Why does gravity slow down time?

Daniel and Jorge explore why time slows down when space is curved

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Episode Transcript

Speaker 1 (00:01):
Hey, Jorhan Daniel here, and we want to tell you
about our new book. It's called Frequently Asked Questions about
the Universe because you have questions about the universe, and
so we decided to write a book all about them.
We talk about your questions, we give some answers, we
make a bunch of silly jokes as usual, and we
tackle all kinds of questions, including what happens if I
fall into a black hole? Or is there another version

(00:22):
of you out there that's right? Like usual, we tackle
the deepest, darkest, biggest, craziest questions about this incredible cosmos.
If you want to support the podcast, please get the
book and get a copy, not just for yourself, but
you know, for your nieces and nephews, cousins, friends, parents, dogs, hamsters,
and for the aliens. So get your copy of Frequently
Asked Questions about the Universe is available for pre order now,

(00:46):
coming out November two. You can find more details at
the book's website, Universe f a Q dot com. Thanks
for your support, and if you have a hamster that
can read, please let us know. We'd love to have
them on the podcast. Hey Daniel, I've noticed something pretty

(01:10):
strange about how time works. Oh yeah, you have time
to make your own theory of time. Well that's the thing, right, Like,
I noticed that time seems to speed up as you
get closer to a deadline. That's true. Deadlines seem to
be infinitely far in the future, until all of a
sudden there on top of you. Right. I think the
only thing that can explain it is that time itself

(01:31):
bends when it's closed to some procrastination. Well, time does
slow down near a massive object, like a black hole.
I think maybe I just need less massive deadlines maybe,
or maybe your two do list is so dense it's
becoming a black hole. Definitely sucks you in. Hi am Jorge.

(02:04):
I'm a cartoonist and the creator of PhD comics. Hi
I'm Daniel. I'm a particle physicist and a professor you
see Irvine, and I'm just part of Jorges to do list.
That sounds kind of inappropriate, Daniel, I mean I have
to be like Daniel, or have to be Daniel. I
mean that I just contribute elements to your to do list. Oh,
I see you add to my things to do. I'm

(02:26):
on your to do list. But anyways, welcome to our
podcast Daniel and Jorge Explain the Universe, a production of
My Heart Radio, in which our to do list is
to explain the entire universe bit by bit, concept by concept,
puzzle by puzzle to you, our wonderful listeners who deeply
and desperately want to understand the nature of the universe.

(02:47):
We find ourselves in the whole project of physics, starting
with ancient man looking up at the sky and extending
to the Greeks trying to understand the nature of reality.
Ends here on the podcast when we try to break
down on the nature of the universe and explain all
of it to you and hopefully a timely matter, right,
because you know, time is money and money is gravity.

(03:11):
Money is dough and everybody needs it. Yeah, sometimes we
eat too much dough. But yeah, it is a big,
beautiful universe and we like to talk about all the
things in it, even time itself, because time is part
of the universe. Right. Time is part of the universe
that we don't understand. Is it a fundamental element of
the universe? Is it something that bubbles up and just
sort of appears from other fundamental things like ice cream

(03:34):
and lava and hurricanes? Or is it really deeply ingrained?
And on this podcast we don't shy away from asking
those really big, deep and difficult to grapple with questions,
you know, like what is time anyway? How do you
even define it? How do you even ask crisp and
precise questions about this slipperyst but most essential of concepts. Yeah,

(03:55):
because time always seems to slip by you no matter
what do you do, and asking these questions, it's not
just in ourder to do list, but it's also in
our want to do list, right, Daniel, I mean, that's
kind of what you're getting, kid. That is what I
get paid for, not on a day to day basis,
and mostly I'm cracking open particles at the large age
on collider, But these are the big goals of physics,

(04:15):
is to understand the nature of our experience. You know,
some people think physics is sort of like abstract and
separated from humanity. But it's taken me a long time
to realize that it's the most human of sciences because
it asked these questions to the very core of the
context of what it's like to be alive. You know,
you notice things slipping into the past through this weird

(04:35):
instantaneous slice called the present. It's definitely something we'd like
to understand. Are you saying, physics out humanities? The humanities?
Daniel out philosophies the philosophers. Yeah, you know, I used
to think that physics was the most interesting science because
it was probably the most fundamental, the most universal, that
somehow it escaped humanity, that if aliens came, they wouldn't

(04:56):
be interested in our biological advances because they wouldn't be
relevant to them, but they would be interested in what
we've learned about physics. I'm not so sure about that anymore.
And can hear all those mathematicians out they're laughing at
you right now. You're like, you're pure, We're the purest,
even mathematicians. Man, Mathematics is just codification of the logic
and human brains. And I suspect that aliens might not

(05:18):
even do math, and if they do, we might not
even recognize it. Oh man, you're saying physics supersedes math
from a human point of view. Oh boy, we just
started in an academic warrior. You just did that. That's
not what I'm saying at all. I'm saying that everything
we do is based in human thought and contextualized by
human questions. But that doesn't make it worthless. It means
that we get to ask really interesting questions about what

