What are boson stars?

Daniel and Jorge talk about whether its possible to have stars made out of bosons!

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Episode Transcript

Speaker 1 (00:08):
Hey, Daniel, what does it take to be a star?
I think you either have to have enormous natural talent
or rich and famous parents. I bet that helps. But
what about a star out there in the universe. Oh,
that's easier. All you need is like a lot of gas,
Like a lot of gas, like the bourbon kind, or
the other kind of gas, any kind of gas that

(00:31):
kind of make a star out of methane. I don't
really know how that would smell, but I bet it
would burn pretty well. How about a starman out of
laughing gas? That would be pretty funny. I am more

(00:58):
handmade cartoonists and the creator of PhD comics. Hi, I'm Daniel.
I'm a particle physicist and I'm no kind of star.
But I bet you're a gas. I try to make
people laugh. Well. Welcome to our podcast, Daniel and Jorge
Explain the Universe, a production of our Heart Radio in
which we try to gas you up about all the
incredible things out there in the universe, the things we understand,

(01:21):
the things we don't understand, and the things that scientists
are still puzzling over, and the things that make you
curious about how our universe works. We try to wrap
it all up and ject some jokes and make it
all understandable to you. Yeah, because there is a lot
out there in the universe to be curious about, including
things that may or may not exist. Those are the

(01:42):
best things to be curious about. Like is there a
tiny little teacup between here in Venus? Yes? Can you
prove it? Though I have faith, Daniel in the teacup hypothesis? Well, fortunately,
we have better ways of exploring the universe than just faith.
We have science, and science encourages us to think creatively,

(02:02):
but then ask ourselves questions like how do we know
that's true? And how could we approve it? Yeah, because
the universe is full of surprises. Sometimes we think some
things are impossible, even though the math says that it's possible,
and then we find those things out there in the cosmos. Yeah,
we have surprises, both experimentally and theoretically. Sometimes we look

(02:23):
out into the universe with a new kind of telescope
and we see something totally weird than we never expected
and didn't understand, like the Fermi bubbles or all sorts
of other weird stuff like pulsars. And then there are
theoretical surprises where we find something in the math that
says this thing could exist in the universe, maybe even
should exist. Let's go and see if it's real. Yeah,

(02:45):
because I feel like there's no there's nothing in the
universe that says that what our expectations are of it
or what our experience of it is has to dictate
what is actually out there. Thank God for that, right,
it's nice that the universe is filled with surprises. It
would be borning of the universe was just like the
surface of the Earth and not much else. Well, you
know that old cars that says, I hope you have

(03:07):
an interesting life. Yes, I want to live in interesting
scientific times. Actually, I want to live in times when
we discover crazy things that totally upend our knowledge of
the universe and our understanding of our place in it.
That's the power of science. That's the joy of it,
is revealing the truth and stripping away our intuition and
the ideas that came about just from like living on

(03:29):
the surface of the Earth. Well, today we're going to
talk about what such thing the scientists think that could
be out there. At least the math says it's possible,
but we don't really know if it does exist. It
would be pretty wild if it does. That's right. It
turns out that even though we've talked about neutron stars
and magnet oars and crazy white dwarves and all sorts
of other kinds of stars. We've done the biggest stars

(03:51):
in the universe, the weirdest stars in the universe, it
turns out there are even weirder, stranger kinds of stars
we haven't even scratched the surface of. So is this
the weirdest Star Daniel episode in our Extreme series? This
is the most hypothetical list Star I see, I see,
the most imaginative star, maybe the most bunkers is Star. Yeah,

(04:11):
because each of these sort of extreme examples tells something
a little bit about what matter can and cannot do. Right,
they're sort of like, you know, extreme examples of where
you can take physics. Yeah, these are really useful thought experiments.
You say to yourself, is this possible? And if the
laws of physics say that it is, then you go
out there and you hunt in the universe to see

(04:33):
if you find it. And if you find it, cool
you've learned something about the universe. And if you don't
find it, if you can prove that it doesn't exist
somehow that it should exist in the universe, but we
don't see any of it. There's a clue, there's a
hint that there's something about physics you don't yet understand.
And those clues are super valuable because those are the
ones that lead us down the path to revealing something

(04:55):
true about the universe we didn't know before. Yeah, and
that's the whole point of signs, to find out things
we didn't know before. What It's not just to make
us feel good or make us laugh sometimes, or do
you have interesting careers. Science makes me feel good. You know.
I had a tasty breakfast this morning because of science.
I slept in a warmhouse last night because of science.
I'm alive because of science. So, yes, science makes me

(05:16):
feel good. I think you mean engineering, Daniel. I don't
think a scientists, you know, fix your A C system.
This scientist certainly didn't fix my own AC system. That's true. Well,
today on the podcast will be asking the question what
is a Boson star now, Daniel? That doesn't just refer

(05:37):
to the press the button when they discovered the Higgs Boson, right,
That would be me. Yes, I'm a star in the
Higgs boson. Uh no, I was Did you press the button?
I pressed lots of buttons, actually, yes, because I spent
time in the control room at the large Hadron Collider,
which looks a lot like you know, the way they
depict the control room at NASA where they're launching the
shuttle or whatever. It's a bunch of monitors and people

