April 25, 2019 • 43 mins
Why can't we change the direction of time? What are the past, present and future really?
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Speaker 1 (00:07):
Hey or hey, I think it's time we confronted a
really tricky topic. Do you think we have time for that?
You know, there's no time like the present. It could
be a good time. It might be a fun way
to pass some time. All right, I'll make time for it.
It's about time. That's so corny. How many time puns
(00:28):
can one fit into a single podcast? That's right? How
much time do you have? Um, I've got time. I
think it's a timely set of puns. Well, you use
that pun time after time, commending else. Now is the
time to start the podcast. Now is the time to
tackle this tricky topic. I am and I'm Daniel. It's
(01:07):
time for our podcast, Daniel and Jorge Explain the Universe,
a production of I Heart Radio in which we take
everything about the universe, the now, the future, the past,
the deep, deep, deep past and try to explain it
to you. That's worry. We take the time to take
a little bit of your time and maybe get you
to understand a little bit more about this amazing and
(01:28):
spacious place that we live in called the universe. That's right.
And sometimes we have to grapple with topics that are
sort of obvious. You know sort of things that are
right in front of you and understand like does physics
know how to make sense of it? Does physics have
a good definition of it? Do we understand why it's
this way and not that way? That's right? And so
today we're tackling a topic that is probably in everybody's
(01:49):
mind all the time. That's right. This is something you
think about every day from when your alarm clock goes
off to when it's time to go to bed. To
stay on the podcast, we will be talking about out time.
What is time anyway? Yeah? And why does it only
go forwards? Why do you remember the past and not
(02:12):
the future? Can you remember the future? That would be
pretty cool. Well, that's the whole question. Right on one hand,
very intuitive understanding of time. We all know what time
is right, yesterday was yesterday, tomorrow's tomorrow, right now is now,
But when you get down to it, we don't really
understand why we have it right. Why is it like that?
Why does it only go forwards? Why is the future
(02:33):
different from the past. Yeah, it's weird to think that
we are in the pressing right now and there's a
past going backwards in time, and there's a future going
forwards in time? But what's really the difference between those
two things? Yeah? Why can't I see or feel or
have any kind of memory about the future? Yeah? Exactly
what makes it different? Why is the past fixed? Right,
(02:54):
unchangeable unless you believe in crazy science fiction time travel stories,
which I don't, and the future undetermined? Right? What's the difference?
Can physics reveal that? Is there some understanding of time?
From physics? It tells us why one is different from
the other? Yeah? Why is there an arrow of time
like a one way sign in the universe, in the
(03:14):
highway of time of the universe? Yeah, this is one
of those wonderful questions because on the surface of it
it seems kind of dumb, Like somebody asks you what
is time? You're gonna tell him, Oh, it's three o'clock.
And if they dig deeper and ask you though, like
what does time even mean? Man? Sometimes I say it's
two o'clock. Depending on the circumstances, I always give the
same answer no matter what. That way, people stop asking.
(03:38):
My kids are aways what time is it? And they
have like a watch on their hand. There's like two
watches on the wall, Like why are you asking me?
So I would just say it's three o'clock no matter what,
so that they stop. It sounds like it sounds like
the same parenting strategy my my wife has when we
going road trips to always ask how long are we
do we get there? Is it? Are we there yet?
And she always answers, nope, scant twenty more hours, no
(04:01):
matter what. You're like pulling into the place, and she's like,
twenty more hours, yeah, no matter what, she always says
twenty It's a great strategy. You know, me and your wife,
we would really get along. It seems like we see
eye to eye about how to handle these approaches. How Tom,
you would have some very um sarcastic kids probably exactly. Um. Anyway,
(04:23):
I love these questions you know that are like, on
the surface so simple, when you dig deep, they reveal
like enormous gaps in our knowledge about the universe. Those
questions are the best because those are the opportunities to
reveal that the universe is different from the way we understood.
Like the way we think about the universe reflects just
our experience, the way we've lived and grown up, and
(04:44):
not something fundamental, something universal, right, And that's the whole
goal of physics. Yeah, it's one of those questions, like
it makes you kind of look almost inwards or look
in your like you one day you just wake up
and you don't even know where you're standing, or how
how the house that you live in was built. Who
don't lose grasp of reality there? Or or hey, we're
(05:05):
trying to dig deeper, trying to make you go crazy.
Doesn't it make it kind of question at like the
very nature of the universe. It does. It's sort of
like if you say a word like a hundred times,
like say the word marriage a hundred times, it starts
to turn into a really weird word. Right, you take
it apart, you look at you like, that's really strange.
It's the same thing with the concept of time, Like
(05:27):
you know, pick up your hand and rub your fingers together, right,
you're feeling that right now? Right? But what does now mean?
Right now? Sort of like it's always it's infinitely short,
and it's slipping constantly into the past. You can never
grasp onto it, you can never hold it, right, it's
always you're always losing it. It's a really strange concept
of concept of now. Yeah. Yeah, although I don't know
(05:50):
if that's what makes marriage weird. But there's lots of
things that make marriage weird. Um, saying it a hundred
times is not it. But I remember reading a book
about consciousness back when I was really interested in the
science of consciousness, which we can talk about another podcast.
But the basic idea in that book, as by Daniel
Dennit it's called Consciousness Explained, is that there is no
(06:12):
now that you're the consciousness is basically an illusion that
all you're ever doing is remembering the immediate past. And
I don't know if I believe that, but it really
made me think about what now is and how you
can never really grasp it. It's always just like sliding
away from you and you know, whether it really exists,
whether there is a now, right, whether there's a special
instant that that differentiates between the past and the future
(06:35):
right right, because sort of technically in physics of time
could go both ways right like you could in time
is kind of just like an arrow you can flip
back and forth in the equations that you guys use, right. Yeah,
time is not really very central to physics. Yeah, I mean,
on one hand, it is. On one hand, it isn't.
Tom you could write a lot of physics down without
ever thinking about time. On the other hand, time is
(06:57):
essential property physics because we're trying to use physics to
predict the future. So um, this time is both like
not part of physics and deeply entwined in it at
the same time. So it's, uh, it's an important topic. Yeah, no,
And it's a topic that everybody experiences right at every
second of the day, every day of the year, every
year their lives. They're experiencing time, and they're moving forwards
(07:19):
in it, and they feel it, right. Yeah, they think
about it, they think about the future, they remember the past,
and so it kind of makes you wonder how many
people actually think about or know what time actually is. Yeah, exactly.
So I was wondering do people know why time goes forward? Like,
(07:39):
I mean, physicists don't really, but it's always fun to
ask general public questions that scientists don't know the answer to,
just to see what they come up with. So I
walked around the US of Irvine campus and I asked
people why does time only go forwards? Here's what people
had to say. Do you know why time only goes forwards? Um?
Not really, honestly, I have like some sort of like
(08:02):
intuition why it only goes forward? But I don't think
we can't explain that. Yeah, okay, cool, I have no idea. No,
I don't any guess. If I had to go, I
just said gravity or something. Okay, No, I don't any idea,
best guess. Sorry. I think time is lack of force
dimensional right, So actually I don't know, creaky right, Okay,
(08:29):
I'm not sure. Okay, Well, when I think of time,
I think of it kind of almost as a linear progression,
meaning that it's a constant in the universe, and it's
always kind of moving forward, and that's something that we
accept as being a fact of the universe, at least
as we understand it now. So in that regards, I
(08:49):
think that's why we can only see it as moving forwards,
because it doesn't kind of make sense to us or
it's not even physically possible currently for us to move
backwards in time, so we don't consider it in that way.
I think it might be a matter of observation. So
relative to us, time moves forward, and the way that
you experienced the world biologically is designed to be in
a manner that is sort of one dimentional and a
(09:12):
forward mind all the time. But in reality it might
be that the fourth dimension of time in space time
might just be a way of describing the universe that
the math predicts a from physics and my option perhaps
other related science fills. Okay, cool, Yeah, so I got
a pretty good mix of answers there, right from the
(09:33):
standard physics, like gravity, which is you know, that's a
good answer. You never know, it might be, right, gravity.
Somebody actually said gravity. I don't know gravity, It could
be it could be. Yeah, well, gravity is like the
answer to half the questions in physics, the way like
pie is the answer to half the questions in math. Right,
so if you have no idea, you could just say
gravity and have the time. You'll be right. Not the
(09:55):
movie the fourth right, yeah, the fabric of space time itself.
That's right, it's gravity. Yeah. And then you know, some
other folks had some more elaborate discussions, But people had opinions.
People did have opinions. Yeah, some of them were sort
of longer word salad. Right, you know, some of them
hit on some of the scientific concepts time being a
fourth dimension, etcetera. Um, but um, yeah, a lot of
(10:19):
people want to ask them this question. You could just
see in their faces that they had not ever considered this.
They're not even question they had imagined, but as soon
as they thought about it, they realized they didn't know.
And that's my favorite part of this this topic is
that nobody, nobody has really thought about it in terms
of general public, but as soon as they do, they're
very curious. Right, this is the best questions. It's amazing
(10:40):
how little you actually kind of need to know about
the universe to live a full and happy life, you
know what I mean, that's the protective happy life. There's
so much you don't need to know. Ignorance is bliss,
you know, that does have a meeting. People have been
living in this universe for hundreds of thousands of years,
depending on how you count the beginning of humanity, and
(11:01):
there's been a lot of happiness and joy without really
any understanding. I mean, I would say people five years
ago basically knew nothing about the way the universe actually works.
