Time’s Arrow

Arthur C. Clarke 1952

THE RIVER WAS DEAD AND THE LAKE ALREADY DYING WHEN the monster had come down the dried-up watercourse and turned onto the desolate mud-flats. There were not many places where it was safe to walk, and even where the ground was hardest the great pistons of its feet sank a foot or more beneath the weight they carried. Sometimes it had paused, surveying the landscape with quick, birdlike movements of its head. Then it had sunk even deeper into the yielding soil, so that fifty million years later men could judge with some accuracy the duration of its halts.

For the waters had never returned, and the blazing sun had baked the mud to rock. Later still the desert had poured over all this land, sealing it beneath protecting layers of sand. And later-very much later-had come Man.

“Do you think,” shouted Barton above the din, “that Professor Fowler became a palaeontologist because he likes playing with pneumatic drills? Or did he acquire the taste afterward?”

“Can’t hear you!” yelled Davis, leaning on his shovel in a most professional manner. He glanced hopefully at his watch.

“Shall I tell him it’s dinnertime? He can’t wear a watch while he’s drilling, so he won’t know any better.”

“I doubt if it will work,” Barton shrieked. “He’s got wise to us now and always adds an extra ten minutes. But it will make a change from this infernal digging.”

With noticeable enthusiasm the two geologists downed tools and started to walk toward their chief. As they approached, he shut off the drill and relative silence descended, broken only by the throbbing of the compressor in the background.

“About time we went back to camp, Professor,” said Davis, wristwatch held casually behind his back. “You know what cook says if we’re late.”

Professor Fowler, M.A., F.R.S., F.G.S., mopped some, but by no means all, of the ocher dust from his forehead. He would have passed anywhere as a typical navvy, and the occasional visitors to the site seldom recognized the Vice-President of the Geological Society in the brawny, half-naked workman crouching over his beloved pneumatic drill.

It had taken nearly a month to clear the sandstone down to the surface of the petrified mud-flats. In that time several hundred square feet had been exposed, revealing a frozen snapshot of the past that was probably the finest yet discovered by palaeontology. Some scores of birds and reptiles had come here in search of the receding water, and left their footsteps as a perpetual monument eons after their bodies had perished. Most of the prints had been identified, but one-the largest of them all-was new to science. It belonged to a beast which must have weighed twenty or thirty tons: and Professor Fowler was following the fifty-million-year-old spoor with all the emotions of a big-game hunter tracking his prey. There was even a hope that he might yet overtake it; for the ground must have been treacherous when the unknown monster went this way and its bones might still be near at hand, marking the place where it had been trapped like so many creatures of its time.

Despite the mechanical aids available, the work was very tedious. Only the upper layers could be removed by the power tools, and the final uncovering had to be done by hand with the utmost care. Professor Fowler had good reason for his insistence that he alone should do the preliminary drilling, for a single slip might cause irreparable harm.

The three men were halfway back to the main camp, jolting over the rough road in the expedition’s battered jeep, when Davis raised the question that had been intriguing the younger men ever since the work had begun.

“I’m getting a distinct impression,” he said, “that our neighbors down the valley don’t like us, though I can’t imagine why. We’re not interfering with them, and they might at least have the decency to invite us over.”

“Unless, of course, it is a war research plant,” added Barton, voicing a generally accepted theory.

“I don’t think so,” said Professor Fowler mildly. “Because it so happens that I’ve just had an invitation myself. I’m going there tomorrow.”

If his bombshell failed to have the expected result, it was thanks to his staff’s efficient espionage system. For a moment Davis pondered over this confirmation of his suspicions; then he continued with a slight cough:

“No one else has been invited, then?”

The Professor smiled at his pointed hint. “No,” he said. “It’s a strictly personal invitation. I know you boys are dying of curiosity but, frankly, I don’t know any more about the place than you do. If I learn anything tomorrow, I’ll tell you all about it. But at least we’ve found out who’s running the establishment.”

His assistants pricked up their ears. “Who is it?” asked Barton, “My guess was the Atomic Development Authority.”

“You may be right,” said the Professor. “At any rate, Henderson and Barnes are in charge.”

