When Einstein began his research as an amateur scientist, there were two major problems:
- Light was known to be a wave but had to be considered as made up of lumps — not waves — to explain the ultraviolet catastrophe (the observation that hot objects emit less ultraviolet light and more light of other colors).
- In mechanics, the results of experiments are identical in motion or at rest (all motion is relative, and there is no absolute motion). Not so in electromagnetism, because you can be at rest in the ether (there is absolute motion).
Scientists were struggling to make existing theories work, but more and more they were becoming aware of their inadequacies. The stage was set for Einstein to make history, and in 1905, he did just that.
What did Einstein achieve during his year of miracles? He wrote and published five scientific papers that would change physics forever:
1. March 17: "On a heuristic point of view concerning the production and transformation of light." This paper laid the foundation for quantum theory with the introduction of the concept of quanta of energy, or photons.
2. April 30: "A new determination of molecular dimensions." This was Einstein's PhD dissertation, which the University of Zurich accepted in July. Although not revolutionary, this paper helped establish the existence of molecules.
3. May 11: "On the motion of small particles suspended in a stationary liquid." This paper not only explained the zigzag motion of a speck in a liquid (called Brownian motion), which had puzzled scientists for a long time, but also showed the reality of molecules.
4. June 30: "On the electrodynamics of moving bodies." This was Einstein's first paper on the theory of relativity.
5. September 27: "Does an object's inertia depend on its energy content?" This second paper on the theory of relativity contained Einstein's most famous equation: E = mc2.
Even before the first paper was published, Einstein suspected that what he was about to do was of great importance. In May of 1905, he wrote to one of his closest friends:
I promise you four papers . . . the first of which I might send you soon, since I will be receiving the free reprints. The paper deals with radiation and the energy properties of light and is very revolutionary, as you will see . . .
The first paper of 1905 certainly was revolutionary. It laid the foundation for quantum theory. Einstein won the Nobel Prize in physics several years later for this work.
As if that weren't enough, the fourth paper and fifth papers that Einstein published that year were also revolutionary. The other two papers were also very important because they helped to establish the existence of atoms and molecules, which were not yet universally accepted. But unlike the other three, they didn't turn the scientific world upside down.
Defining the nature of light
Einstein solved the first major problem in physics with thefirst paper of his miracle year, the paper on the light quantum.
German physicist Max Planck had used a mathematical trick to explain radiation in the ultraviolet part of the spectrum; he bundled light into quanta of energy. In the first paper of 1905, Einstein made Planck's quanta a property of light and of all electromagnetic radiation (radio waves, x-rays, ultraviolet and infrared light, and so on). It isn't that light is lumpy in some instances. Light is always lumpy, like a particle. It comes in bundles. The light emitted by hot objects isn't somehow split into these bundles. Light is made up of these bundles, these photons as they are called, that can't be split.
By making lumpiness a property of light, Einstein paved the way for the development of quantum theory that would take place in the 1920s. Quantum theory would later explain that light is both a wave and a particle. Light behaves like a wave under certain conditions, and under other conditions, it behaves like a particle. Quantum theory integrates both behaviors seamlessly.
Even though Einstein's first paper was read with a great deal of interest, most physicists didn't believe his idea of photons of light, including Planck himself initially. For the next 15 years, Einstein was almost the only one who believed in the light quantum idea. But quantum theory, developed by other physicists in the 1920s based on Einstein's work, would become the most successful physics theory ever.
Einstein's first paper of 1905 also explained a phenomenon called the photoelectric effect in a clever but simple way. In 1921, after Einstein had already become world famous, the Nobel committee awarded him the Nobel Prize in physics for this discovery.
Eliminating the ether
A key contradiction between mechanics and electromagnetism was the existence of absolute motion. According to Newton, all motion is relative — absolute motion can't exist. But according to Maxwell, it can.
Einstein sided with mechanics. In his fourth paper of 1905, commonly referred to as the relativity paper (even though the word relativity doesn't appear in the title), Einstein reformulated electromagnetism so that it would also remain unchanged whether the person observing was at rest or moving at a constant velocity. In other words, he modified electromagnetism so that its description would depend only on relative motion, without any need for the ether. Light does not need a substance to move through. It can move in the empty space between the stars.
With the publication of this paper, the ether was gone from physics. According to Einstein, absolute motion does not exist. When you are on an airplane, you have no way to tell, without looking out the window, whether you are moving or at rest. All the laws of physics, those of mechanics and those of electromagnetism, are the same everywhere in the universe, no matter how you move (provided that you don't accelerate).
Einstein extended the idea of relative motion to light itself. Anybody, anywhere in the universe, whether at rest or in motion with a constant velocity, always measures the same speed of light.
All of thephysics known at the timefollowed the simple principles that Einstein put forward in his relativity paper. And all the physics discoveries since then have followed those principles. Einstein's paper didn't just fix the problems with electromagnetism; it actually created a new way of looking at the world.
Introducing E = mc2
Einstein's final paper of 1905, which was also the last of his revolutionary papers, contained the famous E = mc2 equation. This paper was more of a follow-up to the first relativity paper than an introduction to a new equation.
In this beautiful three-page paper, Einstein used electromagnetic equations from his first relativity paper to explain that energy has mass. Two years later, he realized that the opposite should also be true, that mass of any kind must have energy. According to Einstein, mass and energy are equivalent. An object's mass is a form of energy, and energy is a form of mass.
Here are a few examples of how this tiny little equation has changed our lives in big ways:
- Scientists spent more than 40 years finding a way to demonstrate the reality of E = mc2. World events made this demonstration very dramatic with the development of the nuclear bomb, which was first tried in the desert in Alamogordo, New Mexico, in July of 1945. One month later, the bomb was dropped for real in Hiroshima and Nagasaki, Japan. The energy released by the bomb comes from nuclear fission, the splitting of the uranium-235 nucleus.
- E = mc2 gives the recipe for the conversion of part of the uranium nucleus into energy. The same recipe applies to a nuclear reactor, except that the production of energy is controlled with very precise procedures.
- Together with the later development of quantum physics, E = mc2 helped explain another long-standing problem: understanding how the sun burns its fuel and generates the energy that makes possible life on earth.
