The Many-Worlds Interpretation of Quantum Mechanics

Daniel talks to Sean Carroll about the quantum multiverse, and whether it is real

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Episode Transcript

Speaker 1 (00:08):
What if your idea of how the universe works is
just wrong? I mean, you live in a world that
seems to make sense of you, that seems to follow
rules that you're familiar with. But what if that's just wrong?
Could reality what's actually out there beyond our brains and
our senses. Could it be something so strange and bizarre

(00:30):
that we would hardly recognize it. Could it be dramatically
different from the glimpses we get through our senses and experiments.
There's a vital clue that might just point us in
that direction, something that has puzzled physicists and philosophers for
nearly one hundred years, and that may take another hundred
years to solve. Solving it might require us to swallow

(00:52):
a picture of reality that is mind bending lee strange
to our little human brains. I'm Daniel, I'm a particle physicist,

(01:15):
and I'm drawn to the possibility that the universe might
be very different from the way we imagine it. What
is the goal of physics, anyway, if not to reveal
the true nature of reality to us. We build mathematical
stories in our minds and apply them to our experiences.
But why immediately it's because we want to predict what

(01:36):
will happen when we throw a stone or jump a river.
But going deeper it gives us the chance to ask
questions about how the universe works. If our mathematical story
describes the universe, then we can look at that math
and ask why the universe seems to follow that and
what it all means. And sometimes the universe out there

(01:58):
seems to insist on a math modical story that we
find very weird, shocking, almost And that is the goal
of physics, not just to give us the power to
throw rocks and jump rivers, but to reveal the truth.
And the most exciting moments are when the truth and
our intuition clash dramatically, when the universe says to us, know,

(02:18):
your ideas about the universe are just wrong. And that's
the goal of this podcast. Daniel and Jorge Explain the Universe,
a production of I Heart Radio in which we tackle
the biggest and hardest and nastiest and funnest questions of
the universe. The ones that make your brain twist, the
ones that slip away from you just as you thought
you had figured them out. The ones that might elude

(02:40):
humanity for centuries or forever. We don't shy away from
any questions on the podcast, but we seek to approach
them and explain our knowledge and our ignorance to you.
My friend and co host Jorge is on a break,
but I have a special treat for you today. We
are very lucky to have as a guest one of
my favorite physicists and one of my writers about physics.

(03:01):
Today we'll be talking to Professor Sean Carroll about some
of the problems at the heart of quantum mechanics and
a potential solution. So today on the podcast, we'll be
answering the question what is the many worlds interpretation of
quantum mechanics. So it's my great pleasure to introduce Professor

(03:23):
Sean Carroll. He's a theoretical physicist at cal Tech, and
he's known for his work on cosmology, general relativity, and
the foundations of quantum mechanics. He's also the author of
several widely acclaimed and widely read books, including Something People
Hidden and The Big Picture, and is the host of
the podcast Mindscape, which might actually be nearlier than this podcast. Today,

(03:44):
Sean is here to talk to us about the many
worlds interpretation of quantum mechanics and the measurement problem in
quantum mechanics. Sean, Welcome to the podcast. Thanks very much
for having me here. Wonderful to have you. So I
want to die right in and before we talk about
what the many worlds interpretation is, I want to get
review on what problem it solves, Like why do we
need so many interpretations of quantum mechanics. What problem is

(04:06):
it that they are trying to address. I think there's
actually two problems. I mean, this is the right question,
because are we just wasting our time or it's not. Honestly,
it's not a lot of time compared to other physicists
thinking about other things. The foundations of quantum mechanics is
a minority pursuit. But I think there are two problems,
and there's such looming large problems, and quantum mechanics is
so important to modern physics that I do wish we

(04:27):
were spending more time on them. So in Quantum Mechanics,
I'll try to give my briefest version of quantum mechanics
that we can we talk about objects in the universe,
whether it's an electron or whatever, in a different way
than we talked about them in classical mechanics and Isaac
Newton's view the universe. And Newton's view, we would have
a position a location in space for a particle, and

(04:48):
we would also have a velocity. And if you knew
the positions of velocities of everything in the universe, in principle,
you could predict what would happen, and you could measure
what would happen as much as you want. In quantum
mechanics we say no, no, no, that's not how we
describe reality. There's something called the wave function, which, for
a single particle like an electron, is just very wave
like basically at every point in space that has a value.

(05:11):
But positions and velocities are not properties of the electron anymore.
There are things we can observe about it, and the
wave function tells us the probability that we'll get different
answers if we observe like the position or the momentum.
The momentum is just the mass times the velocity. So
this raises two big questions. One is what is the

(05:34):
wave function? Is it supposed to be the real world?
Is it that somehow we're not measuring the real world
exactly when we do our measurements, but it really is
described by this weird thing called the wave function that
we don't have direct access to. Or is it just
part of the world, And there's other extra variables in
addition to the wave function, hidden variables we sometimes called them.

(05:55):
Or does the wave function have nothing to do with
the real world, That it's just a way of printdicting
the experimental outcomes, and the real world is something much
more definite than that. So all three of these options
are very much on the table. This is what I
call the reality problem, Like what is the real world?
Is that the wave function or something weirder, something else,
I should say, not necessarily weirder. The other problem with
that brief version of quantum mechanics I just gave you

(06:17):
is that it involves the word measure or observe. Right,
No other fundamental theory of physics uses those words at all.
They just assume you can measure whatever you want. But
in quantum mechanics it seems to be the case that
you need separate rules for describing systems when you're not
measuring them and describing them when you are measuring them.

