that could be heat radiation, or it could be
light radiation. And even when something is invisible light to
humans radiation, UM, it's still quote unquote light. So dark
radiation I imagine. I think it has something to do
with dark energy. Probably I have to give all credit
(13:09):
to no Degress Tyson for why I know this. But
dark radiation is the mediator of dark matter particle interaction.
Say that five times fast. The best analogy for it is, um,
it's the dark matter equivalent of a photon. I think
dark radiation is radiation coming from dark matter right away.
It makes me think of dark matter and dark energy,
(13:31):
and I know they both interact with regular matter only
through gravity. So I would guess that it has something
to do with how dark matter radiates away or evaporates
in the same manner regular mass does with like a
black hole. All right, some pretty good answers here, mostly questions.
(13:53):
I feel like people answered back with some questions too. Yeah,
and people get the sense that it has something to
do with dark matter or dark inner g right, because
people have this concept that radiation energy, and so maybe
it falls into the like energy category rather than the
dark matter category. So yeah, some great answers. Yeah, I
feel like you've really sort of branded that word dark,
(14:13):
Like people automatically anything you associated with the word dark,
people know it's oh dark matter or dark energy. Yeah.
The funny thing is that the word dark and physics
just means we don't know anything about it. It's like
hidden from us, something we can't see, and so like
the only relationship between dark matter and dark energy really
is that we're pretty clueless about both of them. Well,
my question is, what's gonna happen when you do discover
(14:35):
what it is and when you can see it. Do
you need to change the name. Oh, that's a great question.
It depends, I suppose, on whether it remains dark or not.
You know, if we find some way to communicate with
the dark sector with these dark particles, then they won't
any longer be totally dark. So yeah, because it depends
specifically on what we figure out about it. Well, you
just said that it's you call it dark when you
(14:56):
don't know anything about it. So if you do know
something about it and can't see it, maybe think about
changing the name to like, you know, Horhi matter or
cam matter, just saying, you know, some suggestions. We'll put
that on the list, right next to white chocolate matter. Yeah,
I just realized its kind of running for a guy
named white sun. It sounds like you have some father issues. Yeah,
self loathing matter, that's what it is. That's that's pretty dark.
(15:18):
That's pretty dark. I'm gonna go in some more dark
chocolate and now feel better. Alright, So Daniel, we're going
to answer this question. What is dark radiation? Which is
sounds very tantalizing, it sounds cool. It sounds maybe dangerous
dark radiation, or maybe dark radiation is good for you
because it's the opposite of regular radiation. It's not like
entire radiation like that. What you need is a dose
(15:39):
of dark radiation that'll cure Peter Parker, right, he'll no
longer be Spider Man. The you know, damage right out
of you. No, dark radiation is an analog to radiation,
but in dark matter. So in our kind of matter,
we have you know, electrons and quarks, and they can
radiate things like photons because there are forces between these particles,
(15:59):
and so dark radiation would imagine that between dark matter
particles there might be some new dark force, and that
new dark force would be capable of generating dark radiation.
But you know, I think technically in physics you use
the word radiation for basically any particle, any quantum particle
moving in a quantum field, right, I think that's isn't
(16:21):
that the technical definition? Unfortunately, we're not totally consistent that
when we talk about radiation. When we talk about radiation
sort of chemically like what is producing radiation, that can
include things like photons, but also, as you say, high
energy particles. Here, when we talk about radiation and we
distinguish it from matter, we're talking about the particles that
are associated with forces rather than the particles that are
(16:43):
associated with matter. So for example, a photon would be radiation,
whereas a cork would be matter. Even if a cork
moving at high speed, a chemist would call it radiation.
Oh I see, So if we're going with the chemist definition,
then radiation is just any particle moving kind of what
if you're going, if you're going with the day annual interpretation. Here,
radiation just means a force particle or like a particle
(17:04):
associated with a force moving through space. The idea is
that maybe dark matter also produces dark radiation. That's the idea,
and it's a really interesting area to study because you know,
we don't know what's going on with dark matter. We
just know that there's a lot of it in the universe.
But it's tempting to extrapolate from the kind of matter
that we have, right, the kind of matter we have
(17:25):
does all sorts of interesting crazy stuff. It feels forces,
it forms complex objects like ice cream and chocolate and pizza.