But you know, people had birthday parties and eight cakes
and had moments of joy. So yeah, I mean like
a hundred only a hundred years ago, right in the
nine hundreds, early nineteen hundreds, people didn't know the concept
(11:25):
of Adams or quantum physics, right, Yeah, that was about
a hundred years old. We didn't know how long the
universe had been around, right, so pretty basic stuff about
the way the universe works is pretty modern. Right, So
we didn't know there were other galaxies, right, No, they didn't. Um,
So what's your point here? Your point is you don't
need physics to be happy? Is that what you're trying
to say is irrelevant happiness? You think of how much
(11:46):
of a richer life you can lead if you know physics. Okay,
I like the way you spun that back, right, using
all this technology to talk to me. That's based on physics,
not not like not like money rich, but you know,
like spiritually right right? Um? Isn't your career based on
technology that was developed with the help of physicists? Where
(12:07):
would you be? Isn't your science dependent on technology? Yes?
I am a tiny little mite standing on the shoulders
of giants, absolutely, But so are we all? So are
we all? Yeah? Well maybe it's time we got back
on topic here, and um, so let's break it down
(12:28):
for people, Daniel, what how do physicists see time? Like
if if you're at a conference and you guys are
talking and somebody said, hey, guys, what is time? What
do you think people would generally say, I think there'd
be a lot of disagreements because we don't really have
a great description of time, you know, one that really
makes sense. And you know, the way physics approaches the
problem is first you try to build like a model
(12:50):
of the problem. You're trying to describe it in a
way that we can grapple using the language that we're
familiar with, which is mathematics. You try to find an
equation that tells you what what's what's happening? Yeah, exactly,
you know, something's happening in the real world. Maybe it's
like a ball is being thrown the air, and we
try to understand it. So we write down a bunch
of equations to describe it um and that lets us
(13:13):
build that model in our heads and manipulate it and
then use information from the model to describe what's happening
in the world. Cool. How do we do that with time? Right?
How do we ask this question about like why can
I remember the past and not the future? Why does
ice melt but not unmelt? Right? These kinds of questions
about time, it's pretty hard to to wrap your mind around.
(13:36):
One approach is to think about the universe in terms
of a bunch of snapshots. Like you think about a movie. Right,
what is a movie? It's not really a continuous experience
in time. If you're looking at the screen, you're not
really seeing smooth motion. You're seeing a bunch of snapshots.
You're seeing like a little cells in a film, or
like a bunch of screenshots. Yeah, it's a bunch of screenshots,
(13:58):
and they're just packed so tightly together that you don't
notice that they're not actually smoothly varying. Right. Your brain
does the interpolation for you. It tells you a story.
So sometimes we think about the universe that way. We
think about like all the universe at this moment, and
then all the universe at the next moment, and all
the universe at the next moment, like snapshots of the universe.
(14:19):
By snapshots, I mean like what is every particle doing
and where is it going? Yeah, exactly and time. Then
is a way to sort of order those snapshots is
to say this one first, that one next, that one next,
um and to sort of put them together. This one
can go in front of the other one. Yes, you
know what I mean. It limits um the ordering. Yeah,
(14:41):
And in that sense, physics. The job of physics is
to take all the snapshots in the past and predict
the ones in the future and to say, Okay, according
to the laws of physics, the next snapshot will look
like this, or if you want to talk quantum mechanically,
there's a probability distribution of the next snapshots. These are likely,
these are unlikely, these are impossible. Right, So physics um
(15:04):
takes sort of that view of time, and there's a
bunch of caveats there. Like, you can't have a snapshot
of the whole universe, right because time is not universal.
It means different things for different people. You can't know
everything about the universe. But you know, you take a
small enough model or a single particle or something, you
can also have quantum and you also have quantum states,
right Like it' isn't that prevent you from getting an
(15:25):
exact snapshot of the universe? Yeah, exactly. So you can't
really get an exact snapshot, but you could do a
quantum mechanical version. You could say, well, what's the quantum
state of all the particles or you know, what's the
wave function. What's the state of the wave function right
now that you can talk about, and that determines probabilities.
I see, So you describe me kind of how physicists
see the universe. They don't see it as um. They
(15:46):
see it as as this kind of sequence of snapshots
of how things are arranged. Yeah. For example, think about
the Shortinger equation. The Shortinger equation famous equation describes how
a particles a wave function um moves through time, right,
and it tells you how it moves. It says, if
you have this wave function now and you experience this,
(16:07):
then your way function will be that later. Right. So
that's that's what physics tries to do, or or go
back to them making the mechanical example of a ball
going through the air right tells you, well, you you
shot your ball in this direction. Where is your ball
going to be in the future. So that's the role
of time in physics, right to predict the future snapshots. Oh,
I see. So to you guys, time is like a
(16:27):
like an input that tells you what the universe is
likely to do. You you know what I mean? Like
you do you It's an input. It's not like an
output or something that you can vary or that it
is affected by all the other variables. Yeah, exactly. And
so for example, you want to predict the flight of
your ball, it's an input. You can dial that knob
(16:48):
and say I want to know where the ball is
going to be in one second or five seconds or
eight point two nine seconds or whatever. So it's sort
of that knob. The thing we don't the thing that
that doesn't explain at all is like why it only
go forwards and who's controlling how fast that knob is turning?
Or why do we have time at all? Why don't
we just have a static universe? It's just sort of
like there, you know, it's the first step. Of course,
(17:12):
they just try to describe it, and then you can
build from that and try to answer some of these questions.
But you mean, like, how come we don't have a
static universe, Like, um, why do we have time at all?
Boom right, I don't know, you don't know, nobody knows, Like, well,
I know, there's this idea that time is the fourth dimensions.
We have three dimensions up, down, left and right, backwards
(17:32):
and forwards, and maybe time is just another direction of
the universe. And so we and I think the days
that we do sort of exist in this a state
of being still in all four dimensions, but it just
so happens that we somehow feel one of the mensitions.
Do you know what I mean? Yeah? I love the
(17:53):
it just so happens part of that explanation, right, Okay,
here's the hardest part. I'll just YadA, YadA, YadA over
that bit. That's that's my that's my Nobel Prize, Nobel
Prize winning paper officer. It just so happens. I was
going ninety miles an hour. Um, you know, I can't
explain it now, You're right. Um, time is sometimes thought
of as the fourth dimension. And so let's dig into
(18:13):
that a little bit. What does that mean? Why do
we think of it that way? We think of it
that way because it's helpful mathematically. Like when Einstein was
developing his theory of special relativity and we talked all
about time dilation and stuff like that on a previous episode,
he discovered that the equations have a certain symmetry. They
look really nice like that you can write them down
really simply and compactly if you include time as the
(18:37):
fourth dimension of this larger concept he called space time.
So he took three dimensions of space. He tacked on
a fourth dimension, which is time, and he constructed this
thing called space time. And because your perception of time
depends on where you are in space and how fast
you are moving through space, the equations get much simpler
if you think about all four dimensions in that way, right,
(19:00):
if you think about time parallel to space in that way.
And that's a clue when you write down equations, and
they become simpler if you think about them a certain way.
That's a clue from the universe that it's maybe the
right way to think about things. Okay, so that's um,
that's time is a different dimension. Let's get more into it.
Let's go down that rabbit hole. But first let's take
a quick break. So I think it's fun to think
(19:33):
about time as the fourth dimension. But it's also kind
of a trick, right, It's not. It doesn't really fit.
So wait, So the idea that it's a fourth dimension
you're saying, came from mathematical convenience by Einstein, do you
know what I mean? Like, it's not. It was just
easier to treat it like a dimension. Yeah, And I
(19:54):
wouldn't you blow that off? Mathematical convenience is not a
small thing, you know. The whole goal of physics, remember,
is to like write down an equation of the universe
in one line. And so if you can write things
more simply, that's a clue that it's probably more correct,
it's more and it's like a deeper understanding. So yeah,
he discovered that if you write things down with time
(20:14):
is sort of the fourth column of your vector, then
a lot of the equations are simpler to write down.
There's it's there's a connection. There's a symmetry there. Right.
We talked about symmetry in another episode. You can treat
time the same way you treat space in many ways.
And that's that's tempting because you think, oh, well, that
answers the question time is just another dimension, right, But
it doesn't answer the question because it's not just another dimension. Right.
(20:37):
You know time is not space? Right? Time? What does
that mean? Time is not space? Well, for money, you
can move forwards and backwards in space, right, you can
go left, you can go right, you have some control
over it. You can't do that with time. But isn't
it just sort of a matter of perspective, Like I think,
like in the grand scheme of the history of the universe, Um,
(21:01):
you know, I existed for this amount of time, but
while I'm living it, I can only go forward. I
kind of get the experience of moving forward in time. Yeah,
that's true. Technically, I sort of existed all throughout my
life in space time, you existed all throughout your life, Yeah,
but you don't exist all throughout time. Right, There's no
limit to where you can visit. In space, you can
(21:21):
go from here to there to the other places. But
in time, you can only visit between your birth and
your death, and you can only go to each time once. Right,
that's not true for space. I imagine there are lots
of places you go to many many times, like your
refrigerator or your bed. Right, Um, you visit those places
many times. So space is quite different from time. Right,
(21:43):
there's a special loops in space, but I can't do
loops in time exactly. And time time has this special difference.
