1 00:00:00,000 --> 00:00:02,385 - [Voiceover] We've seen in several videos so far 2 00:00:02,385 --> 00:00:06,751 that most of the waves that humans have encountered 3 00:00:06,751 --> 00:00:09,886 in nature--waves in the water, sound waves, 4 00:00:09,886 --> 00:00:12,286 or just waves traveling along a rope-- 5 00:00:12,286 --> 00:00:15,781 they were disturbances traveling through a medium. 6 00:00:15,781 --> 00:00:19,566 And so when light displays or has wave-like properties 7 00:00:19,566 --> 00:00:21,239 a very natural assumption 8 00:00:21,239 --> 00:00:23,989 was, well, light must also be a disturbance 9 00:00:23,989 --> 00:00:26,184 traveling through some type of a medium 10 00:00:26,184 --> 00:00:29,314 even if that medium wasn't so easy to detect. 11 00:00:29,314 --> 00:00:32,194 But they conjectured that there is some type of medium 12 00:00:32,194 --> 00:00:34,090 that light is disturbance traveling through. 13 00:00:34,090 --> 00:00:36,163 They called it the luminiferous ether. 14 00:00:36,163 --> 00:00:38,800 And much of physics in the 19th century 15 00:00:38,800 --> 00:00:42,821 was all around proving that the luminiferous ether existed 16 00:00:42,821 --> 00:00:47,745 and also figuring out what our relative velocity was 17 00:00:47,745 --> 00:00:50,851 in regards to that luminiferous ether. 18 00:00:50,851 --> 00:00:52,218 And why do we feel confident, 19 00:00:52,218 --> 00:00:53,751 or why did they feel confident, 20 00:00:53,751 --> 00:00:56,827 that there was a relative velocity? 21 00:00:56,827 --> 00:00:58,917 Well, we talked about that in the last video. 22 00:00:58,917 --> 00:01:02,131 The earth is spinning and then it's spinning around, 23 00:01:02,131 --> 00:01:04,288 it's orbiting around, the sun at a nice clip 24 00:01:04,288 --> 00:01:06,289 and then the whole solar system is orbiting 25 00:01:06,289 --> 00:01:08,391 around the center of the galaxy at a nice clip. 26 00:01:08,391 --> 00:01:10,527 The galaxy itself might be moving, 27 00:01:10,527 --> 00:01:12,791 so if you have some absolute frame of reference 28 00:01:12,791 --> 00:01:14,753 that's defined by the ether, 29 00:01:14,753 --> 00:01:18,667 well we are going to be moving relative to it. 30 00:01:18,667 --> 00:01:20,815 And if we're moving relative to it 31 00:01:20,815 --> 00:01:22,752 well maybe you just measure the speed of light 32 00:01:22,752 --> 00:01:26,501 in different directions and see whether the speed of light 33 00:01:26,501 --> 00:01:28,257 is faster or slower in a certain direction 34 00:01:28,257 --> 00:01:30,105 and then that might help you identify-- 35 00:01:30,105 --> 00:01:32,692 well, one, validate that the ether exists-- 36 00:01:32,692 --> 00:01:35,791 but also think about what our velocity is 37 00:01:35,791 --> 00:01:38,125 relative to the ether, 38 00:01:38,125 --> 00:01:41,211 relative to that absolute frame of reference. 39 00:01:41,211 --> 00:01:43,027 But the problem in the 19th century 40 00:01:43,027 --> 00:01:44,852 is that we didn't have any precise way 41 00:01:44,852 --> 00:01:48,985 of actually measuring--or a precise enough way 42 00:01:48,985 --> 00:01:51,586 of measuring--the speed of light where we could detect 43 00:01:51,586 --> 00:01:54,419 the relative difference due to the light going 44 00:01:54,419 --> 00:01:58,567 for or against, or into or away from, 45 00:01:58,567 --> 00:02:01,983 the actual direction of the ether wind. 46 00:02:01,983 --> 00:02:04,792 And so the experiment that is usually cited 47 00:02:04,792 --> 00:02:06,987 with first kind of breaking things open, 48 00:02:06,987 --> 00:02:10,679 starting to really make a dent in this whole idea 49 00:02:10,679 --> 00:02:15,679 of a luminiferous ether, is the Michelson-Morley Experiment. 