1 00:00:12,820 --> 00:00:15,490 MARKUS KLUTE: Welcome back to special relativity. 2 00:00:15,490 --> 00:00:19,480 Bending of light is one of the spectacular consequences 3 00:00:19,480 --> 00:00:22,240 of general relativity, a prediction, if you want, 4 00:00:22,240 --> 00:00:23,890 [? of ?] [? general ?] relativity, 5 00:00:23,890 --> 00:00:26,470 which was experimentally confirmed. 6 00:00:26,470 --> 00:00:28,630 But let's analyze the situation first 7 00:00:28,630 --> 00:00:32,990 before we look at the actual evidence. 8 00:00:32,990 --> 00:00:35,770 They use Alice and Bob again, where 9 00:00:35,770 --> 00:00:37,540 Alice is an elevator operator. 10 00:00:37,540 --> 00:00:41,670 She's stationary and operating this elevator. 11 00:00:41,670 --> 00:00:44,910 Bob is a passenger and riding within the elevator. 12 00:00:44,910 --> 00:00:47,010 So Alice is injecting a light beam 13 00:00:47,010 --> 00:00:49,890 and watching this light beam in the elevator from outside. 14 00:00:49,890 --> 00:00:52,110 And the light beam, as you would expect, 15 00:00:52,110 --> 00:00:54,150 is going in a straight line. 16 00:00:54,150 --> 00:00:57,090 We'll see pictures here show the elevator at three different 17 00:00:57,090 --> 00:01:00,670 times-- [? t1, ?] [? t2, ?] and t3. 18 00:01:00,670 --> 00:01:04,069 For Bob, the very same situation looks completely different. 19 00:01:04,069 --> 00:01:06,970 He sees the light entering the elevator. 20 00:01:06,970 --> 00:01:09,370 And then after some time, the light pulse 21 00:01:09,370 --> 00:01:11,870 is maybe on the height of his head. 22 00:01:11,870 --> 00:01:18,130 And after some additional time, he sees the light at the end-- 23 00:01:18,130 --> 00:01:19,930 or at the bottom part of the elevator. 24 00:01:19,930 --> 00:01:23,740 So if you would draw a line on the back of the elevator 25 00:01:23,740 --> 00:01:26,800 where he was able to observe the light pulses, 26 00:01:26,800 --> 00:01:28,580 you could draw this line. 27 00:01:28,580 --> 00:01:33,640 So for Bob, who's stationary in this accelerating reference 28 00:01:33,640 --> 00:01:38,450 frame, light is bending. 29 00:01:38,450 --> 00:01:42,290 As we discussed in the first section in this chapter, 30 00:01:42,290 --> 00:01:44,870 there's an equivalence between accelerating 31 00:01:44,870 --> 00:01:48,720 and the gravitational field causing an acceleration. 32 00:01:48,720 --> 00:01:53,060 And so you can use heavy objects, like the Sun, 33 00:01:53,060 --> 00:01:55,130 in order to bend light. 34 00:01:55,130 --> 00:01:57,410 And this led to the first observation 35 00:01:57,410 --> 00:02:00,690 of the rotational bending of light. 36 00:02:00,690 --> 00:02:02,750 And so in order to do this, we want 37 00:02:02,750 --> 00:02:05,660 to have a star very close, or starlight 38 00:02:05,660 --> 00:02:07,790 passing very close to the Sun. 39 00:02:07,790 --> 00:02:11,360 And because the Sun is very bright, you want to use solar-- 40 00:02:11,360 --> 00:02:14,820 a total solar eclipse in order to test this effect. 41 00:02:14,820 --> 00:02:18,030 And this was first achieved in 1919 42 00:02:18,030 --> 00:02:20,420 with a solar eclipse in Brazil, also 43 00:02:20,420 --> 00:02:24,680 at the West Coast of Africa, by Arthur Eddington. 44 00:02:24,680 --> 00:02:32,840 And so this then led to Einstein really becoming famous. 45 00:02:32,840 --> 00:02:34,790 I talk about this later. 46 00:02:34,790 --> 00:02:37,130 You might remember the last partial eclipse here 47 00:02:37,130 --> 00:02:40,820 in Massachusetts in 2017. 48 00:02:40,820 --> 00:02:43,790 I have a vivid memory of this, showing this to my kids 49 00:02:43,790 --> 00:02:49,550 up into the sky, obviously, with proper eye protection. 50 00:02:49,550 --> 00:02:51,950 So the idea is, again, that you have a star, 51 00:02:51,950 --> 00:02:53,930 and light passes by the Sun. 52 00:02:53,930 --> 00:02:56,960 And because of the gravitational bending of the Sun, 53 00:02:56,960 --> 00:02:58,710 there's a bending effect. 54 00:02:58,710 --> 00:03:02,030 So there seems to be an offset of the actual start 55 00:03:02,030 --> 00:03:04,910 position due to the bending. 56 00:03:04,910 --> 00:03:08,060 And this effect, again, was discovered-- 57 00:03:08,060 --> 00:03:13,580 or was measured in 1919 by Arthur Eddington, and let's-- 58 00:03:13,580 --> 00:03:16,190 maybe because of an article in The New York Times 59 00:03:16,190 --> 00:03:18,410 to the fame of Albert Einstein. 60 00:03:18,410 --> 00:03:23,150 So at the time, there was no science reporters 61 00:03:23,150 --> 00:03:25,370 for newspapers. 62 00:03:25,370 --> 00:03:29,030 So a former sports reporter reported 63 00:03:29,030 --> 00:03:30,950 on the scientific endeavor. 64 00:03:30,950 --> 00:03:33,550 And the way he writes about it is rather interesting. 65 00:03:33,550 --> 00:03:35,300 You might want to read the entire article. 66 00:03:35,300 --> 00:03:37,130 But here's just the headlines. 67 00:03:37,130 --> 00:03:40,790 And it read like, "Lights All Askew in the Heavens," "Men 68 00:03:40,790 --> 00:03:44,150 of Science More or Less Agog Over Results 69 00:03:44,150 --> 00:03:46,070 Of Eclipse Observations," "Einstein's 70 00:03:46,070 --> 00:03:50,120 Theory Trumphs," Stars not Where They Seemed or Were Calculated 71 00:03:50,120 --> 00:03:54,200 to Be, But Nobody Need to Worry," "A Book for 12 Wise 72 00:03:54,200 --> 00:03:57,650 Men," "No more in all the world could comprehend it," 73 00:03:57,650 --> 00:04:01,910 said Einstein, when his daring publisher accepted it. 74 00:04:01,910 --> 00:04:07,490 So, again, this populous writing made Einstein very popular. 75 00:04:07,490 --> 00:04:09,850 He was then later invited to come and visit 76 00:04:09,850 --> 00:04:12,680 to the United States and made a tour, which he also 77 00:04:12,680 --> 00:04:14,630 used for political reasons. 78 00:04:14,630 --> 00:04:17,570 But, really, it made him very, very famous-- 79 00:04:17,570 --> 00:04:20,530 made him a pop star of the time.