Published 25 June 2010 by Ashutosh Jogalekar
Pigeon waste, cosmic melodies and noise in scientific communication
There it was, that darned noise again.
Nobody could possibly be happy cleaning pigeon droppings. Yet Arno Penzias and Robert Wilson were being forced to do it. As good scientists they simply could not avoid it, since they had to discount the role of this "white dielectric substance" in the noise that was plaguing their equipment. When they finished with the cleaning and dispatched the pigeons by mail to a faraway place, the noise still did not disappear. And it seemed to come from all directions. The implications of this annoying constant background hum, corresponding to a temperature of only 3 degrees above absolute zero, signified one of the most momentous discoveries in twentieth-century physics, notable even among Nobel Prize-winning discoveries.
Arno Penzias, the German-born physicist who fled from the Nazis with his family in 1939, is at Lindau this year along with his co-recipient Robert Wilson. There are two other physics Nobel laureates this year at Lindau- George Smoot and John Mather. The four men whose work is separated by forty years share a very close connection. Smoot and Mather’s work put the capstone on that of Penzias and Wilson’s. In a way the work of these gentlemen eclipses the work of all Nobel laureates since it deals with something that happened long before any of the others made their discoveries. In fact, it was so long before that it dealt with the beginning of time itself…or so we think. Penzias and Wilson discovered what is called the"cosmic microwave background radiation", which is nothing less than the notes from the harp that the universe played when it fired up. Just like you can guess the identity of the great conductor Herbert von Karajan from listening to the thundering notes of his performance of Beethoven’s 9th symphony, so can you discern the violent birth throes of the universe from the noise this cosmic cataclysm left behind.
It was noise, quite literally. And it was bothering Penzias and Wilson when they were working on a radio antenna inthe fall of 1964 in a small town called Holmdel in New Jersey. There, Bell Laboratories which owned the instrument had turned it over to Penzias and Wilson for pure science. That was a different age, when companies like BellLabs gave free rein to researchers to pursue their own ideas; in the ensuing decades the institution would produce a steady stream of Nobel laureates,an astonishing feat for an industrial laboratory. But at that point the lab was just getting warmed up.
The antenna was about twenty feet wide and shaped like a horn to minimize reception from the sides. It was originally designed to detect transmitted signals from Echo, the first communications satellite. But now Penzias and Wilson were using it to detect radio waves from celestial bodies. The receiver was tuned to detect only frequencies in the microwave region of the spectrum.
But there it was, that darned noise again. No matter what they did to the antenna, it was steadily humming in the background, like one of those overenthusiastic teenagers deliberately trying to annoy you. Like good scientists, Penzias and Wilson started to eliminate all possible sources of noise they could find. Radio noise could arise from any number of sources. It could come from the ground which was warmed up by the sun. It could arise from the earth’s atmosphere, or from the gas between the stars, or from the emission of molecules in outer space. Noise could be generated by waves bouncing off some of the sharp edges in their antenna. Oneby one the scientists investigated and discounted all these sources. They covered up the sharp edges with aluminum tape. They turned the antenna towards and away from major cities like New York. And of course, finally in desperation, they cleaned up waste from a family of pigeons that was roosting in a small part of the antenna horn. They dispatched these pigeons to the main offices of Bell Labs (like good homing pigeons the birds promptly showed up two days later). The noise still would not disappear. Like constant mumbling from disgruntled voters at a campaign speech, it simply would not die.
The two men were accomplished scientists and they did not doubt their own skills. Penzias had worked with Isidor Rabi (whom we met earlier) and Charles Townes, the inventor of the maser, at Columbia University. Wilson had gotten his PhD. at Caltech a few years ago. Their temperaments also complemented each other. Penzias was a big picture man, trying to tie together facts in an overarching thread. Wilson was much more detail-oriented and could patiently tinker with equipment until he got it right. Together the scientists were confident in their analysis and believed they had discarded all sources of noise. Yet it remained. For the moment they decided to take a break.
