Albert Einstein's Legacy

Albert Einstein was born over 100 years ago on March 14, 1879. There was no indication at the time that the few kilograms of gray matter in the mewling infant would ignite into an intellectual rocket whose mighty blast would shake the very foundations of physics and illumine the farthest reaches of space and time.

Most people are aware that Albert Einstein's legacy included something to do with the invention of nuclear energy, because of his famous equation E = mc². Others may know of his special theory of relativity, which brought us the famous twin-astronaut paradox, or of his general theory of relativity in which curved space replaces the more familiar notion of gravitation. Any one of these would have guaranteed his place in history, but Einstein contributed far more to our understanding of the physical world.

Indeed, Einstein's Nobel Prize was not awarded for either one of his relativity theories—the Nobel Committee thought them too speculative at the time. Rather, Einstein won the prize for explaining the operation of electric eyes, those gadgets that sit in the doorways of small shops and ring bells or open doors when you approach. The eminent physicist began his professional career by publishing an amazing series of four scientific papers in 1905. Barely twenty-six years old—before he had obtained his Ph.D—Einstein explained the photoelectric effect, calculated the dimensions of invisible molecules, demonstrated the existence of a fundamental limit to the sensitivity of any signal detector, and in 9,000 words outlined the entire special theory of relativity.

Now, over 100 years after his birth, we see evidence of Albert Einstein's legacy all around us. His theories of atomic noise have taught electrical engineers how to develop more sensitive FM stereo receivers. Nuclear power plants demonstrate Einstein's ideas for converting mass into energy. His laws of light have given us the laser, now used to cut men's suits, line up tunnel bores, and read videodiscs. Weather and communication satellites in orbit cells above us are powered by Einstein photoelectrons from solar cells, while scientists search the heavens for Einstein-begotten black holes using ultrasensitive Einstein masers and detectors of Einstein gravitational waves.

His body may be dead, but the products of Einstein's brain are alive and well. Those insights are the very tools we shall use to explore the future.

A Look at the Future

Imagine it is 2079—the two-hundredth anniversary of Einstein's birth. New constellations sprinkle the sky—dotted chains of gigantic solar-power stations in synchronous orbit. They deploy novel types of solar cells consisting of thin, layered photosensitive material, carefully designed to extract maximum energy in accordance with Einstein's photoelectric equations. This energy is converted into powerful beams of light by laser arrays using Einstein's laws for stimulated emission of radiation. Some laser light is sent down to power aircraft, ships, and electric-conversion stations on the earth's surface. Other beams shoot outward, impinging upon the billowing light sails of interplanetary freighters and interstellar spacecraft.

Radio messages from deep-space probes, red-shifted by the velocity effects of Einstein's special relativity theory, are picked up by new receivers, using the latest low-noise maser techniques. Except for deep-space work, however, radiocommunication systems on Earth are becoming obsolete. New gravitational-wave transmitters now allow direct point-to-point communication anywhere on Earth. Another Einsteinian equation!

Out in the asteroid belt, scientists are playing with a miniature black hole they found trapped in the center of an asteroid. By applying the theory of gravity they will soon learn how to convert a scientific curiosity into a highly efficient power plant that can use any form of matter as fuel.

As we look around us in the year 2079, Albert Einstein's legacy remains all-pervasive. We will continue to reap the benefits from that intelligence for centuries to come.

A Look at the Past

Now go back in time. Perhaps to when Einstein is twenty-five and has just finished his college education in Switzerland. The normal procedure for a new graduate is to obtain employment as an assistant to a former professor. In this sheltered first job, the new graduate can gain experience while carrying out research, publishing papers, or writing a Ph.D dissertation. However, Einstein was a scientific ugly duckling. Although all of his cohorts obtained positions in the school, no one wanted Einstein.

But then, young Albert himself may have been partly to blame. Einstein knew he was smarter than all of his contemporaries and his professors. Being a brash young man, he did not disguise this fact, did not even bother to attend many of his classes. He preferred to read scientific journals by himself in his room. Later, just before an exam, he would read over the class notes of his friends. The response of Albert's professors was understandable. Why should they go out of their way to accommodate a smart aleck who considered himself above coming to class?

