World Year of Physics 2005

Introduction
1905: Atoms and Quanta
     Quantized Light
     The Reality of Atoms
1905: Relativity
2005: Frontiers
     Atoms and Quanta
     Relativity and Unification
Resources
Site Credits

Introduction
In the space of just six months, from March to September of 1905, a 26-year-old patent clerk named Albert Einstein published four exceedingly strange research papers-and gave us a whole new way to think about light, matter, energy, space and time.

Indeed, his ideas about atoms, quanta and relativity laid the foundations for most of modern physics, not to mention microchips, lasers and a host of modern technologies.

So join us as the National Science Foundation celebrates the centennial of that miraculous year, an event known across the globe as the World Year of Physics. Explore what Einstein actually did in 1905-and all that has come of it since...

1905: Atoms and Quanta
In it, we see the oxygen and copper atoms in a solid compound called cuprite, which forms the basis of many high-temperature superconductors. We also see that the atoms are a bit fuzzy. This is because the electrons orbiting in their outer regions are forced to spread out because of quantum effects, forming a kind of diffuse cloud known as a wave function.

Finally, we notice that the electron clouds reach out to connect the oxygen and copper atoms in pairs. These fuzzy quantum links are chemical bonds. Indeed, this image is one of the first ever taken of chemical bonding.

Photo Caption:
Einstein's work in 1905 helped lay the foundation for quantum mechanics, one of the most far-reaching theories of Twentieth Century physics. What we see here is an actual image of quantum mechanics in action.

Credit: Arizona State University Research Magazine
End Photo Caption

The Quest
How did Einstein do it? How did an isolated patent clerk suddenly come up with a string of ideas that would shake the very foundations of science?

Ultimately, of course, that's a question without an answer. The emergence of an Albert Einstein is as mysterious in its own way as the emergence of a Michelangelo, a Bach or a Mozart.

Yet that hasn't kept historians from sifting through Einstein's life in search of clues. His habit of relentlessly asking "Why?" surely had something to do with it. So did his stubborn refusal to quit without an answer. But arguably the most important factor of all-and the thread that ties together everything that Einstein achieved in 1905 (and later)-was his search for the underlying unity of nature.

Witness his inquiry into the nature of light...

Photo Caption:
This computer visualization takes us inside the nucleus of a deuterium atom, also known as "heavy hydrogen." The deuterium nucleus consists of precisely one proton and one neutron, which are subject to strong quantum effects because of their very tight confinement. The visualization shows one of the resulting wave patterns.

Credit: Argonne National Laboratory
End Photo Caption

Quantized Light
At some point after he started his job at the Swiss patent office in 1902, Einstein found himself wondering about an odd kind of dis-unity in the way physicists understood nature.

If you looked at the smallest scales, he knew, every speck of solid, liquid or gas in the universe was made out of atoms. So matter was fundamentally discrete and lumpy. But if you looked in the vicinity of a bar magnet, or an electric current flowing along a wire, or even a Leiden jar that had stored a lot of charge, you would find that the surrounding space was threaded by invisible "lines of force"-or as physicists liked to call them, "fields." And as far as anyone could tell, these electric and magnetic fields were not discrete and lumpy. Nor were light waves, which were known to consist of oscillating electric and magnetic fields. All of them seemed to be completely smooth and continuous.

Einstein found this lumpy/not-lumpy distinction ugly and artificial. If matter came in the form of discrete atoms, he wondered, then why shouldn't light come in the form of discrete atoms, as well?

No reason at all, he realized. And in a paper whose German title translates as "On a Heuristic Point of View about the Creation and Conversion of Light," he showed exactly how such "atoms" of light might be detected.

Einstein began by analyzing a phenomenon known as "blackbody radiation," which was essentially just the glow coming from a red-hot oven, or a bed of burning charcoal. This simple-seeming radiance loomed as a major mystery in 1905, largely because the colors predicted by the conventional wave theory of light weren't anything like the colors that physicists actually measured in the laboratory.

Just five years earlier, in fact, in 1900, the German physicist Max Planck had become so obsessed with finding an explanation for the difference that he'd tried a bizarre new way to calculate the radiation. Just for the sake of argument, he said, let's don't assume that the charcoal or the oven walls can radiate energy freely, as called for in the conventional theory. Let's assume that it can radiate energy only in tiny, discrete steps.

Drawing on a Latin word meaning "how great," Planck had called each step a "quantum" of energy-"quanta" in the plural. And he'd discovered, much to his own astonishment, that this ridiculous-sounding assumption gave the right answer: Planck's theoretical curve fit the laboratory data perfectly. The only problem was that no one, Planck included, knew what to make of this fact. Was the quantum assumption just a mathematical trick-or something physicists should take seriously?

Einstein, for one, took it very seriously. He showed in his paper that the observations were exactly what you'd expect if the blackbody radiation was not a collection of waves, but was actually a gas of discrete particles. Or to put it another way, Einstein showed that light radiation had a kind of atomic structure, just as matter did. The unity of nature was preserved.

Then at the end of the paper, almost as an afterthought, Einstein pointed out a even more explicit test of his idea. If a light beam was really a swarm of particle-like objects-today we'd call them "photons"-then you had a very simple and straight-forward explanation for the "photoelectric effect" (see animation), another much-studied phenomenon that could not be explained by the wave theory.

Einstein's paper was published in March 1905 in the prestigious German physics journal, Annalen der Physik (Annals of Physics), and is now regarded as one of the watersheds of 20th century science. After Planck, five years earlier, physicists had scratched their collective heads. But after Einstein, they had to accept that quanta were real. Indeed, Einstein's paper not only laid the foundation for the quantum theory of light, but for quantum theory in general.

It was also the work for which he would win the Nobel prize in 1921.

Photo Caption:
This image shows a light-emitting device created from organic compounds that have been deposited on a textile material using inkjet printing. The device itself is only a few nanometers thick.

Einstein's insights into the quantum nature of light and matter are critical to the development of electronic and photonic components like this one. Potential applications include photo-conductive films; LEDs (light-emitting diodes) and other forms of solid-state lighting; flexible electronics; biologically-compatible devices; and chemical sensors.

Credit: Prof. Ghassan E. Jabbour, Optical Sciences Center, University of Arizona
End Photo Caption

Photoelectric Effect
The basics of the photoelectric effect were very well known by 1905: when you shine a light on a metal surface, electrons come flying out.

So were the basics of the explanation: light gives electrons a kick, because light waves carry energy. And since the electrons in a metal are free to move around, instead of being bound to a single atom, they can easily get kicked off into space.

The mystery was in the details. For example, if you tried this experiment with light of one color, meaning a light wave of one frequency, then all the electrons would emerge with the same energy-no matter how strong or feeble the light might be. A brighter light would produce more electrons, but it would not produce faster ones.

Then if you tried it with light of lower and lower frequencies, shifting the color down from, say, ultraviolet through violet, blue, green and so on, the electron energy would go down as well. Eventually, in fact, at some cutoff point that depended on the precise kind of metal you were using, the electron energy would fall to zero. Below that point, the electrons would stay put no matter how much you cranked up the brightness.

The problem was that none of this seemed to make any sense at all. According to the conventional wave theory of light, a brighter wave would pack more energy than a feeble wave. So you would expect it to kick the electrons harder, and make them fly out faster. But not so. Likewise with the cutoff: even if you assumed that the electrons needed an extra boost to break free of the metal itself-think of the extra whack that's needed to hit a golf ball out of a deep sand trap-a bright enough light beam ought to have ample energy for the job. But again, not so.

