Modern physics

Key points: Four fundamental forces; fundamental particles; matter and antimatter

There are four fundamental forces - they account for all forces known to physics. We have already met three of them: 1.) Gravitational (very, very weak) 2.) Electromagnetic (moderate) 3.) Nuclear (very strong) In addition, there is 4.) the "weak" nuclear force that is required to explain some nuclear reactions. (illustration from CERN)

Although gravity is very, very weak, it has the advantage that it acts at long range, and that there is only one sign for the force - unlike electric charges, there are no plus and minus gravity charges to cancel each other out. Thus, gravity is the dominant force over distances of a few kilometers and greater.

The electric force is an inverse r squared force, just like gravity, and it is much stronger. However, it is canceled out over large distances by the existence of plus and minus charges - if there is enough charge of one sign to exert a long-range force, the first thing it does is attract particles with the opposite charge, and they cancel much of the force. Nonetheless, it is generally dominant over distances from the separation of the proton and electron in an atom, about 10^-13 kilometers = 10^-10 meters, up to a few kilometers.

The nuclear forces are short range, and act over the size of an atomic nucleus only, about 10-13 meters. To the right is an artist's concept of the strong nuclear force bonding the protons and neutrons in an atomic nucleus. (from Judy Racz, http://home.vicnet.net.au/~richard/racz.htm)

Nuclear Particles

Modern physics has discovered far more particles than just the photon, proton, neutron, and electron that we have discussed We will learn a bit more about quarks and the early Universe soon. They are thought to be the basic building blocks for all matter There are six of them, and physicists with some measure of whimsy have named them up, down, top, bottom, strange, and charm. They have fractional electric charge (either +2/3 or -1/3), and in various combinations account for some 200 separate larger particles. They are held in these various combinations by the strong nuclear force, and in fact are highly unstable unless confined this way. As an example, the proton is made of two "up" quarks, each with charge of +2/3, and one "down" quark, with a charge of -1/3. Neutrons are one "up" quark and two "down" quarks. If quarks are so unstable, how to we know they are real? Well, their existence was first predicted by the standard model, which has had many other successes in explaining the behavior of nuclear particles, in a simple and unifying way. Although these concepts seem very abstract, the predictive power of the standard model is very convincing in a scientific sense.

Another particle is the neutrino. It is the product of certain reactions we will learn about when we study the energy output of the sun, for example. Neutrinos have little or no mass and hence travel at virtually the speed of light. They almost do not react at all with other forms of matter, so they can escape from the very core of the sun and make it to earth.

The electromagnetic, nuclear, and weak nuclear forces are all described by the "standard model" of particle physics, but this theory does not account for the behavior of gravity.

Higgs Boson

Matter and Antimatter

Einstein's E = mc² implies that it might be possible for nuclear particles to get converted to energy, and energy to nuclear particles. And so it is. However, understanding how it happens leads to a new perspective on matter. It shows that there is a whole zoo of nuclear particles just like the ones we have discussed except reversed in key aspects, such as electric charge. Thus, there is an anti-proton, with the same mass as a proton but negative charge. There is an anti-electron (sometimes called a positron), with the same mass as an electron but a positive charge, and so forth.

When a particle encounters its anti-particle, they annihilate each other and the energy predicted by Einstein's equation emerges as two gamma rays. Similarly, if gamma rays come together in just the right way, they can disappear with the creation of a particle and its anti-particle. (figures from J. Schombert, http://zebu.uoregon.edu/~js/ast123/lectures/lec18.html)