Speaker 1: I want you to just imagine you're lying on a
Speaker 1: gentle hillside at night.
Speaker 2: Oh that sounds lovely right now.
Speaker 1: Yeah, it really does. The grass is, you know, cool
Speaker 1: and soft beneath you, right, and you're just looking up
Speaker 1: at this vast, quiet canopy of stars.
Speaker 2: Just breathing in that cool night air.
Speaker 1: Exactly. I want you to take a slow, deep breath
Speaker 1: with us, because tonight this isn't a lecture, no, not
Speaker 1: at all.
Speaker 2: It's more of a like a slow comforting exhale.
Speaker 1: Yeah, quiet exploration. We're just two friends whispering about the
Speaker 1: strangeness of existence under the night sky.
Speaker 2: And you know, we really want you to just let
Speaker 2: go of all those racing thoughts from the day.
Speaker 1: Just let them drift away, because for our time together
Speaker 1: today I have gathered this gentle stack of notes and sources.
Speaker 2: Some really beautiful mysteries in those nights.
Speaker 1: You really are We're going to dive into a deep
Speaker 1: exploration of a mystery that has just baffled the greatest
Speaker 1: minds for over a century.
Speaker 2: We're talking about the building blocks of reality itself, right,
Speaker 2: we are the very bottom of everything. And what we're
Speaker 2: going to ponder tonight is something called the double slit experiment.
Speaker 1: Which sounds a bit technical, but it's really just a
Speaker 1: story about the strange dual nature of light and matter.
Speaker 2: Yeah, and it leads to this really beautiful kind of
Speaker 2: haunting question, like is the universe truly there? When no
Speaker 2: one is looking right?
Speaker 1: And I know, you know, sometimes physics can feel so.
Speaker 2: Heavy, oh, absolutely overwhelming, full of dense math and equations.
Speaker 1: Yeah exactly. But tonight, for this deep dive, we are
Speaker 1: completely leaving the math behind.
Speaker 2: That's a relief. Just simple ideas tonight.
Speaker 1: Just simple pictures, things you can hold softly in your mind,
Speaker 1: like a spinning coin or a pebble dropped in a pond.
Speaker 2: I love that using those simple images to understand the
Speaker 2: whole universe.
Speaker 1: So let's start there. Let's set the baseline of everyday reality.
Speaker 2: Let's do it. We're going back to eighteen oh one.
Speaker 1: Actually, right, Thomas Young, he did this classic setup. I
Speaker 1: want you to picture just a solid wall, a simple
Speaker 1: dark wall, and in this wall, someone has cut two
Speaker 1: two narrow vertical slits, just two little doors, right side
Speaker 1: by side, and behind that wall, a little ways back,
Speaker 1: there's a blank screen like canvas resting in the dark.
Speaker 2: Okay, I have that picture in my mind.
Speaker 1: So imagine standing in front of this wall and you
Speaker 1: have like a handful of tiny physical objects, let's say,
Speaker 1: tiny pellets or little.
Speaker 2: Bullets, just solid everyday things.
Speaker 1: Right, and you start firing them one by one towards
Speaker 1: the wall with the slicks.
Speaker 2: Well, what happens is exactly what you'd expect, right, They
Speaker 2: travel in.
Speaker 1: Straight lines, right straight through the air.
Speaker 2: Yeah, and most of them bounce off the solid wall.
Speaker 2: But the ones that go through the two slits, they
Speaker 2: just keep going straight until they hit the backscreen.
Speaker 1: So you just end up with two distinct bands, two
Speaker 1: piles of marks directly behind the.
Speaker 2: Little doors, exactly two stripes on the backscreen. Because that
Speaker 2: is how solid matter behaves. It's simple, it's predictable.
Speaker 1: It makes sense. But now let's change the picture. Let's
Speaker 1: take that same wall with the two slits and place
Speaker 1: it in a quiet pond.
Speaker 2: Well, I like this one, just still dark water.
Speaker 1: Yeah, and you drop a pebble in the water.
Speaker 2: You create a wave, a gentle ripple, spreading out.