This time the bomb exploded effectively; so much so that Davis nearly drove the jeep off the road-not that that made much difference, the road being what it was.

“Henderson and Barnes? In this god-forsaken hole?”

“That’s right,” said the Professor gaily. “The invitation was actually from Barnes. He apologized for not contacting us before, made the usual excuses, and wondered if I could drop in for a chat.” ,

“Did he say what they are doing?”

“No; not a hint.”

“Barnes and Henderson?” said Barton thoughtfully. “I don’t know much about them except that they’re physicists. What’s their particular racket?”

“They’re the experts on low-temperature physics,” answered Davis. “Henderson was Director of the Cavendish for years. He wrote a lot of letters to Nature not so long ago. If I remember rightly, they were all about Helium II.”

Barton, who didn’t like physicists and said so whenever possible, was not impressed. “I don’t even know what Helium II is,” he said smugly. “What’s more, I’m not at all sure that I want to.”

This was intended for Davis, who had once taken a physics degree in, as he explained, a moment of weakness. The “moment” had lasted for several years before he had drifted into geology by rather devious routes, and he was always harking back to his first love.

“It’s a form of liquid helium that only exists at a few degrees above absolute zero. It’s got the most extraordinary properties-but, as far as I can see, none of them can explain the presence of two leading physicists in this corner of the globe.”

They had now arrived at the camp, and Davis brought the jeep to its normal crash-halt in the parking space. He shook his head in annoyance as he bumped into the truck ahead with slightly more violence than usual.

“These tires are nearly through. Have the new ones come yet?”

“Arrived in the ‘copter this morning, with a despairing note from Andrews hoping that you’d make them last a full fortnight this time.”

“Good! I’ll get them fitted this evening.”

The Professor had been walking a little ahead; now he dropped back to join his assistants.

“You needn’t have hurried Jim,” he said glumly. “It’s corned beef again.”

It would be most unfair to say that Barton and Davis did less work because the Professor was away. They probably worked a good deal harder than usual, since the native laborers required twice as much supervision in the Chief’s absence. But there was no doubt that they managed to find time for a considerable amount of extra talking.

Ever since they had joined Professor Fowler, the two young geologists had been intrigued by the strange establishment five miles away down the valley. It was clearly a research organization of some type, and Davis had identified the tall stacks of an atomic-power unit. That, of course, gave no clue to the work that was proceeding, but it did indicate its importance. There were still only a few thousand turbo-piles in the world, and they were all reserved for major projects.

There were dozens of reasons why two great scientists might have hidden themselves in this place: most of the more hazardous atomic research was carried out as far as possible from civilization, and some had been abandoned altogether until laboratories in space could be set up. Yet it seemed odd that this work, whatever it was, should be carried out so close to what had now become the most important center of geological research in the world. It might, of course, be no more than a coincidence; certainly the physicists had never shown any interest in their compatriots so near at hand.

Davis was carefully chipping round one of the great footprints, while Barton was pouring liquid perspex into those already uncovered so that they would be preserved from harm in the transparent plastic. They were working in a somewhat absentminded manner, for each was unconsciously listening for the sound of the jeep. Professor Fowler had promised to collect them when he returned from his visit, for the other vehicles were in use elsewhere and they did not relish a two-mile walk back to camp in the broiling sun. Moreover, they wanted to have any news as soon as possible.

“How many people,” said Barton suddenly, “do you think they have over there?”

Davis straightened himself up. “Judging from the buildings, not more than a dozen or so.”

“Then it might be a private affair, not an ADA project at all,”

“Perhaps, though it must have pretty considerable backing. Of course, Henderson and Barnes could get that on their reputations alone.”

“That’s where the physicists score,” said Barton. “They’ve only got to convince some war department that they’re on the track of a new weapon, and they can get a couple of million without any trouble.”

He spoke with some bitterness; for, like most scientists, he had strong views on this subject. Barton’s views, indeed, were even more definite than usual, for he was a Quaker and had spent the last year of the War arguing with not-unsympathetic tribunals.

The conversation was interrupted by the roar and clatter of the jeep, and the two men ran over to meet the Professor.

“Well?” they cried simultaneously.

