# Tag Info

27

You're right; the Schrödinger's equation induces a unitary time evolution, and it is deterministic. Indeterminism in Quantum Mechanics is given by another "evolution" that the wavefunction may experience: wavefunction collapse. This is the source of indeterminism in Quantum Mechanics, and is a mechanism that is still not well understood at a fundamental ...

24

First of all, "loopholes" is no disrespect. It's standard nomenclature. Given a law, a "loophole" is a way to circumvent it. Bell's inequalities, in their mathematical formulation, are laws that prevent superdeterminism, so if we believe it should exist, we have to find loopholes in the assumptions. It might be that the loophole is so big that the whole law ...

20

No. Shor's algorithm isn't demonstration of MWI. MWI is a way to think about Shor's algorithm, just like other interpretations of QM. Deutsch advocates MWI as a way to think about quantum algorithms because it is an easy way. where else did all the exponentially many combinations happen? Why do you need many worlds for exponentially many combinations ...

19

The idea of my latest paper is simple. I experienced in other blogs that most people refuse to go with me all the way. I'll give my argument step by step and you may choose where you want to step out. Consider superstring theory, in its original, completely quantized version. Many people believe it might have something to do with the world we live in. It ...

16

Nobody has explained to me how Shor's quantum factorization algorithm works under the transactional interpretation, and I expect this is because the transactional interpretation cannot actually explain this algorithm. If it can't, then chances are the transactional interpretation doesn't actually work. (I have looked at some of the papers that purport to ...

14

Every working interpretation of quantum mechanics, and the Copenhagen interpretation and Consistent Histories in particular, correctly predicts that Shor's algorithm works much like the other algorithms. The question As he often asked, where else did all the exponentially many combinations happen? is loaded because the computational power of a quantum ...

13

You say "ridiculous sounding ideas often end up becoming standards science" - but take into account that "ridiculous sounding ideas" much more often, by a large factor, end up becoming no science at all. I agree that "that form of idealogical bullying should never exist" - except that it's not ideological, but just a practical matter of deciding which ...

12

I agree with Luboš that this question has a lot to do with psychology. I think the tic-tac-toe analogy is relevant. There are an infinite number of games that are precisely equivalent to tic-tac-toe, but humans are probably terrible at playing most of these games. Chess is even worse. You can teach a child the rules of chess in a few minutes, but imagine ...

12

There are observables corresponding to the light going through both slits. You can write down a basis: "it went through slit A + slit B", and "it went through slit A - slit B". Although maybe you can't detect these observables easily with an experiment, they're perfectly good observables, they're orthogonal, and a clever enough experiment should be able to ...

12

Dear Jack, there is no physical phenomenon that could be called the collapse. The collapse of the wave function, as first emphasized by Werner Heisenberg and then many others, is just the event when we learn something about a physical property of a physical system. When we learn that Osama bin Laden is located in a building in Pakistan, his wave function - ...

12

I realize I'm late to this discussion. For whatever it's worth, I disagree in the strongest terms with Ron Maimon and Dmytry, when they criticize Yudkowsky for being "too conceptual." As I see it, that's exactly what you should and must do if your goal is to explain QM to an audience of non-physicists! Indeed, most popularizations of QM go off the rails ...

12

Bell's theorems indeed rule out simple theories where hidden variables obey local equations. However, no matter how you reason, it's always at some point where you need another assumption. In its simplest form, it is the assumption that two observers, Bob an Alice, have the "free will" to choose along which axis they will measure the spin of a particle ...

11

The paper does not go into details about what interpretations would be disproved by their results. There's a good reason for this: There are no interpretations that would be disproved by their results. They are disproving a straw-man. Here is the central result proved by the paper, phrased in a less obscure way: "If a system is in the state $|+_Z\rangle$ ...

11

Generally, when you make a quantum calculation, you have to make some sort of measurement of the qubits at the end of the algorithm where the result you're looking for is a very probable (but not necessarily certain) result. In any interpretation that actually agrees with the basic results of quantum mechanics, these probabilities will still hold and the ...

11

Except in very elementary examples (single particles), the QM wave function has nothing to do with a wave (apart from the historical origin). For a system consisting of $N>1$ particles, the wave function is a function in configuration space (with 3N variables), not one in 3-space (whose coordinates are positions $x$ with 3 components). This can be read ...

9

Laplace's determinism is not physically correct over long periods of time. That is, it neglects chaos/"sensitive dependence on initial conditions"/exponential growth of microscopic perturbations already in Newtonian dynamics, which was seriously thought about only in the 20th century. Being true, this also will not be overcome. Stochasticity enters ...

9

This is essentially a philosophical rather than a physical question. (I don't mean that as a pejorative statement, by the way, just a descriptive one.) There are different philosophical approaches regarding the meaning of probability. Broadly speaking, some people think of probability in a frequentist sense, meaning that probabilities refer to the frequency ...

9

I think at least some readers should have noted by now that many of these arguments, particularly the more pathetic ones, are questions of wording rather than physics. Once you made your model simple enough, you can map anything onto anything. Now this was my starting point: if a system is sufficiently trivial, you can do anything you like. Now how can we ...

9

Can someone explain the quantum physics-consciousness connection? There is none. There's a whole lot that can be said about this, but at a basic level, the explanation is that an observation in quantum mechanics is best thought of as an interaction with another particle (technically: a classical system). It doesn't have to involve a conscious observer. ...

8

There are currently two different accounts that give a larger picture of what happens when a quantum system is measured. One of them is the fact that many random interactions between the system (which might be a 1-body or N-body quantum system) and the environment (which is considered for most purposes a pseudo-classical system with infinite degrees of ...

8

The normal state of subsystem is entangled, as follows from the Schroedinger equation, which turns a separable state immediately into an entangled state if there is some interaction. The state of a subsystem is obtained from the state of the universe by tracing out all other degrees of freedom. This leaves a density matrix, which is a perfectly good state ...

8

The statistical interpretation of quantum mechanics is alive, healthy, and very robust against attacks. The statistical interpretation is precisely that part of the foundations of quantum mechanics where all physicists agree. In the foundations, everything beyond that is controversial. In particular, the Copenhagen interpretation implies the statistical ...

8

Your running into circles will stop once you commit yourself to a choice. What to regard as postulate is always a matter of choice (by you or by whoever writes an exposition of the basics). One starts from a point where the development is in some sense simplest. And one may motivate the postulates by analogies or whatever. The CCR are a simple ...

8

As Dan points out in his excellent answer, most alternative "interpretations" reproduce standard quantum mechanics by construction. However, collapse interpretations such as the GRW theory, or Penrose's gravitational collapse seem to prohibit coherence in quantum systems larger than some certain size or mass. Of course, these breakdown scales are larger than ...

8

Why is the Copenhagen interpretation the most accepted one? I would say the answer is this: it's the oldest more or less "complete" interpretation hence you'll find it in many (all?) early text books, which is basically from where people writing modern text books copy from. the overwhelming majority of physicists doesn't really care about the ...

8

You are right, it is wrong to think that in gauge theory "gauge transformations are just a redundancy". This becomes true only if one abandons locality, ignores all boundary effects, all instanton effects, hence most of what is interesting about gauge theory. Of course forming gauge equivalence classes (say of observables) is something one wants to do every ...

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