Speaker 1: Right, and that wave hits the wall and it passes
Speaker 1: through both of those slits at the same time.
Speaker 2: And when it does that, it creates two new sets
Speaker 2: of ripples on the other side.
Speaker 1: Two new waves spreading out from the slits.
Speaker 2: Yeah, exactly. And here's the beautiful part. Those two new
Speaker 2: sets of ripples they spread out and they start to
Speaker 2: overlap in the water. They weave together, they do, and
Speaker 2: where two peaks of the waves meet, the water grows taller.
Speaker 2: But where a peak meets a valley.
Speaker 1: Oh, they cancel out right exactly.
Speaker 2: The water goes perfectly flat. There.
Speaker 1: So on the back screen or the back wall of
Speaker 1: the pond, you don't just get two stripes, No.
Speaker 2: You get this beautiful repeating pattern. It's called an interference pattern.
Speaker 1: An alternating sequence of like strong waves and completely still water.
Speaker 2: Yes, multiple bands spreading across the back. It's a signature
Speaker 2: of waves overlapping.
Speaker 1: So we have this great divide in our everyday experience.
Speaker 1: Things are either solid objects like the tiny.
Speaker 2: Bullets, which give you two stripes.
Speaker 1: Right, or they are spread out waves like the pawn ripple.
Speaker 2: Which give you that beautiful overlapping pattern.
Speaker 1: And in our normal world, these things don't mix. A
Speaker 1: bullet isn't a ripple.
Speaker 2: No bullet is a bullet. A wave is a wave.
Speaker 1: So this brings us to this incredibly gentle but baffling question.
Speaker 1: What happens if we take the absolute smallest pieces.
Speaker 2: Of the universe, like tiny specks of light or pieces
Speaker 2: of atoms?
Speaker 1: Right, what if we send those through the two slits?
Speaker 2: Well, this is where the universe starts to look very,
Speaker 2: very strange.
Speaker 1: Because scientists did this right.
Speaker 2: They fired light at the slits, they did, but to
Speaker 2: really understand it, they had to isolate it. They dimmed
Speaker 2: the light source down so low that it was only
Speaker 2: firing one single tiny speck of light at.
Speaker 1: A time, a photon. Right, just one solitary little piece
Speaker 1: of reality.
Speaker 2: Yes, exactly, just one speck traveling alone in the dark.
Speaker 1: But wait, if it's just one single speck traveling completely
Speaker 1: on its own, it has to choose, right, you would think. So,
Speaker 1: I mean, it has to go through either the left
Speaker 1: slit or the right slit, just like a solitary little bullet.
Speaker 1: It can't overlap with itself.
Speaker 2: That is exactly what common sense tells us. And at first,
Speaker 2: when they ran the experiment, it looked like common sense
Speaker 2: was right.
Speaker 1: Oh really, what did it look like?
Speaker 2: Well, a single spike is fired, ping it hits the backscreen,
Speaker 2: there's one tiny dot, like a single bullet mark right,
Speaker 2: and then another speck is fired, ping it lands somewhere
Speaker 2: else do It just looks random, completely random, like a
Speaker 2: scatter of dots in the dark. But as they kept
Speaker 2: firing them one by one, dot by dot, taking all
Speaker 2: the time in the.
Speaker 1: World, a pattern starts to emerge.
Speaker 2: Yes, a very specific pattern. Slowly, those individual dots build
Speaker 2: up into the exact same alternating pattern of waves.
Speaker 1: The interference pattern, the pond.
Speaker 2: Ripples, exactly the pond ripples made out of individual bullets.
Speaker 1: Oh wow, So each individual speck, even though it's traveling
Speaker 1: entirely on its own, it's interfering with itself.
Speaker 2: That's the staggering implication. For that beautiful alternating pattern to
Speaker 2: appear on the back screen, there has to be an overlap.
Speaker 1: But there's only one spec I know.
Speaker 2: It means that the single particle cannot be traveling as
Speaker 2: a solid little deed.
Speaker 1: It can't just be going through one door, right.
