SCIENTISTS and science fiction writers have an important attribute in common: they both make up stories about the past.

Science fiction writers are fond of time machines and the perils of messing with them. There are many time travel tales we could cite, but Pat's favorite is Ray Bradbury's short story A Sound of Thunder. A decade before Edward Lorenz coined the term "butterfly effect," Bradbury used the idea in a story about a wealthy adventurer who travels back to the time of the dinosaurs, inadvertently kills a butterfly, and thereby transforms his own time. He altered the future by making a tiny change in the past.

Scientists' stories take a different form—journal articles, theories, discussions at conferences. Scientists piece those stories together using bits of evidence collected by many people, often over many decades. They are based on evidence, but still they are stories.

Mind you, we're not saying that there's anything wrong with making up stories. Quite the contrary. Storytelling in all its forms is a way of making connections and finding patterns, something humans are both good at and slightly obsessed with. (The obsessive aspect of this explains why people see the man in the moon, animals in the clouds, and so many rocks that look like human faces. This tendency to see something meaningful in random images is so prevalent it even has a name: Pareidolia.)

Scientists make up stories to help make sense of the world, to figure out how the world works. And they do a great job of that. Such a good job that many of the stories they make up about the past have taken on the semblance of fact. Pat was reminded of this recently when she was writing a lesson about dinosaurs for third graders.

How exactly do we know what we know about dinosaurs? When you get right down to it, we know about them from the stories that scientists tell. The distant past is an imaginary land, a place that people have constructed by looking at whatever evidence they can find.

In this column, we will consider some of the ways that scientists have learned about this imaginary land of the past. We will consider how footprints in mud changed our fundamental understanding of the dinosaurs, how the rings of trees that live a few hundred years have shown that California's drought is the worst in 1,200 years, and how the quest for a lost isotope led to the discovery of fossilized nuclear reactors.

Our examples are animal, vegetable, and mineral. Given Pat's affection for dinosaurs, we'll start with the animals.

Pat is so old that she remembers when dinosaurs dragged their tails. As a child, she saw the dinosaurs in the Peabody Museum of Natural History at Yale University. Enormous skeletons assembled from fossil bones stood in the museum's Great Hall. Pat remembers the stegosaurus, the triceratops, and the brontosaurus. (And yes, it was a brontosaurus, not apatosaurus, back in those ancient times.) All of the great beasts were posed in lifelike positions—with tails dragging on the ground behind them.

Othniel Charles Marsh had collected those skeletons during what came to be known as the Bone Wars (or the Great Dinosaur Rush). From the 1870s to the turn of the century, two notable paleontologists engaged a great scientific pissing match. O. C. Marsh (of the Peabody Museum) and Edward Drinker Cope (of the Academy of Natural Sciences in Philadelphia) engaged in a bitter competition to discover and describe species of dinosaurs.

This was the era of westward expansion following the Civil War. Marsh and Cope, heading rival teams, dug in the rich bone beds of Colorado, Nebraska, and Wyoming. In their efforts to best one another, they resorted to bribery, theft, destruction of bones, and personal attacks in the scientific literature.

When these eminent paleontologists passed away, they left behind boxes of bones for future researchers, a half-told story about the great prehistoric animals that had been dubbed dinosaurs or "terrible lizards," and a public eager to learn more.

Dinosaurs, as Marsh and Cope told the story, were lumbering lizards that trudged about their steamy world, cold-blooded and slow moving, dragging their tails in the mud. The scientists had assembled the fossil skeletons to match that story. And they convinced everyone (including Pat and Paul) to believe their story. But in constructing this view of the prehistoric beasts, the great paleontologists had overlooked some very important evidence.

Back in 1802, well before Marsh and Cope began their famous feud, a teenager named Pliny Moody was plowing a field in Massachusetts. In that field, Moody found a slab of stone that bore large three-toed footprints. The teenager described them as the footprints of a large bird and thought they were interesting. No one had seen anything like them before. Moody dragged the stone slab home to use as a doorstep and it became a local curiosity.

In 1839 (when O. C. Marsh was just nine years old), that stone slab ended up in the hands of Professor Edward Hitchcock of Amherst College, who had become interested in the strange stone footprints that could be found in Massachusetts and Connecticut. Hitchcock entered into a lifelong study of the tracks—the ones Moody found, and many others from the same area—and amassed an enormous collection of these tracks.

He was certain that they were the tracks of ancient birds. This conviction emerged from a detailed analysis. Hitchcock examined the tracks of living animals and compared them to the fossil tracks. From those fossil tracks, he worked out how many toes and toe bones the track makers had—and the manner of their locomotion. Both the number and position of toe bones and the upright posture that he postulated matched only one class of animals: birds.

