You Can’t Predict a Game of Pinball

20 Min Read

Jeffrey Heninger, 29 March 2023


When fascinated with a brand new thought, it helps to have a specific instance to make use of to achieve instinct and to make clear your ideas. Video games are significantly useful for this, as a result of they’ve effectively outlined guidelines and objectives. Lots of the most spectacular talents of present AI programs might be present in video games.

To show how chaos idea imposes some limits on the ability of an arbitrary intelligence, I may even take a look at a recreation: pinball.

On this web page, I’ll present that the uncertainty within the location of the pinball grows by an element of about 5 each time the ball collides with one of many disks. After 12 bounces, an preliminary uncertainty in place the scale of an atom grows to be as giant because the disks themselves. Since you can not launch a pinball with greater than atom-scale precision, and even measure its place that exactly, you can not make the ball bounce between the disks for greater than 12 bounces.

The problem is just not that we now have not discovered the foundations that decide the ball’s movement. The principles are easy; the ball’s trajectory is decided by easy geometry. The problem is that the chaotic movement of the ball amplifies microscopic uncertainties. This isn’t an issue that’s solvable by making use of extra cognitive effort.

The Sport

Let’s take into account a physicist’s recreation of pinball.

Overlook about a lot of the board and deal with the three disks on the high. Every disk is an ideal circle of radius R. The disks are organized in an equilateral triangle. The minimal distance between any two disks is L. See Determine 1 for an image of this setup.

Determine 1: An idealization of the three disks close to the highest of a pinball desk. Drawn by Jeffrey Heninger.

The board is frictionless and flat, not sloped like in an actual pinball machine. Collisions between the pinball and the disks are completely elastic, with no pop bumpers that come out of the disk and hit the ball. The pinball strikes at a relentless pace the entire time and solely adjustments path when it collides with a disk.

The purpose of the sport is to get the pinball to bounce between the disks for so long as potential. So long as it’s between the disks, it will be unable to get previous your flippers and go away the board.

An actual recreation of pinball is extra difficult than this – and correspondingly, tougher to foretell. If we are able to set up that the physicist’s recreation of pinball is unattainable to foretell, then an actual recreation of pinball shall be unattainable to foretell too. 


When the ball approaches a disk, it is not going to usually be aimed immediately on the heart of the disk. How far off heart it’s might be described by the affect parameter, b, which is the space between the trajectory of the ball and a parallel line which passes by the middle of the disk. Determine 2 reveals the trajectory of the ball because it collides with a disk.

The floor of the disk is at an angle relative to the ball’s trajectory. Name this angle θ. That is additionally the angle of the place of the collision on the disk relative to the road by the middle parallel to the ball’s preliminary trajectory. This may be seen in Determine 2 as a result of they’re corresponding angles on a transversal.

Determine 2: A single bounce of the pinball off of one of many disks. Drawn by Jeffrey Heninger.

On the collision, the angle of incidence equals the angle of reflection. The whole change within the path of the ball’s movement is

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We can not goal the ball with good precision. We are able to calculate the impact of this imperfect precision by following two barely completely different trajectories by the collision as an alternative of 1.

The 2 trajectories have barely completely different preliminary areas. The second trajectory has affect parameter b + db, with db ≪ b. Name db the uncertainty within the affect parameter. We assume that the 2 trajectories have precisely the identical preliminary velocity. If we had been to additionally embody uncertainty within the velocity, it will additional lower our potential to foretell the movement of the pinball. A diagram of the 2 trajectories close to a collision is proven in Determine 3.

Determine 3: Two close by potential trajectories of a pinball bounce off of one of many disks. Drawn by Jeffrey Heninger.

The affect parameter, radius of the disk, and angle are trigonometrically associated: b = R sinθ (see Determine 2). We are able to use this to find out the connection between the uncertainty within the affect parameter and the uncertainty within the angle: db = R cosθ dθ.

After the collision, the 2 trajectories will not be parallel. The angle between them is now 2 dθ. The 2 trajectories will separate as they journey away from the disk. They should journey a distance of at the least L earlier than colliding with the subsequent disk. The gap between the 2 trajectories will then be at the least L 2 dθ.


