It is a fair claim that one of the greatest scientific discoveries of the twentieth century was made by telephone. In fact, not by one telephone call but by two.
In April 1965, Arno Penzias phoned Bernie Burke, a prominent American radio astronomer at the Carnegie Institution’s Department of Terrestrial Magnetism in Washington DC. Penzias’s call was prompted not by the problem with the 20-foot antenna but by another matter altogether, and he would never have even mentioned the irritating static had Burke not asked him in passing how the experiment on Crawford Hill was going. Immediately, Penzias launched into a long complaint about the irritating signal that would not go away and about how frustrating it was trying to track down its source.
Burke sat up. One of his colleagues, Ken Turner, had told him about a search which was under way for just such a signal at Princeton. Could that be what Penzias and Wilson had picked up?
He tried to recall what Turner had told him. Turner had been to a talk the previous month given by Jim Peebles, a friend from his days as a graduate student at Princeton (Turner’s supervisor had been none other than Bob Dicke). The talk Peebles had given was at a meeting of the American Physical Society held at Columbia University in New York. As far as Burke could remember from what Turner had told him, it was about fireball radiation being an unavoidable consequence of a hot Big Bang. Peebles had argued that if the Universe’s helium was indeed produced in the Big Bang, then today’s Universe should be filled with microwaves with a temperature of less than ten degrees above absolute zero. This tepid afterglow of creation was detectable with current technology. In fact, Dicke’s group at Princeton had already embarked on a search for it.
Burke immediately alerted Penzias to the possibility that the anomalous signal might be the leftover glimmer of the Big Bang. It was music to Penzias’s ears. By now he was desperate to find an explanation – any explanation – for the 3.5-degree excess temperature. He got on the phone to Dicke immediately.
When the phone rang in Dicke’s office at Princeton, Dicke had company. Seated in a circle round his desk, sipping cups of coffee and eating sandwiches, were his three disciples – Wilkinson, Roll and Peebles. ‘Every week we used to have these brown-bag lunches to chat about how our experiment was going and talk about what we ought to be doing next,’ says Wilkinson. ‘Arno’s call came during one of those gatherings.’
Dicke’s telephone conversation was rather one-sided. He mostly listened, now and then nodding and repeating phrases which were familiar to the others in the office. Wilkinson’s ears pricked up the moment he heard Dicke mutter the words ‘horn antenna’.
Peebles remembers the conversation vividly. ‘I seem to recall it involved such mysterious things as pigeon droppings,’ he says.
Nobody in the Princeton group knew Arno Penzias or Robert Wilson, but the team was well aware of the 20-foot antenna Bell Labs had built out at Holmdel for the Echo project. Roll and Wilkinson had learnt about it while scouring the microwave journals before starting on their experiment. ‘It was abundantly clear to us that Bell Labs had the best antenna around,’ says Wilkinson.
Roll and Wilkinson had come across Ed Ohm’s papers in The Bell System Technical Journal and had read them carefully. They had concluded that there were clear signs the 20-foot antenna was picking up something unusual from all over the sky. But Ohm did not have a cold load. Without that, there was no way to tell whether he was really seeing the cosmic background radiation or simply a spurious radio signal from a more mundane source.
On the telephone, Dicke continued to repeat familiar microwave phrases. Then suddenly he said ‘cold load’.
‘As soon as we heard those words, we knew the game was up,’ says Wilkinson.
Moments later Dicke hung up the phone. He turned to Peebles, Roll and Wilkinson. ‘Well, boys,’ he said, ‘we’ve been scooped!’
The next day, Dicke, Roll and Wilkinson drove the 30 miles over to Holmdel to take a look at the Bell Labs apparatus. They were met by Penzias and Wilson on Crawford Hill.
Although the two groups of astronomers had never met, Penzias and Wilson knew the name of Bob Dicke. ‘I was considerably in awe of him,’ says Wilson. ‘He was the grand old man of microwaves.’
Once the introductions were over, Penzias and Wilson led their visitors over to the 20-foot antenna and began to show them the equipment. ‘I don’t remember them being unusually inquisitive,’ says Wilson.
If they were not very inquisitive, it was because Dicke had already asked most of the pertinent questions on the telephone the day before. ‘Before we went over to Bell Labs, we were pretty convinced that they’d found the Big Bang radiation,’ says Wilkinson. ‘You see, the experiment to look for it is a rather simple one if you have the right apparatus. There are half a dozen things that you have to do right, then the temperature of the background just pops out.’
