# Does Heisenberg's uncertainty principle imply no free will?

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## Main Question or Discussion Point

Does the Heisenberg uncertainty principle mean that some particles do not have law-determined properties like position and momentum, or does it mean that their properties cannot be measured accurately? In other words, do all particles have a certain position and momentum at time t?
This question relates to the question of free will that's still being debated.

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phinds
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The HUP is not a measurement problem, it is a description of the fundamental nature of particles.

As far as I know this has absolutely nothing to do with free will (although I confess, I consider free will to be a purely theological concept removed from science and really don't pay any attention to discussion of it).

fresh_42
Mentor
I've seen Hawking on tv discussing the issue, but similar as @phinds I didn't pay any attention to it, regardless whose speculation it was. I even have a problem with the anthropological principle, as I don't think our very special (and short) evolutionary background has anything to do with the principles of nature. To me, free will is a philosophical topic at best, a religious at worst, and not existent from a biological point of view. To draw a connection to HUP is completely artificial in my opinion and in the tradition to view ourselves as the center of life and universe - a desperate attempt to hold on tight what long had been lost.

Uncertainties etc. all come from the Statistical Interpretation of the wave function. It is in fact a probabilistic interpretation. E.g. |Ψ(x,t)|2 is the probability density etc. ...
That's all that's official (+measurement selection rules etc.). Anything else is speculation.

PeterDonis
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do all particles have a certain position and momentum at time t?
No. As @phinds pointed out, this is due to the fundamental nature of particles--more precisely of quantum particles and the possible states that they can have in quantum mechanics. It turns out that there simply are no quantum states of particles that have both a definite position and a definite momentum. (The technical reason for this is that the operators for the position and momentum observables do not commute, but that's getting beyond the "B" level of this thread.)

This question relates to the question of free will that's still being debated.
Actually, it doesn't. The statement I made above has nothing to do with free will, and would remain the same regardless of which "theory" of free will turns out to be correct.

Here's the question I should have asked:
Does every molecular, atomic, and sub-atomic interaction in brains have a predetermined result, or could there be more than one result?

No. As @phinds pointed out, this is due to the fundamental nature of particles--more precisely of quantum particles and the possible states that they can have in quantum mechanics. It turns out that there simply are no quantum states of particles that have both a definite position and a definite momentum. (The technical reason for this is that the operators for the position and momentum observables do not commute, but that's getting beyond the "B" level of this thread.)
Actually, it doesn't. The statement I made above has nothing to do with free will, and would remain the same regardless of which "theory" of free will turns out to be correct.

hilbert2
Gold Member
Described in scientific language, "free will" would mean something like "when an individual is exposed to some stimulus, their response is not a completely deterministic function of the properties of themselves and the stimulus in question, but there remains some uncertainty in the way they respond". This kind of thing is not really approachable with the scientific method that is used in physics, as we cannot handle wavefunctions describing macroscopic things like a human being.

Described in scientific language, "free will" would mean something like "when an individual is exposed to some stimulus, their response is not a completely deterministic function of the properties of themselves and the stimulus in question, but there remains some uncertainty in the way they respond". This kind of thing is not really approachable with the scientific method that is used in physics, as we cannot handle wavefunctions describing macroscopic things like a human being.
I've restructured my question as: Does every molecular, atomic, and sub-atomic interaction in brains have a predetermined result, or could there be more than one result?
Of course this can't be analyzed with macro-physics.
My reasoning is if every physical interaction in brains is deterministic, then we do not have free will. Is that reasonable?

hilbert2
Gold Member
Yes it's reasonable to say that there can't be free will in something that is pre-determined. A more difficult question is whether indeterminism "implies" what we call free will.

Yes it's reasonable to say that there can't be free will in something that is pre-determined. A more difficult question is whether indeterminism "implies" what we call free will.
Very good point! I guess we'll never know.
Sam Harris is convinced we don't have free will, and he uses everyday examples to illustrate his point. Not totally convincing.

beastforever
I've restructured my question as: Does every molecular, atomic, and sub-atomic interaction in brains have a predetermined result, or could there be more than one result?
Of course this can't be analyzed with macro-physics.
My reasoning is if every physical interaction in brains is deterministic, then we do not have free will. Is that reasonable?
If we can take into consideration of what's said, that if we know how things are going to behave in a sub atomic level, how can we make an assumption that combinations of many pre-defined results will in turn be pre-defined?

If we can take into consideration of what's said, that if we know how things are going to behave in a sub atomic level, how can we make an assumption that combinations of many pre-defined results will in turn be pre-defined?
In a computer there are billions of subatomic interactions per second, but when I press the space key it makes a space. If all interactions in the universe are deterministic, then all macro interactions are also deterministic.
It's only because of the HUP that we're not sure about determinism.
See Michio Kaku's explanation...

PeterDonis
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Does every molecular, atomic, and sub-atomic interaction in brains have a predetermined result, or could there be more than one result?
How would you test this experimentally? If it can't be tested experimentally, it's not really a question of physics.

According to our best current model of such interactions, the answer is no, the interactions do not have a pre-determined result, because our best current model relies on quantum mechanics, and quantum mechanics only gives probabilities for different possible results, it doesn't pre-determine a single result. But even then it is still possible to have interpretations of QM, such as the MWI, in which the evolution of the wave function is entirely deterministic, so in that sense there would be predetermined results.

