March 12, 2019 • 36 mins
Why does time slow down when you go fast?
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Episode Transcript
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Speaker 1 (00:07):
Hey, Daniel, do you somebody's feel when you're bored time
slows down? That never happens to me when I'm talking
to you, or Hey, every conversation with you is riveting,
It goes back super fast. That's right. Time just flies by,
you know, and call you up, and then all of
a sudden it's hours later. Do you know what I mean? Like,
it's the idea that maybe time is relative in our heads. No,
(00:29):
I think that's true. When you're waiting for something, time
feels like it goes really slow. When you're listening to
your favorite podcast hosts, it just whizzes right by. We're
pretty bad at measuring time as a species, are we really?
Like our brains are not good at estimating time? Oh yeah,
they do all these experiments where they ask people to
estimate how long something has been, and they always over underestimated.
(00:50):
So psychologically time can be relative. That's proven. But what
about physics can that actually happen? Yeah? It turns out
that physics also doesn't provide a bedrock layer of truth
that time can slow down. Huh, So there's no universal clock, like,
there's no impartial, godlike measure of time. That's right. So
(01:12):
if next time you're late for a meeting, you can
just say, hey, I got my own clock and my
observer says I was on time. Physics gives you an
excuse to be late for your meeting. And I'm Daniel.
(01:40):
Welcome to our podcast Daniel and Jorgey explain the universe,
in which we take plenty of time to explain to
you how time works in the universe. Hey, yeah, we're
here to help you. Whatever it is you're doing, communing
to work on a subway, going out for a jog.
We're here to help the time move a little bit
faster for you. That's right. We're here to kill some
time for you, and out's time to get on with it.
(02:01):
So today's topic is why do clocks run slower when
they are moving fast? That's right. This is a really
popular thing for people to get confused about in relativity.
It's technically called time dilation, the fact that clocks that
(02:22):
move fast run slow, and it's a topic that confuses
people from here to infinity. Yeah, it's the idea that
if you're going really fast, maybe it close up the
speed of light, then time slows down for you. That's right.
And the thing we want to understand today is not
just does it happen, but why does it happen? What
is it about the universe that makes that the way
(02:43):
it really works? And the thing that I love about
this is that it's one of the best examples of
how the universe doesn't work the way you think it does,
that it doesn't make sense to our sort of intuitive
understanding of the way the world should work, right, especially
at these extreme conditions. Right, that's right, because we're not
used to those extreme conditions. So we've like operated in
(03:05):
you know, on the surface of the Earth, which is
pretty slow speeds for thousands and thousands of years. We
build up these intuitive models for how we think the
universe behaves right, what the rules are, And one of
those rules is that we think that there is a
absolute history, right, We think that there is a reality
out there and that something actually happens and in the end,
(03:26):
everybody should agree if they're honest observers about what happened.
It turns out that's just not true. And hopefully this
is kind of a topic that a lot of people
have hopefully I think maybe have heard about. It's sort
of permeated out into popular culture a little bit, right, like,
it's the basis of the movie Interstellar. I knew you
were going to talk about Interstellar Interstellar reference, but it's
(03:48):
it's a big example of it in pop culture, right,
Like I think, really Interstellar was a pretty big movie
regardless of what your physicists thinks of it, and my
year and my year physicist is that how used to
refer to me? No, Interstellar actually did a pretty good
job of modeling the relativity portion. My issues with the
Interstellar are more about the time travel and going inside
(04:08):
a black hole. But from the point of view of relativity,
that made a lot of sense. I mean, I thought
it was good that they actually built into the plot
what would happen to various people's clocks. And that's the
key thing, is that relativity is all about comparing clocks.
How fast is my clock going to your compared to
your clock? So we're going to assume that you have
heard of this concept that if you are in a
spaceship going really fast and time will slow down for you.
(04:32):
But we were wondering how many people out there know why?
So as usual, I tortured the undergraduates of you see
Irvine by walking around and asking them random questions without
any preparation. And remember, for those of you who think
that these answers are silly, these are hard questions to
answer on the top of your head, so give them
some slack. It's tortured. This called the physics boarding, enhanced
(04:55):
physics exactly, physics boarding, you know, exactly. I'm gonna cover
your face with a towel and poor physics on your face. Um. Eventually,
I think the undergrads are gonna recognize me, and they're
gonna be like, don't let that guy talk to you.
He'll embarrass you. Run away. We're not trying to embarrass
these people. No, it's great. I think I would be
totally flabbergasted if you asked me these questions out of
(05:17):
the street. All right, I'll plan the ambush you one day.
So before you listen to these answers, think for yourself.
Do you know what time dilation is? Can you explain
why moving clocks run more slowly? Here's what people had
to say. Have you heard of time dilation? Do you
know that clocks that move really fast go slower? This
(05:37):
is trying your information to me. Yes, do you know
why that is? No? I don't know the real reason.
I'm sorry, No, no, there is no time, thanks very much.
All right, So maybe I was a little wrong. Not
a lot of people have heard about this concept. That's right,
and there are even people out there that deny the
(05:57):
time exists. Right, there is no time, there is no
concept of time, or that there was not enough time
to explain it to you. You know, I was so
flabbergasted by the response. I couldn't even formulate a follow
up question. I was like still processing, like, what does
that even mean? Wow? Did he did he just dropped
or she dropped the bike? There is no time? Great phase.
(06:20):
She said it with a lot of finality. Yeah, So
there wasn't a whole lot of opening there for interrogation
or follow up questions. It was like, this is a
clearly known fact, there is no time. Well, I mean
she was just going to rush and she's like, I
have no time for this. No. I think it was
definitely more of the time and time is an illusion.
So an answer, Yeah, I've heard of that time is
an illusion. Yeah, Well, I think we have to do
(06:42):
a whole other podcast episode about what is time and
how does it work? And why does it only go
forward and all that kind of stuff. But this is
kind of related. This topic is related to that idea
that time is not what we think it is. That's right.
We've met a lot of progress in the last hundred
years in understanding time, and we've connected it to space.
You've probably heard the concept of space time, right. We
(07:02):
have three dimensions at least of space and one dimension
of time, and Einstein's relativity tied them together and showed
us how time and space are connected. But that doesn't
mean that time can be simply understood as a fourth
dimension of space. It's much more complicated. It's different from
the other dimensions of space. Yeah, they're all tied together, time, speed, space,
It's all one big molasses of a universe. It's all
(07:26):
one big tangle. The amazing thing is that it actually
all does work. You know. We have this new version
of our understanding of the universe, not that new anymore
as a hundred years old, but this revised version of
our understanding the universe, and it actually hangs together. I mean,
the answers that gives you don't make any sense to
your intuition, Like they fly in the face of what
you think should happen. They require you to like throw
(07:47):
out the way you think the universe works, but they
actually do hang together mathematically and they are correct. Like
every time we make a ridiculous prediction from relativity and
God and check it, the universe is like, yep, that
ridiculous thing actually happens. Well, let's bring it down for people.
So what does it mean for clocks to run slower
when they move fast? So that's that's what we're exploring,
(08:11):
and that's what we're trying to explain, is why when
you're moving really fast, your clock is going to actually
slow down. Right, And so let's be very careful and
how we say this, because a lot of people get confused.
People think that if you are moving fast, that your
clock slows down. That like, if you're looking at your
watch and running that you can see the seconds tick slower.