Hitchcock passed away in 1864, well before the Bone Wars began.

You might think that the paleontologists who were so interested in collecting dinosaur bones might have also been interested in the tracks Hitchcock had collected. But if you thought that, you would be wrong. Hitchcock was regarded as a bit of a comic character. After all, he thought those dinosaur tracks belonged to birds. How foolish!

So we have a story of lumbering, reptilian, tail-dragging dinosaurs, represented by the mounted skeletons in Peabody Museum and many an illustration. And we have another story of giant birds, told by a guy who spent decades collecting fossil tracks. Most people embraced the first story and ignored the second.

More than a hundred years after Hitchcock's death, that changed. In 1975, Robert Bakker published an article in Scientific American titled "Dinosaur Renaissance." In this article, he began a new draft of the dinosaur story, writing,

       Dinosaurs have a bad public image as symbols of obsolescence and hulking inefficiency; in political cartoons they are know-nothing conservatives that plod through miasmic swamps to inevitable extinction….
       Recent research is rewriting the dinosaur dossier….
       In this article I shall be presenting some of the evidence that has led to reevaluation of the dinosaurs' role in animal evolution. The evidence suggests, in fact, that the dinosaurs never died out completely. One group still lives. We call them birds.

Among other evidence, Bakker's argument used studies of fossil bones—the concentration of blood vessels in dinosaur bones were like that of warm-blooded mammals and birds, rather than that of cold-blooded reptiles. He also cited the speed and stride length of dinosaurs.

And that brings us back to those fossil footprints that Hitchcock favored. Dinosaur bones are a bit like a snapshot; they preserve the dinosaur as it was at the moment it died. But dinosaur tracks are more like a movie; they preserve a record of movement. If you know how to read them, tracks can provide an estimate of how fast the animal is moving.

Think about what happens when you run. You move faster—usually by taking bigger steps. Speed walkers aside, people take bigger steps when they want to move faster. It's not just people. Every animal that's been tested—from elephant to ostrich—takes larger steps when running than when walking.

The guy who tested all those animals was a British zoologist named Robert McNeill Alexander. Back in the 1960s, he studied animal locomotion and footprints. Working at the London and Whipsnade zoos, he examined the footprints and measured the speed of many different animals—including ostriches and elephants and camels and dogs and lizards and people and too many others to list. With all that data, he came up with a mathematical formula that related an animal's speed to the length of its legs and the length of its stride. Given leg length and stride length, he could provide a good estimate of the animal's speed.

Alexander applied his formula to the dinosaurs. That's where Hitchcock's work comes in. Footprints gave Alexander the stride length of different species. Examination of the bones provided the leg length. Put those numbers into his formula and voilà! You have the dinosaur's approximate speed—and a new revision of the dinosaur story.

No longer were they slow-moving tail-draggers. Now we had the speedy carnivores of Jurassic Park. And by the way, the footprints also revealed that dinosaurs rarely dragged their tails. With just a few exceptions, the footprints that Hitchcock collected lacked the marks that would have been left by a dragging tail.

Enough about dinosaurs! Though it's hard to leave the story of such charismatic megafauna, we have other tales to tell.

For the past three years, California (where Pat and Paul live) has been experiencing a severe drought. People talk about the "drought of the century" as if that's a big deal. But one hundred years is small potatoes when you are talking about climate. It seems like a long time to us because it's longer than a human lifespan.

People have trouble conceiving of the effects of a megadrought that lasts for decades because these events are so rare no one alive has ever experienced one. In fact there are not even written stories of people experiencing a megadrought. At least, not stories that are written on paper. Paleoclimatologists have to read stories of past drought in the rings of trees.

A tree growing in a temperate climate has a ring for each year it grows. Just about any elementary school student can tell you that. But dendrochronologists, the scientists who study tree rings, know that a tree's growth rings record much more than the tree's age. A tree with wide, evenly spaced rings—what a dendrochronologist would call a "complacent series"—has experienced years of easy living, with good soil and abundant water. A tree growing in an area where temperature and precipitation vary will have rings that vary in width, what's called a "sensitive series."

You could think of a tree as a time machine made of wood. It tells you only about the stuff that matters to a tree—sunlight, water, insect attack, fires, and the like. That's okay if you're mostly interested in climate. Unfortunately the tree-based time machine can only take you back so far. Cut down a tree and you can project growing conditions backward in time from that day until the tree was born. You might think that your knowledge of the past was limited by the life span of the tree.

