In 1940, when the idea of anti-matter was nothing more than a mass of mathematical equations, the Russian scientist Vladimir Rojansky suggested the possibility of the existence in outer space of contraterrene meteorites (contraterrene, CT, or its phonetic transcription Seetee are obsolete terms for anti-matter, and were once very popular in science fiction; the ordinary matter was called terrene). Rojansky also said that such a meteorite 'would be entirely radiated away before reaching the sea-level'.
During the same year, The New York Times reported on 15 September: 'As the 22-foot cutter-type sailboat Rockit II was crossing Long Island Sound near Bridge port, Conn., yesterday morning with four peaceful persons aboard, a shell screeched across her bow and exploded in the water 100 yards away.' A passenger recalled: 'It was a most disquieting experience ... The screech came first - an unholy noise. Then, a split second later, the explosion, about two points off the starboard bow. It blew up a great tower of water, twenty or thirty feet in the air. It was the strangest thing in the middle of the peaceful Sound. Why, there wasn't even a boat in sight! And not an airplane overhead!'
Authorities investigated the incident and found that no artillery shell could have exploded near the boat. As a meteorite on striking water would not explode, many astronomers of the time speculated that the explosion may have been due to the fall of a tiny contraterrene meteorite. In a comment on the Rockit II mystery in Popular Astronomy, Samuel Herrick Jr, an astronomer at the University of California, supported the contraterrene meteorite hypothesis and said that Dirac and other scientists 'are to be congratulated on one of the most ingenious (and entertaining) hypotheses of recent years'. He also warned his fellow astronomers that they 'will have to distinguish between the highly explosive fireballs or bolides from which no material reaches the ground, and which accordingly may be contraterrene, and those which are the source of terrestrial meteorites'.
This debate on contraterrene meteorites prompted Lincoln La Paz, a leading American meteorite expert who took a keen interest in Kulik's expeditions to Tunguska and co-translated many of his papers into English, to suggest in 1941 that the Tunguska meteorite was contraterrene in nature because of the great amount of energy released, the absence of impact craters, and the absence of nickel-iron positively attributed to meteorites. 'If a contraterrene iron meteorite of a size comparable to those of the largest irons conjectured to have fallen should strike the Earth', he said, 'an extremely powerful explosion would result, since, in addition to the large store of heat energy resulting from the transformation of the kinetic energy of motion of the meteoritic mass, a vast amount of energy would be liberated by its annihilation'. He pointed out that no original meteorite material would remain at the site of the explosion.
Herrick and La Paz's explanations generated a somewhat angry response from Harvey N. Ninniger, a noted meteorite expert and the President of the American Society for Research on Meteorites. He said that both phenomena could be explained by demonstrated facts without assuming the existence of any such 'purely hypothetical material', and 'we are surely courting a return to the days of "spirits and mystery" when we shrink from painstaking (or even back-breaking) investigations and seek refuge in untried hypotheses, especially when those hypotheses rest entirely on assumptions!'. (In 1928, Ninniger had urged American scholarly associations to send an expedition to Siberia 'to secure what is yet available of this greatest message from the depths of space that has ever reached this planet'. No one showed any interest.)
When scientists as distinguished as Willard Libby, who had developed the carbon-14 dating technique, and his colleagues Clyde Cowan and C.R. Alturi suggested in 1965 that the Tunguska object had been composed of anti-matter, they were probably not in danger of returning to the days of 'spirits and mystery'. Since Ninniger's warning in 1941 a lot had been discovered about anti-matter. Although its existence had yet to be experimentally proved, anti-matter was no longer considered a 'purely hypothetical material'.
In their detailed research paper in Nature, the three American scientists ruled out the possibility of a nuclear fission or fusion reaction and argued in favour of the anti-matter hypothesis. They said that neither fission nor fusion could explain the observed effects of the Tunguska explosion. To start a fission chain reaction (in which a heavy atomic nucleus splits into lighter nuclei), a critical mass of a fissionable material such as uranium or plutonium is required. The multi-megaton blast at Tunguska would require a large initial mass - well above the critical mass - which seems unlikely. On the other hand, fusion (in which lighter atomic nuclei combine to form a heavier nucleus) requires a sufficient amount of tightly packed deuterium that must be heated to several million degrees Celsius. Such a high temperature could not be obtained just by entry into the atmosphere.
The anti-matter hypothesis could explain the high nuclear energy yield of the Tunguska blast, but the researchers were quick to point out that 'several objections immediately arise' to this hypothesis. Two main objections were: (a) the lack of evidence for the existence of anti-matter; and (b) the anti-matter object would start disintegrating the moment it entered the atmosphere, and its largest yield of energy would be somewhere towards the middle of the path, rather than towards its end. 'A second look at the process tempers these conclusions, however', they said. Of the three models for nuclear explosion, they decided in favour of the annihilation of an anti-rock in the atmosphere. Their calculations showed that if the Tunguska explosion had been due to an anti-rock, it should have behaved like a 35-megaton fission or fusion bomb. The explosion would also have generated trillions of radioactive carbon-14 atoms.
As the feasibility of the American trio's anti-matter hypothesis depended upon the discovery of large amounts of radioactive carbon-14 in trees, they analysed carbon-14 content in sections of a 300-year-old fir tree that fell in 1951 in Tucson, Arizona, and an oak tree cut in 1964 near Los Angeles. They took nearly 90,000 counts of carbon-14 in tree rings from 1870 to 1930, which showed that the count peaked in 1909. However, the increase was much smaller than they had predicted. Their conclusion: although there are uncertainties, 'the data do yield a positive result'.
Recent measurements also show a rise in carbon-14, but not enough to support the idea of annihilation caused by a nuclear explosion, whether it be fission, fusion or anti-matter.
A decade after the publication of the anti-matter hypothesis, Hall Crannell of the Catholic University of America looked at other ways of measuring the antimatter content of the Tunguska object. He said that silicon, and to some extent aluminium, are abundant elements in rocks, and when the Tunguska anti-rock hit the ground, ordinary aluminium was converted into radioactive aluminium-26. If the aluminium-26 content of rocks or soil is measured as a function of the distance from the centre of the explosion, he suggested, the highest concentration of aluminium-26 should be found near the centre. No one has yet carried out such measurements.
The British astronomer David Hughes rejected the anti-matter hypothesis on the ground that 'it is hard to understand how it penetrated to such a depth in the atmosphere and why the explosion maximised at the end of the trajectory and not midway along it'.
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