January 12, 2021 • 48 mins
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Episode Transcript
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Speaker 1 (00:08):
I like to think sometimes about what it was like
to be a caveman or cave one. You know, they
probably didn't travel nearly as much as we do, and
so they just didn't have a global sense for where
they lived in the world. They just saw a little
patch and all they could do was wonder about what
lay beyond. Now, of course, we know so much more
about the world, and so we can look at the
(00:31):
bones of Stone Age people and feel like we might
know more about their world than they did. There might
have been people who lived near the ocean but never
saw it, things that were right around the corner they
didn't even know about. But aren't we still cave men
and cave women in that way? We still see our
little patch of the universe, and we don't know what's
(00:52):
around the corner. What if there's some crazy awesome thing,
but it's just a little bit further than we can see.
And so that makes me wonder will future humans look
back at us in the same way, chuckling to themselves
about how clueless we are? I sure hope so, because
it means physicists have been doing their jobs. Hi. I'm Daniel,
(01:29):
I'm a particle physicist and Welcome to the podcast Daniel
and Jorge Explain the Universe our production of My Heart
Radio today and the podcast is just me. Jorge can't
be here today to join us, and so I'm gonna
be taking you on a tour of all the amazing
things in our universe, all the incredible things that we
want to understand, all the distant places that we can
(01:51):
just barely prob with our telescopes, all of the tiny
particles that we seek to understand. This podcast is about
all of those things and everything. We want to explain
the entire universe in a way that makes you come
away thinking I get it, and maybe once or twice
even makes you chuckle. And one of the things that
we cherish on this podcast is not just the things
(02:14):
that science has been working on, but the things that
you have been wondering about. Because physics is not something
which belongs to a tiny group of people who happen
to get to be professors of physics, but it's something
we can all do. Every time you wonder about the universe,
every time you think to yourself, how does that work?
Every time you hear two things you don't quite know
(02:35):
how to fit them together in your mind. You are
doing physics, you are being a physicist, and we love
that here on the podcast, and we want to encourage that.
And one way we do that is by answering your questions.
So if you have questions about the way the universe works,
you have lots of ways to get in touch with us.
You can send us an email to questions at Daniel
(02:57):
and Jorge dot com. You can engage with us and
Twitter at Daniel and Jorge, or you can come and
ask me in person. Come to one of my public
office hours where I hang out on zoom and answer
questions from anybody and everybody about physics. For information on that,
go to my website to sites dot you see I
dot edu, slash Daniel and you'll find all the relevant
(03:20):
connection information. Sometimes I'll get emailed the question from a
listener and it's such a good question, such a fun topic,
that I think I'm not just gonna write back and
said them the answer. I want to talk about this
on the podcast, So we'll ask them to send me
an audio clip of themselves asking the question, because I
think all of you might be interested in the answer.
(03:41):
And that's exactly what we're gonna do on today's episode. Today,
I'll be catching up with our backlog of listener questions
while Jorge is on a break and answering questions from
real people. Questions about physics, questions about which corner of
the universe we're in, questions about how mirrors work, Questions
about the potns and gravity, all the kind of things
(04:02):
that thinking people wonder about when they try to understand
the world around them. All right, so let's get to it. First.
Up is a question about the universe? Are Daniel and Hord.
My name is Elia, and I'm a big fan of
your podcast. I've always wondered whether or not it is
possible to know which corner of the universe we're in,
depending upon what we believe the shape of the universe is,
(04:26):
and if it's shaped like a doughnut, for example, is
it possible to cross through the donut hole and re
enter the universe provided inflation suddenly stopped. Thank you so
much for teaching and inspiring us. All right, thanks very much,
Ilia for that super fun question. I like this question
a lot, not just because it talks about doughnuts and
that makes me hungry and it makes me wonder was
(04:48):
Ilia eating a doughnut when he asked me this question,
why a donut? Why not like a pastry shaped universe,
or you know, a pie shaped universe. But we'll dig
into that and take a bite out of that part
of the question. But I also love this question because
it touches on something really deep and fascinating, not just
about the shape of the universe, but about where we
are in the universe. Are we in an interesting part
(05:12):
of the universe? Are we near the center of the universe?
Are we near some weird edge or corner of the universe?
I love when you take a deep question about the universe,
how big is it? What shape is it? And you
make it a personal question, like where are we in
the universe? So let's dig into it. Where are we
in the universe? What corner of the universe are we in?
(05:35):
And you're totally right ilia that the answer to this
question depends a lot on what we think about the
shape and the size of the universe. So let's answer
it in a few different ways. Let's start with the
most likely configuration for the universe and imagine that the
universe is infinite. That means that it goes on forever
in every direction, there's no edge, there's no point at
(05:57):
which the universe stops. It just goes on forever. And
it's sort of euclidian in a sense that you can
go on forever without looping around on yourself, and the
two parallel lines will go on forever without crossing. Why
do I say this is the most likely scenario, the
most likely set up for a universe. I just think
it's the most natural because it has no edges, because
(06:18):
it's sort of simple. If you have an edge, then
you have to explain why that edge. If you have
an edge, you have to answer why is this edge
over here and not somewhere else? You need a special case.
And I like explainations that don't have special cases, that
are simple and that our universal. And so this one
that the universe just sort of goes on forever in
(06:39):
every direction is nice because it makes no place in
the universe special. Every point in the universe has an
infinite amount of universe in every direction, and so it's
very similar everywhere. And that's the key thing. That's a
crucial idea in cosmology recently. This cosmological principle, the one
that says that every location in the universe is equivalent.
(07:03):
There's no center, there's no edge, there's no corners. Every
place in the universe is basically like every other place
in the universe. Now, let's unpack that a timely a
little bit. What do we really mean when we say
every place in the universe is like every other place
in the universe. Because my house is not the same
as your house, and the Earth is not the same
(07:24):
as Jupiter, and there are galaxies, and there are places
where there are no galaxies. So like, at the very
microscopic level, it's not true that every place in the
universe is the same as every other place in the universe.
What we mean when we say that is that the
laws of physics are the same, that they always apply
in the same way. That you don't have different laws
and different constants over there than you have over here. Now,
(07:48):
if you have the same laws of physics, how is
it you can even have any difference? How is it
that you can have a planet here and not a
planet there, or you have all these galaxies over here
and no galaxies over there. Well, the current understanding is
that this comes from quantum randomness in the very early
universe when it was hot and dense, but still infinite, right,
(08:08):
still goes on in every direction, but hotter and denser
and younger. That there were quantum fluctuations that made some
spots in the universe more dense, with stuff in some
spots less dense. Now does that mean that those spots
are different from the other spots, Well, they got different
random numbers, but they had the same opportunity. They're playing
the same game and they just had a different outcome.
(08:30):
And quantum mechanics is amazing because the rules are the same.
It gives the same probability distribution for every scenario that's identical,
but individual outcomes can be different. You know. Quantum mechanics
tells us that if you poke a particle the same
way a hundred times, you might get a hundred different outcomes.
Or you might have a scenario where half the time
(08:50):
it goes left and half the time it goes right,
even if the initial conditions are exactly the same. So
that's what's happening in the very early universe. The initial
conditions great energy density is the same everywhere, but different
things can happen in different places. They are following the
same rules, so there's no special place, but they have
a different outcome, and then those little pockets of over
(09:12):
density get expanded into massive macroscopic effects, which seed the
structure of the universe when the universe expanded very rapidly
in the early Big Bang, So you have hot, dense
stuff in the very early universe. Random quantum fluctuations make
some of it a little bit more dense. Inflation expands
those tiny little microscopic things into big macroscopic things, and
(09:35):
then gravity takes over. So that's how you can have
a universe where the rules of physics are the same everywhere,
but you don't actually have the same outcome everywhere. You
have galaxies here and no galaxies over there. So in
that scenario, the universe is the same everywhere. There's no corners,
there's no center, and everywhere in the universe follows the
(09:56):
same rules. And so where are we in the universe? Well,
where he but here is sort of the same as
there so here, or there there or here. It sounds
sort of like a Doctor Seuss book, right, but it's
actually real. And this I think is the most prevalent
idea for where we are in the universe because it
doesn't require any special cases. Now, for those of you
(10:16):
who are thinking back about how I described the Big
Bang and thinking, hold on a second, wasn't the Big
Bang at a certain point? I described it as a
moment when the universe was hot and dense and young. Right,
That doesn't mean that it was hot and dense and localized.
The way I think about the Big Bang is not
as a tiny dot which then exploded out to fill
(10:39):
up the universe with stuff. The more modern view of
the Big Bang is that it was a change from
high density to low density. I'm using different words there
because I mean something different. I mean that the universe
was high density but still infinite, So not a tiny
little dot of stuff like many people used to describe
the Big Bang, but an universe filled with hot, dense
(11:02):
stuff which then expanded. So now that hot dense stuff
becomes cold and dilute, right, it spreads out. Space itself
is expanded. You create new space between stuff in order
to go from a hot, dense universe to a cold,
dilute universe. So the Big Bang was not an explosion
as much as an expansion of space. It's stretched space everywhere,
(11:26):
So the Big Bang happened everywhere, all at once. So
that's the answer. In the scenario that the universe is infinite,
that it goes on forever in every direction, and in
that scenario, doesn't really make sense to talk about where
we are in the universe because everywhere is the same place.