(05:59):
at desk looking at screens, and you got buttons in
front of you, and so yeah, sometimes you actually get
to press a button. Yeah, did you press any buttons?
Was your role there monitoring the collisions or something? Yeah,
you monitor collisions, You make sure the data that's coming
in looks reasonable, and then in very rare circumstances, there
might be an emergency. I was actually on shift the
large hingge On collider when they first turned it on,

(06:21):
very early on. It was two thousand eight, if I
remember correctly, when we had that accident when there was
a spot that was welded poorly and there was an
arc and liquid helium was ejected and the whole thing broke,
and this this big red button in the control room
that you have to hit in the case of an emergency.
And I had said at that desk for weeks looking
at that button, wanting to press that button, because you know,

(06:42):
buttons they have to be pressed right, And so I
actually got to press that button. Wow. Now it wasn't
just a coincidence that things went wrong when you weren't
on the shift. It was a hud of coincidence that
things went wrong when I was on shift. Absolutely nothing
to do with me at all. It's not like I
knocked coffee onto a critical control panel or something. Right

(07:03):
with t But yeah, we're asking the question what is
a Boson star? And I have to say, I've never
heard of this concept a boson star. I mean, I've
heard of the Higgs boson, and I think I know
what a boson is. It's a kind of particle. But also,
but put together with the word star, it's a whole
new thing. Yeah. It's fun to just like take two
science words and stick them together and say, hey, is

(07:23):
this a thing in the universe. I wonder if that's
how they came up with this idea. Let's come up
with a few quantum black hole that is a thing?
Man there, remote dynamic firm me on teleporting hamsters, all right, Yeah,
this is an interesting concept in physics. Is it kind

(07:43):
of like a new thing, or is it an old
thing that people are rediscovering? What's the context here? It's
not that old an idea. It's something people have been
thinking about in the last few decades, but it's received
a little bit of attention recently because one of the
ingredients you need to make it a particularly weird kind
of Boson has sort of seen a resurgence of interest

(08:03):
as a candidate for what might explain the dark matter.
All right, well, let's see if people on the internet
know what it is. As usual, Daniel went out there
and as well, if they knew what a Boson star is. Yeah,
and so my deepest gratitude as usual for people who
are willing to volunteer to speculate without any preparation on
tough physics concepts even Jorge hasn't heard about. So if

(08:25):
you would like to participate in the future, please write
to me. Two questions at Daniel and Jorge dot com.
All right, well here's what people have to say, no idea,
the clowns of the star. Do you know there's bosons
and fermions. Those are two types of particle. I think
one adds up to a different charge and the other.

(08:48):
So maybe a boson star is just a start, just
completely made out of bosons. That's my best guess. I
thought we are done with boson. We find the piggs
boson then and that's it. Well and on, No, I
don't boson stars. No, sorry, Well I've never heard of them.
But my assumption would be that if you can have

(09:09):
a star that's only made of neutrons, then you'd be
looking at a star that's only made of bosons. However,
what that would look like or how it behaves has
completely lost all me. Boson stars are stars to give
off a lot of bosons, um, and I'm gonna have
to back up a few podcasts to remember what bosons are.

(09:31):
All right, A lot of good guesses. I like the
one about clowns, Like I wonder how many boson stars
can you fit into a small car? A lot? Actually
a lot. You can seek me a lot of them
into there because they're bosons. No. I love hearing these
folks try to work it out on the fly. That's
my favorite thing about this. It's not like I got
your question. I like hearing people think about it and

(09:53):
apply their knowledge of physics and try to put these
things together and figure it out, you know, in fifteen seconds.
So thanks to everybody for your great ideas. Yeah, well,
there are a lot of good ideas here. Some people
are saying they're stars that give off a lot of bosons,
and some people may be saying, we're thinking that there
are stars made out of bosons. Could it be a
star that eats bosons? Yeah, and there's one person who

(10:17):
suggested that maybe neutron stars are made out of bosons,
which is a cool idea. Neutron stars are super awesome,
but neutrons are not actually bosons. Even though you can
have objects we call stars made out of only neutrons,
that doesn't qualify as a boson star, but good try.
All right, well let's get into it, Daniel, that us
three it. What is a boson star? I guess maybe

(10:39):
start with the word boson? What does that mean? Yeah,
So there are two kinds of particles out there in
the universe that we've discovered. There are fermions and there
are bosons. And these are not just like cool names
for things. These actually have meetings, and the meetings are
important because fermions and bosons are very very different kinds
of particles. What's the difference. Well, fermions tend to be

(11:00):
the kind of particles that make up matter, and bosons
tend to be the kind of particles that transmit forces. So,
for example, electrons are fermions, Quarks are fermions. Even when
you put three quirks together to make a proton or
a neutron, you still get a fermion. And so all
the stuff that we're made out of, me and you,
and amsters and most of the stars in the universe

(11:22):
are made out of fermions. Right, So all the matter
in the universe are made out of fermions. Were fermion
fellas we are, yes, and all the other kind of
stuff like light beams and higgs bosons and the weak
nuclear force and the strong force. These use particles to
communicate between fermions. Like what happens when an electron repels

