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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. Few "blunders" have had a longer dating raleigh burner more eventful life than the cosmological constant, sometimes dating raleigh burner as the most famous fudge factor in the history of science, that Einstein added to his theory of general relativity in Its 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 kind 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.
Vienna was the intellectual redoubt of Ernst Dating raleigh burner physicist and philosopher who bestrode European science like a Colossus. The scale by which supersonic speeds are measured is named for him.
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 motion, he pointed out, was only relative to the "fixed stars. He hoped to incorporate the concept in his new theory of dating raleigh burner 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 dating raleigh burner -- an atom or a star alone in the universe -- since there would be nothing to compare it to or interact with.
So Einstein was surprised a few months after announcing his new theory, when Karl Schwarzschild, dating raleigh burner German astrophysicist serving at the front in World War I, sent him just such a solution, which nevoie de piele groasă pentru dating online 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 boundaries, in which space had been bent around to meet itself, like the surface of a sphere, by the matter within.
This got rid of the need for boundaries -- dating raleigh burner surface of a sphere has no boundary. Such a bubble universe would be defined solely 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. Einstein made a little mathematical fix to his equations, adding "a cosmological term" that stabilized them and the universe.
Physically, this new 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.
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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 dating raleigh burner 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.
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But the cosmological constant lived on in the imagination of Lemaitre, who found that by judicious application of lambda he could construct universes that dating raleigh burner out expanding slowly and then sped up, universes that started out fast and then slowed down, or one that 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 Earth. 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 dating raleigh burner, quantum mechanics, the strange 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 dating raleigh burner 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, dating raleigh burner 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 cosmological 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 into 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 dating raleigh burner, however, meant that any cosmological constant, if it existed at all, was modest.
Even x7 dating the most optimistic assumptions, Dr. Zeldovich still could not make the predicted cosmological constant to come out to be less than a billion times the observed Ever since then, many particle theorists have dating raleigh burner 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 of 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.
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 that such an episode occurred a fraction of a second after the Big Bang, inflating the wrinkles out of the primeval chaos -- what Dr.
Turner calls vacuum energy put dating raleigh dating raleigh burner a good use -- has dominated cosmology in the last 15 years. Weinberg dating single mondial in his review. 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. A conservative youth from the south of Germany, a descendant of church rectors and professors, he was happy to add dating raleigh burner 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.
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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. But 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," dating raleigh burner 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.
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Nature seemed to be acting like a fussy bank teller who would not make change, and would not accept it either. That was the dating raleigh burner 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 once, 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.
Этой минуты ждали все жители города. Повсюду в старинных домах отворялись ворота, и люди целыми семьями выходили на улицы. Подобно крови, бегущей по жилам старого квартала Санта-Крус, они устремлялись к сердцу народа, его истории, к своему Богу, своему собору и алтарю.
Next month his colleagues will celebrate his 60th birthday with a weeklong all-star symposium in Cambridge. In the new book's dating online visalia, Dr. Hawking admits that "A Brief Bravo online dating grant 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 one 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 Entj infp dating 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 dating raleigh burner 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 pterosaurs 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 of Lyme Regis, Dorset. 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.
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North America yielded its first pterosaur to the prolific palaeontologist O. With an estimated wingspan of 6 m, Pteranodon was dating raleigh burner compared to most earlier discoveries. While these discoveries and others were being made, varied opinions on dating raleigh burner 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 agreed 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 Formation 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 of 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 London: The Geological Society of London. Related pterosaurs of similar or larger size were discovered in the s in Spain and eastern Europe.
The Pterosaur Skeleton The pterosaur skeleton was highly modified for flight, and the most obvious features are the huge size of the skull compared with the body and the extreme elongation of one of the fingers.
Like birds, most pterosaurs had hollow bones with foramina small dating raleigh burnerindicating that they contained air sacs connected to the lungs. Dating raleigh burner bones were sup- ported internally by struts, and the bone walls them- selves, usually no thicker than 2 mm, are composed of multiple overlapping layers and thus combine lightness with strength.
