# How do we formally define "indeterminism" of physical expressions?

I recently read Emperor's New Mind, by Sir Roger Penrose. In it he talks at length about "determinism" in science.

How does the Uncertainty Principle bring about the notion of so called indeterminism, in the sense that how do we know it is a fundamental law of nature that doesn't allow us to make certain observations, unlike the average Newtonian chaotic system (turbulent flow of fluids and the like) which is labelled deterministic chaos and not regarded as a fundamental principle?

Edit : I got interested (also confused) towards this question after reading G 't Hooft's SE question and subsequent responses.

The Heisenberg uncertainty principle reflects the fact that, according to quantum mechanics, certain combinations of the observable properties of particles are fundamentally incompatible (the technical term is 'non-commuting'), so that the particle cannot posses both properties at once. If a particle has a definite position say, its momentum is undefined. Conversely, if a particle has a definite momentum its position is undefined. There is no exact classical analogy, but you might consider, for example, the viscosity of liquid water and the shear strength of an ice crystal. They are both properties of water, but they are not properties that any given volume of water can possess simultaneously.

Quantum mechanics says that if you make a measurement of one property of a particle, and then make a measurement of another property that does not commute with the first, the result will be unpredictable, although there is a rule (the Born rule) which allows you to calculate the relative probabilities of one result compared to any other. It is the unpredictable nature of this effect which makes for an indeterministic outcome.

The effect is quite different in principle from classical chaos.

• It would be great to know more on how classical chaos differs from heisenberg's principle
– Maan
Nov 12, 2019 at 21:09
• Hi Maan, the difference is that in classical theory you could in principle pin down all of the positions and momenta of the particles that make up a turbulent liquid- what stops you is that there are just too many to deal with. In quantum theory you cannot in principle pin them down, because a particle cannot possess both a position and a momentum simultaneously- it either has one or the other. Nov 12, 2019 at 21:54
• QM does not say that a particle cannot have both momentum and position, but only that you cannot prepare a state where both properties are exactly known. This is a consequence of the fact that different properties require different experimental arrangements (like different orientation of a magnetic field in the case of spin preparation). Nov 13, 2019 at 5:51
• @Maan Because you asked for it here, I added a new answer and tried to give a more thorough account on (classical) chaos. Jan 5, 2021 at 22:55
• @Andrei You are right that quantum theory doesn't forbid a particle to have both a momentum and a position. But I would consider this to be in the domain of "unspeakable" properties that Bell talks about. There is no way of making verifiable statements about a particle's position and momentum, and I personally content myself with the fact that speaking about a definite position or momentum in the absence of measurement is a void statement. Jan 5, 2021 at 22:55

In this context how does the Heisenberg Uncertanity Principle

Physics is the discipline that uses mathematical models, called Physics Theories, to describe observations and data, and, very important to predict future behavior. (A model that only fits existing data is a map, not a theory). So quantum mechanics, the theory developed from observations and measurements, has introduced indeterminacy because it is necessary to describe the data.

It was found that the differential equations that describe wave functions can be used to model the observations and data, if extra axioms pick up the correct solutions that can be descriptive and predictive of data. Principles, laws, postulates are the names used for these axiomatical statements. The Heisenberg Uncertainty Principle (HUP) was deduced from data that were incompatible with the microscopic world, where mostly the new quantum theory is needed, can be shown to emerge from the commutation relations of the complicated theory of Quantum Mechanics.

bring about the notion of so called indeterminism ,in the sense that how do we know it is a fundamental law of nature

In the final theory, the indeterminacy comes from the wavefunction postulate

$$Ψ(x,t)$$ = single valued probability amplitude at $$(x,t)$$

$$Ψ^*(x,t)Ψ(x,t)$$ = the probability of finding the particle at $$x$$ at time $$t$$ provided the wave function is normalized

This is what makes for the basic indeterminacy in quantum mechanics, and the theory was developed in order to explain the data of that time: photoelectric effect, black body radiation, spectra of atoms. It prevailed because it was predictive of new data.

that doesn't allow us to make some observations and unlike the average Newtonian chaotic system (turbulent flow of fluids and the like) which is labelled deterministic chaos.

The concept of probability in both classical mechanics and quantum mechanics is the same, the same with the simple probabilities of throwing a dice.

