December 26, 2019 • 43 mins
Find out how relativity affects our perception of time with Daniel and Jorge
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Speaker 1 (00:08):
Ay, Daniel, what's the most mind blowing thing that physics
has taught us about the universe? Wow? It is hard
to pick just one. Could I do a top five? Maybe? No?
I want your single all time one thing that you
think is the most bonkers that we've learned about Who
(00:28):
had to challenge? All Right? I got one. There is
one thing about the universe that, to me is the
most brain scrambling thing I've ever learned. The hardest thing
for me to get my brain around is that people
can disagree about the order in which events happened. We
don't all see the same things happening sometimes in the universe. Yeah,
(00:50):
you can see things happening in one order, and then
I can see things happening in a totally different order,
and we can both be correct. Hi'm poor hammade cartoonists
(01:16):
and the creator of PhD Commics. Hi. I'm Daniel. I'm
a particle physicist, sometimes in a podcast host other times,
and everybody disagrees about when I do what. Welcome to
our podcast. Daniel and Jorge agree to disagree about the universe,
in which we explained to you the craziest, the most
amazing but true things about our universe. Yeah, and this
(01:40):
is again a production of I Heart Radio, and in
our podcast we try to take you to the forefront
of science to talk about the craziest things that scientists
are thinking about today, and also delve into the history
of science to tell you why we think the universe
is so nuts and how we figured that out. Is
there a history of science, Daniel, or is that also
open for this agreement people disagree about in what order
(02:02):
we discover things, or what was the most important, or
how we change people's minds. Yeah. I think in our
podcast we also not just talk about scientific discoveries or
scientific theories, but also a little bit about how that
affects how we see the universe, you know, and how
it challenges what we believe about how the universe works. Yeah. Absolutely.
(02:26):
I think a lot of the deepest questions in physics
come from a sort of a philosophical motivation. We want
to know how the universe works because it matters to
us that we can make sense of the universe. And
when we ask the universe questions and it gives us
answers that force us to totally change the way we
think about the universe. That has pretty deep philosophical implications,
(02:46):
and so that's sometimes the most fun topic. Yeah, And
so basically, we're just trying to blow your mind in
the gentlest, most podcasts friendly way possible. Or we're trying
to share with you how the minds of physics this
have been blown, and we want your mind to be
blown just as much. But yeah, a little bit more gently. Yes, please,
I'm not sure my brain can handle this much mind
(03:08):
blowing is in in one hour. But you know, when
I was learning physics as an undergrad, I remember these
moments when I finally really deeply understood a concept, and
that concept would force me to relax or destroy or
get rid of some assumption I had about the universe,
and I realized that that just wasn't true, that the
way I'd been thinking about the world was just fundamentally wrong. Yeah,
(03:31):
it's amazing. And so today we'll be talking about one
such topic that honestly gives me a headache every time
we talked about it, Daniel. We've written about it in
actually a couple of chapters in our book We Have
No Idea, A Guide to the Unknown Universe, And I
have to admit I never really got my head around it.
I authored the book, but didn't actually understood what was
(03:53):
in it. Yeah, and this is a topic that people
email us about, they ask us about, they listen to
some of our podcasts about it, and still right in
with questions. I think there's a lot of popular misconceptions
about how this works and some consequences that I think
a lot of people have not really realized. There's some
really deep philosophical implications to these ideas that I don't
(04:13):
think have been widely enough appreciated. Yeah, So to the
on the podcast, we'll be talking about how does relativity
affect our understanding of time? Time? Or I guess the
short title is is there a consistent order of events
(04:36):
in the universe? Yeah, and spoiler alert, the answer might
be it's relatively surprising that the answer is not what
you expect. We've talked to this podcast before about time dilation,
the effect of speed on the rate of which clocks
move forward, and that's weird and that's hard for people
to get their mind around, and there's some misconceptions there.
(04:57):
But you know, time and relativity is even weirder than that.
It's more than just making things run slowly or making
things run quickly. It's about how things happen here versus
how things happen there. Do you think age also affects
how you perceive time? Like I feel like my kids,
I have no patients at all, and that the world
just moves at a snail's pace for them, whereas for me,
(05:20):
as I get older, things he's moved fast. Well, I
mean I moved slower, but the world seems to move fast. Yeah,
And I think that Actually you raise an interesting point,
because we are all imperfect observers if you just sort
of watch something and describe it later. There's all these
studies about how people give terrible accounts, you know, even
eyewitnesses give different accounts of the same events, And so
(05:42):
that's certainly one problem. It's like people are unreliable observers.
But even in the case when everybody was a perfect observer,
everybody had a video camera, I've already had, you know,
perfect measuring devices, and everybody accounted for transmission of light
and all that stuff. Even in those cases, it's hard
to make sense of the universe. Like if we were
all robots but the exact same you know, clock in
(06:04):
our chips, and we were all measuring things exactly, we
it would still be kind of a weird universe where
not everyone would agree maybe on the order of events, Yeah, precisely.
And you have to separate those because we're interested not
in the question of our people good eyewitnesses, because we
already know the answer to that is no um, but
the question of how does the universe actually work, because
(06:26):
we want to know the answer to the question, not
just for humanity, but in general, you know, the deep
fundamental questions, so that when the aliens come, when we
talk to them about physics, we can make sense of
what they've learned. And we don't want to learn human physics,
we want to learn fundamental physics. And so this idea
of time dilation, this idea that time move slowly depending
on maybe how fast you're going or or where you are,
(06:50):
is kind of weird and pretty mind blowing. And so
we were wondering out there how many people really sort
of understood what it means and what the ramification, what
it means for our basic understanding of time. So I
walked around campus that use the irvine, and I asked folks,
not people in the physics department, not people who have
(07:11):
taken my class on special relativity, just random people on campus,
willing to answer questions from a scrafty looking physicist. So
I think for a moment, what do you know about
how relativity affects our understanding of time? Here's what people
had to say. Do you know how relativity affects our
understanding of time? No? I do not. Actually, no, I'm
(07:32):
not share Newton's theory of relativity. I'm sorry, I don't.
Is this physics like a physics question, I'm not. I
don't know much about physics. Okay, I don't know how
to answer that. I'm an English Ready, if there was
a whole like demonstration of that, where if like you
go at like in a spaceship, like near the speed
(07:53):
of light or something, and then you can come back
and then like some amount of time has changed on
Earth while like no, really time has changed for you
or somewhere about and thinking Planet of the Apes when
they're in the spaceship and then by the time they
come back, they've only aged a couple of years, but
timeline Earth has passed a lot because they're traveling at
a different speed. Remain Planet Apes is a well known
physics documentary. Right, yes, yes, I believe relativity. That's from
(08:19):
Einstein is the idea that like time doesn't flow at
the same rate like across the universe. I think something
like I think gravity or something affects like how it
flows in other places. All Right, a couple of knows,
but a couple of people did have a lot of
pretty good ideas, or at least they said a lot
(08:39):
of physics terms. And I mean like Newton's theory of relativity,
like the Planet of the Apes. You know, I've seen
that movie. Yeah, Planet of the Apes. That's a that's
a documentary about physics. Yeah, there you go. Now, you're right.
People had the people who knew anything about it definitely
knew that relativity and time connected and that relativity changes
(09:02):
have time moves. And I think already that's a big success, right,
the idea that time is not universal, that we don't
have clocks everywhere in the universe ticking forward at the
same rate. That's a big step forward to to break
that assumption. You don't think you didn't have a theory
of relativity. I mean he could have had one about
his relatives or something, but just not not the one
(09:22):
that caught up right. His theory relativity was don't bring
up politics at Christmas dinner. Yeah, also also useful things
to live by. But yeah, I think it seems like
maybe there is a general sense right that relativity effects
our understanding of time and makes things weird, but maybe
(09:44):
a lot of people don't know how or what. Yeah.
And I think also there's a lot of misunderstandings based
on the emails that we get from listeners who are
asking us questions about relativity and tweets and stuff that
people have posted. I have the feeling that people think
that if you move fast and your clock will slow down,
which is not the right way to think about time
(10:04):
and relativity. That's how I thought time and relativity works.
So we're in for a long conversation. So, um, so
people have been writing with questions about time. Yeah, a
lot of people ask the questions like what is it
like to be a photon? Or how does a photon
move at the speed of light? If moving at the
speed of light means that it's time is frozen, they're
(10:25):
asking what would be the human experience of moving at
these velocities where physics gets really weird. Yeah, and so
let's clear that up. First of all, the key thing
to understand about relativity and time and clocks. Is that
the speed at which o'clock moves depends on its relative
velocity to you. Now, these things are always relative. So
(10:47):
if you're holding a clock and it's not moving relative
to you, it's just going to go forward at one
second per second. Like you always experience time the same
way because you're not moving relative to yourself. Right, it's
all the relative motion that distorts time. If you see
a clock moving fast, and you're watching the face of
that clock, then it runs slowly, but only according to you.
(11:10):
I think maybe we need to step back a little
bit paint the picture of Like I'm standing here and
I'm holding a clock in front of me. You're saying
that because the clock is not moving relative to me,
I'm not going to see or feel anything strange about that.
