December 31, 2020 • 45 mins
Daniel answers questions from listeners like you! Got questions? Come to Daniel's public office hours: https://sites.uci.edu/daniel/public-office-hours/
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Episode Transcript
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Speaker 1 (00:08):
Isaac Newton was very clearly a smart guy. He made
huge leaps and understanding of optics and gravity and calculus.
But one of the biggest mental steps he took was
applying the laws that we have down here on Earth
to what's going on up there in space. His biggest
idea was probably that there should only be one idea,
(00:31):
one set of physical laws, but that it should cover everything.
And here we sit on a tiny, isolated rocking space
trying to make rules that explain the whole universe, only
able to see a tiny little bit of it and
study with our hands and our tools an even smaller bit.
(00:51):
It's sort of like visiting the zoo and only seeing
the insect exhibit, but then making laws that are supposed
to describe how elephants and amphibian's work. So ask yourself,
how likely is it that our ideas are actually universal? Hi?
(01:22):
I'm Daniel. I'm a particle physicist, and I have an
infinite list of questions about our probably infinite universe. And
welcome to the podcast. Daniel and Jorge explain the universe
of production of My Heart Radio. We talk about things
happening far far away, and we talk about things happening
under your feet. We ask questions about the very beginning
(01:43):
of time, and we ask questions about the very nature
of time and the end of time. We ask questions
about the entire universe because we think that curiosity and
asking questions is universal. We think everybody out there has questions,
and everybody deserves to have their question is explored, if
not answered, because not every question has an answer to
(02:04):
it so far, but questions really are at the heart
of science, and that's why on today's program, while my
friend collaborator and co host Jorge can't be here today,
I'm gonna take the opportunity to gather up a bunch
of questions asked from listeners and try to answer them.
We're always asking people please send us your questions. If
(02:25):
there's something you don't understand about the universe, something you've
read and didn't quite follow, something you've thought through that
didn't quite make sense to you, please send it to
us because we cherish those questions. Those questions are our
opportunity to help people understand what we do and what
we don't know about the universe. And remember that everybody
(02:46):
out there who was asking questions is basically an armchair physicist.
If you are trying to wrap your mind around the universe,
you're trying to make one holistic sense of understanding of
how things work. If you've read something somewhere and you're
trying to make it agree with something else you used
to think, or something you read somewhere else, or something
your friend told you, that's doing physics. You're trying to
(03:07):
unify your understanding. You're saying, I need to have one
set of ideas that describes the entire universe that explains everything. Now,
of course, we don't know if it's possible to describe
the entire universities in one set of laws. We don't
know if it's possible for humans to do it, or
maybe take some super alien intelligence or some artificial intelligence
(03:30):
with super incredible powers. But that doesn't stop us from trying,
because it's our dream that we could encapsulate the entire
workings of the universe somehow inside the puny human mind.
So please, if we haven't answered a question that's in
your mind, send it to us two questions at Daniel
and Jorge dot com. We answer all of our emails
(03:50):
and sometimes we put those questions here on the podcast.
But If you don't like writing emails or you don't
want to engage with us on Twitter, we have other
ways to get your questions answered. You can check out
Daniel's public office hours, look at the website for the podcast,
or go to sites dot you see I dot E,
d U slash Daniel, you'll see a link. Therefore, when
(04:12):
Daniel has public office hours, he hangs out on Zoom
and answers questions about physics and life and the universe
and everything from people like you, people who have thought
about stuff and have a nagging little question that they
can't find the answer to using Google and they just
have to know how it works, all right, And today
on the podcast will be answering questions from listeners from
(04:34):
all over the world. Our first question comes to us
from Germany. I have a question concerning doc energy. Does
it violate the law of energy conservation? It seems to
come out of nothing and getting bigger and bigger. Thanks
a lot, all right, Thank you and Juras from Germany.
This is a beautiful example of what I was just
(04:55):
talking about about applying our ideas about how things work
in the universe and taking them to the extreme and
saying does this really work everywhere. Is this a universal law?
Is there some part of the universe that seemed to
break this rule which would make it not universal? And
one of the most fundamental things we thought we understood
about the universe was this idea of energy conservation. Of course,
(05:18):
a hundred years ago, we thought other things were conserved,
like mass. We thought that stuff was conserved, that you
could move it around, you could switch it up, you
could rearrange it like lego bricks, but you couldn't create
or destroy mass. Now, of course we know that's not true.
And the lesson we learned from that, from the lack
of mass conservation, is that mass is not a fundamental
(05:40):
element of the universe. It can be created, it can
be destroyed. You can have more mass, you can have
less mass. It's not something which we should consider sort
of on the list of fundamental descriptors of the universe.
And that's important because what we're doing with physics is
trying to drill down to the most fundamental, the simplest description,
Because we imagine if one day we are looking at
(06:03):
a list of the fundamental elements of the universe, of
things that define the universe and completely explain the universe
that we will somehow be revealing the nature of the universe.
So we don't want anything on that list which isn't fundamental.
You don't want that list to have like strings and
energy and then ice cream, right because ice cream can
be described by all the other elements already on the list.
(06:24):
So we want to strip it down to a sort
of most minimal set of rules, most minimal set of
things you need to describe the universe. Having left our
list of things that describe how the universe works tells
us something about what the universe isn't. It isn't a
place that cares so much about mass. However, energy seems
to have retained its exalted stature as a quantity which
(06:48):
is conserved. And you know what is energy conservation anyway.
Energy conservation is the statement that you can calculate this
thing about nature. You add up all the energy in
a system, all the ways that things can move or
wiggle or store energy, and then you let a bunch
of stuff happen. Things collide, things explode, things slash around, whatever,
and you add up all the energy again and it
(07:10):
should be the same. So it's sort of a statement
that energy is fundamental to the universe. You can move
it around, that you can change it from one thing
to another, but that you can't get rid of it,
that it's inherent, that it's fundamental, that it's a deep
part of what makes the universe the universe. So if
energy is not conserved, then that tells you what It
(07:31):
tells you that maybe energy isn't actually important to the universe.
Maybe energy isn't on that list of fundamental elements we
think are needed to define and describe the universe. So
Andreas is doing exactly what a physicists should be doing,
thinking about ways energy conservation might be violated. We know,
for example, that if you roll a boulder up a hill,
(07:53):
you're spending energy in your muscles, and that energy then
goes into the position of the boulder. It's gravitation in
a location is more distant from the Earth, And if
you let go of the boulder and run it back
down the hill, the energy goes from that potential energy
of the boulder into its motion, into its kinetic energy.
So we have lots of examples of where energy is conserved,
(08:13):
and people probably expect to hear that energy is conserved
everywhere in the universe and there's some way you can
do the calculation to figure out that energy is actually
conserved in the case of dark energy. So what is
he talking about. Remember that dark energy is not something
that's very well understood. It's not a theoretically well formulated idea.
(08:34):
It's more an observation. It's an observation that the universe
is expanding and that that expansion is accelerating. So we
look out into the universe and we see that the
universe is expanding, and not only are things moving away
from us, but the speed at which they're moving away
from us is increasing. You might expect the opposite to
(08:54):
be happening. In fact, physicists expected the opposite to be
happening for a long time, that things were moving away
from us, but that speed might be decreasing as gravity
very slowly pulls on things and tugs them back together.
After the Big Bang, what we actually found about years
ago now is that things are moving away from us
faster and faster, and we don't have an explanation for
(09:17):
why this is. All we have is the observation that
it is happening. There are a few sort of proto explanations.
There are ideas for what might describe it, but none
of those ideas really work so far. One of those
ideas is that there is energy in empty space, that
all of space has energy in it. For example, the
(09:37):
Higgs Boson field is a quantum field that's in all
of space, and even when it's at its most relaxed,
at its lowest level, it doesn't have zero energy in it.
That means that when you create a piece of space,
you're creating a Higgs Boson field that has energy in it.
And this is what Andreas is talking about. That dark
energy creates more space because it's not just moving things
(10:01):
through space. It's creating new space between galaxies. It's stretching
that space. It's making new space. And when you make
new space, it comes with new energy. So it seems
an awful law like dark energy is in fact violating
conservation of energy because as you make more space, you
(10:21):
are increasing the total volume of the universe. And if
every cubic meter of space has a certain energy, then
by increasing the volume of space, you're increasing the total
energy in the universe. How does that not violate conservation
of energy? Well, in fact it does, and as energy
conservation is not guaranteed in our universe, and this is
(10:43):
one example. As space expands, the energy increases because you
get more dark energy, which means overall more energy. There's
also another example, which is that energy can decrease when
space expands. If you have a photon flying through space,
for example, from the cosmic microwave background radiation, then what
happens when space expands. When space stretches, Well, that photon
(11:07):
gets red shifted. Its wavelength gets longer because space has
gotten stretched. Right, Imagine you draw a wiggle on a
sheet of paper and then you stretch that paper, the
wavelength gets longer. But for photons, the energy and the
wavelength are very closely connected. One defines the other. Higher
energy photons are those with shorter wavelengths, and so if
(11:29):
you stretch the wavelength of a photon, then you decrease
its energy. Where does that energy go. It doesn't go anywhere.
