1 00:00:00,000 --> 00:00:19,871 *36c3 preroll music* 2 00:00:19,871 --> 00:00:22,920 Herald: Ok, I have to say, I'm always deeply impressed about how much we already 3 00:00:22,920 --> 00:00:31,430 learned about space, about the universe and about our place in the universe, 4 00:00:31,430 --> 00:00:37,230 our solar system. But the next speakers will explain us how we can use 5 00:00:37,230 --> 00:00:44,320 computational methods to simulate the universe and actually grow planets. The 6 00:00:44,320 --> 00:00:49,530 speakers will be Anna Penzlin (miosta). She is PHC student in computational 7 00:00:49,530 --> 00:00:55,230 astrophysics in Tübingen and Carolin Kimmich (caro). She is a physics master's 8 00:00:55,230 --> 00:01:02,630 student at Heidelberg University. And the talk is entitled "Grow Your Own Planets 9 00:01:02,630 --> 00:01:07,691 How Simulations Help us understand the universe." Thank you! 10 00:01:07,691 --> 00:01:14,743 *applause* 11 00:01:14,743 --> 00:01:24,720 caro: So hi, everyone. It's a cool animation right? And the really cool thing 12 00:01:24,720 --> 00:01:28,729 is that there's actually physics going on there. So this object could really be out 13 00:01:28,729 --> 00:01:35,380 there in space but was created on a computer. So this is how a star is 14 00:01:35,380 --> 00:01:41,680 forming, how our solar system could have looked like in the beginning. Thank you 15 00:01:41,680 --> 00:01:47,440 for being here and that you're interested in how we make such an animation. Anna and 16 00:01:47,440 --> 00:01:54,060 I are researchers in astrophysics. And we're concentrating on how planets form 17 00:01:54,060 --> 00:01:58,789 and evolve. She's doing her PHD and in Tübingen and I'm doing my masters in 18 00:01:58,789 --> 00:02:04,030 Heidelberg. And in this talk, we want to show you a little bit of physics and how 19 00:02:04,030 --> 00:02:13,390 we can translate that in such a way that a computer can calculate it. So, let's ask a 20 00:02:13,390 --> 00:02:19,420 question first. What is the universe or what's in the universe? The most part of 21 00:02:19,420 --> 00:02:23,650 the universe is something we don't understand, yet. It's dark matter and dark 22 00:02:23,650 --> 00:02:28,680 energy and we don't know what it is, yet. And that's everything we cannot see in 23 00:02:28,680 --> 00:02:35,120 this picture here. What we can see are stars and galaxies, and that's what we 24 00:02:35,120 --> 00:02:39,980 want to concentrate on in this talk. But if we can see it, why would we want to 25 00:02:39,980 --> 00:02:48,590 watch a computer? Well, everything in astronomy takes a long time. So each of 26 00:02:48,590 --> 00:02:54,299 these tiny specs you see here are galaxies just like ours. This is how the Milkyway 27 00:02:54,299 --> 00:02:59,560 looks like. And we are living in this tiny spot here. And as you all know, our earth 28 00:02:59,560 --> 00:03:04,080 takes one year to orbit around the sun. Now, think about how long it takes for the 29 00:03:04,080 --> 00:03:10,459 sun to orbit around the center of the galaxy. It's four hundred million years. 30 00:03:10,459 --> 00:03:16,159 And even the star formation is 10 million years. We cannot wait 10 million years to 31 00:03:16,159 --> 00:03:23,520 watch how a star is forming, right? That's why we need computational methods or 32 00:03:23,520 --> 00:03:29,730 simulations on a computer to understand these processes. So, when we watch to the 33 00:03:29,730 --> 00:03:35,970 night sky, what do we see? Of course we see stars and those beautiful nebulas. 34 00:03:35,970 --> 00:03:42,319 They are a gas and dust. And all of these images are taken with Hubble Space 35 00:03:42,319 --> 00:03:51,060 Telescope. Oh, so there's one image that does belong in there. But it looks very 36 00:03:51,060 --> 00:03:56,810 similar, right? This gives us the idea that we can describe the gases in the 37 00:03:56,810 --> 00:04:04,840 universe as a fluid. It's really complicated to describe the gas in every 38 00:04:04,840 --> 00:04:10,069 single particle. So, we cannot track every single molecule in the gas that moves 39 00:04:10,069 --> 00:04:15,489 around. It's way easier to describe it as a fluid. So remember that for later, we 40 00:04:15,489 --> 00:04:22,310 will need that. But first, let's have a look how stars form. A star forms from a 41 00:04:22,310 --> 00:04:29,500 giant cloud of dust and gas. Everything moves in that cloud. So, eventually more 42 00:04:29,500 --> 00:04:38,699 dense regions occur and they get even denser. And these clams can eventually 43 00:04:38,699 --> 00:04:47,160 collapse to one star. So, this is how a star forms. They collapse due to their own 44 00:04:47,160 --> 00:04:53,810 gravity. And in this process, a disc forms. And in this disc, planets can form. 45 00:04:53,810 --> 00:04:59,710 So why a disc? As I said, everything moves around in the cloud. So it's likely that 46 00:04:59,710 --> 00:05:05,120 the cloud has a little bit of an initial rotation. As it collapses, this rotation 47 00:05:05,120 --> 00:05:11,840 gets larger and faster. And now you can think of making a pizza. So when you make 48 00:05:11,840 --> 00:05:16,870 a pizza and spin your dough on your finger, you get a flat disc like a star, 49 00:05:16,870 --> 00:05:25,110 like a disc around a star. That's the same process, actually. In this disc, we have 50 00:05:25,110 --> 00:05:31,330 dust and gas. From this dust in the disc the planet can form. But how do we get 51 00:05:31,330 --> 00:05:40,759 from tiny little dust particles to a big planet? Well, it somehow has to grow and 52 00:05:40,759 --> 00:05:46,320 grow even further and compact until we have rocks. And even grow further until we 53 00:05:46,320 --> 00:05:53,390 reach planets. How does it grow? Well, that dust grows we know that. At least 54 00:05:53,390 --> 00:06:00,720 that's what I observed when I took those images in my flat. Well, so dust can grow 55 00:06:00,720 --> 00:06:06,680 and grow even further and compact. But when you take two rocks, we're now at this 56 00:06:06,680 --> 00:06:11,729 in this stage. When you take two rocks and throw them together, you don't expect them 57 00:06:11,729 --> 00:06:21,060 to stick, right? You expect them to crash and crack into a thousand pieces. So, 58 00:06:21,060 --> 00:06:28,300 we're standing on the proof that planets exist. How does this happen? And it's not 59 00:06:28,300 --> 00:06:34,699 quite solved yet in research. So, this is a process that is really hard to observe 60 00:06:34,699 --> 00:06:39,379 because planets are very, very tiny compared to stars. And even stars are only 61 00:06:39,379 --> 00:06:45,380 small dots in the night sky. Also, as I said, planets form in a disc. And it's 62 00:06:45,380 --> 00:06:52,389 hard to look inside the disc. So this is why we need computation to understand a 63 00:06:52,389 --> 00:06:58,670 process that how planets form and other astronomical processes. So let's have a 64 00:06:58,670 --> 00:07:09,530 look at how this simulated on a computer. miosta: OK. So, somehow we have seen 65 00:07:09,530 --> 00:07:15,949 nature. It's beautiful and it's just like a tank of water and a bubbly fluid we 66 00:07:15,949 --> 00:07:21,000 already have. So, now we have this bubbly fluid and here in the middle demonstrated. 