The Physics Bug That Stumped Everyone Is Finally Gone!

Finally! This amazing technique solves the  problem of this physics glitch where objects   just clip through water. By the end of the video,  I’ll tell you how it works in very simple words,   but it turns out, there’s more to this. There  is excellent life advice in this research paper.   And you will also see if this can get close to  reality or not. That is an incredibly tough test. Spoiler alert: there is no AI in here.  No neural networks guessing the pixels.   None. This is physics, fueled by nothing but  human brilliance and the beauty of nature. Okay, now, there are research  works on how to simulate liquids   moving around in a computer program. And  yes, they can do this and this and this.  But very few people know that almost all  of them are missing one key ingredient. Let me show you. Here is a  propeller put into water,   and…wow. That is a beautiful  turbulence simulation. Wait what? What is going on in this one?  This one isn’t even in the water. What   are we expecting to happen here? Ooooh!  Air-driven turbulence. That is crazy!  Let’s slow it down a bit. Absolute  pandemonium. And it is beautiful.   What incredible detail! I really  want to know how this is possible. This technique can pull off this  sort of miraculous simulation,   and you’ll see a bunch of others  as well. Like this, this and this. And look at this one. It is raining... cows? And  bunnies? I have heard of raining cats and dogs,   but this is ridiculous! But look closely.  Beautiful bubbles and swirls appear.   Words fail me to describe how beautiful  these are. And not just beautiful! The   physics remains rock solid, despite many  of them being of different densities. Also, here is an airplane ditching  into the water. I learned a new word   here - this is a controlled landing. If  you look at the top of the container,   the splash actually hits the ceiling! That is  the level of energy we are dealing with here. Okay, question: why is this so hard  to do? Well, because water is heavy,   and air is light. About 800 times lighter,  to be exact. In the simulation world,   this is a nightmare. It is like a sumo wrestler  trying to dance with a mouse without stepping on   it. Usually, researchers resort to cheats  or ignore these effects to keep the math   stable. Otherwise it blows up. But not here! These  researchers said “No sir! we do it the hard way!” Okay, so most computer games are a bit like a  mosh pit. This is where rockers do….whatever   these rockers are doing, just let them  be and keep your distance if your life   is dear. This is how solids and fluids behave  in most programs. They don't know the rules,   they step on each other's toes,  and they crash into each other.   When you’re 20 years old. Really cool.  When you are 40, life hazard. No thanks! Now this technique turns this crazy  mosh pit into a synchronized ballet.   The input is your 3D objects and water, and  the output is… the most well-mannered physics   you’ve ever seen. Of course it is, it was  developed in France! Partially. Mon dieu! Now this ballet is what experts like to  call two-way coupling. And it allows us   to simulate this incredible situation.  Look. Notice the air bubble forming in   front of the windshield? That is not some  fake effect. That is the air particles   realizing they have no room to dance and  getting out of the way naturally. Note   that I haven’t forgotten about the testing  against reality part, that is coming soon. And in the wake of the car, you get  these incredible flow patterns too.   It can simulate disk sliding, stone skipping. It  can simulate how a coin flutters when thrown into   the water. I am getting really excited here.  I think I need a little break to cool down. Okay, I am back! Let’s unpack the secrets of this  paper. This uses the Lattice Boltzmann Method.   Traditional techniques are a bit like a manager  shouting to the whole crowd with a megaphone. But   this one instead whispers instructions  into the ears of every single dancer. It operates with two steps: one  particles moving freely, and two,   interaction. Ever have a day at work where  you have hours and hours of meetings,   and the time you actually have left to work is  in small 15 minute blocks? It’s terrible right?   Of course it is! You cannot flow and collide  at the same time. Just like these particles,   you need a block of time carved out to  move forward alone. And then, separate   time slots to interact with others. Once again,  excellent life advice right there. Loving it. Then, it has a hybrid moving  bounce-back technique that   teaches the particles to dance properly.  This says "If you collide with someone,   here is what you need to do. Bounce back with  exactly this much energy, and take some of the   object's momentum with you”. This is an  etiquette guide for dancers, if you will. Now that is incredibly important. Why? Because  this creates something called two way coupling.   That is the magic of this work: the water  starts pushing the object, but in return,   the object also pushes the water. There is a  two-way communication between them. That is   actually incredible life advice. Successful  relationships require two-way coupling. Both   parties need to be able to influence each  other. You cannot just push through with   every decision without hearing out the other  one. Power needs to be shared, or it is lost. Now, what if we don’t do that correctly?  Well, here is a previous technique that   does not implement this properly. Now, hold on  to your papers Fellow Scholars and look at the   new one! Oh goodness, now we’re talking! That’s a  dance. So how much do we have to pay for this new   one in terms of simulation time? It takes how  much longer? 10x longer? Let’s see…excuse me,   what? This is not only so much  better than the previous method,   but it is also 4x faster at the same  time! This sounds like a dream. Wow. And it not only looks better, but it can  perform things that previous techniques   can’t even dream of! For instance, this was  the stone skipping across the water. Most   simulations cannot do this. Why? Because they  are too sticky. But this method simulates the   air layer between the stone and the water.  Our little dancers can get to have their own   personal space on the dance floor. This way,  the stone can actually bounce multiple times. Okay, and now, the toughest test.  This is almost impossible to do well,   so I am really excited for this. Let’s  compare it to the ultimate judge: reality. Phase 1: the breach. As the key slices through  the surface, I am loving this already. There is no   clipping here. The water is forced to part ways. Phase 2: the veil. This is the critical   moment. Oh my. Look at that long pocket of  air trailing behind the key. It looks like a   bridal veil made of bubbles. Beautiful  beyond words. Just like real life.  And now, phase 3: the end of the wedding. This  is where the math shows its muscles. That smooth   veil you saw a moment before now becomes  unstable. The water pressure crushes it,   and it becomes a cloud of bubbles.  An incredible story in three phases. And I see absolutely nobody talking about this  work. Nobody. Think about what a tragedy this is.   This is why I am doing Two Minute Papers.  To give a voice to incredibly brilliant   works like this. And if we don’t do it  here, I am worried that no one else does. Also, seeing things like this is why I go out  there in nature to go and watch a mountain stream.   Or any stream. Seriously. Go and observe how water  flows around rocks. This is the original two-way   coupling that mother nature computes millions  and millions of times each moment. Whether we are   there or not. And nature does all this perfectly,  and with the speed of light. Isn’t it incredible?   What a time to be alive! Try it out, this will  teach you patience and beauty. And if you enjoyed   this, don’t forget to subscribe, hit the bell, and  leave a really kind comment. Thank you so much!