Comment by mcarvin | Hacker News Reader

Comment by mcarvin | Hacker News Reader

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I understand the question quite differently. The question of objective probabilities is a question of whether the world is deterministic.

If you believe the world is governed by physical laws and you further believe you understand those laws (physics), you consequently must believe that with sufficient information about the past you can predict the future (determinism). So if I was to throw a dart at a dartboard, with sufficient information about arm speed, humidity, wind etc you could perfectly predict where on the dartboard it would land. The problem is that the mathematical / computational complexity necessary to achieve that is still beyond our means. Enter Probability, which we use to reduce the computational burden of predicting the future. As probability will always be model dependant (ie subject to someone's view of the world and necessarily wrong some % of the time) - it follows that an objective probability cannot be consistent with a deterministic world.

So is the world deterministic?

Your premises are wrong: you can believe in physics, but the laws of physics could not deterministic.

To the best of our knowledge this is the case[1], and we have strong indications [2] that this is a fundamental feature of nature: the non deterministic aspects do not come from our lack of understanding and knowledge.

[1] http://en.wikipedia.org/wiki/Quantum_mechanics [2] http://en.wikipedia.org/wiki/EPR_paradox

Certain irony about saying that a LAW of physics is not deterministic. Conceded though that determinism does not hold up at the quantum level.

Disagree though that quantum non-determinism translates to non-determinism at the scale we experience [1]. Have never seen a Feynman diagram for a soccer ball. Which is why I feel comfortable saying that if you believe the macro world is deterministic then you cannot also believe there are objective probabilities (unless, as you indicated you are talking about the subatomic world).

Chaotic macroscopic system + non-determinism at atomic scale = non-determinism at macroscopic scale.

So, e.g., the temperature at the tip of the Eiffel tower on Jan 1, 2014 1pm is non-deterministic.

From a practical approach, the physical world can seem fairly deterministic. If you drop something in a vacuum and nothing hits it, you can say with a great deal of accuracy how long it will take to hit the ground. I don't think that this practical determinism translates to a philosophical determinism.

Determinism does hold up at the quantum level. Quantum mechanics is completely deterministic. Collapsing the wave function to a classic state, is the probabilistic aspect, if any.

I think that quantom mechanics makes us think differently about probability and determinism by giving the mathematical (human created) concept of probability a physical meaning. Similar to the example in intuition 2, a coin toss could be considered determinstic even if we do not try to predict the result. It is pre-determined that given a large enough number of coin tosses, very close to 50% of them would come out heads. Qunatom mechanics makes us consider probability as a physical property inherent in all physical objects. This way of thinking helps reasoning about genetic predisposition as well (which is always probabilistic). Just like any coin is "free" to make a choice, and yet it is certain that they would (almost) precisely split down the middle, so too can genetic predisposition can be said to be both constraining and not constraining at all at the same time.

Admittedly beyond at the limits of what this armchair quantum mechanic can say with confidence but to borrow from Hawkings... probabilities in quantum mechanics "reflect a fundamental randomness in nature" [The Grand Design]. So while quantum theories are testable, they are not always predictive (necessary for determinism).

There are alternative uses of probability. For example, you might measure the weight of 50 people eating one of two diets, and determine an average weight for each group. You want to know if the different diets affect weight. It is highly unlikely that the average weight will be exactly identical for each group, so now you use statistical methods to determine the probability that the diet caused the observed difference in weight. You now assign a "probability" to the hypothesis that diet influenced weight -- is that probability objective?

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