My first query is why does he claim the position and period of an electron to be unobservable “in principle”? There was theoretically no reason (at THAT time) to doubt that these quantities could be measured, though certainly they were indeterminate practically.

Werner Heisenberg obviously disagreed with this assumption of yours and it just happened that his ability to disagree made him a founder of quantum mechanics.

He has spent several years by trying to develop “quantized planetary” models of the helium atom etc. before he understood that this failing project is failing for fundamental reasons. Such a helium with well-defined positions would be described by a chaotic 3-body problem and there would be no way how it could be consistent with the known regular behavior of the helium atom (and other atoms and other coherent systems), including the sharp spectral lines.

So Heisenberg was able to see in 1925 something that you can’t see now: that the electrons can’t be going along any particular trajectories while they’re in the atoms. Instead, what is observed is that they have a totally sharp energy from a possible list, the spectrum – something we can really observe via the photons that atoms emit or absorb. To conclude that electrons can’t be going along particular classical trajectories in the atoms, he didn’t have to wait for measuring apparatuses that would be sufficiently accurate. He was able to make this conclusion out of the available data by “pure thought”, and he was right.

— Lubos Motl

2014.10.04 Saturday ACHK

Does string theory have rivals? The answer by the most cited theoretical physicist (and perhaps scientist) is that there are not any interesting competing suggestions. Be sure that people attending my popular talks (and sometimes radio hosts etc.) often ask the same question and I give the same answer. One reason, as Witten reminds us, is that ideas that actually have something good about them, like twistors and noncommutative geometry, are gradually identified as aspects of string theory itself and absorbed by string theory. It’s just how the things are.

— An interview with Edward Witten at a bizarre place

— Lubos Motl

2014.10.01 Wednesday ACHK

Quantum Liouville equation

The analog of Liouville equation in quantum mechanics describes the time evolution of a mixed state. Canonical quantization yields a quantum-mechanical version of this theorem, the Von Neumann equation. This procedure, often used to devise quantum analogues of classical systems, involves describing a classical system using Hamiltonian mechanics. Classical variables are then re-interpreted as quantum operators, while Poisson brackets are replaced by commutators.

— Wikipedia on Liouville’s theorem (Hamiltonian)

2014.09.28 Sunday ACHK

The Higgs mechanism explains only a small fraction of the mass in the universe.

Most popular science articles give the Higgs model broad credit for lending mass to everything in the universe. However, the Higgs field gives mass only to elementary particles such as quarks and electrons. Most of the visible universe is made of composite particles such as protons and neutrons, which contain quarks. Just as a loaf of raisin bread weighs more than the sum of its raisins, protons and neutrons get their mass from more than just the quarks inside them. The strong nuclear force that holds those quarks together does most of the mass-giving work.

— Ten things you may not know about the Higgs boson

— Kathryn Jepsen

2014.09.16 Tuesday ACHK