(06:40):
And so this raises what we call the measurement problem.
Which is what's up with that? Which includes like, what
do you mean measure? Does that? You know what? What
does the definition of your measurement is? Doesn't need to
be a conscious creature? Could it be a robot or
a video camera? What if you just measure it badly?
Does the same thing happen? When does it happen? How
quickly does it happen? Why is there even a separate

(07:00):
set of rules. So a whole bunch of questions get
swept under the rug of the measurement problem in quantum mechanics.
And I think both these are big, really big problems.
If we want to think the quantum mechanics is the
right theory of how reality works, we need to know
what reality is, and we need to know why this
measurement process plays such a special role. And you make
a really interesting distinction there. You say, an electron, we
can observe these properties of it, or we can observe

(07:22):
these quantities, but it no longer has these properties that
its velocity's position are not like aspects of the electron.
You've like separated the electron from these things we can
learn about it. Yeah, and actually in doing that, I've
already cheated I've already sort of slipped into my favorite
way of thinking about quantum mechanics because there are people

(07:43):
who would say that the wave function is just a
way to predict the outcomes of what you measure, and
there really is something called the position, something called the velocity.
We just don't know how to predict what it's going
to be until we measure them. Whereas someone like myself
is a way function realist, my point of view is, look,
every version of quantum mechanics uses something like the wave function, Okay,

(08:07):
either way function or something completely equivalent to it. So
the simplest, most minimal version of quantum mechanics would only
use the wave function, right, Like, not as a route
to get to somewhere else or as part of the story,
but the whole story. Like why not imagine that the
wave function is actually what the world is. And when
that's true in that perspective, it becomes the case that

(08:29):
things like positions and velocities are not features of the
wave function, there possible experimental outcomes. And that's the biggest
conceptual hurdle here, because we all look at things and
we think they have positions, and we think they have velocities,
and if you're this wave function realist kind of version
of quantum mechanics, no longer is that true. So let's
move from positions velocities to something that's more binary, because

(08:51):
I think it's easier to think about Let's talk about
the electron and it's spin like, maybe it's spin up
or maybe it's spin down. So then what's the problem
with the orthodox the Copenhagen approach to quantum mechanics where
you say, I have a way function that describes the
probabilities of this electron being spin up or spin down,
and then when I make a measurement, when I poke
it with my finger, the universe rolls the diet and says, okay,

(09:13):
well you had a sixty chance of being spin up,
so you've got to spin up or nope, you've got
to spin down this time. What's the problem with taking
that approach? Well, you already said when I measure the
spin or when I poke it, I want something more
definite than that. If what I'm talking about here is
a really fundamental theory of physics, I should not be
able to rely on weasel words about poking and measuring,

(09:35):
or at least I should give a super duper rigorous
definition of what exactly that is and the originators of
quantum mechanics and it's conventional textbook form. The Copenhagen interpretation
resolutely refused to do this. The strategy they adopted was
to say that observers like you and me just aren't

(09:56):
subject to the rules of quantum mechanics. We are classical.
We are as if quantum mechanics never happened. Okay, you
and I, and this is perfectly compatible with our everyday
experience of the world. But then they say, but individual
particles or atoms obey the rules of quantum mechanics. And
you come along and say, well, but I'm made of atoms.
How could it be with my atoms obey quantum mechanics

(10:17):
and I obey classical mechanics. And so the real problem
with this sort of conventional textbook version of quantum mechanics
is that it's just not a deaf and physical theory.
Is it's not even something you can compare to other things.
It assumes that we're in a regime where this division
between quantum and classical is good enough to get us by.
And I think that when it comes to fundamental physics,

(10:39):
we need to do better than that. And for example,
if I'm poking something with my finger, you could say, well,
I'm classical, so it should collapse the wave function. But
then you can imagine the very very tip of my
finger is just a quantum particle, and that shouldn't collapse
the wave function. And so at what point is that
wave function collapsing happening? Is it two layers of quantum particles?
Is it ten? Isn't it gets to my neuron? As

(11:00):
you said, there's no good answer to that. Yeah, exactly right.
And you know what if you missed the particle, or
what if you like graves it. You know, all of
these questions you can ask are just you're told you're
not allowed to ask them in the conventional way of
thinking about quantum mechanics. Well, I mean I am a
professional particle physicist. I think I know how to poke
a particle when I want to, but I won't take
on ridge at your example. Um, So then what is

(11:21):
the solution offered by the many world interpretation? How does
that solve this problem? So many worlds came about from
a graduate student, Hugh Everett. So this is always what
you should aspire to do as a graduate student overthrow
the fundamental nature of reality. And interestingly, Everett was working
with John Wheeler, who was a very famous physicist who
was an acolyte of Neil's Bore, the grandfather of the

(11:44):
Copenhagen interpretation, and Wheeler gave him the following thesis problem
quantized gravity. This turns out to be very hard quantizing gravity.
We use the word quantized as a verb to turn
an existing classical theory into a quantum theory. And what
happens with gravity gravity we understand classically pretty well in
a theory called general relativity given to us by Einstein.

(12:07):
And the point is that the reason why it becomes
a problem for quantum mechanics is both technical, like when
you try to quantize gravity, run into infinities and other
things you don't like, But there's also conceptual problems. Everett said, Look,
in the Copenhagen view of quantum mechanics, it's crucial that
I have the quantum system I'm looking at and the

(12:28):
outside observer poking it. But if I'm quantizing the universe,
then I don't have an outside observer. That I have
the whole universe, and I should include all the possible
observers in there. So he started thinking about that. He said,
what happens if we just include the quantum state of
observers as well. I don't know why it took twenty

(12:49):
years for people to guess this, but he has a
very natural place to go. What happens if you let
the observer be part of the way function rather than
treating them differently. So consider that electron that we have
at where you could get either spin up or spin down, right,
and consider the following possibility. Since you're a particle physicist,
we're going to assume that you're pretty good and measuring

(13:10):
the spin of the electron. And what that means is
if the electron was absolutely spin up, we're going to
grant you that you would always measure it to be
spin up. So that means that you, as a physical system,
would evolve into a state where your brain says, I
measured its spin up, and likewise for spin down, if
the electron was a spin down, we're going to grant

(13:30):
you that you would say, yep, I measured that to
be spin down. Now you're saying that I can do
something which is unfamiliar to me, which is I can
be in a quantum state, I can have a superposition
of having measured one thing and the other thing. Well,
I haven't said that yet. I'm about to say that,
but it was so far saying is if the electron
was a percent spin up and you measured its spin,

(13:52):
you would find it to be spin up. You're not
in a superposition of anything, right, And likewise, if it's
a hundred percent spin down, you would be spin down.
Let's assume that. Let's assume we want to be getting
the right answer when the answer is definite and known already. Okay,
I mean that's the least that we can ask. I'm
a reliable measuring device so far, yeah, exactly. But then
if you are a quantum system, that's all you need

(14:15):
to know because quantum mechanics, in the technical jargon, it's linear.
So what that means is if the electron is definitely
spin up, you always get spin up. If it's definitely
spin down, you always get spin down. Then when it's
in a combination, when it's in a superposition of both,
we know what you're going to evolve into the wave
function of you, plus the electron will evolve into part

(14:38):
of it where the electrons spin up and you measured
its spin up, and a part where the electron is
spin down and you measured it'spin down. So this is
taking advantage of the quantum mechanical feature of entanglement that
you don't separately say, well, here's the wave function for you,
here's the way function for the electron, etcetera. There's only
one way function for everything. And in fact, ever it

(14:59):
referred to his own theory, not as many worlds, but
as the theory of the universal wave function. And so
everyone agrees with this. By the way, everyone agrees that
if you treat you as a quantum system and you
measure that spin, you evolve into an entangled superposition, part
of which says the electron has spin up and that's
what you saw likewise for spin down. Everett's only move

(15:21):
is to say, and that's okay, there's nothing wrong with that.
So the immediate visceral responses that can't be right, because
I've measured electrons before and I've never felt like I
was in a superposition. And Everett says, that's fine, because
you've misidentified yourself in the wave function. You think that

(15:41):
you're this combination of having measured spin up and having
measured spin down. But that's not right because you're entangled
with the electron. There's two parts of the wave function,
one of which is very consistent. The electron has spin
up and you measured to be spin up. The other
part is also very consistent, the electron has been down
and you measure to be spin down. And Everett points
out that in the future evolution of the wave function,

(16:03):
these two parts of the wave function will never interfere
or interact with each other. Ever, again, they have no
influence on what each other are doing. If I poke,
as you say, if I change or alter what's going
on in part of the wave function where the spin
was up, let's say, the part where spin is down
doesn't know it is not influenced by that. So it

(16:23):
is as if these two parts of the wave function
are now describing separate worlds. And the crucial thing to
keep in mind, whether or not do you like many worlds,
is that ever didn't put in a bunch of worlds.
All he said was that we're going to take way
functions seriously and include observers in them. The world's come
along for free. Once you believe that the electron can

(16:43):
be in a superposition of spin up and spin down.
You've got to be able to believe that observers can
be in the superposition of I measured spin up and
I measured spin down. I see. So he avoids this
distinction between quantum observers which don't collapse the wave function,
and classical observers, which do collapse it by saying everything
is quantum, nothing ever collapses the way function, that the

(17:04):
wave function is the universe that just keeps going, but
you experience one outcome rather than the other because you
are no longer every part of the way function. You
are part of the way function that experienced spin up,
or you are part of the way function that experience
spin down. Is that a fair summary? That is precisely right. Actually,
that's an excellent summary. So I think that if we're

(17:24):
being fair, we should all agree on the pros and cons.
At this point of view. The pro is it's just
quantum mechanics already taken at face value. There's a wave
function for everything. Like you said, everything is quantum, and
it obeys one single equation, the Schrodinger equation or some
equivalent version thereof. You don't need new variables, you don't

(17:47):
need new dynamical laws. You don't need any extra stuff.
So at the level of writing down the theory, it's
the simplest possible version of quantum mechanics, but of course
the cons are at the level of coming to terms
with it, it's the biggest possible imaginative leap, right because
we're saying that every single time we measure the spin

(18:08):
of an electron, a new world is created. And you know,
you gotta give the skeptics a fair nod to say, yeah, like,
that's a lot to buy, and we proponents of it
will say, but you already bought it when you bought
quantum mechanics, like you Nigally, We're already there. We're just
putting your face in it. Well, what do you think
that moment was like for Everett when he you know,

(18:28):
followed this line of thinking and then had this perhaps
moment of understanding where he realizes, hold on a second,
maybe the universe has all these different layers and he's
much vaster and much more complex than we imagine. What
was that moment? Like, did he ever write about that,
you know, moment of epiphany or or realization or understanding?
As far as I know, he didn't directly write about that, no,

(18:49):
but he wrote a lot. In fact, you know, Wheeler,
who is his advisor, was trying to pretend Wheeler himself
was in a superposition of effort has done something radical
and interesting, and Everett is just going along with the
conventional Copenhagen interpretation. Because Wheeler didn't want to annoy his
own mentor Neil's bore, so he had Everett both visit

(19:10):
Copenhagen literally, and you know bores people visit Princeton where
they were living themselves. And letters went back and forth.
So there is a lot of writing about this. And
the one thing I will say is that you know, again,
as as working physicists, both of us, some physicists are
just lucky. Sometimes right you're in the right place at
the right time, you get either the right experimental data,

(19:31):
you get the right idea, and and good for you,
and you get credit for that, but we do inevitably
separate that out from how good you are, how smart
you are, how brilliant you are. Right like, I mean,
there's brilliant people who just never were in the right
place at the right time, and there's some people got lucky.
And if you didn't know any better, which I didn't
when I first started thinking about this. H Everett would

(19:53):
be the classic example of someone who got lucky, right,
someone who's just in the right place at the right time.
You only at one idea. He left physics to graduate
school and moved on to other things. But you read
what he wrote about this stuff and you realize, oh, nope,
Actually he was brilliant. He completely understood what he was doing.
And this is what I mean by being brilliant, because
very often, like someone will have an idea in theoretical physics,

(20:15):
and someone else, super duper smart, will understand and appreciate
the implications of that idea and spell it all out.
And Ever did both. He really appreciated exactly what he
was saying, and in these letters going back and forth
to the giants of quantum mechanics back in Europe, Everett
more than held his own and in fact, he kind
of ran rings around him. So I don't know how

(20:38):
he felt when he first came up with the idea,
but I do give him credit for really thinking through
the implications of that idea. Wonderful. Well, I want to
talk more about the implications of many worlds and what
it means. But first, let's take a quick brick. All right,

