New Eyes on the Universe: 400 Years of Physicist Astronomers
By Michael S. Turner
Four-hundred years ago Galileo turned a 2-cm telescope to the sky and increased the sensitivity of human eyes on the Universe by a factor of 100–an increase only matched since by that of the Hubble Space Telescope. His discoveries, including the phases of Venus and the resolution of the Milky Way into stars, established the Copernican paradigm and profoundly changed our view of the Universe. Since then, the sensitivity of optical telescopes has improved by another factor of 100 million, and we have added radio, infrared, UV, x-ray, gamma-ray, cosmic-ray, and neutrino eyes on the Universe. These new and more powerful eyes have continued to deliver stunning discoveries.
Because physicists have contributed so significantly to the improvement of the performance of “our eyes on the Universe” and in shaping the science of astrophysics, it is appropriate that our Society take part in the celebration of this anniversary (the International Year of Astronomy) through the theme–New Eyes on the Universe: 400 years of telescopes–and content of the April Meeting. Galileo was not only the first physicist to turn a telescope to the sky, but the telescope and microscope were the first instruments of science that extended our ability to explore the physical world.
The telescope Galileo used was a simple refractor. The sheer weight of a large glass lens makes a refracting telescope larger than about a 1 meter in diameter impractical. The introduction of the reflecting telescope by Newton in 1669 paved the way to virtually all modern telescopes. Newton made a few other contributions relevant to astronomy including his invention of the fields of gravitational physics and spectroscopy.
George Ellery Hale, the MIT-educated solar-physicist, was the pre-eminent telescope builder of the 20th century. After starting the first Department of Astronomy and Astrophysics at the University of Chicago, Hale took astronomy to the mountains of California where the seeing (image quality) is much better. (When observing point-like objects, sensitivity improves as the point-spread function squared. On the best mountaintops seeing can be 0.3 arcseconds which results in a hundredfold gain.) Four times Hale built the largest telescope in the world: the 40-inch refractor at Yerkes Observatory in Wisconsin and the 60-and 100-inch reflectors at Mt. Wilson and the 200-inch reflector at Mt. Palomar, and he left a great scientific legacy in the discoveries made by these telescopes.
Edwin Hubble used the 100-inch Hooker telescope to solve the riddle of the mysterious nebulae that had been catalogued for more than 100 years. He showed that they were “island universes” and not gas clouds within our own Milky Way galaxy, thereby enlarging the known Universe by a factor of 100 billion. Hubble went on to discover the expansion of the Universe, revealing our big bang origin. The 200-inch was used to discover quasars, now known to be super-massive black holes powered by the accretion of matter. This discovery, featured on the cover of Time Magazine in March 1966, opened our eyes to the “extreme Universe” of relativistic objects like neutron stars and black holes.
Physicist Albert A. Michelson not only showed that the speed of light is frame independent, but he also introduced interferometry to astronomy. The use of an interferometer to combine the light from two telescopes to create a telescope of larger effective diameter–the separation of the two telescopes–vastly increases its resolving power. Today, optical interferometers are in operation at Mt. Wilson and on the European Southern Observatory’s Cerro Paranal in Chile, where the light of four 8-meter telescopes can be combined. These interferometers will soon image distant planetary systems, black hole accretion disks, and the surfaces of stars.
Michelson interferometers are also being used in the quest to directly detect gravitational waves. At the heart of the two U.S. LIGO detectors (in Hanford, WA and Livingston, LA) are 4-km Michelson interferometers which are used to detect the tiny (10-15 cm) changes in the separation of the mirrors due to passing gravitational waves. Soon, LIGO and other gravitational-wave detectors around the world will be “listening” to the collisions of black holes and neutron stars across the Universe. By monitoring the coalescence of two black holes into one larger one, some of the most fundamental predictions of general relativity will be tested.
Nowhere has the impact of physics on astronomy been greater than with the introduction of spectroscopy. In the mid-nineteenth century, Sir William Huggins compared the spectra of the sun and distant stars and showed that both are made largely of Hydrogen, making clear that our sun is just a star up close and that we are made of the same stuff as the cosmos. The ability to remotely analyze the composition of objects across the Universe and thereby to begin to understand their inner workings created the field of astrophysics. It is an interesting footnote to history that in 1835, Auguste Comte, a prominent French philosopher, stated with great authority that “humans would never be able to understand the chemical composition of stars.” Never say never when physicists are involved!
The first president of the APS, Henry Rowland, revolutionized spectroscopy with his gratings, still the workhorse of spectroscopy today. Astronomers take spectra of just about anything they can see (and even things that they can’t see) in the Universe–from the brightest stars to the faintest galaxies. Spectra are used to determine velocities, compositions, and more generally to get at how things work. The redshift of a galaxy determines how big the Universe was when its light was emitted and through Hubble’s law how far away it is; the spectrum of a supernova reveals the exploding shell of material and the newly formed heavy elements. Very stable spectrographs were used to detect the small (of order meters per second) periodic wobbles in nearby stars caused by the exoplanets that orbit them. Hundreds of planets have been discovered, and in the future, spectra of the exoplanets themselves may reveal the chemical signatures of life that exists on them.
