Forces of Nature

So the question is what are the the 'basic building blocks' from which everything is made? Since the wavelength of light is much larger than the size of an atom, we cannot expect to look at at the parts of an atom in the ordinary way. We need to use something with a much smaller wavelength. Quantum mechanics tells us that all particles are waves, and the higher the energy of a particle, the smaller the wavelength of the corresponding wave. The best answer to our question depends on how high a particle energy we have at our disposal, because it determines on how small a length scale we look. These particle energies are measured in electron volts (the energy that an electron gains from a electric field of one volt is what is called an electron volt.) Recently discovered was the use of electromagnetic fields to generate thousands of millons of electron volts. And so we know that particles that were thought to be elementary are in fact made up of smaller particles. We may be able to go to even higher energies, and even smaller particles could be found.

Using wave particle theory, everything in the universe, including light and gravity can be described in terms of particles. These particles have a property called spin. One way of thinking of spin is to imagine particles as little tops spinning about an axis. This can be misleading, because quantum mechanics tells particles do not have a well defined axis. What the spin of a particle really tells us is what the particle looks like from different directions. A particle of spin 0 is like a dot: it looks the same from every direction. A particle of spin 1 is like an arrow: it looks different from different directions. Only if one turns it round a complete revolution 360 degrees does the particle look the same. A particle of spin 2 is lie a double headed arrow it looks the same if rotate in a half revolution 180 degrees. Similarly higher spins look the same if rotated in smaller fractions of a revolution. There are particles that require 2 revolutions to look the same and particles are said to have spin 1/2.

All the known particles in the universe can be divided into two groups: particles of spin 1/2, which make up the universe, and the particles of spin 0,1 and 2, which give rise to forces between the matter particles. The matter particles obey the Pauli exclusion principle: - two similar particles cannot exist in the same state, that is they cannot have the same position and the same velocity, within the limits of the Heisenberg uncertainty principle. The exclusion principle is important because it explains why matter particles do not collapse to a state of very high density under the influence of the influence of the forces produced by the particles of spin 0, 1 and 2: if the matter particles have very nearly the same positions, they must have very different velocities. If the world had been created without the exclusion principle, quarks would not have form separate, well-defined protons, neutrons, electrons and form separate well-defined atoms. They would all collapse to form roughly uniform, dense 'soup'.

Dirac's theory was described mathematically and was consistent with quantum mechanics & special theory of relativity and explained why the electron had a spin of 1/2 and didn't look the same unless rotated by 720 degrees. It also predicted that the electron should have a partner: an antielectron or positron. We now know that every particle has an antiparticle, with which it can annihilate. Thus antiworlds & antipeople made out of antiparticles can be formed. However if you meet your antiself you would not want to shake hands! As you would both vanish.

In quantum mechanics, the forces or interactions between matter particles are all supposed to be carried by particles of integer - 0, 1 or 2. What happens is that a matter particle, such as an electron or a quark, emits a force-carrying particle. The recoil from the emission changes the velocity of the matter particle. The force carrying particle then collides with another matter particle. The force-carrying particle then collides with another matter particle and is absorbed. This collision changes the velocity of the second particle, just as if there had been a force between the two matter particles.

It is an important property of the force-carrying particles that they do not obey the exclusion principle. This means that there is no limit to the number that can be exchanged, and so give rise to a strong force. However if the force-carrying particles have a high mass, it will diffcult to produce and exchange them over a large distance. So the forces that they carry will have only a short range. If the force-carrying particles have no mass of their own, the forces will be long range. The force-carrying particles exchanged between matter particles are said to be virtual particles because they cannot be detected directly by a particle detector. Particles of spin 0, 1 or 2 do exist in some circumstances as real particles, when they can be detected directly. They appear to us as waves such as light or gravitional waves. They may sometimes be emitted when matter particles interact with each otherby exchanging virtual force-carrying particles. (For eg, the electric repulsive force between two electrons is due to the exchange of virtual photons, which can never be directly detected; but if one electron moves past another, real photons may be given off, which can be detect as light waves.)

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