Scientific American has featured countless contributions from notable scientist-authors in its 167-year history. Among them, nearly 150 Nobel Prize winners have written for the magazine, contributing more than 200 articles altogether. In the July issue, we featured 12 excerpts of those articles written by past winners of the Nobel Prize in Physics to coincide with the 62nd annual Nobel Laureate Meeting in Lindau, Germany, which this year focuses on physics.

To give a more complete picture of the tales of discovery contained in the magazine's archives, we have selected three additional articles penned by physics Nobelists. All three, excerpted below, relate in some way to the research that earned their respective authors a Nobel.

The 1930 account of William H. Bragg, for instance, describes how x-rays can provide a window into crystalline structure. He had shared the 1915 Nobel Prize in Physics with his son for advancing that field of inquiry. Likewise, laureate Donald A. Glaser won in 1960 for the invention detailed in his 1955 article, "The Bubble Chamber." And the most recent physics Nobel, in 2011, went to a trio of researchers who played a leading role in the discovery that the universe's expansion is accelerating, thanks to "dark energy." One of those researchers, Adam G. Riess, co-authored a 2004 article in Scientific American, excerpted below, about the effort to determine just when the universe began to speed up.


X-Ray Fingers Feel Out the Atomic Structure of Matter
By William H. Bragg (Nobel Prize in 1915)
Published December 1930

Man, having the power to forecast the result of overcoming difficulties and the wish to try to overcome them, has devised various ingenious methods to help him in his task. Taking first of all the difficulties that depend on the inadequacy of his vision he has invented the microscope which gives him the power of seeing details thousands of times too fine to be perceived by the naked eye.

But there is a point which the microscope can not pass. With its aid we perceive what is very small, but not the "very very" small. There are details of the structure of the living cell, essential features in the composition of metals, cotton, silk, rubber, paint, bone, nerve, and a thousand other things which are hidden even from the microscope, and must always remain so hidden because the failure does not lie with the skill of the optician but with the incapacity of light itself.

The nature of radiation is in many respects a mystery, but we know enough about it to understand that we may talk of it in many of its important aspects as waves in some medium which we call the ether. If the radiation falls on any object, it is turned aside and modified in various ways. When our eyes are directed towards the object, they take in the modified rays, and we have learned by long practice to know, from these modifications, the nature of the object that has made them. That is "seeing."

The central point of the process is the act of scattering and modification. Now waves have a certain wavelength, and common experience of such waves as may be seen, for example, on the surface of the sea tells us that an object which is very much smaller than the length of the wave has no appreciable effect upon it. In just the same way there may be objects which are so small that they can not affect a ray of light, and such objects are forever invisible in the ordinary sense. The length of the light wave which our eyes can perceive lies within a short range on either side of a fifty-thousandth of an inch.

The X rays break down the barrier for us, and admit us to this immense field in which we want to be. They do so by virtue of their character as light waves, 10,000 times or so smaller than visible waves, but of exactly the same nature.

If a substance is such that all the atoms which compose it are arranged on one and the same pattern, so that the straight rows run through from side to side, the substance is a single crystal; crystalline character meaning, simply, perfect arrangement. But most substances, and especially those we handle every day, such as the metals, must be described as masses of small separate crystals.

However we try to deform [a bar of multi-crystalline material], there are always some crystals which resist being deformed in that particular way. And the various crystals back each other up according to some principle which we do not fully understand. Thus the properties of the bar depend upon its crystalline character. It is only the X ray that can tell us the internal arrangement of the crystal.

The X rays are of short enough wavelength to be turned aside or scattered by the atoms, when longer light waves are not. A single atom can, however, do very little. Here is where the regularity of crystal arrangement comes in. The unit of pattern is repeated an enormous number of times even in a crystal just visible to the naked eye. Whatever one of these units does in the way of scattering, all the others do in regular order. The combined amount is perceptible, and so the crystalline character is detected.

Of course this is an indirect way of examining the structure. We do not perceive the individual atoms; we discover only their arrangements. But the knowledge so gained can be combined with other knowledge that we already possess and we have actually found ourselves able to decipher the patterns of Nature to an extent we did not dream of a few years ago.