We are able to now iterate this. How does the uncertainty within the affect parameter change because the pinball bounces round between the disks?

Begin with an preliminary uncertainty within the affect parameter, db₀. After one collision, the 2 trajectories shall be farther aside. We are able to use the brand new distance between them because the uncertainty within the affect parameter for the second collision. The brand new uncertainty within the affect parameter is expounded to the outdated one based on:

db_1 geq L 2 dtheta = 2 L frac{db_0}{R costheta} geq frac{2L}{R} db_0 ,.

We additionally used 1 / cos θ > 1 for π/2 < θ < π/2, that are the angles that might be concerned in a collision. The ball is not going to go by the disk and collide with the inside of the far aspect.

Repeat this calculation to see that after two collisions:

db_2 geq frac{2L}{R} db_1 geq left(frac{2L}{R}proper)^2 db_0 ,,

and after N collisions,

db_N geq left(frac{2L}{R}proper)^N db_0 , .

Plugging in reasonable numbers, R = 2 cm and L = 5 cm, we see that

db_N geq 5^N , db_0 ,.

The uncertainty grows exponentially with the variety of collisions.

Suppose we had began with an preliminary uncertainty concerning the measurement of the diameter of an atom, or 10−10 m. After 12 collisions, the uncertainty would develop by an element of 5¹², to 2.4 cm. The uncertainty is bigger than the radius of the disk, so if one of many trajectories struck the middle of the disk, the opposite trajectory would miss the disk fully.

The exponential progress amplifies atomic-scale uncertainty to change into macroscopically related in a surprisingly quick period of time. In case you needed to foretell the trail the pinball would comply with, having an uncertainty of 2 cm can be unacceptable.

In apply, there are numerous uncertainties which might be a lot bigger than 10−10 m, within the manufacturing of the pinball, disks, & board and within the mechanics of the launch system. In case you managed to unravel all of those engineering challenges, you’ll ultimately run into the elemental restrict imposed by quantum mechanics. 

It’s in precept unattainable to arrange the preliminary location of the pinball with a precision of lower than the diameter of an atom. Heisenberg’s Uncertainty Precept is related at these scales. In case you tried to arrange the preliminary place of a pinball with extra precision than this, it will trigger the uncertainty within the preliminary velocity to extend, which might once more make the movement unpredictable after an analogous variety of collisions.

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Once I point out Heisenberg’s Uncertainty Precept, I count on that there shall be some individuals who wish to see the quantum model of the argument. I don’t assume that it’s important to this investigation, however in case you are , you could find a dialogue of Quantum Pinball within the appendix.


You can’t put together the preliminary place of the pinball with higher than atomic precision, and atomic precision solely permits you to predict the movement of the pinball between the disks with centimeter precision for lower than 12 bounces. It’s unattainable to foretell a recreation of pinball for greater than 12 bounces sooner or later. That is true for an arbitrary intelligence, with an arbitrarily exact simulation of the pinball machine, and arbitrarily good manufacturing & launch programs.

This habits is just not distinctive to pinball. It’s a widespread function of chaotic programs.

When you’ve got infinite precision, you can precisely predict the longer term. The equations describing the movement are deterministic. On this instance, following the trajectory is an easy geometry downside, solvable utilizing a straightedge and compass. 

However we by no means have infinite precision. Each measuring gadget solely offers a finite variety of correct digits. Each idea has solely been examined inside a sure regime, and we shouldn’t have good purpose to count on it can work exterior of the regime it has been examined in.

Chaos shortly amplifies no matter uncertainty and randomness which exists at microscopic scales to the macroscopic scales we care about. The microscopic world is filled with thermal noise and quantum results, making macroscopic chaotic movement unattainable to foretell as effectively.


It’s in precept unattainable to foretell the movement of a pinball because it strikes between the highest three disks for greater than 12 bounces. A superintelligence is perhaps higher than us at making predictions after 8 bounces, if it may design larger decision cameras or extra exact ball and board machining. Nevertheless it too will run into the low prediction ceiling I’ve proven right here.

Maybe you assume that this argument proves an excessive amount of. Pinball is just not utterly a recreation of probability. How do some individuals get significantly better at pinball than others?

