WEBVTT 00:00:07.619 --> 00:00:10.660 Howdy Stargazers and welcome to the season finale 00:00:10.660 --> 00:00:14.400 of Star Trails. My name is Drew and I'll be your 00:00:14.400 --> 00:00:16.920 guide to the night sky for the week of November 00:00:16.920 --> 00:00:21.519 30th to December 6th. This week we're building 00:00:21.519 --> 00:00:25.239 a time machine, or at least looking at the science 00:00:25.239 --> 00:00:28.320 that it might take to create one. Along the way, 00:00:28.440 --> 00:00:30.660 we'll look at some famous time machines in pop 00:00:30.660 --> 00:00:34.000 culture, plow through Einstein's two theories 00:00:34.000 --> 00:00:37.340 of relativity, which provide us with the equations 00:00:37.340 --> 00:00:40.939 to calculate our time travel efforts, and we'll 00:00:40.939 --> 00:00:44.039 verge into some wild theoretical territory that 00:00:44.039 --> 00:00:47.700 reads more like science fiction than fact. The 00:00:47.700 --> 00:00:50.920 winter's night sky is nearly upon us and in the 00:00:50.920 --> 00:00:53.439 second half of the show I'll preview what you 00:00:53.439 --> 00:00:56.020 can expect to see in the night sky for the next 00:00:56.020 --> 00:00:59.240 month while we're on a holiday break. Whether 00:00:59.240 --> 00:01:01.759 you're tuning in from the backyard or the balcony, 00:01:02.140 --> 00:01:04.579 I'm glad you're here. So grab a comfortable spot 00:01:04.579 --> 00:01:07.140 and let's see what the universe holds for us 00:01:07.140 --> 00:01:17.680 this week. As a kid growing up in the 80s, it's 00:01:17.680 --> 00:01:20.599 safe to say time travel movies were some of my 00:01:20.599 --> 00:01:23.560 favorites. There was Back to the Future, which 00:01:23.560 --> 00:01:26.340 taught us about the dangers of creating a paradox 00:01:26.340 --> 00:01:30.280 by going back in time. There was Star Trek IV, 00:01:30.560 --> 00:01:32.920 The Voyage Home, which showed us we could go 00:01:32.920 --> 00:01:35.780 back in time by slingshotting around the sun 00:01:35.780 --> 00:01:39.640 in a Klingon battle cruiser. Oh, and that humpback 00:01:39.640 --> 00:01:42.980 whales are apparently an intelligent alien species. 00:01:43.200 --> 00:01:47.180 Who knew? In middle school we read the original, 00:01:47.340 --> 00:01:51.219 The Time Machine by H .G. Wells, and I also became 00:01:51.219 --> 00:01:55.120 a fan of the 1960 film of the same name. That's 00:01:55.120 --> 00:01:58.359 the one where Rod Taylor manages to go forward 00:01:58.359 --> 00:02:02.599 in time some 800 ,000 years in a brassed out 00:02:02.599 --> 00:02:05.920 steampunk sled. Once he gets there, he falls 00:02:05.920 --> 00:02:09.000 in love with an Eloi girl and presumably brings 00:02:09.000 --> 00:02:12.379 her back home to civilized Victorian era London. 00:02:12.699 --> 00:02:16.460 Talk about a goal to aspire to. And later, as 00:02:16.460 --> 00:02:19.939 a teen, Bill and Ted's excellent adventure. A 00:02:19.939 --> 00:02:23.439 gonzo trip through time with two California goofs 00:02:23.439 --> 00:02:26.400 who gain access to a time machine to help them 00:02:26.400 --> 00:02:29.919 write a history term paper, of all things. I 00:02:29.919 --> 00:02:33.340 also have to mention Disney's Star Wars knockoff, 00:02:33.479 --> 00:02:36.659 The Black Hole. Not because there was time travel, 00:02:36.759 --> 00:02:39.580 but because of its willful ignorance of general 00:02:39.580 --> 00:02:45.210 relativity. More on that in a moment. The 1980s, 00:02:45.409 --> 00:02:48.169 especially as a child, felt like an era where 00:02:48.169 --> 00:02:50.889 anything was possible if you threw enough science 00:02:50.889 --> 00:02:53.830 at it, especially with the rise of computers 00:02:53.830 --> 00:02:56.969 and advancements in space travel, like the space 00:02:56.969 --> 00:03:01.050 shuttle. Magazines of that era were wild. As 00:03:01.050 --> 00:03:05.150 a kid, I read Omni, Discover, Astronomy, Popular 00:03:05.150 --> 00:03:08.300 Science, and more. And one day, after reading 00:03:08.300 --> 00:03:10.919 an article about the possibility of time travel, 00:03:11.180 --> 00:03:14.199 I decided to grab a pack of graph paper and start 00:03:14.199 --> 00:03:17.539 designing my own interstellar ship. One that 00:03:17.539 --> 00:03:20.659 used theoretical propulsion that could either 00:03:20.659 --> 00:03:23.919 take us to the speeds nearing that of light itself, 00:03:24.379 --> 00:03:26.699 or at least provide us with the means to reach 00:03:26.699 --> 00:03:31.599 the event