So we don't know what's going on with the dark matter.
There's various possibilities all the way from like it's totally
inert it feels no forces. It just sort of sits
there and feels gravity too. It has like seventy five
new forces we haven't even imagined. They're capable of forming intricate,
(17:47):
complex things that would blow our minds. And in that scenario,
those forces would be mediated by dark particles, and so
those would be dark radiation, right, Because I think all
we know about dark matter is that it it sort
of looks like it's a big giant, fuzzy clump. But
that's just what we can see of it. You know,
it could be something really detailed and organized. We just
(18:08):
have a very unfocused view of it. That's right, And
that's because so far, our only way to learn things
about dark matter, to interact with dark matter so as
to figure out like where it is and what it's doing,
is through gravity, and gravity is the weakest force in
the universe we've discovered, and not by a little bit,
but by like ten to the thirty six. It's like
(18:28):
super duper crazy week. So it's like trying to look
at dark matter, but you're looking through very very dark glasses,
so you can just barely see it. You can only
see huge clumps of it, and that gives us a
very fuzzy view of what's going on. The glasses are
made out of dark matter. I'm a little confused here.
They filter dark radiation. I think you mean like fuzzy glasses. Yeah, yeah,
(18:50):
all right, Well maybe for the people who are not
familiar with dark matter, maybe give us a quick refresher
of what it is and how we know it's there.
So we don't know that much about dark matter, but
we do know that it makes up most of the
stuff in the universe, and it's some kind of matter
by which we mean that it provides gravity. And you know,
we invented the idea of dark matter to explain why
(19:11):
there was gravity in the universe that we couldn't otherwise explain. So,
for example, we see that galaxies are rotating really really fast,
and there's apparently enough gravity to hold them together to
keep the stars from going out into interstellar space being
tossed out. But we can't explain where that gravity is
coming from, so we say probably dark matter. And if
we want to understand the structure of the universe, how
(19:33):
it got to have these galaxies and these superclusters and
all of this stuff. If you run a simulation of
the universe without dark matter, then it just doesn't form
galaxies and stars this quickly. You need more gravity to
pull that stuff together. So we need dark matter not
just to explain how galaxies are spinning, but also the
whole structure of the universe. And if we look in
(19:54):
the very early ever beginnings of the universe and we
see like how wiggles in the first plasma of the
universe formed, and how those wiggles propagated, they don't really
make sense unless you add dark matter to our calculations.
So we have a lot of evidence that dark matter
is a thing, that it's out there, that it clumps
together in these big structures that shaped our whole universe.
(20:15):
We just don't really know what it's made out of
other than something that gives gravity. Yeah, we we know
it's there and it has to be there to explain
what we see. But I think that's the crazy things
that we don't know what it is, Like, we really
have no idea, Like it could be a totally new
kind of thing that's not even a particle, or even
something sort of like cohesive. It could be anything, right, Yeah,
And we've done a lot of podcast episodes diving into
(20:37):
some of those examples. Sort Of the mainstream idea is
a whim weakly interacting massive particle that's sort of like,
you know, a generic idea like what's the simplest possible
explanation for dark matter? But there's lots of other more
exotic and fascinating ideas, from things like axions, these weird
heavy photons, to things like primordial black holes formed very
(20:59):
very early in the universe to things like super duper
long wave photons generated by dark stars. So we just
don't really know what it is. And as you say,
it might not even be a particle. It might be
that this whole idea of particles and fields and quantum
forces only applies to the little bit of the universe
that we've been studying for the last few hundred years,
(21:20):
and that you can't extrapolate to the rest of the
universe the same way you can't, like look at an
elephant's tail and assume the rest of the elephant is
like the tail. So it might be that we have
dramatic mind blowing lessons to be learned about dark matter,
or it could be super boring and just be one
particle that doesn't do anything. Some pretty extreme possibilities there.
But but are you saying that maybe, like maybe even
(21:41):
quantum mechanics only applies to a small part of the universe.
