Dianna led us across the hangar/museum to a metal stairway that led upward to a balcony. She opened one door to reveal another flight of stairs that spiraled upwards. “This was originally an air-traffic control tower,” she told me. At the top was a door with an electronic lock. She punched in a code, and the door opened. I hadn’t been sure what to expect in the control room, but the one thing I had taken for granted was that it would be a quiet place, probably manned by a few older men crouched silently over a computer screen. What I found instead was a room filled by 25 people who bustled about, talking loudly and sometimes arguing. With a ceiling fifteen feet high, the room was just big enough for the noise to produce noticeable echoes. Except for one white-haired man, none of the staff looked over 35. Even more surprisingly, most of the staff were from the Orient, predominately Indians and Japanese.

I was simultaneously dazzled and confused by the machinery. The walls were virtually lined with computer screens, which displayed either indecipherable equations and lines of computer code or weird shapes. Several showed false-color images of landscapes viewed from above. The room was dominated by a three-dimensional layered LED display, fourteen feet tall, twenty feet wide and four feet thick, that stood in the center. It had obviously been used originally by air-traffic controllers to plot the positions of planes, but now it was nearly filled by a computer-generated geometric form that looked like macaroni stretched to infinity. “What is that?” I said.

The white-haired man turned around and said: “That is the Earth, represented four-dimensionally.” He held out his hand. “I am Dr. Julius Werner. I designed the Temporal Displacement Device. I also trained this lovely young lady.” Dianna blushed. “And you must be Ted Flockman. Dr. Wrzniewski, I already know. This is only the third time that applicants have come in here, so please excuse our lack of decorum.”

“That’s fine,” I said, smiling at the implied compliment. “So, what’s this about representing the Earth in four dimensions?”

“It is a visual representation of string theory,” Werner said. “The underlying premise is that any object, at a given point in time, is only one part of a four-dimensional string, or, to use a more colorful analogy, one segment of a worm. In this case, the object being modeled is our planet as it moves through space. We call this model the Earth-Worm. What you see is only a small portion. When preparing for a temporal displacement, we plot where, and when, we want to send the expedition on the model.”

“It’s my understanding that only a part of the apparatus actually moves through time,” I said. “What does that look like, and how does it move?”

“We call the mobile part of the device the time bell, after a diving bell,” Werner explained. “Like a diving bell, it has no motive power of its own, but must instead be moved by an external agency. The time bell is very simple: a square platform, with a pole on each corner. The poles contain machinery that generates a temporal displacement field. The power, both for going to the past and coming back to the present, is produced by the anti-matter generator. The course, for lack of a better word, is programmed by us. Since there is no contact between the time bell and the rest of the device, all time travelers will have to adhere to a plan more rigid than that of a space flight. You must return to the present after a fixed amount of time. In fact, the machine will do so automatically. You must have a specific amount of mass aboard. If anything goes wrong, we will have no way to help you.”

I felt a little intimidated. “Is it dangerous?”

“Of course,” Werner said. “That is why we need the very best staff for an expedition. But rest assured, we will do everything possible to keep you safe. We will provide you with the best weapons, the best vehicles and the best supplies to accomplish your mission. We will hire paleontological consultants to instruct you on what is known about past environments, and we will use “light probes” to map where you go ahead of time. All in all, you will face less risk working for Naughtenny Moore, Ltd. than you will crossing the street.”

“Not that I’m paranoid. . .but why couldn’t you help us?” I asked. “If nothing else, couldn’t you load a rescue party aboard, and send the time bell back to when it left?”

Werner shook his head. “That’s not an option. At the present time, all displacements have an uncertainty factor of plus or minus 50 years. There’s no way we could send a time bell back to exactly when it left.”

I examined the “Earth-worm” thoughtfully. “You seem to be able to travel in space, as well as time. Could this be used for space travel, as well as time travel?”

Werner laughed. “NASA has been very interested in that possibility,” he said. “Unfortunately, again, the answer is no. We can easily go to different places on the Earth’s surface, but two problems prevent travel to other planets. One is differential velocities. This is a problem even for the Earth. Because the Earth moves in different directions during its orbit, and because its rotation rate has steadily slowed over time, the time bell invariably will undergo an instantaneous change in velocity on arrival in the past. The time bell has massive shock absorbers, but if it landed at the wrong place at the wrong time, it would be smashed like a bug on a windshield. Most of the calculations we have to make are simply to prevent that from happening. If the target were a different planet, even in our own solar system, the difficulties, and the potential for disaster, would be infinitely greater.

