The official from the Safety and Civil Reassurance Administration gave a dismissive shrug. “They’re perfectly safe,” he said. He glanced up at Zaphod and suddenly said with uncharacteristic frankness, “There’s worse than that on board. At least,” he added, tapping at one of the computer screens, “I hope it’s on board.”
The other official rounded on him sharply.
“What the hell do you think you’re saying?” he snapped.
The first shrugged again. He said, “It doesn’t matter. He can say what he likes. No one would believe him. It’s why we chose to use him rather than do anything official, isn’t it? The more wild the story he tells, the more it’ll sound like he’s some hippy adventurer making it up. He can even say that we said this and it’ll make him sound like a paranoid.” He smiled pleasantly at Zaphod, who was seething in a suit full of sick. “You may accompany us,” he told him, “if you wish.”
*
“You see?” said the official, examining the ultra-titanium outer seals of the aorist rod hold. “Perfectly secure, perfectly safe.”
He said the same thing as they passed holds containing chemical weapons so powerful that a teaspoonful could fatally infect an entire planet.
He said the same thing as they passed holds containing zeta-active compounds so powerful that a teaspoonful could blow up a whole planet.
He said the same thing as they passed holds containing theta-active compounds so powerful that a teaspoonful could irradiate a whole planet.
“I’m glad I’m not a planet,” muttered Zaphod.
“You’d have nothing to fear,” assured the official from the Safety and Civil Reassurance Administration. “Planets are very safe. Provided,” he added – and paused. They were approaching the hold nearest to the point where the back of the starship Billion Year Bunker was broken. The corridor here was twisted and deformed, and the floor was damp and sticky in patches.
“Ho-hum,” he said, “ho very much hum.”
“What’s in this hold?” demanded Zaphod.
“By-products,” said the official, clamming up again.
“By-products...” insisted Zaphod, quietly, “of what?”
Neither official answered. Instead they examined the hold door very carefully and saw that its seals were twisted apart by the forces that had deformed the whole corridor. One of them touched the door lightly. It swung open to his touch. There was darkness inside, with just a couple of dim yellow lights deep within it.
“Of what?” hissed Zaphod.
The leading official turned to the other.
“There’s an escape capsule,” he said, “that the crew were to use to abandon ship before jettisoning it into the black hole,” he said. “I think it would be good to know that it’s still there.” The other official nodded and left without a word.
The first official quietly beckoned Zaphod in. The large dim yellow lights glowed about twenty feet from them.
“The reason,” he said quietly, “why everything else in this ship is, I maintain, safe, is that no one is really crazy enough to use them. No one. At least no one that crazy would ever get near them. Anyone that mad or dangerous rings very deep alarm bells. People may be stupid, but they’re not that stupid.”
“By-products,” hissed Zaphod again – he had to hiss in order that his voice shouldn’t be heard to tremble – “of what?”
“Er, Designer People.”
“What?”
“The Sirius Cybernetics Corporation were awarded a huge research grant to design and produce synthetic personalities to order. The results were uniformly disastrous. All the ‘people’ and ‘personalities’ turned out to be amalgams of characteristics which simply could not coexist in naturally occurring life-forms. Most of them were just poor pathetic misfits, but some were deeply, deeply dangerous. Dangerous because they didn’t ring alarm bells in other people. They could walk through situations the way that ghosts walk through walls, because no one spotted the danger.
“The most dangerous of all were three identical ones – they were put in this hold, to be blasted, with this ship, right out of this universe. They are not evil, in fact they are rather simple and charming. But they are the most dangerous creatures that ever lived because there is nothing they will not do if allowed, and nothing they will not be allowed to do...”
Zaphod looked at the dim yellow lights, the two dim yellow lights. As his eyes became accustomed to the light, he saw that the two lights framed a third space where something was broken. Wet, sticky patches gleamed dully on the floor.