I think that's true. Yeah, All of the tests that
we've done of quantum mechanics use our kind of matter
and our kind of forces, and so we have a
pretty good quantum description of quantum particles, but we don't
really have a quantum description of gravity and gravity is
our poor to dark matter. The only thing we know
(22:01):
about dark matter is that it does feel gravity, and
so it could be some other new, weird kind of
thing that generates gravity but doesn't otherwise follow the rules
of quantum mechanics. Well, all right, so that's a dark matter. Now,
what would you say is a dark force? Besides the
obvious Star Wars reference, a dark force would be anybody
(22:22):
who tries to force me to eat white chocolate, the
dark force in my life. Now, a dark force would
be in the assumption that dark matter is made of
particles that follow the rules of quantum mechanics like our particles,
then you might imagine that they also feel some dark
forces between them. You know that too. Dark matter particles
(22:43):
can exchange some like dark photon or dark z boson
or dark higgs boson that they can like push or
pull on each other. They can do more than just
feel each other's gravity. Maybe there's some additional dark forces
and they're passing particles back and forth as a way
to mediate those forces. Rights. Even the matter in our
universe is sort of selective about which forces it feels
(23:04):
or which forces they give out, right, Like some particles
in our type of matter only feel like the magnetic forces,
or they don't feel the strong force and things like that. Right,
that's exactly right. Neutrinos, for example, only feel the weak force.
They ignore the strong force, they ignore electromagnetism. Electrons they
do feel electromagnetism and the weak force, but they ignore
(23:25):
the strong force. Only the corks feel all of the forces,
including the strong force. And so for an individual particle,
you can ask, like whether it has a charge for
that force. So electromagnetism only interact with particles that have
electric charges. For example, we know that dark matter doesn't
have electric charges, and it doesn't have weak charges, or
it doesn't have strong charges, but it might have charges
(23:47):
for other forces we haven't yet discovered. Wow, like there
could be a whole different category of forces that we
just don't happen to feel in our kind of matter,
but that maybe dark matter it does feel exactly So
there could be like one or forty new forces that
we haven't yet discovered because our particles don't feel those,
or maybe more, not just forty, maybe four thousand exactly.
(24:11):
All right, Well, let's dig into what this new kind
of force might be, this dark force and what it
could all mean and whether or not we can never
hope to see it or feel it. But first let's
take a quick break. All right, we're talking about dark radiation,
(24:36):
and in this case, the word radiation sort of is
associated with the word force. So really we're asking does
dark matter feel dark forces that maybe are out there,
but that we in our kind of matter can feel
or touch or detect. But then maybe it is maybe
organizing dark matter into structures or maybe you know, even
making dark matter particles and Adams exactly. And it's part
(24:57):
of this imagination game we play, you know, we wonder
what's going on out there in the dark universe. Is
it simple, is it fascinating, is it complex? What is
going on? What is they're out there to learn? And
usually when we go out and explore the universe, we
find that things are crazier and more fascinating and richer
than we ever imagined. And so we're at this point
(25:19):
where we really don't know about most of the universe.
But at some point in the future, I hope a
hundred years, five hundred years from now, we will know
what's going on in the dark sector and those people
will look back at our ideas as ridiculous. Probably you
might think of us as the Dark Ages physics. They
like the really dark ages, not that the trial run
in the Middle Ages, but more serious. Yes, back when
(25:40):
people ate white chocolate, that's right. Maybe we're looking forward
to an age of dark enlightenment, in the age of oxymorons.
But yeah, So in our kind of matter, we feel
the electromagnetic force, which is what basically holds our atoms together.
And what you know when you touch something, when you
touch the table or you hold the banana, that's the
(26:01):
force you're feeling. And that force is transmitted by a
particle called the photon. So the photon is not just
lighted Tosso what transmits the electromagnetic force? And so the
question daniel I guess is does dark matter have the
equivalent of a photon or at least an electromagnetic force field.