Einstein's great insight was that all of these phenomena did make perfect sense-if you gave up the idea that light was a continuous wave. Instead, he said, let's imagine that the light beam is actually a swarm of particle-like "quanta," each with an energy proportional to the light's frequency. And then let's imagine that each of these light quanta enters the metal surface and strikes an electron there.

In that case, Einstein said, the electrons would each get one unit of energy and no more. So you would expect to see each electron flying out with the same energy-exactly as observed. Likewise, he said, a brighter light beam would have more quanta, so more electrons would get knocked out. But none would receive more than that one quantum of energy, so none would be going any faster-again, exactly as observed.

And as for the cutoff, said Einstein, a light beam with too low a frequency would have quanta with too low an energy. No matter how many quanta you poured into the metal, no single one of them would have what it took to boost an electron free. So the electrons would stay put-exactly as observed.

The Reality of Atoms
In principle, of course, the unification of light and matter could have worked the other way around. Instead of light's being discrete and atom-like, matter could have been continuous and field-like.

Indeed, this was still considered a serious possibility in 1905. Granted, the idea of atoms gave us a beautifully simple way to account for much of the complexity in the universe. Chemists, for example, could explain all the millions of compounds they were discovering (and the nearly infinite number of reactions among those compounds) as the combination and recombination of fewer than 100 types of atomic building blocks, or "chemical elements." And physicists, meanwhile, could explain the ebb and flow of heat in terms of the microscopic motions of atoms and molecules.

Yet some influential physicists objected. If these hypothetical atoms were too small to see, they argued, then how could you be sure they existed at all? If there was no way to detect atoms, or measure them, how could you even call them "science"? Far better to stick to concepts like density and temperature: continuous, laboratory-scale variables that could be measured.

This debate over the existence of atoms had gotten quite heated by 1905, to the point where many physicists were reluctant even to talk about atoms. Einstein, however, wasn't about to give up that easily. He was so convinced atoms were real that he'd chosen to do his PhD dissertation about them, analyzing a new way to determine the size of atoms. (This work would also be published in 1905, in April.) But what he needed was a way to prove the reality of atoms for sure.

His solution, "On the Motion, Required by the Molecular Kinetic Theory of Heat, of Small Particles Suspended in a Stationary Liquid," appeared in Annalen der Physik in May 1905. The article's title referred to a discovery made back in 1827, when the Scottish botanist Robert Brown noticed that tiny grains of pollen suspended in a water droplet would undergo slow, random motions that never stopped. Researchers soon verified that any small particle would move this way, and in any liquid. But until Einstein, no one could understand why.

What Einstein showed was that this mysterious motion could be explained quite easily-if the liquid were made of discrete molecules instead of being a continuous fluid. (See animation.) Furthermore, he showed, the numbers worked out: the observed rate of Brownian motion was in perfect accord with independent estimates of quantities like the size and mass of atoms.

Once again, this paper was a landmark. Before Einstein, atoms were a beautiful idea, but only an idea-and a controversial one, at that. After Einstein, atoms were an experimental reality.

Photo Caption:
Einstein's solution to the mystery of Brownian motion grew from his deep insight into how atoms and molecules functioned in large groups. Here we see water up close. A water molecule in isolation is just an oxygen atom wearing two little hydrogen atoms like Mickey Mouse ears. But a water molecule in the liquid state has zillions of companions. They tumble over one another incessantly, constantly making and breaking bonds. Together they define what it means to be liquid.

Credit: Nicolle Rager Fuller, National Science Foundation
End Photo Caption

Brownian Motion
Brownian motion is what happens when a pollen grain, a dust mote or any other small particle is suspended in water-or indeed, any other fluid: it will undergo a slow, random movement that never stops.

Einstein's explanation began with a disconcerting fact: the still, quiet loveliness of a droplet of water (or any other fluid) hides a hurricane of activity at the atomic level. The water molecules are actually flying around and slamming into one another at a very high rate of speed-something like 1000 miles per hour at room temperature. Indeed, Einstein pointed out, that's what temperature is in atomic theory: a measure the molecules' average energy.

Likewise, Einstein continued, any object that's immersed in a fluid is going to be pummeled by fast-moving molecules from every side. That's what pressure is in atomic theory: a measure of the average force exerted by all those impacts.

However, said Einstein, if the object is small enough-say, the size of a pollen grain or a dust mote-then the atomic-scale impacts won't quite average out. Sometimes, just by chance, one side of the object will feel a few more impacts than the others. So it will experience a little kick on that side. Then an instant later, just by chance, it will feel a kick from another direction-and then another, and another.

The result, said Einstein, would be random movements that never stop: the Brownian motion.

1905: Relativity

Relativity

Photo Caption:
Einstein's 1905 relativity papers were the start of his decade-long inquiry into the nature of space, time and gravity-an inquiry that culminated in his greatest achievement, the general theory of relativity.

General relativity was the basis for the image shown here, which comes from a series of computer simulations depicting two black holes in orbit around one another. The image shows the black holes just before they merge, when they are whipping around at a ferocious rate and emitting copious quantities of the space-time ripples known as gravitational waves.

The simulations were carried out in 2002 by a team of German scientists using the facilities of the National Center for Supercomputing Applications at the University of Illinois, Champagne-Urbana.

Credit: Scientific contact, Edward Seidel, Louisiana State University; simulations, Max Planck Institute for Gravitational Physics (Albert Einstein Institute); visualization, Werner Benger, Zuse Institute, Berlin, and AEI.
End Photo Caption

Meanwhile, even as he was working out his ideas about atoms and quanta, Einstein was puzzling over another apparent dis-unity in physics.

Suppose you take a metal wire and move it through a magnetic field, he thought to himself. You get a surge of electric current in the wire. No surprises there: the basic principle was well understood by 1905. (Indeed, it's the operating principle of the dynamo, the device that still generates most of the world's electric power.) And now suppose that you hold the wire at rest, and move the magnetic field past it. No surprises there, either: you get another surge of current.

But that was precisely the problem, thought Einstein: the result was the same in both cases, yet the textbook explanations were very different. (In the first case, the current arose purely from the motion of the wire through the magnetic field; there was nothing else. But in the second case, the moving magnetic field produced an electric field, and that drove the current.)

Once again, Einstein found this distinction ugly and artificial. Why should it make any difference whether the wire was moving or the field was moving? If the results were the same, then the physics should be the same; only the relative motion should matter.

This principle of relativity, as he called it, seemed so fundamentally right to Einstein that he took it as a given-one of the very foundation stones of physics.

Yet that quickly led him into a serious conundrum. By 1905 it was well-established that oscillating electric and magnetic fields would together produce a wave moving freely through space. Depending on the frequency of the oscillations, we would experience that wave as a radio signal, a beam of infrared, visible or ultraviolet light, or even a burst of x-rays. But whatever the wave's frequency, we would always see it traveling at the same velocity: 186,000 miles per second (300,000 kilometers per second), also known as "the speed of light."

And that was the conundrum: speed relative to what?

For any other type of wave the answer was obvious. With sound, for example, it would be speed relative to the air, because it was air that did the vibrating; anyone moving through the air would see the sound wave moving at a different relative speed. (Theoretically, in fact, you could have moved fast enough to catch the wave and "break the sound barrier"-although no one would actually manage that feat until 1947.) Likewise with an ocean wave, the answer would be speed relative to the water's surface, and so on. The speed was always relative to the "medium" that was waving.

But light didn't seem to have a medium-just electric and magnetic fields oscillating in space. So Einstein felt he had no choice: if the laws of physics were the same for every observer, as the relativity principle demanded, then he had to postulate that the speed of light was the same for every observer.

