Editor's Note: Andrew May holds a PhD in astrophysics from the University of Manchester, UK, and for 30 years has been active in academia, government and the private sector before becoming a science writer, writing for magazines such as Fortrean Times, How It Works, All About Space, Popular Science and others.
Albert Einstein (1879-1955) was one of the most famous scientists of all time, and his name has become almost synonymous with the word "genius." While much of his widespread reputation in the world is due to his eccentric appearance and occasional remarks on philosophy, world politics, and other non-scientific topics, what really makes him famous is his contributions to modern physics that changed our entire view of the universe and helped shape the world we live in today.
01
spacetime
Einstein's special theory of relativity changed the way we think about space and time—and established the universal speed limit of the speed of light.
Source: NASA
One of Einstein's earliest achievements at the age of 26 was his theory of special relativity— so called because it involves relative motion in special cases that ignore gravity. It may sound like nothing, but it was one of the greatest scientific revolutions in history that revolutionized the way physicists think about space and time. In effect, Einstein merged these into a single space-time continuum. One reason we think space and time are completely separate is because we measure them in different units, such as miles and seconds, respectively. But Einstein showed how they are actually interchangeable, interconnected by the speed of light — about 186,000 miles per second (300,000 kilometers per second).
Perhaps the most famous result of special relativity is that nothing can travel faster than the speed of light. But it also means that as we approach the speed of light, things start to get very strange. If you can see a spaceship flying at 80% of the speed of light, it will look 40% shorter than when stationary. According to Georgia State University's HyperPhysics website, if you can see inside, everything seems to be moving in slow motion, and the clock takes 100 seconds to walk through a minute. This means that the crew of the spaceship will actually get older as they travel slower.
02
E = mc2
E = mc2 is probably the most famous equation in the world
图源：VICTOR HABBICK VISIONS/SCIENCE PHOTO LIBRARY via Getty Images
An unexpected branch of special relativity is Einstein's famous equation E = mc2, which may be the only mathematical formula to achieve cultural icon status. The equation represents the equivalence of mass (m) and energy (E), two physical parameters that were previously considered completely independent. In traditional physics, mass measures the amount of matter contained in an object, while energy is a property that an object has due to its motion and the forces acting on it. In addition, energy can exist in completely no matter situations, such as in light or radio waves. However, Einstein's equation says that mass and energy are essentially the same thing, as long as you multiply the mass by c2 — the square of the speed of light, which is a very large number — to ensure that it ends up with the same result units as energy.
This means that the object increases mass as it moves faster, simply because it is gaining energy. This also means that even inert, stationary objects have a lot of energy locked in it. In addition to being an exciting idea, the concept also has practical applications in the field of high-energy particle physics. According to the European Commission for Nuclear Research (CERN), if enough energetic particles collide together, the energy of the collision can produce new matter in the form of additional particles.
03
laser
The stage of stimulated emission in the laser cavity.
Image credit: Encyclopædia Britannica/ Getty Images
Lasers are an important part of modern technology for everything from barcode readers and laser pointers to holograms and fiber optic communications. Although people often think that lasers have nothing to do with Einstein, it was ultimately his work that made it possible. According to the American Physical Society, the term laser was coined in 1959 to represent "light amplification of stimulated radiation" — a concept proposed by Einstein more than 40 years before that. In 1917, Einstein wrote a paper on the quantum theory of radiation, which described how photons passing through matter could trigger the emission of more photons.
Einstein realized that new photons travel in the same direction, at the same frequency, and in phase as the original photons. As more and more nearly identical photons are produced, this leads to cascading effects. As a theorist, Einstein did not delve further into the idea, while other scientists were slow to realize the enormous practical potential of stimulated ejection. But then the whole world discovered the immense value of lasers, and to this day people are still looking for new applications for lasers, from anti-drone weapons to ultra-high-speed computers.
04
Black holes and wormholes
In 1935, Einstein and Nathan Rosen described the possibility of a shortcut from one point in space-time to another—known as the Einstein-Rosen Bridge.