67 00:07:21,000 --> 00:07:25,819 But now we have to teach our computer to deal with the bubbly fluid. And that's way 68 00:07:25,819 --> 00:07:31,601 too much single molecules to simulate them, as we already said. So there are two 69 00:07:31,601 --> 00:07:37,759 ways to discretize it in a way that we just look at smaller pieces. One is the 70 00:07:37,759 --> 00:07:47,080 Lagrangian description, just like taking small bubbles or balls of material that 71 00:07:47,080 --> 00:07:52,370 have a fixed mass. They have a certain velocity that varies between each particle 72 00:07:52,370 --> 00:07:57,180 and they have, of course, a momentum because they have a velocity and a mass. 73 00:07:57,180 --> 00:08:01,629 And we've created a number of those particles and then just see how they move 74 00:08:01,629 --> 00:08:08,260 around and how they collide with each other. That would be one way. And that was 75 00:08:08,260 --> 00:08:12,639 described last year in a very good talk. I can highly recommend to hear this talk if 76 00:08:12,639 --> 00:08:18,099 you're interested in this method. However, there's a second way to also describe 77 00:08:18,099 --> 00:08:23,169 this. Not just going with the flow of the particles, but we are a bit lazy, we just 78 00:08:23,169 --> 00:08:30,039 box it. So we create a grid. And as you see down here in this grid, you have the 79 00:08:30,039 --> 00:08:38,789 certain filling level, a bit of a slope. So, what's the trend there? And then we 80 00:08:38,789 --> 00:08:44,910 just look for each box, what flows in what flows out through the surfaces of this 81 00:08:44,910 --> 00:08:51,280 box. And then we have a volume or a mass filled within this box. And this is how we 82 00:08:51,280 --> 00:08:57,230 discretize what is going on in the disc. And actually, since we are usually in the 83 00:08:57,230 --> 00:09:04,220 system of a disc, we do not do it in this nice box way like this. But we use boxes 84 00:09:04,220 --> 00:09:09,710 like those because they are already almost like a disc and we just keep exactly the 85 00:09:09,710 --> 00:09:14,780 same boxes all the time and you just measure what goes through the surface in 86 00:09:14,780 --> 00:09:22,890 these boxes. So, this is how these two methods look like if you compute with both 87 00:09:22,890 --> 00:09:30,030 of them. So, one was done by me. I'm usually using this boxing method and the 88 00:09:30,030 --> 00:09:35,960 other was done by my colleague. You see this like when you look at them, at the 89 00:09:35,960 --> 00:09:40,490 colors, at the structure here, you have the slope inwards, you have the same slope 90 00:09:40,490 --> 00:09:46,600 inwards here. You have even this silly structure here. The same here. But what 91 00:09:46,600 --> 00:09:52,640 you notice is you have this enlarge dots that are really the mass particles we saw 92 00:09:52,640 --> 00:09:57,980 before, these bubbles. And here you have this inner cutout. This is because when 93 00:09:57,980 --> 00:10:05,450 you create this grid, you have the very region at the inner part of the disc where 94 00:10:05,450 --> 00:10:11,410 the boxes become tiny and tinier. And well, we can't compute that. So, we have 95 00:10:11,410 --> 00:10:19,130 to cut out at some point in inner part So, here when you go to low densities, these 96 00:10:19,130 --> 00:10:24,580 bubbles blow up and distribute their mass over a larger area. So, it's not very 97 00:10:24,580 --> 00:10:30,840 accurate for these areas. And here we have the problem we can't calculate the inner 98 00:10:30,840 --> 00:10:38,590 area. So both methods have their pros and cons. And are valid. But now, for most we 99 00:10:38,590 --> 00:10:51,460 will focus on this one. Just so we have this nice stream features. So, again, 100 00:10:51,460 --> 00:11:00,150 going back to the boxes, we have to measure the flow between the boxes. This 101 00:11:00,150 --> 00:11:06,580 flow, in physics we call it flux, and we have a density row one, density row too. 102 00:11:06,580 --> 00:11:12,280 And the flux is the description of what mass moves through the surface here from 103 00:11:12,280 --> 00:11:22,100 one box to the next. So, if we write this in math terms, it looks like this. This 104 00:11:22,100 --> 00:11:36,560 says the time derivative of the density, meaning the change over time. So how much 105 00:11:36,560 --> 00:11:43,540 faster or slower, the velocity would be a change in time. And then this weird 106 00:11:43,540 --> 00:11:50,440 triangle symbol it's called nabla is a positional derivative. So, it's like a 107 00:11:50,440 --> 00:12:00,620 slope. So, how do we change our position, actually. So, if we change, look at the 108 00:12:00,620 --> 00:12:10,200 density over time, it should correlate to what inflow we have over position. That is 109 00:12:10,200 --> 00:12:16,100 what that says. So and then we have in physics a few principles that we have 110 00:12:16,100 --> 00:12:21,920 always to obey because that is just almost common sense. One of them is, well, if we 111 00:12:21,920 --> 00:12:29,760 have mass in a box. Well, like this, the mass should not go anywhere unless someone 112 00:12:29,760 --> 00:12:35,200 takes it out. So, if we have a closed box and mass in that box, nothing should 113 00:12:35,200 --> 00:12:42,620 disappear magically. It should all stay in this box. So, even if these particles jump 114 00:12:42,620 --> 00:12:48,050 around in our box with a certain velocity, it's the same number of particles in the 115 00:12:48,050 --> 00:12:57,870 end. That's again, the same equation just told in math. So, a second very 116 00:12:57,870 --> 00:13:04,220 rudimentary principle is if we have energy in it, in a completely closed box. So, for 117 00:13:04,220 --> 00:13:10,480 example, this nice chemicals here and we have a certain temperature. So, in this 118 00:13:10,480 --> 00:13:18,150 case, our temperature is low, maybe like outside of around zero degree Celsius. And 119 00:13:18,150 --> 00:13:24,070 then we have this nice chemicals down here and at some point they react very heavily. 120 00:13:24,070 --> 00:13:30,770 We suddenly end up with much less chemical energy and a lot more thermal energy. But 121 00:13:30,770 --> 00:13:36,940 overall, the complete energy summed up here, like the thermal and the chemical 122 00:13:36,940 --> 00:13:47,290 energy, also the energy of the movement and the energy of potential added up to 123 00:13:47,290 --> 00:13:53,720 this variable "U". That should not change over time if you sum up everything. 124 00:13:53,720 --> 00:13:59,500 Because our energy is conserved within our clothed box. And then the third thing is I 125 00:13:59,500 --> 00:14:09,860 think you all know this. If you have like a small mass with a certain velocity, a 126 00:14:09,860 --> 00:14:14,181 very high velocity in this case and it bumps into someone very large, what 127 00:14:14,181 --> 00:14:21,330 happens? Well, you get a very small velocity in this large body and the 128 00:14:21,330 --> 00:14:28,260 smaller mass stops. And the principle here is that momentum is conserved, meaning 129 00:14:28,260 --> 00:14:36,680 that the velocity times the mass of one object is the same as then later for the 130 00:14:36,680 --> 00:14:42,700 other one. But since it's larger, this product has to be the same. That doesn't 131 00:14:42,700 --> 00:14:49,381 change. And we have also in our simulations to obey these rules and we 132 00:14:49,381 --> 00:14:54,730 have to code that in so that we have physics in them. So you say, ok, this is 133 00:14:54,730 --> 00:14:59,450 really simple, these rules, right? But actually, well, it's not quite as simple. 134 00:14:59,450 --> 00:15:03,880 So, this is the Navier-Stokes equation, a very complicated equation is not 135 00:15:03,880 --> 00:15:10,550 completely solved. And we have here all that is marked red are derivatives. Here 136 00:15:10,550 --> 00:15:16,230 we have our conservation law that was the nice and simple part. But now we have to 137 00:15:16,230 --> 00:15:25,700 take other physical things into accounting for pressure, accounting for viscosity, 138 00:15:25,700 --> 00:15:33,370 for compression. So squeezing. And like how sticky is our fluid? And also gravity. 