WEBVTT - Why Does the Universe Have a Speed Limit? (feat. Sean Carroll) 0:00:00.920 --> 0:00:04.560 Hey, welcome to Sign Stuff, a production of iHeartRadio. I'm 0:00:04.600 --> 0:00:08.000 hoorhe cham and today we are answering the question can 0:00:08.039 --> 0:00:12.000 we go faster than lights? Like the fastest thing in 0:00:12.039 --> 0:00:14.960 the universe? And what would happen if you tried to 0:00:15.040 --> 0:00:18.079 go faster. We're going to talk to a couple of 0:00:18.200 --> 0:00:23.599 experts about this, including a famous theoretical physicist and a 0:00:23.800 --> 0:00:28.760 historian of special relativity. So buckle up, put your helmets on, 0:00:29.120 --> 0:00:32.760 because we are going to ludicrous speed to answer the 0:00:32.840 --> 0:00:38.680 question can we go faster than light? 0:00:44.520 --> 0:00:44.720 Hey? 0:00:44.760 --> 0:00:48.800 Everyone? Okay, So today's question is a pretty deep one. 0:00:49.360 --> 0:00:51.960 It goes to the very heart of what the laws 0:00:51.960 --> 0:00:55.280 of physics in our universe are, and it has huge 0:00:55.320 --> 0:00:59.120 consequences for what it means to have an orderly and 0:00:59.240 --> 0:01:04.720 logical existence and reality. It's such a huge question that 0:01:04.800 --> 0:01:08.800 it even stumped Einstein. So I knew I needed a 0:01:08.840 --> 0:01:11.520 lot of help, which is why I called two very 0:01:11.520 --> 0:01:15.160 smart friends of mine. And according to them, there are 0:01:15.319 --> 0:01:19.160 three ways to answer this question. The first way is 0:01:19.200 --> 0:01:22.760 to think about how fast light can go. Here's my 0:01:22.880 --> 0:01:27.119 conversation with my first friend, famous theoretical physicist, New York 0:01:27.120 --> 0:01:30.960 Times bestselling author and host of the Mind Skate Podcast, 0:01:31.360 --> 0:01:38.280 Professor Sean Carroll. Cool, and I might crack some jokes. 0:01:38.440 --> 0:01:41.560 That's the tone of the episode. That's okay, that's allowed. 0:01:41.560 --> 0:01:44.759 We're allowed to entertain the podcast. All right, Well, thank 0:01:44.760 --> 0:01:46.840 you doctor Carrol for joining us. Thanks very much for 0:01:46.880 --> 0:01:48.360 having me. Please tell us who you are and what 0:01:48.440 --> 0:01:50.840 you did. My name is Sean Carroll. I'm a physicist 0:01:50.840 --> 0:01:54.559 and philosopher at Johns Hopkins University. I think about where 0:01:54.600 --> 0:01:56.680 the universe came from, what it's made of, what the 0:01:56.760 --> 0:01:59.559 laws of physics are awesome. Have you ever gone faster 0:01:59.600 --> 0:02:02.120 than this of light? I never have. I've gone stoller 0:02:02.120 --> 0:02:04.240 in the speed of light many many times. 0:02:04.760 --> 0:02:05.080 I guess. 0:02:05.120 --> 0:02:07.400 Just to start us off, what is the speed of light? 0:02:07.800 --> 0:02:10.920 It's kind of something you can think about at different 0:02:11.000 --> 0:02:14.160 levels of sophistication. To be honest, the speed of light 0:02:14.280 --> 0:02:18.160 is the speed that light travels at in empty space. 0:02:18.639 --> 0:02:21.000 So one very quick thing to get straight is that 0:02:21.120 --> 0:02:24.440 light itself can travel at different speeds. Light travels at 0:02:24.480 --> 0:02:27.240 a different speed through air or water than it does 0:02:27.280 --> 0:02:29.680 through empty space. So when we're being a little bit 0:02:29.720 --> 0:02:32.880 more careful, we say the speed of light in vacuum. 0:02:32.919 --> 0:02:35.639 That's the special thing that's what people really care about. 0:02:35.800 --> 0:02:37.800 That means that it's a speed of light when there's 0:02:37.840 --> 0:02:40.519 nothing in the way, kind of nothing for the light 0:02:40.560 --> 0:02:43.880 to bump into. Okay, so the first thing to know 0:02:44.000 --> 0:02:48.240 about how fast light moves is that it has a speed. 