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Albert Einstein
Albert Einstein Head.jpg
Albert Einstein, 1947
Born (1879-03-14)14 March 1879
Died 18 April 1955(1955-04-18) (aged 76)
Nationality German, American
Alma mater
Known for
Spouse(s) Mileva Marić (1903–1919)
Elsa Löwenthal (1919–1936)
Children "Lieserl" (1902–1903?)
Hans Albert (1904–1973)
Eduard "Tete" (1910–1965)
Scientific career
Fields Physics
Thesis Folgerungen aus den Capillaritatserscheinungen (1901)
Doctoral advisor Alfred Kleiner
Other academic advisors Heinrich Friedrich Weber
Notable students
Albert Einstein signature 1934.svg

Albert Einstein (14 March 1879 – 18 April 1955) was a German-born theoretical physicist who developed the general theory of relativity, one of the two pillars of modern physics (alongside quantum mechanics).

He received the Nobel Prize in Physics in 1921, but not for relativity. His theories of special and general relativity are of great importance to many branches of physics and astronomy. They have been given experimental confirmation by many experiments and observations.

Einstein is well known for his theories about light, matter, gravity, space, and time. His most well known equation is E=mc^2. It means that energy and mass are different forms of the same thing.

Einstein published more than 300 scientific papers and over 150 non-scientific works. He received honorary doctorate degrees in science, medicine and philosophy from many European and American universities.

Near the beginning of World War II, he warned President Franklin D. Roosevelt that Germany might be developing an atomic weapon, and recommended that the U.S. begin nuclear weapons research. That research, begun by a newly established Manhattan Project, resulted in the U.S. becoming the first and only country to have nuclear weapons during the war.


Einstein was born [at Ulm] in Württemberg, Germany, on 14 March 1879. His family was Jewish, but was not very religious. However, later in life Einstein became very interested in his Judaism. Einstein did not begin speaking until after age two. According to his younger sister, Maja, "He had such difficulty with language that those around him feared he would never learn". When Einstein was around four, his father gave him a magnetic compass. He tried hard to understand how the needle could seem to move itself so that it always pointed north. The needle was in a closed case, so clearly nothing like wind could be pushing the needle around, and yet it moved. So in this way Einstein became interested in studying science and mathematics. His compass inspired him to explore the world of science.

When he became older, he went to a school in Switzerland. After he graduated, he got a job in the patent office there. While he was working there, he wrote the papers that first made him famous as a great scientist.

Einstein had two severely disabled children with his first wife Mileva. His daughter "Lieserl" (her real name may never be known) was born about a year before their marriage in January 1902. She spent her very short life (believed to be less than 2 years) in the care of Serbian grandparents where it is believed she died from scarlet fever. Some believe she may have been born with the disorder called Down syndrome but it has never been proved. Her very existence only became known to the world in 1986 when a shoe box containing 54 love letters (mostly from Einstein), exchanged between Mileva and Einstein from late 1897 to September 1903, was discovered by Einstein's granddaughter in an attic in California. Their son, Eduard, was diagnosed with schizophrenia. He spent decades in hospitals, and died in the Zurich sanatorium in 1965.

In 1917, Einstein became very sick with an illness that almost killed him. His cousin Elsa Lowenthal nursed him back to health. After this happened, Einstein divorced Mileva, and married Elsa on 2 June 1919.

Just before the start of World War I, he moved back to Germany, and became director of a school there. He lived in Berlin until the Nazi government came to power. The Nazis hated people who were Jewish or who came from Jewish families. They accused Einstein of helping to create "Jewish physics," and German physicists tried to prove that his theories were wrong.

In 1933, under death threats from the Nazis and hated by the Nazi-controlled German press, Einstein and Elsa moved to Princeton, New Jersey in the United States, and in 1940 he became a United States citizen.

During World War II, Einstein and Leó Szilárd wrote to the U.S. president, Franklin D. Roosevelt, to say that the United States should invent an atomic bomb so that the Nazi government could not beat them to the punch. He was the only one who signed the letter. He was, however, not part of the Manhattan Project, which was the project that created the atomic bomb.

