I've heard this pop up in a discussion with my physicist/engineer roommates, but didn't care to ask at the time. Now I'm mighty curious about it. Wikipedia doesn't really seem to say much on this issue.

From what I understand about the Uncertainty Principle, it says that there are certain properties of electrons and stuff that cannot be measured, and are therefore uncertain. Then Wikipedia (under indeterminism) states that Sir Arthur Eddington says that the Uncertainty Principle isn't really so because we can't measure these properties, but because turns out nature is indeterministic. At least that's what I took from those paragraphs. Even without my biased wording, it sounds more like an assertion than evidence.

I've also read a few things about how other scientific conventions perceive the issue, like how a ball on the peak of a perfect mound might randomly roll down in any direction, and I'm still unconvinced. My belief of determinism is generally that if you knew every single variable that existed as a factor at the very beginning and birth of the universe, you could correctly determine all properties of any individual particle at any point in time.

Could anyone provide some more background about this? Especially regarding quantum mechanics?

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    You should look at a similar question I asked on the subject not too long ago. It may help clarify the confusion regarding fundamental nature of quantum physics in regards to the very idea itself being a positive claim as opposed to a "lack of knowledge" claim. – stoicfury Sep 2 '11 at 15:27
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    Thanks for pointing me to your question. At first it wasn't evident, but reading these answers brought up that question for me. – glifchits Sep 3 '11 at 21:50
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    Although the uncertainty principle was originally proposed by Heisenberg as a limitation on measurement, it's now understood to be a limitation on what there is to be known about a physical system. – Ben Crowell Jun 5 '13 at 21:49
  • I found the other day this video: youtube.com/watch?v=DMNZQVyabiM This was a problem inside the scientific community, specially between Alberts Einstein and Niels Bohr, this is known as the Bohr–Einstein debates: en.wikipedia.org/wiki/Bohr%E2%80%93Einstein_debates – user50618 Aug 20 '15 at 12:21
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    Searle's Third Law: "Anything philosophers say about quantum mechanics is B.S. and quantum physicists aren't much better." – Mr. Kennedy Dec 28 '16 at 0:31

15 Answers 15

up vote 30 down vote accepted

I thought I would give a physicist's perspective here.

There are two types of evolutions in quantum mechanics: unitary (or free) evolution and measurement. Free evolution is fully reversible and deterministic; a given operator takes a specific wave functions and maps it to a specific other wave function. The uncertainty comes from the non-unitary measurement evolution.

Unfortunately, if you want to approach this problem from a realist point of view (how most people think of classical mechanics, etc) it becomes difficult to solve the measurement problem: i.e. what constitutes a measurement, where is the system and where is the measurement device? Isn't the measurement device + orginal system just a bigger system that should be undergoing unitary transformations? This question has puzzled many, with some notable scientists even linking measurement to the acts of conscious observers. But this is not a standard view.

Most researchers on the foundations of quantum mechanics, however, usually side-step this question by taking the operationalist point of view. Tagline: "all we have is some procedures for setting up an experiment and the results of experiments". In this framework, you can derive Bell's theorem, which says that any phenomenon that is both deterministic and local must satisfy the Bell inequality. Quantum mechanics violates the Bell inequality (and there have been many experiments that mostly confirm this violation, there are some technical loopholes that need to be addressed in some of the experiments). This means that you must give up at least one: locality or determinism. Since without locality it becomes impossible to talk about causality, most people prefer not to give it up, and instead give up determinism.