0:02:49.600 --> 0:02:50.559 It may seem. 0:02:50.280 --> 0:02:53.440 Instantaneous when we turn on a light or a flashlight, 0:02:53.600 --> 0:02:56.840 but it does take time for light to go places, 0:02:57.120 --> 0:03:00.079 and this is something you can measure. For example, you 0:03:00.120 --> 0:03:02.280 can shoot a laser at the moon and then see 0:03:02.280 --> 0:03:05.160 how long it takes before it bounces back. If you 0:03:05.200 --> 0:03:08.640 did that, you'd see it takes about two point six seconds. 0:03:09.080 --> 0:03:09.920 Or you could send the. 0:03:10.080 --> 0:03:14.040 Radio signal, which is also light, to Mars and you 0:03:14.120 --> 0:03:17.440 see it takes about fifteen minutes on average for the 0:03:17.480 --> 0:03:21.320 signal to go there and come back. The second thing 0:03:21.360 --> 0:03:24.040 to know about how fast light moves is that it 0:03:24.120 --> 0:03:27.799 depends on what it's moving through. If it's moving through 0:03:27.840 --> 0:03:32.120 nothing or a vacuum, then it moves at its maximum speed, 0:03:32.560 --> 0:03:37.200 which is about three hundred thousand kilometers per second. At 0:03:37.200 --> 0:03:39.640 that speed, light can go all the way around the 0:03:39.720 --> 0:03:44.440 Earth seven times in one second. But if it moves 0:03:44.440 --> 0:03:50.040 through air or water or something transparent, then light slows 0:03:50.160 --> 0:03:54.240 down and the denser that thing is the slower light 0:03:54.440 --> 0:03:57.840 will move. So in a vacuum, light moves at about 0:03:57.840 --> 0:04:01.800 three hundred thousand kilometers per second, but through water it 0:04:01.880 --> 0:04:05.840 moves two hundred and twenty five thousand kilometers per second. 0:04:06.280 --> 0:04:09.520 If it moves through glass, it's two hundred thousand kilometers 0:04:09.520 --> 0:04:13.120 per second, And if it's a diamond, light moves at 0:04:13.160 --> 0:04:16.800 one hundred and twenty four thousand kilometers per second, or. 0:04:16.800 --> 0:04:19.320 Almost two and a half times slower. 0:04:19.720 --> 0:04:23.000 And that's because even though these things are transparent, the 0:04:23.040 --> 0:04:28.200 atoms still interact with the light. So when we think 0:04:28.240 --> 0:04:32.800 about light traveling through water or glass or air, we 0:04:32.880 --> 0:04:35.520 think it's transparent, it's just going right through, right, But 0:04:35.640 --> 0:04:39.039 in fact, the light is being affected by the medium 0:04:39.040 --> 0:04:43.200 it's moving in, by the very gentle electromagnetic interactions with 0:04:43.240 --> 0:04:46.120 all the atoms that it might bump into. That's why 0:04:46.120 --> 0:04:48.440 when you pass light through a prism, it will be 0:04:48.520 --> 0:04:52.039 broken up into different wavelengths. Right, it'll become it'll be 0:04:52.080 --> 0:04:54.680 the pink Floyd Dark side of the Moon album cover, 0:04:55.279 --> 0:04:58.200 and white light becomes all the colors of the rainbow. 0:04:58.360 --> 0:05:01.000 It gets psychedelic. You know, I'm not going to say 0:05:01.040 --> 0:05:03.240 how you should best experience dark side of the moon. 0:05:03.320 --> 0:05:07.520 That up to personal choices. Now, a lot of people 0:05:07.600 --> 0:05:11.080 think the reason that light slows down when going through 0:05:11.080 --> 0:05:14.839 something like air or water is that it's bumping into 0:05:14.880 --> 0:05:18.240 all the atoms that are there. But that's not quite right. 0:05:18.800 --> 0:05:22.320 The reason is a little technical, but basically, light is 0:05:22.360 --> 0:05:26.159 an electromagnetic wave, and when it goes through water or 0:05:26.200 --> 0:05:29.680 a diamond, the wave makes the electrons in the atoms 0:05:29.680 --> 0:05:33.760 that are there wiggle, and the wiggling causes a kind 0:05:33.760 --> 0:05:37.479 of counter electromagnetic wave that when you add it to 0:05:37.520 --> 0:05:42.880 the original light wave, it slows it down. And it's 0:05:42.920 --> 0:05:45.960 a good problem set for undergraduates learning this stuff for 0:05:46.000 --> 0:05:49.680 the first time, because it's actually quite miraculous. It's the 0:05:49.720 --> 0:05:53.080 net effect of all of the atoms has a sort 0:05:53.120 --> 0:05:56.960 of simple calculable effect on what the light is doing. 