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Physics Stack Exchange users whose comments are worth studying include Lubos Motl and Ron Maimon (now at http://www.quora.com/Ron-Maimon and http://www.physicsoverflow.org/user/Ron+Maimon). Also see http://arxiv.org/abs/hep-ph/0207124 for a review of physics since the standard model.


Jul
29
comment Discreteness and Determinism in Superstrings?
't Hooft's theory doesn't have a wavefunction at a fundamental level, just the CA. But he's saying that the dynamics of his CA is the same as the dynamics of the beables in a particular Bell-Bohm theory. This Bell-Bohm theory is the one which can be described as a basis permutation in Hilbert space, but 't Hooft is saying that if you look at the eigenvalue dynamics induced by this permutation, it equals the CA.
Jul
27
comment Why aren't we Boltzmann brains in an infinite universe?
What @Mad scientist really wants to know is, why don't BBs dominate the counting of observers? Well, any argument that BBs dominate depends on particular physical and cosmological hypotheses. So one or more of the hypotheses is wrong, and quantum cosmology is still mostly speculation, so this is hardly surprising.
Jul
27
comment Why aren't we Boltzmann brains in an infinite universe?
"Why aren't we Boltzmann brains in an infinite universe?" Because BBs usually die or disintegrate immediately after their creation. That's how we know that this is an ultra-exponentially unlikely hypothesis...
Jul
27
comment Why aren't we Boltzmann brains in an infinite universe?
"Are they all the same brain" - no, because when you apply a diffeomorphism to move one BB to where the other one is, the other one will also be moved by the diffeomorphism to a new position. So you'll still have two of them after the move.
Jul
27
comment Discreteness and Determinism in Superstrings?
... because all the dynamics is in the wavefunction part of the ontology. Similarly, the QFT observables that 't Hooft defines in terms of his ultimate CA beables (in part 6 of 1205.4107 and part 5 of 1207.3612) are the second-order beables out of which the macroscopic world is constructed. The mysterious part is that the CA dynamics is also supposed to produce the right dynamics for measurements of all observables.
Jul
27
comment Discreteness and Determinism in Superstrings?
See prac.us.edu.pl/~ztpce/QM/Bell_beables.pdf page 8 for what I think is 't Hooft's real philosophy. In this paper Bell constructs a hidden variables theory in which there is a "basic local beable" (he uses fermion number density) out of which all space-time objects are constructed. Bell remarks that any observable capable of specifying the positions of objects at mesoscopic resolution could play the role of basic local beable. Your experiment might be measuring spin, but even if your ontology only contains position beables, it will still describe the experiment correctly...
Jul
27
comment Discreteness and Determinism in Superstrings?
+1 for the remarks about ghosts, scalars, and SUSY. But you must be wrong to say that 't Hooft wants superpositions of his ontological basis states to play any role (such as representing states of imperfect knowledge). He explicitly says, e.g. in 1112.1811 page 1, that such superpositions do not occur...
Jul
25
awarded  Organizer
Jul
23
comment Which universe had a beginning? The universe or the observable universe?
Equating existence with observability is quite unnecessary. This idea took hold only as a reaction to the human tendency towards dogmatism and unexamined assumptions. Asking for evidence is always a good corrective. But when a philosophy leads an otherwise intelligent person to claim that whole galaxies, with all their hundreds of billions of stars and planets, literally cease to exist when they become unobservable to us, you know that the corrective philosophy has itself become a problem.
Jul
23
comment Why doesn't the anthropic principle select for N=2 SUSY compactifications with an exactly zero cosmological constant?
Maybe we are in an N=2 universe. arxiv.org/abs/hep-th/0109168 physics.stackexchange.com/questions/27421/…
Jul
22
comment Can quantum mechanics really be the same as underlying deterministic theory?
In the paper after this one, 't Hooft constructs his alleged mapping from a CA to a QFT. The CA is really simple but the mapping is a little nontrivial; at least, I haven't grasped the essence of it yet. It will be hard to say anything concrete about how or even whether 't Hooft can account for observed superpositions, until someone understands this further stage of his recent work.
Jul
22
comment Can quantum mechanics really be the same as underlying deterministic theory?
In Bohmian mechanics with a specific pilot wave, you can substitute the specific pilot wave into the equations of motion for the classical objects of the theory, and you end up with a pseudo-classical theory in which a classical equation of motion is augmented by a nonlocal potential. It must be possible to explain superposition in terms of this nonlocal potential, because the theory is still identical to Bohmian mechanics with a specific pilot wave, but no-one has ever taken this route and exhibited what such explanations look like...
Jul
22
comment Can quantum mechanics really be the same as underlying deterministic theory?
't Hooft thinks there is an ontologically preferred basis and superpositions of those basis states are not real. The Bohmian treatment of measurement interactions shows that you can have a preferred basis and still describe other observables correctly - though the Bohmians also still have superpositions in their ontology, as states of the pilot wave (unless they take the "nomological" path, and treat the specific pilot wave of an individual system as a dynamical law rather than a physical state)...
Jul
12
comment What Observations could undeniably support string theory?
I think Argus is asking for possible experiments which would clearly show a stringy effect. If you had a particle collider operating at the GUT scale, maybe you'd see excited string states. But in my opinion the best hope for proving string theory is for people to figure out the right vacuum (e.g. right shape of the extra dimensions) and show that it predicts the particle masses and mixing angles.
Jul
9
comment Is there a concise-but-thorough statement of the Standard Model?
This is the earliest occurrence (1978) of the phrase "standard model" that I have found: prd.aps.org/abstract/PRD/v17/i1/p275_1
Jun
30
answered Is the only diffeomorphism invariant anthropic principle the final anthropic principle?
Jun
28
comment Natural macroscopic amplification of quantum randomness
... but the problem with this example is that you have a persistent stochastic input, rather than stochastically determined initial conditions followed by smooth evolution. So the role of QM would be to produce a constant level of "noise". Still, that might be enough - to specify a mesoscopic system in which there's a stochastic noise term with a quantum origin.
Jun
28
comment Natural macroscopic amplification of quantum randomness
"It's only in artificial setups like the Schrodingers' cat that you'll see an effect from an individual quantum event". But the real world does contain chaotic dynamics, capable of macroscopically amplifying a very small difference in initial conditions. An example might be turbulence in a medium where the microscopic dynamics includes quantum scattering events. You wouldn't have macroscopic coherence, but you would have mesoscopic stochastic dynamics reflecting the outcome of individual collisions...
Jun
28
comment Natural macroscopic amplification of quantum randomness
Schrodinger's cat would be an example, where you have two or more macroscopically distinct outcomes with a quantum cause. But it's an artificial setup. I want examples from nature. If you follow the link, the guy is asking for situations where distinct quantum outcomes show up in the mesoscopic world. Radioactive decay would be an example, but there has to be something more striking than that.
Jun
28
comment Natural macroscopic amplification of quantum randomness
"If you believe in decoherence there is no quantum non-determinism" - that is not true; decoherence is wavefunction dynamics and is independent of interpretation. Anyway, this is not a question about quantum ontology. I am looking for concrete examples of physical systems in which macroscopic outcomes are the result of quantum events. This is true for measuring devices, for example, but those aren't natural systems... The question is in response to squid314.livejournal.com/314580.html