0:00 You drop a glass, it shatters into a hundred pieces on the floor.0:05 You will wait for the entire age of the universe, and you will never, ever see those hundred pieces of glass spontaneously jump back together to reform the glass.0:17 Right.0:18 And that seems so obvious it’s almost not worth saying.0:22 But it reminds me of what we were exploring last week, Admin, with the slipperiness of measuring a feeling.0:31 Our experience of time is just as fundamental, just as one-way, but the 'why' of it...0:38 that's a whole other kind of puzzle.0:40 It's the central puzzle, I think.0:43 Because here's the thing that physicists have been wrestling with for over a century.0:50 Almost all of our fundamental laws—Newton's, Einstein's, even quantum mechanics—they are perfectly time-symmetric.0:59 Wait, what does that mean, symmetric?1:02 It means the laws work the same forwards and backwards.1:06 If you watch a video of two billiard balls colliding, you can't really tell if I'm playing the tape forward or in reverse.1:16 Both versions look physically plausible.1:19 The laws governing that collision don't have a preferred direction.1:24 Oh, wow.1:25 Okay, but my life is not a billiard ball collision.1:29 I remember yesterday.1:30 I don't remember tomorrow.1:32 I get older, not younger.1:34 So how can the basic rules be directionless when...1:38 well, when EVERYTHING we experience has a direction?1:43 Exactly!1:43 That is the question.1:45 That is the Arrow of Time.1:47 Why does the macroscopic world of glasses and eggs and people have such a strict, unbreakable forward-only rule, when the microscopic world it's built from...1:59 doesn't?2:00 So what's the leading theory?2:02 There has to be one.2:04 There is.2:05 And it's all about mess.2:07 Um, the formal term is entropy.2:09 The Second Law of Thermodynamics says that in a closed system, the total amount of disorder, or entropy, can only increase or stay the same.2:20 It never goes down.2:22 An egg unscrambling itself.2:24 An egg unscrambling is a massive decrease in entropy.2:28 It’s moving from a disordered, messy state—the scramble—to a highly ordered, separated state.2:35 The universe just doesn't tend to do that.2:39 It tends to go from ordered to messy.2:41 Your tidy bedroom gets chaotic; your hot coffee cools to room temperature.2:47 That increase in disorder gives time its arrow.2:51 Okay.2:51 So time is just a side effect of the universe getting progressively messier.2:57 That feels...2:58 a little bleak, but it makes a certain kind of sense.3:03 It makes a lot of sense.3:05 It's called the Thermodynamic Arrow of Time.3:08 But then you have to ask a deeper question.3:11 If the universe is always moving toward a state of maximum messiness...3:17 why wasn't it already there?3:19 Huh.3:20 For entropy to be increasing, it must have started out in a state of incredibly low entropy.3:27 Unimaginably ordered.3:28 The Big Bang had to be a moment of near-perfect order.3:33 And nobody knows WHY.3:34 So we've just replaced one mystery with an even bigger one.3:39 Welcome to cosmology!3:41 And it gets worse.3:42 Because that's just one arrow.3:45 There's also the Cosmological Arrow—the fact the universe is expanding, not contracting.3:51 And maybe a Quantum Arrow, related to how making a measurement seems to be an irreversible process.3:59 And are they all the same arrow?4:02 We don't know!4:03 That's the core of it.4:05 We don't know if the thermodynamic arrow and the cosmological arrow are two sides of the same coin, or if it's just a coincidence they happen to point in the same direction in our current era.4:20 There are theories where, in a contracting universe, time could run backwards.4:26 Whoa.4:26 So you could have a universe where people remember the future and get younger?4:33 It’s...4:33 theoretically debated.4:35 The problem is, these aren't just abstract physics questions.4:40 The Arrow of Time is what allows for cause and effect.4:44 It's the reason we can have memories, the reason we can learn.4:49 It's woven into the fabric of our consciousness.4:53 Because you can't have a memory of something that hasn't happened yet...4:58 unless the arrow was pointing the other way.5:02 Precisely.5:03 And so for me, this isn't just about why glass breaks.5:07 It's about why we can experience a 'now' at all.5:11 Why we seem to be moving from a known past into an unknown future.5:16 The universe began in a special, highly-ordered state for reasons we cannot explain.5:23 And every single moment of your life, every choice, every memory, is a downstream consequence of that one, primordial mystery.
