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How to Travel Through Time

Lots of people like to talk about time travel. It’s probably the most popular topic within science-fiction. What does verified science say about it, though?

Well, to begin with, we should define what time travel is. I’m going to define four different kinds of time travel:
1. Being able to observe the past
2. Being able to interact with the past
3. Being able to observe the future
4. Being able to interact with the future

If you don’t want to read about physics, here’s the short summary: Kinds 1, 3 and 4 are theoretically possible, while kind 2 is not. If you do want to learn the how and why of it, keep on reading.

So, why are kinds 1, 3 and 4 possible, and why theoretically? It’s all thanks to the special theory of relativity, by good ol’ Albert Einstein. The core concepts we need to understand is that light has a limited speed, and that the flow of time is dependent on the speed of whoever or whatever is measuring the time.

We all learn about the speed of light in school. 300 000 000 metres per second, or 1 080 000 000 kilometres per hour, or 671 000 000 miles per hour for the imperially inclined. I’m sure we all can agree that light is pretty fast. It’s so fast, that a lightning bolt striking the earth two kilometres from me, which is roughly a 15-20 minute walk, emits light that reaches my eyes in just 0,000007 seconds. What we often don’t consider is the effect this has at really large, astronomical distances. Let’s get back to the lightning example, but instead of looking at light, let’s consider sound. Sound travels at around 340 metres per second in air, meaning that it would take a little under 6 second for me to hear the thunder. When the pressure waves reach my ears, I am hearing the thunder from 6 seconds ago. Now, let’s look at the Sun. Don’t actually look at it, it’s not good for your sight, but the Sun is some 150 000 000 000 metres away from us. Dividing that by the speed of light, we get 500 seconds, or 8 minutes and 20 seconds. This is the time it takes for the light from the Sun to reach us. Put another way, when we observe the Sun from Earth, we see it as it was 8 minutes and 20 seconds ago. We are observing the Sun from the past. The closest star to the Sun is Alpha Centauri, at 4,37 light years. A light year is a measure of distance, defined as the distance light travels in one year. It’d take you a while to measure it with a ruler, at 9,5 * 1015 meters. If someone decided to observe the Earth from Alpha Centauri, because of the distance of 4,37 light years, the light that would reach them at the time would be the light emitted from Earth 4,37 years ago. If I looked at Earth in mid-November 2020, I would see the earth as it was around the start of July 2016.

This idea is well established in physics, and it’s a factor astronomers have to take into account when observing distant objects. When we observe a star blow 20 million light years away, it means that for the last 20 million years there’s pretty much been a cloud of gas there. If we were 65 million light years away from Earth, we could have a first-class seat at the livestream of the extermination of dinosaurs. At 2000 light years, we may have gotten to see Romans leading some people towards a hill, dragging large pieces of wood behind them.

Of course, it’s not so simple. I mean, I haven’t said anything that contradicts modern understanding of our universe. What keeps it in the real of pure hypotheticals is the observation part. First off, the Earth does not emit light. Or at least it hasn’t been up until some 150 years ago. We know of many more starts than planets in the universe because planets are really damn hard to spot. Furthermore, at distances that we’re talking about, our entire galaxy would be little more than a blob of light. Trying to focus on some small living beings on the surface of a small planet orbiting around an average-sized star in a random spot of a random galaxy is, as far as our current technology’s concerned, completely impossible. The biggest hurdle is still ahead of us, though. The speed of light is small at an intergalactic scale, but it is still blazing fast, all things considered. The fastest objects built by humankind don’t even scratch 1% of it. We can’t get anywhere faster than the light from our origin point does. As far as we know, nothing can move faster than light, and nothing that has mass can move at the speed of light. That’s because, as we move, we build up kinetic energy. The mass-energy equivalence, expressed in E = mc² tells us that… mass is equivalent to energy (took me quite a while to figure this one out). At everyday speeds we don’t notice it much, as our rest energy (energy from our mass when we stand still) is absolutely humongous. But when we start to approach the crazy speeds like the speed of light, the kinetic energy leads to us getting more massive, which in turn requires more kinetic energy to keep us going, which in turn makes us more massive, and so on. It’s a self-feeding loop that, by the time we reach the speed of light, would require an infinite amount of energy to make us move, which, needless to say, is more than we have. We could move at, for example, 90% the speed of light just fine, but then we wouldn’t be able to observe the past.

