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Episode Transcript
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Speaker 1 (00:08):
So much of physics is about a journey into the impossible.
We spend a lot of time in physics understanding what
we see in the universe. How does the Sun produce energy,
Why is the universe expanding? How does light get from
point A to point B. We do all this by
distilling what we observe and describing it mathematically. But there's
(00:28):
another side to that. We can also push on the
limits of what is possible to try to break down
barriers and create something new that's never been done before.
We can take our mathematical understanding of the universe and
find the corners, explore the nooks and crannies, and see
if we can do something that's never been done before. Truly,
(00:51):
physics is about exploring the impossible. Hi, I'm Daniel. I'm
(01:11):
a particle physicist and my personal scientific fantasy is to
do something people once thought was impossible. And Welcome to
the podcast Daniel and Jorgey Explain the Universe, a production
of I Heart Radio, a podcasting which we explore what
is possible in the universe and what might be possible
(01:31):
and what's downright impossible. We mix it all up and
we talk about it. We try to figure it out.
We apply our minds to understanding what's out there in
the universe and try to bring your intuition up to
speed so that all of it makes some sense to you.
And in the end, everything that we do in physics,
and everything that we do in science starts with a question,
(01:53):
the question how does that work? Or could we even
do that? Or why is it this way and not
the other way? And questions are wonderful because they are
at the heart of science. They are the engine of
our curiosity. They are the reason that science moves forward.
People often think of science as this big, monolithic institution
that rolls forward at a steady pace year after year.
(02:16):
But instead you should imagine it as a big swarm
of people pushing with their individual little hands on some envelope,
expanding the sphere of knowledge by individual effort, by curiosity,
by lonely pursuits sometimes. And so it's those questions asked
by individuals that have resulted in everything we know about
the universe. And that's why you should keep asking questions,
(02:39):
and you should wonder about the universe, and you should
put value in those questions. You should really cherish your
curiosity because it's those moments of curiosity that have led
us to where we are today. And that's why on
this show we really value answering your questions, not just
the questions that I find exciting or the whore he
is willing to talk out, but also the questions that
(03:01):
real people are wondering about, people like you who think
about the universe and wonder how does this idea fit
with that idea or I've heard about this a lot
of times, but I've never really understood it. So that's
what we're here for, to answer your questions, to make
sure you actually understand the universe. Hey, the name of
the podcast is Explain the Universe after all. And as
(03:23):
you might have figured out already, Jorge is not here today,
so I'll be taking the opportunity to catch up on
our backlog of listener questions. If you have a question
about the universe you'd like to hear us break down,
please send it to us two questions at Daniel and
Jorge dot com. We answer every email, we respond to
every tweet. We will eventually answer your question. And some
(03:44):
of these questions are fascinating and tricky enough that we
promote them right onto the podcast and answer them directly
because we think it might be a question lots of
people are interested in hearing the answers to, so on
today's program, we are tackling listener questions about time like curves,
(04:06):
force fields, and the ends of the Earth. Before we
dig into today's questions, I want to say thank you
to everybody who came to my public office hours recently.
If you have questions about the way the universe works,
if you're not into writing emails and you don't tweet,
and you might like to ask a follow up question,
then come hang out with me at my public office hours.
You can find the instructions on my website sites dot
(04:29):
U c I, dot E, d U slash Daniel, and
you'll find directions for how to sign up for my
public office hours, where I hang out with folks, talk
physics and answer questions. This last one was super fun
and I even got asked a question I had never
heard before, which for me is amazing because it means
I get to think about something new. I get to
put two ideas together I had never had in my
(04:50):
head before. So, without further ado, here's our first listener
question of the episode. Hi Daniel and Horror. Hey, I'm Autumn,
and my question for to you is about closed time
like curves and what are they. I've heard a bit
about them and how their solutions to the theory of
general relativity, and that supposedly they allow for time travel,
(05:13):
but other than that not a lot. Thank you guys,
and take care all right. Thank you very much Autumn
for writing about closed timelike curves. This is indeed the
kind of thing we like to dig into because it's
something you might have heard of if you're interested in
science fiction or science and time travel and all that
kind of stuff. Because the possibility of actual time travel
(05:36):
being allowed by the laws of physics is something that
would blow our minds and hey open the possibility to
fix all those mistakes you made in life. So let's
dig into it. She asks, what is a closed timelike
curve and is it possible to use it for time travel?
Let's break it down first. Let's understand what is meant
(05:57):
by curve in this context before we talk about what
a closed timelike curve is. Let's make sure we're using
this word in a way that makes sense to everybody.
When we say curve here, we don't mean the shape
of the bowld you're eating your cereal in, or the
smooth shape of the surface of the Earth. We're talking
about something very specific. We're talking about how a particle
(06:17):
moves through space and time. So if you imagine your head,
some chunk of space, and there's a particle in it,
and that particle moves through space as time goes forward,
then the path of that particle sometimes it's called a
worldline of the particle, or sometimes it's called it's curve,
and this is just the path of the particle as
(06:38):
it moves through space. So that's pretty simple, closed timelike
curves a special case of other kinds of curves, other
kinds of worldlines, paths that particles can go in. So
that's what curves mean. But there's a little bit more
to it, because a particle has to follow rules as
it moves through space. You can't just have any curve.
(07:00):
You can't have a curve with discontinuities in it. For example,
you can't be here and then one instant later be
an Andromeda. There are rules. Physics tells us how the
present can turn into the future, and very specifically, you
have a limit to where you can be in the
universe based on where you are, because there is a
(07:21):
limit to the speed anything can move through the universe,
which is why, of course, you can't appear in Andromeda
in a moment because it's too far away. So imagine
now your particle flying through space. Where can it exist
in the future. You can exist in the future nearby
where it is right, because it can move, and it
can move quickly, but only up to a certain speed.
(07:44):
So this defines what we call a cone, the cone
of future possibilities for where this particle can be. If
it's at a certain place in a moment, then that
cone is projected forward in time and tells you where
it's possible to reach if you're traveling at the speed
of light or less. The surface of the cone tells
you where you could reach if you're traveling at the
(08:04):
speed of light. So, for example, if you move one
second forward, you have a circular slice of that cone
with radius of one light second. If you move one
year forward, then the cone, of course expands and your
slice of that cone is now one light year in radius.
So that's why it's a cone. It starts the tip
is at you or the particle, and then it expands
(08:25):
forward in time. So that's the light cone. It describes
all the places that you could be in the future,
given that you are where you are right now. And again,
the surface of the cone is as far as you
could get if you're moving at the speed of light.
If you're moving less than the speed of light, then
of course the number of possibilities shrinks. You can technically
go anywhere in that cone if you could travel up
(08:45):
to the speed of light, but the cone tells you
sort of the maximum possible places that you can go.
And this is important because we're talking about what's possible
to do in the universe. How is it possible to move?
Can you move through space and through time? Now in
normal space and flat space, the kind of space you
imagine when you think about just space and darkness and
(09:06):
spaceships floating out there. Light cones are pretty simple, and
they're just cones. You're in a spaceship, but a certain
location in space, then where you can go is described
by your light cone. But what happens in space is weird.
What happens near a massive object, well, then space curves
and these cones get a little bit more complicated because light,
for example, would change its path near a massive object,
(09:29):
light would change its path. Near a black hole, it
would curve, for example, So as you get near a
massive object, your light cone is not a simple geometric
cone that you would imagine. It actually bends a little
bit the possible places that you could go, even if
you're traveling at the speed of light change. And this
makes sense if you think about, for example, what happens
(09:50):
near a black hole. As you get near a black hole,
your cone tends to tilt towards the black hole, and eventually,
once you cross the event horizon, your cone isn't higherly
inside the black hole. That's what we mean when we
say that space inside a black hole is bent so
much that all of your futures, all of your possible paths,
lead towards the center of the black hole. Your entire
(10:12):
light cone has now tilted towards the center, so that
every single place in that cone is now towards the singularity.
So your singularity is in every possible future. There's no
part of your light cone that exists outside the black hole.
So it makes sense to us now to think about
the trajectories of a particle. That's what we call its
curve or its worldline as it moves through space. And
(10:33):
we can also imagine the possible curves for a particle,
like how it might move through space, and these are
dictated by the light cone, which again depends on the
mass and the energy around you, because that bends space itself,
which is what determines the shape of this light cone. Okay,
so we understand the curve part of it. What does
it mean to have a closed curve? Well, this is
(10:55):
actually pretty simple. It just means that it turns back
on itself. But not like you walk around in a
circle and you're in the same location you were here.
We mean something more specific. We mean that it turns
back on itself, returning to the same point in space
and in time. So a closed curve would be one
that returns to where and when you started. How is
(11:19):
that possible? Well, are closed time like curves possible? It's
possible if you can take these cones that we talked about,
and if they can tilt sort of more than ninety degrees.
