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Episode Transcript
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Speaker 1 (00:08):
When I checked the news and I see some story
trending about a crazy news science discovery, like NASA discovers
a parallel universe or Chinese scientists teleport matter into space.
These are real headlines. But when I see these, I
feel a couple of things at the same time. First,
(00:29):
of course, excitement. I love science because it has the
potential to teach us crazy new stuff about the universe,
to blow our minds and show us that reality is
different from what we imaginement, or the things we thought
were impossible can now be done. But the second thing
I feel is skepticism. There's a lot of clickbait out
(00:53):
there where journalists have taken an interesting study and given
it a bonker's headline just to get eyeballs. And some
days the headlines are real. You know that scientists actually
have taken pictures of black holes and discovered new particles
and landed robots on the surface of distant moons. So
(01:14):
there's no shortage of ways to get your mind blown
for real. Hi, I'm Daniel, I'm a particle physicist and
(01:36):
I'm always looking for ways to get my mind blown.
And Welcome to the podcast. Daniel and Jorge explain the
Universe a production of I Heart Radio, our podcast in
which we explore all the amazing and crazy things about
the universe. We love the bonkers new ideas about how
the universe might work, and we unpack what we do
(01:57):
know already about how the universe do us actually work
And this podcast, curiosity is the driving force, and we
let it run wild. We ask crazy questions about the
nature of the universe, and then we bring it home
and try to explain all of it to you. Now
you'll notice that today on the podcast it's again just
me Daniel, my friend and co host Jorge can't be
(02:19):
here today, so I'm taking the opportunity, as I do occasionally,
to catch up on listener questions. We mean it when
we say we want to answer every question from listeners
because we think everybody's curiosity is valuable. If you are
wondering something about the universe, we want to help you
figure it out. Everybody out there that's thinking deep thoughts
(02:40):
about the way the universe works and the way tiny
particles fit together is doing physics, and physics is fun
and physics is awesome, and we want to encourage it
and we want to enable it. So on these podcasts.
When Jorge isn't here, I dig into our backlog of
questions from listeners, real people like you, thinking about the
universe and sending in their questions. A lot of times
(03:03):
these questions are not just hey, man, tell me about
black holes. They're more like, I've been thinking about this
thing and I don't quite understand it. Can you help
me figure it out? Or I've been googling and reading
articles about this topic and none of it makes any sense.
And that's our job to make these things explainable to you.
So if you have questions that you'd like answered, please
(03:23):
send them to us two questions at Daniel and Jorge
dot com. We answer every email, we respond to every tweet,
and we will get to your question, we promise. And
if you don't like writing tweets or emails, you could
also come to my public office hours. I'm a professor
at a public university, and i think it's important to
be available sometimes to the public. So I'm hanging out
(03:46):
on zoom answering questions from anybody and everybody. Check it out.
You can go to sites dot u, c I, dot
du slash Daniel and you find information there about my
next upcoming public office hours where you can come and
ask a physicist any question you like about the universe.
I won't offer relationship advice. On today's episode of the podcast,
(04:07):
we're doing even more listener questions. That's right, and today's
questions are super fun. It's deep fundamental physics, it's particle physics,
it's how to get around the solar system. But first,
it's about science headlines. So here's a great question from
a listener, dan An wal Hey, I know that science
(04:27):
headlines are often sensationalized. So when I see a headline,
what are some things that I can look out for
when evaluating the paper behind or the article alongside the headline. Thanks.
This is a great question because we should all be
informed and educated critical readers of science journalism. Remember that
(04:48):
the goal of science journalism is to educate. They want
to take work that scientists have done and explain it
to the general public. But also they have an interest
in entertainment, in splashy stories to get to to clip
on their headline, to get you to read the article,
to get a little bit of attention. So it's a
great idea to develop some tricks, some tools, and I'm
(05:08):
gonna give you some tips into how to read science
journalism and know whether it makes any sense. Now, first
of all, if it seems like a crazy, big deal,
then you'll read about it in lots of places, and
you'll read about it in places with a good reputation. So,
for example, if you hear that NASA has discovered a
parallel universe, whoa, you should see that as a huge
(05:28):
headline in the New York Times and in other places.
Otherwise you might start to suspect this is not really
something which is a scientific consensus or has really penetrated
deep into the community. And the idea of a scientific
consensus is really important. Any scientists can make some claim
and even maybe even write a paper and maybe even
get it published, but for a big idea to really
be accepted has to be accepted by a broad segment
(05:51):
of the science community, people who don't necessarily have an
interest in getting that paper published, who just want to
dig into the truth. And so the most valuable thing
you can look for when you're reading coverage of a
new science result is whether there are discussions or quotes
from other scientists not involved in the study, but experts
in the field reacting to it. You'll see this in
(06:14):
the best science journalism, and they'll say things like we
asked professor Michelle Blah blah blah, who is not involved
in this study but as an expert on the topic,
what she thought, and if she says, Wow, this is
groundbreak and this is revolutionary, this is a huge leap forward,
then you know this is really something to get excited about.
But if it's mostly just parroting the claims from the
people who did the science and wrote the paper, then
(06:36):
that doesn't necessarily mean it's wrong. It means that it
hasn't received the same level of review of other experts
in the community who don't have the same interests. So
that's my number one thing, is to look for quotes
from other scientists in the field who are not involved
in the study. And it really it comes down to trust,
because often you can't digest the science in these articles.
(06:57):
I read science journalism very broadly, and there's lots of
topics I don't know more than the science journalist about
neuroscience are all sorts of crazy stuff, and I'd like
to believe them. So what I've done is trying to
develop a set of trusted sources, meaning people or magazines
whose articles seem credible. For example, there's a magazine called
Quantum Magazine, which I really like, and every time I
(07:20):
read an article in that magazine that's about my field,
I find it well written and fair and accurate. So
that allows me to evaluate it. I think, well, they
do a good job. They hire good science journalists who
actually dig into it and try to represent these results
fairly and not in a sensationalist way. And so I
trust the articles in that magazine, even when they're not
in my field. It's earned my trust. And you might
(07:42):
find this about particular journalists, people who's writing you like
and who develop a credibility with you, and you can
look to them and say, well, if this really is
such a big deal, what is my favorite journalist? Can
changing in the New York Times, for example, say about this?
Or maybe you'll discover that another journalist always blows things
out of purports, and so when you see an article
by that person, you disregarded. So you have to develop
(08:04):
sort of a network of trusted sources, locations science journalists
you trust, and also look to see that they have
asked other people for their opinion. And the last thing
is you know if you see a really big headline,
ask yourself why you never heard of this before if
it's such a big deal. Sometimes it's not the answer
that they're blowing out of proportion, but the question, like, yeah,
(08:25):
maybe the scientists have accomplished something. It's just not that
significant or not as interesting as everybody said. So not
that the experiment didn't work, but that they didn't achieve
what they were set out to do. But maybe what
they set out to do isn't actually that important. I mean,
it's not like you've heard necessarily of people trying to
do that for years in the past. In contrast, you
(08:48):
know there are other things that you might be aware of,
so that when you hear about progress in them, you
understand to be impressed. Like the moment that a computer
first beat a human world champion at chess. That was
a big deal for the world because people have been
working up to it for decades of sort of a
long standing challenge. Or when people really did walk on
the moon that was something everybody acknowledged was important and hard,
(09:10):
and so when it was achieved whow we could all
be impressed. Or when we discovered the Higgs boson it
had been a decades long search and been sort of
in the cultural zeitgeist already. People knew it was something
to look for, saying with pictures of black holes. So
when you see a result that makes a big claim
about something you never heard of before, you have to
wonder if maybe the question itself is getting blown out
(09:33):
of proportion. All right, I hope that was helpful, but
I think it's great. Read science journalism, get some trusted sources,
and look to see if those sources are asking other
scientists not involved in the study. All right, but let's
get back to our bread and butter, which is answering
science questions. So here's a really fine question about navigating
(09:54):
the Solar System and maybe protecting the Earth. Hello, Daniel
and Hall, Hey, I have a question. What's the sling
shots affect and how does it work? Do we use
it to our advantage with space probes? And could we
ever use it to deflect asteroids or even planets? Thank you?
All right, what a fun question. I love this topic.
(10:16):
This is about gravitational slingshots or gravitational assists, and the
basic idea is using the gravity of a planet or
of the Sun to help navigate the Solar System. Without
spending as much fuel. Remember that fuel is expensive, not
just because it costs money to make the fuel, but
it costs fuel to bring fuel. Every pound of fuel
(10:40):
that you want to bring on your spacecraft, if you're
on a mission out to Neptune, for example, requires you
to bring more fuel in order to push that fuel
along with you. So fuel needs fuel to bring it,
and the mast fuel needs more fuel, and pretty quickly
it gets crazy. So what you really want to do
is minimize the amount of fuel you need to bring.
(11:00):
It's expensive and it blows up very quickly, requiring more
and more fuel just to bring that fuel along. So
the idea is if you could somehow get your spaceship
to change directions or to pick up speed or even
slow down somehow to navigate the Solar System without using fuel,
then you can save cost and it's also a lot simpler.
(11:21):
And that's the idea of a gravitational slingshot or gravitational assist.
You're using the gravity of a planet or the gravity
of the Sun, either to change the direction of your
spacecraft or to speed it up or to slow it down.