139 00:15:33,370 --> 00:15:38,790 So, we have a lot of additional factors, additional physics we also have to get in 140 00:15:38,790 --> 00:15:45,470 somehow. And all of these also depend somehow on the change of position or the 141 00:15:45,470 --> 00:15:51,850 change of time. And these derivatives aren't really nice for our computers 142 00:15:51,850 --> 00:15:57,370 because they well, they don't understand this triangle. So, we need to find a way 143 00:15:57,370 --> 00:16:03,920 to write an algorithm so that it can somehow relate with these math formula in 144 00:16:03,920 --> 00:16:14,920 a way that the computer likes. And one of the way to do this is, well, the simplest 145 00:16:14,920 --> 00:16:24,690 solution actually is just we say, OK, we have now this nasty derivatives and we 146 00:16:24,690 --> 00:16:32,160 want to get rid of them. So, if we look just at one box now and we say that in 147 00:16:32,160 --> 00:16:42,170 this box, the new value for the density in this box would be the previous density, 148 00:16:42,170 --> 00:16:49,590 plus the flux in and out times the time stepover which we measure this flux, 149 00:16:49,590 --> 00:16:58,260 right? So, and we have to somehow get to this flux and we just say, OK, this flux 150 00:16:58,260 --> 00:17:06,220 now is if we start here and the slope of this curve, the trends so to say, where 151 00:17:06,220 --> 00:17:10,550 this curve is going right now, it would look like this. So, in our next step, time 152 00:17:10,550 --> 00:17:19,140 step, we would have a density down here. And well, then we do this again. We again 153 00:17:19,140 --> 00:17:25,620 look at this point, where's the trend going, where's the line going? And then we 154 00:17:25,620 --> 00:17:36,539 end up here. Same here. So, again, we just try to find this flax and this is the 155 00:17:36,539 --> 00:17:43,130 trend at this position in time. So, this goes up here. And then if we are here now, 156 00:17:43,130 --> 00:17:48,399 look at this point, it should go up here. So this is what our next trend would be. 157 00:17:48,399 --> 00:17:55,269 And we do this over all the times. And this is how our simulation then would 158 00:17:55,269 --> 00:18:02,929 calculate the density for one box over a different time steps. So, that kind of 159 00:18:02,929 --> 00:18:09,250 works. So, the blue curve is the analytical one, the red curve, well it 160 00:18:09,250 --> 00:18:17,740 kind of similar, it works. But can we do better? It's not perfect, yet, right? So, 161 00:18:17,740 --> 00:18:23,259 what we can do is we refine this a bit, taking a few more steps, making it a bit 162 00:18:23,259 --> 00:18:31,000 more computationally heavy, but trying to get a better resolution. So, first we 163 00:18:31,000 --> 00:18:36,309 start with the same thing as before. We go to this point, find the trend in this 164 00:18:36,309 --> 00:18:43,689 point. That point like the line would go in this direction from this point. And 165 00:18:43,689 --> 00:18:51,529 then we go just half a step now. Sorry! And now we look at this half a step to 166 00:18:51,529 --> 00:18:57,650 this point now. And again, the same saying, OK, where's the trend going now? 167 00:18:57,650 --> 00:19:07,539 And then we take where this point would go and added to this trend. So that would be 168 00:19:07,539 --> 00:19:14,179 that. The average of this trend, of this exact point and this trend, this dark 169 00:19:14,179 --> 00:19:19,360 orange curve. And then we go back to the beginning with this trend now and say this 170 00:19:19,360 --> 00:19:24,260 is a better trend than the one we had before. We now use that and go again and 171 00:19:24,260 --> 00:19:34,700 search the point for half a time step. And then again, we do the same thing. Now we 172 00:19:34,700 --> 00:19:42,459 again try to find actually the trend and average it with the arrow before. So it's 173 00:19:42,459 --> 00:19:46,321 not exactly the trend. It's a bit below the trend because we averaged it with the 174 00:19:46,321 --> 00:19:51,880 arrow before. And now we take this averaging trend from the beginning to the 175 00:19:51,880 --> 00:19:57,080 top like this. Okay. This is already quite good, but we can still do a little bit 176 00:19:57,080 --> 00:20:02,570 better if we averaged with our ending point. So, we go here, look, where is the 177 00:20:02,570 --> 00:20:10,740 trend going that would go quite up like this and we average this and this together 178 00:20:10,740 --> 00:20:15,110 and then we end up with a line like this. This is so much better than what we had 179 00:20:15,110 --> 00:20:22,920 before. It's a bit more complicated, to be fair. But actually it's almost on the 180 00:20:22,920 --> 00:20:29,059 line. So, this is what we wanted. So, if you compare both of them, we have here our 181 00:20:29,059 --> 00:20:34,690 analytical curve. So, over time in one box, this is how the densities should 182 00:20:34,690 --> 00:20:39,909 increase. And now with it both of the numerical method, the difference looks 183 00:20:39,909 --> 00:20:46,050 like this. So, if we have smaller and smaller time steps, even the Euler gets 184 00:20:46,050 --> 00:20:55,749 closer and closer to the curve. But actually the Runge Kutta this four step process 185 00:20:55,749 --> 00:21:00,620 works much better and much faster. However, it's a bit more computationally 186 00:21:00,620 --> 00:21:08,370 and difficult. caro: When we simulate objects in 187 00:21:08,370 --> 00:21:15,039 astronomy, we always want to compare that to objects that are really out there. So, 188 00:21:15,039 --> 00:21:20,489 this is a giant telescope, well consisting of a lot of small telescopes. But they can 189 00:21:20,489 --> 00:21:27,010 be connected and used as a giant telescope and it takes photos of dust in the sky. 190 00:21:27,010 --> 00:21:34,159 And this is used to take images of discs around stars. And these discs look like 191 00:21:34,159 --> 00:21:41,049 this. So, these images were taken last year and they are really cool. Before we 192 00:21:41,049 --> 00:21:46,121 had those images, we only had images with less resolution. So, they were just 193 00:21:46,121 --> 00:21:52,120 blurred blobs. And we could say, yeah, that might be a disc. But now we really 194 00:21:52,120 --> 00:21:58,659 see the discs and we see rings here, thin rings and we see thicker rings over here. 195 00:21:58,659 --> 00:22:05,590 And even some spiraly structures here. And also some features that are not really 196 00:22:05,590 --> 00:22:11,990 radial symmetric like this arc here. And it's not completely solved how these 197 00:22:11,990 --> 00:22:24,259 structures formed. And to find that out a colleague of mine took this little object 198 00:22:24,259 --> 00:22:30,799 with the asymmetry here. And so, this is image we just saw. And this is his 199 00:22:30,799 --> 00:22:37,590 simulation. So, this is how the disc looked like in the beginning, probably. 