Time travel has spawned legions of sci-fi fans who wish rocketing back to the past or forward to the future were something more than just fiction, but it may only be perceived that way because we haven’t taken a ride with the Doctor. Yet.
Traversing through time may not be so alien when you realize how it could be powered by physics. General relativity and quantum theory have much to say about what the universe possibly would and wouldn’t allow in terms of defying the passage of time as we know it. Some theories are just on the edge of reality. Some are so exotic that the only place they exist (for now) is on paper. There are even types of time travel—on a micro scale—that have already happened, such as the manipulation of gravity that can send subatomic particles into the future.
Set your warp drive to the speed of light and travel forward (or backward) through these seven possibilities that seem so out there they could be fiction, but are actually science.
Turning quantum mechanics inside out
What if time was a crystalline structure in which different versions of you were forever trapped? While this sounds like a creepy phenomenon from The Dark Tower, the thought of different freeze-frames of yourself preserved somewhere in time is not so sci-fi to some theoretical physicists. Quantum physics views time as a series of individual static units. These are like animation cells that don’t move on their own, but give the illusion of time moving forward when viewed as a series—theoretically making time travel possible if one of these cells was somehow identified in the future.
What stands in the way of hypothetical trips forward in time is the Planck constant, which argues that the ultimate limit units of time can shrink to is 10-43 seconds, which makes it impossible to separate any two events by less. You just know someone is going to find a crack in a seemingly rigid theory like this, and someone did. No one said anything about events being separated by larger units of time until Mir Faizal threatened to flip quantum mechanics on its head.
Though Planck time is too fleeting for its existence to be proven by any experiment, Faizal insists that there are methods which can reveal a unit of time to exceed Planck time. Never mind that you would have to do things like measure the spontaneous rate of emission of a hydrogen atom to convince any raised eyebrows that the revamped theory is science and not fiction.
“If this view is taken seriously, then our conscious precipitation of physical reality based on continuous motion becomes an illusion,” insisted Faizal, echoing Plato by saying that “true reality exists independent of our senses.”
Mind (and senses) blown.
Hurtling through space at the speed of light
Say you blasted off into deep space at warp speed while another astronaut stayed grounded in mission control. You plunged into the darkness for what your watch measured as a decade, so you’ve predictably aged that much without plastic surgery. Meanwhile, your colleague is shocked when you finally put a boot on terra firma and take off your helmet to reveal you’ve actually aged 32 years. If you were preparing for takeoff when SONG first invaded the radio, you’d be shocked to hear it on the retro rock station now.
The reason you’d hypothetically end up that much older on Earth is because time is relative rather than absolute. You would have been travelling with relativistic speed, and if someone on the home planet was actually able to observe your spaceship through the lens of a telescope, the observer would end up with one Earth time, and one “shooting across the night sky at an unimaginable speed” time that would register as significantly earlier. Neither one is an error. Moving at extremely high speeds has that effect because time measured by a static observer increases as velocity skyrockets.
This actually happened to ISS cosmonaut Sergei Krikalev, who holds the record for spending the longest time ever in space, except that instead of aging 10 years from existing in the bubble of a high-speed facility for over 800 days, he aged a grand total of .02 seconds when he landed back on Earth. Time runs a fraction of a millisecond faster on an orbiting space station that zooms around at close to 5 miles per second. The speed of light is 186,282 miles per second.
So we still have a way to go before we reach the speed of light.
You can probably hear Morgan Freeman’s otherworldly voice echoing through the cosmos already at the mention mention of these shortcuts through spacetime, but the possibility for their existence extends beyond your TV screen. But if they exist, wormholes probably aren’t the gaping chasms that haunt science fiction. Most scientists who theorize they are somewhere out there also believe that these shortcuts through spacetime are infinitesimal. Astrophysicist Eric W. Davis challenges them to think in another dimension.
"You can go into the future or into the past using traversable wormholes," said Davis, insisting that physicists have become so obsessed with time machines that theories in the realm of spacetime geometry (which can offer plenty of time-machine phenomena) have almost fallen into oblivion. He also believes they are unavoidable in spacetime.
Before you start scanning the universe for the nearest exit to nobody-knows-where, the other side of such theories is nebulous. Even Davis admits that making travel-via-wormhole actually possible would require an unearthly level of effort. It would take an immense amount of exotic matter to stabilize a wormhole enough for travel. Except science doesn’t understand exotic matter well enough for that yet. Not to mention that while microscopic amounts have been observed, its very existence contradicts general relativity. Then there’s the issue of a quantum back reaction. Turn a wormhole into a time machine, and the energy buildup would be so intense that it would obliterate the wormhole before you even had a chance to be trebucheted through time.
So with Einstein tossing in his grave and quantum physics shaking its proverbial head, don’t expect to travel through the wormhole anytime soon.
Taking advantage of defects in spacetime
The aftermath of the Big Bang may have left some strange anomalies in spacetime. Cosmic strings are one-dimensional defects believed (by those who do believe) to have formed during the dawn of the universe, when regions of spacetime underwent phase changes during which energy accumulated at their boundaries, kind of like the cracks in a mystic quartz crystal. No one has actually ever proven their existence, but these invisible imperfections could theoretically catapult you back in time to an altogether alien when and where.