That's not true. If you're holding a clock and the
(08:34):
clock is not moving relative to you, you'll always see
it moving at one second per second. Okay, no matter
how fast you're going relative to anything else, your clock
always runs the same way. It's not like I get
on a spaceship hit the warp speed and then that's
not what I experienced. That's right. You never notice your
own time changing, right, you experienced time at one second
(08:55):
per second, no matter what. Okay, the thing that does
happen is that clock's moving relative to you. Right. So,
if I'm standing still and Horge has a clock and
he runs because he's a pretty zippy guy, if he
runs a half speed of light because I'm running late,
probably if I notice that he's moving really fast, then
(09:16):
I will see his clock running more slowly. Right. So,
now from his point of view, he will see his
clock running normally, but I will see his clock running
more slowly. If I'm zooming past you and you just
look at my clock as I'm zooing by, it's not
going to be running at the same speed as your clock,
that's right. If I'm watching you as you go by,
and I'm watching your clock's hands tick forward, right, then
(09:37):
they don't agree with mine. Yours mark the seconds more
slowly than my clock does. And that's the key thing
is that the observation of time depends on your relative
velocity to the clock. So it's not that it actually
slowed down for me at least, it just you saw
it run slower. Right. I love how you try to
(09:58):
use the word actually right because you're in your imagining,
there's some true version of the effect of the effect, right,
I'm just this is an illusion or it looks this way,
but it's not actually happening. The problem is there is
no what actually happened. Okay, I observe one thing, you
observe something else. We can both be right even if
those accounts disagree. WHOA Okay, So then I'm running past
(10:22):
you really fast with a clock and you see it
run slower than I do, or it's running slower than
your clock. Then what happens if I stopped, like if
I stopped a few paces after you, Does that mean
our clocks are going to be out of sync? Yes,
our clocks will definitely be out of sync, exactly, And
there's a lot of interesting effects there. Right. So I'm
a I'm standing still from my point of view, right,
(10:43):
and you're running past me. I see your clock moving
more slowly. I see my clock running normally. Right, what
do you see? Well, you see your clock running normally, right,
because everybody sees their own clock running normally. But you
also see my clock running slowly because even though you're
doing doing the running, I'm moving relative to you and
your clock. So then what happens if I stopped, Because
(11:05):
I'm gonna think time moved normally, But when when I
compare my clock to your clock, your clock will have
skipped ahead because right or no, we'll have skip back.
This is really tricky, okay, and we should probably avoid
this topic. Well maybe we won't. Let's dig into it.
(11:26):
This is called the twin paradox, right, This is people say, well,
how do you reconcile this? Right? So the classic framing
of this is, say we're twins, right, and I stay
on We draw straws for Hugas to go to Alpha Centauri,
and you lose. So you have to go to Alpha
Centauri or you win. You have to go to Alpha Centauri.
I stand on Earth and you take a rocket shipped
Alpha Centauri, and I see your clock running more slowly. Okay,
(11:48):
well we sink First of all, we sink clocks like
before I take off. Yes, we're just going to sink clocks.
Time zero start go now, that's right, and I see
your clock running more slowly. Meanwhile, you have telescope. You're
looking at my clock. You see mine running more slowly. Right,
so we both see the other person as aging more slowly. Right.
So now after a hundred years, you see me as
(12:10):
only being ten years older, and I see you as
only being ten years older, right, right, So then I
come back and then what happened? Who's older? All right,
let's break this down really carefully because it is tricky.
So when the Earth twin is watching the ship twins clock,
he of course sees time moving more slowly on the ship, right,
that's relativity. The same way when the ship twin watches
(12:31):
the Earth twins clock, he also sees time moving more
slowly on Earth. So far everything is symmetric, right, and
that's why we like it. Because you could be in ship,
you could be in the Earth. That shouldn't really matter,
right now. If they just kept going this way, nothing
would change. Of course, they would have conflicting views of
whose clock is moving slower, right, But that's okay. In relativity,
you can have two people with conflicting but both correct
(12:54):
views of the same situation, right, Because there is no
ultimate truth, right, your answer is depend on your speed
and your location. So what happens to break the symmetry?
The symmetry is broken when the space twin turns around.
That's an acceleration, right, That changes everything. Well, only the
space twin does any acceleration, so that makes his case different, right,
(13:15):
And during that acceleration, the time on Earth seems to
zoom forward really fast from the point of view of
the space twin. So on the way back, yeah, he
sees Earth moving fast and Earth clock going slowly. But
during that acceleration Earth time has leapt forward really far,
so that when the space twin gets back to Earth,
he's younger than his twin on Earth. Right, And the
(13:37):
reason they're no longer symmetric is that only the space
twin has done any acceleration, so you shouldn't expect them
to be the same from each point of view. It's
the coming back that then lets me stay younger. Yeah, exactly. Okay,
let's let's dig into it even more. But let's take
a quick break, all right. So this idea that time
(14:10):
moves slower when you're going fast, it's what's cool is
that it's always happening, right. It doesn't just happen when
you're going at the speed of light or close to
the speed of light. It's it happens like on your
on a daily basis, That's right. It applies all the time.
It always applies to things that have velocity relative to you.
But it's a really tiny effect if you're not going
(14:31):
really really fast. So when you're driving sixty in the freeway, yeah,
your clock is running a little bit slower, but you'll
never notice it. Wow, but it's there. Like if we're
all feeling relativity and time dilation all the time everywhere,
there's no way to escape relativity. It is everywhere. It's
relatively everywhere. It's absolutely every Absolutely thing about relativity is
(14:54):
that it's everywhere. Okay, so it only happens when you
get it's only noticeable. You're saying, when you're going to
close the speed up. But I heard it happens to
astronauts here on Earth, like the Space Shuttle. The clocks
get out of sync with the clocks on Earth. That's right,
though it takes a lot of precision to measure it.
It's something like every six months or so they lose
(15:15):
like less than a hundredth of a second. So it's
something we can measure, and they have really precise clocks
precisely to measure this to verify these predictions, but it's
not something that people have really like qualitatively experienced. We
haven't had an astronaut come back, you know, deep into
the future and still feel young. And it goes really nonlinear, right,
as you get faster and faster, the effect gets stronger
(15:36):
and stronger. But they did do the twin experiment with astronauts, right, Like,
they sent to one twin into space for a whole year,
and then he came back and technically he was point
oh one seconds younger. That's right. I wonder if he
was the one who was originally born first or the
one who's ovisually born second, because it be interesting and
be like, well you used to be the older twin,
but now I'm the older twin. A should have stayed
(15:58):
up there for ten more years. Do you think growing
up twins still argue by that kind of stuff. Well,
I'm the older brother, so you have to listen to me.
What do you think they're both austers. They're probably trying
to one up each other. Well, yeah, I'm an engineer.
Oh yeah, I'm a pilot. Oh yeah, I'm a I'm
an auster. Not me too, yeah, probably it never ends, right,
(16:19):
but technically that's true. He went out into space, went
around the Earth for a whole year. When he came back. Technically,
time for him moved one hundred of a second slower. Yeah,
I wonder what he's gonna do with all that extra time,
you know, scratches nose or something. Right, it's gone, he
lost it. He used it up. Yeah, But as you
go faster and faster, the effect gets stronger and stronger,
(16:40):
and as you approached the speed of light, time gets
so slow that we can say that if you go
the speed of light, time would actually stop. So if
I'm zooming running past you at the speed of light,
you would see my clock totally frozen, exactly. Now, caveats
are important here. Nothing that has mass actually go the
speed of light, right, and only massless things can go
(17:03):
the speed of light, So you can never go the
speed of lights. Are saying I have much mass, Daniel.
You're pretty massive, dude, Yeah, by which I mean you're funny,
and you're massively awesome thanking, and you have a massively
excellent podcast, and you are brilliant. But you're not entirely
made of light. But that all right, Yeah, I was
just fishing for relative compliments. You're absolutely wonderful. So if
(17:28):
you have somebody in a spaceship and they're going super
duper fast and they're approaching the speed of light, and
time slows down further and further, to the point where
it almost stops. But remember, for them, time doesn't slow down.
It's not like they're living in molasses. It's just our
observation of their time. So if somehow a photon had
a clock, yeah, our view of the photons clock would
(17:48):
be that it was frozen. Right for us, Photons don't
move forward in time. They are frozen in time. But
if you were a photon, what would your experience be. Well,
it's hard to answer that because you're not a photon,
and you have to have that concept of like ascension photon,
which seems impossible, but it's always basically an impossible question
to answer. But they remember, your time always moves forward
(18:10):
um at one second per second. Okay, so that's the
effect of time dilation. Time moves seems to move slower
when you're going faster. So let's get into why it happens.
