My greater question regarding time travel does not ask whether it is possible but whether it is practical. I touch on this in my novel The Roswell Swatch. But I leave a lot to be explored.
Spoiler: Stephen Hawking was wrong, by the way.
For the sake of exploring this idea, let’s skip over talking about quantum theory, warping spacetime, wormholes, negative energy, or all those extra dimensions in string theory. That’s not going to get us anywhere anytime soon. For the sake of this discussion, let’s just assume time travel is possible. Then we can get into my concern regarding its practicality.
First I must ask if physics applies more or less universally across the universe. Of course it does, with some extraordinary and, most likely, irrelevant exceptions. The physical properties and dynamics that happen on Earth are not unique to Earth. Nor is Earth exempted from any basic ones. Our world is not the center of the universe. Sure, we live here and we love it. We’re Earth-centric. The laws of physics are not. Those laws see Earth as a random speck of matter in the universe, like Pluto, only bluer, and with better beer.
One of those laws says the universe is expanding, has been since the Big Bang, and will continue to do so forever. That means the Earth is not where it was a few billion years ago, or 100 years ago, or last February.
When we imagine traveling in time, we imagine going to a specific time and to a specific place. Traditionally, that’s the same place we were before traveling. Where is that place? We mustn’t mistake physical objects for places. Physics doesn’t. Universally speaking, the Earth is an object we relate to, not a place. My living room recliner is an object, not a place. The place that was furnished with my living room recliner a year ago is now out in the middle of outer space, because the universe is expanding. Do we even know where the Earth was a year ago? Can we calculate that within an expanding universe?
And concern is even more complicated, of course. While everything is flying away from the point of the Big Bang, Earth also is spinning. It also is revolving around the sun. The sun and our whole solar system are revolving around the hub of the Milky Way galaxy. I ask again, where was the Earth a year ago? Where was my living room recliner 31,536,000 seconds ago? Or, if we’re going into the future, where will the Earth be a year from now? And my recliner? Those are the kinds of vector calculations that made me despise trigonometry class.
Let’s bring this problem down to Earth, metaphorically. Forget for a moment all I was just saying about astrophysics. Let’s say you are in the back seat of a car traveling 60 mph down a lonely highway, somewhere in the boonies of west Texas. I’m in the front passenger seat. I have a time-machine ray gun. I turn around and zap you, sending you one minute into the future. Where do you reappear a minute from now? That exact location in space you were occupying when you disappeared? That would not be in the back seat of the car. Physics doesn’t care about the car you were sitting in. The car is a thing, a speck of matter, not a place. A minute into the future, the car will be a minute farther up the road from that place you had occupied. At 60 mph, that’s a mile away. So you would reappear in thin air, about two feet above the pavement where the car used to be.
(And then you’d fall onto a vacant highway and probable bounce and tumble at 60 mph until friction of the asphalt finally brings a stop to the bloody pulp that used to be your body. That’s because the matter that is you should still possess the kinetic energy of traveling at 60 mph in the backseat of the car when you reappear. But that’s not my point. That’s another problem.. Let’s skip over it.)
Let’s rerun our scenario. Now let’s say my ray gun is really high-tech. Cutting edge. It’s a smart time-machine ray gun. I can program it to not only transport you in time, but to transport you in space. I can actually send you due west—our highway, of course, heads due west—exactly one mile. So you should reappear where the car should be a minute from now.
Pop. You are gone. I count to 60 Mississippi. Pop. You’re back in the back seat.
That’s assuming my smart ray gun is really good, and the driver is really steady, and the speedometer is really accurate, and the topography of the road is really predictable, and there are no unforeseen circumstances. If we are off by just a few inches, you’ll reappear in the seat cushion. Off by two feet, you’re in the trunk. Off by six feet, you’re tumbling along the asphalt. Oops. Sorry.
This is not a new concern. Even H.G. Wells hinted that, for his time traveler’s reappearance, unforeseen circumstances could be hazardous. Remember Marty McFly? Remember when he and the DeLorean vanished from the mall parking lot? What did he find there 30 years earlier? A farm. The DeLorean reappeared in the barnyard. Its kinetic energy caused it to crash into the barn. What if the exact location where the DeLorean left in 1985 had contained a tree in 1955? What if a cow was standing there? Would a DeLorean and a cow survive co-occupation of the same place? What a mess.
Even co-occupation with a car’s backseat cushion would be messy.
Now, let’s get back to the realization that physics isn’t Earth-centric. The Earth is spinning. The Earth is revolving around the sun. The solar system is revolving around the galaxy. The galaxy is flying away from the Big Bang in an expanding universe. All that. Calculating where the car’s empty back seat will be within the universe a minute later would require math just short of magic.
Calculating where my recliner was in the universe a year ago probably would require data that no one is bothering to gather. Not even I am sure where the chair was 365 days ago, because we occasionally rearrange the furniture. If my great-great-great-great granddaughter were to pop into my living room for a visit, she’d need that information so that she doesn’t co-occupy with my chair. And, by the way, my dog sleeps wherever the hell she wants. There’s no data on such random complications of a dynamic world.
This is all to address Hawking’s theorem that if no time traveler has ever presented himself, then time travel probably isn’t possible.
Surely, Hawking asserted, if physics allows for time travel, then humans will inevitably—one day in the future—invent it. Right? And if it is invented, then travelers would come from the future. They would already be here now. And be here then. Hawking raised this logic to suggest we should be experiencing time tourists. Of course, we should assume that our descendants would be careful, and authorities would set important protocols to make sure travelers do the time-travel equivalent of leaving only footprints and taking only pictures. But one certainty of the human race: sooner or later, someone breaks the rules, or screws up. in his 1999 lecture, “Space and Time Warps” Hawking offered this: “Even if there were sound reasons for keeping us in ignorance, human nature being what it is, it is difficult to believe that someone wouldn’t show off, and tell us poor benighted peasants the secret of time travel.” Since that hasn’t happened, he surmised, time travel must not ever be invented.
The problem with that theorem is, it is essentially based on either/or logic. On or off. Yes or no.
There is another prospect. We invent time travel, but it comes with more problems than it’s worth, such as the Earth not being anywhere near where it used to be, or where it will be.
Time travel almost certainly would have to involve space travel. First, outer space might be the only safe place to pop back into existence. If your calculations aren’t exactly right, there is little risk of a cow being there, or a tree, or a seat cushion. Second, chances are we’ll be popping back into existence where the Earth used to be, or where it hasn’t gotten to yet.
So time travel most likely would have to be done in a space ship. The whole vessel, with everyone inside, would have to be blasted with my ray gun, as was done in “Fantastic Voyage,” or operated with an onboard lever, like Wells’ time traveler, or with a gas pedal, like Doc Brown’s DeLorean.
Pop out, pop in. We’re in a new point in time, but we’re in outer space. Now, where the hell is Earth?
One day’s time travel to reach Earth wouldn’t be bad. When we pop back in, we’d still be able to see Big Blue. A week, a month, even a year wouldn’t require such a bad space trip to get back home. But I assume no one is inventing time travel during the next year, let alone making it accessible enough that the community of time travelers would inevitably include the guy who screws up. No; it’ll more likely be in 50 years, 100, maybe longer. Any idea where the Earth will be in 100 years? Let us just accept that it will be very far from here, somewhere in the spread of the expanding universe. A long space trip, without a reliable map.
Doable? Let’s say yes, at least eventually. Practical? Depends on how badly someone wants to make the trip.