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As long ago as 340 BC the Greek philosopher Aristotle, in his book On the Heavens, was able to put forward two good arguments for believing that the earth was a round sphere rather than a flat plate.
As we shall see, the concept of time has no meaning before the beginning of the universe.
in 1929, Edwin Hubble made the landmark observation that wherever you look, distant galaxies are moving rapidly away from us. In other words, the universe is expanding.
For example, Aristotle’s theory that everything was made out of four elements, earth, air, fire, and water, was simple enough to qualify, but it did not make any definite predictions.
The eventual goal of science is to provide a single theory that describes the whole universe.
Today scientists describe the universe in terms of two basic partial theories – the general theory of relativity and quantum mechanics.
if there really is a complete unified theory, it would also presumably determine our actions. And so the theory itself would determine the outcome of our search for it!
his value for the speed of light was 140,000 miles per second, compared to the modern value of 186,000 miles per second.
The fundamental postulate of the theory of relativity, as it was called, was that the laws of science should be the same for all freely moving observers, no matter what their speed.
object approaches the speed of light, its mass rises ever more quickly, so it takes more and more energy to speed it up further. It can in fact never reach the speed of light, because by then its mass would have become infinite, and by the equivalence of mass and energy, it would have taken an infinite amount of energy to get it there. For this reason, any normal object is forever confined by relativity to move at speeds slower than the speed of light. Only light, or other waves that have no intrinsic mass, can move at the speed of light.
We must accept that time is not completely separate from and independent of space, but is combined with it to form an object called space-time.
(This is rather like watching an airplane flying over hilly ground. Although it follows a straight line in three-dimensional space, its shadow follows a curved path on the two-dimensional ground.)
not until 1919 that a British expedition, observing an eclipse from West Africa, showed that light was indeed deflected by the sun, just as predicted by the theory. This proof of a German theory by British scientists was hailed as a great act of reconciliation between the two countries after the war.
In the theory of relativity there is no unique absolute time, but instead each individual has his own personal measure of time that depends on where he is and how he is moving.
The old idea of an essentially unchanging universe that could have existed, and could continue to exist, forever was replaced by the notion of a dynamic, expanding universe that seemed to have begun a finite time ago, and that might end at a finite time in the future.
IF ONE LOOKS at the sky on a clear, moonless night, the brightest objects one sees are likely to be the planets Venus, Mars, Jupiter, and Saturn.
The nearest star, called Proxima Centauri, is found to be about four light-years away
Our sun, for comparison, is a mere eight light-minutes away!
The present evidence therefore suggests that the universe will probably expand forever, but all we can really be sure of is that even if the universe is going to recollapse, it won’t do so for at least another ten thousand million years, since it has already been expanding for at least that long.
In other words, one has a singularity contained within a region of space-time known as a black hole.
quantum mechanics does not predict a single definite result for an observation. Instead, it predicts a number of different possible outcomes and tells us how likely each of these is.
ARISTOTLE BELIEVED THAT all the matter in the universe was made up of four basic elements – earth, air, fire, and water.
(The word atom means ‘indivisible’ in Greek.)
A star is formed when a large amount of gas (mostly hydrogen) starts to collapse in on itself due to its gravitational attraction.
In other words, the singularities produced by gravitational collapse occur only in places, like black holes, where they are decently hidden from outside view by an event horizon.
he may be able to avoid hitting the singularity and instead fall through a ‘wormhole’ and come out in another region of the universe. This would offer great possibilities for travel in space and time,
cosmic censorship hypothesis states that in a realistic solution, the singularities would always lie either entirely in the future (like the singularities of gravitational collapse) or entirely in the past (like the big bang).
The rate of energy loss in the case of the earth and the sun is very low – about enough to run a small electric heater. This means it will take about a thousand million million million million years for the earth to run into the sun, so there’s no immediate cause for worry!
It is a matter of common experience that disorder will tend to increase if things are left to themselves. (One has only to stop making repairs around the house to see that!)
