Daniel Whiteson, Time Traveler

In this episode of Stuff to Blow Your Mind, Robert and Joe welcome back Daniel Whiteson, particle physicist and co-host of Daniel and Jorge Explain the Universe. Tune in for a discussion of time travel, wormholes and more that will actually transport you roughly an hour into the future. Plus Daniel will discuss the new book "Frequently Asked Questions About the Universe."

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Episode Transcript

Speaker 1 (00:00):
My welcome to Stuff to Blow Your Mind, the production
of My Heart Radio. Hey you welcome to Stuff to
Blow Your Mind. My name is Robert Lamb and I'm
Joe McCormick. And for today's episode, we're going to be

(00:20):
chatting with Daniel Whitson, who is a particle physicist and
science communicator and one of the hosts of the podcast
Daniel and Jorge Explain the Universe. This is Daniel's third
time hopping on the show with us. The previous episodes
were in September of twenty nineteen and April of And

(00:40):
for this episode, we're gonna be talking about a book.
Daniel and his co host and co author Jorge cham
have a new book called Frequently Asked Questions about the Universe.
So it was a real pleasure to have Daniel on
the show for the hat trick, and I guess without
any further delay, we will go right into the interview. Daniel,

(01:01):
welcome back to the show. We're so glad you're here.
Thanks very much for having me back. Always fun to
talk to you guys about things that blew my mind. Awesome,
So um, the podcast Daniel and Jorge Explain the Universe
still going strong um. How how far are you into
explaining the universe in its entirely? We have explained zero
point zero zero zero zero zero zero zero zero one

(01:23):
percent of the universe so far. Nice. I actually I
was looking at your recent episodes and I saw did
you recently do one that was an interview with Sean
Carroll about the uh, the many worlds interpretation of quantum mechanics.
I know, I know he favors that, right. Yeah, we
actually have a series where I interview an expert on
each of the interpretations of quantum mechanics. We did one

(01:45):
on Copenhagen interpretation with annam Becker, We did one on
the relational interpretation of quantum mechanics with Carlo Rovelli, and
then we talked to Sean about many Worlds interpretation. And
just a couple of weeks ago we did one about
the pilot wave theory of quantum mechanics, which totally blew
my mind. Really much overlooked and unnecessarily maligned interpretation of

(02:07):
quantum mechanics, in my opinion, malign, like people are being
mean to it. Well, there's this famous proof by John
von Neuman like seventy years ago demonstrating that it was
essentially impossible, and because von Neuman is such a giant
of the field, everybody thought, well, that's that. Turns out
he was wrong, though, and and it took people years

(02:30):
to figure it out. It was Bell actually who figured
out that Noeman was wrong, and that it's possible to
have a theory of quantum mechanics with hidden variables that's deterministic,
that's not random at all um. But still to this day,
nobody really takes pilot wave theory seriously, to Bell's great restoration.
And I think it's because Neuman sort of threw shade

(02:52):
on it decades ago and it never really recovered. I
guess that's always dangerous when there's like a famously smart
person who has an opinion absolutely and I find that
physics Nobel Prize winners are especially guilty of this imagining
that they are experts in every corner of everything and
opining on economics or you know, social politics or whatever.

(03:12):
If you have Nobel Prize winner in front of your name,
you're an expert. Uh So, today we wanted to talk
about a couple of chapters that are in a book
of yours. Did that come out earlier this year? Tell
us a bit about the book. Yeah, so the book
is called Frequently Asked Questions about the Universe that I
wrote together with my co host on the podcast and
longtime collaborator Or Cham who's also famous for being the

(03:35):
genius behind PhD comics. And the book comes from noticing
that people who write into our podcast often ask similar
types of questions. There are a few things that seems
like everybody just wants to know about or understand or
the things that people grapple with. You know, I'm a
professional particle physicist in my day job, um, and so

(03:56):
I like asking questions about, you know, the deep nature
of the universe, usend, how our space and time really related.
But you don't have to be a professor of physics
to find these things interesting. And we feel like, in
a sense, you know, curiosity is democratic. Everybody wonders about
these things. So we wanted to try to attack some

(04:16):
of these really big questions that everybody wonders about in
an approachable way, in a way that doesn't require you
to really have any knowledge of modern physics at all. Yeah.
I've really been enjoying the chapter as I was reading.
One thing I like that you do in this book
is um is that you know, it's not like a
continuous narrative that has to you have to have read

(04:37):
everything that came before in order to understand. Like, the
chapters can be consumed pretty much on their own, right. Yeah,
we figured, you know, each chapter should be like one
long bathroom break. So I mean, I'm not telling you
where to read it, but if you're looking for reading
while you're busy sitting down doing something else, each chapter
you know, should entertain you while you're doing your business.

(04:57):
Just you know, don't get so distracted that you forget
to flush right now. Obviously this edition would lack the
wonderful illustrations that are in the print and the Kendall version.
But um, but you guys put together an audio version
as well, right, yes we did. We got to record
the audio version of the book, which is out now also,
and the chapters are read by me and by Jorge alternating,

(05:19):
which is a lot of fun just sort of hear
your words come to life. But yes, the audiobook does
miss some other the real genius of Jorge's drawings. Um.
Jorge and I started working together on science communication more
than ten years ago when I reached out to him
because I thought that cartoons would be a really great
medium for communicating science because they don't take themselves seriously,

(05:42):
you know, they're a cartoon is different from like a
figure into science paper, you know, which is very official
and formal. A cartoon like makes fun of itself and
is easy to you know, hang out with and accessible.
And Jorge was great at that kind of stuff. So
he and I started working together on explaining science using
cartoon is a long time ago. Um, And one thing

(06:02):
I really value about his cartoons is not just that
they are good visual explainers. He has a real visual
skill for explaining something simply on the page, but also
that there's sort of a second voice there you can hear,
like in the text is the voice of me as
a physicist, and then the cartoons you can hear sort
of his response to some of the crazy ideas UM.