Do we know why this happens. We do know why,
and it's a consequence of another really strange, counterintuitive thing
(18:32):
that we've observed about the universe that makes very little sense.
So it's a conundrum built on a conundrum. It's a bizarre,
counterintuitive consequence of something really weird about the universe and
really weird about light. Actually, it's the fact that everybody
always observes light going at the same speed, no matter
(18:53):
how fast they're going relative to the source of the light. Okay,
let's break it down what that means. So light travels
through space, but nobody can see it move faster than
that speed that light losing. That's right. There's a certain
speed of light that goes through space three times ten
to the eights per second. And we'll call it the
speed of light. Of course. And if I'm standing on
(19:14):
a planet and I turn on a flashlight, then the
light leaves me and travels at three times tended them
per second away from me. Right, And if I'm and
if you're shooting it at me, then I see moving
towards me at three times ten to the eight meters
per second. Okay, that's right, even if you're even if
you're on a rocket ship and you're moving towards me, right,
(19:34):
say you're moving towards me at half the speed of light.
I shoot my laser beam at you're my flashlight at you,
you still see that flashlight, the light from it coming
at you at the speed of light, right, And that's
different from you know, sound waves or rocks or something,
or a baseball. Like that's weird. Right, Like, if you
throw a baseball at me and I'm running full speed
towards you, that baseball is going to be it's gonna
(19:57):
look like it's moving really fast towards me. That's right.
If I threw a baseball at you at a hundred
miles an hour, which I promise I can totally do,
this is very realistic, and you're running towards me at
fifty miles an hour, which I'm sure you're totally capable of. Um,
then obviously I see the baseball is moving away from
me at a hundred miles per hour, but you see
the baseball is coming towards you at a hundred fifty
(20:17):
miles per hour. Right. That's the way it works for
normal things. But you're saying that if that baseball instead
of a baseball, it was a beam of light, that
wouldn't happen. That's right. Light doesn't follow those rules. Everybody
who measures it measures it as traveling at that fixed
speed of light. No matter what. I mean, there are
caveats here, like it slows down and it travels through
air or water, whatever, But let's just talk about it
(20:38):
in a vacuum. The crazy thing is that doesn't matter
how fast you're going, everybody sees light is traveling at
this maximum speed of the universe, no matter what. And
that's crazy, right and right there is where your concept
that everybody sees the same thing the same way, or
that there is one absolute truth that we're all observing
in different ways breaks down because your description of events
(21:00):
and my description events are going to be very different.
If we see light moving at different speeds. It's the
weird It's like a weird rule of the universe. Not
that light something can't travel at this faster than the
speed of light. It's a weird rule that says nothing
can be seen to travel faster than the speed of light,
not even light. That's right. Light always travels at the
speed of light, and nothing can travel faster than the
(21:21):
speed of light. And these two things are connected, right,
because if light operated the same way as baseballs, then
you could see light moving as faster than the speed
of light just by moving towards me. When I'm shooting
a laser at you, right then that light would be
moving relative to you. It's faster than the speed of light.
But it doesn't right, you move towards me, and you
still measure light as moving at the same speed. It
doesn't matter if you're running away from me or towards me. Okay,
(21:44):
So if you shoot a light beam at me and
I'm running towards you really really, really fast, You're going
to measure the light moving at the speed of light.
And I should measure the light moving faster towards me.
But I'm also going to measure it moving at the
speed of light, even if I'm moving towards you or
away from you or to the side of you exactly,
no matter how fast and moving, I'm always going to
(22:05):
measure it and moving at the speed of light exactly.
And that's crazy, right, It's bonkers, doesn't make any sense.
And it's a famous experiment, Michael said. Morally, the experiment
that that did this, they shot beams of light in
two directions, and because the earth is moving, they figured, well,
one of them is going to go slower than the
other one because the earth is moving, right, So they
did the experiment different times of year, and the light
came back took the same amount of time to go
(22:28):
in these two perpendicular directions every single time. And that
told them that that the speed at which light travels
is not dependent on how fast you are moving the
observer is moving. It's always the same speed. It's crazy.
Is it like some kind of um just like fundamental
limit in this stuff of the universe itself? You know,
(22:49):
like nothing can propagate through this thing we call space
faster than the speed of light. Is that kind of
what it's related to. Yeah, definitely, But it's it's a
deep question and we don't have a solid answer to too.
Why does light always travel at this speed regardless of
the speed of the observer. We don't know the answer
to that. That's just like Einstein postulated that. He said, Okay,
let's start from this crazy assumption and build the math
(23:12):
up from there, and if everything then works, and we'll say, well,
that assumption must be true. And if you start from
that assumption, you get all sorts of crazy predictions which
all turn out to be true. Right, So that is
a deep truth of the universe. But the answer is,
we don't know why. We don't know why light always
travels at the same speed, or a light is always
observed at the same speed, no matter who is doing
(23:33):
the measuring and how fast they're going. It's a weird
rule about the universe. It's a weird, weird rule. And
when I meet the people who wrote the simulation, I'm
gonna ask them, why did you do that, man that
made everything so complicated about the universe? One star on
Yelp for that for that bit of load. No, I
think it's it's there's a fascinating angle there, you know,
(23:56):
because we grew up sort of as a species in
an environment where nothing was near the speed of light,
and so we never noticed this, and so we assume
things like everybody's clock runs at the same time because
we've always thought it did. And so they're not allowed
us to assume things like there must be some sort
of absolute sense of time and absolute history and there's
a really universe out there, and this is I mean,
(24:18):
this shakes the very foundations of how we even think
about the universe that's out there and whether it makes
sense at all. So it's it's pretty crazy stuff. I
feel like we're kepped with weirdness at both ends of
the specter. What I mean is like, if you move
it close to the speed of light, things get weird.
But also if you don't move at all, things good weird.
Right because of quantum physics and the uncertainty principle. Right,
(24:41):
Like if you try to discertain things that zero velocity,
things get weird too. Things always get weird. I think
that's the takeaway, right. The universe is weird, like I
think I said last week, is weirder and stranger and
hotter and nastier and wetter than you could ever even imagine.
And the I think the craziest surprises about the way
the universe work are still yet to come. You know,
we've made these discoveries that showed us to the universe
(25:02):
is so different from the way our ancestors have imagined.
There must be more of those discoveries coming. It's certainly
not the case that we figured them all out. There
are crazy revelations in our future. Well, that's good for
our podcast topics, that's right. Okay, let's so let's get
into how this affects time. But first let's take a
quick break. So we know that the speed of light,
(25:37):
you can observe going anything faster than the speed of light.
So how does this affect how we view time or
how we experience time? Right, So it comes directly the
fact that light can't travel at any other speed is
what directly affects how time passes and how we measure time.
And it can be a complicated topic, but I think
the best way to do is to think about maybe
how a simple clock operates. So let's build a simple
(26:00):
clock that uses light. You mean like an Apple watch? Er, No,
I mean let's imagine that you're you have to build
a clock, and all you have as a laser, right,
And so you know, for example, that light takes about
ten nanoseconds to go ten feet right, it goes about
one ft every nanosecond. So what you do is you
(26:20):
measure very precisely, you know, ten ft. You put a
mirror at the end, and then you shoot your laser.
You shoot a laser pulse, and you say, however long
it takes to go um there and back. That's two seconds, right,
ten feet there and ten feetback. And you set up
the mirrors in the floor and in the ceiling, right,
so that being is bouncing up and down right right,
(26:42):
So you you shoot the laser up towards the ceiling
and back, and you know how far it is to
your ceiling and how far it is back to your floor,
and so you say that's twenty ft, so that's two
seconds round trip. Oh I see. So then the way
you turn that into a clock is you count how
many times the light bounces up and down the ceiling,
and that sort of gives you a sense of how
(27:04):
time is moving. That's right. You want to say, well,
how long does it take my cat to finish his lunch?