It just goes away. So we have two examples of
the violation of the conservation of energy, both coming from
space expanding. And that's the clue as the clue that
tells us why energy might not be conserved, and most
(11:52):
of the conservation laws in physics, most of the things
that are conserved that are not changed when you let
things bang around and smash into each other come from
some kind of symmetry. This is very deep result in
physics called Noether's theorem from Emily Norther, who developed it
more than a hundred years ago, and she discovered that
every time you have a symmetry, like every time you
(12:13):
can take space and rotate it and still get the
same laws of physics, or move your coordinate system over
by ten kilometers and still expect the same law of physics,
or fast forward things by a hundred years and still
expect the same laws of physics. Every time you can
apply some sort of translation or rotation to the universe
and not see any change in the law of physics.
(12:35):
That's a symmetry, and every symmetry has some kind of
conservation law that comes from it. So, for example, the
fact that space is the same everywhere, that the laws
of physics apply here and somewhere else, that gives you
the law of conservation of momentum, and the fact that
you can rotate space, that there's no preferred direction, that
physics should work the same in every direction. That's why
(12:57):
we have conservation of angular moment them and it's the
symmetry of the universe with respect to time is what
gives us conservation of energy. The fact that it seems
like the universe should work the same now as it
does in a hundred years and a thousand years ago
is what gives us conservation of energy. But that only
works if we expect the same rules to apply now
(13:20):
and in a hundred years and in a thousand years.
That only works if space is essentially static, if it's
not changing, if space is the same now and in
a hundred years and a thousand years ago. But we
know that it's not because we know that space is expanding.
So conservation of energy is something we expect to apply
in a static universe where space is not changing. In
(13:43):
our universe, however, space is expanding, and it's expanding quite rapidly.
Expansion is not a small thing in our universe. Seventy
percent of the energy budget of the universe goes towards
the expansion of space time, so when space is expanding,
energy is not concerned. Now, we did a whole podcast
episode about this conservation of energy, and there is one
(14:05):
way that you can sort of rig up a calculation
in which you get negative energy from gravity that might
account for some of this, but most cosmologists think it's
sort of a band aid and theoretically doesn't hold together.
And you're interested in more details and not check on
our whole podcast episode about conservation of energy. But congrats
to you Andrea's for figuring this out, for applying your
(14:27):
understanding of physics to crazy scenarios far beyond your living
room and coming up with a contradiction. And those contradictions
are what leads to questions, and those questions are would
lead us to deeper understandings about the universe. So keep
asking questions. Thanks very much for sending that in. All right,
I have more questions from listeners I want to get
(14:48):
to but first let's take a quick break. All right,
we're back, and this is Daniel and I'm here today
on my own answering questions from listeners. I love when
(15:11):
people write to us. They send us amazing questions, things
that they have been thinking about the universe and that
they can't figure out via Google, and they don't happen
to know a physicists they can ask these questions of,
so they send them to us. And I have a
bit of an embarrassing backlog of questions from listeners that
I really want to get to, and so while Jorge
isn't here, I'm gonna plow through a bunch of these
(15:33):
and try to catch up to our backlog. So thanks
very much to everybody who's senting questions. Here's the next
question we're gonna answer today. Hey guys, this is Jeff
from Los Angeles. My question relates to the period of
inflation after the Big Bang. I know you said the
universe expanded by a factor of ten to the thirty
in the small amount of time of ten to the
minus thirty. How do you explain that if nothing can
(15:56):
travel faster than the speed of light. I also want
to know if the edges of the universe were expanding
at this crazy fast speed, and it was expanding through nothing,
then what's slowing it down? Why isn't the universe still
expanding at that crazy rate of inflation? I look forward
to the answer. Thanks a lot, all right, Jeff from
(16:16):
l A who's basically an amateur cosmologist. Thank you for
thinking deeply about the universe and for trying to reconcile
what you've heard about the early days of the universe
with what you understand about how the universe works. Again,
that's exactly what doing physics means, so let's get to it.
The first part of your question was if the universe
expanded by a factor ten to the thirty intended to
(16:39):
minus thirty questions, how is that possible? Given that we
know that there's a very hard limit on how fast
things can move through space, which is the speed of light.
It's a great question. Another way to think about this
question is how did the universe get so big? I mean,
the universe is about fourteen billion years old, but the
(17:00):
size of the observable universe, the distance to the furthest
things that we can see, you might expect to be
fourteen billion years times the speed of light, which would
be fourteen billion light years, but it's not. It's much
much further than that. We can see things that are
about forty five billion light years away, So the size
(17:22):
of the observable universe is about ninety billion light years wide.
How is that possible? How is it possible to see
things which are further away than the speed of light
times the age of the universe. How did that stuff
get there so far? How did the universe expand faster
than the speed of light. So it's a wonderful question.
(17:42):
And the key concept you need to know to understand
this is that there's a difference between moving through space
and expansion of space. So moving through space is the
kind of thing you're familiar with. You move through space
every day. When you get out of your bed and
you go for a glass of water in the middle
of the night, you are moving through space. When you
(18:02):
throw a baseball really really fast. When you get on
your spaceship and you try to travel to a nearby star,
you are moving through space. When you turn on your
flashlight and you shine it at the moon, you are
sending photons through space to the moon. And there is
in fact a very hard limit on the speed at
which things can move through space, and that's the speed
(18:22):
of light. In a vacuum. Nothing, no information at all,
can move through space faster than the speed of light.
That includes neutrinos, that includes everything, that includes quantum information.
It's a very tough rule, and breaking it when undermine
special relativity, which we're pretty sure as an accurate description
of space time in our universe. All right, So that's
(18:44):
moving through space, but that's different from the expansion of space.
The expansion of space means stretching space itself. So imagine,
for example, you are one meter apart from your friend.
You have a meter stick, and you measure exactly how
far apart you are. You could take a step back.
That would be moving through space. But you could also
(19:05):
expand the space between you. You could take the very
universe and stretch it so that now you guys are
two meters apart without having moved through space at all. Right, Remember,
space is not just the backdrop on which things happen.
It's not the stage on which the acts of the
universe are played out on. It's a dynamical thing. It's stretchy,
(19:28):
it's like goo. It responds to the presence of mass
and energy. It bends, it twists, it can expand, and
it tells things how to move. So space is really
part of the universe. It's not just like some fuzzy
abstract concept, some set of glowing axes in our mind
that we just impose on the universe to try to
(19:50):
make sense of it. Space really can do a bunch
of weird things. You already know this because you know
that gravity is not just a force. It's actually the
curving of space. Right, The reason that the Earth goes
around the Sun is not because gravity is a force
which is tugging on it, but because the presence of
the Sun changes the shape of space in its vicinity,
(20:11):
so that an object moving in a straight up inertial
path will move in a circle around the Sun. That's
because space can bend and twist, changing the relative distances
between things. So, for example, a photon a beam of
light always takes the shortest path between two things, But
the shortest path between two things isn't always what you
(20:31):
imagine to be a straight line, because the shape of
space can be complicated between two points. The same way
an airplane going from l A to London takes the
Great Circle route right, which seems like a curve, is
actually the shortest distance between two places on a curved surface,
which is why gravity can influence even things that don't
have mass. All right, So that tells us that space
(20:53):
can do things, and it can stretch, and it can expand,
and so that's exactly what happened in the very early universe.
It wasn't an explosion like a tiny dot of stuff
and then everything exploded out from the center. Instead, it
was an expansion of space itself. Huge amounts of new
space were made everywhere all over the universe simultaneously. So
(21:17):
that doesn't make it easier to understand the fact. It
makes it even more buying boggling that this happened, That
every unit of space was blown up by a factor
of ten to the thirty intend to the minus thirty seconds.
It's an incredible moment in the history of the universe.
It's an incredible idea to even have in your mind.
Imagine coming up with this Bonker's notion and then realizing
(21:39):
that actually it's the story that makes the most sense
in the universe. And you might ask, well, how do
we know. How do we know that the universe expanded
in this way, that it didn't just explode from a
tiny dot and spread out through the universe. Well, answer
number one is that it would be impossible. As you say,
it's not possible for things to travel that are in
(22:00):
that short distance because of the limitation of the speed
of light. It's against the rules. The only way to
get things that far apart in that short amount of
time is to create space between them, is to expand
the space between everything. But it's more than that, because
Explosions and expansion look different. Explosions are like a bomb.
(22:23):
You push everything out from one central location and send
it flying in every direction. And if there is an explosion,
you could look at the direction things are flying and
you could track them backwards, and you could point back
to the center. Right. If you come upon an explosion,
you can look at the path of the debris and
you can figure out where the bomb was. That's not
true for an expansion, and expansion is more like a
(22:46):
loaf of bread rising in the oven, where everything is
growing at every point simultaneously, assuming you're not a terrible baker, right,
and that your loaf is expanding smoothly. And that's what
we see when we look out into the universe. We
see these galaxies and they are rushing away from us.
They are moving away from us, and they're moving away
from us faster and faster every year. So either we
(23:07):
are at the exact center of the universe by some
incredible cosmic coincidence there was an explosion and we happen
to be right at the center of it, or it
looks this way because it's an expansion, and it would
look this way at any point in the universe. See.