That's right. Like it's it's gonna I'm gonna hear it ticking,
and I'm gonna it's gonna sound like it's taking at
(11:32):
one second. That's right. And you always experience time the
same way, Like you don't experience time going slowly or
going quickly. But if I'm zipping past you at half
the speed of light and I look at your clock,
that it looks to me like your clock is running slowly. Now,
according to you, the clock is not moving. You're holding it,
so the clock runs at the normal pace one second
(11:53):
per second like usual. You don't experience time slowing down
just because I'm moving past you and I see your
time slowing down. But again, if i'm is it being
passed you at half the speed of light, then it
looks to me like your clock is running slowly. Well,
first of all, how would you look at my clock
if you were moving back? Remember would you have time? Daniel? Really,
(12:14):
I'm super robot observer. Remember I can observe anything accurately. Oh,
I see you're running at a basilian giga hurt. I
am super physics scratched. No, you have to imagine I
have like some awesome telescope and I'm watching the face
of your clock or something. All right, So I'm in
my little space here and I have my clock, and
it's taking away and you're zooming bye. And as you're
(12:36):
zooming by, we're going to go into super matrix slow
motion and you'r and so you're looking at my clock,
and my clock is moving relative to you like you're
seeing it go by. And if you were to measure
its ticks you, it would to you it would seem
like it's ticking slower like tick take right, is that
(12:58):
kind of what you mean by that? My clock seems
slower to you precisely, and your clock seems slow to me,
But it doesn't seem slow to you. Right, It looks
to me like you are aging slowly because your time
is running slowly. In a year, for me, less time
will have passed for you, and I will see you
aging slowly. But for you time just moves forward because again,
(13:21):
you're not moving relative to you. The thing to remember
is moving clocks run slowly, so you're not moving relative
to yourself, So your clock doesn't run anymore slowly, but
they only move slowly relative to the person who's not moving. Yeah,
everybody has a frame of reference that's centered at them,
and things are moving or not moving in their frame.
And if they're moving fast, then their clocks run slowly.
(13:44):
If they're not moving, then their clocks run normally, all right,
And that's that's basic general relativity is that time seems
to slow down in a pocket of space that's moving
fast relative to you. So that's special relativity or relativity
has to do with how space is bent by mass.
Special relativity is all these effects of light speed and
(14:06):
clocks and stuff. And it's not just that it appears
to go slowly, like your time really does move slowly
according to me. And that's the crux is that I
have a story about the universe that I'm telling based
on my perfect observations, and that's a true story. And
you're telling a different story about the order in which
things happened, in the rate at which time flowed, and
(14:28):
that's your story. And your story is different because you're
in a different place and moving in a different speed
relative to me. But both our stories can be correct, meaning, um,
I can experience my clock taking it one second per second,
but you would say that my clock is not taking
it one second per second precisely. And it's not just
that it appears that way, it actually is. I guess
(14:51):
it depends on what the deffer nition of is. Thank you, Bill.
And so you know a lot of people write in
they say things like, you know, a photon is moving
at the speed of light, which is true. And if
a photon had a clock on it, somehow, I don't
know how you could have a photon with a clock
on it. But imagine you could then wouldn't that clock
be frozen? Yes? True, because something moving at the speed
(15:13):
of light, it's time slows down to an effective rate
of zero. But that doesn't mean that photon is not
experiencing time. It's experiencing time maybe normally, but by the
time it blinks, it's made it across the whole length
of the universe. Yeah, because from the point of view
the photon, I guess the whole universe is moving past
(15:34):
it at the speed of light, and so the whole
universe is like length contracted down to zero. We'll have
to do a whole other episode about the effect of
relativity on length and distances and crazy stuff like that.
But the idea is that moving clocks runs slowly. That's
the thing to keep in mind, because all these statements
have to be made in a relative sense. You can't
(15:55):
say I'm moving really fast, and therefore my clock has
run slowly. You have to say I'm moving with fast
relative to what. And it's for that observer that your
clock is running slowly, not for you, right, Because I'm
sitting here in my pocket of space and I'm watching
you sit by and if I look at your clock,
I would say that your clock is moving slowly, and
(16:15):
you would be correct. And I would look at your
clock and say your clock is moving slowly and I
would be correct. Well, it would be a really short conversation.
Then if you're moving past, have dispeeded, like, hey, that
well he's gone already, forget it. If this podcast was
in stereo, we could do some cool effects. They're resooming
back and forth. Make it happen, engineers, you started to
(16:39):
treat engineers the same way I have now. Just engineers
are just people out there to make things happen. I
think people who make things happen are awesome. By the way,
my caveat to day engineers. All right, So that's that's
special relativity. That's the general idea that, uh, time seems
to move totally depending on how fast you're going relative
(17:03):
to other people who might be watching your clock. Yeah,
I would say time moves slowly for people that are
moving fast relative to you. I think that's pretty close
to what it's just said. I'm sure it's not. We
can disagree on that, all right. So that's special relativity,
and it's it creates a weird situation for arguments about
(17:24):
what happened and in what order things happen, and just
in general um about time and in the universe. So
let's get into that, but first let's take a quick break,
(17:47):
all right, Daniel, Let's maybe get into the details here
of how time or how special relativity messes with our
understanding of time. So what what's them? Are we going
to paint a picture for people? Are are were gonn
to be on trains on rockets? Or Alice and Bob
going to be involved? Or yeah, let's let's mix it
up a little bit, because usually special relativity is like
(18:09):
people on trains, because when they invented special relativity, trains
are like the fastest thing around. But that's old and
usually quantum mechanics thought experiments involved two experimenters named Alice
and Bob. But let's pick it up. Let's use Alice
and Bob for a special relativity thought experiment. Let's use
Alicia Andrew berta perfect though, just to mix it up,
(18:31):
all right, So if um, Alicia and Roberto are having
a race, say, it's like some family events right there
there with their relatives, so it's all about relativity and
they've all had their you know, Christmas Ham or whatever,
and now they're going to go out and make an
ill advised hundred yard dash in the backyard. And they're
both really fast, and people are watching the race, and
(18:52):
then afterwards, of course they're going to argue about who one.
But let's say that everybody who's watching the race is
a perfect observer. Everybody has um a clock that they
can start and stop, and everybody's really paying attention and
knows what they're doing. Then the question is who wins
the race? So they're both at the starting line, you know,
we said go, they start running towards the finish line,
(19:15):
and I'm sitting there at the finish line waiting to
see who gets there first, precisely and all right. The
amazing thing is that there is no one, single correct
answer for who wins this race. Now, if you're sitting
at the finish line, and my money is on Alicia,
to be honest, you've always liked her more than Roberto,
(19:37):
and frankly, Roberto is pissed about it. Well, she's my cousin,
So it's Roberto, man, what's wrong with you? Anyway? Um,
if you are sitting at the finish line, so you
have um no velocity relative to the ground. Then you
might see Alice. You might see Alicia beat Roberto in
(19:58):
the race. Cool, and might think, well, that's it, that's
the answer, right, she won time to celebrate. So so
I'm sitting in a chair by the finish line, and
to me, Alicia was running faster than Bob, so she
got to the finish line first, precisely, And you might think, well,
that's it, right. Other people might see from another angle,
or other people might be sitting somewhere differently, or you know,
(20:19):
maybe even if somebody is driving by in a car,
they might see the velocity of Alicia and Roberto be different.
But everybody should agree about the basic facts because there's
a single thing that happened. That's the way you grew
up thinking about the universe. That's what sort of makes
intuitive sense to you. It's true, right, Like if there's
somebody sitting on the opposite side of the finish line,
sitting in a chair as well, you're saying that they
(20:41):
could also see a different result, even if they're not
moving like me. Actually, the other observer would have to
have a different velocity and you not just be separated
in distance. But you're right, The order of events that
you see depends on two things. Your location relative to
Alicia and Roberto and your speed relative to them. So
(21:03):
if you are in a to make it simple, if
you are in a car, and your car is super
fast and you're going at like half the speed of
light or something, then you could see the race differently
and you could see you have a different outcome. You
could see Roberto cross the finish line before Alicia. This
is not some trick where like the life from you
takes longer to get there or anything like that. But
(21:26):
if you have a different relative distance to Alicia and Roberto,
and you're moving at a certain velocity, then you can
actually see the events happen in a different order. I
am sitting in my chair, I'm going to measure who wins.
But if you're moving, driving by at really fast velocities,
you might see something totally different than what I see. Yeah,
(21:48):
I could see Roberto across the finish line before Alicia,
and I would also be right. But do you have
to drive towards them perpendicular to them? You know what
I mean? Like what's actually happening there to make you
see something different? Now, let's imagine It's pretty simple. Let's
say I'm moving in the same direction as Alicia and Roberto. Right,
(22:10):
you started way before them and or way back, and
by the time they cross the finish line, you're crossing
the finish line too, but you're going really fast. Right,
I started on the moon or something like that, so
that I'm passing the race at exactly the same moment
that Alicia and Roberto are actually running, so that I'm
parallel with them but moving at some very high velocity.