To call this idea "strange" would be an understatement. If you climbed into a rocket ship and tried to chase down a beam of light-well, you couldn't. According to Einstein, it wouldn't matter whether you cranked up your ship to 10 miles per hour or to 185,000 miles per second: you'd still see the light beam pulling ahead at precisely the same rate, 186,000 miles per second.

But that was only the beginning. If the speed of light was truly the same for every observer, Einstein showed, then moving clocks would appear to slow down-a phenomenon that has come to be called "time dilation." (Animation 1.) Likewise, moving objects would appear to shrink in the direction of motion. ("Length contraction.") And perhaps most bizarre of all, observers moving relative to one another would disagree on what they meant by "now." (Animation 2.) One observer might perceive two events as happening at the same time, while another observer might see them as happening at different times.

These conclusions, comprising most of what we now know as the Special Theory of Relativity, were the subject of Einstein's paper, "On the Electrodynamics of Moving Bodies," which appeared in the Annalen der Physik in June 1905. In this case, the word "landmark" hardly does the paper justice. Not only did relativity lead us toward a far deeper understanding of space, time, cosmology, the origin of the universe and indeed, the fundamental nature of reality, but the very idea of relativity would resonate through the wider culture. Literature, philosophy, art-everything was touched by it.

Of course, this first relativity paper was missing one important piece. But Einstein himself filled the gap in September of 1905, with the publication of a four-page follow-on paper entitled "Does the Inertia of a Body Depend upon its Energy-Content?" By "inertia" he meant the object's mass. And his conclusion, we now know, was Yes: the mass of an object changes when it gains or loses energy of any kind. Or to put it another way, mass and energy are equivalent-just two aspects of the same thing. Moreover, the relation between them (although Einstein didn't give the equation in quite this form), can be written very simply in terms of a constant "c" that stands for the speed of light:

E=mc2

-undoubtedly the most famous scientific equation in history.

Photo Caption:
This image, oddly reminiscent of Vincent van Gogh's "Starry Night," is the Hubble Space Telescope's view of an expanding halo of light around a distant star named V838 Monocerotis. The illumination of interstellar dust comes from the red supergiant star at the middle of the image, which gave off a flashbulb-like pulse of light two years ago. V838 Mon is located about 20,000 light-years away from Earth, at the outer edge of our Milky Way galaxy.

Credit: NASA and The Hubble Heritage Team, Space Telescope Science Institute/Association of Universities for Research in Astronomy.
End Photo Caption

Animation 1: Time Dilation
Let's imagine two identical rocket ships floating stationary in space a certain distance apart. And let's also imagine that the crew of one ship sends out a laser pulse toward the other. The pulse travels across the gap at the speed of light, bounces off a mirror on the second ship, and returns along the same path. You can think of the round-trip time as being one tick of a simple clock. Indeed, by bouncing pulses back and forth, the crew can measure any interval of time they want.

Next, however, let's imagine what this same laser clock looks like from a different perspective-one in which the rocket ships are moving past at a high rate of speed.

One thing that doesn't change is the ship-to-ship distance, since that distance is perpendicular to the direction of motion and doesn't undergo any length contraction. Another thing that doesn't change is the speed of the laser pulse itself, since the speed of light is the same for all observers.

But what does change is the path of the laser pulse. Instead of just going down and back, the pulse now moves along two much longer diagonals.

Thus the phenomenon of time dilation. From the new perspective, the laser pulse is moving at precisely the same rate of speed, but traversing a longer path. So the pulse takes longer to finish its round trip. Or to put it another way, the ticking of the laser clock is slower when the ships are moving than when they are stationary.

Animation 2: Simultaneity
Let's imagine two observers, one seated in the center of a speeding train car, and another standing on the platform as the train races by. Let's also imagine that lightning strikes the train at both ends just as it's passing the platform.

And now let's ask the question: did the two strikes happen at the same time?

The observer on the platform says they did. After all, he knows that each lightning strike produces a flash of light, with one flash coming forward from the rear of the train, the other coming backward from the front of the train. He knows that both flashes have to travel the same distance-namely, half the length of the train car. And he knows that both flashes are moving at the same speed-namely, the speed of light. So when he sees that the two flashes reach him at the same time, he concludes that the two strikes must have happened at the same time. They were simultaneous.

The observer on the platform also notices that his friend on the train sees the two light flashes at different times. But from his point of view, that's to be expected. He sees that train is rushing toward the flash that's coming from the front. So of course his friend sees it first: the light doesn't have to travel as far. And he sees that the train is rushing away from the flash that's coming from the rear. So of course his friend sees it later: the light takes longer to catch up.

However, the observer on the train sees the same events in a very different way. From her point of view, the two lightning strikes did not happen at the same time. After all, she too knows that both flashes of light have to travel the same distance-namely, half the length of the train car. And thanks to Einstein, she too knows that both flashes are moving at the same speed-namely, the speed of light. So when the two flashes reach her at different times, she has to conclude that the two strikes must have happened at different times. They were not simultaneous: the front strike occurred first.

So which observer's interpretation is correct? Did the strikes happen simultaneously, or did the front strike happen before the rear strike?

The answer is that there is no answer: Special relativity tells us that both observers are right-but only within his or her own frame of reference. Indeed, this was Einstein's greatest and strangest insight about relativity. From different reference frames, there can never be agreement on the simultaneity of events. The very notion of now is relative.

2005: Frontiers

Photo Caption:
Einstein's insights into atomic and quantum theory laid the foundation for our modern ability to manipulate atoms and materials almost at will-a skill that is critical to the emerging field of nanotechnology.

This image, for example, shows an array of feathery nanostructures that are pure zinc oxide except for the beads of tin at each tip. For scale, each bead is about 100 nanometers across. The nanostructures could be the basis for new kinds of ultra-responsive sensors.

Credit: Zhong Lin Wang, Georgia Institute of Technology.
End Photo Caption

As miraculous as 1905 had been for Einstein, it was only the beginning.

True, he would remain at the Swiss patent office in Bern for several more years. But as other physicists began to take notice, Einstein moved on to a series of ever more prestigious academic appointments in Zurich, Prague and, in 1914, Berlin. And all along the way, he continued to do groundbreaking work on the theory of atoms and quanta.

In 1906, for example, he showed that the quantum principle also applied to electrons moving through solid materials: the electrons, like the atoms that emitted blackbody radiation, could release or absorb energy only in certain, discrete steps. Looking back on it, we can see that he had taken a first giant step toward our modern understanding of matter in the solid state-an understanding that would eventually point the way toward transistors, microchips, LEDs, the CCD chips in digital cameras and indeed, most of the technology of today's digital age.

Then in 1917, in a stunningly innovative analysis of the radiation process, Einstein predicted that a quantum of light emerging from an atom would obey a kind of subatomic herd instinct, and would try to follow any other light quanta in the vicinity. He was right. This kind of "stimulated emission" would eventually find application in a device known as the laser-short for Light Amplification by the Stimulated Emission of Radiation.

And then in 1924, building on an idea put forward by a young Indian physicist named Satyendra Nath Bose, Einstein pointed out that certain types of particles and atoms would exhibit the same herd instinct. Or to put it in modern terminology, the class of particles and atoms known as "bosons" will always obey "Bose-Einstein statistics"-a concept that has allowed physicists to understand not just the laser, but superconductors, superfluids and most recently, a weird quantum state of supercold atoms known as a Bose-Einstein condensate.