horizon of a black hole. I was so naive, 00:03:31.740 --> 00:03:34.729 but I had the science to back it up. Reading 00:03:34.729 --> 00:03:37.889 those articles in Omnia and Discover taught me 00:03:37.889 --> 00:03:41.889 a few important things. One, you can't go back 00:03:41.889 --> 00:03:45.669 in time, only forward. Two, if you could travel 00:03:45.669 --> 00:03:48.550 near the speed of light, you can travel forward 00:03:48.550 --> 00:03:52.550 faster in time. Three, if you could orbit a black 00:03:52.550 --> 00:03:55.409 hole, you could go even further forward in time. 00:03:55.689 --> 00:03:59.469 Sounds pretty easy, right? These time travel 00:03:59.469 --> 00:04:02.469 parameters were already old news even in the 00:04:02.469 --> 00:04:06.680 1980s. See, Albert Einstein figured this stuff 00:04:06.680 --> 00:04:15.860 out back in the 1930s. He was Doc Brown way before 00:04:15.860 --> 00:04:18.899 Michael J. Fox jumped into a DeLorean and fired 00:04:18.899 --> 00:04:22.779 up the flux capacitor. So in this episode, we'll 00:04:22.779 --> 00:04:26.660 examine the two surefire ways to dilate time 00:04:26.660 --> 00:04:30.040 based on Einstein's special theory of relativity 00:04:30.040 --> 00:04:34.120 and his general theory of relativity. The former 00:04:34.120 --> 00:04:37.500 deals with high speeds. The latter deals with 00:04:37.500 --> 00:04:41.139 regions of massive gravity. We know these principles 00:04:41.139 --> 00:04:43.699 are accurate because we use technology linked 00:04:43.699 --> 00:04:48.160 to it every day. Those GPS satellites that float 00:04:48.160 --> 00:04:51.240 overhead that we use for navigation, they're 00:04:51.240 --> 00:04:55.279 time travelers in a sense. They orbit fast and 00:04:55.279 --> 00:04:58.459 high above our planet. So much that if we don't 00:04:58.459 --> 00:05:01.699 factor in their time offsets using super accurate 00:05:01.699 --> 00:05:05.439 atomic clocks, we'd be sent off course down here 00:05:05.439 --> 00:05:09.600 on Earth. So if the science actually works, what 00:05:09.600 --> 00:05:12.560 would it realistically take to make a time machine? 00:05:13.199 --> 00:05:16.639 One we can ride around in and fly forward ages 00:05:16.639 --> 00:05:24.740 into the future. Let's find out. Before we get 00:05:24.740 --> 00:05:27.500 any ideas of going back in time to play with 00:05:27.500 --> 00:05:30.899 dinosaurs, assassinate Hitler, or play Johnny 00:05:30.899 --> 00:05:33.720 B. Goode at a prom to make our parents fall in 00:05:33.720 --> 00:05:36.420 love, we need to know that time is a one -way 00:05:36.420 --> 00:05:40.100 street. We're not going back in time, sadly. 00:05:40.939 --> 00:05:44.439 Time only moves forward. This isn't philosophy, 00:05:44.720 --> 00:05:48.120 it's physics. The arrow of time points toward 00:05:48.120 --> 00:05:51.939 increasing entropy, toward disorder. toward future 00:05:51.939 --> 00:05:56.000 moments that haven't happened yet. You age forward. 00:05:56.180 --> 00:05:59.399 Your memories accumulate forward. The universe 00:05:59.399 --> 00:06:04.240 expands forward. Einstein's equations might be 00:06:04.240 --> 00:06:06.839 symmetrical, but everything we observe in nature 00:06:06.839 --> 00:06:11.040 has a preferred direction. Backwards simply doesn't 00:06:11.040 --> 00:06:14.250 work. Yes, there are some theories where time 00:06:14.250 --> 00:06:17.050 may go backwards, but as you'll find out later, 00:06:17.389 --> 00:06:20.230 all these theories generally have a giant loophole 00:06:20.230 --> 00:06:23.949 that breaks them. The big takeaway is simply 00:06:23.949 --> 00:06:27.769 this. The rate at which time flows is flexible. 00:06:28.170 --> 00:06:31.029 You can stretch it. You can compress it. And 00:06:31.029 --> 00:06:33.410 if you're clever, you can make your journey into 00:06:33.410 --> 00:06:37.329 the future faster. That's where relativity comes 00:06:37.329 --> 00:06:43.480 in. Special relativity tells us that the faster 00:06:43.480 --> 00:06:47.019 you move, the slower your clock ticks relative 00:06:47.019 --> 00:06:51.220 to someone staying still. Go fast enough, really 00:06:51.220 --> 00:06:55.600 fast, and your timeline becomes syrupy. A five 00:06:55.600 --> 00:06:58.420 -year journey for you might be decades for the 00:06:58.420 --> 00:07:01.379 people left on Earth. The problem is the speeds 00:07:01.379 --> 00:07:06.730 required are insane. Take the Parker Solar Probe, 00:07:06.750 --> 00:07:09.629 for instance, the fastest object humans have 00:07:09.629 --> 00:07:13.209 ever built. It falls toward the Sun at about 00:07:13.209 --> 00:07:18.470 .058 % the speed of light. That's blistering 00:07:18.470 --> 00:07:22.310 for us and virtually meaningless for space -time. 00:07:23.129 --> 00:07:26.490 After years of flying, the Parker Probe has accumulated 00:07:26.490 --> 00:07:29.329 about as much time dilation as you'd get from 00:07:29.329 --> 00:07:38.769 a long nap. To get dramatic effects, the kind 00:07:38.769 --> 00:07:42.230 that movies show, you need to be going a significant 00:07:42.230 --> 00:07:46.930 fraction of the speed of light. 