Speaker 1: What if every single decision you ever made, I mean,
every left turn or right turn, every yes or no,
didn't just disappear into the past, but actually created an
entirely new, fully formed version of reality.
Speaker 2: And what if those other realities, those other yous, aren't
just a fun thought experiment. What if they're physically real
in or influencing you. Right now, it.
Speaker 1: Sounds like something straight out of a movie marathon, not
a physics lab.
Speaker 2: Exactly. Welcome to Thrilling Threads. Today, we're diving into source
material that takes that very concept and argues it's not
science fiction but a cold hard science. We're looking at
the work of Oxford physicist David.
Speaker 1: Deutsch, right, and his argument is pretty bold. He claims
that parallel universes aren't just some speculative, optional theory.
Speaker 2: No, he says they are the only necessary explanation for
the most fundamental, most baffling experiments in all of quantum mechanics.
Speaker 1: So that's our mission for this deep Dives. We're going
to start with a surprisingly simple experiment, just a laser
and a few holes, really the kind.
Speaker 2: Of thing you might see at a high school physics.
Speaker 1: Class, and we're going to show how It's bizarre results
point to what Deutsch calls invisible elements of reality.
Speaker 2: From there, we'll explore the implications. You know, what this
means for who you are, your identity, and even the
mind bending possibility of time travel, which gets a total
rewrite in the multiverse.
Speaker 1: The ideas here really do challenge common sense at every turn.
They suggest our reality is just one layer in a
much much larger structure of existence.
Speaker 2: To even begin to get your head around that, you
have to start where it all began with light.
Speaker 1: Okay, let's set the stage with this core experiment. It's
famously known as the double slit experiment, and if you
haven't heard of it, well get ready.
Speaker 2: It really is the most mysterious demonstration in all of science.
So simple, but the implications are just vast.
Speaker 1: It's the cornerstone of the whole multiverse argument we're discussing.
Speaker 2: Today, and to understand it we actually have to go
way back, all the way back to eighteen oh one.
Speaker 1: Before electricity, before we even knew what an atom was exactly.
Speaker 2: There was this English polymath Thomas Young, and he wanted
to settle a very old debate. Is light made of
tiny particles like little bullets or is it a wave
like ripples in a pond.
Speaker 1: Newton had argued for particles, so that was the dominant
theory at the time it was.
Speaker 2: So Young sets up this brilliantly simple experiment. He takes
candlelight and shines it at a screen, But in between
the candle and the screen he puts a barrier with
two very thin, very close together slits in it.
Speaker 1: Okay, so, if light is made of particles like tiny
little paintballs, you'd expect to see two bright lines on
the screen behind it, two.
Speaker 2: Lines mirroring the two slits. Simple.
Speaker 1: But that's not what he saw, not at all.
Speaker 2: What he saw was a whole series of alternating bright
and dark stripes spread across the screen, a classic interference pattern.
Speaker 1: Which is something only waves do. Right When two waves overlap,
they can either add up and get bigger, or they
can cancel each other out.
Speaker 2: Precisely, the bright stripes are where the waves reinforce each
other constructive interference, and the dark stripes are where they
cancel out destructive interference. So Young proved it light is a.
Speaker 1: Wave case closed for a while anyway, for.
Speaker 2: About a century. Then you know Max Planck and Albert
Einstein came along and showed that light also behaves like
it's made of discrete packets.
Speaker 1: Of energy particles, which they called photons.
Speaker 2: So suddenly we have this paradox, this duality. Light is
somehow both a wave and a particle depending on how
you look at it. And that's where really our every
day intuition just completely breaks down.
Speaker 1: Einstein himself said, this simple setup contains the deepest mystery
of physics.
Speaker 2: Because the act of just looking at it seems to
change what happens.
Speaker 1: So let's bring that into the modern day. This is
the demonstration David Dortch uses to build his entire case.
Speaker 2: Right, instead of a candle, we use a laser, And
instead of two slits, let's say we have a piece
of cardboard with ten tiny pinprick.