The only way to get around this is to decrease the distance that we travel. Perhaps a wormhole 0cc could help us out, perhaps quantum entanglement could let us teleport around faster than light. This is, sadly, a realm of very fringe physics intermingled with science fiction, however, and nothing that we could use now or anytime soon. So in short, observing the past events on Earth is theoretically possible, but practically unreachable to us. It is fun to imagine some aliens in another part of the Milky Way sitting down to watch the newest episode of the Crusades, right as they occur, though.

There is another aspect of special theory of relativity that we can use to achieve time travel, and that is time dilation. The idea is that, as you move faster, time moves slower. This effect doesn’t really have any “real life” analogues as far as I’m aware, which is a shame, and it’s actually one of the more mind-bending aspects of the theory of relativity that people talk about. As far as the math is concerned, the rate at which time slows down is called the Lorentz factor, and is calculated with the following formula: 1/(sqrt(v^2/c^2)), where v is the speed of the object and c is the speed of light. Because we are dividing our speed by the speed of light squared, you can see why time dilation is not easy to spot on Earth. A car moving at 100 km/h will, by the above equation, will experience time flowing 0.0000009% slower than a car standing still. That’s equivalent to having one extra minute in over 20 years. But if the car or, more likely, a space shuttle was moving at half the speed of light, 150 000 000 meters per second, would experience their time flowing 86,6% slower than if they were still. If I had two clocks showing the exact same time, left one on Earth and took one with me on a shuttle going at half the speed of light, if I came back to earth by the time my clock showed that a year has passed, the clock left on Earth would be saying that roughly 7,5 years have passed. This may sound weird, one would think that time should be the same always, but it isn’t. It’s relative to the observer, which is exactly where the theory of relativity gets its name from.

We have the mechanism, so now let’s apply it. Let’s say that we have a space shuttle capable of flying at, again, 150 000 000 meters per second, 50% of the speed of light. This is also science fiction, but building something fast at least seems more feasible than finding a wormhole/teleportation device and a telescope that would make Hubble look like a kid’s toy. We’re going to use that space shuttle to travel from Earth to Alpha Centauri and back again. The distance is, like I’ve mentioned earlier, 4,37 light years. Since we travel at half the speed of light, it’s going to take us 8,74 years to get to the star, and another 8,74 years to get back. 17,48 years spent in the shuttle, at 86,6% slower flow of time, results in roughly 130 years passing on Earth. It may not be as quick and convenient as Dr. Who’s phone booth, but we’ve just travelled 113 years into the future using nothing more than physics we’ve known for nearly a century and have experimentally shown to be true many times. We have travelled to the future, and can both observe it and interact with it. There’s no way back to our time, though…

So, how about travelling to the past? Since time slows down as we approach the speed of light, and it actually stops completely at the speed of light, some people theorize that going past the speed of light would actually lead to reversal of time flow. This is likely to be the idea behind the ending scene of the 1978 Superman movie, where he flew so fast around the Earth that he reversed time. Speed isn’t the only thing that affects time, gravity does as well, and very strong gravitational fields have an effect of slowing down time, as well. This effect was showcased, for example, in the 2014 movie Interstellar. Black holes have some of the strongest gravitational fields in the universe, hence why they’re often parts of hypothetical time machines, like the one used by the fictional John Titor. The idea of using time dilation to reverse time is purely in the realm of science fiction, though. After all, exceeding the speed of light is impossible, and creating and sustaining a black hole isn’t something we can practically do at the moment.