200 00:22:37,590 --> 00:22:43,980 And we put in three planets and let the simulation run. And so, what we see here 201 00:22:43,980 --> 00:22:52,029 is that the star is cut out as Anna said. So, the grid cells in the inner part are 202 00:22:52,029 --> 00:22:56,690 very, very small. And it would take a long time to compute them all. So, that's why 203 00:22:56,690 --> 00:23:06,779 we're leaving out that spot in the middle. And what we see here is three planets 204 00:23:06,779 --> 00:23:16,309 interacting with the material in the disc. And we can see that these planets can make 205 00:23:16,309 --> 00:23:24,440 this thing here appear so that in the end we have something looking very similar to 206 00:23:24,440 --> 00:23:30,700 what we want to have or what we really observe. So, we can say three planets 207 00:23:30,700 --> 00:23:37,379 could explain how these structures formed in this disc. It's a little bit 208 00:23:37,379 --> 00:23:42,409 elliptical, you see that. That's because it's tilted from our side of line. It 209 00:23:42,409 --> 00:23:47,430 would be round if you watched at it face on. But it's a little bit tilted. That's 210 00:23:47,430 --> 00:23:55,269 why it looks elliptical. miosta: So, we already saw we can put 211 00:23:55,269 --> 00:24:02,080 planets in the gas and then we create structures. One very exciting thing that 212 00:24:02,080 --> 00:24:08,740 we found in the last year - or two years ago it started but then we found more - is 213 00:24:08,740 --> 00:24:15,690 this system PDS 70. In this system, for the very first time, we found a planet 214 00:24:15,690 --> 00:24:24,249 that was still embedded completely within the disc. So, the gas and dust. Usually, 215 00:24:24,249 --> 00:24:32,259 because the gas and dust is the main thing that creates this signal of some radiation 216 00:24:32,259 --> 00:24:37,749 because of heat. We only observe that and then we can't observe the planet embedded. 217 00:24:37,749 --> 00:24:41,629 But in this case, the planet was large enough. And in the right position that we 218 00:24:41,629 --> 00:24:48,940 actually were able to observe some signature of accretion on this planet that 219 00:24:48,940 --> 00:24:57,440 was brighter than the rest of the disc. And then later, just this year, just a few 220 00:24:57,440 --> 00:25:03,739 months ago, we actually found out well, this is not the only object here. This is 221 00:25:03,739 --> 00:25:10,850 very clearly a planet. But actually, like this spot here is also something. So, 222 00:25:10,850 --> 00:25:17,299 we can see it in different grains. Every picture here is a different set of grains 223 00:25:17,299 --> 00:25:26,950 observed. And we can see this in five different kinds of 224 00:25:26,950 --> 00:25:32,799 observations. So, there is a planet here. And then there is also something we don't 225 00:25:32,799 --> 00:25:37,710 know what it is yet, but its point like and actually creates the feature that we 226 00:25:37,710 --> 00:25:43,240 reproduce in different kinds of observational bands or different kinds of 227 00:25:43,240 --> 00:25:52,070 signals here. This is very interesting. For the first time, we actually see a 228 00:25:52,070 --> 00:25:58,030 planet forming right now within the disc. And so a colleague of mine also is very 229 00:25:58,030 --> 00:26:04,929 interested in the system and started to simulate how do two planets in a disc 230 00:26:04,929 --> 00:26:13,149 change the dynamics of a disc? So here we have, of course, this disc is again tilted 231 00:26:13,149 --> 00:26:20,230 because it's not phase on, it's like 45 degrees tilted, not like this, but like 232 00:26:20,230 --> 00:26:27,289 this. And so he had it face on. This is what a simulation looks like. So, there 233 00:26:27,289 --> 00:26:33,880 are two planets: these blobs here, again, as in this simulation. Here we have a 234 00:26:33,880 --> 00:26:39,289 close up. You can actually see this little boxes are actually our simulation boxes in 235 00:26:39,289 --> 00:26:47,429 which we have our own densities. And then he just looked at how the planets would 236 00:26:47,429 --> 00:26:52,620 change the structure and the gas and also how the gas would interact with the 237 00:26:52,620 --> 00:26:59,249 planets, shifting them around. And it's interesting. So, the planets tend to clear 238 00:26:59,249 --> 00:27:05,259 out an area, open a gap, and within the disk, that block has a lot of gas around 239 00:27:05,259 --> 00:27:11,039 here. So, you have the brighter ring here again and then clearing out more and more. 240 00:27:11,039 --> 00:27:23,390 And at some point in the simulation you saw they get a bit jumpy. So it's very nice. 241 00:27:23,390 --> 00:27:29,570 You also see that planets induce in the whole disc some kind of features like 242 00:27:29,570 --> 00:27:36,740 spiral features. And so a single planet will change the symmetry and the 243 00:27:36,740 --> 00:27:40,989 appearance of a whole disc. caro: So, the reason why the planet is 244 00:27:40,989 --> 00:27:46,489 staying at this point is because we're rotating with the planet. So it's actually 245 00:27:46,489 --> 00:27:53,499 going around the disc, but the like camera is rotating with the planet. So, it's 246 00:27:53,499 --> 00:28:00,360 staying at that fixed place we put it in. miosta: Exactly. But there's more because 247 00:28:00,360 --> 00:28:04,600 as I already said, in the Navier-Stokes equation, we have a lot of different kinds 248 00:28:04,600 --> 00:28:08,970 of physics that we all have to include in our simulations. One of the things, of 249 00:28:08,970 --> 00:28:15,149 course, is we maybe don't have just a star and a disc. We have planets in there and 250 00:28:15,149 --> 00:28:20,600 maybe two stars in there. And all of these larger bodies have also an interaction 251 00:28:20,600 --> 00:28:27,360 between each other. So, if we have the star, every planet will have an 252 00:28:27,360 --> 00:28:32,600 interaction with the star, of course. But then also the planets between each other, 253 00:28:32,600 --> 00:28:40,381 they have also an interaction, right? So, in the end, you have to take into account 254 00:28:40,381 --> 00:28:48,820 all of these interactions. And then also we have accretion just looking like this. 255 00:28:48,820 --> 00:28:59,350 So, accretion means that the gas is bound by some objects. It can be the disc, the 256 00:28:59,350 --> 00:29:07,009 planet or the star that takes up the mass, the dust or the gas and bounce it to this 257 00:29:07,009 --> 00:29:14,840 object. And then it's lost to the disc or the other structures because it's 258 00:29:14,840 --> 00:29:22,309 completely bound to that. So, the principle of this would be the simulation 259 00:29:22,309 --> 00:29:29,279 I did last year and published, we have here a binary star. So, these two dots are 260 00:29:29,279 --> 00:29:38,809 stars. I kind of kept them in the same spot. But every picture will be one orbit 261 00:29:38,809 --> 00:29:42,759 of this binary, but since we have interactions, you actually see them 262 00:29:42,759 --> 00:29:48,539 rotating because of the interactions, with each other. And then also we have here a 263 00:29:48,539 --> 00:29:52,669 planet and here a planet. And the interesting thing was that these two 264 00:29:52,669 --> 00:30:00,361 planets interact in such a way that they end up on exactly the same orbit. So, one 265 00:30:00,361 --> 00:30:06,179 star's further out, the orange one, and then very fast they go in. And they end up on 266 00:30:06,179 --> 00:30:28,419 exactly the same orbit. If it now play nicely. So, another thing is with the accretion here, 267 00:30:28,419 --> 00:30:36,600 we actually see clouds from above dropping down onto the new forming star here. So, 268 00:30:36,600 --> 00:30:44,409 all of this, what you see here would be gas, hydrogen. And it's a very early phase 269 00:30:44,409 --> 00:30:49,499 so that disc is not completely flat. It has a lot of material. And then we 270 00:30:49,499 --> 00:30:55,779 actually have this infall from above towards the star and then the star keeps 271 00:30:55,779 --> 00:31:01,767 the mass. And we have to take this also into account in our simulations. Another 272 00:31:01,767 --> 00:31:07,220 thing we have to take into account up till now, we just cared about masses and 273 00:31:07,220 --> 00:31:12,929 densities. But of course what we actually see is that stars are kind of warm, 274 00:31:12,929 --> 00:31:21,759 hopefully. Otherwise, temperatures here would also not be nice. And different 275 00:31:21,759 --> 00:31:27,929 chemicals have different condensation points. And this is also true in a system. 