(05:41):
it's like to be alive. Yeah, and so sometimes those
questions get into the idea of time itself, because I
think maybe most people have a conception of the university's
sort of existing outside of time, or like it's a
universe moving through time, but actually physicists think of time
as part of the universe, right, it's like another thing
in it. Yeah, we don't really understand. Physicists like to

(06:02):
divide elements of the universe into things that are fundamental,
meaning they're like basic, they're essential, they have to be there.
They define what the universe is, and then other things
that are emergent that sort of come out of the
interplay of those fundamental objects. And we don't know if
time is fundamental, if it's like absolutely essential it's part
of the nature of the universe, or if it bubbles

(06:24):
up from something else. We don't know if space and
matter sort of sit in a framework of time which
is external to them, or if the whole thing is
just you know, comes up from some other weird, deep nature.
We haven't even imagined, like, is it hard coded into
the circuitry of the universe or is it like a
program running on top of the chip of the universe
that's kind of what you mean, right, Or is it

(06:47):
just the way the humans think, you know? It might
be that our tendency to think in terms of stories
and narratives which have cause and effect, might bias us
to see things as flowing forwards in time, when in
reality the truth might be much more complex. We just
an episode about how causality, cause and effect might not

(07:07):
even be an essential element of the universe, and so
it might be that time itself could be something that's
just sort of very human. Yeah, it's sort of like
this podcast. We have no narrative, right, We are just
jumping around in time to whatever comes into our minds here.
But it's these kind of basic questions that give us
a launching off point for asking questions and doing studies

(07:29):
and trying to like find a way to grapple with
these things scientifically, because we could sit here and smoke
man in appeals and talk about the nuture of time
forever without making progress. But we want to do experiments.
We want to use science to add to our body
of actual knowledge about time. Yeah, so we'll be exploring
a little bit about this weird concept of time and

(07:50):
especially about how time behaves according to our theories about
the universe. So today on the podcast, we'll be asking
the question why does gravity slow down time? Or should
I should I ask it slow down time? Or maybe

(08:12):
you should use your gravest voice, you know, your James
Earl Jones voice. You're trying to make a gravity joke.
That's kind of heavy, man, don't make light of the situation.
I'm a massive fan of that movie. You know. Now,
this question kind has two components to it, Like you
just said, it has gravity and time in it. And
because we know from physics that somehow gravity affects time
or is related to time, because in the time can

(08:34):
be affected. It's not like some sort of absolute thing.
That's right. We do not have like a single clock
for the universe. The simplest model of how time works
might be imagining that the universe all over the place
is in one state. You know, particles are going in
some direction or they're in some location, and then things
sort of tick forwards and everything takes a step forward
in unison and in that picture, the whole universe has

(08:57):
like a single clock. But we've learned over the last
time of years that that conception of time is not valid.
That time flows differently in different parts of the universe,
and differently four different observers and moving at different speeds.
So time is much weirder, more local, and less universal
than we ever imagined. In addition, it's also weirdly bent

(09:17):
by heavy objects. Yeah, things that are really heavy, like
black holes or even just our planet. They have gravity,
and that somehow makes time slowed down. Now, that's a
that's a pretty weird concept. Bread. I guess that was
in that movie Interstellar. It was exactly in that movie Interstellar.
Every time they visited a really heavy planet or came
near a black hole. When they left, they found that

(09:40):
the rest of the universe had experienced a lot more
time than they have. And so I think a lot
of people are maybe familiar with this part of relativity,
where like, if you're moving past, time slows down. But
also it also happens when you're near a heavy object.
That's right. These are two completely separate effects with different
sources and importantly different mechanisms and different consequences. Yeah, so
as usually when we were wondering how many people out

(10:01):
there even knew that gravity slowed down time or much less,
I have thought about why it slows down time. So
Daniel went out there into the internet to ask people
why does gravity slow down time? I like the way
you say I went out there into the internet. It
makes me feel like I got sucked into my computer
and went and visited these people like tron like the

(10:22):
m eighties movie plotline and you got, you know, salkt
into your CRT monitor. Yeah, I'm writing those tron cycles
around the Internet, gathering information from our lisitors, dodging calls
and tweeting left and right. But if you out there
would like me to beat myself into your inbox with
crazy questions about the nature of the universe so you
can hear yourself speculate about them on the podcast, Please

(10:45):
don't be shy. Right to me two questions at Daniel
and Jorge dot com. Yeah, and you actually answer every email, right,
I do answer every email with every question from everybody.
You take the time. It slows me down, but I
love it all. Right. Well, here's what people had to say. Well,
gravity is basically the same as acceleration, and when you're accelerating,

(11:11):
the speed of light stays the same for everybody, and
so it must be time that's slowing down. And when
we're going at our normal speeds, you don't really notice it. Um.
So gravity being equivalent to acceleration causes the same effect.
Gravity slows down time because as particles with mass moved

(11:33):
through the Higgs field, they're slowed down. The stronger the
Higgs field, the gravity is the sole of the quarks
and leptons move, causing quantum interactions to take place at
a different rate depending on the strength of the field.
Scaled up into the macro world, that's what we experienced
this time. We would never notice if our particles were
moving at a faster slower speed, if everything we observed
was experiencing the same Higgs field along with us. I

(11:54):
guess that the reason it's because of gravity bench down
of pace time and thinks the length of space faith
changes in order for the speed of life we're being constant.
Was the time of flowing differently. First of all, time
is like space dimensions, so the gravity can affect time.

