Legend has it that Henry A. Rowland, Johns Hopkins University’s first physics professor, once declared under oath he was the world’s greatest physicist. The tale grew from proceedings over a lawsuit in which the court noted he was “the highest known authority in this country upon the subject of the laws and principles of electricity.”
Whether the opinion was Rowland’s or a judge’s, it wasn’t an exaggeration. Rowland’s work in the late 1800s provided the foundation for a generation of spectroscopy research worldwide and helped usher in a new field: astrophysics. More than a century after his death, he continues to influence modern physics in ways direct and indirect, particularly in the fields of astronomy, condensed matter physics, and particle physics, three main focuses of the Krieger School’s Henry A. Rowland Department of Physics and Astronomy, which is named for the pioneer.
In a display case on the second floor of the department’s Bloomberg Center for Physics and Astronomy, a primitive-looking machine no bigger than a table saw marks Rowland’s most famous contribution to science: the ruling engine.
Two centuries before Rowland’s work, Sir Isaac Newton showed that the sun’s white light could be broken up into a continuous series of colors, which he called a spectrum. His early spectroscope—a small aperture and a lens to focus the light, and a glass prism to disperse it—launched the field of spectroscopy, which didn’t advance much until the early 1800s, when discoveries led to studies of the spectrum beyond visible light—into the infrared and ultraviolet portions. Eventually, astronomers started using reflective plates etched with thousands of fine lines to analyze the light from stars, which gave them insight into stars’ chemical compositions and temperatures. But their equipment was imprecise and, therefore, the field was hamstrung.
Rowland, hired at the age of 27 by founding university president Daniel Coit Gilman, began engineering a “perfect screw” in 1882 that ultimately made precise those etched plates—called diffraction gratings—thereby producing spectra of superb resolution and accuracy. His ruling engine also employed concave plates for the gratings, instead of flat pieces of metal, as the machine’s predecessors had done.
“Rowland realized that if you could do that, you would both effectively disperse the light into its component wavelengths and focus it at the same time,” explains Paul Feldman, astronomy professor and former department chair. “That’s a very powerful and very useful accomplishment.”
Useful, indeed: The new gratings enabled astronomers to crack open and dissect light from stars, planets, and other sources, a process that eventually revealed new information about their composition, distance, and surroundings. They came to identify spectroscopic signatures made unique by the light sources and all the material those sources passed through.
This opened up the universe to scientists as never before. The territory beyond the solar system, once just distant and mostly unknowable points of light in the night sky, became in a relatively short time a very colorful and lively cast of many different kinds of stars, galaxies, and other characters, all with discernible patterns of birth, growth, death, and renewal.
“Astrophysics really was born with the modern spectroscopy made possible by Rowland’s gratings,” says Holland Ford, professor of physics and astronomy and principal investigator for the Hubble Space Telescope’s Advanced Camera for Surveys. “Once scientists realized that individual elements had unique spectroscopic signatures, that’s when we began to really unravel what the universe is like.”
Rowland supplied his gratings at cost to spectroscopists around the world, and today, astronomers continue to use gratings inspired by Rowland’s innovative designs, including on the Hopkins-built and run Far Ultraviolet Space Explorer (FUSE). For his contributions to physics and astronomy, the American Physical Society, of which Rowland was the founding president, recently announced it would designate the Bloomberg Center as one of its first five national historic sites. The society will likely install a plaque next to Rowland’s ruling engine in recognition of the physical and astronomical advances his work made possible.
Messages from the Cosmos
For astrophysicist Charles Bennett, who joined the Physics and Astronomy Department last January, spectroscopy is an essential tool for answering questions about the biggest topic humanity has ever considered: the universe.
"Rowland was a very prominent physicist of his day...he recognized that studying what was in the sky was a way of probing physics. You could use what's out there to learn about physics here on earth, and that continues."
“My research looks back to the dawn of time and the very earliest moments of the universe, where our two models of physics—gravity physics and quantum mechanics—break down and conflict with each other,” explains Bennett. “It also looks to the end of the universe, to the very far future and its ultimate fate.”
Like Rowland’s gratings, Bennett’s research would be impossible without remarkable dedication to precision. Bennett’s insights into the origins and fate of the universe come from precise study of a form of radiation that permeates the universe. Located in the microwave portion of the electromagnetic spectrum, this radiation is an “echo” of the Big Bang, the explosive moment of the universe’s creation.
Bennett has been a leading contributor to two of the most ambitious attempts to map very slight variations in this echo across the vastness of the universe. The maps help Bennett and other astrophysicists learn more about the earliest stages of the universe and the history of its development, revealing, for example, the precise age of the universe: 13.7 billion years.
For his remarkable contributions to research, Bennett’s fellow scientists earlier this year elected him to the National Academy of Sciences and awarded him the academy’s prestigious prize, the Henry Draper Medal.
Bennett is only the third Hopkins physicist to receive the medal; Rowland was the first.
“Rowland was a very prominent physicist of his day,” Bennett says. “He recognized that studying what was in the sky was a way of probing physics. You could use what’s out there to learn about physics here on Earth, and that continues today.”