Another reason that Dicke’s group asked so few questions was that they already knew most of the answers. The Bell Labs apparatus turned out to be remarkably similar to the experiment Roll and Wilkinson were building back at Princeton. In particular, Penzias’s helium cold load was almost identical to the one Peter Roll had designed. ‘The similarities meant we caught on very quickly,’ says Wilkinson.
Dicke’s group was rapidly convinced that Penzias and Wilson were first-rate radio astronomers. ‘I was really impressed that they had hung in there on a problem that wasn’t central to what they were doing,’ says Wilkinson. ‘Here was this thing that they really wanted to understand. And they’d been working on it for a year, worrying at it and never letting go. They had taken enormous care to rule out the more obvious explanations for their puzzling signal.’
What worried the Princeton team most was that unwanted radio waves from the ground might be somehow finding their way into the 20-foot Holmdel antenna. ‘It was impossible to shield the horn from the ground because it was such a big thing,’ says Wilkinson. But Penzias and Wilson were able to convince their visitors that when the Holmdel antenna was pointed at the sky, very little ground radiation could bend its way into the 20-foot opening of the icecream-cone antenna.
Wilkinson and the others pored over Penzias and Wilson’s data – wiggly red lines on chart recorders. By now, they were satisfied with what they had seen. ‘Penzias and Wilson were looking at a wavelength where there shouldn’t have been any signal at all, so we were convinced they must be seeing the cosmic background,’ says Wilkinson.
The effect they had measured was small – no more than a few degrees. Any other instrument in the world would have missed it, but the Holmdel antenna was uniquely suited for distinguishing a weak background signal from other, much stronger sources. There on the chart recorder was a cryptic message from the very beginning of time.
If they were right, it was the most important discovery in cosmology since 1929, when Edwin Hubble had found that the Universe was expanding. Permeating every pore of the Universe was a tepid radiation, the ‘afterglow’ of the titanic fireball in which the Universe was born. Before the Holmdel antenna had intercepted it, the radiation had been streaming across empty space for an incredible 13.7 billion years. Penzias and Wilson had stumbled on the oldest ‘fossil’ in creation, carrying with it an imprint of the Universe as it was soon after the creation event itself.
The temperature of the background radiation was the temperature the Universe had had long ago, greatly reduced by the enormous expansion the Universe had undergone since. When the radiation broke free of matter, the Universe was at a temperature of about 3,000 degrees.1 But while it had been flying to us across space, the Universe had expanded about a thousand times in size, diluting the temperature of the radiation by exactly the same amount, so that today it appeared to be only about three degrees above absolute zero.
The temperature of about three degrees above absolute zero is the temperature of the Universe. Although the stars are very hot and very numerous, when their temperatures are averaged over all of space their contribution to the temperature of the Universe is completely negligible compared with the fireball radiation.
The cosmic background radiation came from the time when it became cool enough for atoms to form for the first time. At this instant, about 380,000 years after the Big Bang, the rapidly cooling fireball suddenly became transparent to light. Photons which had bounced from particle to particle in the fog of the fireball were suddenly able to move freely. And they have been doing so ever since, gradually losing energy as the Universe has grown in size.
It may seem peculiar that the cosmic background radiation is arriving at the Earth only today, 13.7 billion years after the Big Bang. After all, in a sense we were in the Big Bang (or at least the particles of matter that would one day condense to form the Earth were) and the fireball radiation was all around us. Surely it should have already passed us by now?
Well, radiation which in the Big Bang was emitted by matter in our immediate neighbourhood has already passed us. Forgetting for a moment that the Universe has expanded a lot since the Big Bang, it is true to say that radiation emitted 13.7 billion light years from us is just arriving at the Earth today.2 On the other hand, radiation that was emitted about 9 billion light years away would have arrived 9 billion years after the Big Bang – or just as the Sun and the Earth were forming 4.6 billion years ago.
The expansion of the Universe complicates matters a little because when those photons of the Big Bang radiation arriving at the Earth today broke free of matter, the Universe was only about a thousandth of its present size. The photons have therefore taken 13.7 billion years to cross a gap that was originally only 13.7 million light years wide. It is as though you were trying to sprint in a 100-metre race on a running track that has grown a thousand times longer while you are running.
