From The Archives

Fifty Years

Comment for Sidereal Times, November 2012

by Freeman Dyson, Institute for Advanced Study, Princeton, New Jersey

Fifty years ago, when AAAP began, the piece of the universe that we had explored was tiny, just a little blob with us in the middle. We could see a lot of stars and galaxies, but the volume of space that we could see was only about a tenth of one percent of the universe. With our biggest telescopes we could measure distances of galaxies about a tenth of the way to the edge, if the universe had an edge. We did not imagine that within our lifetimes we would be able to see out all the way to the edge. Now, fifty years later, everything has changed. The whole shebang is pretty well explored. In any direction, if we look for faint objects, we can see almost all the way back to the beginning of time. Some huge gaps remain, but the entire universe is now within our field of view.

How has this change happened? It did not happen because we are smarter now than we were fifty years ago. It happened because we have better tools. The most important new tools were radio telescopes. Fifty years ago, we already had radio telescopes, and they had discovered large numbers of sources of radio waves in the sky, but only a few of them could be identified with visible objects. It seemed that the radio telescopes and the optical telescopes were looking at different universes. There was a curious lack of connection between the two universes. Amateur astronomers could only look at visible objects. They had not much reason to be interested in these radio sources that nobody understood. Nobody could tell how far away a radio source was if it did not have an optical identification. There was only one fact that suggested that radio sources might be at huge distances. If they were objects randomly distributed in space and time, we ought to have seen more faint sources. There were too few faint sources to be a random distribution. The radio astronomers explained the lack of faint sources by saying that they had hit the edge of the universe. If they were seeing sources all the way out to the edge of the universe, then the lack of faint sources made sense. The optical astronomers mostly did not buy this argument. The optical astronomers thought it was a weak argument for making such a big claim. If the sources were really out near the edge of the universe, they would have to be absurdly powerful.

The first big breakthrough happened in 1963, one years after AAAP began. Maarten Schmidt, an astronomer working at the Palomar observatory, photographed the spectrum of an optical object that coincided with a bright radio source called 3C273, and found absorption lines of hydrogen with a red-shift of 0.18. The optical object is magnitude 13, bright enough to be seen by amateurs with a six-inch telescope, and the red-shift says that it is two billion light-years away. That means that the object really is absurdly powerful. Both in visible light and in radio waves, it is putting out about a hundred times the power of a big galaxy. So the radio astronomers were right. The radio sources are absurdly powerful, and a lot of them are close to the edge of the universe. This discovery had two major consequences. Optical and radio astronomers started to work together, finding optical identifications for radio sources and measuring their red-shifts, which soon confirmed that optical sources can be seen as far away as radio sources. Also, the only plausible theory to explain a super-powerful source was a super-massive black hole at the center of a galaxy, sucking in gas which radiates away prodigious amounts of energy as it falls into the hole. Black holes quickly jumped from being esoteric theoretical toys to being big players in the evolution of the universe.

The second big breakthrough was the discovery of the cosmic microwave background radiation by Arno Penzias and Robert Wilson in 1965. It was another jump outward, with radio astronomers reaching further than optical astronomers could see. The background radiation gives us a picture of the universe about half a million years after the big bang, when the cooling matter first became transparent to its own radiation. We were lucky to have David Wilkinson, one of the world’s leading radio astronomers, here in Princeton. He organized the design and construction of two space missions, COBE, short for Cosmic Background Explorer, and MAP, short for Microwave Anisotropy Probe, to observe the fine details of the microwave radiation. To our great sorrow, David died soon after MAP was launched, and MAP became WMAP, short for Wilkinson Microwave Anisotropy Probe. The local variations in brightness observed by WMAP give us a direct view of the early stages of the evolution of everything in the universe.

