All right, so you know how like, if we always
talk about these big physics ideas, but the world
just feels so normal, right? Like you were listening
to this and it's like a thing you were doing.
It is. But what if this was like just one possibility?
Okay. And there are a bunch of other realities
where like all sorts of other crazy stuff is
happening right now. Right. Kind of wild. Yeah,
I mean, it sounds like science fiction. Totally.
But this all comes from like... the most basic
stuff about how the universe works at the smallest
level. Oh yeah, quantum mechanics. Right, quantum
mechanics. And it is so different than the physics
we see in the world around us. Exactly. Like
if I throw this pen, I can like kind of tell
where it is gonna land. But if you look at like
an electron, all bets are off. Totally unpredictable.
Yeah, and so the big question is like when and
how does that quantum weirdness become the normal
world that we see. Yeah, that's the big mystery.
And one of the craziest answers to that is the
many -worlds interpretation. And it starts with
this thing called superposition, which is already
a pretty wild idea. Yeah, and many -worlds really
just takes it to the extreme. Okay, so superposition,
for folks who haven't gone deep into the quantum
world, imagine a tiny particle. Okay. And it's
not in one specific place. It's like it's kind
of everywhere at once. Yeah, all at the same
time. Or at least it's in all the places it could
be until we look at it. Until you try to measure
it exactly. One of our sources describes it as
like probability clouds of where the particle
might be. That's a great way to think about it.
Yeah, it's wild. It is. So it's like all these
possibilities are just kind of hanging out. Just
swirling around. And the best way to see this
is with the double slit experiment. Right. Actually,
we did a whole deep dive on that a while back.
Yeah, that was a good one. If you want all the
details on that, you should definitely check
it out. Yeah. But the main point is you have
these particles and you... shoot them at a wall
with two slits. Too little open. And they don't
just go through one or the other. Right, they
act like waves. Yeah. And create this interference
pattern. It's like they went through both slits
at the same time. Yeah, it's like they took every
path at once. Exactly. And quantum mechanics
describes that by saying that each particle is
a superposition of all those possible paths.
Yeah, like all those paths are real possibilities.
But here's the thing. If you try to figure out
which slit the particle actually went through.
Right. that interference pattern goes away? It
just disappears. And it looks like the particle
just picked one path. Yeah. So what's up with
that? Well, this is where the Copenhagen interpretation
comes in. Ah, the Copenhagen interpretation.
It says that when you measure something, you
force the wave function to collapse. In your
cuff shoes. So that probability cloud becomes
just one definite. outcome one reality and that
was like the bridge between the quantum world
and the normal world exactly from all those possibilities
to the one we actually experience but a lot of
physicists even like the people who came up with
quantum mechanics oh yeah weren't really happy
with this idea of like collapsing wave functions
just because someone is watching yeah it does
seem a little strange doesn't it yeah and Schrodinger
He came up with this thought experiment with
a cat to show why? Oh yes, the famous Schrodinger's
cat. So imagine you have a cat in a box. In a
sealed box. And in the box with the cat, there's
a vial of poison, and it's connected to a device
that's triggered by a radioactive atom. Okay,
so if the atom decays, the poison is released.
Yeah, and here's where it gets really weird.
Quantum mechanics says that until we look at
it, That atom is in a superposition of states.
Both decayed and not decayed. At the same time.
Yeah. So if that tiny quantum event is in this
blurry state, doesn't that mean the cat is too?
The cat whose life literally depends on the atom.
Is the cat both alive and dead until we open
the box? It seems absurd when you think about
it on a larger scale. Yeah, it is like totally
crazy. And that was Schrodinger's point. Yeah,
he was saying, like, why does it have to be a
human observer? Right, what's so special about
us? And what about all the other stuff happening
in the universe that no one is watching? Exactly,
it raises a lot of questions. But the stuff we've
been looking at says that there are some newer
interpretations of the Copenhagen interpretation.
Yeah. Like this thing called decoherence. Right,
decoherence. So it's like saying that quantum
superposition doesn't really last that long?
Not at the macroscopic level. Especially when
you're talking about like a cat which is interacting
with its environment. Right, all the atoms in
the cat in the box are constantly interacting
with each other. Yeah, so it's like all that
quantum fuzziness gets kind of smoothed out by
the everyday world. And those different possibilities
become more like definite outcomes. So it's like
the universe kind of settles on an answer like
the cat being alive or dead based on which possibilities
were more likely. But even with decoherence,
the Copenhagen interpretation is still saying
that the choice of which outcome happens is random,
right? Yeah, fundamentally random. Like the universe
is rolling dice. Yeah. OK, so let's talk about
something even wilder. OK. The many -worlds interpretation.
Right. What if the wave function never collapses?
Never ever. What if all those possibilities are
always real? Love them. If we can imagine a cat
being both alive and dead? Yeah. Why not just
go all the way with it? All the way. What if
all those possibilities extend to us? To the
observers. And to the whole universe? The entire
universe, yeah. So if we open the box and the
cat is alive... Many worlds says it's not because
the wave function collapsed, it's because we
are now in a timeline. A branch of reality. Where
the atom didn't decay, but there's another reality
out there where it did decay and another us is
dealing with that. With a very different outcome.
