Tuesday, October 11, 2011

Weinberg on directions in physics

Famous physicist Steven Weinberg has a new essay on symmetry. He says:
It is still true in Special Relativity that making observations from a moving laboratory does not change the form of the observed laws of nature, but the effect of this motion on measured distances and times is different in Special Relativity from what Newton had thought. Motion causes lengths to shrink and clocks to slow down in such a way that the speed of light remains a constant, whatever the speed of the observer. This new symmetry, known as Lorentz invariance,4 required profound departures from Newtonian physics, including the convertibility of energy and mass.

[footnote 4] Lorentz had tried to explain the constancy of the observed speed of light by studying the effect of motion on particles of matter. Einstein was instead explaining the same observation by a change in one of nature's fundamental symmetries.

The advent and success of Special Relativity alerted physicists in the twentieth century to the importance of symmetry principles. But by themselves, the symmetries of space and time that are incorporated in the Special Theory of Relativity could not take us very far.
This distinction between Lorentz and Einstein was not recognized by anyone at the time. You cannot find it in the original papers. Just try comparing Einstein 1905 to Lorentz 1892, Lorentz 1895, Lorentz 1899, and Lorentz 1904.

Einstein did not say anything about changing one of nature's fundamental symmetries. That was done by Poincare and Minkowski. Einstein never even expressed any disagreement with Lorentz's theory, and others called it the Lorentz-Einstein theory.

Weinberg treats symmetry as the most important concept in 20th century physics. There is no mention of Poincare, who introduced symmetry groups to relativity. He was the first to show that the Lorentz transformations form a group, to show the covariance of Maxwell's equations for electromagnetism, and to search for laws of physics invariant under a group.

Weinberg also has a new paper, Collapse of the State Vector, that starts:
There is now no entirely satisfactory interpretation of quantum mechanics. The Copenhagen interpretation assumes a mysterious division between the microscopic world governed by quantum mechanics and a macroscopic world of apparatus and observers that obeys classical physics. During measurement the state vector of the microscopic system collapses to one of a number of classical states, in a way that is unexplained, and cannot be described by the time-dependent Schroedinger equation. The many-worlds interpretation and decoherent histories approach assume that the state vector of the whole of any isolated system does not collapse, but evolves deterministically according to the time-dependent Schroedinger equation, but in this interpretation it is hard to see where probabilities come from. Also, the branching of the world into vast numbers of histories is disturbing, to say the least.

Faced with these difficulties, one is led to consider the possibility that quantum mechanics needs correction. There may be an inherently probabilistic physical collapse of the state vector, not limited as in the Copenhagen interpretation to measurement by a macroscopic apparatus, but occurring at all scales, though presumably much faster for large systems.
I am surprised to see him so negative about the interpretations of quantum mechanics.

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