50 to 99 percent. 00:07:52.410 --> 00:07:55.149 Right now, we don't have any technology that 00:07:55.149 --> 00:07:58.490 could even begin to approach those numbers. Traditional 00:07:58.490 --> 00:08:01.819 rockets forget about it. One proposed method 00:08:01.819 --> 00:08:04.860 that's been theorized since the 1960s involves 00:08:04.860 --> 00:08:08.279 detonating a series of nuclear bombs off behind 00:08:08.279 --> 00:08:12.040 your ship and riding the shockwave. That was 00:08:12.040 --> 00:08:15.100 Project Orion, and I mention it because it could 00:08:15.100 --> 00:08:18.620 get us in the theoretical range of 5 % of the 00:08:18.620 --> 00:08:23.360 speed of light. This project was a real government 00:08:23.360 --> 00:08:26.540 -funded proposal decades ago, with the idea being 00:08:26.540 --> 00:08:30.100 we could travel to Mars in weeks and stars in 00:08:30.100 --> 00:08:34.500 decades. This was published everywhere in science 00:08:34.500 --> 00:08:37.559 magazines during the early space age and it echoed 00:08:37.559 --> 00:08:41.039 through sci -fi art and articles for years after. 00:08:41.820 --> 00:08:45.000 Omni especially adored Project Orion because 00:08:45.000 --> 00:08:48.360 it embodied that mid -century utopian futurism. 00:08:48.549 --> 00:08:51.669 What if we used nuclear bombs to explore the 00:08:51.669 --> 00:08:55.710 stars? And even physicists like Freeman Dyson 00:08:55.710 --> 00:09:00.509 worked on it seriously. Strangely, this is the 00:09:00.509 --> 00:09:03.590 propulsion method used by my middle school graph 00:09:03.590 --> 00:09:07.210 paper time machine. Evidently, I read about Project 00:09:07.210 --> 00:09:12.149 Orion somewhere. I also augmented my design with 00:09:12.149 --> 00:09:16.059 something called a Bussard ramjet. This was another 00:09:16.059 --> 00:09:18.500 theoretical propulsion scheme that involved a 00:09:18.500 --> 00:09:21.879 giant scoop at the front of a starship to collect 00:09:21.879 --> 00:09:24.720 wandering hydrogen, convert it to energy via 00:09:24.720 --> 00:09:27.919 fusion, and propel the ship forward using the 00:09:27.919 --> 00:09:31.980 nuclear exhaust. Sounds great, right? Infinite 00:09:31.980 --> 00:09:35.240 fuel for a long journey. Heck, even Star Trek 00:09:35.240 --> 00:09:39.419 borrowed this idea once. The problem is, interstellar 00:09:39.419 --> 00:09:42.720 space contains hydrogen, but just barely. The 00:09:42.720 --> 00:09:46.700 typical density is around 0 .1 atoms per cubic 00:09:46.700 --> 00:09:50.100 centimeter. So if we do some math, we learn that 00:09:50.100 --> 00:09:53.259 the Boussard scoop would need to be tens of thousands 00:09:53.259 --> 00:09:56.860 of kilometers wide, maybe millions for the concept 00:09:56.860 --> 00:10:00.820 to work. It's not practical at any scale. And 00:10:00.820 --> 00:10:04.299 another problem arises. Eventually, this massive 00:10:04.299 --> 00:10:07.179 scoop would encounter a certain amount of drag 00:10:07.179 --> 00:10:10.340 as it rams into these hydrogen atoms, becoming 00:10:10.340 --> 00:10:13.399 more like a parachute than a stellar supercharger. 00:10:14.890 --> 00:10:18.370 So, assuming we managed to get our Project Orion 00:10:18.370 --> 00:10:22.190 engine fired up and we could achieve 5 % of the 00:10:22.190 --> 00:10:24.590 speed of light, what does that really get us? 00:10:25.269 --> 00:10:28.830 Let's set our sails for Proxima Centauri, the 00:10:28.830 --> 00:10:32.309 closest star to us outside of our own solar system. 00:10:32.870 --> 00:10:34.850 Because if we're pushing towards the speed of 00:10:34.850 --> 00:10:37.070 light, we might as well go somewhere interesting. 