Into the Universe with Stephen Hawking 1x2

1. Hello.

2. My name is Stephen Hawking...

3. physicist, cosmologist...

4. and something of a dreamer.

5. Although I cannot move...

6. and I have to speak through a computer...

7. in my mind, I am free.

8. Free to explore the universe and ask the big questions...

9. such as, is time travel possible?

10. Can we open a portal to the past?

11. Or find a shortcut to the future?

12. Can we ultimately use the laws of nature

13. to become masters of time itself?

14. Check it out.

15. Time travel was once considered scientific heresy.

16. I used to avoid talking about it

17. for fear of being labeled a crank.

18. But these days, I'm not so cautious.

19. In fact, I'm more like the people who built Stonehenge.

20. I'm obsessed by time.

21. If I had a time machine...

22. I'd visit Marilyn Monroe in her prime.

23. Or drop in on Galileo

24. as he turned his telescope to the heavens.

25. Perhaps I'd even travel to the end of the universe

26. to find out how our whole cosmic story ends.

27. To see how this might be possible,

28. we need to look at time as physicists do.

29. As the fourth dimension.

30. It's not as hard as it sounds.

31. All physical objects, even me and my chair,

32. exist in three dimensions.

33. Everything has a width and a height and a length.

34. But there is another kind of length...

35. a length in time.

36. While a human may survive for 80 years,

37. these stones will last much longer...

38. for thousands of years.

39. And the solar system will last for billions of years.

40. Everything has a length in time as well as space.

41. Traveling in time

42. means traveling through this fourth dimension.

43. To see what that means,

44. let's do a bit of normal everyday traveling

45. just to get a feel for it.

46. A fast car makes it a bit more fun.

47. Drive in a straight line,

48. and you're traveling in one dimension.

49. Turn right or left, and you add the second dimension.

50. Drive up or down a twisty mountain road,

51. and that adds height.

52. So that's traveling in all three dimensions.

53. But how on earth do we travel in time?

54. How do we find a path through the fourth dimension?

55. Let's indulge in a little science fiction for a moment.

56. Time-travel movies often feature a vast energy-hungry machine.

57. The machine creates a path through the fourth dimension,

58. a tunnel through time.

59. A time traveler...

60. a brave, perhaps foolhardy individual

61. prepared for who knows what, steps into the time tunnel

62. and emerges who knows when.

63. The concept may be far-fetched.

64. And the reality may be very different than this.

65. But the idea itself is not so crazy.

66. Physicists have been thinking about tunnels in time, too.

67. But we covered it from a different angle.

68. We wonder if portals to the past or the future

69. could ever be possible within the laws of nature.

70. As it turns out, we think they are.

71. What's more, we've even given them a name.

72. Wormholes.

73. The truth is that wormholes are all around us,

74. only they're too small to see.

75. Wormholes are very tiny.

76. They occur in nooks and crannies in space and time.

77. You might find it a tough concept, but stay with me.

78. Nothing is flat or solid.

79. If you look closely enough at anything,

80. you'll find holes and wrinkles in it.

81. It's a basic physical principle, and it even applies to time.

82. Take this pool table.

83. The surface looks flat and smooth.

84. But up close, it's actually anything but.

85. It's full of gaps and holes.

86. Even something as smooth as a pool ball

87. has tiny crevices, wrinkles, and voids.

88. Now, it's easy to show that this is true

89. in the first three dimensions.

90. But trust me, it's also true of the fourth dimension as well.

91. There are tiny crevices, wrinkles, and voids in time.

92. Down at the smallest of scales,

93. smaller even than molecules, smaller than atoms,

94. we get to a place called the quantum foam.

95. This is where wormholes exist.

96. Tiny tunnels or shortcuts through space and time

97. constantly form, disappear, and re-form

98. within this quantum world.

99. And they actually link two separate places

100. and two different times.

101. Unfortunately, these real-life time tunnels

102. are just a billion trillion trillionths

103. of a centimeter across.

104. Way too small for a human to pass through.

105. But here's where the notion of wormhole time machines

106. is leading.

107. Some scientists think it may be possible to capture one

108. and enlarge it many trillions of times

109. to make it big enough for a human

110. or even a spaceship to enter.

111. Given enough power and advanced technology,

112. perhaps a giant wormhole could even be constructed in space.

113. I'm not saying it can be done, but if it could be,

114. it would be a truly remarkable device.

115. One end could be here near the earth,

116. and the other far, far away, near some distant planet.

117. Theoretically, a wormhole could do even more.

118. If both ends were in the same place

119. and separated by time instead of distance,

120. a ship could fly in and come out still near the earth,

121. but in the distant past.

122. Maybe dinosaurs would witness

123. the ship coming in for a landing.

