Photons, the fundamental particles of the electromagnetic field that permeates the universe, don’t have mass. They don’t have charge. Because they travel at the cosmic speed limit, they don’t experience time.
They sound completely alien. And yet photons are the quanta of light that allow us to observe the world around us—both up close and from afar.
“Photons, in one way or another, have always been a key element in the way we can look at nature and understand it and manipulate it.”
“Photons, in one way or another, have always been a key element in the way we can look at nature and understand it and manipulate it,” says Claudio Pellegrini, distinguished professor emeritus at the University of California, Los Angeles, and adjunct professor at the Department of Energy’s SLAC National Accelerator Laboratory. “The whole story is incredible.”
Understanding and harnessing light
In the 1600s, scientist Galileo Galilei manipulated light by building a telescope and pointing it at the sky. There, he saw stars, moons and the phases of Venus—all of which contributed to his revelation that the Earth was not, in fact, the center of the universe. Not content to stop there, he also built his own microscope, zooming in on the smallest details of nature. “It opened up a whole new world,” Pellegrini says.
That same century, scientists began to pay attention to the nature of light itself. In 1675, Isaac Newton proposed that light could be broken down into small particles, which he called corpuscles. But a universe of individual particles would also contain vast swaths of empty space. And if space were empty, how could light travel from the moon to Earth?
By the 19th century, the corpuscle theory had fallen out of favor, and many scientists believed in the idea of an ether, a ubiquitous medium through which light could travel. “The idea of continuity and continuous substances was now key,” says Paul Halpern, professor of physics at St. Joseph’s University and an expert on the history of physics.
Light, many scientists came to believe, was in fact a wave. In the mid-19th century, this was due in large part to the efforts of physicist and mathematician James Clerk Maxwell. Maxwell showed that the wavelength of photons determined their place on the electromagnetic spectrum, which included waves that lie beyond what human eyes can see, from radio waves to gamma rays.
The book was not closed on photons yet. By the 20th century, physicist Max Planck had proposed that light was both a particle and a wave—an idea that gave him key insights into the nature of thermal radiation. But Albert Einstein disagreed; he proposed in one of his four famous 1905 papers that light was made up of discrete energy packets—another notch for the particle theory.
In the 1920s, scientist Arthur Compton’s experiments on photons that had been scattered by electrons could only be explained if photons had features of both particles and waves. Once Louis de Broglie proposed that other particles, like electrons, also had wave properties, this dual nature of particles was accepted.
Photons today
Understanding this wave-particle duality has helped scientist think about photons in different ways while conducting experiments—sometimes it is helpful to think about them as particles, and other times as waves. It has also led to technological advancements in photonic devices such as lasers, microscopes, and even quantum computers—developments that have fueled research itself.
Since the days of Galileo, scientists have greatly improved upon the first telescopes. Now, they can peer into modern instruments such as the James Webb Space Telescope, which uses a large mirror to collect infrared light, allowing scientists to peer back in time to more than 13.5 billion years ago.
Scientists’ ways of looking at the very small have evolved as well. For example, over the past 35 years, scientists at SLAC’s Stanford Synchrotron Radiation Lightsource have used intense beams of X-ray light to image everything from human cells to solar cells at the molecular level. And at SLAC’s Linac Coherent Light Source, that X-ray capability is pushed further, allowing researchers to track even a single atom during a chemical reaction. “Now, not only can we see the details of the moon and Venus, but we can watch chemical reactions as a movie,” Pellegrini says.
Photons have helped Uwe Bergmann, a professor at the University of Wisconsin-Madison and visiting faculty at SLAC, to extend his physics research into the humanities as well. Bergmann has used SLAC’s light sources to reveal ancient texts that had been erased and overwritten. In 2005, he used SSRL to reveal the text of a copy of Archimedes’ mathematical theories. The parchment had been scraped and re-used as a prayer book in the 12th century, but the powerful X-rays revealed the iron in the original ink, allowing researchers to read the original text for the first time. Since then, he has used similar imaging techniques to reveal the chemistry of traces left in fossils, parts of an opera that were erased by composer Luigi Cherubini, and to understand the elements used in early Korean prints made by moveable type.
“This work allows me to collaborate with people from very different backgrounds and contribute to understanding history,” Bergmann says. “It has been absolutely amazing.”
Photons have also been instrumental in understanding and developing quantum technologies. Because of their wave-particle duality, they are polarized—either in a clockwise or counter-clockwise state— and they can be used like computer bits and carry information.
But their true power comes from quantum entanglement—a phenomenon that allows two particles to share a quantum state, such as photons’ polarization. From the 1970s to the 1990s, scientists showed how photons could be entangled, which paved the way for the latest quantum information science revolution.
“Photons have given us proof of the strangest properties of quantum mechanics,” Pellegrini says. Recently Pellegrini and Bergmann have teamed up with SLAC scientist Alex Halavenau and others to exploit these properties at LCLS and other new X-ray lasers.
Photons are some of the most obvious and abundant particles in the universe, and humans have a long history of both studying them and harnessing them to examine everything from celestial objects to fundamental particles. As researchers continue to upgrade their tools to study the universe on the smallest and largest scales, photons will continue to light the way.
