WEBVTT - BrainStuff Classics: What Was Stephen Hawking's Final Project? 0:00:01.920 --> 0:00:06.480 Welcome to brain Stuff production of I Heart Radio. Hey 0:00:06.559 --> 0:00:09.360 brain Stuff Lauren Vogbaum here with another classic from the 0:00:09.360 --> 0:00:12.399 podcast archives. In this one, we take a look at 0:00:12.440 --> 0:00:16.079 what turned out to be theoretical physicist Stephen Hawkings final 0:00:16.160 --> 0:00:22.200 research paper about the multiverse. Hey brain Stuff, Lauren vog 0:00:22.239 --> 0:00:26.800 Obam here. Days before his death, on March fourteen, famed 0:00:26.840 --> 0:00:31.040 theoretical physicist and cosmologist Stephen Hawking completed what would be 0:00:31.160 --> 0:00:34.479 his final research paper. It has since passed peer review 0:00:34.520 --> 0:00:36.920 and was published online in the Journal of High Energy 0:00:36.960 --> 0:00:41.680 Physics on April. Written with co author Thomas Hertog, a 0:00:41.800 --> 0:00:45.640 theoretical physicist at the University of louisn Belgium, the paper 0:00:45.720 --> 0:00:48.920 adds another facet to our understanding of this universe that 0:00:48.960 --> 0:00:53.199 we live in, and needless to say, it's complicated. Titled 0:00:53.400 --> 0:00:57.440 a Smooth Exit from Eternal Inflation, the publication discusses an 0:00:57.520 --> 0:01:01.760 enigmatic problem facing cosmologists. But before we delve into the 0:01:01.800 --> 0:01:03.960 crux of the study, let's go back to when our 0:01:04.040 --> 0:01:07.560 universe was a baby, some thirteen point eight billion years ago. 0:01:08.880 --> 0:01:11.960 A lot of evidence suggests that our universe originated from 0:01:11.959 --> 0:01:15.520 a singularity, an infinitely dense point from which all the 0:01:15.600 --> 0:01:18.440 universe as we know it was born. We call that 0:01:18.520 --> 0:01:21.880 event the Big Bang. But how the Singularity came to 0:01:21.920 --> 0:01:24.600 be and why the Big Bang happened isn't of concern 0:01:24.720 --> 0:01:28.080 right now. We're interested in what happened immediately after our 0:01:28.160 --> 0:01:32.920 universe was spawned, a period known as inflation. Cosmologists predict 0:01:33.000 --> 0:01:36.800 that inflation occurred over a vanishingly small period right after 0:01:36.840 --> 0:01:39.920 the Big Bang, during our universe's very first ten to 0:01:40.000 --> 0:01:44.960 thirty two seconds. During inflation, the universe expanded exponentially and 0:01:45.240 --> 0:01:48.280 much faster than the speed of light. After only a second, 0:01:48.480 --> 0:01:53.000 the energy from this inconceivably gargantuan explosion condensed to form 0:01:53.240 --> 0:01:58.760 sub atomic particles that, over millions of years, created the stars, galaxies, planets, and, 0:01:59.040 --> 0:02:01.840 after another few years, a life as we know it. 0:02:02.680 --> 0:02:07.000 Once this inflationary period ended, the universe's rate of expansion slowed, 0:02:07.200 --> 0:02:10.880 but it continues to expand to this day. Because inflation 0:02:10.919 --> 0:02:14.880 powered a faster than light speed expansion, the observable universe 0:02:14.919 --> 0:02:18.919 that we see today is not the entire universe. Rather, 0:02:19.080 --> 0:02:22.079 we exist inside a region of the cosmos that light 0:02:22.120 --> 0:02:25.200 has had time to reach. It's like dropping a pebble 0:02:25.240 --> 0:02:28.520 into a calm swimming pool, The first circular ripple to 0:02:28.560 --> 0:02:31.760 propagate from the splash travels at a fixed speed across 0:02:31.800 --> 0:02:34.360 the surface of the pool. If we imagine that the 0:02:34.400 --> 0:02:38.080 limit of our observable universe is that ripple traveling across 0:02:38.080 --> 0:02:40.320 the pool at the speed of light, it's not that 0:02:40.360 --> 0:02:44.440 nothing exists beyond that ripple. There's more pool or universe 0:02:44.440 --> 0:02:47.960 beyond it. We just can't see it yet. So the 0:02:48.040 --> 0:02:50.760 consequence of inflation is that there should be a lot 0:02:50.840 --> 0:02:53.480 more universe beyond what we can see, even with our 0:02:53.480 --> 0:02:57.640 most powerful telescopes. And cosmologists have been grappling with the 0:02:57.639 --> 0:03:01.480 possibility that our universe is not the only universe. In fact, 0:03:01.800 --> 0:03:03.919 we could be nothing more than a single bubble in 0:03:04.000 --> 0:03:08.200 an infinite, frothy ocean, a concept known as the multiverse. 