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Entropy and the Arrow of Time — Why Yesterday Is Different From Tomorrow · Talking Science

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You know, there is this this question that is
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just so obvious, so deeply baked into our everyday
0:05
existence that we almost never bother to ask
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it. Right. And that question is simply this.
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Why does time only flow in one direction? It
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really is the ultimate unquestionable fact of
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reality, isn't it? I mean, you remember yesterday,
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but you have absolutely no memory of tomorrow.
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Exactly. Think about dropping like a delicate
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wine glass onto a hard tile floor. Oh, yeah.
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It shatters into a hundred pieces. And you could
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just sit there on the floor staring at those
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pieces and wait until the literal end of the
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universe. And you'll never see those shugs spontaneously
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leap back up and, you know, reassemble into a
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perfect unbroken glass in your hand. Right. Yeah.
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Time flows one way, past a future. It's a one
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-way street. It feels entirely intuitive to us
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as human beings living in this macroscopic world.
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But when you look at the source material we're
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diving into today, you realize this everyday
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experience completely masks a genuine mind -bending
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paradox. Which is incredibly counterintuitive
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when you first hear it. Right. Because almost
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all the fundamental laws of physics, like the
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mathematical rules governing how particles move
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and interact, they work perfectly well in either
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direction of time. Okay, give me an example.
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Well, if you film two billiard balls rolling
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across a table and colliding, and then you play
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that film backward for a physicist, the backward
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version obeys the exact same laws of motion as
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the forward version. Oh, wow. Yeah. There's absolutely
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nothing in the underlying physics of those particles
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or the atomic collisions taking place that says,
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hey, this is forward and this is backward. So
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if the underlying rulebook of the universe doesn't
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actually care about the direction of time, why
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doesn't reality work in reverse? That is the
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big question. Right. Where does that one -way
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arrow come from? That is our mission for today's
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deep dive. We are going to explain the arrow
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of time for you by unpacking one of the most
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profound yet heavily misunderstood ideas in all
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of science. Yeah, we are talking about entropy.
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Entropy. And entropy carries a lot of baggage
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as a word. People usually hear it and immediately
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associate it with like disorder or decay. Okay,
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let's unpack this. We really need to redefine
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entropy. stripping away all that scientific jargon
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and the common misconceptions. Right. Because
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they can really lead you astray. Because if you
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look at older textbooks, they often use this
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analogy of a teenager's messy bedroom to explain
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entropy. Right. The classic messy room. But that
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implies there is some invisible malevolent force
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out there actively tearing things apart or making
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a mess of the universe. Yeah. And the messy bedroom
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metaphor is highly misleading. I mean, it assumes
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a human value judgment on what messy even means.
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We really need a cleaner, more rigorous way to
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see it. Entropy isn't actually about messiness
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or decay at all. It's fundamentally about math.
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Just math. Specifically, it's a measure of probability.
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It is about counting how many microscopic ways
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you can arrange a system while keeping the same
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macroscopic appearance. Okay, so to make this
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concrete, our sources use a really brilliant
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analogy. Imagine a brand new deck of playing
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cards. A fresh deck. Yeah, when you first open
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the box, the deck is in perfect factory order.
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It's a ranged ace through king, grouped perfectly
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by suit. Beautifully organized. Right. And in
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the entire universe of mathematical possibilities,
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there's exactly one way for that deck to be in
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that specific pristine order. Just one single
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microscopic arrangement out of everything possible.
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But now take that deck and give it a thorough
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shuffle. Okay. How many possible arrangements
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can those 52 cards fall into? The number is staggering.
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It really is. It is roughly 80 unvigentillion.
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Which is a ridiculous word. It is. That is an
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80 followed by 67 zeros, a 68 -digit number of
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possible shuffled orders. What's fascinating
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here is the underlying mechanism. When you shuffle
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those cards, why does the deck become disordered?
