Quantum Information and Spacetime

John Preskill, Caltech
QIP 2017, 15 Jan 2017


What Happens When Energy Goes Missing?

Article: Performance of algorithms that reconstruct missing transverse momentum in √s = 8 TeV proton-proton collisions in the ATLAS detector
Authors: The ATLAS Collaboration
Reference: arXiv:1609.09324

The ATLAS experiment recently released a note detailing the nature and performance of algorithms designed to calculate what is perhaps the most difficult quantity in any LHC event: missing transverse energy. Missing transverse energy (MET) is so difficult because by its very nature, it is missing, thus making it unobservable in the detector. So where does this missing energy come from, and why do we even need to reconstruct it?



Eugene Paul Wigner’s Nobel Prize

Y.S. Kim
In 1963, Eugene Paul Wigner was awarded the Nobel Prize in Physics for his contributions to the theory of the atomic nucleus and the elementary particles, particularly through the discovery and application of fundamental symmetry principles. There are no disputes about this statement. On the other hand, there still is a question of why the statement did not mention Wigner’s 1939 paper on the Lorentz group, which was regarded by Wigner and many others as his most important contribution in physics. By many physicists, this paper was regarded as a mathematical exposition having nothing to do with physics. However, it has been more than one half century since 1963, and it is of interest to see what progress has been made toward understanding physical implications of this paper and its historical role in physics. Wigner in his 1963 paper defined the subgroups of the Lorentz group whose transformations do not change the four-momentum of a given particle, and he called them the little groups. Thus, Wigner’s little groups are for internal space-time symmetries of particles in the Lorentz-covariant world. Indeed, this subgroup can explain the electron spin and spins of other massive particles. However, for massless particles, there was a gap between his little group and electromagnetic waves derivable Maxwell’s equations. This gap was not completely removed until 1990. The purpose of this report is to review the stormy historical process in which this gap is cleared. It is concluded that Wigner’s little groups indeed can be combined into one Lorentz-covariant formula which can dictate the symmetry of the internal space-time time symmetries of massive and massless particles in the Lorentz covariant world, just like Einstein’s energy-momentum relation applicable to both slow and massless particles.


Werner Heisenberg and the German Uranium Project 1939 – 1945

Myths and Facts

Klaus Gottstein
The results of a careful analysis of all the available information on the activities of Heisenberg and of his talks during the years 1939 to 1945 can be summarized in the following way. Like several other German physicists Heisenberg was drafted by German Army Ordnance when war began in Europe in September 1939 to investigate whether the energy from splitting Uranium nuclei by neutrons could be used for technical and military purposes. Heisenberg found that this is possible in principle but that military use would require such enormous industrial expenditures that it would take many years and would be impracticable while the war lasted. The project was therefore dropped by the Nazi government in 1942. Heisenberg even refrained from calculating a precise value for the critical mass of U 235. He was relieved that he was thus spared a moral decision between obeying an order to build the bomb or risking his life by refusing to be involved in the project or sabotaging it. He was happy to be confined to a project of building a small test reactor under civilian administration that the government had approved. In 1941 Heisenberg tried to get the opinion of Niels Bohr in Copenhagen on what the international community of nuclear physicist could possibly do or prevent regarding the long-range technical feasibility of making nuclear weapons. Bohr completely misunderstood the cautious approach of Heisenberg.