# Table TenetJanuary 12, 2022 11:38 AM   Subscribe

Physics simulations of bounching balls, simulated forward and backward from a perfect moment in time. More where that came from. [via Kottke]
posted by cortex (24 comments total) 24 users marked this as a favorite

Thanks. I needed that.
posted by brachiopod at 11:48 AM on January 12

time is a bunch of flat circles bouncing on a plane
posted by OverlappingElvis at 11:51 AM on January 12 [1 favorite]

From the video description: "Each sequence is obtained by joining two simulations, both starting from the time in which the balls are arranged regularly. One simulates forward in time, one backwards."

Neat.
posted by escape from the potato planet at 12:10 PM on January 12 [1 favorite]

One simulates forward in time, one backwards.

These make sense from the "perfect moment" on, but I'm not sure what's happening before that (the backwards in time part?)
posted by ChurchHatesTucker at 12:26 PM on January 12

Every now and then I like to think how, if the universe is some astronomical number of bouncing balls like this, we are currently right there in the moment that they all align to make a pretty pattern.
posted by aspo at 12:26 PM on January 12 [1 favorite]

These make sense from the "perfect moment" on, but I'm not sure what's happening before that (the backwards in time part?)

From a reply to a comment on one of the videos:
It's a standard simulation starting from the desired configuration, where the elasticity of each shape is set above one (precisely, to the reciprocal of the real elasticity of the shape); this implies that shapes gain energy at each collision. When played back in reverse, the result is equivalent to the expected dynamics, with the end state being the desired one.

This works because, as long as the balls only bounce and fly, the dynamics of the system is time reversible. That is not true once a ball comes to rest and stops bouncing (you can't uniquely determine what happened before). However, this never happens during the first half of each simulation.
posted by OverlappingElvis at 12:38 PM on January 12 [12 favorites]

elasticity of each shape is set above one

So it simulates flubber, backwards.
posted by paper chromatographologist at 12:41 PM on January 12 [5 favorites]

So theoretically, you could set up the initial conditions for the balls (locations, velocities) so that they would make that perfect pattern automatically, after bouncing a while. But that would be really hard to get those conditions just right! Instead they cheat by finding those conditions by reversing time via reciprocal elasticity.
posted by TreeRooster at 1:44 PM on January 12 [3 favorites]

Very pleasing! The clicky sounds really sell it for me.

I wouldn't call this approach "cheating," it's the most efficient way of finding the initial conditions that lead to the desired ordered state. Indeed, from a physical perspective it really doesn't matter if your "initial" conditions are at time t=0 or some other arbitrary point in time. Time is reversible anyway, aside from increasing entropy which doesn't matter in a fully determined system.
posted by biogeo at 2:00 PM on January 12 [3 favorites]

In Sean Carroll’s book “From Eternity To Here,” he discusses the problem of the “arrow of time” and the related physical law that the entropy of a closed system always increases instead of decreasing. This physical law makes the cosmological prediction that the Big Bang must have been a low-entropy state, which some folks find troubling.

The entropy law, as taught to graduate students, is not inevitable, but is instead overwhelmingly likely. Your randomly-shuffled deck of cards is never going to have the top thirteen cards be all the spades, in order from ace to king. Not because such an arrangement is impossible, but because there are 1021 possible orderings for the first thirteen cards in a deck, and you’re not going to live long enough to shuffle a deck of cards 1021 times. The universe tends towards disorder, not because disorder is special, but just because “disordered” describes more states than “ordered” does.

As a corollary, the internal fluctuations which drive an isolated system from a high-entropy to a low-entropy state aren’t required to evolve that way; they’re just overwhelmingly likely to evolve that way. If you have an isolated system which has reached its maximum-entropy state, these internal fluctuations don’t cease, and therefore must occasionally cause the system’s entropy to decrease a little. A mathematically-minded person can come up with a statement of how often you should expect an entropy decrease of a given size, and how long it should last before evolving back to maximum entropy again.

One of the proposals Carroll makes in his book is that our low-entropy Big Bang was one of these low-entropy fluctuations in an otherwise-high-entropy system. If we could look back “before” the entropy minimum at the Big Bang, he suggests, we would find the entropy start to increase again; inhabitants of that region would believe their “arrow of time” points opposite to ours.