Not so fast. Scientists are not just good storytellers. Like science fiction writers, they are clever as well. In this case, the clever scientist was Andrew Ellicott Douglass, an astronomer looking for a natural record that might reveal ancient cycles in sunspot activity.

Back in 1902, Douglass visited lumberyards in Flagstaff, Arizona, and measured the widths of rings in the logs. With this data, he established the close association between ring width and rainfall.

Then he figured out how to travel farther back in time. In core samples from living trees, he found identifiable patterns of wide and narrow rings and figured out when those rings formed. Then he examined the rings of standing dead trees, searching for patterns in the rings that overlapped those of the living trees. Since he knew the dates of the overlapping pattern, he could work backward, extending the tree-ring record of climate backward in time.

Over the years, Douglass collaborated with archaeologists from the American Museum of Natural History and the National Geographic Society, examining wood from beams found in the ruins of prehistoric cliff and mesa dwellings. Eventually, he developed two separate records of climate based on patterns of tree rings, each covering more than 500 years.

One of these chronologies started with living trees. In this record, each ring could be assigned to a specific calendar year. The other chronology was based on ancient beams. Douglass could find no overlap between the chronologies.

Finally, in 1929, Douglass set out on an expedition targeting samples that could bridge the gap between the two chronologies. On June 22, 1929, a beam from a prehistoric settlement in New Mexico connected the two timelines. With that, Douglass had a continuous record of tree-ring data dating back to A. D. 700. Using this chronology, dates could be assigned to Southwestern ruins with certainty. A continuous record of climate in the area had been established.

The technique Douglass pioneered, known as cross dating, has been widely applied in the American Southwest. An historical archaeologist once took Paul hiking in the Dark Canyon primitive area in Utah. The group climbed up to a difficult-to-reach pueblo cliff dwelling. Few had visited this pueblo; tiny corncobs dating from when these dwellings were occupied could still be found tucked away in corners of the rooms. But the cottonwood beams above the door openings in these cliff dwellings had modern corks stuck in them. Numbers were printed on each cork, marking where archaeologists had drilled core samples in the beams to use the tree rings to date the beams.

Archaeologists and paleoclimatologists all over the world have made use of tree rings and cross dating. In Europe, a chronology created using oak and pine trees extends over 12,460 years. In the U.S., the North American Drought Atlas provides data on soil moisture extending back as far as 1,992 years, based on 835 separate tree-ring chronologies.

The story told by the tree rings isn't a happy one. It tells of severe droughts over the past thousand years, including megadroughts that lasted for decades rather than years. Megadroughts may be to blame for the collapse of past civilizations, include the Anasazi of the American Southwest and the Maya of Central America.

The current situation in California is dry indeed. In terms of soil moisture, this drought is the worst in 1,200 years. It seems that California is suffering from a combination of little rain and record high temperatures. According to the National Weather Service, 2014 was the warmest year in the historical record. The high temperatures mean that more water evaporates from plants and the soil, from streams and rivers.

Bad news for us Californians (and the rest of you in the West), but very useful for science fiction writers. Nothing like a disaster to fuel a good story.

Speaking of disasters, how about a runaway nuclear reaction? Better yet, how about a naturally occurring nuclear reaction that happened so long ago that it's now fossilized? (Yeah, we know it sounds unlikely, but strange things happen in the imaginary land of the past.)

This story begins in 1972 with a lab technician at a nuclear fuel processing facility in France who was worried about some missing uranium. He had discovered an anomaly while counting isotopes of uranium using a mass spectrometer. The isotope of Uranium 235 (U235) made up only 0.717 percent of the uranium atoms in the ore the technician was analyzing. The rest of the sample was Uranium 238.

Here's what made that weird. Every other natural sample of uranium is 0.720 percent U235. Since U235 is the isotope of uranium used to make atomic bombs, this small deviation from the norm set off a hunt for the missing U235.

Things got worse in a hurry. As French scientists started the search, they found that some samples contained only 0.44 percent U235. That meant there was a lot more missing uranium. Big trouble.

In their search, French scientists followed the sample back to its source: the Oklo uranium mine in Gabon, Africa. And that's where things got really strange. Analyzing the uranium in the mine, scientists discovered large deposits of ore that did not contain the usual amount of U235.

Where did the U235 go? Scientists eventually uncovered a paper from 1953 that suggested a possibility: perhaps the uranium in the mine had spontaneously and naturally formed a nuclear reactor.

In a nuclear reactor, a neutron passes through a U235 nucleus. This triggers a nuclear reaction that causes the U235 nucleus to split roughly in half, becoming two nuclei. For example, you could end up with one nucleus of barium and one of krypton (watch out, Superman), plus two or more neutrons and a lot of energy. This process is called "fissioning."

