Copyright � 2026 by Shane
Tourtellotte
The desire to travel across time has been with humanity probably since we first understood the concepts of past and future. The point when we progressed from inchoate wishing to a more concrete imagining of how we could accomplish this is not well pinned down, and in practical terms doesn’t matter much. Any ideas by, say, an ancient Greek about how to travel through time would have been even more uninformed than his ideas about how to fly to the Moon.
We began getting more practical by the 19th century, with the idea of time as a dimension akin to the spatial ones. H.G. Wells could claim some scientific verisimilitude for his concept of The Time Machine. Though Wells wouldn’t impress today’s physicists with his light gloss of science, he was still far ahead of every other writer imagining a way to travel across time. They might as well have been back in ancient Athens.
One common method was simply to traverse a strange patch of ground, perhaps misty, perhaps dark, and find oneself magically transported to another era. (Readers of a certain age who recall the Choose Your Own Adventure books just had a rush of nostalgia, mixed with a slight urge to go spelunking.) No explanation is given; no mechanism is proposed. It just happens, so the writer can get on with the story.
Another method that didn’t require any serendipitous discoveries was wishful thinking. Technically it was auto-hypnosis, but it amounts to the same thing. Tell yourself that you’re traveling to the past, or the future, convincingly enough, and you will do it. This has been used in a few novels, one by noted science fiction author Richard Matheson which was adapted into a 1980 movie1.
You are welcome to attempt this method, as its potential for harm is slight. The equivalent idea of believing you’re a pterodactyl so you can fly, I cannot shrug off as disinterestedly. Whoever is below you when you make your attempt deserves better.
You may also try the related traumatic method of time-travel, in which some violent impact could send a recipient reeling through time as much as through space. Mark Twain notably used this for A Connecticut Yankee in King Arthur’s Court, but one doubts he cared about the science as much as having some excuse for the story he wanted to tell. Oddly, this concept also endured well into the 20th century, with no less an author than Robert Heinlein employing it to have a hit from a Soviet nuclear weapon in World War III blast a bomb shelter more than two millennia into the future.
There is also the well-trodden method of sleeping one’s way into the future. Whether Rip Van Winkle or Buck Rogers, a character could bypass decades or centuries by not being conscious for them (although Van Winkle did inconveniently age while he was asleep). The idea of cryogenic hibernation gave this method a scientific grounding and a fresh life, and the time dilation aspects of relativity made it stronger still, without even needing to sleep through the intervening time. Those discoveries could not remove the great inconvenience of it being a one-way trip, with no way to reverse the process and return to the present.
While writers of science fiction and fantasy still make up their methods for time travel, serious scientists have been getting into the game for a while. Their rules are different. They want something that works in reality, not fiction. The theories they’ve produced over the last several decades have sprung from physics rather than sheer imaginative invention. This doesn’t make them less fantastic, just fantastic in a different sense.
One of the theories revolves, unsurprisingly, around black holes2. A rotating black hole is required, but since whatever body collapsed to produce the black hole was liable to have some angular momentum, any randomly chosen black hole should qualify. Most people with a moderate knowledge of the physics of black holes know that at the center of a black hole is a singularity, the point where all the matter comprising the body collapsed to a single point of infinite density. This, however, is a simplified model, based on a non-rotating black hole, the type that I just noted would be quite uncommon.
A rotating black hole (called a Kerr hole after a mathematician who studied them) would have a ring singularity at its core. Some solutions to gravitational field equations suggest that the ring could act as a portal to regions of space-time otherwise inaccessible to us. Some could be different universes; others could be the past or the future of our universe. A ship flying through the ring singularity at the heart of a black hole could, in theory, travel back or forward in time.
This is an amazing theory, but in practical terms there is so much wrong with it that it’s hard to know where to start. I’ll begin by noting there’s a very broad range of possible results for this experiment, and no way of knowing which one you’ll get. Getting a specific answer would probably require solving some very complicated equations of quantum gravity. We don’t have a theory of quantum gravity yet, and may never have one.
Assume the equations do get solved, and we can figure out where and when passing through the ring singularity would take us. That seems to leave us with one destination per black hole, which though it technically might be time travel isn’t the kind we were hoping for. It could be that differences in speed and angle when reaching the ring singularity could change the destination, in both space and time. Good luck maintaining so precise a course while plunging through the extreme gravitational forces of a black hole.
This raises my next objection: you’re required to fly into a black hole. Those gravitational forces I just mentioned have an important element: they’re stronger the closer you get to the singularity. A black hole’s gravity is so strong that, close in, there will be a notably stronger pull on the front of something entering it than its back. Strong enough to stretch, then tear apart, that object, such as a spaceship, or a person inside the spaceship. You might still reach your destination time -- as a spaghettified debris field.