0:05:57.000 --> 0:06:00.720 It changes its speed. Okay, what does mean for the 0:06:00.760 --> 0:06:04.119 first way of answering the question can we go faster 0:06:04.240 --> 0:06:07.240 than light? Is that you can imagine setting up an 0:06:07.520 --> 0:06:11.560 unfair race. On one side, you would shoot light through 0:06:11.600 --> 0:06:15.919 a long piece or rod of a dense material like 0:06:16.080 --> 0:06:18.920 glass or diamond, so that the light would be going 0:06:19.000 --> 0:06:23.039 at half or less of its normal speed, and next 0:06:23.040 --> 0:06:25.839 to it, you would hop into a rocket with nothing 0:06:25.880 --> 0:06:29.880 in front of it but empty space. Now, you still 0:06:29.920 --> 0:06:33.280 have to go pretty fast, thousands of times faster than 0:06:33.400 --> 0:06:37.119 any human has ever gone before. But technically you could 0:06:37.200 --> 0:06:39.920 beat the light going through that rod. If you had 0:06:40.040 --> 0:06:43.040 enough fuel, you could get up to a speed that's 0:06:43.160 --> 0:06:47.279 faster than that light beam, Which means that technically, one 0:06:47.320 --> 0:06:50.240 way to answer the question can we go faster than light? 0:06:50.760 --> 0:06:58.040 Is yes, in an unfair race. All right, you're probably thinking, sure, Jorge, 0:06:58.240 --> 0:07:01.880 but that's cheating. And I imagine you clicked on this 0:07:01.960 --> 0:07:04.680 episode to find out if you could go faster than 0:07:04.760 --> 0:07:08.520 light without cheating. That is, can you go faster than 0:07:08.520 --> 0:07:12.160 the speed of light in a vacuum? After all, that's 0:07:12.200 --> 0:07:14.880 the question that's going to make it practical or not 0:07:15.360 --> 0:07:18.240 if he could ever go to distant stars or even 0:07:18.320 --> 0:07:23.160 to other galaxies. But here the answer is a lot trickier, because, 0:07:23.360 --> 0:07:26.040 as it turns out, the speed of light isn't just 0:07:26.120 --> 0:07:31.800 how fast light can go. It sets a universal speed limit. 0:07:32.400 --> 0:07:38.920 Here's how Professor Carroll puts it. The slightly more sophisticated 0:07:38.960 --> 0:07:42.280 way of saying it is the universe has a speed 0:07:42.320 --> 0:07:45.480 limit built into the laws of physics, there is a 0:07:45.680 --> 0:07:50.320 fastest speed at which anything can go, and unlike any 0:07:50.360 --> 0:07:54.320 other speed, this speed is absolute. It is not relative. 0:07:54.360 --> 0:07:57.640 It doesn't matter how you're moving through the universe, you 0:07:57.680 --> 0:08:01.320 are going to measure the same speed limit relative to you. 0:08:01.800 --> 0:08:04.320 It just so happens that light travels at this speed 0:08:04.360 --> 0:08:06.960 when it's moving in vacuum. But other things do as well. 0:08:07.320 --> 0:08:10.680 Gravitational waves, for example, also move at the speed of light. 0:08:11.160 --> 0:08:15.160 So it's really a feature of space time itself. It's 0:08:15.200 --> 0:08:18.040 a crucially important feature. It says that there are things 0:08:18.080 --> 0:08:21.480 to our past, there are things to our future, and 0:08:21.520 --> 0:08:24.080 there are things sort of sideways to us, things that 0:08:24.200 --> 0:08:26.800 are neither in our past and our future, because we 0:08:26.840 --> 0:08:29.440 can't get there without going faster than the speed of light, 0:08:29.480 --> 0:08:33.080 which is not allowed. All right, when we come back, 0:08:33.240 --> 0:08:36.600 we're gonna dig into everything Professor Carroll just said, and 0:08:36.640 --> 0:08:40.240 we're gonna go into the mind of Albert Einstein as 0:08:40.240 --> 0:08:44.400 he wondered the same question we are asking today, what 0:08:44.440 --> 0:08:48.000 would happen if he could catch up to a lightbeam. 