Professor Fowler looked at them thoughtfully, his expression giving no hint of what was in his mind. “Had a good day?” he said at last.

“Come off it, Chief!” protested Davis. “Tell us what you’ve found out.”

The Professor climbed out of the seat and dusted himself down. “I’m sorry, boys,” he said with some embarrassment, “I can’t tell you a thing, and that’s flat.”

There were two united wails of protest, but he waved them aside. “I’ve had a very interesting day, but I’ve had to promise not to say anything about it. Even now I don’t know exactly what’s going on, but it’s something pretty revolutionary-as revolutionary, perhaps, as atomic power. But Dr. Henderson is coming over tomorrow; see what you can get out of him.”

For a moment, both Barton and Davis were so overwhelmed by the sense of anticlimax that neither spoke. Barton was the first to recover. “Well, surely there’s a reason for this sudden interest in our activities?”

The Professor thought this over for a moment. “Yes; it wasn’t entirely a social call,” he admitted. “They think I may be able to help them. Now, no more questions, unless you want to walk back to camp!”

Dr. Henderson arrived on the site in the middle of the afternoon. He was a stout, elderly man, dressed rather incongruously in a dazzling white laboratory smock and very little else. Though the garb was eccentric, it was eminently practical in so hot a climate.

Davis and Barton were somewhat distant when Professor Fowler introduced them; they still felt that they had been snubbed and were determined that their visitor should understand their feelings. But Henderson was so obviously interested in their work that they soon thawed, and the Professor left them to show him round the excavations while he went to supervise the natives.

The physicist was greatly impressed by the picture of the world’s remote past that lay exposed before his eyes. For almost an hour the two geologists took him over the workings yard by yard, talking of the creatures who had gone this way and speculating about future discoveries. The track which Professor Fowler was following now lay in a wide trench running away from the main excavation, for he had dropped all other work to investigate it. At its end the trench was no longer continuous: to save time, the Professor had begun to sink pits along the line of the footprints. The last sounding had missed altogether, and further digging had shown that the great reptile had made a sudden change of course.

“This is the most interesting bit,” said Barton to the slightly wilting physicist. “You remember those earlier places where it had stopped for a moment to have a look around? Well, here it seems to have spotted something and has gone off in a new direction at a run, as you can see from the spacing.”

“I shouldn’t have thought such a brute could run.”

“Well, it was probably a pretty clumsy effort, but you can cover quite a bit of ground with a fifteen-foot stride. We’re going to follow it as far as we can. We may even find what it was chasing. I think the Professor has hopes of discovering a trampled battlefield with the bones of the victim still around. That would make everyone sit up.”

Dr. Henderson smiled. “Thanks to Walt Disney, I can picture the scene rather well.”

Davis was not very encouraging. “It was probably only the missus banging the dinner gong,” he said. “The most infuriating part of our work is the way everything can peter out when it gets most exciting. The strata have been washed away, or there’s been an earthquake-or, worse still, some silly fool has smashed up the evidence because he didn’t recognize its value.”

Henderson nodded in agreement. “I can sympathize with you,” he said. “That’s where the physicist has the advantage. He knows he’ll get the answer eventually, if there is one.”

He paused rather diffidently, as if weighing his words with great care. “It would save you a lot of trouble, wouldn’t it, if you could actually see what took place in the past, without having to infer it by these laborious and uncertain methods. You’ve been a couple of months following these footsteps for a hundred yards, and they may lead nowhere for all your trouble.”

There was a long silence. Then Barton spoke in a very thoughtful voice.

“Naturally, Doctor, we’re rather curious about your work,” he began. “Since Professor Fowler won’t tell us anything, we’ve done a good deal of speculating. Do you really mean to say that—”

The physicist interrupted him rather hastily. “Don’t give it any more thought,” he said. “I was only daydreaming. As for our work, it’s a very long way from completion, but you’ll hear all about it in due course. We’re not secretive-but, like everyone working in a new field, we don’t want to say anything until we’re sure of our ground. Why, if any other palaeontologists came near this place, I bet Professor Fowler would chase them away with a pick-axe!”

“That’s not quite true,” smiled Davis, “He’d be much more likely to set them to work. But I see your point of view; let’s hope we don’t have to wait too long.”

