Authors: Ben Bova
“You can?” The writer perks up.
“Certainly,” replies the mathematician. “Please remember that a mathematician named Newton showed three hundred years ago that an artificial satellite could be established in orbit. Mathematicians can tell you what the stars will look like a billion years from now, or what interactions a mu-meson will undergo in its first millionth of a second of lifetime, or...”
“Okay, okay,” says the science fiction writer. “What about starships?”
“First,” the mathematician says, “it’s not necessary to travel exactly at the speed of light. If the ship could get to within a few percent of light speed, then time would begin to change aboard the ship.
“This all stems from Einstein’s theory of relativity,” he adds. “Although most people claim Einstein was a physicist, he was really quite a mathematician as well.”
“Spare us the commercial,” the medical doctor mumbles. Sniffing slightly, the mathematician goes on, “The physicist told you that strange things begin to happen to matter and energy when you get close to light speed. Well, strange things happen to time, as well.
“The mathematics of relativity,” he explains, “show that if a ship were to approach the speed of light, time aboard the ship would slow down. A clock aboard the ship would tick slower and slower as the ship’s speed got closer and closer to the speed of light. Everything aboard the ship, the human crew included, would slow down with respect to time on Earth. But aboard the ship itself, nothing would seem to change. Everything would seem quite normal, even though years of time might pass on Earth before a second elapses on the ship.
“This is the basis of the famous ‘twin paradox’ of relativity. If one twin brother stayed on Earth while the other flew to a star at nearly the speed of light, when the flying twin returned to Earth, he would be younger than the brother he left behind.
“The German mathematician Eugen Sanger once gave the following example: A ship flying at more than 90 percent of the speed of light travels 1,000 light-years to Polaris, the North Star. Ignoring such details as the time spent accelerating to top speed and decelerating to landing speed again, the ship could make the flight to Polaris and back to Earth in a
subjective
time of 20 years. That is, to the crew on board the ship, only 20 years will have passed. But when they return to Earth, our planet will be 2,000 years older than when they left!”
“That’s wild,” says the science fiction writer, looking a little groggy at this point.
The mathematician nods happily. “So you see, if we could travel at speeds close to the speed of light, we could reach the stars. There’s no need to break the so-called ‘light barrier’ to get to the stars.”
“Not to all the stars,” says the astronomer. “Just to a handful of stars, the nearest ones. Even at light speed, the stars are too far away.”
The mathematician disagrees. “Come now. Sanger showed you could fly across the entire known universe in a subjective time of only 40 years, if you fly at 99 percent of the speed of light.”
“And return to an Earth that’s billions of years older than when you left it,” the astronomer retorts. “Who would go on such a venture? How could you know that the Earth would still exist after such a time?”
“Wait... wait...” The writer puts an end to their argument before it can go any further. “If it’s mathematically possible to cover such distances, could we really build ships to do the job? I mean, sticking to these ideas that there is a ‘light barrier’ and that nothing can go faster than light, can we someday build starships that will go at least
close to
the speed of light?”
“You might not have to,” says the engineer. “There’s always the possibility of an interstellar ark. You know, a huge ship with a completely self-sufficient colony aboard. They’d sail out toward the stars at speeds not much more than solar escape velocity—that 58,000 kilometers per hour we were talking about a few minutes ago.”
“But that would take thousands of years... millions...”
The engineer shrugs. “Sure, it would take generations and generations. People would be born aboard the ship, live out their lives, and die. Their great-great-many-greats-grandchildren would eventually get to the star they were aiming for. But that would be the simplest kind of ship to build. Awfully big, of course—like a moving city. But it could be built. I think.”
The psychiatrist, who’s been silent up to now, says, “I doubt that normal, well-adjusted human beings would ever embark on such a journey. How could they, in good conscience? They’d be dooming many generations of their offspring to live and die aboard the ark. How do they know that the children who finally reach their destination star will want to live there?”
“Or,” the astronomer adds, with a twinkle in his eye, “that another group in a faster ship hasn’t beaten them to it?”