Zaphod and the official walked cautiously toward the lights. At that moment, four words came crashing into the helmet headsets from the other official.
“The capsule has gone,” he said tersely.
“Trace it,” snapped Zaphod’s companion. “Find exactly where it has gone. We must know where it has gone!”
Zaphod approached the two remaining tanks. A quick glance showed him that each contained an identical floating body. He examined one more carefully. The body, that of an elderly man, was floating in a thick yellow liquid. The man was kindly looking, with lots of pleasant laugh lines round his face. His hair seemed unnaturally thick and dark for someone of his age, and his right hand seemed continually to be weaving forward and back, up and down, as if shaking hands with an endless succession of unseen ghosts. He smiled genially, babbled and burbled like a half-sleeping baby, and occasionally seemed to rock very slightly with little tremors of laughter, as if he had just told himself a joke he hadn’t heard before, or didn’t remember properly. Waving, smiling, chortling, with little yellow bubbles beading on his lips, he seemed to inhabit a distant world of simple dreams.
Another terse message suddenly came through his helmet headset. The planet toward which the escape capsule had headed had already been identified. It was in Galactic Sector ZZ9 Plural Z Alpha.
Zaphod found a small speaker by the tank, and turned it on. The man in the yellow liquid was babbling gently about a shining city on a hill.
He also heard the Official from the Safety and Civil Reassurance Administration issue instructions to the effect that the missing escape capsule contained a “Reagan” and that the planet in ZZ9 Plural Z Alpha must be made “Perfectly safe.”
TIME TRAVEL IN THEORY AND PRACTICE
Stan Love
We are all time travellers.
But time travel as it’s commonly practiced is not as much fun as it ought to be. We’re doing it all the time, so it gets monotonous. We can’t control our routing very well, so there are surprises and disappointments. We can’t change our speed, a stately 3600 seconds per hour, except at the infinitesimal end of a long string of decimal places. Not exactly adventure travel. We are all stuck on the same train, and for practical purposes it never speeds up, slows down, or goes backward.
Those are the uninteresting facts as we know them today. But science fiction stirs some fun into the facts by asking, “what if?” What if we could visit the future and see what will become of ourselves and the things we know? Better yet, what if we could travel into the past, view history with our own eyes, and maybe even use the gift of hindsight to adjust events to our advantage?
Those questions have provided fertile ground for writers of short stories, books, and movies, going back at least as far as Mark Twain. An incomplete list of written works on time travel might feature H. G. Wells’s The Time Machine, Robert A. Heinlein’s The Door into Summer, Ray Bradbury’s A Sound of Thunder, Fritz Leiber’s The Big Time, Lester Del Rey’s Tunnel Through Time, Kurt Vonnegut’s Slaughterhouse-Five, Clifford D. Simak’s Mastodonia, Anne McCaffrey’s Dragonflight, Frederik Pohl’s Gateway, Julian May’s Pliocene Exile series, and the literally unsurpassable The Restaurant at the End of the Universe by Douglas Adams. Time travel movies and TV shows include Time Bandits, The Terminator, Back to the Future, Groundhog Day, 12 Monkeys, Meet the Robinsons, The Girl Who Leapt Through Time, a double handful of Star Trek episodes and movies, and decades of Doctor Wh
o.
Plenty of exciting, inspiring, and even funny reading and viewing. But is it all just fantasy? Is our steady journey together into the future really all there is? Might science somehow, someday imitate art and make time travel – real time travel, with the ability to make big changes in speed and direction – possible?
Let’s take a look.
Fast Forward
Travelling forward in time is allowed by physics. And simulating it is downright easy. You can get all the main effects without the need for any special equipment. Just go on a long trip. You’ll return to find a lot of undone work and an overflowing mailbox. Stay away long enough (maybe for an overseas deployment or a prison sentence) and when you return you’ll be disoriented about current events. Technology will have advanced. Everyone you know will have changed, and they may not recognize you.