And so you count how many times it takes the
laser pults to go up to the ceiling and back,
and that's the number of two second intervals it takes
your cat to eat his lunch. Okay, so that's where
it seems like a pretty impractical clock. But hey, this
this is how we do things in physics. Man, be like,
(27:26):
can we build this thing using lasers? What's the most
inconvenient wristwatch we can build with lasers at home? The
way we toast bread for breakfast as we use lasers,
of course, and cats cats, I feel like it's a key. No,
that's just how we toast our cats. Man. Let's say
(27:47):
you cooked the food for the I want to I
want to officially distance myself from that joke, because nobody
should ever fire a laser at a cat. Well, you
can fire a laser near a cat to entertain it,
of course, but I don't actually hit your your cat
with a laser, please, oh man? All right, So now
we have our clock, right, that's the way a clock
that uses light. Okay, so this is a thought experiment.
We're going to build a clock where we measure time
(28:07):
by measuring counting how many times it bounces off the
ceiling up and down. That's right, And it doesn't have
to be a thought experiment. You've got mirrors, you've got lasers.
Go ahead, build your self o'clock. Um. But the interesting
thing is that what happens if you put that clock
on a spaceship right right on the train. Let's make
it even more inconvenient. Let's put this on a spaceship.
(28:29):
And so that's when things start to get interesting, right, Like,
that's when we start to see how time slows down. Yeah,
Or let's put the clock on the back of your cat, right,
and then see what happens when you're well, I guess
you need the ceiling on the back of the cap.
Maybe that doesn't. Actually, I feel like trying to get
a cat to do what you want is even more
difficult than getting on a spaceship. Einstein had no idea
how to get your cat to do what you want.
(28:50):
You can master. He was the master of the cosmos,
but not of cats. All right. So we have a
clock where you measure time by counting how many times
it bounced off the ceiling on the floor, and we
stick it in a spaceship, and then we start going.
What happens then, right, So if you're in the spaceship,
nothing changes. It doesn't matter that you're going at half
(29:13):
the speed of light or nine tenths of speed of light.
You're in the spaceship. You have no velocity relative to
the clock. So things work the same way on the
spaceship for you as they did when you tested your
clock in your living room. As you just see the
beam go up and down, bounce up and down, and
you count, and that's your time. That's right. And since
you brought your cat along, it takes your cat the
same amount of time to eat his lunch in your
spaceship as it does at home, assuming he's not wearing
(29:35):
a silly cat space suit. The interesting thing is that
when since you left me, Um, you left me on Earth,
you didn't invite me on your awesome spaceship. Thanks by
the way, that would have been too inconvenient. And I'm
so heartbroken that I'm spying on you. Have a massive
telescope and I'm watching your cat eat lunch on a spaceship. Um.
Now I'm looking at your clock, okay, and I'm wondering
(29:56):
how long does it take his cat to eat lunch, etcetera.
And I'm watching your clock. Okay, you're trying to count
how many times it bounces off the ceiling to exactly.
So I see the laser pulse go up, and I
see the laser pulse come down. Right. The problem is
that I don't see the laser pulse going ten feet
up and ten feet down. I see the laser pulse
(30:18):
as going further because you're moving, which means the laser
pulse is not just going up and down. It's also
going sideways in the direction of your motion. So you
have the up and down and the sideways. So the
light for me, the laser is going in a diagonal right,
diagonal up hit the ceiling, and diagonal back down to
hit the floor, because you see it hit the ceiling
and then on its way down, the spaceship is awesome moving,
(30:41):
so it's kind of moving diagonally to hit the floor
where it's going to be exactly. And so the mirrors
and the clock move with the laser. Obviously they're all
going at the same speed sideways, and so the laser
beam hits the mirror on the top and it hits
the receiver or whatever on the bottom. But I see
you traveling further than you do, right, And this is
where the the absolute speed of light kicks in because
(31:03):
you say, okay, travel ten feet, right. It traveled those
ten feet at the speed of light, So I know
it takes ten seconds, but I see it traveled further. Right,
Depending on how fast you're going, it could have traveled
like fourteen fifteen feet, right, because light always travels at
the same speed. Right, I see it taking longer because
it's gone further and it can go faster than the
(31:24):
speed of light. It can go faster than the speed
of light. So I see your clock running slower. It
literally takes longer to count off seconds for you because
the clock is built on the premise that it takes
light a certain time to go a certain distance. But
now that distance is further and the speed can't change
because it's going up and down. The light is going
up and down, and it's trying to go forward to exactly,
(31:45):
so it has to take longer exactly. Now, if you
built this same kind of clock using something else, like
sound waves, right, you had a speaker, then this wouldn't
happen because the sound waves don't have that same property
that they always travel at the speed at a certain
speed relative to observers. Right. Um, sound waves, as we
know from Doppler shifts, change their speed based on how
(32:06):
fast you're going. So if this is a sound wave,
if we use sound waves instead, then I would just
see the sound waves is traveling faster. But light can't
do that. Light always travels to the same speed no
matter who the observer is. So it slows your clock down.
So let's let's go back a step. Um. Okay, So
so I'm gonna be in my spaceship counting how many
times it bounces off the ceiling, and for me, it's
going to be like bounce, bounce, bounce, bounce, bounds. But
(32:29):
for you, you're saying because it has to travel up
and down and forward. You're going to count it slower,
right like for you, it's going to be bounds, bounds, bounds,
And so that's kind of the definition of time. Yeah, exactly, exactly,
and any clock. And you're thinking, okay, well, that's just
one example, right, But the same argument holds for if
(32:51):
you're going in another direction, right, you don't have just
be going sideways. If you're shooting in the direction of
the mirror, right, then the light has longer to go
in one direction, but it can't make up the time.
And if you're going at an angle, the same thing happens.
And also for any clock. This is just one example.
It's the clearest example because we've built it out of
something that only uses light, but the true the same
(33:13):
happens for every physical process, for any kind of clock. Okay.
So that's kind of the explanation is that time, the
definition of time is sort of tied to the speed
of light, and the universe has this weird rule about
the speed of light, which is that nobody can never
see it move faster than the speed of light. That's right,
And the other important thing to understand is that your
(33:35):
definition of time and your definition of what happens depends
on how fast you are going. There is no absolute
sense of time. It's not like the universe has a
big clock out there and it's keeping track of what's
going on, and we're trying to make measures of measurements
of it, and they're kind of sloppy sometimes when we
get them wrong, Like there is no absolute sense of time, right,
And you can do crazy experiments where you know, depending
(33:57):
on how fast people are going relative to experiment, they
can see the order of events changing, like I can
see A happen before B, and you can see B
happen before A because you're zooming in the other direction.
And you might think, well, that's impossible either A happened
before B or BE happened before A. There is a
real truth, right, answer is there is no truth. The
(34:17):
truth is not out there. X Files was a lie.
So I feel like we should get into that in
another episode. But I think the conclusion we're reaching here
is that basically there is no time. After all, there's
no time to talk about this anymore. And that's true. No,
there is time, but time is a different thing that
you than you thought it was, right, It's something weirder
and more malleable now in our safe, little slow worlds,
(34:40):
it doesn't. You can pretend that time is the way
you thought it was right and you'll get by just fine.
But in reality, if you want to understand the way
that that physics works at its deepest level, whether universe
is actually put together, what the real rules are, that
it turns out time is really different than you thought
it was. So that person who answered there is no
time sort of, I feel like she skipped ahead to
(35:02):
the end of this count. She's probably a physicist visiting
from the future from Alpha centri. Is it her? Or
she's your cat evolved for thousands of years into a
future physicist and then traveled back in time to deliver
that message. I think she's your twin Danny. Maybe she is,
maybe she is. Well, speaking of time, I think we're
(35:25):
out of time for this episode, so it's time to
wrap it up. Thank you everyone for your patients and
for listening, And if you have questions about how things
work in the universe or anything else crazy, please send
them to us on Twitter or email us at feedback
at Daniel and Jorge dot com. We love listener questions. Yeah,
hopefully We made time move a little bit faster for you,
(35:45):
whatever it is you're doing, unless you were bored, which
is probably canceled out. If you still have a question
after listening to all these explanations, please drop us a line.
We'd love to hear from you. You can find us
(36:07):
at Facebook, Twitter, and Instagram at Daniel and Jorge That's
one word, or email us at Feedback at Daniel and
Orge dot com.
