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
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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