Einstein, a Jew but not an Israeli citizen, was offered the presidency in 1952 but turned it down, stating "I am deeply moved by the offer from our State of Israel, and at once saddened and ashamed that I cannot accept it." Ehud Olmert was reported to be considering offering the presidency to another non-Israeli, Elie Wiesel, but he was said to be "very not interested".

He taught physics at the Institute for Advanced Study at Princeton, New Jersey until his death on 18 April 1955 of a burst aortic aneurysm. He was still writing about quantum physics hours before he died. He was awarded the Nobel Prize in Physics.

Theory of special relativity

The theory of special relativity was published by Einstein in 1905, in a paper called "On the Electrodynamics of Moving Bodies". It says that both distance measurements and time measurements change near the speed of light. This means that as you get closer to the speed of light (nearly 300,000 kilometres per second), lengths appear to get shorter, and clocks tick more slowly.

Special relativity also relates energy with mass, in Albert Einstein's E=mc2 formula.

Mass-energy equivalence

E=mc2, also called the mass-energy equivalence, is one of the things that Einstein is most famous for. It is a famous equation in physics and math that shows what happens when mass changes to energy or energy changes to mass. The "E" in the equation stands for energy. Energy is a number which you give to objects depending on how much they can change other things. For instance, a brick hanging over an egg can put enough energy onto the egg to break it. A feather hanging over an egg does not have enough energy to hurt the egg.

There are three basic forms of energy: potential energy, kinetic energy, and rest energy. Two of these forms of energy can be seen in the examples given above, and in the example of a pendulum.

A pendulum converts potential energy to kinetic energy and back.

A cannonball hangs on a rope from an iron ring. A horse pulls the cannonball to the right side. When the cannonball is released it will move back and forth as diagrammed. It would do that forever except that the movement of the rope in the ring and rubbing in other places causes friction, and the friction takes away a little energy all the time. If we ignore the losses due to friction, then the energy provided by the horse is given to the cannonball as potential energy. (It has energy because it is up high and can fall down.) As the cannonball swings down it gains more and more speed, so the nearer the bottom it gets the faster it is going and the harder it would hit you if you stood in front of it. Then it slows down as its kinetic energy is changed back into potential energy. "Kinetic energy" just means the energy something has because it is moving. "Potential energy" just means the energy something has because it is in some higher position than something else.

When energy moves from one form to another, the amount of energy always remains the same. It cannot be made or destroyed. This rule is called the "conservation law of energy". For example, when you throw a ball, the energy is transferred from your hand to the ball as you release it. But the energy that was in your hand, and now the energy that is in the ball, is the same number. For a long time, people thought that the conservation of energy was all there was to talk about.

When energy transforms into mass, the amount of energy does not remain the same. When mass transforms into energy, the amount of energy also does not remain the same. However, the amount of matter and energy remains the same. Energy turns into mass and mass turns into energy in a way that is defined by Einstein's equation, E = mc2.

Albert Einstein (Nobel)
A picture of Einstein after winning his Nobel Prize, 1921

The "m" in Einstein's equation stands for mass. Mass is the amount of matter there is in some body. If you knew the number of protons and neutrons in a piece of matter such as a brick, then you could calculate its total mass as the sum of the masses of all the protons and of all the neutrons. (Electrons are so small that they are almost negligible.) Masses pull on each other, and a very large mass such as that of the Earth pulls very hard on things nearby. You would weigh much more on Jupiter than on Earth because Jupiter is so huge. You would weigh much less on the Moon because it is only about one-sixth the mass of Earth. Weight is related to the mass of the brick (or the person) and the mass of whatever is pulling it down on a spring scale — which may be smaller than the smallest moon in the solar system or larger than the Sun.

Mass, not weight, can be transformed into energy. Another way of expressing this idea is to say that matter can be transformed into energy. Units of mass are used to measure the amount of matter in something. The mass or the amount of matter in something determines how much energy that thing could be changed into.

Einstein 1921 portrait2
Albert Einstein, 1921

Energy can also be transformed into mass. If you were pushing a baby buggy at a slow walk and found it easy to push, but pushed it at a fast walk and found it harder to move, then you would wonder what was wrong with the baby buggy. Then if you tried to run and found that moving the buggy at any faster speed was like pushing against a brick wall, you would be very surprised. The truth is that when something is moved then its mass is increased. Human beings ordinarily do not notice this increase in mass because at the speed humans ordinarily move the increase in mass in almost nothing.

