This article presents a very inaccurate view of the realist approach. The universe does not "split" when you make a measurement.
The measurement problem is a solved problem. The solution is that measurement and entanglement are the same physical phenomenon. Measurement is just entanglement extended to a macroscopic system through a process called "decoherence". The net result is that, when you do the math, you recover classical behavior by taking "slices" of the wave function (the mathematical operation is called a "trace"). This has been known for decades now, and provides a coherent and easy-to-understand picture of what is "really" going on. It is astonishing to me that the physics community still bifurcates into two camps: those who think this is common knowledge, and those who are completely unaware of it (or think it's a crazy idea).
A measurement is an interaction of the quantum system under test/consideration with another system / component that is classical (non-quantum, e.g. big).
Why? Because otherwise we would have to specify the exact details of every measurement device. Model number wouldn't be enough. What is the full quantum state of photon detector with its enormous number of atoms? Unusable.
Entanglement isn't some spooky thing. In fact it's required for us to be able to measure anything. The measuring device's quantum state must become entangled with the quantum system it's measuring. We can't really consider each one separately, but we try anyway, and with great success if we do it right.
I like James Binney's take on this: QM is adult physics. It's an admission that when you measure something, you disturb it. When you measure something really small, you disturb it alot, and can't idealise that away as you can for classical macroscopic systems.
EDIT> I should add that for a measuring device, we want it to be correlated to the system under test. I.e. I could make a thermometer that always read 5C (uncorrelated to environment), but it would be a bad thermometer. Quantum entanglement is how this correlation is set up. But it means that now the future evolution of the quantum system depends on the (unknown to us) exact state of the measuring device.
I would love to consider measurement something ontologically special, but it's not possible because there is no well-defined definition for what a measurement is.
The definitions I have found always invoke the presence of a "classical system"/"observer".
But that just kicks the can down the road, because there is no well-defined definition of a "classical system" either.
At this stage I don't think very many philosophers consider measurement as it takes meaning in quantum mechanics to have anything in particular to do with living creatures or brains or consciousness or anything.
A measurement is an interaction that crosses the Heisenberg cut separating the quantum from the classical world. Eg you prepare an atom in a superposition of states relative to some 'quantization axis' determined by a Stern-Gerlach apparatus. The state will randomly 'collapse' into one of the available final states, resulting in a macroscopic dot on some screen. You and all your physicist buddies will agree on the placement of the dot.
There is no agreement on how this happens, complicated by the fact that there is no such thing as a classical world I alluded to above, ie any interactions with the apparatus and the conscious observers supposedly obey the laws of quantum mechanics.
Ancient greek's could surely measure some things, how else did they build those buildings?
You've declared we can measure everything important so reality is objective, but quantum physics suggests the things we can't measure may be astronomically large. The physicists who subscribe to the many world's theory believe there's a near infinite assortment of humans we cannot percieve or effectively measure, for example, and our own experience of self is inaccurate as we don't percieve ourselves to be branching in time.
I don't think measurements solve the problem of the uncertainty of reality.
Measurement problem is a problem of our understanding of the quantum world, but otherwise a feature of the entire universe, not a human limitation - and definitely not a license for magic under the hood.
I believe gus_massa was just saying that we simply define out of existence this supposed problem with measurement (for macro objects). This is done because even though the 'furniture of the world'/everyday objects/macroscopic objects are still quantum entities, at this level, error rates are such that they can be treated in a classical manner.
Couldn't it be that the measurement device (or any other interacting object) "falls" into all of the possible states of the measured system, therefore we see the measurement device itself as a quantum object when we don't interact with it, but every possible "outcome" of the measurement device sees the system as classical. At a higher scale, a human being itself is an object interacting with the measurement device, and every possible "outcome" of the human being sees the device as a classical system, showing a classical system.
At least this explanation does not involve "quantum brains" nor enthropocentrism.
I think confusion arises when people hear 'measurement' and jump to thinking "wait, what's so special about measurement? Would the world not exist if we weren't there to measure it?" This is due to an imprecise understanding of measurement, and should be corrected by pointing out all particle interactions are measurements. Our classical world exists without superposition because there are enough particles interacting with each other that everything is constantly being measured.
This article's comments section links to reviews of the paper, and, basically, the second reviewer nails it.
In one of the outcomes authors examine, which gives the rise to the apparent contradiction, measurements do no commute, which means, according to QM it is impossible to get that set of results in real world.
The mistake in this line of thinking is in the tacit assumption that a quantum system possesses a certain property prior to or independent from a measurement. It does not. Anything we measure is essentially an artifact of the measurement process itself, and the randomness of the result comes from bringing together two incompatible worlds - quantum and classical. All we are left with at the quantum level is wave-functions; but what matters for us is, of course, what we can experience, being ourselves classical, and QM has so far provided us with a perfectly good way of finding that out.
Sufficiently few that it is beginning to be said by a small number of physicists that it is a point of embarrassment that the Measurement Problem is about as unresolved as when it first came up a hundred years ago.
I'm not sure what exactly the problem is with measurement (Sounds like it is that physically we don't know what it means?) but current quantum categorical logic research usually does have a working definition of what they call measurement.
No, it isn't. The mathematics that describe the results of measurement are clearly defined, but what constitutes a measurement--i.e., when you need to apply that process--is not. There is no mathematics to back it up--it's purely ad-hoc.
That is what "measurement is just entanglement" provides.
The measurement is the theoretical duct tape between the "quantum world" and the "classical world".
But there is no such thing as a "classical world", it's just a useful approximation.
And therefore, there is also no such thing as a "measurement", it's also an approximation.
(Maybe not even an approximation, but maybe more like a projection...)
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