(22:32):
Then I can see the order of events differently. I
can see the race start, and then I can see
Roberto finish the race before Alicia. Even if you sitting
on your plastic chair eating leftovers while you're watching sees
Alicia passed the finish line first. Yeah, no, I understand
that what is maybe happening, But I guess I'm trying
to just understand maybe for the people listening to this
(22:56):
is why, like, is there any way that we can
understand with this example, like how special relativity makes it
so that we see different things? Well, special relativity tells
you that the flow of time is dependent on velocity, right,
and also on distance, So the way the clocks work
depends on how far away they are and how fast
(23:17):
they are moving. So we talked earlier about how the
speed that a clock will move depends on how far
how fast it's moving relative to you. That's true, but
there's another factor we didn't get into, which is that
it also depends on where the clocks are. And so
if you see two clocks moving at the same speed
relative to you, but there's a distance between them, then
(23:41):
you see a different effect on the two clocks. It's
not just dependent on the velocity, it's also dependent on
the distance to the clocks. Because fundamentally, what's happening is
that the universe has sort of like a clock at
every location and those and the way those clocks flow
depends on your velocity rel tipped to them. Okay, so
(24:01):
now you're talking about like a third person. The key
thing is this separation between Roberto and Alicia. If Roberto
and Alicia are like literally on top of each other,
then you don't get this effect. But if there's a
gap between that's a different that's a different picture there
that that doesn't happen in family gatherings. But if there's
a gap between them, right, if there's a their meter apart,
(24:22):
five ms apart or something. Then their clocks are going
to run differently. But how does that affect who I
see getting to the finish line first? Like, like, it
shouldn't mattered to me that their clocks are running slower.
How does that affect how I see them and how
you would see? Well, if you see if you see
Alicia's clock running more slowly than Roberto's, then she's got
(24:43):
not going to be running as fast. I only know
that they're clocks are running slower because they're moving relative
to me. So how why imagine that there's a clock
floating in space next to Alicia and Roberto? Right, So
some drone robot clock that hangs out right next to them,
and it can perfectly synchronize it's motion and location relative
to them to which one one for each of them.
(25:04):
Then you would see Alicia's clock running at a different
rate than you see Roberto's clock, because not only do
they have a velocity relative to you, but there's a
distance between them. You have a different location, and so
the way at which time flows depends not just on
your relative velocity but also where you are relative to
that observer. All right, So you're saying that I'm going
to measure. I might measure Alicia or Roberto winning sitting
(25:27):
by the finish line, but as somebody else you on
your car, running towards the race at an at an
angle maybe, or going at a certain speed closest speed
of light, you might see something different than what I see. Precisely,
if you are sitting there and you have no velocity
relative to the ground, maybe you see them tie because
they've been training forever and they've been working on this,
(25:49):
and they're both really really fast. But then if I
am zipping past in a car or moving really fast,
then I could see Alicia reached the finish line before Bob.
Reach is the finish line. And you know, it's tricky
when you're thinking about special relativity. It's really easy to
get yourself confused. But the most the cleanest way to
think about it is in terms of events and when
(26:10):
those events happen. And so you see Alicia reached the
finish line before Roberto. Now you might think, how is
that possible, Like they're running at the same speed. Well,
you know, it's there's also a question of did you
see them leave the starting point at the same moment? Right,
because if you're moving at a high speed relative to
the race, then time is distorted for you, your your
(26:31):
view of their time is distorted. So you're saying that
it has to do a little bit with the idea
that time for Alicia and Roberta are going to be
moving differently relative to me and to you, and so
that's going to affect kind of how what we see
who we see crossing the line. First, precisely, if you
are sitting there in a plastic chair, finishing your dinner
(26:52):
and watching Alicia and Roberto across the finish line at
the same moment, and you see their clocks moving at
the same speed, and you see them also starting their
race at the same moment, right, I think maybe that's
the key insight, whereas you maybe saw them start at
different times. Yeah, I see Roberto win, but might also
look to me like Roberto cheated, like he left the
starting point at a different moment. Right. This concept of
(27:14):
simultaneity of things happening at the same moment depends on
your relative speed to those events and your distance from them.
So like in the Jorge Olympics Backyard Olympics um, Alicia
would win, but in the Daniel moving at the really
in a really fast car Olympics, somebody else, not only
would they, somebody else would win, but somebody else would
(27:37):
have cheated. Yeah, but you know, according to me, they've
cheated it. According to you they haven't. And then we
have a third cousin, you know, um, and she is
driving her even faster car the other direction, right than that,
different religio velocity can cause a different distortion of how
time works from her perspective, and she could see Alicia
(27:57):
totally blow Roberto away instead of just winning by little bit. Well,
I think at the end of the day, you know,
you're going to be by the time they finished, you're
going to be, you know, hundreds of thousands of miles away,
And so who cares? What do you think? And if
you think they cheated, it's how I would put it.
Whereas I'm right there with the metals ready to hand
them out, you know, precisely, yea. So in the end,
your point of view is most important because you're handing
(28:19):
out the trophies. But the point is that the folks
in the car driving past on high speeds, they see
different order of events, and they're not wrong. It's not
like they messed it. Up because they're moving fast, or
it took light more time to get to them, or
any sort of trick like that. If we're assuming perfect
robotic observers, they just see the order of events differently
(28:39):
because time is moving differently according to them, and their
account doesn't have to be wrong. It's just different, just
like the astronaut or the or just like the photon.
It's not like the photon is wrong. It's just that
they experience things differently because they're moving at a different speed. Right,
And you know, we as humans grew up in a
world where things move pretty slowly, and so we can
(29:01):
in the end reconcile usually a single series of events
like this happened, then that happened, then this other thing happened.
And sometimes it's hard to disentangle people's emotions and they're
bad eyewitnesses. But you know, if we had video cameras
that everybody's location, usually we can disentangle this stuff. But
as you get next to the speed of light, as
you get things moving really quickly, then that breaks down
(29:23):
and time flows differently in such a way that it
could actually change this order, which means that people have
a different account of what happened first, and what happened second?
And I remember the moment in my you know, junior
special relativity class when I learned this. It just blew
my mind that there's not like a universally agreed upon
order of events for stuff happening. But mostly we would
(29:44):
be all right, we would be or I don't, I
don't want to say disagreeing, but we would be seeing
different things. But that usually sort of happens more prominently
when you get closer to the speed of light. Yeah,
you can't notice these effects at slow speeds, even for
the Examp Bowl of Alicia Roberto. If they're running at
fairly low speeds, you know, ten per second, which actually
(30:06):
pretty fast, and they're running only a hundred you'd have
to be going really fast to change the order of
events by even nano seconds. So these effects are very
small unless you're approaching the speed of light. Yeah, okay, Yeah,
it's not like Roberto can argue that he should have
gone in the metal, because the likelihood that there's an
observer going to near the speed of light near my
(30:28):
backyard is um maybe not zero, but unlikely. Well, it
depends on who's listening to his argument. I might be
more sympathetic to it. I might be like, you know,
you're right, Roberto, there is some universe in which you
did win this and you were jilted from getting the metal. Yeah,
but really you would be like, yes, Robert, but I
think you're and then you'd be gone. Daniel thinks you
(30:50):
won and he's your medal, but he's now by Alpha centauri,
so you're out of yea, So so go catch up
if you want your your metal. All right, that's pretty mine,
bend sing um that so many things can be happening
in my in my backyard, or that I have new
cousins called Alicia and Roberto. But let's talk about now
whether you know how that makes any sense or how
(31:10):
physicists are able to wrap their heads around these kinds
of things. But first let's take a quick break, all right, Daniel,
(31:30):
So according to physics, my backyard Olympics are totally arbitrary,
meaning that to me they the outcome might be one thing,
but to you, moving at close to the speed of light,
the outcome might be different. And you're saying that this
really throws into question this idea of simultaneity in the universe,
Like things happening at the same time, because things happening
(31:53):
at the same time might depend on who's measuring whether
they happen at the same time. And it's a kind
of thing that we have to do in physics all
the time, is we think the universe works a certain way,
and then the universe shows us the Nope, it doesn't,
and that momentarily throws us for a loop thing. What
you know, um, you could do the same experiment twice
and get different outcomes because quantum mechanics is really probabilistic,
(32:15):
or you know, time doesn't work the same way for everybody.
But we don't give it up. We don't say the
universe therefore makes no sense. What we do is we
sort of retreat to a looser sense of what sense means.
We said, all right, we can't make those assumptions. What
assumptions can we make? What can we say about the universe,
and we find another way for it to make sense.
(32:36):
They sort of don't make sense in the sort of
like Newtonian you know, high school physics or you know
what we learned as babies how the world works. It
doesn't make sense in that way, just like maybe quantum
mechanics doesn't make sense to us, but it does make
sense if you sort of expand and you understand the
sort of the equations that are happening. Yeah, Like in
(32:58):
quantum mechanics, you can't say that the universe is deterministic.