But as we've talked about on the podcast a few times,
(11:46):
we don't know that the universe is infinite. We can't
know that the universe is infinite. We could only see
a finite patch of the universe, just like ancient people's
who thought about the universe but never traveled very far
from their home. They only saw a tiny little patch,
and they could only imagine what was beyond the edge
of their knowledge. In the same way, there's a patch
(12:08):
of the universe about ninety billion light years across that
we can see. We call this the observable universe, and
beyond it, we don't know what's there. Why can't we
see it, Well, it's a physical limitation. It's because of
the speed of light. Light just hasn't had enough time
to get to us from there since the beginning of
(12:28):
the universe. It's on his way. It's been flying our
direction the whole time. The universe has been doing its
universe thing. But it started out so far away that
even though it's traveling at light speed, it hasn't reached
us yet, and it will. And every year that goes
by the size of the observable universe, the portion of
the universe that we can see gets bigger and bigger,
(12:52):
and we can see more and more stars and more
planets around them, and all sorts of amazing interesting stuff.
And of course, first we see the stuff that's happened
were teen billion years ago, because that's when the light
left it to get to us. So as we look
further out into the universe, we actually see the past
more than we see the present. So if there was
something crazy happening just past the edge of the observable universe,
(13:15):
we wouldn't see it for a while. If it's just
happening now, if there's some crazy dancing banana party happening
on a planet just past the edge, it's going to
take billions of years before those awesome images get to us,
so we can embarrass the aliens by posting them on
social media. So what lies beyond the edge of the
observable universe we don't know, and that's one reason why
(13:35):
we don't know the shape of the universe. There are
a few scenarios for how the universe could be shaped.
One of course, is that it's infinite. Another is that
it's not infinite, but it doesn't have an edge, something
like the surface of a sphere. As you walk along
the Earth, you're not aware of there being any edges.
I mean, there are oceans and all sorts of stuff
and walls and fences, but sort of geo magically only
(13:58):
imagine yourself walking just on the surface of a perfect sphere.
You wouldn't notice any edges. You could go on forever
without bumping into anything, and eventually you could even come
back to where you started. So that's a universe that's
not infinite, has a finite amount of area the surface
of a sphere to make sort of the leap from
the two dimensional surface of a sphere where you're walking
(14:19):
around on it now into three dimensional space. How does
that work? Are we saying that three dimensional space is
sort of like plastered onto the surface of a four
dimensional sphere. No, we're just saying that the universe could
be connected differently. We don't know if our three dimensional
space is like embedded in some higher dimensional space four
or eleven or twenty six. Though, if you're interested in
(14:42):
that kind of stuff. We have a whole podcast episode
about how many dimensions there are in space. But even
if we just assume that space has three dimensions, it's
still possible for it to curve, because space can be
bent and connected in weird ways. You can loop around itself,
you can have all sorts of distortions. And we'll talk
about this actually later today in the same podcast. But
(15:02):
you can imagine space being complex in such a way
that bits of it are connected to each other, so
that if you did embed them in a four dimensional space,
it would look like a weird shape. It would look
like the surface of a sphere, or it would look
like a donut or a banana or a pastry or
something else. And now I'm getting too hungry to continue
answering this question. But we just don't know the shape
(15:23):
that all of these things are possible, because we just
can't see enough of the universe to rule some of
this out. Now, it's also possible that the universe isn't
infinite and that it does have some weird edge. Remember,
space is not something that we understand very well. Only
for the last hundred years or so have we even
understood that space is a thing that it can bend
(15:44):
and distored and expand and grow and do all sorts
of weird stuff. So we're at the really the very
beginning of our understanding of the nature of space. And
it might be the space has some weird edge beyond
which there is no more space. What would be there
beyond space? Is that nothing? Is that a thing without space?
We just don't know. But it's possible for space to
(16:06):
have weird connections so that if you get to the
edge of it, you just can't go that way anymore.
And you might think, well, what does that mean? How
is that possible? Well, remember that in black holes, for example,
space can be distorted so that you can only move
in one direction. Inside a black hole, space is so
distorted that every motion takes you towards the center. It's
an example of a non simple arrangement of space. So
(16:29):
now imagine the edge of the universe where there just
is no direction out. There's no direction you can go
that would take you beyond the edge of the universe.
There's only directions in towards the universe, sort of like
when you're in a black hole. There is no path
out of the universe. But there is an edge to
the black hole, there is an event horizon, so it
(16:50):
is possible to have these weird edges, these discontinuities in space,
places where you just cannot go. Is the universe like that?
We just don't know. Maybe future societies, as they eat
their donuts will laugh at us for thinking that this
was real. Or maybe future societies, as they eat their
healthy snack will be thinking, boy, they were really onto something.
(17:11):
They should have dug even deeper. All right, so let's
get to the last part of Ilia's question, which is
about the donut hole. What if the universe is not
infinite and it's not like the surface of a sphere
where you can sort of move around in every direction.
What if it's connected like a donut. You know, the
surface of a donut is connected differently from the surface
of a sphere. Right on a sphere, if you go north,
(17:33):
you come back to where you started, and if you
go east, you come back to where you started. On
the surface of a donut, it's a little bit different
because on the surface of a donut, if you go north,
for example, you come back to where you started, but
you don't get to the other side of the donut.
To do that, you have to go east right, So
there's no trivial mapping from a surface of a donut
(17:54):
to the surface of a sphere. A donut has a
hole in the middle, and a sphere just doesn't. So really,
this awesome question is is it possible to go through
the donut hole? Is it possible to get to the
other side of the universe without going the long way around? Well,
I think to answer your second question, first, you could
get to the other side of the universe using a wormhole. Right.
(18:16):
If the universe is topologically complex, then you can allow
other wrinkles. Instead of just being sort of hooked together
like a bunch of chain links. But in the shape
of a donut, you could have a connection directly between
one side of the donut and the other. That's what
a wormhole would be. That's exactly what a wormhole is.
It's a connection between two points in space that are
otherwise very distant. And so you can imagine using a
(18:39):
wormhole to get from one side of the donut to
the other. Could you actually be inside the donut? Could
you like leave the universe and fly through the donut
hole to get to the other side. I think Unfortunately,
the answer to this question is no. Even just imagining
the universe as having a hole in the middle, that
that donut hole is a real thing means that you're
(19:00):
taking this analogy one step too far. You're imagining the
universe as embedded in some high dimensional space where that
whole exists. Remember that this donut is an analogy. It's
an analogy of a two D universe that's sort of
painted onto the surface of a three D donut. But
our universe is three dimensions. And again we don't think
(19:21):
that it's painted onto the surface of a four D universe.
We just think that the connections between the points in
the three D universe might resemble the relationships of the
surface of a three D donut. Right, And so if
you want to visualize it, you can imagine putting it
into four D space to sort of arrange it in
(19:42):
your head and think, oh, and in that four D
space you get a three D donut. But that doesn't
mean that four D space exists, And so that doesn't
mean that that place, the whole inside the donut, is
a real thing. Right. Said another way, if the universe
is a donut, then the whole in the inside doesn't
exist as a place you can go anymore than the
(20:03):
part of the outside of the donut exists. There's no
space there, there's no way to be there, no way
you can go there, and so it just doesn't exist.
So sorry, Iliot, you can't take a short cut across
the donut hole, no matter how many donuts you eat.
All right, that was a super fun question, Thanks very much, Ilia.
I want to answer some more questions, but first let's
(20:24):
take a quick break. All right, we're back and this
is Daniel and I'm answering listener questions. We talked about
the universe as a donut, and I hope that during
that break you all went off and got a snack,
(20:45):
healthy or unhealthy. Up to you. And now we're back
and we're talking more physics. And here's an awesome question
from another listener. Hey, Daniel, and explained the universe amazing
programs you have there all the way is always inspurring. Um.
I was wondering the other day how mirrors work, and
(21:07):
then I remember that everything at the smallest scale, it's
even weirder. So how do mirrors actually work at the
quantum level? Are photos bouncing off a mirror? Or are
they being observed and remit it? How does that work?
Thank you love your podcast. All right, this is a
(21:31):
really fun question because it digs really deep into understanding
what light actually is. And I love the whole spirit
of this kind of exercise of looking at things around
us in the world and wondering what's really going on
at the microscopic level. I mean, that's why I'm a
particle physicist, because I have this feeling like everything around
(21:52):
us in the world is not the true world, sort
of build up of tiny little things which, acting together
in vast amounts in aggregate, make these weird effects, these
effects like people and weather and bananas. But that's not
the true stuff of the universe, right, Bananas are not
fundamental to the universe, no matter how much horror Hey
likes to snack on them. The true universe is made
(22:14):
out of the smallest, tiniest things. And it's my sense
that if you understood the universe the smallest, the tiniest bits,
then you're understanding the deep truth and you could always
work your way up from that deep truth to anything
else like supernovas and bananas. And that's why every time
I see something weird in the world. I want to
understand it in terms of the microscopic effects, like what's
(22:37):
going on at the particle level that makes this happen.
And so I think that's the motivation for this question,
this question about how a mirror works, because you could
imagine it like, well, what if light is just sort
of like photons anything of photons as particles, And what
happens when a photon hits a mirror bounces off? And
you have an image in your head of like a
(22:58):
tennis ball bounce off of a wall and it bounces
off and it sort of reflects off the wall. No
big deal, right, But what it really happens microscopically because
if you zoom in, you realize, well, the wall is
not smooth, right, The wall is man out of a
lattice of atoms, and the photon is also small, and
so it's sensitive to these little defects. The wall is
(23:21):
not perfectly smooth, So how can it bounce off of it?