Bennett’s colleague in the department, Gerhard H. Dieke Professor Warren Moos, is the principal investigator of FUSE, an orbiting NASA observatory launched in 1999 to look at light far into the ultraviolet portion of the electromagnetic spectrum (the name for the entire range of light energy) that can’t be observed from ground-based telescopes.
As the first orbital NASA observatory administered from a university campus, the 18-foot tall, 3,000-pound satellite, now entering its sixth year of observations, is a landmark in both the history of NASA and of the Rowland Department of Physics and Astronomy.
FUSE observes the universe in the hopes of answering such long-standing fundamental questions as: What were conditions like in the first few minutes after the Big Bang? How are chemical elements dispersed throughout the galaxies, and how does this affect the way galaxies evolve?
FUSE’s principal scientific instruments are four devices known as Rowland circle spectrographs, which produce various light fre-quencies in circles instead of the customary long rectangles. Circle spectrographs are both easier to make and more efficient, Moos says. The efficient design, which reduces the number of times light has to be reflected, is ideal for FUSE.
Since its launch, FUSE has yielded several groundbreaking findings, including the first-ever observation of molecular nitrogen outside our solar system; confirmation of a hot gas halo surrounding the Milky Way galaxy; and a rare glimpse into molecular hydrogen in the Mars atmosphere.
Photo by Will Kirk/HIPs
"The sheer magnitude and amount of scientific work that is being produced using FUSE is beyond what we had imagined."
“The sheer magnitude and amount of scientific work that is being produced using FUSE is beyond even what we had imagined,” Moos said on the occasion of FUSE’s fifth anniversary, at which point the satellite had collected more than 47 million seconds of data on more than 2,200 unique objects in the cosmos.
Like any good astronomer, Moos likes to find answers. But those answers are much more exciting when they immediately lead to new questions.
Sometimes those new questions are detail-oriented, but they can also be very big, fundamental questions. Adam Riess, a Space Telescope Science Institute astronomer who recently accepted a professorship at Hopkins, was a leading member of a team of researchers that used spectroscopic observations of distant exploding stars to uncover a completely unexpected aspect of the development of the entire universe.
Nothing travels faster than light, but even the speed of light is finite. Light from distant stars and galaxies has to travel for millions or billions of years before it reaches Earth, so as astronomers observe light from distant stars and galaxies, they’re actually looking at them as they existed millions and billions of years ago.
Riess and his colleagues discovered that the universe’s expansion, which began with the Big Bang and continues today, was inexplicably speeding up.
Riess and others theorized that this acceleration may be caused by a new form of energy they call dark energy.
“We think dark energy pervades the vacuum of space and pushes the universe apart so strongly that it overcomes the gravitational pull all the parts of the universe exert on each other,” he explains. “Trying to understand what dark energy might be is one of the biggest mysteries in physics right now.”
Bennett is also working on the mystery of dark energy. He and his colleagues have used maps of the background radiation from the Big Bang to put together a “recipe” for the universe—an inventory of how much of the different types of matter and energy are present in the universe. That recipe is dominated by dark energy and dark matter, a mysterious type of matter only discernible to astronomers because of its effects on the structure and movements of distant galaxies.
Bennett, Riess, and other astronomers are working on new ways to learn more about these exotic forms of matter and energy. The fact that these so-far inscrutable enigmas make up the bulk of the universe might seem daunting, but Bennett takes encouragement in looking back at how far our understanding of the universe has already progressed.
“Just think about it—standing on a speck of dust, we can look out across vast distances and tell what things are made of, weigh them, and determine with considerable detail what’s happening in places far away in space and time,” he says. “It’s just astonishing.”
Rowland’s work in advancing spectroscopy aids other fields in physics, too. Professor Collin Broholm, a condensed matter physicist, is one of those reaping the benefits.
Photo by Will Kirk/HIPs
"Neutron spectroscopy gives us unique insights into the nature of materials that can't be generated any other way."
Using a technique called neutron spectroscopy, Broholm fires subatomic particles called neutrons at materials to learn more about how their atoms are locked together into crystals. These structures affect the material’s properties, such as its response to magnetism or heat.
The subatomic firing tech-nique might seem far removed from breaking up light from the sky to learn about distant corners of the universe, Broholm notes, but there are striking similarities. Absorption spectroscopy relies on changes put into light after it passes through a cloud of gas or dust. Broholm looks for changes in the neutrons’ energy and direction after they pass through target compounds.
“Essentially, changes in the energy and momentum of the neutrons after they emerge from the target crystal allow us to learn more about the crystal structure and about atomic scale motion within them,” he explains.
Broholm specializes in exotic materials—with multisyllabic chemical names—that should be magnetic but are not for interesting reasons. Neutron spectroscopy reveals magnetic fluctuations that help explain why introducing impurities into them can make them magnetic.
Getting the neutrons they need for their experiments—neutrons that have the right energy and momentum prior to their entry into the target—is no small feat, and Broholm and his colleagues recently turned to the creative genius of Henry Rowland’s work for help and inspiration.
