June 27, 2019 • 35 mins
How can we travel through the universe without running out of fuel? Find out today with Daniel and Jorge.
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Episode Transcript
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Speaker 1 (00:09):
Hey, Daniel, you like the outdoors. You're an outdoorsy kind
of person. Are you a fan of sailing? Oh? Man,
I love sailing. There's so much to appreciate, the wind,
the quiet, But mostly it's because it's about applied physics.
That's the part you love the most about it is
the physics of sailing. Yeah. You know, you've got the
wind coming in one way and the boat is pointing
another way, and you have the you know, the keel
(00:30):
underneath and the rudder. It's a it's a big complicated calculation,
all happening in real time under you. Yeah, and you
might die or or get strounded out and see if
you if you mess it up, that's right, that's right.
So physics can save your life. It's good to understand vectors.
But no, yeah, I always find it really confusing. You know.
It's like if you want to go one way, you
actually have to point the boat the other way. But
(00:51):
then you have to move the sail in line with
the wind, and so there's like the Bernoulli effect that
I just I found it. What we're really confusing? It
is confusing basically, Um, you know, all you need to
know is if you want to go one way. Make
sure you point the boat a different direction, right, That's
the only way to get there. Never never point to
where you're going. No, that's classic groupie mistake. Him Orgey,
(01:26):
I'm a cartoonist and the creator of PhD Comics. Hi,
I'm Daniel. I'm a particle physicist and the co author
of our book Together We Have No Idea, A Guide
to the Unknown Universe. Right now, you're listening to our podcast,
Daniel and Jorge Explain the Universe, a production of I
Heart Radio, in which we try to find weird, crazy, wacky, interesting,
(01:47):
amazing things about the universe that will blow your mind
and explain them to you in a way that makes
you understand them and also laugh along the way. That's right.
We take you to all the corners of the universe
in rockets, in spaceships, in warp drives potentially, and also
maybe using sailboats. That's right. We want to navigate your
brain from here to there and all around, and we're
(02:09):
willing to do in any sort of mental capsule that's possible.
And so today on the program, we're going to be
answering a listener question, who was wondering if there are
better ways to travel through space. That's right. You guys
might be familiar with the question of how do we
get from here to there? Right? And if you want
to fly from Earth to Mars, or Earth to Jupiter,
or even do another star, you gotta push yourself there,
(02:32):
or you gotta get your ship going at a certain speed,
and that requires energy. And most of the technologies that
we have currently are basically just like a fire spewing
out the back, and that requires some sort of fuel.
That's right, you need fuel, And unfortunately there are no
gas stations between here and Mars, and so that you
kind of have to bring it all with you. But
there might be Tesla supercharger stations for all we know. Right,
(02:53):
isn't that Elon Musk's plan? How does that company doing
these days? I haven't really checked my stock portfolio, but
you know, the guy thinks big. So probably as SpaceX
goes to Mars, he's gonna stop along the way and leave,
you know, charging stations. Yeah. So to the end the program,
we'll be talking about an interesting idea about traveling to
the stars. We'll be talking about solar sales. How do
(03:19):
they work? Do you really sale our solar cells on sale. Right,
how did what does this thing even mean? That'd be
nice if they're on sale, everyone can get one, that's right.
That's right, more than just our listeners in Delaware. Remember
that podcast when we were going to cover the state
of Delaware with solar cells. Oh? Yeah, do we get
any feedback from delawareans? Oh? Actually we did. Um. First
(03:40):
of all, they commented that we need to also include Maryland.
And they weren't defended at all. Actually, they were just
glad to have any sort of Delaware shout out. They
say that they don't often get mentioned. Never happens. I
guess exactly. So we love you, Delaware. Keep listening. Yeah,
so it's kind of an interesting idea that you can
make a sale to sale all the deep blue blackness
(04:02):
of space. Yeah, it's really kind of an awesome idea.
I mean, you're we're familiar with the wind, right, and
the wind is a wonderful way to use a sailboat
because you don't have to bring any fuel with you. Right,
you go on a motor boat, you've got to bring
all that fuel with you. It makes your boat heavier,
it's loud, it's noisy, is obnoxious. Right, One of the
things I love about sailing is that it's quiet. Right.
(04:22):
First of all, you don't have to fuel up because
your fuel is just the wind. And then when you're sailing,
there's no obnoxious motor sound to listen to. So the
idea of traveling from one planet to another through space
using something similar like being propelled without bringing along your
own fuel, it's a beautiful idea. Yeah, it's kind of
cool to think that you can ride something out in space, right,
Like there's something already going there and you can just
(04:45):
kind of latch onto it and will push you to
where you want to go. That's right. And you might
be tempted to say, what's pushing the sail, Like obviously
on Earth here it's air, But what's pushing your sail
out in space? Because there's no air in space and
you're right, right if we're not sailing in that same way.
And then you might be tempted to say, what about
the solar wind? These guys have told us about all
the particles shooting out from the Sun, Is that the
(05:07):
solar wind that a solar sale would be riding. But
that's not even the case. It's not it's not the
solar wind that a solar sale works off of though
that would be some sort of poetic you know, beautiful parallel. Wait,
so it's not the solar wind that you would sail
on in space. No, it seems tempting, right, you'd like
to have a solar sail boat using the solar wind
(05:27):
for this beautiful analogy for our whiskey fueled adventures on
the high seas here. But it's not actually the solar
wind that pushes along a solar sail. Oh man. The
solar wind does it give you a sunburn and chapped
lips at the same time? That's right, It gives you
that solar wind touseled hair. Also you have deep fried
probably exactly not recommended, folks, not recommended. It's like a
(05:51):
Brazilian blowout. But um, Chris here, if you're listening to
this podcast on the space station, do not stick your
head at the window to get a solar wind tewseled look. Yeah, instead, right,
is because that'd be really cool to hear from you,
exactly exactly. Yeah, So we'll be talking to me about
the solar sales, the idea that you can go between
planets or stars or galaxies using a sale to catch
(06:14):
something out there in space to take you there. And so,
as we mentioned, this question came to us from Derek
from Cleveland, and so here is Derek asking his question. Hey,
Daniel Jorge, this is Derek from Cleveland, Ohio. My question
is how does solar sales work if photons are massless?
How can they impart momentum to a satellite? And how
is the conservation of momentum upheld? I quickly want to
(06:34):
say hi to my brother in law, Tim, who I'm
sure we'll listen and discuss your answer with me later.
Thanks guys, All right, thank you Derek. That's a pretty
interesting question. I mean, it's it's sort of um. It
sounds like he's heard of solar sales and he knows
they work by catching light, but his question was how
does that work if photons have no mass? Yeah, so
many good questions here, right, like how does the solar
(06:55):
sale work? He seems to understand that it catches light,
but it doesn't understand like does light half momentum because
he's probably thinking that momentum is mass times velocity, right,
which is the case for slow moving massive objects. So
he's wondering if the mass term is zero, how can
you have any momentum, Like, how do things with no
mass have any momentum? And if they have meantum, if
(07:16):
they don't have momentum, how can they push along a
solar sale? It's a great question, right, I mean, I
think it means, like, you know, if light has nothing
to it in terms of like mass, how can it
push you? Exactly? That's the question. And so I was
wondering what people thought about this. So people aware of
solar sales do they understand photons and momentum and all
this kind of stuff. And so I went around and
(07:37):
I asked people, um what they knew about solar sales.
And before you listen to these answers, think to yourself,
do you know how a solar sale works? Do you
understand how light can have momentum if it doesn't have mass.
Here's what they had to say. I feel like I've
heard of it, but I'm not sure. I mean, I've
seen like six once, but I haven't seen I know
that they're kind of transitioning like testas doing the roof
(07:58):
tiles and stuff that I was sore, but I don't
I don't know if I've seen a big pike. I
guess kind of like sheet of sunlight. Yes, actually I
have a solar sale is basically a really big sale
that doesn't get pushed by wind. It gets pushed by
energy from the sun. So basically energy has a little
bit of movement in it, and that big movement, a
(08:22):
little bit of movement can push it because it's not
too much movement, but it's a lot of energy in
a big area that that will push it. I'm honestly
not sure. I couldn't even I couldn't mean guess like
maybe the stilted price on solar system or something like that.