276 00:31:27,929 --> 00:31:35,019 So, we start with the start temperature at the surface of the star. We have a 277 00:31:35,019 --> 00:31:41,479 temperature around 4.000 Kelvin. And then we go a bit into the disc. And there is a 278 00:31:41,479 --> 00:31:47,889 point where we for the first time reach a point where we have any material at all. 279 00:31:47,889 --> 00:31:52,169 Because it starts to condensate and we actually have something solid like iron. 280 00:31:52,169 --> 00:31:58,179 For example, at a 1500 Kelvin. And then if we go further in, we reach a point where 281 00:31:58,179 --> 00:32:07,690 we have solid water and this is at 200 Kelvin. This is what we then would need 282 00:32:07,690 --> 00:32:12,590 actually to have a planet that also has water on it. Because if we don't get the 283 00:32:12,590 --> 00:32:18,889 water in the solid state, it will not fall onto a terrestrial planet and be bound 284 00:32:18,889 --> 00:32:24,899 there, right? So, this is important for our Earth, actually. And then if we go 285 00:32:24,899 --> 00:32:33,340 even further out, we have also other gases condensating to solids like CO2 or methane 286 00:32:33,340 --> 00:32:40,591 or things like that. And since we only get water on a planet when we have a 287 00:32:40,591 --> 00:32:48,009 temperature that is low enough so that the water actually forms is solid and it's 288 00:32:48,009 --> 00:32:54,269 important for us to think about where that is in our forming disc. Where do we start? 289 00:32:54,269 --> 00:32:59,769 We have a planet like Earth that could have some water, right? But it's not just 290 00:32:59,769 --> 00:33:07,570 the simple picture, where we have all these nice ring structures, where we have a clear 291 00:33:07,570 --> 00:33:13,619 line. Actually, it gets more complicated because we have pressure and shocks. And 292 00:33:13,619 --> 00:33:19,539 thermodynamics is a lot like pogo dancing, right? You crash into each other. And it's 293 00:33:19,539 --> 00:33:25,629 all about collisions. So, the gas temperature is determined by the speed of 294 00:33:25,629 --> 00:33:31,299 your gas molecules. Like you bouncing and crashing into each other, exchanging 295 00:33:31,299 --> 00:33:39,340 momentum. So, there's two ways to heat up such dance. First thing is you get a large 296 00:33:39,340 --> 00:33:45,944 amount of velocity from the outside like a huge kick, a shock into your system. A 297 00:33:45,944 --> 00:33:51,519 second way would be if we have a higher pressure, like more molecules, then also 298 00:33:51,519 --> 00:33:55,909 you of course have more collisions and then a higher temperature. So, if you 299 00:33:55,909 --> 00:34:02,529 change - because you have a planet now in the system - the pressure at some point, 300 00:34:02,529 --> 00:34:08,700 you actually get a higher temperature. So, that is not then we lose this nice line 301 00:34:08,700 --> 00:34:19,136 because suddenly we have different pressures at different locations. And a 302 00:34:19,136 --> 00:34:24,700 colleague of mine also simulated this. So, this is the initial condition we 303 00:34:24,700 --> 00:34:28,860 just assumed: OK, if we have no disturbance whatsoever, we have our nice 304 00:34:28,860 --> 00:34:36,890 planet here at 1au. So, same distance as earth to the sun. Here, too. But here we 305 00:34:36,890 --> 00:34:46,670 assume that less heat gets transferred from the surface of the disc. And here we 306 00:34:46,670 --> 00:34:52,030 have the planet far out like Jupiter or something. And now we actually let this 307 00:34:52,030 --> 00:34:59,590 planet change the structure of the disc. And what happens is - we found these spirals 308 00:34:59,590 --> 00:35:05,800 and within these spirals, we change pressure. And with this actually, if you 309 00:35:05,800 --> 00:35:11,590 see this orange, everywhere where it's orange it's hotter than the iceline. So, 310 00:35:11,590 --> 00:35:17,020 we don't have water where it's orange. And where it's blue we can have water. And the 311 00:35:17,020 --> 00:35:22,350 interesting thing is, even if we put a planet out here like Jupiter, we still 312 00:35:22,350 --> 00:35:32,569 form these regions in the inner system where we have less water. 313 00:35:32,569 --> 00:35:38,022 caro: One problem in astrophysical simulations is that we don't always know 314 00:35:38,022 --> 00:35:47,940 how to shape our boxes or how small these boxes have to be. So, we use a trick to 315 00:35:47,940 --> 00:35:54,670 reshape the boxes as we need them. It's called adaptive mesh. And this is a 316 00:35:54,670 --> 00:35:58,890 simulation of the red fluid flowing in this direction and the blue fluid in the 317 00:35:58,890 --> 00:36:06,581 other one. So, at the boundary, the two fluid shear and they mix up somehow and we 318 00:36:06,581 --> 00:36:12,990 don't know how in advance. So, we start a simulation and as the simulation starts, 319 00:36:12,990 --> 00:36:19,640 we reshape those boxes here. So, in the middle we don't need much. We reshape 320 00:36:19,640 --> 00:36:25,400 because it's not that complicated here. It's just the flow. But at the boundary we 321 00:36:25,400 --> 00:36:35,060 see those mixing up of the two fluids. And so, we reshape the cells as we need them. 322 00:36:35,060 --> 00:36:44,760 This is done in a program, in an astrophysical program called AREPO. We 323 00:36:44,760 --> 00:36:52,750 will later show you some more programs to use for simulations. But another 324 00:36:52,750 --> 00:36:59,020 simulation I want to show you first is also done with AREPO and it's a simulation 325 00:36:59,020 --> 00:37:04,710 of the universe. So, from here to here, it's very big. It's 30 million light 326 00:37:04,710 --> 00:37:12,210 years. So each of these dots you see here is the size of a galaxy or even more. And 327 00:37:12,210 --> 00:37:17,840 here you can actually see that at some regions it's very empty. So, we're 328 00:37:17,840 --> 00:37:23,420 rotating around this universe, this simulated universe here. And these regions 329 00:37:23,420 --> 00:37:28,990 here are empty. And we don't need a lot of boxes there. The big boxes are enough 330 00:37:28,990 --> 00:37:35,010 here. But in this dense regions where we have a lot of material, we need smaller 331 00:37:35,010 --> 00:37:42,380 boxes. And this method I showed you where we reshape the boxes as we need them is 332 00:37:42,380 --> 00:37:53,420 used for this simulation. miosta: So, actually, you see the 333 00:37:53,420 --> 00:37:56,340 beginning of the universe there. caro: Yes! 334 00:37:56,340 --> 00:38:01,000 miosta: Basically, the initial mass collapsing to the first galaxies and first 335 00:38:01,000 --> 00:38:07,030 supernovae starting. Very beautiful simulation. 336 00:38:07,030 --> 00:38:19,820 caro: So, there are different programs, as I already mentioned, in astrophysics. 