0:08:48.440 --> 0:08:50.400 Stay with us, we'll be right back. 0:08:59.520 --> 0:09:00.240 We come back. 0:09:01.320 --> 0:09:04.640 Okay, we're answering the question, can we go faster than light. 0:09:05.040 --> 0:09:08.000 And as I mentioned, there are three ways to answer 0:09:08.040 --> 0:09:12.120 this question. The first is to cheat. You can slow 0:09:12.200 --> 0:09:15.640 down light by making it go through something like water 0:09:15.880 --> 0:09:19.199 or a diamond, and you could then technically beat it 0:09:19.400 --> 0:09:22.080 in a race. The second way to answer this question 0:09:22.280 --> 0:09:24.560 is to ask if we can go faster than the 0:09:24.720 --> 0:09:28.000 fastest light can go, which is the speed of light 0:09:28.280 --> 0:09:31.840 in a vacuum. And here the answer is trickier because 0:09:32.000 --> 0:09:35.280 if you ask any physicists on Earth this question, they'll 0:09:35.280 --> 0:09:38.920 probably tell you the answer is no. And so for 0:09:39.000 --> 0:09:41.319 this segment, I thought i'd take you through the history 0:09:41.480 --> 0:09:45.520 of how physicists arrived at this answer, because it definitely 0:09:45.600 --> 0:09:49.160 wasn't by trying to go faster than light. 0:09:52.000 --> 0:09:54.440 It sort of came in waves. It wasn't always clear 0:09:54.480 --> 0:09:58.040 to people for all time that light even had a 0:09:58.120 --> 0:10:01.200 finite speed. Some people wonder if light simply was instant, 0:10:01.400 --> 0:10:03.800 as if it traveled at an infinite speed, and a 0:10:03.840 --> 0:10:05.600 bunch of people wondered, maybe that's not right. 0:10:06.520 --> 0:10:10.400 That's my other friend, Professor David Kaiser. He's a theoretical 0:10:10.440 --> 0:10:14.720 physicist and a historian of science at the Massachusetts Institute 0:10:14.720 --> 0:10:18.439 of Technology or MIT. According to doctor Kaiser, it took 0:10:18.480 --> 0:10:21.920 a few hundred years since humans started doing serious science 0:10:21.960 --> 0:10:25.480 in the fifteen hundreds, with people like Galileo to figure 0:10:25.480 --> 0:10:29.520 out that light wasn't instantaneous and then it moved at 0:10:29.559 --> 0:10:32.520 a certain speed. And that's at the stage for a 0:10:32.559 --> 0:10:37.960 young teenager named Albert Einstein to ask a very simple question. 0:10:39.679 --> 0:10:43.120 That's what sets up a pretty amazing intellectual journey that 0:10:43.200 --> 0:10:45.840 leads to people like a young Albert Einstein and many others. 0:10:46.080 --> 0:10:48.280 Right around the turn of the twentieth century. In the 0:10:48.320 --> 0:10:52.320 early nineteen hundreds, Einstein was wondering, even as a teenager 0:10:52.640 --> 0:10:55.200 really before he was doing very formal schooling. In this 0:10:55.880 --> 0:10:58.000 he wondered, what would happen if I could catch up 0:10:58.080 --> 0:10:59.920 to a light wave? What would I see? 0:11:00.960 --> 0:11:05.440 What teenager Einstein wondered was, well, if light doesn't move 0:11:05.640 --> 0:11:09.080 infinitely fast, if it moves at a set speed, what 0:11:09.160 --> 0:11:13.040 would happen if you went faster and faster, could you 0:11:13.120 --> 0:11:16.480 eventually catch up to a beam of light? And if 0:11:16.520 --> 0:11:18.440 you did, what would it look like? 0:11:19.760 --> 0:11:22.240 And so he had these wonderful, very visual, kind of 0:11:22.520 --> 0:11:25.360 imaginary thought experiments in his head as a sixteen year old, 0:11:25.400 --> 0:11:28.240 and he realized, well, if there's a surfer on an 0:11:28.280 --> 0:11:30.720 ocean wave, and they're going at the same speed. If 0:11:30.760 --> 0:11:33.600 they look from side to side, at least momentarily, they'll 0:11:33.640 --> 0:11:36.200 see this wave kind of frozen, right, because they'll be 0:11:36.200 --> 0:11:39.160 moving with the crest of the wave. There's some time 0:11:39.280 --> 0:11:42.560 they would see basically a wave frozen in time, not 0:11:42.720 --> 0:11:46.080 zipping past them. And Einstein wondered, well, why can't I 0:11:46.120 --> 0:11:48.120 do that with a LightWave, or what would happen if 0:11:48.120 --> 0:11:49.960 I if I zoomed up and traveled at the same 0:11:50.000 --> 0:11:52.199 speed as a light wave, but I see a frozen 0:11:52.280 --> 0:11:54.560 wave like that frozen ocean wave as a surfer. 