Can Physics predict the future? Is the universe fundamentally predictable?
Sir Isaac Newton found that his laws could explain most things through mathematical laws. Pierre-Simon Laplace. He imagined a super-intelligent being, Laplace’s demon, who if he had Newton’s equations and unlimited brainpower, could use them to compute the past and the future of the entire universe with perfect accuracy. What’s the problem?
Well, there’s a catch. Knowing “perfect information” about every little particle is practically impossible. This is Chaos theory, where a tiny difference in initial conditions, can lead to a totally different outcome down the line. This is called the butterfly effect. It’s “sensitive dependence on initial conditions.” Even though the weather obeys deterministic equations, in practice we can’t measure the starting conditions perfectly enough to predict the distant future. Other systems are also chaotic: the turbulent flow of water, the stock market. So, even though Newton’s equations are deterministic, our ability to use them as a crystal ball is limited.
But even though systems like the double pendulum, weather, and turbulent fluid flow, are unpredictable, they are unpredictable only because we cannot measure the smallest variations in their initial conditions. But if we had perfect information or perfectly precise measurement tools, all these systems would be predictable. So the future behavior of macroscale chaotic phenomena like these are predictable in principle.
A big problem to predictability is from quantum mechanics. At a microscopic scale, nature is not deterministic but probabilistic. It appears to be a fundamental feature of the universe, not lack of knowledge. One can experiment with an atom or electron exactly the same way each time, but get different results in a random pattern. We can calculate the odds of each result, but we cannot predict the behavior of individual particles.
Take radioactive decay. We can measure a half-life – the time by which half of them will have decayed on average – but we have no way to know when an individual atom will decay. This fundamental randomness is a built-in limit on predictability.
Also, the Heisenberg uncertainty principle, shows that we also have a limit on the precision with which we can know certain pairs of properties. We can’t know a particle’s position and momentum with infinite precision. If you can’t know them exactly now, you can’t plug them into an equation to predict the future exactly either.
It gets weirder: quantum mechanics, says a particle doesn’t even have a definite position or outcome until you measure it. It exists in a sort of superposition of possibilities. We can call it a wave of probability. The Schrödinger equation lets us predict how this wave of probability evolves over time, so that part is deterministic.
Some scientists argue that we are just missing information in a fundamentally predictable universe. However, there’s no experimental evidence of these hidden variables.
And experiments on Bell’s theorem have demonstrated that A local hidden variable theory is likely wrong. So if there are hidden variables, they’d have to be non local hidden variables, like particles influencing other particles faster than light, or particles would need to be intertwined with everything in the universe in a conspiracy-like way.
But just because quantum phenomena cannot be predicted in advance, it doesn't mean that macro-scale phenomena are also unpredictable. Quantum effects typically become negligible at everyday scales due to decoherence, Things like planetary orbits, solar eclipses, and tides are completely predictable years and decades in advance because they are based on deterministic physics, where quantum effects are negligible. But other phenomena that we consider chaotic today, the stock market, are also predictable in principle because they are based on deterministic physics. We need better models and more precise data.
00:00 - The future from numbers?
01:17 - Laplace's Demon and Predictability
04:35 - Chaos: the first "problem"
06:48 - BUT Chaos is predictable in theory
08:18 - Quantum Mechanics and predictability
10:20 - Heisenberg Uncertainty principle: limit on precision
10:58 - Even weirder quantum mechanics
12:04 - How quantum mechanics be deterministic
13:54 - "known" laws of physics could be wrong
16:03 - Verdict on macro scale predictability