The way an expansion looks is that it always looks
like you're at the center of it. No matter where
(23:29):
you are in a loaf of bread, if you look
around you, everything is growing away from you. Imagine putting
a bunch of chocolate chips into your loaf of bread
and tracking their emotion everywhere inside the loaf of bread,
of course, except for the crust. You would see this
chocolate chips moving away from you. So that's what we see.
We see that the universe is expanding, not that it's
exploding out from a tiny dot. And this expansion is
(23:52):
actually really important to sort of the state of the
universe as we know it, because when the universe began,
we think it began very very smooth, like totally homogeneous.
Everywhere was exactly like everywhere else. And why wouldn't it
be right when the universe is created, Why would you
have one spot that's like denser than another spot. The
problem is, however, a universe like that that's created perfectly smoothly,
(24:15):
nothing very interesting ever happens in that universe. There's nothing
for gravity to do in that universe because there's no
spots that are heavier or denser than anything else, which
is what gravity needs to sort of like seed the
structure to start coalescing things together into stars and planets
and galaxies and all that good stuff. So how did
the universe get any structure? Well, it was perfectly smooth,
(24:38):
except down to the quantum level. Of the quantum level,
there are always random fluctuations. Every point in the universe
gets a different random fluctuation, so you get these really
super duper tiny little variations in the density of the
universe due to quantum mechanics, and then inflation steps in.
Inflation takes those tiny little quantum fluctuations and it blows
(25:00):
them up to the macroscopic scale, makes things which were
invisibly small somehow suddenly now huge. Right, takes a meter
stick sized thing and it blows it up to a
trillion light years. It takes something which was sub atomic
and it makes it macroscopic. So now those random quantum
fluctuations are not small. They're pretty big, and they're big
(25:21):
enough to see the structure of the universe. So the
reason that we have a galaxy over here and then
over there it's empty space is because of a random
quantum fluctuation in the very early universe, which was expanded
out into something macroscopic that see to the structure of
that galaxy and allow gravity to pull stuff together to
(25:42):
make something interesting, to make me and to make you,
and to make the sun that warms our toes. Now,
the second part of Jeff's awesome question is that if
the edges of the universe were expanding through nothing, what's
slowing it down? Why isn't it still expanding at that
crazy rate? So lots of really good angles on the question.
First of all, we don't know if there is an
(26:02):
edge to the universe. I think in his mind Jeff
might be imagining an explosion, an explosion which has a
wavefront which is moving through the universe and then slowing down.
But we don't know that there was an edge. We
don't know if there is an edge. I think the
cleanest way to think about these things is to think
that the universe is infinite. We don't know that's true,
but it seems somehow more natural to have an infinite
(26:24):
universe than to have an edge. And then you can grapple.
Instead of thinking about the whole universe, just think about
a chunk of it and think about sort of the
density of that part of the universe. So I imagine
the whole universe created infinite at its birth as a
very very dense place, and then expanded suddenly, very rapidly,
using inflation. So there's no edge there. Everything is moving
(26:47):
away from everything else. He also asks, if there's no edge,
what's slowing it down? Why isn't it still expanding at
that crazy rate? Awesome question. I wish I knew the answer.
There was this incredible moment of inflation in the very
early universe, this rapid expansion in a very short amount
of time. We don't know what caused it. We have
(27:07):
ideas about ideas were sort of proto ideas for what
might have caused it, crazy particles and field called the
infloton field, but those are sort of placeholders to have ideas.
We don't really have any well worked out, super well
formulated ideas that actually come together mathematically to explain inflation.
So because we don't know what started it and what
(27:28):
sustained it, we also don't know why it stopped. We
just know that it started and we know that it stopped,
but the expansion itself has not stopped. It was very
rapid in the very beginning, and then it was very
slow for a while, but about five billion years ago
it started to pick up again. Dark energy took over
and it started to accelerate the expansion of the universe
once again. And this expansion is very similar to what
(27:51):
happened in the inflationary period of the universe. It's not
nearly as rapid, but the sort of stretching of space
is the same concept. We don't know if there's a
relationship between the mechanism or the reason for why space
is expanding now and why space expanded in the very beginning.
We're pretty clueless about what dark energy is. Again. We
have a few basic ideas for what might explain it,
(28:14):
but none of them hold together mathematically, so most of
this is just an observation. We see that this happened,
we can't explain why. So it is still expanding at
a crazy rate, not as crazy, but we don't know
the answer to the question why the universe stopped inflating
and why it's not inflating at that crazy rate today. So, Jeff,
the answer to your question is that the universe expanded
(28:36):
so rapidly, not by things moving through space, but by
expanding the nature of space itself, by creating new space
between stuff and why did that stop. We don't know
why inflation itself stopped, but the expansion has not stopped.
The universe is still expanding, and it's expanding faster and
faster every day. All right, Thanks for that super awesome question.
(28:57):
I love all of these ideas. I have one more
question I'm an get to today, but first let's take
another quick break. Okay, we're back and this is Daniel
(29:17):
and I'm answering questions about the nature of the universe
and how it expanded and whether it violates conservation of energy.
And our next question is a tiny bit more concrete. Hi,
Daniel and JOI I'm Tristan from Melbourne, Australia. Congratulations on
the awesome podcast. My question is about spice dust. We
(29:37):
hear about it all the time, but what exactly is
spice dust? Is it tiny gas molecules or really minute
dust particles like here on Earth or is spice dust
just a relative term? And they'm all like basketball sized
or cast sized or bigger? All right, Thanks very much
for that fun question. This is actually a surprisingly fascinating
topic when you think about do you think it's like
(30:00):
dirt and something you want to get rid of. It's
an annoyance. It gets in your way. If you zoom
in on it, you discover that, like a lot of
the dust in your house is actually left over dried
bits of human skin that makes you want to throw
up a little bit. So space dust is sort of similar.
For a long time, people thought space dust was just
like an annoyance. It was this stuff floating in space
(30:21):
which like blocked your view. I mean, if you look
out into space, it's incredible how far we can see.
You're standing on the top of a rock in space
and you're peering out in your eyeballs are absorbing photons
that have traveled billions of light years, mostly unimpeded to
get to you. It's incredible that space is as clear
as it is, so we shouldn't be complaining. We have
(30:43):
the best view in the universe. The kind of things
we see with hubble are just eye dropping the gorgeous.
But sometimes there are things that are obscured by space dust.
If you look at the center of the galaxy, for example,
they're huge clouds of dust to make it harder to
see what's going on there. And for a long time
astronomers treated space dust that way, like an annoyance, like, oh,
(31:06):
these things are shrouded in dust, so we can't see
them what's going on inside there? And if you're like me,
your curiosity is only heightened when something is hidden, something
is behind a veil, like things inside a black hole.
It just makes me want to know even more what's there.
So for a long time space dust was treated that way.
It's just something that gets in our way, something to
(31:27):
be annoyed about. However, now we see that space dust
is just sort of part of the dynamics of space,
part of the astrophysical soup that's constantly churning, making new
stars and all their crazy things, and it can actually
help us understand the structure of the galaxies and how
things work. For example, we can see space dust. It
(31:47):
doesn't just blocklight, it actually gives off its own light.
This is something I think is not widely enough understood
that everything in the universe glows. Everything in the universe
that's made that of our kind of stuff atoms, that's
only five of the universe, but all that stuff glows,
and it glows based on its temperature. The hotter you are,
(32:09):
the more energetic photons you give off, which is why.
For example, if you heat up metal, it starts to
glow and it glows at different frequencies, different colors as
it gets hotter. Everything is actually like that. Even you glow,
and if you put on infrared goggles then you can
see your body heat because of your temperature. But it's
(32:29):
not just living things. Even rocks glow, even if they're
very very cold. They glow at some wavelength. And space
dust is out there and it glows as well, so
you can see it. It's not giving off visible light.
But if you have a special camera like night vision
goggles for the universe and what we call an infrared telescope,
then you can see it. And if you point an
(32:51):
infrared telescope, for example, at the Andromeda galaxy, you can
see where in Andromeda there is this space dust. The
space does glows at different frequencies than the stars. The
stars admitted visible light and you can see them. That's
super fascinating. But then if you turn on your night
vision goggles for space, you can see the other stuff,
(33:12):
the colder stuff, which is glowing at longer wavelengths in
the infrared, and you can see it totally differently, and
Dromeda looks different in the infrared, you can see where
the space dust is, and that helps us understand like
how did and Drameda form, what's going on over there,
what are the dynamics, what things are moving against the
other stuff. So these days space dust isn't just like
(33:34):
an annoyance. It isn't just a cloud that gets in
your way. It's another thing out there that we can study.
And it turns out that there's a lot of different
kinds of space dust. Most generally, what is space dust.
