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[Note I am asking up to, but not including, consciousness as this bleeds into philosophy and is a much messier question]

Assuming that the laws of physics have remained constant across space & time since the big bang, has the way the universe evolved been entirely predetermined? While impossible to know the physical parameters & interactions across all space and time, these unknown states would have been acted upon by consistent forces since their onset. Thus, while not knowable, the universe grew along only a single possible path forward, meaning from the instant of the big bang to (cosmic) now, everything has been entirely determined?

Phrased another way, was there any physical process that introduced true randomness that caused the universe to have multiple different potential paths? Note that I don't mean things that seem random - such as paths of specific gas molecules when dispersing - since these are ultimately entirely predictable given sufficient knowledge of states & environment.

Or is my premise somehow wrong?

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    $\begingroup$ There is not ‘sufficient knowledge of states’ that can predict when a particular U235 atom will spontaneously fission. So, no. $\endgroup$
    – Jon Custer
    Nov 22, 2023 at 18:53
  • $\begingroup$ @JonCuster I note that Newtonian mechanics instilled the false impression in people that if we know enough about starting conditions, ALL future states are predictable. Once physicists started investigating quantum mechanics, and again when physicists started investigating chaos, that assumption revealed itself to be false. $\endgroup$ Nov 22, 2023 at 19:08

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Newtonian mechanics is theoritically predictable from initial conditions. However there are some gocthas.

You need to know the initial conditions infinitely precisely to predict the future accurately. You might think that if you make a $0.001$% error in the initial velocity of a particle you make a $0.001$% error in the distance it travels. You might be happy with that. But the problem is much worse than that.

Imagine a frictionless pool table. If you make a small error in the initial angle of one of the balls, it will cause an error in the angle that the balls bounce when they collide. The error of rebound angle will be bigger than the initial error. Those two balls will collide with other balls, causing the error to grow again. Pretty soon a collision that was supposed to happen will be a miss. Errors will continue to grow until the prediction will look entirely different from reality.

This is the Butterfly effect in a different setting. For a pool table, if the initial error was caused by neglecting the gravitational pull of a single electron at the edge of the galaxy, the prediction was be completely incorrect in about a minute. You can still say that given infinitely precise knowledge of initial conditions, an infinitely precise outcome can be predicted.

A second gotcha is there are corner cases where Newtonian mechanics is not predictable. See Norton's Dome.

But the biggest gotcha is that the universe only approximately follows Newtonian mechanics. Reality is quantum mechanical. Quantum mechanics outcomes are not precisely predictable, even in theory. Electrons are not particles. They are quantum objects that have some particle like properties and some wave like properties. Like a wave, they have no precise position. They have no precise velocity. They have no precise trajectory. It is possible to calculate probabilities of future positions, but you not make precise predictions. See The more general uncertainty principle, regarding Fourier transforms for more.

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No, and indeed if inflation did happen the unpredictability started only $10^{-33}$ seconds after the Big Bang.

If the universe underwent an inflationary stage then during this stage a cosmological horizon formed and that horizon emitted Hawking radiation. As far as we know Hawking radiation is a random process so during this stage the universe was perturbed in a random way and this ultimately led to the random perturbations we observe today in the cosmic microwave background.

It is true that in Newtonian mechanics time evolution is deterministic, so if the evolution of the universe had been purely Newtonian it would be deterministic as you suggest. However any time that quantum effects become significant they introduce a fundamentally non-deterministic element to the evolution.

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