And what is bouncing really mean for a photon? Because
what happens when a photon appurchase the wall is that
it like interacts with the electrons and the atoms in
that lattice of the wall. Right. And photon don't like
touch the wall. They don't bounce off the wall the
(23:42):
way a tennis ball bounces off the wall. The only
way a photon can bounce or touch anything is through
its interactions. That means that it like gets absorbed by
the atom and then gets re emitted. And if it's
getting absorbed and re emitted, how can it bounce off
in the right direction? Right? And so I think that's
(24:02):
the underlying question here. How do we understand reflection? How
do we understand how mirrors bounce photons off in the
right direction? If mirrors are just collections of particles and
photons are just streams of particles. So I think to
understand this question and to keep your handle around it
at the microscopic level, there's two important things to understand,
(24:24):
two effects that we have to have in our minds
to make a clear picture of what's going on. And
the first is what's going on between the photon and
this particle in the mirror, this bit of the mirror
that it's interacting with. So photon approaches the mirror and
you can think of the photon as a particle, or
you can think about it as a wave. Doesn't really matter.
But remember, if you're thinking about it as a particle,
(24:46):
its motion is determined by its quantum waves. So it's
really the wave like effects that dominate here. Now I'm
not saying a photon is a wave. I'm not saying
it's a particle. If you've been listening to the podcast,
you know that. I think it's neither a particle nera wave.
It's some weird other quantum mechanical thing which can't be
perfectly described by anything we have intuition for. But sometimes
(25:09):
some pictures are more useful than others. So what happens
when the photon approaches the bit of the mirror, Well,
photons can only really do one thing, which is that
they can get absorbed by charged particles and then they
can get re emitted. And in this case it's mostly
the electrons. Like there are charges also in the atomic nucleus,
but the nucleus is surrounded by electrons, and so when
(25:30):
the photon approaches the service of the mirror really just
sees the electrons. It sees this like wall of electrons.
And that's okay. Photons can talk to electrons. They're happy
to do that. But the only way for that to
happen is for the electron to absorb the photon, then
it has more energy and then it can give off
that photon. It emits that photon, and then the question
(25:51):
is how does it know which direction to emit that photon?
When an electron which is spinning around an atom gives
off a photon, doesn't it just emit it sort of randomly?
And you can imagine all of these atoms is just
like tiny little sources of light because they've absorbed the photon.
Now it's there's why don't they just shine them all
in different directions? Well, that is the right way to
(26:12):
think about it. You can think about each of these
atoms which receives a photon as a little sort of
point source is giving off a little bit of light.
And this happens in lots of other examples too. For example,
when light goes through a little slit, as it famously
does in the double slit experiment, when it comes out
of that slit, it's not going necessarily in the same
direction as when it went into the slit. Now it's
(26:35):
like a little point source at the slit, and that's
what you need to have, Like interference between two slits.
You need a light to be coming out in lots
of different directions, so we can add up constructively or destructively.
And that same picture also works for what's happening at
the surface of a mirror. The photon is absorbed by
an atom and then the atom emits, and it emits
(26:56):
in all directions. So why does the photon end bouncing
at the correct angle? Right? Because if the photon comes
in at a very steep angle, comes out at a
steep angle. If photons come in a very shallow angle
to come out at a shallow angle, well, the way
it works is that the photon comes out not just
with the direction, but also with a phase. So the
(27:16):
quantum wave function of the photon that determines where it's
going has a number in it called the phase, sort
of like how much it's spun. And the atom actually
emits photons in all directions, but with different phases, and
those phases determine whether or not the photons add up
constructively or destructively. And the phase that comes out depends
(27:39):
on the angle that the photon came in. And so
what happens is photon comes in, gets absorbed by the atom,
and the atom emits photons sort of in every direction
belt with different phases, and only in the right direction,
in the direction that sort of corresponds to the angle
of photon came in. Do those photons not cancel each
other out, So you get destructive interference for all the
(28:02):
other directions, and constructive interference in the direction that corresponds
to the same angle that came in. And that's what
mirrors do. They bounce photons off, so the angle they
leave at is the same as the angle they came in.
Becoming at ninety degrees, you leave at ninety degrees, if
you come in at two degrees, you leave at two
degrees in the other direction. So the atom is absorbing
(28:24):
that photon and then it is sort of emitting in
every direction, but there is one direction where the phase
is right for the photons to not get canceled out
on top of each other. So that's why one direction dominates.
And you can think about that either in terms of
like lots of photons coming in in parallel, hitting the
surface and then sprang out in all different directions, and
(28:45):
the ones that are sort of in the wrong directions
end up canceling each other out, and that's why you
only get photons coming out in parallel, or you can
actually also think about it in terms of one photon.
This is sort of mind blowing the same with the
double slit experiment. Is the single photon hitting a mirror
doesn't have like neighboring photons on either side to do
(29:06):
the destructive and constructive interference for it. So how does
that happen? Well, when a photon is absorbed by the
atom and then emitted, there's a probability distribution there for
where it's going to go, and it's those probabilities that
can interfere with each other. Just like a single particle
going through the double slit experiment has probabilities to go
through either slit, a single photon hitting a mirror has
(29:28):
probabilities to go in lots of different directions, and those
probabilities can interfere with themselves. So single particles probabilities interferes
with itself, and that was determines the direction that the
particle can go in. Alright, So I said that there
were two things you have to understand to figure out
how mirrors work. When we just talked about how adams
(29:48):
absorbed and re emit the light and how it ends
up going in the correct direction, because the quantum mechanical
phase of the wave function ensures that that direction is
the only one that doesn't get destructive inference. The second
is that conductors make better mirrors than non conductors. Things
that do conduct electricity make good mirrors. That's why metals,
(30:10):
for example, make good mirrors a good sheet of aluminum,
and the reason for that is that conductors don't let
photons penetrate deep into their surface, and instead they mostly
reflected just from the very very thin sheet of the surface,
which gives you a coherent image. If a particle went
deep into a material and was absorbed before being re emitted,
(30:31):
like a centimeter inside the material, it might never emerge,
and if it did, it might not be as coherent
because they will have interactive with lots of different atoms.
To get a nice, really crisp picture, you want to
reflect just off the very surface of the material, so
you're reflecting from a very flat plane, not getting the
distortions you would get if you deflected off of a
surface that was not smooth, and so if particles can
(30:54):
penetrate different levels of the material, you're basically reflecting off
a not smooth mirror and you get models mess So
metals are very good mirrors because they don't let photons
penetrate very far into them. And that's because they are conductors,
because they can rearrange their electrons to basically cancel out
any electric field. If you have an electric field and
(31:16):
you put a conductor in it, it will move its
electrons around to balance that electric field, because electric fields
act on those electrons, and if you have a positive
voltage in one direction, it will pull electrons in that
direction to cancel it out. And so conductors, because they
have these electrons to sort of slosh around inside them
and reconfigure pretty easily, can cancel any electric field inside
(31:40):
of them. That's why, for example, it's very difficult to
get a phone call inside an elevator. Right in an elevator,
you're surrounded by a metal box, and a phone call
is just radio waves. It's a fluctuation in the electromagnetic spectrum,
and that will just induce electrons inside the elevator walls
to move round to cancel it. So a Faraday cage
(32:03):
is nothing but a metal box, and it can pretty
effectively cancel almost any electromagnetic signal. The other side of
that is that mirrors don't allow photons to go very
deep into the material because they can rearrange the electrons
to avoid those e M fields going into the material.
And that makes them good reflectors because the photons reflect
(32:23):
just from the surface rather than deep in the material
where the image might get muddled. All right, So next
time you're looking in the mirror and you're wondering, wow,
why am I so good looking, it's because of quantum
mechanics and because of electrons zooming around to give you
a nice, crisp, clear picture of your smiling face. All right,
I hope that answered your question. I have one more
(32:45):
question I really want to get to, but first let's
take another break. All right, we are back today. It's
just me, Daniel, the particle of hysicist, answering your questions
(33:07):
about the nature of the universe, whether the universe is
as shapely and as hasty as a donut, whether photons
bounce off a mirror in the same way that a
tennis ball bounces off a wall, and how does that
all work? And now we're gonna get to the last
question to day, which is a classic a question I
love talking about. So here it is. Hello, Daniel, and JAYI.
(33:29):
My name is David Somerville. I live in Fort Worth, Texas,
but I'm originally from Yorkshire in England. I love the
way you guys bound stuff off each other on the podcast.
It really works well, so please keep up the good work.
I got a question for you. Black holes arenknown to
have such huge gravitational pull that nothing can escape, not
(33:52):
even light. However, photons are massless, so how can the
gravitational force of a black hole have any effect online
at all? No matter whether you use eth equals m
A or eth equals g mm over on squad, if
the mass of the photon is zero, then it means
that that force on the light is zero, meaning it
(34:14):
will carry on on its merry way unaffected. Obviously this
is wrong, but I don't understand why. If you could
explain that on the podcast, I would really appreciate it.
Thank you all right, thank you very much for this
wonderful question. This is a great question, not just because
it's fun to talk about black holes and photons and
all that stuff, but because it shows somebody being a
(34:36):
physicist in action. You're taking one idea black hole with
crazy power from gravity, and then applying your understanding of
gravity to it and saying, hold on a second, this
doesn't work, This doesn't connect in my head. These two
ideas clash, and so help me understand it. And that's
what we're here for. We are here to help you
(34:57):
understand the universe and resolve all of these ideas is
in your head. So this short I answer you to
your question is that you're right, it doesn't work because
you're using Newton's physics, Newton's law of gravity, which cannot
describe what happens inside a black hole, and you've got
to upgrade to Einstein's theory of gravity, which can describe it.
(35:18):
All right, but let's talk a little bit more about
what that means. You were walking through your explanation and
your question and you were thinking, well, gravity is a force,
and it's a force between things that have mass. Right,
that's what we typically think about as gravity. You are
held onto the Earth because the Earth has a lot
of mass and you have mass, and gravity is a
(35:38):
force between objects with mass. That was Newton's picture of gravity.
