Fundamental Forces: A History of Unifying Forces

FROM THE LECTURE SERIES: UNDERSTANDING THE MISCONCEPTIONS OF SCIENCE

By Don Lincoln, Ph.D.Fermi National Accelerator Laboratory (Fermilab)

How could we unify the known forces and demonstrate that they have a common origin? In order to see that, it might be helpful to take a look at unifications that we’ve already accomplished. To unify forces has a mysterious sound to it, but it’s really not an exotic thing. It has been done many times in the past.

detail from 'The School of Athens' by Raphael, showing Plato and Aristotle in the center
The Ancient Greek philosophers had their own theories about the universe. These were eventually updated in more modern times. (Image: Raffaello Sanzio da Urbino/ Public domain)

Early Understandings of Gravity

In the 4th century BC, Aristotle tried to describe gravity as being the nature of matter. His theory revolved around a geocentric model, one in which the Earth was in the center of the universe. The natural location for all matter was the center. Hence, when things fell, it was explained simply as matter trying to get to its natural location.

Aristotle’s theory of what we might call terrestrial gravity, that is to say the behavior of gravity here on Earth, was simply an instance of objects trying to get to their natural place. The sky was a different thing. The sky was governed by the gods. There was no concept of what we might call celestial gravity, which is, of course, the gravity of the heavens.

Learn more about untangling how quantum mechanics works.

A Heliocentric Theory

In 1543, Polish astronomer Copernicus shook up the astronomical community when he postulated that the Sun and not the Earth was at the center of the universe. The Catholic church was not at all happy with this idea, but it’s stood the test of time.

German astronomer Johannes Kepler made additional contributions when he used precise astronomical observations made by his mentor, Tycho Brahe, to determine that he could explain the data if the planets orbited the Sun in elliptical orbits. He also was able to work out the speed of the various planets and found that they moved at different speeds while they orbited.

The penultimate hero in our study of gravity is Italian astronomer and physicist Galileo Galilei. He studied both the behavior of matter here on Earth and also in the heavens. Legend has it that Galileo dropped heavy and light objects from the Leaning Tower of Pisa and demonstrated that they fell at the same rate. This story is probably apocryphal.

Galileo also studied the heavens and used the new-fangled telescope, which he didn’t invent but did lead the way in using it in astronomy. He found that Jupiter has moons, which weighed heavily towards the idea of the Sun, and not the Earth, as being the center of the universe. But again, there was no unifying explanation between how things fell on Earth and how objects moved in the sky.

This is a transcript from the video series Understanding the Misconceptions of ScienceWatch it now, on The Great Courses Plus.

Newton’s Idea of Gravity

Illustration showing the effect of gravity on an apple and between the Moon and Earth
Newton’s theory of gravity explained both gravity on the Earth as well as planetary motion. (Image: Inna Bigun / Shutterstock)

It was British polymath Sir Isaac Newton who made the truly crucial advance. What he did was come up with a model that successfully demonstrated that celestial and terrestrial gravity were one and the same. We call his gravity model Newton’s Law of Universal Gravity for just that reason.

He worked out the idea that 2 objects would feel a mutual gravitational force, governed by the mass of the 2 objects and the distance between them—well, the distance squared, actually. Using that idea and the mathematical discipline of calculus, which he also invented, he could explain both how things fall to the ground on the surface of the Earth, and also the exact motion of the moons, planets, asteroids, comets, and all of the myriad of objects available to an astronomer’s curious gaze.

And he did this in 1670 or thereabouts. The fact that celestial and terrestrial phenomena could be unified by universal gravity was a big deal. And that’s the kind of thing that we’re hoping to do now.

Learn more about the reality of atoms.

Electricity and Magnetism

There is another example of unification that is familiar enough now that it’s hard to remember that it was a unification. In the years prior to 1870, a horde of scientists explored the mysteries of electricity and magnetism. Electricity, of course, governs such things as the strike of a lightning bolt and the spark of static electricity you might feel on a cold and dry winter’s day. We now know it to be the stuff that comes out of the wall and powers electrical equipment.

People kind of knew about that in the late 1700s and early 1800s; batteries had been invented, and people could make shocks that way as well. During that same time, scientists were also fascinated by magnets. Magnets, as you know, can pick up iron objects. You can use them to make compasses. Magnets attract or repel other magnets. They’re a real curiosity. But they don’t make shocks, nor does it seem possible to conduct magnetism through wires. So it’s not at all obvious that electricity and magnetism are the same thing. However, scientists in the early 1800s were coming to realize that they were somehow connected. An electrical current in a wire could make a magnet, and a moving magnet could make an electrical current. That there were connections was growing increasingly obvious, but the detailed connection was elusive.

This is a transcript from the video series Understanding the Misconceptions of ScienceWatch it now, on The Great Courses Plus.

A Unified Electromagnetism

Engraving of James Clerk Maxwell by G. J. Stodart from a photograph by Fergus of Greenock
James Clerk Maxwell unified electricity and magnetism. (Image: George J. Stodart/ Public domain)

It was in about 1870 that British physicist James Clerk Maxwell was able to take the observations, theories, and equations developed by many people over the previous decades and put them all together, when he wrote down what are now called Maxwell’s equations.

Maxwell’s equations showed the deep connections between electricity and magnetism. These connections are so deep that we don’t think of 2 distinct phenomena; there is but one phenomenon, called electromagnetism. And electromagnetism was more than the simple unification of electricity and magnetism. Electromagnetism also explains how both light and chemistry work. This brilliant unification makes possible most of our modern technological world.

But we still have the task of unifying the four forces that we still need to explain the universe.

Common Questions About the Unification of Fundamental Forces

Q. What was Aristotle’s theory of gravity?

Aristotle tried to describe gravity as being the nature of matter. His theory revolved around a geocentric model, one in which the Earth was in the center of the universe. The natural location for all matter was the center. Hence, when things fell, it was explained simply as matter trying to get to its natural location.

Q. Who were the first scientists to talk about the heliocentric idea?

Copernicus first postulated the heliocentric idea. Johannes Kepler used precise astronomical observations made by Tycho Brahe to determine that he could explain the data if the planets orbited the Sun in elliptical orbits.Galileo found that Jupiter has moons, which weighed heavily towards the idea of the Sun, and not the Earth, as being the center of the universe.

Q. How did Newton unify terrestrial and celestial gravity?

Newton’s Law of Universal Gravity declared that 2 objects would feel a mutual gravitational force, governed by the mass of the 2 objects and the distance between them. Using that idea he could explain both how things fall to the ground on the surface of the Earth, and also the exact motion of the moons, planets, asteroids, comets, and other heavenly objects.

Q. How were electricity and magnetism unified?

James Clerk Maxwell’s equations showed the deep connections between electricity and magnetism. These connections are so deep that we don’t think of 2 distinct phenomena; there is but one phenomenon, called electromagnetism. Electromagnetism was more than the simple unification of electricity and magnetism. Electromagnetism also explains how both light and chemistry work.

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