50 00:02:15,765 --> 00:02:17,901 Michelson-Morley Experiment. 51 00:02:17,901 --> 00:02:20,489 They recognized, okay, we can't measure the speed of light 52 00:02:20,489 --> 00:02:23,217 with enough precision to detect has it gotten slowed down 53 00:02:23,217 --> 00:02:26,724 by the ether wind or sped up by the ether wind, 54 00:02:26,724 --> 00:02:27,951 but what we could do, 55 00:02:27,951 --> 00:02:29,953 and this is what Michelson and Morley did do, 56 00:02:29,953 --> 00:02:33,088 and I'm gonna do an oversimplification of the experiment, 57 00:02:33,088 --> 00:02:35,598 is that, okay, you have a light source, 58 00:02:35,598 --> 00:02:38,490 you have a light source right over here. 59 00:02:38,490 --> 00:02:41,241 So, you have a light source. 60 00:02:41,241 --> 00:02:45,352 And so that's going to send light in this direction. 61 00:02:45,352 --> 00:02:47,688 It's going to send light just like that. 62 00:02:47,688 --> 00:02:50,590 And what you do is you have a half-silvered mirror 63 00:02:50,590 --> 00:02:53,831 that allows half the light to pass directly through it 64 00:02:53,831 --> 00:02:56,811 and half of it to be reflected. 65 00:02:56,811 --> 00:02:59,984 So let's put a half-silvered mirror right over here. 66 00:02:59,984 --> 00:03:04,417 So, there's a half-silvered mirror. 67 00:03:04,417 --> 00:03:08,159 And so half of this light will bounce off like this, 68 00:03:08,159 --> 00:03:10,361 and this is just a simplification of it. 69 00:03:10,361 --> 00:03:12,886 Let me do it a little neater than that. 70 00:03:12,886 --> 00:03:16,184 So half will bounce off like that. 71 00:03:16,184 --> 00:03:19,388 And then the other half will be able to go through it. 72 00:03:19,388 --> 00:03:20,781 Will be able to go through it. 73 00:03:20,781 --> 00:03:22,887 It's a half-silvered mirror. 74 00:03:22,887 --> 00:03:25,889 And then we make each of those light rays-- 75 00:03:25,889 --> 00:03:28,048 we've essentially taken our original light ray 76 00:03:28,048 --> 00:03:29,627 and split it into two-- 77 00:03:29,627 --> 00:03:34,297 well then we'll then bounce those off mirrors. 78 00:03:34,297 --> 00:03:36,848 Bounce those off mirrors that are equidistant. 79 00:03:36,848 --> 00:03:37,893 And there are some adjustments 80 00:03:37,893 --> 00:03:39,089 when you actually have to factor in everything, 81 00:03:39,089 --> 00:03:40,617 but just as a simple notion, 82 00:03:40,617 --> 00:03:43,259 these things are just now going to bounce back. 83 00:03:43,259 --> 00:03:46,693 So, this one is now going to bounce back. 84 00:03:46,693 --> 00:03:49,154 It's half-silvered, it can go through, 85 00:03:49,154 --> 00:03:51,511 or part of it can go through, that mirror. 86 00:03:51,511 --> 00:03:52,823 So that's that ray. 87 00:03:52,823 --> 00:03:55,261 And then this one is going to bounce back. 88 00:03:55,261 --> 00:03:56,492 This one's going to go bounce back. 89 00:03:56,492 --> 00:04:00,509 And part of it is going to bounce into this direction. 90 00:04:00,509 --> 00:04:02,927 And then you can detect what you see. 91 00:04:02,927 --> 00:04:05,450 You can detect what you see. 92 00:04:05,450 --> 00:04:08,949 So this right over here is a detector. 