We just don't know, and so we're playing this game
of trying to extrapolate from what we do know. And
(26:22):
in the same way that we imagine maybe dark matter
is made of a particle, because we're made of particles,
we wonder if there are dark forces that look like
our forces. And so one of the simplest forces to
understand is electromagnetism because it just has like one particle
and it interacts with lots of stuff, and so a
simple proposals to say, well, what if dark matter has
(26:44):
a dark electromagnetism, meaning a dark photon. This would be
a particle that has a lot of properties similar to
our photon, but it would be different. You know, those
dark matter particles would have like a dark electric charge
and the dark photon, but only interact with particles with
a dark electric charge, not ones with our electric charge
(27:05):
and our normal vanilla photons wouldn't interact with dark matter
because they don't have an electric charge, only a dark
electric charge. But it wouldn't necessarily be the same or
the sort of the analog of light. Right you're just
saying like, maybe it's a force, and let's why not
let's qual it the dark electromagnetic force. It doesn't have
to be, and it could be the dark matter fields
(27:26):
forces that are more similar to like our weak force,
or forces that are more similar to our strong force,
or forces that are totally different from these forces and
have other new weird capacities. Right before we discovered the
strong force, who could have imagined this crazy force, you know,
where it gets stronger as things get further apart, and
it has like eight different kinds of gluons pulling it together.
(27:46):
It's bonkers. Nobody would have written that into the script.
So it's possible that there are other kinds of forces.
But if there is a dark force that's very similar
to electromagnetism, then that dark photon would have properties very
similar to our photon, because those properties come from the
structure of the force, all right. And also it makes
for a cool name, dark photon, because it kind of
(28:07):
it trips up your brain, right, Like, how can a
photon which transmits light be dark? That's right, because it
transmits dark light into dark eyeballs. Oh yeah, I feel
like you should maybe just call it something different. What
would you call it the dark on the invisible photon?
I don't know, dark on is good, But then what
do you find another force? Yeah, we'll have to be
(28:27):
even more creative with our names. We'll just have, you know,
a brainstorming session with lots of chocolate, and we'll come
up with something good and then and then who knows
what you'll get out of it? Maybe a heart attack,
all right? So dark matter might feel this new kind
of or strange new kind of force, a dark force.
So how would we ever detected Daniel, would we ever
hope to feel it or you know, know it's there
if we can't touch it or see it. So in
(28:49):
the end, this question is really asking what dark matter
does to itself? You know, can we speculate about what's
going on in the dark sector beyond these particles existing
can like touch each other and become sticky and do stuff,
And so we have sort of two ways to probe this.
The first category is sort of like indirect ways to
look at it, and broadly speaking, we can tell that
(29:10):
dark matter is doing things because of its gravity. And
even though it's not a great way to study dark matter,
we can use its gravitational effects to ask questions like
is it clumping? Is it interacting with itself? Because if
it interacts with itself, it forms sort of different structures
that have different gravity. So we can sort of ask
questions about whether dark matter is feeling itself by trying
(29:30):
to look to see how it organizes itself in the universe.
I see, we can't maybe see it directly or touch
dark matter, but if we can somehow kind of know
what it's doing, And if we see it doing stuff
to itself, then we know that it's there must be
some kind of force that it does interact with exactly.
For example, if dark matter felt a version of the
strong force, if it interacted with itself very very strongly
(29:53):
and could like clump together very very very sticky ways,
then it would form much denser objects than we see
currently in the universe. Currently, dark matter looks really big
and fluffy, like the dark matter in our galaxy isn't
nearly as compact as the normal matter in our galaxy.
It spreads out much further out. It's a big, fluffy
cloud hasn't collapsed nearly as much, and we think that's
(30:16):
because it's not as sticky as our matter. We think
that it doesn't feel like a super powerful force that
sticks it together, because that would help it like form
denser objects like dark planets and dark stars. So we
already know very broadly something about how dark matter can't
feel itself because it stays sort of fluffy and doesn't
seem to clump as much as normal matter. Right, But
(30:38):
I wonder you know it sort of looks fluffy to
us because we don't have a good way of seeing it.