(12:19):
But how I'm thinking right now, like the gravity does
to also the objects that contracts them. And this is
what I'm thinking, That it contracts time. It squeezes it
like the space and doing this it is slowing it down.

(12:41):
I guess gravity could slow down time because gravity makes
those wells and like the space time fabric, and so
as that material of the universe gets stretched out, the
time would slow down as it gets deeper, sort of

(13:02):
like if you're driving over hills versus driving over flat things.
I can only imagine it's got something to do with
space time. Like gravity isn't just a force, but as
Einstein says, it's a warping of space time. And if

(13:22):
you've got a ton of gravity staying a black hole,
warping space space is not really a thing, but space
time is a thing. So you can't affect one without
affecting the other, is what I'm guessing. I know time
moves slower relatively speaking for someone that travels faster, So
maybe gravity slows town time because it also necessarily makes

(13:46):
an objectile person move faster. All right, some pretty good
answers here. A lot of people sort of knew that
time can slow down, and I guess they assume that
gravity is someone related to really activity and moving fast
and so why not. Yeah, a lot of really interesting
and insightful and thoughtful questions here, some confusion about the
role of the Higgs field, but also a lot of

(14:08):
good concepts about the connections between space and time and
how masses have to bend both of them. All right, well,
let's get into this idea of time slowing down, and
I guess let's recap maybe the one people are most
familiar with, maybe, which is the one that when you're
moving fast and especially close to the speed of light,
time slows down for you. So maybe walk us through that,

(14:28):
and then we'll get into the one about gravitation. That's right,
So the one you're probably more familiar with. We call
velocity time dilation. This is something that happens in special relativity.
And to think about this, you should imagine an empty universe,
one without really massive objects that are gonna make space
curvy and all sorts of weird stuff. In this universe,
light travels in straight lines, and spaceships zoom around and

(14:51):
all the time. Dilation just comes from the relative velocity
of objects. And the most important thing to remember is
that movie clocks run slowly, so people often make the
mistake of saying, oh, I'm going fast in a spaceship,
so my time should slow down, right, Well, it's only
moving clocks that slow down. So if you're in a
spaceship and you're holding the clock, then the clock is

(15:14):
not moving relative to you, so you're not going to
see it slow down. So you never experience velocity time
dilation because you're never moving a relative to yourself. Somebody
else out there on a planet that you're zipping by
could see your clock running slowly because your clock is
moving for them, and so moving clocks run slowly, meaning
that your clock would run slow. You don't experience it,

(15:37):
but they see your clock moving slowly. Right, Like, if
your time is moving slowly, you don't notice it because
you know your brain is also sort of moving slowly
in a way, right, So like everything about you is
moving slowly as well, and so you don't notice that
you're actually moving slowly. But even that suggested some sort
of like universal picture of what really happened. And in

(15:57):
the universal picture, it makes sense for you to feel
like time move normally even though it actually moved slowly.
But there is no like what actually happened. Some observers
seeing you go by the planet at the speed of light.
They see your time as moving slowly. You see your
time as moving normally. You can try to unify those
into one picture of what actually happened, but there is

(16:18):
no what actually happened. There's just what different observers observe,
and what you see depends on where you are and
how fast things are moving relative to you. For example,
if you're on the space ship and you're looking at
the clock on Earth, you see Earth moving past you
at high speed, and so you see Earth clock running slowly.
So Earth sees your clock running slow. You see Earth

(16:40):
clock running slow. What actually happened, Well, both of those
things happened. Is just what happened depends on where you are.
I guess maybe stem me through it. So I'm here
on Earth, I'm watching you on a spaceship go by
the speed of light, Like, what does it mean for
me to see your time slow down? Like? I see
your the clock inside of your spaceship take but it's

(17:00):
not taking as fast as my clock exactly. So you
get a telescope. It's super powerful. So you can look
at a clock that's inside my ship and you watch
it and you see it's ticks going and you compare
it to a clock that's right in your hand, not
moving relative to you. And so every time my clock
ticks on the spaceship, you see the clock in your
lap taking ten times. So time is moving faster for

(17:22):
you in your lap. Then you see it moving for
me on this ship. And this, of course already takes
to an account the fact that it takes light time
to get to you to the telescope from the ship.
We sort of factor that out already. So what does
it mean. So I have a telescope and I'm pointing
it at you, but you're zooming by, so I have
to kind of track. You have to move my telescope,
and so I track you as you're going by, and

(17:44):
I'm moving my telescope and I'm measuring your ticks and
they're not taking as fast as my clock. That's right.
You see my clock as running slowly. So you see
me aging slowly, You see me moving slowly. You see
me like waving back to you in super slow motion.
So you see my clock is running slow relative to
your clock. And that's the sort of a consequence of

