Pokrewne
- Strona Główna
- Carter Stephen L. Elm Harbor 01 Władca Ocean Park
- Stephen King Mroczna Wieza t3 Ziemie jalowe (rtf)
- Clarke Arthur C. Baxter Stephen Swiatlo minionych dni (2)
- McIntosh Fiona Trojca 02 Zemsta (CzP)
- Spellman Cathy Cash Malowane wiatrem 02
- Terry Pratchett 11 Kosiarz
- Dukaj Jacek Czarne oceany (SCAN dal 758) (2
- Stefan Zweig Magellan
- Reymont Chlopi I
- Rice Anne Sluga kosci
- zanotowane.pl
- doc.pisz.pl
- pdf.pisz.pl
- metta16.htw.pl
[ Pobierz całość w formacie PDF ]
.Yet so strong was the belief in a static universe that it persisted into the early twentieth century.Even Einstein,when he formulated the general theory of relativity in 1915, was so sure that the universe had to be static thathe modified his theory to make this possible, introducing a so-called cosmological constant into his equations.Einstein introduced a new antigravity force, which, unlike other forces, did not come from any particularsource but was built into the very fabric of space-time.He claimed that space-time had an inbuilt tendency toexpand, and this could be made to balance exactly the attraction of all the matter in the universe, so that astatic universe would result.Only one man, it seems, was willing to take general relativity at face value, andwhile Einstein and other physicists were looking for ways of avoiding general relativity s prediction of anonstatic universe, the Russian physicist and mathematician Alexander Friedmann instead set about explainingit.Friedmann made two very simple assumptions about the universe: that the universe looks identical inwhichever direction we look, and that this would also be true if we were observing the universe from anywhereelse.From these two ideas alone, Friedmann showed that we should not expect the universe to be static.Infact, in 1922, several years before Edwin Hubble s discovery, Friedmann predicted exactly what Hubble found!The assumption that the universe looks the same in every direction is clearly not true in reality.For example, aswe have seen, the other stars in our galaxy form a distinct band of light across the night sky, called the MilkyWay.But if we look at distant galaxies, there seems to be more or less the same number of them.So theuniverse does seem to be roughly the same in every direction, provided one views it on a large scale comparedto the distance between galaxies, and ignores the differences on small scales.For a long time, this wassufficient justification for Friedmann s assumption as a rough approximation to the real universe.But morerecently a lucky accident uncovered the fact that Friedmann s assumption is in fact a remarkably accuratefile:///C|/WINDOWS/Desktop/blahh/Stephen Hawking - A brief history of time/b.html (3 of 9) [2/20/2001 3:14:24 AM] A Brief History of Time - Stephen Hawking.Chapter 3description of our universe.In 1965 two American physicists at the Bell Telephone Laboratories in New Jersey, Arno Penzias and RobertWilson, were testing a very sensitive microwave detector.(Microwaves are just like light waves, but with awavelength of around a centimeter.) Penzias and Wilson were worried when they found that their detector waspicking up more noise than it ought to.The noise did not appear to be coming from any particular direction.First they discovered bird droppings in their detector and checked for other possible malfunctions, but soonruled these out.They knew that any noise from within the atmosphere would be stronger when the detectorwas not pointing straight up than when it was, because light rays travel through much more atmosphere whenreceived from near the horizon than when received from directly overhead.The extra noise was the samewhichever direction the detector was pointed, so it must come from outside the atmosphere.It was also thesame day and night and throughout the year, even though the earth was rotating on its axis and orbiting aroundthe sun.This showed that the radiation must come from beyond the Solar System, and even from beyond thegalaxy, as otherwise it would vary as the movement of earth pointed the detector in different directions.In fact, we know that the radiation must have traveled to us across most of the observable universe, and sinceit appears to be the same in different directions, the universe must also be the same in every direction, if onlyon a large scale.We now know that whichever direction we look, this noise never varies by more than a tinyfraction: so Penzias and Wilson had unwittingly stumbled across a remarkably accurate confirmation ofFriedmann s first assumption.However, because the universe is not exactly the same in every direction, butonly on average on a large scale, the microwaves cannot be exactly the same in every direction either.Therehave to be slight variations between different directions.These were first detected in 1992 by the CosmicBackground Explorer satellite, or COBE, at a level of about one part in a hundred thousand.Small though thesevariations are, they are very important, as will be explained in Chapter 8.At roughly the same time as Penzias and Wilson were investigating noise in their detector, two Americanphysicists at nearby Princeton University, Bob Dicke and Jim Peebles, were also taking an interest inmicrowaves.They were working on a suggestion, made by George Gamow (once a student of AlexanderFriedmann), that the early universe should have been very hot and dense, glowing white hot [ Pobierz całość w formacie PDF ]