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    Thanks for this fantastic answer! I'm only unclear what you mean by the two evolutions in quantum mechanics; are you referring to branches of opinion or distinct sub-fields within the topic? – glifchits Sep 6 '11 at 3:01
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    Thanks, I meant evolution as in a transformation of state from time t to time t + 1. – Artem Kaznatcheev Sep 6 '11 at 3:04
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    @ArtemKaznatcheev I don't see that locality is required for determinism. 'Action at a distance' makes perfect sense and is compatible with determinism, even if it seem incredible. So it isn't the violation of Bell's Inequality that makes systems non-deterministic, it is rather the statistical interpretation of the wave function presented originally by Born that is incompatible. A deterministic non-statistical interpretation has been presented by Bohm. – adrianos Jan 6 '12 at 14:10
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    Determinism works fine w/ a realist perspective if you consider that Many Worlds is strictly simpler than Collapse. EDIT: Not that you can determine/discriminate, from the starting point, whether you will end up in World1 or World2 because no matter which one your theory predicts, there will be a version of you in the other world thinking "Well, I guess that theory was wrong." – medivh Jul 29 '13 at 7:47
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    Nice answer. For my two cents I would give up locality before determinism and feel there is no other way forward. For me QM is just further proof of the doctrine of the Upanishads, just as it was for Schrodinger and is for Mohrhof and others today. This keeps determinism but loses locality for the idea that space-time is not metaphysically real. – PeterJ Nov 22 '17 at 15:16

Quantum Physics doesn't disprove determinism.

What Quantum Physics does do is significantly complicate the task of arguing for determinism.

Put in the simplest possible terms, the Uncertainty Principle indicates that: 1) our observation of an event has a significant effect on the event, and 2) it is impossible for a single observation to observe all relevant properties of an event. This means that any argument for determinism can no longer have simple recourse to the notion of observation.

So, when you say:

My belief of determinism is generally that if you knew every single variable that existed as a factor at the very beginning and birth of the universe, you could correctly determine all properties of any individual particle at any point in time.

you instantly run into trouble, because we can't know every single variable that existed as a factor at any point in time (including the initial state) through any type of observation.

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    Does the Uncertainty Principle state that only OUR observations have effects on the event, or that all possible observations would have? Is it impossible that ever any new technique is invented to measure the 'real' events? – bonifaz Sep 2 '11 at 22:41
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    I don't think that arguments that depend upon beings unbound by the laws of physics are going to be of much help in arguing how the universe (with its concomitant laws of physics) works. In terms of the broader question, I personally find the whole "free will vs determinism" debate singularly uninteresting and tiresome for two reasons: 1) it is *impossible to come up with a probative argument in either direction, for reasons indicated by Wittgenstein on rule-followin, and 2) it makes absolutely no difference in terms of anything that follows. – Michael Dorfman Sep 3 '11 at 11:08
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    @stoicfury: I still believe in an ethical subject, regardless of free will. I said that it makes absolutely no difference, because it certainly appears to us that we have free will, so it is incumbent for us to live as if we had free will; if, in actuality, this view is mistaken and our decisions were all pre-determined, nothing has changed-- we have acted precisely the way we would have, regardless. – Michael Dorfman Sep 5 '11 at 13:09
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    This answer gives a much weaker statement than what is commonly implied by quantum mechanics. Quantum mechanics does not simply make an epistemic statement as @MichaelDorfman's answer seems to imply it makes an ontological statement about the impossibility of local realism. – Artem Kaznatcheev Sep 5 '11 at 17:50
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    @Artem: The impossibility of local realism isn't a problem for determinism, as it is concerned with the global case (i.e., the entire universe) and isn't bound by the speed of light--so I think the epistemological case is more relevant to the topic at hand. – Michael Dorfman Sep 5 '11 at 19:56

The Uncertainty Principle is not directly problematic for determinism; it just says you can't measure your states that accurately. You could always assume that the states were there, but you just couldn't measure them. Einstein preferred this view, and together with Podolsky and Rosen devised a paradox that would show that uncertainty is not fundamental. Unfortunately for Einstein, the experiments delivered the seemingly paradoxical result, showing that uncertainty is fundamental and determinism, if true, is not local. (Actually, it even shows that causality is not local.)

But the more telling blow to determinism is the success of entangled/superimposed states that are stochastically collapsed under certain conditions. The double-slit experiment is the most famous of these, but it's really Bell's inequality and experiments (that failed) to confirm it that made determinism look like a bad model of reality. The experiments are too technical and detailed to describe here, but so far Bell's inequality has been routinely violated, and hence, there is no room for a deterministic model where the relevant state stored locally. (Of course, a computer simulation with all state stored globally can reproduce anything, in principle, but that doesn't make it a parsimonious way to explain results in physics.)