All right, I love these answers or I feel like
they're all correct. Like is it a boat powered by
(08:46):
solar power? Yes, technically that would be a solar That
would be like a boat with an electric engine on
it that's powered by solar cells. That's pretty cool. Yeah, sure, yeah,
that's like a solar boat for sure. Or like a discount,
I don't think is it? Is it like a discount
on solar panel? I think like a solar sale. I
think that guy might have thought I was actually trying
(09:06):
to sell him solar panels, like he looked kind of wary.
He's like, are you another one of those shady solar
salesman discounts. They are pretty shady quote unquote, that's right exactly.
Their business wouldn't stand up to sunlight. You have to
be pretty bright. And there was one folk, one guy
who actually had a pretty solid answer. Was this your
physics student sale and slash sale instructor? It was not.
(09:29):
Actually that was my eleven year old son, who I
think overheard me talking about solar sales to somebody else
before I asked him, so he had a preview of
the answer and just wound wanted to sound smart. You're
saying your son cheated, I'm saying I asked him the question,
and he knew the answer, and so he answered it.
That's right, it's it wasn't a test. Yeah, he had
(09:51):
a bit more of a head start than the rest
of the other folks that we asked. So, um, it's
not something that a lot of people seem to know
about or recognize the name of. That's right. It's not
a in concept. And that surprised me a little bit
because I thought that people had heard more about like
how to move through space or the kinds of engines
you might use. But I guess not which makes an
excellent topic for today's podcast. Yeah. Well, I have to
(10:12):
admit my only reference to a solar sale powered spaceship
is from Star Wars. Do they do solar sales? And
Star Wars? I don't even remember that. I thought they
all had ion engines and stuff. Yeah, I know. I mean,
if you if you've watched the prequels, which I'm sure
we can get into a whole episode about. But count
Dooku with that character has rise around in a ship
(10:33):
that has solar sales or what looks like what looked
like solar sales. Oh my gosh. Why you have like
an encyclopedic knowledge of all the ships in every science
fiction movie ever. That's that's impressive. And I have an
encyclopedic knowledge of all NERD movies. So that's what That's
what you do it for In the afternoon when you're
not feeling in the mood to do cartooning, it's like
(10:54):
turn on some science fiction movie. Yeah, isn't that what
everybody does? Yes, yes, that is exactly what everybody does
in the middle of the afternoon. Um, no, but you're right,
these things are in science fiction. Um. I've read lots
of books with solar sale powered chips in them. Uh,
it's a pretty common idea in terms of like how
to get from here to there. And you know what,
(11:15):
we don't have that many ideas for how to get
from one planet to another or one star to another,
and so it's on a pretty short list. Well, this
is how it kind of worked in Star Wars. So
count get on this ship. He takes off, he flies
up into orbit or space, and then he like unfurls
this huge giant sail, this like giant sheet that looks
(11:35):
kind of like it's made out of gold, and then
then he starts going. So it's like he's it's like
he opened a giant umbrella and then that pushed him forward. Yeah,
that's basically a solar sales are giant umbrellas. Podcast over,
you nailed it Star Wars for the wind. Yeah, let's
get into it, Daniel. What is a solar sale? So
(11:55):
a solar sale is an enormous flat sheet, just like
a sail on a sale boat. Um, and the thing
that it captures is not wind like in air, because
there is no wind in space and it's not the
solar wind because the solar wind member is protons being
shot out by the Sun and those could push the sail,
but there's just not that much of them. There's just
not that many of them. There's not enough of them.
(12:17):
What is capturing is light. So the light from the
sun is zooming out from the Sun and it's carrying
energy with it, and we'll dig into that in a moment.
But photons have momentum, and so what happens when they
hit the sail is that they give it a little push,
and you get enough little pushes and you can actually
get going somewhere. So this is a real thing, like
people have built these, Like there are functioning solar wind
(12:42):
sailboats out there in space. Um, yes, this is a
real thing. It's like physics says, there's no reason why
I shouldn't work. People have big plans to make huge
solar sales that can go all around the Solar System
or maybe even to other stars. And somebody has actually
built one of these things and made it work. The
Japanese they launched one um several years ago and showed
(13:03):
that it actually worked. Like the physics isn't just theoretical,
Like there is one thing out there that's solar sailing
right now, that's pretty cool the Japanese. The Japanese, nice job.
But the thing about solar sales is that it's gentle, right,
It's not really really powerful winds of light out there
for you to capture with your sales. So if you
have like even a really big sale, like eight hundred
(13:26):
meters by eight hundred meters, which is you know, enormous, right,
then that only gets five newton's of force, And five
newtons is not that much. It's like how much you know,
a fourth there isn't like a one pound object on Earth,
So like a one pound rock, you know, think about
how much that feels in your hand. That's the force
of the Earth's gravity on a one pound rock. That's
(13:48):
the same amount of force that the Sun would push
on a eight hundred meter by eight hundred meters sail
about the distance from of the Earth, which is huge, right,
I mean it's a several city blocks. Yeah, that is
not a small thing to build and to unferral and
to keep from getting shredded right by like micro meteorites
or whatever. But you're saying if I made a tarp
(14:10):
that was about a kilometer on each side over Manhattan,
it would get pushed down not just by the weight
of it, but also by the sun that's hitting it. Yes, exactly,
And every time a photon hits you, you're getting pushed. Like.
Photons are actually pushy. They are kind of rude, aren't they.
They just like zoom along wherever they want to go.
(14:31):
They don't ask questions, they don't take turns, or they
just going straight lines wherever they need to know. Photons
care momentum, and every time they hit something, they give
it a little push. So when you turn, when you
flip the switch and turn on the light, you get
pushed down a tiny little bit. And you might think, what,
I can't feel it, And you're right because it's a
small effect, which is why solar sales are very gentle,
(14:53):
and you need a huge one to add up to
any measurable force. Well, let's step us through a little
bit of how this works. I mean, so you're saying,
if I take a flashlight and I flash it at you,
I am technically kind of pushing you with it. You're
basically punching me in the face with light. Yeah. Yeah,
that's that's really me because we don't think of light
as having substance to it, right, Like you I mean,
(15:14):
we know we sort of associated with energy and heat
and warmth, but not um force. That's right. We don't
think of photons is having mass because they don't, but
they do have energy, right, Photons are energies is just
pure energy. And if you have energy, that means you
have momentum. And it's complicated because we're only used to
thinking about things having momentum when they have mass. We
(15:36):
think about you give something momentum because it's heavy and
it's hard to get going, and once you've pushed it,
it keeps going. It has momentum. Right, So something that's
like light and fluffy and you know, weighs nothing, it's
hard to understand how that can have momentum. Well, I
think I think we're all pretty familiar with like if
I shine a flashlight on you or a laser, like
your body is gonna absorb that energy or you know,
(15:58):
warm up a little bit or something. But you're saying
that if I have an atom on my skin and
you're flashing a flashlight on it, the photons actually sort
of hit it and the atom wants to go the
other way. Yes, exactly. Think about what happens with a mirror, right,
it's a little bit more complicated when the light gets absorbed.
So let's think about a mirror where the light gets reflected,
the light zooming in one direction, it hits the mirror,
(16:18):
and the lights going in the other direction. Right, how
does the light change direction? Right? What changes the direction
of the light. It's the mirror. Instead of thinking about
light in a mirror, think about a ping pong ball
and a paddle. Right, if you get hit. If a
ping pong paddle gets hit by the ball, then the
ball bounces the other direction, but the paddle gets pushed
back a little bit. Right, That balances the momentum. Momentum
(16:41):
is conserved. So in the same way, when the light
hits the mirror, it gives that mirror a little push,
and the mirror pushes back, and that's what turns the
light the other direction. And so when light reflects off
of something, it's giving it a little push. So pure
energy can push you basically exactly. It can result in
something wanting to move the other way. That's right. And
(17:01):
this is one of the cool things in physics when
you learn that a concept you thought you had a
firm grasp on, like momentum, is actually just a special
case of sort of a larger concept. Like we're familiar
with momentum is mass times velocity. But that's not it's definition.
That's just how you calculated for slow moving things with mass.
You can calculate momentum for things that don't have mass,
(17:22):
because momentum is a larger concept. It's just it captures
like the quantity of motion of something, not just it's
mass times velocity. It's just that it's more like the
total energy. Yeah, but it's fascinating because energy is just
a number. It doesn't have a direction, right, and physics
energies is a scaler, not a vector, like it doesn't
have a direction, just a magnitude, whereas momentum has a direction.