0:11:55.600 --> 0:11:59.120 In other words, Einstein wondered, as a sixteen year old, 0:11:59.320 --> 0:12:03.040 if you can catch to light, would you essentially freeze 0:12:03.160 --> 0:12:06.360 light or make it look like it's standing still? And 0:12:06.400 --> 0:12:12.040 this question bothered Einstein. Okay, so then Einstein had this 0:12:12.200 --> 0:12:15.240 idea that he nurtured since he was sixteen. And by 0:12:15.240 --> 0:12:17.360 the way, when I was sixteen, I was definitely not 0:12:17.440 --> 0:12:21.000 thinking about the nature of the universe. Not too loud, 0:12:22.240 --> 0:12:23.120 that's right here we are. 0:12:23.400 --> 0:12:25.959 So then what happened and then he got bothered by 0:12:26.000 --> 0:12:28.600 that he was reading for fun as a teenager and 0:12:28.880 --> 0:12:32.000 early college did there's still pretty new theory worked out 0:12:32.040 --> 0:12:35.240 by people like James Clark, Maxwell and Hendrick Herritt's and others. 0:12:35.559 --> 0:12:39.160 That was just scoring like win after win in describing 0:12:39.200 --> 0:12:42.439 all these other things about light. It was like better 0:12:42.480 --> 0:12:45.160 than anyone even imagined. It would match all these experiments 0:12:45.200 --> 0:12:48.160 to high accuracy. It was a theory not only of 0:12:48.400 --> 0:12:52.280 optics but also electric field magnetic fields. But in none 0:12:52.320 --> 0:12:54.079 of those solutions was there a sense that you could 0:12:54.120 --> 0:12:57.560 slow light down to zero. Wow, there was no mathematical 0:12:57.600 --> 0:13:01.640 solution of these otherwise beautiful and very powerful equations that 0:13:01.720 --> 0:13:03.640 showed a LightWave frozen in space. 0:13:04.640 --> 0:13:08.200 So for young Einstein, this idea of going faster than 0:13:08.280 --> 0:13:11.280 light and catching up to a lightbeam and seeing it 0:13:11.360 --> 0:13:15.600 frozen wasn't allowed according to the other theories of the day. 0:13:15.880 --> 0:13:19.440 It seemed to be true according to lots of experiments, 0:13:19.720 --> 0:13:22.679 and so he came up with a pretty radical solution. 0:13:24.040 --> 0:13:26.560 And so Einstein got bugged by that, and he wouldn't 0:13:26.600 --> 0:13:28.880 let it go. Whereas other teenagers might think about other things. 0:13:28.880 --> 0:13:32.600 For one, he got there and he realized one way 0:13:32.640 --> 0:13:36.360 to remove this uncomfortable paradox would be to say, what 0:13:36.480 --> 0:13:39.880 if the speed of light always has the same value, 0:13:39.920 --> 0:13:43.439 even if I'm moving fast. What if every person who's 0:13:43.480 --> 0:13:46.440 even in motion would measure the speed of light to 0:13:46.440 --> 0:13:48.640 have that same speed of speed of light in vacuum. 0:13:48.679 --> 0:13:50.439 If that's the case, that if he was careful to 0:13:50.440 --> 0:13:52.840 say if whoa, then no one could ever catch up 0:13:52.880 --> 0:13:53.480 with a light wave. 0:13:53.840 --> 0:13:57.080 That's a huge leap. I mean it almost sounds like 0:13:57.120 --> 0:13:58.240 a crazy idea. 0:13:58.559 --> 0:14:00.760 It is not what most people were I think, that's 0:14:00.800 --> 0:14:01.200 for sure. 0:14:02.280 --> 0:14:04.959 Okay, here's Einstein's idea. What if you did try to 0:14:05.000 --> 0:14:08.160 catch up to a lightbeam, you might start going faster 0:14:08.280 --> 0:14:10.640 and faster behind it, trying to catch up to it, 0:14:10.880 --> 0:14:13.000 and you would expect it to get closer to you. 0:14:13.520 --> 0:14:17.120 But what if instead it never does. What if the 0:14:17.200 --> 0:14:20.560 light beam always moves away from you at the speed 0:14:20.600 --> 0:14:23.560 of light, no matter how fast you go. 0:14:24.000 --> 0:14:25.760 And this is very strange. 