As if this work on atoms and quanta were not enough, however, Einstein was also continuing his work on relativity. The theory he'd outlined in 1905 had been a triumph. Yet he still wasn't satisfied.

After all, he reasoned, the 1905 theory was just a special case: it applied only when observers were moving at constant velocity. A truly general theory of relativity ought to apply equally well to observers who were accelerating, rotating, or changing their velocity in any other fashion.

Einstein's quest to generalize relativity took him a full ten years, and led him down a far stranger path than he had imagined. By the time he had placed the theory in its final form, in 1915, he had arrived at a profound new understanding of space, time and the universal force of gravity. Indeed, he had come to see that gravity was not a force, as physicists usually understood that concept. It was actually a kind of curvature in the basic fabric of space and time.

Einstein's general theory of relativity, as it is now known, was not only his greatest achievement, but is arguably the most elegant and beautiful theory in all of science. Certainly it is the foundation of modern cosmology and astrophysics, helping us understand everything from black holes to the Big Bang itself.

And it was the theory that made Einstein world-famous. In 1919, photographs taken during a total eclipse showed that starlight grazing the limb of the Sun was deflected by the Sun's gravity in precisely the way general relativity predicted. The news was reported around the globe, and from then on, the "halo-haired Dr. Einstein," as one writer later described him, was an international celebrity.

But Einstein, of course, was indifferent to fame. What he cared about was physics, and his ongoing quest for the ultimate unity of nature.

Ironically, that quest would soon lead him away from the mainstream of his field. By the end of the 1920s, physicists such as Niels Bohr, Werner Heisenberg, Erwin Schrödinger and Paul Dirac had taken up the quantum ideas that Einstein helped pioneer, and had forged them into quantum mechanics: a mathematical theory that could explain the behavior of atoms and light with remarkable precision.

To do that, however, these scientists had been forced to postulate that things were fundamentally uncertain at the atomic scale-and that the outcome of any experiment was largely a matter of chance. And that was just too much for Einstein. "God does not play dice," he famously said. Even if quantum mechanics worked in a mathematical sense, he argued, it could not possibly the whole story. Instead, he tried to extend relativity once again. This time, he wanted a theory that would account for gravity, electricity, magnetism and quantum behavior all at once.

That quest would occupy him for the last 30 years of his life, until his death in 1955. During that period, in 1933, Einstein would be forced to flee the rising Nazi threat in Germany and settle in the United States, at the Institute for Advanced Study in Princeton. He would become a leading advocate for international peace-especially after Hiroshima and Nagasaki-and a strong supporter of the new state of Israel.

But Einstein never quit striving for that ultimate unification. And indeed, while his own approach proved to be a dead end, his spirit still animates the modern quest for unification, from the "standard model" of particle physics to the theoretical frontiers of superstring theory and quantum gravity.

In short, Albert Einstein's work during the first half of the 20th century laid out much of the agenda that the physical sciences have followed ever since. So click on the icons to the right to explore what has come of his ideas in the realms of Atoms and Quanta, and Relativity and Unification.

Photo Caption:
Einstein's insights into space, time and gravity laid the foundation for our modern science of cosmology. In particular, his general theory of relativity is built into virtually all modern computer simulations of the cosmos.

Here, for example, we see a snapshot from a computer simulation of the "large-scale structure" of the universe - that is, the way galaxies and clusters of galaxies seem to be distributed into knots and filaments on a scale of millions of light years, presumably as the result of gravity acting on fluctuations in the density of dark matter. These fluctuations were tiny in the immediate aftermath of the Big Bang, but grew rapidly. This simulation shows a cubical chunk of the universe measuring some 209 million light years on a side at present, and models both dark matter and ordinary matter (in the form of gas.) The large structure in the center is a massive galaxy cluster that could contain on the order of 1,000 galaxies. The faint translucent shapes trace gas density, increasing in magnitude from blue to green to red to yellow. Spheres represent a sampling of the simulated gas particles; the larger and bluer the spheres, the lower the density.

Credit: Simulations by Paul Shapiro and Hugo Martel, Galaxy Formation and Intergalactic Medium Research Group, University of Texas. Visualization by the Center for Computational Visualization, University of Texas, using image rendering software created by Chandra.
End Photo Caption

2005 Frontiers

Introduction

The revelations of Einstein's "Miracle Year" changed physics and the way we understand the universe forever.

Where have these concepts taken us, and where will they lead us in the foreseeable future?

Click on the links above for a sampling of the ramifications of the paradigm shifts brought about through Einstein's genius.

Atoms and Quanta
The realm of atoms and quanta is a realm where our intuitions are violated wholesale. This is a world where an electron can be in many places at once, where an atomic nucleus can be spinning clockwise and counterclockwise at the same time- where matter itself dissolves into a ghostly blur of possibilities as soon as you try to look at it.

Indeed, this is a world is so bizarre that Einstein called it "spooky," and refused to accept its reality. And yet, for all his objections, quantum spookiness has become a very real and practical matter in the modern world, where it has proven critical to a host of modern technologies.

Click the circles on the left for more information.

Circle: The Quantum Theory of Light
The quantum theory of light describes how atoms absorb and emit the discrete packets of light energy known as "photons." That process, in turn, provides the basis for applications that range from atomic clocks, to magnetic resonance imaging (MRI) scanners, to the myriad uses of lasers.

Photo Circle 1: Lasers
A state-of-the-art femtosecond laser amplifier system.
Although the pulse energies from this laser are modest (on the order of millijoules), and the size is relatively small, the laser system crams all of the energy into a tiny, 20 femtosecond pulse. The result is a peak power close to a terawatt (a terawatt is roughly the continuous electrical generating capacity of the United States). The amplifier system allows the researchers to generate coherent EUV (extreme-ultraviolet wavelengths) beams by focusing the laser into a hollow fiber called a waveguide. The green light in the picture comes from the pump lasers that are used to amplify the femtosecond pulses to the terawatt level.
Credit: Image courtesy of the University of Colorado and NSF.

Photo Circle 2: Fiber Optics
A "waveguide" coaxes light waves into traveling at the same speed.
This picture shows a schematic of the waveguide, the modulated hollow-core fiber used to efficiently generate shorter wavelength EUV light. The wall of the hollow fiber is modulated - wavelike, with indentations 10-micrometers (µm) deep, periodically spaced every 0.5 mm (in the highest-intensity waveguide). The average inner diameter of the fiber is 150µm. By modulating the diameter of the fiber, the researchers modulate the intensity of the initial laser beam, and therefore also modulate the process that produces the EUV light. The researchers adjusted the period of the wavelike modulations to restrict the EUV emission to regions where the light waves will be in phase - this is how the researchers were able to generate shorter wavelength light more efficiently than was previously possible.
Credit: The University of Colorado and NSF

Photo Circle 3: CDs and DVDs
A convergence of technologies, such as lasers, mechanics, electronics, and coding technology.
In 1979 Philips and Sony decided to join forces, setting up a joint taskforce of engineers whose mission it was to design the new digital audio disc. Prominent members of the taskforce were Kees Immink and Toshi Doi. After a year of experiment and discussion the taskforce produced the 'Red Book', the Compact Disc standard. Philips contributed the general manufacturing process, based on the (unsuccessful) video Laserdisc technology. Philips also contributed the Eight-to-Fourteen Modulation, EFM, which offers both a large playing time and a high resilience against disc handling damage such as scratches and fingerprints; while Sony contributed the error-correction method, CIRC. The Compact Disc Story, According to Philips, the Compact Disc was thus "invented collectively by a large group of people working as a team." The Compact Disc reached the market in 1983, and this event is often seen as the 'Big Bang' of the digital audio revolution. The new audio disc was enthusiastically received and its handling quality received particular praise. From its origins as a music format, Compact Disc has grown to encompass other applications. Two years later, in 1985, the CD-ROM (read-only memory) was introduced. With this it was now possible to disseminate massive amounts of computer data instead of digital sound. A user-recordable CD for data storage, CD-R, was introduced in the early 1990s, and it became the de facto standard for exchange and archiving of computer data and music.
Credit: photos.com