337 00:38:19,820 --> 00:38:24,970 Three of them, those three are all open source, so you can download them and use 338 00:38:24,970 --> 00:38:31,090 them on your own machine, if you like. But there are more, a lot more. Some of them 339 00:38:31,090 --> 00:38:38,630 are open source, some of them are not. Sometimes it's hard to get them. In the 340 00:38:38,630 --> 00:38:43,700 following, we will present the tool FARGO3D and PLUTO in a detailed version or 341 00:38:43,700 --> 00:38:53,160 a more detailed vision than AREPO because we usually use those two for our 342 00:38:53,160 --> 00:38:58,380 simulations. What I want to show you with this slide is that depending on what you 343 00:38:58,380 --> 00:39:04,520 want to simulate, you need to choose a different program. And one thing is that 344 00:39:04,520 --> 00:39:10,250 in astrophysics we sometimes call the whole program code. So, if I use the word 345 00:39:10,250 --> 00:39:19,170 code. Sorry about that. I mean, the whole program. So, let's have a look at FARGO3D. 346 00:39:19,170 --> 00:39:27,870 It's a hydro dynamics code and what you see here is an input parameter file. There 347 00:39:27,870 --> 00:39:35,180 you define how the disc looks like. How much mass does it have? How big is it? And 348 00:39:35,180 --> 00:39:43,140 what planet? So, here at Jupiter, do you see that? Jupiter is put in. And we also 349 00:39:43,140 --> 00:39:51,280 define how big our boxes are. This program is written in C, which is quite 350 00:39:51,280 --> 00:39:57,500 nice because a lot of astrophysical programs are still written in Fortran. So, 351 00:39:57,500 --> 00:40:05,600 this is good for me because I don't know any Fortran. We can run this and what's 352 00:40:05,600 --> 00:40:11,010 typical for FARGO3D. So that's a compilation actually on my computer. So, I don't need 353 00:40:11,010 --> 00:40:18,840 a fancy computer. I just did it on my small laptop and now we run it. Now, 354 00:40:18,840 --> 00:40:26,130 typical for FARGO3D, as you will see are lot of dots. So, here it will print out a lot 355 00:40:26,130 --> 00:40:33,810 of dots and it will create at certain times some outputs. And these outputs are 356 00:40:33,810 --> 00:40:38,300 huge files containing numbers. So, if you look at them they are not really 357 00:40:38,300 --> 00:40:44,290 interesting. They just are a numbers in something like a text file. So, a big part 358 00:40:44,290 --> 00:40:50,430 of astrophysics is also to visualize the data. Not only to create it but also to 359 00:40:50,430 --> 00:40:57,080 make images so that we can make movies out of them. For that, I prefer to use Python 360 00:40:57,080 --> 00:41:01,600 but there are a lot of tools Python Matplotlib, but there are a lot of 361 00:41:01,600 --> 00:41:09,290 different tools to visualize the data. So, this is actually that output. That first 362 00:41:09,290 --> 00:41:16,350 one we just saw. The Jupiter planet in the disc that I defined in this parameter file 363 00:41:16,350 --> 00:41:23,280 and it's already started to do some spirals. And if I would have let it 364 00:41:23,280 --> 00:41:33,680 run further than the spirals were more prominent. And yeah, now we have a planet 365 00:41:33,680 --> 00:41:45,230 here on our computer. miosta: OK, so we also have PLUTO. PLUTO 366 00:41:45,230 --> 00:41:53,590 somehow has a bit more setup files. So, what I need is three files here. Looks a 367 00:41:53,590 --> 00:41:59,320 bit complicated to break it down. This file defines my grid and initial values. 368 00:41:59,320 --> 00:42:04,770 And this simulation time here we input actually what physics do we want to need? 369 00:42:04,770 --> 00:42:13,020 What is our coordinate system? So, do we want to have a disc or just like spherical 370 00:42:13,020 --> 00:42:20,660 boxes or like squared boxes? And how is the time defined? And here we then 371 00:42:20,660 --> 00:42:26,720 actually write a bit of code to say, OK, now how do I want a gravitational 372 00:42:26,720 --> 00:42:34,580 potential? So, what's the source of gravity or what will happen at the inner 373 00:42:34,580 --> 00:42:39,890 region where we have this dark spot? We have somehow to define what happens if gas 374 00:42:39,890 --> 00:42:45,090 reaches this boundary. Is it just falling in? Is it bouncing back or something? Or 375 00:42:45,090 --> 00:42:50,890 is it looping through the one end to the next? This is also something we then just 376 00:42:50,890 --> 00:43:01,530 have to code in. And if we then make it and let run, it looks like this. So, 377 00:43:01,530 --> 00:43:08,590 again, our nice thing we hopefully put in or wanted to put in: the time steps, what 378 00:43:08,590 --> 00:43:14,890 our boundaries were, parameters of physics. Hopefully, the right ones and 379 00:43:14,890 --> 00:43:21,396 then nicely we start with our time steps and then we see this. It's hooray! It 380 00:43:21,396 --> 00:43:27,240 worked actually. Because it's actually not quite simple usually to set up a running 381 00:43:27,240 --> 00:43:32,000 program. A running problem, because you have to really think about what should be 382 00:43:32,000 --> 00:43:38,170 the physics. What's the scale of your problem? What's the timescale of your 383 00:43:38,170 --> 00:43:44,990 problem? And specify this in a good way. But in principle, this is how it works. 384 00:43:44,990 --> 00:43:49,320 There are few test problems if you actually want to play around with this to 385 00:43:49,320 --> 00:43:56,390 make it easy for the beginning. And this is how we do simulations. So, as I already 386 00:43:56,390 --> 00:44:02,320 set, we can just start them on our laptops. So, here this is my laptop. I 387 00:44:02,320 --> 00:44:07,859 just type a dot slash FARGO3D and that should run, right? And then I just wait 388 00:44:07,859 --> 00:44:16,450 for ten years to finish the simulations of 500 timesteps or outputs. Well, that's not the best 389 00:44:16,450 --> 00:44:27,660 idea. So, we need more power. And both of us, for example, are using a cluster for 390 00:44:27,660 --> 00:44:36,880 Baden-Württemberg and that takes down our computation time by a lot. Usually, like a 391 00:44:36,880 --> 00:44:45,050 factor of maybe 20, which is a lot. So, I would need on my computer maybe a year and 392 00:44:45,050 --> 00:44:53,040 then I just need maybe 5 hours, a few days or a week on this cluster, which is 393 00:44:53,040 --> 00:44:56,380 usually the simulation time about a week for me. 394 00:44:56,380 --> 00:45:04,440 caro: So, what you see here is that we use GPUs, yes. But we do not or mostly not use 395 00:45:04,440 --> 00:45:09,630 them for gaming. We use them for actually actual science. Yeah, would be nice to 396 00:45:09,630 --> 00:45:20,614 play on that, right? That just said! miosta: So, back to our Earth, actually. 397 00:45:20,614 --> 00:45:27,670 So, can we now? We wanted to grow our own planet. We can do that, yes of course. Can 398 00:45:27,670 --> 00:45:31,600 we grow Earth? Well, Earth is a very special planet. We have a very nice 399 00:45:31,600 --> 00:45:37,720 temperature here, right? And we have not a crushing atmosphere like Jupiter, like a 400 00:45:37,720 --> 00:45:43,440 huge planet that we could not live under. We have a magnetic field that shields us 401 00:45:43,440 --> 00:45:53,760 from the radiation from space and we have water. But just enough water so that we 402 00:45:53,760 --> 00:46:00,170 still have land on this planet where we can live on. So, even if we fine tune 403 00:46:00,170 --> 00:46:05,230 simulations, the probability that we actually hit Earth and have all the 404 00:46:05,230 --> 00:46:12,800 parameters right is actually tiny. It's not that easy to simulate an Earth. And 405 00:46:12,800 --> 00:46:17,320 there are a lot of open questions, too. How did we actually manage to get just 406 00:46:17,320 --> 00:46:24,240 this sip of water on our surface? How did we manage to collide enough mass or 407 00:46:24,240 --> 00:46:30,060 aggregate enough mass to form this terrestrial planet without Jupiter is 408 00:46:30,060 --> 00:46:35,740 sweeping up all the mass in our system? How could we be stable in this orbit when 409 00:46:35,740 --> 00:46:42,660 there are seven other planets swirling around and interacting with us? All of 410 00:46:42,660 --> 00:46:48,660 this is open in our field of research actually, and not completely understood. 