(06:22):
And that sort of mirrors the way the podcast works.
On the podcast, I'm talking about physics and Jorges you
know something like that doesn't make any sense, or how
could that possibly be? Or what you got to explain
that again? Because so it sort of tries to capture
those two voices. Yeah, I really liked that the the
illustrations almost seem kind of riffing on the written contents
of the book. Well, so, the parts of the book

(06:43):
that we wanted to focus on today, I think we're
mostly centered around the idea of time and so maybe
maybe a good place to start is you have a
chapter in the book where you talk about time travel
and you make some arguments about which types of time
travel are plausible from a physics perspective and which are not.
So maybe that would be a good place to start

(07:04):
to give us the way of the land, like what
types of time travel are the least consistent with the
known laws of physics and which are the most consistent. Yeah. Sure,
So for those of your listeners who are busy building
their time travel devices, of this useful advice. Well, you know,
the kind of time travel that's most inconsistent with the

(07:25):
law of physics is the kind that most people want
to do, you know, which is I want to go
back in time and change something. I want to not
spill my coffee on my lap, or I wanted to
go you know, not um make a mistake, or I
want to go ask that person out in high school,
which I was too timid to do. And now I
realized I should have that kind of thing. It's not

(07:45):
just that it's ruled out by the laws of physics.
In my view, it's not even sort of internally self
consistent what it means um, you know, and and a
lot of people think about time travel is like I
want to go back in time, as if time was
a place, like if it's a somewhere you can go.
It's just sort of like along a different direction or something.

(08:08):
And it's tempting to think about it that way because
we we hear a lot about modern physics telling us
that space and time are related and time is like
a fourth dimension of space, and so it makes you
want to think about time as a direction in which
you can move and maybe you could just rewind it somehow, right,
But the problem is that time. You know, first of all,

(08:29):
we don't understand time like at all, and we can
dig into that in a minute if you like. Um.
But the problem is that time sort of reflects how
the universe changes. And so you know, I think about
time is like you have a timeline. That timeline is
the universe changing, Like you have the universe at one moment,
and you have the universe at another moment. The next

(08:50):
moment comes later in time, and things can't change without time.
Time is that change. So the self consistency problem is
that going back in time to change it changes the
timeline itself. So like, how does the timeline change If
the timeline is the change, how does the timeline itself change?

(09:11):
It would need like its own time, Like the timeline
is now moving through time because it was a time
before you changed in a time after you changed it.
So it needs like a second dimension of time. I mean,
it just sort of all becomes very complicated and falls
apart as soon as you start thinking about it carefully.
So going back and changing something in the past really

(09:32):
just makes no sense from a physics point of view. Yeah,
I love this because I have long kind of been
skeptical about the idea of time travel into the past.
And one of the reasons I had doubts about this
is that wouldn't we expect to have already encountered lots
of time travelers at some point in history? And there's
no unambiguous evidence of that. I mean, obviously some people,

(09:54):
you know, they're weird little things people think or time travel,
but nothing that looks really clear. So It kind of
makes me think that if if time travel into the
past ever happens in the future, it will be of
a very limited nature. Yeah, I love that as an
experimental proof, you know, like, if time travel exists any

(10:14):
time in the future, then you would expect to see it. Now.
I love that. It's just such a powerful argument. It
sort of reminds me of Stephen Hawking's famous invitation to
time travelers where he threw a party and then he
posted the invitation later after the party. The idea of
being the time travelers, you know, they should be able
to get there anyway, And of course nobody showed up

(10:35):
to his party. Well we know he might have dispensed
with them, or maybe he is a time traveler. Oh
that's a good premise for a sci fi movie like
The Time Traveler Hunters, trying to eliminate all evidence of
the time travelers. Well that that kind of plays into um,

(10:58):
you know, someone, what you're talking about about it being
if it if it does exist in the future, then
it must be limited in scope. And I guess you
could look at it a couple of different We could
basically just scify the hell out of it in multiple directions.
But you know, you could say, like, well, maybe travel
into the past. It has a range, and we haven't
reached the point to where time machines of the future
can reach us. Or it's just so tightly policed that

(11:19):
nobody can make it back. You know, we have time
cops or or something that are that are keeping people
from making too much of a show of the whole thing.
There's so many of those science fiction depictions of like
a big time bureaucracy, you know that's managing the time flow,
like you saw that in Loki and in Umbrella Academy
and and It and in um that book recently, this

(11:42):
is how you win the time war. And those can
be a lot of fun, but also I feel like
they're they just make no sense at all. You know,
how do you have this weird administration that's separated from
time and also weirdly frozen in like nineties bureaucracy. It's
you know, it's fun, but not not if you think
about it really at all. What also reminds me a

(12:02):
lot of something I was actually chatting with you about
a couple of weeks ago, when I interviewed you for
a short freelance piece for How Stuff works dot com
about UM about the zoo hypothesis. Uh you you spoke
about about that for the interview, and uh you mentioned
that one of the strong arguments against it is that
if there is actually this, um, this conspiracy of of

(12:25):
aliens to avoid contact with humans and and and keep
us in the dark about the uh you know, the
galactic civilizations just outside of our view, UM, the main
argument against it is that governments as we know them,
by the only model that we know the the the
human model, are not really good at keeping secrets. They're

(12:45):
not good at advanaging secrets. And it seems like you
could also apply that to the idea of intelligent beings
or humans and the future managing the timeline and so forth. Exactly,
you know, some version of Elon Musk and the future
is get his hands on it, and he's gonna launch
a bunch of crazy, you know, missions, and somebody's gonna
mess something up. So it's hard to imagine that people

(13:06):
in the future having time travel and somehow keeping a
secretor slipping into the past unnoticed and nobody ever, you know,
breaking the protocol or something. It's it just becomes totally
implausible the more you think about it. Picking up off that.
I mean, this is another one of the weird things
about time is it seems like time is actually one
of the arguments against the idea of a coherent galactic civilization,

(13:31):
if this makes any sense, because like you think, a
civilization in order to organize itself, has to have some
pretty close to synchronous uh, you know, thing going on,
Like things have to be happening pretty close to around
the same time for them. But does it even make
sense for I don't know, one planet in a galactic
civilization to be part of a civilization with one on

(13:54):
the other side of the galaxy. I mean, is there
you know, can they say, uh, is there such a thing?
Is what's happening right now on a planet on the
other side of the galaxy? Yeah. You make a great point,
because there's a speed limit to information moving through the universe,
which puts an effective limit on like how well you
can coordinate and organize things. Actually, think about this in

(14:16):
cosmology all the time, because there's a like the largest
thing that can exist in the universe just from very
simple arguments like the age of the universe and the
speed of light. You can't have an object that's like
ten thousand billion light years wide, that's like coordinated, that
has like a structure that's like gravitationally bound on itself,

