The Physics Bug That Stumped Everyone Is Finally Gone!

0:00 Finally!

0:00 This amazing technique solves the problem of this physics

0:05 glitch where objects just clip through water.

0:07 By the end of the video, I’ll tell you how it works in very simple words,

0:12 but it turns out, there’s more to this.

0:15 There is excellent life advice in this research paper.

0:18 And you will also see if this can get close to reality or not.

0:23 That is an incredibly tough test.

0:25 Spoiler alert: there is no AI in here.

0:28 No neural networks guessing the pixels.

0:30 None.

0:30 This is physics, fueled by nothing

0:33 but human brilliance and the beauty of nature.

0:37 Okay, now, there are research works on how

0:39 to simulate liquids moving around in a computer program.

0:43 And yes, they can do this and this and this.

0:47 But very few people know that almost all of them are missing one key ingredient.

0:53 Let me show you.

0:54 Here is a propeller put into water, and…wow.

0:57 That is a beautiful turbulence simulation.

1:02 Wait what?

1:03 What is going on in this one?

1:05 This one isn’t even in the water.

1:07 What are we expecting to happen here?

1:12 Ooooh!

1:13 Air-driven turbulence.

1:14 That is crazy!

1:17 Let’s slow it down a bit.

1:19 Absolute pandemonium.

1:20 And it is beautiful.

1:22 What incredible detail!

1:23 I really want to know how this is possible.

1:27 This technique can pull off this sort of miraculous simulation,

1:30 and you’ll see a bunch of others as well.

1:34 Like this, this and this.

1:36 And look at this one.

1:38 It is raining...

1:39 cows?

1:40 And bunnies?

1:41 I have heard of raining cats and dogs, but this is ridiculous!

1:47 But look closely.

1:49 Beautiful bubbles and swirls appear.

1:52 Words fail me to describe how beautiful these are.

1:55 And not just beautiful!

1:57 The physics remains rock solid,

1:59 despite many of them being of different densities.

2:04 Also, here is an airplane ditching into the water.

2:07 I learned a new word here- this is a controlled landing.

2:10 If you look at the top of the container, the splash actually hits the ceiling!

2:15 That is the level of energy we are dealing with here.

2:19 Okay, question: why is this so hard to do?

2:23 Well, because water is heavy, and air is light.

2:27 About 800 times lighter, to be exact.

2:30 In the simulation world, this is a nightmare.

2:33 It is like a sumo wrestler trying to dance with a mouse without stepping on it.

2:39 Usually, researchers resort to cheats or ignore

2:42 these effects to keep the math stable.

2:45 Otherwise it blows up.

2:47 But not here!

2:48 These researchers said “No sir!

2:50 we do it the hard way!” Okay, so most computer games are a bit like a mosh pit.

2:57 This is where rockers do….whatever these rockers are doing,

3:00 just let them be and keep your distance if your life is dear.

3:04 This is how solids and fluids behave in most programs.

3:08 They don't know the rules, they step on each other's toes,

3:12 and they crash into each other.

3:14 When you’re 20 years old.

3:16 Really cool.

3:17 When you are 40, life hazard.

3:19 No thanks!

3:20 Now this technique turns this crazy mosh pit into a synchronized ballet.

3:26 The input is your 3D objects and water,

3:30 and the output is… the most well-mannered physics you’ve ever seen.

3:35 Of course it is, it was developed in France!

3:39 Partially.

3:40 Mon dieu!

3:41 Now this ballet is what experts like to call two-way coupling.

3:45 And it allows us to simulate this incredible situation.

3:48 Look.

3:49 Notice the air bubble forming in front of the windshield?

3:53 That is not some fake effect.

3:56 That is the air particles realizing they have no

3:59 room to dance and getting out of the way naturally.

4:03 Note that I haven’t forgotten about the testing against reality part,

4:08 that is coming soon.

4:09 And in the wake of the car, you get these incredible flow patterns too.

4:14 It can simulate disk sliding, stone skipping.

4:17 It can simulate how a coin flutters when thrown into the water.

4:22 I am getting really excited here.

4:24 I think I need a little break to cool down.

4:28 Okay, I am back!

4:30 Let’s unpack the secrets of this paper.

4:33 This uses the Lattice Boltzmann Method.

4:35 Traditional techniques are a bit like a manager

4:38 shouting to the whole crowd with a megaphone.

