Albert Einstein is thought to have been a genius, and he is considered one of the world's greatest thinkers. Although he isn't known for inventions, as with Thomas Edison or Nikola Tesla, Einstein's theories and ideas related to physics continue to exert influence today.

He spent much of his life researching theories of relativity, investigating space, time, matter, and energy. So, what were Albert Einstein's most significant theories? As we look back on this innovative thinker, here are some of Albert Einstein's most significant achievements.

**1. Quantum Theory of Light**

Einstein proposed his theory of light, stating that all light is composed of tiny packets of energy, called photons. He suggested these photons were particles but also had wave-like properties, a totally new idea at the time.

He also spent some time outlining the emission of electrons from metals as they were hit with large electric pulses, like lightning. He expanded on this concept of the photoelectric effect, which we'll discuss later in this article.

**2. Special Theory of Relativity**

In Einstein's studies, he began to notice inconsistencies of Newtonian mechanics in their relation to the understanding of electromagnetism, specifically to Maxwell's equations. In a paper published in September 1905, he proposed a new way of thinking about the mechanics of objects approaching the speed of light.

This concept became known as Einstein's Special Theory of Relativity. It changed the understanding of physics at the time.

Einstein’s revelation was that observers in relative motion experience time differently. He realized that it is possible for two events to happen simultaneously from the perspective of one observer, but occur at different times from the perspective of the other. And both observers would be right.

Understanding the Special Theory of Relativity can be a little difficult, but we'll boil it down to a simple situation.

He began with the idea that light always travels at a constant 300,000 km/s, and asked what would happen to our ideas of space and time if that was the case?

Now, imagine that you once again have an observer standing on a railway embankment as a train goes by, and that each end of the train is struck by a bolt of lightning just as the train’s midpoint is passing the observer. Because the lightning strikes are the same distance from the observer, their light reaches his eye at the same instant. So the observer would say the two strikes happened simultaneously.

However, there is another observer, this on the train, sitting at its exact midpoint. Because the train is moving, the light coming from the lightning in the rear has to travel farther to catch up, so it reaches this observer later than the light coming from the front. This observer would conclude the one in front actually happened first. And both observers would be correct.

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Einstein determined that motion through space can also be thought of as motion through time. In essence, space and time affect each other, both being relative concepts in relation to the speed of light.

## 3. Avogadro's Number

For anyone that has made it through a high school chemistry class, Avogadro's number might ring a bell.

While Einstein was working to develop his mathematical model for explaining Brownian motion, the erratic movement of particles in a fluid, he also proved the existence of atoms, and laid the foundation for calculating Avogadro's number, the number of atoms in one mole of molecule or an element.

Einstein's work on Brownian motion suggested the existence of tiny indistinguishable particles. This theory was later proven by Jean Perrin, who carried out experiments using a high precision microscope to verify Einstein's mathematical work. This allowed Perrin to calculate Avogadro's number and prove the existence of atoms — for which he received the Nobel Prize in 1926.

## 4. The Bose-Einstein Condensate

In 1924, Einstein was sent a paper from physicist Satyendra Nath Bose. This paper discussed a detailed way to think of photons of light as a gas. Einstein generalized Bose’s theory to an ideal gas of identical atoms or molecules for which the number of particles is conserved.

Einstein worked with Bose to extend this idea to atoms, which led to a prediction for a new state of matter: the Bose-Einstein Condensate. The first example of this state was produced in 1995.

He also predicted that at sufficiently low temperatures, the particles would become locked together in the lowest quantum state of the system. This phenomenon is called Bose-Einstein condensation.

A Bose-Einstein condensate is essentially a group of atoms that are cooled very close to absolute zero. When they reach that temperature, they hardly move in relation to one another. They begin to clump together and enter into exactly the same energy states. This means that, from a physical point of view, the group of atoms behaves as if they were a single atom.

We now know that this only happens for “bosons” — particles with a total spin that is an integer multiple of *h*, the Planck constant divided by 2 pi.

## 5. General Theory of Relativity

In 1916, Einstein published his G*eneral Theory of Relativity*. This paper generalizes the concepts of Special Relativity and Newton's Law of Universal Gravitation, describing gravity as a property of space and time. This theory has aided our understanding of how the large-scale structure of the universe is set up.

The Theory of General Relativity can be explained like this:

Newton helped quantify gravity between two objects as a tugging of two bodies, independent of how massive each one is or how far apart they are.

Einstein determined that the laws of physics hold constant for all non-accelerating observers, that the speed of light is constant no matter how fast the observer travels. He found that space and time were interwoven and that events that occur at one time for one observer could occur at a different time for the next.

This led to his theory that massive objects in space could distort spacetime.

Einstein's predictions have helped modern physicists study and understand black holes and gravitational lensing.

## 6. The Photoelectric Effect

Einstein's theory of the Photoelectric Effect discusses the emissions of electrons from metal when light shines on it, as we alluded to before. Scientists had observed this phenomenon but had been unable to reconcile the finding with Maxwell's wave theory of light.

His theory of photons aided the understanding of this phenomenon. He theorized that, as light hits an object, there is an emission of electrons, which he deemed photoelectrons.

This model formed the basis of how solar cells work — light causes atoms to release electrons, which generate a current, thus creating electricity.

## 7. Wave-Particle Duality

Albert Einstein's work on the development of the quantum theory was some of the most impactful he ever accomplished. During his early career, Einstein persisted in asserting that light should be treated as both a wave and a particle. In other words, photons can behave as particles and as waves at the same time. This became known as wave-particle duality.

He is quoted as saying this on the subject, "We are faced with a new kind of difficulty. We have two contradictory pictures of reality; separately, neither of them fully explains the phenomena of light, but together they do."

As we think about all of Einstein's work, we have to also consider how it has influenced those who came after him. Einstein's work has influenced advanced modern quantum mechanics, the model of physical time, the understanding of light, solar panels, and even modern chemistry. He relentlessly questioned the world around him. This is what made him great, his infinite curiosity about the world.

The important thing is not to stop questioning. "Curiosity has its own reason for existing", remarked Einstein. Albert Einstein's accomplishments have unequivocally influenced our understanding of physics as we know it today.