Image source: Shutterstock
Einstein's special theory of relativity shows that space-time can do some very strange things even without a gravitational field. But this is just the tip of the iceberg, as Einstein discovered when he finally succeeded in adding gravity to the mixture in his general theory of relativity. He found that massive objects like planets and stars actually distort the structure of space-time, and it is this distortion that produces the gravitational effect we perceive.
Einstein explained general relativity through a complex set of equations with a wide range of applications. Perhaps the most famous solution to Einstein's equation comes from Karl Schwartzchild's solution of 1916, the black hole. Even stranger is the solution developed by Einstein himself in 1935 in collaboration with Nathan Rosen, which describes the possibility of shortcuts from one point in space-time to another. Originally known as the Einstein-Rosen Bridge, all science fiction fans are now more familiar with the term "wormhole."
05
The expanding universe
Schematic diagram of the expansion of the universe.
图源：MARK GARLICK/SCIENCE PHOTO LIBRARY via Getty Images
Back in 1915, the first thing Einstein did with the equations of general relativity was to apply them to the entire universe. But in his opinion, the answer was wrong. This implies that the structure of space itself is in a state of constant expansion, pulling galaxies along with them, so the distance between them is increasing. Common sense told Einstein that this couldn't be true, so he added something to his equation called the cosmological constant to produce a well-behaved static universe.
But in 1929, Edwin Hubble's observations of other galaxies showed that the universe was indeed expanding, apparently as predicted by Einstein's primordial equations. It looked like the end of the cosmic constant, which Einstein later called his biggest mistake. However, this is not the end of the story. Based on more precise measurements of the expansion of the universe, we now know that it is accelerating, rather than slowing down without a cosmological constant. So it seems that Einstein's "mistake" was not a simple "mistake".
06
atomic bomb
In 1945, Alamogordo, New Mexico, united States, tested the atomic bomb for the first time.
Image source: Universal Historical Archive/Universal Photo Group Getty Images
Einstein is sometimes considered the "inventor" of nuclear weapons because of his equation E = mc2, but according to the Einstein online website of the Max Planck Institute for Gravitational Physics, the connection between the two is insignificant. The key factor was the physics of nuclear fission, in which Einstein was not directly involved. Nevertheless, he played a crucial role in the actual development of the first atomic bomb. In 1939, some colleagues reminded him of the possibility of nuclear fission and the horrors that would have been caused if Nazi Germany had acquired such weapons. Eventually, according to the Atomic Heritage Foundation, he was persuaded to give a letter to U.S. President Franklin W. Bush. D. Roosevelt conveyed these concerns in a letter. The end result of Einstein's letter was the launch of the Manhattan Project, which built the atomic bomb used against Japan at the end of World War II.
Although many prominent physicists were involved in the Manhattan Project, Einstein was not among them. According to the American Museum of Natural History (AMNH), he was denied the necessary security clearance because of his left-leaning political views. For Einstein, that wasn't a big loss — his only concern was denying the Nazi monopoly on the technology. In 1947, Einstein told Newsweek magazine, "If I knew the Germans wouldn't succeed in developing an atomic bomb, I would never have moved a finger," a statement from Time magazine.
07
gravitational wave
Gravitational waves, neutron stars
图源：R. Hurt/Caltech-JPL
Einstein died in 1955, but his vast scientific legacy continues to make headlines even in the 21st century. This happened in an astonishing way in February 2016, when scientists announced the discovery of gravitational waves — another result of general relativity. Gravitational waves are tiny ripples that propagate through the structure of space-time, and it is often bluntly said that Einstein "predicted" their existence. But the reality is not so clear.
Einstein was never entirely sure whether his theory predicted or ruled out gravitational waves. Astronomers have spent decades looking for ways to decide this thing in one way or another. Eventually they succeeded, using mega-facilities such as the Laser Interferometer Gravitational-Wave Observatory (LIGO) in Hanford, Washington, and Livingston, Louisiana. The discovery of gravitational waves is not only another triumph of Einstein's theory of general relativity (though he himself is less certain), but also provides astronomers with a new tool for observing the universe — including rare events like black hole mergers.