There are now known to be four fundamental physical “forces,” although, at the beginning of this century, only two were known. What I mean by forces is the scientific term that implies a push or a pull. In physics, a force is any influence that causes an object to undergo a certain change, in either its speed or direction of motion, or geometrical construction.
In other words, a force is that which can cause an object with mass to change its velocity (which includes to begin moving from a state of rest), that is, to accelerate, or which can cause a flexible object to deform. Force can also be described by intuitive concepts such as a push or pull. A force has both magnitude and direction, making it a vector quantity. Newton’s second law, F = ma, was originally formulated in slightly different, but equivalent terms: the original version states that the net force acting upon an object is equal to the rate at which its momentum changes. (Momentum is a quantity equal to mass times velocity.)
As I stated, there are four “fundamental” forces known in nature. These are forces that act at a distance. They should not be confused with forces that occur with contact such as: frictional force, tension force, normal force, air (or fluid) resistance, applied force, or spring force (and there are others).
Historically, the first fundamental force to be understood is the force of gravity, and we credit Sir Isaac Newton (1642-1727) with its discovery. Prior to Newton’s time, the focus had been on understanding the motion of the planets and stars. For thousands of years, humankind was misdirected by assuming the earth was the center of all this motion. Although many give a religious reason for this incorrect understanding, it was really more the case that people sensed that the earth didn’t move. After all, it was well known that motion produced various pushes and pulls (acceleration) and it was obvious to the observers that the earth was standing still.
That’s not the first time that observations have been in error. Among Newton’s many discoveries and explanations of the laws of force were an understanding of how it would appear that the earth was stationary even though it is undergoing all kinds of motion from rotation on its axis to revolution around the sun to motion as part of the galaxy.
Using Tycho Brahe’s (1546-1601) extremely accurate and complete observations of the motion of Mars and other planets, Johannes Kepler (1571-1630) developed mathematical laws that described the motions within the solar system, correctly placing the sun at the center of this system with the planets revolving around the sun, and the earth’s moon revolving around the earth.
What Newton did was to develop a set of laws that described both gravity and rules for motion. These laws or formulas, when solved, predicted exactly the motions that Kepler had discovered. (There was a small amount of error when Newton’s laws were applied to Mercury, the innermost planet of the solar system. We’ll return to that small error later in this story.)
This is Newton’s “Universal Law of Gravitation.”
• F is the force between the masses,
• G is the gravitational constant,
• m1 is the first mass,
• m2 is the second mass, and
• r is the distance between the centers of the masses.
G is a constant that makes the units work out right. It is a fundamental characteristic of our universe.
There were several conceptual breakthroughs included in Newton’s work. It is said that he was watching an apple fall out of a tree when he had his great a-ha moment. He realized that the apple fell to earth because the earth had an attraction for the apple: gravity. But he also realized the apple attracted the earth! Everything that has mass attracts everything else that has mass. Of course, since the earth’s mass is so much greater than the apple’s, the apple falls to earth, although you could argue the earth moves too, but just the tiniest little bit.
Further, he looked at the moon, and realized that it too was falling, just like the apple. It is just that the moon had a linear velocity that caused it to “fall” around the earth. From this comes orbital mechanics and an explanation of how masses can revolve around each other.
Writing this as an equation, he calculated the “force of gravity.” It is actually the weakest of the four fundamental forces, but the first to be discovered because it is a prime element in the motion of the planets and stars. It literally holds the macro universe together. “Macro,” in this sense, means “large” or “what we see.” Certainly the universe is large. The range or active distance of gravity is infinite, however, the force decreases with distance as the square of the distance.
That means that at twice the distance, the force is one over two squared or one-fourth the effect. We say that the force of gravity is proportional to the inverse of the distance squared. You can see that in the “r” value in the denominator of the equation for the force of gravity. This fact can be deduced easily with simple solid geometry. As the distance from a point increases, the surface of the sphere increases as the square of the distance. This “spreading” out of the force weakens its effect on a given mass.
Newton also developed three laws that deal with motion and contact forces. He had to invent a new form of mathematics called “The Calculus” to perform the calculations, and he is considered one of the greatest physicists and mathematicians of all time. He also did work with light and optics, which is a good segue to our second force.
All through the seventeenth and eighteenth and nineteenth centuries discoveries were made regarding electricity and magnetism. Even our Benjamin Franklin had a hand developing electrical principles during the time of the American Revolution. Grade schoolers are taught Franklin’s experiments with kites and lightning, an experiment in which we’re lucky one of our founding fathers didn’t experience an early end to his existence.
The big breakthrough occurred when James Clerk Maxwell (1831-1879) combined some of the equations found earlier by these pioneers and added a little bit of his own simply to make the set of equations symmetrical. These are now known as Maxwell’s equations and they predicted the existence of a force, now known as electro-magnetic force, as well as the existence of waves we now call radio waves and light. Experimental proof of the existence of radio waves would be delayed several years, and radio is a major part of the “Second Scientific Revolution” that begins with the new century.
Maxwell’s equations combined the known attractive force of magnetic materials and the electrostatic attraction demonstrated when a plastic comb is rubbed and then attracts small bits of paper. Unlike the force of gravity, which is always an attractive force, that is it draws masses together, the electro-magnetic force can be an attraction or drawing together as well as a repulsion or pushing apart.
In terms of the electrostatic force, we know there are two polarities, positive and negative, that we are familiar with from electrical circuits and batteries. We know that like charges repel. Two positive charges will repel each other as will two negative charges. But unlike charges attract.
There is a similar situation with magnetism where the two polarities are called “north” and “south” after the earth’s magnetic poles, and, again, like poles repel and unlike pole attract.
The electro-magnetic force is much, much stronger than the force of gravity. Think of how the comb can pick up pieces of paper held on the desk by the gravity created by the mass of the entire earth. All that gravitation force overcome by a small comb. Or consider a magnet able to pick up large weights and lift them against gravity. Obviously, gravity is the lesser of the two forces.
We also know that the electro-magnetic force extends to infinity, but decreases by the square of the distance, exactly like gravity. There are many similarities between the two forces, and the existence of “gravitational waves” similar to radio waves has been hypothesized. Although gravitational radiation has not been directly detected, there is indirect evidence for its existence.
For example, the 1993 Nobel Prize in Physics was awarded for measurements of the Hulse-Taylor binary system which suggests gravitational waves are more than mathematical anomalies. Various gravitational wave detectors exist. However, they remain unsuccessful in detecting such phenomena.
It was very near the end of the nineteenth century when a pair of experimenters by the names of Albert Michelson and Edward Morley. The Michelson–Morley experiment was performed in 1887 in the United States. Its results are generally considered to be the first strong evidence against the theory of a luminiferous aether (ether). The result of the experiment was to establish the speed of electromagnetic waves — light if you will — as a fixed speed often called the “speed of light.”
The most immediate effect at the time was to put an end to the Vortex theory, which said that atoms were vortices in the ether. So that would beg the question, “just what are atoms.” For the answer, we must move into the twentieth century. The experiment has been referred to as "the moving-off point for the theoretical aspects of the Second Scientific Revolution."
And this is what I really intend to speak on, the “Second Scientific Revolution” and a young man that lead that revolution. So now the stage is set. Two of the fundamental forces of nature are known and can be calculated and put to use. What happens next, at the beginning of the twentieth century, will reshape every aspect of life on this planet. This is exciting.