“Even leaving that possibility aside,” the psychiatrist continues, “no group of human beings who could be considered to be normal would ever contemplate such a mission. Why, they would have to be a group of exiles. Or religious fanatics.”
“Like the Pilgrims or Quakers?” somebody asks.
The engineer says, “I’m assuming that the rocket engines aboard the ark will be based on nuclear fusion. You know, the hydrogen fusion process, such as the Sun and stars use. Hydrogen atoms come together to make a helium atom, and release energy.”
“No one’s built a fusion rocket,” the physicist points out. “In fact, even the fission rockets—the kind that use uranium or plutonium, where the atoms are split to release energy—have never gone beyond the testing stage. Nobody’s flown one. And the only way we’ve been able to release fusion energy here on Earth is in hydrogen bombs.”
“I know,” the engineer admits. “But progress in fusion research has been very encouraging over the past few years. I think we can safely agree that fusion power will be available before the end of this century.”
“Perhaps,” the physicist says reluctantly.
“Fusion rockets will make tremendous propulsion systems,” the engineer says glowingly.
The engineer goes on to explain about a study undertaken by Dwain F. Spencer and Leonard D. Jaffe at the California Institute of Technology’s Jet Propulsion Laboratory. “Spencer and Jaffe assumed that fusion rockets could be built, and then they tried to design a starship that uses fusion power. The ship they came up with—on paper— had five stages, each one powered by fusion rockets. It can make a round-trip flight to Alpha Centauri in a total elapsed time of 29 years. The ship would accelerate at 32 feet per second, every second, for several months. This is the same force that we feel here on Earth due to our planet’s gravity. So, during the ship’s acceleration period, the crew would feel 1
g,
their normal Earth weight.
“After several months of this acceleration, the ship would be traveling at a relativistic speed—fast enough for time effects to come into play. It would then shut down its engines and coast the rest of the way to Alpha Centauri. The same procedure would be followed for the return trip: a few months of 1
g
acceleration, then coasting flight back to Earth.
“The 29 years would seem slightly shorter to the ship’s crew,” the engineer says, “because of the relativistic time-dilation effect.”
“And that’s using power that we know we can harness,” the science fiction writer adds excitedly. “Why, maybe early next century we could reach Alpha Centauri! People alive today might make the trip!”
“Excuse me,” says the astronomer. “Have any of you heard of the Bussard interstellar ramjet?”
“R. W. Bussard was a physicist at the Los Alamos Scientific Laboratory when he thought of the interstellar ramjet idea,” the astronomer explains.
“Bussard realized that one of the main drawbacks to any rocket engine is that it must carry all of its propellant with it. Spencer and Jaffe’s five-stage fusion rocket, for example, must be more than 90 percent hydrogen propellant—allowing very little payload for such a huge vehicle. The rocket must also spend a considerable amount of its energy just lifting its own propellant mass. The situation becomes a vicious circle. As long as you must carry all the rocket’s propellant along with you, any increase in speed must be paid for by more propellant mass. When you’re considering flight at close to the speed of light, this becomes a serious obstacle. It poses a fundamental limitation on the amount of energy you can get out of the fusion rocket.
“But suppose the interstellar ship didn’t have to carry any fuel at all? It could carry much more payload. And its range would be unlimited—it could go anywhere, at close to light speed, as long as it could somehow find propellant to feed to its engines.
“Interstellar space is filled with propellant for a hydrogen fusion rocket—hydrogen gas. There is enough hydrogen gas floating freely among the stars to build billions of new stars. This is an enormous supply of propellant.
“However,” the astronomer admits, “when I use the word
filled
I’m being a little overly dramatic. The hydrogen gas is spread very thinly through most of interstellar space... no more than a few atoms per cubic centimeter. By contrast, there are more than 10
19
atoms per cubic centimeter in the air we’re breathing. That’s ten million trillion atoms in the space of a sugar cube. Out among the stars, there are fewer than ten atoms per cubic centimeter.