(14:37):
because there hasn't been time for like a photon to
even cross over the entire size of that object. So
there's like a limit to how big the universe can
even build like a thing, not to mention like the
close coordination required to like organize a galactic empire. And
so yeah, absolutely, um, I think that the sheer size

(14:57):
of space definitely limits our ability to exploit it unless
it breaks down into you know, lots of different unorganized entities,
Like maybe we send humans in an arc off to
another star and they start their own human colony and
we're not in touch and a part of them, you know,
political nation state. But at least we're humans here and
there are humans there. Yeah, I think that's a great

(15:19):
way to conceptualize it. So, I guess coming back to
time travel for a minute, I wanted to talk about
some of the specifics you offer about physically plausible ways
of traveling into the past. Uh. So you mentioned a

(15:39):
couple of things. You mentioned the idea of wormholes, and
then you also mentioned one that might be less familiar
to people. The idea of an infinitely long cylinder of
spinning dust, which could potentially, at least maybe depending on
something about whether something about relativity is true or not,
could potentially allow time travel into the past through something

(16:00):
called time loops. Could could you explain how this would work? Like,
what would this experience be like for the time traveler? Yeah, well,
the short answer is, we just don't know. Uh, this
is a realm where we are like on the cutting edge. Theoretically,
people are looking at the rules of how space and
time bend and twist because you know, general relativity, our

(16:22):
theory for space and time itself essentially tells us that
space and time bend in response to mass and then
tell masses how to move. So, for example, you have
an empty universe and you put a star in it,
it bends the space around the star, and then the
bending of that space tells things how to move, and
not just through space but also through time. So you

(16:44):
go near a black hole, for example, time is slowed down.
So there's definitely some deep connection there between space and
time and what people have done is trying to explore
extreme scenarios about what happens if you do this, what
happens if you do that? Is this allowed? Is that allowed?
And so it's sort of like exploring the universe, but
just inside our own heads. We can't necessarily yet go

(17:06):
out there and build these things in space and say,
let's see what happens experimentally, but we can do similar
like fought experiments, where we say, what would happen if
you did this, and let's just let's assume the equations
are correct and see what happens. And so there's a
couple of fund scenarios there. One, as you said, is wormholes.
These aren't really crazy because they are like connections between

(17:28):
different points in space. And when you think of space,
you probably think of like just sheer emptiness, you know,
the back drop the stage on which the universe happens.
But now we know that space is more complex. It
can bend and it can twist, and that might be
something that you can put in your head. You can
imagine like space bending around the sun. But because space
is like a thing with an arrangement, it could also

(17:49):
do other really weird things, like be connected non trivially,
so you have like a chunk of space over here.
It can be directly connected to a chunk of space
over there. What does that mean. Well, you're used to
the space around you being connected to the space right
next to it. That's what it means to be right
next to it. Right, you take a step to the left,

(18:10):
you move to the next sort of piece of space.
Think of it sort of like pixels on a screen. Right, Well,
a wormhole is a connection between two points in space
that are otherwise really distant. And so you take a
step from a from one pixel and now you're in
a pixel on the other side of the screen. And
so that seems weird and impossible, but remember space can

(18:31):
have all sorts of strange connections, and according to the
equations of general relativity, the ones that define how space
is organized, that is allowed. It is possible. And so
a couple of folks at cal Tech we're thinking about, well,
you know what about time? Is it possible for one
end of the wormhole to be in one place and
the other end to be in another time? Because, as

(18:54):
you were mentioning earlier, like the notion of simultaneity, like
when is now depends really on where you are. Also,
so they have this idea to take one end of
the wormhole and you accelerate it near the speed of light.
That effectively it can be sort of back in time.
And this all works theoretically, but it also sort of
contradicts other things we know, like if you go through

(19:17):
this wormhole and you come out in the past, you know,
doesn't that break things like causality? And you come out
in the past and kill yourself before you um do
the experiment, Then you don't do the experiment, you don't
come out in the past. So it appears to create paradoxes,
and nobody knows like how to resolve that. Does that
mean that these things are impossible? Does that mean if

(19:38):
you did that, the universe would like disappear in a
puff of logic. Nobody really knows what would happen. So
that's a bit of a contradiction in the theory itself
that it predicts something which seems to be disallowed by
other parts of the theory. And it's a similar idea
for these closed timelike curves. People said, if you create

(19:58):
these infinitely long the cylinders of spinning dust, which doesn't
sound easy to do, then it bends time in this
way that the time then as you move forward in
time you're actually moving sort of like sideways through space
time in a way that's similar to the experience of
going into a black hole. Outside a black hole, time

(20:20):
always moves forwards. Inside a black hole, space has bent
so much that space only moves towards the center of
the black hole. It's like one direction to space. So
if you imagine space being distorted, not quite as much
as a black hole, but sort of in a similar
direction that it sort of bends space sideways, then you

(20:40):
can create these paths where something can move in a
loop through time. Um, but you would be trapped on
that loop, so you wouldn't be able to like change anything.
It's like a fixed loop, sort of like Harry Potter
style loop through time, where every time you go through
it's exactly the same thing happening. And these are really
fun because nobody knows like if these are actually possible

(21:01):
and what would happen if you actually went through them.
So so we don't really know, for instance, like what
conceivable reason there would be for a civilization to conceivably
construct one of these, yeah, because we don't know practically
what you could achieve. And also, an infinite cylinder of
spitting dust sounds like an expensive project. You know, the

(21:21):
word infinite seems to raise some doubts. And when it
comes to wormholes, people know how to calculate whether a
wormhole is allowed by the theory of general relativity. Nobody
knows how to build a wormhole. You know. It's sort
of like saying, Okay, it's possible to have an apple pie,
but but we we don't have a recipe for making one. Right.