As speeds get closer to the speed of light, then the changes in mass become impossible not to notice. The basic experience we all share in daily life is that the harder we push something like a car the faster we can get it going. But when something we are pushing is already going at some large part of the speed of light we find that it keeps gaining mass, so it gets harder and harder to get it going faster. It is impossible to make any mass go at the speed of light because to do so would take infinite energy.

Sometimes a mass will change to energy. Common examples of elements that make these changes we call radioactivity are radium and uranium. An atom of uranium can lose an alpha particle (the atomic nucleus of helium) and become a new element with a lighter nucleus. Then that atom will emit two electrons, but it will not be stable yet. It will emit a series of alpha particles and electrons until it finally becomes the element Pb or what we call lead. By throwing out all these particles that have mass it has made its own mass smaller. It has also produced energy.

In most radioactivity, the entire mass of something does not get changed to energy. In an atomic bomb, uranium is transformed into krypton and barium. There is a slight difference in the mass of the resulting krypton and barium, and the mass of the original uranium, but the energy that is released by the change is huge. One way to express this idea is to write Einstein's equation as:

E = (muranium – mkrypton and barium) c2

The c2 in the equation stands for the speed of light squared. To square something means to multiply it by itself, so if you were to square the speed of light, it would be 299,792,458 meters per second, times 299,792,458 meters per second, which is approximately
(3•108)2 = (9•1016 meters2)/seconds2=
90,000,000,000,000,000 meters2/seconds2
So the energy produced by one kilogram would be:
E = 1 kg • 90,000,000,000,000,000 meters2/seconds2
E = 90,000,000,000,000,000 kg meters2/seconds2
E = 90,000,000,000,000,000 joules
E = 90,000 terajoule

About 60 terajoules were released by the atomic bomb that exploded over Hiroshima. So about two-thirds of a gram of the radioactive mass in that atomic bomb must have been lost (changed into energy), when the uranium changed into krypton and barium.


The idea of a Bose-Einstein condensate came out of a collaboration between S. N. Bose and Prof. Einstein. Einstein himself did not invent it but, instead, refined the idea and helped it become popular.

Zero-point energy

The concept of zero-point energy was developed in Germany by Albert Einstein and Otto Stern in 1913.

Momentum, mass, and energy

Statue of Albert Einstein in the Israel Academy of Sciences and Humanities.

In classical physics, momentum is explained by the equation:

p = mv


p represents momentum
m represents mass
v represents velocity (speed)

When Einstein generalized classical physics to include the increase of mass due to the velocity of the moving matter, he arrived at an equation that predicted energy to be made of two components. One component involves "rest mass" and the other component involves momentum, but momentum is not defined in the classical way. The equation typically has values greater than zero for both components:

E2 = (m0c2)2 + (pc)2


E represents the energy of a particle
m0 represents the mass of the particle when it is not moving
p represents the momentum of the particle when it is moving
c represents the speed of light.

There are two special cases of this equation.

Albert Einstein in later years
Einstein in his later years, c. 1950s

A photon has no rest mass, but it has momentum. (Light reflecting from a mirror pushes the mirror with a force that can be measured.) In the case of a photon, because its m0 = 0, then:

E2 = 0 + (pc)2
E = pc
p = E/c

The energy of a photon can be computed from its frequency ν or wavelength λ. These are related to each other by Planck's relation, E = hν = hc/λ, where h is the Planck constant (6.626×10−34 joule-seconds). Knowing either frequency or wavelength, you can compute the photon's momentum.

In the case of motionless particles with mass, since p = 0, then:

E02 = (m0c2)2 + 0

which is just

E0 = m0c2

Therefore, the quantity "m0" used in Einstein's equation is sometimes called the "rest mass." (The "0" reminds us that we are talking about the energy and mass when the speed is 0.) This famous "mass-energy relation" formula (usually written without the "0"s) suggests that mass has a large amount of energy, so maybe we could convert some mass to a more useful form of energy. The nuclear power industry is based on that idea.