“The second, even more fundamental problem rises from the nature of space-time. For generations, it has been recognized that large objects distort not only space, but time. The Earth-worm is not merely a plot of the Earth’s position over time, but a representation of a four-dimensional structure created by the Earth’s passage. All “points” on this structure are interconnected. That is why time travel is possible. As long as we travel to Earth’s past, space-time works in our favor. But if we aim at a point outside the structure, space-time is against us. To reach any such point would require energy expenditures millions of times greater than what is already necessary. The chances of such a point being occupied by a planet would be infinitesimal, even with the most careful calculations. Picture trying to shoot a gnat from a million miles away. That is what it would be like to try to go to another planet in another solar system. And if you do make it, there’s still the velocity problem…”

The whole time, Carlos was gazing thoughtfully at the model. Finally he spoke, with surprising softness, “Perhaps. . .perhaps this is the face of the Earth Mother.”

I finally raised what was perhaps the most vital question. “If we travel back in time, what are the risks of changing the past?” I was grimly recalling my disastrous attempt to replay history.

“One of the two expeditions that have already gone performed an experiment to test that,” Werner said. “Ten people went, on an experimental time bell slightly bigger than a freight elevator. Dr. Gonzalez was one of them, incidentally. They went to a desolate region of Israel, in the second century C.E. Their main objective was to study the Dead Sea scrolls when they were still intact. Nothing was collected. They simply went to the caves where the scrolls had been found, took them out, photographed them and then put them back. Utmost care was taken to make no changes, except one.

“In a carefully chosen spot that had never been surveyed by archeologists, the team bored a hole 20 meters deep and 20 centimeters wide. Into this hole, they dropped an airtight plastic canister, holding a titanium plate inscribed with the names of everyone in the expedition. Then they carefully refilled the hole. While the expedition was in the past, a whole crew of construction workers searched for the canister. They found it. It is now on display in the museum.”

I glanced at Dianna. She had a somber expression. It was Carlos who spoke: “What does that prove, exactly? Since it was planted in a place no one ever looked before, you don’t know whether the past was changed. To make the test conclusive, you should have dug it up before the expedition.”

“We know it is not entirely conclusive,” Werner said. “But we did make progress. We proved that a time traveler can have a tangible effect on the present world. That had been a subject of some debate. One school of thought has held that any trip to the past would create a timeline separate from the one from which the time traveler came. That view is entirely refuted. If it were true, the canister would exist in the new timeline, but not in ours. Another view is that the universe would somehow obliterate any changes, in which case seemingly chance circumstances would have conspired to destroy the canister, or even prevent it from being planted at all. They did not, even with 2000 years to work with. Most scientists have come to the conclusion that no changes occurred. The canister was always there.

“As you say, the conclusive test would have been to dig for it before the expedition left. But that was considered potentially hazardous to make such a direct test. The universe may be flexible in areas of uncertainty, such as ground where no one has dug, but not so where we have prior experience. UNCOST has passed a measure which will prevent any similar experiments. We are now prohibited from sending expeditions to any place and time where humans exist. They also made me install this.” He pointed to a large green button on a large console in front of the 3-D display. “That is a lockout button. If pushed, it will automatically lock down the controls. It was installed against the hypothetical event of an attempted hijacking. Quite nonsensical, since the typical band of terrorists couldn’t operate this machine if we gave them an instruction manual and a tech support hotline. There would be a greater risk of a chimpanzee flying away with a space shuttle!”

Werner folded his hands. “The question I am about to ask, gentlemen, is your final test. For all intents and purposes, you have earned your jobs. The only remaining question is, knowing what you do now, do you still want them? Two men have come this far before, only to say no. How about you?”

“Hell yes!” Carlos said. Werner smiled and looked at me. Intellectually, I could see that this would be a dangerous job, perhaps the most dangerous that has ever existed. Yet, when I thought about all I had gone through, and especially when I glanced at my lovely new co-worker, I didn’t feel like there was any question at all.

I said yes.