0:14:25.360 --> 0:14:28.600 And different than what we experience in our everyday life. 0:14:28.920 --> 0:14:32.520 In our daily lives, we're used to speeds adding up. 0:14:32.800 --> 0:14:35.680 For example, if you're driving down the highway at sixty 0:14:35.760 --> 0:14:38.200 miles per hour and your kid in the back seat 0:14:38.440 --> 0:14:41.080 throws a toy at you at ten miles per hour, 0:14:41.520 --> 0:14:44.080 then to someone outside the car, they would see that 0:14:44.200 --> 0:14:49.520 toy going at seventy or sixty plus ten miles per hour. 0:14:49.960 --> 0:14:53.960 But Einstein said, what if that doesn't happen for light? 0:14:54.440 --> 0:14:56.960 For example, what if you're going sixty miles an hour 0:14:57.000 --> 0:15:00.360 in your car and you turn on the headlights. Expect 0:15:00.400 --> 0:15:02.680 the light from your headlights to go the speed of 0:15:02.720 --> 0:15:05.880 light plus to sixty miles an hour that the car 0:15:06.000 --> 0:15:10.680 is moving. But Einstein said, nope, that light still only 0:15:10.720 --> 0:15:13.360 goes at the speed of light. You and the car 0:15:13.400 --> 0:15:15.480 would see the light go at the speed of light, 0:15:15.680 --> 0:15:18.960 and someone standing on the highway would also see it 0:15:19.000 --> 0:15:21.840 going at the speed of light. And this weird but 0:15:21.920 --> 0:15:25.040 simple idea that light always seems to go at the 0:15:25.080 --> 0:15:28.000 speed of light no matter who looks at it basically 0:15:28.120 --> 0:15:31.120 revolutionized our understanding. 0:15:30.600 --> 0:15:31.520 Of the universe. 0:15:32.280 --> 0:15:37.600 Although it wasn't immediately a big hit, the. 0:15:37.560 --> 0:15:40.680 First reaction Einstein's work was no reaction at all. He 0:15:40.840 --> 0:15:43.760 was out of the way. He wasn't in an academic position. 0:15:44.000 --> 0:15:46.200 He was, you know, frankly, kind of a nobody put 0:15:46.200 --> 0:15:47.760 it this way. He didn't even get a promotion at 0:15:47.840 --> 0:15:48.680 the patent office. 0:15:51.640 --> 0:15:55.120 But eventually other people did notice Einstein's work. 0:15:56.400 --> 0:15:59.600 Nonetheless, more time goes by, he's able to get a 0:15:59.600 --> 0:16:02.080 fewer people to read the work and take it seriously, 0:16:02.120 --> 0:16:04.680 and a few of them actually were beginning to do 0:16:04.720 --> 0:16:07.080 things like write their own review articles and pay more attention. 0:16:07.440 --> 0:16:10.320 So over a period that stretches nearly to a decade, 0:16:10.840 --> 0:16:13.720 more and more of the professional scientists begin to pay attention. 0:16:14.560 --> 0:16:16.640 Okay, you might be wondering, what does this have to 0:16:16.680 --> 0:16:20.240 do with going faster than lights? Well, it turns out 0:16:20.240 --> 0:16:24.240 that Einstein's sort of simple idea has some really weird 0:16:24.400 --> 0:16:28.120 consequences for how the universe works. You might have heard 0:16:28.200 --> 0:16:31.160 that time slows down when you're going really really fast. 0:16:31.600 --> 0:16:34.800 That comes from Einstein's idea. Or that things look shorter 0:16:35.040 --> 0:16:40.680 when they're going super fast. That's also from Einstein's crazy idea. 0:16:41.200 --> 0:16:43.400 And he's very clear, I'm going to assume this right 0:16:43.560 --> 0:16:45.480 and then say what will follow from that? And even 0:16:45.480 --> 0:16:47.960 more stream sounding things followed from that, even in that 0:16:48.000 --> 0:16:51.680 first nineteen to oh five paper. So from this postulate 0:16:51.800 --> 0:16:54.760 he deduces other consequences. If you're zipping by on the 0:16:54.760 --> 0:16:57.520 train at some high speed, you and I will disagree 0:16:57.960 --> 0:17:00.240 on whether two events happened at the same time. This 0:17:00.280 --> 0:17:03.400 becomes known as the relativity of simultaneity. Another thing he 0:17:03.440 --> 0:17:05.440 begins to realize is that we're going to disagree on 0:17:05.480 --> 0:17:08.720 the measurements of lengths if we're moving at a relative speed. 