Photo Circle 4: Laser Surgery
A laser makes clean, high-precision surgical cuts in the human cornea.
The laser beam is focused into a tiny spot of energy that passes harmlessly through the outer layers of the cornea until reaching its exact focal point within the central layer of the cornea.
Credit: IntraLase

Circle: Atoms, Molecules & Materials Quantum theory is the foundation for our modern mastery of atoms, molecules and materials. This is no accident: quantum effects are dominant on the atomic scale because quantum effects set the scale. To see how, imagine an electron moving in the electric field of an atomic nucleus. As the nucleus tries to pull the electron closer and closer, Heisenberg's quantum uncertainty principle forces it to fight back by moving faster and faster-and more randomly. The result is a standoff in which the electron occupies a kind of diffuse cloud known as a wave function. The size of that cloud is the size of the atom.

Much the same thing happens in molecules: the chemical bonds that hold the atoms together typically involve electrons whose wave functions reach out to embrace several atomic nuclei. By analyzing these bonds, chemists can understand why molecules have the structure they do, and why they react the way they do-an understanding that pays off in applications ranging from pharmaceuticals, to synthetic fibers, to new household cleansers, to the cellular regulation of DNA.

Quantum effects can be important even when zillions of atoms and molecules are collected into solids, liquids and gases: the kind of large-scale, bulk matter that makes up the everyday world around us. Indeed, there is a whole branch of condensed matter physics devoted to the study of such materials.

For example, quantum theory helps us understand how electrons flow through metals and semiconductors-which makes it the basis of virtually all of today's microchip technology, not to mention such applications as light emitting diodes and photovoltaic solar cells. And quantum theory is critical to our understanding of various "super" phenomena, including superconductivity, superfluidity, and the recently discovered, ultra-cold state of matter known as a Bose-Einstein condensate.

Photo Circle 1: Molecular Modeling
Simulating quantum molecular interactions.

The chemical bonds that hold molecules together are best understood through quantum theory: bonds are created when electrons have quantum wave functions that reach out and embrace several atomic nuclei, instead of just one. By analyzing these bonds, chemists can understand why molecules have the structure they do, and why they react the way they do—an understanding that pays off in applications ranging from pharmaceuticals, to synthetic fibers, to new household cleansers, to the cellular regulation of DNA. And these days, not surprisingly, one of their most powerful tools is the computer, which can simulate quantum molecular interactions that are far too complex for pencil and paper solutions.

Here, for example, we see a computer-generated prediction for the shape of villin, a protein that is especially active in the cells that line the digestive tract. Credit: folding@home, Stanford University.

Photo Circle 2: Computer Chips
Computer chips are a kind of semiconductor, which are components of most electronic circuits.

Computer chips like this one are used in research at the Alabama Experimental Program to Stimulate Competitive Research (EPSCoR). The Alabama EPSCoR program was recently awarded a Research Infrastructure Improvement Program grant of $9 million from the National Science Foundation, which will include funding for further developments to the Internet 2 high-speed computer network.
The EPSCoR is an NSF-supported program fun by NSF's Education and Human Resources Directorate.
Credit: University of Alabama

Photo Circle 3: Superconductors
Microchips may use "spintronics" to carry information using the electrons' intrinsic spin.

As the ever-increasing power of computer chips brings us closer and closer to the limits of silicon technology, many researchers are betting that the future will belong to "spintronics": a nanoscale technology in which information is carried not by the electron's charge, as it is in conventional microchips, but by the electron's intrinsic spin.

One key challenge is how to control such a spintronic device. One proposal for how to do so comes University of Notre Dame physicist Boldizsar Janko and his colleagues, and is illustrated here: use superconductivity.

The idea is to create the device as a series of layers, each only a few dozen nanometers thick. At the bottom is a layer of semiconductor—for example, gallium arsenide doped with manganese atoms. Each manganese atom contributes an extra electron to the material, and thus an extra electron spin. Above that is a layer of insulator, and then a layer of superconductor.

When a magnetic field is applied to this device, quantum effects guarantee that it can punch through the superconducting layer only pinching down into an array of nanoscale flux tubes (green columns). Then, as the intensely focused field of each flux tube passes on through the semiconductor layer, it forces a patch of electron spins to fall into line (red arrows). From there, standard laboratory techniques allow the researchers to move the magnetic flux tubes at will—with the spin patches moving with them. Credit: Dr. Ovidiu Toader, University of Toronto

Photo Circle 4: LEDs
Light-emitting diodes glow when voltage is applied.

Light emitting diodes, or LEDs, are another example of quantum mechanics at work: the light is emitted by electrons making a quantum transition from one energy state to another inside a certain type of semiconductor. Indeed, researchers can tailor the emissions very precisely by tailoring the composition and crystal structure of the semiconductor.

Because LEDs are also very energy-efficient, they are finding an increasing wide range of applications, ranging from long-lived flashlights to colorful architectural lighting. Shown here is the vivid LED-based illumination scheme created for The Corner House, a shopping, leisure and entertainment complex in Nottingham, England.
Credit: Intelligent solid-state lighting by Color Kinetics, photos by Louise Strickland.

Photo Circle 5: Liquid Crystals
"Banana-phase" liquid crystal being studied at the University of Colorado.
Liquid crystals are an odd class of materials that are sort of like solids, in the sense that they're made of long, thin molecules that all tend to line up in the same direction. But they are also sort of like liquids, in the sense that those nicely aligned molecules are constantly moving around to different positions. This dual nature gives them some very useful optical properties, which are exploited most famously in applications such as Liquid Crystal Displays (LCDs).

Shown here is a microscope's view of a new type of "banana-phase" liquid crystal being studied at the University of Colorado. (The name comes from the fact that the molecules are shaped like tiny bananas.) Although applications are still a long way off, one possible use for the material might be as a read-write head for holographic information storage.
Credit: Renfan Shao, Liquid Crystal Materials Research Center, University of Colorado, Boulder.

Photo Circle 6: Modern Materials
A single-crystal diamond made using a chemical vapor deposition process.
Researchers at the Carnegie Institution of Washington, D.C. have produced ten-carat, half-inch thick single-crystal diamonds at rapid growth rates (100 micrometers per hour) using a chemical vapor deposition (CVD) process. The size is approximately five times that of commercially available diamonds produced by the standard high-pressure/high-temperature (HPHT) method and other CVD techniques.

In addition, the team has made colorless single-crystal diamonds, transparent from the ultraviolet to infrared wavelengths with their CVD process. Credit: Carnegie Institution

Circle: Nanotechnology
Quantum effects often play a defining role in the new frontier of nanotechnology, which deals with matter in clusters of a few hundred atoms or molecules at a time. This turns out to be a strange, in-between kind of realm, in which electrons can roam much further than they can in an atom or molecule, but not nearly as far as they can in bulk materials. The result is a multitude of unique quantum effects, which in turn give rise to properties such as color and chemical reactivity that are unique to each individual nanoparticle-and that are critically dependent on its precise size and shape.

Indeed, much of modern nanotechnology is about understanding those properties, and learning how to control them.