When these neutrons penetrate other U235 nuclei, they can cause them to fission and release more energy and another two neutrons. Suppose those two neutrons caused two more U235's to fission, releasing four neutrons. Suppose each of those four neutrons caused another U235 to fission. Suppose this happened again and again.

This is the recipe for a nuclear chain reaction—a bomb, if you will. But there was no evidence a bomb had gone off in Gabon. To make a reactor rather than a bomb, something had to slow the reaction down.

When neutrons come out of a fissioning U235 atom, they come out fast. Fast neutrons zip through an atomic nucleus so quickly that they don't have time to react with the nucleus. Neutrons trigger fission hundreds of times more strongly if they can be slowed down. So reactors need something that will slow down those fast neutrons. That something is called a "moderator."

Water is the moderator of choice in most nuclear reactors. But water comes with a problem. When neutrons collide with the hydrogen nuclei in water, they lose energy in the collision and slow down. That's good, because it means the neutrons can trigger fission. But the hydrogen nuclei in water can also react with the neutrons and capture them. That slows things down and stops the nuclear reaction.

So there's a delicate balance required—you need enough water to slow things down, but you also need enough U235 to keep things going. In a modern reactor, the fuel must be enriched in U235 until it makes up three percent of the fuel before enough neutrons escape capture by the water to create a nuclear chain reaction.

About 1.7 billion years ago, water dissolved uranium-containing minerals and deposited them as ore bodies in the sediments that were to become rock at Oklo.

Back then, there was more U235 around. U235 radioactively decays by emitting alpha particles, eventually becoming lead. U238 also radioactively decays to lead with a half-life of 4.5 billion years. Since the Earth is about 4.5 billion years old, about half of the uranium that the Earth had when it was formed has now turned into lead. (Remember U238 makes up most of the Earth's uranium.)

U235 has a shorter half-life—a mere 700 million years. So the 1.7 billion years since the Oklo sediments were deposited is a little over two half lives. That means that U235 made up three percent of the uranium when the sediments formed. Exactly the amount needed to make a water-moderated nuclear reactor.

When water seeped into the sedimentary rock, the water slowed the neutrons so that they could start a nuclear chain reaction. This nuclear reaction heated the rock and water to hundreds of degrees Celsius. The increase in temperature made the water boil away—removing the moderator and turning off the nuclear reactor.

The reactor left traces of its operation in the nuclear waste it produced. This nuclear waste reveals that the reactor would turn on for thirty minutes and boil away the water, then stay off for one hundred fifty minutes while the rock cooled and the water came back. Then the reactor would start up again.

The pulsed boiling of water is like a nuclear-powered Old Faithful. No one was around to watch this Very Old Faithful—this was 1.7 billion years ago, when life as we know it was getting started with a few single-celled organisms in the world's oceans. But from the nuclear waste left behind, scientists figure this cycle kept repeating for over 100,000 years.

In the Oklo sediments, there were at least eighteen natural nuclear reactors. Each one produced an average of 100 kilowatts of power, enough to light a thousand hundred-watt light bulbs.

The nuclear waste products of these reactors have remained locked in the ground at the Oklo site for nearly two billion years. So nature handled its own nuclear wastes. And the wastes, when analyzed, tell us the story of a natural nuclear reactor. Readers will be relieved to know that since so much U235 has decayed over the last few billion years, a natural water-moderated nuclear reactor is no longer possible.

In a poem titled "The Speed of Darkness," notable feminist poet Muriel Rukeyser wrote a particularly memorable line: "The universe is made of stories, not of atoms." We think Rukeyser was on the right track, but we'd modify that statement, just a touch.

The universe is made of stories. Some of those stories tell of atoms. Other stories tell of nuclear reactions captured as fossils, of measuring time with trees, of terrible lizards that turn out to be birds, and of many other unknown worlds, reconstructed from painstakingly gathered evidence.

Some people think science fiction is about the future, but we think those people have misunderstood the genre. Science fiction is about the great sweep of time—how things might be in the future and how they might have been in the past. The stories told by scientists and science fiction writers let us explore unknown worlds, including the imaginary world of the distant past.
 

Paul Doherty works at The Exploratorium, San Francisco's museum of science, art, and human perception—where science and science fiction meet. For more on Paul's work and his latest adventures, visit www.exo.net/~pauld. Pat Murphy is a science writer, a science fiction writer, and occasionally a troublemaker. She works at Mystery Science, developing hands-on lessons for elementary school. You can learn more about what she's up to at www.brazenhussies.net/Murphy.