The Many Worlds of the Quantum Multiverse | Space Time | PBS Digital Studios


The weird rules of
the subatomic world are very, very
different to those of the familiar
large-scale universe. A huge outstanding
question is when and why does the weirdness
of quantum mechanics give way to classical physics. One answer to this
question suggests that the entire universe is so
much weirder than we imagined, or should I say the multiverse. [MUSIC PLAYING] One of the strangest
features of the quantum description of reality is
the idea of superposition. We can’t describe the
most fundamental building blocks of our universe with
defined singular properties. Instead, they seem to
behave as probability clouds of all properties
they might have were we to try to measure them. Mathematically,
this is encapsulated in the wave function
of a quantum particle or system of particles. The best illustration
of why we need to describe the
quantum world this way is the famous
double-slit experiment. We did an episode on it. Check it out if you
aren’t familiar. But to summarize, a stream of
photons or electrons, or even molecules, travels
from some point to a detector screen
via pair of slits. These particles arrive
at the screen distributed like the interference
pattern you would expect from a simple wave. Quantum mechanics
very successfully predicts this result by
describing each particle’s journey as a superposition
of all possible trajectories. In other words, the
particle simultaneously takes all possible
paths, which means it passes through both slits. It tries out all histories
between launch and landing. And those many maybe
histories somehow interact with each
other to determine the most likely
final destination when a measurement is made. In a sense, different
possible superposed histories appear to converge
on one final outcome. But what causes
that convergence? In the original
Copenhagen interpretation of quantum mechanics,
the act of measurement was thought to collapse
possibility space into a single reality,
at least with respect to the measured property. It collapses the wave function. That collapse signifies the
transition between the quantum and classical realms. One of the founders of quantum
mechanics, Erwin Schrodinger, found this ridiculous. And he proposed his famous
Schrodinger’s cat thought experiment to highlight
the absurdity. It goes like this– a cat is in
a box with a flask of poison. A machine containing
a radioactive element is set to shatter the
flask in the event that the radioactive
element decays. If that happens, the cat dies. That radioactive decay is
a purely quantum process. And so until it’s
observed, it exists in a superposition of states. It has both decayed
and not decayed. But doesn’t that mean that
the entire macroscopic system attached to that quantum event
is also in superposition? If so, then the cat should be
simultaneously alive and dead until we open the box. But why can’t the cat collapse
its own wave function? And from its point of view,
is the physicist outside also a quantum blur until
the box is opened? And what about the entire
rest of the universe that’s not currently being observed
by physicists or cats? Many adherents to Copenhagen now
have a more sensible resolution to the paradox of
Schrodinger’s cat. It’s that quantum
superposition doesn’t extend to macroscopic scales. It disappears when different
quantum scale histories diverge. This is called decoherence. When the wave functions
describing quantum systems overlap sufficiently–
in other words, they are coherent– it’s
possible to get interference in the double-slit experiment
and spookily correlated quantum entanglement measurements. But when these systems interact
with their environment, coherence is lost and
parallel histories fall out of alignment. They can no longer
interact with each other. By the Copenhagen
interpretation, we might say that the universe
chooses the final outcome of all those histories. It doesn’t exactly
choose a single history. Instead, it chooses an end
result– say, particle location on a screen or cat
alive or deadness– based on those histories. If a larger number
of possible histories lead to a given
result, then it’s more likely that the universe
will select that outcome. The Copenhagen
interpretation says that this selection happens
in a fundamentally random way. The universe plays dice,
even if the dice are weighted towards certain results. It is what we would call a
nondeterministic interpretation because there’s no
underlying predictability behind the selection. However, there is
another way to interpret the transition between the
quantum and classical worlds. What if the wave
function never collapses? If we can imagine a cat in
a superposition of states, alive and dead, why
stop at the cat? What if the family
of possible states extends beyond the radioactive
decay, beyond the cat, and includes the observer and,
indeed, the entire universe, too. If we open the box and
find that the cat is alive, it’s because we’re part
of an entire quantum timeline in which
the radioactive decay and subsequent poisoning
never happened. But there’s an equally valid
timeline in which it did, and another version of
us experiencing that. This sounds outrageous, but it’s
a very serious interpretation of the mathematics
of quantum mechanics. It was proposed by Hugh
Everett in his 1957 PhD thesis entitled “The Theory of
the Universal Wave Function.” It’s come to be known as the
many worlds interpretation. To outline the idea without
killing so many cats, let’s talk about what
this means in the context of the double-slit experiment. The Copenhagen