(18:05):
just how the universe is or or to the somehow
the limitations of the speed of light. Yeah, it's really interesting.
You can start from lots of different places to derive this.
You can say like, well, we've seen that nothing can
move faster than the speed of light. That's a hard
limit on the speed of information. And from that you
can derive these things, these time dilation effects, and you
can walk yourself through an example. We actually have it

(18:26):
worked out in detail in our book We have No Idea,
a Guide to the Unknown Universe. And you can think
about how a photon clock would tick in a spaceship
as it moves up and down, and if it's moving
really fast and it has to go like a little
bit diagonal for example, And because light can't move faster
than the speed of light, when it moves on a diagonal,
it takes longer to get from one side of the

(18:48):
clock to the other. So it's basically the constancy of
the speed of light and the fact that nothing, even
light can move faster than the speed of light directly
lead to this consequence. The time goes slower for moving
clock right. And so there's this sort of famous scenario
called the twin paradox for the twin experiment where like
you take a pair of twins here on Earth, and
you put one of them in a spaceship that goes

(19:10):
out into space at the speed of light and then
comes back, and supposedly when they come back there are
a different age than the one that stayed on Earth.
This is a wonderful paradox because it gets to the
heart of like what actually happened. Because in the example
we're talking about, it feels awfully symmetric, right like I'm
looking at you through my telescope and I'm seeing your
clock take slowly. But maybe on the ship you're also

(19:33):
looking at me on Earth and you see my clock
taking slowly. So you want to feel like, well, what
actually happens? And the way to bring that to a
point is to like bring those two people back together.
So if one twin goes off on their spaceship twin
on Earth sees the spaceship twin aging slowly, and the
spaceship twin sees the Earth twin aging slowly, and so
you want to feel like, well, which one is actually younger?

(19:55):
And so you turn the spaceship twin around bring it
back to Earth, and you ask like, well, which and
is younger? And what you discover is that the spaceship
twin is younger, and so that sort of breaks this symmetry.
And you wonder, like, hold on a second, if this
is supposed to be symmetric, if it just depends on
relative velocities, why is it that one of them is
now younger than the other. So that means it's not

(20:16):
just like a perception thing, it's like time actually slowed
down for the space twin. In that case, it is
because they've broken special relativity. One of the rules of
special relativity is no acceleration. You can fly at high
velocity and then you can make all these measurements and
do these calculations, and things are just as we described.
But as soon as you accelerate, then you're out of

(20:37):
the bounds of special relativity. It's something you can do.
We can you can do it, we can calculate it.
But it makes things more complicated, and the simple rules
we described earlier of moving clocks run slow get much
more tricky. So here, for example, when the spaceship twin
turns around to come back to Earth so that he
or she can compare her age with her twin, then

(20:58):
she's accelerating because changing your direction means accelerating, And what
happens when you accelerate is you break the symmetry. Now
one of the twins is accelerating, the other one is not,
And when you accelerate, weird things happen to time. Specifically,
when the twin in the spaceship turns around and accelerates,
time jumps forward for the rest of the universe. So
time runs a little slower for the accelerating twin jumping

(21:21):
forward for the rest of the universe, which is why
the twin in the spaceship now is younger when they
arrive at Earth than the twin that stayed home. You
just kind of accelerated a little too fast there for
my brain, I guess. One question is, but isn't acceleration
also relative? Like if I'm accelerating away from you, I
see you accelerating away from me, So why is an
acceleration also like kind of symmetric. And the second question

(21:46):
is you're saying it's the acceleration that causes time to
slow down, so that's sort of a different scenario. Yeah,
these are great questions. Acceleration is actually absolute. Velocity can
only be measured relative to other stuff, like if you're
an empty universe. You can't measure your velocity because there's
nothing to move past. Right, velocity is only defined only

(22:07):
has meaning relative to other objects. That's not true for acceleration.
Acceleration is something you can measure, even in an empty universe. So,
for example, put yourself in a spaceship. You're in an
empty universe. If you're moving, your motion has no meaning.
There's no experiment you can do inside your spaceship to
measure your actual velocity because there's nothing outside to measure

(22:29):
relative too. That's not true for acceleration. You can measure
inside your spaceship whether or not you're accelerating. For example,
you can tell do I feel a force by being
pressed by one side of the spaceship. You'll feel those
g forces if you're accelerating. So acceleration is different from velocity.
You can't have an absolute acceleration, you can measure it.
And so that's why the rules are different for acceleration

(22:51):
and for velocity. And so then you're saying that the
twin who went out into space will actually be younger
when they come back, they will actually be younger. Exactly,
So when the twin comes back to Earth and is
now in the same reference frame moving no velocity relative
to the other twin. Then you can ask real questions
about in this reference frame, who is younger and who
is older. And because the twin that went into space

(23:13):
also did some acceleration, their time slowed down a lot
during that acceleration, or equivalently, time for the rest of
the universe jumped forward during that acceleration. So that breaks
the symmetry because only one twin accelerated, and so the
twins stay at home actually is older. And you know
this isn't just like a thought experiment. There actually are
a pair of twin astronauts. One of them went and