It's basically anything that's out there in space that's very,
very small, right, So you wouldn't call Earth a big
speck of space dust. This is one of those arbitrary
(33:56):
categorizations in astronomy and astrophysics. Space does is basically anything
that's out there that's smaller than like a millimeter, and
it can go down all the way to like a
few molecules, but the upper edge is generally agreed to
be like a millimeter or half a millimeter, maybe a
tenth of a millimeter. Anything that size or smaller that's
floating out in space, we call it space dust. Bigger
(34:18):
than that, like a basketball or car sized thing, we
would call that an asteroid. Or a comet, or even
a proto planet or a moon or right a sun
if it gets big enough. So there's this whole spectrum
of sizes of stuff in space and things on the
smaller edge we call space and dust. And you might wonder, like, well,
why is there space dust? After all, you have these
(34:40):
huge clouds of things formed after the Big Bang, and
some of it gathered together to make stars, and some
of it gathered together to make planets, but not everything
instantly gets clean together. Right. Gravity is very patient, but
it's also very very weak, And the gravity on very
small objects, tiny little specks of up floating out in space,
(35:01):
is very weak. And other things I'm much more powerful.
One thing that prevents gravity from gathering stuff together is
angular momentum. If something is moving in a circle around
something with gravity, then it doesn't necessarily fall in the
same way the Earth orbits the Sun without falling in,
even though there is gravity tugging on the Earth from
the Sun. The reason we don't fall in immediately is
(35:24):
because we're moving in a circle. The same reason the
whole galaxy doesn't collapse into the central black hole is
because of angular momentum. That's just one example and so
that keeps some space dust from collapsing into larger objects,
and so you end up with this whole spectrum of
really dense stuff that's sort of cleared out the space
around them than a whole distribution of smaller bits, which
(35:45):
we call space dust. And we can study this stuff.
NASA actually sends planes up into the high atmosphere to
gather space dust and these big collectors under the wings
to pull it together and say like, well, what's in there.
And actually there's a huge amount of space dust out there.
It's not very rare. The Earth is traveling through a
cloud of this stuff is like one particle per million
(36:09):
cubic meters, but there's a lot of cubic meters out there,
and there's a lot of square meters on the surface
of the Earth, and so a lot of space dust
actually falls onto the Earth every year. Some of the
stuff that's lying around your house might be dead bits
of skin, but some of it might be space dust,
because there are thousands of tons of space dust that
(36:30):
reached the surface of the Earth every year. Yeah. I
just about fell out of my chair when I read
that tidbit that fact thousands of tons of the stuff
if you could like sweep up all the space dust
that hits the Earth and make a pile of it,
it would be a huge, huge mountain of space dust,
all right. So then what is this stuff? Right? What
(36:51):
is this stuff that's out there, that's floating through the
universe that didn't get gathered together into planets and rocks
and other kinds of stuff. Well, it's a big mix, right.
It's basically a big soup of leftover stuff either from
the very early universe that never got gathered together into
something else, or that has had a chance to be
part of a star or a planet and then got
(37:12):
blown into little smithereens. So one category of stuff is
star dust. Star dust are little pellets made on the
outside of stars, a little grains, for example, of oxygen
or carbon rich elements that are floating out near the
outside of stars, and that gets blown out away from
the star and so they get frozen into these little pellets.
(37:33):
Remember that in the first population of stars, we had
only hydrogen, but those hydrogen stars burned helium, and in
later generations of stars fuse that helium into heavier and
heavier stuff. So stars are the engines to make these
heavier elements, and eventually they can gather together. A lot
of this we call it ash because it's the product
of fusion. A lot of it falls to the center
(37:55):
of the star, makes a denser and denser core, which
eventually leaves the star to collapse, not all of it,
so if it gets blown out into the outer edges
of the star, and then it can get pushed even
further out and float out into space. So these little
grains of stuff produced inside stars, this is called star dust,
and this is floating out there, and a good amount
(38:15):
of the space dust are actually these kinds of grains,
and a lot of them came together to form our
star and our planet. So a lot of what makes
me and you and the Sun are actually these bits
of other stars. Now, of course, inside the Earth and
inside the Sun, they've all been melted down to their
basic elements and maybe even fused into other stuff. But
(38:36):
space dust hasn't. It's frozen. It's like a little time
capsule that tells you where it came from. And if
you can capture one of these grains of star dust,
and you can look at the relative fractions of stuff,
like how much iron is there, how much carbon is there?
Then you can get an idea from what kind of
star came from. You can read like it's ancient history
just by looking at what's inside of it. So each
(38:59):
of these is like a little time capsule that tells
us what happened. And these events are billions of years old.
You know the star dusk grains that helped form our sun,
well that was five billion years ago, So they were
produced more than five billion years ago, and they're still
floating around. Some of them coalesced into like micro meteorites,
but they don't necessarily lose their elemental structure. They just
(39:21):
sort of got like get stuck together like a big
pile of rice grains. And so if you're careful, you
can tear them apart and look at the individual grains
and still study these little time capsule from other stars. Now,
sometimes you look at these space dust capsules and you
see something really strange, and people actually predicted that you
would see this. You see things produced in supernovas. Supernovas, remember,
(39:43):
are these very special occasions. When it starts, gravity overcomes
its pressure and it collapses very very rapidly, this implosion,
which then leads to an explosion where it throws crazy
stuff through space. Well, in those moments of implosion are
intense moments of fusion, and these are situations that allow
for the creation of other kinds of elements and different
(40:04):
mixtures of elements than what you would expect from the
normal production you get in a star. And sometimes these
things get thrown out during the supernova and they're like
little time capsules, a little like samples from what's going
on deep inside is supernova. So these supernova grains are
super awesome vines because they're not created nearly as often,
(40:25):
and they're this little time capsule from this incredible moment
during one of the most violent acts in our universe,
so they're super fun. And then a lot of the
other space dust is just floating tiny rocks. Basically. Some
of them are carbonaceous, you know, other ones have iron
or sulfur or nickel. Some of them are silicates, which
means they're basically bits of sand, and they have all
(40:46):
sorts of irregular shapes, you know, just like any random rocks.
Some of them are kind of fluffy, loose amalgams. Some
of them are very compact. Some of them accumulate little
layers of ice around them, so you might expect them
to be like super mini comets, and and the sizes
of them differ. Right, they go all the way down
from the tiny, tiny little grains up to you know,
less than a millimeter or so. And this is important
(41:08):
because the size determines how you can see them. Like
pretty big grains actually reflect light, so if the sun
is behind you, you could see them the way you
see the moon, like comes from the sun bounces off
of them and then back to you. But if they're
really really small, then they don't reflect light. They just
sort of deflected a little bit, which means it's only
(41:29):
easy to see them if the light is behind them.
They have to be back lit, and this is why,
for example, we didn't really know that Jupiter had rings
and had rings made of dust until we got cameras
out past Jupiter and you could look back and you
could see those rings of dust back lit by the
sun because they only deflect the light a little bit.
(41:51):
So it's important how big they are, and it's also
important their shape. We think this space dust is not
just around the Solar System, and not just in the galaxy,
but also between the galaxies. It's basically spread out everywhere,
and it's actually really valuable because these grains are not spheres.
They're like weird, all blong shapes. So what they do
(42:12):
is they tend to align with magnetic fields. They're like
tiny little needles and they tend to line up with
magnetic fields. And people have been studying magnetic fields through space,
wondering like, is their magnetic fields all through the galaxy?
Are there magnetic fields between galaxies? Are there magnetic fields
in deep space that were created during the Big Bang?
(42:32):
This is called the primordial magnetic field. We have a
whole podcast episode about it. If you listen to that.
What you learn is that these dust grains line up
with magnetic fields, which is important because it changes how
light moves through it because these dust greens are now polarized,
and so we can use space duft to sort of
track the magnetic fields in otherwise empty portions of space,
(42:56):
sort of like sprinkling magnetic filings on a sheet to
see if the the magnetic field there. It's actually good
that space dust is sort of everywhere, because if space
was truly empty, it would be much much harder to
study it. Alright, So I hope that answers your question,
what is space dust? It's tiny little grains of stuff,
some of them created within stars, some within supernovas, some
(43:18):
of them aligning with magnetic meals to tell us where
things are. We don't know what their future is. Maybe
one day some of them will gather together to make
a new star, a new planet, even a new race
of intelligent aliens that make a podcast even better than ours.
All right, so thank you very much everybody who's sent
in questions, and thanks also to those of you who
have sent in listener questions audio and not yet had
(43:40):
your questions answer. I promise we will get through our
backlog and we will answer all of your questions, because
I think that everybody out there should be asking questions
about the universe, should be tapped into their innate curiosity.
Discovering how the world works. Asking these questions and knowing
that there's an answer is one of the most satisfying experience.
Is it tells you that maybe the human mind is
(44:02):
capable of gaining not a full understanding, but at least
a foothold into our universe of ignorance, cracking that open
a little bit and revealing a tiny slice of how
the universe works. It's certainly we're doing, even if it
doesn't immediately lead to applications and better lasers and pants
with better zippers and stuff like that. I view the
deep exploration of the nature in the universe to be
(44:24):
on par with the creation of art. It's part of
what makes life worth living. So thanks everyone for lending
us your questions and your curiosity. It's been a wonderful ride,
and tune in next time for more questions from listeners.
(44:45):
Thanks for listening, and remember that Daniel and Jorge Explain
the Universe is a production of I Heart Radio or
more podcast from my Heart Radio. Visit the I Heart
Radio Apple Apple Podcasts, or wherever you listen to your
favorite shows.
