The zen koan has no answer, of course. It’s intended to get your mind out of its usual ruts, to make you think differently. You do this by seeking an answer and never, necessarily, finding one. And by observing your own mind in action.
One answer to the question is: there is no sound. You have to start somewhere. So there is no sound. But then neither is there a clap.
Is there a hand? Isn’t that where we started? No, not really. We started with an event: one hand clapping. If there’s no event, if that event is impossible, then none of it is real. If nothing happens, then nothing is real. This is a physical statement, as true as any of Newton’s or Einstein’s laws.
Could something just exist, without anything happening to it? Imagine an isolated particle in space. Big black space. There’s nothing else. Is the particle moving? How fast? Things only move in reference to other things. An isolated particle is not moving, it has no speed. (And you can’t escape this by suggesting a frame of reference, which is an abstraction, a derivation of the mind that must reference real things to have meaning.) What is the particle’s energy? Its kinetic energy exists only in relation to its speed. If there’s no other particle, it has no kinetic energy. Other forms of energy also (electrical, chemical, nuclear, etc) are only in relation to forces that occur through interactions with other things. And mass only has meaning with respect to interactions with other mass and energy. No thing has any meaning on its own. It doesn’t exist.
Going one step further: when, in time, does it exist? Quantum mechanics tells us that fundamental events don’t happen until they happen. Before an event, there are only probabilities. The superpositions and uncertainties of the quantum world describe particles that are not yet real, and won’t be until they interact in an event. The Schrödinger equation, a fundamental equation of quantum mechanics that defines how a physical state evolves with time, tells us how those probabilities change over time. It tells us that the probabilities change in an oscillating pattern, the mathematics of a circular or spherical motion, which is the smallest possible path for something not yet settled. Physicists call the output of the Schrödinger equation a “wave function” because it resembles the mathematics of waves of water and electromagnetic energy, which are governed by similar equations. But it predicts probabilities, not certainties at any specific space and time. The probabilities evolve deterministically, but the events they predict do not.
What’s real? The wave function? It’s been proven in experiments. But it is an equation, a description, not a thing. What was there before the event is uncertain. After the event, everything is uncertain again except what is past, which is no more. Only the event is real.
In the real world, we can only use probabilities, and never certainties, in extrapolating future events from past ones. There is an inherent randomness about what will happen at every moment. Before the clap, there is no hand, only a probability of a hand being there.
Particles, mass, charge and other properties are useful abstractions, but only have meaning in relation to events. They come out of a fog of probabilities and disappear into a fog of future possibilities. They tell us how events are related to each other. They are connections between events. We have it backward when we think of events as connections, or collisions, between things with mass and charge. Events are fleeting but the connections set up patterns that repeat, making them seem permanent, but nothing is. The sound of one hand clapping is meaningless. So is the clap. So is the hand, without a clap.
Now let’s get back to simulations. Computer simulations are deterministic, they don’t evolve as interacting probabilities. But we know that real events, before they happen, are only probabilities that can go either way, happen or not happen. Many events, including those that give us life, are connected in ways that result in very high probabilities for events that are important to us. It makes life feel deterministic. But in the universe, which is much larger than us, many events are connected with greater uncertainties than we experience. Science allows us to see some of these things, but our day by day experience is built on things that happen and happen again and again with high probability. As organisms, most things we need to deal with to survive seem determined precisely by past events. We can be fooled.
The idea of simulating the universe occurs because simulations of real things we see in VR headsets seem so real. Those convincing simulations seem like “real life.” But that’s because our experience of “real life” is very, very limited, much as we dislike admitting it. We are alive because we only have to deal with probabilities that are near certainty for most of what we do. At least as far as those things we have to react to and reason about (i.e. make crude simulations in our minds) in a certain timescale, in order to continue living. For many other things, we accept uncertainty and just deal with things when they happen, and those of our ancestors for whom this has been good enough have survived, adapting through evolution. It doesn’t mean our simplified, deterministic understanding of how the world works is right. It just works for us, enough of the time.
We do manage to live with some things that are highly uncertain, like the weather. We’ve learned to accept this uncertainty, and prepare for the range of possibilities that usually occur. This keeps us, unless we are meteorologists, from thinking about how much uncertainty there really is in the chains of causal events that result in the weather we experience. Which doesn’t mean simulations of systems with lots of uncertainties are useless. Weather simulations keep getting more and more accurate. They’ll just never get all the way to deterministic, and the farther they go the slower their progress will get, and ultimately slow to a hard limit on their precision in predicting future events.
Some everyday things we experience are far beyond our ability to simulate. Like ping pong. One of the early simulation games was Pong, a simulation of ping pong. The simulation of trajectory reflection rules for elastic objects’ momentum gave an impressive simulation of reality, a warm feeling that simple equations could capture reality. But a real ping pong game involves two or four humans. A real ping pong game would require simulating the human perceptions and reactions that control the actual movements of the paddles. It would include things like subconscious signals arising from childhood disappointments in the distant past. No computer yet made could predict a real game of ping pong. The name given the early simulation game is Pong, just half of ping pong. Like one hand clapping.
Could a deterministic simulation of the universe include artificial probability simulations, using pseudo-random variables, just to fool us? That would be hard, for a whole universe. But it might not be impossible, given a big enough computer. It might have to be a computer with a different timescale, to simulate all the randomness and have events occur on the timescale of our experience. It might have to be a computer literally larger than our universe. But why would this occur? One could imagine an infinite regress of computers simulating universes that contain computers that simulate more universes ad infinitum. But why? Occam’s razor should be unsheathed here.
A simulated universe? Maybe. But, probably, not.