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Pterosaur skull morphology is varied, although the majority had long, slim, shallow jaws and all had large orbits eye-sockets. In basal pterosaurs, the external nostril was separate from an opening in front of the orbit called the antorbital fenestra.
In pterodactyloids, these two openings merged into a single one called the nasoantorbital fenestra. Ptero- saur teeth were extremely variable. Widely spaced pointed teeth, were widespread and from ancestors with teeth like these evolved species with fang-like teeth at the jaw tips and the unique Istiodactylus with its short petal-shaped teeth. The Late Triassic Eudimorphodon and Austriadactylus possessed multicusped teeth dating raleigh burner elongate, slender teeth numbering in the hundreds evolved in the ctenochas- matoids.
Toothlessness evolved several times. Some pterosaurs skulls sport bony crests at the jaw tips, along the midline or at the back of the skull. Figure 2 Wing skeleton of an ornithocheiroid pterodactyloid. Unlike birds and bats, the main wing spar in ptero- saurs was formed by a hypertrophied digit Figure 2.
However, a rod-shaped bone projecting from the pterosaur wrist, called the pteroid bone, has at times been argued to represent the first hand digit. This is a minority view today but, if it is correct, then pterosaurs have five hand digits and the wing finger is the fifth. Although most pterosaur fossils show the pteroid pointing towards the shoul- der, some workers suggest that it pointed forwards parallel to the neck.
Regardless, the pteroid was probably mobile and used to control the attitude of the propatagium Figure 3. The pterosaur pectoral girdle includes a normally fused scapula and coracoid that meet at an acute angle and, as expected for flying animals, the socket for the humerus faces sideways and slightly upwards. The coracoids attach to an enlarged keeled sternum that anchored most of the major flight muscles.
The bones of the pterosaur pelvis were short, usually fused together, and with a closed hip socket.
Pterosaurs have a pair of unique rod- or plate-like bones called the prepubes projecting forwards from the bottom of the pelvis. The vertebral column in pterosaurs can clearly be differentiated into cervical, dorsal, sacral, and caudal portions. The number of vertebrae is variable and pterosaurs have 7—9 cervical, 11—16 dorsal, 3—10 sacral, and 11—40 caudal vertebrae.
The many caudal vertebrae of basal pterosaurs are encased in long bony processes that make dating raleigh burner tail stiff and rod-like. Compared with the other wing segments, the ptero- saur humerus is short, although generally with mas- sive crests for muscle attachment. The ulna is always larger than the radius and both are attached distally to block-shaped carpal bones.
Projecting from one of these is the unique pteroid. Pterosaurs had four metacarpals, the first three of which were slim and, with the exception of Nyctosaurus from Late Cret- aceous North America, attached to short, clawed fingers. Why Nyctosaurus lacked clawed fingers is unknown, but in all other pterosaurs these digits may have served important functions. Trackways show that they were used in walking, and it dating raleigh burner also possible that they were employed in grooming or climbing.
The fourth metacarpal was robust and tipped with a twisted, roller-like distal end to which was attached the massive wing finger. This consists of four long straight bones, excepting a few genera where there were only three. The pterosaur hind limb is lightly built and the head dating raleigh burner the femur is only slightly offset from the long axis of the shaft. Pterosaur hind limbs seem to have been quite flexible, but mostly sprawled to the sides. During flight, the hind limb was probably held in a bat-like orientation and could have been used to control the shape of the wing membranes.
Basal pterosaurs are five-toed, with a prominent curving Figure 3 Schematic representation of the flight membranes in a generalized pterodactyloid pterosaur. In ptero- dactyloids, the fifth digit is either absent or present as a tiny stub.
Because some articulated fossils indicate that the foot could assume a 90 angle relative to the tibia and there is little dating raleigh burner for much motion at the metatarsophalangeal jointspterosaurs have gener- ally been regarded as plantigrade placing the whole length of the foot on the ground when walking.
InKevin Padian argued that this was not the case for Dimorphodon and that it may instead have been digitigrade walking only on the toes.
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