Einstein said that it was not a good idea to use the classical formula relating momentum to velocity, p = mv, but that if someone wanted to do that, he would have to use a particle mass m that changes with speed:

mv2 = m02 / (1 – v2/c2)

In this case, we can say that E = mc2 is also true for moving particles.

The General Theory of Relativity

General relativity
G_{\mu \nu} + \Lambda g_{\mu \nu}= {8\pi G\over c^4} T_{\mu \nu}
Einstein field equations

The General Theory of Relativity was published in 1915, ten years after the special theory of relativity was created. Einstein's general theory of relativity uses the idea of spacetime. Spacetime is the fact that we have a four-dimensional universe, having three spatial (space) dimensions and one temporal (time) dimension. Any physical event happens at some place inside these three space dimensions, and at some moment in time. According to the general theory of relativity, any mass causes spacetime to curve, and any other mass follows these curves. Bigger mass causes more curving. This was a new way to explain gravitation (gravity).

General relativity explains gravitational lensing, which is light bending when it comes near a massive object. This explanation was proven correct during a solar eclipse, when the sun's bending of starlight from distant stars could be measured because of the darkness of the eclipse.

General relativity also set the stage for cosmology (theories of the structure of our universe at large distances and over long times). Einstein thought that the universe may curve a little bit in both space and time, so that the universe always had existed and always will exist, and so that if an object moved through the universe without bumping into anything, it would return to its starting place, from the other direction, after a very long time. He even changed his equations to include a "cosmological constant," in order to allow a mathematical model of an unchanging universe. The general theory of relativity also allows the universe to spread out (grow larger and less dense) forever, and most scientists think that astronomy has proved that this is what happens. When Einstein realized that good models of the universe were possible even without the cosmological constant, he called his use of the cosmological constant his "biggest blunder," and that constant is often left out of the theory. However, many scientists now believe that the cosmological constant is needed to fit in all that we now know about the universe.

A popular theory of cosmology is called the Big Bang. According to the Big Bang theory, the universe was formed 15 billion years ago, in what is called a "gravitational singularity". This singularity was small, dense, and very hot. According to this theory, all of the matter that we know today came out of this point.

Einstein himself did not have the idea of a "black hole", but later scientists used this name for an object in the universe that bends spacetime so much that not even light can escape it. They think that these ultra-dense objects are formed when giant stars, at least three times the size of our sun, die. This event can follow what is called a supernova. The formation of black holes may be a major source of gravitational waves, so the search for proof of gravitational waves has become an important scientific pursuit.


Many scientists only care about their work, but Einstein also spoke and wrote often about politics and world peace. He liked the ideas of socialism and of having only one government for the whole world. He also worked for Zionism, the effort to try to create the new country of Israel.

Einstein's family was Jewish, but Einstein never practiced this religion seriously. He liked the ideas of the Jewish philosopher Baruch Spinoza and also thought that Buddhism was a good religion.

Even though Einstein thought of many ideas that helped scientists understand the world much better, he disagreed with some scientific theories that other scientists liked. The theory of quantum mechanics discusses things that can happen only with certain probabilities, which cannot be predicted with better precision no matter how much information we might have. This theoretical pursuit is different from statistical mechanics, in which Einstein did important work. Einstein did not like the part of quantum theory that denied anything more than the probability that something would be found to be true of something when it was actually measured; he thought that it should be possible to predict anything, if we had the correct theory and enough information. He once said, "I do not believe that God plays dice with the Universe."

Because Einstein helped science so much, his name is now used for several different things. A unit used in photochemistry was named for him. It is equal to Avogadro's number multiplied by the energy of one photon of light. The chemical element Einsteinium is named after the scientist as well. In slang, we sometimes call a very smart person an "Einstein."


Most scientists think that Einstein's theories of special and general relativity work very well, and they use those ideas and formulas in their own work. Einstein could not agree that phenomena in quantum mechanics can happen out of pure chance. He believed that all natural phonomena have explanations that do not include pure chance. He spent much of his later life trying to find a "unified field theory" that would include his general relativity theory, Maxwell's theory of electromagnetism, and perhaps a better quantum theory. Most scientists do not think that he succeeded in that attempt.

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