That night, much midnight oil was burned at the main camp. Barton was frankly skeptical, but Davis had already built up an elaborate superstructure of theory around their visitor’s remarks.

“It would explain so many things,” he said. “First of all, their presence in this place, which otherwise doesn’t make sense at all. We know the ground level here to within an inch for the last hundred million years, and we can date any event with an accuracy of better than one per cent. There’s not a spot on Earth that’s had its past worked out in such detail-it’s the obvious place for an experiment like this!”

“But do you think it’s even theoretically possible to build a machine that can see into the past?”

“I can’t imagine how it could be done. But I daren’t say it’s impossible-especially to men like Henderson and Barnes.”

“Hmmm. Not a very convincing argument. Is there any way we can hope to test it? What about those letters to Nature?”

“I’ve sent to the College Library; we should have them by the end of the week. There’s always some continuity in a scientist’s work, and they may give us some valuable clues.”

But at first they were disappointed; indeed, Henderson’s letters only increased the confusion. As Davis had remembered, most of them had been about the extraordinary properties of Helium II.

“It’s really fantastic stuff,” said Davis. “If a liquid behaved like this at normal temperatures, everyone would go mad. In the first place, it hasn’t any viscosity at all. Sir George Darwin once said that if you had an ocean of Helium II, ships could sail in it without any engines. You’d give them a push at the beginning of their voyage and let them run into buffers on the other side. There’d be one snag, though; long before that happened the stuff would have climbed straight up the hull and the whole outfit would have sunk-gurgle, gurgle, gurgle …”

“Very amusing,” said Barton, “but what the heck has this to do with your precious theory?”

“Not much,” admitted Davis. “However, there’s more to come. It’s possible to have two streams of Helium II flowing in opposite directions in the same tube-one stream going through the other, as it were.”

“That must take a bit of explaining; it’s almost as bad as an object moving in two directions at once. I suppose there is an explanation, something to do with Relativity, I bet.”

Davis was reading carefully. “The explanation,” he said slowly, “is very complicated and I don’t pretend to understand it fully. But it depends on the fact that liquid helium can have negative entropy under certain conditions.”

“As I never understood what positive entropy is, I’m not much wiser.”

“Entropy is a measure of the heat distribution of the Universe. At the beginning of time, when all energy was concentrated in the suns, entropy was a minimum. It will reach its maximum when everything’s at a uniform temperature and the Universe is dead. There will still be plenty of heat around, but it won’t be usable.”

“Whyever not?”

“Well, all the water in a perfectly flat ocean won’t run a hydro-electric plant-but quite a little lake up in the hills will do the trick. You must have a difference in level.”

“I get the idea. Now I come to think of it, didn’t someone once call entropy ‘Time’s Arrow?’”

“Yes-Eddington, I believe. Any kind of clock you care to mention-a pendulum, for instance-might just as easily run forward as backward. But entropy is a strictly one-way affair-it’s always increasing with the passage of time. Hence the expression, ‘Time’s Arrow.’ “

“Then negative entropy-my gosh!”

For a moment the two men looked at each other. Then Barton asked in a rather subdued voice: “What does Henderson say about it?”

“I’ll quote from his last letter: ‘The discovery of negative entropy introduces quite new and revolutionary conceptions into our picture of the physical world. Some of these will be examined in a further communication.’”

“And are they?”

“That’s the snag: there’s no ‘further communication.’ From that you can guess two alternatives. First, the Editor of Nature may have declined to publish the letter. I think we can rule that one out. Second, the consequences may have been so revolutionary that Henderson never did write a further report.”

“Negative entropy-negative time,” mused Barton. “It seems fantastic; yet it might be theoretically possible to build some sort of device that could see into the past….”

“I know what we’ll do,” said Davis suddenly. “We’ll tackle the Professor about it and watch his reactions. Now I’m going to bed before I get brain fever.”

That night Davis did not sleep well. He dreamed that he was walking along a road that stretched in both directions as far as the eye could see. He had been walking for miles before he came to the signpost, and when he reached it he found that it was broken and the two arms were revolving idly in the wind. As they turned, he could read the words they carried. One said simply: To the Future; the other: To the Past.