Speaker 2: It must be traveling as a wave of possibilities. It's
Speaker 2: moving through both slits at the exact same time.
Speaker 1: Let that idea just linger softly for a second.
Speaker 2: It's hard to hold in your mind.
Speaker 1: It really is a single piece of reality. Spreading out
Speaker 1: like a ghost.
Speaker 2: Just taking every possible path through the dark.
Speaker 1: Yeah, going left and going right simultaneously, until it finally
Speaker 1: lands on the screen.
Speaker 2: And when it lands it becomes a solid dot again. Yeah,
Speaker 2: but while it was traveling, it was everywhere it could be.
Speaker 1: It's so beautiful. But also, you know, my mind immediately
Speaker 1: wants to solve it.
Speaker 2: Oh, of course, we want to know how the trick
Speaker 2: is done, right.
Speaker 1: I think, well, if it's going through both slits at once,
Speaker 1: why don't we just look closely?
Speaker 2: Why not just watch it happen.
Speaker 1: Yeah, let's just put a tiny little camera or you know,
Speaker 1: a detector right next to the doors to see which
Speaker 1: way it actually goes.
Speaker 2: And that is exactly what the scientists thought too. They
Speaker 2: wanted to peek behind.
Speaker 1: The curtain, so they added a detector right right at
Speaker 1: the slits.
Speaker 2: They did. They set up a way to gently watch
Speaker 2: and see does this speck go left or does it
Speaker 2: go right?
Speaker 1: And what happened?
Speaker 2: Well, when the detector is turned on, we do see
Speaker 2: this back. We see it go through just one slit
Speaker 2: or the other.
Speaker 1: Okay, so it does choose a door, it does.
Speaker 2: When we look, But the moment we look something heartbreaking
Speaker 2: happens on the backscreen. The pattern changes, the beautiful wave pattern,
Speaker 2: the ripples, it completely disappeared.
Speaker 1: Wait, really, it just vanishes.
Speaker 2: It vanishes. The dots stop forming the wave pattern. They
Speaker 2: collapse into just two plain, boring bands, exactly like the
Speaker 2: solid bullets.
Speaker 1: Wow. So just the act of watching it, yeah, forces
Speaker 1: it to act like a solid object.
Speaker 2: It's as if the universe knows we are paying attention.
Speaker 1: It's like it's shy. It was doing this beautiful, ghostly
Speaker 1: dance through both doors, but the second we look, it
Speaker 1: just drops to the floor and pretends it's a regular bullet.
Speaker 2: Exactly. The magic vanishes, the mom bites observed.
Speaker 1: That is just it's wild. Let's try to picture this
Speaker 1: with an analogy, because the physics terms for this can
Speaker 1: be so heavy.
Speaker 2: Yeah, let's keep it grounded.
Speaker 1: In the notes, there's this concept called quantum superposition. But
Speaker 1: let's just imagine a spinning coin.
Speaker 2: A coin spinning rapidly on a tabletop.
Speaker 1: Right, imagine looking down at it while it's spinning. It's
Speaker 1: just a blur, isn't it.
Speaker 2: Yeah, it's just a hum of motion.
Speaker 1: It's not heads and it's not tails. It's a beautiful
Speaker 1: mixture of both possibilities at once.
Speaker 2: I love that the unobserved particle is exactly like that
Speaker 2: spinning coin. It exists in this fluid state of pure possibility, its.
Speaker 1: Heads and tails, left door and right door, all at
Speaker 1: the same.
Speaker 2: Time, right until we decide to look.
Speaker 1: And looking looking at the particle with our little camera
Speaker 1: or detector, that's like slamming your hand down on the
Speaker 1: spinning cord.
Speaker 2: No, that's exactly what it's like. You slam your hand.
Speaker 1: Out stops instantly. The soft fluid wave is gone.
Speaker 2: The beautiveruse is forced to make a choice, heads or tails,
Speaker 2: left slit or right slit.
Speaker 1: Just the act of touching it, of measuring it, collapses
Speaker 1: all those beautiful possibilities into one single, cold fact.