That was Newton's description, and he formulated it mathematically. Force
of gravity is big G, which is just a number
times the two masses involved. Divided by the distance between
them squared. So what does that tell us. That's analyze
that for a moment. It means that the more mass
(36:00):
as you have, the more force of gravity F equals
g m m over our squared. The bigger the ms,
the bigger the F and so the more force you have.
And that's why, for example, you feel stronger pulled towards
a heavier object like the Earth than you do to
a grapefruit that's even closer to you. And the one
in the bottom though, the bit on the bottom of
the equation is the one over our square, and that
(36:21):
tells you that gravity gets weaker as you get further,
and if you're twice as far away, it gets four
times weaker because it's an R squared. So you're right
that if you use Newton's idea of gravity, and you
plug in the mass of a photon being zero, then
you get zero force no matter what the other masses,
the other masses ten billion sons. It doesn't matter according
(36:44):
to Newton, because Newton thought that gravity was just a
force between two objects. But Newton was wrong in a
couple of really important ways. One is that he thought
that gravity was instantaneous he thought that information about gravity
try I will through the universe with no delay. He thought,
for example, if the Sun disappeared, you would stop feeling
(37:06):
its gravity just like that, there would be no delay. Now,
of course, we know that it takes time for all
information to propagate through the universe, and that if the
Sun disappeared, you wouldn't feel it for eight minutes, because
that's the maximum speed of information through the universe. So
that's one thing about gravity that Newton got wrong. He
thought it was instantaneous, and Einstein showed us that it
(37:28):
can't be. But Einstein also showed us something much much deeper.
He showed us that the right way to think about
gravity is not that it's a force at all. Instead
that it's a bending of space, and it's this bending
of space that affects the way things move in such
a way that it looks as if there was a
force we call gravity. Now, you may have heard that
(37:49):
a few times, so I don't want to really sink
that into your brain. I want to make sure you
guys really understand what that means. So let's walk through
a little analogy. Say, for example, you're on the surface
of the Earth and you're on the equator and your
friend is somewhere else on the equator, and you both
have perfect compasses, and you say, all right, we are
going to head north. Everybody walk north. Now, if you're
(38:09):
walking north and your friend is also walking north, then
you're walking in parallel directions. Right, If you both walk
north on the surface of the Earth, you're walking in
parallel directions. And you imagine that two people walking in
parallel should never meet. Right. If you take two beams
of light and you make them perfectly parallel, they will
never touch each other. That's true unless space is curved,
(38:33):
just as it is on the surface of a sphere
and on the surface of the Earth. What happens if
you start on the equator and you walk north. Doesn't
matter how far apart you are, everybody will end up
at the north pole. Those lines will cross. So parallel
lines on a curved surface do in fact cross. Parallel
(38:53):
lines on a flat surface will not cross. Right, So
what is it like for somebody on that surface If
you aren't walking north and your friends started out, you know,
maybe ten tho kilometers or even just ten ms away
from you, you notice them gradually getting closer and closer
to you, what is that like. It's like something is
(39:13):
pulling you together. It's as if there was a force there,
pulling everybody together, so that by the time you got
to the north pole boom, you're clumped together into a
little dot. So you can see there how a curved space.
Moving in a curved space can make us feel as
if there's an apparent force there. And if you didn't
(39:34):
understand that you were on the surface of a larger object.
If you thought, hey, this is my universe, I'm a
two D person, you might come up with some force
of gravity to explain why things are moving towards each
other even if they take off in parallel directions. That's
a helpful analogy, but there's also an important element of
it missing as we try to take our minds from
(39:56):
moving on the surface of a sphere like the Earth,
to moving in our or universe. Right, the service of
a sphere is too D and we wrap it around
a three dimensional sphere in order to understand it. But
our universe is three D. Does that mean that we're
wrapping it around some four dimensional sphere? No, Because the
curvature that we talk about when we say that mass
(40:17):
bends space, when we say that general relativity describes gravity
as a curvature of space. The curvature we're talking about
there is not extrinsic curvature. It's not curvature relative to
some four dimension not wrapped around the surface of some
higher dimensional sphere. It's an intrinsic curvature. What does that mean,
(40:38):
intrinsic curvature. How do you have curvature if it's not
relative to like some outside ruler. Well, an intrinsic curvature
is just a change in the relationship between points in space.
It says, if you have a flat space and you
arrange a grid, then all the points are equidistant. But
then if you distort it, if you add mass to it,
(41:00):
curve that space, then you change the relationship between those points.
Some of those points are now closer to each other
than they were a moment ago. Some of the points
are now further from each other than they were a
moment ago. So what happens, for example, to light traveling
through space. Well, light always follows the shortest path between
two points, and if space is flat, then light just
(41:22):
follows a straight line right and goes from point A
to point B, and the shortest path is a straight line. Now,
imagine a sheet of graph paper, right, you right point
A in one spot and point B in the other spot,
and you measure the shortest path between point A and
point B. It's going to be a straight line between
those points. And the grid is square right space is flat.
(41:43):
Now what if I took that graph paper and I
distorted it. I made some of those points closer together
and other points not as close together, And then I
asked you, all right, now find the shortest path from
A to B. Some of those points don't cost as
much to go between as other points, and so now
the shortest path between A T V might not be
(42:04):
the same as it was before. It might require some
sort of curve or a wiggle or two. That's the
kind of intrinsic curvature we're talking about. What happens when
you have a blob of mass inside some space. Well,
it bends space, and it bends space intrinsically so that
the relative distances between points change. And that's why, for example,
(42:26):
the Earth goes around the Sun, not because there's a
force of gravity on the Earth pulling it, but because
the shape of space around the Sun is curb so
that an object moving inertially with no forces on it,
We'll move in a circle around the Sun because that
is the shortest path, that is the natural path for
(42:47):
an object in that curved space. All right, so we
understand that gravity is not just a force g mm
over our squared is a curvature of space. Now, it
happens to me that you can't in take Newton's picture
and it mostly works. That is, you can assume that
space is flat, that there's no curvature, and you can
treat gravity as if it was a force, and you
(43:08):
can write this mathematical equation and mostly it does work,
and that's cool, And that just shows you how a
lot of times in physics of the universe you can
have different ways to see the same effects. But it
only mostly works. It breaks down in some cases. It
breaks down when the masses get really really large, for example,
in the vicinity of a black hole, because what's going
(43:30):
on in a black hole. A black hole is a
huge amount of mass, but it's not that it has
an incredibly high value for the force of gravity. Just
because the m value is so large, it's a distortion
of space that's so great that space becomes one directional
inside the event horizon. Past this point there are only
(43:51):
paths that move towards the center of the black hole.
It's not just that light is being pulled by a
very very strong force, because if you use Newton's law
to calculate the force, you would get zero. It doesn't
matter how much mass you have, because gravity bends space
so much that even photons cannot escape. And the reason
(44:13):
is because there is no path out. There's no direction
you can go if you're inside the event horizon, which
will lead you out. It doesn't matter how fast you move,
doesn't matter what the forces are on you. The direction
of space requires you to move towards the center of
the black hole. Right, And so that's a key understanding.
If you think about gravity not as a force but
(44:35):
as a distortion of space, then black holes can make sense.
And it's not even just in black holes that photons
are affected by gravity, right. Photons are also affected just
by the changing of shape and space in other non
extreme scenarios. For example, we see things like gravitational lensing.
If you have a galaxy in between you, and that
(44:57):
galaxy is a huge blob of dark matter, for example,
then that dark matter can act like a lens because
it's mass changes the shape of space, and its light
passes through it, it gets distorted in just the same
way as if there was a huge lens in space.
Even though light has no mass, it's the shape of
space itself that has changed, so that now the shortest
(45:19):
path for light to follow from that galaxy to your
eyeball is a sort of a curved path. So this
example gravitational lensing shows us how light can be affected
by gravity. And that's how we know that Einstein is
more right than Newton, because we found these scenarios where
their predictions are different, and lots of situations they give
exactly the same predictions. You can think about the shape
(45:41):
of space being curved, or a force between two objects
and flat space, and you get exactly the same answer.
But in some scenarios, and this is what Einstein cooked
up to test his theory, you can see differences. And
one example scenario is when light gets bent around a
heavy object. Because Newton would say it should not be
bent all in Einstein would say space is curved, and
(46:03):
even massless objects will be influenced by the curvature of space.
All right, I hope that answers your questions and explains
why photons feel gravity even if they have no mass.
It's because gravity is the distortion of space itself, and
photons have to fly through that space, and so they
are affected by gravity. And thanks to everybody who's been
(46:25):
sending in questions, and mostly thank you to everybody who's
been thinking deeply about physics and wondering about the universe
on this program. We love to wonder, we love to think,
We love that there are questions, and sometimes we just
love to luxuriate in our ignorance to know that there
are answers out there to these questions. That for these
really big science questions, there are factual, objective answers that
(46:48):
one day humanity will uncover. If we keep doing science
and we keep pushing forward on the forefront of knowledge,
we will get answers to some of these really big,
deep questions about the universe, things that we cannot imagine
what it's like to know. People in the future will
know those things, and they will look back at us
and they will wonder what was it like to be
(47:09):
a caveman and not know that the ocean lay just
twenty miles away or fifty miles away. What is it
like to be a modern human today and not understand
the size of the universe, whether shape or the universe,
or how the universe was created. These answers are out there,
and the way to getting those answers is to keep
doing science, to keep asking questions, to keep wondering how
(47:30):
the universe works. So thanks everybody for lending us your
curiosity and taking this ride with us. Please send us
your questions, Thanks for listening, and remember that Daniel and
Jorge explained the universe. Is a production of I Heart Radio.
