It is a pleasure to welcome back to the program.
Sean Carroll, who is a theoretical physicist,
cal tech host of the podcast mindscape and also author of the forthcoming book, something
deeply hidden, which is out in September.
Um, maybe we start there, Sean.
I mean, what is it to be with this idea of something being hidden is very interesting
when we talk about your work.
What is it that you allude to there as being hidden from us?
Well, it's a quote from Einstein actually, and the book is about quantum mechanics and
the fact that for the whole 20th century physicists kind of punted the attempt to truly understand
what was going on in quantum mechanics.
So something deeply hidden refers to Einstein's, which I share that there's something going
on behind the operation of the natural world.
When we talk about quantum mechanics, we've often not really pressed our best theoretical
theories on what is happening.
When we make a quantum measurement.
I think it's time to actually face this problem head on.
Why would it be that we don't press as much as we might in that area?
Well, it's hard, you know.
Um, the quantum mechanics is mysterious mostly because it seems to imply that what truly
exists, the true nature of physical reality is different from what we see when we look
at it.
Right?
There's a difference between measurement outcomes and physical reality, and that's something
that no other physical theory has ever featured.
So way back in the 1920s and thirties, physicists settled on a coping mechanism, which they
called the Copenhagen interpretation, which asked us to sort of not ask too carefully
what happens when you make a quantum measurement.
And I think that we can do better than that by now.
How do you explain to lay people not only in the simplest possible terms, what, what
quantum mechanics is, but also why it even matters?
Because I think sometimes there's a contingent that when faced with something as complicated
as quantum mechanics, it can be easy to say, if I don't even understand why that matters
to me or why that matters to human life, I'm not even gonna think about it too much.
Like why?
Why is this stuff important?
Yeah.
Quantum mechanics is mysterious because we're used to like seeing particles, right?
We say that there's the particle, it has a position, it has a velocity, and quantum mechanics
says that's not what is really happening.
Particles have a wave function and you can observe their position, you can observe their
velocity, but that's not what is really going on.
And nevertheless, quantum mechanics is at the center of all of modern physics.
It is the most important theory we have and it is incredibly important to understand things
like Adam's electricity, transistors, lasers, all of modern technology is based on quantum
mechanics in one way or another.
To me, the most vivid fact about quantum mechanics is that without it matter would be completely
unstable.
Electrons would not stay orbiting the nuclei of atoms.
The chair you're sitting on would not be solid.
It would all just collapse into a black hole and a tiny fraction of a second.
So quantum mechanics is kind of really important to how we understand the universe right now.
So to play pseudo intellectual devil's advocate, is it important to understand those things
for the lay person?
As long as they work,
you know, you don't have to understand anything really to get through life except you know
how to eat and how to sleep and so forth.
But I think that what makes us human is something very different, right?
I think that human beings like to do more than simply survive.
Of course, you can get through your daily life as long as your daily life is not that
of being a physicist without knowing that much about quantum mechanics.
But it is best theory we have of the fundamental nature of reality and it is understandable.
So if you give into the human impulse to be curious, to understand the world around us,
then understanding quantum mechanics should be very high on your list of priorities.
Yeah.
Very uh, very well said.
Can you talk a little bit about some of the ways in which a better understanding of quantum
mechanics could be a game changer over the next, I mean, whatever timeline you think
is relevant.
I mean, whether it's 20 years or 50 years or a hundred years, in what ways will a better
understanding of quantum mechanics change the way in which humans interact with the
world around us in our environment?
Well, I think that there's two big ways it's, you know, it's very interesting to imagine
that we've been using quantum mechanics for 90 years or so and it's our best theory of
physics and yet we don't understand it.
There are other things we also don't understand.
For example, we don't understand how gravity fits into the quantum mechanical paradigm
and no one seems to have, I had the idea that maybe the fact that we don't understand quantum
gravity is related to the fact that we don't understand quantum mechanics itself.
So I think that understanding quantum mechanics itself will be really, really helpful in our
pursuit of a theory of everything once and for all final theory of the way the world
works at a much more down to earth level.
Technology is improving to the point where quantum mechanical effects are becoming manifest
in the devices that we build.
We are building not only quantum computers, but quantum cryptography, quantum eavesdropping,
a whole bunch of applications of quantum mechanics, and so we're going to be needing to know what
the rules are of the quantum world.
We're going to need to do better than the kind of slapdash recipe we put together the
1920
can you talk a little bit about what the many worlds theory is?
Well, when you make a measurement of a quantum system, the system can be in what is called
a superposition.
It's neither here nor there.
It's in the superposition of every possible place it could be, but when we look at it,
we never see a superposition.
We see every single possible measurement outcome, but only one at a time.
And so what many worlds says is what you're missing.
The trick is to realize that you yourself are a quantum mechanical system.
That means that you can be in super positions, so when you measure a particle and it could
have many different possible locations, it's not according to many worlds that once you
observe it, it collapses to a single location.
Rather the particle is still in many possible locations, but you are in many possible measurement
outcomes.
There's a world where the particle was there and you saw it there.
There's another world where the particle is over here and you saw it over here and this
is a straightforward prediction of the equations of quantum mechanics.