The detection of the cosmic background radiation by Penzias and Wilson meant that the Big Bang was triumphant. If Martin Ryle’s work at Cambridge on radio galaxies had sent the steady-state theory reeling, the discovery of the afterglow of creation dealt it a knockout blow.
For the second time in its history scientists at Bell Labs in Holmdel had made a great scientific discovery serendipitously. Back in 1931, a 26-year-old Bell Labs physicist named Carl Jansky, who had been investigating possible sources of radio interference, detected a weak static that seemed to be coming from the Milky Way, and thus invented the science of radio astronomy.
By rights Dicke’s group should have been sick that they had been scooped. But if they were, it was not the impression Wilson got. ‘I don’t remember them appearing deflated,’ he says. ‘That didn’t come across strongly at all.’
‘At the time, it didn’t bother me that we had been scooped,’ says Wilkinson. ‘Peter and I were too busy getting our experiment going to worry. Also, I was young. I thought this was just one of a series of wonderful things that was going to happen to me in my career. But, of course, discoveries like this come along only every decade or so.’
Ironically, it had never occurred to Wilkinson and Roll to ask Bell Labs if they could use the 20-foot antenna, despite the fact they recognised it as the only instrument in the world that could detect the fireball radiation. ‘If they had come and asked, I’m sure Bell Labs would have given them permission,’ says Wilson. ‘Arno and I would have been left standing watching on the sidelines.’
Peebles remembers Dicke and the others coming back from Bell Labs and pronouncing themselves impressed by what they had seen. ‘I don’t remember feeling particularly excited by the discovery nor deeply disappointed that it had not been a Princeton discovery,’ he says. ‘You see, it was by no means obvious that this was radiation from the Big Bang. It could still have turned out to be something quite mundane.’
Penzias and Wilson were both slow to accept the cosmological origin of their mysterious signal. ‘They’d spent so long focusing on all the mundane explanations, like pigeon droppings,’ says Peebles, ‘that I think it took them a while to realise just how great a discovery they had really made.’
In fact, it was at least a year before the two astronomers would accept that their anomalous signal came from the Big Bang. ‘We had made a measurement which we thought would hold up,’ says Wilson. ‘But we weren’t so sure that the cosmology would.’
Wilson had another reason for dragging his feet. ‘I’d rather liked the steady-state theory,’ he says. Inadvertently, he had helped to destroy it.
But though Penzias and Wilson were a bit dubious about the Big Bang idea, both astronomers were very pleased indeed finally to have an explanation for the problem that had been troubling them for so long. ‘When we came along, they were at a complete loss for any other explanations,’ says Peebles. ‘They were feeling driven against a wall.’
‘They desperately wanted to use the antenna to do some radio astronomy,’ says Wilkinson.
This is certainly illustrated by Penzias’s immediate reaction to the Princeton explanation. According to Peebles, in one of their early telephone conversations Penzias said: ‘Well, that’s a big relief. We understand this thing at last. Now we can forget it and go and do some real science!’ But rarely had there been a scientific result that was less likely to be forgotten.
The parallels with the twentieth century’s other great cosmological discovery were striking. Both the expansion of the Universe and the fireball radiation had been found by scientists who were completely unaware that predictions of the phenomena had been made many years before in the scientific literature. It made you wonder whether scientists ever read the scientific literature at all.
The Princeton and Bell Labs groups decided to announce the discovery in two scientific papers published side by side in Astrophysical Journal Letters. Two weeks before the papers were due to appear in print, Wilson finally began to realise how important a discovery he and Penzias had made. The phone rang out at Crawford Hill, and on the other end was Walter Sullivan, the science reporter of The New York Times.
Sullivan had been on the trail of another story entirely when he had happened to call the offices of the Astrophysical Journal. ‘For some unknown reason they leaked our paper to him,’ says Wilson. Sullivan grilled Penzias about the work with the 20-foot antenna.
At the time of the phone call, Wilson’s father was visiting him from Texas. A habitual early riser, the next day he got up well before his son to walk down to the local drugstore. When he came back, he had a copy of The New York Times. He thrust it in the face of his bleary-eyed son. There on the front page was a picture of the 20-foot horn with a description of the Astrophysical Journal Letters paper. ‘For the first time, I really got the impression the world was taking this thing seriously,’ says Wilson.
