Over many centuries physicists have developed ways of understanding reality, from the tiny to the enormous, the freezing to the fiery. Their quest? To explain of all nature with just one set of rules. It’s been a long journey, with many dead ends and setbacks. But some are hoping we’re nearly there.
Today physicists know that there are four fundamental forces of nature. All other forces can be built up as combinations of these four basic interactions. Some are familiar from everyday existence, while others are only relevant on tiny scales.
Electromagnetism is the force that governs how electricity flows and magnets move. The force is transmitted by particles of light, called photons. All charged particles feel the force of electromagnetism.
Gravity tells us why objects fall and planets orbit stars. Nobody knows for certain how it is transmitted. Many believe that it is carried by a messenger particle called a graviton. However, unlike the photon, this has never been discovered. Gravity affects the structure of time and space and is much weaker than all the other forces.
The other two forces act on a subatomic level. The weak force accounts for radioactive decay. We harness it in nuclear power stations. The strong force describes how particles stick together inside atomic nuclei despite their electromagnetic repulsion. These forces are also transmitted by messenger particles.
Currently, our best theories suggest that the universe is composed of tiny particles. These behave according to quantum mechanics. This famously counterintuitive theory works with incredible accuracy. Through quantum field theory we can obtain a quantum mechanical version of electromagnetism, the weak force and the strong force.
But we do not have the correct quantum theory of gravity. One of the biggest tasks in modern research is to find the missing piece of the puzzle that reconciles gravity with quantum mechanics. What is this elusive idea and could it be string theory?
This section is a story of the jigsaw pieces we’ve located so far, and the reasons why they don’t quite fit together.