0:00 What if I told you that every time you look up at the night sky, you’re actually a time traveler looking into the past?0:08 Okay, I'm listening.0:10 That sounds like way more fun than my last flight.0:13 It's SO much better!0:15 Last week, we talked about the Standard Model, that amazing recipe for everything in the universe.0:22 But the thing that carries all the information between those particles, the cosmic messenger?0:29 That’s light.0:30 And light plays by one very, very weird rule that changes everything.0:35 The speed of light, right?0:37 I remember that from school, but it was just a number in a textbook.0:42 Right!0:42 A reaaally big number.0:44 About one hundred and eighty-six thousand miles per SECOND.0:48 That’s fast enough to circle the entire Earth seven and a half times in the time it takes to snap your fingers.0:56 Whoa.0:56 Okay, that’s… that’s hard to even picture.1:00 It is!1:00 But here's the part that breaks your brain.1:03 It’s not just fast.1:04 It’s the same speed for everyone, no matter what.1:08 It’s a universal constant.1:10 We call it 'c'.1:11 Wait, hold on.1:12 What do you mean, the same for everyone?1:15 If I’m on a train going a hundred miles an hour and I throw a baseball forward, someone on the ground sees it going faster than I do.1:25 Exactly!1:25 That’s how we think everything should work.1:28 But light is the universe’s one big exception.1:32 If you’re on a rocket ship going at, say, half the speed of light, and you turn on your headlights… The light should go one and a half times the speed of light, right?1:44 NO!1:44 That’s what’s so wild!1:46 You, inside the rocket, would see the light beam shoot away from you at the speed of light.1:53 And someone watching you from Earth would ALSO see that same light beam moving at the exact same speed of light.2:01 Not faster.2:02 That… makes no sense.2:03 How can it be moving away from you at full speed if you're already moving?2:08 One of those measurements has to be wrong.2:12 Or… something else has to change to make the math work.2:16 And that something else is time itself.2:18 Time?2:19 You’re telling me time slows down or speeds up?2:22 It does!2:23 This is the core of Einstein's theory of relativity.2:27 Because the speed of light MUST be constant for everybody, time has to be flexible.2:33 The faster you move through space, the slower you move through time relative to a stationary observer.2:40 So, like, the twin paradox?2:42 Where one twin goes on a space journey and comes back younger than the one who stayed on Earth?2:49 That’s the one!2:50 It’s not a paradox; it's just how the universe works.2:54 The astronaut twin literally experienced less time because they were moving so fast.3:00 Time stretched out for them.3:02 My brain is starting to feel a little stretched out.3:06 Good!3:06 It means you’re getting it!3:08 Now, let’s connect this back to you being a time traveler.3:13 Since light has a finite speed, it takes time for it to travel from an object to your eye.3:19 Okay, that makes sense.3:21 Like a delay.3:22 A delay, exactly.3:23 But over cosmic distances, that delay becomes… immense.3:27 The Sun is about ninety-three million miles away.3:31 It takes light about eight minutes and twenty seconds to travel from the Sun to us.3:37 So when I look at the Sun—which, don't worry, I'm not staring at it—I’m seeing it as it was eight minutes ago?3:45 PRECISELY!3:45 If the Sun were to just… vanish right this second, we wouldn’t know about it for another eight minutes.3:53 We’d be orbiting nothing, in total darkness, with no idea anything was wrong.3:59 Oh, that is an unnerving thought.4:01 But it’s also… kind of amazing.4:03 What about the stars?4:05 Oh, this is where it gets REALLY good.4:08 The nearest star system to us, Alpha Centauri, is about four point three light-years away.4:14 So a light-year is the distance light travels in a year?4:18 Yep!4:19 So when you look at Alpha Centauri, you are seeing it as it was four point three years in the past.4:26 You're seeing four-year-old starlight.4:29 Wow.4:29 So we’re never really seeing the universe as it is right now.4:33 We're always looking at a photograph from the past.4:37 ALWAYS.4:38 And the farther away you look, the older the photograph is.4:42 The Andromeda Galaxy is the most distant thing you can see with your naked eye.4:48 It’s two and a half MILLION light-years away.4:51 Wait.4:51 Two and a half million years?4:53 So the light we see from it tonight left that galaxy when our earliest human ancestors were first walking on Earth.5:02 Yes.5:02 Isn’t that just the most beautiful, mind-bending thing?5:06 That starlight traveled across intergalactic space for two and a half million years, past countless other stars and planets, and ended its journey right at this moment, in your eye.5:19 Every time you look up, you are connecting with a past so deep it’s almost unimaginable.5:26 You’re not just seeing light; you’re seeing history.5:30 You’re seeing time itself.