The simplest way of travelling backwards in time would be to have access to the fourth dimension. “Simplest” here is not meant to imply that getting to the fourth dimension is easy (it’s quite impossible from what I know), but that, if we did have access to it, travelling in time in any direction would be no harder than walking forward or jumping. There are different kinds of dimensions. In mathematics you may come across quaternions, which one can think of as four-dimensional numbers, just as complex numbers are two-dimensional and real numbers are one-dimensional. The fourth dimension of quaternions isn’t time, however. When I talk about four dimensions, I mean the three spacial dimensions and the fourth dimension of time. Think of it like this. Imagine a simple square. It has a given length and width, and you place it at height 0 of some arbitrary coordinate system. Height 0 could mean that it’s lying on a desk. You, as a being able to operate in three dimensions can raise it to height 1, which could be analogous with holding it 10 cm above the desk, for example. In-between those two points you could also put the square at height 0,5 or 0,33 or 0,25 or 0,31415 or any other fraction you can think of. How does this relate to a cube? If the cube has the same length and width as the square, and a height of 1, one could think of it as a three-dimensional representation of every possible state the two-dimensional square could be in. And since we can manipulate the third dimension, we could for example cut a slice of the cube off in the middle, revealing the square at height 0,5.

Now, take your cube and move it across the desk, again an arbitrary distance of 1 (this arbitrary distance thing is something I got from linear algebra, it’s a pretty useful trick in my opinion). You have a clearly-defined 3D object at time 0, at time 1 and at every time in-between. And so, a four- dimensional cube, a hypercube, would be a representation of every possible state of the three-dimensional cube. Some magic 4D scissors could cut the hypercube at a random location, giving us the 3D cube at that specific point in time. Just like using scissors or walking forward (moving in a special dimension) is easy for a 3D being like a human, a 4D being would have the ability to move along the time dimension, forward and backwards. If I wanted to travel to Earth in the year 1960, I could just take my giant, interplanetary four-dimensional, time-cutting scissors and cut off the bit of Earth that I want, corresponding in the time dimension to the year 1960. This is all, of course, pure science fiction, and even borderline fantasy. We have no idea how to reach the fourth dimension, and even if we did, it’s unknown how “cutting off” a 3D slice of a 4D object would really work. It’s interesting to think about what a 4D world would look like. If time is just a dimension, would that mean they don’t experience time in any way similar to ours? Does time move only if they want it to move? If time is always flowing, no matter what, would that be like being constantly moving? I’ll leave that for you to ponder.

There’s another reason why the fourth dimension would be the best place to look for travelling to the past, and that is the multiverse. You see, travelling to the past is very dangerous in that it’s rife with paradoxes. Let’s suppose that you go to the past, and do something, intentionally or not, that stops someone in your lineage from having offspring. Maybe it’s your parents, grandparents or some ancestor from the middle-ages. That would necessarily lead to you never being born, would it not? But if you were never born, then you were never able to travel back in time and do what you did. And so the world would continue paradox-free, everything would occur as it should, you would be born and raised… at which point you’d travel back in time and start the problem again. Another example: Let’s say that a friend of yours gives you a locket. She claims that she received it from a time traveller. You take the locket, and many years later, you discover time travel. You decide to go back in time, and hand your friend the locket. She gets the locket from you, but you were able to give it to her only because she gave it to you, first. At the same time, she was able to give it to you only because you had given it to her, first. So what’s with the locket? When was it created? If I decided to not go back in time and just keep the locket, that means I never would’ve given it to her, which means she would’ve never given it to me, and so I should not have the locket. Lots of people say “If I could talk to myself when I was younger”, hoping that it would prevent their bad decisions. But let’s think about it. If you talked to your younger self and they heeded your advice, that means the current you – them from the future – never made the bad decision. As such, you would have no reason to think about that bad decision and go back in time to warn yourself about it. Without that warning, you end up making the mistake, and decide to warn your past-self against it. It’s a never ending cycle.

The multiverse comes in to save us. Anytime we do something in the past, what happens is that we create another universe. If I warned myself of a bad decision, I now created a new timeline, a new world in which I never made that mistake. Likewise, killing a grandparent, instead of making me become a time criminal, would just create a new timeline without my grandparent, and by extension, without me in it. And since four-dimensional space allows us to move in time without affecting 3D space, as well as move in 3D space without affecting time, it is the perfect place to seek such a multiverse.

Regardless, that’s the gist of time travel from the perspective of current physics, with a bit of science fiction and philosophy tossed in. I don’t have a morale or some deeper insight to share with you, I honestly just wanted to tap at the keyboard about something I find interesting, but that many people may not know about.

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