(17:44):
Like you know, pointed in one direction is different from
pointed in the other direction. Right, Uh, light has momentum.
Light has momentum, has energy, and it has momentum and
it has no mass. One way that some people like
to think about it is you can think about momentum
sort of the difference between the energy and the mass.
Right because in real and relativity we think of the
(18:04):
total energy as the energy store in the mass plus
the energy of the momentum. So you can sort of
invert that equation and say the momentum is the difference
between the total energy and the mass. And in the
case of the photon, there is no mass. Momentum just
comes from the energy, whereas in the case of another particle, right,
it has a bunch of energy. Some of that is
in the mass and some of it is in its motion.
(18:25):
But for the photon, all the energy is just in
its motion, so it just has momentum and no mass.
All right. Solar sales are a real thing, and they
work by taking the momentum that's in light, absorbing it
and then using that to push you along in space.
That's right, And it's the light from the Sun that's
doing this, not the protons from the solar wind. And
everybody actually can see this if you just have seen
(18:47):
a comet. You know, comet is just a big ice
ball and it falls towards the Sun and as it
does so it gets a big tail, right, and that's
the tail of it, like the stuff melting and dripping
out the back, and the tail points in a specific
direction away from the Sun, not in the direction of motion.
People think, like, you know, it's like a fuzzy lines
behind something that's moving fast. But the commet actually always
(19:08):
points away from the Sun, and that's because of this
effect of light. That's light pushing the tail of the commet. Really,
it's not like a trail of a dust that you
leave behind. No, Because as the common is moving towards
the Sun, the tail is behind it. But as the
comet goes around the Sun and then leaves, the tails
in front of it. Wow, so it shouldn't be called
the tail once again? Yeah, I guess. Or maybe it's
(19:32):
like backing away from the Sun as it leaves. It's
like whoa, that was close. All right, let's get into
will it work and how how can we make it work?
But first let's take a quick break. All right. So
(19:55):
that's solar sales Daniel. That's pretty cool. The question now,
I think is does it work? If you build it,
will it take us to other planets and maybe other galaxies? Yeah,
it's it seems like it will. And you know, we
know a lot about it. We can do these calculations.
We know how much light there is, we can calculate
how it will reflect and how much will be absorbed,
and we know all the numbers we need. And so
(20:15):
people have plugged all this stuff into calculations, and they said,
if you build a solar sale that's like a kilometer
by a kilometer and you have a really light ship,
you know, not too heavy, how long would it take
to get to Mars or to get to Jupiter. And
the thing you have to understand is that the strength
of the light, of course, gets weaker as you get
further from the Sun. So the solar sale is very
(20:36):
powerful close to the Sun because it's capturing more light,
and then as you get further and further away from
the Sun, it gets weaker and weaker. So solar sales
are great for navigating around the inner parts of a
solar system. Well, the question I have is, you know
these sales are catch the rays from the Sun, right,
they catch the light from the Sun and push it
get pushed away from the Sun. But what if you
(20:58):
want to travel towards the Sun are perpendicular to the Sun.
How would you do that? If you want to move
towards the Sun, It's not hard, right, you just need
to use gravity. Right, the Sun is going to pull
you towards the Sun, so that part is not hard.
But the only way you can use the sail to
build energies is moving away from the sun. So you
can play this game of gravity versus the solar sail
(21:18):
depending on which way you want to go. If you're
facing the sun, can you go side? What can it
push you sideways? Because gravity is pushing me towards the
sun and the solar sales pushing me away from the sun.
How do how do I gain speed sideways? WHOA, that's
a really good question. I haven't even thought about that.
How do you steer it? Because the photons are only
coming out from the sun, so can you move in
(21:40):
a direction that's not out away from the sun. My
first thought is, let's think about how a sale works
on a sail boat. Right, on a sail boat, you
can move in a different direction than the wind if
you put the sail across from the wind. But the
problem is a sailboat has a keel, right, has something
underneath it that keeps it from sliding sideway is and
(22:00):
so it can only use the part of the wind
that it wants by pointing in the right direction. A
solar sale, of course, on a spaceship doesn't have a keel,
So I first thought you might think, oh, a solar
sale can only take you in the direction the photons
are going. Right. Well, let's think about it for a moment.
What happens if you tilt the sale so that it's
(22:21):
not pointing um flat away from the sun right, if
it's at an angle with respect to the photons. You
might think, well, it doesn't really matter because the photons
hit it and bounce off, and they're giving a push
the direction they were going. Right. It's actually a tiny
bit more complicated. If the photons hit it at an angle,
then they also bounce off at an angle right. So
(22:45):
if the photons are coming directly out from the sun
and they hit flat on the solar sail, then they
bounced back the opposite direction, and the momentum they have
given is in the direction they were going. But if
they come out from the sun and they hit the
solar cell at an angle right, then they were flecked off.
Think about light hitting a mirror right. It doesn't come
back the way it was going. It reflects off at
an angle, and so the push that it gives is
(23:07):
actually in the direction that the solar sale was facing.
So I think that means that you can steer a
solar sale you just turn it through the way it
wants to go, and it will catch that part of
the photons momentum that pushes it in the direction it's facing.
You can't just point in that direction and drive the
way you came with your car. You have to take
(23:28):
an account all these limitations. But you can do it,
and you can take a pretty heavy load, you know,
a few tons to mercury, and it just takes a
few years. If you want to go to Venus, it
takes you know, about a year. If you want to
go the other direction, it starts taking longer because the
radiation from the Sun drops, right, So if you want
to go to Mars, it takes like a year and
a half maybe two. Takes like two years to get
(23:50):
to Jupiter, three to get to Saturn, and then like
six years and nine years to get out to the
outer planets. It's a lot of whiskey drinking on board.
That's your fuel, after all, right, there's no there's no
solar whiskey, and you'd be using it up. You could
have burned it to evade that asteroid, but no, you
had to drink it. Yeah, well, you know, it depends
(24:11):
on how well you get along with your crewmates, depending
that determines how much whiskey you have to drink. Can
you imagine being stuck in a spaceship with your physics
student for nine years? Tan? No, but I also can't
imagine being a spaceship by myself for nine years, right, Like,
what do you think would be easier to handle total
isolation or being stuck with one person in a tin
can for nine years? Oh? For me, would definitely be
(24:32):
by myself. Yeah, you and a big stack of science
fiction movies, right all you need and a cartooning pad, obviously,
so I can document my descent into madness, your further
descent into madness, you ment, surely, that's thank you for
reminding me. But yes, but what about beyond the Solar system?
Could you use the solar sale to leave the Solar system?
(24:55):
You could a little bit. I mean what you can
do is try One strategy is to just try to
get going as fast as you can using solar sales
and gravity, and then zoom off in the direction of
another star. So you're like you sling shot around the
Sun to get some speed up, and then as you're
leaving the Sun, you unfurl your sails and it pushes
you away from the sun is as hard as you can.
(25:16):
But then once you leave the solar system, it's really
going to stop pushing you very much, and so your
speed doesn't pick up anymore. But you know, if you
do it right and you have a light enough object,
you could get up to a reasonable speed UM and
head off in that direction. Yeah. I mean, at some
point when you leave the solar system, it's like the
there's no wind. It's like being on a sailboat with
no wind. There's nothing to push you anywhere. Yeah, and
(25:37):
you know there is radiation out there, and there are particles,
but it's very low density. It's not enough to really
really boost you. UM. One idea people have, and I
love this idea because it's so crazy plus ambitious, which
is my favorite combination, is to basically beam light at
these things. Like if you're in a solar sailboat and
you're leaving the solar system, maybe we could like build
(25:58):
a huge laser and beam the energy at your solar
sale to give you a push. What if you punch
a hole through the sale with their laser? Yeah, exactly.
It can't be too intense. UM. But you know these
lass these sales will be very reflective, right. They are
essentially the big sheets of aluminum or maybe plastic that's
covered with aluminum so that they're super reflective. So if
(26:21):
it's reflective enough, then even a really really powerful laser
won't punch a hole, right, because punching a hole means
the energy gets absorbed and so like melts and destroys
the sail. If it's reflective enough, then you should be
able to shoot even a really powerful laser and have
all the energy to be transferred into the momentum of
the sail. It's like blowing on a little sailboat. Yeah, exactly.