(47:52):
For more podcast from my Heart Radio, visit the I
Heart Radio Apple Apple Podcasts, or wherever you listen to
your favorite shows.
I like to think sometimes about what it was like
to be a caveman or cave one. You know, they
probably didn't travel nearly as much as we do, and
so they just didn't have a global sense for where
they lived in the world. They just saw a little
patch and all they could do was wonder about what
lay beyond. Now, of course, we know so much more
about the world, and so we can look at the
(00:31):
bones of Stone Age people and feel like we might
know more about their world than they did. There might
have been people who lived near the ocean but never
saw it, things that were right around the corner they
didn't even know about. But aren't we still cave men
and cave women in that way? We still see our
little patch of the universe, and we don't know what's
(00:52):
around the corner. What if there's some crazy awesome thing,
but it's just a little bit further than we can see.
And so that makes me wonder will future humans look
back at us in the same way, chuckling to themselves
about how clueless we are? I sure hope so, because
it means physicists have been doing their jobs. Hi. I'm Daniel,
(01:29):
I'm a particle physicist and Welcome to the podcast Daniel
and Jorge Explain the Universe our production of My Heart
Radio today and the podcast is just me. Jorge can't
be here today to join us, and so I'm gonna
be taking you on a tour of all the amazing
things in our universe, all the incredible things that we
want to understand, all the distant places that we can
(01:51):
just barely prob with our telescopes, all of the tiny
particles that we seek to understand. This podcast is about
all of those things and everything. We want to explain
the entire universe in a way that makes you come
away thinking I get it, and maybe once or twice
even makes you chuckle. And one of the things that
we cherish on this podcast is not just the things
(02:14):
that science has been working on, but the things that
you have been wondering about. Because physics is not something
which belongs to a tiny group of people who happen
to get to be professors of physics, but it's something
we can all do. Every time you wonder about the universe,
every time you think to yourself, how does that work?
Every time you hear two things you don't quite know
(02:35):
how to fit them together in your mind. You are
doing physics, you are being a physicist, and we love
that here on the podcast, and we want to encourage that.
And one way we do that is by answering your questions.
So if you have questions about the way the universe works,
you have lots of ways to get in touch with us.
You can send us an email to questions at Daniel
(02:57):
and Jorge dot com. You can engage with us and
Twitter at Daniel and Jorge, or you can come and
ask me in person. Come to one of my public
office hours where I hang out on zoom and answer
questions from anybody and everybody about physics. For information on that,
go to my website to sites dot you see I
dot edu, slash Daniel and you'll find all the relevant
(03:20):
connection information. Sometimes I'll get emailed the question from a
listener and it's such a good question, such a fun topic,
that I think I'm not just gonna write back and
said them the answer. I want to talk about this
on the podcast, So we'll ask them to send me
an audio clip of themselves asking the question, because I
think all of you might be interested in the answer.
(03:41):
And that's exactly what we're gonna do on today's episode. Today,
I'll be catching up with our backlog of listener questions
while Jorge is on a break and answering questions from
real people. Questions about physics, questions about which corner of
the universe we're in, questions about how mirrors work, Questions
about the potns and gravity, all the kind of things
(04:02):
that thinking people wonder about when they try to understand
the world around them. All right, so let's get to it. First.
Up is a question about the universe? Are Daniel and Hord.
My name is Elia, and I'm a big fan of
your podcast. I've always wondered whether or not it is
possible to know which corner of the universe we're in,
depending upon what we believe the shape of the universe is,
(04:26):
and if it's shaped like a doughnut, for example, is
it possible to cross through the donut hole and re
enter the universe provided inflation suddenly stopped. Thank you so
much for teaching and inspiring us. All right, thanks very much,
Ilia for that super fun question. I like this question
a lot, not just because it talks about doughnuts and
that makes me hungry and it makes me wonder was
(04:48):
Ilia eating a doughnut when he asked me this question,
why a donut? Why not like a pastry shaped universe,
or you know, a pie shaped universe. But we'll dig
into that and take a bite out of that part
of the question. But I also love this question because
it touches on something really deep and fascinating, not just
about the shape of the universe, but about where we
are in the universe. Are we in an interesting part
(05:12):
of the universe? Are we near the center of the universe?
Are we near some weird edge or corner of the universe?
I love when you take a deep question about the universe,
how big is it? What shape is it? And you
make it a personal question, like where are we in
the universe? So let's dig into it. Where are we
in the universe? What corner of the universe are we in?
(05:35):
And you're totally right ilia that the answer to this
question depends a lot on what we think about the
shape and the size of the universe. So let's answer
it in a few different ways. Let's start with the
most likely configuration for the universe and imagine that the
universe is infinite. That means that it goes on forever
in every direction, there's no edge, there's no point at
(05:57):
which the universe stops. It just goes on forever. And
it's sort of euclidian in a sense that you can
go on forever without looping around on yourself, and the
two parallel lines will go on forever without crossing. Why
do I say this is the most likely scenario, the
most likely set up for a universe. I just think
it's the most natural because it has no edges, because
(06:18):
it's sort of simple. If you have an edge, then
you have to explain why that edge. If you have
an edge, you have to answer why is this edge
over here and not somewhere else? You need a special case.
And I like explainations that don't have special cases, that
are simple and that our universal. And so this one
that the universe just sort of goes on forever in
(06:39):
every direction is nice because it makes no place in
the universe special. Every point in the universe has an
infinite amount of universe in every direction, and so it's
very similar everywhere. And that's the key thing. That's a
crucial idea in cosmology recently. This cosmological principle, the one
that says that every location in the universe is equivalent.
(07:03):
There's no center, there's no edge, there's no corners. Every
place in the universe is basically like every other place
in the universe. Now, let's unpack that a timely a
little bit. What do we really mean when we say
every place in the universe is like every other place
in the universe. Because my house is not the same
as your house, and the Earth is not the same
(07:24):
as Jupiter, and there are galaxies, and there are places
where there are no galaxies. So like, at the very
microscopic level, it's not true that every place in the
universe is the same as every other place in the universe.
What we mean when we say that is that the
laws of physics are the same, that they always apply
in the same way. That you don't have different laws
and different constants over there than you have over here. Now,
(07:48):
if you have the same laws of physics, how is
it you can even have any difference? How is it
that you can have a planet here and not a
planet there, or you have all these galaxies over here
and no galaxies over there. Well, the current understanding is
that this comes from quantum randomness in the very early
universe when it was hot and dense, but still infinite, right,
(08:08):
still goes on in every direction, but hotter and denser
and younger. That there were quantum fluctuations that made some
spots in the universe more dense, with stuff in some
spots less dense. Now does that mean that those spots
are different from the other spots, Well, they got different
random numbers, but they had the same opportunity. They're playing
the same game and they just had a different outcome.
(08:30):
And quantum mechanics is amazing because the rules are the same.
It gives the same probability distribution for every scenario that's identical,
but individual outcomes can be different. You know. Quantum mechanics
tells us that if you poke a particle the same
way a hundred times, you might get a hundred different outcomes.
Or you might have a scenario where half the time
(08:50):
it goes left and half the time it goes right,
even if the initial conditions are exactly the same. So
that's what's happening in the very early universe. The initial
conditions great energy density is the same everywhere, but different
things can happen in different places. They are following the
same rules, so there's no special place, but they have
a different outcome, and then those little pockets of over
(09:12):
density get expanded into massive macroscopic effects, which seed the
structure of the universe when the universe expanded very rapidly
in the early Big Bang, So you have hot, dense
stuff in the very early universe. Random quantum fluctuations make
some of it a little bit more dense. Inflation expands
those tiny little microscopic things into big macroscopic things, and
(09:35):
then gravity takes over. So that's how you can have
a universe where the rules of physics are the same everywhere,
but you don't actually have the same outcome everywhere. You
have galaxies here and no galaxies over there. So in
that scenario, the universe is the same everywhere. There's no corners,
there's no center, and everywhere in the universe follows the
(09:56):
same rules. And so where are we in the universe? Well,
where he but here is sort of the same as
there so here, or there there or here. It sounds
sort of like a Doctor Seuss book, right, but it's
actually real. And this I think is the most prevalent
idea for where we are in the universe because it
doesn't require any special cases. Now, for those of you
(10:16):
who are thinking back about how I described the Big
Bang and thinking, hold on a second, wasn't the Big
Bang at a certain point? I described it as a
moment when the universe was hot and dense and young. Right,
That doesn't mean that it was hot and dense and localized.
The way I think about the Big Bang is not
as a tiny dot which then exploded out to fill
(10:39):
up the universe with stuff. The more modern view of
the Big Bang is that it was a change from
high density to low density. I'm using different words there
because I mean something different. I mean that the universe
was high density but still infinite, So not a tiny
little dot of stuff like many people used to describe
the Big Bang, but an universe filled with hot, dense
(11:02):
stuff which then expanded. So now that hot dense stuff
becomes cold and dilute, right, it spreads out. Space itself
is expanded. You create new space between stuff in order
to go from a hot, dense universe to a cold,
dilute universe. So the Big Bang was not an explosion
as much as an expansion of space. It's stretched space everywhere,
(11:26):
So the Big Bang happened everywhere, all at once. So
that's the answer. In the scenario that the universe is infinite,
that it goes on forever in every direction, and in
that scenario, doesn't really make sense to talk about where
we are in the universe because everywhere is the same place.