(21:43):
It's a different thing to say, like I know how
to put it together than to say it's technically allowed
to exist in the universe. You know. It's like if
you say, well, the sun is allowed by the laws
of physics, but I don't know how to make it
happen if I just start from a cloud of gas,
for example, And so that's a big le. Nobody really
knows how to build a wormhole or even keep one open. Um,

(22:04):
if you did manage to build one, Are there any
reasons to suspect that wormholes exist naturally? Oh, great question,
Not yet, No, um. Some people wonder if there are
wormholes that connect the super massive black holes at the
hearts of all of our galaxies, But there's not like
any evidence out there anything that can't be explained without wormholes,

(22:26):
that you would need wormholes to explain. Um, that would
be super cool, though no, Um, I'm not aware of
any evidence like that. Well, Daniel, you also have a
chapter in the book that I really liked on the
question of will time ever stop? And I think this
is one of those great questions because it's a yes
or no question, and like many big questions in physical cosmology,

(22:49):
it's a binary. But no matter which answer it is,
it's mind boggling, like it is impossible to imagine time
either stopping or going on forever. Uh So, so what
are your thoughts here about whether time will ever stop?
I go, you're feeling there, And I also think it's
really fascinating to go back through history and read about

(23:10):
which concept felt more natural to people. Initially, it felt
to people like time should go on forever. Obviously, that
was like a hundred fifty years ago, before we knew
that the universe was expanding. People looked out in the
stars and they looked like they were just sort of
hanging out, and they thought, maybe the universe is just
sort of there, and so obviously it's been there forever, right,

(23:32):
And that was like this you know, de facto assumption
in science until Hubble discovered that the universe is expanding,
and that gave the universe sort of like a direction.
It's like things are changing, and as you look back
in time, that suggests, you know, something a moment when
the universe was like crazy infinitely dense. So it suggested

(23:52):
a beginning. And that must have been an incredible sort
of mind bending mental gymnastics to execute to go from thinking, oh,
it makes sense for the universe to be infinite in time,
to go into like, oh, the universe had a beginning
and now let's trying to figure out what that beginning was. Um.
So I think that's really interesting. And you know, I

(24:13):
think the thing that's really cool about this question is
not just that it's tangible because it makes you wonder
like am I going to go on forever as the
universe always going to be here? But because it really
gets at the heart of a the deepest problem in
physics right now, like the fundamental conflict we have between
two ideas in physics, which are quantum mechanics and general relativity.

(24:36):
You know, we have a quantum mechanical description of how
like particles bounce off each other and we you know,
have a lot of questions about how that works. But
we have a pretty good theory for you know, understanding
quantum particles, and we've been talking about general relativity, you know,
how space bends and how it affects time and what
happens in black holes and all that stuff also very successful.
The problem is that these two theories nobody knows how

(24:58):
to bring them together end critically. They have very different
stories to tell about what time is. They treat time
totally differently, with huge consequences for the answer to this question,
will time ever stop? And so to me, this is
a fun question because it puts its finger on right
on that conflict. Yes, so there is. There's a concept

(25:20):
that you introduce in this chapter about uh sort of
time as we experience it being a sort of special
case or special circumstance of a hypothetical substance you refer
to as meta time. Can you explain something like what
are you getting at here? Well, one of the basic
questions is is time fundamental or is it emergent? You know,

(25:45):
a deep question in modern physics is like what are
the essential ingredients to the universe? What did it start with?
And then what sort of arises out of that, out
of the complexity of the possible interactions. You know, for example,
if you're playing with legos, the fund mental ingredients are
the basic pieces, and from that you can make complicated
things dinosaurs or pirates or spaceships or whatever. But those

(26:07):
spaceships they're emergent, you know, they're not necessary. They don't
have to exist. You can take it apart and just
have the legos. In the same way in our universe
there are complicated things like ice cream and hurricanes, but
those don't have to exist in the universe, right, You
can imagine a universe without hurricanes or without ice cream,
as sad as that is. So then the question is

(26:27):
what are the basic elements of the universe. And for
a long time, you know, people like Newton thought that, well,
obviously space and time are fundamentals of the universe. They're
just like you gotta have that, right, And now people
are wondering, like, well, is that really true? Is it
possible to have a universe without space or without time?
You know, we gotta when you're really digging deep into

(26:49):
the nature of the universe, you gotta push hard on
the fundamental assumptions. So There are a lot of ideas
now about how space could be emergent, you know, how
it could be that the universe that self, that space
is not a natural thing, that like ice cream, you
could have a time in the universe where there wasn't
any space. That space is like just briefly, the stitching

(27:10):
together of these um separated pixels of space using quantum
entanglement to sort of weave together this idea of space,
these relations between different locations that we experience, and we
could talk about that for an hour um. But even
moving beyond that, now folks are wondering, like, is time
also emergent? Is it possible that time is not a

(27:33):
fundamental property of the universe but it just sort of
something that exists now. And it's really hard to even
think or talk about it because, like I just said,
it exists now I'm using time to talk about when
time is. It's very complicated and confusing. But there are
some theories that tell us that time might be not

(27:54):
an illusion, right, not in the sense that it doesn't exist,
but it might not be fundamental, that it might rise
from complex interactions of smaller, more fundamental elements of the universe,
and so that's this idea of meta time. You have
to imagine some like deeper laws of physics that control
those fundamental bits that I'm being vague about because we

(28:15):
have no idea what they would be or what they are,
what the rules are. And And if this seems sort
of like frustrating, it's because we're at the very beginning
of even talking about the answers, because we're just formulating
the questions. You know, sometimes it takes like a hundred
years to figure out Okay, the important question to ask
is is there always time in the universe? What does

(28:35):
that mean? And how do you even think about a
universe without time? Then you can start to make progress
on the crazy ideas that might explain it. Uh. This
may be kind of a tangent, but this actually makes
me wonder about a question that's come up on the
show before. Do you have a view on what the
present is, on whether something special is actually happening in

(28:57):
the present? Uh? Like, does only the present exists or
does all of time exist? It's a really great question.
We don't understand that at all. I don't understand it.
I don't even know if it's a question of science
or if it's a question of philosophy because it goes
into the nature of consciousness. You know, does the whole
timeline exists and we only experience part of it? Or

(29:19):
you know, does the only this moment exist um? Physics
doesn't have a great way to even define what the
present is um, and so it's it's pretty hard to
put your finger on it um. And I love that
because these are questions that like, we don't even really
know how to attack these questions. And what that suggests
is that there's something wrong in the way we're organizing

(29:40):
our thinking. You know. It's like if you're asking a
question and you're just using the wrong language, we're using
the wrong notation, then your question seems really complicated and confusing.
And if you learn a new perspective and then suddenly
would make sense. You know, I'm reminded of that Far
Side cartoon where the scientists are trying to understand dolphins
and they're writing down phonetically with the dolphins are saying,

