by Richard Muller
from
Muller'sGroup Website
LUIS ALVAREZ walked into my
office looking like he was ready for a fight.
"Rich, I just got a crazy paper from
Raup and Sepkoski. They say that great catastrophes occur on the
Earth every 26 million years, like clockwork. It's ridiculous."
I recognized the names of the two
respected paleontologists. Their claim did sound absurd. It was
either that, or revolutionary, and one recent revolution had been
enough. Four years earlier, in 1979, Alvarez had discovered what had
killed the dinosaurs. Working with his son Walter, a geologist, and
Frank Asaro and Helen Michel, two nuclear chemists, he had shown
that the extinction had been triggered 65 million years ago by an
asteroid crashing into the Earth. Many paleontologists had initially
paid no attention to this work, and one had publicly dismissed
Alvarez as a "nut," regardless of his Nobel Prize in physics.
Now, it seemed, the nuts were sending
their theories to Alvarez.
"I've written them a letter pointing
out their mistakes," Alvarez continued. "Would you look it over
before I mail it?"
It sounded like a modest request, but I
knew better. Alvarez expected a lot. He wanted me to study the
"crazy paper," understand it in detail, and then do the same with
his letter. He wanted each of his calculations redone from scratch.
It would be a time-consuming and tedious task, but I couldn't turn
him down. He and I depended on each other for this kind of work. We
knew we could trust each other to do a thorough job. Moreover, we
had enough mutual respect so that we didn't mind looking foolish to
each other, although neither of us liked looking foolish to the
outside world. So I reluctantly accepted the task, as I had many
times before.
The Alvarez theory had slowly been gaining acceptance in the
scientific world. The astronomers had been the most receptive,
perhaps because their photographs often showed large asteroids and
comets floating around in space in orbits that crossed the path of
the Earth. They knew that disastrous impacts must have taken place
frequently in the past. Many geologists had likewise been won over.
But a majority of paleontologists still seemed opposed to the
theory, which was disruptive to their standard models of evolution.
Alvarez took pride in the fact that some of the most respected
paleontologists nevertheless liked his theory, including Stephen Jay
Gould, Dale Russell, David Raup, and J.John Sepkoski.
I began my assignment by reading the paper by Raup and Sepkoski.
They had collected a vast amount of data on family extinctions in
the oceans, far more than had previously been assembled. That fact
disturbed me; I hate to dismiss the conclusions of experts,
especially conclusions based on such minute study. Their analysis
showed that there were intense periods of extinctions every 26
million years.
It wasn't surprising that there should
be extinctions this often, but it was surprising that they should be
so regularly spaced. Alvarez's work had shown that at least two of
these extinctions were caused by asteroid impacts, the one that
killed the dinosaurs at the end of the Cretaceous period, 65 million
years ago, and the one that killed many land mammals at the end of
the Eocene, 3539 million years ago. (The age was uncertain because
of the difficulty of dating old rocks.)
Astrophysics was a field I thought I knew; my work in it had earned
me a professorship in physics at Berkeley and three prestigious
national awards. But the paper beggared my understanding. I found it
incredible that an asteroid would hit precisely every 26 million
years. In the vastness of space, even the Earth is a very small
target. An asteroid passing close to the sun has only slightly
better than one chance in a billion of hitting our planet. The
impacts that do occur should be randomly spaced, not evenly strung
out in time. What could make them hit on a regular schedule? Perhaps
some cosmic terrorist was taking aim with an asteroid gun. Ludicrous
results require ludicrous theories.
I hurried to the end of their paper, like a reader cheating on a
mystery novel, to see how Raup and Sepkoski would explain the
periodicity. I was disappointed to find that they had no theory,
only facts. Physicists have a wry saying: "If it happens, then it
must be possible." Many discoveries had been missed because
scientists ignored data that didn't fit into their established mode
of thinking, their paradigm, and I didn't want to fall into that
trap. Maybe it would be best to review their data, I thought, and
try to judge them independently of theory. On a chart, they had
plotted the varying extinction rate for the last 250 million years.
The big peaks in the rate were spaced 26 million years apart.
Next I turned to Alvarez's letter. He thought there were several
mistakes in the way that Raup and Sepkoski had analyzed their data.
Several of the apparent peaks, he argued, should be removed from the
analysis because of their low statistical certainty. Likewise, both
the Cretaceous and Eocene extinctions should not be considered as
part of a periodic pattern, since they were due to asteroid impacts
and therefore must be random in time. This had been as obvious to
Alvarez as to me. With these extinctions removed, the remaining ones
were so widely separated that it looked like all evidence for
periodicity had vanished.
Alvarez's approach was convincing, but was it right? It was my job
to be the devil's advocate, to defend the conclusions of Raup and
Sepkoski. I went back to their paper and looked at the chart again.
I mustn't be too skeptical, I thought. I replotted their data,
substituting the conventions of physicists for those of
paleontologists. I gave each extinction an uncertainty in age as
well as in intensity. The new chart looked more impressive than I
had expected. It was a rough version of the one shown on page 6. I
had placed the arrows at the regular 26-million-year intervals.
Eight of them pointed right at extinction peaks; only two missed.
The peaks certainly seemed to be evenly spaced.
Maybe they were right. I realized I had better reexamine 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 11C 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 Raup 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.
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