The bartender says: 鈥淲e don鈥檛 serve time travellers in here.鈥� A time traveller walks into a bar.
OK, yes, I will almost certainly regret starting this article with such a hoary old joke. At some point, most of us have wished we could travel back in time to repair some misstep or faux pas. But it鈥檚 impossible, right?
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Well, not necessarily. Albert Einstein鈥檚 general theory of relativity suggests that time travel may indeed be possible. We know that matter can bend space-time, and if you bend it enough, you may be able to create a time loop. Caveats abound, of course, and researchers are yet to announce a working time machine. But that hasn鈥檛 put them off exploring the possibilities.
Here are five ways that time travel might just be feasible, from science fiction staples to surprising new ideas 鈥� as well as some pesky practical obstacles we would need to overcome.
1. Rig up a galactic ring of lasers
The main problem with time travel is that nothing can travel faster than the speed of light, which clocks in at 299,792,458 metres per second. This speed limit preserves causality, the idea that cause must always come before effect. Travelling faster than the speed of light would scramble that, thanks to a quirk of Einstein鈥檚 special relativity and the fact that space and time are inextricably connected. If we could go faster than light, we would be travelling back in time. But we can鈥檛.
The next best thing, then, is to manipulate the fabric of space-time. at the University of Connecticut found that a rotating ring of lasers may be able to warp space-time so much that it curves in on itself, creating a This works because light can produce a gravitational pull just like matter can. 鈥淭he rotating light would pull space-time along with it,鈥� says at Tufts University in Massachusetts, who wasn鈥檛 involved in the research.
Sadly, though, it isn鈥檛 just a case of stringing together a bunch of laser pointers 鈥� your laser ring would need to be of galactic proportions. Plus, even if that were possible, such a cosmic laser vortex would create a singularity, a point where space-time is so warped that the laws of physics break down, says Olum.
That would be a problem. 鈥淲hen you allow singularities, all hell breaks loose and anything is possible,鈥� says at Imperial College London. Physicists don鈥檛 understand singularities, but what is clear is that we would be unlikely to survive an encounter with one.
2. Swing on a cosmic string
On to the next idea, then, which could at least be survivable, in theory. This one involves hypothetical filaments running through the fabric of space-time, called cosmic strings, and some seriously strange matter.
The idea is that cosmic strings could have been created moments after the big bang as the newborn universe changed phase 鈥� just like water does when it freezes 鈥� from higher to lower energy. If they exist, and we are yet to confirm that, they would essentially be long, thin strands of extremely high energy density left behind from the fiery first throes of the universe.
In 1991, J. Richard Gott at Princeton University calculated that two moving, infinitely long cosmic strings , if they were oriented correctly. The time loop would arise as the strings crossed each other while moving in opposite directions. To go back in time, you would have to travel around them at 99.99 per cent the speed of light. Simple!
Or not. 鈥淵ou could not construct such a time machine,鈥� says Olum, who has demonstrated that this would require infinite amounts of matter with negative energy density, meaning the matter would have less energy than empty space. Now, we have produced this strange form of matter in experiments at the quantum scale, but not in anywhere near the sorts of volumes we would need.
Such a cosmic laser vortex would create a singularity. That would be a problem
鈥淚t creates only a very small amount,鈥� says at Queen Mary University of London. What鈥檚 more, she says, it requires huge amounts of positive energy to maintain the effect, so creating a macroscopic amount may not be plausible, let alone an infinite supply.
3. Wangle a warp drive
So far, our putative time machines appear to be plagued by practical snags. Would that also be true of the most famous mode of time travel, the warp drives popularised by Star Trek, which would let us travel faster than light after all?
A warp drive is a hypothetical engine that creates its own personal bubble of space-time. They don鈥檛 require any kind of traditional engine or propulsion, relying instead on compressing space-time itself. The engine would consist of a shell of matter that could somehow shrink the space-time ahead of it while expanding that behind, changing its location without technically moving.
Here鈥檚 the thing: the cosmic speed limit only applies to objects moving through space-time, so a spacecraft within the safety of a warp drive bubble could reach speeds faster than light without breaking the laws of physics. Moreover, it wouldn鈥檛 kill you 鈥� you wouldn鈥檛 even feel it, says physicist , who studied warp drives while at the University of Alabama in Huntsville.
That would allow for time travel, in principle. Alas, as with the cosmic string time machine, a warp drive that could enable this faster-than-light travel would require huge amounts of
4. Jump through a wormhole
Maybe this whole project would be easier if, instead of trying to build your own time machine, you used one lying around in the universe: a wormhole.
These hypothetical tunnels through space-time have two or more connecting mouths, usually black holes. If you could travel through the tunnel, you could theoretically end up in a distant place both in space and time.
The good news is that traversable wormholes 鈥渃ould already exist in our universe鈥�, says Wiseman. The most plausible way one might arise, without the need for negative energy, is between two tiny, charged black holes orbiting each other. Unfortunately, there鈥檚 another snag: the tunnel would be subatomic, much too small for a human to enter, so it probably isn鈥檛 the best mechanism for time travel. And even if we found a wormhole we could squeeze through, it is unclear whether we would survive the journey 鈥� the incredible acceleration and radiation may well be deadly.
That said, there could be a safer option out there: ring wormholes, which wouldn鈥檛 be tunnels but rather flat portals made of negative energy. The negative energy would warp space-time to permit travel just like a regular wormhole, but there would be no lethal black holes involved. 鈥淚f you went through one, you wouldn鈥檛 experience anything in particular,鈥� says at the University of Alberta in Canada. 鈥淚t would be like walking through a door.鈥�
5. Send a message to the past
If jumping into the inky blackness of a wormhole sounds too terrifying, there is an altogether safer option. Quantum mechanics may offer us a way to send a message back to the past, which is surely the next best thing to time travelling yourself.
The equations of quantum mechanics, which describes the world of atoms and subatomic partices, work equally well backwards as forwards. This makes it possible to interpret some quantum effects as involving signals sent back in time, though it is unclear whether that is what is actually going on.
Last year, researchers led by at the Hitachi Cambridge Laboratory in the UK showed that the experimentally proven process of quantum teleportation 鈥� sending a particle between two points in space 鈥� could just as well be interpreted as sending a particle between two points in time. They are now working on an experiment to test this by sending a real particle effectively back in time.
This process wouldn鈥檛 work with anything as big as a person. 鈥淚t seems technologically impossible because there鈥檚 so many particles involved in a human to teleport,鈥� says Arvidsson-Shukur. But with the right set-up, you could imagine sending a series of particles back in time and encoding patterns in their properties. Think of it like sending quantum Morse code into the past 鈥� which isn鈥檛 hugely glamorous, but probably our best bet for now.