4:41 But this one instead whispers instructions into the ears of every single dancer.

4:48 It operates with two steps: one particles moving freely, and two, interaction.

4:54 Ever have a day at work where you have hours and hours of meetings,

4:59 and the time you actually have left to work is in small 15 minute blocks?

5:06 It’s terrible right?

5:08 Of course it is!

5:09 You cannot flow and collide at the same time.

5:13 Just like these particles,

5:14 you need a block of time carved out to move forward alone.

5:19 And then, separate time slots to interact with others.

5:23 Once again, excellent life advice right there.

5:26 Loving it.

5:27 Then, it has a hybrid moving bounce-back

5:30 technique that teaches the particles to dance properly.

5:34 This says "If you collide with someone, here is what you need to do.

5:39 Bounce back with exactly this much energy,

5:41 and take some of the object's momentum with you”.

5:44 This is an etiquette guide for dancers, if you will.

5:48 Now that is incredibly important.

5:50 Why?

5:51 Because this creates something called two way coupling.

5:55 That is the magic of this work: the water starts pushing the object,

6:00 but in return, the object also pushes the water.

6:04 There is a two-way communication between them.

6:07 That is actually incredible life advice.

6:10 Successful relationships require two-way coupling.

6:14 Both parties need to be able to influence each other.

6:18 You cannot just push through with every

6:20 decision without hearing out the other one.

6:23 Power needs to be shared, or it is lost.

6:27 Now, what if we don’t do that correctly?

6:29 Well, here is a previous technique that does not implement this properly.

6:33 Now, hold on to your papers Fellow Scholars and look at the new one!

6:38 Oh goodness, now we’re talking!

6:39 That’s a dance.

6:40 So how much do we have to pay for this new one in terms of simulation time?

6:48 It takes how much longer?

6:51 10x longer?

6:52 Let’s see…excuse me, what?

6:54 This is not only so much better than the previous method,

6:58 but it is also 4x faster at the same time!

7:03 This sounds like a dream.

7:06 Wow.

7:06 And it not only looks better,

7:08 but it can perform things that previous techniques can’t even dream of!

7:13 For instance, this was the stone skipping across the water.

7:17 Most simulations cannot do this.

7:19 Why?

7:20 Because they are too sticky.

7:22 But this method simulates the air layer between the stone and the water.

7:27 Our little dancers can get to have their own personal space on the dance floor.

7:33 This way, the stone can actually bounce multiple times.

7:37 Okay, and now, the toughest test.

7:40 This is almost impossible to do well, so I am really excited for this.

7:46 Let’s compare it to the ultimate judge: reality.

7:49 Phase 1: the breach.

7:51 As the key slices through the surface, I am loving this already.

7:56 There is no clipping here.

7:58 The water is forced to part ways.

8:01 Phase 2: the veil.

8:03 This is the critical moment.

8:05 Oh my.

8:06 Look at that long pocket of air trailing behind the key.

8:11 It looks like a bridal veil made of bubbles.

8:15 Beautiful beyond words.

8:17 Just like real life.

8:19 And now, phase 3: the end of the wedding.

8:23 This is where the math shows its muscles.

8:26 That smooth veil you saw a moment before now becomes unstable.

8:31 The water pressure crushes it, and it becomes a cloud of bubbles.

8:37 An incredible story in three phases.

8:40 And I see absolutely nobody talking about this work.

8:44 Nobody.

8:45 Think about what a tragedy this is.

8:48 This is why I am doing Two Minute Papers.

8:51 To give a voice to incredibly brilliant works like this.

8:54 And if we don’t do it here, I am worried that no one else does.

8:59 Also, seeing things like this is why I go out

9:02 there in nature to go and watch a mountain stream.

9:05 Or any stream.

9:06 Seriously.

9:07 Go and observe how water flows around rocks.

9:10 This is the original two-way coupling that mother

9:14 nature computes millions and millions of times each moment.

9:17 Whether we are there or not.

9:20 And nature does all this perfectly, and with the speed of light.

9:25 Isn’t it incredible?

9:27 What a time to be alive!

9:29 Try it out, this will teach you patience and beauty.

9:32 And if you enjoyed this, don’t forget to subscribe,

9:35 hit the bell, and leave a really kind comment.

9:38 Thank you so much!