Einstein's former teachers neither wanted him nor were willing to give him a strong recommendation. As a result, the young genius was unable to obtain a position at any university. Finally, through family friends, he found a job as a patent examiner (second-class). Traditionally a position for lesser mentalities, the job in the patent office turned out to be beneficial for Einstein. His work consisted of examining patent applications and extracting from the confused writing of the inventor the essence of the invention. From this he learned to peel back the surface of a physical concept and arrive at the principles underlying the idea. He would later use this ability to ask similarly penetrating questions of nature. The patent office also taught Einstein how to express complex scientific and engineering concepts in clear language (he often rewrote applications for the inventor), an ability that can be seen in all his writings. The patent office gave Einstein sufficient money to live on and eight hours a day and weekends free to spend thinking about physics. It was during these off-hours that he developed the ideas that were later to shake the world.

The Growth of the Automatic Door

Although automatic door openers had not even been invented in 1905, the basic concept was under study in many physics laboratories. Physicists were investigating what they called the "photoelectric effect." It was found that when a light beam fell upon certain metals, particles of electricity called electrons, were emitted. If these electrons came flying off the surface, the material was called photoemissive. If the electrons stayed in the material, as they do in solar cells, the material was called photovoltaic, for under the influence of the light, a voltage would appear across the photovoltaic material, turning it into a miniature battery.

However, there were features to the behavior of these devices that made no sense to physicists of that time. They knew that to knock the electrons loose from the metal required energy. It was obvious that the light beam supplied the energy. By using more light in the beam, they reasoned, they could generate more energy. With more energy being put into the material, the electrons would come flying out at higher speeds. But it didn't work that way.

If you used a beam of green light, scientists eventually discovered, all the electrons came out at the same speed. If you changed the color to blue or ultraviolet without changing the intensity, there would be fewer electrons, but they would come out at higher speeds. If you increased the intensity of the beam, the speed of the emitted electrons would stay the same, but the number of electrons would increase.

Two other features were even more puzzling. If you changed the color of the light beam more and more toward the red end of the spectrum, the electrons would come out with slower and slower speeds, until at some color in the deep red or infrared spectrum, no electrons would be emitted at all, regardless of how much energy there was in the beam. If you decreased the intensity of a green beam until the energy in the beam was so low that only one electron was emitted every second, and then spread the beam over a meter-wide plate of material so that any portion of the plate received only a small fraction of the total energy needed to emit an electron, you found that still one electron would be emitted every second!

None of this made sense to the experimentalists of that time, who still visualized light as a wave. Some theorists were on the right track, especially Planck, who proposed that nature acted by using "quanta" of energy. But it was the young, unknown Einstein who explained everything and started the field of quantum mechanics with his paper on the photoelectric effect.

A Lesson in Light

Einstein showed that light does not consist of continuous waves, nor of small, hard particles. Instead, it exists as bundles of wave energy called photons. Each photon has an energy that corresponds to the frequency of the waves in the bundle. The higher the frequency (the bluer the color), the greater the energy carried by that bundle. Instead of spreading out as it travels, as an ordinary wave would, the waves in a photon stay bunched together. When the photon hits the photoelectric surface, all of its energy is delivered to one place. Using these very novel concepts, Einstein was able not only to explain the behavior of photoelectricity but also to give mathematical formulas that others could use to calculate other features of the behavior of light. His ability to observe nature and correctly deduce the sometimes very bizarre behavior of the physical world made Einstein a Natural Philosopher. Those insights alone would have been a great contribution. However, Einstein went even further. He was able to translate his insights into the cold, rigid, logical system of thought we call mathematics. Once the beautiful ideas were in that form, they could then be manipulated by ordinary people and their calculating machines.

Einstein on Atoms and Molecules

The next two 1905 papers by Einstein were on the theory of atoms and molecules and their behavior. There were still many scientists in 1905 who did not believe in atoms or molecules! Matter to them was a continuous but porous substance, and heat was a weightless fluid that moved through the pores. Einstein, with his unique insight, observed that a solid melted into a liquid. The liquid then evaporated into a gas. The gas then behaved like a collection of small particles or molecules that bounced around in the otherwise empty space of a container holding the gas. When you changed the volume or the temperature of the container, the pressure of the gas would change by a simple law that had been deduced from empirical facts many decades before by the inventors of steam engines.

Einstein took these pieces of ideas and put them together into a coherent picture of all matter consisting of tiny, indivisible atoms or molecules. Heat to Einstein was just the random motion or vibration of those atoms exchanging energy according to the mathematical laws that Einstein had previously worked out for photons. In a mathematical tour de force, he concluded one paper by calculating the size of the invisible molecules! (According to Einstein's 1905 calculations, the diameter of a sugar molecule is 9.8 angstroms, or about a billionth of a meter.)