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Right. Why? It's not because your hands are applying
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a magical destructive force to the paper. It
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becomes disordered purely because there is only
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one way to be perfect and a mind -bendingly huge
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number of ways to be mixed up. So it's just statistical
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probability. If you pull a random card order
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from that 68 digit number of possibilities, it's
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going to look to the human eye like a random
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meaningless jumble. Exactly. Mess is overwhelmingly
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more probable simply because there is vastly
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more of it to land on, like trying to roll a
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million dice and getting all sixes versus just,
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you know, a random mix of numbers. Yeah, that's
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a great way to think about it. And if you prefer
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a physics example, imagine a single drop of dense
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black ink gently squeezed into a glass of perfectly
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still water. Okay, I can picture that. Initially,
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all the ink molecules are clumped together in
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one tight little sphere right in the center of
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the water. That is a state of low entropy. Because
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there's only a few ways to organize them like
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that. Right. There are very few ways for the
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molecules to arrange themselves and still maintain
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that tight, perfect spherical shape. But the
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water molecules in the glass, they're constantly
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jittering and bouncing around, right? They bump
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into the ink molecules. Exactly. As they bump,
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the ink molecules drift. And over time, they
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spread out until the entire glass is just this
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uniform, pale gray. Yeah. Why doesn't the gray
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water ever spontaneously pull all the ink back
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into a single dark drop? Because the odds are
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just terrible. Yes, because there are trillions
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upon trillions of microscopic ways for the ink
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to be evenly spread out. and only a tiny handful
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of ways for it to be perfectly clumped together.
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So high entropy simply means there are vastly
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more ways to be like this. That's the perfect
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way to summarize it. The shuffled deck, the gray
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water, they have high entropy because there are
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so many microscopic paths to achieve that overall
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look. The sorted deck, the tight drop of ink,
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they have low entropy. It really is just a game
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of astronomical probabilities. Which brings us
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to one of the most bulletproof statements in
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all of science. The second law of thermodynamics.
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Okay, here we go. Now, I know thermodynamics
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can sound kind of intimidating, but the second
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law just formalizes the math we just discussed.
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It states that in any isolated system, meaning
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a system completely sealed off, where no energy
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or matter can enter or escape entropy, tends
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to increase over time. So things inevitably move
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from less probable ordered arrangements toward
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more probable disordered ones? Not because the
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universe actively wants to be chaotic. Right.
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Simply because the disordered states are so overwhelmingly
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more numerous that landing in one is a statistical
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certainty at the macroscopic level. Let's bring
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this down to earth with a perfect everyday example
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from our sources. Think about a cup of hot coffee
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sitting on a table in an ordinary room. Ah, a
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very relatable everyday scenario. Absolutely.
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When that coffee is freshly poured, you have
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a highly specific arrangement of energy. Because
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temperature is essentially just a measure of
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how fast atoms and molecules are moving. Right.
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So in your fresh cup of coffee, you have highly
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energetic, fast -moving molecules tightly concentrated
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inside the ceramic cup. Meanwhile, out in the
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rest of the room the air molecules are moving
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much slower. And in the language of thermodynamics,
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this is a highly ordered, unusual separation
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of energy. The fast stuff is all corralled in
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one tiny area, and the slow stuff is everywhere
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else. It's segregated. Exactly. This is a low
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entropy state, exactly like the drop of ink before
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it diffuses. But if you just leave that coffee
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alone, we all know what happens. The fast -moving
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molecules in the coffee are constantly bouncing
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around. They slam against the ceramic wall of
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the mug. Yeah, doing physical work. Right. That
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kinetic energy causes the ceramic molecules to
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vibrate faster. And they, in turn, slam into
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the slower air molecules hovering right next
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to the mug. So they are sharing their kinetic
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energy through billions of microscopic collisions
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every fraction of a second. Just bumping and
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bumping. Yeah. And as they share that energy,
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the fast molecules slow down and the slow molecules
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speed up. The heat diffuses until the coffee,
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the mug, and the room are all the exact same
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lukewarm temperature. The energy is spread out
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completely evenly. Hold on, I have to stop you
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there. Okay. Because we just established at the
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beginning of our deep dive that the fundamental
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laws of physics are reversible. So when a hot
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coffee molecule hits the mug wall and transfers
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its momentum, that specific individual collision
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works perfectly in reverse according to the equations
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of motion. The underlying mechanics of the collision
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are completely time symmetric. Yes. So... Why
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can't the reverse happen? I mean, if the equations
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don't have a time direction baked in, saying
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it's just probability feels almost like a cop
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-out. It feels wrong, I know. It really feels
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like we're missing a fundamental law. You were
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telling me there is no hard physical barrier
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preventing the lukewarm air in my office from
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spontaneously bumping into my mug in exactly
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the right sequence to reheat my coffee and leave
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the room freezing cold. It sounds totally absurd,
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but it's true. There is absolutely no underlying
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law of physics. that forbids the room's ambient
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energy from spontaneously concentrating back
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into your cup. The laws of motion completely
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allow for your coffee to reheat itself. But I
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have stared at a lot of cold cups of coffee in
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my life. Haven't we all. And not once has one
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spontaneously started boiling on my desk. And
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it never will. This is where probability at the
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scale of Avogadro's number, the sheer quantity
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of atoms in a cup of coffee becomes an ironclad
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law. Because the numbers are so big. Exactly.