They learned nothing from Professor Fowler, which was not surprising; next to the Dean, he was the best poker player in the College. He regarded his slightly fretful assistants with no trace of emotion while Davis trotted out his theory.

When the young man had finished, he said quietly, “I’m going over again tomorrow, and I’ll tell Henderson about your detective work. Maybe he’ll take pity on you; maybe he’ll tell me a bit more, for that matter. Now let’s go to work.”

Davis and Barton found it increasingly difficult to take a great deal of interest in their own work while their minds were filled with the enigma so near at hand. Nevertheless they continued conscientiously, though ever and again they paused to wonder if all their labor might not be in vain. If it were, they would be the first to rejoice. Supposing one could see into the past and watch history unfolding itself, back to the dawn of time! All the great secrets of the past would be revealed: one could watch the coming of life on the Earth, and the whole story of evolution from amoeba to man.

No; it was too good to be true. Having decided this, they would go back to their digging and scraping for another half-hour until the thought would come: but what if it were true? And then the whole cycle would begin all over again.

When Professor Fowler returned from his second visit, he was a subdued and obviously shaken man. The only satisfaction his assistants could get from him was the statement that Henderson had listened to their theory and complimented them on their powers of deduction.

That was all; but in Davis’s eyes it clinched the matter, though Barton was still doubtful. In the weeks that followed, he too began to waver, until at last they were both convinced that the theory was correct. For Professor Fowler was spending more and more of his time with Henderson and Barnes; so much so that they sometimes did not see him for days. He had almost lost interest in the excavations, and had delegated all responsibility to Barton, who was now able to use the big pneumatic drill to his heart’s content.

They were uncovering several yards of footprints a day, and the spacing showed that the monster had now reached its utmost speed and was advancing in great leaps as if nearing its victim. In a few days they might reveal the evidence of some eon-old tragedy, preserved by a miracle and brought down the ages for the observation of man. Yet all this seemed very unimportant now; for it was clear from the Professor’s hints and his general air of abstraction that the secret research was nearing its climax. He had told them as much, promising that in a very few days, if all went well, their wait would be ended. But beyond that he would say nothing.

Once or twice Henderson had paid them a visit, and they could see that he was now laboring under a considerable strain. He obviously wanted to talk about his work, but was not going to do so until the final tests had been completed. They could only admire his self-control and wish that it would break down. Davis had a distinct impression that the elusive Barnes was mainly responsible for his secrecy; he had something of a reputation for not publishing work until it had been checked and double-checked. If these experiments were as important as they believed, his caution was understandable, however infuriating.

Henderson had come over early that morning to collect the Professor, and as luck would have it, his car had broken down on the primitive road. This was unfortunate for Davis and Barton, who would have to walk to camp for lunch, since Professor Fowler was driving Henderson back in the jeep. They were quite prepared to put up with this if their wait was indeed coming to an end, as the others had more than half-hinted.

They had stood talking by the side of the jeep for some time before the two older scientists had driven away. It was a rather strained parting, for each side knew what the other was thinking. Finally Barton, as usual the most outspoken, remarked:

“Well, Doc, if this is Der Tag, I hope everything works properly. I’d like a photograph of a brontosaurus as a souvenir.”

This sort of banter had been thrown at Henderson so often that he now took it for granted. He smiled without much mirth and replied, “I don’t promise anything. It may be the biggest flop ever.”

Davis moodily checked the tire pressure with the toe of his boot. It was a new set, he noticed, with an odd zigzag pattern he hadn’t seen before.

“Whatever happens, we hope you’ll tell us. Otherwise, we’re going to break in one night and find out just what you’re up to.”

Henderson laughed. “You’ll be a pair of geniuses if you can learn anything from our present lash-up. But, if all goes well, we may be having a little celebration by nightfall.”

“What time do you expect to be back, Chief?”

“Somewhere around four. I don’t want you to have to walk back for tea.”

“O.K.-here’s hoping!”

The machine disappeared in a cloud of dust, leaving two very thoughtful geologists standing by the roadside. Then Barton shrugged his shoulders.

“The harder we work,” he said, “the quicker the time will go. Come along!”