Isaac Newton was very clearly a smart guy. He made
huge leaps and understanding of optics and gravity and calculus.
But one of the biggest mental steps he took was
applying the laws that we have down here on Earth
to what's going on up there in space. His biggest
idea was probably that there should only be one idea,
(00:31):
one set of physical laws, but that it should cover everything.
And here we sit on a tiny, isolated rocking space
trying to make rules that explain the whole universe, only
able to see a tiny little bit of it and
study with our hands and our tools an even smaller bit.
(00:51):
It's sort of like visiting the zoo and only seeing
the insect exhibit, but then making laws that are supposed
to describe how elephants and amphibian's work. So ask yourself,
how likely is it that our ideas are actually universal? Hi?
(01:22):
I'm Daniel. I'm a particle physicist, and I have an
infinite list of questions about our probably infinite universe. And
welcome to the podcast. Daniel and Jorge explain the universe
of production of My Heart Radio. We talk about things
happening far far away, and we talk about things happening
under your feet. We ask questions about the very beginning
(01:43):
of time, and we ask questions about the very nature
of time and the end of time. We ask questions
about the entire universe because we think that curiosity and
asking questions is universal. We think everybody out there has questions,
and everybody deserves to have their question is explored, if
not answered, because not every question has an answer to
(02:04):
it so far, but questions really are at the heart
of science, and that's why on today's program, while my
friend collaborator and co host Jorge can't be here today,
I'm gonna take the opportunity to gather up a bunch
of questions asked from listeners and try to answer them.
We're always asking people please send us your questions. If
(02:25):
there's something you don't understand about the universe, something you've
read and didn't quite follow, something you've thought through that
didn't quite make sense to you, please send it to
us because we cherish those questions. Those questions are our
opportunity to help people understand what we do and what
we don't know about the universe. And remember that everybody
(02:46):
out there who was asking questions is basically an armchair physicist.
If you are trying to wrap your mind around the universe,
you're trying to make one holistic sense of understanding of
how things work. If you've read something somewhere and you're
trying to make it agree with something else you used
to think, or something you read somewhere else, or something
your friend told you, that's doing physics. You're trying to
(03:07):
unify your understanding. You're saying, I need to have one
set of ideas that describes the entire universe that explains everything. Now,
of course, we don't know if it's possible to describe
the entire universities in one set of laws. We don't
know if it's possible for humans to do it, or
maybe take some super alien intelligence or some artificial intelligence
(03:30):
with super incredible powers. But that doesn't stop us from trying,
because it's our dream that we could encapsulate the entire
workings of the universe somehow inside the puny human mind.
So please, if we haven't answered a question that's in
your mind, send it to us two questions at Daniel
and Jorge dot com. We answer all of our emails
(03:50):
and sometimes we put those questions here on the podcast.
But If you don't like writing emails or you don't
want to engage with us on Twitter, we have other
ways to get your questions answered. You can check out
Daniel's public office hours, look at the website for the podcast,
or go to sites dot you see I dot E,
d U slash Daniel, you'll see a link. Therefore, when
(04:12):
Daniel has public office hours, he hangs out on Zoom
and answers questions about physics and life and the universe
and everything from people like you, people who have thought
about stuff and have a nagging little question that they
can't find the answer to using Google and they just
have to know how it works, all right, And today
on the podcast will be answering questions from listeners from
(04:34):
all over the world. Our first question comes to us
from Germany. I have a question concerning doc energy. Does
it violate the law of energy conservation? It seems to
come out of nothing and getting bigger and bigger. Thanks
a lot, all right, Thank you and Juras from Germany.
This is a beautiful example of what I was just
(04:55):
talking about about applying our ideas about how things work
in the universe and taking them to the extreme and
saying does this really work everywhere. Is this a universal law?
Is there some part of the universe that seemed to
break this rule which would make it not universal? And
one of the most fundamental things we thought we understood
about the universe was this idea of energy conservation. Of course,
(05:18):
a hundred years ago, we thought other things were conserved,
like mass. We thought that stuff was conserved, that you
could move it around, you could switch it up, you
could rearrange it like lego bricks, but you couldn't create
or destroy mass. Now, of course we know that's not true.
And the lesson we learned from that, from the lack
of mass conservation, is that mass is not a fundamental
(05:40):
element of the universe. It can be created, it can
be destroyed. You can have more mass, you can have
less mass. It's not something which we should consider sort
of on the list of fundamental descriptors of the universe.
And that's important because what we're doing with physics is
trying to drill down to the most fundamental, the simplest description,
Because we imagine if one day we are looking at
(06:03):
a list of the fundamental elements of the universe, of
things that define the universe and completely explain the universe
that we will somehow be revealing the nature of the universe.
So we don't want anything on that list which isn't fundamental.
You don't want that list to have like strings and
energy and then ice cream, right because ice cream can
be described by all the other elements already on the list.
(06:24):
So we want to strip it down to a sort
of most minimal set of rules, most minimal set of
things you need to describe the universe. Having left our
list of things that describe how the universe works tells
us something about what the universe isn't. It isn't a
place that cares so much about mass. However, energy seems
to have retained its exalted stature as a quantity which
(06:48):
is conserved. And you know what is energy conservation anyway.
Energy conservation is the statement that you can calculate this
thing about nature. You add up all the energy in
a system, all the ways that things can move or
wiggle or store energy, and then you let a bunch
of stuff happen. Things collide, things explode, things slash around, whatever,
and you add up all the energy again and it
(07:10):
should be the same. So it's sort of a statement
that energy is fundamental to the universe. You can move
it around, that you can change it from one thing
to another, but that you can't get rid of it,
that it's inherent, that it's fundamental, that it's a deep
part of what makes the universe the universe. So if
energy is not conserved, then that tells you what It
(07:31):
tells you that maybe energy isn't actually important to the universe.
Maybe energy isn't on that list of fundamental elements we
think are needed to define and describe the universe. So
Andreas is doing exactly what a physicists should be doing,
thinking about ways energy conservation might be violated. We know,
for example, that if you roll a boulder up a hill,
(07:53):
you're spending energy in your muscles, and that energy then
goes into the position of the boulder. It's gravitation in
a location is more distant from the Earth, And if
you let go of the boulder and run it back
down the hill, the energy goes from that potential energy
of the boulder into its motion, into its kinetic energy.
So we have lots of examples of where energy is conserved,
(08:13):
and people probably expect to hear that energy is conserved
everywhere in the universe and there's some way you can
do the calculation to figure out that energy is actually
conserved in the case of dark energy. So what is
he talking about. Remember that dark energy is not something
that's very well understood. It's not a theoretically well formulated idea.
(08:34):
It's more an observation. It's an observation that the universe
is expanding and that that expansion is accelerating. So we
look out into the universe and we see that the
universe is expanding, and not only are things moving away
from us, but the speed at which they're moving away
from us is increasing. You might expect the opposite to
(08:54):
be happening. In fact, physicists expected the opposite to be
happening for a long time, that things were moving away
from us, but that speed might be decreasing as gravity
very slowly pulls on things and tugs them back together.
After the Big Bang, what we actually found about years
ago now is that things are moving away from us
faster and faster, and we don't have an explanation for
(09:17):
why this is. All we have is the observation that
it is happening. There are a few sort of proto explanations.
There are ideas for what might describe it, but none
of those ideas really work so far. One of those
ideas is that there is energy in empty space, that
all of space has energy in it. For example, the
(09:37):
Higgs Boson field is a quantum field that's in all
of space, and even when it's at its most relaxed,
at its lowest level, it doesn't have zero energy in it.
That means that when you create a piece of space,
you're creating a Higgs Boson field that has energy in it.
And this is what Andreas is talking about. That dark
energy creates more space because it's not just moving things
(10:01):
through space. It's creating new space between galaxies. It's stretching
that space. It's making new space. And when you make
new space, it comes with new energy. So it seems
an awful law like dark energy is in fact violating
conservation of energy because as you make more space, you
(10:21):
are increasing the total volume of the universe. And if
every cubic meter of space has a certain energy, then
by increasing the volume of space, you're increasing the total
energy in the universe. How does that not violate conservation
of energy? Well, in fact it does, and as energy
conservation is not guaranteed in our universe, and this is
(10:43):
one example. As space expands, the energy increases because you
get more dark energy, which means overall more energy. There's
also another example, which is that energy can decrease when
space expands. If you have a photon flying through space,
for example, from the cosmic microwave background radiation, then what
happens when space expands. When space stretches, Well, that photon
(11:07):
gets red shifted. Its wavelength gets longer because space has
gotten stretched. Right, Imagine you draw a wiggle on a
sheet of paper and then you stretch that paper, the
wavelength gets longer. But for photons, the energy and the
wavelength are very closely connected. One defines the other. Higher
energy photons are those with shorter wavelengths, and so if
(11:29):
you stretch the wavelength of a photon, then you decrease
its energy. Where does that energy go. It doesn't go anywhere.