A light cone in flat space, if you take your
units to all be one, has a forty five degree
line defined by the speed of light. Right now, that
(11:39):
cone tilts as you get towards a black hole, so
that one edge of it sort of turns more than
forty five degrees eventually towards ninety degrees. But if you
built some weird thing in space, something which distorted the
fabric of space so much that your light cone tilted
past ninety degrees, that would open up the possibility to
effect really move backwards in time. And if you had
(12:03):
a series of these cones, you stack them sort of
on top of each other, then you could curve your
worldline back through time and space back to where you started.
So it's essentially a circle in space time, a worldline
which doesn't move forward through space and time, but instead
curves through space and time back to itself. And this
(12:23):
works because if you imagine the individual particle, it can
go anywhere inside its light cone. Right that's moving into
its local future. Somebody else looking at it from the outside, remember,
might have a different sense of time, and so they
would see this particle moving into the past. The particle
itself is always sort of experiencing its own time. Just
(12:44):
like if you get on a spaceship and you travel
really really fast. Other people might see your clocks go slow,
but you experience your clock moving forward normally in the
same way, this particle would be experiencing its own normal time,
but from the outside we would see it moving back
words in time. So what is a closed time like curve.
It's a series of light cones tilted so that they
(13:07):
loop back on themselves and construct a path through space
time that returns to the original point in time. Now,
is this possible? Is this something which can actually happen?
So far we've just sort of been describing what the
phrase means. We haven't worried too much about whether the
laws of physics actually allow for this, So answering this
question is a bit of a theoretical exploration. What we
(13:29):
need to do is ask is there a way to
construct a universe and gives us a space time that
works this way? And that's how general relativity works. General
relativity describes how space and time are bent by mass
and energy, and then how objects move through that bent space.
(13:50):
So you can do it in a couple of directions.
You can say, here's my mass and energy. General relativity
tell me how is space bent? What happens to space?
If I have this mass and energy configuration. You can
also try to go the opposite direction. You could say, hey,
I'd like to have a universe that's bent in this
certain way. Is that possible? Is there some way to
(14:12):
construct a configuration of mass and energy that gives me
this space? And that's tricky. These equations are always very
very hard to solve, and they've only ever been solved
for a very few very simple configurations, such as an
empty universe or a universe uniformly filled with stuff, or
a flat universe with a black hole in it. So
(14:33):
it's in general very hard to solve these equations. But
there was somebody who several decades ago came up with
a solution that allows for closed time like curves. In
general relativity in a very weird configuration of mass and
energy in the universe. So if you have an infinite
spinning cylinder of dust to something which goes on forever
(14:57):
and is spinning, and it's this sort of compactified collection
of tiny objects dust basically, and it's spinning, then you
could generate in space the kinds of distortions you would
need to have light cones bend past ninety degrees and
construct a closed time like curve. So that's the solution.
In general relativity is it actually possible to assemble the
(15:21):
mass in such a way that would give you those curves.
We don't know. I don't personally think it's very feasible
to build an infinite spinning cylinder of dust, but it
opens up the possibility suggests that maybe there are ways
to assemble mass and energy and space so that it
bends the fabric of space and time to allow for
this kind of motion. Does this actually allow for time travel?
(15:44):
Could that really happen? Well, First of all, this isn't
like arbitrary time travel. It's not like back to the future.
We just dial in when you want to go. This
would be a very specific path, and this object would
move along this path. But it could only move along
this path. It's not like you could go to any
specific time or any arbitrary time. You can only move
(16:04):
along this worldline through time and space, and it would
be a closed loop. So basically you should be on
it forever. You can go back and change the past,
because that would be off of this closed time like curve.
Nor for the curve to exist, it has to reinforce itself,
has to be a complete solution, So it doesn't allow
for arbitrary time travel. But even still, wouldn't that be
(16:27):
weird to have a particle doing a loop in space
and time, even if it's stable, even if it doesn't
get to go back and kill its own grandparent particle.
Is that actually something which could happen on our universe?
Doesn't that feel like it violates causality and all sorts
of things. Well, theorists are not sure. It seems to
work according to general relativity, but people suspect that in
(16:47):
practice it probably wouldn't happen, that there's something preventing that.
And we know that general relativity is a great theory.
We also know it's not a perfect theory. We know
that cannot describe the universe as it is because it
breaks down it's singularities, like the Big Bang in the
center of black holes and all sorts of crazy stuff.
And so maybe that this is just an artifact of
(17:09):
that theory, a mathematical construction which works most of the
time but sometimes gives nonsense answers, And some future theory
of space time like quantum gravity, would prevent this from happening.
So most theorists, if you ask them, think that there's
something out there that would block this from really existing.
But to date we do not know, and our best
theory about how the universe works. General relativity, which determines
(17:33):
how space and time bend in the presence of mass,
does not prohibit closed timelike curves. So the jury is
still out. So thanks for that awesome question, Autumn. I've
been looking forward to digging into time travel and closed
timelike curves. Thanks very much for sending that in. I
want to get to some more questions, but first let's
take a quick break. All right, we're back and we're
(18:09):
answering questions from listeners. We've talked about time travel and
now we're gonna dig into something even more awesome and futuristic.
So here's a great question about force fields. H R.
Daniel And if a science fiction inspired question, our force
fields possible? Like the ones in Star Trek, everything comes
down to particles and arrangement of them in gifin that,
(18:31):
for example, of brick walls, just a certain arrangement of particles.
Could we mimic that in the air? Okay? I love
this question, and mostly because I love the role of
science fiction in pushing forward science and inspiring our curiosity.
You know, science fiction authors are like the theorists. Of theorists,
they think about new crazy things we might be able
(18:51):
to do, or ways the universe might work, and they're
not limited by mathematics or practicality or anything. And sometimes
you read something that they write and you go, I
wonder if that is possible or maybe we could actually
do that. And so thank you science fiction authors for
injecting crazy ideas into the minds of everybody out there
and being on the vanguard of creative thinking. So this
(19:13):
question is about force fields. Could we build force fields?
First of all, we have to talk about what we
mean by a force field. And if I was going
to commission a force field for my spaceship, for example,
because I was about to go into intergalactic war or whatever,
here's what I would want. I would want it to
be invisible, or at least mostly invisible, because I want
to be able to see through it. If I have
(19:35):
a spaceship, for example, and a force field around it,
I don't want turning on the force field to mean
that I can't see anything outside in the universe. Then
I'd be a huge strategic disadvantage. So it should be
invisible or at least translucent, And then it should block weapons, right,
it should be able to absorb radiation weapons like lasers,
and it should be able to stop not our weapons
(19:57):
like kinetic energy weapons, you know, bullets or other kind
of momentum driven weapons. So that's what I'd like, and
you know, as a bonus and be cool if you could,
for example, be used to hold prisoners. Right, you could
like trap somebody who you've captured and put them in
the hold view ship and use a force field so
you didn't have to build cells and easily configure it
and all this kind of stuff. So that's my wish
(20:17):
list for a force field. It should be invisible, you
should be able to block radiation weapons, and you should
be able to block matter weapons, and if possible, you
should also be able to touch it without actually being damaged.
And that's a long list of requirements. So let's talk
about what is actually possible and what might be possible
in terms of force fields. So I did a little
bit of research in this, and there are people out
(20:39):
there actually doing research on force fields. Some of the
things don't really seem like the kind of force fields
we're talking about. For example, there's a company out there
building electric armor. The idea here is to turn the
skin of your spaceship or your tank or whatever into
something which responds to a bullet. So if somebody shoots
a bullet at you, it's not just an in your
(21:00):
wall of matter which absorbs that energy and maybe gets destroyed,
but it responds to it. So the way they do
this is by making two layers of armor and having
a huge electrical gap between them, and when something impacts
on that armor, it basically closes that gap and results
in a huge electric discharge which pushes back on the bullet.
(21:20):
It's sort of reactive armor which responds when you're hit
with a force backwards. So that's pretty cool. But it's
not really a force field. I mean, it's not invisible.
You need to build it. You couldn't like turn it
on or off, and only works on matter weapons, doesn't
stop lasers, for example. But you know, it's something that
people are actually doing, and so it's something you might
(21:40):
actually see out there in the world soon. But let's
talk about what might be possible. You know, when I
think about a force field, first, I think about the
Earth's force fields, because the Earth actually does have a
force field. We have a huge magnetic field that protects
the Earth's from particles. The big swirling masses of melted
rock and metal in the core of the Earth are
(22:01):
providing an enormous magnetic engine, which creates a huge magnetic
field with sort of vertical field lines that go from
the north to the south or the south to the north.