411 00:46:48,660 --> 00:46:54,620 This is the reason why we still need to do astrophysics and even in all our 412 00:46:54,620 --> 00:47:01,010 simulations there is no planet B. And the earth is quite unique and perfect for 413 00:47:01,010 --> 00:47:06,570 human life. So, please take care of the Earth and take care of yourself and of all 414 00:47:06,570 --> 00:47:12,270 the others people on the Congress. And thank you for listening and thank you to 415 00:47:12,270 --> 00:47:20,380 everyone who helped us make this possible. And to the people who actually coded our 416 00:47:20,380 --> 00:47:24,210 programs with which we simulate. Thank you! 417 00:47:24,210 --> 00:47:37,370 *applause* 418 00:47:37,370 --> 00:47:42,320 Herald: Thank you for the beautiful talk and for the message at the end, the paper 419 00:47:42,320 --> 00:47:47,970 is open for discussion, so if you guys have any questions, please come to the 420 00:47:47,970 --> 00:47:57,160 microphones. I'm asking my Signal Angel? No questions right now. But microphone two 421 00:47:57,160 --> 00:48:00,160 please! Mic2: Oh, yeah. Thank you very much. 422 00:48:00,160 --> 00:48:05,690 Really beautiful talk. I can agree. I have two questions. The first is short. You are 423 00:48:05,690 --> 00:48:10,980 using Navier-Stokes equation, but you have on the one hand, you have the dust disc 424 00:48:10,980 --> 00:48:14,940 and on the other hand, you have solid planets in it. And so are you using the 425 00:48:14,940 --> 00:48:18,620 same description for both or is it a hybrid? 426 00:48:18,620 --> 00:48:23,550 miosta: It very much depends. This is one of the things I showed you that for PLUTO, 427 00:48:23,550 --> 00:48:31,300 we write this C file that specifies some things and about every physicist has 428 00:48:31,300 --> 00:48:39,090 somewhat his or her own version of things. So, some usually the planets, if they are 429 00:48:39,090 --> 00:48:47,030 large, they will be put in as a gravity source. And possibly one that can accrete 430 00:48:47,030 --> 00:48:54,090 and pebbles are usually then put in a different way. However, also pebbles are 431 00:48:54,090 --> 00:48:57,540 at the moment a bit complicated. There are special groups specializing in 432 00:48:57,540 --> 00:49:04,080 understanding pebbles because as we said in the beginning, when they collide, 433 00:49:04,080 --> 00:49:10,450 usually they should be destroyed. If you hit two rocks very together, they don't 434 00:49:10,450 --> 00:49:14,870 stick. If you hit them hard together, they splatter around and we don't end up with an bigger object 435 00:49:14,870 --> 00:49:23,390 caro: Just to explain pebbles are small rocks or like big sand stones or something 436 00:49:23,390 --> 00:49:28,710 like that. Yeah. So bigger rocks, but not very big, yet. 437 00:49:28,710 --> 00:49:33,190 miosta: Yes! caro: It depends on which code you use. 438 00:49:33,190 --> 00:49:38,370 Mic2: Thank you. Very short, maybe one. Do you also need to include relativistic 439 00:49:38,370 --> 00:49:46,520 effects. Or is that completely out? miosta: It's a good question. Mostly if 440 00:49:46,520 --> 00:49:54,680 you have a solar type system, you're in the arrange where this is not necessary. 441 00:49:54,680 --> 00:50:00,010 For example, with the binaries, if they got very close together, then at the inner 442 00:50:00,010 --> 00:50:05,200 part of the disc, that is something we could consider. And actually, I know for 443 00:50:05,200 --> 00:50:10,500 PLUTO, it has modules to include relativistic physics, too, yes! 444 00:50:10,500 --> 00:50:14,000 Mic2: Thank you! Herald: OK, we have quite some questions, 445 00:50:14,000 --> 00:50:19,700 so keep them short. Number one, please! Mic1: Thank you. Yeah. Thank you very 446 00:50:19,700 --> 00:50:24,490 much for your interesting talk. And I think you had it on your very first slides 447 00:50:24,490 --> 00:50:31,780 that about 70 percent of the universe consists of dark matter and energy. Is that 448 00:50:31,780 --> 00:50:37,000 somehow considered in your simulations or how do you handle this? 449 00:50:37,000 --> 00:50:43,020 caro: Well in the simulations we make, we are doing planets and discs around stars. 450 00:50:43,020 --> 00:50:47,440 It's not considered there. In the simulation we showed you about the 451 00:50:47,440 --> 00:50:52,670 universe at the beginning, the blueish things were all dark matter. So, that was 452 00:50:52,670 --> 00:50:56,260 included in there. Mic1: OK, thank you. 453 00:50:56,260 --> 00:51:00,510 Herald: OK. Microphone 3. Mic3: Hi, thanks. Sorry, I think you 454 00:51:00,510 --> 00:51:05,740 talked about three different programs. I think PLUTO, FARGO3D and a third one. So, 455 00:51:05,740 --> 00:51:09,620 for a complete beginner: which program would you suggest is like you more use 456 00:51:09,620 --> 00:51:12,570 like if you want to learn more? Which one is user friendly or good? 457 00:51:12,570 --> 00:51:18,590 miosta: I would suggest FARGO3D first. It's kind of user friendly, has a somewhat good 458 00:51:18,590 --> 00:51:26,240 support and they are always also very thankful for actual comments and additions 459 00:51:26,240 --> 00:51:32,030 if people actually are engaged in trying to improve on that. Because we are 460 00:51:32,030 --> 00:51:37,328 physicists. We're not perfect programmers and we're also happy to learn more. So 461 00:51:37,328 --> 00:51:42,720 yeah, FARGO3D I would suggest, it has some easy ways of testing some systems and 462 00:51:42,720 --> 00:51:45,440 getting something done. caro: And it also has a very good 463 00:51:45,440 --> 00:51:53,567 documentation and also a manual "How to make the first steps on the Internet". So, 464 00:51:53,567 --> 00:51:56,980 you can look that up. Mic3: Awesome. Thank you. 465 00:51:56,980 --> 00:52:00,150 Herald: Let's get one question from outside, from my Signal Angel. 466 00:52:00,150 --> 00:52:05,600 Signal Angel: Thank you for your talk. There's one question from IRC: How do you 467 00:52:05,600 --> 00:52:09,510 know your model is good when you can only observe snapshots? 468 00:52:09,510 --> 00:52:17,770 caro: Oh, that's a good question. As we said, we're in theoretical astrophysics. 469 00:52:17,770 --> 00:52:25,170 So, there are theoretical models and these models cannot include everything. So, 470 00:52:25,170 --> 00:52:32,610 every single process, it's not possible because then we would calculate for years. 471 00:52:32,610 --> 00:52:37,480 Yeah, to know if a model is good you have to… 472 00:52:37,480 --> 00:52:46,430 miosta: Usually, you have a hypothesis or an observation that you somehow want to 473 00:52:46,430 --> 00:52:54,064 understand. With most of the necessary physics at this stage to reproduce this 474 00:52:54,064 --> 00:53:01,660 image. So, also from the observation we have to take into the account what our 475 00:53:01,660 --> 00:53:07,650 parameters kind of should be, how dense this end of the simulation should be and 476 00:53:07,650 --> 00:53:13,150 things like this. So, by comparing two observations, that's the best measure we 477 00:53:13,150 --> 00:53:21,790 can get. If we kind of agree. Of course, if we do something completely wrong, then 478 00:53:21,790 --> 00:53:26,600 it will just blow up or we will get a horribly high density. So, this is how we 479 00:53:26,600 --> 00:53:34,270 know. Physics will just go crazy if we do too large mistakes. Otherwise, we would 480 00:53:34,270 --> 00:53:39,330 try to compare two observations that it actually is sensible what we did. 481 00:53:39,330 --> 00:53:44,440 caro: Yeah, that's one of the most complicated tasks to include just enough 482 00:53:44,440 --> 00:53:52,400 physics that the system is represented in a good enough way. But not too much that 483 00:53:52,400 --> 00:53:57,400 our simulation would blow up in time. Herald: Number two, please. 