But as we've talked about on the podcast a few times,
(11:46):
we don't know that the universe is infinite. We can't
know that the universe is infinite. We could only see
a finite patch of the universe, just like ancient people's
who thought about the universe but never traveled very far
from their home. They only saw a tiny little patch,
and they could only imagine what was beyond the edge
of their knowledge. In the same way, there's a patch
(12:08):
of the universe about ninety billion light years across that
we can see. We call this the observable universe, and
beyond it, we don't know what's there. Why can't we
see it, Well, it's a physical limitation. It's because of
the speed of light. Light just hasn't had enough time
to get to us from there since the beginning of
(12:28):
the universe. It's on his way. It's been flying our
direction the whole time. The universe has been doing its
universe thing. But it started out so far away that
even though it's traveling at light speed, it hasn't reached
us yet, and it will. And every year that goes
by the size of the observable universe, the portion of
the universe that we can see gets bigger and bigger,
(12:52):
and we can see more and more stars and more
planets around them, and all sorts of amazing interesting stuff.
And of course, first we see the stuff that's happened
were teen billion years ago, because that's when the light
left it to get to us. So as we look
further out into the universe, we actually see the past
more than we see the present. So if there was
something crazy happening just past the edge of the observable universe,
(13:15):
we wouldn't see it for a while. If it's just
happening now, if there's some crazy dancing banana party happening
on a planet just past the edge, it's going to
take billions of years before those awesome images get to us,
so we can embarrass the aliens by posting them on
social media. So what lies beyond the edge of the
observable universe we don't know, and that's one reason why
(13:35):
we don't know the shape of the universe. There are
a few scenarios for how the universe could be shaped.
One of course, is that it's infinite. Another is that
it's not infinite, but it doesn't have an edge, something
like the surface of a sphere. As you walk along
the Earth, you're not aware of there being any edges.
I mean, there are oceans and all sorts of stuff
and walls and fences, but sort of geo magically only
(13:58):
imagine yourself walking just on the surface of a perfect sphere.
You wouldn't notice any edges. You could go on forever
without bumping into anything, and eventually you could even come
back to where you started. So that's a universe that's
not infinite, has a finite amount of area the surface
of a sphere to make sort of the leap from
the two dimensional surface of a sphere where you're walking
(14:19):
around on it now into three dimensional space. How does
that work? Are we saying that three dimensional space is
sort of like plastered onto the surface of a four
dimensional sphere. No, we're just saying that the universe could
be connected differently. We don't know if our three dimensional
space is like embedded in some higher dimensional space four
or eleven or twenty six. Though, if you're interested in
(14:42):
that kind of stuff. We have a whole podcast episode
about how many dimensions there are in space. But even
if we just assume that space has three dimensions, it's
still possible for it to curve, because space can be
bent and connected in weird ways. You can loop around itself,
you can have all sorts of distortions. And we'll talk
about this actually later today in the same podcast. But
(15:02):
you can imagine space being complex in such a way
that bits of it are connected to each other, so
that if you did embed them in a four dimensional space,
it would look like a weird shape. It would look
like the surface of a sphere, or it would look
like a donut or a banana or a pastry or
something else. And now I'm getting too hungry to continue
answering this question. But we just don't know the shape
(15:23):
that all of these things are possible, because we just
can't see enough of the universe to rule some of
this out. Now, it's also possible that the universe isn't
infinite and that it does have some weird edge. Remember,
space is not something that we understand very well. Only
for the last hundred years or so have we even
understood that space is a thing that it can bend
(15:44):
and distored and expand and grow and do all sorts
of weird stuff. So we're at the really the very
beginning of our understanding of the nature of space. And
it might be the space has some weird edge beyond
which there is no more space. What would be there
beyond space? Is that nothing? Is that a thing without space?
We just don't know. But it's possible for space to
(16:06):
have weird connections so that if you get to the
edge of it, you just can't go that way anymore.
And you might think, well, what does that mean? How
is that possible? Well, remember that in black holes, for example,
space can be distorted so that you can only move
in one direction. Inside a black hole, space is so
distorted that every motion takes you towards the center. It's
an example of a non simple arrangement of space. So
(16:29):
now imagine the edge of the universe where there just
is no direction out. There's no direction you can go
that would take you beyond the edge of the universe.
There's only directions in towards the universe, sort of like
when you're in a black hole. There is no path
out of the universe. But there is an edge to
the black hole, there is an event horizon, so it
(16:50):
is possible to have these weird edges, these discontinuities in space,
places where you just cannot go. Is the universe like that?
We just don't know. Maybe future societies, as they eat
their donuts will laugh at us for thinking that this
was real. Or maybe future societies, as they eat their
healthy snack will be thinking, boy, they were really onto something.
(17:11):
They should have dug even deeper. All right, so let's
get to the last part of Ilia's question, which is
about the donut hole. What if the universe is not
infinite and it's not like the surface of a sphere
where you can sort of move around in every direction.
What if it's connected like a donut. You know, the
surface of a donut is connected differently from the surface
of a sphere. Right on a sphere, if you go north,
(17:33):
you come back to where you started, and if you
go east, you come back to where you started. On
the surface of a donut, it's a little bit different
because on the surface of a donut, if you go north,
for example, you come back to where you started, but
you don't get to the other side of the donut.
To do that, you have to go east right, So
there's no trivial mapping from a surface of a donut
(17:54):
to the surface of a sphere. A donut has a
hole in the middle, and a sphere just doesn't. So really,
this awesome question is is it possible to go through
the donut hole? Is it possible to get to the
other side of the universe without going the long way around? Well,
I think to answer your second question, first, you could
get to the other side of the universe using a wormhole. Right.
(18:16):
If the universe is topologically complex, then you can allow
other wrinkles. Instead of just being sort of hooked together
like a bunch of chain links. But in the shape
of a donut, you could have a connection directly between
one side of the donut and the other. That's what
a wormhole would be. That's exactly what a wormhole is.
It's a connection between two points in space that are
otherwise very distant. And so you can imagine using a
(18:39):
wormhole to get from one side of the donut to
the other. Could you actually be inside the donut? Could
you like leave the universe and fly through the donut
hole to get to the other side. I think Unfortunately,
the answer to this question is no. Even just imagining
the universe as having a hole in the middle, that
that donut hole is a real thing means that you're
(19:00):
taking this analogy one step too far. You're imagining the
universe as embedded in some high dimensional space where that
whole exists. Remember that this donut is an analogy. It's
an analogy of a two D universe that's sort of
painted onto the surface of a three D donut. But
our universe is three dimensions. And again we don't think
(19:21):
that it's painted onto the surface of a four D universe.
We just think that the connections between the points in
the three D universe might resemble the relationships of the
surface of a three D donut. Right, And so if
you want to visualize it, you can imagine putting it
into four D space to sort of arrange it in
(19:42):
your head and think, oh, and in that four D
space you get a three D donut. But that doesn't
mean that four D space exists, And so that doesn't
mean that that place, the whole inside the donut, is
a real thing. Right. Said another way, if the universe
is a donut, then the whole in the inside doesn't
exist as a place you can go anymore than the
(20:03):
part of the outside of the donut exists. There's no
space there, there's no way to be there, no way
you can go there, and so it just doesn't exist.
So sorry, Iliot, you can't take a short cut across
the donut hole, no matter how many donuts you eat.
All right, that was a super fun question, Thanks very much, Ilia.
I want to answer some more questions, but first let's
(20:24):
take a quick break. All right, we're back and this
is Daniel and I'm answering listener questions. We talked about
the universe as a donut, and I hope that during
that break you all went off and got a snack,
(20:45):
healthy or unhealthy. Up to you. And now we're back
and we're talking more physics. And here's an awesome question
from another listener. Hey, Daniel, and explained the universe amazing
programs you have there all the way is always inspurring. Um.
I was wondering the other day how mirrors work, and
(21:07):
then I remember that everything at the smallest scale, it's
even weirder. So how do mirrors actually work at the
quantum level? Are photos bouncing off a mirror? Or are
they being observed and remit it? How does that work?
Thank you love your podcast. All right, this is a
(21:31):
really fun question because it digs really deep into understanding
what light actually is. And I love the whole spirit
of this kind of exercise of looking at things around
us in the world and wondering what's really going on
at the microscopic level. I mean, that's why I'm a
particle physicist, because I have this feeling like everything around
(21:52):
us in the world is not the true world, sort
of build up of tiny little things which, acting together
in vast amounts in aggregate, make these weird effects, these
effects like people and weather and bananas. But that's not
the true stuff of the universe, right, Bananas are not
fundamental to the universe, no matter how much horror Hey
likes to snack on them. The true universe is made
(22:14):
out of the smallest, tiniest things. And it's my sense
that if you understood the universe the smallest, the tiniest bits,
then you're understanding the deep truth and you could always
work your way up from that deep truth to anything
else like supernovas and bananas. And that's why every time
I see something weird in the world. I want to
understand it in terms of the microscopic effects, like what's
(22:37):
going on at the particle level that makes this happen.
And so I think that's the motivation for this question,
this question about how a mirror works, because you could
imagine it like, well, what if light is just sort
of like photons anything of photons as particles, And what
happens when a photon hits a mirror bounces off? And
you have an image in your head of like a
(22:58):
tennis ball bounce off of a wall and it bounces
off and it sort of reflects off the wall. No
big deal, right, But what it really happens microscopically because
if you zoom in, you realize, well, the wall is
not smooth, right, The wall is man out of a
lattice of atoms, and the photon is also small, and
so it's sensitive to these little defects. The wall is
(23:21):
not perfectly smooth, So how can it bounce off of it?