And what this does is when charged particles hit a
magnetic field, they get curved, right. This is the Lorenz
law in physics. And so an electron, for example, generated
in the Sun and given a huge amount of energy
(22:21):
and shot towards the Earth, which might otherwise penetrate through
the atmosphere and give you cancer instead is bent around
these magnetic field lines and funneled up to the north
pole or down to the south pole. So that's cool
because it's real and it exists. It doesn't really satisfy
all of our requirements. I mean, it is invisible and
it can deflect matter, but with a couple of big caveats, like,
(22:44):
it only works on charged particles, right, It works on electrons,
works on protons. It does not work on neutral particles
because neutral particles don't feel those magnetic fields. It depends
on the charge of the particle. Remember that electricity and
magnetism are very tightly woven to get that. They're actually
just two sides of the same coin. So these magnetic
fields only deflect charge particles. Plus they don't really deflect it,
(23:07):
they just sort of focus it on the North Pole
and the South Pole. People think, wow, aurora boreality is
really cool. It's cool, but it's radiation. So if you
live near the North Pole or the South Pole, you
actually get more cosmic radiation than anywhere else on Earth
because the magnetic field sort of funnels it up there
and funnels it down to the South Pole. So it's
protecting the rest of the Earth, but at the expense
(23:29):
of the North and the Southern caps, so not even
really deflecting those charged particles. And then of course it
doesn't work on radiation. A magnetic field will not stop
a laser beam. A laser beam is made of photons,
and photons don't feel magnetic fields, which you know, is
kind of weird because photons are partially magnetic fields. They
are oscillating electro magnetic waves, right, the electrical component oscillating
(23:53):
to the magnetic component and then back. But photons are neutral,
they have no electric charge, and so Again, they are
not selected by magnetic fields. So a magnetic field kind
of like a force field, but not really and also
kind of impractical. If you wanted to have a really
powerful magnetic force field, you need to be able to
generate that inside your spaceship, and you don't really want
(24:13):
to have enormous, massive currents of iron and nickel slashing
around in the inside of your spaceship to generate this
magnetic field. All right, So what else could we do? Well?
The third possibility is a plasma shield. Plasma is another
state of matter. Basically, you just take gas and you
make it even hotter, and then the electrons have so
(24:34):
much energy that they whiz off, they leave their protons,
and now they're free. So it's ionized gas. You take
all the atoms and gas and you break apart the
electrons and the nuclei, and that's what a plasma is.
It's nothing special or fancy or science fiction. It's just
gas that's been heated up so much that the electrons
now run free. But it is super duper hot and
(24:55):
it's electrically charged, and that means it has the capability
to basically vaporize anything. I mean, imagine basically a slice
of the Sun. The Sun is plasma, it's super duper hot,
and it's ionized. So imagine, for example, having a shield
around your spaceship that was a slice of the Sun.
You threw anything in it, boom, it would melt. Also,
(25:16):
it's opaque. Two lasers, right, A laser can't penetrate the
Sun because it's filled with charged particles which would interact
with the light in the laser, unless, of course, you
tune the laser to be a specific frequency that the
plasma didn't absorb. But these excited particles, these free particles,
are not limited to absorbing only very specific frequencies. It
(25:36):
would be very difficult to get your laser beam through
a slice of the Sun through a plasma shield. So
that's pretty cool. And actually we have a little bit
of a plasma shield already on Earth. The part of
the atmosphere we call the ionosphere is basically plasmas filled
with ions, and it blocks a lot of radiation from
the Sun, neutral radiation, photons, etcetera. And our ionosphere is
(26:00):
not very dense but mostly blocks very long wavelength radiation.
But if you made a denser plasma, more like a
slice of the Sun, then you could block shorter wavelength.
So this is the kind of thing you could actually build. Now,
how do you make a plasma shield? How is that possible?
Can you actually do that? It would be pretty tricky.
I mean you need to have basically a shell of gas,
(26:22):
and then you need to have that gas get excited
to get very very hot. You need to dump a
huge amount of energy into it. So you can imagine,
for example, puffing out a shell of gas and then
zapping it with a bunch of lasers to turn it
into a plasma shield. Like essentially deposit so much energy
in this shield around you that anything else that comes
into it we get absorbed or interacted with or diffused.
(26:43):
This is not something that's very easy to do, not
something we have the technology to do at all already.
It's difficult for us to make and control plasmas. This
essentially is the task of fusion. We are trying to
replicate what's happening on the Sun in laboratories on Earth,
not just so we can build fourth fields, but so
that we can generate essentially limitless energy through nuclear fusion,
(27:05):
the same process that happens in the center of the Sun,
and it's tricky. People have been working on it for decades.
They're trying to make a donut of plasma, and they're
containing it with magnetic fields. Because the stuff is so
volatile it would vaporize any container you put it in.
It's so hot and nasty and interactive, so they use
a magnetic bottle to contain this plasma. They have basically
(27:27):
a donut of the sun, and it's hard to get
it to go, it's hard to keep it straight. This
is a big engineering challenge, and there's a project going
on right now called Eater, which is the biggest and
very promising application of this, but it costs billions of
dollars and it's not simple to set up. So this
would be a very difficult engineering challenge, and you'd have
to somehow create this shield around your spaceship, get it
(27:47):
zapped up to turn it into a plasma, and then
keep it in line with very strong magnets. So possibly
you could do that potentially sometime in the future. It
might be possible, but it sort of violates one of
the first principles we asked for for force field, which
is that it's invisible if you surrounded yourself with sort
of a sheet of the Sun in a sphere around
(28:08):
your ship. Then you wouldn't be able to see anything
outside that sphere, and you'd be stuck inside basically a
little slice of the sun, which I guess would get
pretty hot. So that's sort of the best idea that's
out there that I'm aware of to build an actual
force field. But it has a lot of engineering challenges
ahead of it, and even if we could solve all
of those, it doesn't actually satisfy all the requirements we have.
(28:31):
So fun question. Maybe somebody in the future will come
up with a new way to think about how to
actually build a force field. And I shouldn't spy anybody
from imagining and from wondering and from being creative. So
keep using force fields in the science fiction that you write,
and keep an eye out in science fiction for other
cool ideas that we might actually turn one day into reality.
(28:52):
All right, I want to get to one more question,
but first let's take another break. All Right, we are back,
and we're having a lot of fun talking about the
(29:12):
future and time travel and force fields and future technologies.
And now I want to take a trip even deeper
into the future, wondering how long humans can survive on Earth, Pie,
Daniel and Joey. My name is Gavin and I live
in South Wales in the United Kingdom. In the last
podcast What's Inside the Earth, You've got me thinking. You
(29:33):
told us that the big lump of moon sized core
inside the Earth is getting bigger by one millimeter every year,
and that at some point in the future the core
will stop turning on the Earth will load its magnetic
force like we think happened to Mars. My question is, well,
this happened before I was set and goes supernova. All right,
(29:54):
Thank you very much for that question, and also on
behalf of all of humanity, thank you so much for
thinking ahead, for worrying because we don't have time to
about the deep future, and starting to make plans today
for what we might have to do to prepare for it.
So he's wondering which of these two calamities will we
have to deal with first, the Sun exploding and fiddling
(30:16):
out or the earth magnetic field dying, And the two
are related, of course, because the earth magnetic field is
important for protecting us from the Sun's radiation, so if
it disappears then we will be fried. But hey, if
it's gonna last longer than the Sun, then we don't
need to worry about it. So which one will kill
us first is basically the name of the game in
(30:37):
this question. So first let's remind ourselves what the timeline
is for our sun. How long is this thing? And
to keep burning and keeping us toasty and keeping us
keeping on. So our son is currently about five billion
years old. It's about the age of the Solar system,
because it basically is the Solar system. Remember that a
coalesced together from a huge cloud of gas and dust
(31:00):
and leftovers from population three in population two stars had
gathered together in some gravitational event, slowly slurping together, And
that most of the stuff in the Solar system is
in the Sun. About of everything that's in the Solar
system is the Sun, So we're just like a little
detail on top of the Sun. Now, that happened about
(31:21):
four and a half to five billion years ago, and
since then, what's the Sun been doing? Well, it's been
burning hydrogen. Gravity gathers together all this material, mostly hydrogen,
but also some helium left over from burning from previous generations,
and a few other heavier elements, but still overwhelmingly hydrogen
and that's good because that's the fuel for the Sun.
(31:44):
Gravity squeezes together this hydrogen, and when it gets it
close enough, then it confused. Remember that hydrogen is essentially
just a proton with an electron around it, but when
things get hot, it's basically just a proton. And to
make fusion happen, you've got to squeeze to these protons together.
But protons are both positively charged and so they resist.
And until you get them closed enough together, when the
(32:06):
strong force can take over and do nuclear fusion and
release a bunch of energy, they will resist. And that's
why fusion is hard to make happen. You need really
high temperatures and pressures. That's why it's difficult for them
to engineer it here on Earth. But anyway, it happens
in the Sun, and it's been happening now for about
five billion years. So what's going to keep the Sun
from burning on forever? Well, eventually it's going to run
(32:29):
out of fuel. It's basically a huge hydrogen thermonuclear device.