00:10:40.289 --> 00:10:45.970 Proxima Centauri is 4 .24 light years away. Cruising 00:10:45.970 --> 00:10:49.289 at 5 % the speed of light, it would take nearly 00:10:49.289 --> 00:10:54.409 85 years to get there, one way. We're going to 00:10:54.409 --> 00:10:57.850 use something called the Lorentz factor to calculate 00:10:57.850 --> 00:11:01.269 the time dilation. This equation tells us that 00:11:01.269 --> 00:11:05.029 if it takes us 84 .8 years to get to Proxima 00:11:05.029 --> 00:11:08.830 Centauri, we'd only experience about 39 less 00:11:08.830 --> 00:11:15.509 days than clocks back on Earth. So at 5 % of 00:11:15.509 --> 00:11:18.110 light speed, you're doing something insanely 00:11:18.110 --> 00:11:21.830 fast by any human standard. And relativity basically 00:11:21.830 --> 00:11:26.250 says, cool story, here's a one month bonus. Time 00:11:26.250 --> 00:11:28.990 is stubborn until you get really close to the 00:11:28.990 --> 00:11:32.909 speed of light. And that is some sad astrophysics. 00:11:36.490 --> 00:11:40.269 To make this fun, let's assume we can move at 00:11:40.269 --> 00:11:43.970 99 % of the speed of light. Because remember, 00:11:44.090 --> 00:11:46.549 we can never achieve the speed of light itself. 00:11:47.409 --> 00:11:50.590 The journey to Proxima would still take 4 .28 00:11:50.590 --> 00:11:53.370 years, but things get more intriguing when we 00:11:53.370 --> 00:11:56.370 plug that into the Lorentz equation. At this 00:11:56.370 --> 00:11:59.490 rate, we're about 4 .5 years younger than people 00:11:59.490 --> 00:12:02.649 back home. And because our local time is slowing 00:12:02.649 --> 00:12:05.919 down, Our one -way journey feels more like seven 00:12:05.919 --> 00:12:12.240 months. If you could achieve 99 .999 % of the 00:12:12.240 --> 00:12:15.080 speed of light, the effect would even be wilder. 00:12:15.620 --> 00:12:17.879 At that speed, Earth would see the trip take 00:12:17.879 --> 00:12:20.720 just over four years, but from our perspective 00:12:20.720 --> 00:12:23.100 on the ship, it would only take about a week. 00:12:23.740 --> 00:12:27.000 So a week for us and four years for everyone 00:12:27.000 --> 00:12:30.929 back home. do a round trip, and Earth ages eight 00:12:30.929 --> 00:12:33.750 and a half years while we age about two weeks. 00:12:34.909 --> 00:12:37.029 At that point, you're not just traveling through 00:12:37.029 --> 00:12:39.730 space, you're surfing along the edge of time 00:12:39.730 --> 00:12:49.669 itself. So speed is one way to bend time, but 00:12:49.669 --> 00:12:53.649 it's not the easiest way. Gravity does something 00:12:53.649 --> 00:12:57.429 extraordinary. It slows down time. The deeper 00:12:57.429 --> 00:13:00.870 you fall into a gravitational well, the slower 00:13:00.870 --> 00:13:03.970 your clock runs compared to someone farther away. 00:13:05.269 --> 00:13:08.490 This means that technically your head ages faster 00:13:08.490 --> 00:13:12.250 than your feet, really. Sadly, nothing in our 00:13:12.250 --> 00:13:15.110 solar system is massive enough to give you sci 00:13:15.110 --> 00:13:19.159 -fi levels of time dilation. If you orbit the 00:13:19.159 --> 00:13:22.480 Sun closely for a year, you might gain a couple 00:13:22.480 --> 00:13:26.059 seconds compared to interstellar space, and you'd 00:13:26.059 --> 00:13:29.860 probably fry. Orbit Jupiter and you might gain 00:13:29.860 --> 00:13:34.580 a few microseconds. Real gravitational time machines 00:13:34.580 --> 00:13:38.059 need something stronger, preferably a black hole 00:13:38.059 --> 00:13:41.539 or a neutron star, where we can achieve substantial 00:13:41.539 --> 00:13:46.279 dilation, minutes to hours in a day. But let's 00:13:46.279 --> 00:13:49.159 get crazy. Let's journey to the center of the 00:13:49.159 --> 00:13:52.460 Milky Way to the supermassive black hole there, 00:13:52.720 --> 00:13:56.759 Sagittarius A. Here we can seriously mess with 00:13:56.759 --> 00:14:11.620 time. Sagittarius A is four million times the 00:14:11.620 --> 00:14:15.500 mass of the Sun. It curves space -time so violently 00:14:15.500 --> 00:14:18.320 that time practically sticks to it like glue. 00:14:19.360 --> 00:14:22.639 If we park our spacecraft just outside the innermost 00:14:22.639 --> 00:14:25.860 stable circular orbit, a place where the tidal 00:14:25.860 --> 00:14:29.360 forces won't tear us apart, our personal clocks 00:14:29.360 --> 00:14:31.899 would slow dramatically compared to clocks back 00:14:31.899 --> 00:14:35.919 on Earth. We calculate it using the Schwarzschild 00:14:35.919 --> 00:14:40.320 time -dilation equation. Depending on the specific 00:14:40.320 --> 00:14:44.059 orbit, one hour for me might equal days, weeks, 00:14:44.279 --> 00:14:47.820 or even years back home. If you've seen the movie 00:14:47.820 --> 00:14:50.600 Interstellar, you've seen this type of dilation 00:14:50.600 --> 00:14:53.759 accurately depicted when the crew momentarily 00:14:53.759 --> 00:14:56.659 landed on Miller's planet, then returned to their 00:14:56.659 --> 00:14:59.320 ship to find years had elapsed for the scientists 00:14:59.320 --> 00:15:03.600 who remained behind. Of course, there's one tiny 00:15:03.600 --> 00:15:08.080 detail. Sagittarius A is 26 ,000 light years 00:15:08.080 --> 00:15:11.019 away. Getting to the thing that lets you time 00:15:11.019 --> 00:15:14.200 travel efficiently requires a level of time travel 00:15:14.200 --> 00:15:21.240 we don't have. Remember when I mentioned Disney's 00:15:21.240 --> 00:15:24.299 Black Hole movie earlier? Well, what bugs me 00:15:24.299 --> 00:15:26.679 about that film is that they're orbiting right 00:15:26.679 --> 00:15:29.480 at the edge of a black hole, seemingly ignorant 00:15:29.480 --> 00:15:33.090 to its effects on time. Remember, Einstein worked 00:15:33.090 --> 00:15:36.570 this out nearly 50 years earlier. Not sure it 00:15:36.570 --> 00:15:38.769 matters because they all fall into the black 00:15:38.769 --> 00:15:41.110 hole at the end of the film and the bad guys 00:15:41.110 --> 00:15:44.450 emerge in some existential version of hell and 00:15:44.450 --> 00:15:46.990 the protagonists pop out near a nice -looking 00:15:46.990 --> 00:15:51.149 planet, presumably to repopulate Eden or something. 00:15:56.590 --> 00:15:59.730 So why does this happen? Why does moving fast 00:15:59.730 --> 00:16:04.129 or floating in a gravity well affect time? To 00:16:04.129 --> 00:16:07.590 the universe, time isn't an independent thing. 00:16:07.990 --> 00:16:11.490 Time is woven into space. It's literally part 00:16:11.490 --> 00:16:14.850 of the geometry. When you move or when gravity 00:16:14.850 --> 00:16:18.309 bends space -time, you're changing that geometry. 00:16:19.029 --> 00:16:21.710 Your path through space -time gets stretched 00:16:21.710 --> 00:16:25.389 or squeezed and your clock ticks along that path. 00:16:25.929 --> 00:16:28.289 When you approach the speed of light, you're 00:16:28.289 --> 00:16:31.269 using more of your motion budget to move through 00:16:31.269 --> 00:16:35.029 space and less to move through time. Relativity 00:16:35.029 --> 00:16:38.330 says there's a fixed speed you move through space 00:16:38.330 --> 00:16:41.570 -time, always the speed of light, but divided 00:16:41.570 --> 00:16:45.850 between space and time. If you move faster through 00:16:45.850 --> 00:16:50.059 space, you must move slower through time. So 00:16:50.059 --> 00:16:53.200 your personal time that is the time your body 00:16:53.200 --> 00:16:56.659 experiences slows down But you don't feel it 00:16:56.659 --> 00:16:59.759 to you everything feels normal your heart beats 00:16:59.759 --> 00:17:02.799 normally your brain fires normally It's only 00:17:02.799 --> 00:17:05.279 when you compare with someone who stayed behind 00:17:05.279 --> 00:17:12.359 that you see the difference Gravity slows time 00:17:12.359 --> 00:17:16.170 because it bends space -time Remember, general 00:17:16.170 --> 00:17:19.269 relativity redefined how science sees gravity. 00:17:19.630 --> 00:17:22.329 It isn't just a force pulling downward. It's 00:17:22.329 --> 00:17:26.210 a curvature of space -time itself. The closer 00:17:26.210 --> 00:17:29.049 you are to a massive object, the more space -time 00:17:29.049 --> 00:17:32.309 is bent. And the deeper you sit in that curvature, 00:17:32.789 --> 00:17:36.650 the slower time passes for you. That's why clocks 00:17:36.650 --> 00:17:39.670 tick slower at sea level than on mountaintops. 00:17:40.130 --> 00:17:43.190 It's why GPS satellites tick faster than your 00:17:43.190 --> 00:17:46.890 phone. And near a black hole, space -time is 00:17:46.890 --> 00:17:50.410 curved so violently that time practically congeals. 00:17:50.829 --> 00:18:02.150 Even light struggles to escape. We understand 00:18:02.150 --> 00:18:06.089 the how. The equations match experiments perfectly. 00:18:06.400 --> 00:18:10.299 GPS, particle accelerators, atomic clocks flown 00:18:10.299 --> 00:18:14.099 on planes, or nuclear -powered clocks on mountaintops, 00:18:14.200 --> 00:18:18.019 and so on. But the deeper why is more philosophical. 00:18:19.000 --> 00:18:22.019 The best answer physics gives us is because space 00:18:22.019 --> 00:18:25.859 -time has geometry, and time is one of its dimensions. 