Speaker 1: Holes in it, and we shine the laser through those
holes at a detector screen.
Speaker 2: Now, even with just one hole open, the result is
already a bit strange. You don't just get a single dot.
Because light is a wave, it defracts, it spreads out,
and you get this sort of fuzzy pattern of rings.
Speaker 1: Okay, that still makes sense with a wave, But.
Speaker 2: Then you open a second hole and the pattern on
the screen changes completely. It becomes that classic interference pattern
of stripes.
Speaker 1: And if you open a third, or a fourth.
Speaker 2: Or all ten, every single time you add a hole,
you get a totally new, unique, and very complicated interference pattern.
Speaker 1: So the key thing here is that the final pattern
on the screen depends on the exact number and arrangement
of all the holes that are open.
Speaker 2: Yes, every available path of the like contributes to the
final outcome, which again makes total sense if you're thinking
of a big continuous waves washing through all ten holes
at once.
Speaker 1: But now we get to the part that just breaks
your brain, the single photon paradox.
Speaker 2: This is where physicists really push the experiment to its
absolute limit. They decided to see what would happen if
they sent light through one particle at a time.
Speaker 1: Do you do that?
Speaker 2: You use an incredibly dense filter, so dense that it
only lets a single photon, one indivisible particle of light
pass through at a time, sometimes with minutes or even
hours between each one.
Speaker 1: So there's absolutely no chance they are bumping into each
other or interfering with each other. Each photon makes the
journey completely alone.
Speaker 2: Utterly alone. Now, when you start this. The first few
photons hit the detector screen and they just look like
random dots. One land's here, another one over there.
Speaker 1: It's just static, as you'd expect. It's a particle. It
has to land in one specific spot.
Speaker 2: But you let this run for hours, days, and as
thousands and thousands of these individual isolated dots accumulate, something
impossible starts to happen.
Speaker 1: They form a pattern.
Speaker 2: They form the exact same complex wave like interference pattern
you saw when the full laser beam was shining through
all ten holes.
Speaker 1: Wait, how that makes no sense. If each photon is
a particle and it's traveling alone, it has to go
through one hole, just one.
Speaker 2: Logically, yes, it'd have to pick a path whole number one,
or whole number seven, or whole number three.
Speaker 1: So if that were true, the final pattern should just
be ten slightly fuzzy clusters of dots on the screen
right behind each of the ten holes.
Speaker 2: That's what classical physics would predict. But that's not what happens. Instead,
each individual particle lands in a position that is statistically
determined by the existence of the other nine holes it
did not go through.
Speaker 1: It's as if each photon somehow knew that the other
nine holes were open.
Speaker 2: It acts as if it passed through every single hole,
simultaneously interfered with itself, and then chose a final landing
spot based on that interference.
Speaker 1: That's the part that is just it's a logical contradiction.
Speaker 2: It is. Think about it. If we have all ten
holes open, we get pattern A. But if we close
just one of them, say whole number five, and run
the experiment again, we get a totally different pattern B.
Speaker 1: Even though the photon we detect still had to go
through one of the other nine holes.
Speaker 2: Right, the presence or absence of a path that never
took changes the outcome. So the source material forces a
conclusion on us. Something must be going through those other.
Speaker 1: Holes, something we can't see.
Speaker 2: Something we can't see, but something that is physically real
enough to shove our photon aside as Deutsch puts it,
causing the interference.
Speaker 1: So if our one visible photon went through hole one,
there must have been nine other somethings, yeah, that went
through the other nine holes exactly.
Speaker 2: And when you do tests to figure out what this
something is, you find it behaves exactly like light. It
travels at the speed of light. It reflects, it defracts,
it does everything a photon does, except except we can't
detect it. It's invisible to our instruments in this reality.
Speaker 1: So if it looks like light and acts like light,
that is invisible, the.