(26:41):
Or I'm sure I've seen some science fiction movie where
somebody like reflects a laser blaster with a mirror. Right, Oh,
you just have like a pure mirror shoot at it
and then that pushes you forward. Yeah, if somebody shoots
you with a laser blaster and you have a mirror,
you can just reflect their you know, energy pulse or
whatever back at them. All Right, Well, let's let's get
into how you actually build a solar sale, even if
(27:02):
it wasn't on sale. But first let's take a quick break.
That would be a pretty awesome idea for merch like
solar sale. Daniel and Jorge solar sale explore the universe
(27:24):
instead of explain the universe. That'd be pretty cool. What
do you mean, what will we sail sale sell? We'd
sell the solar sale like you are. We have a
shop where we sell t shirts and coffee mugs with
you know, Daniel and Jorge is playing the universe on it.
We should add to our Internet merch account a solar
(27:44):
sale that you can buy. Okay, cool, done, let's do it.
And then you just get a sheet of aluminum foil?
Is that the idea? That one kilometer by one kilometers
wherever loominum foil with your picture on it. There you go,
and you can sail way into bankruptcy, right, and you
can shoot a laser at that picture from Earth dual
(28:05):
purpose aluminum foil. You can wrap your leftovers and go
into space exactly. But that's basically what a solar sailing is. Right.
You wanted to be really really big, but you also
wanted to be really really light. You want your solar
sailboat to be as light as possible so that all
that momentum gets turned into speed um. The more massive
your sailboat, the slower it's going to go for the
(28:25):
same amount of momentum as you want it really big
and really thin. I mean, does it matter in the
long run how much here solar sail ways? Sure it does. Yeah.
I mean you want to get up to like half
the speed of light or a quarter of the speed
of light or something, it's gonna take a lot longer.
And if you don't have a laser push, then you
have a limited amount of time to get energy from
the sun as you want to get up to as
high as speed as possible. Oh, I mean, but that's
(28:47):
just in general the same for any acceleration, right, Absolutely,
the lighter you are, the faster it is to go faster. Yeah, exactly.
And that's like why people who you know, climb mountains
on their bicycles or move every tiny little thing that's
not necessary because they don't want to carry that mass
up the mountain. You know, I met a guy who's
climbing Matu Picchu and he like sawt his toothbrush in
(29:09):
half because he didn't need the bottom half of it.
And you didn't want to carry that thing up the mountain.
So yeah, you wanted these your spaceship to be as
light as possible so you can translate all of that
momentum into speed. So basically that's all you need is
just a thin aluminum foil. But usually they showed them,
show them in movies and and pictures. Is gold It
looks like gold foil. Does that give you a better
(29:31):
advantage if it's gold or golden? No? Um, you can't
just have aluminum foil because aluminium is too brittle. Like
you try to make a huge sheet of aluminum foil,
I mean, I can't even get like a one foot
square of aluminum foil to stay together without tearing. And
so you try to like fold and then unferral a
one kilometer square of aluminum foil, It's not gonna work.
So usually what they do is they have some sort
(29:51):
of plastic and they spray aluminium coating on it. Um
And so sometimes that plastic is gold. But I don't
think there's a reason why that's gold. I think that's
just to look cool. Right. Well, where are we at then,
in terms of projects that are trying to use solar
sales to get us out there? Yeah, So the best
solar sale ever so far is the Icarus project. Launched
(30:11):
in by Japan and they had a fourteen by fourteen
meter solar sale and that sounds small compared to what
we're talking about like eight eight hundred, but fourteen meters
is not small, and that's like, you know, the size
of an apartment. Can I just comment on their choice
of names, like have they read the whole story of Icarus?
Do they know how that story ends? I don't know.
(30:33):
I love I love the hubris in choosing the name
Icarus for a solar sale project. Right, it's they chose
a story about Hubris and they did it kind of hubristically.
They're like thumbing their nose in history. I love it.
That's not going to happen to us. We're gonna fly
really close to the sun and nothing bad is going
to happen to Maybe they're trying to regain the name Acharis.
(30:53):
They're claiming it back, you know the way some people
claim back bad words. That we're going to turn this
new a positive stereotype. Now, they want to redeem Icarus himself.
That's it exactly. Maybe they're like the descendants of Acharis himself. Yeah,
so they they're building it. They build it. They're going
to build it, they built it, they launched it. It worked.
(31:16):
It's solar sailing. It's zooming around the sun right now. Really,
it's out there. It's out there, sailing away. It's sipping
whiskey and looking down on us. And that's the most
successful solar sale so far. But it's not like a ship,
right It's like a satellite or a little vessel. Yeah,
it's it's a little vessel. It's basically just two experiments
that can we do this thing. Let's see if it
(31:36):
actually works. Because you know, sometimes in physics, you do
the calculations, they sound great, and then you know it
doesn't actually work the way you expected because you forgot
to account for something. But in this case, it's pretty
much worked as they expected. And that's a huge boost
for all the solar sale enthusiasts. I didn't know that
was such a thing. I'm a solar sale enthusiast. I'm
a big fan. First of all, I love sailing, but
(31:57):
I'm a big fan of anything that's going to take
us to other planets or take us to other stars
and it isn't going to require a huge ridiculous amount
of fuel cool Well, uh, and then you were telling
me there's another cool project called the Breakthrough star Shot project. Yeah,
they have this awesome idea. They say, let's not send
one big solar sale, let's build a hundred of like
a thousand nano sales. So there would be a bunch
(32:20):
of really small ones and they would push them along
with lasers between here and Alpha Centauri. The problem with
building a laser that pushes a big solar sales you
need a big laser. So they were like, okay, well,
let's just have a small solar sale that can be
pushed by a small laser and then just have like
a tiny little craft. And so they imagine that that
it would take twenty years to get to Alpha Centauri
(32:43):
with these little nano sailors, And so they would have
a giant laser gun on Earth shooting at these things,
pushing them along, a thousand laser guns pushing on them. Oh,
each one has their own little laser gun. Yeah, exactly,
each one needs its own little boost. But the cool
thing about solar sailing is that you know it's a
small effect, but it just adds up, and if you're
traveling for a long time, it's just like more and
(33:03):
more acceleration. Eventually these things can get up to a
respectable fraction of the speed of light, you know, so
they can cross the distances between stars, and then they
would communicate back what they find. But they can't come back,
can they. They cannot come back. No, because there's no
laser system over there for to shoot them back. I
mean that we're aware of. Maybe they'll get there and
find Elon Musk has built some you know station over
(33:25):
there for recharging your solar your solar lasers or something.
But but no, they it's just a it's a one
way trip. Well, that's cool. It's like instead of building
a giant boat, you build a whole bunch of little
rubber duckies into the river. Kind of exactly. That's gonna be.
Humanity's first contact with an alien species is the equivalent
(33:46):
of a rubber ducky. Hi, look at us, We're so impressive.
This is a little squeezed toy. And then the aliens
are like, this is a sign of war. That's sorry.
Maybe the rubber ducky is like holy to them and
it's offensive that we like through on in their face.
Plus we probably all of our lasers will be like
burning their eyeballs down for a twenty years while the
while the natocrafts are getting there. But if we fill
(34:07):
them up with whiskey, maybe they'll they'll like it. They'll
either love it or hate it. They'd be like, what
Scottish whiskey? I only drink irish? This needs war. Wars
have been fought over less, I'm sure. All right. So
that answers Derek's question, who wanted to know how do
sol of our sales work? And hopefully you guys enjoyed
(34:27):
that and are inspired to maybe think about taking off sailing.
That's right, and you know, maybe someday we'll go out
there and we'll sail among the stars. I think it's
really beautiful and poetic to imagine using the life from
the stars themselves to push ourselves from one to the other.
I really wish that could happen. All right, Thanks for listening, guys,
see you next time. If you still have a question
(34:56):
after listening to all these explanations, please drop us a line.
We'd love to hear from you. You can find us
at Facebook, Twitter, and Instagram at Daniel and Jorge that's
one word, or email us at Feedback at Daniel and
Jorge dot com. Thanks for listening and remember that. Daniel
and Jorge Explain the Universe is a production of I
heart Radio. For more podcast from my heart Radio, visit
(35:18):
the i heart Radio app, Apple Podcasts, or wherever you
listen to your favorite shows.