The Bubble Chamber
By Donald A. Glaser (Nobel Prize in 1960)
Published February 1955

In their exploration of the submicroscopic world of atomic nuclei, physicists are like men groping in a dark cave with a flashlight that goes on for only an instant and each time lights only a tiny corner of the cave. Occasionally the flash catches some activity or event—either a familiar particle behaving in a familiar way or some strange new particle whose behavior is altogether baffling. From these scanty glimpses nuclear physicists are attempting to identify the particles and the forces at play in the dark, violent world of the nucleus of the atom. It would help if they had a better flashlight.

Let us look for a moment at the events they are trying to observe and at the observing devices that have been available up to now. Physicists are probing the nucleus by bombarding it with particles, preferably particles with enough energy to break up the nucleus into its constituent parts.

He has had two ways of seeing and measuring these happenings. The first is the Wilson cloud chamber. In a chamber supersaturated with a vapor, a flying charged particle leaves a visible trail of liquid droplets, which condense on the ions the particle has produced by hitting vapor and gas atoms in its path.... Sometimes the particle breaks down ("decays") into lesser particles which make divergent tracks. But these interesting events occur only rarely in a vapor-filled chamber, because collisions in the gas are infrequent.

The second device for recording nuclear events is the photographic emulsion. A particle charging into the dense emulsion has a high probability of colliding with nuclei; hence there is a good chance that the emulsion will show interesting events, including scattering, disintegrations and the formation of new particles. However, the emulsion also has its drawbacks. Its very density makes collisions so frequent and the particle path so crooked that the effect of a magnetic field cannot be measured. And from the moment it is manufactured an emulsion begins to collect random particle tracks from cosmic rays and terrestrial radioactivity

Could some compromise be found which would eliminate the defects and combine the respective virtues of the cloud chamber and the emulsion? In May, 1952, I began to try a new approach to the problem, and I soon decided to explore the possibility of a liquid medium.

What kind of reversible process in a liquid could show the path of a flying particle and quickly erase the track after its passage? It would have to be a process that magnified the tiny effect of the atomic particle itself, as the condensation of droplets in supersaturated vapor magnifies the ionization produced by a particle in a cloud chamber. It occurred to me that a superheated liquid, like a supersaturated vapor, might provide the desired unstable equilibrium that could be triggered by a small stimulus to yield a large effect. Physical chemists have long known that in a clean, smooth-walled vessel a very pure liquid may be heated above its usual boiling point without boiling. I wondered whether a flying particle might, under suitable conditions, trigger the formation of the microscopic bubbles that start the boiling process. If so, it might make a visible track in a superheated liquid.

I used a bulb (half an inch in inside diameter) filled with ether; it was connected by a capillary tube to a piston-fitted cylinder with a hand crank which could quickly lower the pressure. High-speed movies, at the rate of 3,000 pictures per second, were made of the happenings in the bulb when the pressure was reduced. Sure enough, the pictures disclosed a track of tiny bubbles when a particle darted through the superheated ether. The bubble type of chamber soon proved to be a very sensitive recorder. Even fast mu mesons, which ionize only lightly, made visible tracks in the superheated liquid.

Having demonstrated that the bubble chamber idea worked, we proceeded to the task of building one large enough for practical laboratory use. We first built a two-inch chamber of duralumin and glass, with a diaphragm, actuated by compressed air, which could fully expand the chamber in five thousandths of a second. The liquid remained sensitive for seven thousandths of a second. We then incorporated the same design features in a larger pentane-filled version in which the liquid volume is six inches long, two inches wide and three inches high. This chamber is now in use with the Cosmotron at the Brookhaven National Laboratory. We have made 400 excellent pictures of tracks of protons from this accelerator. These track photographs are as easy to read as the best cloud chamber records and are about 10 times as accurate.


From Slowdown to Speedup
By Adam G. Riess (Nobel Prize in 2011) and Michael S. Turner
Published February 2004

From the time of Isaac Newton to the late 1990s, the defining feature of gravity was its attractive nature. Gravity keeps us grounded. It slows the ascent of baseballs and holds the moon in orbit around the earth. Gravity prevents our solar system from flying apart and binds together enormous clusters of galaxies. Although Einstein’s general theory of relativity allows for gravity to push as well as pull, most physicists regarded this as a purely theoretical possibility, irrelevant to the universe today. Until recently, astronomers fully expected to see gravity slowing down the expansion of the cosmos.

