Physics and Predestination

Great Debates
1

OK, I am not entirely sure if this is a Great Debate, but this has been bothering me for the last day or so, and I don’t think it has a clear answer.

I have heard many times Physicists say that if one knows the momentum of every single particle in the universe, or even in a closed system, then one can predict not only what happened back in time exactly, but what will happen forward in time.

I was thinking about this, and then it struck me. If this is true, then to what extent do I have any sense of free will provided the following:

  1. We assume that human intelligence is a biological function, and that there is no “soul” component that does not lend itself to being described in terms of “momentum”.

  2. We assume that the Heisenberg uncertainty principle does not enter into play.

  3. There is no Goddess, or supreme being.

If these conditions are true, then it seems I have no free will at all, as my fate (even the typing of this post) could have been predicted in theory by someone who knew the momentum of every particle in the solar system yesterday.

Now, it is specious to think that there is such a way to know the momentum of every particle in a person, let alone a planet, or solar system. And knowing how every particle interacts exactly seems like it would be impossible, especially if you consider things like radioactive decay, which (I think) are a statistical function?

But do you see what I’m getting at? Conceptually, if everything could be predicted from particle momentum and interactions with other particles (I’m not saying humans could actually do so in reality, but if the possibility exists that these things could be predicted), then I have no free will?

Or are my assumptions above unreasonable enough that they invalidate the any attempt at prediction?

I hope this doesn’t sound as dumb to all of you as it looks now to me. But this is bothering me somewhat. :frowning:

2

Well, I do think your assumption that quantum uncertainty doesn’t play a role is incorrect. It’s in fact has turned out to be a very fundamental part of how the universe works.

Physicist/mathematician Roger Penrose wrote a controversisal book called “The Emperor’s New Mind,” in which he makes the suggestion that quantum effects may explain human consciousness. Penrose was pretty roundly criticized for the book - many thought he was making too big a leap in logic, and new nothing about brain physiology… but the idea still seems like an interesting one to me.

3

Well, I’m a determinist, so I don’t believe in free will in the first place. Me lack of belief in free will centers around my belief in cause and effect. However, I don’t believe in fate either. To me, things don’t happen because they are meant to happens, it’s more that they couldn’t help but happening.

Regarding the Uncertainty principle, I think it has more to do with being unpredictable, not being random. That the very act of observing makes it impossible know what would happen if it weren’t being observed.

4

What physicists have you heard this from? The “clockwork universe” model went out quite some time ago, I believe. Quantum physics is a funny thing; while it may be 99.99999% certain that an electron is right by its neutron, it could also be across the room, and for quantum calculations to be accurate scientists have to take this into acount. In a complete vaccuum, electron and anti-electron pairs occaisionally (and for no apparent reason) pop out of nowhere and usually immediately annilate themselves, but sometimes survive long enough to set of an electron detector. Electrons spontateously emit and reabsorb photons for no apparent cause. Scientists have found absolutely no way of predicting quantum decay of an atom; they can simply say that given X time period, 50% of the atoms in a sample will decay, but there is no way of knowing which or when (and in suprising support of the old saying “a watched pot never boils”, if you excite atoms so that they would all drop down an energy level after a certain amount of time, if you keep “looking” at them they never will). The fundamental nature of reality seems to depend on probabilities, not the certainties that would be required for a clockwork world.

5

Which physicists? This is not quite a true statement, as some of the assumptions implicit in it (and that you make below) are not valid. It is a decent approximation for big systems, for example stars and planets, but it is still not correct - chaos theory has taught us how very small differences in original conditions will give us large divergences in final results.

A demonstration of this is the Newtonian three body problem. In Newtonian physics, it is possible to solve exactly for the behaviour of a two body system (ignore Einstein for now.) However, when you introduce a third body to the system, it becomes impossible to solve. It is necessary to approximate the situation.

Likewise, in Quantum Mechanics it is possible to solve exactly for the behaviour of the Hydrogen atom (a two-body system). However, it becomes impossible to solve for the Helium atom (a three-body system) and approximations are necessary.

**Statement 1:**So, in a macroscopic situation if you knew all the exact positions and momenta of the planets/stars, you could calculate the behaviour into the future. However, if you are off even by one part in one billion, your calculations will diverge from reality eventually.

Assumption 2 is not valid. The HUP in fact does enter into play, by asserting that it is impossible to know exactly the position and momentum of an entity. Thus, Statement 1 will always fall victim to the ‘however’ clause, because of the HUP, if not long before that due to imprecise measuring equipment.

Yes, I see what you are getting at. Fortunately the HUP rescues us from predestination, but even before HUP, chaos will rescue us. The behaviour of complex systems is unpredictable due to their complexity. Your consciousness, assuming it is ‘brain induced’, stems from a very complex system - the human brain. It is thus unpredictable.

6

Gaudere and douglips, I have read it in a few “pop Physics” type books, and I believe Hawking talks of it when he discusses something called “conservation of information”, where he claims a black hole violates that because we cannot know the momentum of the particles past the event horizon.

I’m not a physicist. I’m a mechanical engineer who used to like to read about physics, but at heart I don’t have education or experience enough to argue the points. It is nice that chaos and the HUP seem to combine to give me free will - perhaps that will make me feel better.

I guess there’s not much of a debate here then - perhaps this should be kicked to GQ and I should stay out of GD for a while.