George Gamow, by now retired, read the story in The New York Times. To his dismay, he saw no mention of his name, nor those of Ralph Alpher or Robert Herman. It is fair to say that he awaited the publication of the scientific papers with intense interest.
The papers duly came out. The title of Penzias and Wilson’s gave nothing away: ‘A Measurement of Excess Antenna Temperature at 4080 Megacycles per Second’. Rarely can such an important scientific discovery have been disguised so well.
In the paper, the two Bell Labs astronomers wrote: ‘Measurements of the effective zenith noise temperature of the 20-foot horn-reflector antenna at the Crawford Hill Laboratory, Holmdel, New Jersey, at 4080 megacycles per second have yielded a value of about 3.5 degrees higher than expected.’
And that was basically all Penzias and Wilson said. Nowhere in their brief paper did they mention that the radiation they had picked up might have come straight from a hot Big Bang. They merely noted: ‘A possible explanation for the observed excess noise temperature is the one by Dicke, Peebles, Roll and Wilkinson in the companion letter in this issue.’
‘I think they were rather over-cautious,’ says Wilkinson.
‘Their paper was written in such a way that it could have been almost anything they’d found,’ says Dicke.
‘In contrast, our group really went out on a limb,’ says Wilkinson. ‘In our paper, we were interpreting a single microwave measurement as proof of the existence of the Big Bang radiation.’
‘In fact, Penzias and Wilson weren’t even going to write a paper at all until we told them we were writing one,’ says Dicke.
Wilson says the reason he and Penzias did not write about the Big Bang theory of the origin of the background radiation was because they were not involved in that work. ‘We also thought that our measurement was independent of the theory and might outlive it,’ he says.
‘We were pleased that the mysterious noise appearing in our antenna had an explanation of any kind, especially one with such cosmological implications. Our mood, however, remained one of cautious optimism for some time.’
The moment the two scientific papers were published, Gamow had made a beeline for his library. He had raced through the two papers, becoming increasingly angry. Nowhere was there a mention of his groundbreaking work in the 1940s. Gamow, Alpher and Herman had not only published the results of their hot Big Bang calculations in a series of technical articles in the Physical Review, but they had written numerous popular accounts of their work as well. For instance, in 1952 Gamow published a book for lay readers called The Creation of the Universe in which he talked about the cooking of helium in a hot Big Bang and how this was connected to the temperature of the Universe. Four years later, Gamow aired his ideas in an article in the popular magazine Scientific American.
But all these accounts were missed entirely by Dicke’s team at Princeton. ‘We absolutely didn’t know about Gamow’s work,’ says Wilkinson. ‘When Jim Peebles and I were searching through the scientific literature to see what had already been done, we read only the microwave journals, so we never saw any of Gamow’s stuff.’
One of the problems was that before Penzias and Wilson’s discovery of the cosmic background radiation, cosmology was not really a distinct field. ‘There was no cosmology literature,’ says Wilkinson. ‘The scientific papers that were published – and there were not many – were published all over the place. I’m still finding papers on the cosmic background radiation that I never knew existed.’
But though it is easy to understand how Wilkinson and Peebles missed Gamow’s work, it is harder to explain how Dicke could have missed it. Several years earlier he had actually attended a talk Gamow had given at Princeton about making elements in a hot Big Bang. ‘Gamow spoke about a Universe in which you start with a mass of cold neutrons which suddenly explode in a Big Bang,’ he says. ‘But that’s all I can recall about what he said.’
And the connection between Dicke and Gamow does not end there. It turns out that the very same issue of the Physical Review that contained George Gamow’s first 1940s paper on the hot Big Bang also contained a paper by Dicke. That might not seem too much of a coincidence but, in a throwaway remark in his paper, Dicke actually made a comment about the possibility of a microwave background in the Universe.
As part of his wartime radar work, Dicke and his colleagues had gone to Florida to measure the radio waves coming from water vapour in the moist atmosphere. As an aside, he had wondered whether the sky might be glowing uniformly with microwaves. If such a uniform glow existed, it would have to be coming from the Universe as a whole, since nearby sources, such as a planet or the Milky Way, would fill only a small part of the sky.