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The number of ways for the energy to be spread
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out in the room is so unfathomably larger than
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the number of ways for it to be concentrated
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in the cup that the odds of the reverse happening
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are functionally zero. Wow. If you waited trillions
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of times the current age of the universe, you
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still wouldn't see it happen. The coffee cools
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for the exact same reason the deck becomes messy
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when you shuffle it. The math is very incredibly
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stacked against it that statistical drift becomes
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indistinguishable from a physical law. So what
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does this all mean for our big mystery? We started
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out trying to figure out why time only moves
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forward. Well, understanding that the cooling
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coffee is a one -way street of probability brings
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us right back to our original riddle. This statistical
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drift is what gives us the arrow of time. Okay,
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I see. The individual laws of physics, the microscopic
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billiard ball collisions, they don't distinguish
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past from future. But entropy does. The future
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is literally defined as the direction in which
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entropy is higher. Exactly. The future is the
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direction of more probable, more spread out arrangements
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of matter and energy. The past is defined as
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the direction of lower entropy, the more concentrated
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ordered states. The shattered glass on the floor,
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the diffused ink in the water, the lukewarm coffee
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on the desk, they define the direction of the
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future because they represent an increase in
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entropy. Okay, so we've established the physical
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environment, but if I can push a step further,
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how does this mathematical probability actually
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force my psychological reality to experience
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time moving forward? Like, why do I only remember
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yesterday? This raises a really important question,
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and it takes us right into the biology and physics
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of the human brain. Oh, interesting. Because
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the two are deeply intertwined. Forming a memory
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is not some abstract, ethereal concept. It is
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a physical process. To write information into
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your brain, your neurons have to change state.
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They have to pump ions across membranes, forge
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new synaptic connections, and literally alter
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their physical structure. It's like writing a
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1 or a 0 onto a computer's hard drive. It requires
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physical work. Exactly that. There is this principle
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in physics known as Landauer's principle, which
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states that any mathematically irreversible operation
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like erasing a piece of information or locking
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a new memory state into place has a minimum thermodynamic
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cost. Meaning what exactly? It physically requires
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energy, and in doing so, it generates a tiny
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bit of heat. So to write a memory of this conversation
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into my physical neurons right now, my brain
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has to expend metabolic energy. My brain physically
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heats up by some microscopic fraction of a degree.
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Yeah, and your skull radiates that heat out into
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the surrounding room. By generating that heat
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and releasing it into the wider environment,
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you are actively increasing the overall entropy
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of the universe. That is wild. The very mechanism
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of recording the past fundamentally requires
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dumping heat into the future. So I physically
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cannot form a memory without contributing to
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the universe's drift toward disorder. The arrow
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of time we experience psychologically is just
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the arrow of entropy. Remembering, aging, the
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fact that a cause always proceeds in effect.
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Every single process that distinguishes before
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from after rides on the back of entropy climbing.
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Time, in a very real physical sense, points from
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order toward mess. Which is profound, but it
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naturally forces another massive question. If
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the universe is a one -way street constantly
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drifting toward higher entropy, that implies
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the past had much lower entropy. It does. So
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if you rewind the clock all the way back, why
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was the universe ordered to begin with? If we
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connect this to the bigger picture, it leads
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us to the very birth of the cosmos. Our universe
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began 13 .8 billion years ago in the Big Bang.