zanotowane.pl doc.pisz.pl pdf.pisz.pl agnieszka90.opx.pl
.Yet so strong was the belief in a static universe that it persisted into the early twentieth century.Even Einstein,when he formulated the general theory of relativity in 1915, was so sure that the universe had to be static thathe modified his theory to make this possible, introducing a so-called cosmological constant into his equations.Einstein introduced a new antigravity force, which, unlike other forces, did not come from any particularsource but was built into the very fabric of space-time.He claimed that space-time had an inbuilt tendency toexpand, and this could be made to balance exactly the attraction of all the matter in the universe, so that astatic universe would result.Only one man, it seems, was willing to take general relativity at face value, andwhile Einstein and other physicists were looking for ways of avoiding general relativity s prediction of anonstatic universe, the Russian physicist and mathematician Alexander Friedmann instead set about explainingit.Friedmann made two very simple assumptions about the universe: that the universe looks identical inwhichever direction we look, and that this would also be true if we were observing the universe from anywhereelse.From these two ideas alone, Friedmann showed that we should not expect the universe to be static.Infact, in 1922, several years before Edwin Hubble s discovery, Friedmann predicted exactly what Hubble found!The assumption that the universe looks the same in every direction is clearly not true in reality.For example, aswe have seen, the other stars in our galaxy form a distinct band of light across the night sky, called the MilkyWay.But if we look at distant galaxies, there seems to be more or less the same number of them.So theuniverse does seem to be roughly the same in every direction, provided one views it on a large scale comparedto the distance between galaxies, and ignores the differences on small scales.For a long time, this wassufficient justification for Friedmann s assumption as a rough approximation to the real universe.But morerecently a lucky accident uncovered the fact that Friedmann s assumption is in fact a remarkably accuratefile:///C|/WINDOWS/Desktop/blahh/Stephen Hawking - A brief history of time/b.html (3 of 9) [2/20/2001 3:14:24 AM] A Brief History of Time - Stephen Hawking.Chapter 3description of our universe.In 1965 two American physicists at the Bell Telephone Laboratories in New Jersey, Arno Penzias and RobertWilson, were testing a very sensitive microwave detector.(Microwaves are just like light waves, but with awavelength of around a centimeter.) Penzias and Wilson were worried when they found that their detector waspicking up more noise than it ought to.The noise did not appear to be coming from any particular direction.First they discovered bird droppings in their detector and checked for other possible malfunctions, but soonruled these out.They knew that any noise from within the atmosphere would be stronger when the detectorwas not pointing straight up than when it was, because light rays travel through much more atmosphere whenreceived from near the horizon than when received from directly overhead.The extra noise was the samewhichever direction the detector was pointed, so it must come from outside the atmosphere.It was also thesame day and night and throughout the year, even though the earth was rotating on its axis and orbiting aroundthe sun.This showed that the radiation must come from beyond the Solar System, and even from beyond thegalaxy, as otherwise it would vary as the movement of earth pointed the detector in different directions.In fact, we know that the radiation must have traveled to us across most of the observable universe, and sinceit appears to be the same in different directions, the universe must also be the same in every direction, if onlyon a large scale.We now know that whichever direction we look, this noise never varies by more than a tinyfraction: so Penzias and Wilson had unwittingly stumbled across a remarkably accurate confirmation ofFriedmann s first assumption.However, because the universe is not exactly the same in every direction, butonly on average on a large scale, the microwaves cannot be exactly the same in every direction either.Therehave to be slight variations between different directions.These were first detected in 1992 by the CosmicBackground Explorer satellite, or COBE, at a level of about one part in a hundred thousand.Small though thesevariations are, they are very important, as will be explained in Chapter 8.At roughly the same time as Penzias and Wilson were investigating noise in their detector, two Americanphysicists at nearby Princeton University, Bob Dicke and Jim Peebles, were also taking an interest inmicrowaves.They were working on a suggestion, made by George Gamow (once a student of AlexanderFriedmann), that the early universe should have been very hot and dense, glowing white hot [ Pobierz całość w formacie PDF ]