Time Travel: The Science Of Yes and Maybe
Time Travel is one of the most cherished tropes of science fiction and when it’s done well it’s pure magic. Traveling into the future or past taps into numerous age-old desires like finding out what the future holds and experiencing historic milestones including seeing long-dead loved ones. Seeing the methods used and the technology employed to time travel is also a blast as well. Like much of the technology in science fiction though, real science often takes a backseat to other considerations like plot, budget, visual splendor, and too often, scientific misconceptions. So what does modern science have to say about the feasibility of Time travel?
To the Future
Traveling into the future is quite natural. We travel into the future one hour per hour every day after all (sorry, that’s my standard time travel joke, I couldn’t resist). Any real time machine worth its salt though should be able to take you into the future much faster than it normally takes. Moving ahead a day in a minute would be nice. Months or years in a similar amount of time would be even awesomer. The cool thing is that physics is totally cool with that. In a word (or two), it’s all about Time Dilation.
Time dilation is an outgrowth of Einstein’s Special Theory of Relativity. In order for the speed of light to remain constant (which it always does no matter how fast you’re going), space and time needs to do some wacky shit. That includes slowing down time the faster you go (relative to the place you left). This is called Relative Velocity Time Dilation and it is a bona-fide fact that has been proven over and over again.
For example, I just flew home from Dragon Con. The pilot was aided in his navigation by a suite of GPS satellites. Since they are traveling quite fast relative to a plane or car (14,000 km per hour), the atomic clocks on board lose microseconds per day. Those satellites are time machines traveling into the future faster than we are and so that has to be taken into account in order to properly calculate the planes position above the earth. Even more dramatic examples involve subatomic particles. Many of these particles are unstable and last only a paltry thousandth or millionth of a second. For example, muons are created high in our atmosphere by super velocity cosmic rays all the time. They typically live only about 1.5 microseconds meaning they shouldn’t get too deep into our atmosphere before decaying. Yet they have been detected as far down as sea level meaning they must have lived to the ripe old age of 170 microseconds. They lived so long and penetrated so deeply because they were traveling at close to the speed of light, making them time dilation time machines.
So what does this mean to a flesh and blood would-be time traveler? Unfortunately, you won’t be able to step into a time-machine in your room or a Delorean and travel into the future. You’ll need to board a ship and travel into space, but I’m ok with that. Ludicrous speed may not be enough however. You’ll need to hit holy-crap speed. The relativistic, time traveling effects of high velocity don’t amount to much until until you’re traveling at…well……relativistic velocities. That means a significant fraction of the speed of light. If you look at the curve of time dilation image here, you’ll see that you need to hit at least 98 or 99% the speed of light before you seriously start time traveling. From there, as you mount the decimals 99.9%, 99.99% etc, your ship’s clock starts getting progressively lethargic more and more quickly. Keep in mind however that this slowing would be unnoticed by the traveler. Everything on the ship is slowing at the same time so everything would seem normal. If you could somehow instantly view earth however, the clocks would be moving at quite a pace.
Say you accelerate away from earth at 1G in your “time machine”. This force of acceleration would be quite comfortable since it duplicates the force of earth’s gravity. It also means that you’d be going faster and faster, getting closer and closer to the speed of light (but never quite reaching it), and going more and more quickly into the future.
Here I list 3 popular tourist spots to show how dramatic the time-traveling effects of time dilation can be:
- Alpha Centauri: Ship time: 2.3 years…Earth time: many months in the future
- Andromeda Galaxy, Ship time: 15 years…Earth time: millions of years in the future
- Edge of the Observable Universe: Ship time: 24 years…Earth time: Billions of years in the future
Of course pulling off a time-traveling journey like this would entail horrific technical challenges. Conventional fuel costs alone would melt your brain. We are nowhere near being able to pull that off. There’s also the problem of hitting a mote of dust in space while traveling relativistically. That mote would pack the punch of a speeding freight train. Yet it can, in principle, certainly be done.