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And for reasons physicists are still actively
13:11
debating today, The initial state of the Big
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Bang was incredibly dense, incredibly hot, and
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possessed an astonishingly low entropy. The universe
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started as the ultimate, pristine, factory -ordered
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deck of cards. Beautiful way to put it. And it
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has been shuffling itself ever since. All the
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complex phenomena we see today, galaxies forming,
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stars igniting, planets coalescing, they are
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just the universe unwinding from that highly
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concentrated initial state, slowly spreading
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its energy out across the extending void. But
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when you follow that arrow far enough into the
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future, it leads somewhere incredibly dark. Physicists
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in the 19th century were actually the first to
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grasp this implication, and it remains the ultimate
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logical endpoint of the second law. Because if
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entropy always increases, then everything in
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the cosmos is essentially running down. The hot
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stars are radiating their concentrated heat into
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the freezing vacuum of space. The massive differences
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in temperature that drive the cosmos are forever
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being smoothed away. If you wind that forward
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far enough, you reach a truly bleak endpoint,
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a universe where all the stars have burned out,
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all the black holes have evaporated, and all
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the energy is perfectly evenly spread out across
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a massive expanding void. Just a giant dark equilibrium.
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Yeah, where everything is the exact same temperature
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everywhere. In physics, this uniform, unchanging
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equilibrium was christened the heat death of
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the universe. Because when everything is the
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exact same temperature, absolutely nothing interesting
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can ever happen again. Right. You need a gradient.
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Exactly. To do work, to power an engine, to drive
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a chemical reaction or to sustain biological
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life, you need a gradient. You need a difference
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in temperature. You need a hot place and a cold
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place so energy can flow between them. Without
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differences, the universe is just a static, featureless
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soup. Time itself effectively loses its meaning
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because... Nothing changes from one moment to
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the next. It is the ultimate consequence of probability.
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It really is. The heat death is simply the most
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mathematically likely arrangement of a universe's
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worth of energy. But to put this into perspective,
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we are talking about time scales that defy human
15:19
comprehension. Right. No need to panic yet. No,
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we're talking trillions upon trillions of years
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into the future. The current 13 .8 billion year
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age of the universe is not even a microscopic
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fraction of a percent of the time it will take
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to reach thermodynamic equilibrium. Okay, so
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the heat death is an unimaginably distant problem.
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But here's where it gets really interesting,
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because looking at the world around me right
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now, I find myself wanting to mount a massive,
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hopeful counter -argument to this whole concept
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of relentless decay. Oh, let me guess. You're
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thinking about biology. Yes. Look at you and
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me. Look at the listener going about their day.
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If the universe is constantly breaking down into
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a featureless, chaotic soup, how do we explain
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the complex, highly ordered existence of life?
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A living organism is exquisitely ordered. It
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is arguably the most complex, low -entropy, localized
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structure we know of in the entire universe.
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We build complex, highly structured biological
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bodies out of simple inputs. Like, think about
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a towering redwood tree. Okay. It starts as a
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tiny seed. And it builds a massive, structurally
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perfect, living wooden tower using nothing but
16:27
thin air, liquid water, and some photons from
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the sun. That's incredible. It is taking chaos
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and turning it into structure. Aren't living
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things actively decreasing entropy? Aren't we
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walking, talking defiances of the second law
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of thermodynamics? It is a wonderful observation,
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and it's the ultimate intuitive pushback against
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the bleakness of entropy. But the answer is deeply
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illuminating. Okay, get me. To understand it,
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we have to look closely at the fine print of
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the rules. Remember the crucial caveat I mentioned
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when I first defined the second law? You said
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it applies to an isolated system, a sealed box
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where nothing gets in or out. Yes. Yeah. And
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a living thing is the exact opposite of an isolated
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system. You are an open system. You interact
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vigorously with your environment. I am constantly
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taking things in and pushing things out. Exactly.
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You maintain your beautiful, highly ordered internal
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state. Only by constantly absorbing incredibly
17:21
low entropy energy from the outside, you eat
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complex carbohydrates and proteins. Right. The
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redwood tree absorbs highly concentrated, low
17:29
entropy sunlight. This is premium structured
17:32
fuel. And what do we do with that premium fuel?
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Your cellular machinery extracts the energy it
17:36
needs to maintain your internal structure, repair
17:39
your DNA, pump your blood. But the thermodynamic
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cost of doing all that work is high. Once you've
17:45
extracted the useful energy, you dump massive
17:47
amounts of higher entropy waste back into your
17:50
environment. We exhale carbon dioxide, we produce
17:52
physical waste, and most importantly, we radiate
17:55
body heat. And that heat you radiate is dissipated,
17:58
disorganized energy. You are an island of complex
18:01
order, yes. But you essentially purchase that
18:04
internal order by acting as an engine of chaos.