And what happens when it burns hydrogen is that it
creates ash. That ash is helium, and it starts at
the core because that's where the fusion starts. That's the hottest,
densest part. So hydrogen first burns out at the core,
and you get this helium core now surrounded by hydrogen,
(32:50):
and the fusion is now happening in the sort of
outer layers surrounding the core. This pushes out on the Sun,
making it bigger and bigger. It fluffs it out, larger
and larger, and actually even makes it brighter and brighter.
So every year the Sun gets a little bit hotter.
In four billion years, for example, the Sun will be
about brighter than it is today, and just that is
(33:12):
enough to like boil all of the oceans on Earth
and turn them into vapor. So right there you can
see that in about four billion years, our sun will
not make Earth a very cozy place to live in.
But as the Sun grows, it's gonna get bigger and bigger.
And you might think, well, how much bigger can it get.
It's already huge, right, it's already a million times the
volume of the Earth. Well it's gonna get bigger by
(33:33):
a lot. It's gonna grow by about a factor of
two hundred. And that's bad news, because I mean it's
gonna get so big that it's radius is going to
match the size of Earth's orbit. Right, It's gonna envelop
all the inner planets, and the Earth will be right there,
right about on the edge of the Sun. What does
that mean to be like inside the sun or right
(33:54):
just past the edge of the Sun. Well, it's gonna
be really hot, and we're gonna be surrounded by these
huge sheats of burning hydrogen, which is not going to
be a good place to live in. Now, inside the Sun,
you now have a helium core which is getting bigger
and bigger because as hydrogen burns, it creates more and
more helium. Eventually you can even fuse this helium together
(34:14):
to make something even heavier carbon. And so this is
awesome because it burns hydrogen for millions and millions and
billions of years, and then all of a sudden it
starts to burn helium. It passes this critical point you
can get this helium flash, and the amount of energy
generated by the Sun in this moment is actually brighter
than all the stars in the galaxy put together, although
(34:35):
you don't get to see it because it's absorbed by
the inner layers of the Sun. But you know, maybe
that's good because otherwise it would fry everything on Earth.
So it starts helium fusion, creating carbon, and our sun
is not heavy enough. It's not massive enough to then
take the next step and fuse carbon into heavier elements.
You could eventually make oxygen and neon and silicon and
(34:55):
all sorts of crazy stuff if you had a bigger star.
Our star is not big enough to do that. So
what's gonna happen is it's going to accumulate helium, which
will burn into carbon, But then it's sort of stuck there.
The carbon is inert and it's like having a lot
of ash in your fire. It makes it harder for
things to burn, and so the sun will just sort
of decrease in brightness from there, basically fizzling out and
(35:17):
turning into a white dwarf. A white dwarf is just
a hot blob of carbon. It's not fusing anymore, it's
not producing any more energy, but it's still really really hot,
right it glows. White dwarfs are called white because they
do give off light again, not because they're actually fusing
they are creating new energy. They're just glowing the same
(35:37):
way that anything that's hot glows. It's a white hot
lump of carbon in the universe, and so it glows
white the way like a really hot piece of metal
that you stick into a blacksmith's furnace will also glow
white hot, and eventually it will cool. And scientists think
that white dwarfs, if given enough time, will eventually radiate
off all of their energy and turn into something else,
(36:00):
something weird, something called a cold black dwarf. But that
will take trillions of years. So let's review the timeline.
We're about five billion years into the life cycle of
the Sun. It will keep burning for about five billion
more years, after which it will envelop the Earth, will
have the helium flash, and it will convert into a
white dwarf, which won't give off enough heat to keep
(36:22):
the Earth warm. But the Earth will have already been
fried at that point, and then eventually the white dwarf,
in trillions of years, will cool off. So timeline for
the Sun to fry us is a few billion years.
Now let's turn to the other side of the question,
how long will we have a magnetic field for? To
answer this question, we need to think about what causes
the magnetic field and why that might come to an end.
(36:44):
So as we talked about a few minutes ago, the
magnetic field we think comes from internal flows of molten
rock and metal. And again it's good idea to sort
of turn back to the clock and remember how the
Earth was formed and why it is the way it is.
We think the Earth came together basically a bunch of
bits of rock which gathered together into larger bits of rock,
into larger bits of rock. So the early Earth was
(37:06):
just a collection of rock basically squeezed together, and it
wasn't actually melted in the middle yet. Gravity then took
over and squeezed it further and further. That plus radioactive
decay from certain heavy metals that were embedded in the rock,
helps melt the center of the Earth. This might have
taken a few hundred million years to melt the center
of the Earth to make it molten rather than just
(37:28):
hot rock. And this was crucial because it let all
the heavy elements, the iron, the nickel, sort of melt down. Right.
Things get hot, they get molten, they get liquid. Now
you have a fluid instead of rocks, so it's easier
for things to sort of like slide around and rearrange themselves.
So the heavier elements sink to the core, and now
you've get the structure that we have today, which is
(37:48):
a solid inner core, very very dense, surrounded by that
liquid outer core. And then on top of that is
the mantle, which isn't exactly liquid, but it is rock
that's sort of flowing. And then on top of that
is the crust, the things we actually live on. And
we think that it's this molten motion inside the earth.
Essentially this liquid outer core that's generating the magnetic field
(38:10):
that it's spinning, that there's electric currents in there, and
that motion of electric charges creates magnetic fields. Because remember
there's a very tight connection between electricity and magnetism. There
really are just one thing, electro magnetism, which is why
moving particles can generate magnetic fields and why magnetic fields
can bend the path of charged particles. So what happens
(38:31):
is this stuff is swirling around. It's basically a current,
and that creates a magnetic field, and the magnetic field
enhances the current. Right, the magnetic field pushes these particles
in a circle, and then moving in a circle makes
a bigger magnetic field. This is called a dynamo effect. Basically,
builds on itself. It makes a stronger and stronger magnetic field,
but it relies on the flowing of this liquid molten
(38:54):
rock and metal in the center of the Earth outside
the inner core. You have to have the ability for
this thing to slash around. You need the motion in
order to have a magnetic field. And as the question mentioned,
this inner core is growing. The Earth is cooling. Basically,
it's freezing very slowly, and this core is growing by
one millimeter per year, and that's just because things cool, right,
(39:17):
We are giving off heat into space the same way
that a white dwarf will eventually radiate out all of
its energy into space and become a cold black dwarf.
Entropy tells us that he should spread out. So you
have an isolated hot blob of something in cold space.
Eventually it will radiate out its energy. So that's what's
happening to Earth. And the inner core is growing, and
(39:37):
eventually it will grow and it will cool down the Earth.
So how long will this take? Will it be faster
than the Sun enveloping the Earth and frying us, or
will it be slower. It's difficult to predict these things
because it requires projecting out pretty far in time, and
our understanding of how these things work inside the Earth
is still a little speculative, all right. We've never drilled
(39:58):
down to the Earth. A lot of this stuff is
reconstructed from seismographs, from basically bouncing waves off the inside
of the Earth and understanding how those waves are reflected
as they hit various layers inside the Earth. So there's
still a lot of guesswork involved. But the best estimate
I found is that it will take about ninety billion
years for the Earth to lose this magnetic field, essentially
(40:20):
for the Earth to freeze internally and to stop having
the flowing liquid rock and metal necessary to have a
magnetic field. So that's nine one billion years. That's a
lot longer than the Sun's expected lifetime. The Sun, we
think will fizzle out in about five billion years, So
the Earth, if it survives that, if it doesn't just
(40:40):
get like melted and slurped into the Sun, would continue
on for a long time for tens of billions of
years with a magnetic field spinning and reflecting charged particles happily,
even if we're now orbiting a white dwarf. So we
think the Sun will expand and it will fry the
Earth in a few billion years, after which you'll call
laps to a white dwarf, and then the Earth, with
(41:02):
its magnetic field, will orbit this white dwarf for billions
of years until eventually it cools and solidifies and loses
its magnetic field in something like ninety billion years. So
that's definitely something to think about, something to plan for,
something to wonder about. I think personally, we'd be lucky
if we made it a billion years on this Earth,
(41:22):
so that we have to worry about things like the
Sun getting bigger and frying us. We have more immediate
problems that we need to tackle, not force fields, not
closed timelike curves, but just sort of taking care of
the planet so we can last that long. Anyway, it's
fun to think about these things, and fun to understand
the physics that goes into making a son or making
a magnetic field so that you can understand how long
(41:44):
they will last. All right, Thanks everybody for sending in
these questions and for letting us ride with you on
your curiosity journey, for wondering about the nature of a space,
for wondering if it's possible to go backwards in time
to build force fields and how long this planet Earth
will be around for us to live on. It's a
joy to get your emails and enjoyed to think about
the ideas that are in your head, so please don't
(42:07):
be shy send us your questions. Tune in next time.
Thanks for listening, and remember that Daniel and Jorge Explain
the Universe is a production of I Heart Radio. Or
more podcast from my heart Radio visit the I heart
(42:27):
Radio app, Apple Podcasts, or wherever you listen to your
favorite shows. Ye
So much of physics is about a journey into the impossible.