Photo Circle 1: Nanotubes
Carbon nanotubes may prove very useful in nanotechnology applications.
A view from within a flattened twisted carbon nanotube. A team led by Vincent Crespi, associate professor of physics, has simulated carbon nanotubes that are smaller and stronger than any other nanotube.

Using supercomputers in California, Michigan, and Texas to model the electronic states and total energies of various carbon molecules, Crespi and his colleagues discovered a tetrahedral carbon atom that creates tight and stable bonds to form tiny tubes only six atoms across--the smallest diameter theoretically possible. Crespi believes they may prove very useful in nanotechnology applications. This work was supported under National Science Foundation grant DMR 95-20554.
(Year of image: 1996)
Credit: Crespi, Penn State Physics

Photo Circle 2: Nanostructures
A photo- micrograph of a 3-dimensional nanostructure.
A three-dimensional nanostructure grown by controlled nucleation of silicon carbide nanowires on Gallium catalyst particles. As the growth proceeds, individual nanowires 'knit' together to form 3D structures. This photomicrograph was taken by Ghim Wei Ho, a Ph.D. student studying nanotechnology at Cambridge University. Ghim Wei--who works with Professor Mark Welland, head of Cambridge's Nanoscale Science Laboratory--makes new types of materials based on nanotechnology (these 'flowers' are an example of new material).

Nanometer scale wires (about one thousandth the diameter of a human hair) of a silicon-carbon material (silicon carbide) are grown from tiny droplets of a liquid metal (Gallium) on a silicon surface, like the chips inside our home computers. The wires grow as a gas containing methane flows over the surface. The gas reacts at the surface of the droplets and condenses to form the wires. By changing the temperature and pressure of the growth process the wires can be controllably fused together in a natural process to form a range of new structures, including these flower-like materials. Researchers are investigating possible applications for these structures, such as water repellant coatings and as a base for a new type of solar cell.
Note: All the images in the "Three-Dimensional Nanostructure" series were taken with a scanning electron microscope. Image color was modified using Adobe Photoshop.
(Year of image: 2004)
Credit: (c)Ghim Wei Ho and Prof. Mark Welland, Nanostructure Center, University of Cambridge.

Photo Circle 3: Nanomaterials
Preparing a precursor material for use in the synthesis of novel inorganic nanomaterials.
Dr. Vladimir Kolesnichenko, a University of New Orleans research specialist with the nanomaterials team of Louisiana's micro- and nanotechnologies consortium for advanced physical, chemical and biological sensors, is shown preparing a precursor material for use in the synthesis of novel inorganic nanomaterials. These materials are sensitive to moisture and oxygen in the air and must be synthesized and handled in a glove box where the atmosphere can be controlled. This work was supported by the Louisiana EPSCoR (Experimental Program to Stimulate Competitive Research) project under a grant from the National Science Foundation EPSCoR Program. Credit: (c)2001 Paul Taylor

Photo Circle 4: Nanocrystals
The calculated valence electron density of a silicon nanocrystal.
This plot shows where all the electrons tend to be moving within the silicon nanocrystal. Each electron in the molecule exists in a strangely shaped orbit, but when looking at all the electrons and their orbits, the average is this. Note how the plot looks like the electrons form clouds around all the atoms and in between neighboring atoms. This plot shows the bonds that exist between atoms that hold the molecule together. (Year of image: 2002)
Credit: Zack Helms, Quantum Simulations Laboratory, North Carolina State University; simulations completed using computational resources provided by the National Center for Supercomputing Applications.

Relativity & Unification

It is surely one of the greatest ironies of modern science.

In the century that has passed since 1905, the quest to understand the fundamental workings of nature has led scientists on a long journey inward-first to the structure of the atom, then to the powerful and dangerous forces inside the atomic nucleus, then to a bewildering array of subatomic particles with names like quark, gluon, and lepton, and most recently, to the theoretical frontiers of superstrings and quantum gravity.

The quest has also led scientists on a long journey outward-to studies of how the stars shine, how black holes behave, how galaxies form, and how the universe is expanding.

And that is precisely the irony: the journey inward and the journey outward now seem to be leading to the same place. The physics of the tiniest particles turns out to be intimately intertwined with the universe as a whole, and how the Big Bang brought it all into being some 13.7 billion years ago. A discovery about one is almost inevitably a discovery about the other.

Where these journeys will finally take us is still anybody's guess. But for a hundred years now, perhaps the most reliable guide we've had has been Einstein's theory of relativity.

Circle: E=mc2

Soon after Einstein pointed out the relativistic equivalence of mass and energy-a relation made famous by the equation, E=mc2-physicists began to realize that it was critical for understanding radioactive decay: a recently discovered phenomenon in which certain atoms would spontaneously emit one or more types of high-energy particles.

The mass-energy equivalence didn't explain the why and the how of these emissions; that would have to wait until the 1930s and a better understanding of the atomic nucleus. But it did explain what powered them: each decaying atom somehow lost a tiny fraction of its mass, and in the process gained enough energy to create a radiation particle and hurl it outward.

Eventually, of course, the development of nuclear weapons made the mass-energy equivalence a major factor in human affairs. The discovery of nuclear fission in 1939 revealed that prodigious amounts of energy could be released when the nuclei of uranium atoms split apart. And from there, given the ferocity of World War II, the drive to build an atomic bomb took on a relentless life of its own.

Happily, however, that was not the whole story. The same mass-energy relation that became so horribly real over Hiroshima and Nagasaki has also given us radiation therapy for cancer, PET scans of the brain, nuclear power plants for electricity, and much else besides.

It is the ultimate source of solar energy: the Sun and all the other stars like it are powered by thermonuclear reactions in the core, where hydrogen nuclei give up some of their mass by fusing into helium.

Indeed, the mass-energy relation is the ultimate source of just about everything. Earth, Sun, planets, stars, galaxies-every bit of stuff in the universe can trace its ancestry back to the Big Bang, when the inconceivably fierce energies of creation first began to condense into the particles of matter we know today.

Circle Photo 1: Nuclear Weapons
The awesome energy released by a nuclear weapon. The discovery of nuclear fission in 1939 revealed that prodigious amounts of energy could be released when the nuclei of uranium atoms split apart. And from there, given the ferocity of World War II, the drive to build an atomic bomb took on a relentless life of its own. Ever since, the relativistic equivalence of mass and energy has been a major factor in human affairs.
Credit: Getty Images.

Photo Circle 2: Radiotherapy
Radiation therapy involves treating disease with penetrating beams of high-energy radiation. Radiation therapy is used to treat cancer - alone or in conjunction with surgery and/or chemotherapy. Radiation therapists are highly skilled members of the cancer management team. They are responsible for accurately recording, interpreting and administering the treatment prescribed by radiation oncologists. During treatment, therapists help physicians use fluoroscopy, X-ray films or CT scans to localize and outline anatomical areas requiring treatment. Credit: Mayo Clinic

Circle Photo 3: Nuclear Energy
Nuclear energy supplies a significant portion of the world's electricity. The same mass-energy relation that gave us nuclear weapons has also given us nuclear energy. While controversial, it's a technology that nonetheless produces a substantial fraction of the world's electric power. Credit: Photos.com

Circle Photo 4: The Energy Source of the Sun & Stars
Solar storms send bubbles of gas pelting into Earth, causing magnetic storms. Magnetic structures on the Sun are linked to solar storms that can set off disturbances when they reach the upper atmosphere, affecting satellites, ground-based communications systems and power grids on Earth. Credit: NASA

Circle: General Relativity

Imagine that you place a bowling ball in the middle of a trampoline: the heavier the ball, the deeper the depression it makes in the surface. And now imagine that you roll a marble across the trampoline: the closer it passes to the bowling ball, the further it dips into the depression and the more its path is deflected. So you could say that the ball is exerting a "force" on the marble-even though the marble is actually just responding to a distortion in the surface.