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You increase the entropy of your surroundings
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far more than you decrease it within your own
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skin. So life doesn't violate the second law
18:13
of thermodynamics. We are spectacular local exceptions
18:17
funded by a cosmic credit card. That's a great
18:19
analogy. When you add up the whole ledger you
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plus your surrounding environment, the total
18:23
entropy of the system still goes up, exactly
18:26
as the mathematical probability demands. Life
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is actually a spectacularly efficient machine
18:31
for obeying the second law. We consume concentrated
18:34
energy and rapidly convert it into dispersed
18:37
heat. And the ultimate source funding all of
18:40
this local order on Earth, the bank issuing that
18:42
cosmic credit card, is the sun. The sun is a
18:45
massive ball of low entropy energy. It pours
18:49
incredibly concentrated visible light out into
18:52
the solar system. Life on Earth intercepts a
18:55
tiny fraction of that pristine flow. uses it
18:58
to build forests and ecosystems and civilizations,
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and then the Earth eventually radiates all that
19:02
energy back out into deep space as diffuse, high
19:06
entropy infrared heat. The universe's books always
19:09
balance in entropy's favor. Always. We are just
19:11
spectacular little biological machines surfing
19:14
this massive cosmic wave of energy dispersal.
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Yeah. Let's gather up all these threads for you,
19:19
the listener, because this has been quite the
19:20
journey today. It really has. We started with
19:23
the mundane mystery of time flowing in one direction,
19:25
and we found our answer by fundamentally reframing
19:28
entropy. Entropy is not some invisible, destructive
19:31
force of decay. It is simply math. Just math
19:35
and probability. Right. It is the inescapable
19:37
statistical probability that there are vastly
19:39
more ways for a system to be mixed up and spread
19:41
out than there are for it to be highly concentrated
19:44
and arranged. And that statistical drift is so
19:47
incredibly reliable, backed by numbers so unfathomably
19:51
large, that it governs the physical universe
19:54
as an ironclad law. Out of that mathematical
19:57
near certainty. comes the direction of time itself.
20:00
It is the reason the past is fixed and the future
20:03
is open. It's the reason the dropped wine glass
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shatters instead of mending, the reason the ink
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spreads in the water, and the reason we all get
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a little bit older, not younger, with every passing
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day. I think the most beautiful takeaway from
20:17
this deep dive is how it changes your perspective
20:19
on the mundane world around you. The next time
20:22
you are sitting at your desk and you watch your
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coffee go cold, you aren't just watching a hot
20:26
drink equalize with the room temperature. Right.
20:28
It's so much bigger than that. You are witnessing
20:30
the exact same cosmic principle that points the
20:33
entire universe from its ordered Big Bang past
20:35
toward its mixed up dispersed future. You are
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watching the arrow of time, one lukewarm cup
20:41
at a time. But I want to leave you with one final,
20:44
slightly provocative thought to mull over, building
20:47
on what we've unpacked today about human memory
20:49
and heat. We learned that our brain's physical
20:52
ability to record the past relies entirely on
20:55
expelling heat and increasing the universe's
20:57
disorder. Right. Landauer's principle. Exactly.
21:00
So does that mean our deeply felt conscious experience
21:03
of time passing isn't actually a fundamental
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feature of reality? Is it just an artifact of
21:09
our biological hard drives surfing on a wave
21:11
of increasing messiness? That is a deep thought.
21:14
Because if a being existed outside of this...
21:17
Thermodynamic drift -like, a theoretical entity
21:20
whose consciousness wasn't dependent on chemical
21:22
reactions and entropy generation, how would they
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experience existence? Could they experience all
21:28
of time at once? past, present, and future laid
21:30
out simultaneously like a vast, static landscape
21:33
rather than a one -way street. It's a concept
21:36
that completely bends the mind, and it is exactly
21:38
why we love doing these deep dives. Remember
21:40
to keep questioning the obvious things around
21:42
you. The falling glass, the ticking clock, the
21:45
cooling coffee. There's a lot of magic in the
21:47
ordinary. Sometimes the most mundane things hold
21:49
the keys to the most profound cosmic secrets.
21:53
Thanks for joining us on this deep dive, and
21:55
we'll catch you next time.