0:03:08.840 --> 0:03:12.560 The idea here is that inflation didn't happen once, It's 0:03:12.639 --> 0:03:16.679 always happening via some infinitely vast chain reaction known as 0:03:16.960 --> 0:03:21.480 eternal inflation. One universe will appear, and inflation will take over, 0:03:21.560 --> 0:03:24.679 expanding that universe, and that universe will have its own 0:03:24.760 --> 0:03:28.000 quantum instabilities that will spawn more singularities that go on 0:03:28.120 --> 0:03:32.080 to create more universes. It's like blowing up a party 0:03:32.120 --> 0:03:35.320 balloon that itself spawns many other party balloons that are 0:03:35.360 --> 0:03:38.520 rupped from its rubbery surface, seemingly at random. If this 0:03:38.600 --> 0:03:43.320 situation sounds chaotic, it is. Proponents of this hypothesis think 0:03:43.440 --> 0:03:48.760 that eternal inflation is unstoppable, vastly complex, and continually generating 0:03:48.800 --> 0:03:52.440 new universes, and the math of this situation suggests that 0:03:52.520 --> 0:03:56.360 the multiverse acts like a fractal, fractals being sets of 0:03:56.440 --> 0:04:00.800 data that contain repeating patterns at every scale. Visually, this 0:04:00.920 --> 0:04:03.560 means that complex shapes look pretty much the same at 0:04:03.600 --> 0:04:06.240 a wide range of scales, like a small piece of 0:04:06.240 --> 0:04:08.800 it looks pretty similar to the whole structure. Think of 0:04:08.800 --> 0:04:11.880 a head of cauliflower. Any given segment will resemble the 0:04:11.880 --> 0:04:14.760 whole head, and if you zoom in further, each cluster 0:04:14.840 --> 0:04:19.080 of buds resembles the larger segments. It's worth noting that 0:04:19.120 --> 0:04:22.040 in this theory, each success of universe and the multiverse 0:04:22.120 --> 0:04:25.800 doesn't likely share the same physics as our universe. One 0:04:25.920 --> 0:04:28.919 universe might not have gravity, another may not support the 0:04:28.960 --> 0:04:32.240 forces that hold matter together. There would be a lot 0:04:32.279 --> 0:04:35.720 of stillborn universes that just don't amount too much. We 0:04:35.960 --> 0:04:38.280 humans are simply lucky to have a universe that has 0:04:38.320 --> 0:04:41.640 the right environment to create what we see, a philosophical 0:04:41.720 --> 0:04:45.760 argument known as the anthropic principle. The problem with eternal 0:04:45.800 --> 0:04:48.680 inflation is that it's messy and infinite, and that the 0:04:48.760 --> 0:04:54.040 hypothesis is ultimately untestable. So what does Hawking and her 0:04:54.120 --> 0:04:58.839 Tog's research have to do with this unrelenting multiverse In 0:04:58.880 --> 0:05:02.039 the multiverse? Are you? Niverse is merely a pocket universe 0:05:02.040 --> 0:05:05.919 where inflation has ended, and despite the odds, it found 0:05:06.040 --> 0:05:08.880 enough calm to create a bounty of stars and galaxies 0:05:08.920 --> 0:05:11.280 and a bunch of humans living on some random rock 0:05:11.400 --> 0:05:14.680 pondering the cosmos. What's going on beyond our pocket of 0:05:14.760 --> 0:05:19.279 calm is, however, somewhat different, Hawking said in an interview. 0:05:19.279 --> 0:05:24.039 In the usual theory of eternal inflation predicts that globally, 0:05:24.120 --> 0:05:27.599 our universe is like an infinite fractal with a mosaic 0:05:27.640 --> 0:05:31.880 of different pocket universes separated by an inflating ocean. The 0:05:31.920 --> 0:05:34.520 local laws of physics and chemistry can differ from one 0:05:34.520 --> 0:05:38.120 pocket universe to another, which together would form a multiverse. 0:05:38.560 --> 0:05:40.720 But I have never been a fan of the multiverse. 0:05:41.080 --> 0:05:43.640 If the scale of different universes in the multiverse is 0:05:43.760 --> 0:05:47.800 large or infinite, then the theory can't be tested. The problem, 0:05:47.880 --> 0:05:51.480 according to Hawking in her Dog, allies with the incompatibility 0:05:51.560 --> 0:05:54.960 of Einstein's general relativity that governs the evolution of the 0:05:55.040 --> 0:05:58.760 universe and quantum mechanics that seeds the creation of new 0:05:58.839 --> 0:06:03.760 universes through quantum fluctuations. The eternal inflation model of the multiverse, 0:06:03.920 --> 0:06:06.760 as her Tog said in a press release quote, wipes 0:06:06.800 --> 0:06:10.880 out separation between classical and quantum physics. As a consequence, 0:06:11.080 --> 0:06:15.640 Einstein's theory breaks down in eternal inflation. Their study doesn't 0:06:15.640 --> 0:06:19.440 go as far as reconciling general relativity with quantum physics, 0:06:19.480 --> 0:06:22.720 a quest that has so far been unsuccessful, but they 