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The article misses that we do not know how classical world that we perceive arises. Ultimately according to QM the whole universe is just single wave function w
by _0w8t 4y ago
The article misses that we do not know how classical world that we perceive arises. Ultimately according to QM the whole universe is just single wave function with no probabilities. But this contradicts our experience of the classical world including that we measure probabilities.
So we need to explain that to avoid various very ad-hock notions of classical measurement device and similar routes.
There are attempts to explain the classical world using a notion of decoherence, but that have problems with little progress in the last, say, 20 years to address them.
Then there are ideas based on Everett works that perhaps it is a conscious itself that perceives the classical world and the real world is the wave function. But then such ideas do not explain the exact numerical probabilities.
The interesting resent suggestion was that the probabilities were not really physical but rather represent the lack of information in Bayesian sense.
In past that was used to construct various hidden variable theories, but the new take is that the classical notion that the state of a system can be fully described by its properties at some moment in time is wrong. One also needs to know some information about future to fully describe evolution of the system. This is very speculative, but at least it is fully compatible with relativity and explains probabilities. They simply represent lack of knowledge of boundary conditions in the future.
- FeepingCreature 4y agoIn the single wavefunction, beings who perceive classical reality arise from mere wavefunction mathematics. How does this not answer the question? Once you can locate yourself in a system, the system is adequate as a physical theory of your phenomenological state.
- _0w8t 4y agoThis does not explain how a classical observer gets probabilities that are precisely measured. In universe-as-a-wave-function there are no probabilities. So how exactly do they arise and what they mean?
- layer8 4y agoMy understanding is that this is just the law of large numbers. Most branches match the mathematical probabilities very closely, and hence that’s what we observe. Yes, people on one of the rare branches where things behave differently wouldn’t observe that, but we simply happen to not be on one of those rare branches.
- goatlover 4y agoSo you’re endorsing Many Worlds with an appeal to the anthropic principle. That’s fine, but you do have to assume MWI is the true interpretation of the mathematical formalism.
- layer8 4y agoAll the other interpretations require additional assumptions on top of the mathematical formalism, hence MW seems the most plausible to me. The GP wrote “[in a] universe-as-a-wave-function”, which basically is the MWI.
- DiogenesKynikos 4y agoThe Many-Worlds Interpretation is not really an interpretation. It just says that the known laws of Quantum Mechanics are correct, and that any measurement process itself has to obey the laws of Quantum Mechanics. In other words, nothing un-Quantum-Mechanical happens during measurement.
- Jensson 4y agoMVI doesn't solve wave function collapses, if it did it would be able to tell exactly when the universe will split into many worlds, but it can't. So it doesn't solve anything, the difficulty with the theory is still there.
- layer8 4y agoThe perceived wave function collapse is explained by decoherence. There is no extra “split” that happens. “World” is just a term referring to the parts of the universal wave function that are entangled with each other. As to “exactly when the universe will split into many worlds”, it happens zillions of times per second pretty much everywhere. It’s even not clear whether the branching is a discrete and countable occurrence, it could really be a continuous fanning out.
- aatd86 4y agoIs the bayesian sense equivalent to locally non-deterministic (lack of information, partial function execution within global state) versus globally deterministic (superdeterminism? The function execution has global state as argument)? In any case, none of the crutches of the article seem contradictory to me.
- pdonis 4y ago> There are attempts to explain the classical world using a notion of decoherence, but that have problems with little progress in the last, say, 20 years to address them. What problems? Note that Aaronson, in the article (as well as a number of commenters in the comments) gives decoherence as precisely the answer to the issue you raise.
- _0w8t 4y agoOne problem is the preferred basis. Why decoherence selects a particular basis? Another is that decoherence still does not explain the origin of probabilities. I.e. there are models that shows how a pure classical system may arise. But we still have no explanation how a classical device interacting with a quantum system gets the value it measures.
- consilient 4y agoThe preferred basis is (roughly) the eigenbasis for the interaction term between the thermal environment responsible for decoherence and the decohering system in question, since those are the states that are robust to thermal noise. Outside the perturbative regime, there isn't a preferred basis because there aren't any classical observers.
- superposeur 4y agoTo add to this, the states robust to entanglement with environment were dubbed “pointer states” by Zurek since they are the quantum states most closely analogous to the “pointer” of a measuring apparatus showing one particular result.
- pdonis 4y ago> Why decoherence selects a particular basis? consilient answered this one. > decoherence still does not explain the origin of probabilities The amplitudes corresponding to each decoherent branch are the right ones to give the Born rule probabilities. Which, given that the Born rule is empirically observed to work, is probably as good as we're going to get unless (as I doubt will ever happen) someone manages to find a way to derive the Born rule from unitary dynamics.
- pdonis 4y ago> the new take is that the classical notion that the state of a system can be fully described by its properties at some moment in time is wrong. One also needs to know some information about future to fully describe evolution of the system. Do you have any references that give more details on this?
- _0w8t 4y agoSee https://en.m.wikipedia.org/wiki/Lawrence_Schulman https://en.m.wikipedia.org/wiki/Lawrence_Schulman and the book Time’s Arrow and Archimedes’ Point by Huw Price
- pdonis 4y agoSo basically he's using the path integral formalism, and since that requires you to specify both the start and end points of the path, he's claiming that physics has to work that way? That seems like a very extravagant claim given that we can compute the same results without using the path integral formalism, and without having to know anything about future end points.
- pdonis 4y ago> Time’s Arrow and Archimedes’ Point This looks something like the Transactional Interpretation of QM, which made use of advanced as well as retarded waves to explain things like EPR correlations. I think it's fair to say this is still a very open area of research and it's way too early to say how it will end up.
- gaze 4y ago> just single wave function with no probabilities this is definitely not true and contradicts relativity. (Type-III Von Neumann algebras have no pure states.)
- consilient 4y ago> Type-III Von Neumann algebras have no pure states All this means is that you're not going to recover the state of the entire universe from local observables. Disappointing, but hardly surprising.