###### By Dan Hooper, Ph.D., University of Chicago

## Relativity and quantum mechanics were the areas in which Einstein made his greatest and most lasting contributions. But these were not the only areas in which Einstein was interested. For a period of three decades, Einstein searched—with very little success—for unified field theory. What comprises a unified theory, and what is its significance?

A unified field theory, Einstein hoped, would combine and merge the theory of general relativity with the theory of electromagnetism, fusing them together into a singular physical and mathematical framework. The theory that Einstein had hoped to discover would be far more powerful, and more far-reaching, than either of these individual theories could ever be alone.

Unification has played a very important role in the history of physics. In fact, arguably, many of the greatest accomplishments in physics are examples of unification. By unification in this context, it means uniting two or more ideas that were thought to be as completely distinct, proving their different aspects as the same underlying phenomenon.

Learn more about Einstein and gravitational waves.

### Unification by Newton’s Universal Gravitation

Consider, for example, what we call gravity. Prior to Isaac Newton, gravity was seen as a force that pulls things downward, and toward the Earth. And also, independently, it had been shown by Johannes Kepler and others that planets followed elliptical orbits around the Sun. But at the time, no one knew why planets followed these orbits. It was just known that they did.

But Isaac Newton changed all of that. With his theory of universal gravitation, Newton combined or unified these seemingly very different phenomena with a single overarching principle. His proposal was that all kinds of mass attract one another with a strength proportional to their masses, and inversely proportional to the square of the distance separating them. With this simple relationship, Newton found an idea that could explain both why massive objects are pulled toward the Earth, and why planets, and moons and comets, for that matter, follow the trajectories that they do as they move through the solar system.

This is a transcript from the video seriesWhat Einstein Got Wrong.Watch it now, on The Great Courses Plus.

### Unification of Electricity and Magnetism

Another important example of unification in physics took place almost two hundred years later, in the 19th century. Prior to this, electricity and magnetism were conceived of as unrelated phenomena. Electricity, on the one hand, was responsible for things like lightning, and static charge. On the other hand, magnetism caused compass needles to point north. They were seen as entirely different forces, which acted in different ways, and acted on different things.

But by the mid-1800s, the work of physicists such as Michael Faraday and James Clerk Maxwell showed that electricity and magnetism were related. In fact, they discovered that a magnetic field is itself nothing more than a moving or changing electric field. In other words, magnetism is just electricity in motion. Its effects might seem different; but beneath it all, it’s really just another aspect of the same underlying thing. Today, physicists talk about ‘electromagnetism’ as a singular aspect of nature, and from a modern perspective that’s what it is.

But before the unifying work by Faraday and Maxwell, the phrase ‘electromagnetism’ wouldn’t have made any sense at all. Only after the unified theory of electromagnetism was discovered could one see any reason to think about electricity and magnetism as being connected to one another in any meaningful way.

Learn more about quantum entanglement.

### What Is the Significance of a Unified Theory?

Sometimes, when two ideas are found to be deeply connected, they reveal new and surprising things in the process. In the case of electromagnetism, the equations that were discovered to relate electricity and magnetism to one another also described the nature and behavior of light waves. We found that light was an electromagnetic wave—a combination of oscillating electric and magnetic fields that move together through space.

Every time that physicists manage to successfully unify a set of seemingly unrelated phenomena, they are left with a more powerful theory. A unified theory can explain more, and do it with less. Newton’s unified theory of gravity explains why objects fall down and why planets move in elliptical orbits. It explains all of this more simply than the theories that preceded it could. But in addition, Newton’s theory can be used to understand and predict many things that the preceding theories simply couldn’t.

From Kepler’s equations, one can’t tell you how heavy a bowling ball will be on the moon, or predict the trajectory of a satellite around the Earth. Now, one can predict these things using Newton’s theory because it unifies multiple ideas into one, it can explain more. It’s more powerful.

Learn more about what Einstein got right: Special Relativity

### Einstein’s Search for a Unified Theory

Einstein was also looking for a similarly powerful theory. His quest for a unified field theory started only a couple of years after he completed his general theory of relativity, in or around 1918 or so. At the time, there were two fundamental theories that were central to how physicists understood their universe. One of these theories was Einstein’s new general theory of relativity, which explained the phenomenon of gravity, and its relationship to space and time.

Then, there was the theory of electromagnetism, usually written as a set of four equations, known as Maxwell’s equations. Maxwell’s equations can be used to describe a wide range of phenomena associated with electricity and magnetism, including that of light. So from Einstein’s perspective, there were two different facets of nature before him. Both of these theories were individually powerful and quite mathematically elegant.

Einstein admired James Clerk Maxwell a great deal, and he held Maxwell’s equations of electromagnetism in very high regard. Einstein also, of course, was quite fond of his own theory of general relativity. These two theories were among the greatest accomplishments in all of physics. As far as anyone could tell, the theories of general relativity and electromagnetism seemed to be basically unrelated to one another. But Einstein wasn’t so sure that this was really the case.

After all, electricity and magnetism seemed to be unrelated until Faraday and Maxwell showed us otherwise. Einstein wanted to do something similar with general relativity and electromagnetism. Like many of the greatest physicists before him, Einstein wanted to make a more powerful and widely applicable theory, that could predict and explain more than its predecessors could.

### Common Questions about Einstein’s Unified Field Theory

**Q: What does a unified theory mean?**

A Unified Theory tries to unite in a single mathematical framework the electromagnetic and weak forces with the strong force or with the strong force and gravity.

**Q: What are the four main unified theory of electromagnetism?**

The four main forces that can be unified into different unified theories are gravitation, electromagnetism, weak Interaction, and strong Interaction. The main fundamental forces are mediated by fields which result from the exchange of gauge bosons in the Standard Model of Quantum Field Theory.

**Q: Who discovered the Unified Field Theory?**

The first successful classical Unified Field Theory was developed by James Clerk Maxwell. He successfully combined electricity and magnetic field theory into Electromagnetic Field Theory and led the way forward for different unified theories.

**Q: What were Einstein’s goals?**

Einstein, in the latter part of his career, wanted to unify the theories of general relativity and electromagnetic field into one unified theory.

He wasn’t able to achieve any significant success in this goal though.