(21:04):
we're back, and we're talking about the mind blowing idea
that maybe the universe is more than just what we see,
that there are many universes out there, part of this
quantum multiverse, where the universe is constantly splitting based on
the various possibilities of what could happen every time quantum
particles interact. And I think that the question that probably
most listeners have, it's probably a question you hear a lot,

(21:26):
is are these other worlds real? In what sense? Are
they real? Like? Is this a calculational tool to help
us understand our experience or are those worlds like in
some sense really out there? Yeah? I think they're real.
You know, this gets into deep philosophical questions about what
you mean by real right away. But you know, here's
how I think about it. If we have our best

(21:48):
explanation for what we observe, and that explanation takes the
form of some physical theory, and that physical theory predicts
the existence of stuff we don't observe, then we take
that stuff seriously until will we have a better physical theory? Right? So,
I mean, we often discover new things in the universe
by doing this, whether it's you know, new planets or
dark matter or whatever. And so if you take Everett's

(22:12):
version of quantum mechanics seriously, he solves the both the
reality problem and the measurement problem. The answer to the
reality problem is the wave function directly describes reality. The
answer to the measurement problem is when a quantum mechanical
system becomes entangled with big macroscopic things, that's what counts
as a measurement. And a prediction of his resolution to
these two problems is that these other worlds are real.

(22:34):
So if you want to get the benefit of his solutions,
but you find the other world's distasteful, that's okay. But
then you have to come up with a better theory,
and you have to come up with a disappearing world's
theory where you get rid of the other world. And
you can do that. People have done that. I mean,
I'm saying it in a sort of facetious voice, but

(22:54):
it is in fact an ongoing research program to do
exactly that. And the problem is there's a couple of problems.
One is it is inevitably more complicated, right, I mean,
you're adding something to a very clean, crisp formalism to
get rid of parts of it that you find distasteful.
And number two, it's hard to make it work. Everett
is very plug and play, and especially when you go

(23:17):
from a quantum mechanical theory of particles to a theory
of fields and then to a theory of quantum gravity,
which was initially his initial motivation. Every Ready in quantum
mechanics is perfectly happy doing any one of those, whereas
when you try to mess with it, adding more variables
or adding more rules or whatever, you kind of find
you have to mess with it in different ways for
every version of the theory, and who knows what will

(23:40):
end up being. So, both from a philosophical point of view,
and I think from a physicist point of view, the
simplicity and success of the Evert interpretation speaks to not
working so hard to get rid of the other worlds.
So I think that you're responding to sort of implied
criticism in my question, which is about the nature of

(24:00):
what he's real, which is totally reasonable. And you know,
I think that a lot of people argue that if
you can't measure it directly, if you can't interact with it,
then it's not real physically, that it's can only be
real philosophically. And I think that your response is probably
if it's required by your theory, and your theory is
the only one you have that describes what you can observe,

(24:21):
then what's required by your theory but not observed is
still real. Is that fair? Yeah? I think that's exactly right.
And honestly, if all you wanted to do was to say,
I believe everything that EVERT says, but I don't believe
the other worlds are real, you know, knock yourself out.
It's a free country, right, Like, I don't know what
you get from that. I think if you face up

(24:42):
to that perspective, you're going to have to change the physics.
You're gonna have to change the equations to literally get
rid of those other worlds, and you're going to get
in trouble doing that. But if it's just kind of
an attitude like I don't care about the other worlds
because I'm not in them, then that's fine. Whatever. I
do think. You know, one thing just to put on
the table that maybe we'll get back to you later,
is that it's not just philosophy, you know, I really

(25:05):
do think that one of the reasons why we still
struggle to understand quantum gravity, for example, as a field
of theoretical physics, is exactly because we had not struggled
hard enough to understand quantum mechanics. And I think that
rather than sort of putting the heads in the sand
and denying the existence of these other worlds, if you
do take the formalism seriously, it provides new angles of

(25:28):
fruitful approach to longstanding problems in physics. So you know,
it's a reason to not give in to your first
impulse to be worried about all those other worlds. Well,
we were chatting with Carlo Rovelli a few weeks ago,
and he said that every interpretation of quantum mechanics has
a cost. And I think that a lot of people
would see, you know, these infinite other universes is maybe

(25:49):
like a cost of the many worlds interpretation. But to me,
it's exactly what I got into physics to do, is
to blow my mind and shake up my intuition about
the universe. I don't get into physics to have the
universe say, yeah, Daniel, what you thought about the universe?
That's basically it I'm hoping to peel back a layer
of reality and see something shocking, which at first is

(26:10):
difficult to accept because it's different from my intuition, but
that eventually, guided by mathematics, that can help some new
intuition be like, Wow, the universe is different from what
I imagined, and it works in this incredibly beautiful way.
So to me, it's not a cost, it's a it's
a feature, it's it's the goal of digging into quantum mechanics.
But my question is, you just said that we've been
hesitant to dig into the foundations of quantum mechanics. Why

(26:31):
do you think that is? Why do you think that
for such an important problem at the core of modern physics,
that progress has been so slow. Why haven't we taken
this question more seriously? I think it's a large number
of reasons. And this is a very very good question
also that I've talked about other people who talked about.
But it's sort of a more sociology, psychology, history question, right,
So it's a little on shakier ground here, So forgive me.

