Physicists recognize four fundamental forces: strong nuclear, weak nuclear, electromagnetic, and gravity. The Standard Model characterizes three, while at high energies, the weak nuclear and electromagnetic forces combine. The strong nuclear force holds protons and neutrons together, while the electromagnetic force is responsible for chemical reactions and physical properties. The weak nuclear force mediates beta decay, and gravity is the weakest but most pervasive force. It has not yet been integrated with the others in a rigorous mathematical way.
Since the early 20th century, physicists have recognized four fundamental forces, or interactions, that encompass all known phenomena in nature. Three out of four have been rigorously and mathematically characterized by the Standard Model, formulated in the early 1920s. The four forces are the strong nuclear force (also known as the color force), the weak nuclear force (average beta decay), the electromagnetic force, and gravity.
At very high energies, the weak nuclear force and the electromagnetic force combine (begin to behave interchangeably), while at even higher forces, the strong force is believed to combine with the electroweak and eventually the strong electroweak force merge with gravity. All four are believed to have been brought together an instant after the Big Bang, in the early stages of the formation of the universe.
The strong nuclear force holds protons and neutrons together in the atomic nucleus. More specifically, it is mediated by the exchange of gluons between quarks that make up protons and neutrons. It is 100 times stronger than the electromagnetic force. When nuclei are crushed in nuclear reactions, energy is released from this force. Described by the theory of physics called quantum chromodynamics, it loses all its strength at distances much larger than the atomic nucleus.
The electromagnetic force is what people are most familiar with, and it is responsible for all chemical reactions and most recognizable physical properties, such as light. It is mediated by photons, which make up all electromagnetic radiation, from cosmic rays to visible light to extremely low frequency radio waves. Both heat and light are made up of photons.
The interactions of electromagnetic forces are determined by the electric charge. The reason people don’t fall through a chair while sitting in it is that the negative charge of the atomic electron shells that make up the body is repelled by the negative charge of the electron shells that make up the chair. Photon waves decrease in intensity as the square of the distance from their source.
The weak nuclear force is responsible for a relatively small range of fundamental interactions. It mediates beta decay, which is what happens when a neutron breaks down into a proton and an electron or positron. Mediated by the W and Z bosons, it is about a hundred billion times weaker than the electromagnetic one. It only works over short distances.
Gravity is the weakest of all forces, but the most pervasive in the universe because it is generated by all bodies with mass. Gravity is 1036 times weaker than the electromagnetic force, making mathematical analysis difficult. The particles believed to be mediators of gravity, gravitons, have not yet been detected. Gravity is also distinct from other forces in that it has not yet been integrated with the others in a rigorous mathematical way. Physicists have been searching for a theory to unify gravity with other forces for nearly a century, with no luck so far.
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