Hey, Daniel, you like the outdoors. You're an outdoorsy kind
of person. Are you a fan of sailing? Oh? Man,
I love sailing. There's so much to appreciate, the wind,
the quiet, But mostly it's because it's about applied physics.
That's the part you love the most about it is
the physics of sailing. Yeah. You know, you've got the
wind coming in one way and the boat is pointing
another way, and you have the you know, the keel
(00:30):
underneath and the rudder. It's a it's a big complicated calculation,
all happening in real time under you. Yeah, and you
might die or or get strounded out and see if
you if you mess it up, that's right, that's right.
So physics can save your life. It's good to understand vectors.
But no, yeah, I always find it really confusing. You know.
It's like if you want to go one way, you
actually have to point the boat the other way. But
(00:51):
then you have to move the sail in line with
the wind, and so there's like the Bernoulli effect that
I just I found it. What we're really confusing? It
is confusing basically, Um, you know, all you need to
know is if you want to go one way. Make
sure you point the boat a different direction, right, That's
the only way to get there. Never never point to
where you're going. No, that's classic groupie mistake. Him Orgey,
(01:26):
I'm a cartoonist and the creator of PhD Comics. Hi,
I'm Daniel. I'm a particle physicist and the co author
of our book Together We Have No Idea, A Guide
to the Unknown Universe. Right now, you're listening to our podcast,
Daniel and Jorge Explain the Universe, a production of I
Heart Radio, in which we try to find weird, crazy, wacky, interesting,
(01:47):
amazing things about the universe that will blow your mind
and explain them to you in a way that makes
you understand them and also laugh along the way. That's right.
We take you to all the corners of the universe
in rockets, in spaceships, in warp drives potentially, and also
maybe using sailboats. That's right. We want to navigate your
brain from here to there and all around, and we're
(02:09):
willing to do in any sort of mental capsule that's possible.
And so today on the program, we're going to be
answering a listener question, who was wondering if there are
better ways to travel through space. That's right. You guys
might be familiar with the question of how do we
get from here to there? Right? And if you want
to fly from Earth to Mars, or Earth to Jupiter,
or even do another star, you gotta push yourself there,
(02:32):
or you gotta get your ship going at a certain speed,
and that requires energy. And most of the technologies that
we have currently are basically just like a fire spewing
out the back, and that requires some sort of fuel.
That's right, you need fuel, And unfortunately there are no
gas stations between here and Mars, and so that you
kind of have to bring it all with you. But
there might be Tesla supercharger stations for all we know. Right,
(02:53):
isn't that Elon Musk's plan? How does that company doing
these days? I haven't really checked my stock portfolio, but
you know, the guy thinks big. So probably as SpaceX
goes to Mars, he's gonna stop along the way and leave,
you know, charging stations. Yeah. So to the end the program,
we'll be talking about an interesting idea about traveling to
the stars. We'll be talking about solar sales. How do
(03:19):
they work? Do you really sale our solar cells on sale. Right,
how did what does this thing even mean? That'd be
nice if they're on sale, everyone can get one, that's right.
That's right, more than just our listeners in Delaware. Remember
that podcast when we were going to cover the state
of Delaware with solar cells. Oh? Yeah, do we get
any feedback from delawareans? Oh? Actually we did. Um. First
(03:40):
of all, they commented that we need to also include Maryland.
And they weren't defended at all. Actually, they were just
glad to have any sort of Delaware shout out. They
say that they don't often get mentioned. Never happens. I
guess exactly. So we love you, Delaware. Keep listening. Yeah,
so it's kind of an interesting idea that you can
make a sale to sale all the deep blue blackness
(04:02):
of space. Yeah, it's really kind of an awesome idea.
I mean, you're we're familiar with the wind, right, and
the wind is a wonderful way to use a sailboat
because you don't have to bring any fuel with you. Right,
you go on a motor boat, you've got to bring
all that fuel with you. It makes your boat heavier,
it's loud, it's noisy, is obnoxious. Right, One of the
things I love about sailing is that it's quiet. Right.
(04:22):
First of all, you don't have to fuel up because
your fuel is just the wind. And then when you're sailing,
there's no obnoxious motor sound to listen to. So the
idea of traveling from one planet to another through space
using something similar like being propelled without bringing along your
own fuel, it's a beautiful idea. Yeah, it's kind of
cool to think that you can ride something out in space, right,
Like there's something already going there and you can just
(04:45):
kind of latch onto it and will push you to
where you want to go. That's right. And you might
be tempted to say, what's pushing the sail, Like obviously
on Earth here it's air, But what's pushing your sail
out in space? Because there's no air in space and
you're right, right if we're not sailing in that same way.
And then you might be tempted to say, what about
the solar wind? These guys have told us about all
the particles shooting out from the Sun, Is that the
(05:07):
solar wind that a solar sale would be riding. But
that's not even the case. It's not it's not the
solar wind that a solar sale works off of though
that would be some sort of poetic you know, beautiful parallel. Wait,
so it's not the solar wind that you would sail
on in space. No, it seems tempting, right, you'd like
to have a solar sail boat using the solar wind
(05:27):
for this beautiful analogy for our whiskey fueled adventures on
the high seas here. But it's not actually the solar
wind that pushes along a solar sail. Oh man. The
solar wind does it give you a sunburn and chapped
lips at the same time? That's right, It gives you
that solar wind touseled hair. Also you have deep fried
probably exactly not recommended, folks, not recommended. It's like a
(05:51):
Brazilian blowout. But um, Chris here, if you're listening to
this podcast on the space station, do not stick your
head at the window to get a solar wind tewseled look. Yeah, instead, right,
is because that'd be really cool to hear from you,
exactly exactly. Yeah, So we'll be talking to me about
the solar sales, the idea that you can go between
planets or stars or galaxies using a sale to catch
(06:14):
something out there in space to take you there. And so,
as we mentioned, this question came to us from Derek
from Cleveland, and so here is Derek asking his question. Hey,
Daniel Jorge, this is Derek from Cleveland, Ohio. My question
is how does solar sales work if photons are massless?
How can they impart momentum to a satellite? And how
is the conservation of momentum upheld? I quickly want to
(06:34):
say hi to my brother in law, Tim, who I'm
sure we'll listen and discuss your answer with me later.
Thanks guys, All right, thank you Derek. That's a pretty
interesting question. I mean, it's it's sort of um. It
sounds like he's heard of solar sales and he knows
they work by catching light, but his question was how
does that work if photons have no mass? Yeah, so
many good questions here, right, like how does the solar
(06:55):
sale work? He seems to understand that it catches light,
but it doesn't understand like does light half momentum because
he's probably thinking that momentum is mass times velocity, right,
which is the case for slow moving massive objects. So
he's wondering if the mass term is zero, how can
you have any momentum, Like, how do things with no
mass have any momentum? And if they have meantum, if
(07:16):
they don't have momentum, how can they push along a
solar sale? It's a great question, right, I mean, I
think it means, like, you know, if light has nothing
to it in terms of like mass, how can it
push you? Exactly? That's the question. And so I was
wondering what people thought about this. So people aware of
solar sales do they understand photons and momentum and all
this kind of stuff. And so I went around and
(07:37):
I asked people, um what they knew about solar sales.
And before you listen to these answers, think to yourself,
do you know how a solar sale works? Do you
understand how light can have momentum if it doesn't have mass.
Here's what they had to say. I feel like I've
heard of it, but I'm not sure. I mean, I've
seen like six once, but I haven't seen I know
that they're kind of transitioning like testas doing the roof
(07:58):
tiles and stuff that I was sore, but I don't
I don't know if I've seen a big pike. I
guess kind of like sheet of sunlight. Yes, actually I
have a solar sale is basically a really big sale
that doesn't get pushed by wind. It gets pushed by
energy from the sun. So basically energy has a little
bit of movement in it, and that big movement, a
(08:22):
little bit of movement can push it because it's not
too much movement, but it's a lot of energy in
a big area that that will push it. I'm honestly
not sure. I couldn't even I couldn't mean guess like
maybe the stilted price on solar system or something like that.