You know, we talked earlier about having we kind of
have a fuzzy lens when we look at dark matter
because we can only see it through gravity. You know,
is it maybe even possible that there are dark matter
stars out there, which is don't have the resolution to
see them. You know, absolutely, there's a very big loophole
in this argument that you allude to, which is that
(31:00):
you can see sort of the bulk of dark matter,
but we can't tell what it's doing in detail. In
an episode recently about where is the dark matter, and
we talked about how we can tell sort of the
dark matters in these big fluffy clouds, but we also
can't really tell if it is forming clumps. So you
could imagine that dark matter maybe it's made out of
a few different kinds of things. So most of it,
(31:20):
maybe like you know, half of the dark matter or
three fourths of the dark matter, is some big, fluffy
stuff that hardly interacts with itself. But there could be
a component of the dark matter that does do interesting,
complicated things and has crazy interactions and forms dark stars
and dark planets and dark life, and that would not
mess up the distribution of dark matter, because remember, there's
like so much dark matter out there that even like
(31:43):
if a quarter of it does more interesting stuff, that's
still more than all of our kind of matter. So
there's plenty of room to have like a component that
has complex interactions without missing up these constraints about the
big picture of dark matter. So I feel like dark
matter could be in the shape of I don't know,
like rubber duckies. It's just that they're sort of distributed
out in space, these rubber duckies, and to us it
(32:06):
just seems like a big fluffy cloud. Yeah, most of
it has to be pretty big and fluffy to form
that big fluffy cloud. It can't stick to itself. But
it could be that inside that big fluffy cloud there
are like rubber duckies made out of like a special
kind of dark matter, you know, the way they like
our matter. There's lots of different kinds is electrons, is
different kinds of quarks. Right, we have twelve different matter particles.
(32:27):
Dark matter could have like fifty different kinds of particles,
and maybe most of them don't interact and make a
big fluffy cloud, but one or two of them form
dark rubber duckies that are floating out there in the universe,
and we couldn't tell the difference because the only way
we can see them is through their overall gravitational interaction.
I'm pretty sure that's what dark Vader uses in his bathtuff.
But I guess the question is, you know, isn't the
(32:48):
fact that dark matter does seem to be sort of
fluffy and doesn't seem to stick to itself. Is that
evidence that maybe it doesn't interact with itself other than
with gravity. It's evidence that it doesn't interact with itself
very very strongly. But again, the loophole is that some
component of it might be able to Now. The way
to probe that a little bit more deeply is to
do experiments. Is to take like two big clouds of
(33:11):
dark matter and throw them against each other and see
if they pass right through each other, or if maybe
they stick together and make them like dark explosions. That
kind of experiment is pretty tough to do, but we
were lucky and the universe did it for us. It
collided two huge clusters of galaxies, each of which have
their own dark matter associated with them, smashed them together
(33:32):
millions of years ago, and we got to see what
happened and what did happen, and so this is called
the bullet cluster. You can go ahead and look at
these pictures online if you're interested. It's really beautiful. What
happened is that the stars in these galaxy clusters mostly
passed through themselves because stars are pretty sparse, so it's
like two clouds of sand passing through each other. The
gas and the dust that were in these galaxies smashed
(33:53):
together and admitted a lot of light and radiation. But
the dark matter looks like it basically passed right through itself.
You might ask school like, how could we know where
the dark matter is? We can see the dark matter
because it distorts the stuff behind it. From the dark
matter is matter, it feels gravity, so it bends space
a little bit, which creates like gravitational distortions in the
light that passes through it. So from this bullet cluster,
(34:16):
it looks like these two clouds mostly passed right through
each other. So not it doesn't have any strong interactions
with itself, but it could still have some interactions with itself,
which would maybe give rise to something like a dark photon.
That's right, and so people often cite the bullet cluster
is evidence that dark matter can't feel anything with itself,
and that's not exactly true, because, as you say, there's
still room in there for some kind of interaction. Because
(34:38):
remember that the stars, which do definitely feel each other,
also passed right through themselves. Right. The star is hardly collided,
and that's just because it's pretty sparse, and so could
be that dark matter forms the structures, but they didn't
collide with each other because just like the stars, they're
pretty sparse and space is pretty big. And it could
also be that some components of the dark matter did
(34:58):
collide with itself, did get stuck sort of in the
middle there. But we don't really have the resolution to tell.