This is basically how Einstein accounted for the force we experience as gravity. The larger the mass of an object, according to his general theory of relativity, the stronger the curvature it produces in the surrounding space-time. And the stronger the curvature, the further any other object gets deflected as is passes by. Or, as the physicist John Wheeler liked to put it, matter tells space how to curve, and space tells matter how to move.

In most cases, of course, this is a distinction without a difference. Einstein's equations make almost exactly the same predictions that Sir Isaac Newton would have made 300 years ago, whether they're applied to the fall of an apple or the motions of the moon.

Sometimes, however, that "almost exactly" isn't good enough. If the designers of the Global Positioning System hadn't taken account of general relativity, for example, we'd be facing a slow, steady degradation in the accuracy of that system.

And in certain situations, the differences can be dramatic. In the immediate vicinity of a black hole, for example, the curvature becomes so strong that nothing can escape, not even light. Anything that falls into a black hole is on a one-way trip, doomed to be crushed by a "singularity" so extreme it almost certainly can't be described by the known laws of physics.

Indeed, some gravitational events shake the fabric of space-time so violently that the ripples can-in principle-be felt in far-distant galaxies. That's why the National Science Foundation has funded the world's largest and most ambitious gravitational-wave observatory, LIGO. One day soon, with any luck, LIGO will be making routine observations of space-time ripples caused by colliding black holes, merging neutron stars, and many other ultra-violent events.

Circle Photo 1: Gravity Waves
The LIGO facility at Hanford. The LIGO project is spearheading the completely new field of gravitational-wave astronomy.

The National Science Foundation (NSF) provides funding for large, multi-user facilities that provide researchers and educators with access to the latest technological tools and capabilities. NSF also supports far-reaching areas of science and engineering that hold promise for breakthroughs that will enhance the nation's future in profound, and possibly unpredictable, ways. The Laser Interferometer Gravitational-Wave Observatory (LIGO) is an example of both. Credit: Caltech; National Science Foundation

Circle Photo 2: Black Holes
A simulation of the merger of two black holes and the ripples in spacetime.
This numerical simulation is part of a series depicting orbiting black holes and represents the first time that three-quarters of a full orbit has been computed.

The simulations show the merger of two black holes and the ripples in spacetime--known as gravitational waves--that are born of the merger. These simulations were created on the National Center for Supercomputing Applications (NCSA) Itanium Linux Cluster by researchers from the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) in Potsdam, Germany, and visualized by Werner Benger of the Albert Einstein Institute and the Konrad-Zuse-Zentrum in Berlin. The simulation was completed in the Spring of 2002.

National Science Foundation support was used for this project both through an NRAC proposal for computer time at NSF computing facilities--including NCSA, and also indirectly through NSF grant PHY 99-79985. [Image 4 of 5 related images.] See Related
(Year of image: 2002)

Credit: Scientific contact Ed Seidel (eseidel@aci.mpg.de); simulations by Max Planck Institute for Gravitational Physics (Albert-Einstein-AEI); visualization by Werner Benger-Zuse Institute, Berlin (ZIB) and AEI; computation performed on NCSA's Itanium Linux cluster

Circle Photo 3: GPS
Global positioning system (GPS) receivers are used to collect research data.
Michigan State University graduate students Scott Bearer (left) and Guangming He calibrate Global Positioning System equipment as they evaluate bamboo growth in the Wolong Reserve in Sichuan Province. 
Scientists from Michigan State (MSU) and Stanford universities, in a fresh look at world population dynamics, have revealed evidence that increased numbers of households, even where populations are declining, are having a vast impact on the world's biodiversity and environment.

Reduction in household size has led to a rapid rise in household numbers around the world and has posed serious challenges to biodiversity conservation, write Jianguo (Jack) Liu of MSU and Stanford colleagues Gretchen C. Daily, Paul R. Ehrlich and Gary W. Luck in the Jan. 12 advance online publication of the journal Nature. Biodiversity is threatened severely not only by increased numbers of households, but also by less efficient per capita consumption of natural resources, the researchers say. A systems ecologist, Liu received his NSF CAREER award in 1997. He already had acquired significant background on the impact of household dynamics on giant pandas in southwest China's mountainous Wolong Nature Reserve when he approached Stanford's Paul Ehrlich, renowned for his population studies, with an idea: Expand the probe into a worldwide look at the impact of households on global biodiversity and environment. "The numbers of households increased much faster than the size of the population at Wolong," Liu said. "This has important implications because given the same population size, more households mean a need to consume more resources, and therefore, a greater impact on the environment. What was discovered from the panda reserve helped me to conclude that considering population size and growth alone is not enough, and made me want to find out whether other areas in the world have similar phenomena." In China's Wolong, a reduced average household size was tied directly to an increase in household numbers and a rise in the amount of fuel wood consumed by the local populace for cooking and heating, which has contributed to deforestation and loss and fragmentation of habitat for giant pandas.

For the complete text of this story, see NSF Press Release PR 03-06. (Year of image: 2002) Credit: Sue Nichols, Michigan State University

Circle: Cosmology

Walk out into a clear, dark night, and look up at the stars. This is when we come face to face with the question of questions: Where did it all come from?

Science can't completely answer that question. Yet. But it has given us at least one big piece of the answer. Indeed, our modern account of creation dates back 1917, when Einstein showed that his equations of general relativity didn't apply just to gravity, but to cosmology. His theory could describe the shape and evolution of the universe as a whole. And (despite some early confusion) the implications were startling.

Some 13.7 billion years ago, according to the currently accepted version of this story, the universe sprang to life in a single, vast eruption known as the Big Bang. Everything was born in that instant-matter, energy, space, and even time itself, all ballooning outward from an infinitesimal point. The universe has been expanding and cooling ever since. And slowly, over those billions of years, the primordial matter that emerged from the Big Bang has been organizing itself into galaxies, stars, planets, and most recently, us.

However, even as observations have confirmed and filled in the details of this account, scientists have found themselves confronting new mysteries. One of the most astonishing is the fact that almost everything in the universe is utterly invisible.

Astronomers stumbled onto this fact only a few decades ago, when they began to realize that the stars, galaxies, and nebulae they can see through their telescopes are just a tiny fraction of what's actually out there. Far more prevalent is dark matter: a kind of cosmic ectoplasm that makes itself known only though its immense mass, which produces an equally immense gravitational field.

Without that field, individual galaxies like our own Milky Way would fly apart like broken pinwheels, and clusters of galaxies would disperse into the void. Without it, in fact, none of those galaxies and clusters would have formed in the first place; they don't contain nearly enough mass to clump up on their own. It was the dark matter that began to do so, starting right after the Big Bang itself; the ordinary matter simply got carried along, like bright flecks of foam on a dark ocean.

No one knows for sure what dark matter is. But the best guess is that it's a haze of massive, weakly interacting elementary particles left over from the Big Bang. (It's often referred to as "cold dark matter" because the particles are thought to be moving fairly slowly, at much less than the speed of light.) In an effort to learn more about it, astronomers are planning a new generation of telescopes to observe the earliest phases of galaxy and cluster formation. And physicists are building ambitious new detectors to search for the dark matter particles directly.