All right, I love these answers or I feel like
they're all correct. Like is it a boat powered by
(08:46):
solar power? Yes, technically that would be a solar That
would be like a boat with an electric engine on
it that's powered by solar cells. That's pretty cool. Yeah, sure, yeah,
that's like a solar boat for sure. Or like a discount,
I don't think is it? Is it like a discount
on solar panel? I think like a solar sale. I
think that guy might have thought I was actually trying
(09:06):
to sell him solar panels, like he looked kind of wary.
He's like, are you another one of those shady solar
salesman discounts. They are pretty shady quote unquote, that's right exactly.
Their business wouldn't stand up to sunlight. You have to
be pretty bright. And there was one folk, one guy
who actually had a pretty solid answer. Was this your
physics student sale and slash sale instructor? It was not.
(09:29):
Actually that was my eleven year old son, who I
think overheard me talking about solar sales to somebody else
before I asked him, so he had a preview of
the answer and just wound wanted to sound smart. You're
saying your son cheated, I'm saying I asked him the question,
and he knew the answer, and so he answered it.
That's right, it's it wasn't a test. Yeah, he had
(09:51):
a bit more of a head start than the rest
of the other folks that we asked. So, um, it's
not something that a lot of people seem to know
about or recognize the name of. That's right. It's not
a in concept. And that surprised me a little bit
because I thought that people had heard more about like
how to move through space or the kinds of engines
you might use. But I guess not which makes an
excellent topic for today's podcast. Yeah. Well, I have to
(10:12):
admit my only reference to a solar sale powered spaceship
is from Star Wars. Do they do solar sales? And
Star Wars? I don't even remember that. I thought they
all had ion engines and stuff. Yeah, I know. I mean,
if you if you've watched the prequels, which I'm sure
we can get into a whole episode about. But count
Dooku with that character has rise around in a ship
(10:33):
that has solar sales or what looks like what looked
like solar sales. Oh my gosh. Why you have like
an encyclopedic knowledge of all the ships in every science
fiction movie ever. That's that's impressive. And I have an
encyclopedic knowledge of all NERD movies. So that's what That's
what you do it for In the afternoon when you're
not feeling in the mood to do cartooning, it's like
(10:54):
turn on some science fiction movie. Yeah, isn't that what
everybody does? Yes, yes, that is exactly what everybody does
in the middle of the afternoon. Um, no, but you're right,
these things are in science fiction. Um. I've read lots
of books with solar sale powered chips in them. Uh,
it's a pretty common idea in terms of like how
to get from here to there. And you know what,
(11:15):
we don't have that many ideas for how to get
from one planet to another or one star to another,
and so it's on a pretty short list. Well, this
is how it kind of worked in Star Wars. So
count get on this ship. He takes off, he flies
up into orbit or space, and then he like unfurls
this huge giant sail, this like giant sheet that looks
(11:35):
kind of like it's made out of gold, and then
then he starts going. So it's like he's it's like
he opened a giant umbrella and then that pushed him forward. Yeah,
that's basically a solar sales are giant umbrellas. Podcast over,
you nailed it Star Wars for the wind. Yeah, let's
get into it, Daniel. What is a solar sale? So
(11:55):
a solar sale is an enormous flat sheet, just like
a sail on a sale boat. Um, and the thing
that it captures is not wind like in air, because
there is no wind in space and it's not the
solar wind because the solar wind member is protons being
shot out by the Sun and those could push the sail,
but there's just not that much of them. There's just
not that many of them. There's not enough of them.
(12:17):
What is capturing is light. So the light from the
sun is zooming out from the Sun and it's carrying
energy with it, and we'll dig into that in a moment.
But photons have momentum, and so what happens when they
hit the sail is that they give it a little push,
and you get enough little pushes and you can actually
get going somewhere. So this is a real thing, like
people have built these, Like there are functioning solar wind
(12:42):
sailboats out there in space. Um, yes, this is a
real thing. It's like physics says, there's no reason why
I shouldn't work. People have big plans to make huge
solar sales that can go all around the Solar System
or maybe even to other stars. And somebody has actually
built one of these things and made it work. The
Japanese they launched one um several years ago and showed
(13:03):
that it actually worked. Like the physics isn't just theoretical,
Like there is one thing out there that's solar sailing
right now, that's pretty cool the Japanese. The Japanese, nice job.
But the thing about solar sales is that it's gentle, right,
It's not really really powerful winds of light out there
for you to capture with your sales. So if you
have like even a really big sale, like eight hundred
(13:26):
meters by eight hundred meters, which is you know, enormous, right,
then that only gets five newton's of force, And five
newtons is not that much. It's like how much you know,
a fourth there isn't like a one pound object on Earth,
So like a one pound rock, you know, think about
how much that feels in your hand. That's the force
of the Earth's gravity on a one pound rock. That's
(13:48):
the same amount of force that the Sun would push
on a eight hundred meter by eight hundred meters sail
about the distance from of the Earth, which is huge, right,
I mean it's a several city blocks. Yeah, that is
not a small thing to build and to unferral and
to keep from getting shredded right by like micro meteorites
or whatever. But you're saying if I made a tarp
(14:10):
that was about a kilometer on each side over Manhattan,
it would get pushed down not just by the weight
of it, but also by the sun that's hitting it. Yes, exactly,
And every time a photon hits you, you're getting pushed. Like.
Photons are actually pushy. They are kind of rude, aren't they.
They just like zoom along wherever they want to go.
(14:31):
They don't ask questions, they don't take turns, or they
just going straight lines wherever they need to know. Photons
care momentum, and every time they hit something, they give
it a little push. So when you turn, when you
flip the switch and turn on the light, you get
pushed down a tiny little bit. And you might think, what,
I can't feel it, And you're right because it's a
small effect, which is why solar sales are very gentle,
(14:53):
and you need a huge one to add up to
any measurable force. Well, let's step us through a little
bit of how this works. I mean, so you're saying,
if I take a flashlight and I flash it at you,
I am technically kind of pushing you with it. You're
basically punching me in the face with light. Yeah. Yeah,
that's that's really me because we don't think of light
as having substance to it, right, Like you I mean,
(15:14):
we know we sort of associated with energy and heat
and warmth, but not um force. That's right. We don't
think of photons is having mass because they don't, but
they do have energy, right, Photons are energies is just
pure energy. And if you have energy, that means you
have momentum. And it's complicated because we're only used to
thinking about things having momentum when they have mass. We
(15:36):
think about you give something momentum because it's heavy and
it's hard to get going, and once you've pushed it,
it keeps going. It has momentum. Right, So something that's
like light and fluffy and you know, weighs nothing, it's
hard to understand how that can have momentum. Well, I
think I think we're all pretty familiar with like if
I shine a flashlight on you or a laser, like
your body is gonna absorb that energy or you know,
(15:58):
warm up a little bit or something. But you're saying
that if I have an atom on my skin and
you're flashing a flashlight on it, the photons actually sort
of hit it and the atom wants to go the
other way. Yes, exactly. Think about what happens with a mirror, right,
it's a little bit more complicated when the light gets absorbed.
So let's think about a mirror where the light gets reflected,
the light zooming in one direction, it hits the mirror,
(16:18):
and the lights going in the other direction. Right, how
does the light change direction? Right? What changes the direction
of the light. It's the mirror. Instead of thinking about
light in a mirror, think about a ping pong ball
and a paddle. Right, if you get hit. If a
ping pong paddle gets hit by the ball, then the
ball bounces the other direction, but the paddle gets pushed
back a little bit. Right, That balances the momentum. Momentum
(16:41):
is conserved. So in the same way, when the light
hits the mirror, it gives that mirror a little push,
and the mirror pushes back, and that's what turns the
light the other direction. And so when light reflects off
of something, it's giving it a little push. So pure
energy can push you basically exactly. It can result in
something wanting to move the other way. That's right. And
(17:01):
this is one of the cool things in physics when
you learn that a concept you thought you had a
firm grasp on, like momentum, is actually just a special
case of sort of a larger concept. Like we're familiar
with momentum is mass times velocity. But that's not it's definition.
That's just how you calculated for slow moving things with mass.
You can calculate momentum for things that don't have mass,
(17:22):
because momentum is a larger concept. It's just it captures
like the quantity of motion of something, not just it's
mass times velocity. It's just that it's more like the
total energy. Yeah, but it's fascinating because energy is just
a number. It doesn't have a direction, right, and physics
energies is a scaler, not a vector, like it doesn't
have a direction, just a magnitude, whereas momentum has a direction.
