1 00:00:00,600 --> 00:00:08,200 In the videos on massive stars and on black holes, we learned that if the remnant of a massive star is 2 00:00:08,200 --> 00:00:15,800 massive enough, the gravitational contraction, the gravitational force, will be stronger than even the 3 00:00:15,800 --> 00:00:20,898 electron degeneracy pressure, even stronger than the neutron degeneracy pressure, 4 00:00:20,898 --> 00:00:23,067 even stronger than the quark degeneracy pressure, 5 00:00:23,067 --> 00:00:27,400 and everything would collapse into a point. 6 00:00:27,400 --> 00:00:32,979 And we called these points "black holes". 7 00:00:32,979 --> 00:00:36,056 And we learned there is an event horizon around these black holes. 8 00:00:36,056 --> 00:00:40,067 And if anything gets closer or goes within the boundary of that event horizon, 9 00:00:40,067 --> 00:00:44,046 there is no way that it can ever escape from the black hole. 10 00:00:44,046 --> 00:00:46,325 All it can do is get closer and closer to the black hole. 11 00:00:46,325 --> 00:00:49,067 And that includes light, and that's why it's called a black hole. 12 00:00:49,067 --> 00:00:54,079 So, even though all of the mass is at the central point, this entire area, 13 00:00:54,079 --> 00:01:02,933 or the entire surface of the event horizon -- I'll do it in purple, although it's supposed to be black -- 14 00:01:02,933 --> 00:01:08,533 This entire surface will appear black. It will emit no light. 15 00:01:08,533 --> 00:01:11,364 Now, these type of black holes that we described, 16 00:01:11,364 --> 00:01:16,101 we call those stellar black holes. 17 00:01:16,101 --> 00:01:21,200 And that is because they are formed from collapsing massive stars. 18 00:01:21,200 --> 00:01:30,933 And the largest stellar black holes that we have observed are on the order of 33 solar masses, give or take. 19 00:01:30,933 --> 00:01:33,287 So, very massive to begin with, lets just be clear. 20 00:01:33,287 --> 00:01:35,800 And this is what the remnant of the star has to be. 21 00:01:35,800 --> 00:01:43,333 So a lot more of the original star's mass might have been pushed off in supernovae (the plural of supernova). 22 00:01:43,333 --> 00:01:47,833 Now, there is another class of black holes here, and they are somewhat mysterious. 23 00:01:47,833 --> 00:01:57,133 They are called supermassive black holes. 24 00:01:57,133 --> 00:02:02,467 To some degree the word super isn't big enough. 25 00:02:02,467 --> 00:02:05,933 They are not just a little bit more massive than stellar black holes -- 26 00:02:05,933 --> 00:02:07,826 they are a lot more massive. 27 00:02:07,826 --> 00:02:18,033 They are on the order of hundreds of thousands to billions 28 00:02:18,033 --> 00:02:19,877 of solar masses. 29 00:02:19,877 --> 00:02:25,333 A hundred thousand to billions of times the mass of our sun. 30 00:02:25,333 --> 00:02:28,747 And what's interesting about these, other than the fact that they're super-huge, 31 00:02:28,747 --> 00:02:31,867 is that there doesn't seem to be black holes in between. 32 00:02:31,867 --> 00:02:34,200 Or at least we haven't observed black holes in between. 33 00:02:34,200 --> 00:02:36,667 The largest stellar black hole is thirty-three solar masses. 34 00:02:36,667 --> 00:02:40,667 And then there are these supermassive black holes that we think exist. 35 00:02:40,667 --> 00:02:44,321 And we think they mainly exist in the centers of galaxies. 36 00:02:44,321 --> 00:02:47,267 And we think most, if not all, centers of galaxies 37 00:02:47,267 --> 00:02:50,513 actually have one of these supermassive black holes. 38 00:02:50,513 --> 00:02:52,176 But it's kind of an interesting question: 39 00:02:52,176 --> 00:02:57,667 If all black holes were formed from collapsing stars, wouldn't we see things in between? 40 00:02:57,667 --> 00:03:02,267 So one theory of how these really massive black holes form 41 00:03:02,267 --> 00:03:05,282 is that you have a regular stellar black hole 42 00:03:05,282 --> 00:03:08,085 in an area that has a lot of matter than can accrete around it, 43 00:03:08,085 --> 00:03:10,538 (So let's imagine you have a regular . . . 44 00:03:10,538 --> 00:03:13,593 So I'll draw the event horizon around it. 45 00:03:13,593 --> 00:03:14,954 The actual black hole's going to be in the center of that, 46 00:03:14,954 --> 00:03:18,025 or the mass of the black hole will be in the center of it.) 47 00:03:18,025 --> 00:03:20,267 And then over time you just have 48 00:03:20,267 --> 00:03:24,067 more and more mass just falling into this black hole. 49 00:03:24,067 --> 00:03:26,936 Just more and more stuff just keeps falling into this black hole, 50 00:03:26,936 --> 00:03:31,067 and then it just keeps growing. 51 00:03:31,067 --> 00:03:33,867 And so this could be a plausible reason . . . 