Jeans-Jeffreys tidal hypothesis November 8, 2009

Jeans-Jeffreys tidal hypothesis

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Schematic representation of the Jeans-Jeffreys tidal hypothesis. (a) A tidal bulge is induced. (b) A filament of material is drawn out in which condensations form. (c) The produced protoplanets orbit the Sun with high eccentricities. Image credit: Stephen Oxley, doctoral thesis, Univ. of York, 1999

Championed by James Jeans and Harold Jeffreys, this explained the origin of the solar system as a result of a close encounter between the Sun and a second star. However, it differed significantly from the other major catastrophic hypothesis of the 20th century, the Chamberlin-Moulton planetesimal hypothesis.

As a result of a detailed mathematical analysis, Jeans concluded in 1916 that the tidal interaction between the Sun and a passing star would raise tides on the Sun resulting in the loss of a single cigar-shaped filament of hot gas, rather than separate streams of gas as in the Chamberlin and Moulton scenario. This hot gas would then condense directly into the planets instead of going through a planetesimal stage. The central section of the “cigar” would give rise to the largest planets –Jupiter and Saturn – while the tapering ends would provide the substance for the smaller worlds.

This model had important repercussions for the possibility of life elsewhere in the universe because if planetary systems came about only as a result of freak stellar encounters, there would be relatively few extrasolar worlds to provide biological platforms. In his 1923 lecture “The Nebular Hypothesis and Modern Cosmogony, Jeans said:

Astronomy does not know whether or not life is important in the scheme of things, but she begins to whisper that life must necessarily be somewhat rare.

By the late 1920s, this opinion was shared by many astronomers. However, in 1935, Henry NorrisRussell raised what would become fatal objections to the Jeans-Jeffreys hypothesis. He pointed out that it was hard to see how a close stellar encounter could leave the Sun, which is a thousand times more massive than the planets, with such a tiny share of the solar system’sangular momentum. Furthermore, he could not understand how the planets could condense out of hot material ejected from the Sun. The former objection was put into stronger form by Russell himself in 1943, while the latter was strengthened by Russell’s student, Lyman Spitzer, in 1939.

Germ theory October 29, 2009

Germ theory

Pasteur demonstrated that fermentation is caused by the growth of microorganisms, and that the emergent growth of bacterium in nutrient broths is not due to spontaneous generation but rather to biogenesis (Omne vivum ex ovo).

Bottle en col de cygne (Swan neck duct) used by Pasteur

Institut Pasteur de Lille
He exposed boiled broths to air in vessels that contained a filter to prevent all particles from passing through to the growth medium, and even in vessels with no filter at all, with air being admitted via a long tortuous tube that would not allow dust particles to pass. Nothing grew in the broths unless the flasks were broken open; therefore, the living organisms that grew in such broths came from outside, as spores on dust, rather than spontaneously generated within the broth. This was one of the last and most important experiments disproving the theory of spontaneous generation. The experiment also supported germ theory.
While Pasteur was not the first to propose germ theory (Girolamo Fracastoro, Agostino Bassi, Friedrich Henle and others had suggested it earlier), he developed it and conducted experiments that clearly indicated its correctness and managed to convince most of Europe it was true.[6] Today he is often regarded as the father of germ theory and bacteriology, together with Robert Koch.
Pasteur’s research also showed that the growth of microorganisms was responsible for spoiling beverages, such as beer, wine and milk. With this established, he invented a process in which liquids such as milk were heated to kill most bacteria and molds already present within them.[8] He and Claude Bernard completed the first test on April 20, 1862. This process was soon afterwards known as pasteurization.
Beverage contamination led Pasteur to the idea that microorganisms infecting animals and humans cause disease. He proposed preventing the entry of microorganisms into the human body, leading Joseph Lister to develop antiseptic methods in surgery.[6]
In 1865, two parasitic diseases called pébrine and flacherie were killing great numbers of silkworms at Alais (now Alès). Pasteur worked several years proving it was a microbe attacking silkworm eggs which caused the disease, and that eliminating this microbe within silkworm nurseries would eradicate the disease.
Pasteur also discovered anaerobiosis, whereby some microorganisms can develop and live without air or oxygen, called the Pasteur effect.