93 00:04:08,949 --> 00:04:09,820 And you might be saying 94 00:04:09,820 --> 00:04:11,224 okay, Sal, well what's the big deal? 95 00:04:11,224 --> 00:04:13,384 You've taken a light source, you've bounced, 96 00:04:13,384 --> 00:04:16,053 you've split the light rays, you've put them back together, 97 00:04:16,053 --> 00:04:17,922 you've bounced them around a little bit. 98 00:04:17,923 --> 00:04:21,994 But think about if there was a luminiferous ether 99 00:04:21,994 --> 00:04:25,693 these light waves that are going in orthogonal directions 100 00:04:25,693 --> 00:04:27,698 will be going at different velocities. 101 00:04:27,698 --> 00:04:29,918 Let's say if that luminiferous ether, 102 00:04:29,918 --> 00:04:34,337 if that luminiferous ether wind was doing something 103 00:04:34,337 --> 00:04:36,551 like, let me see, if the ether wind 104 00:04:36,551 --> 00:04:39,511 were in this direction, 105 00:04:39,511 --> 00:04:42,588 if the ether wind were in that direction, 106 00:04:42,588 --> 00:04:45,525 when the light wave is going that way 107 00:04:45,525 --> 00:04:48,184 it should be going faster, and when the light wave 108 00:04:48,184 --> 00:04:53,184 is coming back it should be going slower. 109 00:04:53,277 --> 00:04:55,350 And so what Michelson and Morley did is they said 110 00:04:55,350 --> 00:04:58,319 okay, let's assume... 111 00:04:58,319 --> 00:05:01,074 let's adjust our apparatus right over here, 112 00:05:01,074 --> 00:05:04,925 so when then these two lights rays bounce off 113 00:05:04,925 --> 00:05:07,653 and come back together, if there were no ether 114 00:05:07,653 --> 00:05:10,432 you would have some basic interference pattern. 115 00:05:10,432 --> 00:05:12,420 So, what do I mean by interference pattern? 116 00:05:12,420 --> 00:05:16,356 Well let's say that you have maybe this one 117 00:05:16,356 --> 00:05:17,377 bouncing from up here. 118 00:05:17,377 --> 00:05:19,084 Let me do that in a different color. 119 00:05:19,084 --> 00:05:21,297 So the one bouncing from up here, 120 00:05:21,297 --> 00:05:26,237 the one bouncing from up here, 121 00:05:26,237 --> 00:05:29,396 let's say that looks like this. 122 00:05:29,396 --> 00:05:31,356 I'll just draw it as a longitudinal wave, 123 00:05:31,356 --> 00:05:32,981 just like this. 124 00:05:32,981 --> 00:05:35,257 Best I had in hand-drawn longitudinal wave. 125 00:05:35,257 --> 00:05:39,348 And then the one coming from the other direction, 126 00:05:39,348 --> 00:05:43,284 the one that bounces here, and then comes back like this, 127 00:05:43,284 --> 00:05:47,727 is another longitudinal wave, like this. 128 00:05:47,727 --> 00:05:50,490 And when they overlap they are going to interfere 129 00:05:50,490 --> 00:05:53,034 with each other, either constructively interfere 130 00:05:53,034 --> 00:05:54,821 or destructively interfere. 131 00:05:54,821 --> 00:05:58,851 So you could have something like this. 132 00:05:58,851 --> 00:06:02,090 So let me copy and then let me paste it. 133 00:06:02,090 --> 00:06:04,818 So depending on how far or how fast 134 00:06:04,818 --> 00:06:06,466 each of these traveled, you're going to have 135 00:06:06,466 --> 00:06:08,488 different levels of interference. 136 00:06:08,488 --> 00:06:10,056 And you would have a difference, 137 00:06:10,056 --> 00:06:11,391 depending on the orientation, 138 00:06:11,391 --> 00:06:13,957 depending on what the actual ether wind is doing. 