But time travel the old-fashioned way is too slow to satisfy the purist. Part of the allure of travelling to the future is being able to see it before our friends do, and to get there without ageing. What we want is a shortcut. Fortunately, Albert Einstein showed us not just one but two shortcuts. According to Dr. Einstein, there is a Special way to move quickly into the future, and a General way.
Special Relativity
The classroom clock crawls
As we speed through our studies.
Relativity.
Einstein’s Theory of Special Relativity predicts that things moving at almost the speed of light experience an array of strange effects. Lengths contract. Masses increase. Things that happen simultaneously as seen by one observer happen at different times as seen by another. And, crucially, time slows down.
The Twin Paradox is a famous “thought experiment” that has been used since the early 1900s to illustrate the time-distorting effects of travel at relativistic speeds. In the experiment, which has to be done in thought because we can’t do it for real yet, one twin travels out into space and then back to Earth on a ship moving at relativistic speed. Because of the immense speed she’s travelling at, time runs slow for her. If her brother (they don’t have to be identical twins) watches her through a powerful telescope, he will see her moving in slow motion, the hands of her wall clock turning at a reduced rate, and the light from her reading lamp shifted to longer, redder wavelengths. When she comes back to Earth after her voyage, she will have experienced a fraction of the time that her brother has. She will have aged less than he. She will have effectively travelled into the future.
If you are wondering why that story represents a paradox, bravo for you. It doesn’t. The paradox arises when you consider what the sister sees through her telescope when she looks back at her brother. To her, he is the one who is moving at high speed and whose clock should be running slow. Formally resolving the paradox takes a lot of math: seven pages in my college relativity textbook, the one with the rhinoceroses on the cover. Leaving the calculations as an exercise for the abnormally interested reader, the paradox really does resolve, and the far-travelling twin really does age less than her brother. Robert A. Heinlein’s classic Time for the Stars explores the Twin Paradox in detail, using plenty of actual twins. The story may be getting a little creaky in the joints, but Mr. Heinlein did his physics homework correctly.
The great thing about special relativity is that it is totally fair and square. The theory has been verified by experiment over and over again. Even lettered physicists who love deflating the balloons of science-fiction lovers can’t declare that moving quickly into the future is impossible.
So it’s not impossible... but it is very hard. To dramatically slow down your clock, you must dramatically speed up your self. The speed of light is about 300,000 kilometers per second. To get a meaningful slowing of the clock, you need to go almost as fast as that: say, 240,000 km/s for a time dilation factor of 60%. The fastest speed any human being has ever achieved is about 11 km/s, experienced by the Apollo astronauts whose capsules fell all the way from the Moon. Gaining even that pokey velocity was so difficult and expensive that humanity managed it only a handful of times, back when NASA was enjoying a ten-times-larger share of Federal discretionary spending than it gets today. The fastest we’ve ever made an unpiloted spacecraft go is about 70 km/s, for the Helios solar mission and the Galileo Jupiter entry probe. Neither value is close to 300,000 km/s. In another branch of science, we are able to accelerate things up to within a time-dilated gnat’s eyelash of light speed, but none of those things are bigger than a single atom, and it takes a particle accelerator with the length and power consumption of a small town to do it. If we want to use special relativity for time travel, we’ve got a long way to go in the propulsion department. When we get there, though, we’ll get the stars as a side benefit.
General Relativity
The Theory of General Relativity
Attributes to mass this proclivity:
Sufficient self-gravity
Creates a dark cavity
That holds even light in captivity.
General relativity offers another way to move into the future. Get close to an object with an escape velocity near or equal to the speed of light, and your clock will run slow as seen by an observer out in free space. Again, totally kosher, and confirmed by every experiment that has investigated it, including the recent and exquisitely sensitive Gravity Probe B space mission. Even the gravitational time dilation effect of the Earth, whose 11.2 km/s escape velocity is nowhere near the speed of light, is measurable and known. The GPS unit in your phone has to compensate for general relativity or it wouldn’t work.