Yet even as these efforts get underway, an even greater cosmic mystery has appeared. In 1998, two independent teams of astronomers announced that the expansion of the universe isn't slowing down, as most observers had expected. The expansion is actually speeding up. It is as if some previously unknown force, now known as dark energy, is driving the galaxies apart at an ever-increasing rate.

Scientists have even fewer clues about the nature of dark energy than they have about dark matter. It could be a manifestation of the cosmological constant: a modification to general relativity that Einstein once considered and then rejected. Or it could be something quite different.

But whatever it is, dark energy is incredibly abundant. According to the best current measurements, ordinary matter accounts for no more than 5 percent of the stuff in the universe, on the average, while dark matter amounts to some 20 percent. Dark energy is 75 percent.

The discovery of dark matter and dark energy has electrified the scientific community: here is observational proof that there's something about the cosmic story that we still don't understand.

Circle Photo 1: The Big Bang
According to the Big Bang theory, our universe was born approximately fifteen billion years ago. An artist's conception of our "stop and go" universe, in which the cosmic expansion slowed under the influence of gravity before accelerating again due to an unexplained dark energy. This brief history extends from the Big Bang and the recombination epoch that created the microwave background (bottom), through the formation of galactic superclusters and galaxies themselves (top). The dramatic flaring in the upper reaches of the diagram emphasizes that the universe's expansion currently is speeding up.
Credit: David A. Aguilar, Harvard-Smithsonian Center for Astrophysics

Circle Photo 2: Dark Matter
Dark matter makes up most of the universe, but no one knows how much of it there is.

Darkness Made Visible
By looking at galaxies far in the background, and then tracking how gravity deflects their light as it passes through the galaxies clustered in the foreground, the Hubble Space Telescope has mapped out the cluster's dark matter (shown in blue).
Credit: J.-P. Kneib, Observatoire Midi-Pyrenees, Caltech, et al.; ESA; NASA

Circle Photo 3: Dark Energy
The discovery of dark matter and dark energy electrified the scientific community. Gas dynamics simulations and visualization of large structure formation in the universe. Gravitational collapse of initially-small, amplitude density fluctuations of "dark matter" is thought to have led to formation of the large-scale structure seen in the universe today.

Credit: Simulations by Paul Shapiro and Hugo Martel, Galaxy Formation and Intergalactic Medium Research Group, University of Texas; visualization by the Center for Computational Visualization, University of Texas, using image rendering software created by Chandra

Ultimate Unification

Albert Einstein spent his entire adult life pondering the deepest secrets of matter, energy, space and even time itself. What are they, really? And where is the deep, unifying principle that can help us truly understand them?

In the fifty years since his death, moreover, Einstein's fellow physicists have come a very long way toward answering those questions-although perhaps not along the path he would have chosen.

According to the standard model of particle physics, for example, pretty much everything we can see in the universe is made from the same basic building blocks: elementary particles with names like quark, lepton and gluon. From raindrops to galaxies, it's all the same stuff.

According to Einstein's own general theory of relativity, likewise, a subtle curvature in the fabric of space and time can explain every known effect of gravity: the universal force that pulls apples to the ground and holds the planets in their orbits. Indeed, it's this curvature that determines how universe expanded in the aftermath of the Big Bang, and what happens when matter collapses into a black hole.

For all their successes, however, these two great theories are inconsistent; the standard model and the general theory of relativity cannot both be right. At best they are useful approximations, each one valid in its own domain. The real unifying principle presumably operates at much a much smaller scale-say, 10-35 meter, or a little more than a billionth of a trillionth the size of a proton. (No one knows the scale for sure. But this one is considered the most likely; calculations suggest that it's the scale at which quantum effects cause violent fluctuations in space and time themselves.)

The trick is to figure out what that unifying principle might be. Physicists are exploring a number of intriguing ideas. Easily the most popular is superstring theory, in which the fundamental objects aren't particles, but vibrating threads of energy. Another notable example is loop quantum gravity, in which space itself is built up from a different type of thread.

Stay tuned.

Circle Photo 1: Particle Accelerators
This accelerator helps conduct basic research into particle physics.
This 5,000-ton detector in search of top quarks, takes snapshots of particle collisions in Fermilab's Tevatron accelerator.

The Tevatron can be considered an excellent quark-producing machine. With its help the CDF and DZero experiments discovered the top quark back in 1995. Today, the experimenters are still studying the top quark, but they are also looking for new particles and forces. They are studying many aspects of matter, answering questions like how the building blocks of matter are put together and how they stay together. In addition, the experiments help to reveal how the universe began.
Credit: Fermi National Accelerator Laboratory

Circle Photo 2: The Standard Model
According to the standard model, everything is made from the same basic building blocks.
Physicists call the theoretical framework that describes the interactions between elementary building blocks (quarks and leptons) and the force carriers (bosons) the Standard Model. Gravity is not yet part of this framework, and a central question of 21st century particle physics is the search for a quantum formulation of gravity that could be included in the Standard Model.

Though still called a model, the Standard Model is a fundamental and well-tested physics theory. Physicists use it to explain and calculate a vast variety of particle interactions and quantum phenomena. High-precision experiments have repeatedly verified subtle effects predicted by the Standard Model. Credit: Nicolle Rager Fuller, National Science Foundation

Circle Photo 3: String Theory
An attempt by physicists to describe the forces of nature in a unified way.
One of the most ambitious and exciting theories ever proposed is the long-sought "theory of everything" that eluded even Albert Einstein. String theory-also known as superstring theory-proposes that the fundamental ingredients of nature are inconceivably tiny strings of energy, whose different modes of vibration underlie everything that happens in the universe. The theory successfully unites the laws of the large (general relativity) and the laws of the small (quantum mechanics), breaking a conceptual logjam that has frustrated the world's smartest scientists for nearly a century.
Credit: WGBH

Circle Photo 4: Quantum Gravity
A theoretical structure for connecting quantum mechanics and gravity.
The problem basically is how to quantize gravity. Gravity is associated with the shape and actual fabric of space-time. From experience with the quantum versions of other forces, it is expected that at low energies, for long times and large distances (the realm of our normal experience) the laws of quantum gravity will reduce to the ordinary laws we know describe the universe pretty well - the "classical" version of gravity due to Einstein. It is also known from previous experience roughly where energies become "low", times become "long" and distances "large" for quantum gravity - these are the so-called Planck scales arrived at by combing Planck's constant h with the gravitational constant G and the speed of light.

The energies at which the quantum nature of gravity become important are something like 10^16 times what particle accelerators can achieve, and the length scales and time scales are correspondingly shorter than anything we have investigated up to now. This puts it fairly well beyond experimental investigation for the foreseeable future.
Credit: Jean-Francois Colonna

Resources

American Institute of Physics: Einstein, Image and Impact
http://www.aip.org/history/einstein/

American Museum of Natural History: Einstein Exhibit http://www.amnh.org/exhibitions/einstein/

Einstein Archives Online
http://www.alberteinstein.info/

Einstein's Revolutionary Insights into Radiation http://www.physics.utoledo.edu/~ljc/klepp1.htm

Time Magazine Person of the Century http://www.time.com/time/time100/poc/magazine/albert_einstein5a.html

Albert Einstein Image Credit: Library of Congress, courtesy AIP Emilio Segre Visual Archives

Site Credits

Text:
M. Mitchell Waldrop, National Science Foundation

Animations:
Nicolle Rager Fuller, National Science Foundation (Photoelectric Effect and Brownian Motion) Trent Schindler, National Science Foundation (Time Dilation and Simultaneity)

Site Design:
S2N Media, Inc.

June 30, 2005