(30:02):
and they're saying things like, you know, obla Espanol, and
you know, the scientists don't speak Spanish, so though to
them it's just nonsense. My boys, if they knew the language,
it would all click together. And I feel like that's
the problem we have sometimes that was just not speaking
the right language at the universe yet, and that's why
some of these questions are awkward and really hard to
grapple with. I thought you were gonna say today's physicists

(30:25):
are only equipped with cow tools only for spherical cows, exactly,
you know, all of this um Also, it reminds me
a bit of the Copernican principle to UM. But but
going beyond just the idea of like, you know, there
being some uh I we should not not see that

(30:45):
there's something privileged about about our planet or about humans.
But but could you could you even apply that based
on what you're saying to to the present moment, to
this time that in which from which we are viewing
the universe. Yeah. Probably. I think that's why a lot
of progress could be made if we ever did get
to talk to alien scientists, because I think we would

(31:08):
learn a lot about um, you know, the biases that
creep into our questions and our reference frame for answering
those questions because of our human experience, and alien intelligence
that might have a very different relationship with a concept
of time might have a very different treatment of it
mathematically and physically, and might make a lot more sense,

(31:29):
you know. Um, The problem with alien intelligence, of course
is you know, finding them, talking to them, decoding their language,
and then if they are so fundamentally different that they've
made that they avoid human biases, they might be impossible
to ever understand. And so while it's tantalizing to imagine
that like aliens are out there with the answers the

(31:50):
deep questions about the universe, it might also be that,
uh that we could never understand what they have to say.
I have long thought we should outsource all of our
physics research to like a seven in dimensional octopus. Um,
if you know one, I'd like to meet it because
I got questions, thank uh so. But to come back

(32:15):
to the idea of will time ever stop? You talk
about a couple of possibilities for what that would look like.
Say that you know the far future of our own
universe at least at least what we can reason from
what we know today, and and a couple of these
options are are the Big Crunch and the heat death
of the universe. Do you do you want to talk
about what those would mean as as best we can

(32:36):
guess for time itself. Yes, So remember that there are
two paths to go down if you're asking questions about
the deep future of the universe, and one is quantum
mechanical and the other one is general relativity. And quantum
mechanics is pretty straightforward about this. It says that, look,
time always existed and time will always exist. And there's

(32:56):
a pretty simple argument there because according to quantumy annex,
quantum information can't be destroyed, like when something happens um,
you know, the information about what used to happen is
encoded into the future, and so it suggests the time
has always existed. There's no mechanism in quantum mechanics for
time to start. It should always have existed, and you

(33:19):
flip it around the other direction, it should always exist.
So there should always be a universe and clocks should
always tick forwards according to quantum mechanics. But that assumes
you know that space is flat and simple, and general relativity,
the other pillar of modern physics, tells us that space
is not simple. It's not flat, it's complicated. It's expanding,

(33:40):
and you know, the mechanism by which space is expanding
is not something that we understand hub Will discovered a
hundreds something years ago that the universe is expanding and
things are moving away from us, And then twenty something
years ago we discovered even more mind boggling lye that
that expansion is accelerating. Right, It's not like stuff is
moving through space and gradually slowing down and maybe eventually

(34:04):
gonna stop and turn around and come back um and collapse,
but that it's speeding up, which means that there's some massive,
incredibly powerful force in the universe that's literally tearing it apart.
Because we don't know the mechanism for it, though, we
can't predict what it's gonna do, Like it turned on
about five billion years ago, started tearing the universe apart.

(34:24):
Will it do that forever? If so, you end up
with like a universe where everything is super far apart.
It's just like a bunch of black holes from collapsed galaxies,
separated by you know, unthinkably vast distances, even compared to
the distances we see between our galaxy and other galaxies today.
You know, these galaxies would be so far apart that

(34:45):
they could never even see each other. You know, light
would never reach one from the other. On the other hand.
Dark energy could change its direction, it could stop, it
could turn around, it could cause the universe to collapse
back down into an incredible moment of singularity at the
end of the universe UM, and then we can ask
questions like, well, what happens then, you know, does the

(35:07):
universe stop when you reach another singularity, another moment of
incredible density. We just don't know because general relativity describes
that process. But when you actually get to the singularity,
people think of singularities is like a feature of general relativity.
Really they're like a failure of general relativity. It can't

(35:28):
predict anything that happens there, doesn't know what to do.
It's like, well that's the direction you're going, but once
you get there, I can't tell you what's going to
happen next. So if that happens, we just really don't
know what the fate of the universe would be in
that scenario. But you know, it wouldn't be pleasant for
humans or for seventeen seventeen dimensional octopi. But I guess
with with the other option, with like you know, the

(35:49):
heat death of the universe, everything just expanding and cooling
and reaching some kind of equilibrium where UM where there's
no there's no imbalance to distribute any further. Now, I
think in the book you raised the idea that this
could in a way represent a threat to to our
concept of time, because time would maybe in itself, time

(36:09):
has something to do with entropy, and this would be
a state of maximum entropy. Yeah, we see the universe
proceeding through time and we see entropy increasing. And entropy
is a really tricky topic. You hear people talk about
it a lot, but it's really hard to sort of
grapple with intellectually, and people try to think about it
in terms of like amount of disorder in the universe,

(36:31):
but that can be pretty misleading technically. It's really relates
to the number of different ways you can arrange the
microscopic nature of the universe to be consistent with the
macroscopic nature that you observe. That's a little bit more subtle,
but it's actually a more accurate guide to what entropy is.
And what we notice is that entropy seems to be

(36:52):
increasing through the universe, Like there's something we've observed, and
a lot of places in physics seem to be sort
of like and bivalent about time. The laws will run
the same forward or backwards. It doesn't really matter, if
you know, without friction or air resistance. For example, you
can throw a ball up in the air and it
lands back in your hands. If you played a movie

(37:12):
of that backwards, it would look exactly the same again
without air resistance, because that increases entropy. Um. But entropy
is the one place where in the laws of physics
there seems to be a preference for things moving forwards.
So it's often claimed that entropy might be the reason
time moves forwards. And I think that's a bit of
a step too far. You know, we see that entropy