(23:35):
spent a lot of time up in space and the
other one didn't, and they've compared the two. Really all right, well,
let's get into that real life experiment and then let's
talk about how gravity changes time. But first let's take
a quick break. All right, we're asking the question why

(24:02):
does gravity slow down time? And we were talking first
about the twin experiment where you send a twin out
into space, they go really fast, they come back and
they've aged less. And this age and you're saying is
actually due to the acceleration, it's not actually due to
the speed. Right, there are two effects there. There's the
velocity time dilation, which is the one you're very familiar

(24:22):
with where moving clocks appear to be slow. But that's
not a universal phenomenon. It depends on who is doing
the observing and their relative velocity. Acceleration, however, is different,
and the acceleration does cause an actual slowing of time,
which could be measured by everybody because it's not symmetric,
it's absolute. You can measure somebody's absolute acceleration and that

(24:44):
makes them different, so it breaks the symmetry. So really,
when people say going fast slow is down time, we
really we should be saying accelerating fast causes time to slow.
And is that something that just somehow changes time or
is it because you're pushing all of the particles and
somehow that's blows how they interact, or how how do
you explain exploration changing time? Well, first of all, it

(25:05):
is still correct to say that moving fast slows down time.
It's just that it's slows down time only for observers, right,
observers moving fast relative to those clocks. It's still true,
it still happens. It's it's not like just a perception issue.
It's a true thing about the universe. Acceleration slows down
time in a different way. It's a different mechanism. It's
much harder to understand in terms of it means like

(25:25):
ticks on the photon clock. But you can see it
also comes as a consequence of the speed of light.
All right, well then, so that's acceleration and time dilation
because of exceleraation and because of going close to the
speed of light. But the one we're talking about today
is the one due to gravity. So whenever you're next
to something that's really heavy or massive, time also slows down.

(25:47):
But does it slow down in the same way that
acceleration slows time down, or does it slow down in
the same way that going at a constant speed slows time?
Do you know what I mean? Like is it observer
base or is it actually like time slowing down? Great question.
And so gravity slows down time where the curvature of
space slows down time, and this effect on time is

(26:08):
the same as acceleration slowing down time. And in fact,
it's sort of a deep idea here because in general relativity,
one of the whole inspirational ideas of general relativity is
that gravity is equivalent to acceleration. You know, the experiment
we talked about a minute ago, like, if you were
in space, could you tell if your spaceship was accelerating,

(26:28):
You could, and in fact it would feel like you
were being pressed against one wall of the ship, or equivalently,
it would feel like you were standing on a planet
with gravity. Right, you can in fact make artificial gravity
on a spaceship by providing acceleration, either by spinning or
by zooming off in one direction. So the whole idea
that gave Einstein the inspiration for general relativity was this

(26:51):
one is called the equivalence principle that says that there's
no difference between gravity and acceleration. And so we just
went through the details of how acceleration can cause time
to move slowly, and this is exactly the same effect.
Gravity also makes time move slowly. The curvature of space
around you makes time move more slowly. It's exactly the

(27:13):
same effect. And so it's an absolute effect, not a
relative one like for velocity. So you're saying it's really
sort of acceleration that causes time to slow down, and
gravity is sort of like an acceleration or is it
the other way around? But I guess you can have
acceleration without gravity, So it's more like gravity is kind
of like an acceleration. Yeah, gravity is essentially like a
geometrical interpretation of acceleration. Or you know, said another way,

(27:37):
what happens when you have mass in space, Well, it
changes the curvature of that space, and so what happens
then is that things move differently and they can appear,
for example, to be accelerating. If you aren't aware of
the curvature of space, then it seems like there's a
force there which provides an acceleration towards those masses. And
so that's really what gravity is. Gravity is the bending

(28:00):
of space in a way that appears to provide acceleration.
So gravity and acceleration really are exactly the same phenomenon.
Either acceleration in empty flat space gives exactly the same
effects as curving of space itself. You remember, we did
recently a fun episode about how if you're accelerating there
are times that photons cannot catch you. You have essentially

(28:22):
an event horizon if you're accelerating constantly, And the explanation
there was the same as here is that accelerating constantly
is sort of the same as curving space. And we
know that if you curve space, weird things happen, like
you can be inside a black hole and photons cannot escape.
And so the core idea here is to understand that
accelerating is really the same thing as gravity, And so

(28:45):
if you think of gravity is causing time dilation, it's
really the concept of acceleration causing time dilation the sort
of mentally equivalent. There are a lot of leaps here,
I feel like, and it's kind of hard to keep
track of because I feel like you're saying gravity is
acceleration and gravity is also the curvature of space. Does
that mean that all acceleration is also the curvature of space?