In a third paper, Einstein used his new knowledge of atoms to explain a puzzling microscopic phenomenon. In the earliest days of the compound microscope, nearly 50 years before Einstein was born, the botanist Robert Brown had found that tiny grains of pollen floating in a drop of water never stopped moving!

Normally, you might expect that when you slipped a freshly prepared slide under a microscope, the water would swirl around and the pollen grains would move along with the tide. After a while, friction would slow the motion, and eventually the water and pollen would come to a stop. But it was found that even though one waited a very long time—one experimenter waited a whole year between peeks—the motion of the pollen never stopped. It was as if the pollen had an inexhaustible supply of energy. Which may attribute to all those pesky allergies.

Einstein explained this "Brownian motion" by demonstrating that the continuous random motion of the pollen grains was caused by heat in the water. Heat was not an invisible, weightless fluid but rather the rapid motion of each of the tiny water molecules rushing back and forth, bouncing off each other and the pollen grain, exchanging energy in a random way with each collision (but never, never losing energy). The energy was always conserved—as motion of one particle or another. "Friction" did not exist at the atomic level.

Although the water molecules were thousands of times smaller than the pollen, there were a lot of them. At any one instant there would be 100 million water molecules bouncing off one side of a pollen grain and 100 million (plus or minus 10,000) molecules bouncing off the other side. Although one water molecule could not budge a pollen grain, the pushes from the random 10,000-molecule differences could move the grain far enough for one to see it move under the microscope.

Einstein’s Equations

Again, Einstein not only explained a puzzling phenomenon but wrote down a mathematical equation that described it. The equation predicted that the size of the jumps made by the pollen grain would increase as the grain was made smaller, the liquid less viscous, and the temperature greater. All of these premises were verified by experiment. The same formula, with suitable modifications, is used today to explain noise in radar, radio, and telephone systems. It shows us that at any given temperature there is an ultimate limit to the sensitivity of any detector or amplifier. This limit is caused by the random motion of the atoms and electrons that make up the apparatus itself.

Although Einstein had a strong hand in giving quantum mechanics, the mechanical theory of atoms and light, its mathematical base, he did not contribute significantly to its full development. For one thing, he was dissatisfied with the philosophy implied by his own mathematical inventions. To explain the behavior of atoms and light, Einstein had to use the theory of statistics, which he loathed. The theory of statistics assumes that everything happens by chance. There is no direct relationship between cause and effect. This view was repugnant to Einstein, who is often quoted as having said, "God may be subtle... but He does not play dice." To Einstein, the idea that God created a universe that was not run according to strict rules (which he someday hoped to discover) was unacceptable. He always felt that underneath the seemingly random behavior of atoms and light there was a more logical, predictable base.

Another reason Einstein left the field of quantum mechanics to others was that he had set himself a more important task. That was to deduce the nature of (and write down the mathematical laws for) everything in the entire universe! He began studying those four elements that seemed to make up everything: space, time, matter, and energy. When he finished, the four elements had been reduced to two.

Traveling at the Speed of Light

Einstein's special theory of relativity can be called the Einstein theory of mechanics at high velocities. The old Newtonian laws of mechanics describe how matter moves through space and time. Matter gains and loses energy as various forces act upon it. The Newtonian laws work very well for objects moving at ordinary speeds, but Einstein realized that they weren't going to be adequate when mass velocities began to approach the speed of light.

Thinking unthinkable thoughts, as usual, Einstein asked himself, "What would I see if I could travel at the speed of light and were to look at a photon traveling beside me?" One answer was that he would see the electromagnetic fields in the light standing motionless. Yet it was the vibration of the fields that gave the light its frequency, its energy, its very existence. He asked other knotty questions. "Suppose I were traveling at nearly the speed of light and I sent out a beam of light ahead of me. Being light, it would seem to move away at the speed of light, but since I am sending it from a moving platform, would not some other observer see the light beam moving faster than the speed of light?"

To Einstein the only consistent answer to any of these questions was "No, no matter how fast or slow I am going, to me a beam of light is always traveling at the same speed."