The third big breakthrough happened in 1967, when Jocelyn Bell, a graduate student doing radio observations in England, discovered pulsars, the pulsating radio sources which turned out to be rapidly rotating neutron stars. Once more, radio and optical astronomers talking to each other could understand things much better than either could separately. One of the major mysteries in the old days was the Crab Nebula, an object that is probably familiar to most of the members of AAAP. It is number one on Messier’s famous list of fuzzy objects in the sky. The Crab nebula was known to be a supernova remnant, consisting of debris thrown out by a star that exploded in the year 1054. The supernova was seen by astronomers in Korea and China but not in Europe. The mysterious thing about the Crab Nebula was that it was too bright to be a passively expanding gas cloud. It must have an active source of energy causing it to radiate brightly. As soon as pulsars were discovered and identified as spinning neutron stars, it was obvious that the energy source keeping the Crab Nebula bright might be a pulsar. The pulsar would be unusually vigorous since it was only a thousand years old. As soon as people looked for it they found it, both as a radio source and as an optical source, spinning thirty times a second.

I was lucky then to be a friend of David Wilkinson. Just for fun, David invited me to spend a night observing with him at the Princeton campus observatory on Fitzrandolph Road, looking at the Crab pulsar. David made a shutter spinning thirty times a second and put it on the eyepiece of the one-meter telescope. We could see the pulsar appearing and disappearing as he varied the phase of the shutter. The pulses were so strong that we did not need to worry about all the background light from the town and the football stadium. We could make a quite accurate light-curve showing the shape of the double pulse with a period of thirty milliseconds. This spectacular object could have been discovered fifty years earlier if anyone had had the crazy idea of putting a spinning shutter onto a telescope. But until Jocelyn Bell found the pulsars, no astronomer in his right mind could imagine a star spinning thirty times a second.

Besides radio telescopes, we have many other wonderful new tools since AAAP began. One of the most important new tools is the digital camera, which collects light far more efficiently and measures it more accurately than the old-fashioned photographic plates. Megapixel cameras have now become standard equipment for amateurs as well as professionals. Here in Princeton, Jim Gunn designed and built the top-of-the-line digital camera that was used to do the Sloane Digital Sky Survey. The Sloane Survey was a project to photograph the entire Northern hemisphere sky with high resolution in four colors, and put the images into digital memory. It was a combined effort of a number of universities including Princeton. The Sloane Survey output is available to anyone with enough computer bandwidth to use it. If you have access to the output, you can study the sky in daytime or on cloudy nights, without the trouble and expense of traveling to a big telescope. The Sloane Survey is still going on. The original sweep of the northern sky took four years, and after that the camera came back to look more deeply at areas of sky that are particularly interesting for various reasons. The output is a huge gold-mine of astronomical information waiting to be excavated. With accurate four-color measurements of brightness, you can tell whether a point-like object is a nearby star or a distant galaxy, and you can measure its distance. You can do a rapid computer search and pick out large numbers of distant objects close to the edge of the universe. You have an unbiassed view of everything that shines in the sky, from near-earth asteroids to remote clusters of galaxies.

Another set of great new tools are the telescopes in space. The most famous is the Hubble Space Telescope, which is still up there after 22 years, making important discoveries about once a week. Two Princeton astronomers, Lyman Spitzer at the University and John Bahcall at the Institute for Advanced Study, were the most effective promoters of the Hubble telescope and persuaded the politicians in Washington to pay for it. As soon as it was launched and operating, John Bahcall used it to observe the bright objects that were believed to be super-massive black holes and to prove that they are really at the center of galaxies. The central objects are much brighter than the galaxies, so he needed the superior resolution of Hubble to see the galaxies. Hubble has ten times better resolution than any telescope on the ground, and as a result it can see objects that are about a hundred times fainter. Because Hubble can see ultra-faint objects, it was given the job of taking long exposure pictures of a small patch of sky known as the Hubble Deep Field. The Deep Field pictures give us our clearest view of the universe as it was in the remote past near to the beginning.