You can't say that if you do, if you shoot
an electron at the same atom in exactly the same way,
you'll get the same outcome every time. You won't because
there's a randomness there. But what you can do is
you can say that there are still laws of physics,
and those laws of physics determine which random outcomes are
more likely or less likely. So you've sort of taken
(33:21):
a step backwards. You said, well, I can determine a
specific outcome, but I can say something about the distribution
of outcomes, and you can do something sort of similar
for relativity. You can take a step back and you
can say, well, I can't say that everybody has to
agree about the order in which things happen in the universe. Okay,
what can I say instead? Well, you can say that
(33:41):
everybody sees things happening according to the same laws. So
you know, you sitting in your plastic chair at the
edge of the finish line, you see things happening according
to a certain laws, set of laws of physics. Me
and my high speed Lamborghini, I see things happening according
to the same laws of physics. Now I see different
things happening, but they're following the same laws. Right, You're
(34:04):
basically saying that the laws of the universe account for
this difference of point of view in order of events,
and so then it makes sense. It's like you're saying
the laws of physics don't make sense, and that makes sense.
They make a different sense. They don't make common sense,
(34:24):
but they make mathematically, they certainly don't make common sense.
Now it means that you can use the same laws
to predict what's going to happen. Is I can use
in my frames. So no matter how fast you're going
and where you are in the universe, you should be
able to use the same laws to describe what you
see and to predict what's going to happen, and those
laws should work. So we can still make that requirement.
(34:45):
The events can be different from place to place. In
the description of what happened can be different from place
to place, but each one follows the same set of rules. Right,
there's nothing inconsistent about the universe. It's just that the
rules of the universe allow for this kind of of
different points of view depending on your speed precisely, and
that your point of your direction precisely. And I think
(35:07):
it's a tiny bit deeper than that. It's not just
that we understand how to translate what you see to
what someone else going at high speeds will see. We
do understand that, and that's cool. But the deeper bit
is that the same rules apply no matter how fast
you are going, So you can use the same laws
of physics. Now, you and I can disagree about the
order that we saw the race be run and who
(35:29):
won and who cheated, but we agree on the rules
of physics that describe what we saw, not just how
it translates from what I saw to what you saw.
We see them follow the same rules. And I think
that's pretty deep. And then there's one more thing that
we can still cling to. We can say that even
though things can happen in a different order, sometimes there
(35:49):
are some rules about that, you can't arbitrarily re order
the universe. Oh, I see, Yeah, this is what you're
saying earlier. There's a difference between simultaneity and cold. That's right,
they're both impossible to pronounce words yeah and clug cool. Obviously,
if I can reverse the order there of events, I
(36:11):
would say causality, correct. Yeah. The key thing is that
you can use velocity and distance and all these crazy
relativistic effects to reverse the order of some events, events
that are not causally connected, events where one didn't cause
the other one. But some things you can't. Mean, we
can see different outcomes of a race between Alicia and Roberta,
(36:34):
but we probably wouldn't disagree on a relay raise with Alicia.
Er yeah, yeah, Or for example, there's no speed at
which you can go in order to see Alicia finish
the race before she starts, and that order events definitely happens.
She starts the race, and then later she finishes. Now
when she finishes relative to Roberto, that's a different question.
(36:56):
We can zoom around our spaceships to get whatever answer
we want, But there's no But you'd have to go
faster than the speed of light in order to reverse
two events that are causally connected where one caused the
other one. So that's why you can't, for example, make
the Big Bang happened at the end of the universe,
or you know, all sorts of other crazy stuff. There's
some freedom here to move around the order of events,
(37:17):
but it's not pure total freedom. Don't go crazy people.
Meaning is not that the whole universe is open to interpretation,
because you know, like, um, the Big Bang happened and
Earth formed, and it's not like moving at any speed
will change anything about that. But maybe small things, like
(37:38):
small things that might some people might say happen or
not happen simultaneously. Then people moving at different speed might
have a different point of view. Yeah, And the size
of these things depends on sort of how far away
they are, like things that are in another galaxy, they
are really really far away. Nothing over there is causally
dependent on anything we do, because nothing we do here
(37:59):
can acted for a long long time, because you know,
there's because of the speed of light. So things happening
on the other side of the universe, you know, the
order in which those events happen is almost irrelevant for us.
So we could change those are events by a billion years,
probably because it makes no difference to the causal connections,
because nothing that happens here affects that for a very
long time. Things that are close together, right, it's much
(38:22):
more sensitive. You can causally affect the things close to you.
You can touch things, you can push things over, You
can shine a flashlight and affect things nearby. So you're
saying that that a raise in the backyard in another
galaxy could have happened before or after my race, maybe
because there's a long separation between them. But because they
don't affect each other, then it's it's okay to sort
(38:45):
of move them around in terms of our perception of
when they happen. Yeah, And we talked about this thing
called the light cone, which is this cone of in
space that that opens up at the speed of light
around you. You can only affect things that are in
your light cone. Things that are sort of downstream from you,
you can affect them. Those things are causally dependent on
(39:06):
what you do. Things that are not in your light
cone that they're too far away from you for you
to affect them now or in the near future, there's
no way for you to have any influence over them.
So their relativity can go crazy and it can change
the order events without breaking anything. Right, But we're still
all in the same universe, right, we are all in
the same universe. Yeah. Yeah, it's not like I'm disconnected
(39:27):
from that galaxy or things that are not in my
light cone. Something in my light cone might be in
that other things like cone, and so I'm sort of
still connected to the rest of the universe. Yeah, And
your light cone goes on forever and so eventually will
encompass the entire universe. Right, things you do now could
affect things fifteen billion light years away, but it's going
to take fifteen billion years. So your light cone is growing,
(39:49):
and eventually everything is like cone overlaps, right, So things
do come into contact. And I think my ego needs
any more feeding, Daniel, just to think that I can
affect the entire universe eventually. That's pretty big step up
from handing out medals from the backyard backyard Olympics to
expecting the entire universe. Small steps, my friend, backyard Olympics today,
(40:10):
galaxy collisions tomorrow. No, I think the thing to understand
is that we are all living the same universe, and
that universe has a certain set of rules. But those
rules are not necessarily the rules you thought they were,
and it allows for some fudging and some flexibility and
for some weird stuff to happen. But it's not throwing
everything out the window. It has to be consistent with
the you know, the the universe we've lived and the
(40:31):
things we've observed. You don't get to go really fast
and go back in time. You don't get to go
really fast and you know, change your order from a
chicken sandwich to a burger or whatever. Right. There are
still rules to the universe. They're just not as hard
and fast as you thought. And fundamentally it means that
the universe is different from the one we thought it was.
You know, the way quantum mechanics has deep implications for
(40:53):
the way the universe works, but doesn't really change your
day to day. Right. You if you don't go to work,
you still lose your job. Right. There are some things
that are deterministic. We don't fix climate change. The planet
might get too hot. Right, It's not like we can
disagree about that. No, And this is not an excuse
to say that you can have just whatever facts you want, right,
It's just that the facts are sort of local. Right,
(41:15):
depending on where you are and how fast you're going.
Your account of what happened in the universe is different,
but it's always just blown my mind that people can
have correct but different accounts of the order in which
things happened. Although I think one thing that never changes
is I think Roberto is a cheater. I think he
cheats in any universe, in any galaxy, and no matter
(41:35):
who you're looking, I think the question in my mind
and in listeners minds is do you actually have a
cousin named Roberto? And if so, does he listen to
the podcast? Oh my god, I justly do have a
cousin named Roberto. That's what happened. I don't need to
say you're a cheater talking about our other cousins. Cousin
Roberto Prime, and do you have a cousin named Alicia,
(41:58):
because now she's going to be your favorite. Let me
think I don't have a cousin name. Let me think
who has to say? Let me think, like dozens of cousins,
so one of the thirty six cousins needs a little
bit of a second there to think about, where were
you last night? Honey? Let me think? You know? Never
a very inspiring answer, especially in Latin American culture, where
(42:19):
everyone has a nickname that is totally unrelated to their
actual name. You have to think a little bit. Oh,
so when you're talking about Roberto, that's actually a nickname
for your other cousin, Nicholas or something. Yes, that's right,
something like that. That is how it goes. It sounds
like a Russian novel. All right. Well, I hope that
sufficiently altered or bent your mind there for at least
(42:40):
at the time of this podcast. And remember that the
university is crazy. The university's bonkers, but it's our universe
and we love it, and eventually we hope it will
make sense to us. That's right, just like we love
that crazy cousin we all have who we now have
to apologize to after this podcast. Well, nobody listens to
this podcast, I think so. All right, Thanks everyone for
(43:01):
listening and for asking us crazy questions. And if you
still don't understand special relativity in the flow of time,
don't worry. Nobody else really does either. Thanks for listening.
See you next time. Before you still have a question
after listening to all these explanations, please drop us a line.
(43:23):
We'd love to hear from you. You can find us
on 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 explained. The Universe is a production of I
Heart Radio. From more podcast from my heart Radio, visit
the i heart Radio app, Apple Podcasts, or wherever you
(43:46):
listen to your favorite shows.