In classical deterministic chaos dealing with the many particle states, it is the complexity of the enormous number of particles that displays an emerging chaotic behavior, inability to exactly determine individual particle tracks. Quantum mechanics, by identifying particles with a probability distribution are inherently non deterministic. There are theoretical efforts to define an underlying deterministic layer of physical quantities from which the indeterminacy of quantum mechanics emerges, but they are not successful up to now, that is another long story.

• This is not correct: "deterministic chaos it is the complexity of the enormous number of particles that displays an emerging chaotic behavior". Three degrees of freedom is all one needs for chaos. Nov 18, 2019 at 22:34
• @stafusa OK, I will insert "ffor many particle states", I was not defining deterministic chaos. Nov 19, 2019 at 5:13

Just because it didn't pop up yet, I wanted to add something for completion. Some answers and comments claimed that classical chaos arises because there is no way of telling exactly what a particle's trajectory is because of the complexity of the system. This isn't really the essence of classical chaos.

## Chaos Theory

Equations of motion are relations between initial conditions and final states after some time. Mathematically, these relations can be expressed by maps: starting from an initial state $$X_{0}$$ at time $$t = 0$$, one can ask what the state $$X_t$$ at time $$t > 0$$ is. Experience tells us that there is but one state at time $$t$$, so yes, we are talking about a family of maps $$\Gamma_t$$ that take $$X_0$$ into $$X_t = \Gamma_t(X_0)$$ (notice that we already assumed time translation invariance). The properties of this map are clearly of mathematical and physical interest. One very basic property of a map is its continuity (let's just assume we have some notion of topology on the space of states).

A very brief and incomplete definition of chaos theory is the study of dynamical systems where the map $$\Gamma_t$$ is discontinuous. This is best understood by an example. If you throw darts at a target, and hit the target just a few millimeters below the bull's eye, you will try to imitate your previous shot as perfectly as possible, maybe just aim a wee bit higher. This is because you assume there is a continuous relation between the initial conditions (the way you hold and throw the dart) and the final state (where you hit the target). However, this does not hold true when you analyze the weather, where chaos theory was discovered and why everyone loves talking about insects that cause weather catastrophes by just doing what they do (probably also why that weird "The Butterfly Effect" movie was made. Spoiler: Not much physics in that one.).

The root of chaos is colloquially considered to be nonlinearities (which, right behind infinities, are probably the number two cause of death for well-behavedness in mathematical models). Not all nonlinear systems are chaotic, but you can't get chaos without it. As you can easily verify with your classical mechanics knowledge, classical mechanics permits nonlinear equations of motion. Anything that's not the harmonic oscillator (or some rip-off thereof) is nonlinear. A prominent example of a nonlinear classical set of equations are the Navier-Stokes equations, and turbulence happens exactly when the nonlinear terms in the equations of motion (the $$(\mathbb{v}\cdot \nabla)\mathbb{v}$$ term) become relevant.

Note however that this is not the same as indeterminism. In principle, the mathematical equations allow you to start at an initial state and get to the unique final state that starts at the initial state and adheres to the equations of motion through the dynamical evolution. It is true that this determinism is quite useless: Whenever we measure a system, our measurement has a finite accuracy. That is, we know that the state variables of the system are around certain values, but not exactly those values. If we were to use the initial condition that our measurement provided, a chaotic system may take us to some very different point in state space than the actual evolution of the system, because the measured values differ from the actual values and the non-continuous time evolution takes us to a completely different place in state space.

It is therefore common and well-justified to not bother with the exact nature of state space trajectories. Rather, one looks for other defining features of the system beside the points that a trajectory may or may not pass through. One could say that probability theory is used as a means to consider multiple trajectories simultaneously, but it's not the only viable option at this point.

## Quantum Indeterminism

Much has been said about quantum indeterminism in the answers already. I just want to point out a fine interesting difference: In non-relativistic Schrödinger QM, the time evolution is unitary. That is, the time evolution map is a linear operator that does not change the length of a vector. This means that this map is continuous (an operator with a finite norm is continuous. Leaving aside compactness here, maybe that will be a problem). From this argument, the unitary time evolution in the Schrödinger equation does not allow for chaos (remember, nonlinearity is a crucial ingredient to chaos). The indeterminism is therefore at a different level. It turns out that QM requires some further postulates to connect states to measurement results, the states can not be directly measured (as opposed to classical mechanics). We cannot give anything better than stochastic relations for the state-measurement-connection. Some are discontent with probabilistic laws at the fundamental level of physics, they believe that there are additional ingredients to the state-measurement connections, we just can't measure them in experiments yet (those are Hidden-Variable theories. They follow Einstein's requirement that God doesn't play dice). There are others who are willing to accept probability at the root of physics, but they also don't agree on how and where exactly probability enters the game (they follow Bohr's reply to Einstein, which was roughly: Stop telling God what to do!).

