Time travel is easy. The science is over a hundred years old and not disputed. If someone had lots of money (say a few trillion dollars), I’d estimate that in a the next fifty years they could have built a vehicle to transport themselves a hundred, a thousand, a million or even a billion years into the future. But there’s a catch – you can go forward, but we don’t know how to come back.
Einstein’s relativity theory tells us that if we’re standing still we travel into the future at a sedate 1 second per second. But once we start moving, the rate quickens. Synchronize atomic clocks and put one on a plane, leaving its twin behind on the Earth, and the airborne clock ticks measurably (if microscopically) more slowly. But travel faster, build a spaceship that could travel at a significant fraction of the speed of light, and time dilation kicks in proper.
We can’t quite do that ourselves yet. The fastest we could start work on tomorrow is probably Project Orion from the 1960s, but it’s only capable of reaching around 7% light speed – that’s very fast, but only enough to make a days’ difference every year. We’ll need to go faster, but the methods to do that could be just around the corner.
For instance, the Large Hadron Collider (or LHC) at CERN is accelerating particles into their future every day. In the tunnels beneath France and Switzerland, protons are travelling at 99.9999991% of the speed of light. It took Apollo astronauts three days to reach the moon. Had they gone on a three-day journey at the same speed as those protons, they’d have travelled more than 60 years into the future.
Travelling forwards in time present no philosophical problems. Many people think we live in a rational, causal universe, when one action leads (in time) to another. If we try to reverse our journey, we run into problems: suppose you went backwards in time, killed your grandparents and so prevented yourself ever being born, wouldn’t that make it rather difficult to travel backwards in time to kill your grandparents? That’s just one of those pesky paradoxes that seem able to occur if you could go the other way. Yet if you’re somehow not allowed, doesn’t that present us with another unsavoury philosophical conundrum – that we have no freewill?
Johnny and Clara do go back in time – they’re even responsible for one of the pivotal events in Earth’s history. It’s fair to ask the question, “What if they hadn’t acted as they did?”
Are the paradoxes, which seem able to arise if travelling backwards in time, genuine or imagined? As an early teen (about the same time I was first creating a Johnny Mackintosh style story) I decided the way around any such problems was to extend our 4-dimensional space–time view into a 5-dimensional space–time–probability metric. That our universe was not alone – instead it was one of many, some nearly indistinguishable, resulting from the “many worlds hypothesis” of quantum mechanics. This could mean that, when travelling backwards in time, you’d simultaneously slide across probabilities so the universe you altered was not the one you came from, even if you might think it was.
Has that happened to Johnny and Clara? Readers might be pleased to hear I no longer think it’s necessary. The future they alter is their own (and indeed ours).
Nowadays, I believe the arguments about causality and freewill are illusory. They’re the result of our peculiar perspective that we are 3D creatures who see themselves travelling forwards through time, and so perceive that time itself has a direction of flow. Let’s change out perspective and take a broader view.
Einstein replaced our 3D concept of space with a 4D one of space–time. Once we move up a dimension and consider the universe to be a universal present, time stops “moving” in any meaningful sense and what is sometimes called “the timeline” is fixed. Mostly, creatures such as ourselves would have linear personal timelines. Exceptionally, if to our perspective we travelled backwards, they would bend and curve back. But does this mean that we could alter the past preventing us travelling this way? Absolutely not. Once we view things from a 4D perspective, everything happens “at once”.
Does this mean we lose our freewill? No – we still choose to make our decisions. But from a higher-dimensional perspective they happen simultaneously.
Does this mean an end to causality? No. The chain of cause and effect is a natural side effect when we reduce the actual, higher-dimensional universe to the one we tend to believe we live in.
However, there does seem an even more compelling reason why travelling backwards in time (at least intentionally) isn’t possible: there’s no one from the future here. Even an optimist such as myself can’t believe the future’s so great that no one from there would ever want to visit the past, even for academic study. Can we really accept that our future selves are so disciplined and technologically adept that they would never be noticed if visiting here. Or is Bill Gates really an escapee from the 24th century, who created Microsoft from a broken piece to software someone from his own time had thrown away because it never seemed to work properly?