So you might wonder, well, how does that work? Right, Well,
let's think about it for a minute. You know that
if something is falling towards the Sun, it's going to
(11:43):
get sped up. Imagine a comet, for example. It's falling
in from the outer Solar system, speeding up. As it
gets towards the Sun. It does a quick whip around
the Sun, and that's the moment when it's at its
top speed. It's started in the very outer Solar system,
moving very slowly, but it's fall and in towards the
Sun and it's picked up speed along the way. So
(12:04):
when it whips around the Sun is going at very
very high speed, very small distance from the Sun. These
are very elliptical orbits, not like the Earth's orbit, which
is mostly a circle and the Earth mostly goes in
the same speed all the way around. A comet is
a very elliptical orbit, so it's very slow when it's
far from the Sun, and it speeds up a lot,
and then it whips around super fast around the back
(12:26):
of the Sun. But here's the thing. After it whips
around the Sun, then it starts to slow down because
now it's climbing away from the Sun. Right now, it's
slowing down, and so that when it gets really far
away again it's now slow, so it's sort of stable.
It speeds up and it slows down, and it speeds
up and it slows down. So the question is, how
do you use that to change the direction of your spacecraft,
(12:48):
Because you don't just want to swing by a planet,
speed up while you're by the planet, and then slow
down again. Then there's no point. What you want to
do is accomplish some overall speed up. How can you
do that? It turns out it is possible, and if
you do it in just the right direction, then you
can actually steal some of the energy from that planet. Say,
for example, you're going to the Outer Solar System, which
(13:09):
is really far away, so you want to get there
before all of your human scientists have perished waiting for
you to reach it. So you need a little bit
of a speed up, for example, and Jupiter is on
your way to going to study Neptune, so you can
use Jupiter to help you speed up. Well, how do
you do that? So there's two ways to look at it,
from the point of view of the planet and from
(13:29):
the point of view of the Sun. Now, from the
point of view of the planet, it's just like we
talked about before. The satellite is approaching, you're pulling on it.
It speeds up and whips around, and then it gets
shot off in another direction, but at the same speed
as it approached. Right, it speeds up and then it
slows down. So from the point of view of the planet,
there's no change in the velocity and no speed up.
(13:51):
You sped it up as it approached, but then you
slowed it down as it left, so that doesn't seem
like a wind. But that's if you look at it
from the point of view of the planet. If you
look at it from the point of view of the sun,
you notice something interesting. Not only has it whipped around
the planet, but it's changed direction. Right, it comes in
one direction and it comes out and another direction. Now
(14:12):
if it's new direction is also in the same direction
the planet was moving, then now it's velocity relative the
Sun is actually bigger than it was before because now
it's velocity relative to the Sun gets added to the
planet's velocity. So maybe it was perpendicular to the planet's
velocity before, Now it gets added to the planet's velocity,
(14:33):
so it's actually going faster relative to the Sun. And
it's done this by stealing a little bit of the
energy from Jupiter. Yeah, that's right. So for example, if
you have a one ton spacecraft and it whips around
Jupiter and it gets sped up by a kilometer per second,
which is a pretty big speed up for a spacecraft,
then that slows down Jupiter, but not by a lot
(14:56):
because Jupiter is so massive. Jupiter is so huge it
hardly notices it slows down by ten to the minus
twenty five kilometers per second. See a swing around Jupiter.
You get a little bit of speed up from the
point of view of the Sun, and Jupiter slows down
a tiny bit from the point of view of the Sun.
This isn't a big deal until we get the big numbers,
(15:18):
like if we wanted to send it ten to the
twenty five satellites to use Jupiter for a gravitational assist,
then we might actually have some impact on the orbit
of Jupiter, but hey, who cares anyway. Another way to
get this clear in your head is to imagine what
would happen if you were standing on the platform at
a train station bouncing a tennis ball up and down
right and a train comes by, moving really really fast.
(15:40):
If you decided to throw that tennis ball at the train.
It's gonna bounce off the front of the train and
come back the other direction, and it's gonna be going
faster because now it's added to the train's velocity. If
you threw it at ten meters per second against the train,
it's gonna bounce off at ten per second against the
train from the point of view of the train, but
(16:01):
on the train platform, that ten per second gets added
to the speed of the train, and so now it's
going even faster and it's slowed down the train a
tiny little bit. If you throw a tennis ball against
the front of a train, you are by very tiny
little bit slowing down that train. So this is very
helpful for speeding up without using any fuel or just
(16:24):
changing directions, you can also slow down. Like if you
swing around Jupiter and you end up going the opposite
direction of Jupiter's motion, you could end up with a
smaller velocity relative to the Sun. Imagine, for example, you
were able to change your direction so you were moving
the opposite of the way that Jupiter was moving and
with the same velocity. Then with respect to the Sun,
(16:47):
you would have no velocity. You would be stationary. Right,
So a change in direction relative to the planet can
be a change in velocity relative to the Sun. And
this is pretty awesome, right, but it also has limits.
You can't just say, hey, Jupiter, I need you to
be over there at exactly this moment so I can
sling shot around you. You have to use the planets
(17:08):
where they are and when they are, so when they
make these plans, you might have to spend like a
whole year orbiting the Solar System waiting for a planet
to be just in the right place. So these gravitational
systs can be cool, but they can also add years
and years to missions because it takes a long time
for the plans to just be in the right place.
(17:28):
There was this awesome event in the seventies when all
the planets were in perfect alignment to use one slingshot
to the other and slingshot to the other and slingshot
to the other to get all the way out to
the outer Solar System. This is the Grand Tour of
the Solar System, and it's not going to happen again
for at least another two hundred years. So that's why
they sent the voyager probes out in the seventies because
(17:51):
there was this perfect alignment so the voyager pros basically
slowed down every planet between here and Neptune. But hey,
it was worth it. They got some beauty full pictures. Now,
this is a great history. It was first used in
ninety nine when the Soviet probe Luna three took pictures
of the far side of the Moon and used the
Moon as a slingshot. And then we've done it a
(18:11):
lot of times since Cassini. It passed by Venus twice,
and then Earth and then Jupiter. Even before reaching Saturn,
the messenger probe did a fly by Earth and then
twice past Venus and then three times past Mercury, so
that it could arrive at Mercury with just the right
velocity to enter the atmosphere without having to do a
(18:32):
lot of burns. And you can even use the Sun
itself as a gravitational syst. Now, you can't change your
velocity relative to the Sun, but it can change the
velocity relative to the center of the Milky Way. So
if you wanted to go from our solar system to another,
that's what you'd have to do. So you could use
it and take advantage of the Sun's pretty quick motion
(18:53):
around the center of the Milky Way to change your
velocity with respect to the center of the galaxy and
maybe find your way to other stars. And as the
listener asked, you could also use the same sort of
technique to help deflect an asteroid. This is called a
gravity tractor when you use it in a way to
try to change the direction of the object itself. So
(19:15):
remember we talked about how doing a slingshot past Jupiter
would change the direction of Jupiter, but it wasn't really
a very big effect. Well, that can be a big
effect if the object is smaller. So we're talking about
like a five kilometer rock that's heading towards the Earth.
That's big enough to kill all of humanity if it
strikes the Earth directly, but it's small enough that if
(19:37):
you did a gravitational assist around and you could change
its trajectory. And the key thing to saving humanity from
incoming asteroids is spotting them early so they only need
a tiny little nudge. If you knew that an asteroid
was heading towards the Earth but it was still really
far away, it would only take a tiny little nudge
for it to miss the Earth. It's sort of like
(19:59):
hitting a arget where a high powered rifle really really
far away. The difference between hitting it and missing it
is a tiny change in the direction you point the gun.
So if all we need to do is change the
direction is after it a tiny little bit and it's
not that hard. What you need to do is send
up some heavy probe and send it around the asteroids
so it deflects it gravitationally, right, Or you could even
(20:22):
just have it hang out near the probe and have
it constantly tugging on it with its gravity. Gravity super
duper weak. But again, you only need a really small deflection.
So gravitational assists are a great way to explore the
Solar System, to steal energy from planets, to change directions,
to speed up to slow down, to help navigate the
(20:43):
Solar System without having to pack a lot of extra fuel.
Thanks for that great question. I want to answer some
more listener questions, but first let's take a quick break.
(21:06):
All right, we're back, and this is Daniel hosting today
and answering listener questions from the backlog. I've promised to
get to every single listener question and i will honor
that promise, and I'm using these episodes when Jorge isn't
around to catch up on our backlog. So far, we've
been talking about reading signs, headlines, and gravitational assists. But
(21:27):
here's another question about some recent experimental work. Hey, Daniel
and Jorge, I was wondering if you could explain what
Lawrence symmetry is, what happens if it can be broken
in the search for potential Lawrence symmetry violations. Thanks, all right,
that's a great question, and I happen to know that
came from a listener whose roommate was working on this
question and she wanted to understand it more deeply. So
(21:50):
let's get into it. What is Lawrence symmetry. Well, Lawrence
is a famous Dutch physicist and when the Nobel Prize
in nine two, and he was around during the pivotal
time when relativity was being developed. And that's what Lorenz
symmetry is really all about. It has to do with
how we see the universe and how the universe might
(22:11):
look different to different people, people who are moving at
various speeds or people who are sitting in different locations.
So what Lorentz symmetry actually says is that the same
laws of physics, the ones that we know gravity and
motion and electromagnetism and all those things. The laws of
physics all apply to all observers at every location, moving
(22:34):
at constant speed. So no matter where you are in
the universe and what speed you're moving at, you should
be able to look around you and see that everything
seems to be following the laws of physics. And you
and I should agree. If I'm here and you're at
Alpha Centauri, we should be able to look around us,
and everything and we see happen should follow the same
(22:54):
laws of physics. We shouldn't have to change the laws
of physics because of where we are in the universe.
