0:00 Every single time you use GPS, you are proving Albert Einstein's theory of gravity is correct.0:06 You really are.0:07 Without his equations, your phone would think you're miles away from where you're actually standing.0:15 Which is just mind-bending.0:17 Last week we were talking about the big secrets of the universe, and I feel like this—gravity—is the one we think we know, but we REALLY don't.0:27 I mean, it's just the force that pulls stuff down, right?0:31 An apple falls on Newton's head, the end.0:34 Ah, see, that's the story we all learn!0:37 And for a long time, that was the best explanation we had.0:41 Isaac Newton imagined it as this invisible, mysterious rope pulling everything toward everything else.0:48 The bigger the object, the stronger the pull.0:52 And that works!0:53 I mean, it explains why the moon orbits the Earth and why I'm not floating out of my chair right now.1:00 It works incredibly well.1:02 It's good enough to send rockets to the moon.1:05 But, um, Einstein had a problem with it.1:08 The idea of this "spooky action at a distance," as he called it.1:13 How does the sun instantly tell the Earth where to go?1:17 There's no rope!1:18 Okay, yeah, when you put it like that...1:21 it does sound a little magical.1:23 So what was Einstein's big idea?1:25 He completely reframed the question.1:28 He said, what if gravity isn't a force at all?1:31 What if it's a consequence of the shape of the universe itself?1:36 Whoa.1:36 The shape of the universe?1:38 What does that even mean?1:40 Imagine a big, flat, stretchy sheet, like a trampoline.1:44 That's our universe—or more accurately, what scientists call spacetime.1:49 It’s the fabric of reality, combining the three dimensions of space and the one dimension of time.1:56 Okay, I'm with you.1:57 A giant, four-dimensional trampoline.2:00 Exactly.2:00 Now, let's put a heavy bowling ball in the middle of that trampoline.2:05 What happens?2:06 It sinks down, right?2:08 It creates a big curve, a big dip in the sheet.2:11 Precisely.2:12 That bowling ball is our sun.2:14 It's so massive that it's literally warping and bending the fabric of spacetime around it.2:21 Now...2:21 let's roll a little marble onto the trampoline, not directly at the bowling ball, but near it.2:28 That marble is the Earth.2:30 Ohhh, I see where this is going!2:32 The marble is going to start circling around the dip, right?2:36 It's not being pulled by the bowling ball...2:40 it's just following the curve in the sheet that the bowling ball made!2:45 YOU GOT IT!2:45 That's the core of General Relativity.2:48 The Earth isn't being pulled by the sun.2:51 The Earth is simply moving in what it thinks is a straight line, but it's moving through space that has been curved by the sun.3:00 An orbit is just an object following the straightest possible path through a bent environment.3:07 Gravity isn't a pull, it's the geometry of the cosmos.3:11 That is...3:12 so much more elegant.3:13 It's not a force, it's just...3:15 falling.3:16 We're constantly falling around the sun.3:19 But wait—how do we know this is true?3:21 The trampoline is a great analogy, but what's the proof?3:25 Oh, the proof is beautiful.3:27 For one, Einstein's math perfectly predicted the weird orbit of the planet Mercury, which Newton's laws never quite could.3:36 But the real showstopper?3:38 Gravity bends light.3:40 Hold on.3:40 Light doesn't have any mass.3:42 Newton's gravity shouldn't affect it at all!3:45 Correct!3:46 But light has to travel through spacetime.3:49 And if spacetime itself is bent by a massive star or a galaxy, the light has to follow that curve.3:56 During a solar eclipse in 1919, astronomers saw that the stars behind the sun appeared to be in the wrong place, because their light was being bent as it passed by the sun's immense gravity.4:10 Einstein was right.4:11 So this isn't just an abstract theory about stars and planets.4:16 You said it affects my GPS.4:18 How?4:18 Well, because mass curves spacetime, it also affects the flow of time.4:23 Time literally runs a tiny bit slower in stronger gravity.4:27 Seriously?4:28 So time is running slower for my feet than for my head?4:32 Infinitesimally, but yes!4:34 The GPS satellites are way up high, where Earth's gravity is weaker.4:39 So their clocks tick just a little bit faster than our clocks down here on the surface.4:45 It's a tiny difference—a few microseconds a day—but if the GPS system didn't constantly correct for this effect from General Relativity, its directions would be off by several miles within a single day.4:59 Wow.5:00 So this huge, cosmic idea about the shape of reality is something we're using every day, in our pockets, just to find the nearest coffee shop.5:10 It's not just a theory, it's...5:12 engineering.5:13 It's the language the universe is written in.5:16 From the tiniest wobble in a planet's orbit to the path light takes across billions of years.5:23 It’s all just geometry.5:25 And the most stunning part is that we, these tiny beings on this little rock, managed to read it.5:32 It makes you wonder what other fundamental truths about the universe are just hiding in plain sight, waiting for someone to imagine a different kind of trampoline.