00:18:27.430 --> 00:18:30.970 Einstein didn't explain time dilation so much 00:18:30.970 --> 00:18:33.430 as he showed that it's a natural consequence 00:18:33.430 --> 00:18:36.190 of living in a four -dimensional universe, where 00:18:36.190 --> 00:18:39.430 the speed of light is constant and mass bends 00:18:39.430 --> 00:18:42.950 space -time. Once you accept those two facts, 00:18:43.309 --> 00:18:46.670 everything else follows. Time slows near gravity. 00:18:47.049 --> 00:18:50.690 Time slows at high speeds. It's all a form of 00:18:50.690 --> 00:18:54.920 geometry. We don't know why the universe has 00:18:54.920 --> 00:18:57.799 this geometry instead of some other one, but 00:18:57.799 --> 00:19:00.740 we know that once you plug the geometry in, everything 00:19:00.740 --> 00:19:05.480 behaves exactly as expected. So what use is a 00:19:05.480 --> 00:19:08.980 time machine if we can't go back in time? There 00:19:08.980 --> 00:19:12.400 are theories that enable this, but so far quantum 00:19:12.400 --> 00:19:15.859 physics has sort of put its foot down. In the 00:19:15.859 --> 00:19:19.720 last 15 or 20 years, physicists and quantum information 00:19:19.720 --> 00:19:22.480 theorists have been exploring what happens if 00:19:22.480 --> 00:19:25.359 you allow quantum bits to interact with versions 00:19:25.359 --> 00:19:28.819 of themselves from the future, along a so -called 00:19:28.819 --> 00:19:33.420 closed time -like curve. This isn't a proposed 00:19:33.420 --> 00:19:36.039 machine, more of a thought experiment to test 00:19:36.039 --> 00:19:39.400 the boundaries of causality, and the punchline 00:19:39.400 --> 00:19:43.359 is always the same. The quantum math protects 00:19:43.359 --> 00:19:47.559 itself from paradox. If you try to send contradictory 00:19:47.559 --> 00:19:50.339 information into your own past, the equations 00:19:50.339 --> 00:19:54.410 smooth it out. so no paradox forms. It's like 00:19:54.410 --> 00:19:58.630 the universe is auto -correcting itself. Even 00:19:58.630 --> 00:20:01.829 at the smallest scales, the quantum world, the 00:20:01.829 --> 00:20:04.930 universe seems allergic to backward time travel. 00:20:06.950 --> 00:20:10.569 One modern idea, the Novikov self -consistency 00:20:10.569 --> 00:20:13.569 principle, says that even if you could go backward 00:20:13.569 --> 00:20:16.269 in time, the universe wouldn't let you create 00:20:16.269 --> 00:20:19.930 a paradox. Every choice you try to make in the 00:20:19.930 --> 00:20:22.910 past loops around to become exactly the thing 00:20:22.910 --> 00:20:26.250 that made time travel possible in the first place. 00:20:26.910 --> 00:20:30.589 It's cosmic predestination. The past is locked, 00:20:30.990 --> 00:20:34.529 the future is open, and time refuses to contradict 00:20:34.529 --> 00:20:39.049 itself. Another modern quantum theory leaves 00:20:39.049 --> 00:20:42.769 a tiny conceptual crack open. You might not be 00:20:42.769 --> 00:20:45.910 able to visit your own past, but you could, in 00:20:45.910 --> 00:20:49.019 theory, visit the past of a different branch 00:20:49.019 --> 00:20:52.160 of the universe, one where your arrival doesn't 00:20:52.160 --> 00:20:55.839 break causality. It's not time travel in the 00:20:55.839 --> 00:20:58.279 traditional sense. It's more like switching tracks 00:20:58.279 --> 00:21:02.539 on a cosmic railroad made of probability. So 00:21:02.539 --> 00:21:06.099 far, this idea has been untestable. I can't imagine 00:21:06.099 --> 00:21:11.759 why. For those of us stuck down here on Earth, 00:21:11.880 --> 00:21:14.940 the best time travel we can do is simply observe 00:21:14.940 --> 00:21:17.750 the universe. Feel like I've been saying this 00:21:17.750 --> 00:21:20.609 a lot lately, but every time we look up we're 00:21:20.609 --> 00:21:23.750 looking back into the past Sometimes the recent 00:21:23.750 --> 00:21:27.690 past like the light leaving Jupiter is 40 minutes 00:21:27.690 --> 00:21:32.650 old Light from our Sun is eight minutes old Sometimes 00:21:32.650 --> 00:21:35.730 we look at the deep past like the ancient glow 00:21:35.730 --> 00:21:39.170 of galaxies whose first stars fired long before 00:21:39.170 --> 00:21:43.599 Earth even formed The night sky is a map of moments 00:21:43.599 --> 00:21:46.839 that have already happened, still streaming towards 00:21:46.839 --> 00:21:50.700 us across the vastness. We may never build a 00:21:50.700 --> 00:21:54.799 starship that reaches 99 .999 % of the speed 00:21:54.799 --> 00:21:58.440 of light. We may never orbit a supermassive black 00:21:58.440 --> 00:22:01.859 