Quantum mechanics is the branch of physics that attempts to explain the
behavior of very, very, very small things. This behavior at the smallest scales
appears bizarre because we don't see it in our everyday experiences. Even down to
the scale of things we can see with ordinary microscopes, classical physics
predicts behavior very accurately. The classical physics of Newton and Einstein
is precise, predictable, and deterministic. We're comforted by this. We know
precisely where a basketball is going to go when we throw it up for a shot.
Quantum mechanics, on the other hand, appears to be based on probabilities of
finding an object here or there, we don't know, and can't know, where an electron is
in an atom until we look at it, until we measure it - at least according to one
interpretation. Yes, quantum mechanics is so bizarre, that there are different
interpretations of what might actually be happening. In fact, there is no
majority consensus on any one of the interpretations. There is a particular
interpretation, however, put forth by graduate student Hugh Everett that was
considered preposterous in 1957, when he first proposed it, but it has now become
mainstream, and is gaining huge support. But it requires us to change our
paradigm about our experiences of reality, and consider that there may be
many, many universes, or worlds where every possible quantum outcome happens,
and that we are living in just one of those branches of the universe at any
one moment. One of the biggest proponents of this so-called "Many Worlds
Interpretation" is professor Sean Carroll of Caltech. I spoke with him via
Skype recently, where he answers some of the biggest questions I had as a skeptic.
What is this crazy interpretation? Could we really be living in a universe of
many worlds? What is the mechanismof the branching? How does that occur? The
explanation and answers to questions from Sean Carroll are coming up.
Before we look at the
pros and cons of the many-worlds interpretation, let's take a look at the
most popular and what is considered by many to be the standard interpretation
of quantum mechanics. It's called the Copenhagen Interpretation, championed by
Niels Bohr in the 1920s. In simple terms, the interpretation suggests that
objective reality doesn't exist, until it is observed, or measured. the quantum
world is governed by a set of probabilities and is described by a wave
function that evolves over time in the Schrodinger equation. The act of
measuring forces the set of probabilities to randomly assume one
possible value. So according to this, by observing the quantum world, we force it
to choose one of several possible paths. Prior to this observation, or measurement,
the microscopic world exists in a state of all possible paths at once. An
electron does not spin up OR spin down. It is in a state of spin up AND spin
down, at the same time. When we measure it, it takes one of these states. The
Many-Worlds interpretation, on the other hand, postulated by Hugh Everett, says
that the entire universe is in a state of superposition. A measurement may look
like a particle has some set of properties, but that is not the overall
reality. It posits that the world splits every time we think a quantum
measurement is made. And that although we may see one thing in our world, there's
another world in which another thing has happened. So for example, if we're trying
to measure the spin state of an electron, the many-worlds interpretation says that
the measuring device becomes entangled with the electron. This causes something
called decoherence. The decoherence splits the universe in two, such that in
one universe, the device measures spin up, and in the second universe, the device
measures spin down. Both universes exist. We just happen to experience one of
these universes. We could just as well be in the other universe. And similarly, this
kind of decoherence is happening all the time. And in every instance this happens,
the universe splits. So this is why it's called many worlds, because many such
branches, or splits of the world exist simultaneously. So this raises a lot of
questions, but let's look at why this is getting popular with physicists, when
compared to the standard Copenhagen interpretation. In the Copenhagen
interpretation, there are two sets of rules. One set of rules applies to the
system prior to measurement according to a wave equation, which is essentially a
set of probabilities for the state of a quantum system, at any one time. And a
different set of rules applies to systems after measurement has occurred.
The many-worlds interpretation says, no, this is not the way the universe works,
and that there are only one set of rules for everything, and that rule is that all
systems abide by and evolve over time, according to the Schrodinger equation.
And there is no (randomness) because all possible outcomes are a branch of
the many-worlds. Now you might not believe that branching of the universe
can occur, but I will show you by splitting the universe into two branches
right now. There's an app called universe splitter that I recently bought. I'm
going to put in one of two potential choices, and ask the app to pick one. Now
when I do this, it will send a photon to a partially silvered mirror in a lab in
Switzerland. The photon will have a 50% chance of
taking one of two paths, and reports which path it took. Now I'm going to ask
this app to help me decide between eating this lollipop, or this Apple. Okay,
so I'm going to give it a try now!
