Do weak measurement prove randomness is not inherent?

phyzguy

Fyzik - How can you make this statement?? When a uranium nucleus, which has been happily sitting there for 4 billion years, suddenly decays, why couldn't this be something that 'just happened'? It MAY be that there is some underlying mechanism that caused it to decay, but there is no evidence for this, and no successful model has been proposed to explain this as the result of some underlying deterministic model. So how can you say that something that 'just happens' has never been observed? How can you be so certain that there is not randomness inherent in the operation of the universe, when the evidence suggests very strongly that there is?

Ken G

Actually, I would point out that the decay of the uranium is not actually evidence of randomness "in the operation of the universe." I agree with your main point, that it requires considerable suspension of disbelief to say that the decay is deterministic in the absence of any evidence that it is, but we don't have an either/or situation. We often see the fallacy that "if it isn't random, it must be deterministic, and if I see no evidence that it is deterministic, it must be random." Randomness and determinism are both elements of models we use to describe the operation of the universe, but they are never elements of the operation of the universe. Scientists can only test the success of our models by comparing to the outcomes of experiment. The tests of the operation of the universe are the experiments themselves, not the success of the models-- that's something different.

DrChinese

Great point. I believe this is accurate in physical science. As we factor in new variables (to get more accurate results), it becomes harder and harder to cite any one as the "cause" of the result. And somehow, a new level of indeterminacy firmly creeps in. We used to think of that as relating to "initial conditions" but it doesn't appear that way any longer (at least to me). I don't think the human brain is a deterministic machine either.

DrChinese

Although there are a few questions about my taste in clothes.

Fyzix

My problem with randomness is the TRUE randomness yes, not the "mathematical randomness" / lack of knowledge on the human part.
That's exactly what I am arguing.

Someone mentioned an exampe of a uranium atom decaying after 4 billion years, SOMETHING must cause it.
I can't see any other way around it.
It decaying itself is a mechanism! it's just ignorant of humans to think we already understand enough to say "hey randomness exists, because we don't know everything yet".

People would say the same about EVERYTHING 300 years ago.

Ken G

So we have encountered two logical fallacies:
1) saying that because we don't know what causes something means it is uncaused
(that's called "argument from ignorance")
2) saying that because we cannot imagine something isn't caused means it must be caused
(that's called "argument from incredulity")
Scientific thinking should always avoid logical fallacies, and that's exactly why we must be clear on the difference between the features of our models, and how successful they are when compared with experiment, versus the features of whatever is making the experiments come out the way they do. The only way to avoid fallacies is to be very clear with ourselves what we are really doing when we enter into scientific thought.

unusualname

Randomness exists by Occam's razor.

In Fyzix's world, any event must have a preceding event that caused it, how does the causal mechanism work between two events?

In a random world, any event doesn't need a preceding event to cause it, so we don't need to explain anything further.

However, we know there is some order in the world, so we ought to impose some constraints on the randomness (to explain the world), eg we could insist that the randomness is guided by an evolution equation, like Schrödinger's equation for example.

So we have deterministic evolution of probabilistic states.

There, that's the world.

maverick_starstrider

This is a PHYSICS forum, of course we're concerned with mathematical randomness. I'm getting the strong sense that you don't really have any physics background. Is this the case? The fact is, like it or not, there is an enormous amount of evidence against a deterministic universe and assuming a probabilistic universe has given us the most accurate (in predicting reality) mathematical model ever created. It is from this understanding that we invented the transistor (i.e. the microchip), the laser, modern chemistry, etc. Furthermore, if quantum mechanics were wrong (or just an effective theory of a more general higher order one) and there were a deeper deterministic theory we still have some very strict mathematical limitations on what that deterministic theory must look like and it would have to break a whole lot of rules that every experiment tells us are correct (for example, a deterministic theory CANNOT be local, but locality seems very much to be an inextricable part of reality).