Right then now, right, there's no now in space. I mean,
I'm here, I'm there, you're over here, but there's no
there's no special location in a what time has this
special location? This thing we call now, which exists weirdly
(22:05):
and slides forward weirdly. So I was saying earlier, wh
why isn't the universe static? Why is time move forwards?
Why isn't it just stuck at to equal zero? Nothing happens?
What's pushing it forward? What's turning the engine of the universe? Right?
I guess what I mean is, you know, if time
is you can treat it as a fourth dimension, then
you know, our experience of the universe is at time
(22:27):
is constant and the moves at a steady pace, and that, um,
you're only going forwards in it, right, whereas in space
you're saying we can go backwards. But what if I
just kind of take another dimension and use that as
my kind of ticker? Which dimension would you use as
your clock? I don't know? Up, okay, yeah, so you'd
(22:51):
be like, hey, let's have lunch at x equel's five
meters or something. Yeah, I don't know, or you know,
kind of mathematically, what does that mean? Different? Time and
space are not the same. You can't just swap out
time for space because you don't have the same freedom
and time as you do in space. Right, if if
space were moved always forward, the way time did, Like
(23:12):
you could never go back to anywhere, right, Like, oh yeah, hey,
I had lunch and now is sort of sliding slowly
away from me. You know, Um, it would be pretty weird,
but it's very natural experience. So I see you're more
limited in the time dimension exactly. There's some extra rules
that seemed to apply to the time dimension that don't
apply to the other dimensions. And that's weird. And anytime
(23:33):
we see symmetries where like cool. But then when those
symmetries are broken, we're like, Okay, time is like the
other dimensions, but it's different. Then we ask why is
it different? How the different? What makes it different? Because
that's the clue to solve that, not the other puzzle. Right, Okay,
so time is it is sort of a fourth dimension,
but there are special rules that apply to it that
makes us think that maybe it's not really a fourth dimension.
(23:55):
Maybe it's just like a mathematical um good cluage, right, yeah,
Or maybe there's two kinds of dimensions. There are space
dimensions and time dimensions, right, and yeah, and maybe there
are other people haven't imagined yet, you know. Yeah, that's
that's the kind of mind blowing, like look at the
universe in a different way, discovery that I always hope
(24:17):
to make in science, you know, to like crack something
open that's so deep that it like nobody gets it
but the stoner's man. Okay, so, so what are these rules?
The rules are that you can only you can't visit
two times in your you know, you can't go back
(24:37):
in time, you can make gloops, and it can only
go forward. Are those mainly the main restrictions here with time? Yeah, exactly,
those are the main restrictions. And I think the goal
of physics and the goal of our podcast is is
to try to understand where that comes from. Like can
we look at the laws of physics and say, oh,
that comes from this, you know, like the way we
can look at the laws of quantum mechanics and from
(24:59):
that understand in the way hydrogen works or whatever. We
want to look at the laws of physics and say
do they require this behavior? Is it necessary? What does
it come from? And it's the deepest level, right. The
problem is when we look at the laws of physics,
we don't get those kinds of clues. When when we
look at the laws of physics, we don't so the
equations in your model of the universe don't tell you
(25:21):
that there should be restriction some time. Yeah, let's go
back to those equations about like the ball moving. Right.
Imagine you throw a ball, right, you're talking about basic stuff, Right,
you throw a ball in the air, it goes up
and it comes down. Let's start with the simplest case
with there's no air resistance, Right, it's passed up and
it's passed down. Are gonna be very similar. In fact,
(25:42):
if somebody took a video of that and then plated backwards,
they couldn't tell whether the video is going forwards or backwards. Right,
Physicists analyzing the motion of the ball couldn't tell you
whether the video was going forwards or backwards. So if
you fed it a time, like if you gave it time,
it would give you the same oppu whether you put
(26:02):
negative time or positive time exactly how it breaks down
exactly because the equations work in both directions. Right. The
equations have time in them. They tell you how things
change with time, but they work the same forwards and backwards.
So they don't tell you which way the universe should run, right,
They tell you how the universe changes with time, but
they don't say why it has to go forwards and
(26:23):
not backwards, why it has to go anywhere at all?
Or I can't go forwards and then backwards. Right, So
if the equations of the universe really represent the universe,
then the universe would is okay with traveling backwards and time.
You're saying exactly, the equations sort of saying like, hey,
you want to go backward this time, Sure, here is
what would happen. Yeah, exactly, And it's sort of like
(26:44):
you were saying earlier. It's like an input. You know.
Imagine you're doing some calculation and the calculation is a
function of position and time. Right, you want to know
how many balls are at this location at this time. Right,
you you drop a thousand balls into a box and
you're wondering, like, how many balls are this location the time?
You can ask that question of any point in space,
at any point in time. Right. You can take the
(27:05):
concercurrent configuration and you can evolve it forwards or evolve
it backwards. The laws allow for all of that, but
for some reason that there there's a difference in the
way it actually works in the real world. Right, things
slide forwards in time, and we don't know why. But
there's no restriction on the space parts right there. The
equations are functions of space and time, but there's is
(27:25):
like extra restriction on the time part of it. When
it comes to like actually implementing the universe, right, it's
kind of like, um, we have them now, and if
all we had was now, the equations would let us
tell what happened before in negative time, and we would
let it tell us what happens forwards in time, right,
exactly the equations. Yeah, but but someh our experience. We
(27:49):
only have experience of the stuff that happened in negative time.
That's right. We only remember the past, right, that's the
weird part, right, The past we can remember, we have
access to it. Um in a affects us. The future
we can't. And and and most of the laws of
physics is an asterisk there. Most of the laws of
physics are totally symmetric. Right. They don't care whether the
(28:10):
universe is going forward or backwards. They're happy to do
either one. Wow. So that's the mystery is the time
would be sort of a fourth dimension, but there's some rules,
but they those worlds are sort of not in the
equations of the universe. That's the game with an asterisk,
you're saying, right, Yeah, So the game is figure out
(28:31):
if the laws of physics that we know of, that
we've discovered, that we've written down, if those require things
to move forwards? Um? And if so, you know, does
that give us a clues to why it's happening? Or
maybe there are other laws of physics out there that
are much more sensage to time, that are the ones
making time go forward. What do you mean, Like, so
you're looking for something in the equations that would require
(28:51):
time to only move forwards? What does that mean? Like
if you put negative time, you should break or or
yeah exactly, Like let's think about that. Are there is
there anything in physics that does require time to move forward?
So let's go back to that ball example. Right, I said,
if you throw a ball in the air, Um, you
can't tell whether the video of that ball in the
(29:13):
air goes forwards or backwards. And when we did that,
I said, ignore air resistance. And that's for an important reason, right,
because air resistance changes the flight of the ball, right,
slows it down, so that in reality, if you throw
a ball in the air, right, Um, it's going to
slow down. And so you can tell the difference between
going forwards and going backwards right, right, because there's a
there's like you lose some heat or you lose some
(29:35):
energy in reality, right. Yeah, it's probably similar to think
about like dropping a ball. You know, if you drop
a ball and there's no friction, it's going to come
all the way up to where where you dropped it from.
If you drop a ball, it is air resistance. It's
gonna lose some energy to bumping against all the air molecules. Right,
It's not gonna come up quite as high as you
dropped it. You just keep letting it bounce. Eventually it's
(29:56):
it's gonna dribble down and stop right right. And that one,
that one to you played it backwards, you would be
able to tell the difference exactly that one if you
played backwards. And that's the case for most things, right.
So where does that come from? Well, that comes from entropy, right,
that comes from heat. The heat really there is the key.
It turns out that the universe likes to go from
(30:17):
organized to disorganized, right, And that's this concept we call entropy. Yeah,
I've heard of this before, but before we dive in.
Let's take a short break. What do you mean when
(30:39):
you say that the universe likes entropy or that it
likes to go from order to disorder? What is it?
What is it liking? Me? It means that if the
universe had a bedroom, it would be a mess, right,
and it would never want to clean it up. No,
it's obviously that is our experience of it. Like, maybe
what if there are aliens out there who experienced time
backwards and they're like, why does the universe like order?
(31:03):
Everything we see? It seems to get ordered exactly. So
there's a big flaw in this argument. But let's put
a pin in that and come back to it once
we've done constructing the argument, then we can take it
apart um. Yeah, so you're right that the universe likes it. Well,
it just means that we've noticed, we've observed that disorder increases, right.
For example, the classic example is you have a box
(31:25):
and you put put a bunch of gas particles in
the corner. Right, that's very organized. It's like only one
way to arrange or a few ways to arrange gas
particles in the corner. What happens if you run the
clock forward? If you let that go, Well, if they're
not totally frozen, they're gonna spread out through the box. Right,
It's like we pour water onto a surface that spreads
out things like things tend to go from organized configurations
(31:48):
to disorganized. And that's just an observation. That's not a
deep insight into the universe. It says, here's what we've seen. Right.
