A Simple Guide to Time Travel
While time travel is nothing like we see in the movies, it is possible and here’s how
My guess is that everybody — yes, even you — has at some point thought about time travel. It’s a pretty cool concept, right?! It’d be great to go back to see the dinosaurs or hop forwards and see what life is like a thousand years from now.
While going back in time isn’t possible in the sense that we’d like it to be, going forwards definitely is and this is what we’ll look at. In fact, every human in existence currently time travels every day — we all travel forwards in time at a rate of one second per second (or one hour per hour, or one min… you get the point).
Because we all live on Earth we all experience the passage of time in the same way. We have constructed and accepted a global definition for what a second is and we’ve been able to do this because we all move relatively slowly compared to one another. Because of this common understanding of what a second is and the way we’ve defined it, we can say that we move into the future at a rate of one second per second. This may seem like a tautology but let’s look at it a little more.
When we talk about time travelling into the future what we actually mean is that we want to move forward in time at a rate greater than one second per second. Ideally, we want to be able to pick an amount of time that we want to travel into the future, say one day, and then travel in time at a rate of one day per second for exactly one second so that we’ll have travelled into the future by one day whilst only one second has passed for us. That sentence probably needs rereading a few times but I’ll try to simplify it.
For this to work, we mean that a single person travels in time at a rate of one day per second for exactly one second whilst everybody else continues as normal (at a rate of one second per second). By doing this, everybody else will have experienced a day passing while the person that “time travelled” will have only experienced one second passing. Both groups will have moved forwards in time but just by different amounts.
Now this sounds really cool and the good news is that it is completely possible. If you told me how far into the future you wanted to travel then I could tell you exactly how you could go about doing that. Obviously, things are a little more complicated than I’m making out because otherwise we’d all be doing it by now but the science is pretty solid — it’s just engineering that’s holding us back. Although, the statement, “travel in time at a rate of one day per second” is simple to say, it is far from simple to actually do which is the problem.
Despite the technological hurdles, I’ll explain how this is doable in theory. It’s all because of a bit of theory (in the scientific sense, not the common English sense) introduced by none other than Albert Einstein — special relativity.
I won’t go into the details but special relativity essentially states two postulates that lead to the possibility of time travel. The first states that
the laws of physics are the same in all inertial reference frames
and the second is that
the speed of light in a vacuum is constant in all reference frames.
Now you don’t need to worry about the technical details of those two postulates but what is cool to know is what these postulates infer.
Time Dilation
The first interesting outcome is time dilation. Again, this sounds a bit complicated but all it means is that if you were to move very fast relative to the Earth (say 90% of the speed of light) then you would see time going more quickly on Earth than normal. People and movements would seem to have sped up. Exactly how much faster can be calculated and we’ll get onto that but in essence, this is how you time travel.
All you need to do is move at very fast speeds for a while and once you slow down back to “normal” speeds again, you will have travelled further in time than everybody else. This happens all of the time in our every day life but because we move at such slow speeds compared to the speed of light we just don’t notice it — it isn’t until you move at near light speed that the effect becomes apparent to us.
The problem we have is that getting anywhere close to the speeds necessary to time travel a noticeable amount is quite difficult. We can work out just how close to light speed we would need to go though.
Thanks to a value known as the Lorentz factor (denoted by the Greek letter, gamma) that can be calculated as follows:
Once we’ve calculated the Lorentz factor, all we need to do to work out how much time will dilate at the speed v, is to multiply by the Lorentz factor. This equation can also be rearranged to give an expression for finding the speed v, at which one would need to travel to get a certain Lorentz factor.
It’s this equation that enables us to calculate how fast we need to move to travel a fixed distance into the future. For example, we want to know how fast we need to go if we plan to travel at a speed for exactly one second and be one day into the future when we stop.
Going back to our example of travelling into the future by one day, we know that one day is equal to 86,400 seconds. The ratio between 1 second and 86,400 seconds is (obviously) 86,400. Plugging this value for the Lorentz factor and rearranging the equation tells us that we’d need to travel at 99.9999999933% of the speed of light for one second! That’s fast. If we do that in a big loop so that we end up back where we started then we’ll have time travelled into the future by one day.
If we travel at a high speed for a longer amount of time then we don’t have to go as fast to travel into the future by one day. If we were to travel for an hour at high speed rather than one second then we’d only need to travel at 99.9132% of the speed of light. Still fast but much more doable.
Getting up to these speeds is impossible for us at the moment and made even more difficult by the second cool result that the postulates infer…
Mass Increase
That’s right, time isn’t the only thing that gets scaled by our Lorentz factor, mass does as well.
The faster you travel, the heavier you get. Again, we don’t notice this phenomenon in our day-to-day lives as we all move around too slowly — even Usain Bolt.
Using exactly the same formula as before to calculate the Lorentz factor, we can see that travelling at 90% of the speed of light will mean that we’d be about 2.3 times more massive than normal.
Travelling at 99% of the speed of light would make us just over seven times more massive and travelling at 99.99% of the speed of light would make us nearly 71 times more massive. The Lorentz factor gets rapidly bigger between 99% and 100% due to its asymptotic nature.
This increase in mass means that as we go faster and faster, it gets more and more difficult to continue getting even faster because we are having to move a heavier object. This is a slight setback which is why I saved the third result of the postulates until last — like time dilation, it’s cool in a good way.
Length Contraction
In the same way that time dilates and mass increases, lengths contract when travelling at high speeds. The amount that a length contracts is again given by the Lorentz factor.
Lengths contract in the direction of movement and this means that if we can go fast enough then we can travel any distance in a very short amount of (relative) time.
Take, for example, the Canis Major Dwarf Galaxy that is 25,000 light years from Earth. If a rocket left for this galaxy at 99.9999% of the speed of light then an observer from Earth would have to wait way over 25,000 years before the rocket even gets close.
However, from the perspective of the astronauts in the rocket, lengths contract by the Lorentz factor of around 707 and so the distance of 25,000 light years becomes 35 light years. This is still a pretty long way to travel but it’s a big improvement.
Conclusion
So to conclude, travelling forwards in time is pretty easy in theory but still a long way off on the technological front. Nonetheless, it’s still good fun to wrap our heads around.
Also, special relativity isn’t the only way that we could travel through time. Einstein outlined another theory, general relativity, that discusses how space-time can be warped by massive objects with strong gravitational fields.