(11:44):
another electron is the exchange a photon. That photon is
a boson. So all the particles that represent how matter
particles interact, those are force particles. Those are the boson particles.
So fermion particles are matter particles, and boson particles are
the force particles. Right, So is that the criteria like

(12:05):
what they do. Isn't it technically like from a theory
point of view that they're all just kind of the same.
They're all just like excitations in a quantum field. They
are all excitations of quantum fields. But those fields are different,
and it's not just about what they do, like what
role they serve. They actually have a fundamentally different mathematical
structure because all the firm On particles, which are excitations

(12:27):
of fer me On fields, have a different quantum spin
than all the Boson particles, which are excitations of the
Boson field. Remember we talked about quantum spin once in
an episode. It's not like that the particles are actually spinning.
It's just that they have this property which is really
closely related to angular momentum, and so we call it
quantum spin. But it's a quantum property which means you

(12:49):
can only have a couple of values of it. So,
for example, an electron has one half spin can either
spin one half up or spin one half down. So
for me, on all have these half integer spins half
three halves, five halves, whatever. Bosons all have integer spins
zero one or two. So if you can go sort

(13:11):
of halfway up or halfway down, your fermion, and if
you are on the integer number zero, one or two,
then you're a boson. So they have different sort of
mathematical structures, and that tells us about like the number
of different configurations the field can be in. And so
fermions and bosons really are fundamentally different kinds of particles.
It's like they're part of a different kind of feel altogether, yes,
which probably lets them do different things, yes, exactly. And

(13:34):
there's a very important property that makes fermions and bosons different. Now,
fermions they can't hang out in the same state, like
you can't have two electrons hanging out in the same
quantum state. You can't have them have the same spin
and the same location and the same energy. They just
don't get along. They exclude each other. And that's why,
for example, when you have a complicated atom with eight

(13:57):
electrons around it, for example, they're not all on the
lowest energy state. They stack up on top of each
other like a game of Connect four. So fermions cannot
hang out in the same quantum state, but bosons can.
It's the same for quarts, it's the same for quarks. Yeah, absolutely,
for any kind of fermion. They will not hang on
the same state like this one. In that state, it's done,

(14:18):
it's filled in, it's checked off, and the next one
that comes in has to settle in at some other state,
either higher energy or a different spin or something. But
only they're really close together or in the same exact spot.
Location is part of your quantum state, And so if
you're like isolated in a box, like in a quantum dot,
or in a hydrogen atom or something, then the energy

(14:39):
level or the spin or something else has to distinguish
you from the other electrons. If you're in a different location,
that counts as having a different quantum state. But bosons
can overlap. Bosons can totally overlap. You can have two
bosons in exactly the same quantum state. And you know,
for example, take two flashlights and shine them at each other.
The phote toons don't like bounce off each other. You

(15:02):
can't fill up a room with like light from flashlights
and have it be like stuffed full. But fermions repel
each other, you know, That's why matter has volume, that's
why things fill up. So bosons you can have as
many of them as you like in the same state
and We've done really interesting experiments that we talked about
the podcast, like the bosons dyin condensate, which is an

(15:22):
extreme example of this, when you get a huge number
of bosons all together in the same quantum state. All right,
so then a boson star and then is a star
made out of bosons or that gives off the bosons? Yeah,
a boson star is a star made out of boson.
The Sun, for example, is made out of fermions. It's
made out of quarks and electrons all mixed up in

(15:42):
different configurations, but they're all fermions, And a Boson star
would be a star made out of just boson, like
a star made out of light, pure light. Yes, so
not every boson is capable of making a Boson star,
but yeah, photons are an example of bosons. They're like
the most miss kind of boson. But think about sort

(16:02):
of how hard it is to make a star. You
can't just make a star out of anything. We talked
on the podcast about the conditions for making a star.
It's actually quite tricky, right, even like stars made out
of fermions. You have to have enough mass so that
there's gravity that pulls it together and you make like
an object not just like a big fluffy cloud out
there in the universe. Has to be gravity pulling it together,

(16:25):
but there also has to be something else working in
the other direction. So gravity doesn't like run away and
give you a black hole. In most stars, that's fusion.
Gravity comes together and it makes the core of the
star really really hot, and so you get light and
energy flying out and that pushes against gravity. So the
key thing about making a star is this balance. You
need something pulling in gravity and you need something pushing

(16:48):
out to prevent the collapse. And this is not like
an eternally stable thing, so it's not that easy to
make it happen. Though in most stars you have fusion
and gravity imbalance, and other stars that aren't burning like
white dwarf, you know, other weirder stuff happening. But then
the question about boson stars is like, what can you
get to balance gravity to make a boson star? Right? Well,

(17:08):
I guess the tricky part is that you say that
bosons are the force particles that transmit forces. So are
you talking about like a the idea that you can
make a star out of force particles? Like what does
that even mean? Daniel, Like a star that where you
bring things that are pure force. Yeah, we'll remember that
forces aren't transmitted by particles. But those particles can also

(17:31):
be real. You know, photons are what electrons used to
talk to each other. But photons can also just exist, right.
They can fly across the universe, they can be part
of a laser beam, and they're created all the time.
And so these particles, you know, that's the role they
play in our matter and sort of the story we
tell about nature, but they can also just exist. So, yeah,

