As you sit in front of your computer reading this article, you may be unaware of the many forces acting upon you. A force is defined as a push or pull that changes an object's state of motion or causes the object to deform. Newton defined a force as anything that caused an object to accelerate -- F = ma, where F is force, m is mass and a is acceleration. The familiar force of gravity pulls you down into your seat, toward the Earth's center. You feel it as your weight. Why don't you fall through your seat? Well, another force, electromagnetism, holds the atoms of your seat together, preventing your atoms from intruding on those of your seat. Electromagnetic interactions in your computer monitor are also responsible for generating light that allows you to read the screen.
The first force that you ever became aware of was probably gravity. As a toddler, you had to learn to rise up against it and walk. When you stumbled, you immediately felt gravity bring you back down to the floor. Besides giving toddlers trouble, gravity holds the moon, planets, sun, stars and galaxies together in the universe in their respective orbits. It can work over immense distances and has an infinite range. Isaac Newton envisioned gravity as a pull between any two objects that was directly related to their masses and inversely related to the square of the distance separating them. His law of gravitation enabled mankind to send astronauts to the moon and robotic probes to the outer reaches of our solar system. From 1687 until the early 20th century, Newton's idea of gravity as a "tug-of-war" between any two objects dominated physics.
If you brush your hair several times, your hair may stand on end and be attracted to the brush. Why? The movement of the brush imparts electrical charges to each hair and the identically charged individual hairs repel each other. Similarly, if you place identical poles of two bar magnets together, they will repel each other. But set the opposite poles of the magnets near one another, and the magnets will attract each other. These are familiar examples of electromagnetic force; opposite charges attract, while like charges repel. Scientists have studied electromagnetism since the 18th century, with several making notable contributions.
The nucleus of any atom is made of positively charged protons and neutral neutrons. Electromagnetism tells us that protons should repel each other and the nucleus should fly apart. We also know that gravity doesn't play a role on a subatomic scale, so some other force must exist within the nucleus that is stronger than gravity and electromagnetism. In addition, since we don't perceive this force every day as we do with gravity and electromagnetism, then it must operate over very short distances, say, on the scale of the atom. The force holding the nucleus together is called the strong force, alternately called the strong nuclear force or strong nuclear interaction. In 1935, Hideki Yukawa modeled this force and proposed that protons interacting with each other and with neutrons exchanged a particle called a meson -- later called a pion -- to transmit the strong force.
Uniting Fundamental Forces Science never rests, so the work on fundamental forces is far from finished. The next challenge is to construct one grand unified theory of the four forces, an especially difficult task since scientists have struggled to reconcile theories of gravity with those of quantum mechanics. That's where particle accelerators, which can induce collisions at higher energies, come in handy. In 1963, physicists Sheldon Glashow, Abdul Salam and Steve Weinberg suggested that the weak nuclear force and electromagnetic force might combine at higher energies in what would be called the electroweak force. They predicted that this would occur at an energy of about 100 giga-electron volts (100GeV) or a temperature of 1015 K, which occurred shortly after the Big Bang. In 1983, physicists reached these temperatures in a particle accelerator and showed that the electromagnetic force and weak nuclear force were related.