And that's also true if I'm moving towards you. If
I'm in the spaceship and I'm moving at half the
speed of light towards you and your vacation home in
Alpha Centauri, I should still be able to look out
my window and use the same laws of physics to
observe the universe and describe what I see. Even if
(23:15):
I'm moving relative to you, you and I should be
able to use the same laws of physics. But that's
only true if I'm moving relative to you at constant speed.
If I'm accelerating. If I'm speeding up or if I'm
slowing down, then things get a little wonkier. So we'll
dig into that in a moment. But Lorentz symmetry essentially
is that it says that the same laws of physics
(23:37):
apply for all observers moving at any constant speed relative
to each other. It doesn't mean we all see the
same thing. It means we could all use the same
laws of physics to describe what we do see. All right,
So let's dig into that a little bit more. I mean,
this seems pretty reasonable. We think there should be only
one set of laws of physics that describe the universe.
(24:00):
That's sort of the whole goal of physics, right, is
to find one set of laws that describes everything. You
wouldn't want a set of laws which were dependent on location.
So why is it the Lorentz symmetry only holds if
you're moving at constant speed? Right? This requirement that you
have inertial observers. And the reason is that if you're
not moving a constant speed, if you're accelerating, then you
(24:23):
do see different forces at play. For example, if you
are in an elevator and that elevator is in space,
but it's accelerating, right, it's speeding up. Then what are
you going to feel. You're going to feel a force
from the floor of the elevator, right, You're gonna feel
the force of the floor of the elevator pushing up
on you. It's almost as if there's a force of gravity. Right.
(24:45):
Somebody in an elevator that's moving a constant acceleration sees
the same physics as somebody who's standing on the surface
of a planet and feels the force of gravity. And
that's different physics than somebody who's just floating in space.
We're moving a constant velocity. If you were in a
spaceship moving a constant velocity, you wouldn't feel any gravity,
(25:06):
you wouldn't feel the floor pushing up on you. You
would just be floating, waitless in the middle of your spaceship.
So those people have to use different physics to account
for what they see. The person who's accelerating feels a
new force when they can't otherwise describe. It's as if
they were standing on the surface of a giant planet
pulling down on them. So when they do their calculations,
(25:27):
they have to add this new force to describe what
they see and somebody else, and a spaceship that's not
speeding up, that's moving at constant speed doesn't have to
add that force. So that's like a different set of
laws of the universe. That's why we only talk about
lorn symmetry being relevant to people moving at constant speed,
because if you do have some acceleration, that creates a
(25:51):
fictitious force, an apparent force. The same thing is true
if you're moving around in a circle. Right, say you're
on a merry go around for example, somebody spins. Spinning
moving in a circle is also acceleration because it's changing
the direction of your velocity. You're going around the merrygor round,
you're pointing in one direction. Later you're pointing in another direction.
(26:12):
So if your direction of your motion is changing, your
velocity is changing, that's acceleration. And what do you feel
when you're on a merrygor round, Well, you feel this
weird fictitious force that's trying to throw you off the
merry go round. Is that a real force? There's no particle,
there's no field that's creating. It's not a fundamental force
of the universe. It's a fictitious force because you are
(26:33):
in a non inertial reference frame. Because you are rotating,
you are accelerating. So that's why Lorenz symmetry talks only
about inertial observers, people who are at rest or moving
relative to each other with constant velocity. So what do
we mean when we say you use the same laws
of physics? Because we know we've seen enough special relativity
(26:55):
examples to know that people moving at very high speeds
see things differently from each other. Like if you get
on a spaceship and go really really fast, close to
the speed of light, but it constant velocity, and I
had a telescope and I can look at a clock
on your ship. I'll see your clock moving slowly because
moving clocks run slow. But if you're on the ship
and you look at your clock, you see it running normally. Right,
(27:18):
So you and I see different things when we look
at the universe, even if we don't have any relative acceleration.
So how can we say that observers are all using
the same laws of physics? Because there's an important distinction
between using the same laws of physics and seeing the
same thing. We can see different things happening, but still
(27:38):
have them be described by the same laws of physics.
Here's an example that I think helped Einstein clarify what
was going on in his mind as he developed relativity.
Think about a single electron floating in space. What does
it do? Well, electrons have a charge, so they have
an electric field, right, an the electric field doesn't change.
It's floating in space no velocity relative to you. Now,
(28:00):
say your friend comes by and she's in a hurry,
so she's moving really fast past you. What does she see, Well,
she looks at this electron, and according to her, the
electron is moving. Right. If she's moving relative to you
and the electron is floating in front of you, then
she's also moving relative to the electron, which means the
electron is moving relative to her. Now, what does she see?
(28:20):
She sees a charge in motion. That's a current, right.
Electric currents are just charges in motion, and electric currents
can create magnetic fields. So what does she see She
sees a magnetic field. You see an electric field. She
sees a magnetic field. Right. So you see different things,
but both of you agree, yes, the laws of electromagnetism
(28:42):
are working. You see different things, but you use the
same laws to describe what you do see. And that's
the beautiful thing about Lorenz symmetry is it says you
might not have the same observations, but you can use
the same rules to describe what you are seeing. So
Lorenz symmetry is really, really deeply woven into the very
(29:02):
foundations of physics. Is something we assume is the basis
of special relativity. It's basically the same thing as special relativity.
If Laurent symmetry was violated, then special relativity would be
wrong somehow, and we don't think that it is. It's
been tested out the wazoo and into the wazoo and
around the wazoo, and nearly the speed of light. We're
pretty sure special relativity is correct. People are looking for
(29:24):
violations of this. One way they do this is they
look to see if the speed of light changes as
you move. There's the famous Michaelson Morley experiment that showed
that the speed of light is the same in two
different orthogonal directions, even though the Earth is in motion
around the Sun and the Sun is the motion around
the center of the galaxy. And that tells us that
the speed of light is uniform no matter who is
(29:46):
measuring it and what their speed is. But that's an
experimental result, and so people have been trying to improve that.
They've got this precision down really, really, really fine, so
there's no variation in the measured speed of light down
to like one part and ten to the seventeen, which
is an incredible virtuoso experiment. People also do crazy stuff
(30:07):
like bounce lasers off the Moon. You know, when astronauts
went to the Moon, they left a mirror on the
surface of the Moon, so we could do cool experiments
like shoot laser beams at the Moon, not to blow
it up, of course, but just to measure the flight time.
And this helps us understand the speed of light and
also actually the distance to the moon. The speed of
(30:28):
light is so well known that we can use the
time that a laser takes to get to the moon,
bounce off that mirror and come back as a measurement
of the distance to the Moon, which happens to be
increasing every year by about a centimeter. So people are
looking for violations of this in the way that light moves,
basically looking for violations of special relativity. But there are
also other deeper ways that we study. Lorenz symmetry. Lorenz
(30:52):
symmetry is very closely connected to a symmetry in particle
physics called C P T. Each letter there's stands for
one symmetry. C is for charge, PS for parody, T
is for time, and the combination CPT means all three symmetries.
And what CPT symmetry says is that if you take
(31:12):
a particle physics experiment and you flip the charge of
the particles involved, so like from positive to negative, and
you flip the parody, you like take it in a
mirror and do the mirror inverse experiment, and you flip
the direction of time. So instead of doing it forwards,
you try to do it backwards. That the experiment should
look exactly the same, C, P and T together should
(31:34):
all be conserved. Now we already know that parody is violated.
We have a whole fun podcast episode about how parody
is violated in the weak force and how that was discovered.
Then later people discover that CP is violated the combination
of charge and parody. So if you flip in the
mirror and switch particles to antiparticles, you still get some violations.
(31:56):
But people think that CPT is preserved and the reason
they think it's preserved is that it's required by Lorenz symmetry.
So if you see a violation of CPT somehow, that
would undermine all of modern physics because it would imply
that Lorenz symmetry is also violated. Now, there was an
experiment recently that claimed to violent Lorenz symmetry. The opera
(32:20):
experiment at certain claimed to have sent neutrinos from Certain
to Italy at faster than the speed of light, which
would be a violation of special relativity and a violation
of Lorenz symmetry. Now, those headlines were pretty impressive, and
when I read those, I thought, WHOA, this is kind
of a bonker's result. And it was pretty skeptical when
I read that, because I didn't see a detailed analysis
(32:41):
from anybody else who was not involved in the study.
And so pretty quickly when other folks were not part
of the opera experiment, dug into the details and started
asking questions. The opera folks discovered Oops, they made a
mistake and a cable hadn't been plugged incorrectly, which led
to a wrong calibration constant, which led to a mismeasurement
of the speed of their neutrinos, and turns out their
(33:02):
neutrinos were just ordinary neutrinos traveling at just under the
speed of light, not just over the speed of light.
So so far, nobody's ever seen a violation of learn
symmetry or CPT, and as far as we know, it's
a symmetry about the universe. Then, no matter where you
are in the universe and how fast you are moving,
you can use the same laws of physics to describe
(33:25):
everything that you see, which is pretty cool. Thanks very
much for that awesome question. I hope that helped you
understand it. I have one more question I want to
get to, but first let's take another break. Alright, we're back.
(33:49):
This is Daniel and I am answering questions from listeners.