(29:06):
Or can you have acceloration in not bend space, or like,
can you think of all acceleration even by like electromagnetic forces,
as some sort of curvature of space. Yeah, that's a
deep question, and there are people out there trying to
interpret all acceleration in terms of the curvature space, or
you know, like all forces as being the product of
the curvature space. But that's not necessary. You can think

(29:29):
about acceleration in flat space, you know, just like put
on a rocketship, accelerate your spaceship. Now you're going fast.
But the effects of that on your time and the
way you pursue you the universe are equivalent to if
space was curved around you. So you can think of
acceleration separately from the curvature space and from gravity, but
has exactly the same effect, because that's really kind of

(29:50):
what gravity is, so meaning I feel like then that
you're saying that that it's not really gravity that slowing
down time. It's really the acceleration caused by gravity, or
the bending of space caused by gravity, which is the
same as exceleration. Yeah, there's lots of different ways to
think about it. You can think of acceleration caused by
gravity is really just like motion through curved space. And

(30:12):
one of the other impacts of curved space is that
time also slows down. All right, well, then maybe let's
try it. Let's see why does acceleration slow down time?
Because that seems to be the bigger problem, right, that
seems to be the bigger question. Yeah, so I guess
you can say that gravity slows down time because it's
equivalent to acceleration slowing down time. Why does acceleration slow

(30:34):
down time? That gets back to the speed of light
as the limiting piece of information. You can derive this
in a flat universe using the twins as an example.
It's a little bit more complicated mathematically than like normal
special relativity, where you can do these observing frames with
Lorentz transformations. It gets a little hairy and mathematical, but
the core concept that it comes from is this maximum

(30:56):
speed of light. Everything comes out of that both time
dilation from velocity and also time dilation from acceleration, which
really is equivalent to time dilation from gravity. So I
guess if I'm in a spaceship and I'm accelerating the
limitations of the speed of light, which does that mean that?
Are you sort of saying that it somehow limits how
the things can evolve inside of that spaceship, you know,

(31:18):
move from you know, information between molecules and things like that,
So things sort of evolve slower or they have a
limit to how fast they can evolve. Yeah, I would
say a little bit differently. I would say that requiring
that the speed of light is constant and that everybody
observes the speed of light always to be. The speed
of light restricts the kinds of universes that we can have.

(31:40):
It puts a lot of restrictions on the waste space
and time have to work in that universe. And these
effects that we're talking about, the slowing down of time
by velocity and by acceleration, are consequences of the structure
of that space and time the sort of come out
of that all, right, So you're saying it's just the
way it is saying when things accelerate, there's a limit

(32:01):
in the speed of light, and so that makes time
sort of slow down, makes everything slow down. And I
think it's super cool because it's not symmetric. You know,
like two people can agree on who is accelerating more,
and so that means that they can agree on whose
time is moving more slowly, or you know, said another
way in terms of gravity, like you and I can

(32:22):
agree that if you're near the black hole, then you're
in a part of space that's curved more than my
part of space, and so we can agree that your
time should be moving more slowly. That's not true for
the spaceships, right, If we're in two spaceships passing each other,
it feels symmetric because it is symmetric. I say you're
moving past me, You say I'm moving past you. Everybody's right.
In the case of the black hole, we can agree

(32:44):
it's not symmetric, So we should agree that your time
is moving more slowly. So then a coloration causes time
to slow down. And definitely, when you're near a black hole,
you are being accelerated, probably a lot, because black holes
are very massive. They're pulling you in. And so if
I'm near a black hole, then I'm going to be
moving slower through time than you, who is out way

(33:04):
far from the black hole exactly. And so another cool
thing is that I see your time moving slowly. It
means that you see my time moving more quickly. Right,
This is the real difference with velocity time dilation and
velocity time delation. We both see each other's time moving
more slowly. Here, if you're near a black hole and
you're looking out into the universe, you see the rest

(33:25):
of the universe running forward in time very quickly. And
as you get closer and closer to the black hole
and more and more curvature, you see the universe's clock
speeding up into the future. So then like if I'm
falling into a black hole, like all the stars will
suddenly start speeding up around me, like I'll see the
universe kind of and fast forward exactly. And some people imagine, well,
does that mean that you'll see like the end of

(33:45):
the universe, the end of time. You'll know, like the
final fate of the universe just as you fall into
the black hole. Well that would be super cool, but unfortunately,
it takes a finite amount of time from your perspective
to fall into a black hole, so there isn't time
for all that information from the future universe to get
to you. So you see the fast forward universe for
a while, but you don't see like all the way

(34:07):
into the infinite future. Well, we talked about this last time,
Like when you actually get to the surface of the
black hole, then time actually stands still, right, Like it
slows down more the closer you get to the black hole,
and then it sort of stands still at the surface.
It sort of does, but that's only for somebody far away.
They see your time moving slowly and they see you

(34:27):
sort of smeared across the event horizon. But for you,
you actually fall into the black hole, you don't notice
anything different changing, right, you notice the rest of the
universe's clock speeding up. But from your perspective, you fall
into the black hole, you pass the event horizon, you
get sucked into the singularity, so your time is definitely
finite from your perspective. And this is not something that

(34:48):
we understand very well. There's all sorts of weird paradoxes
and contradictions here about how one person sees you not
falling into the black hole to the end of time,
and you see yourself actually falling in. It's not something
that we know how to reckon. SI. I think what
you're saying is that the person falling in, you're saying
they'll see themselves falling through, But we don't actually know
if that's true, right, Like they might just actually freeze