124. Now, I realize that thinking in four dimensions is not easy

125. and that wormholes are a tricky concept

126. to wrap your head around.

127. But hang in there.

128. I've thought up a simple experiment

129. that could reveal if human time travel through a wormhole

130. is possible now...

131. or even in the future.

132. I like simple experiments and champagne.

133. So I've combined two of my favorite things

134. to see if time travel from the future to the past

135. is possible.

136. I'm throwing a party...

137. a welcome reception for future time travelers.

138. But there's a twist.

139. I am not letting anyone know about it

140. until after the party has happened.

141. Here is the invitation,

142. giving the exact coordinates in time and space.

143. I am hoping copies of it, in one form or another,

144. will survive for many thousands of years.

145. Maybe one day someone living in the future

146. will find the information

147. and use a wormhole time machine to come back to my party,

148. proving that time travel will one day be possible.

149. My time-traveler guests

150. could be arriving any moment now.

151. 5...

152. 4...

153. 3...

154. 2...

155. 1.

156. What a shame.

157. I was hoping a future Miss Universe

158. was going to step through the door.

159. So, why didn't the experiment work?

160. I think one of the reasons

161. might be because of a well-known problem

162. with time travel to the past... the problem of paradoxes.

163. Paradoxes are fun to think about.

164. The most famous one is usually called the grandfather paradox.

165. I have a new, simpler version

166. I call the mad-scientist paradox.

167. I don't like the way scientists in movies

168. are often described as mad.

169. But in this case, it's true.

170. This chap is determined to create a paradox,

171. even if it costs him his life.

172. Imagine somehow he's built a wormhole...

173. a time tunnel that stretches just one minute into the past.

174. It may not sound like much,

175. but even one minute of time travel can cause real trouble.

176. Through the wormhole, the scientist can see himself

177. as he was one minute ago.

178. But what if our scientist uses the wormhole

179. to shoot his earlier self?

180. He's now dead,

181. killed before he'd even finished assembling the pistol.

182. So who fired the shot?

183. It's a paradox.

184. It just doesn't make sense.

185. It's the sort of situation

186. that gives cosmologists nightmares.

187. This kind of time machine would violate a fundamental rule

188. that governs the entire universe...

189. that causes happen before effects

190. and never the other way around.

191. I believe things can't make themselves impossible.

192. If they could, then there'd be nothing

193. to stop the whole universe from descending into chaos.

194. So I think something will always happen

195. that prevents the paradox.

196. Somehow there must be a reason

197. why our scientist will never find himself

198. in a situation where he could shoot himself.

199. And in this case, I'm sorry to say,

200. the wormhole itself is the problem.

201. In the end, I think a wormhole like this one can't exist.

202. And the reason for that is feedback.

203. If you've ever been to a rock gig,

204. you'll probably recognize this screeching noise.

205. It's feedback.

206. What causes it is simple.

207. Sound enters the microphone.

208. It's transmitted along the wires...

209. made louder by the amplifier...

210. and comes out at the speakers.

211. But if too much of the sound from the speakers

212. goes back into the mike...

213. it goes 'round and 'round in a loop,

214. getting louder each time.

215. If no one stops it,

216. feedback can destroy the sound system.

217. I think the same thing will happen with a wormhole,

218. only with radiation instead of sound.

219. As soon as the wormhole expands,

220. natural radiation will enter it and end up in a loop.

221. The feedback will become so strong,

222. it destroys the wormhole.

223. So, although tiny wormholes do exist

224. and it may be possible to inflate one someday,

225. it won't last long enough to be of use as a time machine.

226. That's the real reason no one came to the party.

227. In fact, I believe any kind of time travel to the past,

228. through wormholes or any other method,

229. is probably impossible.

230. Otherwise, paradoxes would occur.

231. So, sadly, it looks like...

232. time travel to the past is never going to happen.

233. A disappointment for dinosaur hunters

234. and a relief for historians.

235. But the story's not over yet.

236. This doesn't make all time travel impossible.

237. I do believe in time travel.

238. Time travel to the future.

239. Time flows like a river.

240. And it seems as if each of us is carried relentlessly along

241. by time's current.

242. But time is like a river in another way.

243. It flows at different speeds in different places.

244. And that is the key to traveling into the future.

245. The idea was proposed by Albert Einstein...

246. over 100 years ago.

247. He realized that there should be places

248. where time slows down

249. and others where time speeds up.

250. He was absolutely right,

251. and the proof is right above our heads...

252. up in space.

253. This is the global positioning system,

254. or GPS...

255. a network of 31 satellites in orbit around the earth.

256. The satellites make satellite navigation possible.

257. But they also reveal that time runs faster up here

258. than it does down on Earth.