Folks who had a question about the universe and roote
me an email and then sent me some audio with
their questions so you could all hear are their questions.
And I've chosen these questions because I suspect that other
folks out there like you, might have the same questions
and might be interested in hearing the answers. So here's
(34:10):
our last question of the day. Hi guys, my name
is George Ray. I'm from southern Ontario and I have
a question for you. It's regarding the speed of light.
Is it possible that in the past the speed of
light was faster or slower? Uh? And if so, how
do we know that? Thanks? By the way, I love
the podcast. All right, what a wonderful question. This question
(34:33):
touches on so many cool things about the universe. One
thing that we've seen in physics is that we have
these equations that describe the universe. They say how things
relate to each other. But those equations have numbers in them,
Like the equations for electromagnetism have some numbers in them
that tell us how fast electromagnetic information moves. That's the
(34:54):
speed of light is determined by these numbers in those equations.
And every time we see numbers in the equations, we
wonder why this number, why not another number? Could it
have been a different number? Is that this number for
a reason? Is it random? Could have been any possible number?
Or is there some deeper theory of physics that explains
(35:16):
these numbers, connects these numbers to other numbers we see
in other equations. So the question you're asking, is this
number the speed of light always been this number, or
has it, for example, changed with time is a really
deep and fundamental question in physics, and it's the kind
of thing we really drill into. It's also really fun
(35:37):
and important because the speed of light affects a lot
of things in the universe. Right, the universe seems really
really big, and one reason is not just that stuff
is far away, but that it takes a long time
to get from here to there. Like, it doesn't really
matter how many billions of kilometers you are away from
other stars. If you could go super duper fast, then
(35:59):
you could get there in a day or in a
half a day, it wouldn't matter. But there's this speed
limit on information of the universe, which of course also
applies to starships and your travel, which means things are
effectively really far away because of this limit of the
speed of light. So it makes us wonder is it
possible for it to change? Could it change in the future. Now,
(36:19):
The first thing to understand is that the speed of
light is not actually one of the fundamental numbers we
talk about when we talk about the parameters of the universe.
You know, the things in the sort of universe control
panety might dial up or down or change. And the
clue to knowing that the speed of light is not
fundamental is that it's the number with units on it. Right,
(36:40):
it's three times tend the meters per second, which means
it's relative to other things like the definition of the
meter and the definition of the second. In particle physics,
for example, we use different units. We use units where
the speed of light is just one, so that we
can erase it from all of our equations, because as otherwise,
(37:00):
we're writing the speed of light all the time and
calculating big numbers. Imagine doing a little thought experiment to
see if you would notice if the speed of light changed. Right, Say,
for example, you change a meter to be a tenth
of a meter, so to change the whole scale of
the universe, and then also change the speed of light
to match, and you change the gravitational constant, which sort
(37:21):
of affects how far apart things are in space where
they balance away from each other. So you could change
all of those numbers and you wouldn't notice anything. The
universe would seem the same to you because those numbers
are all relative to those units and to each other.
So what we've done in physics is isolated the numbers
that don't have any units. We take all the numbers
(37:44):
that we can find, the ones that are connected to
each other, speed of light, gravitational constant, all these other
numbers that do have units, and we divide them against
each other and multiply until we get numbers that have
no units. These are numbers that we can't change just
by changing our units, or by scaling the universe up
or shrinking it down by changing the length of a meter, right,
(38:05):
and so these are the ones that really would control
the nature of physics. And for example, one of them
is called the fine structure constant. It's a weird name.
It comes out of the early days of quantum mechanics
when we were understanding how atomic orbitals work and where
electrons were and how much energy they had. But basically
it's a relationship between the speed of light and planks
(38:28):
constant and the electric charge and this number alpha. The
fine structure constant really does determine sort of the way
the universe looks. You can't change the fine structure constant
without changing the physics of the universe. It's inescapable if
you change the speed of light and planks constant and
the electric charge in such a way to keep the
(38:50):
fine structure constant constant. Then you wouldn't notice any different
Maybe universe would really be bigger or really be smaller,
but then so would we, and so we wouldn't notice
any difference. So that's the key. You have to find
the parameter that actually does make a difference, the one
that would change physics as we see it. And it's
not the speed of light. It's the fine structure constant
(39:12):
one of the other several parameters. We actually have a
whole podcast episode about what are these fundamental parameters and
which ones are really important in the universe. So there
are these fundamental parameters to the universe. There's this whole
list of them, and if you change one of them,
you would change the way the universe worked. Speed of
light not technically one of them, because you can tweak
(39:36):
other parameters to accommodate for a change in the speed
of light and not change anything else. But imagine for
a moment if you just change the speed of light
or the speed of light had been changing on its own,
could you tell any difference. Well, there is one way
that we can tell how the universe worked in the past,
and that's because we can see the past. It's like
(39:59):
out there in space. The finiteness of the speed of
light keeps us from exploring the universe, but it also
means we can look back into the history of the universe,
because light that was created a long time ago with
just now arriving at Earth, if it came from really,
really far away. So as we look deeper out into space,
(40:19):
we see further into the past. And we can't conduct
experiments in the past, but we can see experiments in
the past. We can find them. We can watch things
happening in the past, and we can ask are these
described by the same laws of physics that we know now.
Can we understand galaxy formation and star formation and all
(40:39):
the stuff we see happening in the past in terms
of the same laws of physics, or do we need
to change something like the speed of light? And so far,
all the things we see in the past are very
well described by the speed of light as it is now.
There's no evidence that the speed of light has been
speeding up or going down in the past, and we
(41:01):
would see that happening because it would change the way
things work, It would change how quickly things move, It
would change how rapidly gravitational information was propagated. All sorts
of things would be changed if the speed of light changed,
and so far it seems like it hasn't, but there
are some nuances to that it might be possible, for example,
(41:22):
to reinterpret what we see not in the way we
imagine it now, but as a change in the speed
of light. For example, some people really really don't like
the ideas of cosmic inflation. The ideas of the universe
grew very, very rapidly in the early universe. In the
first few moments, is stretched by ten to the minus
(41:42):
thirty seconds. This crazy stretching of space, a stretching of
space that actually happened faster than the speed of light.
It doesn't violate special relativity because it's a stretching of space,
not a motion through space, and that's an important technicality.
But some people still don't like this con spt and
they wonder if instead maybe it was just that the
(42:03):
speed of light was much much faster. There are these
theories of physics, these alternative theories that are kind of
fringe theories. They're variable speed of light theories that try
to explain what we see in the past, not in
terms of cosmic inflation or expansion, but instead in terms
of a change of the speed of light. And you know,
(42:23):
you can always take the same data and fit another
theory to it, but then you have to ask, how
does that theory look? Is that theory really work And
the problem with these theories, these variable speed of light
theories is, frankly, that they violate special relativity. They violate
Lorenz invariants, and so that makes us not like them
very much. We really do believe in Lawrence invariants. We've
(42:45):
tested it out the wazoo. Now it is potentially possible
that in the early universe things were really different and
Lawrence invariance wasn't as respected. But we have reasons to
believe in Lawrence in variances. It's not just like an
article of faith. It comes out very simply from the
mathematics and from looking at the structure of space itself.
We talked once about Nurther's theorem, which is this idea
(43:08):
that all symmetries are connected to conservation laws, and in
this case we think that Lorenz symmetry is connected to
translation symmetry and rotation symmetry. The fact that everywhere in
the universe seems to be similar. There's no special location
in space. It's not like there's an origin at the
center of the Sun or the center of the Milky
(43:29):
Way that's different from any other place. No matter where
you put your zero zero on your axes, physics should
work the same. So that's a pretty basic assumption about
the way the universe works. To get rid of that,
to toss that out the window would mean tossing out
the window a lot about what we understand about the universe.
That doesn't make it impossible, but it makes it a
(43:50):
big pill to swallows. You need very clear evidence. It's
much simpler to say, well, we think everywhere in the
universe is the same, and Lorenz symmetry makes a lot
sense to us, and the speed of light is a
fixed number. Everything sort of clicks together and works very
very well. You can describe the universe using other theories,
but they don't click together as well. They're not as nice,
(44:11):
they don't have the same beautiful symmetries. They're more complicated,
and so we tend to favor the one that we
have now because it works so well. We don't need
variation of the speed of light to explain what we see,
so to summarize, and we don't think the speed of
light has changed. Actually, you could change it without noticing
anything in the universe if you conspire to change a
(44:32):
few other fundamental constants. The ones you really should be
thinking about are these constants without units, the dimensionless numbers,
the pure numbers that control the universe. But because we
don't know why the speed of light is what it is,
there's nothing necessarily determining its number. It is potentially possible
(44:52):
that it could have changed in the early universe, but
we don't see that. We look out into the data
and we see that the universe is described by the
same laws of physics with the same speed of light.
So it all fits together with us. But thank you
for asking such a deep and fun and wonderful question
about the nature of the universe and whether it is
what it is, and whether that's what it always was.
(45:13):
Thank you to everybody out there who's been thinking deeply
about the universe and wondering how things work, and asking
questions when they don't make sense. So please keep thinking,
keep being curious, keep asking questions, and write to us
two questions at Daniel and Jorge dot com because we
will answer all of your listener questions. Thanks everyone, tune
(45:34):
in next time. Thanks for listening, and remember that Daniel
and Jorge explained The Universe is a production of I
Heart Radio or more podcast from my heart Radio. Visit
the I Heart Radio Apple Apple Podcasts, or wherever you
(45:54):
listen to your favorite shows. No
When I checked the news and I see some story
trending about a crazy news science discovery, like NASA discovers
a parallel universe or Chinese scientists teleport matter into space.