(26:53):
But you know, part of it is just that we
don't know how to answer it. That if there are
different compete eating versions of quantum mechanics that are well
defined physical theories. So you know, pilot wave theories and
I guess Carlo's relational quantum mechanics, etcetera. There are various
spontaneous collapse models on the market, and these are real
physical theories. The problem with Copenhagen this is not a

(27:16):
real physical theory. There's it just doesn't answer certain questions
about what actually happens. But once you like put on
your big boy pants and actually make a theory, then
you can make predictions with that theory and you can
try to experimentally test them. So I think that for
a long time it was just thought to be not
very fruitful to think about these ideas because we didn't
know how to get any experimental data about them. And

(27:37):
the other aspect is, you know, we had other things
going on. Businesses are very good at, you know, pushing
forward in directions they can make progress on. So we're
talking about the thirties, forties, fifties, right, Like, there were
particles in nuclei to understand, there were bombs do bill,
there were superconductors to construct, and quantum field theories to invent,

(27:58):
and normally anything you know, it goes on and on, right,
So there's plenty of work to do that you could
connect directly to experimental progress. So it was kind of
okay to push the foundations of quantum mechanics into the background.
That's sort of like saying, you know, my debit card
still works, so I don't need to check my balance
because probably everything is fine. That's right. I mean, that

(28:19):
is part of it. But also, you know, it's very
common advice when you have an enormous task in front
of you to first do the parts you can do,
rather than fretting about the parts you can't do. I
think that what has changed recently is number one, technology
has grown to the point where this idea of a
division between the classical world and the quantum world as

(28:39):
part of the fundamental description of reality has just become
increasingly untenable. Right, we can make much larger quantum systems
than we could back in the thirties that are in superpositions,
and we need to deal with the reality of entanglement
and so forth when we build quantum computers and things
like that. And the other is that this, you know,
enormous progress we made on under standing particle physics and

(29:01):
field theory has slowed in the past few decades, and
we're in this weird position where we built these amazingly
successful theories that fit all the data, but we know
they're not the final answer, right because gravity is not included,
because there are these naturalness problems, etcetera, etcetera. So one
strategy is just to stubbornly bull forward using the same

(29:22):
tools we've used before. But another strategy is to take
a step back and say, Okay, maybe we have to
think fundamentally in a different way about these questions to
make progress on them. And thinking about the foundations of
quantum mechanics plays into that strategy. So I guess it's
time to check the balance, huh. And we've got to
figure out what's happening down there in the vision we
keep getting turned down at the A T M. So yeah,

(29:43):
So then digging deep into like what it means this
many world's interpretation, The many world's interpretation says essentially that
none of these universes are special or different, that these
branching aspects of the way function, and it sort of
avoids the collapse question that way. But I can't get
around what you said earlier, which is that You're redefining
what it means to be me, right, because this universe

(30:04):
does feel special to me. I mean, I'm in this one.
It's the only one that I can interact with. Is
that sort of a naive objection to the many worlds
interpretation to say that this one must somehow be different?
Can you swap that away by just saying, well, the
other Daniels also think that they're the only one who
interacts with the universe. Pretty much, Yes, that is exactly
how I will slipe it away. The relevant anecdote here

(30:25):
that Everyboden's love this anecdote, so I will just share it.
It is actually about Ludwig Wittgenstein, the philosopher. So one
day one of his former students, Elizabeth Anscombe, was also
an extremely accomplished philosopher in her own right. She comes
across Wittgenstein, you know, standing in the yard at Cambridge
like looking at the sun and Vichenstein was famously a

(30:46):
little idiosyncratic. So she says, what is going on? And
he says, you know, why is it? The people were
reluctant to believe that the Earth rotated, rather than believing
that the sun moved around the Earth. And Anscomb says, well,
it just looks like the Sun moves around the earth, right,
And Vickenstein says, well, what would it have looked like
if the Earth rotated? So the point being that the

(31:09):
question you should be asking is not start with an
impression I see the Sun moving, and then construct a
theory that fits most closely with that immediate impression. The
strategy should be construct theories and then ask what observers
within those theories would observe, and if it's consistent with
what we observe, then it works. So the point is

(31:30):
that in many worlds, if there's you right here and
then you go and do some experiments at CERN and
you observe some particles, the prediction is there are now many,
many branches in which the specific pattern of particles and
a collision are different in every single branch, and the
version of you has now seen different things. And all
of those versions of you exist, and they've seen different things,

(31:53):
and they all think that they're special because they exist,
and the other ones are kind of dubious, but there's
no pointer that says this is the real, real one. Right.
There are other version of the quantum mechanics that try
to do that to try to say like, this is
the real branch and all the other ones are fake,
but it would still be true weave without that pointer
that says this one is real, that all of the

(32:13):
different versions of you on the different branches would feel
equally real. So the experimental empirical prediction of this theory
is exactly what we observe in the world, and I
think that should be the criterion for saying whether or
not it's an adequate explanation. It is interesting. It does
feel somehow like a bit of sleight of hand, like
you've taken the fuzziness of defining a classical object in

(32:37):
terms of quantum mechanical particles and you sort of transformed
it into like, well, I'm just going to redefine what
you are. You aren't who you thought you were. You're
just an element of this quantum wave function instead of
being like the holistic version of you. It feels to
me like, you know, when you make this step you
have this interpretation of quantum mechanics, you need to say, one,
what the wave function is. It's real, it's the universe,

(32:58):
But also something about like the correspondence between the quantum
state of the universe and your experience of it as
an observer. Yeah, no, that's under percent true. And so again,
if we're honest about the pros and cons, the physics
of ever writing quantum mechanics is as simple as it
can be. But the philosophy requires some new moves. And

(33:20):
I'm a you know, on board with people who say
I just can't accept those moves, like it's too much,
or at least say it this way. You know, if
we think we don't agree on what the correct version
of quantum mechanics is, and each of us has our
credence for saying, well, it's probably this, but unlikely that
it's perfectly fair for one of the ingredients that goes

(33:42):
into your choice of your personal credence to say, this
redefinition of who I am is just so dramatic that
I'm going to be skeptical of it. Maybe it's true,
but I'm gonna be a little dubious until I'm forced
into it. I think that's okay, And so I think this,
I'm completely acknowledged jing that the philosophical leaps that required

(34:03):
by many worlds are substantial, and in other versions they're
just not there. They don't bother me as much like
you know, I think that in physics we very often
come across better understandings of the world, including of ourselves.
Right like we might have thought back in the day
that we were a spirit animating a fleshy machine that
housed us, and now we think otherwise. I think that's okay,

(34:23):
as long as again, as long as the model that
we're building, once we understand it turns out to be
completely compatible with the world we observe, then I'm on board.
But then let's talk about how to use many worlds interpretation,
you know, as a functioning theory of quantum mechanics, because
something that is a bit slippery for me is that
in the Copenhagen interpretation, I know what a probability means.