And what is bouncing really mean for a photon? Because
what happens when a photon appurchase the wall is that
it like interacts with the electrons and the atoms in
that lattice of the wall. Right. And photon don't like
touch the wall. They don't bounce off the wall the
(23:42):
way a tennis ball bounces off the wall. The only
way a photon can bounce or touch anything is through
its interactions. That means that it like gets absorbed by
the atom and then gets re emitted. And if it's
getting absorbed and re emitted, how can it bounce off
in the right direction? Right? And so I think that's
(24:02):
the underlying question here. How do we understand reflection? How
do we understand how mirrors bounce photons off in the
right direction? If mirrors are just collections of particles and
photons are just streams of particles. So I think to
understand this question and to keep your handle around it
at the microscopic level, there's two important things to understand,
(24:24):
two effects that we have to have in our minds
to make a clear picture of what's going on. And
the first is what's going on between the photon and
this particle in the mirror, this bit of the mirror
that it's interacting with. So photon approaches the mirror and
you can think of the photon as a particle, or
you can think about it as a wave. Doesn't really matter.
But remember, if you're thinking about it as a particle,
(24:46):
its motion is determined by its quantum waves. So it's
really the wave like effects that dominate here. Now I'm
not saying a photon is a wave. I'm not saying
it's a particle. If you've been listening to the podcast,
you know that. I think it's neither a particle nera wave.
It's some weird other quantum mechanical thing which can't be
perfectly described by anything we have intuition for. But sometimes
(25:09):
some pictures are more useful than others. So what happens
when the photon approaches the bit of the mirror, Well,
photons can only really do one thing, which is that
they can get absorbed by charged particles and then they
can get re emitted. And in this case it's mostly
the electrons. Like there are charges also in the atomic nucleus,
but the nucleus is surrounded by electrons, and so when
(25:30):
the photon approaches the service of the mirror really just
sees the electrons. It sees this like wall of electrons.
And that's okay. Photons can talk to electrons. They're happy
to do that. But the only way for that to
happen is for the electron to absorb the photon, then
it has more energy and then it can give off
that photon. It emits that photon, and then the question
(25:51):
is how does it know which direction to emit that photon?
When an electron which is spinning around an atom gives
off a photon, doesn't it just emit it sort of randomly?
And you can imagine all of these atoms is just
like tiny little sources of light because they've absorbed the photon.
Now it's there's why don't they just shine them all
in different directions? Well, that is the right way to
(26:12):
think about it. You can think about each of these
atoms which receives a photon as a little sort of
point source is giving off a little bit of light.
And this happens in lots of other examples too. For example,
when light goes through a little slit, as it famously
does in the double slit experiment, when it comes out
of that slit, it's not going necessarily in the same
direction as when it went into the slit. Now it's
(26:35):
like a little point source at the slit, and that's
what you need to have, Like interference between two slits.
You need a light to be coming out in lots
of different directions, so we can add up constructively or destructively.
And that same picture also works for what's happening at
the surface of a mirror. The photon is absorbed by
an atom and then the atom emits, and it emits
(26:56):
in all directions. So why does the photon end bouncing
at the correct angle? Right? Because if the photon comes
in at a very steep angle, comes out at a
steep angle. If photons come in a very shallow angle
to come out at a shallow angle, well, the way
it works is that the photon comes out not just
with the direction, but also with a phase. So the
(27:16):
quantum wave function of the photon that determines where it's
going has a number in it called the phase, sort
of like how much it's spun. And the atom actually
emits photons in all directions, but with different phases, and
those phases determine whether or not the photons add up
constructively or destructively. And the phase that comes out depends
(27:39):
on the angle that the photon came in. And so
what happens is photon comes in, gets absorbed by the atom,
and the atom emits photons sort of in every direction
belt with different phases, and only in the right direction,
in the direction that sort of corresponds to the angle
of photon came in. Do those photons not cancel each
other out, So you get destructive interference for all the
(28:02):
other directions, and constructive interference in the direction that corresponds
to the same angle that came in. And that's what
mirrors do. They bounce photons off, so the angle they
leave at is the same as the angle they came in.
Becoming at ninety degrees, you leave at ninety degrees, if
you come in at two degrees, you leave at two
degrees in the other direction. So the atom is absorbing
(28:24):
that photon and then it is sort of emitting in
every direction, but there is one direction where the phase
is right for the photons to not get canceled out
on top of each other. So that's why one direction dominates.
And you can think about that either in terms of
like lots of photons coming in in parallel, hitting the
surface and then sprang out in all different directions, and
(28:45):
the ones that are sort of in the wrong directions
end up canceling each other out, and that's why you
only get photons coming out in parallel, or you can
actually also think about it in terms of one photon.
This is sort of mind blowing the same with the
double slit experiment. Is the single photon hitting a mirror
doesn't have like neighboring photons on either side to do
(29:06):
the destructive and constructive interference for it. So how does
that happen? Well, when a photon is absorbed by the
atom and then emitted, there's a probability distribution there for
where it's going to go, and it's those probabilities that
can interfere with each other. Just like a single particle
going through the double slit experiment has probabilities to go
through either slit, a single photon hitting a mirror has
(29:28):
probabilities to go in lots of different directions, and those
probabilities can interfere with themselves. So single particles probabilities interferes
with itself, and that was determines the direction that the
particle can go in. Alright, So I said that there
were two things you have to understand to figure out
how mirrors work. When we just talked about how adams
(29:48):
absorbed and re emit the light and how it ends
up going in the correct direction, because the quantum mechanical
phase of the wave function ensures that that direction is
the only one that doesn't get destructive inference. The second
is that conductors make better mirrors than non conductors. Things
that do conduct electricity make good mirrors. That's why metals,
(30:10):
for example, make good mirrors a good sheet of aluminum,
and the reason for that is that conductors don't let
photons penetrate deep into their surface, and instead they mostly
reflected just from the very very thin sheet of the surface,
which gives you a coherent image. If a particle went
deep into a material and was absorbed before being re emitted,
(30:31):
like a centimeter inside the material, it might never emerge,
and if it did, it might not be as coherent
because they will have interactive with lots of different atoms.
To get a nice, really crisp picture, you want to
reflect just off the very surface of the material, so
you're reflecting from a very flat plane, not getting the
distortions you would get if you deflected off of a
surface that was not smooth, and so if particles can
(30:54):
penetrate different levels of the material, you're basically reflecting off
a not smooth mirror and you get models mess So
metals are very good mirrors because they don't let photons
penetrate very far into them. And that's because they are conductors,
because they can rearrange their electrons to basically cancel out
any electric field. If you have an electric field and
(31:16):
you put a conductor in it, it will move its
electrons around to balance that electric field, because electric fields
act on those electrons, and if you have a positive
voltage in one direction, it will pull electrons in that
direction to cancel it out. And so conductors, because they
have these electrons to sort of slosh around inside them
and reconfigure pretty easily, can cancel any electric field inside
(31:40):
of them. That's why, for example, it's very difficult to
get a phone call inside an elevator. Right in an elevator,
you're surrounded by a metal box, and a phone call
is just radio waves. It's a fluctuation in the electromagnetic spectrum,
and that will just induce electrons inside the elevator walls
to move round to cancel it. So a Faraday cage
(32:03):
is nothing but a metal box, and it can pretty
effectively cancel almost any electromagnetic signal. The other side of
that is that mirrors don't allow photons to go very
deep into the material because they can rearrange the electrons
to avoid those e M fields going into the material.
And that makes them good reflectors because the photons reflect
(32:23):
just from the surface rather than deep in the material
where the image might get muddled. All right, So next
time you're looking in the mirror and you're wondering, wow,
why am I so good looking, it's because of quantum
mechanics and because of electrons zooming around to give you
a nice, crisp, clear picture of your smiling face. All right,
I hope that answered your question. I have one more
(32:45):
question I really want to get to, but first let's
take another break. All right, we are back today. It's
just me, Daniel, the particle of hysicist, answering your questions
(33:07):
about the nature of the universe, whether the universe is
as shapely and as hasty as a donut, whether photons
bounce off a mirror in the same way that a
tennis ball bounces off a wall, and how does that
all work? And now we're gonna get to the last
question to day, which is a classic a question I
love talking about. So here it is. Hello, Daniel, and JAYI.
(33:29):
My name is David Somerville. I live in Fort Worth, Texas,
but I'm originally from Yorkshire in England. I love the
way you guys bound stuff off each other on the podcast.
It really works well, so please keep up the good work.
I got a question for you. Black holes arenknown to
have such huge gravitational pull that nothing can escape, not
(33:52):
even light. However, photons are massless, so how can the
gravitational force of a black hole have any effect online
at all? No matter whether you use eth equals m
A or eth equals g mm over on squad, if
the mass of the photon is zero, then it means
that that force on the light is zero, meaning it
(34:14):
will carry on on its merry way unaffected. Obviously this
is wrong, but I don't understand why. If you could
explain that on the podcast, I would really appreciate it.
Thank you all right, thank you very much for this
wonderful question. This is a great question, not just because
it's fun to talk about black holes and photons and
all that stuff, but because it shows somebody being a
(34:36):
physicist in action. You're taking one idea black hole with
crazy power from gravity, and then applying your understanding of
gravity to it and saying, hold on a second, this
doesn't work, This doesn't connect in my head. These two
ideas clash, and so help me understand it. And that's
what we're here for. We are here to help you
(34:57):
understand the universe and resolve all of these ideas is
in your head. So this short I answer you to
your question is that you're right, it doesn't work because
you're using Newton's physics, Newton's law of gravity, which cannot
describe what happens inside a black hole, and you've got
to upgrade to Einstein's theory of gravity, which can describe it.
(35:18):
All right, but let's talk a little bit more about
what that means. You were walking through your explanation and
your question and you were thinking, well, gravity is a force,
and it's a force between things that have mass. Right,
that's what we typically think about as gravity. You are
held onto the Earth because the Earth has a lot
of mass and you have mass, and gravity is a
(35:38):
force between objects with mass. That was Newton's picture of gravity.
