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bio website motls.blogspot.com
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Hi, I am a string theorist and a publicist.


Apr
30
revised Tensor product of operators in QM
added 473 characters in body
Apr
30
answered Tensor product of operators in QM
Apr
30
answered How do particles “know” when to decay?
Apr
29
comment What experiment would disprove string theory?
Right. In other words, if string theory were realistically proven wrong, it would almost certainly take the rest of physics to the bottom of the ocean with it. There's nothing wrong about this fact; it just means that string theory has become the state-of-the-art foundation of physics.
Apr
28
awarded  Nice Answer
Apr
28
revised What experiment(s) have or can refute the existence of an electron-particle “system” over the separate existence of a neutron within itself?
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Apr
28
comment What experiment(s) have or can refute the existence of an electron-particle “system” over the separate existence of a neutron within itself?
Dear @docscience, experiments can only refute a "position" if it is sufficiently well-defined to make at least some predictions that are not guaranteed a priori. With any understanding of the "composition" we have seen, and with some knowledge what the composition means according to quantum mechanics, your theory makes the prediction that the neutron will behave like the hydrogen atom or be indistinguishable, and easy experiments are surely enough to refute this prediction. You may refuse the assumptions "what QM implies about composite states" but then you have to write a whole new theory.
Apr
28
revised What experiment(s) have or can refute the existence of an electron-particle “system” over the separate existence of a neutron within itself?
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Apr
28
answered What experiment(s) have or can refute the existence of an electron-particle “system” over the separate existence of a neutron within itself?
Apr
28
comment Quantum Entanglement - What's the big deal?
Entanglement is just the most general correlation described in terms of this more accurate and more compatible-with-experience theory, quantum mechanics. The precise way to calculate the correlations and probability distributions are given by the laws of QM. The laws of classical physics - any classical theory - would be wrong. But it's not "weird" for some theories to be wrong. Most theories people can invent are wrong. This whole ritual of saying that QM or entanglement is "strange" is just an obsessive religious exercise of people who refuse to accept modern physics.
Apr
28
comment Quantum Entanglement - What's the big deal?
But my point is that none of these aspects of quantum mechanics is "weird" in the sense that it would contradict some experience we have really had in the previous centuries. The only thing that these features and predictions of quantum mechanics disagree with is classical physics - an approximate theory that was invented to describe people's observations up to 1925. But classical physics doesn't directly follow from our experience in any way, of course. Quantum mechanics is more, and not less, compatible with our everyday experience than classical physics.
Apr
28
comment Quantum Entanglement - What's the big deal?
Dear @TerryBollinger, after 2+ years :-), let me answer your question. Be sure that I am well aware of Bell's theorem - I have taught this theorem and all these matters at Harvard, too. There is nothing shocking about Bell's theorem. It's just a rudimentary application of quantum mechanics to a very simple problem of 2 spins. What would be "weird" would be if the most elementary properties of Nature such as spins had "hidden variables" behind them. The world just doesn't work like that; it works according to the laws of quantum mechanics.
Apr
27
comment Is there a spacelike curve connection two events in Minkowski space?
Dear Richard, you have only proved that it is a timelike curve if $v\lt c$ at every moment $t$. If $v\gt c$, the curve is spacelike. ... If two points in the Minkowski space are time-like-separated - if the straight line between them is timelike - then you won't find any spacelike trajectory between them.
Apr
27
comment Is there a spacelike curve connection two events in Minkowski space?
Dear @Richard, what is exactly the "problem" you are referring to? What you described, with the $ct$ as the time component, is the most widespread description of a timelike curve. That's exactly what a timelike curve is - a trajectory $\vec x(t)$ which is moving at a speed $v\lt c$ at each moment. Spacelike curves indeed correspond to propagation by $v\gt c$ in your parameterization, which is prohibited for massive objects.
Apr
27
revised Is there a spacelike curve connection two events in Minkowski space?
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Apr
27
answered Is there a spacelike curve connection two events in Minkowski space?
Apr
26
answered Are electrons just incompletely evaporated black holes?
Apr
25
comment The integral is zero! $\int \frac{\mathrm{d}^d k}{(2\pi)^d} = 0$
Dear Marcel, when doing physics and related forms of calculus, the sum isn't interpreted as a limit of partial sums but in a more general, natural, and physical way, and it is often literally written to be equal to the constant for a very good reason. Divergent sums and integrals appear everywhere in quantum field theory (physics) but that doesn't mean that one can assign them with any value he wants. They have to be regularized, renormalized, and the final result may depend on some parameters. In $\sum n$ the dependence goes away and the finite part of the sum is always equal to $-1/12$.
Apr
23
comment Does the path integral measure have dimension?
In principle, one can imagine the logarithm of dimensionful things as well - but they produce things like bizarre additive log(meter) terms. But in quantum mechanics, there is a reason why you are right: one computes the path integral over a spacetime with a thermal circle. And the Euler character of $A\times B$ is $\chi_A\times \chi_B$, and because $\chi$ of a (thermal or other) circle $S^1$ is zero, $\chi=0$. In classical statistical physics, one must be more careful about the normalization factors in front of $Z$ when going in the continuum limit.
Apr
23
answered Does the path integral measure have dimension?