This is a not-great summary of a complicated idea from a book I haven’t re-read for a decade. But it’s totally what’s happening here. Each of these little simulations has one low-entropy instant, where the balls are in a circle or a grid or whatever. We are accustomed to the second half of the simulation, where the “ordered” state collapses towards the floor. We are less accustomed to the first half, where the super-elastic collisions simulate energy gain instead of energy loss. But these simulations are large enough to be chaotic: they will only reproduce the special shape if they start from the middle. If you started the simulations off with any finite precision and ran them forward from the beginning, the middle shape would be overwhelmingly unlikely to happen.

Neat.
posted by fantabulous timewaster at 2:08 PM on January 12 [13 favorites]

intelligent design!
posted by Ten Cold Hot Dogs at 2:51 PM on January 12 [1 favorite]

I wouldn't call this approach "cheating," it's the most efficient way of finding the initial conditions that lead to the desired ordered state.

In practical general terms it's the only way to find those initial conditions with such a complex system, even. Even three body systems under ideal abstract conditions are chaotic; the search space to produce a given desired configuration of n balls by guessing at a non-trivial* starting state of position and velocity of each ball is huge and messy and genuinely incalculable beyond certain small, non-general parts of the problem.

*you could come up with trivial solutions pretty easily: if each ball only needs to move along a simple parabolic arc from its starting point to the desire configuration point, and none of the balls collide with each other or anything else, it's just some fairly basic algebra. But it would also be really sort of trivial and boring to look at and not have any of those good weird bounces and collisions and seeming-chaos that makes these simulations so much fun.
posted by cortex at 3:01 PM on January 12 [4 favorites]

Elasticity of each shape space itself is set above one

Dark Energy
posted by jamjam at 5:17 PM on January 12

Or maybe it is less than one:
The first detection of gravitational waves in 2016 provided decisive confirmation of Einstein’s general theory of relativity. But another astounding prediction remains unconfirmed: According to general relativity, every gravitational wave should leave an indelible imprint on the structure of space-time. It should permanently strain space, displacing the mirrors of a gravitational wave detector even after the wave has passed.
posted by jamjam at 5:27 PM on January 12

For those hung up on practical effects:
1 steep San Francisco street, 250 thousand super bouncy balls.
posted by bartleby at 5:34 PM on January 12

High marks for the title!
posted by TheCoug at 5:45 PM on January 12 [1 favorite]

... bounc(h)ing?
(I'm not sure if I'm the first to notice or if everyone else is just more polite about it, but I figure I should mention)
posted by CrystalDave at 6:19 PM on January 12 [1 favorite]

Oh lord. I'm leaving it. Bounching is my 2022 energy.
posted by cortex at 6:50 PM on January 12 [4 favorites]

> Oh lord. I'm leaving it. Bounching is my 2022 energy.

Heh, classic Jorts
posted by Pronoiac at 6:52 PM on January 12 [3 favorites]

1 steep San Francisco street, 250 thousand super bouncy balls.

Oh jeez, that made me remember seeing this on MeFi back in the day, and it turns out I mean back in the day: it was a post from 2004, and the bouncing ball ad was linked as a quicktime file because Youtube didn't exist yet. The original site for the ad is now a default Wordpress install.
posted by cortex at 6:56 PM on January 12 [2 favorites]

Yeah, I was going to link a version of the video that included some 'making of' bits.
But the little Uploaded: 14 years ago YouTube counter has given me a Chief Inspector Dreyfus level eye twitch.
posted by bartleby at 7:14 PM on January 12

I took "bounching" to be an amalgam of "bouncing" and "bunching", but as my students know, I tend to be an easy grader.
posted by mollweide at 3:47 AM on January 13 [2 favorites]

This is neat! I can't decide whether to be pleased or frustrated that there was no kilroy configuration or something equally silly.

One thing one should never do is dump a box of bouncy balls into the top of a 12 story staircase. Even if you've tried to be safe by posting warnings and covering the windows with cardboard. I'm not going to explain why I believe this is true. But, terminal velocity for a bouncy ball is a lot faster than one could possibly throw it.
posted by eotvos at 10:19 AM on January 13 [1 favorite]

Oh jeez, that made me remember seeing this on MeFi back in the day, and it turns out I mean back in the day: it was a post from 2004, and the bouncing ball ad was linked as a quicktime file because Youtube didn't exist yet. The original site for the ad is now a default Wordpress install.

Oof, I remember watching this on here as well, back in the before times.

This comment would essentially be gibberish to my college-age children: "My QT software is allegedly up-to-date, but I get only the audio & an error saying 'the required compressor could not be found'. Win XP."
posted by Ben Trismegistus at 11:03 AM on January 14