All of this is pretty awful, but it doesn’t really matter, because you won’t be going to that black hole in the first place. Our farthest-flung spaceship has traveled, over several decades, less than one-tenth of one percent of the distance to the star nearest to our solar system. The closest black hole we’ve detected is more than 350 times that distance away. With any technology we have or could soon have, you’re never going to go diving into any black holes, no matter how much they’re spinning3.
The physicists don’t mind if that method is impractical. They have others that are even more impractical. Take the cosmic string idea. Some postulate that if two cosmic strings, moving close to the speed of light, were to pass on parallel courses very close to each other, the spacetime between them could be warped enough to encircle the strings with closed timelike curves. This could allow you, by flying around the string in the right place, to travel in time, as far back as the moment when the timelike path was created, which might have been a very long time ago.
How can you tell when such a curve around the string was created? How can you tell whether such a curve exists around that string? Don’t bother the theorists with such details, or silliness like how you’re supposed to catch up to a cosmic string going close to the speed of light. And don’t even think of mentioning that cosmic strings are still wholly theoretical entities. We’ve detected black holes, but for cosmic strings, we need to trust the equations, which not even all theoretical physicists do.
By now, you’re probably wishing for something more tangible, more concrete in time travel methods. The theorizers can accommodate you, with the Tipler cylinder.
Imagine a cylinder of infinite length, made of hyper-dense material far heavier than anything we have on Earth, spinning on its long axis with a surface speed of at least half the speed of light. The motion of this object could create a closed timelike path on its surface, on which you could move to travel backward in time, as far back as the creation of the cylinder, and then return to your own time.
The obvious objection -- at least the most obvious one -- is needing an infinitely long cylinder. Frank Tipler, who wrote the paper postulating this method, was able only to prove his calculations for a cylinder of infinite length, but he did think a finite cylinder could suffice. Further estimates put the minimum length-to-radius ratio at 10:1 to achieve a closed timelike curve.
There are still theoretical snags. A cylinder of the necessary size might not be able to support itself against the gravity of its own hyperdense material, and could collapse into a sphere, or possibly even a black hole4. If it did hold up, the centrifugal force at the spinning surface would be, for a cylinder twenty kilometers in diameter, a couple hundred billion times the force of gravity on Earth. A cylinder that didn’t collapse on itself might instead hurl itself apart in a trillion shattered pieces. Even if it didn’t, how are you going to hold onto that surface and move across it for the time-travel effect?
(Before I continue, I have to insert this digression somewhere. A recent big-budget film featured a time-travel device made by an ancient Greek -- them again! -- that, when activated, brought protagonists from the 20th century back to the 3rd century BC. It was good for no other destination in time, just the point at which it was created. This struck numerous viewers and reviewers as a strange contrivance, but it actually meshes with the theory underpinning the cosmic string and Tipler cylinder ideas, in which a time machine in the form of a closed timelike loop can only take you back as far as when the loop was created. I don’t know whether this was some screenwriter knowing actual time-travel theory and having the guts to incorporate it into a Hollywood action-adventure movie, or a mere coincidence. I do know that it intrigued and amused me. That suffices.)
The theories of the physicists are remarkable and even beautiful in their way. All the equations balance out. The one primary thing they lack is any correspondence to a human scale. They require things we cannot build, crossing distances we cannot bridge, and finding primordial marvels that may not even exist. It can be done, they tell us. It just can’t be done by us.
There is another theory they had that would permit time travel on a scale that we could put to practical use. Almost inevitably, they throw the most obstacles into the path of this idea. They make enormous difficulties seem almost airy when talking about infinite cylinders and ring singularities, but when the problem approaches a human scale they turn concrete and seemingly insurmountable.
I am talking, of course, about wormholes, which require you to do six impossible things before breakfast if you want to make any use of them. Naturally, the experts got confounded, and we got the time machine.
Footnotes:
Coincidentally, the lead actor in this movie had used another time-travel method in a release two years earlier, flying around the Earth so fast he reversed its rotation and turned back time. You may decide for yourself which technique is more plausible.
No modern theory in advanced physics or cosmology can go very far without invoking them. Black holes’ ability to suck in everything is not limited to mere matter and energy.
If we did somehow come up with the faster-than-light travel necessary to visit those black holes, I strongly suspect those spaceships would have higher-priority uses. For a short rundown of them, watch the opening credits of Star Trek.
Before you suggest it, no, the resultant black hole wouldn’t be big enough to travel through for the other method of time travel, unless you really overbuilt on the original cylinder.
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