It's sort of been lurking in implicitly in the formalism since the Schrodinger equation
came along in the 1920s and we just need to face up that reality.
Now is this for people who are hearing and saying that, is this a multi-verse sort of
theory, the idea that I'm married to person a in the world in which you know, you and
I are currently speaking, but there's another world in which I'm not married.
There's a third world in which a, I'm a tiger.
I mean, you know, however far you want to take it and and sort of cartoonish, is it
concurrent with that idea or is it actually something a little different?
It is a version of that idea.
Absolutely, but it's not just that there is every possible universe.
There are certain universes.
The ones that arise because of how the laws of physics play out and there are not other
universes, but there's a huge challenge that is raised by many worlds because there's the
notion of personal identity that we have.
I mean, in the sentence you just said, you said, is there a world in which I am a tiger?
Okay, so no, there's no world in which you're a tiger.
There might be another world in which other things happened that created a tiger out of
the same descendants that you had, right?
Or there's another world in which there's a version very similar to you in which you
had something different for dinner last night or married a different person or something
like that.
But they're not you.
These people in the other worlds are all separate.
Every person in a world as an individual and all the individuals are different people.
So I think sometimes when people think about the multiverse theories, they share an identity
with that quote, other version of themselves that is in some of these other universes.
That seems, as I hear you saying, it may be a slightly overly simplistic way of thinking
about it.
Yeah.
I think that, you know, the way the philosophers would say it is we need to update our metaphysics
in order to take into account how many worlds works.
If I'm going to measure the spin of an electron and I picked that because there's only two
possible outcomes, it's spinning counterclockwise or it's spinning clockwise, right, so then
there will be two people afterward.
One person saw it's been clockwise, the other solid spin counterclockwise, and it's very,
very tempting for us to say, before we do the measurement, I wonder which one I'm going
to be, but the correct answer is there's no answer to that question.
There is no one that you're going to be.
You are now one person.
You will have two descendants a few moments from now.
They will be separate people.
They'll be related to you in the same way that an ordinary, older version of yourself
is really to the younger version of yourself.
It's just that there are multiple copies of them.
It's a different but completely sensible way of thinking about reality.
So along these lines, I want to talk about entropy a little bit.
Um, one analogy that I have found useful in talking about entropy is to explain, and I'm
curious to your feedback on this as a science educator, the idea that if you build a house
and leave it there and leave it subject to weather and everything else that is going
on around it, there could be a hurricane that will knock some pieces off of it and it might
snow a lot and that would damage of the roof, which would fall off and eventually the house
would basically be rubble.
You are not going to then have those same or other forces happen to get the house back
into the built condition in which it started.
Is that a useful analogy to think about?
Entropy?
Yeah, no, that's an, that's not just an analogy.
That's an example of entropy and action.
There you go.
And so when we think about that as a concept more broadly, not applied to a house but applied
to our universe, what are the sort of important aspects of, of how and and why it matters?
Well that nothing matters more to reality, into life and to our existence here on earth.
Then the fact that the entropy, the disorderliness of the universe was very, very low near the
big bang 14 billion years ago and has been rising ever since.
The fact that we're all born young and we grow older with time we age, that's ultimately
because entropy is increasing.
The fact that you remember the past and you don't remember the future.
That's ultimately because entropy is increasing.
The fact that you can make a choice right now, that will effect what happens in the
future, but you can not make a choice right now that affected what did happen in the past.
All is because entropy is [inaudible].
Okay, so that may seem significant sort of to the individual life, but why is that so
significant when we think about the past and future of the universe?
Well, we think we're not absolutely sure about this, but we think that the fundamental laws
of physics don't actually distinguish between the past and future.
If you look at Newton's laws that we sometimes learn in our physics courses, or if you look
at the Schrodinger equation of quantum mechanics or Einstein's equation of general relativity,
none of them have a preference for one direction of time over the other.
They treat them equally, but the real world seems to have a very strong preference.
And we think that the origin of that strong preference in the real world is because entropy
was low.
So if it wasn't for that difference, if it wasn't for the fact that entropy used to be
low, we would treat the past and future on completely equal grounds.
We would not remember the past any more than we currently remember the future.
Absolutely.
A fascinating Sean.
What, uh, who should read the forthcoming book, something deeply hidden and what in
addition to some of the things we've touched on, uh, will be, it will be part of that book.
Yeah.
It's for everyone.
You know, it does not assume that you know any physics, it does assume that you're willing
to stretch your mind a little bit.
Right?
These ideas are, you know, it's not heavily mathematical, there's no equations in the
book, but they're very, very different from our everyday lives.
So it asks, not expertise of the reader but patients and imagination and a willingness
to sort of sit back and really, really think about the fundamental nature of reality.
And I'll talk about quantum mechanics about many worlds.
And then at the end of the book I talk about quantum gravity and how space time itself
might from a quantum
mechanical wave function of the universe.
The book is out in September.
It is by Sean Carroll.
It is called something deeply
hidden.
Uh, Sean, so great to talk to you again.
Appreciate you making time to talk to me.
Thank you very much David