Referat Space and Time

_{Space and Time}

Because of the non-existence of an absolute rest, the lack of an absolute position in space and time is explained !

A Brief History of Time

'A Brief History of Time' is a book that tries to explain the main theories of today physics in a quite 'non-technical' language so everybody can understand them. This book starts at the beginning of science with the Greek philosopher Aristotle and goes on until the youngest theories about our universe like the superstring-theory which needs 10dimensions.

We go about our daily lives understanding almost nothing of the world. We give little thought to the machinery that generates the sunlight that makes life possible, to the gravity that glues us to an Earth that would otherwise send us spinning off into space, or to the atoms of which we are made and on whose stability we fundamentally depend. Except for children, few of us spend much time wondering why nature is the way it is; where the cosmos came from, or whether it was always here; if time will one day flow backward and effects precede causes; or whether there are ultimate limits to what humans can know. Was there a beginning of time? Could time run backwards? Is the universe infinite or does it have boundaries? These are just some of the questions considered in an internationally acclaimed masterpiece which begins by reviewing the great theories of the cosmos from Newton to Einstein, before diving into the secrets which still lie at the heart of space and time.

This book tries to answer at least some of these questions that can be answered

now. To get some answers we can only follow the theories of Stephen Hawking, which are very good explained in his best-seller.

_{Sinking into space-time}

Einstein

As I already mentioned, Einstein developed mathematical equations to define the nature of time and space. These equations had momentous consequences for cosmologists. To begin with, it emerged that time and space were mathematically one and the same thing. And, as a consequence, Newton's explanation of gravity had to be totally revised, accurate as it seemed to be. Einstein argued that two objects do not directly attract each other as Newton has thought; rather, each of the two objects

'A Brief History of Time' was written by Professor Stephen Hawking, who was born in Oxford, Great

_{Britain, on} ^th _{January 9 2.}

He studied physics at Oxford University and went on to pursue his graduate studies at Cambridge. In his early twenties he was diagnosed as having ALS (Amyotrophic Lateral Sclerosis), known in the UK as Motor Neurone Disease. He holds Newton s chair as Lucasian Professor of Mathematics at Cambridge and is widely considered

to be the greatest scientific thinker since Newton and Einstein. In 9 9 he received an Honorary Doctor of

Science degree from Cambridge University and was made a Companion of Honour.

affects time and space, and any gravitational effects are a consequence of this. That was the moment when he found out that space and time are warped.

The Universe as a Sheet

If this concept is difficult to grasp, imagine a heavy object (such as a cannon ball), representing the sun, being placed in  the middle of a taut rubber sheet [], creating a cone-shaped dent all around it - rather reminiscent of the surface of a vortex of swirling water rushing down a plunge hole.

Einstein argued that whenever something heavy bent space-time like this, it would naturally affect the path of anything lighter travelling nearby. So a smaller ball representing the Earth or one of the other planets could be rolled across the stretched rubber sheet representing space-time, towards the dent around the cannon ball sun.

If it was travelling too slowly, it would fall directly into the dent and quickly reach the surface of the sun (just like Newton's apple falling to the surface of the Earth). If it was travelling too fast, it would have its path deflected towards the cannon ball sun, but would only dip into the dent then climb out of the other side, before continuing on

its journey. But at just the right speed, the small  planet ball would be going fast enough not to fall right into the dent, but too slowly to escape it completely. With nothing else to stop it or slow it down, it would find its level on the 'side' of the dent in space-time, rather like a motorcycle stunt rider going round and round the 'wall of death'. It would have found its static orbit around the sun.

Einstein as Idol

The mathematical formula of Einstein could apart  from even describe the orbit of Mercury, what was not possible with Newton's rather simpler equation. This was impressive evidence that Einstein's theory was correct, or at least an improvement on Newton's explanation of gravity. It was natural for physicists to begin to think: if it fits in with Einstein's theories, it is probably going to be true.

Dynamic Space and Time

It was while studying these equations of Einstein s that Lemaitre, a priest and Belgium's most famous astronomer, discovered something which really excited him. One of the consequences of Einstein s maths was that the universe was not static; it was dynamic.

It is simply enough to see why. If time and space are dented' by anything with mass, then, as one body passes another, it will be drawn closer to it.

If the universe is static, then all objects will eventually be drawn to each other; all mass will congregate together at the bottom of the largest dent in space and time.

This was the same problem which had worried Newton when he came up with his theory of gravity; how could all the matter in the universe still be widely spread out after billions of years? Why hadn't it been pulled together by gravity into one conglomerate lump? But, whereas Newton's idea had confined itself to the attraction of objects, Einstein's theory involved the mathematics of how space and time change when an object with mass affects them. Thus Newton's system had no way for the coming together of all objects to be avoided but Einstein's maths did. Einstein needed

space and time to be able to change in the presence of mass. So space and time had to be dynamic, rather than static.