(17:51):
you can get a huge pile of bosons all together
and then ask questions like what happens to them? Do
they form interesting structures? Right? That's the physics game we play.
We think what happens when you've got a huge pile
of how did yougen together? Oh? Look it does this
cool thing. It makes a star. And now people are
playing that game, like what happens if you've got a
huge number of bosons together? Could you make a star

(18:12):
at them? What would they do? Many? Could you fit
into a small car? Big fundamental questions? All right, well
let's get into how you might actually make a boson
star and if they exist, what would they be like?
But first let's take a quick break. All right, we're

(18:39):
talking about boson stars, a hypothetical, possible, maybe theoretically plausible
kind of star, but that maybe we haven't seen yet
out there in the universe. We talked about how there
are stars that might be made out of bosons. I
guess my first question is, how would you even like
get a bunch of bosons together? Like, do they have mass?

(19:00):
Would gravity bring them together? Or do you need to
capture them somehow or you know, lure them with big
shoes and red noses? How do we bring them together
really comically sized cookies? I think, and that just pulled
them all in. No, that's a fair question. You know.
You can ask two different questions. One is if I
had a huge pile of bosons, would they form a star?
And the other question is could I just get or

(19:22):
should I expect to see in the universe a huge
pile of bosons? Right, it's possible that this thing could
potentially exist if you could assemble the ingredients, but that
it just doesn't happen in our universe because it's not
a consequence of the Big Bang in any way. So
those are two totally interesting but separate questions. Oh, I see.
One is like can it exist? And the other one

(19:43):
is it doesn't exist? Yeah, exactly. And you know, we
talked in this podcast about the infinity of the universe
and everything that can happen will happen, and that's mostly true.
But there's an important coffee out that you had need
to have the right initial conditions. You know, it might
be that even in an infinite universe, there's no way
to start from a hot, dense state that we began

(20:04):
from and end up with like a huge pile of
bosons all in the same place that then, you know,
do whatever they do, maybe make a star. All right, well,
let's play the first game. Then what if you suddenly
have a bunch of bosons all in the same place
or the same vicidity or like volume. What are we
talking about? Yeah, that's exactly what you need to do.

(20:25):
And you need to think about the two ingredients to
make a star. One is gravity and the others outward pressure.
So to have gravity, you need to have these objects
having some appreciable mass, Right, you need to have gravity
be able to work on these things. Now, they're folks
out there probably thinking whole lot of second. I know,
photons don't have mass, but they are affected by gravity

(20:46):
because they can't, for example, escape black holes. And that's true.
And we talked once on the podcast about how you
could like focus enough photons together to maybe make a
black hole, but to make a stable Boson star, you'd
actually need to have a particle with at least little
bit of mass, so gravity has like a little bit
more of a handle to pull it together, right. Well,

(21:06):
But it isn't like mass the same thing as energy, Like,
if I have a lot of photons in one place,
wouldn't that warp the space around it just like it
as if it had a lot of mass. Yeah, absolutely would.
And you could, for example, make a black hole if
you concentrated photons together enough. But a star is a
little bit different. It's actually harder to make than a
black hole. Black hole is just like a bunch of

(21:27):
energy in a super tiny space. A star is a balance, right,
It has to be a balance to have just the
right amount of gravity and just the right amount of
outward pressure. So these two things match, And the calculations
just don't suggest that photons could make a Boson star.
They don't have enough mass to like pull together in
the right density to get the outward pressure you need.

(21:49):
I see. I think what you're saying is that for
something to be called the star, I mean, it can't
be a black hole basically, and it can't be an
explosion either. It has to like shine and shine consistently.
And you're saying thing that you just can't do that
with bosons, like they wouldn't stick together if they don't
have mass. You can't do that with photons. There are
other bosons out there that might be candidates for making

(22:10):
a boson star, but we don't think that photons can
do all right, So which bosons could do it? Well,
let's go through the kinds of bosons there are in
the universe. What's on the menu next up our gluons.
But gluons also have no mass. We've got to scratch
them off. We also need a particle that's stable. We
don't want our boson start to decay instantaneously into other

(22:32):
kinds of stuff, and so that, for example, removes Higgs bosons.
Higgs bosons exist in the universe, but very very briefly,
they very rapidly decay into pairs of fermions, like a
Higgs will decay into two bottom corks or into two
muans or something like that. So we need a stable particle.
So we don't think there are any Higgs stars out there,
which is too bad because that would be kind of awesome. Yeah,

(22:55):
pretty good name recognition right there. And so that removes
the possibility of a Hig star and also a W
star or a Z star. WS and zs are the
bosons associated with the weak nuclear force, and they're also
very massive and they decay very quickly. Z s decay
into a pair of corks. Ws decay also into a
pair of corks or sometimes into leftons, and so you

(23:18):
just can't make them out of those particles because they
would just decay into a Meon star. Well, I'm sad
that we can't have gluon stars, but we have glue balls. Actually,
glue balls are a stable configuration of just gluons, a
particle made out of just gluons, which is pretty but
you can't have a gluon star unfortunately. Yeah. Also that's
a sticky subject. So what does that leave us, which particle,

(23:42):
which boson particle could we used to make a Boson star. Well,
that basically crosses off all the bosons that we know exists.
So all right, we're done, but we're not done because
there are always more particles on the list, this long,
infinite list of hypothetic goal particles, particles that we think