Almost a factor of a million of the gain in sensitivity since Galileo’s time has come from increasing telescope mirror size. Physicists Jerry Nelson and Roger Angel have made innovations in mirror design that have enabled today’s large telescopes. Angel introduced molded honeycomb mirrors that reduce the weight of large mirrors dramatically, and Nelson devised a way to make a large mirror from many smaller and lighter segments. These two innovations are at the heart of the design of the two 30-meter telescope projects–the Thirty Meter Telescope with its four-hundred-ninety-two 1.8-meter hexagonal segments and the Giant Magellan Telescope with its seven 8-meter honeycomb mirrors. These giant telescopes will add another factor of 100 in sensitivity to our eyes on the sky enabling marvelous discoveries.
Even at the best high-mountain sites atmospheric turbulence blurs astronomical images and limits seeing. With the advent of adaptive optics–the use of flexible mirrors with real-time control systems–the blurring can be undone. The critical de-blurring information comes from having a bright guide star near the distant (usually faint) object of interest. Since there are not enough bright stars in the sky for this purpose, artificial guide stars are created by shining high-powered lasers towards the heavens; by exciting atoms in the atmosphere they create the needed guide stars. Next year physics celebrates the 50th anniversary of the invention of the laser.
Photographic plates only capture about 1% of the incident photons, whereas modern quantum devices–CCDs and the like–have efficiencies approaching 100%. Moreover, a digital image can be exploited in ways that a photographic image cannot. For example, the key to discovering the acceleration of the expansion of the Universe in 1998 was finding distant supernovae to use as cosmic mileposts. Supernovae are very bright–as bright as their host galaxies–but very rare–occurring only once in every 100 years or so. They can now be found routinely by taking two images of the sky containing 1000s of galaxies weeks apart; when the two images are digitally subtracted, the supernovae jump right out.
Equally important to the advance of our understanding of the Universe has been the addition of “new eyes” on the Universe which have revealed otherwise invisible objects and have led to stunning discoveries. In the 1930s physicist Karl Jansky of Bell Labs and amateur Grote Reber pioneered radio astronomy by detecting diffuse emission from our galaxy and a few bright individual sources. Today, radio eyes allow us to study rapidly spinning neutron stars, the jets created by the supermassive black holes at the centers of galaxies, and a host of other things invisible to the eye.
Radio astronomy's greatest hit is the discovery of the cosmic microwave background (CMB) in 1964 by physicists Arno Penzias and Robert Wilson. It revealed the hot beginning of our Universe and much more. Today precision measurements of its anisotropy (a part in 105 variations in its intensity across the sky) have given us a glimpse of the Universe when it was only 400,000 years old and before stars and galaxies existed. From measurements of CMB anisotropy the age, shape and composition of the Universe have been accurately measured and information about the earliest moments of creation has been gleaned (e.g., evidence for an early period of inflation).
Physicists Herbert Friedman and Riccardo Giacconi pioneered the field of x-ray astronomy. Without x-ray eyes we would not be able to see most of the baryons in the Universe which reside in hot x-ray emitting gas around galaxies and clusters of galaxies. Black holes of all sizes are studied by the x-ray emission from their accretion disks, and the highly asymmetric shape of the Iron lines seen allow the in falling material to be tracked as it approaches the event horizon.
Masatoshi Koshiba and Raymond Davis opened the field of neutrino astronomy with their large underground experiments which detected neutrinos from the sun and supernova 1987A. Gamma-ray astronomy was secretly born during the Cold War in 1967 with the detection of gamma bursts from black holes forming at the edge of the Universe by the Vela satellites, which were built to monitor the nuclear test ban on Earth. Physicists Robert Leighton and Gary Neugebauer were among the pioneers of infrared astronomy; today IR eyes allow astronomers to see through the dust surrounding the birth of stars and planets as well as the high-redshift universe where the expansion of the Universe has shifted most of a galaxy’s visible light into the infrared. Without devices invented by physicists, x-ray, gamma-ray, infrared, neutrino and radio astronomy would not be possible.
Four hundred years ago the telescope and the microscope were essentially the same device, with one turned to outer space and the other turned to inner space. The paths deep into inner space and into outer space quickly diverged, with exciting, but seemingly unrelated discoveries in these vastly different realms of the physical world. Microscopes discovered microbes, cells and viruses and explored the worlds within them; and most recently, kilometer-sized accelerators revealed the world of quarks, leptons and gauge bosons. At the same time, bigger and bigger telescopes, employing more and more sophisticated detectors, took us to the edge of the Universe and back to the beginning of time. After nearly 400 years of divergence, inner space and outer space have come together again. Today, both astronomers and particle physicists are chasing after dark matter, dark energy, neutrinos, and the birth of the Universe, using telescopes and accelerators. The plenary, invited and contributed talks at the April Meeting illustrate this coming together of quarks and the cosmos with particle physicists talking about the search for dark matter (the particle, that is) at the Large Hadron Collider and astronomers using telescopes to get at the essence of nothing (dark energy, that is).
Michael S. Turner is the Rauner Distinguished Service Professor at the University of Chicago’s Kavli Institute for Cosmological Physics; he is currently Chair-elect of the Division of Astrophysics. He led the National Academies Study From Quark to the Cosmos and coined the term dark energy.