This Physics Breakthrough Looks Impossible

Okay, so here are two different ways of describing  reality. Imagine a silly movie where you have two   cops. One, you have the slow cop that works  by the book. This guy is rigidly organized,   loves structure. Scientists call this the Finite  Element Method. He likes to take something solid,   slice it up into tiny little blocks, and compute  how these blocks interact with each other. This guy is great at a  simple case, one perpetrator,   like simulating solids. But taking care  of a chaotic riot? Like splashing water,   or crumbling sand? This guy just  can’t keep up. Not a chance. He says, yes, we can take care of that, but first,   a little paperwork. By the time the  paperwork is done, it’s too late. So what do we do? Of course, we do  what every silly cop movie does:   we hire another cop. This cop is fast, and  used to be a criminal. Now this guy is crazy,   okay? He is the loose cannon. Doesn’t care about  paperwork, probably even shoots a nice little   hole through it, okay? He is fast, amazing  at taking care of a crazy riot like this,   fluids and sand? No problems. We call  this the Material Point Method. However,   if you ask him to hold everything together nicely,  he falls apart. See how it completely ignored the   interaction between the two? Fine geometry + chaos  equals pandemonium. Can’t be done by this guy. Now we have a huge problem. These  guys would be the perfect duo. Yes!   Wait, no. No, because these guys hate each other. So in video games, when things  try to interact, we get clipping,   where solid objects just ghost through each other  or explode into infinity because the math breaks. Now this research work finally gives us a  way to get these two guys to work together.   Remember this? Well, hold on to your papers Fellow  Scholars, because it suddenly looks like this!   Finally, chaotic interactions and keeping  geometry intact at the same time! Woo-hoo! Now, let’s take a bunch of sand  with about 5.5 million particles,   and wrap it up nicely with a piece of  cloth. This is the perfect gift….for   someone you hate with a passion. Okay, so what happens? The fast   cop takes care of the sand particles,  and the slow cop defines the cloth. And, no cheating, no crazy clipping,   just two cops working together in  harmony. Absolutely loving this. Now this is crazy, because normally, these  two officers really hate each other. Zero   chance of working together. And yet, this  work somehow pulls off the impossible. How?   I’ll tell you in a moment. And I’ll note that  no AI is required here, only human brilliance. That would be great because we can even drop  a snowball onto these elastic mushrooms,   have a wheel imprint itself  into granular soil. Absolutely   incredible. Just think about every video  game and animation movie doing this! Wow. And once again, two worlds collide. They  simulate a rolling pin flattening dough.   It deforms permanently, while the pin stays rigid. And I absolutely love this massive landslide.  Normally, you can simulate the sand particles,   but not the interaction with something elastic.  Now check this out! Oh my. The trees are waving,   and the interaction leaves these little  streaks in the sand at the same time.   It is an absolute miracle that we can do this. Okay, this used to be impossible. But  not anymore - so how on Earth did they   do that? Dear Fellow Scholars, this is Two  Minute Papers with Dr. Károly Zsolnai-Fehér.  Well, the key idea in this paper is creating  a shared bulletin board for the two cops.   This allows two fundamentally opposed  simulation methods to exchange forces   without ever directly touching. And  the result is crash-proof physics. How? Think of it as a way of communicating  between the two cops. The by the book guy,   the Finite Element Method is slow and  methodical. He needs time to fill out   his reports. The fast cop, the material  point method is fast and reactive. He   makes split-second decisions. If they tried  to talk at the same time, it would be chaos. So the researchers created a schedule. The  slow officer takes a big, slow step. Now,   here is the key. Now inside that one big step,  the fast cop takes many tiny, fast steps.   They occasionally update each  other only when necessary. Here,   they effectively agree to disagree  on time, but agree on force. Genius. We can actually look at how this works. Check out  this beautiful simulation view. This is like a   thermal camera showing which officer is sweating.  The blue areas are calm - zero interaction,   low computational cost. Blue means peace.  Blue means the two cops don’t have to argue,   and we can save a ton of compute. Why? Because  everyone is doing what they are good at. But wait,   look at the red area! What’s going on  here? An argument, that’s what’s happening! So what happens then? Well, here, the  slow cop says to the fast cop, hey,   wait a second. Let’s stop and sync up. You give  away a little speed here and get more stability. Okay, now let’s put this all  together and analyze this scene. Look at this. We have highly viscous honey,   controlled by the fast cop, pouring onto a  piece of cloth. This is the slow officer. Normally, this is a crime scene waiting to  happen. Why? Because the cloth is only maybe   half a millimeter thick. But wait, we know what  happens with that! Oh yes, clipping. Usually, the   honey particles would just ignore the thin barrier  and ghost right through it like it doesn’t exist. But here, the slow cop holds the line! There  is communication! Look, the cloth actually   buckles under the weight of the honey. The honey  folds over itself, and starts doing my favorite:   coiling. Oh my, loving it. And it kinda sticks  to the fabric. An absolutely insane simulation. To make these two officers respect  a fraction of a millimeter boundary   without exploding is a mathematical miracle. Not only that, but this is also excellent  life advice. We have two cops here that   are good at different things. Neither of  them can do everything. And you should also   stop trying to be good at everything. Partner  with someone who is strong where you are weak. This is also what I do with my wife Felícia. She  is strong exactly at areas where I am weak. That’s   why we’ve been able to run this channel for  more than a 1,000 videos now. Let’s be honest,   I am the loose cannon here, and I need  someone that makes sure the Papers lab   does not work like a mental asylum. And  it works beautifully. She gave me this   nice white jacket and my lunch slides  in under the door. What’s not to like? Now, all this means we can finally  simulate movie-quality destruction   in one unified system. What a time to be alive! Now, on a more serious note, this is an absolutely  beautifully written research paper. I see   absolutely nobody talking about this. I kind of  understand, I mean it is a really advanced work   in physics simulations. But I am worried that if I  don’t talk about it here, no one else talks about   it. And this thing is a treasure. Subscribe,  hit the bell and leave a kind comment if you   enjoyed this. And make sure to sign up for Lambda,  because they give you these amazing GPUs to run   simulations and anything else on. And they support  us in creating these super fun videos for you.