(35:08):
at the edge, which just don't know. Yeah, we don't
know it is true because nobody's done it and come
to report back. It's possible they actually just freeze the
edge and they think that they're inside, but it's actually
that the inside of a black hole is a hologram
projected from the surface of the black hole. We just
don't really know what's going on inside a black hole.
And so this effect of gravity on time doesn't just

(35:29):
happen in black holes. I mean, black hole is sort
of the extreme example, but it actually happens like every
day and everywhere, like here on Earth, the Earth is
slowing down time and even like I'm slowing down time
for the things around me, right, yeah, absolutely, everywhere there
is curvature, time is slowed down and the Earth curved space, right,
because the Earth has a lot of mass. That's why,

(35:50):
for example, you don't fall off the Earth. You're feeling
it's gravity. So anywhere you're in a situation where you're
feeling gravity, you're also having your time affected. And because
gravity is stronger as you get closer to the Earth
and weaker as you move away from it, that means
that the clocks are variable. Time flows in a variable
way as a function of the distance from the center

(36:10):
of the Earth. And this is something you can measure,
like over your life, your feet will age one second
more than your head, only if you spent a lot
of time standing up Daniel, which cartoon is don't do
a lot of So I guess our feet are still young.
We are still light on our feet exactly. That's why
you lay down all the time, just to keep your
body like in sync, just to keep my feet young.

(36:33):
All right, well, let's get into what this all means.
Does that mean that we're all moving slower through time
than we should? And whether does that mean that also
there's no universal clock actually to measure time in the universe,
So let's get into that. But first let's take another
quick break. All right, Daniel, my favorite question and all

(37:04):
these topics, what does it all mean? Man? So anything
with gravity bends space around it, which causes acceleration, and
accelerating things slowed down in time. So things are always
slowing down in time everywhere all the time. Yeah, everywhere
there's a gravitational field, clocks are being slowed. So if
you're in a gravitational field, then your sense of now

(37:26):
is moving forward differently than other people who are like
out in deep space. And so if you spend a
lot of time near and gravitational objects, then you are
younger than you otherwise would be. Right, But it's not
just gravity too, right, It's like if I get in
my car and I accelerate up to the freeway, I
somehow slowed down time for myself. Yeah, gravity and acceleration.

(37:47):
They're equivalent, and so both of them will slow down time.
Every time you accelerate, the universe sort of leaps forward
a little bit relative to you. If you accelerated a
lot for a long time, you would notice clocks around
you seeming to move forward faster than one second per
second on your clock. And so I guess that means
first of all, that there's no real age to the universe.

(38:09):
Is that really true? That Does that mean that you
know there's no like absolute time? Yeah, there's no absolute time,
which is really frustrating if you'd like to have a
sense that you know, there's truth, that there's something really
going on out there outside of our skulls. It's frustrating
to imagine that. Like, different people can tell different stories
and they can both be correct. But there's an even

(38:29):
deeper problem if you try to extrapolate back to time
equal zero. You know, we say this thing the universe
is thirteen point eight billion years old. Well, according to
what clock, right, is that clock been moving on a spaceship?
Has that clock spent a lot of time near a
black hole? Because if so, it's going to have a
different answer. And so, because different parts of the universe
have different curvature, right like near black holes or near

(38:50):
Sun's or whatever. Then different parts of the universe have
aged differently since it's beginning. So the universe does not
have one single age. Just like your feet and your
head are not the same age, assuming you haven't spent
your whole life in bed, the parts of the universe
have different ages. I do a lot of handstands, so
I'm trying not to go bald, so I'm keeping my head.

(39:11):
I think we all need a video of you doing
a handstand. Let's see that. That's good. But I guess
that confuses me because you told me earlier that acceleration
is absolute, so I can measure exceloration absolutely, and time
is actually bent by exploration. So couldn't I, I don't know,
find a spot in the universe that's never been accelerated

(39:33):
and say that, like, that's the absolute age of the universe. Well,
that's the age of that part of the universe, and
that would be the oldest part of the universe. That
part would have experienced the most time. But you know,
if you had put a clock somewhere else in the
universe and let it run since the beginning, it would
have a different number. So like different parts of the
universe have different ages, and you might reasonably say, well,

(39:57):
the oldest part of the universe, I'm gonna use that
as the a age of the universe. Yeah, nobody cares
how young my feet are. I care, I care, you care.
It sounded a lot like I care though, Right, that
sounded sincere didn't it. But yeah, so there's sort of
an age limit to the universe, right, Like you're you're
saying that there is an absolute age of the universe

(40:17):
by which we can measure all other ages. Yeah, there's
a maximum age to the universe, right, there is a
number beyond which no part of the universe could have
experienced more time than that if you wanted to find
that as the age of the whole universe. I guess
that makes sense, But I think more conceptually it makes
sense to imagine, like how many clock ticks have there
been in a given part of the universe, And that's

(40:39):
not equal all over the universe. It depends on how
much gravity there is nearby, right, Because I guess even
our solar system is being accelerated around the Milky Way,
so therefore our time is sort of being slowed down
in that way too. Yeah, the curvature of the center
of the Milky Way and that supermassive black hole does
affect the motion of the Sun and the curvature nearby,

