Thursday, June 29, 2006

In the quantum world, to be is to be measured

What is quantum mechanics?
The fundamental theoretical framework of contemporary physics.
Why should I be interested?
Because quantum mechanics is trying to tell us something important about the world and ourselves.
I won’t be able to follow the math.
It takes very little math to get the message of the quantum. Besides, you don’t come to understand the physical meaning of a mathematical formula by writing down another formula.
Then why is quantum mechanics said to be shocking, confusing, incomprehensible, and what have you?
Because we make it so. If we ask questions that Nature doesn’t understand, we shouldn’t be surprised if Nature gives answer that we don’t understand. Wrong assumptions lead to meaningless questions. Here as elsewhere, the challenge is to learn to think in ways that do not lead to meaningless questions.
What are the wrong assumptions, in a nutshell?
As the incomparable Sidney Harris said, any philosophy that can be put in a nutshell belongs there. (We are talking philosophy: clarifying the relationship between the mathematical formalism of quantum mechanics and the real world is a philosopher’s job.) But if you held a gun to my head, I’d say that it is wrong to assume that What Ultimately Exists is a multitude (of particles, spacetime points, whatever). What is wrong is the bottom-up approach, which has been paradigmatic for something like 25 centuries.
And what is the message of the quantum, in a nutshell?
Since you keep holding that gun to my head, the nub of it is that What Ultimately Exists is one. Quantum mechanics explains the world from the top down. If you ask it about ultimate constituents and how they interact and combine, it does indeed make little sense. But if you ask it how the One Ultimate Existent—or whatever else you want to call it—takes on the aspect of a multitude, and thereby manifests the world, quantum mechanics makes perfect sense.
How does this solve the measurement problem, which by universal consent is the ultimate stumbling block to beating sense into quantum mechanics?
In a nutshell? The first thing you need to know about the so-called measurement problem is that it is largely spurious. There is a genuine problem, but as a rule this gets swept under the rug and spurious problems are tackled instead.
What is the genuine problem?
The quantum formalism is a probability algorithm. You feed it measurement outcomes, and it returns the probabilities of the possible outcomes of other measurements—measurements that may be made in the future, measurements that are made at the same time in a far corner of the universe, or measurements that could have been made in the past. As far as the math is concerned, that’s all there is to quantum mechanics. The problem is that a probability algorithm presupposes events—things to which probabilities can be assigned. How can such an algorithm be complete? How can it encompass the events which it presupposes?
How come the fundamental theoretical framework of contemporary physics is a probability algorithm?
Have you ever wondered why a stable composite object has spatial extent (it “occupies” space) even though it is “made of” a finite number of objects without spatial extent (they do not “occupy” space)? What “fluffs out” matter is the fuzziness—Heisenberg’s Unschärfe, generally mistranslated as “uncertainty”—of all relative positions and momenta. And what is the proper way of quantifying a fuzzy observable? It is to assign probabilities to the possible outcomes of a measurement of this observable.
And how does your interpretation solve the genuine problem?
Quantum mechanics does not tell us which substructure of its complete theoretical structure (comprising whatever is definable with the help of the mathematical formalism) corresponds to What Exists. The solution consists in making the right choice.
And which is that?
The macroworld. The tricky part is to find a genuine substructure of the theory that agrees with our experience of the macroscopic. This is where the top-down paradigm comes in. The spatial differentiation of the world is incomplete. It doesn’t go all the way down. If in our minds we partition the world into smaller and smaller regions, there comes a point when there isn’t any material object left for which these regions, or the corresponding distinctions, exist. They then exist solely in our heads. This makes it possible to show that macroscopic objects (properly defined) behave in a deterministic manner (properly defined), and this in turn makes it possible to attribute reality to the macroworld.
Can you give an example of the spurious problems that are tackled instead?
The mother of all pseudo-problems—quite literally, since it gives rise to several other spurious problems—is this: why do quantum states have (or appear to have) two modes of evolution, one deterministic and one probabilistic?
Why is this a pseudo-problem?
Because a quantum state is not the kind of thing that evolves. The time dependence of a quantum state is not the time dependence of an evolving instantaneous state but the time dependence of a probability algorithm on the time of the measurement to the possible outcomes of which it serves to assign probabilities. This was clearly seen by Heisenberg, the first to discover quantum mechanics: “There is no description of what happens to the system between the initial observation and the next measurement.”
There is no description of a quantum system between measurements???
Are you surprised? What a physical system does between measurements is by definition speculative. To know what it does, you have to make a measurement. Any story purporting to describe a system between measurements is just that—a story.
Come on, the generally acknowledged fact that a measurement disturbs a quantum system implies that there is something to be disturbed. What can be disturbed can be described in both its disturbed and undisturbed states.
This “generally acknowledged fact” is a myth. A measurement does not disturb the value of an observable—it creates it. There is ample evidence—and I go to some trouble to furnish it—that a property or value is possessed (by a system or an observable) only if, only when, and only to the extent that it is measured. In the quantum world, to be is to be measured.
Do you really believe that the moon exists only when you look at it? (Note: Pascual Jordan recalls Einstein asking him this very question.)
Your question contains two misconceptions. For one, any event or state of affairs from which the truth or falsity of a proposition of the form “system S has property P” can be inferred, counts as a measurement. Contrary to a popular misconception, quantum physics does not involve conscious observers any more than classical physics did. For another, the moon is a part of the macroworld, and this is real per se.
You want me to believe that the properties of the microscopic constituents of the macroworld exist only because they can be inferred from what happens or is the case in the macroworld???
The 25-century old bottom-up paradigm has passed its expiry date. The right questions to ask no longer are, what are the ultimate constituents and how do they interact and combine? The right question to ask is, how does the One Ultimate Existent differentiate itself and thereby manifest the world? Particles and atoms are instrumental in the manifestation of the world rather than its constituent parts or structures. Imagine that you just experienced something the like of which you never experienced before. How are you going to describe it? You are obliged to use words that refer to such experiences as you have had. It is the same with what “happens between” the One Ultimate Existent and its manifestation. Only this time what is missing is not the words but the facts. Even though facts belong to the finished product, the macroworld, we cannot describe its manifestation except in terms of facts, by drawing inferences from property-indicating events and their quantum-mechanical correlations.
This is crazy!
The question (once asked by Niels Bohr) is, is it crazy enough to be true? Quantum mechanics calls for radically new ways of thinking about matter, space, and time. For instance, if you want to think of space as something that exists by itself (in philosophese, a substantial expanse) then it forces you to conceive of it as undifferentiated—it lacks parts. So from where do you get attributable positions? They have to be realized (made real) as properties of macroscopic objects before you can attribute them to what is instrumental in their manifestation. Quantum mechanics helps us understand how attributable positions are realized. If instead you think of space as a storehouse of pre-existent positions, you won’t be able to makes sense of quantum mechanics because you won’t be able to understand what it is trying to explain. Posted in Recommendations, Quantum semantics 4 Comments »

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