# 100 Years of General Relativity: Thought and Action

## Exactly 100 years ago on Nov. 25, 2015, physicist Albert Einstein, then 36, presented a fourth and final lecture to the Prussian Academy of Sciences about his new general theory of relativity.

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Exactly 100 years ago on Nov. 25, 2015, physicist Albert Einstein, then 36, presented a fourth and final lecture to the Prussian Academy of Sciences about his new general theory of relativity. The idea not only redefined the concept of gravity, but also ended up reshaping humanity’s perspective on reality. Here’s a look at the theory in thought and action.

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Einstein was famous for his thought experiments, which often played out for years in his imagination. From the gedankenexperiment, as it is known in German, Einstein grasped fundamental concepts about the physical world that could be verified by observation and experiments. One of his most famous ones began in 1907 when Einstein pondered if a person inside a windowless elevator could tell if he was in a gravitational free-fall, or if the elevator was being hauled up by a constant acceleration. Einstein decided the laws of physics must be the same in both cases. The mathematical equation he derived to explain this so-called principle of equivalence, which equated the effects of gravitation with acceleration in zero-gravity, became the basis for general relativity.

A total solar eclipse on May 29, 1919, gave astronomers an opportunity to verify Einstein’s general theory of relativity by proving that the sun’s gravitational field was bending the light of background stars. The effect was only observable during time when the sun’s light was dim enough for stars to become visible. British astronomer Arthur Eddington led an expedition to the island of Principe, off the West Coast of Africa, to photograph the eclipse, which lasted nearly seven minutes. The images of stars in the region around the sun proved that Einstein’s interpretation of gravity trumped the 200-year old Newtonian model, which interpreted gravity as a force between two bodies. Einstein saw gravity as warps and curves in space and time.

In 1917, Einstein amended his general relativity theory to introduce what he called the “cosmological constant,” a mathematical way to counter the force of gravity on a cosmological scale and stave off the collapse of the universe, which the general relativity theory posited. At the time, astronomers believed that the Milky Way was surrounded by an infinite and static void. In 1923, Edwin Hubble and other astronomers find the first stars beyond the galaxy and by 1929 Hubble provides evidence that space is expanding. Einstein realized the cosmological constant was a blunder. Or perhaps not. In 1998, scientists made the startling discovery that the expansion of the universe is speeding up, driven by an anti-gravity force called dark energy, which in many ways acts like Einstein’s cosmological constant. Pictured here is the Hubble Space Telescope’s extreme deep field view, which contains about 5,500 galaxies. The telescope is named after Edwin Hubble.

One of the first implications of the general relativity theory was the realization that if an object is compressed enough, the dimple it generates in the fabric of space and time will be too strong for even photons of light to escape. Thus, the idea of black holes was born. Though they can’t be directly observed, astronomers have found black holes of all sizes by measuring how they affect nearby stars and gas. Pictured here is an artist’s rendering of a black hole named Cygnus X-1, siphoning matter from a nearby star.

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Like ripples in a pond, scientists believe that gravity transmits in waves, deforming space and time across the universe. It is similar to the movement of electromagnetic radiation, which propagates in waves, except that gravitational waves are moving the fabric of space and time itself. So far, attempts to find gravitational waves, such as those caused by two black holes colliding for example, have been unsuccessful. Next week, the European Space Agency plans to launch a prototype space-based observatory called the evolved Laser Interferometer Space Antenna (eLISA) to test a technology to find gravitational waves. Pictured above is an artist's rendering of two merging galaxies rippling space and time.