We can't very precisely measure how much dark matter passed
through and how much stuck. We can just tell that
a lot of it passed through, but some component could
have gotten stuck and could be doing like crazy stuff
there in the middle. I see, like maybe the Rubert
duckies didn't all crash into each other, but some just
(35:19):
kept going. I think the key point to understand is
that there's so much dark matter out there that even
if a tiny fraction of it is doing interesting things
we couldn't tell, and that tiny fraction would still be
more stuff than all of the stars and gas and
dust in the universe that we know. All right, well,
let's get into other ways that we might be able
to detect this dark radiation and maybe even see these
(35:39):
dark or not see these dark photons. I'm a little
confused about how he might see are not photon But
first let's take another quick break. All right, we're talking
abot dark gradiation and dark photons, which it might be
(36:03):
what dark matter sees and feels with itself when dark
matter looks in the mirror. That's what they would see
dark photons. Yeah, exactly. I hope they haven't eating too
much dark chocolate. But to us it's pretty elusive because
we can see or feel the dark matter or maybe
even these forces. So, Daniel, what are some of the
other ways that we might hope to detect whether or
not dark matter has these dark interactions. There are a
(36:26):
couple other indirect ways, basically again using gravity. One of
them is looking at neutron stars. Neutron stars are these
crazy dense objects that are the remnants of stars that
have blown up and left this very very compact core,
and sometimes neutron stars come in pairs and then those
pairs spin around each other in this sort of like
(36:47):
death spiral before they collide, and when they do that,
they create gravitational waves that we can see here from Earth.
This is like the shaking of space itself. If you're
interested in learning more about that, we have a couple
of episodes about gravitational waves. It's really cool because the
gravitational waves reveal in detail how these stars are spiraling
towards the center, how they're losing energy, and we can
(37:09):
calculate with great precision how fast they should be going
and how much energy they're losing. And so this is
a really nice test of sort of like what's going
on with this kind of matter, Because if these neutron
stars were capable, for example, of feeling this dark radiation
at some level, if the particles in there could feel
this force at all, then they would radiate a little
(37:29):
bit of those dark photons and it would change the
way this inspiral happens, and we would be able to
see that by looking at the gravitational waves. Wait, so
these neutron stars would be made out of regular matter,
the kind we're made out of. But you're saying that
even though the regular matter it might admit this dark radiation,
how is that possible. It might be that our matter
can feel these sort of like new forces, just at
(37:50):
a very very low level. We don't know, and so
this is like, you know, one way to look for
it under the assumption that our kind of matter could
feel these dark forces, which is not something we know.
It's just sort of like a guess. We do this
in physics a lot. We're like, we don't know if
it's possible to see it this way, but let's check,
because if it does feel this force, if neutron stars
can make these dark photons, we would be able to
(38:12):
see that. I see neutron stars are so it's such
an extreme event when they crash into each other that
that's one possible, you know, crazy scenario where it might
make dark gradiations exactly. And it's a place where we
can make very very precise calculations about the gravitational waves
and compare them to very very detailed data. So it's
an opportunity to look for like very small deviations, you know,
(38:33):
things that the neutron stars are doing that we can't
otherwise explain that might be explainable through dark photons. I see,
all right, what are some other ways we could maybe
feel this dark force? All the most generic ways you
just use gravity the kind of tests that we did before,
But there are some other approaches to look for these
dark photons, hoping that there's some connection between our kind
of matter and the dark matter, Like hoping that somehow
(38:56):
maybe these dark photons can turn into normal photon very
very rarely, and then we could detect that what how
can a dark photon turn into a regular photon. It
turns out that if you have another kind of force
that's very similar to the photon, has sort of like
a similar mathematical structure, that these forces like to talk
to each other, that there's like it's very easy to
(39:18):
build a physics model in which another kind of photon
can turn into our photon wha like spontaneously or only
in like high energy completions or you know, moments where
you have kind of this a state of pure energy.
It would be spontaneous, and in order for that to happen,
they would have to be some kind of particle in
the universe that has both kinds of charges. So imagine
(39:41):
some new kind, very very rare dark matter that does
have electric charges and also has some new kind of
dark electric charge. That particle could effectively be like a
portal that connects our photons to these other photons, and
it might be that these particles never really exist in
the universe, the way like top corks almost never exist
(40:02):
in the universe on their own, but they're sort of
like on the list of possibilities. As long as it's
on the list of possibilities, then dark photons can use
that as a portal to become normal photons and vice versa.
This allows them to spontaneously turn from one into the other. Whoa,
So that wouldn't make dark matter not dark, right, It
would mean that you can sort of see them. It would,
(40:23):
but these particles again wouldn't actually have to exist in
the universe. So imagine now the dark sector is some
kind of particle that's actually out there, that most of
the dark matter is made out of this particle. And
then there's the possibility for this other particle that has
electric charges and dark charges, even though it's never really
out there in the universe, as long as the possibility
for it to exist is part of sort of nature's menu.
