Sunday, April 16, 2017

Quantum gravity

Ever since the development of quantum mechanics in the 20s,  there's often thought to be two conflicting pictures of the physical world: the subatomic domain is indeterministic while the macroscopic domain is deterministic. Put another way, Relativity is deterministic while quantum mechanics is indeterministic. Despite some of the best minds in science laboring to reconcile the two theories, the conflict remains intractable. Or so I frequently read. 

In fairness, I've overstated the issue. Some interpretations of quantum mechanics are deterministic. The hidden variables interpretation is deterministic. But from what I've read, Bell's theorem, while it didn't rule out hidden variables, made life very confining for the possibility of hidden variables.

The many-worlds interpretation is deterministic. Every alternate possibility that's physically possible must play out. Hence, the multiverse. That's my understanding. 

But for whatever reason, there are prominent physicists who are dissatisfied with that interpretation.

You can also have antirealists like Stephen Hawking who don't think there's a real conflict because quantum mechanics is just a mathematical model. Likewise, I don't think Bas van Fraassen believes in "theoretical entities" like elementary particles. 

I myself don't have a stake in this issue. Physical indeterminism is compatible with theological determinism. 

What I'd like to briefly discuss is a general principle. Are physical determinism and indeterminism irreconcilable? Can both be true in different respects? 

There are, for instance, situations where the initial state may be indeterministic, but cross a threshold into determinism. For instance, the way a chess game begins doesn't predetermine how it will end. At the outset there may be an infinite number of pathways to victory or defeat. But as the game progresses many pathways are (literally) taken off the table. There comes a turning-point in the game where it's no longer for one player to win. He is bound to lose. In x number of moves, he be checkmated.

Good players can see that coming and concede defeat before it happens. So something that was initially indeterministic can become inevitable.

Another example is gridlock. At one time of day there may be multiple viable routes out of town, but if all the arterials become too congested, there comes a point where the hapless driver can't go forward, backward, right, or left. 

To take a final example: consider the floor plan for a house. Suppose you begin with square footage. Say you have 5000 sf to play with. At that initial stage the possible floor plans are endless. Could be one story, two stories, three stories. Could be square, rectangular, hexagonal, and so on.

However, as you begin to pencil in rooms, that reduces available space for additional rooms. Likewise, the location of some rooms increasingly limits where to put other rooms. As the process continues, you narrow down the range of options. There comes a point at which earlier choices select for the remaining choices. They literally squeeze out alternative configurations. 

So, as a genera principle, I don't seen an inherent conflict between physical determinism and indeterminism. But it may well be that the relationship between Relativity and quantum mechanics isn't analogous to my comparisons.

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