484 00:53:57,400 --> 00:54:03,210 Mic2: I've got a question about the adaptive grids. How does the computer 485 00:54:03,210 --> 00:54:10,800 decide how to adapt the grid? Because the data where's the high density comes after 486 00:54:10,800 --> 00:54:17,660 making the grid... miosta: Yes, this is actually quite an 487 00:54:17,660 --> 00:54:25,470 interesting and also not quite easy to answer question. Let me try to give a 488 00:54:25,470 --> 00:54:34,300 breakdown nutshell answer here. The thing is, you measure and evaluate the 489 00:54:34,300 --> 00:54:39,380 velocities. Or in the flux, you also evaluate the velocity. And if the velocity 490 00:54:39,380 --> 00:54:44,840 goes high, you know there's a lot happening. So, we need a smaller grid then 491 00:54:44,840 --> 00:54:50,420 there. So, we try to create more grid cells where we have a higher velocity. In 492 00:54:50,420 --> 00:54:55,050 a nutshell, this is of course in an algorithm a bit harder to actually 493 00:54:55,050 --> 00:55:00,000 achieve. But this is the idea. We measured the velocities at each point. And then if 494 00:55:00,000 --> 00:55:03,510 we measure a high velocity, we change to a smaller grid. 495 00:55:03,510 --> 00:55:08,640 Mic2: So, you can predict where the mass will go and whether densities are getting high. 496 00:55:08,640 --> 00:55:12,600 miosta: Exactly. Step by step so to say. 497 00:55:12,600 --> 00:55:15,890 Mic2: Thanks Herald: We stay with Microphone 2. 498 00:55:15,890 --> 00:55:20,640 Mic2: Okay. I've got a bit of a classical question. So, I guess a lot relies on your 499 00:55:20,640 --> 00:55:25,201 initial conditions and I have two questions related to that. So first, I 500 00:55:25,201 --> 00:55:30,670 guess they are inspired by observations. What are the uncertainties that you have? 501 00:55:30,670 --> 00:55:33,850 And B, then what is the impact if you change your initial conditions like the 502 00:55:33,850 --> 00:55:41,170 density in the disc? miosta: Yeah, right now my main research 503 00:55:41,170 --> 00:55:46,110 is actually figuring out a sensible initial conditions or parameters for a 504 00:55:46,110 --> 00:55:53,220 disc. If you just let it have an initial set of conditions and a sensible set of 505 00:55:53,220 --> 00:56:00,420 parameters and let it run very long, you expect a system hopefully to convert to 506 00:56:00,420 --> 00:56:05,130 the state that it should be in. But your parameters are of course very important. 507 00:56:05,130 --> 00:56:12,240 And here we go back to what we can actually understand from observations. And 508 00:56:12,240 --> 00:56:17,880 what we need for example is the density, for example. And that is something we try 509 00:56:17,880 --> 00:56:24,900 to estimate from the light we see in these discs that you saw in this nice grid with 510 00:56:24,900 --> 00:56:31,110 all these discs we estimate OK, what's the average light there? What should then be 511 00:56:31,110 --> 00:56:37,790 the average densities of dust and gas in comparable disks. 512 00:56:37,790 --> 00:56:42,890 Mic2: Okay, thanks. Herald: Okay, one more at number two. 513 00:56:42,890 --> 00:56:50,150 Mic2: Yes. Thank you for the talk. When you increase the detail on the grid and 514 00:56:50,150 --> 00:56:59,550 you learn more. When you want to compute the gravitational force in one cell, you 515 00:56:59,550 --> 00:57:05,090 have to somehow hold masses from the all the other cells. So, the complexity of the 516 00:57:05,090 --> 00:57:07,090 calculus grows. miosta: Yes 517 00:57:07,090 --> 00:57:13,820 Mic2: Quadratically, at the square of the... how do you solve that? With more CPUs? 518 00:57:13,820 --> 00:57:20,930 caro: Well, that would be one way to do that. But there are ways to simplify if 519 00:57:20,930 --> 00:57:26,290 you have a lot of particles in one direction and they are far away from the 520 00:57:26,290 --> 00:57:34,400 object you're looking at. So, yeah. So, if you have several balls here and one ball 521 00:57:34,400 --> 00:57:41,710 here, then you can include all these balls or you can think of them as one ball. So, 522 00:57:41,710 --> 00:57:48,770 it depends on how you look at it. So, how you define how many particles you can take 523 00:57:48,770 --> 00:58:02,230 together is when you look at the angle of this... many particles we'll have from the 524 00:58:02,230 --> 00:58:08,040 seen from the object you're looking at. And you can define a critical angle. And 525 00:58:08,040 --> 00:58:14,230 if an object gets smaller or if lot of objects get smaller than this angle, you 526 00:58:14,230 --> 00:58:20,200 can just say, OK, that's one object. So, that's a way to simplify this method. And 527 00:58:20,200 --> 00:58:23,390 there are some, yeah, I think that's the main idea. 528 00:58:23,390 --> 00:58:30,920 Herald: Okay, we have another one. Mic2: Do you have a strategy to check if 529 00:58:30,920 --> 00:58:35,890 the simulation will give a valuable solution or does it happen a lot that you 530 00:58:35,890 --> 00:58:42,060 wait one week for the calculation and find out OK it's total trash or it crashed in 531 00:58:42,060 --> 00:58:45,400 the time. caro: So, that also depends on the program 532 00:58:45,400 --> 00:58:53,240 you're using. So, in FARGO3D, it gives these outputs after a certain amount of 533 00:58:53,240 --> 00:58:58,980 calculation steps and you can already look at those outputs before the simulation is 534 00:58:58,980 --> 00:59:05,210 finished. So, that would be a way to control if it's really working. Yeah, but 535 00:59:05,210 --> 00:59:11,530 I think... miosta: It's the same for PLUTO. So, there 536 00:59:11,530 --> 00:59:18,040 is a difference between timesteps and actually output steps. So and you could 537 00:59:18,040 --> 00:59:23,490 define your output steps not and as the whole simulation, but you can look at each 538 00:59:23,490 --> 00:59:31,150 output step as soon as it's produced. So, I usually get like 500 outputs, but I 539 00:59:31,150 --> 00:59:36,630 already can look at the first and second after maybe half an hour or something like that. 540 00:59:36,630 --> 00:59:39,950 caro: Yeah, but it also happens that you 541 00:59:39,950 --> 00:59:44,060 start a simulation and wait, and wait, and wait and then see you put something wrong 542 00:59:44,060 --> 00:59:48,760 in there and well then you have to do it again. So, this happens as well. 543 00:59:48,760 --> 00:59:53,070 Mic2: Thanks. Herald: Okay. One final question. 544 00:59:53,070 --> 01:00:02,240 Mic2: Yeah, OK. Is there a program in which you can calculate it backwards? So 545 01:00:02,240 --> 01:00:07,180 that you don't have the starting conditions but the ending conditions 546 01:00:07,180 --> 01:00:15,220 and you can calculate how it had started? miosta: Not for hydrodynamic. If you go to 547 01:00:15,220 --> 01:00:22,240 n-body, there is a way to go backwards in time. But for hydrodynamics, the thing is 548 01:00:22,240 --> 01:00:31,580 that you have turbulent and almost like chaotic conditions. So, you cannot really 549 01:00:31,580 --> 01:00:38,810 turn them back in time. With n-body you can it because actually it's kind of... Well, 550 01:00:38,810 --> 01:00:44,890 it's not analytically solved, but it's much closer than like turbulences, 551 01:00:44,890 --> 01:00:50,470 streams, spirals and all the things you saw in the simulations. 552 01:00:50,470 --> 01:00:57,561 Herald: OK, I guess that brings us to the end of the talk and of the session. Thank 553 01:00:57,561 --> 01:01:03,266 you for the discussion and of course, thank you guys for the presentation. 554 01:01:03,266 --> 01:01:16,730 *36c3 postroll music* 555 01:01:16,730 --> 01:01:30,000 Subtitles created by c3subtitles.de in the year 2021. Join, and help us!