hole and return home decades ahead of the people 00:22:01.859 --> 00:22:05.500 we left behind. And we certainly won't be sitting 00:22:05.500 --> 00:22:08.819 on Victorian chairs with bejeweled levers racing 00:22:08.819 --> 00:22:12.279 off to fight the Morlocks, ducking into a police 00:22:12.279 --> 00:22:15.500 call box or driving a DeLorean like we stole 00:22:15.500 --> 00:22:20.099 it to rewrite history. It's not about a machine 00:22:20.099 --> 00:22:23.240 so much as a perspective. Relativity teaches 00:22:23.240 --> 00:22:26.140 us that time isn't a singular river, but more 00:22:26.140 --> 00:22:29.539 like thousands of branching streams. And when 00:22:29.539 --> 00:22:32.650 speed or gravity gets involved, those streams 00:22:32.650 --> 00:22:37.190 can meander. Ultimately, they still all run forward. 00:22:38.170 --> 00:22:41.230 So if you really want to experience time travel, 00:22:41.490 --> 00:22:45.829 go outside tonight. Look up. Find a star. The 00:22:45.829 --> 00:22:49.109 moment you're seeing left that star years ago, 00:22:49.450 --> 00:22:52.809 sometimes centuries, sometimes millions or billions 00:22:52.809 --> 00:22:55.950 of years ago. You're reaching backward with nothing 00:22:55.950 --> 00:22:58.849 but your eyes and the physics of the cosmos. 00:22:59.200 --> 00:23:02.559 those are ancient photons from the past hitting 00:23:02.559 --> 00:23:15.940 our retinas in the present. In the second part 00:23:15.940 --> 00:23:18.519 of this episode, we'll explore what's going on 00:23:18.519 --> 00:23:21.180 overhead for the month of December. That's coming 00:23:21.180 --> 00:23:38.390 up after the break. Stay with us. Welcome back. 00:23:38.990 --> 00:23:41.710 Star Trails is taking a breather in December, 00:23:41.869 --> 00:23:44.269 but that doesn't mean the sky is taking a month 00:23:44.269 --> 00:23:47.890 off. Far from it. December is one of the richest 00:23:47.890 --> 00:23:52.369 observing months of the entire year. So here's 00:23:52.369 --> 00:23:55.829 your December sky strategy. Think of it as a 00:23:55.829 --> 00:23:58.910 guidebook for the whole month. Three simple beats 00:23:58.910 --> 00:24:02.589 to follow. Bright moon season, meteor season, 00:24:02.869 --> 00:24:06.490 and deep winter season. December starts with 00:24:06.490 --> 00:24:09.910 a burst of brightness. We get a full supermoon 00:24:09.910 --> 00:24:13.130 on December the 4th, the final supermoon of the 00:24:13.130 --> 00:24:16.690 year, traditionally called the cold moon. It'll 00:24:16.690 --> 00:24:19.730 look big and bold, climbing the eastern sky just 00:24:19.730 --> 00:24:23.769 after sunset. A week later, on December 11th, 00:24:23.809 --> 00:24:26.230 the moon hits third quarter and lights up the 00:24:26.230 --> 00:24:29.670 sky. But stick with it, because the sky gets 00:24:29.670 --> 00:24:32.690 wonderfully dark by December 19th, when we hit 00:24:32.690 --> 00:24:35.819 the new moon. The new moon window will be your 00:24:35.819 --> 00:24:39.200 best opportunity for deep sky observing. And 00:24:39.200 --> 00:24:41.759 then the month closes out with a first quarter 00:24:41.759 --> 00:24:45.559 moon on December 27th, a bright half moon riding 00:24:45.559 --> 00:24:49.460 high in the afternoon and evening sky. December 00:24:49.460 --> 00:24:53.380 also gives us two meteor showers. One is spectacular. 00:24:53.779 --> 00:24:57.119 The other is subtle, but atmospheric. First, 00:24:57.299 --> 00:25:00.099 the Geminids. They're active from December 4th 00:25:00.099 --> 00:25:02.140 through the 20th, and they peak on the night 00:25:02.140 --> 00:25:05.049 of December 13th, into the early morning of the 00:25:05.049 --> 00:25:09.269 14th. The moon will be just a thin waning crescent 00:25:09.269 --> 00:25:12.250 rising late so the sky will be beautifully dark. 00:25:12.809 --> 00:25:15.230 Under good conditions, the Geminids can deliver 00:25:15.230 --> 00:25:19.109 more than 100 meteors per hour. If you're going 00:25:19.109 --> 00:25:22.089 to commit to one night this month, make it Geminid 00:25:22.089 --> 00:25:25.569 night. Face east, get comfortable, and start 00:25:25.569 --> 00:25:30.250 watching around 10 p .m. The ursids are the second 00:25:30.250 --> 00:25:32.930 meteor shower of the month. These are active 00:25:32.930 --> 00:25:36.210 from December 13th through the 26th, and they 00:25:36.210 --> 00:25:39.509 peak right around the time of the solstice. The 00:25:39.509 --> 00:25:42.509 ursids are gentle, maybe five to ten meteors 00:25:42.509 --> 00:25:45.329 an hour. Look north toward the Little Dipper, 00:25:45.890 --> 00:25:48.170 and let the longest night of the year do the 00:25:48.170 --> 00:25:53.089 rest. December's planetary lineup is straightforward. 