0:17:08.960 --> 0:17:11.840 So it's called length contraction. And the third one is 0:17:11.920 --> 0:17:15.280 time dilation. So if we have identical clocks that we're holding, 0:17:15.400 --> 0:17:18.159 but you're racing past me at some outrageously high speed, 0:17:18.240 --> 0:17:20.440 I will watch your clock tick more slowly. 0:17:21.880 --> 0:17:25.359 The last main consequence of Einstein's idea is that nothing 0:17:25.400 --> 0:17:29.720 can ever go faster than the speed of light. How 0:17:29.720 --> 0:17:32.440 do we go from the speed of light, whether it's 0:17:32.440 --> 0:17:35.800 the same in all directions, to concluding that it's a 0:17:35.880 --> 0:17:37.639 speed limit for everything. 0:17:38.040 --> 0:17:41.439 Partly because from Einstein's own equations, as you try to 0:17:41.560 --> 0:17:44.040 get closer and closer to the speed of light, the 0:17:44.040 --> 0:17:47.120 difference in measured clock rate, all these mismatches in lengths 0:17:47.160 --> 0:17:49.840 of meter six times on clocks, even also things like 0:17:50.080 --> 0:17:53.080 the masses, the way we measure mass, These things all 0:17:53.320 --> 0:17:56.440 rise and in fact become infinite. And that's like saying 0:17:56.440 --> 0:17:59.679 it would take basically an infinite amount of energy to 0:17:59.720 --> 0:18:02.880 go as fast as that light beam does, and I can't. 0:18:02.920 --> 0:18:05.600 I have no source of infinite energy. 0:18:05.880 --> 0:18:10.080 Yeah, infinity can be a bummer, all right. When we 0:18:10.119 --> 0:18:13.440 come back. We're gonna talk more about this explanation, whether 0:18:13.520 --> 0:18:16.560 we've ever actually tried to go faster than the speed 0:18:16.600 --> 0:18:19.159 of light, and then I'm gonna ask our experts what 0:18:19.240 --> 0:18:22.960 they think would happen if we ever did break this 0:18:23.119 --> 0:18:25.200 speed limit. So stay with us. 0:18:25.400 --> 0:18:27.840 You're listening to sign stuff. 0:18:35.119 --> 0:18:35.840 And we're back. 0:18:37.359 --> 0:18:37.679 Okay. 0:18:37.760 --> 0:18:40.640 We're asking the question can we go faster than light? 0:18:41.119 --> 0:18:45.679 And so far we have two different answers. The first 0:18:45.720 --> 0:18:49.359 is yes, you can go faster than light if you 0:18:49.520 --> 0:18:53.480 slow down light by making it go through water or diamond. 0:18:53.920 --> 0:18:57.280 The second answer seems to be no, if you mean 0:18:57.320 --> 0:19:00.000 whether we can go faster than light in a vacum. 0:19:00.680 --> 0:19:03.320 As we learn, it seems that the universe has a 0:19:03.400 --> 0:19:07.040 speed limit, and the speed limit says nothing can ever 0:19:07.119 --> 0:19:10.680 go faster than the speed of light in a vacuum. Now, 0:19:10.800 --> 0:19:13.600 I know what you're thinking right now, which is have 0:19:13.720 --> 0:19:16.520 we actually tried to go faster than the speed of light? 0:19:17.080 --> 0:19:21.200 And the answer is yes. Since about the nineteen thirties, 0:19:21.440 --> 0:19:26.080 scientists have been building particle accelerators. These are huge machines 0:19:26.119 --> 0:19:29.800 that can take small particles like protons or electrons and 0:19:29.880 --> 0:19:33.880 speed them up to go faster and faster. These accelerators 0:19:33.920 --> 0:19:35.480 have been getting bigger and more. 0:19:35.320 --> 0:19:36.719 Powerful over the years. 0:19:36.960 --> 0:19:39.600 They've been pushing the limits of how fast we can 0:19:39.640 --> 0:19:44.080 make something go. The largest accelerator right now, the Large 0:19:44.080 --> 0:19:47.960 Hadron Collider in Geneva, Switzerland, is a seventeen mile long 0:19:48.119 --> 0:19:51.920 circular tunnel that can speed protons up to ninety nine 0:19:52.119 --> 0:19:57.400 point nine nine one percent of the speed of light. 