Pulling a neutron from its natural habitat, the center of the atom, involves such a considerable expenditure and release of energy that it requires a nuclear reactor or a high-energy accelerator. Broholm conducts his studies at a research reactor, the National Institute of Standards and Technology’s Center for Neutron Research (NCNR). Rather than producing electrical power for consumers, this facility is dedicated to producing neutrons for use by physicists, chemists, biologists, and engineers for a wide range of materials science.
Pulling neutrons out of the atomic nucleus is only half the battle. Once they’re out, neutrons self-destruct after about 15 minutes, so scientists have to select the neutrons that have the right kinds of energy and momentum and direct them to the target quickly. They do this with a tool known as a monochromator.
In the mid-1990s, Broholm and his colleagues learned of an opportunity to install a new instrument for neutron spectroscopy at NCNR. As they considered the possibilities for improving monochromator design, Broholm was struck by Rowland’s innovative use of concave surfaces for diffraction gratings, allowing him to focus the light and separate it into components at the same time.
Broholm compares the double payoff to “having your cake and eating it too.” He believed a design inspired by Rowland’s concave diffraction gratings could allow scientists to collect more of the neutrons coming from the reactor and also more precisely select the neutron energy leading to higher resolution and higher intensity spectroscopy.
After years of design and construction, the new monochromator was finally installed at NCNR in late July, much to Broholm’s delight. “Neutron spectroscopy gives us unique insights into the nature of materials that can’t be generated any other way,” he says, “and this instrument should significantly expand the range of materials we can study with this technique.”
Toward a “Grand Unified Theory”
The first thing physics professor Raman Sundrum admits about his work is that many find it very difficult to think about.
Photo by Will Kirk/HIPs
“People like me feel that atomic physics has had its peak, and that has led us to consider a new frontier: the possibility of extra dimensions.”
“People like me feel that atomic physics has had its peak, and that has led us to consider a new frontier: the possibility of extra dimensions,” says Sundrum, a particle physicist. “For many people, these dimensions are impossible to comprehend because they’re so outside of our everyday experience that we can’t come up with a picture of them.”
Sundrum and others like him believe that these extra dimensions will be essential to one of modern physics’ most ambitious goals: putting together the “Grand Unified Theory,” a comprehensive scientific picture of the nature of matter, space, time, and energy. Ideally, the theory would seamlessly explain space and time, how forces are conveyed across space, how matter is created from tiny particles smaller even than protons and electrons, and would unify all the great physics theories and models.
But when physicists try to bring those theories together, they don’t always fit. Sundrum and others like him hope that extra dimensions are the missing pieces that will finally allow scientists to complete the puzzle showing a picture of the origins and nature of the universe.
In Rowland’s time, researchers had only just begun to work on such puzzles. The first subatomic particle, the electron, was discovered in 1897, only four years before Rowland’s death in 1901. However, Rowland’s diffraction gratings and the spectroscopic results they produced would continue to play an important supporting role in the decades of astonishing revelations to come.
Spectroscopic signatures in light are energy emissions from the electrons in atoms. In the classical model of the atom, electrons could take on or shed random amounts of energy. Spectroscopy did away with that model, showing scientists instead that electrons could absorb or emit energy only in tightly defined amounts.
A more scientific term for a tightly constrained amount or packet of energy is a quantum, which is where quantum mechanics gets its name. The various dark and bright lines seen in spectra are a result of differences in the quanta of energy that electrons emit and absorb. Because every element has a unique configuration of electrons orbiting its nucleus, each has a unique spectroscopic signature.
“Looking at the spectroscopic signatures of atoms and molecules was one of the ways that quantum mechanics came about,” notes particle theorist Jonathan Bagger, Krieger-Eisenhower professor and chairman of the department. “Classical mechanics, the theory that preceded quantum mechanics, couldn’t explain why there were distinct spectroscopic signatures.”
This quantum notion led to another major change: Instead of thinking of energy as something transmitted by a wave, there were very good reasons to think of it as something conveyed by particles.
“Because of quantum mechanics, everything shows up as a particle or quantum,” Bagger explains. “Electromagnetic radiation is conveyed by a particle known as a photon, for example, and we believe gravity is conveyed by a particle known as a graviton.”
“We’re still looking for spectra in particle physics,” Sundrum says. “But the spectra we look at are the fingerprints of the extra dimensions that we hope will make a Grand Unified Theory possible.”
At particle colliders, physicists smash elementary particles together. These collisions produce great showers of new and extremely short-lived particles.
“Even though we can’t directly see them, these microscopic particles whirl around the extra dimensions like electrons whirling around the nucleus of an atom,” Sundrum notes. “Quantum mechanics says that whenever you have periodic motion like this, you always have quantized emissions of energy, which are analogous to the spectra of color seen in conventional spectroscopy.”
Just as astronomers spent decades fully understanding how spectroscopic signatures are reflective of distant astronomical objects, particle physicists must decode how the patterns seen in colliders reflect the properties of extra dimensions.
“It’s going to be a huge challenge,” Sundrum says, and it couldn’t be clearer that he’s looking forward to the test.
Michael Purdy is a former news and information officer for Johns Hopkins who now works as a senior medical sciences writer at Washington University in St. Louis.