00:25:53.400 --> 00:25:56.920 Mercury is the prize for early birds. It has 00:25:56.920 --> 00:25:59.259 one of the best morning showings of the entire 00:25:59.259 --> 00:26:03.039 year. Look low in the southeast about 45 minutes 00:26:03.039 --> 00:26:06.160 before sunrise from December 1st through the 00:26:06.160 --> 00:26:09.539 23rd with the greatest visibility around December 00:26:09.539 --> 00:26:13.619 7th. Jupiter is the showstopper of the month. 00:26:13.819 --> 00:26:17.680 Bright, obvious, and unmistakable in Gemini in 00:26:17.680 --> 00:26:20.880 the early evening. If you have a telescope, Jupiter 00:26:20.880 --> 00:26:24.990 is your December anchor. Belts, zones, storms, 00:26:25.210 --> 00:26:28.069 and the dance of its four big moons every single 00:26:28.069 --> 00:26:32.069 night. Saturn hangs in the southwestern sky during 00:26:32.069 --> 00:26:35.130 the early evening hours. It's fading, sinking 00:26:35.130 --> 00:26:38.089 earlier each night, but still makes a wonderful 00:26:38.089 --> 00:26:40.910 warm -up target in Aquarius before you turn your 00:26:40.910 --> 00:26:44.410 attention to the winter constellations. Venus 00:26:44.410 --> 00:26:47.470 and Mars are both tucked too close to the Sun 00:26:47.470 --> 00:26:50.210 this month. If you're hunting planets, your trio 00:26:50.210 --> 00:26:53.670 is Mercury at Dawn, Saturn at dusk and Jupiter 00:26:53.670 --> 00:26:57.609 all night. Now for the real magic, the winter 00:26:57.609 --> 00:27:01.890 sky itself. By 9 or 10 p .m., the whole eastern 00:27:01.890 --> 00:27:04.730 sky is glowing with the constellations that define 00:27:04.730 --> 00:27:08.210 the season. There's Orion rising with his belt 00:27:08.210 --> 00:27:11.210 leading downward towards Sirius and Canis Major, 00:27:11.670 --> 00:27:15.470 the brightest star in the night sky. Above Orion 00:27:15.470 --> 00:27:18.930 is Auriga with brilliant Capella. To the left 00:27:18.930 --> 00:27:23.079 are Castor and Pollux in Gemini. overhead, drifting 00:27:23.079 --> 00:27:26.200 westward, the Pleiades sparkle like a tiny jewel 00:27:26.200 --> 00:27:29.900 box. If you want to keep things simple during 00:27:29.900 --> 00:27:34.039 December, here's an easy plan. Session 1, early 00:27:34.039 --> 00:27:37.339 December around the full supermoon. Watch the 00:27:37.339 --> 00:27:40.660 cold moon rise. Use its glow to trace the winter 00:27:40.660 --> 00:27:43.920 constellations. Let Jupiter and Saturn orient 00:27:43.920 --> 00:27:48.279 you in the evening sky. Session 2, the Geminid 00:27:48.279 --> 00:27:52.589 weekend, December 13th and 14th. Block out one 00:27:52.589 --> 00:27:56.029 night, go somewhere dark if you can, bundle up, 00:27:56.190 --> 00:27:59.910 lean back, and let the meteors pour out of Gemini. 00:28:00.609 --> 00:28:05.150 Session 3 Solstice night, December 21st and 22nd. 00:28:05.390 --> 00:28:08.349 Take advantage of the new moon darkness. Check 00:28:08.349 --> 00:28:11.869 out the Orion Nebula, the Pleiades, the clusters 00:28:11.869 --> 00:28:15.450 in Auriga, and then, after midnight, turn north 00:28:15.450 --> 00:28:18.410 for the soft drizzle of the Ursids on the longest 00:28:18.410 --> 00:28:21.789 night of the year. If you manage even one of 00:28:21.789 --> 00:28:24.329 these sessions, you'll have a memorable December. 00:28:25.029 --> 00:28:27.630 We'll see you again in January. I hope you all 00:28:27.630 --> 00:28:30.410 have a great holiday and maybe some of you will 00:28:30.410 --> 00:28:33.990 unwrap a new pair of binoculars or a smart scope. 00:28:34.329 --> 00:28:40.059 If you do, let me know. If you found this episode 00:28:40.059 --> 00:28:42.299 interesting, please share it with a friend who 00:28:42.299 --> 00:28:44.960 might enjoy it. The easiest way to do that is 00:28:44.960 --> 00:28:48.920 by sending folks to our website, StarTrails .Show. 00:28:49.359 --> 00:28:51.799 And if you want to support the show, use the 00:28:51.799 --> 00:28:54.400 link on the site to buy me a coffee. It really 00:28:54.400 --> 00:28:58.039 helps. Be sure to follow Star Trails on Blue 00:28:58.039 --> 00:29:01.119 Sky and YouTube. Links are in the show notes. 00:29:01.779 --> 00:29:04.759 Until we meet again beneath the stars, be excellent 00:29:04.759 --> 00:29:06.980 to each other. And party on, dudes!