By the early s, it was generally agreed that pterosaurs were bat-like flying reptiles and, in , Harry Seeley — published Dragons of the Air, the first book devoted to pterosaurs. We now know that Gideon Mantell — , best known for the discovery of Iguanodon, found pterosaur remains before this in the Early Cretaceous Wealden strata of Sussex, but had thought that these were from birds.
Sunteți pe pagina 1din Căutați în document There are few scientists dating raleigh burner whom it can be said that their mistakes are more interesting than their colleagues' successes, but Albert Einstein was one. Few "blunders" have had a longer and more eventful life than the cosmological constant, sometimes described as the most famous fudge factor in the history of science, that Einstein added to his theory of general relativity in Dating raleigh burner role was to provide a repulsive force in order to keep the universe from theoretically collapsing under its own weight.
Einstein abandoned the cosmological constant when the universe turned out to be expanding, but in succeeding years, the cosmological constant, like Rasputin, has stubbornly refused to die, dragging itself to the fore, whispering of deep enigmas and mysterious new forces in nature, whenever cosmologists have run into trouble reconciling their observations of the universe with their theories.
This year the cosmological constant has been propelled back into the news as an explanation for the widely reported discovery, based on observations of distant exploding stars, that some dating raleigh burner of "funny energy" is apparently accelerating the expansion of the universe.
How did he and his year-old fudge factor come to be at the center of a revolution in modern cosmology? The story begins in Vienna with a mystical concept that Einstein called Mach's principle.
Cornisoane Pseudoantracnozice | Bivalvia | Physical Cosmology
Vienna was the intellectual redoubt of Ernst Macha physicist and philosopher who bestrode European science like a Colossus. The scale by which supersonic speeds are measured is named for him.
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His biggest legacy was philosophical; he maintained that all knowledge came from the senses, and campaigned relentlessly against the introduction of what he considered metaphysical concepts in science, atoms for example.
Mach argued that we do not see "space," only the players in it. All our knowledge of dating raleigh burner, he pointed out, was only relative to the "fixed stars. He hoped to incorporate the concept in his new theory of general relativity, which he completed in That theory describes how matter and energy distort or "curve" the geometry of space and time, producing the phenomenon called gravity. In the language of general relativity, Mach's principle required that the space- time curvature should be determined solely by other matter or energy in the universe, and not any initial conditions or outside influences -- what physicists call boundary conditions.
Among other things, Einstein took this to mean that it should be impossible to solve his equations for the case of a solitary object -- an atom or a star alone in the universe -- since there would be nothing to compare it to or interact with.
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So Einstein was surprised a few months after announcing his new theory, when Karl Schwarzschild, a German astrophysicist serving at the front in World War I, sent him just such a solution, which described the gravitational field around a solitary star. One of his ideas envisioned "distant masses" ringing the outskirts of the Milky Way like a fence. These masses would somehow curl up space and close it off. His sparring partner de Sitter scoffed at that, arguing these "supernatural" masses would not be part of the visible universe.
As such, they were no more palatable than Newton's old idea of absolute space, which was equally invisible and arbitrary. In desperation and laid up with gall bladder trouble in February ofEinstein hit on the idea of a universe without dating raleigh burner, in which space had been bent around to meet itself, like the surface of a sphere, by dating raleigh burner matter within.
This got rid of the need for boundaries -- the surface of a sphere has no boundary. Such a bubble universe would be defined hit bottom rock dating by its matter and energy content, as Machian principles dictated. But there was a new problem; this universe was unstable, the bubble had to be either expanding or contracting.
The Milky Way appeared to be neither expanding nor contracting; its stars did not seem to be going anywhere in particular. Here was where the cosmological constant came in.
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Einstein made a little mathematical fix to his equations, adding "a cosmological term" that stabilized them and the universe. Physically, this dating raleigh burner term, denoted by the Greek letter lambda, represented some kind of long range repulsive force, presumably that kept the cosmos from collapsing under its Admittedly, Einstein acknowledged in his paper, the cosmological constant was "not justified by our actual knowledge of gravitation," but it did not contradict relativity, either.
The happy result was a static universe of the type nearly everybody believed they lived in and in which geometry was strictly determined by matter. This only came about with the lambda term. Michel Janssen, a Boston University physicist and Einstein scholar, pointed out, "Einstein needed the constant not because of his philosophical predilections but because of his prejudice that the universe is static.