(40:47):
That possibility allows dark photons to turn into photons. It's
called kinetic mixing because we see it happening with regular matter, right,
Like our photons turned into other kinds of stuff all
the time, yes, exactly, Like photons turned into pairs of
particles electrons and positrons, and then those can turn into
z bosons, right because z's also interact with electrons impositron.
(41:08):
So even if there weren't a single electron or positron
in the universe, a photon could still turn into a
z boson. And in the same way, as long as
there's the possibility for some virtual particle that connects these
two forces to exist, then photons could turn into dark
photons and vice versa. And that's kind of our only
hope of seeing this dark force directly. That's the best
(41:29):
way to see these dark forces directly, exactly, And so
people have built really cool experiments to try to like
make this happen more often, try to like create the
scenarios that would help induce dark photons to spontaneously turn
into photons. They have these resonant cavities at Fermilab, for example,
that should enhance the rate of this happening. If you
(41:50):
like build something with the right shape and size, it
creates a situation where this kind of spontaneous transformation is
more likely to happen. And so they look for this
kind of signature. Wait, that's being built right now, Like
there are people building basically dark forest boxes. These things
exist already and they are being run and they're just
building them bigger and bigger. It's sort of like the
(42:11):
way people are looking for a dark matter. They look
in these containers underground, and they started with small ones,
and then they're making bigger and bigger and bigger ones.
Nobody's seeing a dark photon yet, but they're hoping as
they make these cavities more precise and larger to induce
a dark photon to turn into a normal photon. Cool,
and how else can you look for these? Another way
to look for these is to look for particles appearing
(42:32):
where they shouldn't be. So one of my favorite kind
of experiments, it's called a beam dump experiment, where you
take a particle beam you were using for something else
and you dump it, meaning you just like shoot it
into a huge block of concrete or into a mountain side,
just because you know, it's got to go somewhere. You
don't want to like sprayed over the neighborhood because it's dangerous, right,
just shoot it through the earth at people on the
other side of the earth. Is that what you're saying, Yeah, exactly.
(42:54):
And so Sho didn't do a mountain. It was on
the other side of Daniel. It's not California, is I'm
to have to look that up. But if you shoot
this particle beam into a mountain, for example, and then
you put a particle detector on the other side of
the mountain, none of the particles should make it through
the mountain, but you might see particles appearing in your
detector on the other side of the mountain, because dark
(43:17):
photons might make it through the mountain because they basically
see the mountain is invisible as transparent. The ideas if
your beam sometimes occasionally produces dark photons, those dark photons
would make it through the mountain. And so essentially it's
like looking for light shining through walls or I see
so many the ideas that you send this beam out
into the mountain, some of it turns into dark photons,
(43:38):
and then somehow it turns back into regular photons before
it hits your detector. Yeah, that's exactly right. And so
you're hoping that sometimes those dark photons turned back into
normal photons. So you're assuming a lot of things here.
You're assuming that maybe sometimes dark photons are created in
your beam, and that sometimes they turn back into normal photons. Right,
But if they turn back into regular photons, how would
(43:59):
you know they turned into dark photons in the middle. Yeah,
you wouldn't know for sure. You would just know that
something passed through the mountain that typically shouldn't be able to.
So you calculate, like how often should particles be able
to pass through the mountain and turn into photons? And
if that happens more often than you expect, then you
know there's something else and new there, maybe a dark photon,
(44:20):
maybe something else. But you know, that's often the case
with particle physics. We find something new, we're not exactly
sure what it is, and then we study it in detail,
we try to characterize exactly what it is, just like
with the Higgs boson. First thing, we saw some new
particle decaying into two photons, and we weren't sure is
this the Higgs boson or is it something else? We
didn't expect, and over decades of study we sort of
(44:41):
pinned down what it must be because of all of
its behaviors. I see, it's like, the only way it
could have made it through the mountain to and out
into California to hit us it was if it turned
into something invisible like dark photons somewhere in the middle.
Otherwise it wouldn't have made it through. Exactly, I have
to phase through the mountain by turning into something that
doesn't interact with mountains. All right, cool, Well, what are
(45:04):
some other ways we might be able to see these
dark forces? A lot of the other experiments are very similar.