(40:59):
which slows down our time. And I guess you were
trying to tell me earlier that there is sort of
a philosophical question here, which is like, does acceloration change
time or does time change acceleration. Yeah, it's familiar to
think about general relativity is saying that mass causes space

(41:20):
to bend, and then the curvature of space tells masses
how to move. Right, that you get an appearance of acceleration,
this effective force of gravity because space itself is bent.
There's a missing component there, right, which is that time
is also curved by mass. So you have mass somewhere,
it doesn't just curve space, It also curves time, which

(41:41):
is what we've been talking about today, And the curving
of time also contributes to this force of gravity. So
gravity is an apparent force that comes not just from
the bending of space but also from the bending of time.
The two work together because spacetime really is sort of
one thing to create this effect of gravity. You're saying time.
You can also bend time, just like you can bend space,

(42:04):
and somehow that's where gravity comes from. Yeah, exactly. It's
familiar to use like a rubber sheet analogy, where parts
of space are bent and that changes how an object
moves sort of naturally. And you know, that's a little
confusing because in what direction is the rubber sheet bending.
It's bending in some sort of like external direction. You
can measure it in terms of like another dimension in reality,

(42:24):
in our space. It's intrinsic curvature. It just changes the
relationship between points in space their relative distances. So that's
a familiar way to think about how the curvature space
affects the motion of an object. You can do something
similar for a time. You can imagine like different parts
of the universe flowing with different time. Right, It's sort
of like you're moving down a river and different parts

(42:46):
of the river are moving faster than others, and that
will affect the motion of objects. Like you have a
really big object in a river and the river is
flowing faster on the left side than on the right side.
It's going to change the way that object moves it
like tug it towards it's the slow moving part of
the river, and that's part of how the curvature space
and time creates this effect of gravity that objects no

(43:09):
longer move in what we perceive to be straight lines.
I see you're saying, like you can think of it
the other way around, Like me falling to the Earth
or me being an orbit around Earth is actually a
consequence of the differences in time, and like differences in
time cause me to move from one place to another, exactly,
it's space and time being heard, both affect your trajectory. Cool,

(43:31):
So in a way, asking why does gravity slow down time,
you could also mainly ask why does time slowing downtime
cause gravity? Yeah, exactly. Another way to think about it
is that the effective gravity we observe is coming from
the curvature space and the curvature of time, and that
gravitational time dilation is just another aspect of the curving

(43:52):
of space time in respect to mass. So I guess
in the end, you just have to say that it's
all sort of kind of the same thing. It's all
sort of relate it in and it's all you know.
You can look at it from one way, or you
can look at it upside down. But at the end
of the day, and it all comes down to really
I think exceloration, right, Like things that accelerate have to
slow down in time because of the speed limit of

(44:13):
the universe. Yeah, that's a consistent way to think about it.
I prefer the sort of geometrical way to think about it,
that we're living in a universe that's courage and work,
but we don't perceive it directly, and so the way
things move through space and time are affected by these
like invisible warping. That sort of geometrically makes the most
sense to me. But you can also think about it
just in terms of acceleration in flat space. Yeah, all right, Well,

(44:34):
I guess that answer is sort of the question why
does gravity slow down time? The answer is because of
exploration and why does exceleraation slow than time? Well, we
probably need a whole new podcast episode about it. Yeah,
why does gravity slow down time? Because time is bent
in the presence of mass, just like spaces. I think
you just went all the way around. Why does the

(44:56):
excelation slow in time because slowing downtime causes excelraation. It
sounds like a great answer, and it causes podcasts to
go in circles. Yeah, and now you can just hit
replay and listen to this episode all over again and
it should make sense. Right. It's all pretty tricky stuff.
In the end, it all comes down to consequences from
our observation that the speed of light is the maximum
speed of the universe that just happens to be the

(45:18):
universe we live in. And when we build in those
constraints into our theories, these are all the consequences that
come out of it. And maybe that point is what
you were saying earlier than you know. We have the
speed limit and it causes time to do weird things.
So maybe time is not a fundamental thing, right, Like
it's not outside of the universe. Maybe time it's something
that comes out of how the universe works, yeah, or

(45:40):
how we are perceiving it or measuring it. And you know,
there's a whole universe of crazy ideas about time, even
ideas that like time is not essential part of the
universe but comes out of something else, or that there
are multiple dimensions of time, the way there are multiple
dimensions of space, and we're just moving on like a
one D line through threedom mentional time. Man, there's a

(46:02):
whole like crazy, crazy set of really fun ideas to
dig into. Unfortunately we are out of time for this episode.
We'll have to find more time to get into it more.
But we hope you enjoyed that and maybe got you
to think a little bit more about how young your
feet are and how you should maybe do more handstand
I want to see that video if you're doing a handstand,

(46:22):
I can do what actually, but now it's not the time,
all right, Well, thanks for joining us, see you next time.
Thanks for listening, and remember that Daniel and Jorge Explain
the Universe is a production of I Heart Radio or More.
Podcast for my Heart Radio, visit the I Heart Radio

(46:44):
Apple Apple Podcasts, or wherever you listen to your favorite shows.