interpretation tells us that the superposition
of particle trajectories, of histories, merges
into the single timeline of the observer’s reality. Many worlds says this
merging never happens. Those alternative
histories continue, and we find ourselves in
just one of those timelines. Which one? Well, they’re all
equally likely. But some look very
similar to each other. For example, many
histories lead to photons landing on the bright bands
of the interference pattern, and very few to the dark bands. We tend to find ourselves
in the more common families of histories. This is a pretty crazy notion. The many worlds interpretation
invites the idea that reality splits into
different branches every time quantum states diverge into
different possibilities– for example, at every particle
interaction everywhere in the universe. This would lead to an
unthinkably large number of alternate timelines
or worlds that contain all possible
realizations of this universe since the Big Bang. It seems extravagant to propose
uncountable eternally-branching universes just to get out of
collapsing a wave function. It’s like building
an entirely new house to avoid doing the dishes. But remember, the Copenhagen
interpretation itself proposes multiple worlds
in the superposition of paths or properties
of a quantum system. Both many worlds and Copenhagen
create alternate realities. It’s just that
Copenhagen merges them into a single timeline with
its wave function collapsed. The superposition of
states of many worlds can be thought of as
overlayed histories, slices of a universal
wave function that diverge from each other
as the universe evolves, but none ever vanish. Many worlds may, in fact, be
the more pure interpretation of the mathematics
of quantum mechanics because there’s nothing
in that math that requires the collapse
of the wave function. So many worlds is more
economical in the number of unsupported concepts it
adds to quantum mechanics, even if it isn’t particularly
economical in the number of universes it predicts. Now, Everett’s idea
wasn’t taken too seriously when it was first proposed. That may have been
in part because he wasn’t a well-known physicist. He was just a graduate student
who all but disappeared into military research
at the Pentagon right after graduation. But another point
of resistance must be the overwhelming
existential crisis induced by the idea of
near-infinite versions of one’s self. Many worlds may imply that
every possible version of you exists out there. You’re just the one who
happens to be experiencing this branch of reality. Every other possible
life path, including those branching in
different directions from every decision you ever
made, may be just as real. In fact, each may be
real in vast multitudes. There’s no more
evidence for many worlds than there is for other
mainstream interpretations of quantum mechanics. And it is somewhat
mainstream these days, with many noted physicists
being swayed by its parsimony, its economy of ideas. But it remains an
interpretation. And so although it is supported
by the incredibly successful mathematics of
quantum mechanics, it has not yet
added a prediction that might distinguish it
from other equally-supported interpretations. Nor is it complete
in its explanation. There are some
ideas about what’s really happening when these
neighboring coherent histories interact or why the
wave function translates to probabilities
the way it does. But they are far from
generally accepted. Unlike Copenhagen,
many worlds is a deterministic interpretation. Any given timeline is
a predictable chain of cause and effect. It explains the apparent
randomness of quantum mechanics with a sort of observer bias. All possibilities are
chosen at every junction, and we just happen to be
seeing the one that happened in the branch that we occupy. That adds a second possible
cause for philosophical unease. In a purely
deterministic universe, what happens to free will? If we’re going to make all
possible decisions anyway, why sweat any given choice? Well, sure. But of all those countless
future branches of reality, some are going to
be pretty amazing. Think of it as a choose
your own adventure, and steer this version
of you towards one of the more awesome many
world branches of space-time. Well, guys, do
you know who this? This is Dianna from Physics
Girl right here on Space Time. Hey, Dianna. It’s cool to be in space here. Yeah, it’s nice, right? One thing I love
about your show is all of the crazy awesome
experiments you do. They are so much fun. That thing with the water
vortexes blew my mind. You don’t do a lot of
experiments on your show. We could do experiments. OK. Well, then, I have
a challenge for you. I want to challenge you
to prove that the Earth is round using an experiment. Ooh. How long do I have? You’ve got a
year, starting now. Challenge accepted. One thing I really
like about Physics Girl is it explains complex stuff
in such understandable ways. I try. MATTHEW O’DOWD: But I’m a
kind of jargony kind of guy. I love those long,
sciencey words. But it’s hard to keep
track of them sometimes. So I have a challenge for you. OK. I would love you to do
an episode in which you explain the five most jargony
and commonly encountered modern physics words
in simple English. Done. But I want you to use
only the 1,000 most common words in the English
language to do that. That’s going to
be a little harder. Yeah. But challenge accepted. So keep watching PBS
Space Time to make sure that Matt follows up
with that challenge. And keep watching Physics
Girl because we’re going to hold you to that, Dianna. [MUSIC PLAYING]