This article first appeared in Issue 15 of our free digital magazine CURIOUS.
H.G. Wells set off an obsession with his novel The Time Machine, one whose frequent appearances in science fiction testifies to its popularity ever since. Many of the other futuristic concepts Wells and his contemporaries wrote about have long since come true, but we don’t seem to be any closer to time travel, at least of the sort popularly envisaged. So, is it even possible?
Physics offers reasons to think it might be, starting with its status as the fourth dimension. However, it also throws up a lot of obstacles, some of which may yet prove insurmountable.
Not that sort of time travel
Answering a question like this needs to start with what one means by time travel. In one sense we are all traveling in time, approaching the future at the rate of one second every second. Of course, that's not what people mean, but it points to the need to think a bit harder about definitions.
Some forms of time travel are definitely possible, but while they may be closer to popular meanings, they’re still not really there. We know from Special Relativity that time passes differently for someone traveling at a substantial fraction of the speed of light than for someone stationary. Consequently, astronauts on a trip to Mars, particularly a more rapid one than is currently considered, would experience very slight time dilation effects. Looked at one way, they would return to an Earth whose clocks, including biological clocks, are very slightly out of step with their own.
For a short trip like that, the differences would be minor, and overwhelmed in terms of aging by the effects of zero gravity. Faster and more distant journeys, where the time shifts would be more noticeable, may be well outside our current capacity, not to mention budget, but we know they are theoretically possible. The sorts of time travel where space voyagers return to Earth barely changed but their families have aged decades could happen, and if humanity does not derail its progress through war or environmental collapse, they probably will.
What we really mean
Unless some force maintains the traveler's position relative to the planet, you'd find yourself floating helplessly in space waiting for the Earth to catch up.
Pedantry aside, we all know what is meant by time travel: going to a point in either the past or the future and (hopefully) returning safely. Even if this is possible in theory, there are some quite major practical problems to consider.
To pick just one example: fictional representations of time travel almost always assume the traveler ends up in the same location in space, relative to the Earth, but would this be the case? After all, even if you just traveled back in time by a week, so you could bet on a sporting contest for example, the Earth would have moved millions of kilometers in its orbit around the Sun in that time, and the Sun would, in turn, have migrated a small way around the galaxy.
Unless some force maintains the traveler's position relative to the planet, you'd find yourself floating helplessly in space waiting for the Earth to catch up. Ignoring this problem reflects a view of the Earth as the center of the universe disproven by Copernicus.
If you assume time travel involves conservation of momentum, the traveler would likely end up close to their location of origin, but slight changes in direction could still result in relative shifts in location that could prove lethal.
Nevertheless, objections like this don't make time travel impossible, just impractical.
The strangeness of space-time
In addition to raising the possibility of time dilation effects, Einstein’s special theory of relativity implied that space and time are more intimately linked than had previously been assumed. The idea of a four-dimensional space-time was raised soon after, and developed in the general theory of relativity, where Einstein showed that huge mass can curve space-time, including what physicists call worldlines.
One consequence of this is that similar time dilation effects can occur in a powerful gravitational field just as they can when traveling close to the speed of light. If you want to experience time travel at the pico-scale, climb a mountain or get in an airplane to put some distance between you and the Earth’s gravity.
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More significantly, if this distortion becomes sufficiently extreme it should create what is called a “closed timelike curve” (CTC), which followed precisely would take you back to an earlier time.
As the book The Quantum Physics of Time Travel notes: “If time travel is impossible, then the reason has yet to be discovered.”
Nevertheless, there is clearly a lot we don't know about time. If it is merely the fourth dimension, why can we easily change our position in space, but not do so in time, other than letting it carry us forward like a river? The answer could turn out to create physical obstacles to time travel we have yet to identify.
The obvious problem with traveling back in time: accidentally (or deliberately) kill your grandparents and you are never born, so how could you kill them? Science fiction writers usually use this as a plot device to force their characters to undo the changes they have made. The ending produces a world where all the crucial details are similar enough that the trip occurs paradox-free, but things are nevertheless better.
That’s great for fiction, but scientists have to acknowledge that such an outcome is unlikely.
One possible solution to the grandfather paradox lies in the many-worlds interpretation. If true, there are an infinite number of universes branching off from each other every time an event occurs that could turn out in different ways. In this scenario, the universe where your grandfather was mysteriously killed before conceiving your parent is as real as the one you experience, and traveling in time merely means hopping from this universe to that one.
A variation on the grandfather paradox is the Bootstrap Paradox, where time travel creates the conditions for itself to happen. An example might be a person receiving instructions from the future on how to build a time machine, leading to the creation of a world where their future self can send those instructions back. While self-consistent, these situations nevertheless leave open the question of where the information came from in the first place. Once again, this is the sort of thing fiction writers can casually gloss over in films like The Terminator, but physicists have to face directly. Calling them closed causal loops may speed communication but doesn’t solve the problem.
What isn't clear, however, is whether paradoxes actually prevent someone from traveling through time, even if they potentially make it a very bad idea.
A one-way trip?
A return to one's own time would risk the bootstrap paradox, where the information collected on the trip is used to bring the foreseen world into reality.
Most time travel stories today involve efforts to go back in time to try to change the endless things we’re unhappy about. However, Wells’s tale is of a traveler going forward to witness the decline of humanity and eventually the Earth. This avoids the grandfather paradoxes and is quite possibly more plausible.
Such forward traveling might have its appeal, particularly if you expect the future to be better than the past – you‘d get to reap the benefits of those solving today’s problems without having to do any of the work. The problem would come if you started missing your loved ones and wished to return home. At the very least, a return to one's own time would risk the bootstrap paradox, where the information collected on the trip is used to bring the foreseen world into reality.
If so, how?
If we accept that time travel is possible, paradoxes can be resolved, and it's something we are willing to risk, that still leaves the question of how.
The simple-sounding explanation is that one needs to travel faster than the speed of light, but this is easier said than done. It's possible to travel faster than the local speed of light in a medium that slows light down, something we witness in the Cherenkov Effect. This, however, does not get you a ticket to the 19th century. For that, you need to travel faster than the speed of light in a vacuum. Although faster-than-light travel is not inherently impossible, an object with mass cannot travel at exactly the speed of light, which poses something of an obstacle to traveling faster than it. In practical terms, how does one accelerate from slower than light speed to faster without going at the speed of light in between?
According to Stephen Hawking, a prerequisite for time travel is the presence of exotic matter, specifically matter with negative energy. This isn't necessarily as impossible as it might sound, but so far we have not identified the existence of any negative mass particles, in contrast to antimatter, which, while rare, definitely exists.
Why aren't they here?
Ultimately, the strongest evidence against time travel is probably its own equivalent of the Fermi Paradox. If time travel is possible, why have we not encountered any tourists from the future? Hawking even sent them an invite.
Either humanity has never invented time travel, or our descendants have, but have used it very responsibly. Given our record with every other technology, the latter seems unlikely.