That was Newton's description, and he formulated it mathematically. Force
of gravity is big G, which is just a number
times the two masses involved. Divided by the distance between
them squared. So what does that tell us. That's analyze
that for a moment. It means that the more mass
(36:00):
as you have, the more force of gravity F equals
g m m over our squared. The bigger the ms,
the bigger the F and so the more force you have.
And that's why, for example, you feel stronger pulled towards
a heavier object like the Earth than you do to
a grapefruit that's even closer to you. And the one
in the bottom though, the bit on the bottom of
the equation is the one over our square, and that
(36:21):
tells you that gravity gets weaker as you get further,
and if you're twice as far away, it gets four
times weaker because it's an R squared. So you're right
that if you use Newton's idea of gravity, and you
plug in the mass of a photon being zero, then
you get zero force no matter what the other masses,
the other masses ten billion sons. It doesn't matter according
(36:44):
to Newton, because Newton thought that gravity was just a
force between two objects. But Newton was wrong in a
couple of really important ways. One is that he thought
that gravity was instantaneous he thought that information about gravity
try I will through the universe with no delay. He thought,
for example, if the Sun disappeared, you would stop feeling
(37:06):
its gravity just like that, there would be no delay. Now,
of course, we know that it takes time for all
information to propagate through the universe, and that if the
Sun disappeared, you wouldn't feel it for eight minutes, because
that's the maximum speed of information through the universe. So
that's one thing about gravity that Newton got wrong. He
thought it was instantaneous, and Einstein showed us that it
(37:28):
can't be. But Einstein also showed us something much much deeper.
He showed us that the right way to think about
gravity is not that it's a force at all. Instead
that it's a bending of space, and it's this bending
of space that affects the way things move in such
a way that it looks as if there was a
force we call gravity. Now, you may have heard that
(37:49):
a few times, so I don't want to really sink
that into your brain. I want to make sure you
guys really understand what that means. So let's walk through
a little analogy. Say, for example, you're on the surface
of the Earth and you're on the equator and your
friend is somewhere else on the equator, and you both
have perfect compasses, and you say, all right, we are
going to head north. Everybody walk north. Now, if you're
(38:09):
walking north and your friend is also walking north, then
you're walking in parallel directions. Right, If you both walk
north on the surface of the Earth, you're walking in
parallel directions. And you imagine that two people walking in
parallel should never meet. Right. If you take two beams
of light and you make them perfectly parallel, they will
never touch each other. That's true unless space is curved,
(38:33):
just as it is on the surface of a sphere
and on the surface of the Earth. What happens if
you start on the equator and you walk north. Doesn't
matter how far apart you are, everybody will end up
at the north pole. Those lines will cross. So parallel
lines on a curved surface do in fact cross. Parallel
(38:53):
lines on a flat surface will not cross. Right, So
what is it like for somebody on that surface If
you aren't walking north and your friends started out, you know,
maybe ten tho kilometers or even just ten ms away
from you, you notice them gradually getting closer and closer
to you, what is that like. It's like something is
(39:13):
pulling you together. It's as if there was a force there,
pulling everybody together, so that by the time you got
to the north pole boom, you're clumped together into a
little dot. So you can see there how a curved space.
Moving in a curved space can make us feel as
if there's an apparent force there. And if you didn't
(39:34):
understand that you were on the surface of a larger object.
If you thought, hey, this is my universe, I'm a
two D person, you might come up with some force
of gravity to explain why things are moving towards each
other even if they take off in parallel directions. That's
a helpful analogy, but there's also an important element of
it missing as we try to take our minds from
(39:56):
moving on the surface of a sphere like the Earth,
to moving in our or universe. Right, the service of
a sphere is too D and we wrap it around
a three dimensional sphere in order to understand it. But
our universe is three D. Does that mean that we're
wrapping it around some four dimensional sphere? No, Because the
curvature that we talk about when we say that mass
(40:17):
bends space, when we say that general relativity describes gravity
as a curvature of space. The curvature we're talking about
there is not extrinsic curvature. It's not curvature relative to
some four dimension not wrapped around the surface of some
higher dimensional sphere. It's an intrinsic curvature. What does that mean,
(40:38):
intrinsic curvature. How do you have curvature if it's not
relative to like some outside ruler. Well, an intrinsic curvature
is just a change in the relationship between points in space.
It says, if you have a flat space and you
arrange a grid, then all the points are equidistant. But
then if you distort it, if you add mass to it,
(41:00):
curve that space, then you change the relationship between those points.
Some of those points are now closer to each other
than they were a moment ago. Some of the points
are now further from each other than they were a
moment ago. So what happens, for example, to light traveling
through space. Well, light always follows the shortest path between
two points, and if space is flat, then light just
(41:22):
follows a straight line right and goes from point A
to point B, and the shortest path is a straight line. Now,
imagine a sheet of graph paper, right, you right point
A in one spot and point B in the other spot,
and you measure the shortest path between point A and
point B. It's going to be a straight line between
those points. And the grid is square right space is flat.
(41:43):
Now what if I took that graph paper and I
distorted it. I made some of those points closer together
and other points not as close together, And then I
asked you, all right, now find the shortest path from
A to B. Some of those points don't cost as
much to go between as other points, and so now
the shortest path between A T V might not be
(42:04):
the same as it was before. It might require some
sort of curve or a wiggle or two. That's the
kind of intrinsic curvature we're talking about. What happens when
you have a blob of mass inside some space. Well,
it bends space, and it bends space intrinsically so that
the relative distances between points change. And that's why, for example,
(42:26):
the Earth goes around the Sun, not because there's a
force of gravity on the Earth pulling it, but because
the shape of space around the Sun is curb so
that an object moving inertially with no forces on it,
We'll move in a circle around the Sun because that
is the shortest path, that is the natural path for
(42:47):
an object in that curved space. All right, so we
understand that gravity is not just a force g mm
over our squared is a curvature of space. Now, it
happens to me that you can't in take Newton's picture
and it mostly works. That is, you can assume that
space is flat, that there's no curvature, and you can
treat gravity as if it was a force, and you
(43:08):
can write this mathematical equation and mostly it does work,
and that's cool, And that just shows you how a
lot of times in physics of the universe you can
have different ways to see the same effects. But it
only mostly works. It breaks down in some cases. It
breaks down when the masses get really really large, for example,
in the vicinity of a black hole, because what's going
(43:30):
on in a black hole. A black hole is a
huge amount of mass, but it's not that it has
an incredibly high value for the force of gravity. Just
because the m value is so large, it's a distortion
of space that's so great that space becomes one directional
inside the event horizon. Past this point there are only
(43:51):
paths that move towards the center of the black hole.
It's not just that light is being pulled by a
very very strong force, because if you use Newton's law
to calculate the force, you would get zero. It doesn't
matter how much mass you have, because gravity bends space
so much that even photons cannot escape. And the reason
(44:13):
is because there is no path out. There's no direction
you can go if you're inside the event horizon, which
will lead you out. It doesn't matter how fast you move,
doesn't matter what the forces are on you. The direction
of space requires you to move towards the center of
the black hole. Right, And so that's a key understanding.
If you think about gravity not as a force but
(44:35):
as a distortion of space, then black holes can make sense.
And it's not even just in black holes that photons
are affected by gravity, right. Photons are also affected just
by the changing of shape and space in other non
extreme scenarios. For example, we see things like gravitational lensing.
If you have a galaxy in between you, and that
(44:57):
galaxy is a huge blob of dark matter, for example,
then that dark matter can act like a lens because
it's mass changes the shape of space, and its light
passes through it, it gets distorted in just the same
way as if there was a huge lens in space.
Even though light has no mass, it's the shape of
space itself that has changed, so that now the shortest
(45:19):
path for light to follow from that galaxy to your
eyeball is a sort of a curved path. So this
example gravitational lensing shows us how light can be affected
by gravity. And that's how we know that Einstein is
more right than Newton, because we found these scenarios where
their predictions are different, and lots of situations they give
exactly the same predictions. You can think about the shape
(45:41):
of space being curved, or a force between two objects
and flat space, and you get exactly the same answer.
But in some scenarios, and this is what Einstein cooked
up to test his theory, you can see differences. And
one example scenario is when light gets bent around a
heavy object. Because Newton would say it should not be
bent all in Einstein would say space is curved, and
(46:03):
even massless objects will be influenced by the curvature of space.
All right, I hope that answers your questions and explains
why photons feel gravity even if they have no mass.
It's because gravity is the distortion of space itself, and
photons have to fly through that space, and so they
are affected by gravity. And thanks to everybody who's been
(46:25):
sending in questions, and mostly thank you to everybody who's
been thinking deeply about physics and wondering about the universe
on this program. We love to wonder, we love to think,
We love that there are questions, and sometimes we just
love to luxuriate in our ignorance to know that there
are answers out there to these questions. That for these
really big science questions, there are factual, objective answers that
(46:48):
one day humanity will uncover. If we keep doing science
and we keep pushing forward on the forefront of knowledge,
we will get answers to some of these really big,
deep questions about the universe, things that we cannot imagine
what it's like to know. People in the future will
know those things, and they will look back at us
and they will wonder what was it like to be
(47:09):
a caveman and not know that the ocean lay just
twenty miles away or fifty miles away. What is it
like to be a modern human today and not understand
the size of the universe, whether shape or the universe,
or how the universe was created. These answers are out there,
and the way to getting those answers is to keep
doing science, to keep asking questions, to keep wondering how
(47:30):
the universe works. So thanks everybody for lending us your
curiosity and taking this ride with us. Please send us
your questions, Thanks for listening, and remember that Daniel and
Jorge explained the universe. Is a production of I Heart Radio.
(47:52):
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Daniel Whiteson
Kelly Weinersmith
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