You must realize that physics and quantum mechanics are a SCIENCE, baseless philosophical pondering devoid of actual knowledge of physics is worthless. Physics is applied math, if you don't understand that math then you can't possible understand the issues. Not liking an extraordinarily accurate theory doesn't mean a thing unless you've got a more accurate theory to supersede it.

Also, one could of course easily make quantum randomness an aspect of the macroscopic world. Take a cathode ray tube (which we'll say sends out only 1 electron at a time), pass the electron through an Sz Stern-Gerlach machine, take the output and put it through an Sx one, take the output and pass it through an Sz again. If it comes out spin up, cleave a random person's head off with an ax, if it comes out spin down, don't. Wham! Real world consequences of quantum randomness ;)

my_wan

First classical thermodynamics is formulated as a set laws of what was then considered fundamental laws. Statistical mechanics developed later (read 'the statistics of mechanics') was developed later and from which the laws of thermodynamics were found to be derivable from. Statistical mechanics is essentially the kinetic theory of gases.
This kicked of a controversy because:
To continue the above quote does this look familiar in todays context?
Gibbs developed the Gibbs ensemble construction around 1900, providing a more general formal foundation for the whole thing. A few years later (1905) Brownian motion put the final seal on statistical mechanics based on papers over the last 25 years.

Yet here is another interesting and funny bit. The formal definition of Gibbs ensembles define the fundamental bits of the QM formalism on which the many worlds hypothesis was constructed. The many worlds hypothesis is basically the result of postulating every copy of a Gibbs ensembles is existentially real. Hence the many worlds are the Gibbs ensembles.

The only place randomness survives in the theoretically 'pure' form is in subatomic physics.

maverick_starstrider

If by SUBatomic you mean atomic then I suppose. Though I'd ultimately disagree. Statistical mechanics is simply IMPLICITLY "random", yet it is still random. For example, Fermi-Dirac statistics are founded on the Pauli Exclusion Principle. However, the exclusion principle is a direct result of the indistinguishability of particles and Born's rule. Both of these EXPLICITLY relate to the blurred out, probabilistic core of quantum mechanics and the Schrodinger equation. Thus, by taking Pauli Exclusion as axiom, statistical mechanics inherits the underlying assumption of "randomness" even if the behaviour of large ensembles ends up being deterministic. I'd imagine this is particularly obvious in the Mesoscopic regime.

Also, FYI I believe thermodynamics was always a phenomological theory (as opposed to a fundamental one). It was developed around the same time as E&M and I think the notion of an atom was gaining a little bit of traction. The notion that there was ultimately some "under the hood" electromagnetic interaction driving the whole thing was likely in the air. Tragically, Boltzmann committed suicide after his atomistic reduction of thermodynamics continually faced derision.

Ken G

Occam's razor is a technique for deciding on the most parsimonious way to think about reality. It is not a way to establish "what exists." A very wrong way that many people understand Occam's razor is "the simplest explanation is most likely the correct one."
That is wrong for at least two reasons:
1) Occam's razor is a way to choose between theories, not a way to dictate how reality works, and
2) the statement is patently false, contradicted over and over in a wide array of scientific examples.
So the correct way to state Occam's razor is: "since our goal is to understand, and since understanding involves simplification, the simplest theory that meets our needs is the best."
So if we take that correct statement of the razor, and parse your claim, it comes out "randomness exists because it is easier for us to understand randomness." That should expose the problem.

As for your argument that randomness is in fact a simpler description of many of the phenomena we see, including the decay of uranium, I agree.

Correction, that's our simplest description of the world. Big difference. For one thing, you left out the most puzzling part of all-- how a deterministic evolution of probabilistic states gives way to particular outcomes.

unusualname

In the Consistent Histories interpretation this is not a problem, once we have a measurement we can know how the probabilities evolved. There is no way to know this without making a measurement of course.