(24:02):
might exist and if they did, could do other weird
things that the particles were familiar with don't do. And
near the top of that list is a particle which
has gotten a lot of attention recently. Theoretically, it's called
the axion. Yeah, we had an episode about that. Maybe
remind folks what an axon is, And by folks I

(24:23):
mean including myself. Well, an axon is named after a
detergent because it was thought up by Frank Wilcheck and
he was doing a grocery shopping while he was thinking
about the name, and there's a detergent called axon, and
he thought, oh, that's a cool name. So the axon
particle is one that was thought up to sort of
explain a theoretical puzzle in the strong force. People didn't

(24:44):
really understand why the strong force was different from the
weak force in a subtle way, and so they came
up with this axion to explain it. But the reason
that axons are interesting recently is that people think they're
a good candidate for what might be the dark matter particle.
Remember that while we no dark matter is a thing,
we know it's out there. We know it's providing gravity.

(25:04):
Most of the gravity in the universe actually comes from
dark matter. We still don't know what it's made out of.
It could be made out of one particle, or many particles,
or some other weird kind of stuff. But we have
this sort of list of candidates. One of the particles
on that list is a firmion. It's called the WHIMP,
the weekly interacting massive particle, and it's sort of the
leading candidate for a long time, but nobody's founded. We

(25:27):
have all these dedicated experiments looking for whimps and not
seeing them. So recently people have been charted, I think
a little more broadly, dig deeper into that bag of
hypothetical particles to find other things. And the idea that
the axion might be the dark matter is sort of
popular these days. Wow, alright, I'm a little confused now.
So you're saying that dark matter could be made out

(25:49):
of something that's not matter, that's a force a force particle,
and that's an axion, and that if these things exist,
you could potentially put them together to make an action star. Yes, exactly.
You totally understood it, so it must have been perfectly clear.
So we don't know that acidons exist, right, it's an idea.
It would be sort of beautiful theoretically and solvement of

(26:10):
interesting problems. If you're interested in that, go dig into
that podcast episode specifically on that topic. We don't know
that they exist, but they would solve an interesting theoretical
problem about the strong force. They might also be dark matter,
and yes, they would be the perfect ingredient for making
a Boson star because they are boson and they have
a little bit of mass and they are stable. Hey

(26:33):
did I tell you that Frank will Chick retweeted me
the other day or mentioned being a tweet. I didn't
know that. I didn't even know he tweeted you. I
felt like an action star my cell there for a
moment there, alright. So then if a Boson star exists,
it would be potentially made out of actions, which is
you're saying are stable and they do last for a while,

(26:55):
and they do have mass, and so they could get
sort of bunched together by gravity. Yeah, that's the requirements
to be the dark matter, right, you need to have
mass otherwise you can not explain in the dark matter.
And you need to be stable on cosmological time scales,
because we think dark matter sticks around a long time
and still here after all. So axions satisfy both of
those requirements. And then you have to ask the question like, well,

(27:16):
what makes it a star? And I heard you saying earlier, like, well,
it has to shine. And I know we've talked on
this podcast before about the definition of a star versus
a planet, and a star is defined to be something
that has fusion happening at its core. Here though, unfortunately
we're gonna have to be inconsistent and relax that definition
because a boson star doesn't actually shine. How convenient. So

(27:40):
what are we talking about then? That Like, if you
get a whole bunch of axons together, then gravity would
keep them in a in a ball of axions, like
a sphere of acions. What what would happen if I
get a bunch of them together? But like, even even
if I get two of them together, do they attract
each other by gravity? They would attract each other with gravity. Now,
just two particles would have infinitesimal gravitational force, And so

(28:02):
that's why we don't think about gravitationally bound particle systems. Right,
Like the proton and the electron, the gravitational force between
them is basically zero. We should compare to the other forces.
But if you have a lot of axions near each other,
then yeah, you're going to have a lot of mass
and that will make a gravitationally bound system. And so
you can get a huge serving of axions and they

(28:23):
would clump together and they would fall into each other,
and then you have to ask the question, well, like
why wouldn't you just make a black hole? Right? Remember,
a star has to have two conditions, needs to have
gravity to clump it together, and it needs to have
something to resist falling into a black hole. The reason
why our son is not a black holes because it's
resisting that through fusion. The reason that white dwarfs, which

(28:45):
is the future of our Son, aren't black holes is
not because they're burning. It's because they're actually made out
of fermions. And those fermions don't want to sit on
top of each other. Right, Fermions have this exclusion principle,
and so that's like quantum mechanics at work there, and
that white dwarfs don't fall into black holes is because
of quantum mechanics of their fermions. But axons are different.