Okay, so in the universe where I made this video, I am going to eat this Apple.
So that's the video that you are watching. But in another universe I am
eating a lollipop in my video. That universe is just as real as this one. You
may or may not be watching me in that universe, but I'm eating a lollipop there.
So in many worlds, the universe is not random, like it is in
the Copenhagen interpretation. It is deterministic according to the
Schrodinger equation. The randomness is just our perception because we only
experience one of two branches. Now both interpretations are weird but there's
some good points about the many-worlds idea. First it gets rid of the
measurement problem. We use the same equations whether we measure something
or we don't measure something. There's no special role for the observer. Second, it
lets us apply quantum mechanics to the entire universe. This is quite elegant.
And three, there is no chance. There's a 100% chance that the world will be in a
state of superposition of me eating the lollipop or eating the Apple, from an
overall perspective. There is no uncertainty. Both outcomes have occurred.
But this also raises some serious questions. First what is the mechanism of
branching? How does that occur? Second, how is energy conserved? And third, the puzzle
is why do we see only one world, if the other worlds are equally present? Why
can't we experience them? Where the heck are these worlds located? I posed these
questions to the foremost expert on the many-worlds interpretation, Sean Carroll
theoretical physicist at Caltech. His latest book is called "Something Deeply
Hidden - Quantum Qorlds and the Emergence of space-time." The link to his book is in
the description below. I highly recommend it. So first I asked about why the idea
of branching is better than the idea of wave collapse. Take a listen.
Tt seems to me you're replacing the mystery of the collapse of the
wave function, which is mysterious in Copenhagen interpretation,
with the mystery of the branching in the many-worlds interpretation. So, can you
talk about how the branching is less mysterious than the collapse would be?
The problem with Copenhagen is that it is manifestly an incomplete theory. Nobody
can possibly think it's the final answer. You say when you observe the quantum
system its wave function collapses. And if you ask, well when do you observe? What
counts as an observation? What about one atom bumping into it - does that count as
an observation? And also what is the underlying reality that is happening?
What is the real true mechanism that is going on beneath the surface? Copenhagen
just says, we don't know. That's not our job. We're not going to answer those
questions. Okay we know it when we see it - wave function is collapsing.
Whereas many-worlds is a hundred percent, completely specified. There's an equation,
the Schrodinger equation, and that's it. There's nothing that deviates from the
Schrodinger equation at any time. So there's no mystery about when branching
happens in the Evertt interpretation. You have the Schrodinger equation describing
the behavior of physical systems. And those physical systems interact with
each other. And they branch, which is to say, there is an environment.
There is a set of things out there in the world that we don't keep track of -
all the photons, all the molecules of air in your room. You might know some course
features of them, but you don't know the position and velocity of every molecule
or every photon, right? So you don't keep track of that. And therefore, when a quantum
system becomes entangled with that environment, that's when branching
happens. And we can quantify it. We can tell you how quickly it happens. We can
tell you when it happens. There's no loose ends there anymore. So according to Sean
Carroll, in many worlds, the Schrodinger equation always applies. It's the
entanglement of two systems that causes a decoherence, leading to the branching.