0:06:22.880 --> 0:06:25.760 use the math of string theory to help simplify the 0:06:25.839 --> 0:06:29.800 multiverse model. A quick recap. String theory predicts that all 0:06:29.839 --> 0:06:32.960 subtomic particles in our universe are in fact composed of 0:06:33.040 --> 0:06:37.280 one dimensional strings that propagate through space. The vibrational state 0:06:37.320 --> 0:06:40.120 of these strings is what gives these particles their quantum 0:06:40.160 --> 0:06:44.479 state such as charge, spin, and mass. But string theory 0:06:44.560 --> 0:06:48.560 also predicts the existence of the hypothetical graviton, a quantum 0:06:48.600 --> 0:06:51.560 particle that carries the force of gravity. The math that 0:06:51.600 --> 0:06:54.640 suggests that gravitons exist is solid, but no one's been 0:06:54.680 --> 0:06:57.960 able to point to one yet. String theory would therefore 0:06:58.040 --> 0:07:02.880 provide an explanation of how einstein general relativity gravity jibes 0:07:02.960 --> 0:07:07.440 with quantum physics using the mathematical framework of string theory. 0:07:07.760 --> 0:07:11.640 This final study from Hawking simplifies the multiverse. Hawking and 0:07:11.720 --> 0:07:14.680 her dog used the string theory concept of holography to 0:07:14.760 --> 0:07:17.920 reduce our three dimensional universe down to a two dimensional 0:07:18.200 --> 0:07:21.800 surface from which the universe we know and love is projected. 0:07:22.440 --> 0:07:25.160 By doing this, they were able to describe eternal inflation 0:07:25.320 --> 0:07:31.120 without general relativity, creating a timeless state. Her dog explained 0:07:31.160 --> 0:07:33.880 this move in a statement. When we trace the evolution 0:07:33.880 --> 0:07:36.520 of our universe backwards in time, at some point we 0:07:36.600 --> 0:07:40.080 arrive at the threshold of eternal inflation, where our familiar 0:07:40.120 --> 0:07:44.040 notion of time ceases to have any meaning. The math 0:07:44.120 --> 0:07:47.680 is complex, but the result is interesting. The calculations have 0:07:47.760 --> 0:07:51.320 the effect of turning the infinite and fractal multiverse into 0:07:51.360 --> 0:07:56.240 a far simpler and finite situation that eternal inflation does predict. 0:07:57.000 --> 0:07:59.800 Hawking said this about it. We are not down to 0:07:59.840 --> 0:08:03.720 a single, unique universe, but our findings imply a significant 0:08:03.720 --> 0:08:06.840 reduction of the multiverse to a much smaller range of 0:08:06.880 --> 0:08:11.320 possible universes. To put it in perspective, Hawking's final paper 0:08:11.400 --> 0:08:15.200 doesn't revolutionize our understanding of how the universe and indeed 0:08:15.200 --> 0:08:18.440 the multiverse works, but it is a valuable addition to 0:08:18.640 --> 0:08:22.720 a huge field of theoretical work. Specifically, her Tug hopes 0:08:22.760 --> 0:08:25.560 that this study may help a search for ancient gravitational 0:08:25.600 --> 0:08:29.480 waves that were generated by eternal inflation. These ripples in 0:08:29.520 --> 0:08:32.840 space time are far too weak for current gravitational wave 0:08:32.880 --> 0:08:37.079 detectors to detect. However, we need to wait until advanced 0:08:37.120 --> 0:08:41.240 space based observatories such as the European Space Agencies planned 0:08:41.360 --> 0:08:45.520 LIESA mission are launched. Regardless of whether the study leads 0:08:45.559 --> 0:08:48.360 to groundbreaking discoveries about the cosmos that we live in, 0:08:48.720 --> 0:08:51.840 it's a testament to a great scientist who worked tirelessly 0:08:51.960 --> 0:08:54.720 his entire life to answer some of the biggest questions 0:08:54.720 --> 0:08:58.600 that humanity has pondered, and on Hawking's shoulders, other great 0:08:58.640 --> 0:09:01.520 minds will build on this work to hopefully decipher whether 0:09:01.559 --> 0:09:04.400 our universe is unique or if it's just one bubble 0:09:04.480 --> 0:09:13.320 chaotically floating in the ocean of the multiverse. Today's episode 0:09:13.400 --> 0:09:16.319 is based on the article Stephen Hawking's Last Paper Takes 0:09:16.320 --> 0:09:19.560 on the Multiverse on Housetofworks dot com, written by Ian O'Neill. 0:09:20.200 --> 0:09:22.520 Brain Stuff is production of I Heart Radio in partnership 0:09:22.520 --> 0:09:25.000 with housetfworks dot com and it's produced by Tyler clang 0:09:25.640 --> 0:09:28.040 Or more podcasts my heart Radio, visit the I heart 0:09:28.120 --> 0:09:30.720 Radio app, Apple Podcasts, or wherever you listen to your 0:09:30.760 --> 0:09:31.480 favorite shows.