52 00:03:33,867 --> 00:03:35,899 Or at least the mass in the center keeps growing, 53 00:03:35,899 --> 00:03:39,333 and so the event horizon will also keep growing in radius. 54 00:03:39,333 --> 00:03:42,933 Now, this is a plausible explanation based on our current understanding. 55 00:03:42,933 --> 00:03:45,535 But the reason why this one doesn't gel that well is 56 00:03:45,535 --> 00:03:49,400 if this was the explanation for supermassive black holes, 57 00:03:49,400 --> 00:03:52,576 you would expect to see more black holes in between -- 58 00:03:52,576 --> 00:03:57,819 maybe black holes with a hundred solar masses, or a thousand solar masses, or ten thousand solar masses. 59 00:03:57,819 --> 00:03:59,667 But we're not seeing those right now; 60 00:03:59,667 --> 00:04:04,133 We just see the stellar black holes and we see the supermassive black holes. 61 00:04:04,133 --> 00:04:05,667 So another possible explanation -- 62 00:04:05,667 --> 00:04:11,402 my inclination is leaning towards this one because it kind of explains the gap -- 63 00:04:11,402 --> 00:04:14,142 is that these supermassive black holes 64 00:04:14,142 --> 00:04:19,642 actually formed shortly after the Big Bang, that these are primordial black holes. 65 00:04:19,642 --> 00:04:26,877 These started near the beginning of our universe. 66 00:04:26,877 --> 00:04:29,415 Now remember, what do you need to have a black hole? 67 00:04:29,415 --> 00:04:33,945 You need to have an amazingly dense amount of matter, 68 00:04:33,945 --> 00:04:36,421 or a dense amount of mass. 69 00:04:36,421 --> 00:04:38,895 If you have a lot of mass in a very small volume, 70 00:04:38,895 --> 00:04:43,133 then the gravitational pull will pull them closer and closer together, 71 00:04:43,133 --> 00:04:47,000 and they'll be able to overcome all of the 72 00:04:47,000 --> 00:04:51,689 electron degeneracy pressures, and the neutron degeneracy pressures, and the quark degeneracy pressures 73 00:04:51,689 --> 00:04:55,400 to really collapse into what we think is a single point. 74 00:04:55,400 --> 00:04:56,733 I want to be clear here too. 75 00:04:56,733 --> 00:04:58,292 We don't know it's single point; 76 00:04:58,292 --> 00:05:00,729 we've never gone into the center of a black hole. 77 00:05:00,729 --> 00:05:02,667 Just the mathmetics of the black hole -- 78 00:05:02,667 --> 00:05:04,274 or at least as we understand it right now -- 79 00:05:04,274 --> 00:05:09,104 have everything colliding into a single point where the math starts to break down. 80 00:05:09,104 --> 00:05:10,467 So we're really not sure 81 00:05:10,467 --> 00:05:12,933 what happens at that very small center point. 82 00:05:12,933 --> 00:05:16,067 But needless to say, it will be an unbelievably -- 83 00:05:16,067 --> 00:05:21,267 maybe infinite, maybe almost infitely dense point in space 84 00:05:21,267 --> 00:05:23,533 or dense amount of matter. 85 00:05:23,533 --> 00:05:27,000 And the reason why I kind of favor this primordial black hole, 86 00:05:27,000 --> 00:05:28,400 and why this would make sense, 87 00:05:28,400 --> 00:05:30,667 is right after the formation of the universe, 88 00:05:30,667 --> 00:05:35,333 all the matter in the universe was in a much denser space 89 00:05:35,333 --> 00:05:36,736 because the universe was smaller. 90 00:05:36,736 --> 00:05:38,895 So let's say this is right after the Big Bang, 91 00:05:38,895 --> 00:05:41,600 some period of time after the big bang. 92 00:05:41,600 --> 00:05:45,533 Now, what we've talked about before when we talked about cosmic background radiation 93 00:05:45,533 --> 00:05:47,974 is at that point, the universe was relatively uniform. 94 00:05:47,974 --> 00:05:53,222 It was super, super dense, but it was relatively uniform. 95 00:05:53,222 --> 00:05:55,333 So in a universe like this, 96 00:05:55,333 --> 00:05:57,656 there's no reason why anything would collapse into black holes 97 00:05:57,656 --> 00:05:59,548 because if you look at a point here, 98 00:05:59,548 --> 00:06:04,333 sure, there's a ton of mass close to it, but it's very close to it in every direction. 99 00:06:04,333 --> 00:06:05,267 So it would be pulled . . . 100 00:06:05,267 --> 00:06:07,285 the gravitational force would be the same in every direction, 101 00:06:07,423 --> 00:06:11,566 if it was completely uniform. 102 00:06:11,566 --> 00:06:13,267 but if you go shortly after the Big Bang -- 103 00:06:13,267 --> 00:06:15,733 maybe because of slight quantum fluctuaion effects -- 104 00:06:15,733 --> 00:06:18,044 it becomes slightly non-uniform. 105 00:06:18,044 --> 00:06:20,267 So let's say it becomes slightly non-uniform. 106 00:06:20,267 --> 00:06:24,867 But it still is unbelievably dense. 