“Bussard calculated that the ramjet will need a tremendously large scoop to funnel in a continuous supply of hydrogen for the fusion rocket engines. For a ship with a payload of 1,000 tons—about the size of a reasonable schooner—a funnel some 2,000 kilometers in radius would be needed.”
The mathematician smiles. “I’m tempted to say that such a scoop would be
astronomically
big.”
“Yes,” the engineer says, “but there’s plenty of open space out there.”
“And the scoop needn’t be solid material,” the physicist adds. “If you could ionize the hydrogen with laser beams, so that the atoms are broken up into electrically charged ions, then the scoop could be nothing more than an immense magnetic field—it would funnel in the electrified ions quite nicely.”
“Such a ship,” the astronomer goes on, “can reach the nearest stars in a few years—of ship time, that is. The center of the Milky Way would be only about 20 years away, and the great spiral galaxy in Andromeda could be reached in about 30 years. Of course, the elapsed time on Earth would be thousands, even millions of years.”
“Even forgetting that for a moment,” the science fiction writer asks, “don’t you think the crew’s going to get bored? Spending 20 or 30 years traveling isn’t going to be much fun. And they’ll be getting older...”
A polite cough from the other side of the table turns everyone’s head toward the biochemist.
“As long as we’re stretching things,” he says, “we might as well consider the possibility of letting the crew sleep for almost the entire flight—slowing down their metabolism so that they don’t age much at all.”
“Suspended animation?” the writer asks.
With a slightly uncomfortable look, the biochemist replies, “You could call it something like that, I suppose. I’m sure that by the time we’re ready to tackle the stars, a technique will have been found to freeze a human being indefinitely. You could freeze the crew shortly after takeoff and then have them awakened automatically when they reach their destination. They won’t age while they’re hibernating.”
“This is the idea of freezing them at cryogenic temperatures, isn’t it?” the medical doctor asks.
Nodding, the biochemist says, “Yes. Temperatures close to absolute zero. Nearly 400 degrees below zero on the Fahrenheit thermometer.”
“That simply can’t be done,” the doctor says firmly.
“Not now,” the biochemist agrees. “But by the end of this century, we might have learned how to quick-freeze live human beings without damaging their cells.”
The doctor looks unconvinced and shakes his head.
“I must point out,” the psychiatrist says, “that you still have the basic problem of motivation on your hands. Who would want to leave the Earth, knowing that he would return to a world that’s several thousand years older than the one he left?”
“It would be a one-way trip, wouldn’t it?” the writer muses. “Even if the crew comes back to Earth, it won’t be the same world that they left. It’ll be like Columbus returning to Spain during the time of Napoleon.”
“Or Leif Ericson coming back to Scandinavia, next week.”
“The crew members will want to bring their families with them,” the writer points out. “They’ll have to.”
“Nothing man has ever done comes even close to such an experience,” the psychiatrist says.
“Oh, I’m not so sure about that,” objects the anthropologist. He has been sitting next to the psychiatrist, listening interestedly and smoking a pipe through the whole discussion.
Now he says, “The Polynesian peoples settled the islands of the Pacific on a somewhat similar basis. They started in one corner of the Pacific and expanded throughout most of the islands in the central regions of that ocean. And they did it on a somewhat haphazard basis—a mixture of deliberate emigrations into unknown territory plus accidental landings on new islands when ships were blown off course by storms.”
“That’s hardly...”
“Now listen,” the anthropologist insists quietly. “The Polynesians ventured out across the broad Pacific in outrigger canoes. Their travels must have seemed as dark and dangerous to them as interstellar space seems to us. They left their homes behind—purposely, in the case of the emigrants. Usually, when they were forced to emigrate because of population pressure or religious differences, they took their whole families along. But they knew they’d never return to their original islands again. That’s how Hawaii was first settled, and most of the other islands of the central Pacific.”
“That
is
somewhat similar to starflight,” the psychiatrist agrees.
“So we can reach the stars after all,” the science fiction writer says. “It’s not fundamentally impossible.”
“It won’t be simple,” the engineer insists.
“Yes, but imagine a time when we can travel with interstellar ramjets from star to star.”