But the Earth doesn’t slow time very much. Neither does the Sun, which has a surface escape velocity of about 600 km/s; close flybys yield way more scorched paint than temporal displacement. For a meaningful effect on the flow of time, you need a neutron star or a black hole. These are very unneighborly objects. They raise tides strong enough to rip apart any known material, including specifically your soft pink body. They may feature intense high-energy radiation and magnetic fields strong enough to short-circuit your nervous system. You must approach a black hole or neutron star very closely indeed to make time slow down, and somehow hang out there for a while to let the days add up. Landing is not a survivable option, but perhaps you could enter a low orbit, whipping around the monster hundreds of times a second. You will somehow have to endure the tides and radiation. Then, to enjoy your trip to the future, you must get away again, which takes a vehicle that can overcome that near-lightspeed escape velocity! Compared to special relativity, this approach is messy and risky – but both earn a solid endorsement from physics.
Rewind
Moving forward in time would be great, but moving backward would be even better. Making piles of money on the stock market is just one of the attractive possibilities.
Astronomers know that it is possible, indeed unavoidable, to at least see things as they were in the past. Whenever we aim a telescope at a distant object, we’re looking back into history. Even at its dazzling speed of travel, the light that falls on earthly mirrors and detectors takes time to get here. We see the Moon as it was 1.3 seconds ago, the Sun as it was eight minutes ago, the Andromeda Galaxy as it was two and a half million years ago. Out at the limit of observable space, we can see the afterglow of the Big Bang: the infant universe as it was almost fourteen billion years ago.
But at those distances it’s hard to see any details interesting at a human scale, and we are naturally more interested in our own history than that of a distant galaxy. And many of us won’t be satisfied by just looking. We would much prefer to go in person.
So could we do it for real?
The scientific answer is a definite Maybe.
Fair warning: after this point, things are going to get weird, even according to standards that find relativistic time dilation perfectly normal. Moving forward in time is fully authorized by a mature theory that is backed up by experiment to great accuracy. Backward, not so much. Einstein recognized nothing in his work that supported the possibility of going
back in time.
But Einstein was not the only, nor the last, smart person on Earth.
Among the smartest people currently on Earth is a Caltech professor named Kip Thorne. You may not have heard of him, but you have probably heard of the brilliant wheelchair-bound physicist Stephen Hawking. Kip Thorne makes bets about relativity with Stephen Hawking. Sometimes he wins. Dr. Thorne is the world’s authority on practical time machines. He has written a readable book called Black Holes and Time Warps: Einstein’s Outrageous Legacy. Most of what I know on this topic comes from Dr. Thorne’s lectures and book.
It turns out that there are several theoretical possibilities for making a real time machine. None are supported by experiment, and even the theories are contentious. And let us be clear: the engineering would be incredibly difficult. Even if the theory holds up, we will not be ready to build the first working time machine until a far-off future when our technology is almost unimaginably advanced. We’ll be commuting to work at the relativistic speeds we talked about earlier. Our kids will be terraforming planets for Science Fair projects. But let’s say we’ve gotten that far.
One theoretical possibility is to build a massive cylinder of infinite length (not merely as long as the universe is wide, but infinitely long). We set it rotating about its long axis at nearly the speed of light, and then play tag with it in very capable vehicles. Certain flight paths around the beast return to the same point in space, but at an earlier time. Voilá, a time machine.
But infinite cylinders require infinite budgets, and that’s not the way science funding seems to be headed. We might not have to build one, though. Cosmologists have postulated that similar things might have been produced naturally in the early universe: linear black holes called “cosmic string,” which Earthly astronomers might be able to detect because circles drawn around them have fewer than 360 degrees. (I told you it was going to get weird.) I’m not going to say any more about infinite cylinders here because a different method is cooler and has an interesting connection to science fiction.
The Time Traveller's Almanac Page 28