(37:34):
increases as time moves forward, so there's a connection between them.
That doesn't mean necessarily the time has to move forward.
I mean, if time moved backwards, it just means that
maybe entropy would decrease, right it um creates this connection
between entropy and time, It doesn't necessarily imply a direction.
But some people wonder what would happen when you reach

(37:56):
a state of maximum entropy, and maximum entropy would be
as you say, a everything progresses forward and the universe
so spreads out and it becomes maximumly even there's no
like hot spots and cold spots because that allows you
to rearrange the microscopic state as many ways as possible,
so the most freedom to rearrange the microscopic state and
so the most entropy. And in that state, it's called

(38:19):
the heat death of the universe because you have no
hot spots and no cold spots, so no way for
like energy to flow. Nobody like do anything. The way
that you operate as a human being is through energy flows,
and the way that computation happens is through energy transfers,
and so you can't really do anything if there's no
energy ingredients. So that's why it's referred to as the
heat death of the universe. And people who think that

(38:42):
time is deeply connected to entropy, wonder if one entropy
reaches its maximum point, if time then somehow stops, or
maybe time stops and then turns around and entropy starts
to decrease like a bounce in time. And nobody knows
the answer to these questions when he's gonna be around
to know the answer to the questions, even if you're

(39:03):
optimistic about the length of human civilization. But they're really
fun to think about because they make you think about
what time is and you know, and how it relates
to the whole universe. Well, though, on the question of
nobody being around this, this may also be a tangent.
But this makes me wonder do you have opinions on
the alleged Boltzman brain problem. I know we talked about

(39:25):
this on an episode a few years back, and um
so maybe kind of fuzzy on the details, but if
I recall, it has been used to argue against some
types of future eternity ees. But basically the the argument
is if the universe were to go on existing literally forever,
with certain types of properties in play, eventually people whose

(39:47):
brains randomly formed from fluctuations in space would outnumber people
who exist through biological evolution on Rocky Planet, and thus
we would expect to be those brains instead of these
biological brains. Is that roughly right? Yeah. Essentially, it's arguing
that if the universe reaches heat death and then goes

(40:08):
on forever, that most of the time in the universe
is during heat death, right, that really basically randomly sampled
moment in the universe should be when the universe is
spread out and boring and gray. So then Boltsman said, well,
what if you just had a quantum fluctuation while he
was before quantum mechanics. But what if you had a
random fluctuation, Because you know, the law of entropy is statistical,

(40:28):
it's not exact, allows for fluctuations. And so he said, well,
what's the chances of the whole universe then being like
a fluctuation in some vast or heat dead universe that
already has existed for unknown millions of years. So he
was trying to fluctuate an entire universe out of basically nothing.

(40:48):
And so as a counterpoint, people are like, well, you know,
there are smaller but more ridiculous things that you could
fluctuate out of the universe, like a galaxy or even
just like one brain. And so at the point was
made actually to criticize those kinds of cosmological models, because
if your cosmological model seems less likely than you know,

(41:09):
brains forming spontaneously in space and thinking that they're people,
then it seems pretty unlikely. Um, And so I don't
think anybody really takes it seriously. Is like a theory
of the universe. It's sort of just more like a
mental exercise to wonder, like, how likely is your theory
of the universe. Um, you know, is it less likely
than this absurd scenario. So there's another thing you bring

(41:32):
up in your chapter. Will time ever stopped? Is an
idea I was instantly captivated by, which is you point
out that technically, um, time could be stopping and restarting
all the time, frequently without us ever realizing it, because
how would we know, right, Like, our consciousness or experience
of the world is through time, So if time were

(41:53):
to stop, uh and then restart, that might just be
invisible to us. So you know, maybe they're just uh,
these huge gaps in our life. Though. That makes me
wonder if time we're stopping, would it be possible to
measure how long it stopped for? Oh, that's really interesting.
You know, this sort of presupposes some sort of like

(42:15):
meta time, some you know, other rules of the universe
that's controlling our time. And because time itself controls how
our universe changes, then strictly speaking, if time does pause,
you know, according to this meta time, and then pick
up again, nothing should change because that would require our

(42:36):
time to tick forward. Like, no particles can move, no galaxies,
no space can be created, you know, no expansion can happen.
Without time our time ticking forward. That means that there's
nothing in our universe that should change if time doesn't
take forward, which means that there should be no way
to tell. So it could be like a near infinite

(42:56):
amount of time between every tick of our universe could
be passing in sort of like the meta universe. I
think the easiest way to imagine this is in the
simulation hypothesis, the idea that the universe is like a
computer program running on some mega computer, and you know,
if the aliens or super beings running that simulation paused

(43:18):
the simulation to go to the bathroom and come back,
then we don't know that they've paused it. Right. It's
just like the characters in your video game. They're not like,
hey buddy, that was a long number two. You know
what you're doing? Everything okay when you come back, They
have no idea, and to them, you know, the experience
is completely smooth. So I think, no, there's no way
to know how long time has been paused for if

(43:39):
it does get paused. I always wondered with that hypothesis,
would we notice if the resolution on our simulation was downgraded?
You mean, if they lost their funding and had to
had to go to a more course resolution decreased render distance. Yeah,
I don't know. Sometimes it does feel like the resolution

(44:00):
decreases or increases, and depending on what's going on, I'm
gonna blame my failing memory for that, Like you know
what the aliens have just been like cleaning up the cash,
and that's why I can't remember, you know, what happened
last week or when I agreed to clean to the
garage or whatever. Um. But there are always that we
do think we might be able to probe the resolution
of the simulation of the universe under the assumption that

(44:24):
we live in that crazy scenario, because these simulations the
way we do them, at least as we tend to
like divide the universe into huge cubes and stimulate each
cube separately, assuming that like the interactions between cubes are
pretty small, which works pretty well, you know, if you're
in if you're stimulating like a single galaxy at the time,
because mostly you're dominated by what's going on inside the

(44:45):
galaxy and now stuff from other galaxies. But we have
these particles, these crazy high energy particles that whizz through
space at velocities nobody's ever seen before or energies nobody's
ever seen before, much much higher energy than anything like
created by our particle accelerators. And they might be like
tripping up that simulation because they skipped through several of

(45:06):
these simulation pixels faster than anything you should expect. And
you know, there are some things about those particles we
see out there in space that we don't understand, and
so that opens the door to like maybe you could
explain those particles as being like a glitch in the simulation. Now,
speaking of simulations and going back to time travel, does