Also, constructing the Schrödinger evolution at the microscopic level is of course a huge problem, why all the linear group structures in the Standard Model? How does gravity emerge for such an evolution? And the big one - how does human free-will seem to enable us to further guide this evolution beyond (afawk) what exists anywhere else in the universe?

Ken G

All true, but that's why thermodynamics is the deterministic theory (heat flows from hot to cold, etc.) and statistical mechanics is the random (statistical) theory (heat is more likely to flow from hot to cold, etc.). So I would say this is an example of the natural tendency for seemingly deterministic laws to later be reinterpreted as emergent from more fundamentally stochastic laws.
Yes, I too have noticed the appearance of physicist-as-participant-in-physics effects even in classical thermodynamics. It's there in relativity too. The idea that "observer effects" are purely quantum in nature is narrow-minded.
On another thread, I am making the point (to little favor, I might add) that many-worlds is a completely classical concept that picks up nothing particularly special in the quantum context. In both cases, it is only the fact that science has to address the sticky problem that a given observer gets a given observed outcome, that is the actual nature of the problem, not quantum vs. classical. I think you would be sympathetic to that view.
This is where we diverge. I don't think the problem is with the impurity of randomness, because I view all mental constructs (like randomness and determinism alike) as "impure." They are all effective theories, all models, and randomness is the model used in statistical processes like statistical mechanics. Including all the Gibbs ensembles really doesn't remove the need for randomness, because we don't get an ensemble when we do the experiment, we get an outcome. That's where the randomness concept connects most closely to reality, but it is still impure and incomplete, because we still have no idea why we get a particular outcome, when all our theories can only give us statistical distributions. This is a fundamental disconnect between physics and reality that cannot be resolved by imagining the universe is fundamentally random or fundamentally deterministic, because either idea can be made to work with sufficient suspension of disbelief, and anyway there's no reason to imagine the universe is "fundamentally" any of those things.

my_wan

The manner in which you have defined "intrinsic" determinism in the context of thermodynamics is also shared by QM in the underlying wave equations. When thermodynamics was developed the laws were defined in irreversible form. Only when statistical mechanics was further developed it created problems for this assumption, written as law, that such processes were irreversible. Exactly because the real state of the system is defined not by ensembles, but by mechanistic certainties if the particular state each ensemble was actually in was known.

In this context stochastic laws are not fundamental to the system, they are only fundamental to our level of knowledge about the system. Thus saying "fundamentally stochastic laws" is a misnomer of what the physics actually entail, at least in this context.

Now obviously, it is quiet trivial to decompose Gibbs ensembles of a classical medium into distinct physical units. Yet QM is fundamentally quiet different in that respect. Even quantization involves properties rather than parts and do not stay put in any part-like picture ever conceived. Perhaps in the quantum regime "fundamentally" really does belong in front of "stochastic laws", but in thermodynamics it most certainly does not, as illustrated by statistical mechanics. In a classical regime stochastic is merely a consistently 'apparent' property resulting from a limitation in the completeness of our knowledge.

Now the big question. If we as observers have fundamental limits on our knowledge that physical law dictates we cannot 'empirically' get around by any means, would that constitute "fundamental" stochastic laws even if the theory entailed a complete lack of stochastic behavior at the foundational level? That is what we have in classical stochastic behavior, but QM lack a similar underlying mechanism that defines stochastic behavior as purely a product of limited knowledge. That is THE key difference between classical and Quantum mechanics. Saying "fundamentally stochastic laws" requires the presumption that a an ignorance of our ignorance is evidence of a lack of ignorance, i.e., "fundamental". Whereas classically we are aware of our ignorance such that in that context it is not fundamental to the system itself.

Agreed. It is a whole range of these observations that leads me to assume it quiet likely that the conceptual problems in QM is not just ignorance, but an ignorance of our ignorance.

It is quiet likely that I would. Maybe I will check it out shortly.

