Unification and simplification

Search for a more complete theory

Wherever physicists see complexity in nature they look for a more simple explanation. The periodic table is a good example, showing the way to describe large number of elements in terms of a small number of component particles, protons and neutrons.
The first step towards unifying the forces was taken by the Scottish mathematician James Clerk Maxwell in 1864. He'd noticed that there were similarities between electricity and magnetism and he worked out a way to describe them both using the same equations. He called his new theory electromagnetism.

One of the predictions of Maxwell's theory was that the electromagnetic force should be carried by waves. Light is made up of electromagnetic waves with the colour being determined by the wavelength. Red light has a longer wavelength than violet light, for example.

The electromagnetic spectrum extends well beyond what we can see with our eyes. Beyond red, there is infra-red. Hot things emit infra-red which we can't see but special cameras can, allowing pilots, for example, to 'see' things on the ground while flying at night. Beyond infra-red come microwaves and radio waves. At the other end of the spectrum, beyond visible violet, comes ultraviolet and then x-rays and gamma rays.

Forces behave like particles and like waves at the same time, it is called wave-particle duality and is one of the most mysterious things about modern physics. There is even a famous experiment that has shown light behaving like a wave and a particle at the same time. It's based on the famous Young's double-slit experiment. What happens in a normal Young's experiment is that the light waves passing through each slit interfere with each other and produce a pattern of light and dark on a screen behind the two slits. In very sophisticated versions of the experiment it is possible to fire one single particle of light, a photon, at a time. You would think that the particle could only go through one slit or the other so after firing a large number of photons you'd expect to count photons only behind the slits. What actually happens, though, is that the same interference pattern builds up. It's weird, but it really does happen, so scientists have had to get used to the idea that waves and particles are two different ways of looking at the same thing.

Maxwell took the first step on the way to unifying the forces. At the same time, he established that the electromagnetic force behaves as if it is carried by waves. The particle idea came along later.

A hundred years passed before the next step on the way to unification was taken. Three physicists, the Americans Sheldon Glashow and Stephen Weinberg along with Pakistan's most prominent physicist Abdus Salam came up with an idea for describing the weak force in the same framework. Theirs was the idea that if the particles carrying the weak force were massive, then the weak force could be described in the same way as the electromagnetic force. The short range would be explained by the mass.

That's currently where we're at. Electromagnetism and the weak force have been successfully unified in the electroweak theory. There remain the strong force and gravity, the strongest and the weakest of the forces, to unify with electroweak to make a single coherent Theory Of Everything (TOE). That's the main challenge for physicists today.

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