wowt explains: the ins and outs of time travel
Time travel has been a staple of fiction since possibly as far back as the 1600s and became a science fiction mainstay after H. G. Wells’ novel The Time Machine in 1985, which recounts an adventure to the quite literal end of the world. Countless tales, movies, and shows offered a wide variety of scenarios and paradoxes that arise from the ability to travel through time the same way we travel through the three other physical dimensions at will. And this made a lot of scientists curious if it could ever be possible.
As it so turns out, there are ways to move in the fourth dimension, but the trip is probably only one way, requires some very exotic technology, and survival is extremely uncertain if you try to see the past firsthand. There are also a number of interesting ideas on how paradoxes would be resolved, although none of them come with hard proof. In this explainer, we’ll start with the most speculative aspects of time travel and work our way to concepts with far more scientific footing and their implications.
can you actually travel back in time?
If you have a spaceship and can travel to an extremely exotic, theoretical entity we would call a wormhole, it may be. Because each end of the wormhole could be not just in a different region of space but a different point in time, you could arrive in the present, future, or, yes, the past. Of course, there are a few catches. We don’t know if traversable wormholes exist, how to find them if they do, or if we could ever make one given a sufficient understanding of the Casimir force. Though if we could, we’d most likely opt for a warp drive instead.
But that’s not the worst of it. Since the universe as we know it is 13.7 billion years, you couldn’t possibly expect a wormhole to spit you out in August of 1945 to witness the first use of nuclear weapons, or close to September of 2001 to stop the attacks on the World Trade Center and the Pentagon. You shouldn’t be surprised to find yourself millions, if not billions of years off course because in the cosmic chronology, expecting precision to the tune of decades is like expecting accuracy to the femtosecond. It’s almost certainly not happening.
can you kill your own grandfather and survive?
One of time travel’s favorite tropes is the Grandfather Paradox. If you go back in time before your grandfather meets your grandmother and kill him or stop him from meeting her, what will happen to you? Logic says that you’d disappear from existence and the entire timeline would adjust to account for you never having been born. But physics says that you couldn’t do that, or there is some way to allow for you to succeed and still be around because your actions form a closed temporal loop in which you were always going to kill your grandfather.
Basically, if you managed to make it back in time, there was a sequence of events that led up to you being born and making these choices. The universe simply won’t let you violate it, so if you try to do something stupid or meaningful, you’ll be stopped because the key event is otherwise simply not possible. An alternative, that every trip into the past creates its own closed loop in which anything you do is isolated from every other event, is also possible but unlikely since it forces us to make more assumptions and complexity to time and causality.
For example, let’s say you go back in time to try and prevent the 9/11 terrorist attacks. If you succeed, you’re now living in a radically different world, but where does this world exist and how? Does it take up real, physical space? Or does it branch off into its own universe? If it does, what effect does it have on your home universe and how many other universes are there born from other time travelers trying to prevent the attacks and succeeding in different ways, or with others still traveling back to ensure they go off with even more destabilizing effects?
It seems far simpler to work with the assumption that you remained in your own universe and every attempt you made ended in failure, the attacks happened just as they had, and you only wasted your time as a random cop pulled you over, or security tackled you in an airport, or your rental car broke down, what have you, anything that kept you from interfering with the events of the past. This is why most physicists will default to a curt “no” when asked if you can change the past. If you could, it would spawn an infinity of universes or perpetual chaos.
okay, what if instead of traveling we just watched the past?
Again, this is also possible in pure mathematical theory. If you were to find a wormhole which leads into the past, was large and stable enough for you to look into, and its exit was in the past, you would see history unfold with your own eyes at the entrance. But remember, that past is probably going to be millions of years, and in space, so the effect would be exactly like looking at the night sky where the light you’d see shows you what happened eons ago due to the finite limit on the speed of light and the distance the photons had to travel.
You’ll see the past alright, but after expending a lot of effort to build what if effectively a high-tech, high-energy version of a telescope, and only to see what you already do every day. Now, you could potential use quantum effects we know imitate wormholes on subatomic scales to transmit messages to the past or into the future, but in order to make that work, you’d have the build a very specialized device and coordinate with the receiver to receive, decode, and confirm the messages. This is also quite a tall order but could, very theoretically, work.
fine, how about traveling into the future?
Ah, now that we could actually do. We’d need two of three things: a spaceship, a relativistic engine, and a black hole. If you were to board a spaceship capable of traveling at 96% of the speed of light and make a wide loop around the Alpha Centauri system, you’d come back 40 years later despite the trip taking a decade as far as you were concerned. Not exactly much of a jump in the future but a jump, nonetheless. Better still, there are no paradoxes or violation of causality. Time simply moved slower for you than for Earthlings.
But what if you wanted to see the year 7,023 rather than a few decades ahead? You’d need to get back in your spaceship, head to the nearest black hole, and stay as close to is as you could without falling in before flying back home. Its massive gravity well would slow time to a crawl for you when compared with Earth and you could return to a downright alien planet that may be covered by massive arcologies, where humanity has been wiped out by a cataclysm, or lives across the solar system as a nigh unrecognizable species that merged with its machines.
If you were waiting for a catch, here it is. The closest known black hole is 1,600 light years away, meaning that it will take you 456 years to get there and return while traveling at 99% the speed of light in your frame of temporal reference. It’s also only ten solar masses, meaning that it will be about the size of a large city at 30 kilometers across and feature extreme tidal forces that increase exponentially on approach. Your safest bet would be the supermassive black hole at the center of the galaxy which has far less extreme tidal forces and an immense gravity well.
Yes, this does sound like the premise behind the movie Interstellar, but it’s also based on very real and well understood scientific calculations before it descended into a time travel trope for an emotional ending rather than a correct one. And it would be the closest we could come to paradox free, practical time travel: a one-way ticket into the far future courtesy of rocketry and the most extreme objects in the universe as outlined by general relativity.
what can you expect if you land in the far future?
That would depend entirely on what you find when you return to Earth. If previous civilizations have been wiped out by apocalyptic weapons and technology, you might not want to make it a known fact that you’re a time traveler from the distant past. Although you won’t have to try all that hard to be vague in your communication. Languages would have changed dramatically in the span of a thousand years or so, meaning that you won’t be understood as an oddity which can’t communicate with the world.
How well would you fit into life millennia from now, with all your familiar points of reference gone and long forgotten? It seems seductive to look so far into the future of humanity but what if that future is horrifying or dystopian? What if you simply can’t adapt to whatever culture you found at the end of your trip? Will you hop into your spaceship and try again, moving forward in hopes that before the Sun turns Earth into an uninhabitable desert a billion years from now, there will be one you could call home? And what if humanity moved on from Earth long ago?
Time travel is often presented as skipping ahead in the book of history, but the reality would be fraught with danger, uncertainty, and the very real potential to get very lost emotionally and physically in a universe that moved on while you were parked in near a black hole, waiting to find or see something new and different. Maybe a kind stranger living eons from now will lend a helping hand. Maybe a tyrant or criminal born in the equivalent of 83,000 AD will kill you as an odd threat instead. You never know, which is what makes time travel so dangerous.