It was December of that year, and Penzias was coming back on an airplane from a conference. The "noise problem" was on his mind and he mentioned it to a fellow astronomer named Bernie Burke. A short time later, Burke called Penzias back with some interesting news. He had just seen a manuscript written by a physicist named Jim Peebles at Princeton University along with his supervisor Robert Dicke. The manuscript predicted that because of the intense heat during the birth of the universe, there should exist today a remnant of the Big Bang, which would manifest itself uniformly as steady background radiation. Because of the time elapsed since the Big Bang, the temperature of this radiation should be no more than afew degrees kelvin above absolute zero.
Oh yes, and it should be detectable as constant hiss in a good radio telescope.
I leave it to the students at this year’s Lindau meeting to personally ask Penzias what he must have felt when he heard about the Peebles-Dicke paper from Burke. But it surely must have sent his heart racing. Intrigued, Penzias called Dicke. Dicke’s heart must have raced even more because just then he had been trying to construct his own antenna to detect the cosmic noise that his paper predicted. Dicke promptly got into his car and drove the thirty miles or so from Princeton to check out Penzias and Wilson’s data. When he confirmed its authenticity, he must have felt an intense mix of excitement, amazement and chagrin. He had narrowly missed making the discovery that won Penzias and Wilson the Nobel Prize for physics in 1978.
Penzias and Wilson in front of the Holmdel radio antenna
Sounds from a faraway time
The evolution of the idea of the cosmic microwave background is almost a textbook study of ignorance and triumph, and how experimentalists and theoreticians can sometimes unintentionally delay progress by failing to communicate with one another. The Big Bang theory was proposed by Georges Lemaître, a Belgian Roman Catholic priest, although interestingly the term ‘Big Bang’ was pejoratively coined by a detractor of the theory, Fred Hoyle. Later the theory was developed by the brilliant and colorful Russian physicist, scotch lover and popular science writer George Gamow and his colleagues, Ralph Alpher and Robert Herman. After World War 2, Gamow’s colleagues Alpher and Herman predicted the cosmic microwave background. This was based on their ideas about nucleosynthesis- the formation of elements in the universe.
Robert Herman (left) and Ralph Alpher. In the middle is George Gamow, materializing out of a bottle of ‘Ylem’, the primordial soup from which our universe originated.
Big Bang Theory cosmology is based on Einstein’s theory of gravitation. In 1929 Edwin Hubble had already made the groundbreaking discovery that the universe seemed to be expanding. This meant that at some point back in time it must have been infinitely compressed. Compression means high temperature, in this case, a very high temperature of 10 billion degrees when the universe was only one second old. At this temperature matter as we know it ceases to exist, and the universe consists of a thick boiling broth of subatomic particles and electromagnetic radiation in the form of very high frequency gamma rays. Electromagnetic radiation is emitted and absorbed by all electrically charged particles. When there are a sufficient number of particles, this radiation manifests itself as black-body radiation. Black-body radiation can be defined by a certain value of energy at a given frequency range and thus can be completely fixed by the temperature. At ten billion degrees, there would be massive creation of electrons and positrons from the energy of other moving particles. But after one second, Big Bang theory physics predicts that if the number of photons is larger than the number of matter particles, then the initial black-body radiation will continue to be black-body radiation. The only change in this radiation is that it cools with the expansion of the universe. About 400,000 years after the beginning, an epochal event happened. Stable atoms of hydrogen began to form, and the radiation was ‘decoupled’ from the matter and was left free to wander on its own. Thus, even today, almost fifteen billion years later, space should be filled with black-body radiation left over from the Big Bang. Its temperature should be roughly 3 K.