Speaker 2: The gentle wave is gone, just because we asked it
Speaker 2: for a definitive answer.
Speaker 1: It's so wild to think about. It brings up these
Speaker 1: massive philosophical questions, and the notes here talk about these
Speaker 1: philosophers of the quantum world back in the nineteen twenties.
Speaker 2: Ah, Yes, Neils Bohorr and Werner Heisenberg, right, And.
Speaker 1: They came up with something called the copen Hagen interpretation,
Speaker 1: which sounds very.
Speaker 2: Formal, very academic Yeah.
Speaker 1: But really, we can just picture them as a group
Speaker 1: of physicists who dared to ask a very poetic question.
Speaker 2: Would they ask?
Speaker 1: They dared to ask if the moon is there when
Speaker 1: nobody looks?
Speaker 2: That is such a profound way to put it, because
Speaker 2: what they were really suggesting is that reality, at its deepest,
Speaker 2: quietest level isn't made of solid things just waiting around
Speaker 2: to be found.
Speaker 1: So if it's not solid things, what is it made of?
Speaker 2: It's made a problem. It's just a blur of maybes,
Speaker 2: like the spinning coin exactly. They argue that reality itself
Speaker 2: does not take a definite shape until it interacts with
Speaker 2: something else, until the hand slams down.
Speaker 1: But wait, I have to ask, because this is where
Speaker 1: my mind goes, and I think a lot of people's
Speaker 1: minds go here.
Speaker 2: Where's that?
Speaker 1: Well? Does that mean the universe needs us?
Speaker 2: Ah? I see what you're saying, Like.
Speaker 1: Does it need a human brain or a conscious mind
Speaker 1: to look at it to make it real? If I
Speaker 1: close my eyes, does the room I'm in just dissolve
Speaker 1: into a wave of probability?
Speaker 2: It's such a common thought, and it's a very romantic idea,
Speaker 2: isn't it.
Speaker 1: It is it centers us?
Speaker 2: But it's also maybe just a little bit arrogant of
Speaker 2: us to think we're that important.
Speaker 1: Yeah, that's fair.
Speaker 2: Physics actually has a really gentle grounding correction for this.
Speaker 2: There's a famous joke by the physicist John Bell.
Speaker 1: Oh what did he say?
Speaker 2: He was mocking this idea that consciousness is required. He asked,
Speaker 2: did the wave wait billions of years for a single
Speaker 2: celled creature to appear? Or did it wait for a
Speaker 2: highly qualified measure with a PhD?
Speaker 1: That's brilliant? Did the universe stay blurry until someone with
Speaker 1: a degree looked at it?
Speaker 2: Right? It's funny, but it makes a deep point. We
Speaker 2: need to redefine what looking means.
Speaker 1: Okay, so what does it mean to look at a particle?
Speaker 2: Looking just means a physical interaction. It doesn't require eyes
Speaker 2: or a mind.
Speaker 1: So a camera counts, yes.
Speaker 2: Or even just a speck of light hitting a photographic plate.
Speaker 2: That plate is an.
Speaker 1: Observer, or a rocketting another rock exactly.
Speaker 2: The universe doesn't need our human eyes to exist. You
Speaker 2: just need pieces of itself to.
Speaker 1: Touch, to interact.
Speaker 2: Yes, any physical interaction is a measurement. That's what slams
Speaker 2: the hand down on the spinning coin. The universe is
Speaker 2: constantly measuring itself, forcing itself to be real even when
Speaker 2: we aren't around.
Speaker 1: That is a comfort, actually, to know the universe can
Speaker 1: hold itself together.
Speaker 2: It really is. We're just part of it. We aren't
Speaker 2: holding it up with our minds.
Speaker 1: So reality is solidifying all on its own through these
Speaker 1: tiny interactions. Yes, but what about time, because our notes
Speaker 1: here take a really strange turn into time.
Speaker 2: Oh, we are entering the territory of John Wheeler.