Ay, Daniel, what's the most mind blowing thing that physics
has taught us about the universe? Wow? It is hard
to pick just one. Could I do a top five? Maybe? No?
I want your single all time one thing that you
think is the most bonkers that we've learned about Who
(00:28):
had to challenge? All Right? I got one. There is
one thing about the universe that, to me is the
most brain scrambling thing I've ever learned. The hardest thing
for me to get my brain around is that people
can disagree about the order in which events happened. We
don't all see the same things happening sometimes in the universe. Yeah,
(00:50):
you can see things happening in one order, and then
I can see things happening in a totally different order,
and we can both be correct. Hi'm poor hammade cartoonists
(01:16):
and the creator of PhD Commics. Hi. I'm Daniel. I'm
a particle physicist, sometimes in a podcast host other times,
and everybody disagrees about when I do what. Welcome to
our podcast. Daniel and Jorge agree to disagree about the universe,
in which we explained to you the craziest, the most
amazing but true things about our universe. Yeah, and this
(01:40):
is again a production of I Heart Radio, and in
our podcast we try to take you to the forefront
of science to talk about the craziest things that scientists
are thinking about today, and also delve into the history
of science to tell you why we think the universe
is so nuts and how we figured that out. Is
there a history of science, Daniel, or is that also
open for this agreement people disagree about in what order
(02:02):
we discover things, or what was the most important, or
how we change people's minds. Yeah. I think in our
podcast we also not just talk about scientific discoveries or
scientific theories, but also a little bit about how that
affects how we see the universe, you know, and how
it challenges what we believe about how the universe works. Yeah. Absolutely.
(02:26):
I think a lot of the deepest questions in physics
come from a sort of a philosophical motivation. We want
to know how the universe works because it matters to
us that we can make sense of the universe. And
when we ask the universe questions and it gives us
answers that force us to totally change the way we
think about the universe. That has pretty deep philosophical implications,
(02:46):
and so that's sometimes the most fun topic. Yeah, And
so basically, we're just trying to blow your mind in
the gentlest, most podcasts friendly way possible. Or we're trying
to share with you how the minds of physics this
have been blown, and we want your mind to be
blown just as much. But yeah, a little bit more gently. Yes, please,
I'm not sure my brain can handle this much mind
(03:08):
blowing is in in one hour. But you know, when
I was learning physics as an undergrad, I remember these
moments when I finally really deeply understood a concept, and
that concept would force me to relax or destroy or
get rid of some assumption I had about the universe,
and I realized that that just wasn't true, that the
way I'd been thinking about the world was just fundamentally wrong. Yeah,
(03:31):
it's amazing. And so today we'll be talking about one
such topic that honestly gives me a headache every time
we talked about it, Daniel. We've written about it in
actually a couple of chapters in our book We Have
No Idea, A Guide to the Unknown Universe, And I
have to admit I never really got my head around it.
I authored the book, but didn't actually understood what was
(03:53):
in it. Yeah, and this is a topic that people
email us about, they ask us about, they listen to
some of our podcasts about it, and still right in
with questions. I think there's a lot of popular misconceptions
about how this works and some consequences that I think
a lot of people have not really realized. There's some
really deep philosophical implications to these ideas that I don't
(04:13):
think have been widely enough appreciated. Yeah, So to the
on the podcast, we'll be talking about how does relativity
affect our understanding of time? Time? Or I guess the
short title is is there a consistent order of events
(04:36):
in the universe? Yeah, and spoiler alert, the answer might
be it's relatively surprising that the answer is not what
you expect. We've talked to this podcast before about time dilation,
the effect of speed on the rate of which clocks
move forward, and that's weird and that's hard for people
to get their mind around, and there's some misconceptions there.
(04:57):
But you know, time and relativity is even weirder than that.
It's more than just making things run slowly or making
things run quickly. It's about how things happen here versus
how things happen there. Do you think age also affects
how you perceive time? Like I feel like my kids,
I have no patients at all, and that the world
just moves at a snail's pace for them, whereas for me,
(05:20):
as I get older, things he's moved fast. Well, I
mean I moved slower, but the world seems to move fast. Yeah,
And I think that Actually you raise an interesting point,
because we are all imperfect observers if you just sort
of watch something and describe it later. There's all these
studies about how people give terrible accounts, you know, even
eyewitnesses give different accounts of the same events, And so
(05:42):
that's certainly one problem. It's like people are unreliable observers.
But even in the case when everybody was a perfect observer,
everybody had a video camera, I've already had, you know,
perfect measuring devices, and everybody accounted for transmission of light
and all that stuff. Even in those cases, it's hard
to make sense of the universe. Like if we were
all robots but the exact same you know, clock in
(06:04):
our chips, and we were all measuring things exactly, we
it would still be kind of a weird universe where
not everyone would agree maybe on the order of events, Yeah, precisely.
And you have to separate those because we're interested not
in the question of our people good eyewitnesses, because we
already know the answer to that is no um, but
the question of how does the universe actually work, because
(06:26):
we want to know the answer to the question, not
just for humanity, but in general, you know, the deep
fundamental questions, so that when the aliens come, when we
talk to them about physics, we can make sense of
what they've learned. And we don't want to learn human physics,
we want to learn fundamental physics. And so this idea
of time dilation, this idea that time move slowly depending
on maybe how fast you're going or or where you are,
(06:50):
is kind of weird and pretty mind blowing. And so
we were wondering out there how many people really sort
of understood what it means and what the ramification, what
it means for our basic understanding of time. So I
walked around campus that use the irvine, and I asked folks,
not people in the physics department, not people who have
(07:11):
taken my class on special relativity, just random people on campus,
willing to answer questions from a scrafty looking physicist. So
I think for a moment, what do you know about
how relativity affects our understanding of time? Here's what people
had to say. Do you know how relativity affects our
understanding of time? No? I do not. Actually, no, I'm
(07:32):
not share Newton's theory of relativity. I'm sorry, I don't.
Is this physics like a physics question, I'm not. I
don't know much about physics. Okay, I don't know how
to answer that. I'm an English Ready, if there was
a whole like demonstration of that, where if like you
go at like in a spaceship, like near the speed
(07:53):
of light or something, and then you can come back
and then like some amount of time has changed on
Earth while like no, really time has changed for you
or somewhere about and thinking Planet of the Apes when
they're in the spaceship and then by the time they
come back, they've only aged a couple of years, but
timeline Earth has passed a lot because they're traveling at
a different speed. Remain Planet Apes is a well known
physics documentary. Right, yes, yes, I believe relativity. That's from
(08:19):
Einstein is the idea that like time doesn't flow at
the same rate like across the universe. I think something
like I think gravity or something affects like how it
flows in other places. All Right, a couple of knows,
but a couple of people did have a lot of
pretty good ideas, or at least they said a lot
(08:39):
of physics terms. And I mean like Newton's theory of relativity,
like the Planet of the Apes. You know, I've seen
that movie. Yeah, Planet of the Apes. That's a that's
a documentary about physics. Yeah, there you go. Now, you're right.
People had the people who knew anything about it definitely
knew that relativity and time connected and that relativity changes
(09:02):
have time moves. And I think already that's a big success, right,
the idea that time is not universal, that we don't
have clocks everywhere in the universe ticking forward at the
same rate. That's a big step forward to to break
that assumption. You don't think you didn't have a theory
of relativity. I mean he could have had one about
his relatives or something, but just not not the one
(09:22):
that caught up right. His theory relativity was don't bring
up politics at Christmas dinner. Yeah, also also useful things
to live by. But yeah, I think it seems like
maybe there is a general sense right that relativity effects
our understanding of time and makes things weird, but maybe
(09:44):
a lot of people don't know how or what. Yeah.
And I think also there's a lot of misunderstandings based
on the emails that we get from listeners who are
asking us questions about relativity and tweets and stuff that
people have posted. I have the feeling that people think
that if you move fast and your clock will slow down,
which is not the right way to think about time
(10:04):
and relativity. That's how I thought time and relativity works.
So we're in for a long conversation. So, um, so
people have been writing with questions about time. Yeah, a
lot of people ask the questions like what is it
like to be a photon? Or how does a photon
move at the speed of light? If moving at the
speed of light means that it's time is frozen, they're
(10:25):
asking what would be the human experience of moving at
these velocities where physics gets really weird. Yeah, and so
let's clear that up. First of all, the key thing
to understand about relativity and time and clocks. Is that
the speed at which o'clock moves depends on its relative
velocity to you. Now, these things are always relative. So
(10:47):
if you're holding a clock and it's not moving relative
to you, it's just going to go forward at one
second per second. Like you always experience time the same
way because you're not moving relative to yourself. Right, it's
all the relative motion that distorts time. If you see
a clock moving fast, and you're watching the face of
that clock, then it runs slowly, but only according to you.
(11:10):
I think maybe we need to step back a little
bit paint the picture of Like I'm standing here and
I'm holding a clock in front of me. You're saying
that because the clock is not moving relative to me,
I'm not going to see or feel anything strange about that.