You notice that there's a correlation between positive time and
messiness exactly, And that's called the second law of thermodynamics. Right,
it says that entropy always increases. Entropy is a measure
of disorder. Right, So things are basically always spreading out
(32:11):
and um and getting more diffuse, right, And that's the
whole universe is doing that, Right, The universe, like on
a grand scale, is spreading out and getting smoother and
stuff like that. Is it a law law, like an
equation or is it more like an observation, Right, It's
more like we've never seen something good un messy with
positive time. Yeah, that's that's a great question. It's a
(32:33):
really deep question. I love that question. I think the
answer is this, Um, The answer is that it's an observation.
It's something we've noticed and we've never really seen it broken,
and so we thought, well, this must be important, so
let's write that down. Let's call that a law because
it's something we've noticed happens all the time. Now, you
can never get broken. Why is that? Does that reveal
something deeper about the universe? Can we explain that from
(32:55):
something else? And if you dig into like statistical mechanics,
you can and derived the second law of thermodynamics from
simpler assumptions, like if you say every possible configuration of
the universe has equal probability, then it turns out there
are more messy probably messy configurations than unmessy ones, right,
(33:15):
Like there's a thousand ways to have gas particles spread
out through a box, but there's only a couple ways
to have them stuck in the corner. So it's just
more likely to end up with the messy ones because
there are more messy configurations. But that's sort of a
cop out. I mean, it's just sort of another way
to state that, is to say, well, we you can
call this a deeper understanding, but it's also just like
(33:38):
a posture that we came up with so that we
could derive the second law, which we've observed. Right, You're
you're basically down to the same argument you were saying.
You're basically saying that the reason we have entropy is
that when you move forward in times, you go through
the most likely scenario. But then you can ask, why
does when you go forward in time do you go
to the most like exactly, it's just a restatement. Really,
(33:59):
it's a sort of more fundamental statement of the same
basic observation that things go from organized to messy, right,
and um, yeah, exactly. So we were looking for, um, like,
something in the laws of phasics that told us why
going forwards in time is is preferred, And you're saying,
(34:19):
we have this observation that it always does and it's
somehow related to chaos and order and messiness. That's right,
And so this is like the only thing we can
really get our fingers on. You know, everything else in
physics is time symmetric. And there's another asterisk there. There
are some things in particle physics which are time asymmetric.
Those are really really small effects, and we don't really
(34:40):
think that that can explain the direction of time. Um,
But we can cover that in a whole other podcast
topics sometimes, but Basically, entropy is the big one. Entropy
is the only law that like clearly as you say,
has a correlation. But that's key. You made this point,
which I think is very insightful, was that there's a correlation.
It's not causation, right right, we know they're related, but
what's the connection between them. That's what you're looking for,
(35:02):
right exactly. That would be something that explains why time
can only go forward, right exactly. So if you just
take the second law and you add it to the
rest of physics, you can say, okay, there's a correlation
between time and messiness. So either the universe has to
go forward in time and get messier, or the universe
has to go backwards in time and get less messy. Right,
Either the second law of physics allows for both. Right.
(35:25):
So it doesn't actually tell you why time goes forwards.
It just says, well, if you pick forwards, then this
is going to happen. If you pick backwards, that's going
to happen. And there's a difference. So it breaks the
symmetry of forward and backwards, but it doesn't pick one.
Or or if you assume that messiness is kind of
where the universe prefers to be. Then that tells you
that forward time is the preferred thing in the universe. Yeah,
(35:49):
And it doesn't even tell you that entropy determines time, right,
It just tells you that they're connecting. There could be
something else, something deeper, which causes both time and entropy. Right,
that's the whole problem with like correlations. You know, you
can find correlations between between lots of things. It doesn't
explain it, right, what's some of those famous things? Like, Um,
you know global warming is correlated with the decrease in
(36:10):
the number of piracy events worldwide? Right? Does that mean
that pirates kept global warming at bay for hundreds of years? Certainly?
Not right? Um? So we know time and interropy are correlated.
We don't know if entropy causes time or the other way,
or something else deeper that we haven't even thought about yet. Um.
Or maybe it's just random, Like maybe it's a random
connection like pirates and global warming. I think we've observed
(36:34):
in grid enough detail that we know it better than
empirate than global warming. Um. But I love looking at
these correlations. You know, there's like a correlation between the
number of movies Nicolas Cage has done and the number
of people per year that die because they're twisted in
their own bedsheets. So they're correlated. When one goes up,
(36:56):
the other one goes up. Yeah, exactly, or they have right,
that doesn't mean they're going to continue in the few sure, right.
That's the problem with the observational correlations they don't tell
you is if you don't understand the mechanism of it,
you can't really argue that they're connected. They could just
be chance. Right, Um, I mean sometimes there are connections,
Like you know that the consumption of ice cream is
very well correlated with the number of murders. What. Yeah,
(37:20):
Well people kill people more in the summer, I guess
because they're grumpier and they're sweaty and hot. People also
ice cream in the summer. Right, So say some rude
cause cause is in charge of both ice cream consumption
and murder rates. Right. It doesn't mean that ice cream
causes murder, right that we know of that we know
don't understand everything. That's right, the fourth law of thermodynamics,
(37:42):
ice cream causes murder? Um? Okay, So entropies are only
kind of clue you're saying, And there's three possibilities. Either
time an entropy are not at all connected. They just
haven't to go in the same direction, or they are
(38:06):
both the result of something else deeper about the universe,
or maybe one causes the other or right, yeah, exactly
direct connect and we don't know and we don't even
know if there is much of a connection. And if
you know, there's a lot of deep questions. They're like
if time is because of entropy, then like what happens
when the universe reaches maximum entropy, because you know, if
(38:27):
entropy is always increasing and time is going forwards, then
there's no rewind right, the universe marches forward towards messier
and messier, and eventually you reach the heat death of
the universe when everything is perfectly spread out, right, perfectly disorganized,
there's no structures at all. Does time stop? Does time
turn around and go the other way? Like, you know,
(38:47):
just saying time is connected entropy doesn't answer most of
the deepest questions Time quit exactly. Some people have theories
that time stops and turns around, right, and the universe
then goes towards more organized configurations like maybe, um, I
love reading about theories of time because people really go bonkers.
(39:07):
One of my favorites is that is a guy who
asked the question, Okay, we have three or more dimensions
of space, why should we have only one dimension of time? Right? Like,
what if you had two dimensions of time? That's pretty
hard to think about, which is weird because it's not
so hard to think about two dimensions of space to
think about like you know, a plane instead of a line,
(39:29):
but it's pretty hard to think about two dimensions of time,
like I'll meet you at three o'clock in north and
you know four fifteen east west? Like what does that
even mean? What is before and after? Right? Oh? You
could have time coordinates? Yeah, yeah, you could have multiple
dimensions of time. Um, isn't that sort of like a
multiverse argument? Then, like there's maybe parallel universes besides this one.
(39:53):
Well that's a bit of a cop out, right saying
maybe there's every configuration and our universe just happens to
have one core a native time And that's the explanation.
I think there must be a deep reason. There must
be a um, something revealing about the structure the universe
as we've discovered it. There's it's got to be a
clue as the way the things actually work, you know,
and I suspect that it's a it's going to be
(40:15):
a really deep answer that when we figure it out,
it's gonna show us that the universe works really differently
from the way we imagined. That a huge parts of
our life, um, and the way we live and the
things we think are fundamental about the universe are accidents.
Are just constructs, right, they are timely accidents. Sorry, it
took a lot of time, but eventually we we created
(40:37):
this illusion of the universe. Yeah, alright, so then we
we haven't really answered the question. I mean, it sounds
like there is no answer, right, like why does time
only go forwards? We know it's related to entropy, but
we don't really know the connection, and that's that's kind
of as far as we know. I mean, there are
a lot of people out there thought about this much
more deeply than I have. Sean Carroll, Carlo Rovelli, these
folks have written whole books on time. But in my view,
(40:59):
the argument mostly boils down to, you know, either it's
connected to entropy, which I don't find that convincing an
explanation for why it goes forwards, right, or that space
and time itself are illusions and they come out of
something deeper and uh, you know, and we need to
dig deeper to understand like the string theory of the
universe or the quantum loops that make up the fabric
(41:20):
of space and time itself. Um, so we were pretty
pretty clueless as a field. It's a it's a definitely
an open time. Well until we figure it out. I
hope that people out there don't lose it and they
that they make the best use of it. Right. Yeah,
but it's not a waste of time to think about time.
It just takes a little bit of time, that's right.
(41:42):
But it's about time, and now it's time to the
end the podcast. That's right. So thanks for listening, And
if you have questions about big, basic fundamental questions of
the universe, send them to us. We love to dig
into them and explore our ignorance and the ignorance of
modern physics. So send us your questions at feedback at
Daniel and Y dot com. Yeah, thanks for listening. See
(42:03):
you next time or see you last time. We should say, oh,
see you last, see you, see you anything, see anytime.
That's right. Thanks for listening. If you still have a
(42:24):
question after listening to all these explanations. Please drop us
a line. We'd love to hear from you. You can
find us at Facebook, Twitter, and Instagram at Daniel and
Jorge That's one Word, or email us at Feedback at
Daniel and Jorge dot com. Thanks for listening, and remember
that Daniel and Jorge Explain the Universe is a production
of I Heart Radio from More podcast from my heart Radio,
(42:46):
visit the i heart Radio app, Apple Podcasts, or wherever
you listen to your favorite shows.
