The Quantum Thomist

Musings about quantum physics, classical philosophy, and the connection between the two.
Are fundamental particles of matter indestructible?

On physical determinism.
Last modified on Sat Jul 13 18:33:52 2019

In this post I intend to briefly look at a controversial topic, physical determinism. Whether I succeed in being brief (it will be a first) we will find out at the end of this post. It will certainly be brief compared to the vast monographs which have been written on these topics, and which ought to be written were I to do the topic justice.

I will begin, as I always should (but often don't) with my definitions.

Physical determinism is defined as the belief that the laws of physics are such that in principle if one knows the complete state of a closed physical system at one moment in time, and has the correct understanding of physics (and there was no intervention from "outside physics", if such intervention is possible), then one could calculate the complete state of the system at any other moment in time. More specifically, for every physical system in a state A, they will each necessarily evolve into a the same state, which we can label as B, a given time period later. Of course, the calculation might be possible in principle, but in practice we could never know the state of the system well enough to carry it out. The determinism is about the principle rather than the practice; it is a statement about the nature of the laws of physics more than it is about our abilities (or lack of them). As an obvious example, Newton's laws of motion are deterministic.

Indeterminism states that the laws of physics are such that it is not possible even in principle to calculate the state of the universe at the next moment of time, but only to assign probabilities to various outcomes. For example, given a closed physical system in a state A and perfect knowledge of an indeterminate laws of physics, we are saying that the result could be A, B, C or D, but not anything else. It does not imply a complete free-for-all where anything could happen; rather it states that instead of there being only one option, there is a finite number of possibilities. Knowledge of Physics then allows us to compute the probability (or likelihood) that each option is achieved.

Indeterminism is inconsistent with several different forms of causality, especially those which claim there is a necessary connection between cause and effect. If the original state might be preserved, and thus the change to a different state is spontaneous, then it is also inconsistent with those forms of causality which require that every event has a cause. It is not, however, inconsistent with the older view of efficient causality that each substance is caused by another substance, i.e. that the efficient cause of B was A regardless of whether the transition between A and B has a physical explanation (for example, to explain why it happened at one particular time rather than another). This version of efficient causality is the minimum required for the universe to be rational and it to be possible to formulate (useful) theories of physics.

Indeterminism is also not inconsistent with the idea that we can know future events. It states that we cannot predict future events from knowledge of the present, but it might be possible to know future events by some other means. For example, if we had a time machine capable of viewing the future. Or if we were in some way viewing the universe from a position outside time, and could see what is to us (for such a being these terms have no meaning) past, present and future in one glance as we might survey the surface of a wall or floor at one glance. Equally, we can think about past events. We know that at some past time the system was in state A and it later (but still in the past) went on into state B. That we know what happened in no way contradicts the idea that the laws of physics are indeterminate; that just means that we could not calculate that given A it would be B. But by examining our records of the experiment after the event, we see that it did in fact become B. But that's OK, because such knowledge did not come from calculation but from some other means, namely observation.

I should here mention the block (or B) theory of time. The philosophy of time has split into two camps; the presentists (or A theory), who claim that only the present is real, and the block theorists, who see all of the past, present and future as equally real. It views time as being like another dimension of space (although obviously with some differences). We have no difficulty in saying that the location where I am sitting is real and the location of the desk next to me equally real. There are of course also those who combine the A and B theories of time in various different ways, and there are different forms of each school of thought.

The B theory states that we should regard time in the same way; my desk one second in the future is just as real as the space dust located one light second away from me is now. After all, I don't perceive the desk next to me as it is now; I perceive it at the moment in the past corresponding to the time it took for the light to reach me from that desk, and for the signal to pass up my optic nerve to be processed by my brain. An incredibly short time, but still non-zero. In other words, I regard the time about a hundred billionth of a second ago as just as real as the present, so why should I not regard two hours ago as just as real as the present? Or two hours in the future? (While I can't see into the future, people in the future can see me as I am now; so if my moment of time is real to them, so their moment of time must be real to me.)