In 1998, however, researchers discovered the repulsive side of gravity. By carefully observing distant supernovae—stellar explosions that for a brief time shine as brightly as 10 billion suns—astronomers found that they were fainter than expected. The most plausible explanation for the discrepancy is that the light from the supernovae, which exploded billions of years ago, traveled a greater distance than theorists had predicted. And this explanation, in turn, led to the conclusion that the expansion of the universe is actually speeding up, not slowing down. In the past few years, astronomers have solidified the case for cosmic acceleration by studying ever more remote supernovae.

But has the cosmic expansion been speeding up throughout the lifetime of the universe, or is it a relatively recent development—that is, occurring within the past five billion years or so? The answer has profound implications. If scientists find that the expansion of the universe has always been accelerating, they will have to completely revise their understanding of cosmic evolution. But if, as cosmologists expect, the acceleration turns out to be a recent phenomenon, researchers may be able to determine its cause—and perhaps answer the larger question of the destiny of the universe—by learning when and how the expansion began picking up speed.

In Einstein’s theory, the notion of gravity as an attractive force still holds for all known forms of matter and energy, even on the cosmic scale. Therefore, general relativity predicts that the expansion of the universe should slow down at a rate determined by the density of matter and energy within it. But general relativity also allows for the possibility of forms of energy with strange properties that produce repulsive gravity. The discovery of accelerating rather than decelerating expansion has apparently revealed the presence of such an energy form, referred to as dark energy.

Whether or not the expansion is slowing down or speeding up depends on a battle between two titans: the attractive gravitational pull of matter and the repulsive gravitational push of dark energy. What counts in this contest is the density of each. The density of matter decreases as the universe expands because the volume of space increases. Although little is known about dark energy, its density is expected to change slowly or not at all as the universe expands. Currently the density of dark energy is higher than that of matter, but in the distant past the density of matter should have been greater, so the expansion should have been slowing down then.

It is important to look for direct evidence of an earlier, slowing phase of expansion. Such evidence would help confirm the standard cosmological model and give scientists a clue to the underlying cause of the present period of cosmic acceleration. Because telescopes look back in time as they gather light from far-off stars and galaxies, astronomers can explore the expansion history of the universe by focusing on distant objects. That history is encoded in the relation between the distances and recession velocities of galaxies. If the expansion is slowing down, the velocity of a distant galaxy would be relatively greater than the velocity predicted by Hubble’s law. If the expansion is speeding up, the distant galaxy’s velocity would fall below the predicted value. Or, to put it another way, a galaxy with a given recession velocity will be farther away than expected—and hence fainter—if the universe is accelerating.

To take advantage of this simple fact requires finding astronomical objects that have a known intrinsic luminosity—the amount of radiation per second produced by the object—and that can be seen across the universe. A particular class of supernovae known as type Ia are well suited to the task. Over the past decade, researchers have carefully calibrated the intrinsic luminosity of type Ia supernovae, so the distance to one of these explosions can be determined from its apparent brightness.

Finding such ancient and far-off supernovae is difficult, however. A type Ia supernova that exploded when the universe was half its present size is about one ten-billionth as bright as Sirius, the brightest star in the sky. Ground-based telescopes cannot reliably detect the objects, but the Hubble Space Telescope can. In 2001 one of us (Riess) announced that the space telescope had serendipitously imaged an extremely distant type Ia supernova (dubbed SN 1997ff) in repeated observations. Given the redshift of the light from this stellar explosion—which occurred about 10 billion years ago, when the universe was one third its current size—the object appeared much brighter than it would have been if [dust filling intergalactic space simply made the supernovae appear dim, as some researchers had proposed]. This result was the first direct evidence of the decelerating epoch. The two of us proposed that observations of more high-redshift supernovae could provide definitive proof and pin down the transition from slowdown to speedup.

The Advanced Camera for Surveys, a new imaging instrument installed on the space telescope in 2002, enabled scientists to turn Hubble into a supernova-hunting machine. Riess led an effort to discover the needed sample of very distant type Ia supernovae by piggybacking on the Great Observatories Origins Deep Survey. The team found six supernovae that exploded when the universe was less than half its present size (more than seven billion years ago); together with SN 1997ff, these are the most distant type Ia supernovae ever discovered. The observations confirmed the existence of an early slowdown period and placed the transitional “coasting point” between slowdown and speedup at about five billion years ago.