It just goes away. So we have two examples of
the violation of the conservation of energy, both coming from
space expanding. And that's the clue as the clue that
tells us why energy might not be conserved, and most
(11:52):
of the conservation laws in physics, most of the things
that are conserved that are not changed when you let
things bang around and smash into each other come from
some kind of symmetry. This is very deep result in
physics called Noether's theorem from Emily Norther, who developed it
more than a hundred years ago, and she discovered that
every time you have a symmetry, like every time you
(12:13):
can take space and rotate it and still get the
same laws of physics, or move your coordinate system over
by ten kilometers and still expect the same law of physics,
or fast forward things by a hundred years and still
expect the same laws of physics. Every time you can
apply some sort of translation or rotation to the universe
and not see any change in the law of physics.
(12:35):
That's a symmetry, and every symmetry has some kind of
conservation law that comes from it. So, for example, the
fact that space is the same everywhere, that the laws
of physics apply here and somewhere else, that gives you
the law of conservation of momentum, and the fact that
you can rotate space, that there's no preferred direction, that
physics should work the same in every direction. That's why
(12:57):
we have conservation of angular moment them and it's the
symmetry of the universe with respect to time is what
gives us conservation of energy. The fact that it seems
like the universe should work the same now as it
does in a hundred years and a thousand years ago
is what gives us conservation of energy. But that only
works if we expect the same rules to apply now
(13:20):
and in a hundred years and in a thousand years.
That only works if space is essentially static, if it's
not changing, if space is the same now and in
a hundred years and a thousand years ago. But we
know that it's not because we know that space is expanding.
So conservation of energy is something we expect to apply
in a static universe where space is not changing. In
(13:43):
our universe, however, space is expanding, and it's expanding quite rapidly.
Expansion is not a small thing in our universe. Seventy
percent of the energy budget of the universe goes towards
the expansion of space time, so when space is expanding,
energy is not concerned. Now, we did a whole podcast
episode about this conservation of energy, and there is one
(14:05):
way that you can sort of rig up a calculation
in which you get negative energy from gravity that might
account for some of this, but most cosmologists think it's
sort of a band aid and theoretically doesn't hold together.
And you're interested in more details and not check on
our whole podcast episode about conservation of energy. But congrats
to you Andrea's for figuring this out, for applying your
(14:27):
understanding of physics to crazy scenarios far beyond your living
room and coming up with a contradiction. And those contradictions
are what leads to questions, and those questions are would
lead us to deeper understandings about the universe. So keep
asking questions. Thanks very much for sending that in. All right,
I have more questions from listeners I want to get
(14:48):
to but first let's take a quick break. All right,
we're back, and this is Daniel and I'm here today
on my own answering questions from listeners. I love when
(15:11):
people write to us. They send us amazing questions, things
that they have been thinking about the universe and that
they can't figure out via Google, and they don't happen
to know a physicists they can ask these questions of,
so they send them to us. And I have a
bit of an embarrassing backlog of questions from listeners that
I really want to get to, and so while Jorge
isn't here, I'm gonna plow through a bunch of these
(15:33):
and try to catch up to our backlog. So thanks
very much to everybody who's senting questions. Here's the next
question we're gonna answer today. Hey guys, this is Jeff
from Los Angeles. My question relates to the period of
inflation after the Big Bang. I know you said the
universe expanded by a factor of ten to the thirty
in the small amount of time of ten to the
minus thirty. How do you explain that if nothing can
(15:56):
travel faster than the speed of light. I also want
to know if the edges of the universe were expanding
at this crazy fast speed, and it was expanding through nothing,
then what's slowing it down? Why isn't the universe still
expanding at that crazy rate of inflation? I look forward
to the answer. Thanks a lot, all right, Jeff from
(16:16):
l A who's basically an amateur cosmologist. Thank you for
thinking deeply about the universe and for trying to reconcile
what you've heard about the early days of the universe
with what you understand about how the universe works. Again,
that's exactly what doing physics means, so let's get to it.
The first part of your question was if the universe
expanded by a factor ten to the thirty intended to
(16:39):
minus thirty questions, how is that possible? Given that we
know that there's a very hard limit on how fast
things can move through space, which is the speed of light.
It's a great question. Another way to think about this
question is how did the universe get so big? I mean,
the universe is about fourteen billion years old, but the
(17:00):
size of the observable universe, the distance to the furthest
things that we can see, you might expect to be
fourteen billion years times the speed of light, which would
be fourteen billion light years, but it's not. It's much
much further than that. We can see things that are
about forty five billion light years away, So the size
(17:22):
of the observable universe is about ninety billion light years wide.
How is that possible? How is it possible to see
things which are further away than the speed of light
times the age of the universe. How did that stuff
get there so far? How did the universe expand faster
than the speed of light. So it's a wonderful question.
(17:42):
And the key concept you need to know to understand
this is that there's a difference between moving through space
and expansion of space. So moving through space is the
kind of thing you're familiar with. You move through space
every day. When you get out of your bed and
you go for a glass of water in the middle
of the night, you are moving through space. When you
(18:02):
throw a baseball really really fast. When you get on
your spaceship and you try to travel to a nearby star,
you are moving through space. When you turn on your
flashlight and you shine it at the moon, you are
sending photons through space to the moon. And there is
in fact a very hard limit on the speed at
which things can move through space, and that's the speed
(18:22):
of light. In a vacuum. Nothing, no information at all,
can move through space faster than the speed of light.
That includes neutrinos, that includes everything, that includes quantum information.
It's a very tough rule, and breaking it when undermine
special relativity, which we're pretty sure as an accurate description
of space time in our universe. All right, So that's
(18:44):
moving through space, but that's different from the expansion of space.
The expansion of space means stretching space itself. So imagine,
for example, you are one meter apart from your friend.
You have a meter stick, and you measure exactly how
far apart you are. You could take a step back.
That would be moving through space. But you could also
(19:05):
expand the space between you. You could take the very
universe and stretch it so that now you guys are
two meters apart without having moved through space at all. Right, Remember,
space is not just the backdrop on which things happen.
It's not the stage on which the acts of the
universe are played out on. It's a dynamical thing. It's stretchy,
(19:28):
it's like goo. It responds to the presence of mass
and energy. It bends, it twists, it can expand, and
it tells things how to move. So space is really
part of the universe. It's not just like some fuzzy
abstract concept, some set of glowing axes in our mind
that we just impose on the universe to try to
(19:50):
make sense of it. Space really can do a bunch
of weird things. You already know this because you know
that gravity is not just a force. It's actually the
curving of space. Right, The reason that the Earth goes
around the Sun is not because gravity is a force
which is tugging on it, but because the presence of
the Sun changes the shape of space in its vicinity,
(20:11):
so that an object moving in a straight up inertial
path will move in a circle around the Sun. That's
because space can bend and twist, changing the relative distances
between things. So, for example, a photon a beam of
light always takes the shortest path between two things, But
the shortest path between two things isn't always what you
(20:31):
imagine to be a straight line, because the shape of
space can be complicated between two points. The same way
an airplane going from l A to London takes the
Great Circle route right, which seems like a curve, is
actually the shortest distance between two places on a curved surface,
which is why gravity can influence even things that don't
have mass. All right, So that tells us that space
(20:53):
can do things, and it can stretch, and it can expand,
and so that's exactly what happened in the very early universe.
It wasn't an explosion like a tiny dot of stuff
and then everything exploded out from the center. Instead, it
was an expansion of space itself. Huge amounts of new
space were made everywhere all over the universe simultaneously. So
(21:17):
that doesn't make it easier to understand the fact. It
makes it even more buying boggling that this happened, That
every unit of space was blown up by a factor
of ten to the thirty intend to the minus thirty seconds.
It's an incredible moment in the history of the universe.
It's an incredible idea to even have in your mind.
Imagine coming up with this Bonker's notion and then realizing
(21:39):
that actually it's the story that makes the most sense
in the universe. And you might ask, well, how do
we know. How do we know that the universe expanded
in this way, that it didn't just explode from a
tiny dot and spread out through the universe. Well, answer
number one is that it would be impossible. As you say,
it's not possible for things to travel that are in
(22:00):
that short distance because of the limitation of the speed
of light. It's against the rules. The only way to
get things that far apart in that short amount of
time is to create space between them, is to expand
the space between everything. But it's more than that, because
Explosions and expansion look different. Explosions are like a bomb.
(22:23):
You push everything out from one central location and send
it flying in every direction. And if there is an explosion,
you could look at the direction things are flying and
you could track them backwards, and you could point back
to the center. Right. If you come upon an explosion,
you can look at the path of the debris and
you can figure out where the bomb was. That's not
true for an expansion, and expansion is more like a
(22:46):
loaf of bread rising in the oven, where everything is
growing at every point simultaneously, assuming you're not a terrible baker, right,
and that your loaf is expanding smoothly. And that's what
we see when we look out into the universe. We
see these galaxies and they are rushing away from us.
They are moving away from us, and they're moving away
from us faster and faster every year. So either we
(23:07):
are at the exact center of the universe by some
incredible cosmic coincidence there was an explosion and we happen
to be right at the center of it, or it
looks this way because it's an expansion, and it would
look this way at any point in the universe. See.