The B theory was formulated in response to the theories of special and especially general relativity, and in view of these theories I can't see how the A theory can be made to work. Even the statement that two events are at the same time doesn't make sense in the context of relativity (unless they are also at the same location), because it is coordinate system dependent; and which coordinate system we choose is entirely arbitrary. So we can't say (as the A theorist does) that events separated in space are equally real but events separated in time aren't, because there is no unique and objective definition of which events are separated in space but not in time. The A theory assumes Newton and Galileo's presumption (although the notion goes back long before these two thinkers) of an absolute space. But this notion is now outdated and disproven. We are left with two options, either to say that only what is local to me, in both space and time, is real, or adopt a block theory of space and time. (Of course, one may question this view by mentioning that what general relativity describes is not physical space time but a geometrical representation of physical space time. The B theory might apply to the representation but not the physical reality. But this doesn't strike me as tenable; the representation is connected to at least a part of reality by a bijective mapping; if we could not go from space/time to the geometrical representation and back again then physics wouldn't work. I cannot see how one can say that the whole of the geometrical space is real but only one slice of the physical space, since there is a firm connection between each point of the physical space and the geometrical space.)

Most people, whether A theorists or B theorists, seem to regard the B theory as saying that change is impossible. The block universe, after all, is just there; it cannot change because there is no notion of time external to it to measure that change. However, I have never understood why people say this. Change is not a matter of becoming and ceasing to be. Change is the actualisation of a potential; the movement between one state and another; or actual existence at one time becoming potential existence at another time, and potential existence at one time becoming actual existence at the other. Something was one state at one moment in time and is in another state at another moment in time. This is no more inconsistent with the B theory than it was in the A theory. Time itself might be fixed, but objects in time can change from one moment to the next, just as they can change from one location to another. Nor can we discount the definition of time that it is a measure of change. It can play this role in the B theory just as well as in the A theory.

And, of course, to get back to my main discussion, the B theory of time is not inconsistent with indeterminism. If the future is fixed in the B theory, that does not mean that it is predictable. Thus there is no contradiction between the idea that the future is fixed and equally real as the present, and the idea that the future is in principle unpredictable through calculations of the sort done by physicists.

Both deterministic and indeterminate physics are self-consistent and rational. So how can we tell which one holds? We have a few hints from the philosophy of mind. Surely, for example, free will is more compatible with an indeterminate physics than a determinate physics. One cannot choose to deny free will (since the denial of free will contradicts the choice of coming to that belief). If a belief in free will is not through a choice grounded on rational thought and examining the evidence, then it is meaningless and has no guarantee of being true. Thus to deny free will is not only in contradiction to our most basic experience, it is irrational as well. Many modern studies of the brain have been based on the assumption that neurons and synapses are too large for quantum effects to be significant. To suppose this is, I believe, foolhardy. Quantum causes can have macroscopic effects. Equally I have little time for Cartesian or similar dualist philosophies, which might allow for an deterministic physical world and an indeterminate mental world, for the usual reason that they can't adequately explain how one influences the other.

But the test of whether the real world is determinate or indeterminate is experiment. And in principle, a direct experiment is easy. Set up a physical experiment, exactly the same way each time. Repeat the experiment a large number of times. If you get the same result every time for every experiment, then the world is most likely determinate. If on at least one occasion you get a different result, then the world is certainly indeterminate.

However, in practice, it is not as easy as this. Firstly, every experimental measurement carries imprecision. So if your experiments all get the same result, then that means that they agree to within that precision. You can't tell whether or not they are all exactly the same number, or have slight differences but below the resolution of your experiment. You can't tell whether the system is indeterminate, but one outcome is possible but so rare that you by chance haven't measured it in your finite number of experiments. Equally, how do you know that you have exactly the same set up each time? If you see differences in the results beyond the experimental precision, it could be because of differences in the original set-up in qualities you couldn't measure, either because they were below the experimental precision, or for some other reason. So you have to do a bit more work.