Hey or hey, I think it's time we confronted a
really tricky topic. Do you think we have time for that?
You know, there's no time like the present. It could
be a good time. It might be a fun way
to pass some time. All right, I'll make time for it.
It's about time. That's so corny. How many time puns
(00:28):
can one fit into a single podcast? That's right? How
much time do you have? Um, I've got time. I
think it's a timely set of puns. Well, you use
that pun time after time, commending else. Now is the
time to start the podcast. Now is the time to
tackle this tricky topic. I am and I'm Daniel. It's
(01:07):
time for our podcast, Daniel and Jorge Explain the Universe,
a production of I Heart Radio in which we take
everything about the universe, the now, the future, the past,
the deep, deep, deep past and try to explain it
to you. That's worry. We take the time to take
a little bit of your time and maybe get you
to understand a little bit more about this amazing and
(01:28):
spacious place that we live in called the universe. That's right.
And sometimes we have to grapple with topics that are
sort of obvious. You know sort of things that are
right in front of you and understand like does physics
know how to make sense of it? Does physics have
a good definition of it? Do we understand why it's
this way and not that way? That's right? And so
today we're tackling a topic that is probably in everybody's
(01:49):
mind all the time. That's right. This is something you
think about every day from when your alarm clock goes
off to when it's time to go to bed. To
stay on the podcast, we will be talking about out time.
What is time anyway? Yeah? And why does it only
go forwards? Why do you remember the past and not
(02:12):
the future? Can you remember the future? That would be
pretty cool. Well, that's the whole question. Right on one hand,
very intuitive understanding of time. We all know what time
is right, yesterday was yesterday, tomorrow's tomorrow, right now is now,
But when you get down to it, we don't really
understand why we have it right. Why is it like that?
Why does it only go forwards? Why is the future
(02:33):
different from the past. Yeah, it's weird to think that
we are in the pressing right now and there's a
past going backwards in time, and there's a future going
forwards in time? But what's really the difference between those
two things? Yeah? Why can't I see or feel or
have any kind of memory about the future? Yeah? Exactly
what makes it different? Why is the past fixed? Right,
(02:54):
unchangeable unless you believe in crazy science fiction time travel stories,
which I don't, and the future undetermined? Right? What's the difference?
Can physics reveal that? Is there some understanding of time?
From physics? It tells us why one is different from
the other? Yeah? Why is there an arrow of time
like a one way sign in the universe, in the
(03:14):
highway of time of the universe? Yeah, this is one
of those wonderful questions because on the surface of it
it seems kind of dumb, Like somebody asks you what
is time? You're gonna tell him, Oh, it's three o'clock.
And if they dig deeper and ask you though, like
what does time even mean? Man? Sometimes I say it's
two o'clock. Depending on the circumstances, I always give the
same answer no matter what. That way, people stop asking.
(03:38):
My kids are aways what time is it? And they
have like a watch on their hand. There's like two
watches on the wall, Like why are you asking me?
So I would just say it's three o'clock no matter what,
so that they stop. It sounds like it sounds like
the same parenting strategy my my wife has when we
going road trips to always ask how long are we
do we get there? Is it? Are we there yet?
And she always answers, nope, scant twenty more hours, no
(04:01):
matter what. You're like pulling into the place, and she's like,
twenty more hours, yeah, no matter what, she always says
twenty It's a great strategy. You know, me and your wife,
we would really get along. It seems like we see
eye to eye about how to handle these approaches. How Tom,
you would have some very um sarcastic kids probably exactly. Um. Anyway,
(04:23):
I love these questions you know that are like, on
the surface so simple, when you dig deep, they reveal
like enormous gaps in our knowledge about the universe. Those
questions are the best because those are the opportunities to
reveal that the universe is different from the way we understood.
Like the way we think about the universe reflects just
our experience, the way we've lived and grown up, and
(04:44):
not something fundamental, something universal, right, And that's the whole
goal of physics. Yeah, it's one of those questions, like
it makes you kind of look almost inwards or look
in your like you one day you just wake up
and you don't even know where you're standing, or how
how the house that you live in was built. Who
don't lose grasp of reality there? Or or hey, we're
(05:05):
trying to dig deeper, trying to make you go crazy.
Doesn't it make it kind of question at like the
very nature of the universe. It does. It's sort of
like if you say a word like a hundred times,
like say the word marriage a hundred times, it starts
to turn into a really weird word. Right, you take
it apart, you look at you like, that's really strange.
It's the same thing with the concept of time, Like
(05:27):
you know, pick up your hand and rub your fingers together, right,
you're feeling that right now? Right? But what does now mean?
Right now? Sort of like it's always it's infinitely short,
and it's slipping constantly into the past. You can never
grasp onto it, you can never hold it, right, it's
always you're always losing it. It's a really strange concept
of concept of now. Yeah. Yeah, although I don't know
(05:50):
if that's what makes marriage weird. But there's lots of
things that make marriage weird. Um, saying it a hundred
times is not it. But I remember reading a book
about consciousness back when I was really interested in the
science of consciousness, which we can talk about another podcast.
But the basic idea in that book, as by Daniel
Dennit it's called Consciousness Explained, is that there is no
(06:12):
now that you're the consciousness is basically an illusion that
all you're ever doing is remembering the immediate past. And
I don't know if I believe that, but it really
made me think about what now is and how you
can never really grasp it. It's always just like sliding
away from you and you know, whether it really exists,
whether there is a now, right, whether there's a special
instant that that differentiates between the past and the future
(06:35):
right right, because sort of technically in physics of time
could go both ways right like you could in time
is kind of just like an arrow you can flip
back and forth in the equations that you guys use, right. Yeah,
time is not really very central to physics. Yeah, I mean,
on one hand, it is. On one hand, it isn't.
Tom you could write a lot of physics down without
ever thinking about time. On the other hand, time is
(06:57):
essential property physics because we're trying to use physics to
predict the future. So um, this time is both like
not part of physics and deeply entwined in it at
the same time. So it's, uh, it's an important topic. Yeah, no,
And it's a topic that everybody experiences right at every
second of the day, every day of the year, every
year their lives. They're experiencing time, and they're moving forwards
(07:19):
in it, and they feel it, right. Yeah, they think
about it, they think about the future, they remember the past,
and so it kind of makes you wonder how many
people actually think about or know what time actually is. Yeah, exactly.
So I was wondering do people know why time goes forward? Like,
(07:39):
I mean, physicists don't really, but it's always fun to
ask general public questions that scientists don't know the answer to,
just to see what they come up with. So I
walked around the US of Irvine campus and I asked
people why does time only go forwards? Here's what people
had to say. Do you know why time only goes forwards? Um?
Not really, honestly, I have like some sort of like
(08:02):
intuition why it only goes forward? But I don't think
we can't explain that. Yeah, okay, cool, I have no idea. No,
I don't any guess. If I had to go, I
just said gravity or something. Okay, No, I don't any idea,
best guess. Sorry. I think time is lack of force
dimensional right, So actually I don't know, creaky right, Okay,
(08:29):
I'm not sure. Okay, Well, when I think of time,
I think of it kind of almost as a linear progression,
meaning that it's a constant in the universe, and it's
always kind of moving forward, and that's something that we
accept as being a fact of the universe, at least
as we understand it now. So in that regards, I
(08:49):
think that's why we can only see it as moving forwards,
because it doesn't kind of make sense to us or
it's not even physically possible currently for us to move
backwards in time, so we don't consider it in that way.
I think it might be a matter of observation. So
relative to us, time moves forward, and the way that
you experienced the world biologically is designed to be in
a manner that is sort of one dimentional and a
(09:12):
forward mind all the time. But in reality it might
be that the fourth dimension of time in space time
might just be a way of describing the universe that
the math predicts a from physics and my option perhaps
other related science fills. Okay, cool, Yeah, so I got
a pretty good mix of answers there, right from the
(09:33):
standard physics, like gravity, which is you know, that's a
good answer. You never know, it might be, right, gravity.
Somebody actually said gravity. I don't know gravity, It could
be it could be. Yeah, well, gravity is like the
answer to half the questions in physics, the way like
pie is the answer to half the questions in math. Right,
so if you have no idea, you could just say
gravity and have the time. You'll be right. Not the
(09:55):
movie the fourth right, yeah, the fabric of space time itself.
That's right, it's gravity. Yeah. And then you know, some
other folks had some more elaborate discussions, But people had opinions.
People did have opinions. Yeah, some of them were sort
of longer word salad. Right, you know, some of them
hit on some of the scientific concepts time being a
fourth dimension, etcetera. Um, but um, yeah, a lot of
(10:19):
people want to ask them this question. You could just
see in their faces that they had not ever considered this.
They're not even question they had imagined, but as soon
as they thought about it, they realized they didn't know.
And that's my favorite part of this this topic is
that nobody, nobody has really thought about it in terms
of general public, but as soon as they do, they're
very curious. Right, this is the best questions. It's amazing
(10:40):
how little you actually kind of need to know about
the universe to live a full and happy life, you
know what I mean, that's the protective happy life. There's
so much you don't need to know. Ignorance is bliss,
you know, that does have a meeting. People have been
living in this universe for hundreds of thousands of years,
depending on how you count the beginning of humanity, and
(11:01):
there's been a lot of happiness and joy without really
any understanding. I mean, I would say people five years
ago basically knew nothing about the way the universe actually works.