The way an expansion looks is that it always looks
like you're at the center of it. No matter where
(23:29):
you are in a loaf of bread, if you look
around you, everything is growing away from you. Imagine putting
a bunch of chocolate chips into your loaf of bread
and tracking their emotion everywhere inside the loaf of bread,
of course, except for the crust. You would see this
chocolate chips moving away from you. So that's what we see.
We see that the universe is expanding, not that it's
exploding out from a tiny dot. And this expansion is
(23:52):
actually really important to sort of the state of the
universe as we know it, because when the universe began,
we think it began very very smooth, like totally homogeneous.
Everywhere was exactly like everywhere else. And why wouldn't it
be right when the universe is created, Why would you
have one spot that's like denser than another spot. The
problem is, however, a universe like that that's created perfectly smoothly,
(24:15):
nothing very interesting ever happens in that universe. There's nothing
for gravity to do in that universe because there's no
spots that are heavier or denser than anything else, which
is what gravity needs to sort of like seed the
structure to start coalescing things together into stars and planets
and galaxies and all that good stuff. So how did
the universe get any structure? Well, it was perfectly smooth,
(24:38):
except down to the quantum level. Of the quantum level,
there are always random fluctuations. Every point in the universe
gets a different random fluctuation, so you get these really
super duper tiny little variations in the density of the
universe due to quantum mechanics, and then inflation steps in.
Inflation takes those tiny little quantum fluctuations and it blows
(25:00):
them up to the macroscopic scale, makes things which were
invisibly small somehow suddenly now huge. Right, takes a meter
stick sized thing and it blows it up to a
trillion light years. It takes something which was sub atomic
and it makes it macroscopic. So now those random quantum
fluctuations are not small. They're pretty big, and they're big
(25:21):
enough to see the structure of the universe. So the
reason that we have a galaxy over here and then
over there it's empty space is because of a random
quantum fluctuation in the very early universe, which was expanded
out into something macroscopic that see to the structure of
that galaxy and allow gravity to pull stuff together to
(25:42):
make something interesting, to make me and to make you,
and to make the sun that warms our toes. Now,
the second part of Jeff's awesome question is that if
the edges of the universe were expanding through nothing, what's
slowing it down? Why isn't it still expanding at that
crazy rate? So lots of really good angles on the question.
First of all, we don't know if there is an
(26:02):
edge to the universe. I think in his mind Jeff
might be imagining an explosion, an explosion which has a
wavefront which is moving through the universe and then slowing down.
But we don't know that there was an edge. We
don't know if there is an edge. I think the
cleanest way to think about these things is to think
that the universe is infinite. We don't know that's true,
but it seems somehow more natural to have an infinite
(26:24):
universe than to have an edge. And then you can grapple.
Instead of thinking about the whole universe, just think about
a chunk of it and think about sort of the
density of that part of the universe. So I imagine
the whole universe created infinite at its birth as a
very very dense place, and then expanded suddenly, very rapidly,
using inflation. So there's no edge there. Everything is moving
(26:47):
away from everything else. He also asks, if there's no edge,
what's slowing it down? Why isn't it still expanding at
that crazy rate? Awesome question. I wish I knew the answer.
There was this incredible moment of inflation in the very
early universe, this rapid expansion in a very short amount
of time. We don't know what caused it. We have
(27:07):
ideas about ideas were sort of proto ideas for what
might have caused it, crazy particles and field called the
infloton field, but those are sort of placeholders to have ideas.
We don't really have any well worked out, super well
formulated ideas that actually come together mathematically to explain inflation.
So because we don't know what started it and what
(27:28):
sustained it, we also don't know why it stopped. We
just know that it started and we know that it stopped,
but the expansion itself has not stopped. It was very
rapid in the very beginning, and then it was very
slow for a while, but about five billion years ago
it started to pick up again. Dark energy took over
and it started to accelerate the expansion of the universe
once again. And this expansion is very similar to what
(27:51):
happened in the inflationary period of the universe. It's not
nearly as rapid, but the sort of stretching of space
is the same concept. We don't know if there's a
relationship between the mechanism or the reason for why space
is expanding now and why space expanded in the very beginning.
We're pretty clueless about what dark energy is. Again. We
have a few basic ideas for what might explain it,
(28:14):
but none of them hold together mathematically, so most of
this is just an observation. We see that this happened,
we can't explain why. So it is still expanding at
a crazy rate, not as crazy, but we don't know
the answer to the question why the universe stopped inflating
and why it's not inflating at that crazy rate today. So, Jeff,
the answer to your question is that the universe expanded
(28:36):
so rapidly, not by things moving through space, but by
expanding the nature of space itself, by creating new space
between stuff and why did that stop. We don't know
why inflation itself stopped, but the expansion has not stopped.
The universe is still expanding, and it's expanding faster and
faster every day. All right, Thanks for that super awesome question.
(28:57):
I love all of these ideas. I have one more
question I'm an get to today, but first let's take
another quick break. Okay, we're back and this is Daniel
(29:17):
and I'm answering questions about the nature of the universe
and how it expanded and whether it violates conservation of energy.
And our next question is a tiny bit more concrete. Hi,
Daniel and JOI I'm Tristan from Melbourne, Australia. Congratulations on
the awesome podcast. My question is about spice dust. We
(29:37):
hear about it all the time, but what exactly is
spice dust? Is it tiny gas molecules or really minute
dust particles like here on Earth or is spice dust
just a relative term? And they'm all like basketball sized
or cast sized or bigger? All right, Thanks very much
for that fun question. This is actually a surprisingly fascinating
topic when you think about do you think it's like
(30:00):
dirt and something you want to get rid of. It's
an annoyance. It gets in your way. If you zoom
in on it, you discover that, like a lot of
the dust in your house is actually left over dried
bits of human skin that makes you want to throw
up a little bit. So space dust is sort of similar.
For a long time, people thought space dust was just
like an annoyance. It was this stuff floating in space
(30:21):
which like blocked your view. I mean, if you look
out into space, it's incredible how far we can see.
You're standing on the top of a rock in space
and you're peering out in your eyeballs are absorbing photons
that have traveled billions of light years, mostly unimpeded to
get to you. It's incredible that space is as clear
as it is, so we shouldn't be complaining. We have
(30:43):
the best view in the universe. The kind of things
we see with hubble are just eye dropping the gorgeous.
But sometimes there are things that are obscured by space dust.
If you look at the center of the galaxy, for example,
they're huge clouds of dust to make it harder to
see what's going on there. And for a long time
astronomers treated space dust that way, like an annoyance, like, oh,
(31:06):
these things are shrouded in dust, so we can't see
them what's going on inside there? And if you're like me,
your curiosity is only heightened when something is hidden, something
is behind a veil, like things inside a black hole.
It just makes me want to know even more what's there.
So for a long time space dust was treated that way.
It's just something that gets in our way, something to
(31:27):
be annoyed about. However, now we see that space dust
is just sort of part of the dynamics of space,
part of the astrophysical soup that's constantly churning, making new
stars and all their crazy things, and it can actually
help us understand the structure of the galaxies and how
things work. For example, we can see space dust. It
(31:47):
doesn't just blocklight, it actually gives off its own light.
This is something I think is not widely enough understood
that everything in the universe glows. Everything in the universe
that's made that of our kind of stuff atoms, that's
only five of the universe, but all that stuff glows,
and it glows based on its temperature. The hotter you are,
(32:09):
the more energetic photons you give off, which is why.
For example, if you heat up metal, it starts to
glow and it glows at different frequencies, different colors as
it gets hotter. Everything is actually like that. Even you glow,
and if you put on infrared goggles then you can
see your body heat because of your temperature. But it's
(32:29):
not just living things. Even rocks glow, even if they're
very very cold. They glow at some wavelength. And space
dust is out there and it glows as well, so
you can see it. It's not giving off visible light.
But if you have a special camera like night vision
goggles for the universe and what we call an infrared telescope,
then you can see it. And if you point an
(32:51):
infrared telescope, for example, at the Andromeda galaxy, you can
see where in Andromeda there is this space dust. The
space does glows at different frequencies than the stars. The
stars admitted visible light and you can see them. That's
super fascinating. But then if you turn on your night
vision goggles for space, you can see the other stuff,
(33:12):
the colder stuff, which is glowing at longer wavelengths in
the infrared, and you can see it totally differently, and
Dromeda looks different in the infrared, you can see where
the space dust is, and that helps us understand like
how did and Drameda form, what's going on over there,
what are the dynamics, what things are moving against the
other stuff. So these days space dust isn't just like
(33:34):
an annoyance. It isn't just a cloud that gets in
your way. It's another thing out there that we can study.
And it turns out that there's a lot of different
kinds of space dust. Most generally, what is space dust.