Besides Hubble, there are many other telescopes in space that are not so famous or so expensive but equally successful. The most recent is Kepler, which went up three years ago and discovered huge numbers of planets orbiting around other stars. This small telescope totally transformed our view of extra-solar planets. We had imagined that extra-solar planetary systems would be like our own Solar System, but they turn out to be quite different. Other space telescopes have looked at the sky in wave-lengths invisible from the ground, Chandra looking at X-rays, Spitzer looking at infra-red radiation, IUE, short for International Ultraviolet Explorer, looking at ultra-violet. Each of them found new kinds of objects and unexpected behavior of old objects. Together, they gave us a far more complete picture of the complicated ways in which the universe evolves.

One of my most vivid memories is a visit to the Goddard Space Flight Center in Maryland, the day after Hubble was launched. There were two buildings side by side, one containing the command center for Hubble, the other containing the command center for IUE. In the command center for IUE, there were only two people, both of them graduate students, calmly controlling the telescope and observing one ultra-violet object after another without any waste of time. The telescope was in geosynchronous orbit over the Atlantic, so it was working twenty-four hours a day. Observers at Goddard were taking turns with observers in Europe to use it. It was easy to use and produced a steady output of good science. In the command center for Hubble there were three hundred people in a state of total confusion. Nobody knew what had happened to the telescope. It was in a low orbit, spending only a few minutes within range of Goddard each time it passed by. After communications had broken down, it was difficult to regain contact. Three hundred people were all talking at once and nobody seemed to be in charge. After I left, the muddle was gradually sorted out and Hubble started to produce good science too. But the low orbit is still a big problem for anyone using Hubble. The earth is constantly interrupting the observations, and the telescope is actually observing less than a third of the time. A big bureaucratic organization is needed to schedule the operations. In the end, both Hubble and IUE did marvelously well. The low orbit of Hubble made it possible for astronauts in the Shuttle to go up to repair and replace instruments. Pictures from Hubble gave the public spectacular views of the universe. But if you measure cost-effectiveness by the output of scientific papers per dollar of input, then IUE comes out far ahead.

The most recent revolution in astronomy is the discovery that only three percent of all the mass in the universe is visible. All the stuff that we see, stars and planets and gas-clouds and dust-clouds, is only three percent. The remaining 97 percent is invisible. It consists of two separate components, dark matter which is about 27 percent and dark energy which is about 70 percent of the mass. Dark matter was first discovered by Fritz Zwicky in the 1930s, when he did the first sky survey with his little 18-inch telescope on Palomar Mountain. In those days the astronomers did not take Zwicky seriously because he was a physicist and not a member of their club. Now we take dark matter seriously because we can measure its gravitational effects accurately, and we find it to be distributed through the universe in roughly the same way as the visible galaxies. The dark energy was discovered more recently by measuring accurately the rate of expansion of the universe at various times in its history. Quite unexpectedly, the rate of expansion was found to be accelerating with time. The measurement is done by observing large numbers of supernovas exploding at various times, going back billions of years into the past. Zwicky was also the first person to observe supernovas systematically, but in the 1930s he could not observe enough of them to see the acceleration. Our knowledge of the invisible universe comes from our modern tools, big sky surveys and big computers. These tools collect vast amounts of accurate information and process it rapidly, picking out the evidence for gravitational effects of invisible mass from the behavior of the stuff that we can see.

Fifty years after AAAP began, our new tools have given us a new view of the universe. The new universe is full of violent events such as gamma-ray bursts and supernova explosions. It is full of invisible stuff which we do not understand. It is as full of mysteries as it was fifty years ago. But the new mysteries are not the same as the old mysteries. The old mysteries were mostly solved, and the new mysteries mostly discovered, as a result of new ways of observing. In the future, we can be confident that new tools will continue to solve old mysteries and discover new ones. Luckily for AAAP, the new tools are narrowing the gap between amateur and professional astronomers. Amateurs will play an even more important role in the future than they have in the past. Serious amateurs now have wide-field electronic cameras and computers that can produce data of professional quality. They also have one resource that the professionals lack, plenty of observing time.

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