Stories about time travel have been a staple of science fiction ever since H. G. Wells invented the idea. The obvious advantage of the time-travel tale is that it allows the writer to drop a modern human, a character whose knowledge and attitudes will be comfortable for readers, into an unfamiliar world. Also, ancient history is full of drama, not to mention culturally meaningful events.
Nothing in contemporary physics suggests that time travel is possible, so really we’re dealing with fantasy, not science fiction. But all science fiction is fantasy, if you want to get technical about it. SF is a sub-genre of fantasy in which we’re supposed to pretend that the events being depicted could actually take place. With unicorns and vampires, the pretense is transparent, but in SF we’re supposed to convince ourselves that we’re not pretending.
The science fiction genre is jam-packed with pseudo-tech that has no plausible basis in reality. Off-loading your consciousness into a computer so you can live forever, there’s a good one. Utter balderdash, but a few writers have run off toward the goal-posts brandishing it in triumph. Telepathy, same deal. Novels with telepathy are considered SF by convention, but they’re fantasy.
The trick in writing SF is to make it seem believable. If you want to write a time-travel yarn, for instance, you’ll need to come up with an angle on the paradox problem, also called the grandfather paradox. What happens if you go back in time and kill your grandfather before your father was conceived? There are three or four possible routes around this roadblock, and you can choose the one that works best with your story, but you can’t avoid choosing one and sticking to it.
But that’s not what I want to talk about today. Instead, I’m thinking about the messy problem of the inertial frame of reference.
In a typical time-travel story, the lead character hops into the time machine and emerges at some point in the past, either at the same geographical location or at another one that the writer proposes to use as a setting (such as, perhaps, Jerusalem or the London of Jack the Ripper, both of which are popular choices). But where was London in 1888? Saying, “It was right where it is today,” is not quite good enough.
What inertial frame of reference does your time machine use? This is a question in physics.
Let’s suppose it’s the gravitational field of the Earth. Plenty of mass there. You can have some magic physics so your time machine will operate using the Earth as the frame of reference. Unfortunately, the Earth is spinning at a rate of about a thousand miles an hour. Let’s suppose you try sending your time traveler back in time by one hour. Depending on how far your time machine is from the equator, your time traveler is going to step out of the machine (or force-field, or whatever) as much as a thousand miles east of where you expect her to be. If the time machine is in New York or Japan, the time traveler will end up in the ocean.
Okay, you can add a super-computer to your time machine so as to compensate for that. Now you’re moving your time traveler through both space and time, so you get an effective teleporting device for free. That could make for a good story, and you’re welcome to write it if you like.
But wait: Is the Earth really a valid frame of reference? What if the Sun and its planetary system are the frame of reference? In that case, a chrononaut who travels an hour into the past will emerge in outer space, because the Earth is traveling around the Sun at about 67,000 miles per hour. Also, if the solar system is the frame of reference, no matter how good your super-computer is at tracking where the Earth was or will be, you can’t make time jumps of a century at all, because at that time-scale the solar system is chaotic. The gravitational pull of Jupiter and Saturn will have somewhat altered the orbit of the Earth, and there’s really no way to calculate how.
And then there’s the galaxy. What if the galaxy is your frame of reference? In that case, the Earth is moving through the galaxy in a spiral shaped like an enormous corkscrew, and the gravitational fields of nearby stars have to be taken into account. Also, the galaxy itself is in motion relative to … well, relative to other galaxies, which are also in motion.
One of the odd things about the universe is that there really isn’t an inertial frame of reference, other than in a strictly local sense. To quote Alan Watts only slightly out of context, “This is it.”
But don’t let that discourage you. Go ahead and write your time-travel novel. There are some great ones! And nobody but pedants like me will pause and say, “No, but wait….”
Jim Aikin's Oblong Blob 