That's right. Like it's it's gonna I'm gonna hear it ticking,
and I'm gonna it's gonna sound like it's taking at
(11:32):
one second. That's right. And you always experience time the
same way, Like you don't experience time going slowly or
going quickly. But if I'm zipping past you at half
the speed of light and I look at your clock,
that it looks to me like your clock is running slowly. Now,
according to you, the clock is not moving. You're holding it,
so the clock runs at the normal pace one second
(11:53):
per second like usual. You don't experience time slowing down
just because I'm moving past you and I see your
time slowing down. But again, if i'm is it being
passed you at half the speed of light, then it
looks to me like your clock is running slowly. Well,
first of all, how would you look at my clock
if you were moving back? Remember would you have time? Daniel? Really,
(12:14):
I'm super robot observer. Remember I can observe anything accurately. Oh,
I see you're running at a basilian giga hurt. I
am super physics scratched. No, you have to imagine I
have like some awesome telescope and I'm watching the face
of your clock or something. All right, So I'm in
my little space here and I have my clock, and
it's taking away and you're zooming bye. And as you're
(12:36):
zooming by, we're going to go into super matrix slow
motion and you'r and so you're looking at my clock,
and my clock is moving relative to you like you're
seeing it go by. And if you were to measure
its ticks you, it would to you it would seem
like it's ticking slower like tick take right, is that
(12:58):
kind of what you mean by that? My clock seems
slower to you precisely, and your clock seems slow to me,
But it doesn't seem slow to you. Right, It looks
to me like you are aging slowly because your time
is running slowly. In a year, for me, less time
will have passed for you, and I will see you
aging slowly. But for you time just moves forward because again,
(13:21):
you're not moving relative to you. The thing to remember
is moving clocks run slowly, so you're not moving relative
to yourself, So your clock doesn't run anymore slowly, but
they only move slowly relative to the person who's not moving. Yeah,
everybody has a frame of reference that's centered at them,
and things are moving or not moving in their frame.
And if they're moving fast, then their clocks run slowly.
(13:44):
If they're not moving, then their clocks run normally, all right,
And that's that's basic general relativity is that time seems
to slow down in a pocket of space that's moving
fast relative to you. So that's special relativity or relativity
has to do with how space is bent by mass.
Special relativity is all these effects of light speed and
(14:06):
clocks and stuff. And it's not just that it appears
to go slowly, like your time really does move slowly
according to me. And that's the crux is that I
have a story about the universe that I'm telling based
on my perfect observations, and that's a true story. And
you're telling a different story about the order in which
things happened, in the rate at which time flowed, and
(14:28):
that's your story. And your story is different because you're
in a different place and moving in a different speed
relative to me. But both our stories can be correct, meaning, um,
I can experience my clock taking it one second per second,
but you would say that my clock is not taking
it one second per second precisely. And it's not just
that it appears that way, it actually is. I guess
(14:51):
it depends on what the deffer nition of is. Thank you, Bill.
And so you know a lot of people write in
they say things like, you know, a photon is moving
at the speed of light, which is true. And if
a photon had a clock on it, somehow, I don't
know how you could have a photon with a clock
on it. But imagine you could then wouldn't that clock
be frozen? Yes? True, because something moving at the speed
(15:13):
of light, it's time slows down to an effective rate
of zero. But that doesn't mean that photon is not
experiencing time. It's experiencing time maybe normally, but by the
time it blinks, it's made it across the whole length
of the universe. Yeah, because from the point of view
the photon, I guess the whole universe is moving past
(15:34):
it at the speed of light, and so the whole
universe is like length contracted down to zero. We'll have
to do a whole other episode about the effect of
relativity on length and distances and crazy stuff like that.
But the idea is that moving clocks runs slowly. That's
the thing to keep in mind, because all these statements
have to be made in a relative sense. You can't
(15:55):
say I'm moving really fast, and therefore my clock has
run slowly. You have to say I'm moving with fast
relative to what. And it's for that observer that your
clock is running slowly, not for you, right, Because I'm
sitting here in my pocket of space and I'm watching
you sit by and if I look at your clock,
I would say that your clock is moving slowly, and
(16:15):
you would be correct. And I would look at your
clock and say your clock is moving slowly and I
would be correct. Well, it would be a really short conversation.
Then if you're moving past, have dispeeded, like, hey, that
well he's gone already, forget it. If this podcast was
in stereo, we could do some cool effects. They're resooming
back and forth. Make it happen, engineers, you started to
(16:39):
treat engineers the same way I have now. Just engineers
are just people out there to make things happen. I
think people who make things happen are awesome. By the way,
my caveat to day engineers. All right, So that's that's
special relativity. That's the general idea that, uh, time seems
to move totally depending on how fast you're going relative
(17:03):
to other people who might be watching your clock. Yeah,
I would say time moves slowly for people that are
moving fast relative to you. I think that's pretty close
to what it's just said. I'm sure it's not. We
can disagree on that, all right. So that's special relativity,
and it's it creates a weird situation for arguments about
(17:24):
what happened and in what order things happen, and just
in general um about time and in the universe. So
let's get into that, but first let's take a quick break,
(17:47):
all right, Daniel, Let's maybe get into the details here
of how time or how special relativity messes with our
understanding of time. So what what's them? Are we going
to paint a picture for people? Are are were gonn
to be on trains on rockets? Or Alice and Bob
going to be involved? Or yeah, let's let's mix it
up a little bit, because usually special relativity is like
(18:09):
people on trains, because when they invented special relativity, trains
are like the fastest thing around. But that's old and
usually quantum mechanics thought experiments involved two experimenters named Alice
and Bob. But let's pick it up. Let's use Alice
and Bob for a special relativity thought experiment. Let's use
Alicia Andrew berta perfect though, just to mix it up,
(18:31):
all right, So if um, Alicia and Roberto are having
a race, say, it's like some family events right there
there with their relatives, so it's all about relativity and
they've all had their you know, Christmas Ham or whatever,
and now they're going to go out and make an
ill advised hundred yard dash in the backyard. And they're
both really fast, and people are watching the race, and
(18:52):
then afterwards, of course they're going to argue about who one.
But let's say that everybody who's watching the race is
a perfect observer. Everybody has um a clock that they
can start and stop, and everybody's really paying attention and
knows what they're doing. Then the question is who wins
the race? So they're both at the starting line, you know,
we said go, they start running towards the finish line,
(19:15):
and I'm sitting there at the finish line waiting to
see who gets there first, precisely and all right. The
amazing thing is that there is no one, single correct
answer for who wins this race. Now, if you're sitting
at the finish line, and my money is on Alicia,
to be honest, you've always liked her more than Roberto,
(19:37):
and frankly, Roberto is pissed about it. Well, she's my cousin,
So it's Roberto, man, what's wrong with you? Anyway? Um,
if you are sitting at the finish line, so you
have um no velocity relative to the ground. Then you
might see Alice. You might see Alicia beat Roberto in
(19:58):
the race. Cool, and might think, well, that's it, that's
the answer, right, she won time to celebrate. So so
I'm sitting in a chair by the finish line, and
to me, Alicia was running faster than Bob, so she
got to the finish line first, precisely, And you might think, well,
that's it, right. Other people might see from another angle,
or other people might be sitting somewhere differently, or you know,
(20:19):
maybe even if somebody is driving by in a car,
they might see the velocity of Alicia and Roberto be different.
But everybody should agree about the basic facts because there's
a single thing that happened. That's the way you grew
up thinking about the universe. That's what sort of makes
intuitive sense to you. It's true, right, Like if there's
somebody sitting on the opposite side of the finish line,
sitting in a chair as well, you're saying that they
(20:41):
could also see a different result, even if they're not
moving like me. Actually, the other observer would have to
have a different velocity and you not just be separated
in distance. But you're right, The order of events that
you see depends on two things. Your location relative to
Alicia and Roberto and your speed relative to them. So
(21:03):
if you are in a to make it simple, if
you are in a car, and your car is super
fast and you're going at like half the speed of
light or something, then you could see the race differently
and you could see you have a different outcome. You
could see Roberto cross the finish line before Alicia. This
is not some trick where like the life from you
takes longer to get there or anything like that. But
(21:26):
if you have a different relative distance to Alicia and Roberto,
and you're moving at a certain velocity, then you can
actually see the events happen in a different order. I
am sitting in my chair, I'm going to measure who wins.
But if you're moving, driving by at really fast velocities,
you might see something totally different than what I see. Yeah,
(21:48):
I could see Roberto across the finish line before Alicia,
and I would also be right. But do you have
to drive towards them perpendicular to them? You know what
I mean? Like what's actually happening there to make you
see something different? Now, let's imagine It's pretty simple. Let's
say I'm moving in the same direction as Alicia and Roberto. Right,
(22:10):
you started way before them and or way back, and
by the time they cross the finish line, you're crossing
the finish line too, but you're going really fast. Right,
I started on the moon or something like that, so
that I'm passing the race at exactly the same moment
that Alicia and Roberto are actually running, so that I'm
parallel with them but moving at some very high velocity.