This at least was what Alpher and Gamow conjectured. But they were not the first ones to think about it. In fact the microwave background had been indirectly observed in the late 1930s and early 40s by American astronomers. These astronomers saw faint signs of light absorption by certain molecules in outer space and concluded that the molecules were somehow being ‘pumped up’ with energy that came from a temperature of 2.3 K. Unfortunately, there was yet no theoretical framework in which this observation could fit, and it became part of the countless number of other details in astronomy. Now, when Gamow, Alpher and Herman formed their ideas, they did not know about this experimental work. Most incredibly, a couple of years later, two Russian physicists published a paper saying that not only should this background exist, but that the best detecting instrument for it would be the radio antenna in Holmdel, New Jersey! As if to cap this circus of ignorance and lack of communication, Penzias, Wilson and Dicke had all not heard of these earlier predictions; somehow unlike the cosmic background, the signal had been lost in the noise. One can only wonder how things would be if this drama were to be enacted today, in the era of the Internet and Email.
Dicke in fact thought he was the first one proposing the cosmic microwave background. His reasoning was fascinating and harkened back to analogies in Hindu and Buddhist philosophies about the creation of the universe. Dicke was a proponent of the ‘oscillating universe’ which posits that the universe has no end or beginning but cycles through time. According to his logic, if the heavy elements had been created through nucleosynthesis, they could not keep on increasing in proportion indefinitely; after all, the observed universe is largely contained of hydrogen and helium and not heavy elements. Thus, at some point the heavy elements would have to be annihilated to make room for the next batch of matter. This annihilation would produce radiation, and it’s this radiation that would manifest itself as the cosmic background. Dicke’s oscillating universe tome is a perfect example of a scientific ‘model’, a conceptual construction that retains some simple aspects of the universe but which can neatly explain a key observation. As with models, Dicke’s prediction of the microwave background does not uniquely depend on the oscillating universe model. In fact it arises perfectly from the modern day theory of the Big Bang.
Dicke was supremely competent for this kind of theorizing. Today he is the underappreciated man in the cosmic microwave background story. Dicke was a genius at electronic devices, working on important radar components at the famous Radiation Laboratory at MIT during the war. In the early 60s, he did a highly accurate measurement proving the equivalence of inertial mass and gravitational mass, a foundational concept of Einstein’s relativity. Dicke’s thirst was not slaked just by doing experiments. He also provided the first quantum theory of the laser. By the mid 60s Dicke was a legend, the Enrico Fermi of his time, highly accomplished in both theory and experiment. As it turned out though, even Dicke could not escape the vagaries of space and time. Robert Henry Dicke died in 1997.
Robert Henry Dicke (1916-1997)
In 1989, a satellite called COBE was launched by NASA that would measure Penzias and Wilson’s microwave background to unprecedented accuracy. Two of the principal scientists in this endeavor were George Smoot and John Mather who are at Lindau this year. The satellite’s measurements of the black body radiation curve were so remarkably accurate that the error bars in the curve which typically extend on top and bottom were almost lost in the signal.
The COBE satellite black-body radiation curve with the data points (red) plotted on the theoretical spectrum (black). The two match up almost perfectly
The COBE satellite followed by the Wilkinson Probe satellite launched in 2001 put the icing on the microwave background cake. The Big Bang theory has attained the status of one of our most highly refined and validated theories, thanks to the work of Smoot, Penzias, Wilson, Mather and many others. It seems only wonderfully fitting that Penzias, Wilson, Smoot and Mather are all attending this year’s meeting, coming full circle and neatly tying together the ends of a story spread over 40 years, which in turn tells the story of moments in time billions of years ago.
George Smoot and John Mather (Physics, 2006)
The story of the microwave background is an exercise in scientific history, demonstrating how ideas can, and sometimes do not, build on top of each other. It would have been interesting to see how the same story would have played out in today’s age, when the barriers to communication have been all but shattered by the availability of hundreds of years of research that can be accessed through the click of a mouse. Scientific ideas are hostage to the same qualities of human fallibility, determination, creativity, and of course, the uncertainties of space and time, as are other aspects of the human experiment. All we can do is persevere.