How Einstein Challenged Newtonian Physics

From a lecture series presented by The Great Courses

For centuries, Newtonian physics was the dominant worldview. However, Einstein shook up the scientific community with his groundbreaking papers on light quanta; atoms; and, eventually, his theory of relativity.

PHYSICS, SCIENCE. ABSTRACT BACKGROUND WITH DIFFERENT FORMULAS. E=mc2

Newtonian Physics: The Prevailing Worldview

Any discussion of Einstein should begin with what is probably his single greatest contribution to physics—the theory of relativity.

But to understand just how important and revolutionary this theory was, we have to appreciate what came before it. We need to understand not only what Einstein built, but what he overthrew.

Between the late 1600s and the beginning of the 20th century, the field of physics was dominated by the ideas of Isaac Newton. The Newtonian laws of motion and gravitation had, up to that point in time, been the most successful scientific theory in all of history.

Newton’s ideas were, of course, challenged from time to time during those two centuries, but these ideas always seemed to hold up. This is not to say that the field of physics didn’t make any progress during that time—it certainly did.

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

There were many new phenomena that were discovered and that came to be understood in the centuries that followed Newton’s era. Take electricity and magnetism, for example. Until the 19th century, we didn’t really know what electricity or magnetism were, or how they worked. Isaac Newton certainly didn’t have a clue.

But contributions from physicists such as Michael Faraday and James Clerk Maxwell dramatically expanded our understanding of these phenomena. The new theory of electromagnetism did indeed offer new insights into the nature of our world, but this new theory also fit quite nicely into the older Newtonian way of thinking about physics. Electricity and magnetism were newly understood forces, but in many respects, they were not so different from the Newtonian conception of gravity.

To many physicists around the turn of the 20th century, the state of physics seemed very settled. The Newtonian worldview had been very successful, and for a very long time.

To most scientists, it just seemed extremely unlikely that Newtonian physics would be substantially replaced any time soon. When someone wants to describe this state of mind as it existed at the time, they usually quote the eminent physicist Lord Kelvin: “There is nothing new to be discovered in physics now. All that remains is more and more precise measurement.”

Although it’s not clear whether Kelvin actually ever said this, the quote does seem to capture the sentiment of the physics community at the time. Physics seemed to be on very solid footing. Few, if any, were expecting a revolution.

Learn more about how Einstein revolutionized the science world

A Revolution in Physics: Quanta and Atoms

In 1905, however, a revolution in physics did come. And perhaps even more surprising than the revolution itself was where that revolution came from.

In 1905, Albert Einstein was not working as a professor at some prestigious university. He was not famous, or even well-known among other physicists.

Photograph of Albert Einstein by Orren Jack Turner, Princeton, N.J.  [Public Domain]
Albert Einstein worked in the Swiss patent office when he published four groundbreaking papers.

He wasn’t even employed as a physicist at all. Instead, he was working as a third-class technical assistant in the Swiss patent office.

After finishing his degree in 1900, Einstein had spent two years unsuccessfully applying for jobs teaching math or physics. In 1902, after conceding that a teaching job wasn’t in the cards, he accepted the job in the patent office.

The patent office job was a good job, though. It paid well and it seemed to give Einstein enough spare time to continue his research on the side, and to work toward finishing his Ph.D., which he was still pursuing at the University of Zurich.

As of early 1905, however, Einstein had still not finished his doctorate, and he had only published a few papers in scientific journals—and these papers weren’t particularly interesting or noteworthy. At this time—at the age of 25—the prospects for Einstein’s career in physics looked very bleak.

Things didn’t stay this way for long, however. In 1905, Einstein wrote not one or two, but four absolutely groundbreaking papers. Any one of these four papers would have made him a star within the field of physics, and would have certainly secured him a position of prominence in the history of science.