Speaker 1: Now right. This is from nineteen seventy eight. Wheeler had
Speaker 1: this haunting thought experiment. We talked about how the particle
Speaker 1: acts like a wave if we don't look, and acts
Speaker 1: on a bullet if we do look. Right, Well, Wheeler said, what.
Speaker 2: If we wait wait for what?
Speaker 1: What if we let the particle pass through the two
Speaker 1: slits in the dark and we don't decide if we
Speaker 1: want to look at it until after it has already
Speaker 1: gone through.
Speaker 2: Oh, the delayed choice experiment. This is where it gets
Speaker 2: really mesmerizing.
Speaker 1: It's like playing a trick on reality.
Speaker 2: It really is. Imagine a particle coming from a distant star,
Speaker 2: let's say millions of light years away.
Speaker 1: Okay, a tiny speck of starlight traveling through the cold
Speaker 1: dark for millions of years.
Speaker 2: Yes, And on its way to Earth, it passes around
Speaker 2: a massive galaxy, so.
Speaker 1: It could go around the left side of the galaxy
Speaker 1: or the.
Speaker 2: Right side exactly. The galaxy is like the wall with
Speaker 2: the two slits. The starlight is the particle.
Speaker 1: And it has been traveling for millions of years. It
Speaker 1: already passed the galaxy a long, long time ago.
Speaker 2: Right, and it finally reaches us here on Earth tonight. Okay,
Speaker 2: And we decide today, right now in our observatory whether
Speaker 2: we want to measure it as a wave looking for
Speaker 2: the pond ripples, or measure it as a particle, the
Speaker 2: solid bullet.
Speaker 1: But wait, if it has already been traveling for millions
Speaker 1: of years, how can a choice we make tonight on
Speaker 1: Earth change how it behaved millions of years ago when
Speaker 1: it passed that galaxy.
Speaker 2: It feels like it should be possible. It feels like
Speaker 2: we're reaching back into the past.
Speaker 1: Right if we look for a wave today, does the
Speaker 1: universe go back in time and make sure it went
Speaker 1: around both sides of the galaxy.
Speaker 2: For a long time. This was just a quiet thought
Speaker 2: in Wheeler's head, But recently we've actually been able to
Speaker 2: test this in modern labs.
Speaker 1: This is the part that just absolutely blows my mind,
Speaker 1: the fact that we can test this. We have the
Speaker 1: twenty fifteen experiment in Australia at the Australian National University, Yes.
Speaker 2: The a and U experiment, And they didn't just use
Speaker 2: a speck of light, used an actual atom, a single
Speaker 2: relatively massive helium atom.
Speaker 1: So a real physical piece of matter exactly, and they
Speaker 1: dropped it through crossroads of lasers. The lasers act like
Speaker 1: the two slits.
Speaker 2: Right, yes, exactly, they split the atom's possibilities, and then
Speaker 2: at the very last microsecond, while the atom is already falling,
Speaker 2: already past.
Speaker 1: The crossroad, the machine makes a choice.
Speaker 2: A random number generator. It decides at the absolute last
Speaker 2: instant whether to look or not.
Speaker 1: Whether to slam the hand down on the coin or
Speaker 1: let it stay ablur exactly.
Speaker 2: And then ten years later, in twenty twenty five, at
Speaker 2: MIT scientists did the exact same thing, but with ultra
Speaker 2: cold atoms trapped in this beautiful cage of light called
Speaker 2: an optical lattice.
Speaker 1: Just floating there in the dark, and they tested the
Speaker 1: same delayed choice they did, and what did they find
Speaker 1: in both places? Australia and Mit. What happens when we
Speaker 1: make the choice at the last second.
Speaker 2: The result is always the same, and it is breathtaking.
Speaker 2: Even if the choice is made the very last second
Speaker 2: after the particle has crossed the slits, the particle perfectly
Speaker 2: matches our choice.
Speaker 1: It matches it perfectly perfectly.
Speaker 2: If the random machine decides to look, the particle lands
Speaker 2: as a solid bullet, it acts like it only ever
Speaker 2: went through one door.
