Describing measurement as "entangling your brain" is a particular viewpoint that is not widely accepted. Most physicists would agree that the LHC's particle collision data constitute measurements, even though most of those results have never been looked at by a human.

Measurement is at its most mysterious in the MW interpretation, because measurements are probabilistic and probabilities are hard to get out of a purely deterministic theory like MW.

It's not clear what point you're trying to argue. Yes, the particle collision data constitutes a measurement in the sense that it is entangled with whatever medium the data is being stored on (and, in practice, a while lot else), even if a human hasn't looked at it yet. Measurement as we usually mean it when talking about this stuff in the context of QM means a human looking at a thing, but sure that need not always be the case. I mean if you want to split hairs we can go back and forth all day, but that doesn't strike me as an interesting or enlightening avenue of discussion.

> Just call it what it is: entangling the state of your brain with the results of an experiment.

This is a particular viewpoint on the role of the Schroedinger equation in theory, incompatible with the Born interpretation of |\psi|^2 which is the orthodox way to use it. It is a different thought scheme that has no support in experiment. No experiment uses Schr. equation to consider how experimenter's brain gets entangled with their measurement apparatus; the ones that use it at all use it to calculate measurable values and probabilities of measurement results.

The Born rule isn't an interpretation it's a fundamental law derived from observation of how the probabilities work out. If you've got some interpretation of stuff that explains the Born rule I'd really like to hear about it.

And, are you saying that your brain isn't entangled with an experimental result after observing it (at the latest)? If so, what is special about brains, that shields them from it?

There are actually two Born rules - the first one gives probability of configuration as integral of square of \psi - int |\psi(x)|^2dx, and the second one gives probability of result of measurement as square of integral of psi |int \phi_k^*(x)\psi(x)dx|^2. The first one was introduced to give meaning, i.e. interpret, Schroedinger's psi-function, the second one soon followed to connect the theory with the quantum ideas. Of course, the rule has been successful - but as far as the goal is to understand \psi, it is just an interpretation of \psi. It says nothing about what \psi itself actually is, only how to use it to get probabilities.

Regarding the entanglement of brain with experiment, that is totally unfounded extrapolation of applicability of many-particle Schroedinger's equation. It is useful for atoms and molecules, but it is practically intractable for systems of few atoms. For macroscopic objects, like arm indicator of an ammeter, the Born rule assumes that results of measurements are definite, so probabilities can be assigned to them. In the picture where the indicator or brain is just a part of the system that gets entangled, no definite results of measurements are obtained and it makes no sense to talk about their probability in the way all successful applications do.

I wasn't aware brains were made of things other than atoms and molecules. Interesting stuff.

I do not claim that, please read again more carefully. If you do describe the joint system experiment+brain with quantum-theoretic formalism, the brain does get entangled.

However, my objection is to the very idea of using quantum-theoretic formalism to describe macroscopic things. Because: even if we imagine we somehow obtained a solution to Schroedinger's equation for this system, we have no need for it. The purpose of \psi function is to get probabilities of system configuration or results of measurement of some quantity. This is useful for comparing the theory of natural phenomena with the reality.

Much less so for describing the system of experiment+experimenter. Would you want to apply the quantum formalism to obtain probabilities of the end states of the joint system, when you can just ask the experimenter on what was the result of measurement and thus obtain definite answer with more value to physics?

Uhhh.....

> Regarding the entanglement of brain with experiment, that is totally unfounded extrapolation of applicability of many-particle Schroedinger's equation. It is useful for atoms and molecules, but it is practically intractable for systems of few atoms.

It seems like you're saying that a different physics governs reality once the number of particles is large enough. Is your sole objection instead that rigorously describing the state of a brain is computationally intractable? If so, what bearing does that have on this:

> We shouldn't assign mysterious properties to "measurement" as though it's some magical thing that just makes quantum mechanics happen. Just call it what it is: entangling the state of your brain with the results of an experiment.

To answer your question, I don't need formally specify the experimenter in rigorous detail, in order to reason about what is going to happen when I ask the experimenter the result. It's also weird that you mention that I can "just ask the experimenter" when that doesn't really help me make any predictions. (Unless I try to predict what the experimenter will say, at which point we're back to entangled brains again.) What were you getting at with that?

I upvoted you since I don't know why you're so far down the page. What you say is essentially correct. The error most people make when trying to understand quantum mechanics is assuming that the measuring system is independent of the measured system. But the overall composite system can't be factored like that since it evolves unitarily (and deterministically) into a state that isn't representable as a tensor product.