My prior post but one dealt with time travel to the future. This one will consider ways to travel in time not restricted to strictly future-ward possibilities – ways with some scientific credentials and ways with supported only by the science of fiction.
Time Machines
Fiction’s first known time machine is that of H.G. Wells story “The Chronic Argonauts,” which appeared in a student publication in 1888. It was reworked into “The Time Machine” in 1895. Almost all the time machines that owe their existence to writers since have followed Wells in saying little about how their devices work. Wells’s mention of time as a fourth dimension is updated by talk of “space-time.” Sometimes wormholes are said to be involved, and at least one fictional time machine was equipped with a built-in Tipler cylinder generator (of both, more shortly). Part of a time machine film budget goes into the constructing of the machine. Lots of moving parts, spinning wheels, and flashing lights are traditional. The time displacement devices in Terminator franchise are prime examples. The History of Time Travel takes the opposite tack showing a machine it describes as looking like two VCRs, one atop the other, instrumented by an Atari control knob. Then there are also, inter alia, hand held devices,1920 Peugeots, horse drawn carriages, New York subways, DeLoreans, antique dresses, and photo booths.
Wormholes
Wormholes, as everyone knows, connect two different regions of space-time thus, in principle, making a travel shortcut, perhaps even to another universe. There are wormhole friendly solutions to the equations of general relativity. Conceivably, they may be in actual existence all over the place. The problem with the ones with the highest probability of having the desirable property of actual existence is that they are very, very small, down at the “quantum froth” level, and have a life expectancy of very nearly nothing. There are theoretical ideas for sustaining more practicable sized wormholes, but, among other problems, the exotic materials required to sustain them – negative mass, negative energy – are not in evidence.
Moreover, at the current state of wormhole theory, they could only take us to the past, not the future, and only so far into the past as the date on which the wormhole originated.
Still, wormholes are one of the better hopes for heavy duty time travel, and so have been a mainstay of science fiction since 1931 (Jack Williamson, The Meteor Girl). The fiction has been much more generous to wormholes than the science, sometimes permitting travel to the distant past and future. Surmounted are all the wormhole problems – theoretical, engineering, and financial. At least there have been such fictional successes if you count vague, often silly and sometimes incoherent explanations of wormhole dynamics. Owing, no doubt, to the spectacular computer generated graphics to which they lend themselves, wormholes turn up in many of the most popular science fiction films.
Spaceships often seem to come with an app for generating wormholes which the ship then passes through. Typically the ship’s destination is to some reasonable degree in the captain’s control. Other artificial wormholes are generated at a fixed portal. The exit end may also be fixed or may be selectable by the masters of the entrance portal. A little more on portals below. Occasionally fictional wormholes are natural occurrences either discovered or fallen into by accident. They may or may not be well behaved with the respect to the location of their exits.
Storms
Really big storms (wormholes meet seriously low pressure?) can pick up even large items and drop them in another time as when the aircraft carrier Nimitz (launched 1972) gives the Japanese a surprise at Pearl Harbor in The Final Countdown.
Portals
A portal to the past or future in fiction may be artificially created or natural. If there is device that creates it or keeps it open, it is not interestingly different from a time machine except for its lack of mobility. Most time machines can make multi-stop trips. With an artificial portal you usually have only the choice of remaining in your new time or returning to the old.
Natural portals or portals created and left in place by unknown others are common. Portals often connect two times that remain at a fixed temporal distance apart. In Outlander, when, after 20 years back in the 20th century, Claire returns to the 18th, Jamie is 20 years older. Brianna and Roger separately go through to find Claire and Jamie to be just the age they would be had all been on a single timeline. Their transits were all the same: about 202 years. Gellis, by contrast, traveled from 1968 back to the 1730s. The Door (Die Tür) a 2009 German thriller features a suburban tunnel with ends 5 years apart.
Even a portal that was continuously in operation need not have its two ends marching through time at the same rate. More generally, both ends of a portal might jump about in time, although that might make the fictional science, engineering, and plot line a little difficult.
Some portals have special conditions. In Outlander the traveler must have the right genes, and travel is easiest at a solstice, and with the help of gems, fire, or blood sacrifice.
Gödel Universes
There is a respect in which Kurt Gödel was the father of time travel science. He was the first person who gave a serious physical model with closed time-like curves (CTCs), a phrase he coined. A CTC is a path that returns to its starting place and time. He showed the possibility of universes with CTCs that satisfy the equations of general relativity, and so “physically possible” in this important respect. In fact, in a Gödel universe CTCs go through every point of space-time.
Gödel was not, however, much interested in the possibility of time travel. You could not simply stroll along a CTC. To navigate one would require a speed of greater than 70 percent of the speed of light, and to get yourself going that fast would require more than the mass of your spaceship even if all that mass were converted to energy. (Electromagnetic effects, in particular the Lorenz force, might get around this speed-fuel problem, and permit slower CTC travel by a properly charged time traveler.)
