I love a good time travel story. They make for some of the best movies and TV in the sci-fi genre. It seems a real shame to pour a bucket of cold water on the idea that time travel is possible (beyond the extent to which time travel is already impossible for us, that is).
One might argue, quite reasonably, that humankind has never invented time travel at any point in the future. Because if we had at any point invented time travel, we would likely have completely destroyed our history in short order (name one thing that has the potential to make serious money or effect significant political change that hasn’t been corrupted by people shortly after its introduction into reality).
But the real trouble with time travel may have less to do with the fact that it is not possible to achieve, and more to do with the possibility that it wouldn’t work the way we think it would anyway.
Let’s take a moment to think about our expectations of time travel, at least how we use it in our sci-fi stories:
The I reappear where I left time travel:
Or the Back to the Future model of time travel. This suggests that we move through time and reappear in the exact “fourth dimensional location” that we left in our launching point; Marty leaves the Twin Pines Mall parking lot in 1985 and arrives at the precise spot in 1955 on the Peabody farm upon which the mall’s parking lot will later be built. Or even less likely that Marty can reappear on the very same train tracks that he leaves in 1885. (Never mind that Southern California is on a series of fault lines and the exact position of the train tracks was likely to have moved some in the one hundred years between 1885 and 1985 – at least a few inches – certainly enough dislocation of the tracks to derail a train.)
The I reappear anywhere I wish time travel:
It is far more common for sci-fi stories to allow our time traveling heroes (or villains) to be able to pick a place on the planet to reappear in the destination time-line. This is an obvious advantage to the writer as this will certainly enable more robust story-lines for the adventure. Let’s go to 18th century Vienna, for example, is far more interesting for most stories than let’s go to what was once the backyard of my house two hundred years ago (which is the spot in my workshop where I built my time machine). (Never mind that in this scenario, if one were to re-materialize to the other side of the planet, they would likely re-materialize oriented in the same direction as the location on the globe from which they came – or that they usually re-materialize at relative ground-level.)
These two “types” of time travel make up the bulk of the expectations of how time traveling would work, at least as considered by our sci-fi stories about it. The problem is, even if time travel is actually possible, it is not likely to work as it is presented in either of these two scenarios.
Albert Einstein’s Theory of Special Relativity postulates that spacetime is a single thing, not two different things. This makes for the basis of this hypothesis that I am making here that we are not likely to be able to move through time without also having to move through the equal amount of space.
First consider that the Earth is revolving at roughly 1,000 miles per hour at the equator. This rotation is how we experience a day here on the surface of the Earth with differential of daytime and nighttime, and with the sun and stars moving across the sky at a consistent clip from our surface perspective.
But that’s just scratching the surface. In addition to revolving at 1,000 miles per hour at the equator, the Earth is also orbiting the Sun at approximately 67,108 miles per hour, which is how we get our year (and Earth’s axis tilt provides us with our four distinct seasons as we complete each revolution).
In addition to these movements through space, the Sun, along with our entire solar system, is orbiting the center of the Milky Way Galaxy at approximately 447,387 miles per hour.
Also consider that the Milky Way is moving through the universe towards an eventual collision with the Andromeda Galaxy at approximately 252,000 miles per hour. This is not likely to be the entire measurement of the speed at which the Milky Way is actually moving through the universe – it may well be moving faster (or slower), but that is a difficult measurement to pinpoint since all of the objects against which we may be able to measure that speed are so far away and also moving at great speed themselves. There is no known stationary object in the universe against which we can accurately measure our speed of travel through the universe itself, so far we can really only measure with any degree of certainty our galaxy’s speed of travel relative to other objects.
So if space and time are intrinsically linked together as the same thing, as stated in Special Relativity, and we were to travel back in time one hour, wouldn’t we actually re-materialize at the same location in spacetime where that exact launching point on Earth was located in the universe one hour earlier?
With all of these traveling speeds to consider, if we were to travel back in spacetime one hour, we could literally re-materialize hundreds of thousands of miles away from our launching point to a spot in deep space. The very spot where our launching point on Earth appeared in spacetime one hour beforehand.
That then, in a nutshell, could be the trouble with time travel. We can’t travel through space without traveling through time, and likewise, we cannot travel through time without traveling through space.