Speaker 2: Only reasonable conclusion, Deutsch argues, is that it is light,
light of an invisible kind.
Speaker 1: And this isn't just for light, is it. They've done
this with other particles.
Speaker 2: Oh yes, electrons, protons, even whole molecules, everything at the
quantum level exis. It's this same behavior, which implies that
all matter, including the atoms in our bodies, is constantly
being influenced by these invisible.
Speaker 1: Counterparts, which is a staggering thought. It means something that
doesn't exist in our reality can't exert a physical force.
So if our photon is being pushed, the thing doing
the pushing must exist.
Speaker 2: It must be real, and that propels us directly out
of the lab and into the multiverse.
Speaker 1: Okay, so the experiment leaves us with this unavoidable fact.
Invisible things are interacting with the visible things in our universe.
But that feels like a huge leap from invisible light
to infinite parallel universes.
Speaker 2: It is a huge leap, But is where the mathematics
of quantum theory forces you to go. This is where
we get to what's known as the measurement problem.
Speaker 1: Right, So, in quantum mechanics, before you look at a particle,
it doesn't have a definite position. It's described by something
called a wave function.
Speaker 2: And that wave function is essentially a cloud of probabilities.
It described all the places of the particle could be
all at the same time. It's in a state of superposition.
Speaker 1: Like our photon being simultaneously in all ten holes at once.
Speaker 2: Exactly. That's the wave part of its behavior. But here's
the problem that has stumped physicists for a century. The
moment you measure it, the moment the photon hits the screen,
that whole cloud of possibilities just vanishes.
Speaker 1: It collapses.
Speaker 2: It collapses, and the particle instantly snaps into one single
definite location, which.
Speaker 1: Feels a bit like magic. What what causes the collapse?
Why does looking at it change everything?
Speaker 2: That is the central mystery. The mainstream view for a
long time was called the Copenhagen interpretation, and it basically
just said the active measurement causes the collapse.
Speaker 1: Which isn't really an explanation, is it. It's more of
a description.
Speaker 2: It's not. It doesn't explain how or why where do
you draw the line? Does it collapse when it hits
the screen or when the information from the screen hits
a human eye or a computer. It requires this arbitrary,
non physical event to happen just to make the math
fit our single reality.
Speaker 1: Exp and that's where David Deutsch and Many World's Interpretation
or MWI come in.
Speaker 2: Yes, MWI offers a much simpler, though admittedly much stranger explanation.
Speaker 1: It says the wave function never collapses at all.
Speaker 2: Never. Instead, every time a quantum measurement is made, every
time a choice has to resolve, like which hole the
photon goes through, the universe doesn't collapse. It branches.
Speaker 1: So instead of all those possibilities and the probability cloud
disappearing except for one, all of.
Speaker 2: Them happen, every single one. The universe splits into multiple
non interacting copies, and each possibility plays out in its
own separate reality.
Speaker 1: So in our experiment, the universe branched into ten versions.
In one the photon went through hole, one in another hole,
two in another whole.
Speaker 2: Three and so on, all the possibilities described by the
math are physically real. The invisible light we were talking about
is just the photons in those other nearby branches, which
are still close enough to interfere with our branch for
a brief moment.
Speaker 1: The implication, then, is just staggering entire parallel worlds with
their own matter and energy existing right alongside ours.
Speaker 2: And according to the theory, they all came into being
at the Big Bang. They all started out together, almost identical.
The source uses this great analogy of countless books, all
with the same first chapter.
Speaker 1: But then over time, tiny random quantum events and adam
decaying here but not there cause them to fork. They
start to diverge, and.
Speaker 2: The stories in those books start to become different. Our
universe is just one of those stories, one variation out
of an infinite number.
Speaker 1: This leads to Deutsch's core argument for why we have
to take this seriously. He makes a distinction between prediction
and explanation.
Speaker 2: Right, if all you want to do is predict where
the photon will land, you can just use the math
a quantum mechanics. It's probabilistic, but it's incredibly accurate. You
don't need to believe in anything outside this room to
do the calculation.
