Quantum Mechanics and Free Will

In classical physics, the world is deterministic. If you knew everything about the universe at one instant of time you could, in principle, predict exactly what was going to happen for all time, including what someone’s going to eat for breakfast tomorrow. Kinda messes with the idea of free will.

In QM, tho, everything is probabilistic. Even if you could know the exact state of the universe at some time (which you can’t, since the universe is not a repeatable experiment), you would only find out probabilities about the future. “Hmm, Rob will have eggs tomorrow (15% probablility) or cereal (10%) or a bagel (18%) or …” No problems with free will anymore! Heck, you could give numbers like that just by looking at my eating habits over the past couple years.

So, does QM solve the problem of free will and physical law? Discuss.

I’d say the two have nothing to do with each other. Whatever free will may or may not be, it’s not something that occurs on the quantum level.

What is, in YOUR opinion, the “the problem of free will and physical law?” What’s the conflict?

There are so many steps between “particle A interacts with particel B” and “Joe will eat either cereal or a bagel from breakfast” that it is pretty meaningless to talk about one in terms of the other. THEORETICALLY, yes, Newtonian Classical Mechanics would imply that once things get started in motion, the outcome is determined. But even Quantum Mechanics, with it’s probablilistic nature does not have an equation for “human choice”. It just gives probablilities of various outcomes-- you don’t get to select among the outcomes.

Hmm, I thought the problem was obvious. If what you are is a collection of interacting atoms, and those atoms obey deterministic physical laws, then, in principle, I can find out what those atoms are doing and predict exactly what you will be doing 5 minutes or 5 years from now. In other words, you are a robot that behaves in a completely predictable way. You may think you are making choices, but those choices are completely determined by your atomic makeup.

Yeah, OK, there are a lot of steps in between the atomic description and the action. But “a lot of steps” doesn’t remove the basic difficulty: your future is completely determined by what your atoms are doing right now and their interactions with your surroundings. Your “choices” are irrelevant and unneccessary variables.

John: No, there’s no “human choice” variable in QM. But I’m not saying QM describes free will, I’m just saying it leaves room for it. Your future cannot be predicted by knowing what your atoms are doing right now.

Yeah, but doesn’t free will mean you have a choice? When you roll some dice, you don’t know the what the outcome will be, but you don’t get it to “will” a certain outcome.

IANAP(hysicist) and IANAP(hilosopher) but it seems to me that, while QM does rule out determinism, it doesn’t rule in free will. It just implies that your choices, rather than being deterministic, are random (albeit constrained by probabilities).

** FriendRob**


How so? I may be the entity “consciousness” inhabiting this body as a vehicle, in which the body doesn’t “know” anything. So you can only know of my actions after they are made. You may know where I am but not what I will do next since there is usually more then one option.

Quantum mechanics can allow for free will, insofar as it removes absolute determinacy. However, since quantum events are supposedly random (as opposed to exhibiting properties of willfulness), quantum mechanics is insufficient to produce free will.

A machine whose behavior is governed by a random number generator does not have free will. It may exhibit an element of unpredictability, but that is not free will per se.

But what is free will? Can anyone think of a good definition of it?
Is it entirely subjective, or does it have an effect on the outside world? Do we even have free will?

Obviously classical (deterministic) physics rules out the possibility of free will unless you define it in subjective terms only. Quantum mechanics says that even if you can rewind the universe backwards, and then rerun it, different things will happen. Surely this at least allows the possibility of free will?

Sorry for posting again, but another thought has occured to me on this subject…

What is free will? Choice of action surely. But how could you prove someone had choice of action? If a being were perfectly rational, and had clear goals then even if you rewound the universe, that being would choose the same action as it had before. But we’re not perfectly rational, and we don’t have clear goals so if you rewound the universe, then a human might choose a different course of action. This is the only difference i can think of between a human with free will, and one without.

Given that this is the only possible difference free will could make to the universe, then quantum mechanics does allow for free will. Unfortunately there is no way we can rewind the universe, even theoretically, and thus for all intents and purposes free will has no meaning…

What do you guys think about that?

The illusion of free will is good enough for me. I don’t care if my thoughts and decisions are predetermined or not because I still won’t know what they turn out to be until they happen.

The materialist view that if everything could be known then the future could be predicted exactly may very well be true. It doesn’t matter though, because there is no conceivable way that everything could be known and modeled so perfectly. One reason is the limits on certainty that quantum level measurements are faced with. The other is the seeming randomness inherent in modeling complex systems. The only thing that actually performs both of these functions perfectly is the universe itself, and the only way to see what the answers are is to wait for them to happen.