139 00:06:13,957 --> 00:06:16,685 But what Michelson and Morley observed 140 00:06:16,685 --> 00:06:19,378 is that no matter how they oriented this apparatus, 141 00:06:19,378 --> 00:06:21,190 and they did it at different times of the year, 142 00:06:21,190 --> 00:06:23,094 and they rotated it around, and they rotated it 143 00:06:23,094 --> 00:06:26,177 in the vertical direction and the horizontal direction, 144 00:06:26,177 --> 00:06:27,564 no matter what they did 145 00:06:27,564 --> 00:06:30,408 they always got the same interference pattern. 146 00:06:30,408 --> 00:06:32,486 The interference pattern did not change. 147 00:06:32,486 --> 00:06:34,928 And because the interference pattern did not change 148 00:06:34,928 --> 00:06:37,757 it implied that well, maybe this ether 149 00:06:37,757 --> 00:06:40,585 isn't really having an effect on slowing down 150 00:06:40,585 --> 00:06:43,141 or speeding up the light waves. 151 00:06:43,141 --> 00:06:45,984 So this is often called one of the most famous 152 00:06:45,984 --> 00:06:48,317 failed experiments in physics. 153 00:06:48,317 --> 00:06:50,367 So let me write this down. 154 00:06:50,367 --> 00:06:53,356 What's powerful about it is that it was a failed experiment. 155 00:06:53,356 --> 00:06:55,457 Let me get my pen tool out. 156 00:06:55,457 --> 00:06:59,219 It was a failed, failed experiment 157 00:06:59,219 --> 00:07:00,878 but it made people start to question 158 00:07:00,878 --> 00:07:03,653 well, maybe there isn't an ether, 159 00:07:03,653 --> 00:07:06,954 a luminiferous ether, maybe light just somehow travels 160 00:07:06,954 --> 00:07:08,421 through the vacuum. 161 00:07:08,421 --> 00:07:11,790 Maybe there is not this absolute frame of reference 162 00:07:11,790 --> 00:07:15,152 that's defined by the luminiferous ether. 163 00:07:15,152 --> 00:07:17,653 And I wanna be clear, it wasn't this experiment by itself. 164 00:07:17,653 --> 00:07:19,054 This experiment was one of many 165 00:07:19,054 --> 00:07:20,517 that started to put that doubt. 166 00:07:20,517 --> 00:07:22,084 But even after this experiment 167 00:07:22,084 --> 00:07:24,748 and they saw that there was no change 168 00:07:24,748 --> 00:07:26,156 in the interference pattern no matter how 169 00:07:26,156 --> 00:07:27,955 they oriented this thing, whether is was going 170 00:07:27,955 --> 00:07:32,124 in the direction of the hypothetical luminiferous ether 171 00:07:32,124 --> 00:07:33,368 or away from it, whatever, 172 00:07:33,368 --> 00:07:34,984 when they say no matter how they oriented it 173 00:07:34,984 --> 00:07:36,528 they got the same interference patterns, 174 00:07:36,528 --> 00:07:38,293 people tried to come up with other explanations 175 00:07:38,293 --> 00:07:41,915 that still might have been okay with a Luminiferous Ether. 176 00:07:41,915 --> 00:07:43,552 Maybe the length got contracted 177 00:07:43,552 --> 00:07:45,216 in the direction of the motion. 178 00:07:45,216 --> 00:07:47,213 Maybe other things got affected. 179 00:07:47,213 --> 00:07:49,919 But this is a super important experiment in physics 180 00:07:49,919 --> 00:07:53,396 because, once again, it started to show 181 00:07:53,396 --> 00:07:56,883 that hey, maybe there is no luminiferous ether, 182 00:07:56,883 --> 00:07:58,892 that light is just gonna go through that vacuum 183 00:07:58,892 --> 00:08:01,295 and, as we'll see, is going to be traveling 184 00:08:01,295 --> 00:08:04,360 at the same velocity no matter what 185 00:08:04,360 --> 00:08:06,821 frame of reference you look at it from. 186 00:08:06,821 --> 00:00:00,000 But we'll explore that more in future videos.