(34:46):
I'm gonna do an experiment and I'm either going to
get spin up or spin down. I can look at
the way of function. I can say, well projected, you know,
against both of these possible outcomes, and those give me
the probabilities. They just use the Born rule, and whether
or not I'm a quantum or classical observer sort of
separate from that. But in many worlds, everything happens, and
so I can no longer say like, well, the probability
of this happening is six because they both happen in

(35:09):
some universe. How do you define probability if everything that
can happen is going to happen. Yeah, I think this
is the right question to ask. Like I said, there
are objections to many worlds that are not very good,
objections that are just misunderstandings. But there are also puzzles
or problems or things we got to address that arise

(35:30):
only in many worlds that weren't there before. So you know,
I gave Everett credit for solving the reality problem and
the measurement problem, an equal number of new problems arise.
And the reason why I think that's okay is because
I can see the solutions for these problems pretty clearly,
even if we don't have them completely spelled out. One
problem is just and maybe we'll get to this if
you want to, but it's the structure problem, which is

(35:53):
why does the world look so classical to us if
it's really this big quantum wave function. There's a lot
of details in that. And the other one, like you said,
is the probability problem. The benefit of effort is that
the underlying equations are lean and mean and austere, so
there's no room to say, okay, the wave function evolves,
and there's a rule that says the probability forgetting a

(36:13):
measurement is is given by the wave function square. Like,
there's no room to add extra rules like that. So
what you need to do is derive these rules, and
there are different strategies for doing it. And to be clear,
the fact that the probability is given by the wave
functions squared rather than just by the wave function or
by the wave function cube to the logarithm or whatever,

(36:33):
that's not the problem. Of course. It's going to be
given by the wave function squared because the set of
numbers which are given by the wave functions squared are
the unique set of numbers that are all non negative
and they add up to one and they're conserved over time.
That's what you want out of a probability. And it's
just Pythagoras theorem, right. The hypot new squared is the
other two sides squared. That's why you take all of

(36:56):
the different amplitudes in the wave functions squared to add
them up and get one. So that not the tricky part.
The tricky part is, like you said, why are their
probabilities at all? Because it's a completely deterministic theory and
different people have their angles on that. I think that
I've solved it along with my collaborator Chip Saban's, because well,
we borrowed an idea from someone else. I should give
credit to who's leve Widmin. But here's the idea. When

(37:18):
you measure that spin. So there's some amplitude saying this
spin is up. There's some amplitude saying the spin is down.
And you measure, and like you say, with probability one,
there is now a version of you on the branch
where the spin was up, in a version of you
that is on the branch where the spin is down.
But if you be a good physicist and you do
all the details carefully, you can say, well, you know,

(37:41):
EVERT purportedly explains to me when these measurement occurs, it's
just an entanglement process. I can calculate when it happens.
And the answer is, it happens incredibly quickly. The time
scale for the branching to happen is shorter than the
lifetime the Higgs boson for those particle physicists out there
right like less intended mine is on the seconds, and

(38:01):
so your brain doesn't work that fast. You can't actually
know which branch you're on as quickly as the branching happens.
So what that means is, inevitably, when the wave function
does branch, there's a period of time when there are
two copies of you who those the world is not identical,
but those copies of you are identical, okay, And neither

(38:22):
one of those knows what's branches on. So even if
it knows the entire way function of the universe, there's
something about itself that neither one of those copies of
you knows, namely which branch it is on. This is
called self locating uncertainty or indexical uncertainty. And in those cases,
you know there's some fact about the world, but you
don't know it. What do you do as a good

(38:43):
Baysian reason or as as a good modern rational person.
You assigned credences. You assigned non negative numbers that add
up to one, right, that act like probabilities. So it's
a subjective probability. I don't know which branch I'm on,
but I'm going to sign a credence, and you might say, well,
what do I care about the wave function. I'm just
going to assign credences that are fifty fifty, right, because

(39:03):
there's two options, I'm gonna sign equal credence. Turns out
that doesn't work. It's inconsistent because you can then branch
the wave function again. Depending on whether this spin was upper,
spin was down, you have a different number of branches,
and now you have to assign like one third, one third,
one third. So the first guy's probability changes even though
nothing happened in his world. Is that it's kind of
inconsistent over time. Assigning the Born rule probabilities, giving the

(39:27):
probability the credence the subjective probability assigned by the wave
functions squared is the uniquely consistent thing you can do
in this situation. So number one, there are inevitably uncertainties,
and number two, the uniquely rational way to assigned credences
to them is the Born rule. So then the uncertainties
reflect more like our ignorance rather than some fundamental property

(39:51):
of the universe. That's right, and people like me would
go so far as to say, that's always what you
mean by probability. I mean, there's something that happens in
the world but we don't know, so we assign different
credences to it. It's a subjectivist Bayesian version of probability.
I see. So then does Many Worlds interpretation require basian
probability and rule out frequentist probability. It certainly comports way

(40:12):
more comfortably with Bayesian notions of probability. So you don't
need to be as extremist as I am and think
that all probabilities are fundamentally subjective to be an ever
ready in But it doesn't hurt. It helps you sleep
better at night. Let's put it that way. Let's take
a quick break. And so then are these sort of

(40:41):
like philosophical explorations the only way that we can make
progress on these questions of the quantum foundations? I mean,
if we have two interpretations of quantum mechanics, Many worlds
and relational for example, and they both are you know,
actual physical theories unlike Copenhagen, and they both describe everything
that we he as observers, then are we just forced

(41:02):
to make philosophical choices between them? You know, as as individuals.
Are there no experiments we can do to help us
resolve this question. Well, I think there's two answers to that.
One one is that sometimes there are experiments that help
us distinguish between them. You know, Roger penn Rose has
been pushing an idea where there are objective collapses of
the wave function, where the wave function violates the Shorteninger

(41:23):
equation and really does collapse. Other people. There's a famous
theory called the GRW theory Gerardi, Ramini and Webber who
have a similar theory with different equations attached to it.
And these are experimentally testable, and and tests are going on, right,
so we're actually doing them. Theories like hidden variable theories
Bomian mechanics, I think they should be experimentally distinguishable from