(45:28):
anyone out there like make any kind of an argument
for time travel into the past by saying, well, if
we are living within a simulation, then time travel into
the past and the ability to change the past would
be possible within the confines of that simulation. Yeah. You know,
if you're living in a simulation, then the rules are

(45:49):
essentially arbitrary, and then yeah, you could wind time backwards.
I think this goes to the heart of, like I
think a basic confusion about time travel because people imagine,
like you get in a time machine and you and
the time machine does something to you, and then you
end up back in the past. I don't really see
how that could possibly work. What you really want in

(46:09):
time travel is for the whole universe to travel back
in the past and for you to not. So you
gotta like get in the time machine, and it's got
to like rewind the clocks of the rest of the universe, right.
You don't want to be like, Okay, it's still today,
but now I'm ten years younger. I mean maybe some
people want that. That's a whole different thing to look for.
If you want to like unspill your coffee, but you

(46:31):
still want like the ideas, you want to remember having
spilled it on yourself, so you cannot just repeat it.
Then you need to rewind the whole rest of the
universe somehow. It seems like a much bigger job that. Yeah,
that's a great point. Yeah, that so the time machine
would have to change the universe, not you. Yeah, I
never thought of it that way. Yeah. And a lot
of our listeners right in when we talk about time
travel and raise a similar point and a criticism of

(46:53):
science fiction levels, which is that you know, if you
do go back in time somehow, how do you know
where you're going to be? You know, because the Earth
and the sun and the milky way they're all moving, Um,
so how do you know where you're going to end up?
And it's a fun question. Um though, I think if
you're gonna pose it like, okay, you can travel through time,

(47:13):
then ostensibly probably you can travel travel through space time,
so you can appear wherever you want. But the problem
is like, actually, I'm not even really necessarily well defined,
because what does it mean to be here at a
point in space, or there at a point in space,
or now this point in space? Now where is that
point in the future because there is no like marker

(47:34):
to space. Space is all relative. It's not absolutely you
can't like grasp this point of space and give it
a name and say where does this bit go? There's
only stuff moving through space relative to each other. So
it turns out that's not even really well defined. Like
where was the Earth, you know, a million years ago
in our space? Doesn't actually have a meaning? That is

(47:55):
when I had thought of before that always seemed an
insurmountable problem. But this actually reminds me of another thing
I wanted to talk about briefly, which is relating time
to the history of the universe and the Big Bang,

(48:18):
a thing people often ask and I know, there are
theories to address this is is what happened before the
Big Bang. But if you have an understanding that, you know,
you have a singularity at the origin of the Big Bang,
that was the beginning of time itself as we know it.
What are physicists talking about exactly when they try to

(48:39):
envision causes leading to the first instant of the Big Bang?
Mostly they're trying to avoid that singularity because that singularity
is a problem. You know, we don't see things like
singularities in the universe. We don't see infinities, we don't
see things with infinite density, we don't see things of
infinite size. I mean, maybe the universe is itself infinite.

(49:00):
There's nothing that's like infinitely smooth or perfectly circular. These
are sort of abstractions in our mind. And so most
physicists who are working on the very early universe are
trying to avoid that singularity because, as I said earlier,
general relativity breaks down. That's what it means like, if
your theory predicts something infinite, it doesn't know how to
do any calculations beyond that. So instead of having like

(49:22):
a moment of singularity, which is sort of like the
naive general relativistic prediction of increasing density. Instead, they're going
back and saying, well, maybe the Big Bang was just
like a rapid expansion of space from a previously dense
kind of universe that we don't understand at all. So
the basic sketches like you have some kind of weird state.

(49:43):
The universe is filled with like inflotons, some particle we
don't know if it existed, but maybe it did. And
then those inflotons they are causing the rapid expansion of
space and decay then into normal matter. So that's so
now the Big Bang is that moment when the in
photons are expanding and then decay into like our universe.

(50:04):
That's how our universe is sort of created out of
these in photons, and that avoids this moment of singularity.
It's never like a moment when the universe is infinitely dense.
But you know, again, this is very speculative stuff. We
think inflation happened, this crazy expansion in the very beginning
um and this is like a way to avoid having
to put before that this dot, this singularity that breaks

(50:27):
all of the mathematics, instead of replacing it with like
some other weird kind of substance. We don't even really
know what it's like or what it's about. Um, we're
just really beginning to know how to ask questions about it.
And you know, that suggests a really interesting question, which
is like, if there is something before the Big Bang,
what was it? And was there something before that? It

(50:47):
seems like, in one hand, super frustrating because you're just
kicking the can down the road, Like, alright, so the
early universe was this expansion, and before that came something
which caused the expansion, and before that came something which
caused that, which caused the expansion. But is there in
the end something original which caused it. We don't know.
And there's two possibilities. One is that we just keep

(51:08):
digging forever and dig further and further and further and
further back and never get to anything which seems like
could have caused itself. Or it could be that we
get to some state where we're like, this makes sense
to have to be a beginning. It's like it's sort
of the only way things could have happened to me.
It's it's hard to grapple with these ideas, so it's
easier to think about it sort of in a parallel way,

(51:30):
which is like, what is the smallest thing in the universe.
We don't know if as we tear apart, particles will
keep finding things that are smaller and smaller and smaller
and smaller, or if eventually we'll get to one where, like,
you know, what, this one it makes sense to be
a fundamental ingredient to the universe. We can just start
from here and build up. You know, maybe it's like
the smallest fundamental thing. It's at the plank length or something.

(51:52):
We don't know if we'll ever get there, or if
it will be self evident, or if there will be
always people who say, like, I don't know, I want
to dig deeper. In the same way, it might be
that we're doomed to keep digging deeper and deeper back
into the history of the universe, never finding out if
there was an original cause. All right, so I think
we're probably getting close to the end of our time.
But I got to come back to time travel before

(52:14):
we do, because I'm wondering, what what do you think
You mentioned Stephen Hawking's party where the the invitations were
sent out after it happened, But what what is your
personal favorite way to hunt for a time traveler. What
would you do if you wanted to find evidence of
people from the future. Wow, I know I have never
given that any thought about evidence for people from the future. UM.

