Arcadian Functor

occasional meanderings in physics' brave new world

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Marni D. Sheppeard

Saturday, December 01, 2007

Fading Dark Force

Dr Motl reports on a new astro-ph paper on the lack of a Dark Force (cosmological constant). The abstract suggests
a new concordance model with 90% dark matter, 10% baryons, no dark energy and 14.8 Gyr as the age of the universe.
This sounds familiar. It includes a reference to the now published paper by D. Wiltshire. On Wednesday, Dr Motl reported on the difficulties that Fermi initially faced having his ideas accepted. Meanwhile, Louise Riofrio brings us reports from New York, and a small town nearby.


Blogger nige said...

It's brilliant news that at least some mainstream physicists are not fanatical believers in a small positive cosmological constant, just because that appears (superficially) to be the easiest way to incorporate the observed lack of gravitational deceleration of the universe into general relativity's mainstream model of the big bang.

Dr Motl's article about Fermi is good but is missing some points. I read in Dr Eugene Wigner's autobiography that he (Wigner) claimed that several people (including Wigner) had been using Fermi-Dirac statistics long before Fermi, and he wished that Fermi would be given more credit for his theory of beta decay instead of for "Fermi-Dirac" statistics. Fermi's greatest work was that beta decay theory, which was based on Pauli's interpretation of the beta particle energy spectrum, where neutrinos carry off the missing energy between the maximum possible energy of a beta particle and the actual energy in a particular decay event. Pauli's prediction of the neutrino was both the simplest explanation of the data (Bohr suggested a false rival explanation where the missing energy was due to indeterminancy creeping into the law of conservation of energy, which he believed - falsely - was only true in a statistically average way), and was experimentally justified by the nuclear reactor that Fermi invented.

Fermi was not by any means a perfect experimentalist: in 1934 he and his team of Italians irradiated samples of every element known with neutrons, and Fermi wrote a paper claiming to have discovered neutron induced activity in uranium due to simple neutron capture. That work led to Fermi being awarded the 1938 Nobel Prize for physics, and he gave a Nobel lecture called "Artificial Radioactivity Produced by Neutron Bombardment" on 12 December 1938.

He was wrong in claiming to have discovered artificial heavier elements than uranium after subjecting uranium to neutron bombardment. His lecture is online in PDF here

Fermi writes on p3 of that PDF:

"A very striking exception to this [usual neutron capture and induced radioactivity] behaviour is found for the activities induced by neutrons in the naturally active elements thorium and uranium. For the investigation of these elements it is necessary to purify first the element as thoroughly as
possible from the daughter substances that emit beta-particles. When thus purified,
both thorium and uranium emit spontaneously only a-particles, that can be immediately distinguished, by absorption, from the b-activity induced by the neutrons. Both elements show a rather strong, induced activity when bombarded with neutrons; and in both cases the decay curve of the induced activity shows that several active bodies with different mean lives are produced. We attempted, since the spring of 1934, to isolate chemically the carriers of these activities, with the result that the carriers of some of the activities of uranium
are neither isotopes of uranium itself, nor of the elements lighter than uranium
down to the atomic number 86. We concluded that the carriers
were one or more elements of atomic number larger than 92 ...
" [Emphasis added in bold to crucial parts showing errors.]

Nuclear fission was occurring in addition to some neutron capture in uranium that created U-239 (23.5 minutes half life, beta decay into Neptunium-239). Fermi missed the discovery of nuclear fission.

Just at the time that Fermi was giving that misleading Nobel lecture on his work of 1934, in Berlin the German chemists Hahn and Strassmann discovered fission products like barium-140 in the residue of the neutron bombarded uranium, after repeating Fermi's experiment, finding him wrong, and doing a more careful study. Lise Meitner interpreted the results as a fission process, using Bohr's liquid drop model of the nucleus. The electrostatic Coulomb repulsion of the two positively charged "fission fragment" nuclei would accelerate them apart from each other, giving each a kinetic energy of about 100 MeV, hence fission yields about 200 MeV, which was exactly what was predicted from the mass defect between uranium and the masses of the fission fragment atoms, using E=mc^2. In what I've read on the subject, the only man to question Fermi about this massive blunder (which could have led to the Nazis winning WWII if Hitler had funded a German nuclear weapon instead of the V1 and V2 program), was the New York Times science editor William L. Laurence. Laurence reports in his 1959 book "Men and Atoms" that Fermi told him he was glad that he hadn't discovered uranium fission in 1934, because Italy was then a fascist state under Mussolini and it would probably have had tragic consequences. Ideally, Fermi should have got a Nobel prize for the nuclear reactor or beta decay theory, but he got it for an error.

December 01, 2007 10:59 AM  
Blogger Matti Pitk√§nen said...

From the abstract it becomes clear that the model is based on fractality. Many-sheeted space-time gives a fractal hierarchy of Hubble constants, and also my own original model for acceleration was as apparent acceleration based on exactly the same mechanism. Hubble constant depends on the scale of space-time sheet along which radiation arrives and the variation of space-time with scale would give rise to change of Hubble constant. Seems that the material appearing at my homepage becomes "serious" science with a delay of few years;-). I am happy that my homepage is not a show for empty benches.

The recent model which I prefer is based on the observation that the imbeddings of critical and overcritical cosmologies have necessary finite span in time and represent accelerating expansion which can be seen as a direct evidence for the basic prediction of TGD that gravitational interaction is constrained by imbeddability. The mechanism is really elegant and I am still waiting patiently that some brane-theorist discovers this mechanism of accelerated expansion. Perhaps I should improve my communication skills;-).

As I have explained many times, the interpretation would be in terms of quantum transitions increasing gravitational Planck constant.

December 01, 2007 12:57 PM  
Blogger CarlBrannen said...

Strange thing, "Dr. Motl", was recently caught slumming on the Mass post mentioning the variable speed of light and rather shallowly critiquing his reasoning against Louise. (I have a complete proof, but it is too long to fit in the marginal parts of the internet.)

December 02, 2007 5:50 PM  
Blogger Kea said...

LOL, Carl. Maybe someone figured out his password. After all, why would such an esteemed string theorist waste time on the likes of you?

December 03, 2007 7:37 AM  

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