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... alvarez's case, and see if it was flawed. This job was turning out to be more fun than I had expected. On my second reading of Alvarez's letter, I found it particularly dubious that the Cretaceous and Eocene extinctions should be excluded. How do we know that asteroids do not hit the Earth periodically?
I asked. Maybe our failure to arrive at a theory just meant that we hadn't been clever enough. Not finding something is not the same as proving it is not there. I decided to reserve judgment. A few minutes later Alvarez stopped by to see if I had finished, and I told him that I had found a mistake in his logic.
It had been improper to exclude the Cretaceous and Eocene mass extinctions, I said. I presented my case like a lawyer, interested in proving my client innocent, even though I wasn't totally convinced myself. Alvarez rejoined strongly, like a lawyer himself. "To keep those extinctions in the analysis would be cheating, " he said. His belligerent offense threw me momentarily off balance. "You " re taking a no-think approach, " he continued. "A scientist is not allowed to ignore something he knows to be true, and we know those events were due to asteroid impacts. " I knew Alvarez far too well to acquiesce in his onslaught. My approach was not no-think, I said. It was proper to ignore certain "prior knowledge" in testing a hypothesis.
He had no right to assume that the Cretaceous and Eocene extinctions could not be a part of a larger periodic pattern. Maybe if we were clever enough to find the right explanation, we would see that asteroid impacts can be periodic. Alvarez repeated his previous argument, with a little more emphasis on the phrase "no-think. " His body language seemed to say, "Why doesn't Rich understand me? How can he be so dumb?" I repeated my old arguments. We were talking right past each other. He knew he was right.
I knew I was right. We weren't getting anywhere. This was not a question of politics or religion or opinion. It was a question of data analysis, something all physicists should be able to agree on. Certainly Alvarez and I should be able to agree, after nearly two decades of working together. I tried again. "Suppose someday we found a way to make an asteroid hit the Earth every 26 million years.
Then wouldn't you have to admit that you were wrong, and that all the data should have been used?"What is your model?" he demanded. I thought he was evading my question. "It doesn't matter! It's the possibility of such a model that makes your logic wrong, not the existence of any particular model. " There was a slight quiver in Alvarez's voice. He, too, seemed to be getting angry. "Look, Rich, " he retorted, "I've been in the data-analysis business a long time, and most people consider me quite an expert. You just can't take a no-think approach and ignore something you know. " He was claiming authority! Scientists aren't allowed to do that.
Hold your temper, Rich, I said to myself. Don't show him you " re getting annoyed. "The burden of proof is on you, " I continued, in an artificially calm voice. "I don't have to come up with a model. Unless you can demonstrate that no such models are possible, your logic is wrong. "How could asteroids hit the Earth periodically? What is your model?" he demanded again. My frustration brought me close to the breaking point.
Why couldn't Alvarez understand what I was saying? He was my scientific hero. How could he be so stupid? Damn it!
I thought. If I have to, I'll win this argument on his terms. I'll invent a model. Now my adrenaline was flowing. After another moment's thought, I said: "Suppose there is a companion star that orbits the sun. Every 26 million years it comes close to the Earth and does something, I'm not sure what, but it makes asteroids hit the Earth.
Maybe it brings the asteroids with it. " I was surprised by Alvarez's thoughtful silence. He seemed to be taking the idea seriously and mentally checking to see if there was anything wrong with it. His anger had disappeared. Finally he said, "You surprised me, Rich.
I was sure you would come up with a model that brought in dust or rocks from outside the solar system, and then I was going to hit you with a fact I bet you didn't know, that the iridium layer associated with the disappearance of the dinosaurs came from within our own solar system. The rhenium- 187 /rhenium- 185 ratio in the boundary clay is the same as in the Earth's crust. I figured that you didn't know this. But your companion star was presumably born along with the sun, and so it would have the same isotope ratios as the sun. The argument I was holding in reserve is no good. Nice going. " Alvarez paused.
He had been trying to think a step ahead of me, anticipating my moves, like a chess master. He had guessed what my criticism would be and had his answer ready-but I had made a different move. He seemed pleased that his former student could surprise him. He finally said, "I think that your orbit would be too big. The companion would be pulled away by the gravity of other nearby stars. " I hadn't expected the argument to cool down so suddenly. We were back to discussing physics, not authority or logic.
I hadn't meant my model to be taken that seriously, although I had felt that my point would be made if the model could withstand assault for at least a few minutes. He was right that I was ignorant of the rhenium discovery. Alvarez's son Walter, a geologist, had found a clay layer that had been deposited in the oceans precisely at the time the dinosaurs were destroyed. This clay layer, the elder Alvarez hypothesized, had been created by the impact of an extraterrestrial body (such as a comet or an asteroid) on the Earth. Rhenium comes in several forms-among others, rhenium- 185, which is stable, and rhenium- 187, which is radioactive and disappears with a half-life of 40 billion years. In the 4. 5 billion years since the formation of the solar system, approximately 8 % of the rhenium- 187 should have disintegrated.
And, in fact, roughly that amount had. Unless the rhenium in the clay had been produced at the same time as the rhenium in the Earth (i. e. , at the formation of the solar system), the ratios were very unlikely to be so nearly identical. In other words, the extraterrestrial body would appear to have been born at the same time as the sun. Now I took the initiative. "Let's see if you are right that the star would be pulled away from the sun. We can calculate how big the orbit would be. " I wrote Kepler's laws of gravitational motion on the blackboard.