A bigger problem for any hope that we might use Gödel time travel is that it only works in a Gödel universe – one that, inter alia, rotates in the proper way, is not expanding, and is infinite. Infinity has not been ruled out by observation, but non-expansion has long been.
If you are at all into mathematical logic, you know that Gödel is regarded as a genius of the first water. (“The greatest logician since Aristotle.”) When he and Einstein were at the Princeton Institute for Advanced Study, Einstein once said that he used to come to his office, “just for the privilege of walking home with Kurt Gödel.” Given his star power and that his CTCs are compatible with general relativity, it is surprising how rarely Gödel time travel appears in the fiction. The sole instance I have found briefly mentions a spinning universe and its CTCs, without mentioning Gödel. It is the quasi-professional short YouTube video “Live and Let Die.”
Frame dragging and Tipler cylinders
“Frame dragging” is a general relativity phenomenon in which a massive rotating body “distorts” space- time. With the help of exquisitely sensitive instrumentation it has now been empirically confirmed for the Earth, an astrophysical featherweight and not a fast rotator. A black hole in rapid rotation could in principle cause a distortion creating CTCs of practical size. Rotating black holes have the advantage of being actual, unlike Gödel’s rotating universes. Black holes have their own disadvantages, however. One calculation had the CTCs running through the heart of the hole, an unappealing itinerary.
Apparently, two massive black holes nearly merging also create CTCs even without merging. I am not sure how easy the ship engineering and navigating would be, but it sounds like a dangerous neighborhood.
CTCs without the inconveniences of black holes can be achieved with a Tipler cylinder. A quickly rotating cylinder and a strongly accelerating spaceship to follow the CTC made available by the rotational dragging of space-time. The original model involved a cylinder of infinite length, which would be a tough construction. It has been suggested that a sufficiently long cylinder would do the trick, but Hawking seems to have shown that a finite cylinder would only work if negative energy were available, which, again, it apparently isn’t. Even if this problem were finessed, a finite cylinder might be unable to take you farther back in time than the construction date of the cylinder. Despite these drawbacks, the name or general idea of Tipler cylinders has found its way into a couple of handfuls of science fiction works.
Personal power or susceptibility
Some fictional characters can travel in time at will, or with particular limitations, and some find themselves time traveling whether they will or not. Henry in The Time Traveler’s Wife is a paradigm of the “or not” variety. He transits unexpectedly and for unpredictable durations to places and times beyond his conscious control, though generally having some connection to his life and desires, conscious or unconscious. In Henry’s case the travel is paradigmatically personal. He can take nothing with him and arrives naked, a serious inconvenience when the space-time destination is Michigan in the winter. His daughter Alba has the time travel gene, but in her case it expresses itself more as power than susceptibility.
Characters for whom time travel is clearly susceptibility rather than power are Bill Murray’s Phil in Groundhog Day and Sandra Bullock’s, Linda, in Premonition.
Time travel at will is also sometimes genetic, for example, in It’s About Time, it’s a trait that is sex linked. The males of the family need only a dark closet, clenched fists, and a determination to revisit a place and time in their past, and there they are. As in this case, an exercise of will is often the only requisite for the appropriately gifted to travel in time. Sometimes, the will is aided by hypnosis or some other form of suggestion.
Substances into the blood stream
Fictional time travel power or susceptibility is sometimes facilitated by ingesting something. In Synchronic it’s a designer drug marketed as a psychedelic. Inhalation can also do it, as in a science lab lavender smell in The Girl Who Leapt Through Time.
Talismans or tokens
Objects worn or carried by traveler are at least a chief part of the workings of some time travel fiction, for example, the ruby amulet, in the novel of that title by Belinda Murrell and an old key in the Turkish video series Midnight at the Pera Palace. We are left in the dark as to how these talismans achieve their space-time effects.
Trans-temporal communication
Face-timing to a different time has to be regarded as a fiction a little closer to scientific possibility than taking a holiday in the past. We don’t have to worry about fuel consumption to get a photon to go fast. (It may be a problem, however, that they only go fast, and never accelerate.) In any event the electronic communications across time of Durante la tormenta (Mirage) and Frequency, touched on in my 1/13/22 post, do not require too great a suspension of dis-belief. The camera in Time Lapse that takes pictures a day into the future may only co-opt the normal path of photons.
That physical letters placed in a mailbox or an old desk could pass, as in The Lake House from 2004 to 2006 and from 2006 to 2004 or in The Love Letter between the 19th and 21st centuries. The engineering of the mailbox and desk and the science behind them remain unexplained. Moving physical letters through time has got to be more complicated than getting radio waves to do the same trick.
Guides
If Scrooge's trips to those other December 25ths were real, and not dreams, then it was the power of the Ghosts of Christmas Past and Future that made them possible. The Tale would then be in the "First Real Time Travel" competition.
Unexplained
Sometimes time glitches just happen in fiction. In Donnie Darko the creation and breakthrough of a tangent universe, leading to a 28 day repeat, is only hinted at and neither Donnie nor we have inkling why.
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