(41:46):
ever ready in quantum mechanics. But the proponents of those
theories say they're not. So I'm a little suspicious about that,
But I haven't actually, I can't put a good proposal
on the table for how to experimentally distinguish them. But
I still don't quite believe the standard lure in that case.
For things like Ravelli's relational quantum mechanics, I don't understand
it well enough to say I suspect that it will

(42:07):
turn out to be fundamentally equivalent to one of the
other approaches. Either it's just the way function and it's
Everett with the many worlds, or there's got to be
some hidden variables, or is epistemic. I think it's closest
to what we call an epistemic approach. Episdemic approaches. I
haven't even mentioned those yet. Those are the ones where
they really say, the way function is just not reality. Okay,

(42:28):
In Penrose's approach or g r W or hidden variables,
the way function is part of reality, but it's dynamics
are a little bit different than an Everett, whereas in
a truly epistemic approach, the wave function is just a
calculational tool and reality is something very, very different. And
that's fine, But then what is reality? And I'm pretty sure,

(42:48):
at least my strong belief from the current state of
the answers people give me when I ask them, there's
no good theory of what reality actually is in these models,
and that you know, they're like, wait till water, we'll
figure that out. And in principle, that's okay. You know
that we can't ask that every theory as answer every question.
As soon as it's invented. But it's also perfectly fair

(43:09):
to be skeptical of those theories while those questions still
linger out there. But okay, but the other answer to
your question, sorry that it takes me so long to
get to it is the proof of the pudding is
in the tasting. And if I can make progress on
other puzzles and physics by starting from an everady in
perspective and taking it seriously, whereas my friends who are

(43:32):
being epistemic or hidden variably or whatever don't make that progress,
then by the rules of physics, I win, and vice versa. Right,
if the people who are fundamentally pilot wave theories or
epistemic people make progress that I don't, then they win.
That's perfectly fair. So in this situation where we're not
sure what the right answer is, then by all means,
let people do research in their favorite areas, and you know,

(43:53):
whoever actually discover something interesting, we'll get the credit. I
like this idea of measuring competing theories by how much
progress can make, sort of theoretically or philosophically, to show
that it's like a you know, a functioning, working, fertile
intellectual playground. But you raised this interesting question of you
know what reality is? And I want to come back
to something you mentioned earlier, which is why the universe

(44:13):
if it is quantum, If the universe is a wave
function and there's quantum particles and everything is governed by
the shooting equation, why doesn't feel that way to us?
You know? Why we have this emergent experience which is
so drastically not quantum. Is that something we can ever
grapple with? Or is it just like other emergent phenomenon,
like asking like why is there ice cream at some
points in the universe and not other points? I think

(44:35):
there's actually again two aspects of this. There's sort of
the philosophical aspect and the physical aspect. The philosophical aspect
I don't have much to say about, which is just
in a world like ours. Why is it that the
idea of a self, the idea of an agent, the
idea of a conscious creature is attached to just one

(44:56):
branch of the wave function at a time. I already
mentioned the fact that you know, fundum mentally, the different
branches don't interact with each other. So if you tried
to say, well, I'm going to treat reality to me
as two of the branches, not any of the others.
We know the other's gonna take to these two well,
I think that someone would say, yeah, but you have
two things that have literally no impact on each other.

(45:17):
The analogy I use in my book is, what if
there were a ghost world? What if there's a world
that was sort of, you know, the same shape as
the Earth and the same physical location as the Earth
and space, and there were people on it, and they
talked to each other, but there's zero interaction through any
force of nature or any other kind of influence between
us and the people on ghost world. It just doesn't
make sense to call them part of the same reality, right,

(45:39):
I mean, they're two worlds for a long times and
purposes certainly. So that's the sort of philosophical move I
think that we don't have, at least I'm not aware
of a once and for all definition of what a
world is and how you should divide up reality in
that way. But that's the rough idea. You know, a
set of things that interact with each other is a world.
But the other interesting question is, you know I started

(46:01):
that by saying, in a world like ours, with laws
of physics like ours, but Okay, what are the features
of our laws of physics that allow for each individual
branch of the wave function to be mostly classical? You
were pretty good at predicting the positions of planets in
the sky and eclipses and so forth using Newtonian mechanics

(46:22):
long before quantum mechanics came on the scene, right, So
why is classical mechanics a good limit of quantum mechanics,
especially given that you're saying there's all these other worlds
out there, right, And that's a trickier question, And I
think that we're just beginning to make progress on it.
And the answer has things to do with ideas like
entanglement and decoherence and locality, But fundamentally, you know, that's

(46:46):
still a research level problem. Well, I hope that we
make some progress on it in the future. Now, I
want to take a slight turn and ask you a
little bit more of a personal question. You mentioned earlier
that not only are you, you know, talking out there
in the public about science, which are actually at practicing
physics this And you know, in my experience, most people
take one of two paths. They're either a practicing scientist
or they are a science communicator. You know, I don't

(47:08):
know that for example, Bill Nye or Neil de grass
Tyson is still publishing papers. But you have kept your
feet in both worlds. Has it been a challenge for
you to remain part of the scientific community and maintain
that credibility while also being a public intellectual. Yeah, there's
sort of two aspects to the challenge. One is, it's
a lot of work all these things to to write

(47:30):
papers and to advise grad students while also having a
podcast and writing books and so forth. But you know what,
look to be honest, it's not that much work, and
I can compartmentalize it pretty easily, and it's fun for me,
Like I get to do these things that all are
individually very fun, and my personality is such that I'd
like being able to switch gears and do different things

(47:51):
at different times. I would get frustrated if I did
the same exact kind of thing every day, So this
is a good way to do that. The other aspect
of the challenge is, like you said the word credibility,
like what about how other people think of you? And yeah,
that's absolutely a challenge because of course two things Number one,
public outreach and communication is itself undervalued and or devalued,

(48:14):
depending on who you're talking to, Like, it's considered to
be a waste of good brain CPU cycles that you
could be using doing research, right, and doing research is
what really matters. And number two, like you also imply,
there's this idea that even if you think that outreach
and communication are valuable, it's very difficult to imagine being