The major diameter of an elliptical orbit is the period of the orbit, in this case 26 million years, raised to the 2 / 3 power, and multiplied by 2. My Hewlett-Packard 11 C pocket calculator quickly yielded the answer: 176, 000 astronomical units, i. e. , 176, 000 times as far as the distance from the Earth to the sun, about 2. 8 light-years. (A light-year is the distance that light travels in one year. ) That put the companion star close enough to the sun so it would not get pulled away by other stars. Alvarez nodded.
The theory had survived five minutes, so far. "It looks good to me. I won't mail my letter. " Alvarez's turnaround was as abrupt as his argument had been fierce. He had switched sides so quickly that I couldn't tell whether I had won the argument or not. It was my turn to say something nice to him, but he spoke first. "Let's call Rate and Sepkoski and tell them that you found a model that explains their data. " So was born the Nemesis hypothesis, though I had no idea at the time where this would lead me. The issue of the theoretical stability of the Nemesis orbit has been settled, but most astronomers don't know the answer.
Actually, they think they know the answer, but they are incorrect. As the 19 th century humorist Josh Billings said, "The trouble with most folks isn't so much their ignorance. It's know'n so many things that ain't so. " I can guide you to the origin of the confusion. Look at Nature Vol 311, Oct 18, 1984. You will find a host of articles on the stability of the Nemesis orbit.
In addition, you will find an editorial comment by Mark Bailey (on page 602), entitled "Nemesis for Nemesis. " 1. J. G. Hills (page 636) analyzes the stability of the Nemesis orbit. He supports the Nemesis hypothesis and calculates some details.
He speculates that Nemesis may be responsible for the eccentric orbit of Pluto. (Hills was the theorist who originally recognized the possibility of comet showers. ) 2. Piet Hut (page 638) does the most complete and definitive analysis of the Nemesis orbit. He concludes that the results given in the original Nemesis paper are verified: the orbit has a stability time constant of about one billion (10 ^ 9) years. This means that the remaining life of the orbit is a billion years.
When the solar system was created 4. 5 billion years ago, the Nemesis lifetime would have been about 5. 5 billion years, and we have used up 4. 5 of those. The 10 ^ 9 year stability implies that the present orbit is not perfectly periodic, and this is verified by a careful examination of the extinction data. Hut shows that the Nemesis orbit is stable only if it is near the plane of the Milky Way. (Hut is now a fellow at the Institute for Advanced Study at Princeton. ) 3. Torbett and Smoluchowski (page 641) conclude that passing giant molecular clouds would make the Nemesis orbit unstable. However they neglect the fact that these massive clouds are very diffuse; later work (D. Morris and R.
Muller, Icarus v. 65, p. 1 - 12) show that these clouds actually have no effect on the orbit stability. 4. Mark Bailey wrote an editorial review (page 602) entitled "Nemesis for Nemesis, " in which he says, "the Nemesis proposal is extended and shown, in fact, to be quite incapable of producing the strictly periodic sequence for which is was originally designed. " This is a misinterpretation of the original Nemesis paper (Nature vol 308 pp 715 - 717, 1984). We never expected a perfectly periodic signal in an orbit that had only a 10 ^ 9 year lifetime. Bailey goes on to characterize Hut's paper as "a near retraction"! ! ! !
Hut considered his paper to be a vindication of the original Nemesis paper. He contacted Bailey to find out how Bailey could be so wrong in his understanding, and Bailey told Hut that he never wrote those words! "Near retraction" had been inserted by the editor at Nature! Bailey also refers to a paper by Clube and Napier, in which they show that the Nemesis orbit has a stability of 10 ^ 9 years. But Clube and Napier then conclude that this rules out the Nemesis theory, rather than realizing that this stability is exactly what we had said in our original paper.
Apparently they never realized (as did Hut) that the expected lifetime of Nemesis is linear, not exponential, so that that the present stability is not the same as the stability 4. 5 billion years ago. But now for the fascinating sociology of science. I have talked to many astronomers since 1984, and the majority of them believe that the Nemesis theory was ruled out, because the orbit turned out to be unstable. In most of these cases I could track down the origin of their opinion. Frequently the opinion had been obtained from someone else -- often the local planetary scientist. But in every case, the ultimate origin was the altered article by Mark Bailey in Nature.
Why is this? Because Bailey summarized the three articles -- there was no need for a busy scientist to read the actual papers. I never found an expert (i. e. someone that others depended on for their opinion) that had actually read the Hut article. Why bother, when it amounts to a "virtual retraction"?
The trouble with most folks, isn't so much their ignorance... At the time, Piet and I thought we would find Nemesis soon, so he decided not to write a letter to the editor complaining about the error in the Bailey summary. That's half the story of why Nemesis is not believed. The other half is that we predicted we would find it within a few years, and we haven't. So most people think our search found no such star. In fact, the search stalled soon after it started.
There is no reason to believe that Nemesis is not the solution to the mystery of the periodic extinctions, and there is no alternative theory that has survived scrutiny. Bibliography:
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