One way of avoiding this is not to rely on experiment directly and alone, but the combination of experiment and theory. Find a theory that matches the experiments as a whole, and ask whether that theory is determinate or indeterminate. Or rather, is there any deterministic physics that matches up with the whole corpus of your experiments.

Quantum physics, fortunately, allows us to resolve this issue in another way. If physics is indeterminate, then that means that we cannot say what the outcome will be with certainty. That means that we need some measure to parametrise our uncertainty. If the differences we can see in the experimental outcome arise from minute differences in the experimental setup, and the physics is deterministic, then the uncertainty should be governed by classical theory of uncertainty, probability. The number of results you get in each outcomes should be proportional to the number of times you started in each possible initial set-up. Quantum physics is, however, governed by the rules of quantum uncertainty, which is a little bit different. These differences have experimental consequences, particularly when combining two different results.

Experiments up to the twentieth century gave repeatable results, within errors, which strongly suggested physical determinacy, in agreement with Newton's laws. Thus most of the early modern philosophers (baring those who denied the applicability of physical theory entirely, such as Hume and the empiricists) assumed a deterministic metaphysics and built that into their systems. However, subsequent experiments have shown that results aren't exactly repeatable. The same set-up won't lead to the same outcome every time.

Last time, I showed that one leg of the mechanical philosophy which dominated the philosophy of science for most of the modern period was inconsistent with experiment, namely that the fundamental corpuscles are indestructible. This time, I have said that another leg of the mechanical philosophy, namely that physics is deterministic, also contradicts experiment.

It looks like the mechanical philosophy is in bad shape.

But the Aristotelian philosophy, which mechanics replaced, is looking better. Aristotle can cope with one form of matter changing to another type of matter. Aristotle causality is the one solution which allows us to maintain both the rationality of the universe and physical determinacy. Efficient causality, which links one substance with another, still applies. Final causality, which lists the possible outcomes that a physical state tends towards, becomes useful again. Indeed, it is inevitable that physicists will have to introduce final causality back into their work. We use the term 'decay channels' to describe pretty much what Aristotle meant by 'final cause'.

So we need to develop a theory of physics that both allows us to describe the creation and annihilation of particles and physical indeterminacy.

Did science rise in rebellion to the Church?

Reader Comments:

1. Callum
Posted at 13:33:52 Tuesday January 16 2018

Have you read the new anthology &34;Neo Aristotelian perspectives on contemporary science&34;?

2. Nigel Cundy
Posted at 20:00:03 Tuesday January 16 2018

No, I have intended to read it since I first heard about it last year, but I haven't yet done so (baring a few of the previews that are available online). I'll get round to it at some point. It certainly looks like an interesting work, and the list of authors is impressive.

3. Callum
Posted at 20:21:14 Tuesday January 16 2018

I really think you will find it interesting. You can actually download Robert Koons' paper and Alexander Pruss paper in the book directly from their websites. On Google books you can get like 60% of Feser's paper which is really interesting. I've just got hold of your book and looks a good read.

4. Callum
Posted at 20:28:53 Tuesday January 16 2018

Do you think quantum mechanics points towards a PSR that avoids modal collapse? In other words, underlines Pruss' argument that explanans do not logically entail the explanandum (from indetetminism) but yet still operates in a rational way and thereby dodge brute facts?

This is an important question as a PSR for contrastive questions (why this world was chosen rather than another to be created) seems to be problematic for an omniscient God that creates contingently. We seem to need explanans which don't entail the explanandum.

5. Callum
Posted at 20:33:46 Tuesday January 16 2018

One last question, I don't want to spam your blog post, but what do you think of the deterministic versions of QM? The physical interpretations I mean, like Bohm's wave-pilot theory. If I understand correctly, doesn't it maintain classical determinism and have only an epistemic uncertainty?