Speaker 1: But science isn't just about predicting. It's about explaining why exactly.
Speaker 2: If you want to explain why that interference pattern shows up,
if you want to causal reason for what is physically
shoving that photon around, then you need an explanation that
fits the facts we observe.
Speaker 1: And the fact is the photon is being affected by something.
We can see the result of that interaction.
Speaker 2: So you have two choices. You can either say our
photon is being influenced by things that don't exist, which
is just a logical absurdity, or.
Speaker 1: You have to say it's being affected by things that
really do.
Speaker 2: Exist, and that's the only scientifically coherent path. Those invisible
photons and the universes they inhabit must be real. So
the many world's view isn't just an interpretation. It's the
only one that takes the evidence seriously without inventing a mysterious,
unexplained collapse.
Speaker 1: So it's not that we're adding extra stuff to the
theory for fun. We're just taking the math that we
already know works and accepting what it literally tells us
about reality.
Speaker 2: That's the elegance of it. It simplifies the physics by
complicating our picture of reality. It says, if the math
describes a superposition of ten possibility, then ten real things
are happening. The universe just does it all, okay.
Speaker 1: So if we're living in this vast multiverse constantly being
nudged by other realities, the question immediately becomes personal. What
does this mean for me?
Speaker 2: Yeah, this is where it stops being an abstract physics
problem and starts feeling deeply philosophical.
Speaker 1: The version of me who didn't sell his bitcoin in
twenty seventeen is living a very different life right now
in another branch, and he's just as real as I am.
Speaker 2: That's the required mental shift. You have to stop thinking
of yourself as this single entity in one universe. Deutsche says,
we are an entity existing in a multiverse as a whole,
where a collection of all our possible selves.
Speaker 1: So, right at the moment a choice is made, the
copies of us are identical, same memory is same thoughts exactly.
Speaker 2: The source material points out that if two copies are
completely indistinguishable, it's really just a matter of semantics, whether
you call them two separate universes or not.
Speaker 1: But most of them are already slightly different, right, even
by the position of a single atom somewhere in the cosmos.
Speaker 2: Yes, and those tiny differences are what allow for the
ongoing interference at the quantum level. It's why our atoms
are being influenced by their counterparts.
Speaker 1: The source uses a really helpful analogy to explain how
these identical copies grow apart, the identical twin analogy.
Speaker 2: It's a great way to ground it. You have two
identical twins born with the exact same genetic code, indistinguishable.
Speaker 1: But as they live their lives, they make different choices,
have different experiences. One moves to another city, one gets
a different job.
Speaker 2: They row apart, and they each become their own unique person.
They're both equally real, but they have different stories.
Speaker 1: The same thing is happening to all the versions of
you across the multiverse, but on a scale that's almost
impossible to comprehend.
Speaker 2: Every choice, from what you eat for breakfast to who
you marry, and every tiny random quantum event in your
brain causes a divergence. The versions of you start to
branch away from each other.
Speaker 1: Some of those other us are almost identical to you,
just I don't know, blink a second later.
Speaker 2: But other versions, ones that have accumulated many different choices
over many years, could be unrecognizable. They might have completely
different beliefs, a different personality. They might not even like
the same things you do anymore.
Speaker 1: It redefines what a self even is. It's not a
single point. It's this enormous branching tree of all your
possibilities being played out, which is a.
Speaker 2: Beautiful idea in a way. All that potential you feel
you have is actually being realized somewhere.
Speaker 1: But that brings us to the big skeptical question, the
one everyone asks, if all these other worlds and all
these other means are real, why can't I feel them?
Why can't I see them?
Speaker 2: It's the most common objection, and Deutsche's answer is that
our senses are just limited. They haven't evolved to perceive
quantum reality directly.
Speaker 1: I mean, you'd think with infinite universes we'd feel something
like shouldn't there combine gravity? Just crush us.