But you know, people had birthday parties and eight cakes
and had moments of joy. So yeah, I mean like
a hundred only a hundred years ago, right in the
nine hundreds, early nineteen hundreds, people didn't know the concept
(11:25):
of Adams or quantum physics, right, Yeah, that was about
a hundred years old. We didn't know how long the
universe had been around, right, so pretty basic stuff about
the way the universe works is pretty modern. Right, So
we didn't know there were other galaxies, right, No, they didn't. Um,
So what's your point here? Your point is you don't
need physics to be happy? Is that what you're trying
to say is irrelevant happiness? You think of how much
(11:46):
of a richer life you can lead if you know physics. Okay,
I like the way you spun that back, right, using
all this technology to talk to me. That's based on physics,
not not like not like money rich, but you know,
like spiritually right right? Um? Isn't your career based on
technology that was developed with the help of physicists? Where
(12:07):
would you be? Isn't your science dependent on technology? Yes?
I am a tiny little mite standing on the shoulders
of giants, absolutely, But so are we all? So are
we all? Yeah? Well maybe it's time we got back
on topic here, and um, so let's break it down
(12:28):
for people, Daniel, what how do physicists see time? Like
if if you're at a conference and you guys are
talking and somebody said, hey, guys, what is time? What
do you think people would generally say, I think there'd
be a lot of disagreements because we don't really have
a great description of time, you know, one that really
makes sense. And you know, the way physics approaches the
problem is first you try to build like a model
(12:50):
of the problem. You're trying to describe it in a
way that we can grapple using the language that we're
familiar with, which is mathematics. You try to find an
equation that tells you what what's what's happening? Yeah, exactly,
you know, something's happening in the real world. Maybe it's
like a ball is being thrown the air, and we
try to understand it. So we write down a bunch
of equations to describe it um and that lets us
(13:13):
build that model in our heads and manipulate it and
then use information from the model to describe what's happening
in the world. Cool. How do we do that with time? Right?
How do we ask this question about like why can
I remember the past and not the future? Why does
ice melt but not unmelt? Right? These kinds of questions
about time, it's pretty hard to to wrap your mind around.
(13:36):
One approach is to think about the universe in terms
of a bunch of snapshots. Like you think about a movie. Right,
what is a movie? It's not really a continuous experience
in time. If you're looking at the screen, you're not
really seeing smooth motion. You're seeing a bunch of snapshots.
You're seeing like a little cells in a film, or
like a bunch of screenshots. Yeah, it's a bunch of screenshots,
(13:58):
and they're just packed so tightly together that you don't
notice that they're not actually smoothly varying. Right. Your brain
does the interpolation for you. It tells you a story.
So sometimes we think about the universe that way. We
think about like all the universe at this moment, and
then all the universe at the next moment, and all
the universe at the next moment, like snapshots of the universe.
(14:19):
By snapshots, I mean like what is every particle doing
and where is it going? Yeah, exactly and time. Then
is a way to sort of order those snapshots is
to say this one first, that one next, that one next,
um and to sort of put them together. This one
can go in front of the other one. Yes, you
know what I mean. It limits um the ordering. Yeah,
(14:41):
And in that sense, physics. The job of physics is
to take all the snapshots in the past and predict
the ones in the future and to say, Okay, according
to the laws of physics, the next snapshot will look
like this, or if you want to talk quantum mechanically,
there's a probability distribution of the next snapshots. These are likely,
these are unlikely, these are impossible. Right, So physics um
(15:04):
takes sort of that view of time, and there's a
bunch of caveats there. Like, you can't have a snapshot
of the whole universe, right because time is not universal.
It means different things for different people. You can't know
everything about the universe. But you know, you take a
small enough model or a single particle or something, you
can also have quantum and you also have quantum states,
right Like it' isn't that prevent you from getting an
(15:25):
exact snapshot of the universe? Yeah, exactly. So you can't
really get an exact snapshot, but you could do a
quantum mechanical version. You could say, well, what's the quantum
state of all the particles or you know, what's the
wave function. What's the state of the wave function right
now that you can talk about, and that determines probabilities.
I see, So you describe me kind of how physicists
see the universe. They don't see it as um. They
(15:46):
see it as as this kind of sequence of snapshots
of how things are arranged. Yeah. For example, think about
the Shortinger equation. The Shortinger equation famous equation describes how
a particles a wave function um moves through time, right,
and it tells you how it moves. It says, if
you have this wave function now and you experience this,
(16:07):
then your way function will be that later. Right. So
that's that's what physics tries to do, or or go
back to them making the mechanical example of a ball
going through the air right tells you, well, you you
shot your ball in this direction. Where is your ball
going to be in the future. So that's the role
of time in physics, right to predict the future snapshots. Oh,
I see. So to you guys, time is like a
(16:27):
like an input that tells you what the universe is
likely to do. You you know what I mean? Like
you do you It's an input. It's not like an
output or something that you can vary or that it
is affected by all the other variables. Yeah, exactly. And
so for example, you want to predict the flight of
your ball, it's an input. You can dial that knob
(16:48):
and say I want to know where the ball is
going to be in one second or five seconds or
eight point two nine seconds or whatever. So it's sort
of that knob. The thing we don't the thing that
that doesn't explain at all is like why it only
go forwards and who's controlling how fast that knob is turning?
Or why do we have time at all? Why don't
we just have a static universe? It's just sort of
like there, you know, it's the first step. Of course,
(17:12):
they just try to describe it, and then you can
build from that and try to answer some of these questions.
But you mean, like, how come we don't have a
static universe, Like, um, why do we have time at all?
Boom right, I don't know, you don't know, nobody knows, Like, well,
I know, there's this idea that time is the fourth dimensions.
We have three dimensions up, down, left and right, backwards
(17:32):
and forwards, and maybe time is just another direction of
the universe. And so we and I think the days
that we do sort of exist in this a state
of being still in all four dimensions, but it just
so happens that we somehow feel one of the mensitions.
Do you know what I mean? Yeah? I love the
(17:53):
it just so happens part of that explanation, right, Okay,
here's the hardest part. I'll just YadA, YadA, YadA over
that bit. That's that's my that's my Nobel Prize, Nobel
Prize winning paper officer. It just so happens. I was
going ninety miles an hour. Um, you know, I can't
explain it now, You're right. Um, time is sometimes thought
of as the fourth dimension. And so let's dig into
(18:13):
that a little bit. What does that mean? Why do
we think of it that way? We think of it
that way because it's helpful mathematically. Like when Einstein was
developing his theory of special relativity and we talked all
about time dilation and stuff like that on a previous episode,
he discovered that the equations have a certain symmetry. They
look really nice like that you can write them down
really simply and compactly if you include time as the
(18:37):
fourth dimension of this larger concept he called space time.
So he took three dimensions of space. He tacked on
a fourth dimension, which is time, and he constructed this
thing called space time. And because your perception of time
depends on where you are in space and how fast
you are moving through space, the equations get much simpler
if you think about all four dimensions in that way, right,
(19:00):
if you think about time parallel to space in that way.
And that's a clue when you write down equations, and
they become simpler if you think about them a certain way.
That's a clue from the universe that it's maybe the
right way to think about things. Okay, so that's um,
that's time is a different dimension. Let's get more into it.
Let's go down that rabbit hole. But first let's take
a quick break. So I think it's fun to think
(19:33):
about time as the fourth dimension. But it's also kind
of a trick, right, It's not. It doesn't really fit.
So wait, So the idea that it's a fourth dimension
you're saying, came from mathematical convenience by Einstein, do you
know what I mean? Like, it's not. It was just
easier to treat it like a dimension. Yeah, And I
(19:54):
wouldn't you blow that off? Mathematical convenience is not a
small thing, you know. The whole goal of physics, remember,
is to like write down an equation of the universe
in one line. And so if you can write things
more simply, that's a clue that it's probably more correct,
it's more and it's like a deeper understanding. So yeah,
he discovered that if you write things down with time
(20:14):
is sort of the fourth column of your vector, then
a lot of the equations are simpler to write down.
There's it's there's a connection. There's a symmetry there. Right.
We talked about symmetry in another episode. You can treat
time the same way you treat space in many ways.
And that's that's tempting because you think, oh, well, that
answers the question time is just another dimension, right, But
it doesn't answer the question because it's not just another dimension. Right.
(20:37):
You know time is not space? Right? Time? What does
that mean? Time is not space? Well, for money, you
can move forwards and backwards in space, right, you can
go left, you can go right, you have some control
over it. You can't do that with time. But isn't
it just sort of a matter of perspective, Like I think,
like in the grand scheme of the history of the universe, Um,
(21:01):
you know, I existed for this amount of time, but
while I'm living it, I can only go forward. I
kind of get the experience of moving forward in time. Yeah,
that's true. Technically, I sort of existed all throughout my
life in space time, you existed all throughout your life, Yeah,
but you don't exist all throughout time. Right, There's no
limit to where you can visit. In space, you can
(21:21):
go from here to there to the other places. But
in time, you can only visit between your birth and
your death, and you can only go to each time once. Right,
that's not true for space. I imagine there are lots
of places you go to many many times, like your
refrigerator or your bed. Right, Um, you visit those places
many times. So space is quite different from time. Right,
(21:43):
there's a special loops in space, but I can't do
loops in time exactly. And time time has this special difference.