Time travel into the future could also be done by taking advantage of another aspect of time dilation. This one is called Gravitational Time Dilation. It’s not just relative velocity that slows time……propinquity (love that word) to a gravitational field does as well. People at sea level therefore travel through time minutely more slowly than people living on a mountain. Those GPS satellites in orbit have a tug of war of sorts with time. Their distance from earth speeds up their clocks (relative to us) but their velocity slows it down to an even greater degree. To take advantage of gravitational time dilation to seriously time travel however would require something quite extreme and possibly deadly like orbiting a black hole closely at tremendous velocity.
To The Past
So time traveling to the future is fully supported by modern physics. How does travel to the past fair? Pretty crappily I have to say. The classic (and quite persuasive) argument against traveling into the past is that it totally fucks with causality. This is codified in the Grandfather/Grandmother Paradox. If you go back in time and kill a grandparent, then you won’t be born. If you’re not born then you can’t go in the past and prevent your own birth which means …well you get the idea.
There are couple hypotheses that deal with this seeming insurmountable problem. One is called the Novikov Conjecture. It posits that there’s simply a zero probability that a time traveler will create a paradox. So if you try to kill granddad, something innocuous will continually prevent you from doing that. I interpret this as a view of the future that is necessarily the culmination of everything that has happened in the past…including all journeys back in time that have ever happened….or will happen I suppose.
Another option neatly slices away the paradox/causality problem by saying that if you travel to the past then you have simply traveled to a similar alternate universe. Anything you do can’t create a paradox then because you’re not from their future. Though this is cool, it is wholly unsatisfying and no proper time-traveler subscribes to that hypothesis.
That’s all fine and good but what do the physics equations say? Unfortunately, it doesn’t look good, but there is a modicum of hope…and it’s all about Closed Time-like Curves.
General Relativity has certain solutions to its field equations that seem to allow travel into the past if Closed Time-like Curves can be found or created. These buggers are critical to this discussion and they’re weird so let me describe them in multiple ways.
- Space-time bent in such a way that timelines turn back onto themselves forming loops
- Worldlines forming closed loops in spacetime which permit time travel to the past
- Certain geometries of spacetime that allow motion backwards through time
- Spacetime paths that cut through space and time
The best and most fleshed-out example of a closed time-like curve is the iconic and theoretic wormhole. These open-ended topological tubes of space-time could potentially connect distant parts or ages of the universe or maybe even other universes. The variant we’re interested in is called a traversable wormhole since that could allow a person through instead of nothing or just subatomic particles. To hold it open, it might need yet another theoretical component called exotic matter. This can be thought of as matter with a negative energy density.
To use this bizarre beast for time travel into the past, we would need to somehow transport one end of it to relativistic velocities. Once this happens, our good friend time dilation comes into play. A clock in this moving end would tick more slowly than a similar clock in the static end. If the ends were eventually brought back together, you could end up with one clock at the year 2020 and the other at 3020. If a person from the year 3020 went into the into the 2020 end, he would come out the other end in 2020 as well. After that stupendous series of if statements, he will have traveled into the past 1,000 years.
Other closed timelike curves have been bandied about as well. Some involve black holes or primordial super-dense cosmic strings or infinite rotating disks. These are, unsurprisingly, perhaps even less plausible than exotic-matter-supported traversable wormholes.
As science-fictiony and cool as all this sounds, there’s a lot of caveats. Any time machine based on closed timelike curves will not allow time travel earlier than the creation of the time machine itself. This, in fact, potentially explains why we haven’t been inundated by tourists from the future. We simply haven’t reached the point in time yet when they can be created (by humans anyway).
Also, just because something can be described mathematically does not meant that it is necessarily physically realizable. It may just not be possible to create one even if a super-advanced type 3 civilization tries their hardest. And finally, the current state of physics does not allow us to definitively answer the question whether traveling into the past is allowed. We’d probably need to fuse General Relativity and Quantum Mechanics to get a solid answer to that. This so-called quantum gravity is one of the holy grails of physics.
So, we probably just have to wait for quantum gravity to get a final answer about time travel to the past. In the meantime, I’ll continue watching and reading science fiction…and looking longingly at Deloreans.
Image Credit: Teefury.com