Speaker 2: That's where a really key concept comes in, called decoherence.
Think of it as the process that makes the universes
separate and unable to interact on a large scale.
Speaker 1: So they interfere at first, like in the experiment, but
then they stop.
Speaker 2: Why for two branches to keep interfering, they need to
stay in sync, like two perfectly timed waves. But as
soon as a quantum event happens in the macroscopic world,
like you choosing to open a door, that choice involves
interacting with billions and billions of atoms, air, molecules, the doorknob, photons.
Speaker 1: And all those interactions are like tiny measurement countless tiny measurements,
and that complexity immediately scrambles the synchronization between.
Speaker 2: Your universe and the one where you didn't open the door.
They decohere, they become uncorrelated.
Speaker 1: Ah, so the walls between the universes go up, so
to speak. They're still there, but they can't influence each
other in any way we can perceive.
Speaker 2: Precisely, we are only sensitive to the one branch we
happen to be on. The interference we see in the
lab is a glimpse into that brief moment before decoherence
takes full effect.
Speaker 1: Deutsch has a great analogy for this sensory argument, fucos pendulum.
Speaker 2: Yes, our common sense tells us the ground we're standing
on is stationary. We can't feel the Earth spinning in
one thousand miles an hour.
Speaker 1: Right, If you relied only on your senses, you'd conclude
the Earth is still. That was the argument against Galileo
for centuries. If the Earth is moving, why don't we
get thrown off?
Speaker 2: They were using their biological limits as proof. But we
know better. We can build an instrument like a Fuco's pendulum,
which shows the Earth's rotation over time, and we can
indirectly detect that motion.
Speaker 1: So the pendulum is like an extension of our senses, and.
Speaker 2: That's exactly what the quantum experiment is for the multiverse.
Our senses don't operate based on quantum interference, so we
don't feel the other world. But if they did, Deutsch
arg use, we would feel them.
Speaker 1: So we have to build these sophisticated experiments to act
as our quantum senses.
Speaker 2: Our inability to feel the multiverse is a failure of
our biology, not a failure of the physics. The lab
is our telescope for seeing these other realities, and it's
telling us they're there.
Speaker 1: So if we accept this foundation that countless other histories
are playing out in parallel, it completely changes the game
for one of the biggest ideas in science fiction time travel.
Speaker 2: It really does. It takes it from the realm of
logical paradox and makes it well theoretically possible.
Speaker 1: At least, let's start with the physics of it, leaving
aside the paradoxes for a moment. Could we even build
a time machine?
Speaker 2: It's still a very open question, but based on Einstein's
general relativity, the theory that describes gravity and space time,
the answer is maybe this.
Speaker 1: Isn't just about going really fast, this is about actually
warping the fabric of space time itself exactly.
Speaker 2: The math of general relativity allows for solutions where space
time can be so extremely curved, say near a rapidly
spinning black hole or involving some kind of theoretical exotic matter,
that it could actually loop back on itself.
Speaker 1: These are called closed timelike curves CTCs.
Speaker 2: Right, a CTC is basically a path through space time
that ends where it began, not just in space, but
in time. It's a tunnel to the past.
Speaker 1: So if the laws of physics don't explicitly forbid these
from forming, then a pathway to the past is possible.
Speaker 2: In theory, yes, but the moment you say that, you
run headfirst into the paradoxes.
Speaker 1: The most famous one being the grandfather.
Speaker 2: Paradox, the classic I build a time machine, I go
back in time, and I prevent my grandfather from ever
meeting my grandmother.
Speaker 1: If you succeed, your father is never born, which means
you are never born.
Speaker 2: Which means I never existed to build the time machine
to go back in the first place.
Speaker 1: The effect erases its own cause it's a contradiction.
Speaker 2: It's a complete logical breakdown. And that's why in a
single universe model, most physicists conclude that time travel into
the past must be impossible. The universe must have some
law that prevents these paradoxes.