Dicke concluded that there was no such sky-glow that he could measure. In fact, he put it more precisely in his paper in the Physical Review, stating that the temperature of any ‘radiation coming from cosmic matter’ had to be less than 20 degrees above absolute zero.3
Dicke had attempted the first ever measurement of the Universe’s radiation background. But, ironically, he had forgotten all about it, and so, too, had everyone else. ‘Jim stumbled on it only when we were reading through the microwave literature,’ says Wilkinson. In the cosmic background field not only did people often overlook each other’s work, they sometimes even overlooked their own.
But such forgetfulness was hardly likely to console Gamow, Alpher and Herman. The irony was that the last thing anyone wanted to do was to upset Gamow. He was something of an idol to the young radio astronomers at Princeton and Bell Labs.
‘Gamow was one of my heroes,’ says Wilkinson. ‘I read all of his popular books in high school. He was probably the reason I got into science in the first place.’ Wilkinson was not alone. Robert Wilson had also been turned onto science by reading Gamow’s popular books.
All of them realised that Gamow was one of the most intuitive and inventive physicists of the twentieth century. ‘He had the ability to ferret out the essential elements of the most complicated physics,’ says Peebles. ‘It was that ability he used to effect when tackling the problem of the Big Bang and the fireball radiation.’
Peebles and the rest felt guilty they had not given due credit to Gamow’s group. ‘We simply did not do our homework,’ he says. ‘We should have gone through the literature and got every possible reference to this thing. In fact, it was a couple of years before we did that.’ This failure to right the wrong immediately ensured that Gamow, Alpher and Herman would remain bitter about the way they had been treated.
‘I tried to do all I could to bring Gamow into the whole story as much as possible,’ says Wilkinson. Soon after the momentous events of the spring of 1965, he and Peebles decided to write an article about the discovery for the magazine Physics Today. Before putting pen to paper, they went back and read the papers of Gamow, Alpher and Herman. But the article never got past the rough draft stage. ‘Alpher and Herman took issue with our version,’ says Wilkinson. ‘They wrote us a rather strong letter. So in the end we withdrew the article and never published it.’
Perhaps if someone in the Princeton team had actually telephoned Gamow at the outset and asked him just what his group had done and when, then all the misunderstandings would have been avoided.
Arno Penzias tried his best to smooth things over with Gamow, but feelings were simply running too high. ‘I don’t think Gamow ever really forgave Dicke and his group,’ says Wilson. ‘As for us, I don’t know exactly how he felt.’
Gamow remained bitter until his death in 1968, just three years after the definitive proof of the hot Big Bang he had championed. ‘Alpher and Herman never got over it completely either,’ says Wilson.
Alpher and Herman perhaps had reason, for they suffered twin injustices. In the beginning, neither they nor Gamow were credited for their work on the hot Big Bang. But later, when people did give credit, they often cited Gamow alone for predicting the cosmic background radiation. This was particularly galling since this was one consequence of the primordial fireball which he had overlooked and which Alpher and Herman had presented on their own in Nature in 1948.
The controversy was not helped by Gamow himself, who could be rather cavalier himself in giving due credit and who failed in several of his later scientific papers to mention Alpher and Herman when he discussed the fireball radiation. So, when Alpher and Herman got upset at someone for wrongly crediting Gamow with their work, Gamow was often the guilty party for sowing the seeds of confusion in the scientific literature in the first place.
Alpher and Herman speculated a lot about why they were overlooked by the astrophysicists. They thought that the fact that they were outsiders may have had something to do with it. Both spent a considerable part of their scientific careers in industry. Alpher worked at General Electric between 1955 and 1986, and Herman at General Motors from 1956 to 1979. These were precisely the years when cosmology came of age as a science and first caught the attention of the public at large.
Whatever the reasons for being overlooked, nowadays the history books give Alpher and Herman their rightful place. The wounds seem to be healing at long last. ‘These days they will even come to cosmology meetings and talk about it,’ says Wilson.
1. The temperature dropping to about 3,000 degrees also signalled another significant event: the point at which the energy density of radiation, or photons, in the Universe fell below that of matter. From then on, the Universe was dominated by matter and by the force of gravity acting on that matter.
2. A light year is the distance light travels in a year.
3. The technique Dicke used and the receivers available in the 1940s were not capable of detecting a uniform background as cold as three degrees above absolute zero.