To most of us, watching a chess game is about as fun as sitting through a lecture on quantum mechanics. So you might think a video about "quantum chess" would be pretty boring. Unless, perhaps, the players are famed physicist and cosmologist Stephen Hawking and...actor Paul Rudd?
That's right, this short, called Anyone Can Quantum, has everything. Footage from Ant-Man. An immortal Keanu Reeves from 700 years in the future. Cat videos. Trash talk. Fake Twitter commentary from celebrities and the Pope. Anchorman references. Bill and Ted references. The participation of both Bill and Ted (Alex Winter directs and has a brief cameo while Reeves voices his future and present selves). And, of course, quantum weirdness.
Why does this video even exist? Caltech's Institute for Quantum Information and Matter collaborated with Trouper Productions to create the short in celebration of its Quantum Summit, an event where experts discuss the future of various quantum technologies. So the short film serves as advertising and entertainment—but it's also an introduction to the basics of quantum mechanics, which you learn as Paul Rudd does.
"Seven hundred years ago," future-Reeves intones at the beginning of the video, "Paul Rudd changed the course of history by showing the world that anyone could grapple with the concepts of quantum mechanics." Rudd achieves this goal through the clever game of quantum chess, where the pieces must obey both the rules of chess and the laws of quantum mechanics. To understand what's going on in this game, you have to master two quantum principles: superposition and entanglement.
Quantum objects, unlike regular, familiar objects, have the counterintuitive ability to be in multiple states at once. If you flip a normal coin, you know it will land either heads-up or tails-up. But if you flip a quantum coin, and immediately cover it up without checking its status, it will exist in a "superposition" of the heads-up and tails-up positions, a combination of the two states. Until you observe the coin, you can't know its true state, but you can know its "wavefunction," which describes the probability that it's in any given state. Once you peek at the coin, its wavefunction collapses into just one of the possible states, either heads or tails. The most famous example of this phenomenon is "Schrödinger's cat".
So how does this apply to quantum chess? In any given turn, each piece—for example, the queen—has the opportunity to make several different moves. If it's a quantum piece, then it can make multiple moves at once, existing in a superposition of places on the board. Your opponent won't know which move the queen made (and thus where it is) until he or she attempts to intercept it. By doing so, the opponent is "observing" the queen, forcing the piece to appear in just one of its potential positions.
Confused yet? It gets more complicated.
Spoiler alert: After a training montage (of course there's a training montage), Paul Rudd and Stephen Hawking face off in a heated quantum chess battle. Rudd is down to just two pieces, so he tries a risky gambit: He entangles his king and his bishop.
Quantum entanglement is a phenomenon where two particles become linked together and act like a single system. Take the quantum coins from the previous example. If you entangle the coins, then instead of describing each one with a wavefunction, you'd have a single wavefunction for the pair. This means that checking just one of the entangled objects tells you something about its partner's state. In the coin example, let's say the wavefunction tells us that our pair of coins contains one heads-up and one tails-up. If you observe just one of the coins and see that it's heads-up, you will immediately know that the second coin is tails-up EVEN THOUGH you haven't checked its status.
In the chess scenario, Hawking attacks one of the potential positions of Rudd's king. He is thus observing the king...which reveals the position of Rudd's bishop as well. The observation finds that Rudd's king is not in the square the Hawking attacked, but Rudd's bishop is perfectly placed to take Hawking's king and win the game.
Now that you know the basics, you're ready to play quantum chess! Don't you feel ready?