These are real headlines. But when I see these, I
feel a couple of things at the same time. First,
(00:29):
of course, excitement. I love science because it has the
potential to teach us crazy new stuff about the universe,
to blow our minds and show us that reality is
different from what we imaginement, or the things we thought
were impossible can now be done. But the second thing
I feel is skepticism. There's a lot of clickbait out
(00:53):
there where journalists have taken an interesting study and given
it a bonker's headline just to get eyeballs. And some
days the headlines are real. You know that scientists actually
have taken pictures of black holes and discovered new particles
and landed robots on the surface of distant moons. So
(01:14):
there's no shortage of ways to get your mind blown
for real. Hi, I'm Daniel, I'm a particle physicist and
(01:36):
I'm always looking for ways to get my mind blown.
And Welcome to the podcast. Daniel and Jorge explain the
Universe a production of I Heart Radio, our podcast in
which we explore all the amazing and crazy things about
the universe. We love the bonkers new ideas about how
the universe might work, and we unpack what we do
(01:57):
know already about how the universe do us actually work
And this podcast, curiosity is the driving force, and we
let it run wild. We ask crazy questions about the
nature of the universe, and then we bring it home
and try to explain all of it to you. Now
you'll notice that today on the podcast it's again just
me Daniel, my friend and co host Jorge can't be
(02:19):
here today, so I'm taking the opportunity, as I do occasionally,
to catch up on listener questions. We mean it when
we say we want to answer every question from listeners
because we think everybody's curiosity is valuable. If you are
wondering something about the universe, we want to help you
figure it out. Everybody out there that's thinking deep thoughts
(02:40):
about the way the universe works and the way tiny
particles fit together is doing physics, and physics is fun
and physics is awesome, and we want to encourage it
and we want to enable it. So on these podcasts.
When Jorge isn't here, I dig into our backlog of
questions from listeners, real people like you, thinking about the
universe and sending in their questions. A lot of times
(03:03):
these questions are not just hey, man, tell me about
black holes. They're more like, I've been thinking about this
thing and I don't quite understand it. Can you help
me figure it out? Or I've been googling and reading
articles about this topic and none of it makes any sense.
And that's our job to make these things explainable to you.
So if you have questions that you'd like answered, please
(03:23):
send them to us two questions at Daniel and Jorge
dot com. We answer every email, we respond to every tweet,
and we will get to your question, we promise. And
if you don't like writing tweets or emails, you could
also come to my public office hours. I'm a professor
at a public university, and i think it's important to
be available sometimes to the public. So I'm hanging out
(03:46):
on zoom answering questions from anybody and everybody. Check it out.
You can go to sites dot u, c I, dot
du slash Daniel and you find information there about my
next upcoming public office hours where you can come and
ask a physicist any question you like about the universe.
I won't offer relationship advice. On today's episode of the podcast,
(04:07):
we're doing even more listener questions. That's right, and today's
questions are super fun. It's deep fundamental physics, it's particle physics,
it's how to get around the solar system. But first,
it's about science headlines. So here's a great question from
a listener, dan An wal Hey, I know that science
(04:27):
headlines are often sensationalized. So when I see a headline,
what are some things that I can look out for
when evaluating the paper behind or the article alongside the headline. Thanks.
This is a great question because we should all be
informed and educated critical readers of science journalism. Remember that
(04:48):
the goal of science journalism is to educate. They want
to take work that scientists have done and explain it
to the general public. But also they have an interest
in entertainment, in splashy stories to get to to clip
on their headline, to get you to read the article,
to get a little bit of attention. So it's a
great idea to develop some tricks, some tools, and I'm
(05:08):
gonna give you some tips into how to read science
journalism and know whether it makes any sense. Now, first
of all, if it seems like a crazy, big deal,
then you'll read about it in lots of places, and
you'll read about it in places with a good reputation. So,
for example, if you hear that NASA has discovered a
parallel universe, whoa, you should see that as a huge
(05:28):
headline in the New York Times and in other places.
Otherwise you might start to suspect this is not really
something which is a scientific consensus or has really penetrated
deep into the community. And the idea of a scientific
consensus is really important. Any scientists can make some claim
and even maybe even write a paper and maybe even
get it published, but for a big idea to really
be accepted has to be accepted by a broad segment
(05:51):
of the science community, people who don't necessarily have an
interest in getting that paper published, who just want to
dig into the truth. And so the most valuable thing
you can look for when you're reading coverage of a
new science result is whether there are discussions or quotes
from other scientists not involved in the study, but experts
in the field reacting to it. You'll see this in
(06:14):
the best science journalism, and they'll say things like we
asked professor Michelle Blah blah blah, who is not involved
in this study but as an expert on the topic,
what she thought, and if she says, Wow, this is
groundbreak and this is revolutionary, this is a huge leap forward,
then you know this is really something to get excited about.
But if it's mostly just parroting the claims from the
people who did the science and wrote the paper, then
(06:36):
that doesn't necessarily mean it's wrong. It means that it
hasn't received the same level of review of other experts
in the community who don't have the same interests. So
that's my number one thing, is to look for quotes
from other scientists in the field who are not involved
in the study. And it really it comes down to trust,
because often you can't digest the science in these articles.
(06:57):
I read science journalism very broadly, and there's lots of
topics I don't know more than the science journalist about
neuroscience are all sorts of crazy stuff, and I'd like
to believe them. So what I've done is trying to
develop a set of trusted sources, meaning people or magazines
whose articles seem credible. For example, there's a magazine called
Quantum Magazine, which I really like, and every time I
(07:20):
read an article in that magazine that's about my field,
I find it well written and fair and accurate. So
that allows me to evaluate it. I think, well, they
do a good job. They hire good science journalists who
actually dig into it and try to represent these results
fairly and not in a sensationalist way. And so I
trust the articles in that magazine, even when they're not
in my field. It's earned my trust. And you might
(07:42):
find this about particular journalists, people who's writing you like
and who develop a credibility with you, and you can
look to them and say, well, if this really is
such a big deal, what is my favorite journalist? Can
changing in the New York Times, for example, say about this?
Or maybe you'll discover that another journalist always blows things
out of purports, and so when you see an article
by that person, you disregarded. So you have to develop
(08:04):
sort of a network of trusted sources, locations science journalists
you trust, and also look to see that they have
asked other people for their opinion. And the last thing
is you know if you see a really big headline,
ask yourself why you never heard of this before if
it's such a big deal. Sometimes it's not the answer
that they're blowing out of proportion, but the question, like, yeah,
(08:25):
maybe the scientists have accomplished something. It's just not that
significant or not as interesting as everybody said. So not
that the experiment didn't work, but that they didn't achieve
what they were set out to do. But maybe what
they set out to do isn't actually that important. I mean,
it's not like you've heard necessarily of people trying to
do that for years in the past. In contrast, you
(08:48):
know there are other things that you might be aware of,
so that when you hear about progress in them, you
understand to be impressed. Like the moment that a computer
first beat a human world champion at chess. That was
a big deal for the world because people have been
working up to it for decades of sort of a
long standing challenge. Or when people really did walk on
the moon that was something everybody acknowledged was important and hard,
(09:10):
and so when it was achieved whow we could all
be impressed. Or when we discovered the Higgs boson it
had been a decades long search and been sort of
in the cultural zeitgeist already. People knew it was something
to look for, saying with pictures of black holes. So
when you see a result that makes a big claim
about something you never heard of before, you have to
wonder if maybe the question itself is getting blown out
(09:33):
of proportion. All right, I hope that was helpful, but
I think it's great. Read science journalism, get some trusted sources,
and look to see if those sources are asking other
scientists not involved in the study. All right, but let's
get back to our bread and butter, which is answering
science questions. So here's a really fine question about navigating
(09:54):
the Solar System and maybe protecting the Earth. Hello, Daniel
and Hall, Hey, I have a question. What's the sling
shots affect and how does it work? Do we use
it to our advantage with space probes? And could we
ever use it to deflect asteroids or even planets? Thank you?
All right, what a fun question. I love this topic.
(10:16):
This is about gravitational slingshots or gravitational assists, and the
basic idea is using the gravity of a planet or
of the Sun to help navigate the Solar System. Without
spending as much fuel. Remember that fuel is expensive, not
just because it costs money to make the fuel, but
it costs fuel to bring fuel. Every pound of fuel
(10:40):
that you want to bring on your spacecraft, if you're
on a mission out to Neptune, for example, requires you
to bring more fuel in order to push that fuel
along with you. So fuel needs fuel to bring it,
and the mast fuel needs more fuel, and pretty quickly
it gets crazy. So what you really want to do
is minimize the amount of fuel you need to bring.
(11:00):
It's expensive and it blows up very quickly, requiring more
and more fuel just to bring that fuel along. So
the idea is if you could somehow get your spaceship
to change directions or to pick up speed or even
slow down somehow to navigate the Solar System without using fuel,
then you can save cost and it's also a lot simpler.
(11:21):
And that's the idea of a gravitational slingshot or gravitational assist.
You're using the gravity of a planet or the gravity
of the Sun, either to change the direction of your
spacecraft or to speed it up or to slow it down.