No, quantum mechanics allows for the possibility of a non-deterministic outcome. This is not the same as free will. If your behavior is governed by a random number generator, then it’s not truly “free” will, i.e. there is no purposefulness behind that behavior. All you have are pre-programmed actions which are selected by random chance, rather like a computer game “randomly” selecting how it should respond to your keystrokes.

i’d like to point out a common misconception about quantum mechanics and the uncertainty principle.

that is, many people seem to think that what happens on a quantum level is completely random, and that there actually is a probability that particle A is in position x with momentum u, but in fact the uncertainty principle states only that we can determine the position and momentum with limited accuracy, and so we may use probability. note the difference: quantum-level interactions may be completely deterministic, but we consider them probabilistic because we can only measure them up to a certain accuracy (because the measurement affects the measured).

it is not the case that there is a limit on the probability of a particle being in state s, only that there is a limit to the accuracy with which we can measure s.

so no, i don’t think quantum mechanics gives us the free will loophole we might like.

Ramanujan, I think you’ve missed the point. The uncertainty principle is only one tiny aspect of quantum mechanics.

Quantum mechanics teaches that certain quantum events have completely random outcomes. This is entirely different from the uncertainty principle, and it most certainly does imply that the universe is not 100% deterministic. As such, it does allow for the possibility of free will, even though it can not explain how this arises.

JT makes a good point: if we think of the QM universe as being governed by Schroedinger’s equation plus a random number generator that chooses between the possible outcomes, then the future is just as “determined” as in classical mechanics.

If free will somehow lives in the gap caused by that randomness, does that imply that our choices influence the outcome of certain quantum events? Then those events would not be truly random, but would only appear random because we can’t measure the influences. Maybe there would be some experiment that would test if we can influence the collapse of the wavefunction by our choices.

But here’s another point: all of the discussion so far has implicitly assumed that we can give a quantum mechanical description of a human being; a wavefunction for Rob, say. I claim that such a description isn’t possible. A quantum state is defined by a repeatable procedure that always gives the same probabilities for certain measurements. Where is the repeatable procedure that will produce Rob again so we can test those probablilities? I think QM protects free will even more strongly than we’ve been discussing, because there simply is no possible QM description for a human being.

First of all:

  • The Universe in a Nutshell by Stephen Hawking

In other words, the world doesn’t just appear to be random because we can’t measure everything perfectly, it really is random, as JThunder pointed out.


If we take “free will” to mean that our choices are neither predetermined nor random, it is clearly false. It has to be one or the other, which should be obvious. I cut and pasted the following paragraph from something I wrote a few years back in high school. I don’t really think it requires any changes, so I’m sticking it in as is:

One can easily define the term “determined” to mean, “brought about by preceding circumstances.” One can also define “chance” to refer to events that are not completely determined. This seems to be a fair and simple distinction to make. Therefore, for free will to influence our actions, it must be either a part of preceding circumstances or a chance occurrence. There is nothing else that it possibly can be. If free will is a force that determines our actions, then there is no conflict between the concepts of free will and determinism. If free will means that our actions are not determined by circumstances, including all conditions both ethereal and physical, then it means simply that the decisions we make are, in part, random, again with “random” meaning by definition “not completely determined.” QED.

To summarize, “free will” is a contradiction in terms if the term “will” refers to a force that determines our actions and the term “free” denies any such determination.

When I was a child, I understood the concept of free will simply to mean that the mind controls the body, which seems self-evident enough. But people seem to use it to mean that our choices are somehow not subject to causality, which makes no sense to me whatsoever. Is it just me?

If we take “free will” to simply mean that our actions are not determined, then quantum mechanics would seem to imply free will. But I don’t see how the randomness of our choices makes us any more “free”.

Did that make any sense? :dubious:

No, I think that’s an unwarranted conclusion. Free will is purposeful, which neither determinacy nor randomness would allow.

Only if that force is external, i.e. imposed by some outside agent or principle. Will, by its very nature, is internal. It is a manifestation of the self.

Very interesting. Could you give an example, please?

It is perfectly consistent to say that I choose my actions and that they are determined by what I am.

—Free will is purposeful, which neither determinacy nor randomness would allow.—

How so? How is purpose in conflict with determinacy or indeterminancy? What other options are there other than determinancy or indeterminancy?

—Only if that force is external, i.e. imposed by some outside agent or principle. Will, by its very nature, is internal. It is a manifestation of the self.—

Yes, which is precisely the problem. It is the internal force that must somehow be inexpliably “free” from its own nature for “free will” to make sense in the way it is often used.

Try this, using the idea of an omniscient judge:

If this judge judges someone, what exactly is he judging? What makes one person choose one thing (to be good, or even simply to listen to god) and another another (to not be good, or not to listen to god)? Is it because they have different natures? If so, how can they be blamed for their given natures? If they do not, then what explains the difference of their choices? If nothing, then how can they be responsible?

It does no good to assert that people can choose their natures: that simply shifts the question back to whatever agent we claim is doing THIS particular choosing.

The very idea of moral responsibility itself seems to require causality: the ability to trace a choice back to the agent that made it, and the reason for the making.

It’s a dizzying issue.