(29:07):
Bosons are different. They can't do either of those things.
They can't make fusion to have radiation pressure. They can't
rely on the poly exclusion principle because that only applies
to fermions, meaning like if you get a bunch of
electrons at some point they'll repel each other, or protons
or you know, balls of dirt or neutrons. Yeah, I
like to make a planet, but bosons they can't sit

(29:28):
on top of each other. So I guess if you
get a whole bunch of them together, why wouldn't they
just all sit in the same point. That's kind of
what you're saying, right, And if they do, then you
would form a black hole exactly. So you need a
boson which would prevent itself somehow from collapsing. And the
way you do that here is another property of quantum mechanics.
So you're exactly right there. The reason a neutron star

(29:50):
and a white dwarf don't collapse into a black hole
is because the fermi pressure. Right, it's all these particles
not wanting to sit on top of each other. Boson
stars can't do that. But there's an another property of
quantum mechanics, the uncertainty principle, that just prevents axions and
bosons from being too constrained. Right, if you have a
bunch of axions and say they're all within the same

(30:12):
sort of location, then that creates a large uncertainty on
their position, because the uncertainty principle tells us that there's
like a minimum amount of uncertainty in the momentum and
the position of these particles. So if you can strain
their position and their momentum becomes uncertain and then they
fly off, the quantum mechanics sort of resists having these
things collapse. What do you mean They can't merge like

(30:33):
they can be used together like fermions. Well, not all
fermions confuse together, only some, only some, but yeah, but
bosons typically don't interact with themselves, like photons don't interact
with other photons. Photons don't merge together to form something else.
Some bosons do, like gluons or higgs. Bosons have a
self interaction, but most bosons, including axions, don't interact with themselves.

(30:57):
They just like don't even see each other, and unlike
add up, Like if you have two in the same spot,
don't they just add up. They don't add up because
they don't like fill energy levels. Right, they can all
be in the same energy level. That's not a problem
for bosons, and they don't have any interaction. You know
that field, the axon field doesn't interact with itself at all,
just the same way photons don't. Photons, for example, only

(31:18):
interact with charge particles. Right. They will ignore neutrons and
neutrinos and anything else that has zero electric charge, including
their photons, and axions are the same way they ignore
all other axon. If I have two photons in the
same spot, then they just become a bigger photon or
are they still technically to fold doon, there's still two photons. Yeah,
And I should add here that there are actually a

(31:40):
few different flavors of axons, since they are a hypothetical particle.
Right now, we're talking about the ones that don't interact
with themselves or with photons, But there are other versions
of these theories where they have some small self interaction
and they can feel photons a little bit. Those variants
can also make boson stars, and sometimes that self interaction

(32:03):
can help if it's repulsive, because the axons might repel
each other and then form that outward pressure to keep
the axon star from collapsing. All right, well that's our
main imaginary candidate for these imaginary stars, the axon, and
so that's how you would make one. But let's talk
about whether or not we actually see them out there

(32:24):
in space and what they would be like. But first,
let's take another quick break a right. We're talking about
boson stars that may exist out there in the cosmos,

(32:46):
and if they do, they might be made out of axons,
which themselves may or may not exist. I feel like
we're stacking imaginaries here. The imaginary concepts exclude each other, Daniel,
do they add up? Confused? Can they merge? These bonkers
concepts are all boson, so we can have an infinite
number of bonkers nous. I see, they don't exclude each other,

(33:07):
so you can just stack him infinitely exactly until we
get a bozo star. All right, Well, let's say that
the axion does exist, and let's say that you could
somewhere out there and get them all together enough to
make some sort of axon object. That still doesn't tell
me how that makes it a start, Like, why isn't
wouldn't be called an axion planet or an axion you know, Well,

(33:32):
if you were around when they were deciding on the
name of these things, then that would have been a
good idea axion planet. I think that makes more sense,
you know, because the planet is a nonfusing blob of
stuff out there in the universe made out of basically whatever.
So yeah, these axions are also they're not fusing, they're
not glowing, they're not giving off any light. They're just

(33:53):
a stable collection of axions that are resisting collapsing into
a black hole. So I think they called it a
star just to sort of make it sound awesome, not
because it's actually doing it really? Was that a in
the physics paper in there, I don't know. I mean,
in the same way our neutron stars stars, right, they're

(34:14):
just a big collection of neutrons that aren't collapsing into
a black hole yet. But they're not glowing, right, They're
not using there's no radiation being emitted there. So the
same way like white dwarfs, right, we call those stars.
So yeah, astronomy has got support to do. I think
what you're saying. That is it? In physics, there are
no standards for being a star. Anyone can be a star.
It's like our society today. You need an Instagram account. Yeah,

(34:37):
for famous parents. Being a physicist is easy. It's no
big deal. No, I'm saying, I cannot defend the naming
of this thing as a star. It's just there's nothing
I can say about it, and that makes any sense.
But I guess you're saying that it is possible theoretically
to have a whole bunch of axons together in the
same spot without collapsing into a black hole. And what
keeps them from collapsing? Is this on certainty principle? He said, yeah, exactly.