In any event, Einstein's new universe soon fell apart. In another 10 years the astronomer Edwin Hubble in California was showing that mysterious spiral nebulae were galaxies far far away and getting farther -- in short that the universe might be expanding. De Sitter further confounded Einstein by coming up with his own solution to Einstein's equations that described a universe that had no matter in it at all. Calculations showed that when test particles were inserted into it, they flew away from each other.
That was the last straw for Einstein.
In the meantime, the equations for an expanding universe had been independently discovered by Aleksandr Friedmann, a young Russian theorist, and by the Abbe Georges Lemaitre, a Belgian cleric and physicist. A year after his visit with Hubble, Einstein threw his weight, along with de Sitter, behind an expanding universe without a cosmological constant. But the cosmological constant lived on in the imagination of Lemaitre, who found that by judicious application of lambda he could construct universes that started out expanding slowly and then sped up, universes that started out fast and then slowed down, or one dating raleigh burner even began expanding, paused, and then resumed again.
This last model beckoned briefly to some astronomers in the early 's, when measurements of the cosmic expansion embarrassingly suggested that the universe was 4only two billion years old -- younger Dating vamal în irak. A group of astronomers visited Einstein in Princeton and suggested that resuscitating the cosmological constant could resolve the age discrepancy. Einstein turned them down, saying that the introduction of the cosmological constant had been the biggest blunder of his life.
George Gamow, one of the astronomers, reported the remark in his autobiography, "My World Line," and it became part of the Einstein legend. Einstein died three years later. In the years after his death, quantum mechanics, the dating raleigh burner set of rules that describe nature on the subatomic level and Einstein's bete noire transformed the cosmological constant and showed just how prescient Einstein had been in inventing it.
The famous and mystical in its own right uncertainty principle decreed that there is no such thing as nothing, and even empty space can be thought of as foaming The effects of this vacuum energy on atoms had been detected in the laboratory, as early asbut no one thought to investigate its influence on the universe as a whole untilwhen a new crisis, an apparent proliferation of too-many quasars when the universe was about one-third its present size, led to renewed muttering about the cosmological constant.
Jakob Zeldovich, a legendary Russian theorist who was a genius at marrying microphysics to the universe, realized that this quantum vacuum energy would enter into Einstein's equations exactly the same as the old dating raleigh burner constant. The problem was that a naive straightforward calculation of these quantum fluctuations suggested that the vacuum energy in the universe should be about orders of magnitude 10 followed by zeros denser than the matter.
In which case the cosmological constant would either have crumpled the universe 20 80 regulament dating a black hole in the first instant of its existence or immediately blown the cosmos so far apart that not even atoms would ever have formed. The fact that the universe had been sedately and happily expanding for 10 billion years or so, however, meant that any cosmological constant, if it existed at all, was modest.
Even making the most optimistic assumptions, Dr. Zeldovich still could not make the predicted dating raleigh burner constant to come out to be less than a billion times the observed Ever since then, many particle theorists have simply assumed that for some as-yet- unknown reason the cosmological constant is zero.
In the era of superstrings and ambitious theories of everything tracing history back to the first micro-micro second of unrecorded time, the cosmological constant has been a trapdoor in the basement speed dating south gippsland 5physics, suggesting that at some fundamental level something is being missed about the world.
In an article in Reviews of Modern Physics inSteven Weinberg of the University of Texas referred to the cosmological constant as "a veritable crisis," whose solution would have a wide impact on physics and astronomy.
Things got even more interesting in the 's with the advent of the current crop of particle physics theories, which feature a shadowy entity known as the Higgs field, which permeates space and gives elementary particles their properties.
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Physicists presume that the energy density of the Higgs field today is zero, but in the past, when the universe was hotter, the Higgs energy could have been enormous and dominated the dynamics of the universe. In fact, speculation dating raleigh burner such an episode occurred a fraction of a second after the Big Bang, inflating the wrinkles out of the primeval chaos -- captură de captură Dr.
Turner calls vacuum energy put to a good use -- has dominated cosmology in the last 15 years. Weinberg wrote in his review.
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In their efforts to provide an explanation, theorists have been driven recently to talk about multiple universes connected by space-time tunnels called wormholes, among other things. The flavor of the crisis was best expressed, some years ago at an astrophysics conference by Dr. Summing up the discussions at the end of the meeting, he came at last to the cosmological constant. The young Planck didn't mind.