(17:44):
Like you know, pointed in one direction is different from
pointed in the other direction. Right, Uh, light has momentum.
Light has momentum, has energy, and it has momentum and
it has no mass. One way that some people like
to think about it is you can think about momentum
sort of the difference between the energy and the mass.
Right because in real and relativity we think of the
(18:04):
total energy as the energy store in the mass plus
the energy of the momentum. So you can sort of
invert that equation and say the momentum is the difference
between the total energy and the mass. And in the
case of the photon, there is no mass. Momentum just
comes from the energy, whereas in the case of another particle, right,
it has a bunch of energy. Some of that is
in the mass and some of it is in its motion.
(18:25):
But for the photon, all the energy is just in
its motion, so it just has momentum and no mass.
All right. Solar sales are a real thing, and they
work by taking the momentum that's in light, absorbing it
and then using that to push you along in space.
That's right, And it's the light from the Sun that's
doing this, not the protons from the solar wind. And
everybody actually can see this if you just have seen
(18:47):
a comet. You know, comet is just a big ice
ball and it falls towards the Sun and as it
does so it gets a big tail, right, and that's
the tail of it, like the stuff melting and dripping
out the back, and the tail points in a specific
direction away from the Sun, not in the direction of motion.
People think, like, you know, it's like a fuzzy lines
behind something that's moving fast. But the commet actually always
(19:08):
points away from the Sun, and that's because of this
effect of light. That's light pushing the tail of the commet. Really,
it's not like a trail of a dust that you
leave behind. No, Because as the common is moving towards
the Sun, the tail is behind it. But as the
comet goes around the Sun and then leaves, the tails
in front of it. Wow, so it shouldn't be called
the tail once again? Yeah, I guess. Or maybe it's
(19:32):
like backing away from the Sun as it leaves. It's
like whoa, that was close. All right, let's get into
will it work and how how can we make it work?
But first let's take a quick break. All right. So
(19:55):
that's solar sales Daniel. That's pretty cool. The question now,
I think is does it work? If you build it,
will it take us to other planets and maybe other galaxies? Yeah,
it's it seems like it will. And you know, we
know a lot about it. We can do these calculations.
We know how much light there is, we can calculate
how it will reflect and how much will be absorbed,
and we know all the numbers we need. And so
(20:15):
people have plugged all this stuff into calculations, and they said,
if you build a solar sale that's like a kilometer
by a kilometer and you have a really light ship,
you know, not too heavy, how long would it take
to get to Mars or to get to Jupiter. And
the thing you have to understand is that the strength
of the light, of course, gets weaker as you get
further from the Sun. So the solar sale is very
(20:36):
powerful close to the Sun because it's capturing more light,
and then as you get further and further away from
the Sun, it gets weaker and weaker. So solar sales
are great for navigating around the inner parts of a
solar system. Well, the question I have is, you know
these sales are catch the rays from the Sun, right,
they catch the light from the Sun and push it
get pushed away from the Sun. But what if you
(20:58):
want to travel towards the Sun are perpendicular to the Sun.
How would you do that? If you want to move
towards the Sun, It's not hard, right, you just need
to use gravity. Right, the Sun is going to pull
you towards the Sun, so that part is not hard.
But the only way you can use the sail to
build energies is moving away from the sun. So you
can play this game of gravity versus the solar sail
(21:18):
depending on which way you want to go. If you're
facing the sun, can you go side? What can it
push you sideways? Because gravity is pushing me towards the
sun and the solar sales pushing me away from the sun.
How do how do I gain speed sideways? WHOA, that's
a really good question. I haven't even thought about that.
How do you steer it? Because the photons are only
coming out from the sun, so can you move in
(21:40):
a direction that's not out away from the sun. My
first thought is, let's think about how a sale works
on a sail boat. Right, on a sail boat, you
can move in a different direction than the wind if
you put the sail across from the wind. But the
problem is a sailboat has a keel, right, has something
underneath it that keeps it from sliding sideway is and
(22:00):
so it can only use the part of the wind
that it wants by pointing in the right direction. A
solar sale, of course, on a spaceship doesn't have a keel,
So I first thought you might think, oh, a solar
sale can only take you in the direction the photons
are going. Right. Well, let's think about it for a moment.
What happens if you tilt the sale so that it's
(22:21):
not pointing um flat away from the sun right, if
it's at an angle with respect to the photons. You
might think, well, it doesn't really matter because the photons
hit it and bounce off, and they're giving a push
the direction they were going. Right. It's actually a tiny
bit more complicated. If the photons hit it at an angle,
then they also bounce off at an angle right. So
(22:45):
if the photons are coming directly out from the sun
and they hit flat on the solar sail, then they
bounced back the opposite direction, and the momentum they have
given is in the direction they were going. But if
they come out from the sun and they hit the
solar cell at an angle right, then they were flecked off.
Think about light hitting a mirror right. It doesn't come
back the way it was going. It reflects off at
an angle, and so the push that it gives is
(23:07):
actually in the direction that the solar sale was facing.
So I think that means that you can steer a
solar sale you just turn it through the way it
wants to go, and it will catch that part of
the photons momentum that pushes it in the direction it's facing.
You can't just point in that direction and drive the
way you came with your car. You have to take
(23:28):
an account all these limitations. But you can do it,
and you can take a pretty heavy load, you know,
a few tons to mercury, and it just takes a
few years. If you want to go to Venus, it
takes you know, about a year. If you want to
go the other direction, it starts taking longer because the
radiation from the Sun drops, right, So if you want
to go to Mars, it takes like a year and
a half maybe two. Takes like two years to get
(23:50):
to Jupiter, three to get to Saturn, and then like
six years and nine years to get out to the
outer planets. It's a lot of whiskey drinking on board.
That's your fuel, after all, right, there's no there's no
solar whiskey, and you'd be using it up. You could
have burned it to evade that asteroid, but no, you
had to drink it. Yeah, well, you know, it depends
(24:11):
on how well you get along with your crewmates, depending
that determines how much whiskey you have to drink. Can
you imagine being stuck in a spaceship with your physics
student for nine years? Tan? No, but I also can't
imagine being a spaceship by myself for nine years, right, Like,
what do you think would be easier to handle total
isolation or being stuck with one person in a tin
can for nine years? Oh? For me, would definitely be
(24:32):
by myself. Yeah, you and a big stack of science
fiction movies, right all you need and a cartooning pad, obviously,
so I can document my descent into madness, your further
descent into madness, you ment, surely, that's thank you for
reminding me. But yes, but what about beyond the Solar system?
Could you use the solar sale to leave the Solar system?
(24:55):
You could a little bit. I mean what you can
do is try One strategy is to just try to
get going as fast as you can using solar sales
and gravity, and then zoom off in the direction of
another star. So you're like you sling shot around the
Sun to get some speed up, and then as you're
leaving the Sun, you unfurl your sails and it pushes
you away from the sun is as hard as you can.
(25:16):
But then once you leave the solar system, it's really
going to stop pushing you very much, and so your
speed doesn't pick up anymore. But you know, if you
do it right and you have a light enough object,
you could get up to a reasonable speed UM and
head off in that direction. Yeah. I mean, at some
point when you leave the solar system, it's like the
there's no wind. It's like being on a sailboat with
no wind. There's nothing to push you anywhere. Yeah, and
(25:37):
you know there is radiation out there, and there are particles,
but it's very low density. It's not enough to really
really boost you. UM. One idea people have, and I
love this idea because it's so crazy plus ambitious, which
is my favorite combination, is to basically beam light at
these things. Like if you're in a solar sailboat and
you're leaving the solar system, maybe we could like build
(25:58):
a huge laser and beam the energy at your solar
sale to give you a push. What if you punch
a hole through the sale with their laser? Yeah, exactly.
It can't be too intense. UM. But you know these
lass these sales will be very reflective, right. They are
essentially the big sheets of aluminum or maybe plastic that's
covered with aluminum so that they're super reflective. So if
(26:21):
it's reflective enough, then even a really really powerful laser
won't punch a hole, right, because punching a hole means
the energy gets absorbed and so like melts and destroys
the sail. If it's reflective enough, then you should be
able to shoot even a really powerful laser and have
all the energy to be transferred into the momentum of
the sail. It's like blowing on a little sailboat. Yeah, exactly.