It seems that having so many breakthroughs of this magnitude in such a short period of time had never happened before, and has never happened since. In the first of Einstein’s 1905 papers, he proposed that light doesn’t only behave like a wave, but that it is also made up of individual pieces or particles.

At the time, these pieces of light were called “quanta,” and today we call them photons. This insight started us down the road to what would become quantum mechanics.

In the second of Einstein’s 1905 papers, he showed that the random motion of particles in a fluid could be explained and understood if the fluid consisted of a large number of individual atoms or molecules. At the time, atoms were only a hypothetical notion. And most physicists didn’t think that atoms really existed.

But Einstein’s paper provided concrete empirical evidence that atoms were, in fact, real and tangible objects. He was even able to use these arguments to make a pretty good estimate for the size and mass of atoms and molecules. It was a huge step forward.

These two papers—on light quanta and on atoms—made the first half of 1905 an extremely good period of time for Albert Einstein. And as a bonus, he also managed to complete his Ph.D. in April of that year, all while working full-time in the patent office.

Einstein’s Theory of Special Relativity

But as good as the first half of 1905 was, the second half might have been even better for Einstein. In was during those months that he completed and presented for the first time his theory of special relativity. Space and time would never be the same.

By 1905, Einstein had been thinking about the ideas that would eventually lead him to special relativity for about 10 years or so. He would later recall that when he was only 16 years old, and still a high school student, he would sometimes imagine what it might look like to someone who could move alongside a beam of light as it traveled through space.

You might think that if you wanted to do this, you could gradually accelerate until you were moving at the same speed as the light wave, and then, while you were moving in unison alongside the beam, the beam of light would look stationary. This is how it is for other kinds of waves.

For example, in a reasonably fast boat—moving at something like 50 miles per hour—you could move in unison alongside a water wave. From the boat’s perspective, or in that frame of reference, the wave would look approximately stationary.

If you could move quite a bit faster—about 770 miles per hour—you could keep up with and move alongside a wave of sound. In the right frame of reference, water waves and sound waves can be stationary.

Learn more about what Einstein got right: general relativity

Measuring the Speed of Light

But what about light? Einstein had reasons to doubt that light would behave in the same way that water waves and sound waves do.

When we say that sound waves move at a speed of about 770 miles per hour, what we really mean is that they move at this speed relative to the rest frame of the air that they are moving through. In other frames of reference, they can be moving faster or slower speeds.

Plaque showing Maxwell’s equations at the Edinburgh statue.
Maxwell’s equations predicted the speed of light was 670 million miles per hour.

The equations that physicists use to describe the propagation of light waves—what are known as Maxwell’s equations—predict that light should move through space at a speed of about 670 million miles per hour. And more interestingly, these equations don’t make any reference to any medium that the light waves propagate through.

There is nothing in these equations that is analogous to the air that sound waves move through, or to the water that water waves move through.

Unlike other kinds of waves, it wasn’t clear that light needs any kind of medium at all in order to move through space. And this raises a critical question: Without a medium, in what frame of reference does light move at a speed of 670 million miles per hour?

At the time, most physicists side-stepped this question by simply imagining that light really does move through some kind of medium—a medium that they called the luminiferous aether, or just the aether for short.

Although no experiment had ever detected this aether, they argued that it must fill virtually all of space. After all, they argued, the light from a distant star could only reach us if there was a continuous path filled with aether, extending all the way from the star to us.

Eventually, though, physicists discovered that there was no aether. It would be Einstein who would come up with an equation to explain this conundrum.

From the lecture series What Einstein Got Wrong, taught by Professor Dan Hooper

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Big Questions: What Is Reality?
A Search for the Theory of Everything
The Birth of Modern Science

Photograph of Albert Einstein by Orren Jack Turner, Princeton, N.J. Modified with Photoshop by PM_Poon and later by Dantadd. [Public domain], via Wikimedia Commons

Plaque showing Maxwell’s equations at the Edinburgh statue. James Clerk. FF-UK [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons