Einstein's legacy

Physicists call it "Brownian motion" after botanist Robert Brown, who studied the phenomenon in 1827. Einstein attributed the jiggling to molecules bumping into the particles. He showed how to calculate how many molecules hit a grain of pollen and how fast they moved. French physicist Jean Perrin used Einstein's insight to set up experiments that proved once and for all that atoms and molecules exist. For that, Perrin won a Nobel Prize.

Einstein believed the most original idea in his 1905 published work was that light can be a particle as well as a wave. The idea addresses the photoelectric effect by which light shining on certain materials causes an electric current to flow. A photocell used to open a supermarket door illustrates the effect. Einstein explained the phenomenon by treating light as a collection of particles called photons whose energy depends only on the color of the light. He received the Nobel Prize for this insight. The discovery also opened the way for the science of quantum physics that was yet to come.

Think Einstein and you think relativity. Many physicists consider his 1905 relativity insights and their subsequent follow-up to be his greatest achievement. "We have a conception of space and time built into us," which Einstein showed to be an illusion, said physicist and Nobel laureate Steven Weinberg at the University of Texas in Austin. "He for the first time made space and time a part of physics and not of metaphysics."

Newton and physicists after him considered space and time to be primordial absolutes. They were the same for all observers - no questions asked. Einstein, however, said the laws of nature and the speed of light are the absolutes, which must be the same for all observers moving uniformly relative to each other. Observers moving at different speeds would perceive spatial dimensions and clock rates differently.

Those abstract principles have serious consequences. What Einstein called their "most important upshot" became the most famous equation in all of physics: E=mc2. It says that material mass and energy are the same thing. Eventually, that theory set the stage for the development of the atomic bomb and nuclear power plants.

Einstein later added a third relativity principle that extended his theory to include gravity. He assumed that when applying a given force to an object, the kind of mass that determines its acceleration and the mass that produces gravity are the same. In this perspective, gravity is no longer a force between objects. It's the way a mass like Earth actually warps space and affects clock rates.

Today, Gravity Probe B is following a curved path through space produced by Earth's mass. No gravitational "force" holds it in orbit. It just follows the obvious path through space. The satellite is carefully tracing that path to see if it is in accord with Einstein's predictions. Einstein also predicted Earth's rotation would drag space around with it. This has been seen only once before. The Gravity Probe B team hopes to improve the accuracy of that observation 10-fold.

• 1687: Isaac Newton publishes "The Principia," putting forward his famous three laws of motion and theory of universal gravitation. By mathematically stating the motion of visible bodies, Newton establishes a basis for much of modern theoretical physics.

• 1859: Charles Darwin writes "On the Origin of Species." His theory of evolution through natural selection becomes the overarching paradigm for modern biology.

• 1873: James Maxwell publishes his best work on his theories of electricity and magnetism. His set of laws unifies fundamental forces for the first time - and provides the first break with Newton.

• 1885: Louis Pasteur applies his germ theory of disease in successfully inoculating a boy bitten by a rabid dog. The procedure marks the beginning of modern preventative medicine.

• 1905: Albert Einstein publishes five papers on three subjects that prove definitively that atoms exist, show that light comes in discrete packets, and change forever humanity's conception of time and space.

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