Sunteți pe pagina 1din Căutați în document There are few scientists of whom it can be said that their mistakes are more interesting than their colleagues' successes, but Albert Einstein was one.
A conservative youth from the south of Germany, a descendant of church dating covoare antice and professors, he was happy to add to the perfection of what was already known. Instead, he destroyed it, by discovering what was in effect a loose thread that when tugged would eventually unravel the entire fabric of what had passed for reality.
As a new professor at the University of Berlin, Planck embarked in the fall of on a mundane sounding calculation of the spectral characteristics of the glow from a heated object. Physicists had good reason to think the answer would elucidate the relationship between light and matter as well as give German industry a leg up in the electric light business. Dating raleigh burner the calculation had been plagued with difficulties. Planck succeeded in finding the right formula, but at a cost, as he reported to the German Physical Society on Dec.
In what he called "an act of desperation," he had to assume that atoms could only emit energy in discrete amounts that he later called quanta from the Latin quantus for "how much" rather than in the continuous waves prescribed by electromagnetic theory. Nature seemed to be acting like a fussy bank teller who would not make change, and would not accept it either.
That was the first shot in a revolution. Within a quarter of a century, the common sense laws of science had been overthrown. In their place was a bizarre set of rules known as quantum mechanics, in which causes were not guaranteed to be linked to effects; a subatomic particle like an electron could be in two places at dating online grimsby, everywhere or nowhere until someone measured it; and light could be a wave or a particle.
He has moved about in a wheelchair for more than 25 years and now speaks only through a voice synthesizer. Hawking, for whom the word "puckish" seems to have been invented, has often said his disability is an advantage because it frees him to sit and think. Next month his colleagues will celebrate his 60th birthday with a weeklong all-star symposium in Cambridge.
In the new book's introduction, Dr. Hawking admits that "A Brief History of Time" was "not easy going" and laments that some readers got stuck and did not finish it. He has tried, he says, to make this dating raleigh burner easier. Slightly longer than the earlier book, "Nutshell," at pages, is embellished with colorful illustrations that give it a coffee-table- book look. So far the critics are in qualified agreement; one, Bryan Appleyard in the The New he Bivalvia, the second largest class within the Solnhofen Limestone of Eichsta tt, Germany, and was described by Cosimo Collini — in Collini concluded that it was a possible sea creature of unknown affinity, although he did note bat-like features.
Inthe great French anatomist Georges Cuvier — recognized that the creature was a reptile and that its elongated digits must have supported flight membranes.
This later became the generic name Pterodactylus Figures 1 and 4. In the decades that followed, a succession of further dating raleigh burner from the Solnhofen Limestone was an- nounced, many in a spectacular state of preservation and some with their wing membranes intact.
The first recognized British pterosaur, a specimen of the deep-skulled Dimorphodon, was discovered by Mary Anning — in in Lower Jurassic rocks dating raleigh burner Lyme Lent dating sheffield, Dorset.
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We now know that Gideon Mantell —best known for the discovery of Iguanodon, found pterosaur remains before this in the Early Cretaceous Wealden strata of Sussex, but had thought that these were from birds. North America yielded its first pterosaur to the prolific palaeontologist O.
With an estimated wingspan of 6 m, Pteranodon was huge compared to most earlier discoveries.
While these discoveries and others were being made, varied opinions on the nature and life style of pterosaurs were appearing, and they were variously depicted as swimming creatures, as bats, marsupials, or as kin of birds. By the early s, it was generally dating raleigh burner that pterosaurs were bat-like flying reptiles and, inHarry Seeley — published Dragons of the Air, the first book devoted to pterosaurs. South American Cretaceous pterosaurs have proved to be among the most important in the world, but not until was the first pterosaur from the now famous Santana Dating raleigh burner of Brazil discovered.
Since then a significant number of new kinds from around the world around 70 genera are presently recognized have revealed previously unimagined morphologies and maximum sizes. UntilPteranodon sternbergi Figure 1 Life restoration dating raleigh burner the Late Jurassic pterodactyloid, Pterodactylusfrom the German Solnhofen Limestone in a quadrupedal stance.
Note the presence of body hair and the soft tissue head crest. Reproduced with permission from Dino Frey. Geological Society Special Publication