2,985 reputation
11425
bio website staff.science.uu.nl/~hooft101
location Utrecht, Netherlands
age
visits member for 3 years
seen Aug 12 at 7:35

Theoretical physicist, Utrecht University.

University Professor. Elementary particle physics, quantum gravity, black holes, quantum mechanics.

Won some prizes (such as the nobel prize 1999), but please don't hold that against me.


Aug
20
comment Discreteness and Determinism in Superstrings?
@Ron: No, according to my rules, which are exactly as in QM, perturbations don't have to be small, not $\delta\rho$ but $\psi$ is the wave function. Its sign can be positive or negative, and its absolute square is the probability. Earth-Mars interchange acts on $\psi$, not $\rho$.
Aug
20
comment Why do people categorically dismiss some simple quantum models?
@Scary Monster: The claim is that any CA can be cast in the language of QM, although in most cases the QM models you get will be uninteresting; there will be states, and they will obey Schroedinger equations. Now many CA models are computationally universal, so certainly not integrable, and therefore the asociated QM theory is also expected to be non-trivial. But of course the math is much harder; it's much more instructive to search for cases where you can do (perturbative) calculations.
Aug
20
comment Why do people categorically dismiss some simple quantum models?
@ Motl: In ordinary applications of QM you can ignore this, since the templates are good enough, but not in questions of the interpretation of QM.
Aug
20
comment Why do people categorically dismiss some simple quantum models?
@ Motl: Apparently you axiomatize QM by basing it on "postulates". Clearly you won't understand my theory if you are not prepared to make any amendments, since your postulates are imprecise. You said that "experiment has shown that one can superimpose quantum states". Not true, you can only do this with the templates you are using, but not in the real world. When you consider superposition of two states, you ignore the environment of these two states, which are never the same, hence always orthogonal.
Aug
20
revised Why do people categorically dismiss some simple quantum models?
changes: details of my views on some of these questions, in particular where to put the emphasis, continuously change
Aug
20
awarded  Nice Answer
Aug
19
comment Discreteness and Determinism in Superstrings?
@Ron: But you might consider staying on. Once you agree that the beable basis exists (or might exist), just continue doing QM there. Observe however, that you can do the same with any totally classical system such as the planets obeying Newton's laws. Their evolution law (at integer time steps) is also a permutator. You may pause at the question how the "Earth-Mars exchange operator" evolves with time, and conclude that you can understand the physics of the system without solving the problem, but you might also add that operator to your set of observables. It's the same planets you talk about.
Aug
19
comment Discreteness and Determinism in Superstrings?
@Ron, OK, maybe this is again just a question of semantics. I would say that Feynman diagrams represent a causal QFT (if done correctly), no matter which gauge choice or coordinate choice you use. Same for string theory.
Aug
19
revised Discreteness and Determinism in Superstrings?
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Aug
19
revised Why do people categorically dismiss some simple quantum models?
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Aug
19
revised Why do people categorically dismiss some simple quantum models?
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Aug
19
comment In 't Hooft beable models, do measurements keep states classical?
@Ron: Then let me clarify the statement. Call the states in the CA basis $|n\rangle$, where $n=1, ... N$. Whatever wave function $\psi$ I start with, I can call the "probability" that $n$ is realized $\rho_n=|\langle n|\psi\rangle|^2$. Because the evolution is a permutation, this $\rho$ evolves classically. The phase factors in $\langle n|\psi\rangle$ never play any role; they're unobservable. This is because the evolution operator won't let you generate superimposed states from CA states.
Aug
18
revised Why do people categorically dismiss some simple quantum models?
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18
revised Why do people categorically dismiss some simple quantum models?
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18
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Aug
18
answered In 't Hooft beable models, do measurements keep states classical?
Aug
18
comment In 't Hooft beable models, do measurements keep states classical?
Let me add a question, for comparison: My favorite "classical" theory is the planetary system, assuming that planets move as point particles under Newton's laws. Yoy can actually introduce non-commuting operators there as well. The "Earth-Mars exchange operator" puts Mars where Earth is and Earth where Mars is (and some simple rules about their velocities and moons). The eigenvalues of this operator are $\pm 1$. We can calculate how it evolves. Is this an observable?
Aug
18
awarded  Popular Question
Aug
17
awarded  Revival
Aug
17
answered Discreteness of Spacetime and Violation of Lorentz symmetry