Speaker 1: But the Many World's interpretation offers an escape patch.
Speaker 2: It provides this perfect, elegant solution. David Deutsch argues, the
paradox only exists if you assume there's only one timeline.
The multiverse is real. You can't change your own past.
Speaker 1: So time travel isn't about rewriting history.
Speaker 2: It's about jumping tracks. It's about entering the past of
a different universe.
Speaker 1: So if I travel back to, say nineteen fifty, to
stop my grandfather, the moment I arrive, I am not
in my own past anymore.
Speaker 2: You've stepped sideways into a parallel branch, a universe that
was up until that moment identical to your past, but
is now going to have a different future because you're.
Speaker 1: In it, and my original timeline, the one I left.
Speaker 2: It continues on, completely untouched, consistent in that universe. You
were born, you built the time machine, you stepped into it,
and you vanished. The cause and effect are perfectly preserved.
Speaker 1: And in the new universe, I can do whatever I want.
My actions there create a new history for that reality,
but it has no effect on where I came from.
Speaker 2: No contradiction. The effect of you traveling back in time
happens in one universe, while the cause of your existence
remains intact in another. It's a beautiful solution, it's logical, it's.
Speaker 1: Clean, but when you think about the human cost, it's
also incredibly sobering.
Speaker 2: It really is, because if this is how it works,
the journey is strictly one way.
Speaker 1: You can never go home.
Speaker 2: Never. The act of creating and using that pathway through
space time causes such a massive divergence that you were
permanently cut off from your universe of origin.
Speaker 1: Think about what that means for the people you left behind,
your family, your friends. They watch you get in the
machine and you're just gone forever.
Speaker 2: From their perspective, you've vanished, you're lost, you've effectively died.
Speaker 1: And then in the past of the universe you arrive in,
there's another set of problems.
Speaker 2: Because there's already a version of you there, or well,
there will be.
Speaker 1: The original copy of me in that timeline still exists.
So I, the Traveler, just appear out of nowhere.
Speaker 2: Suddenly there are two of you in that world. You've
become this chronological immigrant in a reality that wasn't expecting you.
Speaker 1: So it's not about fixing your mistakes. It's about abandoning
your entire life for a chance to start a new
one somewhere.
Speaker 2: Else, and in the process, leaving a mystery behind in
your old world and creating a complication in your new one.
Time travel becomes less of a fun adventure and more
of a tragedy, the ultimate act of self exile.
Speaker 1: What a journey. We started with a single particle of
light behaving strangely in a simple experiment, and that.
Speaker 2: One experiment, when you really follow its implications, serves as
the bedrock evidence that our reality is being physically shaped
by unseen ones.
Speaker 1: That led us to the multiverse, not as some fun
sci fi idea, but as David Deutsch's necessary conclusion, the
only theory that actually explains the quantum map without just
throwing up its hands and saying it collapses.
Speaker 2: It provides the why, and from there we had to
rethink everything. We realize our own identity isn't some single
fixed thing, but this huge branching tree of all our
potential selves.
Speaker 1: And the reason we don't feel these other worlds is
the same reason we don't feel the earth spinning beneath
our feet. Our senses just aren't built for it. But
the instruments, the experiments, they confirm it's real.
Speaker 2: And finally we saw how all of this provides a stunning,
if a bit tragic, solution to the paradoxes of time travel.
You could visit the past, but it's not your own,
and you can never return.
Speaker 1: It means that every mistake you regret, every road not taken,
it's not a ghost. It's a real story being lived
out by another version of you in a universe just
as real as this one. Our life is just one
book in an infinite library.
Speaker 2: A reality is just one variation.
Speaker 1: So we want to leave you with a final thought
to mull over if time travel really worked like this,
if it were physically possible, But you knew the journey
was a one way ticket, you knew you could never
go back, that you'd abandon everyone you've ever known forever.
Would the chance to step into a new history, maybe
a better one, be worth that ultimate price. Let us
know what you think in the comments.