Hey, Daniel, do you somebody's feel when you're bored time
slows down? That never happens to me when I'm talking
to you, or Hey, every conversation with you is riveting,
It goes back super fast. That's right. Time just flies by,
you know, and call you up, and then all of
a sudden it's hours later. Do you know what I mean? Like,
it's the idea that maybe time is relative in our heads. No,
(00:29):
I think that's true. When you're waiting for something, time
feels like it goes really slow. When you're listening to
your favorite podcast hosts, it just whizzes right by. We're
pretty bad at measuring time as a species, are we really?
Like our brains are not good at estimating time? Oh yeah,
they do all these experiments where they ask people to
estimate how long something has been, and they always over underestimated.
(00:50):
So psychologically time can be relative. That's proven. But what
about physics can that actually happen? Yeah? It turns out
that physics also doesn't provide a bedrock layer of truth
that time can slow down. Huh, So there's no universal clock, like,
there's no impartial, godlike measure of time. That's right. So
(01:12):
if next time you're late for a meeting, you can
just say, hey, I got my own clock and my
observer says I was on time. Physics gives you an
excuse to be late for your meeting. And I'm Daniel.
(01:40):
Welcome to our podcast Daniel and Jorgey explain the universe,
in which we take plenty of time to explain to
you how time works in the universe. Hey, yeah, we're
here to help you. Whatever it is you're doing, communing
to work on a subway, going out for a jog.
We're here to help the time move a little bit
faster for you. That's right. We're here to kill some
time for you, and out's time to get on with it.
(02:01):
So today's topic is why do clocks run slower when
they are moving fast? That's right. This is a really
popular thing for people to get confused about in relativity.
It's technically called time dilation, the fact that clocks that
(02:22):
move fast run slow, and it's a topic that confuses
people from here to infinity. Yeah, it's the idea that
if you're going really fast, maybe it close up the
speed of light, then time slows down for you. That's right.
And the thing we want to understand today is not
just does it happen, but why does it happen? What
is it about the universe that makes that the way
(02:43):
it really works? And the thing that I love about
this is that it's one of the best examples of
how the universe doesn't work the way you think it does,
that it doesn't make sense to our sort of intuitive
understanding of the way the world should work, right, especially
at these extreme conditions. Right, that's right, because we're not
used to those extreme conditions. So we've like operated in
(03:05):
you know, on the surface of the Earth, which is
pretty slow speeds for thousands and thousands of years. We
build up these intuitive models for how we think the
universe behaves right, what the rules are, And one of
those rules is that we think that there is a
absolute history, right, We think that there is a reality
out there and that something actually happens and in the end,
(03:26):
everybody should agree if they're honest observers about what happened.
It turns out that's just not true. And hopefully this
is kind of a topic that a lot of people
have hopefully I think maybe have heard about. It's sort
of permeated out into popular culture a little bit, right, like,
it's the basis of the movie Interstellar. I knew you
were going to talk about Interstellar Interstellar reference, but it's
(03:48):
it's a big example of it in pop culture, right,
Like I think, really Interstellar was a pretty big movie
regardless of what your physicists thinks of it, and my
year and my year physicist is that how used to
refer to me? No, Interstellar actually did a pretty good
job of modeling the relativity portion. My issues with the
Interstellar are more about the time travel and going inside
(04:08):
a black hole. But from the point of view of relativity,
that made a lot of sense. I mean, I thought
it was good that they actually built into the plot
what would happen to various people's clocks. And that's the
key thing, is that relativity is all about comparing clocks.
How fast is my clock going to your compared to
your clock? So we're going to assume that you have
heard of this concept that if you are in a
spaceship going really fast and time will slow down for you.
(04:32):
But we were wondering how many people out there know why?
So as usual, I tortured the undergraduates of you see
Irvine by walking around and asking them random questions without
any preparation. And remember, for those of you who think
that these answers are silly, these are hard questions to
answer on the top of your head, so give them
some slack. It's tortured. This called the physics boarding, enhanced
(04:55):
physics exactly, physics boarding, you know, exactly. I'm gonna cover
your face with a towel and poor physics on your face. Um. Eventually,
I think the undergrads are gonna recognize me, and they're
gonna be like, don't let that guy talk to you.
He'll embarrass you. Run away. We're not trying to embarrass
these people. No, it's great. I think I would be
totally flabbergasted if you asked me these questions out of
(05:17):
the street. All right, I'll plan the ambush you one day.
So before you listen to these answers, think for yourself.
Do you know what time dilation is? Can you explain
why moving clocks run more slowly? Here's what people had
to say. Have you heard of time dilation? Do you
know that clocks that move really fast go slower? This
(05:37):
is trying your information to me. Yes, do you know
why that is? No? I don't know the real reason.
I'm sorry, No, no, there is no time, thanks very much.
All right, So maybe I was a little wrong. Not
a lot of people have heard about this concept. That's right,
and there are even people out there that deny the
(05:57):
time exists. Right, there is no time, there is no
concept of time, or that there was not enough time
to explain it to you. You know, I was so
flabbergasted by the response. I couldn't even formulate a follow
up question. I was like still processing, like, what does
that even mean? Wow? Did he did he just dropped
or she dropped the bike? There is no time? Great phase.
(06:20):
She said it with a lot of finality. Yeah, So
there wasn't a whole lot of opening there for interrogation
or follow up questions. It was like, this is a
clearly known fact, there is no time. Well, I mean
she was just going to rush and she's like, I
have no time for this. No. I think it was
definitely more of the time and time is an illusion.
So an answer, Yeah, I've heard of that time is
an illusion. Yeah, Well, I think we have to do
(06:42):
a whole other podcast episode about what is time and
how does it work? And why does it only go
forward and all that kind of stuff. But this is
kind of related. This topic is related to that idea
that time is not what we think it is. That's right.
We've met a lot of progress in the last hundred
years in understanding time, and we've connected it to space.
You've probably heard the concept of space time, right. We
(07:02):
have three dimensions at least of space and one dimension
of time, and Einstein's relativity tied them together and showed
us how time and space are connected. But that doesn't
mean that time can be simply understood as a fourth
dimension of space. It's much more complicated. It's different from
the other dimensions of space. Yeah, they're all tied together, time, speed, space,
It's all one big molasses of a universe. It's all
(07:26):
one big tangle. The amazing thing is that it actually
all does work. You know. We have this new version
of our understanding of the universe, not that new anymore
as a hundred years old, but this revised version of
our understanding the universe, and it actually hangs together. I mean,
the answers that gives you don't make any sense to
your intuition, Like they fly in the face of what
you think should happen. They require you to like throw
(07:47):
out the way you think the universe works, but they
actually do hang together mathematically and they are correct. Like
every time we make a ridiculous prediction from relativity and
God and check it, the universe is like, yep, that
ridiculous thing actually happens. Well, let's bring it down for people.
So what does it mean for clocks to run slower
when they move fast? So that's that's what we're exploring,
(08:11):
and that's what we're trying to explain, is why when
you're moving really fast, your clock is going to actually
slow down. Right, And so let's be very careful and
how we say this, because a lot of people get confused.
People think that if you are moving fast, that your
clock slows down. That like, if you're looking at your
watch and running that you can see the seconds tick slower.
