One hundred years ago, a patent clerk in Switzerland published a series of scientific papers that would eventually make him so famous that his name and likeness would become registered trademarks. With his tousled hair and moustache as an icon of genius, Albert Einstein's theories on relativity, the photoelectric effect and Brownian motion, all published in 1905, continue to resonate today, say UC Davis scientists.

"Albert Einstein was clearly one of the greatest physicists of the twentieth century, and he accomplished a record amount of outstanding work in the year 1905," said Shirley Chiang, chair of the Department of Physics. The department will mark the centenary this Picnic Day, April 16, with public lectures on hot topics in modern physics, including relativity, earthquake prediction and nanotechnology.

Einstein's Theory of Special Relativity changed our understanding of space and time by showing that measurements in both depend on your movement relative to the object you are trying to observe. It showed that light follows the same rules as the rest of physics, and abolished the notion that space was filled with an "ether" that allowed light waves to propagate.

Ten years later, Einstein published his Theory of General Relativity, incorporating a radical new concept -- that gravity could be described as a curvature of spacetime. General relativity also predicts that changes in gravity can actually travel as waves in the fabric of space.

"It's the greatest single triumph in the history of thought," said mathematics professor Blake Temple.

Einstein's second paper of 1905, on the photoelectric effect, helped create the field of quantum mechanics and is routinely used in laboratories every day, Chiang said.

"It was the first piece of evidence that light can behave like a particle," she said.

The photoelectric effect describes how particles of light (photons) hitting a surface cause electrons to be dislodged. The equations show that light delivers energy in discrete packets, or quanta. The number of packets is determined by the intensity of the light, but -- to the great surprise of physicists at the time -- the energy of each packet depends only on the color of light.

Chiang's laboratory works on advanced microscopes for studying extremely small structures. One routine technique involves shining X-rays onto surfaces and studying the electrons emitted due to the photoelectric effect. "It gives us information about the chemical identity of the atoms on the surface," Chiang said.

"We benefit from the fact that when light interacts with matter, something happens, when we do microscopy, spectroscopy or fluorescence," said Dennis Matthews, director of the Center for Biophotonics Science and Technology. Researchers at the center, a joint program between UC Davis, the Lawrence Livermore National Laboratory and others, are studying ways to use lasers and other forms of directed radiation in medicine, for example, in new ways to detect or treat cancer.

When light travels through body tissues, photons are scattered and travel by different paths -- processes described by special relativity and the photoelectric effect. Using pulses of laser light measured in fractions of a billionth of a second, researchers can develop a "signature" for a tissue based on how light is scattered.

While Einstein's work on the photoelectric effect helped to create quantum mechanics, Einstein himself couldn't quite believe in the theory with its concepts of uncertainty, probability and action at a distance. In fact, reconciling quantum mechanics and general relativity remains what cosmologist Andy Albrecht calls the second-biggest problem in physics.

Quantum mechanics describes how atoms and elementary particles behave. On the other hand, the equations of general relativity describe spacetime and gravity. But the two sets of equations just do not relate to each other, Albrecht said.

Quantum mechanics describes the universe in a probabilistic way, said physics professor Steve Carlip. For example, there is a probability that the pen in my hand will exist at a certain point in time. But that requires the existence of absolute time, and special relativity says that there is no such thing -- the timing of an event depends on your movement relative to it, so different observers will not agree on exactly when an event took place.

If quantum mechanics holds up, then gravity must also be divided into discrete particles, requiring a quantum theory of gravity, Carlip said. That means that space and time must also be quantized. One possible model for this is like a mesh made up of lines joining points separated by nothingness, with new lines constantly popping in and out of existence.

"We're going to end up with a picture of space and time that is very different from what we think," Carlip said.

Einstein's third breakthrough from 1905 was on Brownian motion, named after the Scottish botanist Robert Brown who observed pollen grains jiggling about under a microscope. Einstein showed that these random movements over distances measured in microns and time scales of tenths of a second were driven by much smaller events -- the impacts of molecules measured in nanometers, taking place billions of times a second.

Molecular biologist Jonathan Scholey has a copy of a translation of Einstein's monograph, An Investigation of the Theory of Brownian Movement, on his bookshelf. "It's a fundamental insight into the whole nature of diffusion and Brownian motion," he said. Diffusion, which describes how molecules spread out and mix smoke through air, is a fundamental process in biology and chemistry.

Scholey studies the tiny motors that move things around inside living cells. They move in a jerky fashion, he said: you can only give a probability that a motor protein will take a step forward at a certain time, because every step is subject to Brownian forces -- back and forth movements due to collisions with randomly moving water molecules.

"It's like swimming through syrup for an object our size," Scholey said.

He noted that the last of the five chapters of the monograph is devoted to a simplified version of the theory aimed specifically at biologists and chemists. If Einstein's century-old equations stump some biologists and chemists, attempts to reconcile quantum mechanics and relativity, such as String Theory, have even most physicists foxed.

Which brings us to what Albrecht calls the biggest problem in physics, and "Einstein's Greatest Mistake."

When Einstein developed general relativity, very little was known about the universe, Albrecht said, although just a few years later astronomer Ernest Hubble would find that the universe was expanding.

Einstein's equations showed that the universe could be either expanding or contracting, said astrophysicist Tony Tyson. To make the equations fit with an eternal, unchanging universe, Einstein added a fudge factor to balance the force of gravity, the "cosmological constant." He later rejected the idea, reputedly calling it the "greatest mistake of my life."

But in the late 1990s, astronomers made a startling discovery. The universe is not only expanding, rushing outwards from the Big Bang; the expansion is getting faster. That has lead to a revival of Einstein's cosmological constant to provide a driving force that can push the galaxies apart when gravity should be pulling them together.

"Cosmic acceleration is the deepest mystery in all of physics," Albrecht said.

The cosmological constant could represent "dark energy," a mysterious force that bubbles out of the fabric of space itself. If current theories are correct, dark energy represents about 70 percent of the universe. Dark matter, almost as mysterious, makes up another 25 percent.

The last five percent includes all the stars and galaxies, light and radiation that we can see and think we can understand, adrift like flotsam on an unseen ocean.

A hundred years on, physicists still have big problems to work on, with their roots in Einstein's work. Einstein may be part of the scientific establishment, but his work is also at the heart of the biggest, deepest problems in modern physics, Albrecht said.

Some of those problems may come into focus in the next few years. Caltech researchers are leading the LIGO (Laser Interferometer Gravita-tional Wave Observatory) project to detect gravity waves from deep space. Anyone can join by downloading the Einstein@Home screensaver, which lets computers to process data from LIGO when they aren't in use. New particle accelerators could bring quantum gravity within reach.

"It's a very exciting situation to be a part of," Albrecht said. •