We spend a lot of time in physics understanding what
we see in the universe. How does the Sun produce energy,
Why is the universe expanding? How does light get from
point A to point B. We do all this by
distilling what we observe and describing it mathematically. But there's
(00:28):
another side to that. We can also push on the
limits of what is possible to try to break down
barriers and create something new that's never been done before.
We can take our mathematical understanding of the universe and
find the corners, explore the nooks and crannies, and see
if we can do something that's never been done before. Truly,
(00:51):
physics is about exploring the impossible. Hi, I'm Daniel. I'm
(01:11):
a particle physicist and my personal scientific fantasy is to
do something people once thought was impossible. And Welcome to
the podcast Daniel and Jorgey Explain the Universe, a production
of I Heart Radio, a podcasting which we explore what
is possible in the universe and what might be possible
(01:31):
and what's downright impossible. We mix it all up and
we talk about it. We try to figure it out.
We apply our minds to understanding what's out there in
the universe and try to bring your intuition up to
speed so that all of it makes some sense to you.
And in the end, everything that we do in physics,
and everything that we do in science starts with a question,
(01:53):
the question how does that work? Or could we even
do that? Or why is it this way and not
the other way? And questions are wonderful because they are
at the heart of science. They are the engine of
our curiosity. They are the reason that science moves forward.
People often think of science as this big, monolithic institution
that rolls forward at a steady pace year after year.
(02:16):
But instead you should imagine it as a big swarm
of people pushing with their individual little hands on some envelope,
expanding the sphere of knowledge by individual effort, by curiosity,
by lonely pursuits sometimes. And so it's those questions asked
by individuals that have resulted in everything we know about
the universe. And that's why you should keep asking questions,
(02:39):
and you should wonder about the universe, and you should
put value in those questions. You should really cherish your
curiosity because it's those moments of curiosity that have led
us to where we are today. And that's why on
this show we really value answering your questions, not just
the questions that I find exciting or the whore he
is willing to talk out, but also the questions that
(03:01):
real people are wondering about, people like you who think
about the universe and wonder how does this idea fit
with that idea or I've heard about this a lot
of times, but I've never really understood it. So that's
what we're here for, to answer your questions, to make
sure you actually understand the universe. Hey, the name of
the podcast is Explain the Universe after all. And as
(03:23):
you might have figured out already, Jorge is not here today,
so I'll be taking the opportunity to catch up on
our backlog of listener questions. If you have a question
about the universe you'd like to hear us break down,
please send it to us two questions at Daniel and
Jorge dot com. We answer every email, we respond to
every tweet. We will eventually answer your question. And some
(03:44):
of these questions are fascinating and tricky enough that we
promote them right onto the podcast and answer them directly
because we think it might be a question lots of
people are interested in hearing the answers to, so on
today's program, we are tackling listener questions about time like curves,
(04:06):
force fields, and the ends of the Earth. Before we
dig into today's questions, I want to say thank you
to everybody who came to my public office hours recently.
If you have questions about the way the universe works,
if you're not into writing emails and you don't tweet,
and you might like to ask a follow up question,
then come hang out with me at my public office hours.
You can find the instructions on my website sites dot
(04:29):
U c I, dot E, d U slash Daniel, and
you'll find directions for how to sign up for my
public office hours, where I hang out with folks, talk
physics and answer questions. This last one was super fun
and I even got asked a question I had never
heard before, which for me is amazing because it means
I get to think about something new. I get to
put two ideas together I had never had in my
(04:50):
head before. So, without further ado, here's our first listener
question of the episode. Hi Daniel and Horror. Hey, I'm Autumn,
and my question for to you is about closed time
like curves and what are they. I've heard a bit
about them and how their solutions to the theory of
general relativity, and that supposedly they allow for time travel,
(05:13):
but other than that not a lot. Thank you guys,
and take care all right. Thank you very much Autumn
for writing about closed timelike curves. This is indeed the
kind of thing we like to dig into because it's
something you might have heard of if you're interested in
science fiction or science and time travel and all that
kind of stuff. Because the possibility of actual time travel
(05:36):
being allowed by the laws of physics is something that
would blow our minds and hey open the possibility to
fix all those mistakes you made in life. So let's
dig into it. She asks, what is a closed timelike
curve and is it possible to use it for time travel?
Let's break it down first. Let's understand what is meant
(05:57):
by curve in this context before we talk about what
a closed timelike curve is. Let's make sure we're using
this word in a way that makes sense to everybody.
When we say curve here, we don't mean the shape
of the bowld you're eating your cereal in, or the
smooth shape of the surface of the Earth. We're talking
about something very specific. We're talking about how a particle
(06:17):
moves through space and time. So if you imagine your head,
some chunk of space, and there's a particle in it,
and that particle moves through space as time goes forward,
then the path of that particle sometimes it's called a
worldline of the particle, or sometimes it's called it's curve,
and this is just the path of the particle as
(06:38):
it moves through space. So that's pretty simple, closed timelike
curves a special case of other kinds of curves, other
kinds of worldlines, paths that particles can go in. So
that's what curves mean. But there's a little bit more
to it, because a particle has to follow rules as
it moves through space. You can't just have any curve.
(07:00):
You can't have a curve with discontinuities in it. For example,
you can't be here and then one instant later be
an Andromeda. There are rules. Physics tells us how the
present can turn into the future, and very specifically, you
have a limit to where you can be in the
universe based on where you are, because there is a
(07:21):
limit to the speed anything can move through the universe,
which is why, of course, you can't appear in Andromeda
in a moment because it's too far away. So imagine
now your particle flying through space. Where can it exist
in the future. You can exist in the future nearby
where it is right, because it can move, and it
can move quickly, but only up to a certain speed.
(07:44):
So this defines what we call a cone, the cone
of future possibilities for where this particle can be. If
it's at a certain place in a moment, then that
cone is projected forward in time and tells you where
it's possible to reach if you're traveling at the speed
of light or less. The surface of the cone tells
you where you could reach if you're traveling at the
(08:04):
speed of light. So, for example, if you move one
second forward, you have a circular slice of that cone
with radius of one light second. If you move one
year forward, then the cone, of course expands and your
slice of that cone is now one light year in radius.
So that's why it's a cone. It starts the tip
is at you or the particle, and then it expands
(08:25):
forward in time. So that's the light cone. It describes
all the places that you could be in the future,
given that you are where you are right now. And again,
the surface of the cone is as far as you
could get if you're moving at the speed of light.
If you're moving less than the speed of light, then
of course the number of possibilities shrinks. You can technically
go anywhere in that cone if you could travel up
(08:45):
to the speed of light, but the cone tells you
sort of the maximum possible places that you can go.
And this is important because we're talking about what's possible
to do in the universe. How is it possible to move?
Can you move through space and through time? Now in
normal space and flat space, the kind of space you
imagine when you think about just space and darkness and
(09:06):
spaceships floating out there. Light cones are pretty simple, and
they're just cones. You're in a spaceship, but a certain
location in space, then where you can go is described
by your light cone. But what happens in space is weird.
What happens near a massive object, well, then space curves
and these cones get a little bit more complicated because light,
for example, would change its path near a massive object,
(09:29):
light would change its path. Near a black hole, it
would curve, for example, So as you get near a
massive object, your light cone is not a simple geometric
cone that you would imagine. It actually bends a little
bit the possible places that you could go, even if
you're traveling at the speed of light change. And this
makes sense if you think about, for example, what happens
(09:50):
near a black hole. As you get near a black hole,
your cone tends to tilt towards the black hole, and eventually,
once you cross the event horizon, your cone isn't higherly
inside the black hole. That's what we mean when we
say that space inside a black hole is bent so
much that all of your futures, all of your possible paths,
lead towards the center of the black hole. Your entire
(10:12):
light cone has now tilted towards the center, so that
every single place in that cone is now towards the singularity.
So your singularity is in every possible future. There's no
part of your light cone that exists outside the black hole.
So it makes sense to us now to think about
the trajectories of a particle. That's what we call its
curve or its worldline as it moves through space. And
(10:33):
we can also imagine the possible curves for a particle,
like how it might move through space, and these are
dictated by the light cone, which again depends on the
mass and the energy around you, because that bends space itself,
which is what determines the shape of this light cone. Okay,
so we understand the curve part of it. What does
it mean to have a closed curve? Well, this is
(10:55):
actually pretty simple. It just means that it turns back
on itself. But not like you walk around in a
circle and you're in the same location you were here.
We mean something more specific. We mean that it turns
back on itself, returning to the same point in space
and in time. So a closed curve would be one
that returns to where and when you started. How is
(11:19):
that possible? Well, are closed time like curves possible? It's
possible if you can take these cones that we talked about,
and if they can tilt sort of more than ninety degrees.
