TIME TRAVEL IN THEORY AND PRACTICE
We are all time travelers.
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 Dragon flight, 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 Who.
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.
Traveling 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 traveling 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.
The classroom clock crawls
As we speed through our studies.
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 traveling 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 traveled 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-traveling 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.
The Theory of General Relativity
Attributes to mass this proclivity:
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-light-speed escape velocity! Compared to special relativity, this approach is messy and risky – but both earn a solid endorsement from physics.
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.
Another writer who famously did his homework was Carl Sagan. For his novel Contact, he wanted a physically plausible way for his heroine to travel to the star Vega and back quickly. He came up with a method that an incredibly advanced alien culture might develop, and mailed a description to Kip Thorne to ask his opinion. Dr. Thorne had a better idea. He shared it with Dr. Sagan, who incorporated it into the book.
Dr. Thorne’s suggestion is known to the initiated as an Einstein-Rosen bridge and to producers and consumers of science fiction as a wormhole. (Check out the Wikipedia article on wormholes for details and pictures, including a formally ray-traced image of a wormhole connecting two places on Earth.) Simply connect the throat of a black hole near Vega with that of another near the Earth, and bingo! We’ve built a shortcut to the stars, and the universe is ours.
But wait, it sounds like we’re talking about faster-than-light travel. What does this have to do with time travel?
Everything. Remember, the central tenet of Einstein’s relativity is that space and time are different aspects of the same fundamental thing. Bend one, and you twist the other.
Dr. Thorne and other theorists have suggested that it might be possible to turn a wormhole into a time machine. Leave one mouth of the tunnel at home, and take the other on a Twin Paradox sortie out into space. The traveling mouth experiences less time than the homebound one. When it returns, you can enter the latter and come out the former in the past. It’s a bona fide time machine. (Physicists use the term “closed timelike curves” when discussing them in print, in an attempt to head off media headlines screaming about scientists inventing time machines.)
The wormhole time machine is limited. It’s hard to adjust the time interval between the two mouths. You can do it only by taking one mouth or the other on a high-speed jaunt. And you can never go back to a time before you built the wormhole, a disappointment to people interested in altering the outcomes of still-earlier elections, sporting events, or armed conflicts. But you could still use it to make a fortune on Wall Street, or assassinate an ancestor and finally put to rest all philosophical posturing about the Grandfather Paradox.
General relativity may allow for the possibility of wormholes, but that doesn’t mean they’re a done deal. There are some construction challenges we don’t yet know how to overcome. First, every normal black hole contains an evil singularity in the center. Anything that crosses the hole’s horizon must fall into that singularity, be disrupted by it, and become one with it. Ouch. Next, there is not an obvious way to coax two black holes to connect with one another. Finally, theorists predict that if two holes are somehow spliced together, the resulting tunnel will pinch itself off before anything could pass through. Considerable intellectual energy has been invested in these topics, though, and there could be a way to solve them.
It might be possible to make two connected and singularity-free black holes out of something besides ordinary mass, which could then counteract the natural tendency of the tunnel to collapse. Theory suggests that this requirement would be met by a substance with negative mass and negative pressure. That’s right: to build a traversable wormhole, we’ll need to use something that weighs less than nothing and is emptier than a perfect vacuum. (Did you not believe me when I said it was going to get weird?) Engineers joke about “unobtainium” for applications that demand materials with unrealistic physical properties, but this stuff takes the cake.
Yet off in the fringes of physics there do seem to be things that exert negative pressure. The mysterious “dark energy” that is accelerating the expansion of our universe against the pull of its own gravity might be one. Another is the Casimir effect. It may be possible to build a traversable wormhole using the Casimir effect, so it’s worth covering here.
Physicists believe that at the tiniest-size scales and the briefest flickers of time, our universe is a seething froth of instability, constantly creating pairs of subatomic particles that recombine and vanish before they can be detected. These are called “virtual particles.” Among the virtual particles are photons, the wiggles of electricity and magnetism that make up light, radio, X-rays, and so on. Photons both real and virtual cannot travel very well through electrical conductors such as metals. So if we take two very smooth flat metal surfaces, and place them very close together, they’ll suppress the creation of virtual photons with a wavelength longer than the separation between them. But outside the plates are virtual photons of all wavelengths, which exert a tiny bit more radiation pressure on the back sides of the plates than does the restricted range of wavelengths available between them. If all of this weirdness is really true, then there should be a very tiny force – effectively a negative pressure – pushing the plates toward one another.
This force exists and has been measured in experiments.
There are some difficulties with building wormholes using the Casimir effect. It operates only over very short distances. Also it’s rather weak. It’s not as heavy a hammer for knocking holes in spacetime as, say, the collapsing core of a massive star. But if our kids are terraforming planets and we don’t want to be outdone, we should go for it. We start by building a spherical metal shell with the diameter of the orbit of Pluto, a supersized Dyson sphere. We then build another one surrounding the first, carefully maintaining the gap between them at one Ångstrom unit, roughly the size of an atom. If we accomplish these things, says Dr. Thorne, the Casimir effect will warp space so that we will no longer be able to tell which sphere is the inner one and which is the outer. We will have built a wormhole that allows us to travel the massive distance of one ten-billionth of a meter. Not a practical transportation device, unfortunately. But it’s a real wormhole, and by sending one end on a high-speed trip we might possibly be able to turn it into a real time machine.
Back to the Present
Unfortunately, our own less-than-incredibly-advanced culture won’t be building Matryoshka-doll Dyson spheres and accelerating them to relativistic speeds any time soon. But that doesn’t diminish the appeal of time travel. It remains a fruitful topic for both science fiction and theoretical physics. As in the case of Contact, sometimes the interplay between the two helps make both stronger. And as our train moves inexorably forward at 3600 seconds per hour, the day when we can engineer time machines must be moving just as inexorably closer. Maybe, somewhere up the track, they’re sending people even further along, to still more distant futures where they can send people back.