(35:02):
As he's trying to constrain them to be in a
smaller and smaller location, the uncertainty grows on their energy,
and that essentially resists them being constrained. So the uncertainty
principle resists them from being collapsed too far into a
tiny little spot. So how big would an actium planet
evolve be? Like, if you get a whole bunch of
them together, would it be just super tiny or would

(35:24):
it actually be the size of a planet. It's a
great question, And it depends on the mass of the
boson star, and so a more massive boson star would
be larger have a similar sort of structure two black holes,
right or more massive black hole than just becomes larger
and larger and larger sort of in volume. And so
boson stars in the same way would resist collapsing. And

(35:44):
the more bosons you have, the more resistance there is,
and so they would just sort of like grow larger
and larger. But the question is, like, you know, are
there boson stars and if so, how big are they? Yeah,
because I feel like you're saying that the it's the
uncertainty principle that keeps him kind of from collapsing. Can
you have an uncertainty principle the size of a planet,
like that's a big ball of uncert is a big

(36:07):
ball of uncertainty, Yeah, exactly. But you know it also
applies locally and not just globally, so you can have
like patches of these things where you have bosons lying
on top of each other. So yeah, I think it
certainly could apply to something the size of a planet.
I mean it does also for white dwarves righting, for
neutron stars, there you have, like quantum mechanics at work
preventing particles from overlapping on top of each other, providing

(36:30):
the resistance to collapsing into a black hole. Does that
apply to photons to like? Can photons also be stacked
like that? Does the uncertainty principle also prevent photons from
being on top of each other? Yeah? Absolutely. If you
try to localize photons in the same way, then it
will prevent you from knowing their energy in exactly the
same way. The uncertainty principle applies to all quantum particles.

(36:51):
It might be easier to understand the uncertainty principle if
you think about it in terms of temperature. Quantum mechanics
prevents anything from going to absolute zero temperature because there's
always a minimum energy. Otherwise you'd know a particle's momentum
and location at once because it'd be frozen in place.
So there's a minimum temperature for any collection of particles.
And that's quantum mechanics keeping something from collapsing into a

(37:15):
single dot. So the way you, for example, you would
make a black hole out of photons is not by
trying to squeeze a bunch of photons that already exist
into the same location, but by overlapping laser beams on
top of each other, the photons from different directions are
all coming together in the same place. Well, let's say
that these stars exist, these boson stars exist. What would

(37:36):
they be like? Could we see the one, would we
feel attracted to them? Would they, you know, burn our
eyes if we look at them? They would actually look
a lot like black holes because they are dense gravitational objects.
There contortions in space time right due to the mass
of all the axons, And they're not glowing. They're not
giving off any light or there's no fusion happening inside

(37:58):
of them, but they're not black right. Light can escape them,
so there's no event horizon. But they're sort of like transparent.
In fact, they're more like transparent holes than black holes.
It's weird to think that a whole is transparent, because
the aren't all wholes transparent. Technically, I suppose that it
would be a non weird hole for once. They would

(38:18):
be essentially invisible, but they would distort the light around them,
so it's sort of like just being seeing a big
lens in the sky. It would look a lot like
dark matter, right, dark matter you can't see visually, but
you can detect that it's there because of its gravity,
and so Boson stars would be the same. They would
distort space around them, bending the path of light for example,

(38:39):
so you would see gravitational lensing and all sorts of
other weird stuff. There wouldn't be an event horizon I see,
but wouldn't They wouldn't block or reflect, like like if
I have a bunch of axons there and I shoot
a laser beam into it. With the laser beam, just
shoot right through it. It wouldn't interact with the with
the axons. If you shot a laser beam into a
Boson star, then no, nothing would happen to go right

(39:00):
through because photons and axons don't interact with each other
for some theories of axons. There are other versions of
axons where the photons and axons can interact a bit.
But here we're thinking about axions as dark matter with
no reaction to photons. The only effect would be gravitational.
If you shot your laser beams sort of near the
Boson star, it might curve the path of your laser.

(39:22):
It would bend your laser, but the axions and the
photons is quantum particles don't interact, I see, and what
these things need to be huge? Or could you have
a small Bozon star? We don't know. Actually that's a
great question. I think they might be really huge. In fact,
there's some speculation that some of the black holes at
the center of galaxies might actually just be Boson stars.

(39:43):
But there's also the possibility that you could make them
to be fairly small. In the same way that like
black holes you can make to me, really really really small,
you could also make Boson stars and it's fairly small
helping as long as they were compact enough. All right, Well,
so there would be basically transparent. And it's kind of
like could they have planets other planets, like real planets

(40:03):
orbiting around them? Absolutely they could, and they might also
not be transparent for very long because, for example, think
about what happens if you toss a banana and a
Boson star. What happens, Well, it passes through the axons
and it falls towards the center because of the gravity,
and then it just sort of stays there right like
you just fall into it and not be able to escape.

(40:25):
It has the gravitational pull of something else. With the
same mass, and so things would fall into the core
of it. It would collect normal matter at its core.
It wouldn't get crushed or anything. Yeah, absolutely, it might
get crushed. Your banana might not survive, but it also
wouldn't escape. And so if these boson stars are near
other matter, then that matter might fall into them and

(40:47):
that would be visible. Oh you mean, like in the
same way that dark matter, for example, kind of helps
gather galaxy. Yeah, and acon star could help gather banana
because the way stars tell us where dark matter is,
bananas tell us where both stars are perfect analogy. And

(41:07):
that would be super real because you see a banana,
but it would have like the mass of a black
hole exactly, like the most powerful banana in the universe,
and it would attract other bananas and monkeys and r maybe,
and you would also get other gravitational effects, like it
might have matter swirling around it, the same way that
black holes do. If you have nearby gas, it would