The end of the trench, where Barton was working with the power drill, was now more than a hundred yards from the main excavation. Davis was putting the final touches to the last prints to be uncovered. They were now very deep and widely spaced, and looking along them, one could see quite clearly where the great reptile had changed its course and started, first to run, and then to hop like an enormous kangaroo. Barton wondered what it must have felt like to see such a creature bearing down upon one with the speed of an express; then he realized that if their guess was true this was exactly what they might soon be seeing.

By mid-afternoon they had uncovered a record length of track. The ground had become softer, and Barton was roaring ahead so rapidly that he had almost forgotten his other preoccupations. He had left Davis yards behind, and both men were so busy that only the pangs of hunger reminded them when it was time to finish. Davis was the first to notice that it was later than they had expected, and he walked over to speak to his friend.

“It’s nearly half-past four!” he said when the noise of the drill had died away. “The Chiefs late-I’ll be mad if he’s had tea before collecting us.”

“Give him another half-hour,” said Barton. “I can guess what’s happened. They’ve blown a fuse or something and it’s upset their schedule.”

Davis refused to be placated. “I’ll be darned annoyed if we’ve got to walk back to camp again. Anyway, I’m going up the hill to see if there’s any sign of him.”

He left Barton blasting his way through the soft rock, and climbed the low hill at the side of the old riverbed. From here one could see far down the valley, and the twin stacks of the Henderson-Barnes laboratory were clearly visible against the drab landscape. But there was no sign of the moving dust-cloud that would be following the jeep: the Professor had not yet started for home.

Davis gave a snort of disgust. There was a two-mile walk ahead of them, after a particularly tiring day, and to make matters worse they’d now be late for tea. He decided not to wait any longer, and was already walking down the hill to rejoin Barton when something caught his eye and he stopped to look down the valley.

Around the two stacks, which were all he could see of the laboratory, a curious haze not unlike a heat tremor was playing. They must be hot, he knew, but surely not that hot. He looked more carefully, and saw to his amazement that the haze covered a hemisphere that must be almost a quarter of a mile across.

And, quite suddenly, it exploded. There was no light, no blinding flash; only a ripple that spread abruptly across the sky and then was gone. The haze had vanished-and so had the two great stacks of the power-house.

Feeling as though his legs had turned suddenly to water, Davis slumped down upon the hilltop and stared open-mouthed along the valley. A sense of overwhelming disaster swept into his mind; as in a dream, he waited for the explosion to reach his ears.

It was not impressive when it came; only a dull, long-drawn-out whoooooosh! that died away swiftly in the still air. Half unconsciously, Davis noticed that the chatter of the drill had also stopped; the explosion must have been louder than he thought for Barton to have heard it too.

The silence was complete. Nothing moved anywhere as far as his eye could see in the whole of that empty, barren landscape. He waited until his strength returned; then, half running, he went unsteadily down the hill to rejoin his friend.

Barton was half sitting in the trench with his head buried in his hands. He looked up as Davis approached; and although his features were obscured by dust and sand, the other was shocked at the expression in his eyes.

“So you heard it too!” Davis said. “I think the whole lab’s blown up. Come along, for heaven’s sake!”

“Heard what?” said Barton dully.

Davis stared at him in amazement. Then he realized that Barton could not possibly have heard any sound while he was working with the drill. The sense of disaster deepened with a rush; he felt like a character in some Greek tragedy, helpless before an implacable doom.

Barton rose to his feet. His face was working strangely, and Davis saw that he was on the verge of breakdown. Yet, when he spoke, his words were surprisingly calm.

“What fools we were!” he said. “How Henderson must have laughed at us when we told him that he was trying to see into the past!”

Mechanically, Davis moved to the trench and stared at the rock that was seeing the light of day for the first time in fifty million years. Without much emotion, now, he traced again the zigzag pattern he had first noticed a few hours before. It had sunk only a little way into the mud, as if when it was formed the jeep had been traveling at its utmost speed.

No doubt it had been; for in one place the shallow tire marks had been completely obliterated by the monster’s footprints. They were now very deep indeed, as if the great reptile was about to make the final leap upon its desperately fleeing prey.

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