6. Nigel Cundy
Posted at 23:13:37 Wednesday January 17 2018

Reply to one of the questions

Firstly, no problem in using this site to ask general questions. I'm sorry if it takes me a little while to get round to answering them, it's a busy few weeks for me. I'll answer your easier question first:

I personally don't think so much of the interpretations of QM which maintain determinism. They cost far too much to keep it. You mention Bohm's interpretation. People often mention the lack of locality (which is a serious difficulty; locality is one of the key principles of fundamental physics, and violating it has dramatic consequences), but to my mind the main difficulties with it are the difficulties with reconciling it with Lorentz invariance (special relativity) and in particular quantum field theory. For example, the article at lists four different approaches to the problem. One can't account for fermions, two are indeterministic, and the last one contradicts the standard model's violation of Charge conjugation symmetry. Equally, the best they seem to do is to have Lorentz symmetry emerge dynamically in their experimental predictions. To the modern physicist, symmetries are probably the most fundamental part of their theory. It is very unnatural to regard them as an afterthought. So my natural reaction to Bohm's theory is to be suspicious of it. If I become aware of a version consistent with special relativity and field theory, then I will have to think about it a little more deeply. Though even then, if it doesn't lead to any experimental consequences or predictions about the nature of theory, that would be far from proving it right; we would just have to consider it as an alternative.

The other deterministic theory I am aware of, and the only one that I think could match the physics, is Everett's multi-world interpretation, where the universe branches which each quantum event (instead of being unable to predict the future, we are unable to predict which branch we would end up on). It seems indeterminate to us because we are on only one of those branches; the multiverse as a whole is determinate. This interpretation, again, seemed troubling and unnatural to me. Firstly, of course, there can be no experimental evidence for this branching. Secondly, I don't see any good theoretical or philosophical reason why we should expect the universe to be such that it branches in this way. That there is a single, indeterminate, universe seems to me to be far more plausible option.

7. Callum
Posted at 19:52:29 Thursday January 18 2018

Thanks for the reply. Your comments on the many worlds theory are interesting, Koons and Pruss' aforementioned papers are actually hylemorphic versions of it. I'll have to read again to see of their take is deterministic or indeterministic

8. Nigel Cundy
Posted at 22:00:34 Friday January 19 2018

Thanks again for your comment. I'll need to think about the first question you asked; unfortunately I tied up for the next few weeks with work and personal matters. Maybe I will make it the subject of a future blog post. Also any feedback you have on my own work will be greatly appreciated.

1. Philip Rand
Posted at 16:02:52 Thursday October 11 2018

It's been done...

So what we need to do is construct a new physics that contains a mathematical description of material particles being created and destroyed and has the principles of causality and proportionate causality built in.

This has been done and it does it using concepts more akin to Kant's Noumnenon rather than Aristotle & Aquinas… I use the method.

I am an expert in information-physics and condensed matter physics.

1. Philip Rand
Posted at 16:08:43 Thursday October 11 2018

lt has been done

So what we need to do is construct a new physics that contains a mathematical description of material particles being created and destroyed and has the principles of causality and proportionate causality built in.

Information-physics meets these constraints. Information-physics is the qualitatively and quantitatively akin to Kant's noumenon concept.

9. AC
Posted at 03:13:32 Friday July 10 2020

Sorry, did you said that some theories of mind and free will can work even in a deterministic universe? Is this the case for hilemorphism too? I mean, if physical determinism is true, can we made sense of human freedom since the latter is not only tied to the material?

Also, I may be wrong, but can human freedom work as a "butterfly effect"? I mean that every choice we made can "affect" the world in a significant way? (Maybe causing chaos) That's the issue that I have with determinism, if it is true, we can't say that humans being partly inmaterial change anything, because the results are still deterministic and we aren't working as any "contrary force", so we are "working in the system".

10. AC
Posted at 03:14:06 Friday July 10 2020

PS; sorry for my bad english

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