Could future humans be observing us from spaceships in the sky? (Probably not.)
You can theorize time travel using time quantum physics, which is all about probability.
We’d need to already have a time machine for people to travel back to.
Last summer, a British astronaut made the news when he referred to someone’s theory that unidentified flying objects (UFOs) in our skies may be future humans looking back at us. Tim Peake is a decorated test pilot and was the first European Space Agency (ESA) astronaut from the United Kingdom, with over half a year spent in space including on the International Space Station (ISS); he was answering the question with good humor and skepticism.
But it got me thinking.
Time travel is an iconic storytelling mechanism. It’s hard to say for sure what the very first time travel story was, but people point to H.G. Wells’ novel The Time Machine as the first description of, well, a time machine. Previously, stories like Rip Van Winkle suggested a telescoping of time, but by supernatural rather than mechanical means. In the century-plus since Wells’ influential novel, time-travel fiction has become almost its own entire genre.
People travel through time in machines; they appear to do it without machines; they disappear into pocket dimensions or alternate realities where time is different. Sometimes, like in Stephen King’s story “The Jaunt,” someone disappears into a rift and reappears far older. In the all-time great episode “The Inner Light,” Star Trek’s Captain Picard spends a lifetime inside an anomaly. Time has passed for them normally, but their pocket dimension existed outside of regular time.
People are creative in the ways they handwave away time travel in fiction—but how would it work in real life? Could it even? How might we begin to explain the idea that future humans are visiting us by traveling back in time?
The key question is about directionality. Almost everyone credible agrees that if time travel is possible, it is most possible in a direction going forward in time—basically accelerating the way we experience the passage of time already. And there are some ways the universe appears to naturally “time travel” in this way, like time passing relativistically for a person in space versus on the ground. We have examples around us of the physics that could eventually cause time travel.
But is it possible to travel back in time at all? That requires some more mental gymnastics. We’re surrounded by fiction where people just jump around in time willy nilly, but it’s possible the mechanisms that enable one direction would not enable the other direction, and vice versa. There are some theories of backward time travel, like maybe something far-out that involves a wormhole with one stationary side and one side traveling very, very fast, creating a kind of temporal anomaly. And, of course, there’s quantum physics.
Fabio Costa is a quantum physicist at the University of Queensland in Australia. In 2020, he collaborated with a student on a paper titled “Reversible dynamics with closed time-like curves and freedom of choice.” In the paper, they explained the grandfather paradox, which is a thought experiment about time travel. What if you went back in time to kill your own grandfather? That would then mean you never existed at all, and could therefore not kill your grandfather.
So what if traveling back in time were possible, as it seems it could be within certain frameworks of quantum mechanics? Could we avoid the paradox? Costa and his team found that a free agent making this trip could exercise free will without necessarily affecting what they would find back in the present. The grandfather paradox might simply not be a problem at all. And I had a feeling someone doing research on this topic would entertain my question about future people checking us out from space. (Costa gave me serious information, but with an understanding that we’re just having fun thinking about it.)
“A common feature of all time travel models is that it is not possible to travel back to a time before the first time machine has been invented,” Costa tells Popular Mechanics in an email. “In a sense, traveling back in time requires two doors, one in the future and one in the past. You can only travel back if someone has opened the door to the past. So, people from the future cannot visit us . . . unless someone has already invented a time machine and nobody knows!”
That means time travel could be like the beginning of a relationship, in a way. You start a mental timer when you meet someone new, and if you stay together, you pass milestones while remembering your first dates together. Except if your first date was to finish up your new time machine, you could literally go back and re-experience the date if you wanted. After all this time, you can tell your past self to skip the garlic bread!
If someone had invented that time machine, it makes sense that they wouldn’t broadcast it yet, because there would really be nothing to show until more time had passed. But it’s hard to imagine someone keeping it a secret longer than about a week. (If you could send someone back a week, that would already be a huge deal!) Even if Costa’s paper is right that the agent in the past may not meaningfully affect what has occurred in the present, people would still want to try to disarm mass shooters, let alone more commonplace things like signing bad contracts.
One major group in time-travel thought relies on multiple universes, meaning that decisions ripple into different versions of the universe. By traveling back in time, you simply approach a different version of yourself from a parallel world. Another posits that there may just be one (or no) shared reality, but where the probabilistic parts of quantum mechanics simply doesn’t allow paradoxes to occur. Like, you may go back in time and try to kill your grandfather, but whatever tiny probability that you miss every shot must necessarily come true.
That means if we were indeed seeing future humans flying the friendly skies and trying not to break the Prime Directive, quantum probability could explain why we somehow never see them clearly or find their ships. “That leaves open the question: where is the time machine?” Costa says. Unless it’s somehow also positioned in space, these ships are coming through and taking off from somewhere. If the machine already has to be working today in order for us to entertain future visitors, there’s no way we could have built that machine in the sky.
So that’s another little paradox—but it probably won’t stop our minds from continuing to spin more tales about time travel.
This story was originally published on July 20, 2022.
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