According to any reasonable extrapolation of Newton's laws, this was impossible. Different observers should measure different velocities of light, depending upon whether their own motion is added to or subtracted from the motion of the thing being observed. Yet Einstein accepted his "impossible" answer and then derived mathematical equations describing space, time, matter, and energy that would produce the desired result. Their implications are astounding in the abandonment of common sense, yet all of them have proved true time and time again. They are:

• Space can be converted into time, and vice versa.
• Mass can be converted into energy, and vice versa.
• As you travel near the speed of light, space shrinks, time expands, and mass increases.
• If you travel at the speed of light, space shrinks to zero, time increases to eternity and your mass, if you had any to start with, increases to infinity.

Since it would take an infinite supply of energy to move an infinite mass, it is impossible for any material object (like Einstein himself) ever to attain the speed of light. Light, being a form of pure energy, has no rest mass per se, and so can—and to exist, must—travel at the speed of light. Yet what an odd universe the photon lives in. Since its space has shrunk to zero and its time has expanded to eternity, the photon exists everywhere along its trajectory at all times.

Understanding the Theory of Relativity

The facet of Einstein's special theory of relativity hardest for people to accept is the slowing down of time at high velocities. This is best illustrated by the famous twin paradox:

There are two astronauts who are twins. One astronaut travels off on an interstellar spacecraft and spends a long time traveling at nearly the speed of light. When he returns he will find that his stay-at-home brother has aged considerably, while he himself is still young.

This time-dilation effect is real and is used to advantage every day in large-particle accelerators. The debris from atypical experiment often consists of elementary particles that exist for only a trillionth of a second. If time did not slow down for these particles, they would travel less than a millimeter before complete decay—not far enough for them to get out of the target chamber and into the detectors. However, such particles are traveling so close to the speed of light that their time is slowed by factors of 10,000 or more. They then live long enough in our frame of reference to travel many meters to the detectors, where they can be identified.

Understanding Acceleration and Gravity

After Einstein's success with the special theory of relativity, he turned to the mechanics of acceleration and gravity. A thought experiment Einstein used to explore gravitation and acceleration was to imagine himself in a small elevator with no windows. If the elevator was stopped but sitting on the earth, he would feel his weight on his feet. If he dropped a ball, it would fall to the floor.

Now suppose the elevator were out in space away from the Earth's gravity, but it was being lifted at a constant acceleration equal to the Earth's gravity. He would not be able to tell the difference.

Einstein then made the philosophical leap: Not only would he not be able to tell the difference—there would be no difference!

In one stroke of genius, Einstein threw away the Newtonian concept of gravity as a force by which one large mass attracts other masses. Einstein said there is no such thing as a "gravity force." Rather, a large mass "curves space" near it, and other masses move in that curved space in force-free orbits.

Einstein’s Mathematical Structure

Now one might think that all Einstein did was to produce a complicated description for the more simple Newtonian picture of gravity. But Einstein, as usual, had a complete mathematical structure to backup his philosophical ideas, and mathematical predictions can be checked by experimentalists. The first of these was the calculation of the anomaly in the orbit of Mercury. For centuries, Newtonian law worked well for predicting the behavior of the planets. Of course, as observations improved, theorists had to put in more and more correction terms (the effect of Jupiter's gravity on Saturn, for example) to reconcile certain realities with Newton's theory. One minuscule effect—the orbit of the most insignificant planet, Mercury—seemed to escape them.

The orbit of Mercury is quite elliptical. Whereas most other planets have nearly circular orbits, Mercury's is visibly egg shaped. Because of the perturbation of the other planets, especially Venus, Earth, and Jupiter, you expect the major axis of Mercury's orbit to precess around the sun, and it does—something like 5,600 seconds of arc (about 1.6 degrees) per century. Try as they might, the theorists could only calculate something less than that. For some strange reason not predicted by the Newtonian law of gravity, the precession of the orbit of Mercury was 43 seconds of arc per century (one revolution every 3 million years) more than it should be. When Einstein used mathematics to predict the effect of the curving of space by the sun on the orbit of Mercury, he found that his equations predicted 43 seconds of arc per century more precession than Newton's laws.

Exactly the amount unaccounted for!

Yet another prediction of the general theory of relativity is that clocks should run more slowly in a gravitational field. That means that if you have a clock on the surface of the earth and another on the top of a mountain or in orbit, the one on the ground will tick more slowly (live longer) than the higher one.

Testing the Prediction

As one of the better-justified boondoggles in the annals of science, nothing could beat parlaying a test of Einstein's theories of relativity into a trip around the world.