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    virtual -1 because of "violation of Bell's inequality". Note that Bell's inequality is something that local-and-deterministic theories have to obey. Quantum mechanics violates Bell's inequality, please be careful with your terminology to avoid common misconceptions. – Artem Kaznatcheev Sep 5 '11 at 17:17
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    @Artem Kaznatcheev - Sorry, said it backwards. Fixed now. Thanks for catching that. – Rex Kerr Sep 5 '11 at 17:26
  • If you take the uncertainty principle to refer to an intrinsic uncertainty in the values of conjugate pairs then it is directly responsible for indeterminism. This is more a matter of terminology though but much confusion over QM is because of such confusion. I like your emphasis on the experiments eliminating causality. If I can be even more direct – Vector Shift Dec 16 '16 at 5:07

Once we start using a scientific method, that is, observing nature in order to learn what is really happening, we are already assuming a determinism of some kind, that there are strict rules about how nature works. So it understandable to assume that all our rules about nature are lock-step, undeviating. And if they're not, that's just a failure of effort, to work past the feeble approximations to get to a final exact solution. (use any grade school science here, biology, sociology, physics, etc.). All probabilistic distributions of measurements of natural phenomena are expected to be artifacts of experimental error, not part of nature, and that better experiments would eventually narrow the distribution to a single determined point.

Under the mathematics of Newtonian mechanics, this is a reasonable strategy to pursue.

It just turns out that under investigation of certain physical phenomena, sub-atomic particles, it was experimentally found that even when the experiments were adjusted to the extent that there was -no- variability in the input data (control of single particles), there was still a probability distribution on output of the system. That is, things that we metaphorically think of as discrete particles still act as though they have a probabilistic distribution. There is no determining the outcome exactly, nature has inherent distributions that are not artifacts of the experiment. (I am describing the two slit experiment). Something that we think of as a single particle can have properties that are inherently indeterminate.

At certain scale levels (very small), you really -can't- know, given initial velocity and mass, the end position of the actual particle (or set of particles).

It's not so bad as all that, because we still can quantify that lack of knowledge with a probability distribution.

Anyway, the point is that we seek as much determinism as possible in science (that is what the form of scientific laws follows, but it can turn out that nature doesn't always comply. But really, science has determined enough for us to put people on the moon, make smallpox extinct, and have auto-answer phone-menus for our banks, that a little sub-atomic non-determinism is livable. We certainly -do- know something about the particle

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    And frankly, the newtonian determinism isn't so great philosophically. Sure there's the Lagrangian 'if you had the 3d position/velocity (6 coords), you could determine the course of all later events', except that (forgetting getting the initial data), the simulation on a calculating machine would require a larger universe than the actual universe it is simulating. O solving that system algebraically is currently impossible. There -is- a symbolic solution to the n-body problem but its convergence to a numeric solution is terribly slow. So even Newtonian mechanics has its determinism problems. – Mitch Sep 2 '11 at 13:59
  • But of course (to comment on my comment well after the event, Newtonian mechanics is the start of modern determinism despite my niggling comment) because of the at least conceptual exact solvability... er not solvability... but ability to be simulated. – Mitch Sep 5 '11 at 22:14

The problem is the clear equivocation on the word "determined". Just because a quantum fluctuation for example, is not sufficiently caused or causally determined that does not mean the event was not determined in a sense that the event was "fixed" due to existing tenselessy on a four-dimensional space-time block (the B-Theory of time). If the future is "fixed", then even if there is no sufficient cause for a certain event it still had to be the case due to having a fixed position on The Block. Therefore, even if something is no CAUSALLY determined, it could still be determined in a sense of having a "fixed" position in an objective timeless reality; with temporal becoming only being an illusion.

I'm not a physicist but was a student of philosophy. I am now much more interested in physics and quantum theory then I used to be. I'm not sure if the philosophy of determinism has been properly understood. In order to understand determinism we need to look at the free will v determinism debate. This is where the controversy is. When I was an undergrad the free will side was extremely weak from a logical point of view relying on the idea that we have free will purely because we believe we do. Determinism is more about whether an actor has free will or whether the preceding causes will always have resulted in the same outcome.

There are three main limbs of determinism. The first is that a number of causal factors taken together will result in an action that could never have been avoided. This is largely not controversial. The second limb relates to the idea that there are causual factors in our past from birth to the present that meant that nothing could have turned out diffefently and there was no free will. This is usually where the debate lies. The final limb is that if there was an intelligence with all the knowledge of the universe, every physical law there is in a complete way then all past, present and future actions can be predicted and mapped out.

I read above the overly complex physics explanation and it missed the crux of the position. Those arguing for free will used the advances of quantam particles thoery with respect to charged particles not following a determined path to argue that on the quantam level determinism problematic. This is not considered to be a strong argument at least when I was at uni mainly due to the path being predictable on a correlation. At any rate merely because we cannot predict at the quantuam level with our current knowledge is not an argument against the broad determinism and is not relevant to the other two limbs as they relate to events not quantuam mecha ics.

Summary

Explained determinism more precisely - 3 limbs, the first two relate to cause and effect that can be observed and the notion of free will dispelled. The third relates to a super intellect being able to know all laws of physics being able to map out past present and future events. Explained how quantum charged particles not easily predicted is not an argument that has great weight for the third limb and not relevant to the other two. In essence quantum theory while being clung to by free will proponents merely demonstrates our lack of knowledge rather than lack of determinism.

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    Welcome to Philosophy.SE! A couple suggestions: I would move your last paragraph to the top since it summarizes your argument. I would make more explicit which of your three legs quantum mechanics belongs to. I would also (if applicable) cite which philosopher proposed the three-legs (which I had never heard before and I think is a great way to think about the problem of determinism). I would also use numbered bullet points for the three legs to make it easier to read. – James Kingsbery Apr 4 '16 at 16:58

Certainty is distinct from determinism. To say the world is determined is to say that if the state of the world today implies the state of the world tomorrow. That is, if you re-wound the world to the beginning of today, it would play out again exactly as it did. It says nothing about whether the state of the world today gives an observer certainty about the world tomorrow, or even any predictive power at all.

The Heisenberg uncertainty principle and Bell's Theorem deal a fatal blow to certainty, showing it impossible within a system to be certain of that system's future, but they leave determinism in tact. In fact, it seems un-scientific to reject determinism given the available evidence. After all, at the macro level our measurements directly indicate determinism, and as we near the quantum level the uncertainty principles provide a perfectly good explanation for why measurement itself falls apart.

Heisenbergs uncertainty principle introduced indeterminancy into modern physics, whereby modern I mean Physics from the Italian Renaissance. It was already introduced into the Physics of the Greek atomists as the clinamen which they regarded as an irreducible randomness associated with an atom (they argued it was neccessary in order to get atoms to interact).

It was originally introduced by Heisenberg as an irreducible perturbation on a minute particle. That is the randomness was seen as epistemalogical. The question turned into whether this was in fact epistemological or ontological. That debate is still current today.

For example Quantum Mechanics interpreted under consistent histories it is in fact ontological, as Bohmian Mechanics it is epistemological (but notably locality has to be given up).

In classical mechanics one can determine a trajectory of a particle precisely in spacetime, and this is reversible. In quantum mechanics one can determine the trajectory of a probability wave of the particle exactly and this is reversible. But on an interaction this probability wave collapses to a specific value known to both particles. After this point the trajectory is no longer reversible - how can it be when probability & possibility has collapsed to the known? This state then begins to evolve again.

Determinism is the belief that if all variables are known at enough given times, then all variables can be known for all of time. This is generally true for macroscopic objects. For example, if I drop a ball from a certain height, I can predict the amount of time that elapses until the ball hits the ground. Taking this concept further, if I kick a ball of a known size and density with a measured force while knowing the velocity of wind flow, the Earth's surface gravity and air resistance, and the angle of my foot to the ball at the moment of impact, I should be able to predict the trajectory of the ball as a function of time.

But things get strange in the quantum world. Rather than macroscopic objects (like balls), quantum objects are sub-atomic particles that make up the ball. Atoms consist of a nucleus (protons and neutrons) with "orbiting" electrons. The laws that govern quantum interactions are not definitive because of Heisenberg's Uncertainty Principle, which states that the momentum (speed-dependent) and position of a particle cannot be simultaneously known with infinite precision. This means that if one knows the position of a particle, then one does not definitively know how fast it is moving; conversely, it also means that if one knows how fast a particle is moving, then it is impossible to know its position with infinite precision. This is unlike the case of a kickball.

Taking this further, the total probability of an event occurring is 100% (or 1). When a particle encounters a barrier, there is a probability that the particle will not escape the barrier, but there is also a probability that the particle will penetrate through the barrier; the sum of these probabilities is one.

Albert Einstein and Niels Bohr debated this topic. Einstein reckoned that if a dice-roller knew the initial positions and sizes of the dice, the temperature of the room, the sweatiness of ones palm, and any other necessary variables, then one could compute with infinite precision the outcome of the dice roll. Bohr reckoned that the subatomic world did not necessarily have to obey identical laws.

I prefer to think in terms of flipping a coin. Usually, one tries to guess whether the next flip will be heads or tails assuming equal probability for each outcome. But, the heads side is slightly heavier, causing it to be a slightly more probable outcome than tails. Do you think flipping a coin is deterministic or probabilistic? Interestingly enough, probabilities of nuclear decays are computed using identical math.

One thought is that quantum mechanics is deterministic (ex: Pilot-wave theory, Hidden variables, etc), and that it is imperfections in the human mind/senses and inability to measure with sufficient precision that causes it to appear as though the solutions to all quantum mechanical problems are not inherently deterministic when they actually are. However, the mainstream consensus is that quantum mechanics is inherently probabilistic.

A thought experiment by Schrodinger goes something like this: a cat is trapped in a box with several attached explosives tied to a particle detector. The particle has a 50% chance of firing, in which case it is picked up by the detector and the explosives go off, killing the cat. The particle also has a 50% chance of not firing, in which case the explosives do not go off and the cat is presumably alive. If one does not check the box, how can one determine whether the cat is dead or alive? The idea is that the state of the cat exists in a "superposition of states" in which both states are possible when not being measured; however, the act of measuring by checking the box "collapses the wavefunction" into one of the two observable states. Einstein found this silly; he said that if one doesn't observe the Moon then one can still know definitively where it is in the night-sky. But the Moon is a macroscopic object.

It gets tricky when you consider that subatomic particles firing in human brains are behind most of our actions. As a subset of the universe, humans are governed by the same laws as the universe. If the universe is deterministic, then so are humans and free will goes out the window.

The bottom line is that QM has introduced "uncertainty". All this means is that we, as very large beings relative to quantum particles, have no way to measure the state and locality of a quantum particle at a particular time. If we had the means, for instance, if we figured out a way to measure the wake of spacetime after the particle passed through to determine the state and locality an instant after the particle passed through, we could eliminated the uncertainty and once again have consensus on determinism. People have confused "uncertainty" and "unpredictability" with the phrase "without cause" or the word "random". Although the half life of a single atom is unpredictable, how many photons are released by a mole of atoms per year is quite predictable. This means that there are causal machinations going on even at the quantum level. So no, quantum physics has not disproved determinism. Many site Bell's theorem as proof QM disproves determinism, but Bell's theorem is about the immeasurable nature of quantum physics and humans needing to imply hidden variables to deduce attributes of the particle. If we could measure without effecting the state of the particle, Bell's theorem would be a simple logical mathematical artifact that wasn't applicable to anything. Just because we can't measure the particles doesn't mean they are non-deterministic. And, considering the vast amount of scientific evidence that shows QM to behave predictably, it's safe to say that determinism is still a thing.

There are several different properties that an interpretation of quantum mechanics might have:

  1. determinism The state at one time is determined by its state at a prior time.

  2. wavefunction realism The wavefunction is not merely a mathematical convenience, but refers to a physically real phenomenon.

  3. locality Events are directly influenced only by their immediate surroundings.

  4. counterfactual definiteness It is meaningful to discuss what "would have happened" if properties other than the ones measured had been measured.