The concept of randomness will in fact ALWAYS be needed in science. We can never have perfect knowledge about any system period. We cannot even write down that many decimal places to acquire such knowledge if it was possible. The key difference, that statistical mechanics illustrates, is that classically a perfect Maxwellian Demon could ONLY in principle do away with stochastic behavior altogether, but in QM we have no clue how to construct any model that would allow this Maxwellian Demon to do the same in that regime, even in principle.

SpectraCat

I believe that according to QFT, nuclear decay events are attributed to the same thing that "causes" spontaneous emission of radiation from excited quantum states, namely, interaction of the metastable quantum system with a vacuum fluctuation (or virtual photon, or spaghetti monster tears, or whatever name you want to give to the hypothetical phenomenon). Some sort of interaction is required within the framework of quantum theory for excited molecular or atomic eigenstates to decay, because they are *eigenstates*, and thus their probability density is conserved.

So, the question now is, are vacuum fluctuations (or whatever) truly random? I don't know enough about QFT or quantum cosmology to even approach answering that question. Personally, I have a strong predilection to believe that they are in fact random, but it's just a gut feeling at this point.

Ken G

The determinism of thermodynamics is in the structure of the theory itself. We can predict a deterministic evolution of temperature, for example, in thermodynamics, and first students of thermodynamics are generally not taught that this is just a statistical average they are solving. But quantum predictions are not framed deterministically, instead we speak of testing probability distributions explicitly in QM, via repetition of the same experiment-- a device never used in thermodynamics. In QM, we don't generally test expectation values, whereas in thermo, we are not even taught that the observables are expectation values (even though they are). So thermodynamics is a deterministic theory, and quantum mechanics isn't.
I'm not sure what context you mean. I would place the "fundamental" aspects of a law in the nature of the derivations used for that law, not in the nature of the systems the law is used to predict. That's mixing two different things.
We don't actually know that, because our knowledge is always limited. We have no way to test your assertion. Indeed, in classical chaos, we generally find the stochasticity penetrates to all levels-- no matter what the ignorance is initially, it rapidly expands toward ergodicity. This has a flavor of being more than an apparent aspect of the behavior, instead the behavior is a kind of ode to ignorance. The idea that we could ever complete our information of a classical system is untenable-- ironically, classical systems are far more unknowable than quantum systems, because classical systems have vastly many degrees of freedom. It is that vastness that allows us to mistake expectation values for deterministic behavior, we see determinism in the context where the behavior is least knowable. Determinism is thus a kind of "mental defense mechanism," I would say.
The laws are the theory, so the foundation of the laws is only the structure of the theory, regardless of how successfully they test out. I think you take the perspective that there really are "laws", and our theories are kinds of provisional versions of those laws. My view is that the existence of actual laws is a category error-- the purpose of a law is not to be what nature is actually doing, it is to be a replacement for what nature is actually doing, a replacement that can fit in our heads and meet some limited experimental goals. I ask, what difference does it make the "foundational" structure of our laws? We never test their foundational structure, we only test how well they work on the limited empirical data we have at our disposal. The connection at the foundational level will always be a complete mystery, or a subject of personal philosophy, but what we know from the history of science is that the foundational level of any law is highly suspect.

Yes, that is an important difference. It is not the laws that are fundamental, because that makes a claim about their relationship to reality. It is only the fundamental of the law that we can talk about-- there's a big difference.
I think this is your key point here, the degree of ignorance is worse in QM applications. I concur, but then we are both Copenhagen sympathizers!
Yes, I agree that randomness in our models is inevitable-- chaos theory is another reason.


Yes, I see what you mean, the absence of any concept of a quantum demon is very much a special attribute of quantum theory, although Bohmians might be able to embrace the concept.

Ken G

The one thing we can know for sure is that no scientist will ever know the answer to that question.Knowing QFT would only tell you if the theory of QFT models the fluctuations as truly random, it wouldn't tell you if they are or not.