(22:32):
Then I can see the order of events differently. I
can see the race start, and then I can see
Roberto finish the race before Alicia. Even if you sitting
on your plastic chair eating leftovers while you're watching sees
Alicia passed the finish line first. Yeah, no, I understand
that what is maybe happening, But I guess I'm trying
to just understand maybe for the people listening to this
(22:56):
is why, like, is there any way that we can
understand with this example, like how special relativity makes it
so that we see different things? Well, special relativity tells
you that the flow of time is dependent on velocity, right,
and also on distance, So the way the clocks work
depends on how far away they are and how fast
(23:17):
they are moving. So we talked earlier about how the
speed that a clock will move depends on how far
how fast it's moving relative to you. That's true, but
there's another factor we didn't get into, which is that
it also depends on where the clocks are. And so
if you see two clocks moving at the same speed
relative to you, but there's a distance between them, then
(23:41):
you see a different effect on the two clocks. It's
not just dependent on the velocity, it's also dependent on
the distance to the clocks. Because fundamentally, what's happening is
that the universe has sort of like a clock at
every location and those and the way those clocks flow
depends on your velocity rel tipped to them. Okay, so
(24:01):
now you're talking about like a third person. The key
thing is this separation between Roberto and Alicia. If Roberto
and Alicia are like literally on top of each other,
then you don't get this effect. But if there's a
gap between that's a different that's a different picture there
that that doesn't happen in family gatherings. But if there's
a gap between them, right, if there's a their meter apart,
(24:22):
five ms apart or something. Then their clocks are going
to run differently. But how does that affect who I
see getting to the finish line first? Like, like, it
shouldn't mattered to me that their clocks are running slower.
How does that affect how I see them and how
you would see? Well, if you see if you see
Alicia's clock running more slowly than Roberto's, then she's got
(24:43):
not going to be running as fast. I only know
that they're clocks are running slower because they're moving relative
to me. So how why imagine that there's a clock
floating in space next to Alicia and Roberto? Right, So
some drone robot clock that hangs out right next to them,
and it can perfectly synchronize it's motion and location relative
to them to which one one for each of them.
(25:04):
Then you would see Alicia's clock running at a different
rate than you see Roberto's clock, because not only do
they have a velocity relative to you, but there's a
distance between them. You have a different location, and so
the way at which time flows depends not just on
your relative velocity but also where you are relative to
that observer. All right, So you're saying that I'm going
to measure. I might measure Alicia or Roberto winning sitting
(25:27):
by the finish line, but as somebody else you on
your car, running towards the race at an at an
angle maybe, or going at a certain speed closest speed
of light, you might see something different than what I see. Precisely,
if you are sitting there and you have no velocity
relative to the ground, maybe you see them tie because
they've been training forever and they've been working on this,
(25:49):
and they're both really really fast. But then if I
am zipping past in a car or moving really fast,
then I could see Alicia reached the finish line before Bob.
Reach is the finish line. And you know, it's tricky
when you're thinking about special relativity. It's really easy to
get yourself confused. But the most the cleanest way to
think about it is in terms of events and when
(26:10):
those events happen. And so you see Alicia reached the
finish line before Roberto. Now you might think, how is
that possible, Like they're running at the same speed. Well,
you know, it's there's also a question of did you
see them leave the starting point at the same moment? Right,
because if you're moving at a high speed relative to
the race, then time is distorted for you, your your
(26:31):
view of their time is distorted. So you're saying that
it has to do a little bit with the idea
that time for Alicia and Roberta are going to be
moving differently relative to me and to you, and so
that's going to affect kind of how what we see
who we see crossing the line. First, precisely, if you
are sitting there in a plastic chair, finishing your dinner
(26:52):
and watching Alicia and Roberto across the finish line at
the same moment, and you see their clocks moving at
the same speed, and you see them also starting their
race at the same moment, right, I think maybe that's
the key insight, whereas you maybe saw them start at
different times. Yeah, I see Roberto win, but might also
look to me like Roberto cheated, like he left the
starting point at a different moment. Right. This concept of
(27:14):
simultaneity of things happening at the same moment depends on
your relative speed to those events and your distance from them.
So like in the Jorge Olympics Backyard Olympics um, Alicia
would win, but in the Daniel moving at the really
in a really fast car Olympics, somebody else, not only
would they, somebody else would win, but somebody else would
(27:37):
have cheated. Yeah, but you know, according to me, they've
cheated it. According to you they haven't. And then we
have a third cousin, you know, um, and she is
driving her even faster car the other direction, right than that,
different religio velocity can cause a different distortion of how
time works from her perspective, and she could see Alicia
(27:57):
totally blow Roberto away instead of just winning by little bit. Well,
I think at the end of the day, you know,
you're going to be by the time they finished, you're
going to be, you know, hundreds of thousands of miles away,
And so who cares? What do you think? And if
you think they cheated, it's how I would put it.
Whereas I'm right there with the metals ready to hand
them out, you know, precisely, yea. So in the end,
your point of view is most important because you're handing
(28:19):
out the trophies. But the point is that the folks
in the car driving past on high speeds, they see
different order of events, and they're not wrong. It's not
like they messed it. Up because they're moving fast, or
it took light more time to get to them, or
any sort of trick like that. If we're assuming perfect
robotic observers, they just see the order of events differently
(28:39):
because time is moving differently according to them, and their
account doesn't have to be wrong. It's just different, just
like the astronaut or the or just like the photon.
It's not like the photon is wrong. It's just that
they experience things differently because they're moving at a different speed. Right,
And you know, we as humans grew up in a
world where things move pretty slowly, and so we can
(29:01):
in the end reconcile usually a single series of events
like this happened, then that happened, then this other thing happened.
And sometimes it's hard to disentangle people's emotions and they're
bad eyewitnesses. But you know, if we had video cameras
that everybody's location, usually we can disentangle this stuff. But
as you get next to the speed of light, as
you get things moving really quickly, then that breaks down
(29:23):
and time flows differently in such a way that it
could actually change this order, which means that people have
a different account of what happened first, and what happened second?
And I remember the moment in my you know, junior
special relativity class when I learned this. It just blew
my mind that there's not like a universally agreed upon
order of events for stuff happening. But mostly we would
(29:44):
be all right, we would be or I don't, I
don't want to say disagreeing, but we would be seeing
different things. But that usually sort of happens more prominently
when you get closer to the speed of light. Yeah,
you can't notice these effects at slow speeds, even for
the Examp Bowl of Alicia Roberto. If they're running at
fairly low speeds, you know, ten per second, which actually
(30:06):
pretty fast, and they're running only a hundred you'd have
to be going really fast to change the order of
events by even nano seconds. So these effects are very
small unless you're approaching the speed of light. Yeah, okay, Yeah,
it's not like Roberto can argue that he should have
gone in the metal, because the likelihood that there's an
observer going to near the speed of light near my
(30:28):
backyard is um maybe not zero, but unlikely. Well, it
depends on who's listening to his argument. I might be
more sympathetic to it. I might be like, you know,
you're right, Roberto, there is some universe in which you
did win this and you were jilted from getting the metal. Yeah,
but really you would be like, yes, Robert, but I
think you're and then you'd be gone. Daniel thinks you
(30:50):
won and he's your medal, but he's now by Alpha centauri,
so you're out of yea, So so go catch up
if you want your your metal. All right, that's pretty mine,
bend sing um that so many things can be happening
in my in my backyard, or that I have new
cousins called Alicia and Roberto. But let's talk about now
whether you know how that makes any sense or how
(31:10):
physicists are able to wrap their heads around these kinds
of things. But first let's take a quick break, all right, Daniel,
(31:30):
So according to physics, my backyard Olympics are totally arbitrary,
meaning that to me they the outcome might be one thing,
but to you, moving at close to the speed of light,
the outcome might be different. And you're saying that this
really throws into question this idea of simultaneity in the universe,
Like things happening at the same time, because things happening
(31:53):
at the same time might depend on who's measuring whether
they happen at the same time. And it's a kind
of thing that we have to do in physics all
the time, is we think the universe works a certain way,
and then the universe shows us the Nope, it doesn't,
and that momentarily throws us for a loop thing. What
you know, um, you could do the same experiment twice
and get different outcomes because quantum mechanics is really probabilistic,
(32:15):
or you know, time doesn't work the same way for everybody.
But we don't give it up. We don't say the
universe therefore makes no sense. What we do is we
sort of retreat to a looser sense of what sense means.
We said, all right, we can't make those assumptions. What
assumptions can we make? What can we say about the universe,
and we find another way for it to make sense.
(32:36):
They sort of don't make sense in the sort of
like Newtonian you know, high school physics or you know
what we learned as babies how the world works. It
doesn't make sense in that way, just like maybe quantum
mechanics doesn't make sense to us, but it does make
sense if you sort of expand and you understand the
sort of the equations that are happening. Yeah, Like in
(32:58):
quantum mechanics, you can't say that the universe is deterministic.