It sounds crazy. I know. But it's based on the
math of quantum mechanics. And it was actually
proposed back in 1957 by this guy named Hugh
Everett. Right, in his PhD thesis. And it's this
idea of branching realities that we call the
many worlds interpretation. Exactly. So going
back to the double slit experiment, instead of
those multiple paths merging into one, when we
look at the particle, many worlds says all those
paths keep going in separate realities. They
all exist. And we just happen to be in one of
them. Right. And those bright bands in the interference
pattern, those are just the realities where more
of the particles ended up. More paths leading
to the same outcome. So more populated realities.
More versions of Are you seeing that outcome?
It's a lot to think about. It is mind boggling.
I mean, it seems kind of extreme to say that
the universe splits every time something quantum
happens. Yeah, one of our sources said it was
like building a whole new house just to avoid
doing the dishes. Yeah, compared to just having
the wave function collapse. Yeah, but even the
Copenhagen interpretation has all those possibilities
before the collapse. That's true. So they both
have to deal with those multiple realities in
some way. But the Copenhagen interpretation merges
them all into one. Right, through the collapse.
And many worlds just says they all continue.
They keep branching off. It's like these infinite
parallel timelines all happening at the same
time. Yeah, it's a lot. And one of the things
our sources pointed out was that many worlds
might actually be a simpler explanation mathematically.
Oh, also. Because the equations of quantum mechanics
don't have anything about wave function collapse.
Right, it's just something we added. So many
worlds is kind of just taking the math at face
value. And letting the universe do its thing.
But Everett's idea wasn't exactly popular when
he first proposed it. Yeah, why do you think
that was? Well, he was just a grad student at
the time. Yeah, that doesn't help. And then he
went into military research. So he wasn't really
in the physics world anymore. Plus the whole
idea of infinite copies of yourself living out
every possible choice you could have made. That's
a lot to process. Yeah, it can kind of freak
people out. An existential crisis for sure. And
remember, we still haven't actually proven many
worlds. Right. It's still just an interpretation.
It's a very popular one because it fits the math
so well. And it doesn't need any extra assumptions.
We can't just like hop between universes to check
though. Not yet, at least. And many worlds isn't
completely figured out either. Yeah. There's
still a lot of research being done on how these
branching realities work. Like how they interact
or don't interact. And how the probabilities
that we see actually come from this constantly
branching universe. Right. But there's one really
important difference between many worlds and
Copenhagen. OK, what's that? Many worlds is deterministic.
Deterministic. So in any one branch of reality,
everything happens according to the laws of physics.
Cause and effect. It's like a movie reel that
just keeps playing out. but with infinite branches.
So all that randomness we see in quantum mechanics.
Yeah, like when atom decays. It's not actually
random in many worlds. No, it's just that all
possibilities happen. But we only see the one
in our branch. We are just along for the ride
in our particular timeline. Wow. Yeah. But then
that brings us back to the whole free will question.
Right. If everything is deterministic and all
choices are made in some reality. Yeah. Does
it really matter what we choose here? It's a
deep philosophical question. That's something
that physicists and philosophers are still arguing
about. Yeah. But you know, one of the sources
we read actually had a fun way of putting it.
Oh, yeah. They said that many worlds is like
a giant choose your own adventure book. OK, I
like that. So you, the listener, you get to choose
which branch of reality you experience. You get
to pick your adventure. Yeah. So choose wisely.
And speaking of making complex physics accessible.
Uh -huh. Hold on, did you hear that? What was
that? That sounded like Diana from Physics Girl.
No way. It is. She's beamed in via PBS Space
Time. That's awesome. And she has some challenges
for Matt. Matt from PBS Space Time. Yeah, okay.
First she wants him to prove that the Earth is
round. Okay. with an experiment that he can do
in a year. Wow, that's a tough one. And then
her second challenge is even more interesting.
What is it? She wants him to explain five of
the most complicated physics terms using only
the 1 ,000 most common words in the English language.
Wow, that's going to be hard. Yeah, but he accepted
both challenges. That's amazing. It shows how
important it is to be able to communicate science
clearly. Yeah, so that everyone can understand
it. That's what we try to do here on the Deep
Dive. Exactly. Okay, so let's wrap up. Yeah.
Today we've been talking about the transition
from the quantum world to the normal world. The
classical world. And we've looked at two interpretations.
Two ways of understanding what's going on. The
Copenhagen interpretation, which says that the
wave function collapses when we measure something.
And we end up with one outcome. And the many
-worlds interpretation, which says that all possibilities
continue to exist in separate realities. And
we just experience one of them. And many -worlds
leads to this idea of a multiverse. An infinite
number of universes. Where everything that could
happen does happen. It's a wild idea. But it's
supported by the math of quantum mechanics. It
is. So here's a final thought for you. If every
possibility plays out in some reality, how does
that change the way you think about the choices
you make in this reality? It's something to ponder.
Thanks for listening. See you next time.