So you might wonder, well, how does that work? Right, Well,
let's think about it for a minute. You know that
if something is falling towards the Sun, it's going to
(11:43):
get sped up. Imagine a comet, for example. It's falling
in from the outer Solar system, speeding up. As it
gets towards the Sun. It does a quick whip around
the Sun, and that's the moment when it's at its
top speed. It's started in the very outer Solar system,
moving very slowly, but it's fall and in towards the
Sun and it's picked up speed along the way. So
(12:04):
when it whips around the Sun is going at very
very high speed, very small distance from the Sun. These
are very elliptical orbits, not like the Earth's orbit, which
is mostly a circle and the Earth mostly goes in
the same speed all the way around. A comet is
a very elliptical orbit, so it's very slow when it's
far from the Sun, and it speeds up a lot,
and then it whips around super fast around the back
(12:26):
of the Sun. But here's the thing. After it whips
around the Sun, then it starts to slow down because
now it's climbing away from the Sun. Right now, it's
slowing down, and so that when it gets really far
away again it's now slow, so it's sort of stable.
It speeds up and it slows down, and it speeds
up and it slows down. So the question is, how
do you use that to change the direction of your spacecraft,
(12:48):
Because you don't just want to swing by a planet,
speed up while you're by the planet, and then slow
down again. Then there's no point. What you want to
do is accomplish some overall speed up. How can you
do that? It turns out it is possible, and if
you do it in just the right direction, then you
can actually steal some of the energy from that planet. Say,
for example, you're going to the Outer Solar System, which
(13:09):
is really far away, so you want to get there
before all of your human scientists have perished waiting for
you to reach it. So you need a little bit
of a speed up, for example, and Jupiter is on
your way to going to study Neptune, so you can
use Jupiter to help you speed up. Well, how do
you do that? So there's two ways to look at it,
from the point of view of the planet and from
(13:29):
the point of view of the Sun. Now, from the
point of view of the planet, it's just like we
talked about before. The satellite is approaching, you're pulling on it.
It speeds up and whips around, and then it gets
shot off in another direction, but at the same speed
as it approached. Right, it speeds up and then it
slows down. So from the point of view of the planet,
there's no change in the velocity and no speed up.
(13:51):
You sped it up as it approached, but then you
slowed it down as it left, so that doesn't seem
like a wind. But that's if you look at it
from the point of view of the planet. If you
look at it from the point of view of the sun,
you notice something interesting. Not only has it whipped around
the planet, but it's changed direction. Right, it comes in
one direction and it comes out and another direction. Now
(14:12):
if it's new direction is also in the same direction
the planet was moving, then now it's velocity relative the
Sun is actually bigger than it was before because now
it's velocity relative to the Sun gets added to the
planet's velocity. So maybe it was perpendicular to the planet's
velocity before, Now it gets added to the planet's velocity,
(14:33):
so it's actually going faster relative to the Sun. And
it's done this by stealing a little bit of the
energy from Jupiter. Yeah, that's right. So for example, if
you have a one ton spacecraft and it whips around
Jupiter and it gets sped up by a kilometer per second,
which is a pretty big speed up for a spacecraft,
then that slows down Jupiter, but not by a lot
(14:56):
because Jupiter is so massive. Jupiter is so huge it
hardly notices it slows down by ten to the minus
twenty five kilometers per second. See a swing around Jupiter.
You get a little bit of speed up from the
point of view of the Sun, and Jupiter slows down
a tiny bit from the point of view of the Sun.
This isn't a big deal until we get the big numbers,
(15:18):
like if we wanted to send it ten to the
twenty five satellites to use Jupiter for a gravitational assist,
then we might actually have some impact on the orbit
of Jupiter, but hey, who cares anyway. Another way to
get this clear in your head is to imagine what
would happen if you were standing on the platform at
a train station bouncing a tennis ball up and down
right and a train comes by, moving really really fast.
(15:40):
If you decided to throw that tennis ball at the train.
It's gonna bounce off the front of the train and
come back the other direction, and it's gonna be going
faster because now it's added to the train's velocity. If
you threw it at ten meters per second against the train,
it's gonna bounce off at ten per second against the
train from the point of view of the train, but
(16:01):
on the train platform, that ten per second gets added
to the speed of the train, and so now it's
going even faster and it's slowed down the train a
tiny little bit. If you throw a tennis ball against
the front of a train, you are by very tiny
little bit slowing down that train. So this is very
helpful for speeding up without using any fuel or just
(16:24):
changing directions, you can also slow down. Like if you
swing around Jupiter and you end up going the opposite
direction of Jupiter's motion, you could end up with a
smaller velocity relative to the Sun. Imagine, for example, you
were able to change your direction so you were moving
the opposite of the way that Jupiter was moving and
with the same velocity. Then with respect to the Sun,
(16:47):
you would have no velocity. You would be stationary. Right,
So a change in direction relative to the planet can
be a change in velocity relative to the Sun. And
this is pretty awesome, right, but it also has limits.
You can't just say, hey, Jupiter, I need you to
be over there at exactly this moment so I can
sling shot around you. You have to use the planets
(17:08):
where they are and when they are, so when they
make these plans, you might have to spend like a
whole year orbiting the Solar System waiting for a planet
to be just in the right place. So these gravitational
systs can be cool, but they can also add years
and years to missions because it takes a long time
for the plans to just be in the right place.
(17:28):
There was this awesome event in the seventies when all
the planets were in perfect alignment to use one slingshot
to the other and slingshot to the other and slingshot
to the other to get all the way out to
the outer Solar System. This is the Grand Tour of
the Solar System, and it's not going to happen again
for at least another two hundred years. So that's why
they sent the voyager probes out in the seventies because
(17:51):
there was this perfect alignment so the voyager pros basically
slowed down every planet between here and Neptune. But hey,
it was worth it. They got some beauty full pictures. Now,
this is a great history. It was first used in
ninety nine when the Soviet probe Luna three took pictures
of the far side of the Moon and used the
Moon as a slingshot. And then we've done it a
(18:11):
lot of times since Cassini. It passed by Venus twice,
and then Earth and then Jupiter. Even before reaching Saturn,
the messenger probe did a fly by Earth and then
twice past Venus and then three times past Mercury, so
that it could arrive at Mercury with just the right
velocity to enter the atmosphere without having to do a
(18:32):
lot of burns. And you can even use the Sun
itself as a gravitational syst. Now, you can't change your
velocity relative to the Sun, but it can change the
velocity relative to the center of the Milky Way. So
if you wanted to go from our solar system to another,
that's what you'd have to do. So you could use
it and take advantage of the Sun's pretty quick motion
(18:53):
around the center of the Milky Way to change your
velocity with respect to the center of the galaxy and
maybe find your way to other stars. And as the
listener asked, you could also use the same sort of
technique to help deflect an asteroid. This is called a
gravity tractor when you use it in a way to
try to change the direction of the object itself. So
(19:15):
remember we talked about how doing a slingshot past Jupiter
would change the direction of Jupiter, but it wasn't really
a very big effect. Well, that can be a big
effect if the object is smaller. So we're talking about
like a five kilometer rock that's heading towards the Earth.
That's big enough to kill all of humanity if it
strikes the Earth directly, but it's small enough that if
(19:37):
you did a gravitational assist around and you could change
its trajectory. And the key thing to saving humanity from
incoming asteroids is spotting them early so they only need
a tiny little nudge. If you knew that an asteroid
was heading towards the Earth but it was still really
far away, it would only take a tiny little nudge
for it to miss the Earth. It's sort of like
(19:59):
hitting a arget where a high powered rifle really really
far away. The difference between hitting it and missing it
is a tiny change in the direction you point the gun.
So if all we need to do is change the
direction is after it a tiny little bit and it's
not that hard. What you need to do is send
up some heavy probe and send it around the asteroids
so it deflects it gravitationally, right, Or you could even
(20:22):
just have it hang out near the probe and have
it constantly tugging on it with its gravity. Gravity super
duper weak. But again, you only need a really small deflection.
So gravitational assists are a great way to explore the
Solar System, to steal energy from planets, to change directions,
to speed up to slow down, to help navigate the
(20:43):
Solar System without having to pack a lot of extra fuel.
Thanks for that great question. I want to answer some
more listener questions, but first let's take a quick break.
(21:06):
All right, we're back, and this is Daniel hosting today
and answering listener questions from the backlog. I've promised to
get to every single listener question and i will honor
that promise, and I'm using these episodes when Jorge isn't
around to catch up on our backlog. So far, we've
been talking about reading signs, headlines, and gravitational assists. But
(21:27):
here's another question about some recent experimental work. Hey, Daniel
and Jorge, I was wondering if you could explain what
Lawrence symmetry is, what happens if it can be broken
in the search for potential Lawrence symmetry violations. Thanks, all right,
that's a great question, and I happen to know that
came from a listener whose roommate was working on this
question and she wanted to understand it more deeply. So
(21:50):
let's get into it. What is Lawrence symmetry. Well, Lawrence
is a famous Dutch physicist and when the Nobel Prize
in nine two, and he was around during the pivotal
time when relativity was being developed. And that's what Lorenz
symmetry is really all about. It has to do with
how we see the universe and how the universe might
(22:11):
look different to different people, people who are moving at
various speeds or people who are sitting in different locations.
So what Lorentz symmetry actually says is that the same
laws of physics, the ones that we know gravity and
motion and electromagnetism and all those things. The laws of
physics all apply to all observers at every location, moving
(22:34):
at constant speed. So no matter where you are in
the universe and what speed you're moving at, you should
be able to look around you and see that everything
seems to be following the laws of physics. And you
and I should agree. If I'm here and you're at
Alpha Centauri, we should be able to look around us,
and everything and we see happen should follow the same
(22:54):
laws of physics. We shouldn't have to change the laws
of physics because of where we are in the universe.