What Bell's inequality shows is that it is impossible to satisfy all four of these. The idea is that for all of them to be true, there must be "hidden variables". If we were to measure a particle's spin in the x-axis, there would be some value that would result; counterfactual definiteness means that it is meaningful to ask what the result would be, even if in reality we don't perform the measurement, determinism says that there would be some specific result, not a probabilistic distribution of results, and locality says that this result wouldn't depend on anything that's happening with particles separated from the particle we're measuring; the result must depend solely on the properties of the particle we're measuring, i.e. the particle has some "hidden variable" of what it's "decided" the result will be, even if we never see it.

As an example of how these properties can be tested: suppose there are two types of measurements we have do on a particle, and each measurement can have one of two results. Then there are four different combinations of these results, and given a pair of particles, there are sixteen different combinations. There are four different combination of measurement types we can do on a pair of particles, and for each one there are four different possible combinations of results. If we have a whole bunch of pairs, they will have some distribution over the different combinations, and for each distribution, we can calculate what the distribution of results for each combination of measurements should be. It turns out that whatever the distribution is, a certain inequality should hold. However, there are ways of setting up an experiment such that this inequality doesn't hold. The conclusion is that of the properties I listed at the beginning, at least one must not hold.

However, an argument can be made that even without quantum mechanics, determinism and counterfactual definiteness are incompatible: if we couldn't have made any measurement other than what we did, then what sense does it make to ask what would have happened if we had made another measurement?

Imagine that ball on the top of it's perfect bound, in the dark at close to absolute zero. How accurately can you measure whether it is off centre at all? Only to the nearest photon, the energy of what you use to 'look' at the ball, limits the precision you can know about it with.

The idea that the 'answer' is there but we can't know it, called a https://en.m.wikipedia.org/wiki/Hidden_variable_theory These were, essentially, ruled out by https://en.m.wikipedia.org/wiki/Bell%27s_theorem The information about the ball is literally not there, and we see this manifested in things like wave-particle duality, and https://en.m.wikipedia.org/wiki/Wheeler%27s_delayed_choice_experiment

The Many Worlds interpretation of quantum mechanics (typically accepted by around a third of physicists in polls by Max Tegmark) maintains determinism, by saying the ball rolls in all directions possible within the limit of it's uncertainty, but each outcome happens in a different universe. Some see this as replacing the uncertainty of outcome with uncertainty of which outcome we go into, but if they are all real they are all experienced, and it is just pure chance which we find ourselves looking out of.

Quantum mechanics does not disprove determinism. Although some people would like to use QM for that purpose, it is a very weak argument, if at all. A much stronger basis is, Chaos and/or non-linear functions.
Any system that obeys/follows a non-linear function - is non-deterministic.
The majority of systems, including the Universe, are non-linear systems, therefore non-deterministic.
Since most of the advancement of science is due to work on linear systems, we have developed the mistaken notion that the "linear systems" are the predominant ones, and therefore - the "world" is linear (= deterministic), when just the opposite, is true!

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    Non linear systems can be deterministic. They are just highly sensitive to initial conditions. – CriglCragl Jul 29 at 0:42

Quantum Mechanics did not introduce uncertainty.Uncertainty in measurement of position of a particle was absolutely required when Einstein proved the photon acts like a mass particle even if it has no mass but does have momentum which is preserved under the momentum conservation law! If a photon is going to be used to make a position measurement ;some of its momentum has to be transferred to the target particle which must move before the reflected photon reaches any observer/detector. Conclusion :Uncertainty is guaranteed by nature We don't need Heisenberg"s Uncertainty Principle for the intuitively obvious! -if one accepts the property of the photon predicted by Einstein for which he was awarded the Nobel Prize

A paradigm in which quantum mechanics describes the de Broglie wave in a manner similar to that Maxwell describes the electromagnetic wave should exist.

In this paradigm, two fields should be associated with the particle mass as those associated with charge in Maxwell's theory. In this manner quantum mechanics can be seen as a deterministic theory like Maxwell theory.

  • I made an edit hopefully for clarity. You may roll this back or continue editing. What would be nice to add and a reason to continue editing are references to physicists or philosophers who take the position that you do so it can be supported as more than an opinion. This would strengthen your answer – Frank Hubeny Aug 2 at 3:59

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