(ebook Pdf) Stephen Hawking A Brief History Of Time

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.They may sometimes be emitted when matter particles interact with each other by exchanging virtualforce-carrying particles.(For example, the electric repulsive force between two electrons is due to the exchange of virtualphotons, which can never be directly detected; but if one electron moves past another, real photons may be given off,which we detect as light waves.)Force-carrying particles can be grouped into four categories according to the strength of the force that they carry and theparticles with which they interact.It should be emphasized that this division into four classes is man-made; it isconvenient for the construction of partial theories, but it may not correspond to anything deeper.Ultimately, mostphysicists hope to find a unified theory that will explain all four forces as different aspects of a single force.Indeed, manywould say this is the prime goal of physics today.Recently, successful attempts have been made to unify three of thefour categories of force and I shall describe these in this chapter.The question of the unification of the remainingcategory, gravity, we shall leave till later.The first category is the gravitational force.This force is universal, that is, every particle feels the force of gravity,according to its mass or energy.Gravity is the weakest of the four forces by a long way; it is so weak that we would notnotice it at all were it not for two special properties that it has: it can act over large distances, and it is always attractive.This means that the very weak gravitational forces between the individual particles in two large bodies, such as the earthand the sun, can all add up to produce a significant force.The other three forces are either short range, or are sometimesattractive and some-times repulsive, so they tend to cancel out.In the quantum mechanical way of looking at thegravitational field, the force between two matter particles is pictured as being carried by a particle of spin 2 called thegraviton.This has no mass of its own, so the force that it carries is long range.The gravitational force between the sunand the earth is ascribed to the exchange of gravitons between the particles that make up these two bodies.Although theexchanged particles are virtual, they certainly do produce a measurable effect they make the earth orbit the sun! Realgravitons make up what classical physicists would call gravitational waves, which are very weak and so difficult todetect that they have not yet been observed.The next category is the electromagnetic force, which interacts with electrically charged particles like electrons andquarks, but not with uncharged particles such as gravitons.It is much stronger than the gravitational force: theelectromagnetic force between two electrons is about a million million million million million million million (1 with forty-twozeros after it) times bigger than the gravitational force.However, there are two kinds of electric charge, positive andnegative.The force between two positive charges is repulsive, as is the force between two negative charges, but theforce is attractive between a positive and a negative charge.A large body, such as the earth or the sun, contains nearlyequal numbers of positive and negative charges.Thus the attractive and repulsive forces between the individual particlesnearly cancel each other out, and there is very little net electromagnetic force.However, on the small scales of atomsand molecules, electromagnetic forces dominate.The electromagnetic attraction between negatively charged electronsand positively charged protons in the nucleus causes the electrons to orbit the nucleus of the atom, just as gravitationalattraction causes the earth to orbit the sun.The electromagnetic attraction is pictured as being caused by the exchangeof large numbers of virtual massless particles of spin 1, called photons.Again, the photons that are exchanged are virtualparticles.However, when an electron changes from one allowed orbit to another one nearer to the nucleus, energy isreleased and a real photon is emitted which can be observed as visible light by the human eye, if it has the rightwave-length, or by a photon detector such as photographic film.Equally, if a real photon collides with an atom, it maymove an electron from an orbit nearer the nucleus to one farther away.This uses up the energy of the photon, so it isabsorbed.The third category is called the weak nuclear force, which is responsible for radioactivity and which acts on all matterparticles of spin-�, but not on particles of spin 0, 1, or 2, such as photons and gravitons.The weak nuclear force was notwell understood until 1967, when Abdus Salam at Imperial College, London, and Steven Weinberg at Harvard bothfile:///C|/WINDOWS/Desktop/blahh/Stephen Hawking - A brief history of time/d.html (4 of 8) [2/20/2001 3:14:54 AM]A Brief History of Time - Stephen Hawking [ Pobierz całość w formacie PDF ]

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