A light cone in flat space, if you take your
units to all be one, has a forty five degree
line defined by the speed of light. Right now, that
(11:39):
cone tilts as you get towards a black hole, so
that one edge of it sort of turns more than
forty five degrees eventually towards ninety degrees. But if you
built some weird thing in space, something which distorted the
fabric of space so much that your light cone tilted
past ninety degrees, that would open up the possibility to
effect really move backwards in time. And if you had
(12:03):
a series of these cones, you stack them sort of
on top of each other, then you could curve your
worldline back through time and space back to where you started.
So it's essentially a circle in space time, a worldline
which doesn't move forward through space and time, but instead
curves through space and time back to itself. And this
(12:23):
works because if you imagine the individual particle, it can
go anywhere inside its light cone. Right that's moving into
its local future. Somebody else looking at it from the outside, remember,
might have a different sense of time, and so they
would see this particle moving into the past. The particle
itself is always sort of experiencing its own time. Just
(12:44):
like if you get on a spaceship and you travel
really really fast. Other people might see your clocks go slow,
but you experience your clock moving forward normally in the
same way, this particle would be experiencing its own normal time,
but from the outside we would see it moving back
words in time. So what is a closed time like curve.
It's a series of light cones tilted so that they
(13:07):
loop back on themselves and construct a path through space
time that returns to the original point in time. Now,
is this possible? Is this something which can actually happen?
So far we've just sort of been describing what the
phrase means. We haven't worried too much about whether the
laws of physics actually allow for this, So answering this
question is a bit of a theoretical exploration. What we
(13:29):
need to do is ask is there a way to
construct a universe and gives us a space time that
works this way? And that's how general relativity works. General
relativity describes how space and time are bent by mass
and energy, and then how objects move through that bent space.
(13:50):
So you can do it in a couple of directions.
You can say, here's my mass and energy. General relativity
tell me how is space bent? What happens to space?
If I have this mass and energy configuration. You can
also try to go the opposite direction. You could say, hey,
I'd like to have a universe that's bent in this
certain way. Is that possible? Is there some way to
(14:12):
construct a configuration of mass and energy that gives me
this space? And that's tricky. These equations are always very
very hard to solve, and they've only ever been solved
for a very few very simple configurations, such as an
empty universe or a universe uniformly filled with stuff, or
a flat universe with a black hole in it. So
(14:33):
it's in general very hard to solve these equations. But
there was somebody who several decades ago came up with
a solution that allows for closed time like curves. In
general relativity in a very weird configuration of mass and
energy in the universe. So if you have an infinite
spinning cylinder of dust to something which goes on forever
(14:57):
and is spinning, and it's this sort of compactified collection
of tiny objects dust basically, and it's spinning, then you
could generate in space the kinds of distortions you would
need to have light cones bend past ninety degrees and
construct a closed time like curve. So that's the solution.
In general relativity is it actually possible to assemble the
(15:21):
mass in such a way that would give you those curves.
We don't know. I don't personally think it's very feasible
to build an infinite spinning cylinder of dust, but it
opens up the possibility suggests that maybe there are ways
to assemble mass and energy and space so that it
bends the fabric of space and time to allow for
this kind of motion. Does this actually allow for time travel?
(15:44):
Could that really happen? Well, First of all, this isn't
like arbitrary time travel. It's not like back to the future.
We just dial in when you want to go. This
would be a very specific path, and this object would
move along this path. But it could only move along
this path. It's not like you could go to any
specific time or any arbitrary time. You can only move
(16:04):
along this worldline through time and space, and it would
be a closed loop. So basically you should be on
it forever. You can go back and change the past,
because that would be off of this closed time like curve.
Nor for the curve to exist, it has to reinforce itself,
has to be a complete solution, So it doesn't allow
for arbitrary time travel. But even still, wouldn't that be
(16:27):
weird to have a particle doing a loop in space
and time, even if it's stable, even if it doesn't
get to go back and kill its own grandparent particle.
Is that actually something which could happen on our universe?
Doesn't that feel like it violates causality and all sorts
of things. Well, theorists are not sure. It seems to
work according to general relativity, but people suspect that in
(16:47):
practice it probably wouldn't happen, that there's something preventing that.
And we know that general relativity is a great theory.
We also know it's not a perfect theory. We know
that cannot describe the universe as it is because it
breaks down it's singularities, like the Big Bang in the
center of black holes and all sorts of crazy stuff.
And so maybe that this is just an artifact of
(17:09):
that theory, a mathematical construction which works most of the
time but sometimes gives nonsense answers, And some future theory
of space time like quantum gravity, would prevent this from happening.
So most theorists, if you ask them, think that there's
something out there that would block this from really existing.
But to date we do not know, and our best
theory about how the universe works. General relativity, which determines
(17:33):
how space and time bend in the presence of mass,
does not prohibit closed timelike curves. So the jury is
still out. So thanks for that awesome question, Autumn. I've
been looking forward to digging into time travel and closed
timelike curves. Thanks very much for sending that in. I
want to get to some more questions, but first let's
take a quick break. All right, we're back and we're
(18:09):
answering questions from listeners. We've talked about time travel and
now we're gonna dig into something even more awesome and futuristic.
So here's a great question about force fields. H R.
Daniel And if a science fiction inspired question, our force
fields possible? Like the ones in Star Trek, everything comes
down to particles and arrangement of them in gifin that,
(18:31):
for example, of brick walls, just a certain arrangement of particles.
Could we mimic that in the air? Okay? I love
this question, and mostly because I love the role of
science fiction in pushing forward science and inspiring our curiosity.
You know, science fiction authors are like the theorists. Of theorists,
they think about new crazy things we might be able
(18:51):
to do, or ways the universe might work, and they're
not limited by mathematics or practicality or anything. And sometimes
you read something that they write and you go, I
wonder if that is possible or maybe we could actually
do that. And so thank you science fiction authors for
injecting crazy ideas into the minds of everybody out there
and being on the vanguard of creative thinking. So this
(19:13):
question is about force fields. Could we build force fields?
First of all, we have to talk about what we
mean by a force field. And if I was going
to commission a force field for my spaceship, for example,
because I was about to go into intergalactic war or whatever,
here's what I would want. I would want it to
be invisible, or at least mostly invisible, because I want
to be able to see through it. If I have
(19:35):
a spaceship, for example, and a force field around it,
I don't want turning on the force field to mean
that I can't see anything outside in the universe. Then
I'd be a huge strategic disadvantage. So it should be
invisible or at least translucent, And then it should block weapons, right,
it should be able to absorb radiation weapons like lasers,
and it should be able to stop not our weapons
(19:57):
like kinetic energy weapons, you know, bullets or other kind
of momentum driven weapons. So that's what I'd like, and
you know, as a bonus and be cool if you could,
for example, be used to hold prisoners. Right, you could
like trap somebody who you've captured and put them in
the hold view ship and use a force field so
you didn't have to build cells and easily configure it
and all this kind of stuff. So that's my wish
(20:17):
list for a force field. It should be invisible, you
should be able to block radiation weapons, and you should
be able to block matter weapons, and if possible, you
should also be able to touch it without actually being damaged.
And that's a long list of requirements. So let's talk
about what is actually possible and what might be possible
in terms of force fields. So I did a little
bit of research in this, and there are people out
(20:39):
there actually doing research on force fields. Some of the
things don't really seem like the kind of force fields
we're talking about. For example, there's a company out there
building electric armor. The idea here is to turn the
skin of your spaceship or your tank or whatever into
something which responds to a bullet. So if somebody shoots
a bullet at you, it's not just an in your
(21:00):
wall of matter which absorbs that energy and maybe gets destroyed,
but it responds to it. So the way they do
this is by making two layers of armor and having
a huge electrical gap between them, and when something impacts
on that armor, it basically closes that gap and results
in a huge electric discharge which pushes back on the bullet.
(21:20):
It's sort of reactive armor which responds when you're hit
with a force backwards. So that's pretty cool. But it's
not really a force field. I mean, it's not invisible.
You need to build it. You couldn't like turn it
on or off, and only works on matter weapons, doesn't
stop lasers, for example. But you know, it's something that
people are actually doing, and so it's something you might
(21:40):
actually see out there in the world soon. But let's
talk about what might be possible. You know, when I
think about a force field, first, I think about the
Earth's force fields, because the Earth actually does have a
force field. We have a huge magnetic field that protects
the Earth's from particles. The big swirling masses of melted
rock and metal in the core of the Earth are
(22:01):
providing an enormous magnetic engine, which creates a huge magnetic
field with sort of vertical field lines that go from
the north to the south or the south to the north.