259. Inside each spacecraft is a very precise clock.

260. But despite being so accurate,

261. they all gain around a third of a billionth of a second

262. every day.

263. The system has to correct for the drift.

264. Otherwise that tiny difference would upset the whole system,

265. causing every GPS device on Earth

266. to go out by about six miles a day.

267. You can just imagine the mayhem that that would cause.

268. The problem doesn't lie with the clocks.

269. They run fast because time itself runs faster here

270. than it does down below.

271. And the reason for this extraordinary effect

272. is the mass of the earth.

273. Einstein realized that matter drags on time,

274. slows it down like the slow part of a river.

275. The heavier the object, the more it drags on time.

276. And this startling reality is what opens the door

277. to the possibility of time travel to the future.

278. I admit this is a difficult concept to understand,

279. so let's take a simple example.

280. This is the Great Pyramid of Giza.

281. It weighs over 40 million tons.

282. And, like all heavy things, it's actually slowing down time.

283. The effect is small,

284. billions of times smaller than that of the earth.

285. But if we exaggerate it drastically...

286. you can see the principle at work.

287. Close to the pyramid, everything is slowed down,

288. again, like the sluggish part of a river.

289. Here time itself is passing slower

290. compared to how it's passing further away.

291. But what if people near the pyramid look outwards?

292. They must see the opposite effect.

293. Because they are slowed down,

294. they must see time in the distance as running fast.

295. It's a simple result of the mass of the pyramid.

296. This distortion opens the door

297. to the possibility of time travel.

298. So what we need to really travel in time

299. is something much more massive than a pyramid.

300. And I know just the thing.

301. Right in the center of the Milky Way,

302. 26,000 light-years from us,

303. lies the heaviest object in the entire galaxy,

304. hidden by a vast cloud of gas and stars.

305. It's a super-massive black hole

306. containing the mass of 4 million suns

307. crushed down into a single point by its own gravity.

308. The closer you get to the black hole,

309. the stronger the gravity.

310. Get really close, and not even light can escape.

311. So it's wrapped in a sphere of darkness

312. 15 million miles in diameter.

313. A black hole like this one has a dramatic effect on time,

314. slowing it down far more than anything else in the galaxy.

315. That makes it a natural time machine.

316. I like to imagine how a spaceship

317. might someday be able to take advantage

318. of this spectacular phenomenon.

319. Of course, it would first have to avoid being sucked in.

320. The trick, I think, would be to aim just off to the side

321. so they'd miss it.

322. They'd have to be on exactly the right trajectory and speed,

323. or they'd never escape.

324. Get it right, and the ship would be pulled into orbit,

325. a giant circle 30 million miles in diameter.

326. Here it would be safe.

327. Its speed would be enough

328. to keep it from falling any further in.

329. If a space agency were controlling the mission

330. from Earth or anywhere else far away from the black hole,

331. they'd observe that each full orbit took 16 minutes.

332. But for the brave people on board,

333. close to this massive object...

334. time would be slowed down.

335. And here the effect would be far more extreme

336. than near the pyramid or planet Earth.

337. The crew's time would be slowed down by half.

338. For every 16-minute orbit,

339. they'd only experience 8 minutes of time.

340. 'Round and 'round they'd go, experiencing just half the time

341. of everyone far away from the black hole.

342. The ship and its crew would be traveling through time.

343. Imagine they circle the black hole for five of their years.

344. 10 years would pass elsewhere.

345. When they got home, everyone on Earth

346. would have aged five years more than they had.

347. The crew of the spacecraft would return to a future Earth.

348. They would have made a journey not only in space, but in time.

349. So a super-massive black hole is a time machine.

350. But, of course, it's not exactly practical.

351. It has advantages over wormholes

352. in that it doesn't provoke paradoxes.

353. Plus, it won't destroy itself in a flash of feedback.

354. But it's pretty dangerous.

355. It's a long way away.

356. And it doesn't even take us very far into the future.

357. Fortunately, there is another way to travel in time,

358. and this represents our last and best hope

359. of building a real time machine.

360. Traveling through the fourth dimension

361. will never be a walk in the park.

362. But it turns out

363. there is a surprisingly straightforward way to do it.

364. You just have to travel very, very fast...

365. much faster than even the high speed

366. required to keep out of a super-massive black hole.

367. This is because of another strange fact about the universe.

368. There's a cosmic speed limit.

369. 186,000 miles per second,

370. also known as the speed of light.

371. Nothing can exceed that speed.

372. I realize this sounds weird, but trust me.

373. It's one of the best-established principles in science.

374. Believe it or not, traveling at near the speed of light

375. transports you to the future.

376. To explain why,

377. let's dream up a science-fiction transportation system.

378. Imagine a track that goes right around the earth...

379. a track for a super-fast train.