So the question is, what the heck is decoherence? It's not quite like wave
collapse. It can be thought of as a loss of information to the environment. When
an isolated quantum system, like say an electron, gets entangled with its
environment, which contains things like photons and other molecules that may be
present, this has the effect of a transfer of quantum
information. All the photons and atoms that bounce off the electron are agents
of decoherence, and can fix the electron's position in space, and give it a sharp
outline. It's as if the environment is always observing. This is decoherence, and
it's the cause of the branching. And this is how quantum systems can start
behaving like classical systems. So for the second item, here's what Sean Carroll
had to say about the important question of energy conservation. Energy - energy
conservation - it feels like, you know, where could all the energy come from for
these new worlds? Now in one of your interviews, I saw that you mentioned that
we can't really think of it in terms of new energy being created, but rather the
energy of the Schrodinger equation being fractionalized into new worlds as
they're created. I'll quote you. You said "each world gets a little bit thinner,
although its inhabitants can't tell." So what does it mean when you say that a
world gets thinner? so there's two things going on if you get this. And this is
again, you know, this is not like a question you ask because you don't get
it. This is an important question that we need to sort of think about what it
means. It's a legitimately difficult question that is new to the many-worlds
way of looking at things. In many worlds, there's two different things you might
mean by the energy of the universe. One is the energy of all the branches
of the wave function, okay, like all the worlds at once. That has an energy.
The other thing is, what the energy of each individual branch looks like from
inside, okay. To the perspective of the people inside each branch, what is the
energy? And these are two things. They're different things, but they're related to
each other. The relationship is not that you take the branches - the energy of
each branch from the interior point of view, and add them. That's not what
it is. Sorry, that's just not what the math says. What the math says is - take
the energy from inside each world, that the observers in that world would say,
weight it by the wave function squared, right. Every branch of the wave function comes
with an amplitude, a number. This is where the probability comes from, right - that, that
amplitude squared is the probability that if you find yourself on that branch, the
energy equation works exactly the same way, as the probability - probability of
being on one of the branches somewhere is always going to add up to one. The
energy of the universe that you get by adding the energies of the
individual universes, weighted by their amplitude squared, will always add up to
the energy of the total universe. So the total universe, with all the branches, has
an energy which is 100% conserved. The energy, within any one
branch, almost looks like it's conserved. It's not quite exactly
conserved because there's quantum fluctuations. But it's pretty close to
being conserved. So when you branch the universe in two, the energy that
you perceive in your universe, doesn't decrease by half. It stays is almost
exactly the same. So essentially what Sean carroll is
saying is that the total energy of the universe, and all its branches, is
conserved, analogous to the way that all the individual probabilities inherent in
the Schrodinger equation, add up to one. And finally, something that bugs me a lot
is if there are many worlds, why do we experience just this one world. Here's
what Sean Carroll had to say about that. Why is it that I experienc just this
branch? I'm conscious at this branch. I am not conscious of the other branches.
Look, it's almost exactly like identical twins. At one point, we're the same
single-cell fertilized egg, okay. But rather than just duplicating itself, and
becoming one person, it split into two different people.
Nobody says those identical twins are two copies of the same person. That would
be a weird thing to say. They're different people, right. When the
branching happens, there are now two different people. They had the same past.
But they're different people now. So I think Sean Carroll did a great job of
demystifying the common questions around the many-worlds interpretation, and he
also answered several other questions. If you want to see the interview in its
entirety, you can view it on my Patreon, or youtube membership channel. Now I came
in as a skeptic. And I remain a skeptic, although less so now than before.
I think the math is there to support everything that Sean Carroll says. I don't
think there's any disputing that. But should the universe be interpreted based
strictly on mathematics, regardless of its intuitive aesthetics. Perhaps it
should. Or is there deeper truth that would be more satisfying? No test can
tell us the difference between the outcome of the two interpretations -
Copenhagen versus Many Worlds, because they both give you the same outcome, and
both invoke the Schrodinger equation. Both interpretations seem bizarre. It's a
matter of choosing the one that seems less bizarre to you - a non-deterministic
universe, with probabilistic realities -- or, a deterministic universe, but with near
infinite realities that you can't see, except the one that you're in. The wise
advice, according to some scientists is - "look it doesn't matter what you think or
subscribe to, because the equations give you the correct answer. so stop thinking
and just shut up and calculate." I asked Sean Carroll about that, and he like me,
hates that idea. I myself think that physics is more than calculations. It is
a science that tries to get at the truth about what the true nature of reality
actually is. That's what we should really be after, I feel. And that pursuit, at
least on this channel, will always exist. I'd like to thank my generous members on
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next video my friend!