It's basically anything that's out there in space that's very,
very small, right, So you wouldn't call Earth a big
speck of space dust. This is one of those arbitrary
(33:56):
categorizations in astronomy and astrophysics. Space does is basically anything
that's out there that's smaller than like a millimeter, and
it can go down all the way to like a
few molecules, but the upper edge is generally agreed to
be like a millimeter or half a millimeter, maybe a
tenth of a millimeter. Anything that size or smaller that's
floating out in space, we call it space dust. Bigger
(34:18):
than that, like a basketball or car sized thing, we
would call that an asteroid. Or a comet, or even
a proto planet or a moon or right a sun
if it gets big enough. So there's this whole spectrum
of sizes of stuff in space and things on the
smaller edge we call space and dust. And you might wonder, like, well,
why is there space dust? After all, you have these
(34:40):
huge clouds of things formed after the Big Bang, and
some of it gathered together to make stars, and some
of it gathered together to make planets, but not everything
instantly gets clean together. Right. Gravity is very patient, but
it's also very very weak, And the gravity on very
small objects, tiny little specks of up floating out in space,
(35:01):
is very weak. And other things I'm much more powerful.
One thing that prevents gravity from gathering stuff together is
angular momentum. If something is moving in a circle around
something with gravity, then it doesn't necessarily fall in the
same way the Earth orbits the Sun without falling in,
even though there is gravity tugging on the Earth from
the Sun. The reason we don't fall in immediately is
(35:24):
because we're moving in a circle. The same reason the
whole galaxy doesn't collapse into the central black hole is
because of angular momentum. That's just one example and so
that keeps some space dust from collapsing into larger objects,
and so you end up with this whole spectrum of
really dense stuff that's sort of cleared out the space
around them than a whole distribution of smaller bits, which
(35:45):
we call space dust. And we can study this stuff.
NASA actually sends planes up into the high atmosphere to
gather space dust and these big collectors under the wings
to pull it together and say like, well, what's in there.
And actually there's a huge amount of space dust out there.
It's not very rare. The Earth is traveling through a
cloud of this stuff is like one particle per million
(36:09):
cubic meters, but there's a lot of cubic meters out there,
and there's a lot of square meters on the surface
of the Earth, and so a lot of space dust
actually falls onto the Earth every year. Some of the
stuff that's lying around your house might be dead bits
of skin, but some of it might be space dust,
because there are thousands of tons of space dust that
(36:30):
reached the surface of the Earth every year. Yeah. I
just about fell out of my chair when I read
that tidbit that fact thousands of tons of the stuff
if you could like sweep up all the space dust
that hits the Earth and make a pile of it,
it would be a huge, huge mountain of space dust,
all right. So then what is this stuff? Right? What
(36:51):
is this stuff that's out there, that's floating through the
universe that didn't get gathered together into planets and rocks
and other kinds of stuff. Well, it's a big mix, right.
It's basically a big soup of leftover stuff either from
the very early universe that never got gathered together into
something else, or that has had a chance to be
part of a star or a planet and then got
(37:12):
blown into little smithereens. So one category of stuff is
star dust. Star dust are little pellets made on the
outside of stars, a little grains, for example, of oxygen
or carbon rich elements that are floating out near the
outside of stars, and that gets blown out away from
the star and so they get frozen into these little pellets.
(37:33):
Remember that in the first population of stars, we had
only hydrogen, but those hydrogen stars burned helium, and in
later generations of stars fuse that helium into heavier and
heavier stuff. So stars are the engines to make these
heavier elements, and eventually they can gather together. A lot
of this we call it ash because it's the product
of fusion. A lot of it falls to the center
(37:55):
of the star, makes a denser and denser core, which
eventually leaves the star to collapse, not all of it,
so if it gets blown out into the outer edges
of the star, and then it can get pushed even
further out and float out into space. So these little
grains of stuff produced inside stars, this is called star dust,
and this is floating out there, and a good amount
(38:15):
of the space dust are actually these kinds of grains,
and a lot of them came together to form our
star and our planet. So a lot of what makes
me and you and the Sun are actually these bits
of other stars. Now, of course, inside the Earth and
inside the Sun, they've all been melted down to their
basic elements and maybe even fused into other stuff. But
(38:36):
space dust hasn't. It's frozen. It's like a little time
capsule that tells you where it came from. And if
you can capture one of these grains of star dust,
and you can look at the relative fractions of stuff,
like how much iron is there, how much carbon is there?
Then you can get an idea from what kind of
star came from. You can read like it's ancient history
just by looking at what's inside of it. So each
(38:59):
of these is like a little time capsule that tells
us what happened. And these events are billions of years old.
You know the star dusk grains that helped form our sun,
well that was five billion years ago, So they were
produced more than five billion years ago, and they're still
floating around. Some of them coalesced into like micro meteorites,
but they don't necessarily lose their elemental structure. They just
(39:21):
sort of got like get stuck together like a big
pile of rice grains. And so if you're careful, you
can tear them apart and look at the individual grains
and still study these little time capsule from other stars. Now,
sometimes you look at these space dust capsules and you
see something really strange, and people actually predicted that you
would see this. You see things produced in supernovas. Supernovas, remember,
(39:43):
are these very special occasions. When it starts, gravity overcomes
its pressure and it collapses very very rapidly, this implosion,
which then leads to an explosion where it throws crazy
stuff through space. Well, in those moments of implosion are
intense moments of fusion, and these are situations that allow
for the creation of other kinds of elements and different
(40:04):
mixtures of elements than what you would expect from the
normal production you get in a star. And sometimes these
things get thrown out during the supernova and they're like
little time capsules, a little like samples from what's going
on deep inside is supernova. So these supernova grains are
super awesome vines because they're not created nearly as often,
(40:25):
and they're this little time capsule from this incredible moment
during one of the most violent acts in our universe,
so they're super fun. And then a lot of the
other space dust is just floating tiny rocks. Basically. Some
of them are carbonaceous, you know, other ones have iron
or sulfur or nickel. Some of them are silicates, which
means they're basically bits of sand, and they have all
(40:46):
sorts of irregular shapes, you know, just like any random rocks.
Some of them are kind of fluffy, loose amalgams. Some
of them are very compact. Some of them accumulate little
layers of ice around them, so you might expect them
to be like super mini comets, and and the sizes
of them differ. Right, they go all the way down
from the tiny, tiny little grains up to you know,
less than a millimeter or so. And this is important
(41:08):
because the size determines how you can see them. Like
pretty big grains actually reflect light, so if the sun
is behind you, you could see them the way you
see the moon, like comes from the sun bounces off
of them and then back to you. But if they're
really really small, then they don't reflect light. They just
sort of deflected a little bit, which means it's only
(41:29):
easy to see them if the light is behind them.
They have to be back lit, and this is why,
for example, we didn't really know that Jupiter had rings
and had rings made of dust until we got cameras
out past Jupiter and you could look back and you
could see those rings of dust back lit by the
sun because they only deflect the light a little bit.
(41:51):
So it's important how big they are, and it's also
important their shape. We think this space dust is not
just around the Solar System, and not just in the galaxy,
but also between the galaxies. It's basically spread out everywhere,
and it's actually really valuable because these grains are not spheres.
They're like weird, all blong shapes. So what they do
(42:12):
is they tend to align with magnetic fields. They're like
tiny little needles and they tend to line up with
magnetic fields. And people have been studying magnetic fields through space,
wondering like, is their magnetic fields all through the galaxy?
Are there magnetic fields between galaxies? Are there magnetic fields
in deep space that were created during the Big Bang?
(42:32):
This is called the primordial magnetic field. We have a
whole podcast episode about it. If you listen to that.
What you learn is that these dust grains line up
with magnetic fields, which is important because it changes how
light moves through it because these dust greens are now polarized,
and so we can use space duft to sort of
track the magnetic fields in otherwise empty portions of space,
(42:56):
sort of like sprinkling magnetic filings on a sheet to
see if the the magnetic field there. It's actually good
that space dust is sort of everywhere, because if space
was truly empty, it would be much much harder to
study it. Alright, So I hope that answers your question,
what is space dust? It's tiny little grains of stuff,
some of them created within stars, some within supernovas, some
(43:18):
of them aligning with magnetic meals to tell us where
things are. We don't know what their future is. Maybe
one day some of them will gather together to make
a new star, a new planet, even a new race
of intelligent aliens that make a podcast even better than ours.
All right, so thank you very much everybody who's sent
in questions, and thanks also to those of you who
have sent in listener questions audio and not yet had
(43:40):
your questions answer. I promise we will get through our
backlog and we will answer all of your questions, because
I think that everybody out there should be asking questions
about the universe, should be tapped into their innate curiosity.
Discovering how the world works. Asking these questions and knowing
that there's an answer is one of the most satisfying experience.
Is it tells you that maybe the human mind is
(44:02):
capable of gaining not a full understanding, but at least
a foothold into our universe of ignorance, cracking that open
a little bit and revealing a tiny slice of how
the universe works. It's certainly we're doing, even if it
doesn't immediately lead to applications and better lasers and pants
with better zippers and stuff like that. I view the
deep exploration of the nature in the universe to be
(44:24):
on par with the creation of art. It's part of
what makes life worth living. So thanks everyone for lending
us your questions and your curiosity. It's been a wonderful ride,
and tune in next time for more questions from listeners.
(44:45):
Thanks for listening, and remember that Daniel and Jorge Explain
the Universe is a production of I Heart Radio or
more podcast from my Heart Radio. Visit the I Heart
Radio Apple Apple Podcasts, or wherever you listen to your
favorite shows.
Hosts And Creators
Daniel Whiteson
Kelly Weinersmith
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