0:19:57.880 --> 0:20:01.560 What happens if you try to go faster, uh, not much. 0:20:01.880 --> 0:20:04.040 As you get closer to the speed of light, it 0:20:04.080 --> 0:20:07.679 gets harder and harder to make things go faster. To 0:20:07.880 --> 0:20:10.560 actually reach the speed of light, it will take an 0:20:10.720 --> 0:20:15.520 infinite amount of energy. Here's how doctor Carrol explains it. 0:20:17.920 --> 0:20:20.679 All right, so we have this speed limit, and what 0:20:20.800 --> 0:20:22.879 happens if you try to go faster than that? You 0:20:22.920 --> 0:20:27.800 can try. The thing is that one of the equations 0:20:27.800 --> 0:20:31.719 that Einstein bequeathed to us is a formula for how 0:20:31.800 --> 0:20:34.479 much energy it takes to go at a certain speed. 0:20:34.600 --> 0:20:37.280 We all have heard of equals mc squared. Right, energy 0:20:37.359 --> 0:20:39.800 is vast times the speed of light squared. That's a 0:20:39.800 --> 0:20:42.919 formula for what the energy is for an object with 0:20:43.080 --> 0:20:46.960 zero velocity. If an object is moving with respect to us, 0:20:46.960 --> 0:20:49.440 it also has what we call kinetic energy, and its 0:20:49.520 --> 0:20:52.800 energy becomes bigger, and so there's a different formula as 0:20:52.880 --> 0:20:56.800 that velocity goes closer and closer to the speed of light. 0:20:57.320 --> 0:21:00.680 Einstein's formula for the energy says that the energy goes 0:21:00.720 --> 0:21:05.120 to infinity. So the problem is that it would require 0:21:05.440 --> 0:21:09.440 more than an infinite amount of energy to accelerate yourself 0:21:09.520 --> 0:21:11.560 faster than the speed of light. It's one of the 0:21:11.600 --> 0:21:14.480 ways of thinking about the fact that really you cannot 0:21:14.520 --> 0:21:16.439 go faster than the speed of light. If you tried, 0:21:16.920 --> 0:21:19.760 you would get closer and closer from our reference frame, 0:21:20.119 --> 0:21:21.720 but you would never make it all the way there 0:21:22.080 --> 0:21:23.879 is because you only have a finite amount of energy 0:21:23.880 --> 0:21:26.119 to push you. Would you say then that the reason 0:21:26.160 --> 0:21:28.959 we have a speed limit is because it takes an 0:21:28.960 --> 0:21:31.720 infinite amount of energy to go faster than light. I 0:21:31.720 --> 0:21:33.800 would say that fact, but I would say it the 0:21:33.800 --> 0:21:35.639 other way around. I would say that it takes in 0:21:35.720 --> 0:21:38.080 front of energy because we have a speed limit. It's 0:21:38.080 --> 0:21:41.680 the speed limit that is really just fundamental. The existence 0:21:41.680 --> 0:21:44.639 of a speed limit is really the defining feature of 0:21:44.760 --> 0:21:49.440 space time, according to Einstein. Okay, and this brings us 0:21:49.640 --> 0:21:53.280 to the big question, why does the universe seem to 0:21:53.359 --> 0:21:57.399 have this speed limit. After all, it seems weird that 0:21:57.440 --> 0:22:00.159 we live in a universe where you can't just go 0:22:00.320 --> 0:22:03.640 faster and faster, that there is some sort of arbitrary 0:22:03.760 --> 0:22:08.359 rule that curbs your speed and gives you strange phenomena 0:22:08.840 --> 0:22:13.119 like time flowing differently for different people. I asked doctor 0:22:13.160 --> 0:22:18.240 Carol this question. Okay, so now the question is why 0:22:18.280 --> 0:22:20.760 do we have this speed limit in the universe. That's 0:22:20.800 --> 0:22:24.359 an excellent question, And as we see, it is not 0:22:24.600 --> 0:22:27.639 hard to bump up into questions where the answer is 0:22:28.240 --> 0:22:32.320 there's no necessary reason why, Like maybe there's no such 0:22:32.400 --> 0:22:34.840 thing as why it is a feature of the universe 0:22:35.400 --> 0:22:40.080 that there is this speed limit. It doesn't seem necessary 0:22:40.160 --> 0:22:42.640 if you think about all the possible laws of physics. 0:22:42.720 --> 0:22:46.320 But since relativity came along, we know that the universe 0:22:46.400 --> 0:22:48.679