380. We're going to use this imaginary train

381. to get as close as possible to the speed of light

382. and see how it becomes a time machine.

383. On board are passengers with a one-way ticket to the future.

384. The train begins to accelerate faster and faster.

385. Soon it's circling the earth over and over again.

386. To approach the speed of light

387. means circling the earth pretty fast.

388. Seven times a second.

389. But no matter how much power the train has,

390. it can never quite reach the speed of light,

391. since the laws of physics forbid it.

392. Instead, let's say it gets close,

393. just shy of that ultimate speed.

394. Now something extraordinary happens.

395. Time starts flowing slowly on board

396. relative to the rest of the world,

397. just like near the black hole, only more so.

398. Everything on the train is in slow motion.

399. This happens to protect the speed limit,

400. and it's not hard to see why.

401. Imagine a child running forwards up the train.

402. Her forward speed is added to the speed of the train,

403. so couldn't she break the speed limit simply by accident?

404. The answer is no.

405. The laws of nature prevent the possibility

406. by slowing down time on board.

407. Now she can't run fast enough to break the limit.

408. Time will always slow down just enough

409. to protect the speed limit.

410. And from that fact comes the possibility

411. of traveling large distances into the future.

412. Imagine the train left the station

413. on January the 1st, 2050.

414. It circles the earth over and over again for 100 years

415. before finally coming to a halt on New Year's Day 2150.

416. The passengers will have only lived one week

417. because time is slowed down that much inside the train.

418. When they got out, they'd find a very different world

419. than the one they'd left.

420. In one week,

421. they'd have traveled 100 years into the future.

422. Of course, building a train that could reach such a speed

423. is quite impossible.

424. But we have built something very like the train,

425. at the world's largest particle accelerator

426. at CERN in Geneva, Switzerland.

427. Deep underground, in a circular tunnel 16 miles long,

428. is a stream of trillions of tiny particles.

429. When the power is turned on,

430. they accelerate from zero to 60,000 miles per hour

431. in a fraction of a second.

432. Increase the power, and the particles go faster and faster

433. until they're whizzing around the tunnel

434. at 11,000 times a second,

435. which is almost the speed of light.

436. But just like the train,

437. they never quite reach that ultimate speed.

438. They can only get to 99.99% of the limit.

439. When that happens, they, too, start to travel in time.

440. We know this because of some extremely short-lived particles

441. called pi-mesons.

442. Ordinarily, they disintegrate

443. after just 25 billionths of a second.

444. But when they are accelerated to near light speed,

445. they last 30 times longer.

446. These particles are real-life time travelers.

447. It really is that simple.

448. If we want to travel into the future,

449. we just need to go fast.

450. Really fast.

451. And I think the only way we're ever likely to do that

452. is by going into space.

453. The fastest manned vehicle in history was Apollo 10.

454. It reached 25,000 miles per hour.

455. But to travel in time,

456. we'll have to go more than 2,000 times faster.

457. And to do that, we'd need a much bigger ship...

458. a truly enormous machine.

459. The ship would have to be big to carry a huge amount of fuel,

460. enough to accelerate it to nearly the speed of light.

461. Getting to just beneath the cosmic speed limit

462. would require six whole years at full power.

463. T-minus 10 seconds.

464. 4, 3...

465. 2, 1.

466. The initial acceleration would be gentle

467. because the ship would be so big and heavy.

468. But gradually it would pick up speed

469. and soon would be covering massive distances.

470. In just one week, it would have reached the outer planets,

471. gas giants like Neptune.

472. After two years, it would reach half light speed

473. and be far outside our solar system.

474. Two years later, it would be traveling

475. at 90% of the speed of light speed

476. and passing our closet star system, Alpha Centauri.

477. Around 30 trillion miles away from Earth

478. and four years since launch,

479. the ship begins to travel in time.

480. For every hour of time on the ship, two hours pass on Earth,

481. a similar situation to the spaceship

482. that orbited the massive black hole.

483. But there's more to come.

484. After another two years of full thrust,

485. the ship would reach its top speed...

486. 99% of the speed of light.

487. At this speed, a single day on board

488. is a whole year of Earth time.

489. Our ship would be truly flying into the future.

490. The slowing of time has another benefit.

491. It means we could, in theory,

492. travel extraordinary distances within one human lifetime.

493. A trip to the edge of the galaxy would take just 80 years.

494. But the real wonder of our journey

495. is that it reveals just how strange the universe is.

496. It's a universe where time runs at different rates

497. in different places.

498. Where the most extreme objects imaginable, giant black holes,

499. twist and warp both time and space.

500. It's a place where tiny wormholes exist all around us.

501. And where, ultimately,

502. if we could develop the right technology,

503. we could use our understanding of the laws of physics

504. to become time voyagers through the fourth dimension.