If they do exist, cosmic stings won’t exactly be the easiest objects in space to observe. They are theorized to have the diameter of a proton, and be so dense that it would only take about a mile in length for one to outweigh our planet. This level of density would normally mean intense gravitational forces, but general relativity assures anyone or anything that comes too close to a straight cosmic string will not be shredded to space confetti, because said gravity would supposedly be nonexistent other than the relative deflection of light or matter passing by from the opposite side. Closed cosmic string loops are a different animal. Galactic superclusters were supposedly formed by the pull of their immense gravity throwing matter together with abandon.
There have been some scientists who could have sworn to have found evidence for cosmic strings because of bizarre phenomena in the universe. These include a supposed double quasar and a duo of galaxy images that looked like identical glowing Rorschach blots. The images were determined to be of two different, albeit eerily similar, galaxies, and the question mark on the quasar was erased when it was found that gravitational lensing was responsible for the double vision.
Plunging headfirst into a black hole
Whatever is on the other side of a black hole, you’d never get there in one piece, if any molecule of you even got there at all. “Disemboweled” is an understatement. That’s at least the thinking behind what would happen if you or anything else ventured past the event horizon of a conventional black hole and met your doom at the hands of the immense gravitational forces within. Kerr black holes are anything but conventional.
There is no escape from a black hole’s singularity—that core of unfathomable density that you see in every rendering of a black hole that ever existed—but mathematician Roy Kerr believed there was literally a way around it. Kerr’s proposal was that matter could travel through a black hole without being confetti-fied if that black hole was rotating ridiculously fast. If such a thing exists, it is thought to be the corpse of a neutron star that collapsed into a halo of rotating neutrons rather than the usual ultra-dense singularity. The centrifugal force generated by these neutrons would prevent the singularity from forming and thus give you passage to the other side.
Assuming you remained intact and with all your brain cells after that epic journey, you would be spewed out by a hypothetical “white hole” in some unknown corner of space. No control over this thing means it would be impossible to tell into which where and when you’d emerge. Whether you would end up forwarded a few minutes or centuries into the future, in a parallel universe or just a few light-years away, remains to be seen until someone develops a method to manipulate the white hole, which is impossible until we can prove one actually exists.
Strapping yourself into the universe’s infinite thrill ride
You know how those spiral water slides that seem to go on forever until you finally end up crashing with a great gush of poolwater that may or may not go up your nose? Or roller coasters that basically do the same thing a hundred feet in the air? Space can be a vast amusement park, and the infinite cylinder is the theoretical physicist’s version of the ultimate thrill ride.
Dreamed into theory by Frank Tipler, an infinitely long cylinder that warps spacetime as it whirls and twirls like crazy on its longitudinal axis would supposedly create a frame-dragging effect that tilts the light cones of nearby objects. On a spacetime diagram, these light cones would partly turn on the time axis to point backwards. Try to absorb that for a second. Now imagine a rapidly accelerating spaceship spiraling in the same direction, along a closed timelike curve (CTC) that catapults it back in time if it travels in the same direction the light cones are pointing toward. Meaning, you could theoretically be a hero or knife a villain and change the course of history when you disembarked.
CTCs materialize in some tentacles of general relativity, such as the aforementioned Kerr black hole, and freak most physicists out just about as much as Cthulhu. There is no way we can build something infinite if we can’t measure infinity. The possibility that a cylinder of finite length could churn out CTCs has never been proven. Stephen Hawking argued against the possibility of an infinite cylinder as a portal to the past, because building one in a region with no exotic matter containing negative energy is proved impossible by general relativity. No negative energy, no time machine.
...and the TARDIS
Yes, the acronym for Traversable Acausal Retrograde Domain in Spacetime is TARDIS. No, it has nothing to do with Gallifrey or a blue phone booth.
Rebel mathematician Ben Tippett and astrophysicist David Tsang are convinced of the possibility that there are more than three spatial dimensions to the universe. Their hybrid theory splices together general relativity and the idea that time is actually the fourth dimension of our universe. General relativity predicts that gravity is exerted on passing objects by the same curvature that proves planets orbit stars—if it didn’t exist, star systems would be arranged in straight lines. Because spacetime is curved, theory makes it possible to bend that curve into a loop by simultaneously running time along it. Do that and you’ll defy time.
Tippet and Tsang didn’t need a sonic screwdriver to build a model time machine that was actually able to accomplish this mind-bending bend by using the spacetime curve to their advantage. While not bigger on the inside, their machine creates that elusive loop which could hypothetically take anything moving along it backwards through time. Passengers would even be seen moving in reverse. If you were that passenger, you would see a timeshifted (from a previous time) doppelgänger of you in the same exact box, doing everything you’ve been doing, except backwards. There would be no chance of this happening if time and space were separate.
Unfortunately, Tippet doubts that such a machine could ever exist, because reverse time travel risks opening holes in the universe that could possibly collapse into black holes because of instabilities. Nobody would want to risk being shredded by the gravitational forces of a black hole’s singularity, even for the chance to travel back in time. I’m sorry, I’m so sorry.