Consequently, space-time, and so the universe, could not remain still; and if it had to change it could only really get bigger or smaller. Hence it ought to be gently expanding or contracting.

Gravitational Effects

It is gravity that governs and shapes the large-scale structure of the universe and thus even time.

The laws of gravity were incompatible with the view held until quite recently that the

universe is unchanging in time: the fact that gravity is always attractive implies that the universe must be either expanding or contracting.

According to general theory of relativity, there  must have been a state of infinite density in the past, the big bang, which would have been an effective beginning of time. (Scientists today generally agree on an age of the universe of about 13 billion years.)

Similarly, if the whole universe recollapsed, there must be another state of infinite density in the future, the big crunch, which would be an end of time. Even if the whole universe did not recollapse, there would be singularities in any localised regions that collapsed to form black holes. These singularities would be an end of time for anyone who fell into the black hole.

In every case there are certain locations in space that effect time, seen from an different, innocent, and independent observer.

Solutions?

Schwarzschild calculated a solution to Einstein's equations where the time dilatation is infinite from a certain radius on, out of which evolved the idea of black holes. Wheeler found out that Schwarzschild´s solution included a singularity, but also proved its possibility. (The first black hole was found in 19 4 in the system Cygnus X-

Hubble proved the even expanding of space, which allowed to calculate the Big Bang. Einstein therefore introduced a new term into his theory of relativity, the "cosmic term", which he later thought of as his biggest error, because it would have caused the universe to become unstable.

In the search for "theories of everything", which try to unite relativity and quantum mechanics, the possibility of a cosmic term returned.

A Unifying Theory

When scientists like Stephen Hawking combine quantum mechanics, with general relativity, there seems to be a new possibility for him that did not arise before: that space and time together might form a finite, four-dimensional space without singularities or boundaries, like the surface of the earth but with more dimensions. It seems that this idea could explain many of the observed features of the universe, such as its large-scale uniformity and also the smaller-scale departures from

homogeneity, like galaxies, stars, and even human beings. It could even account for the arrow of time that we observe.

At the end of A Brief History of Time Stephen Hawking concludes that, if we do discover a complete theory that could describe everything, its basic principles and implications should in time be understandable by everyone. And once we all understand the true nature of the universe, we all, philosophers, scientists and just ordinary people, can take part in the discussion of the question of why it is that we and the universe exist. Should we ever resolve this question, he suggests, it will be

'the ultimate triumph of human reason - for then we would know the mind of God'. Perhaps, for many of us, that challenge will seem a step too far. There are millions of

us who have never before got close to discovering the nature of the universe. We may just have not tried; more likely we were convinced that it was beyond our limited capacity to understand.

But I think that our opinion is changing. Changing towards knowing more and more about the world and universe we live in. And that is the reason for me to belief that in no way research in this field will find a sudden end.

_{Already Yogi Berra said :}

_{"It ain't over till it's over"}

Dreams

I  think that these concepts will come to seem as natural to the next generation as the idea that the world is round. Imaginary time is already a commonplace of science fiction. But it is more than science fiction or a mathematical trick. It is something that shapes the universe we live in.

An example of how the human race could cope with the progress made in all scientific directions is given in Star Trek. It shows how much we know already and how much we will be able to do with our knowledge in the near future.

The Physics of Star Trek

It is a popular science book, trying to tell most modern science in a simple language. ' The Physics of Star Trek' is a book to be read many times as long it is up-to-date with our time (till we cross the milky ways of our and other galaxies). It offers a lot of exotic science to anyone who wants to make a small investment of imagination. Perhaps accidentally, Krauss also does a useful job in explaining some important physics, using Star Trek as a pop culture example: the physics of Newton, Einstein and Stephen Hawking all figure in the highly successful analysis. It is a book on physics, but it is written in such a spirit of fun, it might even make you want to watch Star Trek. This book is fun, and Mr. Krauss has a nice touch with a tough subject. Krauss is smart, but speaks and writes the common tongue.

'The  Physics of Star Trek' was written by Lawrence M. Krauss. He is Ambrose Swasey Professor of Astronomy and Chairman of the Department of Physics at Case Western Reserve University. He is the author of two acclaimed books, Fear of Physics: A Guide for the Perplexed and The Fifth Essence: The Search for Dark Matter in the Universe, and over 1 0 scientific articles.

He is the recipient of several international awards for his work, including the Presidential Investigator Award, given by President Reagan in 9 6. He lectures extensively to both lay and professional audiences and frequently appears on radio and television.

In this entertaining book the physics professor Lawrence Krauss looks at how the imaginary science of the Star Trek universe stacks up against the real thing. Krauss speculates on the possibility of alien life, touching on whether any kind of life is such an improbable phenomenon.

There are impressively clear explanations of difficult and up-to-date concepts in information theory, quantum mechanics, particle physics, relativity, mechanics and cosmology. The book goes where not even the show's laudable tradition of scientific evangelism has gone before.