Newton's Universal Law of Gravitation states that, for any two objects, each exerts a gravitational force on the other, that both forces are the same size but point in opposite directions (namely toward the object exerting the force), and that the size of each force is directly proportional to the product of the masses of the two objects and inversely proportional to the square of the distance between them.
Few people these days are surprised to hear that gravity exists. However, many are often surprised to hear that gravity is the weakest of all the forces. This likely stems from the fact that our most common experience with gravity comes from gravity associated with very large objects -- such as the gravitational force exerted on us by the earth. We don't have much experience with other examples of gravity, even though that force exists between ANY two objects - such as between two people. The reason we don't have that experience is simply because the gravitational force between small objects is so small as to go unnoticed in everyday life.
In the 1800's, James Clerk Maxwell showed that electric force and magnetic force are essentially two manifestations of one single force. Certainly it is clear that each causes the other - that's the basic principle of all electric motors. Since I know almost nothing about magnetism, I will focus briefly on the electric force.
The basic concept is that any object has either a positive electrical charge, or a negative electrical charge, or no electrical charge. The standard high school mantra on this is "Like charges repel; unlike charges attract". Electrons, discovered in 1896 by J. J. Thomson, are deemed to each have an electrical charge of -1. (Experiment seems to confirm that all electrons have exactly the same charge.) So a quantity of negative charge on an object can be measured as the number of electrons it has that are extra, compared to what it has when there is no charge. Similarly, a positive charge can be measured as the number of electrons missing from what is needed to make the object have no charge. With that in mind, the forces involved behave very much like gravity, in the sense that any two electrically charged objects exert a force on each other; the sizes of the two forces are equal, but the forces point in opposite directions; and the size of each force varies directly with the product of the sizes of the charges and is inversely proportional to the square of the distance between them.
It's easy to see that the electromagnetic force is much stronger than the gravitational force. Simply hold a magnet just above a paper clip, and notice how the clip jumps up to the magnet - the magnetic force overpowers the gravitational force even though the magnet is much much smaller than the earth.
The strongest of all of nature's forces, this force (sometimes called the colour force) is the one that keeps the nuclei of atoms from splitting up. To clarify this a little, consider the following example.
A helium atom consists of a nucleus with two protons and two neutrons, plus two electrons outside the nucleus. The total charge of -2 on the electrons is exactly offset by the total charge of +2 on the protons, making the atom electrically neutral. Note, however, that the two protons are much closer to each other than they are to the electrons. Yet we know that like charges repel - so why don't the protons push each other away, thus splitting up the nucleus? The answer is the presence of another force in the nucleus - the strong force. Being stronger than the electromagnetic force, it overcomes the electrical repulsion between the two protons, and holds them in place.
That strong force acts on more than just the protons - it also acts on the neutrons - otherwise they would just drift away from the protons. Indeed, all four particles act on each other.
The strong force is like gravity in the sense that each particle acts on the other, exerting an attractive force (the electromagnetic force does that for unlike charges, but for like charges it's a repulsive force). However, while the strength of the gravitational force is stronger when the particles are closer together, the strong force is stronger when the particles are further apart - very much like an elastic band is.
This force is the ultimate cause of radioactive decay. More generally, it is the reason there are so few fundamental particles still surviving in nature. At the time of the Big Bang, there were many more kinds of particles than there are now - without the weak force, there still would be, and the universe would be a very different place from what it is today.
Even more generally, the weak force is responsible for the naturally occurring nuclear reactions that occur in our sun (and in all stars). Without it, all stars would be cold lumps of matter, emitting neither light nor heat.
Of all the forces, this one has the shortest range. Its effect is confined to the interior of things like protons and neutrons.
There are two kinds of electrical charge - positive and negative, each of which is said to be the opposite of the other. Every electron has the same quantity of negative electric charge, and as a result that common charge is taken as the unit of negative charge: : a charge of -1 is the electrical charge on one electron.
The charge on every proton is positive, and the same size as the negative charge on an electron, and so the charge on one proton is +1.
If the charges on all the particles in an object add up to 0, we say the object is electrically neutral with a net charge of 0. Every free atom (i.e.: not chemically bonded to some other atom) is electrically neutral - thus the number of protons it has, equals the number of electrons it has. If an atom picks up one or more extra electrons, it is said to be a negative ion; if it is missing one or more electrons, it is said tobe a positive ion.
There are six kinds of colour charge, but only three of them occur in ordinary matter. Those three are called: red, green, blue. The inspiration for these is that on a monitor for example, those three ordinary colours are prime, in the sense that all colours can be formed from weighted combinations of those three. In particular, an equal combination of all three colours produces white - and any object with equal amounts of red, green, and blue charge is considered to be colour (or strong) neutral.
The other three colour charges are referred to as the colour complements of red, green, and blue. In ordinary colour theory, an equal mixture of a colour and its complement produce white. Similarly, in colour charge theory, an object with equal parts of one colour and its complement is colour neutral. These three complementary colour charges do NOT occur in ordinary matter, but only in antimatter, about which more will be said later.
The weak force also has charges associated with it, but i don't know very much about them. Certainly I have seen references to particles with a positive weak charge, those with a negative weak charge, and also those with a zero weak charge - which is reminiscent of electric charge. However, I have also seen comments saying quite clearly that there is a bigger variety of weak charges than of electrical charges.
To the best of my knowledge, there is no such thing. This makes gravity unique among the four forces of nature. Moreover, quantum theory so far tends to ignore gravity, on the grounds that it is so weak compared to the other forces, that its effects don't enter into the calculations.
Most people think of matter as being composed of molecules, which in turn are composed of atoms, and that atoms are composed of a nucleus with one or more electrons surrounding it, and that the nucleus is composed of one or more protons and zero or more neutrons. Those people are right, of course. However, there is a further common belief that electrons, protons, and neutrons are fundamental in that they can't be broken down into smaller particles. We now know that this is not true of protons or neutrons, and also that electrons can change form into another particle under certain conditions.
Here are some quick facts about electrons:
Are electrons really particles? Please click here to see an important discussion of that and related issues. (This will open a new window in your browser; so when you are done reading it, simply close that window, and you will be back here.)