And what this does is when charged particles hit a
magnetic field, they get curved, right. This is the Lorenz
law in physics. And so an electron, for example, generated
in the Sun and given a huge amount of energy
(22:21):
and shot towards the Earth, which might otherwise penetrate through
the atmosphere and give you cancer instead is bent around
these magnetic field lines and funneled up to the north
pole or down to the south pole. So that's cool
because it's real and it exists. It doesn't really satisfy
all of our requirements. I mean, it is invisible and
it can deflect matter, but with a couple of big caveats, like,
(22:44):
it only works on charged particles, right, It works on electrons,
works on protons. It does not work on neutral particles
because neutral particles don't feel those magnetic fields. It depends
on the charge of the particle. Remember that electricity and
magnetism are very tightly woven to get that. They're actually
just two sides of the same coin. So these magnetic
fields only deflect charge particles. Plus they don't really deflect it,
(23:07):
they just sort of focus it on the North Pole
and the South Pole. People think, wow, aurora boreality is
really cool. It's cool, but it's radiation. So if you
live near the North Pole or the South Pole, you
actually get more cosmic radiation than anywhere else on Earth
because the magnetic field sort of funnels it up there
and funnels it down to the South Pole. So it's
protecting the rest of the Earth, but at the expense
(23:29):
of the North and the Southern caps, so not even
really deflecting those charged particles. And then of course it
doesn't work on radiation. A magnetic field will not stop
a laser beam. A laser beam is made of photons,
and photons don't feel magnetic fields, which you know, is
kind of weird because photons are partially magnetic fields. They
are oscillating electro magnetic waves, right, the electrical component oscillating
(23:53):
to the magnetic component and then back. But photons are neutral,
they have no electric charge, and so Again, they are
not selected by magnetic fields. So a magnetic field kind
of like a force field, but not really and also
kind of impractical. If you wanted to have a really
powerful magnetic force field, you need to be able to
generate that inside your spaceship, and you don't really want
(24:13):
to have enormous, massive currents of iron and nickel slashing
around in the inside of your spaceship to generate this
magnetic field. All right, So what else could we do? Well?
The third possibility is a plasma shield. Plasma is another
state of matter. Basically, you just take gas and you
make it even hotter, and then the electrons have so
(24:34):
much energy that they whiz off, they leave their protons,
and now they're free. So it's ionized gas. You take
all the atoms and gas and you break apart the
electrons and the nuclei, and that's what a plasma is.
It's nothing special or fancy or science fiction. It's just
gas that's been heated up so much that the electrons
now run free. But it is super duper hot and
(24:55):
it's electrically charged, and that means it has the capability
to basically vaporize anything. I mean, imagine basically a slice
of the Sun. The Sun is plasma, it's super duper hot,
and it's ionized. So imagine, for example, having a shield
around your spaceship that was a slice of the Sun.
You threw anything in it, boom, it would melt. Also,
(25:16):
it's opaque. Two lasers, right, A laser can't penetrate the
Sun because it's filled with charged particles which would interact
with the light in the laser, unless, of course, you
tune the laser to be a specific frequency that the
plasma didn't absorb. But these excited particles, these free particles,
are not limited to absorbing only very specific frequencies. It
(25:36):
would be very difficult to get your laser beam through
a slice of the Sun through a plasma shield. So
that's pretty cool. And actually we have a little bit
of a plasma shield already on Earth. The part of
the atmosphere we call the ionosphere is basically plasmas filled
with ions, and it blocks a lot of radiation from
the Sun, neutral radiation, photons, etcetera. And our ionosphere is
(26:00):
not very dense but mostly blocks very long wavelength radiation.
But if you made a denser plasma, more like a
slice of the Sun, then you could block shorter wavelength.
So this is the kind of thing you could actually build. Now,
how do you make a plasma shield? How is that possible?
Can you actually do that? It would be pretty tricky.
I mean you need to have basically a shell of gas,
(26:22):
and then you need to have that gas get excited
to get very very hot. You need to dump a
huge amount of energy into it. So you can imagine,
for example, puffing out a shell of gas and then
zapping it with a bunch of lasers to turn it
into a plasma shield. Like essentially deposit so much energy
in this shield around you that anything else that comes
into it we get absorbed or interacted with or diffused.
(26:43):
This is not something that's very easy to do, not
something we have the technology to do at all already.
It's difficult for us to make and control plasmas. This
essentially is the task of fusion. We are trying to
replicate what's happening on the Sun in laboratories on Earth,
not just so we can build fourth fields, but so
that we can generate essentially limitless energy through nuclear fusion,
(27:05):
the same process that happens in the center of the Sun,
and it's tricky. People have been working on it for decades.
They're trying to make a donut of plasma, and they're
containing it with magnetic fields. Because the stuff is so
volatile it would vaporize any container you put it in.
It's so hot and nasty and interactive, so they use
a magnetic bottle to contain this plasma. They have basically
(27:27):
a donut of the sun, and it's hard to get
it to go, it's hard to keep it straight. This
is a big engineering challenge, and there's a project going
on right now called Eater, which is the biggest and
very promising application of this, but it costs billions of
dollars and it's not simple to set up. So this
would be a very difficult engineering challenge, and you'd have
to somehow create this shield around your spaceship, get it
(27:47):
zapped up to turn it into a plasma, and then
keep it in line with very strong magnets. So possibly
you could do that potentially sometime in the future. It
might be possible, but it sort of violates one of
the first principles we asked for for force field, which
is that it's invisible if you surrounded yourself with sort
of a sheet of the Sun in a sphere around
(28:08):
your ship. Then you wouldn't be able to see anything
outside that sphere, and you'd be stuck inside basically a
little slice of the sun, which I guess would get
pretty hot. So that's sort of the best idea that's
out there that I'm aware of to build an actual
force field. But it has a lot of engineering challenges
ahead of it, and even if we could solve all
of those, it doesn't actually satisfy all the requirements we have.
(28:31):
So fun question. Maybe somebody in the future will come
up with a new way to think about how to
actually build a force field. And I shouldn't spy anybody
from imagining and from wondering and from being creative. So
keep using force fields in the science fiction that you write,
and keep an eye out in science fiction for other
cool ideas that we might actually turn one day into reality.
(28:52):
All right, I want to get to one more question,
but first let's take another break. All Right, we are back,
and we're having a lot of fun talking about the
(29:12):
future and time travel and force fields and future technologies.
And now I want to take a trip even deeper
into the future, wondering how long humans can survive on Earth, Pie,
Daniel and Joey. My name is Gavin and I live
in South Wales in the United Kingdom. In the last
podcast What's Inside the Earth, You've got me thinking. You
(29:33):
told us that the big lump of moon sized core
inside the Earth is getting bigger by one millimeter every year,
and that at some point in the future the core
will stop turning on the Earth will load its magnetic
force like we think happened to Mars. My question is, well,
this happened before I was set and goes supernova. All right,
(29:54):
Thank you very much for that question, and also on
behalf of all of humanity, thank you so much for
thinking ahead, for worrying because we don't have time to
about the deep future, and starting to make plans today
for what we might have to do to prepare for it.
So he's wondering which of these two calamities will we
have to deal with first, the Sun exploding and fiddling
(30:16):
out or the earth magnetic field dying, And the two
are related, of course, because the earth magnetic field is
important for protecting us from the Sun's radiation, so if
it disappears then we will be fried. But hey, if
it's gonna last longer than the Sun, then we don't
need to worry about it. So which one will kill
us first is basically the name of the game in
(30:37):
this question. So first let's remind ourselves what the timeline
is for our sun. How long is this thing? And
to keep burning and keeping us toasty and keeping us
keeping on. So our son is currently about five billion
years old. It's about the age of the Solar system,
because it basically is the Solar system. Remember that a
coalesced together from a huge cloud of gas and dust
(31:00):
and leftovers from population three in population two stars had
gathered together in some gravitational event, slowly slurping together, And
that most of the stuff in the Solar system is
in the Sun. About of everything that's in the Solar
system is the Sun, So we're just like a little
detail on top of the Sun. Now, that happened about
(31:21):
four and a half to five billion years ago, and
since then, what's the Sun been doing? Well, it's been
burning hydrogen. Gravity gathers together all this material, mostly hydrogen,
but also some helium left over from burning from previous generations,
and a few other heavier elements, but still overwhelmingly hydrogen
and that's good because that's the fuel for the Sun.
(31:44):
Gravity squeezes together this hydrogen, and when it gets it
close enough, then it confused. Remember that hydrogen is essentially
just a proton with an electron around it, but when
things get hot, it's basically just a proton. And to
make fusion happen, you've got to squeeze to these protons together.
But protons are both positively charged and so they resist.
And until you get them closed enough together, when the
(32:06):
strong force can take over and do nuclear fusion and
release a bunch of energy, they will resist. And that's
why fusion is hard to make happen. You need really
high temperatures and pressures. That's why it's difficult for them
to engineer it here on Earth. But anyway, it happens
in the Sun, and it's been happening now for about
five billion years. So what's going to keep the Sun
from burning on forever? Well, eventually it's going to run
(32:29):
out of fuel. It's basically a huge hydrogen thermonuclear device.