Right then now, right, there's no now in space. I mean,
I'm here, I'm there, you're over here, but there's no
there's no special location in a what time has this
special location? This thing we call now, which exists weirdly
(22:05):
and slides forward weirdly. So I was saying earlier, wh
why isn't the universe static? Why is time move forwards?
Why isn't it just stuck at to equal zero? Nothing happens?
What's pushing it forward? What's turning the engine of the universe? Right?
I guess what I mean is, you know, if time
is you can treat it as a fourth dimension, then
you know, our experience of the universe is at time
(22:27):
is constant and the moves at a steady pace, and that, um,
you're only going forwards in it, right, whereas in space
you're saying we can go backwards. But what if I
just kind of take another dimension and use that as
my kind of ticker? Which dimension would you use as
your clock? I don't know? Up, okay, yeah, so you'd
(22:51):
be like, hey, let's have lunch at x equel's five
meters or something. Yeah, I don't know, or you know,
kind of mathematically, what does that mean? Different? Time and
space are not the same. You can't just swap out
time for space because you don't have the same freedom
and time as you do in space. Right, if if
space were moved always forward, the way time did, Like
(23:12):
you could never go back to anywhere, right, Like, oh yeah, hey,
I had lunch and now is sort of sliding slowly
away from me. You know, Um, it would be pretty weird,
but it's very natural experience. So I see you're more
limited in the time dimension exactly. There's some extra rules
that seemed to apply to the time dimension that don't
apply to the other dimensions. And that's weird. And anytime
(23:33):
we see symmetries where like cool. But then when those
symmetries are broken, we're like, Okay, time is like the
other dimensions, but it's different. Then we ask why is
it different? How the different? What makes it different? Because
that's the clue to solve that, not the other puzzle. Right, Okay,
so time is it is sort of a fourth dimension,
but there are special rules that apply to it that
makes us think that maybe it's not really a fourth dimension.
(23:55):
Maybe it's just like a mathematical um good cluage, right, yeah,
Or maybe there's two kinds of dimensions. There are space
dimensions and time dimensions, right, and yeah, and maybe there
are other people haven't imagined yet, you know. Yeah, that's
that's the kind of mind blowing, like look at the
universe in a different way, discovery that I always hope
(24:17):
to make in science, you know, to like crack something
open that's so deep that it like nobody gets it
but the stoner's man. Okay, so, so what are these rules?
The rules are that you can only you can't visit
two times in your you know, you can't go back
(24:37):
in time, you can make gloops, and it can only
go forward. Are those mainly the main restrictions here with time? Yeah, exactly,
those are the main restrictions. And I think the goal
of physics and the goal of our podcast is is
to try to understand where that comes from. Like can
we look at the laws of physics and say, oh,
that comes from this, you know, like the way we
can look at the laws of quantum mechanics and from
(24:59):
that understand in the way hydrogen works or whatever. We
want to look at the laws of physics and say
do they require this behavior? Is it necessary? What does
it come from? And it's the deepest level, right. The
problem is when we look at the laws of physics,
we don't get those kinds of clues. When when we
look at the laws of physics, we don't so the
equations in your model of the universe don't tell you
(25:21):
that there should be restriction some time. Yeah, let's go
back to those equations about like the ball moving. Right.
Imagine you throw a ball, right, you're talking about basic stuff, Right,
you throw a ball in the air, it goes up
and it comes down. Let's start with the simplest case
with there's no air resistance, Right, it's passed up and
it's passed down. Are gonna be very similar. In fact,
(25:42):
if somebody took a video of that and then plated backwards,
they couldn't tell whether the video is going forwards or backwards. Right,
Physicists analyzing the motion of the ball couldn't tell you
whether the video was going forwards or backwards. So if
you fed it a time, like if you gave it time,
it would give you the same oppu whether you put
(26:02):
negative time or positive time exactly how it breaks down
exactly because the equations work in both directions. Right. The
equations have time in them. They tell you how things
change with time, but they work the same forwards and backwards.
So they don't tell you which way the universe should run, right,
They tell you how the universe changes with time, but
they don't say why it has to go forwards and
(26:23):
not backwards, why it has to go anywhere at all?
Or I can't go forwards and then backwards. Right, So
if the equations of the universe really represent the universe,
then the universe would is okay with traveling backwards and time.
You're saying exactly, the equations sort of saying like, hey,
you want to go backward this time, Sure, here is
what would happen. Yeah, exactly, And it's sort of like
(26:44):
you were saying earlier. It's like an input. You know.
Imagine you're doing some calculation and the calculation is a
function of position and time. Right, you want to know
how many balls are at this location at this time. Right,
you you drop a thousand balls into a box and
you're wondering, like, how many balls are this location the time?
You can ask that question of any point in space,
at any point in time. Right. You can take the
(27:05):
concercurrent configuration and you can evolve it forwards or evolve
it backwards. The laws allow for all of that, but
for some reason that there there's a difference in the
way it actually works in the real world. Right, things
slide forwards in time, and we don't know why. But
there's no restriction on the space parts right there. The
equations are functions of space and time, but there's is
(27:25):
like extra restriction on the time part of it. When
it comes to like actually implementing the universe, right, it's
kind of like, um, we have them now, and if
all we had was now, the equations would let us
tell what happened before in negative time, and we would
let it tell us what happens forwards in time, right,
exactly the equations. Yeah, but but someh our experience. We
(27:49):
only have experience of the stuff that happened in negative time.
That's right. We only remember the past, right, that's the
weird part, right, The past we can remember, we have
access to it. Um in a affects us. The future
we can't. And and and most of the laws of
physics is an asterisk there. Most of the laws of
physics are totally symmetric. Right. They don't care whether the
(28:10):
universe is going forward or backwards. They're happy to do
either one. Wow. So that's the mystery is the time
would be sort of a fourth dimension, but there's some rules,
but they those worlds are sort of not in the
equations of the universe. That's the game with an asterisk,
you're saying, right, Yeah, So the game is figure out
(28:31):
if the laws of physics that we know of, that
we've discovered, that we've written down, if those require things
to move forwards? Um? And if so, you know, does
that give us a clues to why it's happening? Or
maybe there are other laws of physics out there that
are much more sensage to time, that are the ones
making time go forward. What do you mean, Like, so
you're looking for something in the equations that would require
(28:51):
time to only move forwards? What does that mean? Like
if you put negative time, you should break or or
yeah exactly, Like let's think about that. Are there is
there anything in physics that does require time to move forward?
So let's go back to that ball example. Right, I said,
if you throw a ball in the air, Um, you
can't tell whether the video of that ball in the
(29:13):
air goes forwards or backwards. And when we did that,
I said, ignore air resistance. And that's for an important reason, right,
because air resistance changes the flight of the ball, right,
slows it down, so that in reality, if you throw
a ball in the air, right, Um, it's going to
slow down. And so you can tell the difference between
going forwards and going backwards right, right, because there's a
there's like you lose some heat or you lose some
(29:35):
energy in reality, right. Yeah, it's probably similar to think
about like dropping a ball. You know, if you drop
a ball and there's no friction, it's going to come
all the way up to where where you dropped it from.
If you drop a ball, it is air resistance. It's
gonna lose some energy to bumping against all the air molecules. Right,
It's not gonna come up quite as high as you
dropped it. You just keep letting it bounce. Eventually it's
(29:56):
it's gonna dribble down and stop right right. And that one,
that one to you played it backwards, you would be
able to tell the difference exactly that one if you
played backwards. And that's the case for most things, right.
So where does that come from? Well, that comes from entropy, right,
that comes from heat. The heat really there is the key.
It turns out that the universe likes to go from
(30:17):
organized to disorganized, right, And that's this concept we call entropy. Yeah,
I've heard of this before, but before we dive in.
Let's take a short break. What do you mean when
(30:39):
you say that the universe likes entropy or that it
likes to go from order to disorder? What is it?
What is it liking? Me? It means that if the
universe had a bedroom, it would be a mess, right,
and it would never want to clean it up. No,
it's obviously that is our experience of it. Like, maybe
what if there are aliens out there who experienced time
backwards and they're like, why does the universe like order?
(31:03):
Everything we see? It seems to get ordered exactly. So
there's a big flaw in this argument. But let's put
a pin in that and come back to it once
we've done constructing the argument, then we can take it
apart um. Yeah, so you're right that the universe likes it. Well,
it just means that we've noticed, we've observed that disorder increases, right.
For example, the classic example is you have a box
(31:25):
and you put put a bunch of gas particles in
the corner. Right, that's very organized. It's like only one
way to arrange or a few ways to arrange gas
particles in the corner. What happens if you run the
clock forward? If you let that go, Well, if they're
not totally frozen, they're gonna spread out through the box. Right,
It's like we pour water onto a surface that spreads
out things like things tend to go from organized configurations
(31:48):
to disorganized. And that's just an observation. That's not a
deep insight into the universe. It says, here's what we've seen. Right.