There's one at CERN called Phaser Forward Search Experiment. It's
a pretty tortured acronym, but a really cool idea for
I don't even know what is there an A actually
in the name, there's an A and forward and an
R experiment. Oh man, it's a really cool idea because
(45:27):
they're taking an already existing experiment Atlas where we collide
protons together and they're wondering like maybe dark photons are
created and shoot down the beam. So they built a
detector like really far down along the beam past where
the collisions happen to see if maybe dark photons are
created in those collisions and then fly along the beam
and then turn into something that they can see sort
(45:49):
of downstream. So they added this little bit to the
detector that can do something totally novel and new. It's
actually led by a team here at you see Irvine,
m M, I see. But we don't actually know if
dark matter can switch back and forth that easily, right,
We don't know. And again, it could be that dark
matter is only some inert particle that feels gravity and
nothing else and there are no dark forces and no
(46:11):
way to interact. It could also be that dark matter
does feel some new dark force, but can only interact
with itself, and none of those dark forces can ever
turn into normal matter or normal photons. Here, we're just guessing.
We're trying to like study the various possibilities in the
various ways that those possibilities might manifesto well, And so
(46:32):
what does it all mean? Do you think we will
find it has interactions with itself or do you think
it just represents this whole part of the universe that
we will never be able to see or touch or
even confirm that it's there. You know, like we could
be swimming in dark rubber duckies and never ever ever, Yeah,
part of it comes down to what you think is
more natural. Does it make sense to you to have
(46:54):
a huge component of the universe be sort of simple
and inert and not really doing any being interesting, or
does this seem more natural for it to be complex
and interactive the way that our matter is. Now, what
I know is that the universe doesn't follow what we
think is natural. Our conceptions of how the universe should
work don't seem to be very well aligned with how
(47:14):
it usually does work, and so I suspect there are
some surprises out there. The other side of that question
is what you asked, is might we ever be able
to tell? And it could be that dark matter doesn't
feel any of our forces, and none of these forces
can talk to our forces, and we can never find
these dark rubber duckies. But I have faith in physicists
and engineers to come up with clever ways to probe
these things, ways that we can't even today. Imagine kinds
(47:37):
of experiments that people might think of in ten years
or twenty years, experiments thought up by you know, clever
Listeners to this podcast who aren't constrained by the kind
of thinking that academic physicists have been trained to do.
I think I know how you can do it, Daniel,
Dark Chocolate collisions. You just gotta want it enough. You know,
you have to really want it, because, as Yoda says,
(47:58):
you know, wanting these to pain and pain needs to suffering,
and suffering leads to the dark side, and so that's
that's how you can get there faster. Maybe, Yeah, there
is no try. I should just do it, that's right,
Just I mean, do it? You should, yes, all right.
I don't know why I didn't think of that before.
I'm just gonna go do it right after we're done
with this podcast, or not do it. I guess we're
(48:20):
doing in the dark. Gosh, I'm so confused. But to me,
it really touches on these deep mysteries of physics that
we know the universe out there is telling stories that
we haven't heard yet. We're desperate to hear those stories.
We have hints that we know the shape of those stories,
but we don't know any of the details yet, and
I just hope that one day we'll be able to
fill those in. Yeah, it's almost like there's a whole
universe out there, sitting right in front of us that
(48:42):
we have yet to discover, and that anyone listening to
this could become part of that search. That's right. So
there's plenty more of the universe to find out, and
we desperately need new ideas. And whether the universe counts
as dark chocolate or not, that's for other people to debate.
That's right. And if you love white chocolate, take my apologies.
I love all of them. You love listeners more than
(49:03):
you love your chocolate, exactly, And if you eat white
chocolate while listen to this podcast, I forgive you because
he can't see your or feel you, so it doesn't
matter to exactly. Please accept my dark apologies. All right, Well,
we hope you enjoyed that. Thanks for joining us, See
you next time. Thanks for listening, and remember that Daniel
(49:30):
and Jorge Explain the Universe is a production of I
Heart Radio. For more podcast for my heart Radio, visit
the I heart Radio app, Apple Podcasts, or wherever you
listen to your favorite shows. Yeah,
Hosts And Creators
Daniel Whiteson
Kelly Weinersmith
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