You can't say that if you do, if you shoot
an electron at the same atom in exactly the same way,
you'll get the same outcome every time. You won't because
there's a randomness there. But what you can do is
you can say that there are still laws of physics,
and those laws of physics determine which random outcomes are
more likely or less likely. So you've sort of taken
(33:21):
a step backwards. You said, well, I can determine a
specific outcome, but I can say something about the distribution
of outcomes, and you can do something sort of similar
for relativity. You can take a step back and you
can say, well, I can't say that everybody has to
agree about the order in which things happen in the universe. Okay,
what can I say instead? Well, you can say that
(33:41):
everybody sees things happening according to the same laws. So
you know, you sitting in your plastic chair at the
edge of the finish line, you see things happening according
to a certain laws, set of laws of physics. Me
and my high speed Lamborghini, I see things happening according
to the same laws of physics. Now I see different
things happening, but they're following the same laws. Right, You're
(34:04):
basically saying that the laws of the universe account for
this difference of point of view in order of events,
and so then it makes sense. It's like you're saying
the laws of physics don't make sense, and that makes sense.
They make a different sense. They don't make common sense,
(34:24):
but they make mathematically, they certainly don't make common sense.
Now it means that you can use the same laws
to predict what's going to happen. Is I can use
in my frames. So no matter how fast you're going
and where you are in the universe, you should be
able to use the same laws to describe what you
see and to predict what's going to happen, and those
laws should work. So we can still make that requirement.
(34:45):
The events can be different from place to place. In
the description of what happened can be different from place
to place, but each one follows the same set of rules. Right,
there's nothing inconsistent about the universe. It's just that the
rules of the universe allow for this kind of of
different points of view depending on your speed precisely, and
that your point of your direction precisely. And I think
(35:07):
it's a tiny bit deeper than that. It's not just
that we understand how to translate what you see to
what someone else going at high speeds will see. We
do understand that, and that's cool. But the deeper bit
is that the same rules apply no matter how fast
you are going, So you can use the same laws
of physics. Now, you and I can disagree about the
order that we saw the race be run and who
(35:29):
won and who cheated, but we agree on the rules
of physics that describe what we saw, not just how
it translates from what I saw to what you saw.
We see them follow the same rules. And I think
that's pretty deep. And then there's one more thing that
we can still cling to. We can say that even
though things can happen in a different order, sometimes there
(35:49):
are some rules about that, you can't arbitrarily re order
the universe. Oh, I see, Yeah, this is what you're
saying earlier. There's a difference between simultaneity and cold. That's right,
they're both impossible to pronounce words yeah and clug cool. Obviously,
if I can reverse the order there of events, I
(36:11):
would say causality, correct. Yeah. The key thing is that
you can use velocity and distance and all these crazy
relativistic effects to reverse the order of some events, events
that are not causally connected, events where one didn't cause
the other one. But some things you can't. Mean, we
can see different outcomes of a race between Alicia and Roberta,
(36:34):
but we probably wouldn't disagree on a relay raise with Alicia.
Er yeah, yeah, Or for example, there's no speed at
which you can go in order to see Alicia finish
the race before she starts, and that order events definitely happens.
She starts the race, and then later she finishes. Now
when she finishes relative to Roberto, that's a different question.
(36:56):
We can zoom around our spaceships to get whatever answer
we want, But there's no But you'd have to go
faster than the speed of light in order to reverse
two events that are causally connected where one caused the
other one. So that's why you can't, for example, make
the Big Bang happened at the end of the universe,
or you know, all sorts of other crazy stuff. There's
some freedom here to move around the order of events,
(37:17):
but it's not pure total freedom. Don't go crazy people.
Meaning is not that the whole universe is open to interpretation,
because you know, like, um, the Big Bang happened and
Earth formed, and it's not like moving at any speed
will change anything about that. But maybe small things, like
(37:38):
small things that might some people might say happen or
not happen simultaneously. Then people moving at different speed might
have a different point of view. Yeah, And the size
of these things depends on sort of how far away
they are, like things that are in another galaxy, they
are really really far away. Nothing over there is causally
dependent on anything we do, because nothing we do here
(37:59):
can acted for a long long time, because you know,
there's because of the speed of light. So things happening
on the other side of the universe, you know, the
order in which those events happen is almost irrelevant for us.
So we could change those are events by a billion years,
probably because it makes no difference to the causal connections,
because nothing that happens here affects that for a very
long time. Things that are close together, right, it's much
(38:22):
more sensitive. You can causally affect the things close to you.
You can touch things, you can push things over, You
can shine a flashlight and affect things nearby. So you're
saying that that a raise in the backyard in another
galaxy could have happened before or after my race, maybe
because there's a long separation between them. But because they
don't affect each other, then it's it's okay to sort
(38:45):
of move them around in terms of our perception of
when they happen. Yeah, And we talked about this thing
called the light cone, which is this cone of in
space that that opens up at the speed of light
around you. You can only affect things that are in
your light cone. Things that are sort of downstream from you,
you can affect them. Those things are causally dependent on
(39:06):
what you do. Things that are not in your light
cone that they're too far away from you for you
to affect them now or in the near future, there's
no way for you to have any influence over them.
So their relativity can go crazy and it can change
the order events without breaking anything. Right, But we're still
all in the same universe, right, we are all in
the same universe. Yeah. Yeah, it's not like I'm disconnected
(39:27):
from that galaxy or things that are not in my
light cone. Something in my light cone might be in
that other things like cone, and so I'm sort of
still connected to the rest of the universe. Yeah, And
your light cone goes on forever and so eventually will
encompass the entire universe. Right, things you do now could
affect things fifteen billion light years away, but it's going
to take fifteen billion years. So your light cone is growing,
(39:49):
and eventually everything is like cone overlaps, right, So things
do come into contact. And I think my ego needs
any more feeding, Daniel, just to think that I can
affect the entire universe eventually. That's pretty big step up
from handing out medals from the backyard backyard Olympics to
expecting the entire universe. Small steps, my friend, backyard Olympics today,
(40:10):
galaxy collisions tomorrow. No, I think the thing to understand
is that we are all living the same universe, and
that universe has a certain set of rules. But those
rules are not necessarily the rules you thought they were,
and it allows for some fudging and some flexibility and
for some weird stuff to happen. But it's not throwing
everything out the window. It has to be consistent with
the you know, the the universe we've lived and the
(40:31):
things we've observed. You don't get to go really fast
and go back in time. You don't get to go
really fast and you know, change your order from a
chicken sandwich to a burger or whatever. Right. There are
still rules to the universe. They're just not as hard
and fast as you thought. And fundamentally it means that
the universe is different from the one we thought it was.
You know, the way quantum mechanics has deep implications for
(40:53):
the way the universe works, but doesn't really change your
day to day. Right. You if you don't go to work,
you still lose your job. Right. There are some things
that are deterministic. We don't fix climate change. The planet
might get too hot. Right, It's not like we can
disagree about that. No, And this is not an excuse
to say that you can have just whatever facts you want, right,
It's just that the facts are sort of local. Right,
(41:15):
depending on where you are and how fast you're going.
Your account of what happened in the universe is different,
but it's always just blown my mind that people can
have correct but different accounts of the order in which
things happened. Although I think one thing that never changes
is I think Roberto is a cheater. I think he
cheats in any universe, in any galaxy, and no matter
(41:35):
who you're looking, I think the question in my mind
and in listeners minds is do you actually have a
cousin named Roberto? And if so, does he listen to
the podcast? Oh my god, I justly do have a
cousin named Roberto. That's what happened. I don't need to
say you're a cheater talking about our other cousins. Cousin
Roberto Prime, and do you have a cousin named Alicia,
(41:58):
because now she's going to be your favorite. Let me
think I don't have a cousin name. Let me think
who has to say? Let me think, like dozens of cousins,
so one of the thirty six cousins needs a little
bit of a second there to think about, where were
you last night? Honey? Let me think? You know? Never
a very inspiring answer, especially in Latin American culture, where
(42:19):
everyone has a nickname that is totally unrelated to their
actual name. You have to think a little bit. Oh,
so when you're talking about Roberto, that's actually a nickname
for your other cousin, Nicholas or something. Yes, that's right,
something like that. That is how it goes. It sounds
like a Russian novel. All right. Well, I hope that
sufficiently altered or bent your mind there for at least
(42:40):
at the time of this podcast. And remember that the
university is crazy. The university's bonkers, but it's our universe
and we love it, and eventually we hope it will
make sense to us. That's right, just like we love
that crazy cousin we all have who we now have
to apologize to after this podcast. Well, nobody listens to
this podcast, I think so. All right, Thanks everyone for
(43:01):
listening and for asking us crazy questions. And if you
still don't understand special relativity in the flow of time,
don't worry. Nobody else really does either. Thanks for listening.
See you next time. Before you still have a question
after listening to all these explanations, please drop us a line.
(43:23):
We'd love to hear from you. You can find us
on 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 explained. The Universe is a production of I
Heart Radio. From more podcast from my heart Radio, visit
the i heart Radio app, Apple Podcasts, or wherever you
(43:46):
listen to your favorite shows.
Hosts And Creators
Daniel Whiteson
Kelly Weinersmith
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