107 00:06:24,867 --> 00:06:26,933 So let's say it looks something like this, 108 00:06:26,933 --> 00:06:32,400 where you have areas that are denser, but it's slightly non-uniform. 109 00:06:32,400 --> 00:06:35,645 But extremely dense. 110 00:06:35,645 --> 00:06:38,133 So here, all of a sudden, you have the type of densities 111 00:06:38,133 --> 00:06:39,333 necessary for a black hole, 112 00:06:39,333 --> 00:06:43,667 and where you have higher densities, where it's less uniform, 113 00:06:43,667 --> 00:06:47,115 here all of a sudden, you will have inward force. 114 00:06:47,115 --> 00:06:50,733 The gravitational pull from things outside of this area 115 00:06:50,733 --> 00:06:53,267 is going to be less than the gravitational pull towads those areas. 116 00:06:53,267 --> 00:06:59,200 And the more things get pulled towards it, the less uniform it's going to get. 117 00:06:59,200 --> 00:07:02,348 So you can imagine, in that primordial universe, 118 00:07:02,348 --> 00:07:04,533 very shortly after the Big Bang, 119 00:07:04,533 --> 00:07:07,800 when things were very dense and closely packed together, 120 00:07:07,800 --> 00:07:10,467 we may -- we may -- have had the conditions 121 00:07:10,467 --> 00:07:13,733 where these supermassive black holes could have formed, 122 00:07:13,733 --> 00:07:16,280 where you had so much mass in such a small volume, 123 00:07:16,280 --> 00:07:21,467 and it was just not-uniform enough so that you could have this snowballing effect, 124 00:07:21,467 --> 00:07:29,600 so that more and more mass would collect into these supermassive black holes 125 00:07:29,600 --> 00:07:33,509 that are hundreds of thousands to billions of times the mass of the sun. 126 00:07:33,509 --> 00:07:35,800 And -- this is the even more interesting part -- 127 00:07:35,800 --> 00:07:40,568 those black holes would become the centers of future galaxies. 128 00:07:40,568 --> 00:07:45,933 So you have these black holes forming, these supermassive black holes forming. 129 00:07:45,933 --> 00:07:49,000 And not everything would go into a black hole; 130 00:07:49,000 --> 00:07:55,000 only if it didn't have a lot of angular velocity, it might go into the black hole. 131 00:07:55,000 --> 00:07:56,752 But if it's going past it fast enough, 132 00:07:56,752 --> 00:07:59,167 it'll just start going in orbit around the black hole. 133 00:07:59,167 --> 00:08:02,000 And so you can imagine that this is how the early galaxies, 134 00:08:02,000 --> 00:08:03,672 or even our galaxy formed. 135 00:08:03,672 --> 00:08:05,000 And so you might be wondering, 136 00:08:05,000 --> 00:08:06,830 "Well, what about the black hole at the center of the Milky Way?" 137 00:08:06,830 --> 00:08:10,498 We think there is one. 138 00:08:10,498 --> 00:08:14,585 We think there is one because we've observed stars orbiting very quickly 139 00:08:14,585 --> 00:08:24,267 around something at the center of our Milky Way. 140 00:08:24,267 --> 00:08:25,893 And the only plausible explanation 141 00:08:25,893 --> 00:08:29,400 for things orbitting so quickly around something 142 00:08:29,400 --> 00:08:32,975 is that it has to have a density of either a black hole, 143 00:08:32,975 --> 00:08:36,200 or something that will eventually turn into a black hole. 144 00:08:36,200 --> 00:08:37,600 And when you do the math, 145 00:08:37,600 --> 00:08:40,568 for the middle of our Galaxy, the center of the Milky Way, 146 00:08:40,568 --> 00:08:47,456 our supermassive black hole is on the order of four million times the mass 147 00:08:47,471 --> 00:08:50,667 of the sun. 148 00:08:50,667 --> 00:08:52,735 So hopefully that give you a little food for thought. 149 00:08:52,735 --> 00:08:55,667 There aren't just only stellar collapsed black holes. 150 00:08:55,667 --> 00:08:56,667 Or maybe there are, 151 00:08:56,667 --> 00:08:58,267 and maybe they somehow grow into supermassive black holes, 152 00:08:58,267 --> 00:09:00,067 and everything in between we just can't observe. 153 00:09:00,067 --> 00:09:02,867 Or that they really are a different class of black holes. 154 00:09:02,867 --> 00:09:04,200 They're actually formed different ways. 155 00:09:04,200 --> 00:09:08,000 Maybe they formed near the beginning of the actual universe. 156 00:09:08,000 --> 00:09:10,933 When the density of things was a little un-uniform, 157 00:09:10,933 --> 00:09:13,000 things condensed into each other. 158 00:09:13,000 --> 00:09:14,562 And what we're going to talk about in the next video is 159 00:09:14,562 --> 00:09:17,867 how these supermassive black holes could help generate 160 00:09:17,867 --> 00:09:21,876 unbelievable sources of radiation, 161 00:09:21,876 --> 00:09:25,000 even though the black holes themselves aren't emitting them. 162 00:09:25,000 --> 00:09:27,911 And those are going to be quasars.