Speaker 1: But if the machine decides not to look.
Speaker 2: Then the particle lands as part of a beautiful wave pattern.
Speaker 2: It acts like it went through both doors.
Speaker 1: Even though the choice was made after it already went
Speaker 1: through the doors. Yes, how I mean, how do we
Speaker 1: make sense of that without breaking our brains? Are we
Speaker 1: changing the past?
Speaker 2: No, and that's the gentlest way to understand it. We
Speaker 2: aren't changing the past.
Speaker 1: Okay, so what is happening?
Speaker 2: It's just that the history of the particle simply wasn't written,
Speaker 2: not until the very moment we measured it.
Speaker 1: It didn't have a past exactly.
Speaker 2: The spinning coin was just spinning, floating in time. It
Speaker 2: wasn't heads and it wasn't tails, and it didn't have
Speaker 2: a solid history until the hand came down.
Speaker 1: Wow, it just hovered impossibility.
Speaker 2: It did. The universe leaves things undefined until they absolutely
Speaker 2: have to be defined. It's incredibly efficient in a way.
Speaker 1: It's beautiful. It's like the universe is a blank canvas
Speaker 1: and it only paints in the details right where you
Speaker 1: touch it.
Speaker 2: Yes, exactly that.
Speaker 1: I want to gently bring you, the listener, back to
Speaker 1: that hillside we started.
Speaker 2: On, just feeling the soft grass again.
Speaker 1: Looking up at those stars. How does it feel to
Speaker 1: know that you are part of a universe that is
Speaker 1: so fluid.
Speaker 2: So responsive.
Speaker 1: Yeah, so wonderfully undefined until it touches you, until you
Speaker 1: interact with it.
Speaker 2: It makes you feel very connected to it all, doesn't it.
Speaker 1: It really does. I want you to take another deep,
Speaker 1: slow breath.
Speaker 2: Just let the air out slowly.
Speaker 1: There is no need to figure it all out tonight.
Speaker 1: The universe is just a soft, spinning blur of.
Speaker 2: Possibilities, and right now all you need to do is rest.
Speaker 2: You don't have to observe anything.
Speaker 1: You can just let the waves wash over you. But
Speaker 1: before we drift off completely, there's one final quiet thought
Speaker 1: in our notes.
Speaker 2: A really beautiful step further into the mystery.
Speaker 1: Yeah, building on every thing we've talked about, the spinning coin,
Speaker 1: the hands slamming down.
Speaker 2: What if we ask one more question about that coin?
Speaker 1: Okay, what's the question if.
Speaker 2: When the hand slams down on the spinning coin, the
Speaker 2: universe doesn't actually force it to choose.
Speaker 1: Wait, what do you mean we just said it has
Speaker 1: to choose heads or tails?
Speaker 2: Right? But there is a view in physics called the
Speaker 2: many worldview decoherence.
Speaker 1: Okay, I'm listening.
Speaker 2: What if the universe simply splits splits like in two Yes,
Speaker 2: in one quiet stream of reality, the coin lands on
Speaker 2: heads and you are there to see it. Okay, But
Speaker 2: in another identical universe, just a breath away, the coin
Speaker 2: lands on tails.
Speaker 1: And another version of me is there to see that one.
Speaker 2: Yes, every possibility, every wave of that particle going through
Speaker 2: the less slit or the right slit, it all happens quietly,
Speaker 2: playing out in a vast, endless branching of night skies.
Speaker 1: An endless branching of night skies. That is just it's
Speaker 1: a staggering.
Speaker 2: Thought, every possible universe. Yeah, just resting side by side
Speaker 2: in the.
Speaker 1: Dark makes the night feel so much deeper. It does well,
Speaker 1: I hope that thought brings you a sense of peace tonight,
Speaker 1: leaving you cradled in this vast, gentle mystery of the
Speaker 1: quantum world.
Speaker 2: It's been a wonderful time exploring it with you, it
Speaker 2: really has.
Speaker 1: Thank you for joining us on this quiet exploration. Have
Speaker 1: a peaceful night.