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As with all “This Month in Physics History” articles, I thoroughly enjoyed the February 2016 issue on Amadeo Avogadro’s life, struggles, and achievements. The fact that the Avogadro constant is one of the seven fundamental constants chosen to form the basis of the new International System of units (SI) is a testament to his accomplishments.
I would like to provide one clarification. While I’m extremely encouraged that the members and writers at APS have embraced an SI based on exact values of fundamental constants, the deed is not quite done yet. Barring some incredible event or discovery, the new SI will be officially adopted at the next (26th) meeting of the General Conference on Weights and Measures (CGPM) in the fall of 2018. “Soon the units will be defined by seven physical constants” would be more accurate.
David B. Newell
I agree with statements in “A Brief Comparison of the SSC and LHC Projects” (APS News, February 2016) that the location (Texas) and military-industrial style of management helped to terminate SSC in 1993. But I think it would have been terminated independent of any type of management that was at SSC at that time. There was a great deal of political fighting during the 1992 elections and this termination was one of the results of that fight.
I also agree that if it were at the Fermilab site, it would be more probable that SSC would have survived. I was deeply involved in high-energy accelerator projects in the 1980s. I was project manager of the 3 Tev (UNK) collider under construction in Russia at the Institute for High Energy Physics in Protvino. From 1986 to 1990, progress on UNK construction was very good. U.S. Scientists provided that information to the U.S. Congress. At that time, competition between the USA and the USSR played a very important role in making decisions for new projects in the USA as well as in the USSR.
In 1991, following the collapse of the USSR, the budget for UNK was reduced to zero and construction was terminated by Russia. That political argument to build SSC disappeared. Personally, I believed that the USA would build SSC and moved from Russia to Texas in 1992. I was really surprised that the SSC was under attack and terminated in 1993, in spite of really good progress on construction.
The future of high energy physics and new possible accelerators were considered by the International Committee for Future Accelerators (ICFA) starting around 1976. On several workshops organized by ICFA during the 1970s and 1980s, consensus was achieved that new very big accelerators (VBA) could be built taking into account possible improvements in superconductors and superconducting magnets. ICFA considered the VBA as the next international project, but U.S. physicists took that idea and proposed SSC as a national project without international collaboration. This also simplified cancellation of the SSC later on.
In conclusion, in my opinion:
North Aurora, Illinois
In Emily Conover’s stimulating article, “Gravitational Waves Caught in the Act” she notes, “The researchers also set a bound on the mass of the graviton — the hypothetical particle that transmits the gravitational interaction. …” I believe it is of general interest to point out that Einstein did not believe that there are gravitons, even though, as is well-known, he was the one who proposed light quanta that were later called “photons.”
In general relativity, the so-called gravitational force is not a true force, unlike the Lorentz force in electrodynamics, but a pseudo-force. This is because one can make a coordinate transformation that will eliminate the gravitational pseudo-force at a point, and indeed, as Fermi later showed, it can be made to vanish along an arbitrary world line. Now when a photon strikes an electron, as in the Compton effect, it gives the electron a kick, so to speak, or more technically, a momentum transfer, and hence it exerts a true force that cannot be eliminated by a coordinate transformation.
So if there were gravitons, they too would give kicks to particles they interacted with, and hence would exert true forces. Thus if one wants to stick with the view about the gravitational interaction that emerges from Einstein’s general relativity, one has to reject gravitons. This no-graviton view of Einstein could help to explain why there has been no success in the numerous, and mathematically impressive, efforts to quantize general relativity. For Einstein then, gravitational waves are classical waves that one should not attempt to quantize.
However, this is by no means the end of the story, because these classical gravitational waves that LIGO so remarkably detected are solutions to the linearized Einstein gravitational field equations. If one plugs these linearized solutions into the exact field equations, one finds there are true energy-momentum source terms that result due to the nonlinear structure of the exact equations. The LIGO theoreticians have yet to tell us what is the physical meaning of these classical quantities? Do they really exist, or are they just mathematical artifacts? If they do, can they eventually be detected as well?
Frank R. Tangherlini
San Diego, California
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