That's not true. If you're holding a clock and the
(08:34):
clock is not moving relative to you, you'll always see
it moving at one second per second. Okay, no matter
how fast you're going relative to anything else, your clock
always runs the same way. It's not like I get
on a spaceship hit the warp speed and then that's
not what I experienced. That's right. You never notice your
own time changing, right, you experienced time at one second
(08:55):
per second, no matter what. Okay, the thing that does
happen is that clock's moving relative to you. Right. So,
if I'm standing still and Horge has a clock and
he runs because he's a pretty zippy guy, if he
runs a half speed of light because I'm running late,
probably if I notice that he's moving really fast, then
(09:16):
I will see his clock running more slowly. Right. So,
now from his point of view, he will see his
clock running normally, but I will see his clock running
more slowly. If I'm zooming past you and you just
look at my clock as I'm zooing by, it's not
going to be running at the same speed as your clock,
that's right. If I'm watching you as you go by,
and I'm watching your clock's hands tick forward, right, then
(09:37):
they don't agree with mine. Yours mark the seconds more
slowly than my clock does. And that's the key thing
is that the observation of time depends on your relative
velocity to the clock. So it's not that it actually
slowed down for me at least, it just you saw
it run slower. Right. I love how you try to
(09:58):
use the word actually right because you're in your imagining,
there's some true version of the effect of the effect, right,
I'm just this is an illusion or it looks this way,
but it's not actually happening. The problem is there is
no what actually happened. Okay, I observe one thing, you
observe something else. We can both be right even if
those accounts disagree. WHOA Okay, So then I'm running past
(10:22):
you really fast with a clock and you see it
run slower than I do, or it's running slower than
your clock. Then what happens if I stopped, like if
I stopped a few paces after you, Does that mean
our clocks are going to be out of sync? Yes,
our clocks will definitely be out of sync, exactly, And
there's a lot of interesting effects there. Right. So I'm
a I'm standing still from my point of view, right,
(10:43):
and you're running past me. I see your clock moving
more slowly. I see my clock running normally. Right, what
do you see? Well, you see your clock running normally, right,
because everybody sees their own clock running normally. But you
also see my clock running slowly because even though you're
doing doing the running, I'm moving relative to you and
your clock. So then what happens if I stopped, Because
(11:05):
I'm gonna think time moved normally, But when when I
compare my clock to your clock, your clock will have
skipped ahead because right or no, we'll have skip back.
This is really tricky, okay, and we should probably avoid
this topic. Well maybe we won't. Let's dig into it.
(11:26):
This is called the twin paradox, right, This is people say, well,
how do you reconcile this? Right? So the classic framing
of this is, say we're twins, right, and I stay
on We draw straws for Hugas to go to Alpha Centauri,
and you lose. So you have to go to Alpha
Centauri or you win. You have to go to Alpha Centauri.
I stand on Earth and you take a rocket shipped
Alpha Centauri, and I see your clock running more slowly. Okay,
(11:48):
well we sink First of all, we sink clocks like
before I take off. Yes, we're just going to sink clocks.
Time zero start go now, that's right, and I see
your clock running more slowly. Meanwhile, you have telescope. You're
looking at my clock. You see mine running more slowly. Right,
so we both see the other person as aging more slowly. Right.
So now after a hundred years, you see me as
(12:10):
only being ten years older, and I see you as
only being ten years older, right, right, So then I
come back and then what happened? Who's older? All right,
let's break this down really carefully because it is tricky.
So when the Earth twin is watching the ship twins clock,
he of course sees time moving more slowly on the ship, right,
that's relativity. The same way when the ship twin watches
(12:31):
the Earth twins clock, he also sees time moving more
slowly on Earth. So far everything is symmetric, right, and
that's why we like it. Because you could be in ship,
you could be in the Earth. That shouldn't really matter,
right now. If they just kept going this way, nothing
would change. Of course, they would have conflicting views of
whose clock is moving slower, right, But that's okay. In relativity,
you can have two people with conflicting but both correct
(12:54):
views of the same situation, right, Because there is no
ultimate truth, right, your answer is depend on your speed
and your location. So what happens to break the symmetry?
The symmetry is broken when the space twin turns around.
That's an acceleration, right, That changes everything. Well, only the
space twin does any acceleration, so that makes his case different, right,
(13:15):
And during that acceleration, the time on Earth seems to
zoom forward really fast from the point of view of
the space twin. So on the way back, yeah, he
sees Earth moving fast and Earth clock going slowly. But
during that acceleration Earth time has leapt forward really far,
so that when the space twin gets back to Earth,
he's younger than his twin on Earth. Right, And the
(13:37):
reason they're no longer symmetric is that only the space
twin has done any acceleration, so you shouldn't expect them
to be the same from each point of view. It's
the coming back that then lets me stay younger. Yeah, exactly. Okay,
let's let's dig into it even more. But let's take
a quick break, all right. So this idea that time
(14:10):
moves slower when you're going fast, it's what's cool is
that it's always happening, right. It doesn't just happen when
you're going at the speed of light or close to
the speed of light. It's it happens like on your
on a daily basis, That's right. It applies all the time.
It always applies to things that have velocity relative to you.
But it's a really tiny effect if you're not going
(14:31):
really really fast. So when you're driving sixty in the freeway, yeah,
your clock is running a little bit slower, but you'll
never notice it. Wow, but it's there. Like if we're
all feeling relativity and time dilation all the time everywhere,
there's no way to escape relativity. It is everywhere. It's
relatively everywhere. It's absolutely every Absolutely thing about relativity is
(14:54):
that it's everywhere. Okay, so it only happens when you
get it's only noticeable. You're saying, when you're going to
close the speed up. But I heard it happens to
astronauts here on Earth, like the Space Shuttle. The clocks
get out of sync with the clocks on Earth. That's right,
though it takes a lot of precision to measure it.
It's something like every six months or so they lose
(15:15):
like less than a hundredth of a second. So it's
something we can measure, and they have really precise clocks
precisely to measure this to verify these predictions, but it's
not something that people have really like qualitatively experienced. We
haven't had an astronaut come back, you know, deep into
the future and still feel young. And it goes really nonlinear, right,
as you get faster and faster, the effect gets stronger
(15:36):
and stronger. But they did do the twin experiment with astronauts, right, Like,
they sent to one twin into space for a whole year,
and then he came back and technically he was point
oh one seconds younger. That's right. I wonder if he
was the one who was originally born first or the
one who's ovisually born second, because it be interesting and
be like, well you used to be the older twin,
but now I'm the older twin. A should have stayed
(15:58):
up there for ten more years. Do you think growing
up twins still argue by that kind of stuff. Well,
I'm the older brother, so you have to listen to me.
What do you think they're both austers. They're probably trying
to one up each other. Well, yeah, I'm an engineer.
Oh yeah, I'm a pilot. Oh yeah, I'm a I'm
an auster. Not me too, yeah, probably it never ends, right,
(16:19):
but technically that's true. He went out into space, went
around the Earth for a whole year. When he came back. Technically,
time for him moved one hundred of a second slower. Yeah,
I wonder what he's gonna do with all that extra time,
you know, scratches nose or something. Right, it's gone, he
lost it. He used it up. Yeah, But as you
go faster and faster, the effect gets stronger and stronger,
(16:40):
and as you approached the speed of light, time gets
so slow that we can say that if you go
the speed of light, time would actually stop. So if
I'm zooming running past you at the speed of light,
you would see my clock totally frozen, exactly. Now, caveats
are important here. Nothing that has mass actually go the
speed of light, right, and only massless things can go
(17:03):
the speed of light, So you can never go the
speed of lights. Are saying I have much mass, Daniel.
You're pretty massive, dude, Yeah, by which I mean you're funny,
and you're massively awesome thanking, and you have a massively
excellent podcast, and you are brilliant. But you're not entirely
made of light. But that all right, Yeah, I was
just fishing for relative compliments. You're absolutely wonderful. So if
(17:28):
you have somebody in a spaceship and they're going super
duper fast and they're approaching the speed of light, and
time slows down further and further, to the point where
it almost stops. But remember, for them, time doesn't slow down.
It's not like they're living in molasses. It's just our
observation of their time. So if somehow a photon had
a clock, yeah, our view of the photons clock would
(17:48):
be that it was frozen. Right for us, Photons don't
move forward in time. They are frozen in time. But
if you were a photon, what would your experience be. Well,
it's hard to answer that because you're not a photon,
and you have to have that concept of like ascension photon,
which seems impossible, but it's always basically an impossible question
to answer. But they remember, your time always moves forward
(18:10):
um at one second per second. Okay, so that's the
effect of time dilation. Time moves seems to move slower
when you're going faster. So let's get into why it happens.
Do we know why this happens. We do know why,
and it's a consequence of another really strange, counterintuitive thing
(18:32):
that we've observed about the universe that makes very little sense.