This discrepancy between states and measurable data is, to my mind, at the root of quantum indeterminism. There are many ways in which it pops up, definitely not restricted to non-relativistic QM, and some are formulated as uncertainty principles and the like. But this seems to me to be the core issue.

• "In principle, the mathematical equations allow you to start at an initial state and get to the unique final state..." How would you characterize nonquantum situations in which the final states are not unique? Jan 6, 2021 at 20:37
• @D.Halsey You mean like Burgers' equation, where characteristics intersect? Or are you thinking along the lines of stochastic processes? I guess you are right, mathematical models don't necessarily reflect a uniqueness of final states. But at least in the case of stochastic modelling, we deliberately neglect Newtonian determinism to arrive at meaningful results. Sorry, this is off the top of my head, I will have to think about this, it's a good question. Jan 6, 2021 at 21:08

The HUP is fully compatible with determinism. Quantum mechanics without any special role for measurement (i.e. with the Schrodinger equation as the only law of motion) is deterministic, for example.

• I don't see anything to disagree with here, but this doesn't seem like an answer that will help the OP, nor is it a very complete answer even for someone who is at a higher level of knowledge.
– user4552
Nov 12, 2019 at 20:52
• I don't agree. Quantum mechanics without measurement is not a predictive theory. It doesn't link the mathematical objects to reality. There are two types of time evolution in QM, one is unitary evolution between measurements (Schrödinger eq.), the other is projective evolution (part of which is Born's rule and other measurement postulates). One can do better than the naive projective ebolution by going to POVM theories and the like. But one cannot say that, once one ignores measurement, QM is deterministic. The resulting theory is, but this theory is no longer QM. Jan 4, 2021 at 8:21
• @TBissinger: But it doesn't matter whether the resulting theory is not QM. As long as the resulting theory is deterministic and satisfies the HUP, it serves as a counterexample to the claim that the HUP is incompatible with determinism. Jan 4, 2021 at 12:35
• I think that is a common confusion. Yes, there is a mathematical inequality concerning the variances of two functions related by Fourier transform, but I have never heard a mathematician call it the HUP. The HUP of physics, at least Heisenberg's original idea, is a relation explaining how a measurement of a partile's position with a certain error causes a disturbance in the momentum of the particle. Heisenberg uses certain assumptions about what states result after measurement to proof his principle by relating it to the mathematical inequality. nature.com/articles/srep02221 Jan 4, 2021 at 15:15
• From this standpoint at least, a theory without measurement can't say much about the Heisenberg uncertainty principle, which is a statement about measurement. The presence of Fourier transforms in the position/momentum representation of the Schrödinger picture of quantum theory is a justification of why the HUP may apply. But Fourier transform alone doesn't give HUP. Think about linear hydrodynamics, which is often treated in Fourier space. Consequently, there are some uncertainty relations in hydrodynamics, but not the HUP. There's a recent paper by Matos et al., but I didn't read that yet. Jan 4, 2021 at 15:24

Uncertainty (heisenberg) is a consequence of intrinsic wavelike nature of positions and velocities of particles in the physics model called quantum mechanics.

It is quite fundamental but not necessarily most fundamental since QM is certainly not the most fundamental theory/ model. That's not to say I'd doesn't predict things, it actually makes some of the most precise predictions in all of physics. The things that can't be determined / predicted by QM might be able to be determined by a more fundamental theory.

The only certain fundamental limit of determinism is that of local determinism, meaning that there will be some uncertainty based only on local information like for example in the case of quantum spin which is non local / requires faster than light interaction.

Einstein didn't believe in the (statistical) Copenhagen interpretation of QM. Me neither. He believed in hidden variables, which are pretty hard to find if they are hidden. But what is hidden can be uncovered. I think they are not covered and can be found all around an inside of us: Spacetime! But then again what is spacetime?

In the case of chaos, one leaves from the (Newtonian as you call it, which I doubt) principle that the trajectories of the particles are fixed and not connected with QM. Are you MAAN in the sense of MOON?

• How does this address the OP's question?
– lcv
Dec 29, 2019 at 15:05
• For that, you just have to read both the question as well as this answer thoroughly. Dec 30, 2019 at 3:10