PeterDonis
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it's reasonable to say that there can't be free will in something that is pre-determined.
It might seem reasonable, but it's still not correct. However, this is not something you're going to learn from the physics literature. You would need to look into the philosophy and cognitive science literature. For example, you could try Daniel Dennett's two books on free will, Elbow Room and Freedom Evolves. The short version is that, first, "free will" is not a well-defined term with a single meaning, so before you can even talk about it rigorously you have to figure out a rigorous definition; and second, there are rigorous definitions under which "free will" and "determinism" (which also needs to be rigorously defined, and there's more than one way to do that as well) are perfectly consistent.

It might seem reasonable, but it's still not correct. However, this is not something you're going to learn from the physics literature. You would need to look into the philosophy and cognitive science literature. For example, you could try Daniel Dennett's two books on free will, Elbow Room and Freedom Evolves. The short version is that, first, "free will" is not a well-defined term with a single meaning, so before you can even talk about it rigorously you have to figure out a rigorous definition; and second, there are rigorous definitions under which "free will" and "determinism" (which also needs to be rigorously defined, and there's more than one way to do that as well) are perfectly consistent.
I'm aware of Dennett being a compatibilist, but I think its mainly because of what you implied - that free will isn't definable.

PeterDonis
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I'm aware of Dennett being a compatibilist, but I think its mainly because of what you implied - that free will isn't definable.
I think Dennett would say that "free will" can be defined in different ways, but the definition that matters--the one people actually care about when you pin them down about why "free will" is so important to them--is the one that's compatible with determinism.

I think Dennett would say that "free will" can be defined in different ways, but the definition that matters--the one people actually care about when you pin them down about why "free will" is so important to them--is the one that's compatible with determinism.
I think that compatibilism is a cop out from the inability to find the answer.

beastforever
In a computer there are billions of subatomic interactions per second, but when I press the space key it makes a space. If all interactions in the universe are deterministic, then all macro interactions are also deterministic.
It's only because of the HUP that we're not sure about determinism.
See Michio Kaku's explanation...
In a computer as you said, "we" manipulate those interactions to give us a desired result, in other words, we have made it to do what we want it to, but what about things we don't have control on? In other words, what about results that are independent of any human control? does it have to be that the results are predefined?
If I roll a dice, I get a number as a result, it might or might not be the same number all the time.. so while your computer example supports determinism, this macroscopic example supports indeterminism..

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In a computer as you said, "we" manipulate those interactions to give us a desired result, in other words, we have made it to do what we want it to, but what about things we don't have control on? In other words, what about results that are independent of any human control? does it have to be that the results are predefined?
That's the $64,000 question. In box A a photon hits an electron. In box B the exact same interaction happens. Can the outcome be different in both boxes? beastforever That's the$64,000 question.
In box A a photon hits an electron. In box B the exact same interaction happens. Can the outcome be different in both boxes?
can this be related to schrodinger's cat theory?
and anyway, we both have accepted upon(at least assumed) the fact that sub atomic interactions are in the context, pre defined. It's the issue of macroscopic objects that we deal with.
And as of the question, *can* the outcome be different? yes, in my opinion, it can because the interactions aren't happening in the same box. Let me know if my answer is satisfactory..

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can this be related to schrodinger's cat theory?
and anyway, we both have accepted upon(at least assumed) the fact that sub atomic interactions are in the context, pre defined. It's the issue of macroscopic objects that we deal with.
And as of the question, *can* the outcome be different? yes, in my opinion, it can because the interactions aren't happening in the same box. Let me know if my answer is satisfactory..
Actually, you're right about not being in the same box because nonlocal influences would be different. Using two boxes was only for a thought experiment.
I'm still of the opinion that if sub-atomic interactions are deterministic, then so are macroscopic ones, because the macroscopic supervenes (are built upon) on the sub-atomic.

bhobba
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Does every molecular, atomic, and sub-atomic interaction in brains have a predetermined result, or could there be more than one result?
We don't know.

It would seem QM precludes predetermined results, but we have interpretations of QM that are deterministic (eg BM).

So the answer is your guess is as good as mine - but for the record I don't think it does have a predetermined result - just me of course and just so I cant be accused of being a fence sitter.

Thanks
Bill

bhobba
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can this be related to schrodinger's cat theory?
This is not a thread about Schrodinger's cat, so I will limit myself to saying many people misunderstand Schrodinger's cat. Take standard Copenhagen where QM is a theory about observations in a common-sense classical world. That observation can be taken to have occurred when the particle was detected (Von-Neumann showed it can be placed virtually anywhere - which was the purpose of the thought experiment that if you place it at the observer you get issues - but since it can be placed anywhere placing it where I said is also valid and since there in no way to tell the difference the conclusion it draws is also correct) - everything is classical after that. So the cat is never alive and dead in some weird superposition - its classical so it's alive or dead - period.

Now there is more that can be said - in fact a lot more - but it really requires a separate thread.

Thanks
Bill

We don't know.

It would seem QM precludes predetermined results, but we have interpretations of QM that are deterministic (eg BM).

So the answer is your guess is as good as mine - but for the record I don't think it does have a predetermined result - just me of course and just so I cant be accused of being a fence sitter.

Thanks
Bill
Yes, it's pretty frustrating. I wonder if they can test determinism with experiments. They're pretty good at aiming electrons nowadays

PeterDonis
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2019 Award
I think that compatibilism is a cop out from the inability to find the answer.
No, it's making a claim about the form the answer will take when we find it. Of course just saying "free will is compatible with determinism, when both terms are properly interpreted" doesn't tell you how they are compatible; you still have to find that out. I don't think Dennett would disagree with that. (Also, Dennett has spent considerable time developing a compatibilist model in quite a bit of detail, so he isn't just waving his hands.)