In case you watch a couple of video games {of professional} pinball, the reply turns into clear. The technique usually is to catch the ball with the flippers, then to rigorously hit the balls in order that it takes a specific ramp which scores a whole lot of factors after which returns the ball to the flippers. Skilled pinball gamers attempt to keep away from the elements of the board the place the movement is chaotic. This can be a good technique as a result of, for those who can not predict the movement of the ball, you can not assure that it’ll not fall immediately between the flippers the place you can not reserve it. As a substitute, skilled pinball gamers rating factors largely from the non-chaotic areas the place it’s potential to foretell the movement of the pinball.

Pinball is typical for a chaotic system. The delicate dependence on preliminary circumstances renders long run predictions unattainable. In case you can not predict what is going to occur, you can not plan a method that permits you to carry out persistently effectively. There’s a ceiling in your talents due to the interactions with the chaotic system. With the intention to enhance your efficiency you typically attempt to keep away from the chaos and deal with creating your ability in locations the place the world is extra predictable.

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Appendix: Quantum Pinball

If quantum uncertainty really is essential to pinball, perhaps we needs to be fixing the issue utilizing quantum mechanics. That is considerably extra difficult, so I can’t work out the calculation intimately. I’ll clarify why this doesn’t offer you a greater prediction for the place the pinball shall be sooner or later.

Mannequin the disks as infinite potential partitions, so the wave operate displays off of them and doesn’t tunnel by them. If the pinball does have an opportunity of tunneling by the disks, that will imply that there are much more locations the quantum pinball might be.

Begin with a wave operate with minimal uncertainty: a wave packet with Δx Δp =ℏ/2 in every path. It might be a Gaussian wave packet, or it might be a wavelet. This wave packet is centered round some place and velocity.

So long as the wave operate remains to be a localized wave packet, the middle of the wave packet follows the classical trajectory. This may be seen both by wanting on the time evolution of the typical place and momentum, or by contemplating the semiclassical restrict. To ensure that classical mechanics to be a very good approximation to quantum mechanics at macroscopic scales, a zoomed out view of the wave packet has to comply with the classical trajectory.

What occurs to the width of the wave packet? Similar to how the collisions within the classical downside prompted close by trajectories to separate, the reflection off the disk causes the wave packet to unfold out. This may be most simply seen utilizing the ray tracing technique to unravel Schrödinger’s equation within the WKB approximation. This technique converts the PDE for the wave operate into a group of (infinitely many) ODEs for the trail every ‘ray’ follows and for the worth of the wavefunction alongside every ray. The paths the rays comply with replicate like classical particles, which implies that the area the place the wave operate is nonzero spreads out in the identical method because the classical uncertainty would.

This can be a widespread lead to quantum chaos. In case you begin with a minimal uncertainty wave packet, the middle of the wave packet will comply with the classical trajectory and the width of the wave packet will develop with the classical Lyapunov exponent.

After 12 collisions, the width of the wave packet can be a number of centimeters. After one other collision or two, the wave operate is not a wave packet with a effectively outlined heart and width. As a substitute, it has unfold out so it has a nontrivial amplitude throughout all the pinball machine. There is perhaps some fascinating interference patterns or quantum scarring, however the wave operate is not going to be localized to any specific place. Because the magnitude of the wave operate squared tells you the likelihood of discovering the pinball in that location, this tells us that there’s a probability of discovering the pinball nearly anyplace.

A quantum mechanical mannequin of the movement of the pinball is not going to inform you the placement of the pinball after many bounces. The results of the quantum mechanical mannequin is a macroscopic wavefunction, with nontrivial likelihood of being at nearly any location throughout the pinball machine. We don’t observe a macroscopic wavefunction. As a substitute, we observe the pinball at a specific location. Which of those areas you’ll really observe the pinball in is decided by wavefunction collapse. Alternatively, I might say that there are Everett branches with the pinball at nearly any location on the board.

Fixing wave operate collapse, or figuring out which Everett department it is best to look forward to finding your self on, is an unsolved and doubtless unsolvable downside – even for a superintelligence.


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