(41:30):
get pulled in by the gravitational field. But it doesn't
always collapse in, right, Not everything near a black hole
automatically falls in because it's spinning. So some things instead
of falling in gather into this accretion disc, and a
Boson star might also get an accretion disk, and it
might have radiation from that accretion disc. And the way
that we detect black holes normally is we see like

(41:53):
gravitational influence and an acretion disc and like signals from
that accreation disc of incredible gravity tational stress. Those are
also the signals of a Boson star. So how could
we tell the difference? Or is it even possible that
black holes are made out of bosons? Like you could
throw bozons into a black hole and it would grow too, right, Yeah,

(42:16):
you can throw anything into a black hole, and you
can make a black hole out of anything, So it's
possible that black holes have a lot of bosons or
axons in them. Certainly, how could you tell the difference
between a black hole and a Bozon star. You'd have
to look really directly at it, because a Boson star
doesn't have an event horizon. So, for example, when we
directly image that black hole and we saw the shadow

(42:36):
of the black hole, we saw the back of the
event horizon in the front of it inside the accretion disk.
That's pretty clearly not a Boson star because there's a
black spot in the middle. But if you looked at
one of these things directly and you didn't see the
black spot, if you saw gas all the way through it,
then you think, oh, that's probably a Bozon star. All right, Well,
then how could we see these hypothetical Bozon stars if

(42:57):
they exist? And there are two ways. One is direct
imaging of them, right, just look at black hole candidates
and see if you can see the event horizon. If
you can't, then it might be a Boson star. There's
another way, which is maybe easier, because directly imaging black
holes is hard. You know, We've been working on it
for a long time and only done it for one
and that's it. Looking at the gravitational waves. Gravitational waves

(43:19):
are generated from spinning black holes or from things moving
around black holes, like neutron stars and stuff like that.
So because there's a slightly different structure in the field,
because boson is of a different distribution than like a singularity,
the heart of a black hole is a slightly different
pattern in the gravitational waves, so you might be able
to detect the difference. There's some recent paper is talking

(43:40):
about like exactly how to look for gravitational waves that
come from Boson star collisions rather than black hole collisions.
So this is an imaginary event featuring two imaginary objects
made out of an imaginary particle made out of a
lot of imaginary particles. Yes, exactly. I feel like now
we're going deeper into the rabbit hole here, we're being

(44:02):
incepted to like level four and there's even level five
inception there, which is, like, how did these things get
made in the first place? Even if axions are real,
even if all the laws of physics work the way
we're talking about, so that if you put axons in
the same place they would make a Boson star. Are
their conditions in our universe for that to happen? Should

(44:24):
it arise? So it's not easy to imagine how you
would make that many axions. Really, you got to go
all the way back to the Big Bang and say
maybe during the Big Bang there was some crazy fluctuation
and these things got made primordially and like before most
particles were made, when maybe even early black holes were made,
that you've got these weird collections of bosons created quantum

(44:46):
mechanically during the Big Bang, and those are the seeds
of current Boson stars. Because I guess you can't think
of any circumstances right now in our universe in which
you could get that many bo's. Yeah, and then maybe Daniel,
we're just imaginary imagining these imaginary things. My brain feels
like it's filled with banana sometimes, which might be imaginary

(45:08):
themselves if they weren't so delicious. Good thing. Physics is
so easy, right, all right? Well, so that's a Boson
star and that's pretty interesting. And now are there people
looking for these right now? Is this something that people
are taking seriously or is it still kind of in
the back of the conference room. There, It's definitely in
the back of the conference room, but there are also
some people taking it very seriously, which is sort of

(45:29):
the way in physics. You've got like the mainstream stuff
people are working on, and then you've got the people
thinking in the back of the room, going, m what
about this other weird thing? And sometimes those ideas are right.
I'm really glad that in science were open to all
sorts of crazy ideas. And there are definitely people dedicated
to this topic, you know, running detailed simulations of what
Boson stars would look like and trying to understand like

(45:52):
the plasma loops that might form around them, and how
you would see those signals and gravitational wave detectors of
the future. So it's definitely something people are thinking about.
And how many of those can you fit into a
clown car or how many of you are willing to
get into one for the sake of physics. That's philosophy.
That's philosophy, man, that's not science. See, that's the other
imaginary science. Or maybe it's psychology. I don't know. All right, Well,

(46:16):
it's always interesting to think about what could be out
there in the universe, you know, like we have all
these rules, and if you sort of think about those
rules enough, you sort of come up with these weird
things that may or may not exist. Yeah, And it
could be that we are in the era before the
discovery of Boson stars, when people are just thinking about
what could be out there in the universe. So if
you are a budding astronomer or astrophysicist and you're thinking

(46:39):
the universe has all been discovered, there is still plenty
of crazy stuff out there for you to find. Yeah,
because at some point even thinks like black holes in
dark matter, they're all imaginary. Back at the conference room,
these are now just Nobel prizes waiting to be one. Well,
we hope you enjoyed that. And the next time you

(47:00):
look at into the Start, I think about what you're
not seeing that could be out there sucking bananas and
turning them into smoothies. Thanks for joining us, see you
next time. Thanks for listening, and remember that Daniel and
Jorge explain the Universe is a production of I Heart

(47:22):
Radio or more podcast or my heart Radio. Visit the
I heart Radio app, Apple Podcasts, or wherever you listen
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