So it's a conundrum built on a conundrum. It's a bizarre,
counterintuitive consequence of something really weird about the universe and
really weird about light. Actually, it's the fact that everybody
always observes light going at the same speed, no matter
(18:53):
how fast they're going relative to the source of the light. Okay,
let's break it down what that means. So light travels
through space, but nobody can see it move faster than
that speed that light losing. That's right. There's a certain
speed of light that goes through space three times ten
to the eights per second. And we'll call it the
speed of light. Of course. And if I'm standing on
(19:14):
a planet and I turn on a flashlight, then the
light leaves me and travels at three times tended them
per second away from me. Right, And if I'm and
if you're shooting it at me, then I see moving
towards me at three times ten to the eight meters
per second. Okay, that's right, even if you're even if
you're on a rocket ship and you're moving towards me, right,
(19:34):
say you're moving towards me at half the speed of light.
I shoot my laser beam at you're my flashlight at you,
you still see that flashlight, the light from it coming
at you at the speed of light, right, And that's
different from you know, sound waves or rocks or something,
or a baseball. Like that's weird. Right, Like, if you
throw a baseball at me and I'm running full speed
towards you, that baseball is going to be it's gonna
(19:57):
look like it's moving really fast towards me. That's right.
If I threw a baseball at you at a hundred
miles an hour, which I promise I can totally do,
this is very realistic, and you're running towards me at
fifty miles an hour, which I'm sure you're totally capable of. Um,
then obviously I see the baseball is moving away from
me at a hundred miles per hour, but you see
the baseball is coming towards you at a hundred fifty
(20:17):
miles per hour. Right. That's the way it works for
normal things. But you're saying that if that baseball instead
of a baseball, it was a beam of light, that
wouldn't happen. That's right. Light doesn't follow those rules. Everybody
who measures it measures it as traveling at that fixed
speed of light. No matter what. I mean, there are
caveats here, like it slows down and it travels through
air or water, whatever, But let's just talk about it
(20:38):
in a vacuum. The crazy thing is that doesn't matter
how fast you're going, everybody sees light is traveling at
this maximum speed of the universe, no matter what. And
that's crazy, right and right there is where your concept
that everybody sees the same thing the same way, or
that there is one absolute truth that we're all observing
in different ways breaks down because your description of events
(21:00):
and my description events are going to be very different.
If we see light moving at different speeds. It's the
weird It's like a weird rule of the universe. Not
that light something can't travel at this faster than the
speed of light. It's a weird rule that says nothing
can be seen to travel faster than the speed of light,
not even light. That's right. Light always travels at the
speed of light, and nothing can travel faster than the
(21:21):
speed of light. And these two things are connected, right,
because if light operated the same way as baseballs, then
you could see light moving as faster than the speed
of light just by moving towards me. When I'm shooting
a laser at you, right then that light would be
moving relative to you. It's faster than the speed of light.
But it doesn't right, you move towards me, and you
still measure light as moving at the same speed. It
doesn't matter if you're running away from me or towards me. Okay,
(21:44):
So if you shoot a light beam at me and
I'm running towards you really really, really fast, You're going
to measure the light moving at the speed of light.
And I should measure the light moving faster towards me.
But I'm also going to measure it moving at the
speed of light, even if I'm moving towards you or
away from you or to the side of you exactly,
no matter how fast and moving, I'm always going to
(22:05):
measure it and moving at the speed of light exactly.
And that's crazy, right, It's bonkers, doesn't make any sense.
And it's a famous experiment, Michael said. Morally, the experiment
that that did this, they shot beams of light in
two directions, and because the earth is moving, they figured, well,
one of them is going to go slower than the
other one because the earth is moving, right, So they
did the experiment different times of year, and the light
came back took the same amount of time to go
(22:28):
in these two perpendicular directions every single time. And that
told them that that the speed at which light travels
is not dependent on how fast you are moving the
observer is moving. It's always the same speed. It's crazy.
Is it like some kind of um just like fundamental
limit in this stuff of the universe itself? You know,
(22:49):
like nothing can propagate through this thing we call space
faster than the speed of light. Is that kind of
what it's related to. Yeah, definitely, But it's it's a
deep question and we don't have a solid answer to too.
Why does light always travel at this speed regardless of
the speed of the observer. We don't know the answer
to that. That's just like Einstein postulated that. He said, Okay,
let's start from this crazy assumption and build the math
(23:12):
up from there, and if everything then works, and we'll say, well,
that assumption must be true. And if you start from
that assumption, you get all sorts of crazy predictions which
all turn out to be true. Right, So that is
a deep truth of the universe. But the answer is,
we don't know why. We don't know why light always
travels at the same speed, or a light is always
observed at the same speed, no matter who is doing
(23:33):
the measuring and how fast they're going. It's a weird
rule about the universe. It's a weird, weird rule. And
when I meet the people who wrote the simulation, I'm
gonna ask them, why did you do that, man that
made everything so complicated about the universe? One star on
Yelp for that for that bit of load. No, I
think it's it's there's a fascinating angle there, you know,
(23:56):
because we grew up sort of as a species in
an environment where nothing was near the speed of light,
and so we never noticed this, and so we assume
things like everybody's clock runs at the same time because
we've always thought it did. And so they're not allowed
us to assume things like there must be some sort
of absolute sense of time and absolute history and there's
a really universe out there, and this is I mean,
(24:18):
this shakes the very foundations of how we even think
about the universe that's out there and whether it makes
sense at all. So it's it's pretty crazy stuff. I
feel like we're kepped with weirdness at both ends of
the specter. What I mean is like, if you move
it close to the speed of light, things get weird.
But also if you don't move at all, things good weird.
Right because of quantum physics and the uncertainty principle. Right,
(24:41):
Like if you try to discertain things that zero velocity,
things get weird too. Things always get weird. I think
that's the takeaway, right. The universe is weird, like I
think I said last week, is weirder and stranger and
hotter and nastier and wetter than you could ever even imagine.
And the I think the craziest surprises about the way
the universe work are still yet to come. You know,
we've made these discoveries that showed us to the universe
(25:02):
is so different from the way our ancestors have imagined.
There must be more of those discoveries coming. It's certainly
not the case that we figured them all out. There
are crazy revelations in our future. Well, that's good for
our podcast topics, that's right. Okay, let's so let's get
into how this affects time. But first let's take a
quick break. So we know that the speed of light,
(25:37):
you can observe going anything faster than the speed of light.
So how does this affect how we view time or
how we experience time? Right, So it comes directly the
fact that light can't travel at any other speed is
what directly affects how time passes and how we measure time.
And it can be a complicated topic, but I think
the best way to do is to think about maybe
how a simple clock operates. So let's build a simple
(26:00):
clock that uses light. You mean like an Apple watch? Er, No,
I mean let's imagine that you're you have to build
a clock, and all you have as a laser, right,
And so you know, for example, that light takes about
ten nanoseconds to go ten feet right, it goes about
one ft every nanosecond. So what you do is you
(26:20):
measure very precisely, you know, ten ft. You put a
mirror at the end, and then you shoot your laser.
You shoot a laser pulse, and you say, however long
it takes to go um there and back. That's two seconds, right,
ten feet there and ten feetback. And you set up
the mirrors in the floor and in the ceiling, right,
so that being is bouncing up and down right right,
(26:42):
So you you shoot the laser up towards the ceiling
and back, and you know how far it is to
your ceiling and how far it is back to your floor,
and so you say that's twenty ft, so that's two
seconds round trip. Oh I see. So then the way
you turn that into a clock is you count how
many times the light bounces up and down the ceiling,
and that sort of gives you a sense of how
(27:04):
time is moving. That's right. You want to say, well,
how long does it take my cat to finish his lunch?