(48:35):
productive in both spheres of both doing research and doing
those kinds of things. There are famous counter examples Carl Sagan,
Stephen Hawking, etcetera. Stephen Weinberg right, who recently passed away,
but they're so rare that people almost don't take them seriously.
And especially like there's the feeling that you should first
become a super successful researcher and then you'd be allowed

(48:58):
to do a little bit of writing books and things
like that. I mean, Stephen Hawking invented black hole radiation
long before he wrote A Brief History of Time and
so forth. And Carl Sagan, on the other hand, never
got elected to the National Academy of Sciences because people
thought that he spent too much time doing outreach work.
So that's a challenge. But you know, I'm too old
to worry about that these these days. Um, it has
impacted my career in very tangible, definite ways, but I'm

(49:22):
still having fun doing what I want to do. So
there you go. Well, that's great to hear my experience
a little bit of that. Also, when I talked to
people in my you know, card carrying particle physics world,
and they asked me, oh, are you still doing research
now that you're doing outreach, it's have to remind them
it's possible to do more than the one thing. So
it's interesting. Let me just put it this way because
I like this analogy. Physicists are not so narrow minded

(49:45):
that they won't allow their colleagues to do anything else.
Like if you were a ski jumper or a professional
unicyclist or whatever, or not professional but amateur unicyclist in
your spare time, other physicists would think, oh, that's cookie
and fun. Good for you, and you're probably also still
doing research. But there's something about writing books and giving
talks and making videos and so forth that is different

(50:07):
than writing unicycle or being a ski jumper, because that's
the kind of thing that people think, well, you should
be using that effort doing research, right, Like your unicycling
is just a different kind of thing, so we don't
think that takes away from your research. An enormous number
of professional physicists are rock climbers and mountain climbers, right,
You probably know many yourself, for example. That's fine. But

(50:29):
if instead of going rock climbing, you write a book,
that's actually counts against you and that it's just a
weird thing in my mind, but it's a very definite
syndrome that is strange. And you know this is it's
interesting to explore these career paths, and I wonder is
this sort of the path you envisioned for yourself. Like
if you could go back in time and describe your

(50:50):
life now to fresh faced assistant professor Sean Carroll one
year into your gig at University of Chicago, how do
you think that Sean would re act? Well, the specific
twists and turns of my career were not what I
predicted or wanted, but the ending point is pretty close.
You know, I always wanted to be a broader intellectual

(51:11):
contributor than just a narrow research physicist that you know,
since I was a kid, I wanted that, and that
was always the plan. In some sense, and I was
charmingly naive about how happy academia would be to receive
such a plan. But there was always the plan, And
I do believe that the right way to do that
is to first become an expert at something. Right Like,
you don't start out as an expert in everything you know.

(51:33):
You better get good at your research and your PhD
project and then sort of branch out after that. But
you can't predict all of the ins and outs. But
maybe I would have done things differently if I had
crystal ball and could predict the future. A few tweaks
here and there would have helped. But you know, I
think I can still hold my head high about the
choices that I made along the way. Well, what advice

(51:54):
would you give to young folks now who are excited
about science communication? In my experience, so I followed a
fairly narrow path and got tenure as an experimental particle
physicists before trying to branch out to outreach other types
of things. But I see graduate students in my own
group doing outreach, and I wonder, you know, if I
should advise them. Look, the field is not friendly to

(52:15):
this kind of breath at this age. Wait till you
get tenure, or if I'm telling them not to be
their authentic intellectual selves, and I should encourage them and
the field with it to grow and accept this kind
of activity. What would be your advice or what is
your advice to your students when they try to emulate
the full breadth of your activities. It's enormously good and
important question, and the answer is not obvious. But I

(52:36):
can always fall back on the maxim that I should
tell the truth. So I can tell the truth about
factual statements about the world without necessarily coloring them by
normative statements. So rather than saying don't do that until
you get tenure, I can say, look, it's great that
you like doing a lot of different things outreach and
so forth. I have one recent student of mine who

(52:58):
is actually super duper six US full hot property in
the job market, wrote a musical and performed it, you know,
at cal Tech while he was in graduate school. So
he did okay. But the point is that it's hard
to become a professional academic, right The numbers game is bad.
I'm at cal Tech, and I tell my students that
if you're at a place like cal Tech or Harvard
or whatever, and you get a PhD, maybe one in

(53:21):
four of you will eventually be tenured professors in that field.
I don't know the exact numbers, and it will depend
a lot on sub fields and where you get your
PhD and so forth, but the numbers are against you, certainly.
And the thing you can say with a good amount
of confidence is that all else being fixed, doing outreachy
things will lower the percentage chance that you will someday

(53:44):
become tenured academic. Now you might still decide to do it,
and that's great, but I want you to go in
with your eyes open, right Like I sometimes get in
trouble by being a little bit too candid with my students,
because a lot of my colleagues are like now they're
they're young and impressionable. We have to like juice them
up about physics. And I'm like, well, yeah, but a
lot of them are not going to end up being physicists.

(54:05):
And I think that I love the idea of going
to grad school. I love the idea of getting a PhD.
And I think it's intrinsically worthwhile thing. And I'm not
going to discourage anyone from doing that because it's a
matter of intellectual growth and so forth. I think the
current system where you then have to do somewhere between
five and ten years of post doc afterward is not ideal.
But you know, if you want to get a PhD,

(54:26):
then I think that's the best thing in the world.
And then you should know what the chances are and
what the different aspects are that affect your chances along
the way, and then you make your own decisions. Well,
I agree with that, and I hope that other folks
out there can see that it's possible to be an
academic and to do scientific communication and outreach, and that
encourages the community to accept that into broaden our concept

(54:48):
of what a healthy physicist is. You're allowed to also
be a unicycler or to talk about science to your
friends and neighbors on the internet. All right, we've taken
enough of your time. Thanks very much for joining us
and for telling us about your vision of quantum mechanics.
And it's then do us all the crazy mind blowing
ideas involved in the many world's interpretation. It's been a pleasure.
It's very fortunate that we had to think about these things.
So thanks for having me on. All right, Thanks very much,

(55:18):
thanks for listening, and remember that Daniel and Jorge explained.
The Universe is a production of I Heart Radio or
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