In 1971, American physicists Hafele and Keating borrowed two identical, very accurate portable time standards from the US Naval Observatory and obtained a grant from the Office of Naval Research to pay for three first-class tickets around the world (one seat for each of them and one seat for the clock). The twin clocks were set to the same time in Washington, D.C. One clock stayed in Washington, where it was subjected to the slowing down of time owing to its position in the gravity field of the earth. The other clock took off at 960 kilometers per hour and went around the earth at an average height of nine kilometers.

The time as measured by the moving twin was slowed down by the fact that it was moving at a velocity close to that of light. (Well, at least it was closer to the speed of light than the clock in Washington.) The time as measured by the stay-at-home twin was slowed by the fact that it was subjected to a much greater gravitational field than the elevated twin. (It was 1.001 times greater.) However, the velocity effect was larger than the gravitational effect, so upon return to Washington, the moving clock was found to be slower by exactly the amount predicted by the two theories of relativity. Just to check, the scientists and the by now blasé world-traveling clock went back around the earth the other way, where the rotational speed of the earth subtracted from the airplane's speed rather than adding to it. Again the scientists got the correct result. (This was probably the cheapest test of general relativity ever made; it cost only $8,000, of which $7,600 was spent on air fares.)

With the confirmations of his theory in the early 1920s, Einstein became world renowned. As part of his duties he was called upon again and again to give lectures and write about his discoveries. Unlike many other scientists, who find it impossible to speak without using the familiar (and safe) jargon of mathematics, Einstein was a true genius who could explain his ideas to non specialists.

Despite the demands of fame, Einstein was not finished with his self-imposed goal of completely understanding the universe. He began work on a unified-field theory that would unify space-time, matter-energy, and electromagnetism.

Einstein's intellect finally met its match in the game he played with his God. God had set up a puzzle: "Deduce the rules of the universe, human. When you do, then you shall know me!" Einstein had had more success than any physicist before him, and though he gave his best efforts until his death in 1955, he was unsuccessful.

Foundation of the Laser

Yet, while treading the long, eventually fruitless, path of his search for the unified field theory, Einstein was still able to contribute enormous insights in ways too subtle for many to appreciate. In 1924, he received a paper from Indian physicist S.N. Bose that described light as a gas consisting of photons. This photon gas was a strange type of gas, for the particles in it did not obey the common-sense statistical laws that billiard balls do. If you randomly roll a number of perfectly elastic billiard balls on a frictionless table, sooner or later they will all end up in one pocket or another. If you checked by repeated experiments, you would find that all the balls had an equal probability of falling into any one of the pockets. But if the billiard balls behaved like photons, you would find that if one of the pockets already had a ball in it, the rest of the balls would have a tendency to fall into that pocket. In fact, the more balls already in a pocket, the more likely another ball would choose to join its identical mates.

Now there is no force or attraction involved in this effect. It is just a statistical tendency that causes photons to prefer to travel together. This phenomenon, developed and mathematically expressed by Bose and Einstein, is what makes a laser!

Although Einstein did not invent the laser, his work laid the foundation. It was Einstein who pointed out that stimulated emission of radiation could occur. He used his photon mathematics to examine the case of a large collection of atoms full of excess energy and ready to emit a photon at some random time in a random direction. If a stray photon passes by then the atoms are stimulated by its presence to emit their photons early. More remarkably, the emitted photons go in the same direction and have exactly the same frequency as the original photon. Later, as the small crowd of identical photons moves through the rest of the atoms, more and more photons will leave their atoms early to join in the subatomic parade.

All it took to invent the laser was for someone to find the right kind of atoms and to add reflecting mirrors to help the stimulated emission along. Remember, the acronym LASER means Light Amplification by (using Einstein's ideas about) Stimulated Emission of Radiation.

So as we look back on his life and legacy, Einstein is rightly remembered for his special and general theories of relativity and all the wondrous things, like nuclear energy and black holes, that came from these works. But let us not forget that he also measured the atom, explained the solar cell, described the limits to our senses, and nearly invented the laser. Quite an accomplishment!

Recommended Reading

Einstein's legacy is even greater than could possibly be covered by one article. To further explore the advances made through Einstein's discoveries and equations, read Einstein: His Life and Universe.

Einstein: His Life and Universe by Walter Isaacson was based on personal letters of Einstein, and explores his success in questioning conventional wisdom. The volume looks at how a patent clerk evolved into the man whose inventions still impact our daily lives over 100 years later.