And that's also true if I'm moving towards you. If
I'm in the spaceship and I'm moving at half the
speed of light towards you and your vacation home in
Alpha Centauri, I should still be able to look out
my window and use the same laws of physics to
observe the universe and describe what I see. Even if
(23:15):
I'm moving relative to you, you and I should be
able to use the same laws of physics. But that's
only true if I'm moving relative to you at constant speed.
If I'm accelerating. If I'm speeding up or if I'm
slowing down, then things get a little wonkier. So we'll
dig into that in a moment. But Lorentz symmetry essentially
is that it says that the same laws of physics
(23:37):
apply for all observers moving at any constant speed relative
to each other. It doesn't mean we all see the
same thing. It means we could all use the same
laws of physics to describe what we do see. All right,
So let's dig into that a little bit more. I mean,
this seems pretty reasonable. We think there should be only
one set of laws of physics that describe the universe.
(24:00):
That's sort of the whole goal of physics, right, is
to find one set of laws that describes everything. You
wouldn't want a set of laws which were dependent on location.
So why is it the Lorentz symmetry only holds if
you're moving at constant speed? Right? This requirement that you
have inertial observers. And the reason is that if you're
not moving a constant speed, if you're accelerating, then you
(24:23):
do see different forces at play. For example, if you
are in an elevator and that elevator is in space,
but it's accelerating, right, it's speeding up. Then what are
you going to feel. You're going to feel a force
from the floor of the elevator, right, You're gonna feel
the force of the floor of the elevator pushing up
on you. It's almost as if there's a force of gravity. Right.
(24:45):
Somebody in an elevator that's moving a constant acceleration sees
the same physics as somebody who's standing on the surface
of a planet and feels the force of gravity. And
that's different physics than somebody who's just floating in space.
We're moving a constant velocity. If you were in a
spaceship moving a constant velocity, you wouldn't feel any gravity,
(25:06):
you wouldn't feel the floor pushing up on you. You
would just be floating, waitless in the middle of your spaceship.
So those people have to use different physics to account
for what they see. The person who's accelerating feels a
new force when they can't otherwise describe. It's as if
they were standing on the surface of a giant planet
pulling down on them. So when they do their calculations,
(25:27):
they have to add this new force to describe what
they see and somebody else, and a spaceship that's not
speeding up, that's moving at constant speed doesn't have to
add that force. So that's like a different set of
laws of the universe. That's why we only talk about
lorn symmetry being relevant to people moving at constant speed,
because if you do have some acceleration, that creates a
(25:51):
fictitious force, an apparent force. The same thing is true
if you're moving around in a circle. Right, say you're
on a merry go around for example, somebody spins. Spinning
moving in a circle is also acceleration because it's changing
the direction of your velocity. You're going around the merrygor round,
you're pointing in one direction. Later you're pointing in another direction.
(26:12):
So if your direction of your motion is changing, your
velocity is changing, that's acceleration. And what do you feel
when you're on a merrygor round, Well, you feel this
weird fictitious force that's trying to throw you off the
merry go round. Is that a real force? There's no particle,
there's no field that's creating. It's not a fundamental force
of the universe. It's a fictitious force because you are
(26:33):
in a non inertial reference frame. Because you are rotating,
you are accelerating. So that's why Lorenz symmetry talks only
about inertial observers, people who are at rest or moving
relative to each other with constant velocity. So what do
we mean when we say you use the same laws
of physics? Because we know we've seen enough special relativity
(26:55):
examples to know that people moving at very high speeds
see things differently from each other. Like if you get
on a spaceship and go really really fast, close to
the speed of light, but it constant velocity, and I
had a telescope and I can look at a clock
on your ship. I'll see your clock moving slowly because
moving clocks run slow. But if you're on the ship
and you look at your clock, you see it running normally. Right,
(27:18):
So you and I see different things when we look
at the universe, even if we don't have any relative acceleration.
So how can we say that observers are all using
the same laws of physics? Because there's an important distinction
between using the same laws of physics and seeing the
same thing. We can see different things happening, but still
(27:38):
have them be described by the same laws of physics.
Here's an example that I think helped Einstein clarify what
was going on in his mind as he developed relativity.
Think about a single electron floating in space. What does
it do? Well, electrons have a charge, so they have
an electric field, right, an the electric field doesn't change.
It's floating in space no velocity relative to you. Now,
(28:00):
say your friend comes by and she's in a hurry,
so she's moving really fast past you. What does she see, Well,
she looks at this electron, and according to her, the
electron is moving. Right. If she's moving relative to you
and the electron is floating in front of you, then
she's also moving relative to the electron, which means the
electron is moving relative to her. Now, what does she see?
(28:20):
She sees a charge in motion. That's a current, right.
Electric currents are just charges in motion, and electric currents
can create magnetic fields. So what does she see She
sees a magnetic field. You see an electric field. She
sees a magnetic field. Right. So you see different things,
but both of you agree, yes, the laws of electromagnetism
(28:42):
are working. You see different things, but you use the
same laws to describe what you do see. And that's
the beautiful thing about Lorenz symmetry is it says you
might not have the same observations, but you can use
the same rules to describe what you are seeing. So
Lorenz symmetry is really, really deeply woven into the very
(29:02):
foundations of physics. Is something we assume is the basis
of special relativity. It's basically the same thing as special relativity.
If Laurent symmetry was violated, then special relativity would be
wrong somehow, and we don't think that it is. It's
been tested out the wazoo and into the wazoo and
around the wazoo, and nearly the speed of light. We're
pretty sure special relativity is correct. People are looking for
(29:24):
violations of this. One way they do this is they
look to see if the speed of light changes as
you move. There's the famous Michaelson Morley experiment that showed
that the speed of light is the same in two
different orthogonal directions, even though the Earth is in motion
around the Sun and the Sun is the motion around
the center of the galaxy. And that tells us that
the speed of light is uniform no matter who is
(29:46):
measuring it and what their speed is. But that's an
experimental result, and so people have been trying to improve that.
They've got this precision down really, really, really fine, so
there's no variation in the measured speed of light down
to like one part and ten to the seventeen, which
is an incredible virtuoso experiment. People also do crazy stuff
(30:07):
like bounce lasers off the Moon. You know, when astronauts
went to the Moon, they left a mirror on the
surface of the Moon, so we could do cool experiments
like shoot laser beams at the Moon, not to blow
it up, of course, but just to measure the flight time.
And this helps us understand the speed of light and
also actually the distance to the moon. The speed of
(30:28):
light is so well known that we can use the
time that a laser takes to get to the moon,
bounce off that mirror and come back as a measurement
of the distance to the Moon, which happens to be
increasing every year by about a centimeter. So people are
looking for violations of this in the way that light moves,
basically looking for violations of special relativity. But there are
also other deeper ways that we study. Lorenz symmetry. Lorenz
(30:52):
symmetry is very closely connected to a symmetry in particle
physics called C P T. Each letter there's stands for
one symmetry. C is for charge, PS for parody, T
is for time, and the combination CPT means all three symmetries.
And what CPT symmetry says is that if you take
(31:12):
a particle physics experiment and you flip the charge of
the particles involved, so like from positive to negative, and
you flip the parody, you like take it in a
mirror and do the mirror inverse experiment, and you flip
the direction of time. So instead of doing it forwards,
you try to do it backwards. That the experiment should
look exactly the same, C, P and T together should
(31:34):
all be conserved. Now we already know that parody is violated.
We have a whole fun podcast episode about how parody
is violated in the weak force and how that was discovered.
Then later people discover that CP is violated the combination
of charge and parody. So if you flip in the
mirror and switch particles to antiparticles, you still get some violations.
(31:56):
But people think that CPT is preserved and the reason
they think it's preserved is that it's required by Lorenz symmetry.
So if you see a violation of CPT somehow, that
would undermine all of modern physics because it would imply
that Lorenz symmetry is also violated. Now, there was an
experiment recently that claimed to violent Lorenz symmetry. The opera
(32:20):
experiment at certain claimed to have sent neutrinos from Certain
to Italy at faster than the speed of light, which
would be a violation of special relativity and a violation
of Lorenz symmetry. Now, those headlines were pretty impressive, and
when I read those, I thought, WHOA, this is kind
of a bonker's result. And it was pretty skeptical when
I read that, because I didn't see a detailed analysis
(32:41):
from anybody else who was not involved in the study.
And so pretty quickly when other folks were not part
of the opera experiment, dug into the details and started
asking questions. The opera folks discovered Oops, they made a
mistake and a cable hadn't been plugged incorrectly, which led
to a wrong calibration constant, which led to a mismeasurement
of the speed of their neutrinos, and turns out their
(33:02):
neutrinos were just ordinary neutrinos traveling at just under the
speed of light, not just over the speed of light.
So so far, nobody's ever seen a violation of learn
symmetry or CPT, and as far as we know, it's
a symmetry about the universe. Then, no matter where you
are in the universe and how fast you are moving,
you can use the same laws of physics to describe
(33:25):
everything that you see, which is pretty cool. Thanks very
much for that awesome question. I hope that helped you
understand it. I have one more question I want to
get to, but first let's take another break. Alright, we're back.
(33:49):
This is Daniel and I am answering questions from listeners.
Folks who had a question about the universe and roote
me an email and then sent me some audio with
their questions so you could all hear are their questions.
And I've chosen these questions because I suspect that other
folks out there like you, might have the same questions
and might be interested in hearing the answers. So here's
(34:10):
our last question of the day. Hi guys, my name
is George Ray. I'm from southern Ontario and I have
a question for you. It's regarding the speed of light.
Is it possible that in the past the speed of
light was faster or slower? Uh? And if so, how
do we know that? Thanks? By the way, I love
the podcast. All right, what a wonderful question. This question
(34:33):
touches on so many cool things about the universe. One
thing that we've seen in physics is that we have
these equations that describe the universe. They say how things
relate to each other. But those equations have numbers in them,
Like the equations for electromagnetism have some numbers in them
that tell us how fast electromagnetic information moves. That's the
(34:54):
speed of light is determined by these numbers in those equations.
And every time we see numbers in the equations, we
wonder why this number, why not another number? Could it
have been a different number? Is that this number for
a reason? Is it random? Could have been any possible number?
Or is there some deeper theory of physics that explains
(35:16):
these numbers, connects these numbers to other numbers we see
in other equations. So the question you're asking, is this
number the speed of light always been this number, or
has it, for example, changed with time is a really
deep and fundamental question in physics, and it's the kind
of thing we really drill into. It's also really fun
(35:37):
and important because the speed of light affects a lot
of things in the universe. Right, the universe seems really
really big, and one reason is not just that stuff
is far away, but that it takes a long time
to get from here to there. Like, it doesn't really
matter how many billions of kilometers you are away from
other stars. If you could go super duper fast, then
(35:59):
you could get there in a day or in a
half a day, it wouldn't matter. But there's this speed
limit on information of the universe, which of course also
applies to starships and your travel, which means things are
effectively really far away because of this limit of the
speed of light. So it makes us wonder is it
possible for it to change? Could it change in the future. Now,
(36:19):
The first thing to understand is that the speed of
light is not actually one of the fundamental numbers we
talk about when we talk about the parameters of the universe.
You know, the things in the sort of universe control
panety might dial up or down or change. And the
clue to knowing that the speed of light is not
fundamental is that it's the number with units on it. Right,
(36:40):
it's three times tend the meters per second, which means
it's relative to other things like the definition of the
meter and the definition of the second. In particle physics,
for example, we use different units. We use units where
the speed of light is just one, so that we
can erase it from all of our equations, because as otherwise,
(37:00):
we're writing the speed of light all the time and
calculating big numbers. Imagine doing a little thought experiment to
see if you would notice if the speed of light changed. Right, Say,
for example, you change a meter to be a tenth
of a meter, so to change the whole scale of
the universe, and then also change the speed of light
to match, and you change the gravitational constant, which sort
(37:21):
of affects how far apart things are in space where
they balance away from each other. So you could change
all of those numbers and you wouldn't notice anything. The
universe would seem the same to you because those numbers
are all relative to those units and to each other.
So what we've done in physics is isolated the numbers
that don't have any units. We take all the numbers
(37:44):
that we can find, the ones that are connected to
each other, speed of light, gravitational constant, all these other
numbers that do have units, and we divide them against
each other and multiply until we get numbers that have
no units. These are numbers that we can't change just
by changing our units, or by scaling the universe up
or shrinking it down by changing the length of a meter, right,
(38:05):
and so these are the ones that really would control
the nature of physics. And for example, one of them
is called the fine structure constant. It's a weird name.
It comes out of the early days of quantum mechanics
when we were understanding how atomic orbitals work and where
electrons were and how much energy they had. But basically
it's a relationship between the speed of light and planks
(38:28):
constant and the electric charge and this number alpha. The
fine structure constant really does determine sort of the way
the universe looks. You can't change the fine structure constant
without changing the physics of the universe. It's inescapable if
you change the speed of light and planks constant and
the electric charge in such a way to keep the
(38:50):
fine structure constant constant. Then you wouldn't notice any different
Maybe universe would really be bigger or really be smaller,
but then so would we, and so we wouldn't notice
any difference. So that's the key. You have to find
the parameter that actually does make a difference, the one
that would change physics as we see it. And it's
not the speed of light. It's the fine structure constant
(39:12):
one of the other several parameters. We actually have a
whole podcast episode about what are these fundamental parameters and
which ones are really important in the universe. So there
are these fundamental parameters to the universe. There's this whole
list of them, and if you change one of them,
you would change the way the universe worked. Speed of
light not technically one of them, because you can tweak
(39:36):
other parameters to accommodate for a change in the speed
of light and not change anything else. But imagine for
a moment if you just change the speed of light
or the speed of light had been changing on its own,
could you tell any difference. Well, there is one way
that we can tell how the universe worked in the past,
and that's because we can see the past. It's like
(39:59):
out there in space. The finiteness of the speed of
light keeps us from exploring the universe, but it also
means we can look back into the history of the universe,
because light that was created a long time ago with
just now arriving at Earth, if it came from really,
really far away. So as we look deeper out into space,
(40:19):
we see further into the past. And we can't conduct
experiments in the past, but we can see experiments in
the past. We can find them. We can watch things
happening in the past, and we can ask are these
described by the same laws of physics that we know now.
Can we understand galaxy formation and star formation and all
(40:39):
the stuff we see happening in the past in terms
of the same laws of physics, or do we need
to change something like the speed of light? And so far,
all the things we see in the past are very
well described by the speed of light as it is now.
There's no evidence that the speed of light has been
speeding up or going down in the past, and we
(41:01):
would see that happening because it would change the way
things work, It would change how quickly things move, It
would change how rapidly gravitational information was propagated. All sorts
of things would be changed if the speed of light changed,
and so far it seems like it hasn't, but there
are some nuances to that it might be possible, for example,
(41:22):
to reinterpret what we see not in the way we
imagine it now, but as a change in the speed
of light. For example, some people really really don't like
the ideas of cosmic inflation. The ideas of the universe
grew very, very rapidly in the early universe. In the
first few moments, is stretched by ten to the minus
(41:42):
thirty seconds. This crazy stretching of space, a stretching of
space that actually happened faster than the speed of light.
It doesn't violate special relativity because it's a stretching of space,
not a motion through space, and that's an important technicality.
But some people still don't like this con spt and
they wonder if instead maybe it was just that the
(42:03):
speed of light was much much faster. There are these
theories of physics, these alternative theories that are kind of
fringe theories. They're variable speed of light theories that try
to explain what we see in the past, not in
terms of cosmic inflation or expansion, but instead in terms
of a change of the speed of light. And you know,
(42:23):
you can always take the same data and fit another
theory to it, but then you have to ask, how
does that theory look? Is that theory really work And
the problem with these theories, these variable speed of light
theories is, frankly, that they violate special relativity. They violate
Lorenz invariants, and so that makes us not like them
very much. We really do believe in Lawrence invariants. We've
(42:45):
tested it out the wazoo. Now it is potentially possible
that in the early universe things were really different and
Lawrence invariance wasn't as respected. But we have reasons to
believe in Lawrence in variances. It's not just like an
article of faith. It comes out very simply from the
mathematics and from looking at the structure of space itself.
We talked once about Nurther's theorem, which is this idea
(43:08):
that all symmetries are connected to conservation laws, and in
this case we think that Lorenz symmetry is connected to
translation symmetry and rotation symmetry. The fact that everywhere in
the universe seems to be similar. There's no special location
in space. It's not like there's an origin at the
center of the Sun or the center of the Milky
(43:29):
Way that's different from any other place. No matter where
you put your zero zero on your axes, physics should
work the same. So that's a pretty basic assumption about
the way the universe works. To get rid of that,
to toss that out the window would mean tossing out
the window a lot about what we understand about the universe.
That doesn't make it impossible, but it makes it a
(43:50):
big pill to swallows. You need very clear evidence. It's
much simpler to say, well, we think everywhere in the
universe is the same, and Lorenz symmetry makes a lot
sense to us, and the speed of light is a
fixed number. Everything sort of clicks together and works very
very well. You can describe the universe using other theories,
but they don't click together as well. They're not as nice,
(44:11):
they don't have the same beautiful symmetries. They're more complicated,
and so we tend to favor the one that we
have now because it works so well. We don't need
variation of the speed of light to explain what we see,
so to summarize, and we don't think the speed of
light has changed. Actually, you could change it without noticing
anything in the universe if you conspire to change a
(44:32):
few other fundamental constants. The ones you really should be
thinking about are these constants without units, the dimensionless numbers,
the pure numbers that control the universe. But because we
don't know why the speed of light is what it is,
there's nothing necessarily determining its number. It is potentially possible
(44:52):
that it could have changed in the early universe, but
we don't see that. We look out into the data
and we see that the universe is described by the
same laws of physics with the same speed of light.
So it all fits together with us. But thank you
for asking such a deep and fun and wonderful question
about the nature of the universe and whether it is
what it is, and whether that's what it always was.
(45:13):
Thank you to everybody out there who's been thinking deeply
about the universe and wondering how things work, and asking
questions when they don't make sense. So please keep thinking,
keep being curious, keep asking questions, and write to us
two questions at Daniel and Jorge dot com because we
will answer all of your listener questions. Thanks everyone, tune
(45:34):
in next time. Thanks for listening, and remember that Daniel
and Jorge explained The Universe is a production of I
Heart Radio or more podcast from my heart Radio. Visit
the I Heart Radio Apple Apple Podcasts, or wherever you
(45:54):
listen to your favorite shows. No
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