(52:35):
I try to think about what people would want to do,
Like if I were a time traveler, why would I
come to one? Uh? You know the obvious answers are
like change history. Uh, in which case, you know, I
guess you can blame those time travelers for you know,
the reason things have gone the way they are. Maybe
there because time travelers have come back and tweaked election

(52:57):
results or you know, or something like that. Um. So
I guess the best way to find time travelers with
then to be present at critical hinge moments in history
and look around for suspicious behavior. I suppose I don't
really know. That's a great question. Yeah. I was thinking
about all this in terms of of ancient aliens as well,

(53:18):
because both you have you have people of course who
obsess about the idea of of aliens having visited during
ancient times and so forth, and and you also have,
I guess a more recent phenomenon of people looking back
at old pictures and paintings and you know, playing this
game of basically misinterpreting um things and paintings and photos
of like looking back in an old picture and saying, oh, well,

(53:39):
that person, their their style of dress does not look
archaic enough. They must be traveler. Yeah. Or this painting
she's holding something that looks like an iPhone. Obviously this
is a Renaissance painting of a time travel No. I
think that just says a lot about us, you know,
and who we are, you know, the same way that
like photos of UFO, those seemed to be constantly greeny.

(54:03):
As you know, imaging technology improves, it's always on the
edge of the our ability to capture it. So I
think it says something about our desire to discover weird
things and reveal the truth, which I'm totally sympathetic to.
I also want to peel back a layer of the
universe and wake up to its true nature. I remember
Carl Sagan, and I forget which which book this was,

(54:24):
but it was in one of the books where he
talks a little bit about the idea of ancient aliens,
and I remember him him basically outlining the sort of
ancient account, the sort of myth that one might look
to as as the sort of ancient astronaut account that
could exist. I have such things were possible, And I

(54:45):
wonder if anyone has ever taken a similar approach to
the concept of time travel, like like basically like boiling
it down, saying, Okay, if there is actually evidence, say
in you know, the historical record, all of people having
traveled back in time, you know, well, what ex exactly
what would be looking for? What exactly what they would
they have been doing? Um and uh and and how

(55:06):
would and I guess it would come down to, like
you'd have to imagine, like how truthful are they going
to be? Are they just gonna lie about themselves being
time travels, Because that's then you can basically point to
any pivotal individual or any person in a pivotal period
of time, right, yeah, And would they even be humans? Right?
Like we fantasized about going back to see the dinosaurs.

(55:27):
So if now we're putting ourselves back in the past
and imagining time travelers, we might have to imagine some
like post human apocalyptic, newly intelligent species of you know,
who knows what, penguins or or something coming back in
time to investigate humans, you know, to understand what happened
before the apocalypse or whatever. Um, but I thot machines machine.

(55:50):
Most of the most of our space exploration is uncrewed probes.
Now you would have to imagine that the same would
hold true for time. Yeah, it's probably true, or you know,
after the mission needs have killed us all and they
just have myths about those weird meat creatures that used
to uh roam the earth or something. Um. That's fun,
but it's fundamentally is limited by our imagination. It's the

(56:11):
same problem with trying to look for aliens. We look
for aliens in the way we expect to see them,
although we're pretty sure that if aliens exist, they're not
anything that we expected. So we need to like push
really hard on all the boundaries of our imagination to
make sure we're looking for aliens as broadly as possible
so we don't miss them. We don't just like come by,

(56:32):
we're like, oh, that's not aliens. So it's the same
problem with imagining future time travelers, like who these these
people or things or entities are are well beyond I
think even our most creative science fiction UM writers. Well, so,
like you said, like what's interesting, what would be interesting
about to someone from the future, and we instantly think, too, oh, well,

(56:52):
the you know, the coronavirus or something going on in
you know, geopolitics, or even in the environment. But it
could be something entirely different. It could be you know,
the very beginning of something um that doesn't matter at
all today, right, but but matters, say in exactly which

(57:12):
we could never possibly imagine. I mean, think about people
a thousand years ago trying to anticipate was important to
us today. We couldn't even do that from twenty years ago,
not to mention a thousand Okay, last question, Daniel, what's
your favorite time travel movie? Oh? My favorite time travel
movie has to be Primer, So I think that most
clearly sets out rules, rules that make sense, and then

(57:33):
follows those really carefully with lots of fascinating and unexpected results.
Good answer, good answer, Yeah, that's a good one. I
often gravitate towards the ones that have really goofy time
travel rules. But they but if they still stick to
those rules, then I tend to forgive them. They're always

(57:55):
boundary cases, though they're always cased to were like, I'm
not sure what would happen in this scenario or that scenario.
So I like Prime Room because it has really clear
Chris Brules and those it has cost. You can't just
like pop back in time. You have to like spend
time going backwards um, which has really interesting consequences. So
I found it to be really creative totally. All right, well,

(58:17):
thanks so much for joining us today, Daniel. This has
been a lot of fun. Thank you very much. Always
a pleasure to talk to you guys. All right, well,
thanks well once more to Daniel Watson for jumping on
the old podcast machine and letting us uh pokemon prod
um with various questions about time travel and wormholes and
what have you. Yeah, if you want to learn more,

(58:39):
so if you're not subscribed to Daniel and Jorge Explain
the Universe, you can find that wherever you get your podcast,
but you can also go to www dot Daniel and
Jorge dot com. And you can also find the website
for their new book. Again. The book is called Frequently
Asked Questions about the Universe, and the website for that
is www dot you Diverse f a Q dot com.

(59:02):
And if you'd like to check out other episodes of
Stuff to Blow your Mind, well you can find our
show wherever you get your podcasts. Just look for the
Stuff to Blow Your Mind podcast feed. We run multiple
episodes per week, with core episodes dealing with science and
culture on Tuesdays and Thursdays. On Monday's we do listener mail,
on Wednesday's we do a short form artifact titled the Artifact,

(59:24):
and on Friday's we do something called Weird House Cinema,
which is our time to set aside most serious matters
and just discuss a strange film. So, of course, thanks
again to Daniel for joining us today, and as always
a big thank you to our excellent audio producer Seth
Nicholas Johnson. If you would like to get in touch
with us with feedback on this episode or any other,

(59:44):
to suggest a topic for the future, or just to
say hi, you can email us at contact at stuff
to Blow your Mind dot com. Stuff to All Your
Mind is production of I Heart Radio. For more podcasts
for My Heart Radio, visit the I Heart Radio app,

(01:00:05):
Apple Podcasts, or wherever you listening to your favorite shows.