(26:41):
Or I'm sure I've seen some science fiction movie where
somebody like reflects a laser blaster with a mirror. Right, Oh,
you just have like a pure mirror shoot at it
and then that pushes you forward. Yeah, if somebody shoots
you with a laser blaster and you have a mirror,
you can just reflect their you know, energy pulse or
whatever back at them. All Right, Well, let's let's get
into how you actually build a solar sale, even if
(27:02):
it wasn't on sale. But first let's take a quick break.
That would be a pretty awesome idea for merch like
solar sale. Daniel and Jorge solar sale explore the universe
(27:24):
instead of explain the universe. That'd be pretty cool. What
do you mean, what will we sail sale sell? We'd
sell the solar sale like you are. We have a
shop where we sell t shirts and coffee mugs with
you know, Daniel and Jorge is playing the universe on it.
We should add to our Internet merch account a solar
(27:44):
sale that you can buy. Okay, cool, done, let's do it.
And then you just get a sheet of aluminum foil?
Is that the idea? That one kilometer by one kilometers
wherever loominum foil with your picture on it. There you go,
and you can sail way into bankruptcy, right, and you
can shoot a laser at that picture from Earth dual
(28:05):
purpose aluminum foil. You can wrap your leftovers and go
into space exactly. But that's basically what a solar sailing is. Right.
You wanted to be really really big, but you also
wanted to be really really light. You want your solar
sailboat to be as light as possible so that all
that momentum gets turned into speed um. The more massive
your sailboat, the slower it's going to go for the
(28:25):
same amount of momentum as you want it really big
and really thin. I mean, does it matter in the
long run how much here solar sail ways? Sure it does. Yeah.
I mean you want to get up to like half
the speed of light or a quarter of the speed
of light or something, it's gonna take a lot longer.
And if you don't have a laser push, then you
have a limited amount of time to get energy from
the sun as you want to get up to as
high as speed as possible. Oh, I mean, but that's
(28:47):
just in general the same for any acceleration, right, Absolutely,
the lighter you are, the faster it is to go faster. Yeah, exactly.
And that's like why people who you know, climb mountains
on their bicycles or move every tiny little thing that's
not necessary because they don't want to carry that mass
up the mountain. You know, I met a guy who's
climbing Matu Picchu and he like sawt his toothbrush in
(29:09):
half because he didn't need the bottom half of it.
And you didn't want to carry that thing up the mountain.
So yeah, you wanted these your spaceship to be as
light as possible so you can translate all of that
momentum into speed. So basically that's all you need is
just a thin aluminum foil. But usually they showed them,
show them in movies and and pictures. Is gold It
looks like gold foil. Does that give you a better
(29:31):
advantage if it's gold or golden? No? Um, you can't
just have aluminum foil because aluminium is too brittle. Like
you try to make a huge sheet of aluminum foil,
I mean, I can't even get like a one foot
square of aluminum foil to stay together without tearing. And
so you try to like fold and then unferral a
one kilometer square of aluminum foil, It's not gonna work.
So usually what they do is they have some sort
(29:51):
of plastic and they spray aluminium coating on it. Um
And so sometimes that plastic is gold. But I don't
think there's a reason why that's gold. I think that's
just to look cool. Right. Well, where are we at then,
in terms of projects that are trying to use solar
sales to get us out there? Yeah, So the best
solar sale ever so far is the Icarus project. Launched
(30:11):
in by Japan and they had a fourteen by fourteen
meter solar sale and that sounds small compared to what
we're talking about like eight eight hundred, but fourteen meters
is not small, and that's like, you know, the size
of an apartment. Can I just comment on their choice
of names, like have they read the whole story of Icarus?
Do they know how that story ends? I don't know.
(30:33):
I love I love the hubris in choosing the name
Icarus for a solar sale project. Right, it's they chose
a story about Hubris and they did it kind of hubristically.
They're like thumbing their nose in history. I love it.
That's not going to happen to us. We're gonna fly
really close to the sun and nothing bad is going
to happen to Maybe they're trying to regain the name Acharis.
(30:53):
They're claiming it back, you know the way some people
claim back bad words. That we're going to turn this
new a positive stereotype. Now, they want to redeem Icarus himself.
That's it exactly. Maybe they're like the descendants of Acharis himself. Yeah,
so they they're building it. They build it. They're going
to build it, they built it, they launched it. It worked.
(31:16):
It's solar sailing. It's zooming around the sun right now. Really,
it's out there. It's out there, sailing away. It's sipping
whiskey and looking down on us. And that's the most
successful solar sale so far. But it's not like a ship,
right It's like a satellite or a little vessel. Yeah,
it's it's a little vessel. It's basically just two experiments
that can we do this thing. Let's see if it
(31:36):
actually works. Because you know, sometimes in physics, you do
the calculations, they sound great, and then you know it
doesn't actually work the way you expected because you forgot
to account for something. But in this case, it's pretty
much worked as they expected. And that's a huge boost
for all the solar sale enthusiasts. I didn't know that
was such a thing. I'm a solar sale enthusiast. I'm
a big fan. First of all, I love sailing, but
(31:57):
I'm a big fan of anything that's going to take
us to other planets or take us to other stars
and it isn't going to require a huge ridiculous amount
of fuel cool Well, uh, and then you were telling
me there's another cool project called the Breakthrough star Shot project. Yeah,
they have this awesome idea. They say, let's not send
one big solar sale, let's build a hundred of like
a thousand nano sales. So there would be a bunch
(32:20):
of really small ones and they would push them along
with lasers between here and Alpha Centauri. The problem with
building a laser that pushes a big solar sales you
need a big laser. So they were like, okay, well,
let's just have a small solar sale that can be
pushed by a small laser and then just have like
a tiny little craft. And so they imagine that that
it would take twenty years to get to Alpha Centauri
(32:43):
with these little nano sailors, And so they would have
a giant laser gun on Earth shooting at these things,
pushing them along, a thousand laser guns pushing on them. Oh,
each one has their own little laser gun. Yeah, exactly,
each one needs its own little boost. But the cool
thing about solar sailing is that you know it's a
small effect, but it just adds up, and if you're
traveling for a long time, it's just like more and
(33:03):
more acceleration. Eventually these things can get up to a
respectable fraction of the speed of light, you know, so
they can cross the distances between stars, and then they
would communicate back what they find. But they can't come back,
can they. They cannot come back. No, because there's no
laser system over there for to shoot them back. I
mean that we're aware of. Maybe they'll get there and
find Elon Musk has built some you know station over
(33:25):
there for recharging your solar your solar lasers or something.
But but no, they it's just a it's a one
way trip. Well, that's cool. It's like instead of building
a giant boat, you build a whole bunch of little
rubber duckies into the river. Kind of exactly. That's gonna be.
Humanity's first contact with an alien species is the equivalent
(33:46):
of a rubber ducky. Hi, look at us, We're so impressive.
This is a little squeezed toy. And then the aliens
are like, this is a sign of war. That's sorry.
Maybe the rubber ducky is like holy to them and
it's offensive that we like through on in their face.
Plus we probably all of our lasers will be like
burning their eyeballs down for a twenty years while the
while the natocrafts are getting there. But if we fill
(34:07):
them up with whiskey, maybe they'll they'll like it. They'll
either love it or hate it. They'd be like, what
Scottish whiskey? I only drink irish? This needs war. Wars
have been fought over less, I'm sure. All right. So
that answers Derek's question, who wanted to know how do
sol of our sales work? And hopefully you guys enjoyed
(34:27):
that and are inspired to maybe think about taking off sailing.
That's right, and you know, maybe someday we'll go out
there and we'll sail among the stars. I think it's
really beautiful and poetic to imagine using the life from
the stars themselves to push ourselves from one to the other.
I really wish that could happen. All right, Thanks for listening, guys,
see you next time. If you still have a question
(34:56):
after listening to all these explanations, please drop us a line.
We'd love to hear from you. You can find us
at Facebook, Twitter, and Instagram at Daniel and Jorge that's
one word, or email us at Feedback at Daniel and
Jorge dot com. Thanks for listening and remember that. Daniel
and Jorge Explain the Universe is a production of I
heart Radio. For more podcast from my heart Radio, visit
(35:18):
the i heart Radio app, Apple Podcasts, or wherever you
listen to your favorite shows.
Hosts And Creators
Daniel Whiteson
Kelly Weinersmith
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