And so you count how many times it takes the
laser pults to go up to the ceiling and back,
and that's the number of two second intervals it takes
your cat to eat his lunch. Okay, so that's where
it seems like a pretty impractical clock. But hey, this
this is how we do things in physics. Man, be like,
(27:26):
can we build this thing using lasers? What's the most
inconvenient wristwatch we can build with lasers at home? The
way we toast bread for breakfast as we use lasers,
of course, and cats cats, I feel like it's a key. No,
that's just how we toast our cats. Man. Let's say
(27:47):
you cooked the food for the I want to I
want to officially distance myself from that joke, because nobody
should ever fire a laser at a cat. Well, you
can fire a laser near a cat to entertain it,
of course, but I don't actually hit your your cat
with a laser, please, oh man? All right, So now
we have our clock, right, that's the way a clock
that uses light. Okay, so this is a thought experiment.
We're going to build a clock where we measure time
(28:07):
by measuring counting how many times it bounces off the
ceiling up and down. That's right, And it doesn't have
to be a thought experiment. You've got mirrors, you've got lasers.
Go ahead, build your self o'clock. Um. But the interesting
thing is that what happens if you put that clock
on a spaceship right right on the train. Let's make
it even more inconvenient. Let's put this on a spaceship.
(28:29):
And so that's when things start to get interesting, right, Like,
that's when we start to see how time slows down. Yeah,
Or let's put the clock on the back of your cat, right,
and then see what happens when you're well, I guess
you need the ceiling on the back of the cap.
Maybe that doesn't. Actually, I feel like trying to get
a cat to do what you want is even more
difficult than getting on a spaceship. Einstein had no idea
how to get your cat to do what you want.
(28:50):
You can master. He was the master of the cosmos,
but not of cats. All right. So we have a
clock where you measure time by counting how many times
it bounced off the ceiling on the floor, and we
stick it in a spaceship, and then we start going.
What happens then, right, So if you're in the spaceship,
nothing changes. It doesn't matter that you're going at half
(29:13):
the speed of light or nine tenths of speed of light.
You're in the spaceship. You have no velocity relative to
the clock. So things work the same way on the
spaceship for you as they did when you tested your
clock in your living room. As you just see the
beam go up and down, bounce up and down, and
you count, and that's your time. That's right. And since
you brought your cat along, it takes your cat the
same amount of time to eat his lunch in your
spaceship as it does at home, assuming he's not wearing
(29:35):
a silly cat space suit. The interesting thing is that
when since you left me, Um, you left me on Earth,
you didn't invite me on your awesome spaceship. Thanks by
the way, that would have been too inconvenient. And I'm
so heartbroken that I'm spying on you. Have a massive
telescope and I'm watching your cat eat lunch on a spaceship. Um.
Now I'm looking at your clock, okay, and I'm wondering
(29:56):
how long does it take his cat to eat lunch, etcetera.
And I'm watching your clock. Okay, you're trying to count
how many times it bounces off the ceiling to exactly.
So I see the laser pulse go up, and I
see the laser pulse come down. Right. The problem is
that I don't see the laser pulse going ten feet
up and ten feet down. I see the laser pulse
(30:18):
as going further because you're moving, which means the laser
pulse is not just going up and down. It's also
going sideways in the direction of your motion. So you
have the up and down and the sideways. So the
light for me, the laser is going in a diagonal right,
diagonal up hit the ceiling, and diagonal back down to
hit the floor, because you see it hit the ceiling
and then on its way down, the spaceship is awesome moving,
(30:41):
so it's kind of moving diagonally to hit the floor
where it's going to be exactly. And so the mirrors
and the clock move with the laser. Obviously they're all
going at the same speed sideways, and so the laser
beam hits the mirror on the top and it hits
the receiver or whatever on the bottom. But I see
you traveling further than you do, right, And this is
where the the absolute speed of light kicks in because
(31:03):
you say, okay, travel ten feet, right. It traveled those
ten feet at the speed of light, So I know
it takes ten seconds, but I see it traveled further. Right,
Depending on how fast you're going, it could have traveled
like fourteen fifteen feet, right, because light always travels at
the same speed. Right, I see it taking longer because
it's gone further and it can go faster than the
(31:24):
speed of light. It can go faster than the speed
of light. So I see your clock running slower. It
literally takes longer to count off seconds for you because
the clock is built on the premise that it takes
light a certain time to go a certain distance. But
now that distance is further and the speed can't change
because it's going up and down. The light is going
up and down, and it's trying to go forward to exactly,
(31:45):
so it has to take longer exactly. Now, if you
built this same kind of clock using something else, like
sound waves, right, you had a speaker, then this wouldn't
happen because the sound waves don't have that same property
that they always travel at the speed at a certain
speed relative to observers. Right. Um, sound waves, as we
know from Doppler shifts, change their speed based on how
(32:06):
fast you're going. So if this is a sound wave,
if we use sound waves instead, then I would just
see the sound waves is traveling faster. But light can't
do that. Light always travels to the same speed no
matter who the observer is. So it slows your clock down.
So let's let's go back a step. Um. Okay, So
so I'm gonna be in my spaceship counting how many
times it bounces off the ceiling, and for me, it's
going to be like bounce, bounce, bounce, bounce, bounds. But
(32:29):
for you, you're saying because it has to travel up
and down and forward. You're going to count it slower,
right like for you, it's going to be bounds, bounds, bounds,
And so that's kind of the definition of time. Yeah, exactly, exactly,
and any clock. And you're thinking, okay, well, that's just
one example, right, But the same argument holds for if
(32:51):
you're going in another direction, right, you don't have just
be going sideways. If you're shooting in the direction of
the mirror, right, then the light has longer to go
in one direction, but it can't make up the time.
And if you're going at an angle, the same thing happens.
And also for any clock. This is just one example.
It's the clearest example because we've built it out of
something that only uses light, but the true the same
(33:13):
happens for every physical process, for any kind of clock. Okay.
So that's kind of the explanation is that time, the
definition of time is sort of tied to the speed
of light, and the universe has this weird rule about
the speed of light, which is that nobody can never
see it move faster than the speed of light. That's right,
And the other important thing to understand is that your
(33:35):
definition of time and your definition of what happens depends
on how fast you are going. There is no absolute
sense of time. It's not like the universe has a
big clock out there and it's keeping track of what's
going on, and we're trying to make measures of measurements
of it, and they're kind of sloppy sometimes when we
get them wrong, Like there is no absolute sense of time, right,
And you can do crazy experiments where you know, depending
(33:57):
on how fast people are going relative to experiment, they
can see the order of events changing, like I can
see A happen before B, and you can see B
happen before A because you're zooming in the other direction.
And you might think, well, that's impossible either A happened
before B or BE happened before A. There is a
real truth, right, answer is there is no truth. The
(34:17):
truth is not out there. X Files was a lie.
So I feel like we should get into that in
another episode. But I think the conclusion we're reaching here
is that basically there is no time. After all, there's
no time to talk about this anymore. And that's true. No,
there is time, but time is a different thing that
you than you thought it was, right, It's something weirder
and more malleable now in our safe, little slow worlds,
(34:40):
it doesn't. You can pretend that time is the way
you thought it was right and you'll get by just fine.
But in reality, if you want to understand the way
that that physics works at its deepest level, whether universe
is actually put together, what the real rules are, that
it turns out time is really different than you thought
it was. So that person who answered there is no
time sort of, I feel like she skipped ahead to
(35:02):
the end of this count. She's probably a physicist visiting
from the future from Alpha centri. Is it her? Or
she's your cat evolved for thousands of years into a
future physicist and then traveled back in time to deliver
that message. I think she's your twin Danny. Maybe she is,
maybe she is. Well, speaking of time, I think we're
(35:25):
out of time for this episode, so it's time to
wrap it up. Thank you everyone for your patients and
for listening, And if you have questions about how things
work in the universe or anything else crazy, please send
them to us on Twitter or email us at feedback
at Daniel and Jorge dot com. We love listener questions. Yeah,
hopefully We made time move a little bit faster for you,
(35:45):
whatever it is you're doing, unless you were bored, which
is probably canceled out. If you still have a question
after listening to all these explanations, please drop us a line.
We'd love to hear from you. You can find us
(36:07):
at Facebook, Twitter, and Instagram at Daniel and Jorge That's
one word, or email us at Feedback at Daniel and
Orge dot com.
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Daniel Whiteson
Kelly Weinersmith
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