And what happens when it burns hydrogen is that it
creates ash. That ash is helium, and it starts at
the core because that's where the fusion starts. That's the hottest,
densest part. So hydrogen first burns out at the core,
and you get this helium core now surrounded by hydrogen,
(32:50):
and the fusion is now happening in the sort of
outer layers surrounding the core. This pushes out on the Sun,
making it bigger and bigger. It fluffs it out, larger
and larger, and actually even makes it brighter and brighter.
So every year the Sun gets a little bit hotter.
In four billion years, for example, the Sun will be
about brighter than it is today, and just that is
(33:12):
enough to like boil all of the oceans on Earth
and turn them into vapor. So right there you can
see that in about four billion years, our sun will
not make Earth a very cozy place to live in.
But as the Sun grows, it's gonna get bigger and bigger.
And you might think, well, how much bigger can it get.
It's already huge, right, it's already a million times the
volume of the Earth. Well it's gonna get bigger by
(33:33):
a lot. It's gonna grow by about a factor of
two hundred. And that's bad news, because I mean it's
gonna get so big that it's radius is going to
match the size of Earth's orbit. Right, It's gonna envelop
all the inner planets, and the Earth will be right there,
right about on the edge of the Sun. What does
that mean to be like inside the sun or right
(33:54):
just past the edge of the Sun. Well, it's gonna
be really hot, and we're gonna be surrounded by these
huge sheats of burning hydrogen, which is not going to
be a good place to live in. Now, inside the Sun,
you now have a helium core which is getting bigger
and bigger because as hydrogen burns, it creates more and
more helium. Eventually you can even fuse this helium together
(34:14):
to make something even heavier carbon. And so this is
awesome because it burns hydrogen for millions and millions and
billions of years, and then all of a sudden it
starts to burn helium. It passes this critical point you
can get this helium flash, and the amount of energy
generated by the Sun in this moment is actually brighter
than all the stars in the galaxy put together, although
(34:35):
you don't get to see it because it's absorbed by
the inner layers of the Sun. But you know, maybe
that's good because otherwise it would fry everything on Earth.
So it starts helium fusion, creating carbon, and our sun
is not heavy enough. It's not massive enough to then
take the next step and fuse carbon into heavier elements.
You could eventually make oxygen and neon and silicon and
(34:55):
all sorts of crazy stuff if you had a bigger star.
Our star is not big enough to do that. So
what's gonna happen is it's going to accumulate helium, which
will burn into carbon, But then it's sort of stuck there.
The carbon is inert and it's like having a lot
of ash in your fire. It makes it harder for
things to burn, and so the sun will just sort
of decrease in brightness from there, basically fizzling out and
(35:17):
turning into a white dwarf. A white dwarf is just
a hot blob of carbon. It's not fusing anymore, it's
not producing any more energy, but it's still really really hot,
right it glows. White dwarfs are called white because they
do give off light again, not because they're actually fusing
they are creating new energy. They're just glowing the same
(35:37):
way that anything that's hot glows. It's a white hot
lump of carbon in the universe, and so it glows
white the way like a really hot piece of metal
that you stick into a blacksmith's furnace will also glow
white hot, and eventually it will cool. And scientists think
that white dwarfs, if given enough time, will eventually radiate
off all of their energy and turn into something else,
(36:00):
something weird, something called a cold black dwarf. But that
will take trillions of years. So let's review the timeline.
We're about five billion years into the life cycle of
the Sun. It will keep burning for about five billion
more years, after which it will envelop the Earth, will
have the helium flash, and it will convert into a
white dwarf, which won't give off enough heat to keep
(36:22):
the Earth warm. But the Earth will have already been
fried at that point, and then eventually the white dwarf,
in trillions of years, will cool off. So timeline for
the Sun to fry us is a few billion years.
Now let's turn to the other side of the question,
how long will we have a magnetic field for? To
answer this question, we need to think about what causes
the magnetic field and why that might come to an end.
(36:44):
So as we talked about a few minutes ago, the
magnetic field we think comes from internal flows of molten
rock and metal. And again it's good idea to sort
of turn back to the clock and remember how the
Earth was formed and why it is the way it is.
We think the Earth came together basically a bunch of
bits of rock which gathered together into larger bits of rock,
into larger bits of rock. So the early Earth was
(37:06):
just a collection of rock basically squeezed together, and it
wasn't actually melted in the middle yet. Gravity then took
over and squeezed it further and further. That plus radioactive
decay from certain heavy metals that were embedded in the rock,
helps melt the center of the Earth. This might have
taken a few hundred million years to melt the center
of the Earth to make it molten rather than just
(37:28):
hot rock. And this was crucial because it let all
the heavy elements, the iron, the nickel, sort of melt down. Right.
Things get hot, they get molten, they get liquid. Now
you have a fluid instead of rocks, so it's easier
for things to sort of like slide around and rearrange themselves.
So the heavier elements sink to the core, and now
you've get the structure that we have today, which is
(37:48):
a solid inner core, very very dense, surrounded by that
liquid outer core. And then on top of that is
the mantle, which isn't exactly liquid, but it is rock
that's sort of flowing. And then on top of that
is the crust, the things we actually live on. And
we think that it's this molten motion inside the earth.
Essentially this liquid outer core that's generating the magnetic field
(38:10):
that it's spinning, that there's electric currents in there, and
that motion of electric charges creates magnetic fields. Because remember
there's a very tight connection between electricity and magnetism. There
really are just one thing, electro magnetism, which is why
moving particles can generate magnetic fields and why magnetic fields
can bend the path of charged particles. So what happens
(38:31):
is this stuff is swirling around. It's basically a current,
and that creates a magnetic field, and the magnetic field
enhances the current. Right, the magnetic field pushes these particles
in a circle, and then moving in a circle makes
a bigger magnetic field. This is called a dynamo effect. Basically,
builds on itself. It makes a stronger and stronger magnetic field,
but it relies on the flowing of this liquid molten
(38:54):
rock and metal in the center of the Earth outside
the inner core. You have to have the ability for
this thing to slash around. You need the motion in
order to have a magnetic field. And as the question mentioned,
this inner core is growing. The Earth is cooling. Basically,
it's freezing very slowly, and this core is growing by
one millimeter per year, and that's just because things cool, right,
(39:17):
We are giving off heat into space the same way
that a white dwarf will eventually radiate out all of
its energy into space and become a cold black dwarf.
Entropy tells us that he should spread out. So you
have an isolated hot blob of something in cold space.
Eventually it will radiate out its energy. So that's what's
happening to Earth. And the inner core is growing, and
(39:37):
eventually it will grow and it will cool down the Earth.
So how long will this take? Will it be faster
than the Sun enveloping the Earth and frying us, or
will it be slower. It's difficult to predict these things
because it requires projecting out pretty far in time, and
our understanding of how these things work inside the Earth
is still a little speculative, all right. We've never drilled
(39:58):
down to the Earth. A lot of this stuff is
reconstructed from seismographs, from basically bouncing waves off the inside
of the Earth and understanding how those waves are reflected
as they hit various layers inside the Earth. So there's
still a lot of guesswork involved. But the best estimate
I found is that it will take about ninety billion
years for the Earth to lose this magnetic field, essentially
(40:20):
for the Earth to freeze internally and to stop having
the flowing liquid rock and metal necessary to have a
magnetic field. So that's nine one billion years. That's a
lot longer than the Sun's expected lifetime. The Sun, we
think will fizzle out in about five billion years, So
the Earth, if it survives that, if it doesn't just
(40:40):
get like melted and slurped into the Sun, would continue
on for a long time for tens of billions of
years with a magnetic field spinning and reflecting charged particles happily,
even if we're now orbiting a white dwarf. So we
think the Sun will expand and it will fry the
Earth in a few billion years, after which you'll call
laps to a white dwarf, and then the Earth, with
(41:02):
its magnetic field, will orbit this white dwarf for billions
of years until eventually it cools and solidifies and loses
its magnetic field in something like ninety billion years. So
that's definitely something to think about, something to plan for,
something to wonder about. I think personally, we'd be lucky
if we made it a billion years on this Earth,
(41:22):
so that we have to worry about things like the
Sun getting bigger and frying us. We have more immediate
problems that we need to tackle, not force fields, not
closed timelike curves, but just sort of taking care of
the planet so we can last that long. Anyway, it's
fun to think about these things, and fun to understand
the physics that goes into making a son or making
a magnetic field so that you can understand how long
(41:44):
they will last. All right, Thanks everybody for sending in
these questions and for letting us ride with you on
your curiosity journey, for wondering about the nature of a space,
for wondering if it's possible to go backwards in time
to build force fields and how long this planet Earth
will be around for us to live on. It's a
joy to get your emails and enjoyed to think about
the ideas that are in your head, so please don't
(42:07):
be shy send us your questions. Tune in next time.
Thanks for listening, and remember that Daniel and Jorge Explain
the Universe is a production of I Heart Radio. Or
more podcast from my heart Radio visit the I heart
(42:27):
Radio app, Apple Podcasts, or wherever you listen to your
favorite shows. Ye
Hosts And Creators
Daniel Whiteson
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