You notice that there's a correlation between positive time and
messiness exactly, And that's called the second law of thermodynamics. Right,
it says that entropy always increases. Entropy is a measure
of disorder. Right, So things are basically always spreading out
(32:11):
and um and getting more diffuse, right, And that's the
whole universe is doing that, Right, The universe, like on
a grand scale, is spreading out and getting smoother and
stuff like that. Is it a law law, like an
equation or is it more like an observation, Right, It's
more like we've never seen something good un messy with
positive time. Yeah, that's that's a great question. It's a
(32:33):
really deep question. I love that question. I think the
answer is this, Um, The answer is that it's an observation.
It's something we've noticed and we've never really seen it broken,
and so we thought, well, this must be important, so
let's write that down. Let's call that a law because
it's something we've noticed happens all the time. Now, you
can never get broken. Why is that? Does that reveal
something deeper about the universe? Can we explain that from
(32:55):
something else? And if you dig into like statistical mechanics,
you can and derived the second law of thermodynamics from
simpler assumptions, like if you say every possible configuration of
the universe has equal probability, then it turns out there
are more messy probably messy configurations than unmessy ones, right,
(33:15):
Like there's a thousand ways to have gas particles spread
out through a box, but there's only a couple ways
to have them stuck in the corner. So it's just
more likely to end up with the messy ones because
there are more messy configurations. But that's sort of a
cop out. I mean, it's just sort of another way
to state that, is to say, well, we you can
call this a deeper understanding, but it's also just like
(33:38):
a posture that we came up with so that we
could derive the second law, which we've observed. Right, You're
you're basically down to the same argument you were saying.
You're basically saying that the reason we have entropy is
that when you move forward in times, you go through
the most likely scenario. But then you can ask, why
does when you go forward in time do you go
to the most like exactly, it's just a restatement. Really,
(33:59):
it's a sort of more fundamental statement of the same
basic observation that things go from organized to messy, right,
and um, yeah, exactly. So we were looking for, um, like,
something in the laws of phasics that told us why
going forwards in time is is preferred, And you're saying,
(34:19):
we have this observation that it always does and it's
somehow related to chaos and order and messiness. That's right,
And so this is like the only thing we can
really get our fingers on. You know, everything else in
physics is time symmetric. And there's another asterisk there. There
are some things in particle physics which are time asymmetric.
Those are really really small effects, and we don't really
(34:40):
think that that can explain the direction of time. Um,
But we can cover that in a whole other podcast
topics sometimes, but Basically, entropy is the big one. Entropy
is the only law that like clearly as you say,
has a correlation. But that's key. You made this point,
which I think is very insightful, was that there's a correlation.
It's not causation, right right, we know they're related, but
what's the connection between them. That's what you're looking for,
(35:02):
right exactly. That would be something that explains why time
can only go forward, right exactly. So if you just
take the second law and you add it to the
rest of physics, you can say, okay, there's a correlation
between time and messiness. So either the universe has to
go forward in time and get messier, or the universe
has to go backwards in time and get less messy. Right,
Either the second law of physics allows for both. Right.
(35:25):
So it doesn't actually tell you why time goes forwards.
It just says, well, if you pick forwards, then this
is going to happen. If you pick backwards, that's going
to happen. And there's a difference. So it breaks the
symmetry of forward and backwards, but it doesn't pick one.
Or or if you assume that messiness is kind of
where the universe prefers to be. Then that tells you
that forward time is the preferred thing in the universe. Yeah,
(35:49):
And it doesn't even tell you that entropy determines time, right,
It just tells you that they're connecting. There could be
something else, something deeper, which causes both time and entropy. Right,
that's the whole problem with like correlations. You know, you
can find correlations between between lots of things. It doesn't
explain it, right, what's some of those famous things? Like, Um,
you know global warming is correlated with the decrease in
(36:10):
the number of piracy events worldwide? Right? Does that mean
that pirates kept global warming at bay for hundreds of years? Certainly?
Not right? Um? So we know time and interropy are correlated.
We don't know if entropy causes time or the other way,
or something else deeper that we haven't even thought about yet. Um.
Or maybe it's just random, Like maybe it's a random
connection like pirates and global warming. I think we've observed
(36:34):
in grid enough detail that we know it better than
empirate than global warming. Um. But I love looking at
these correlations. You know, there's like a correlation between the
number of movies Nicolas Cage has done and the number
of people per year that die because they're twisted in
their own bedsheets. So they're correlated. When one goes up,
(36:56):
the other one goes up. Yeah, exactly, or they have right,
that doesn't mean they're going to continue in the few sure, right.
That's the problem with the observational correlations they don't tell
you is if you don't understand the mechanism of it,
you can't really argue that they're connected. They could just
be chance. Right, Um, I mean sometimes there are connections,
Like you know that the consumption of ice cream is
very well correlated with the number of murders. What. Yeah,
(37:20):
Well people kill people more in the summer, I guess
because they're grumpier and they're sweaty and hot. People also
ice cream in the summer. Right, So say some rude
cause cause is in charge of both ice cream consumption
and murder rates. Right. It doesn't mean that ice cream
causes murder, right that we know of that we know
don't understand everything. That's right, the fourth law of thermodynamics,
(37:42):
ice cream causes murder? Um? Okay, So entropies are only
kind of clue you're saying, And there's three possibilities. Either
time an entropy are not at all connected. They just
haven't to go in the same direction, or they are
(38:06):
both the result of something else deeper about the universe,
or maybe one causes the other or right, yeah, exactly
direct connect and we don't know and we don't even
know if there is much of a connection. And if
you know, there's a lot of deep questions. They're like
if time is because of entropy, then like what happens
when the universe reaches maximum entropy, because you know, if
(38:27):
entropy is always increasing and time is going forwards, then
there's no rewind right, the universe marches forward towards messier
and messier, and eventually you reach the heat death of
the universe when everything is perfectly spread out, right, perfectly disorganized,
there's no structures at all. Does time stop? Does time
turn around and go the other way? Like, you know,
(38:47):
just saying time is connected entropy doesn't answer most of
the deepest questions Time quit exactly. Some people have theories
that time stops and turns around, right, and the universe
then goes towards more organized configurations like maybe, um, I
love reading about theories of time because people really go bonkers.
(39:07):
One of my favorites is that is a guy who
asked the question, Okay, we have three or more dimensions
of space, why should we have only one dimension of time? Right? Like,
what if you had two dimensions of time? That's pretty
hard to think about, which is weird because it's not
so hard to think about two dimensions of space to
think about like you know, a plane instead of a line,
(39:29):
but it's pretty hard to think about two dimensions of time,
like I'll meet you at three o'clock in north and
you know four fifteen east west? Like what does that
even mean? What is before and after? Right? Oh? You
could have time coordinates? Yeah, yeah, you could have multiple
dimensions of time. Um, isn't that sort of like a
multiverse argument? Then, like there's maybe parallel universes besides this one.
(39:53):
Well that's a bit of a cop out, right saying
maybe there's every configuration and our universe just happens to
have one core a native time And that's the explanation.
I think there must be a deep reason. There must
be a um, something revealing about the structure the universe
as we've discovered it. There's it's got to be a
clue as the way the things actually work, you know,
and I suspect that it's a it's going to be
(40:15):
a really deep answer that when we figure it out,
it's gonna show us that the universe works really differently
from the way we imagined. That a huge parts of
our life, um, and the way we live and the
things we think are fundamental about the universe are accidents.
Are just constructs, right, they are timely accidents. Sorry, it
took a lot of time, but eventually we we created
(40:37):
this illusion of the universe. Yeah, alright, so then we
we haven't really answered the question. I mean, it sounds
like there is no answer, right, like why does time
only go forwards? We know it's related to entropy, but
we don't really know the connection, and that's that's kind
of as far as we know. I mean, there are
a lot of people out there thought about this much
more deeply than I have. Sean Carroll, Carlo Rovelli, these
folks have written whole books on time. But in my view,
(40:59):
the argument mostly boils down to, you know, either it's
connected to entropy, which I don't find that convincing an
explanation for why it goes forwards, right, or that space
and time itself are illusions and they come out of
something deeper and uh, you know, and we need to
dig deeper to understand like the string theory of the
universe or the quantum loops that make up the fabric
(41:20):
of space and time itself. Um, so we were pretty
pretty clueless as a field. It's a it's a definitely
an open time. Well until we figure it out. I
hope that people out there don't lose it and they
that they make the best use of it. Right. Yeah,
but it's not a waste of time to think about time.
It just takes a little bit of time, that's right.
(41:42):
But it's about time, and now it's time to the
end the podcast. That's right. So thanks for listening, And
if you have questions about big, basic fundamental questions of
the universe, send them to us. We love to dig
into them and explore our ignorance and the ignorance of
modern physics. So send us your questions at feedback at
Daniel and Y dot com. Yeah, thanks for listening. See
(42:03):
you next time or see you last time. We should say, oh,
see you last, see you, see you anything, see anytime.
That's right. Thanks for listening. If you still have a
(42:24):
question after listening to all these explanations. Please drop us
a line. We'd love to hear from you. You can
find us at Facebook, Twitter, and Instagram at Daniel and
Jorge That's one Word, or email us at Feedback at
Daniel and Jorge dot com. Thanks for listening, and remember
that Daniel and Jorge Explain the Universe is a production
of I Heart Radio from More podcast from my heart Radio,
(42:46):
visit the i heart Radio app, Apple Podcasts, or wherever
you listen to your favorite shows.
Hosts And Creators
Daniel Whiteson
Kelly Weinersmith
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