Time Travel is Real.
And why it‘s (partially) impossible.
Is time travel real? Whether complete control over time is a superpower you’ve imagined yourself with or you wished time travel was around right now so you could go back and save yourself from embarrassment at that party last night, time travel has always been an interesting concept. It seemed like The Avengers put it to good use in Avengers: Endgame, but I’m here to tell you that that depiction of time travel (even with their explanation of it) isn’t quite right. In fact, I’ll tell you now that time travel will never exist, though it already does. Let’s see where I’m getting these claims from using a popular time travel paradox — the grandfather paradox.
Being alive without ever having been born.
That’s the premise of the paradox. Let’s say you go back in time to a point where your grandfather hasn’t yet had children and you kill your grandfather. Now you’re a murderer and one of your parents was never born. If one of your parents was never born, then you were never born. So now you’re a murderer and you were never born. But if you were never born, how did you go back in time and kill your grandfather?
It’s a messy and awfully cruel situation that creates an infinite loop of reasoning. To get an understanding of how to navigate and find a solution to this paradox, we first have to understand how time travel to the past might work.
And a short disclaimer before we do just that, this bit of reasoning is a product of my brain and not entirely proven, simply because no objective explanation of time travel to the past has been established. That said, this theory should be pretty cohesive.
Time travel to the past and to the future is very different. A common notion associated with time travelling to the past is initiating an event that would change the future in some capacity. Even something as small as kicking a sharp stone onto the street could have an impact on how the future plays out, right? Not quite. You wouldn’t have noticed a large-scale butterfly effect even if there would’ve been one. Instead of rewriting the future, you’ve simply made an addition to an edited version of the past.
Simply put, you’ve changed the past in a way that this “new” changed past will replace the “original” past which then leads to your future. But, the key is to understand that for you to have travelled to a point in the past, you must have travelled from a point in the future (with respect to the point in the past you travel to). If your past is changed by an event in your future, the change must happen before you get to that point in the future, or else, how would the change have been induced? Paradoxically, that point of change hasn’t been reached yet if the time you spent in the past came before the instant future you went backwards in time. Therefore, chronologically, the point in the future in which you initiate time travel must come before the point in the past you travel to. Then, time can continue linearly again.
Another way to think of that is using the grade 2 math of number lines. We (hopefully should) know that numbers on a number line decrease going left and increase going right.
Let’s draw an analogy between math and time (as if they weren’t already related). Think of a number line as the flow of time with each number being an event or moment in time. 0 represents the instant you were born, negative numbers represent time and events before you were born, and positive numbers represent time and events after your birth. This number line is of your own life and is not related to calendarized time. Yes, this would mean that this depiction of time is personal to you, and no that’s not an error for simplicity — more on this later.
Time travel to the past would change our number line and the arrangement of its numbers, like so:
Normal time flow and arrangement of events: 0, 1, 2, 3, 4, 5, 6, 7, …
Changed time flow and arrangement of events: 0, 1, 2, 5", 3, 4, 5', 6, 7…
As you can see, in our changed number line model, there are two 5s. Let’s say that right now in your life, you are at event 2. Event 5' is the point in the future when you will travel back to the past and event 5" is the point in the past you travel back to. In essence, 5" is its own point in time and it replaces event 3 chronologically. 3 and the rest of the numbers (events) greater than 3 are displaced and changed permanently but continue to progress in the same order once we reach and pass 3.
Now, with you currently living at 2, you are neither aware of, nor affected by changes that have been made by your backwards trip, and this will hold true until you reach 5' and experience the journey to the past yourself. Once you get to 3, the changes would have been completed. Before 5', you might not know of it, but the version of you at 5" has shaped what leads up to your existence at 5' and beyond. When you return to 5' after having completed your roundtrip, time will continue normally again (unless you make any further trips to the past).
What we’ve just discussed has one significant rule. If you were to be present at 5", you must also be present at 5'. If you were to travel to the past, you must also be alive at the moment you travelled back in time. Seems obvious, but this simple fact is often lost in backwards time travel conversations, as you’ll see now.
We can finally revisit the grandfather paradox and find a solution that makes sense. To go to kill your grandfather before he’s had any children would mean you travel to a point before you were born, from a point where you are very much alive. This would be like travelling to a negative number in our number line analogy, let’s call this point -9. If you did this, you must also be present at the positive number from where you travelled back from, which must be a point after your birth. Let’s define that point as 6. If you were born, alive, and able at 6 to travel back in time to -9, and whatever changes you make at -9 affect your future leading up to and past 6, then you must still be alive when you return back to 6. You must still have been born and lived until 6 at the least. Simply put, this means you physically could not have killed your grandfather.
So there is only one solution to the paradox that makes sense: that it simply does not exist. The paradox and its conditions cannot exist within our previously defined laws of time travel to the past. The solution is not finding a break of understanding within its infinite loop of reasoning, rather it's discarding the possibility of that same loop initiating in the first place.
Though structured in a logical manner, this reasoning has been entirely intuitive. In reality, we have no mathematical proof that time travel to the past is even possible, and we certainly do not have the energy to make it happen if we stumbled upon an equation that proves its possibility. Even if we did, the constraints with how much the past can be changed to have it lead to our present state in time are unreasonable to allow for any real change to occur with travel back in time. It is realistically impossible to go back to the past, and even more impossible to change anything that’s not already happened.
However, this doesn’t rule out the possibility of you being able to experience time travel. And I don’t mean looking at a star or planet far away and seeing it with the lag of the time it takes light to reach our eyes from the celestial body. I mean actual time travel. Not to the past, but to the future.
Ever watched Interstellar? The future might be unchangeable with what scientific knowledge we have now, but it’s not unreachable. Maybe not in the way that you might imagine, but we have good proof that you can, in fact, experience the future way early. Really, we have good proof that you already have.
Introducing: time dilation!
And with it comes time being completely personal to you. Your time can be very much different from someone else’s time, and we have a rather familiar man named Albert Einstein to thank for making it all easier to understand.
Einstein’s theories of relativity help explain time dilation — the slowing of time as a result of relative velocities, and/or gravitational potentials. Though we can’t really mess with gravity on a large enough scale just yet to induce time dilation through it, we know how it works. In fact, if you wanted a scientifically-backed example of gravitational time dilation, check out Miller’s Planet from Christopher Nolan’s Interstellar. But another way to experience time dilation is going really fast, and as for going really fast… well we’ve already done that.
Time is very closely related to speed and light. Put those two together and you’ll find that time is also very closely related to the speed of light. The speed of light is the fastest something can go for a good reason, but instead, I like to think of it as the fastest nothing can go. Nothing with mass, at least. Photons, quantum particles of light, have no mass and we know that the lower the mass of something, the faster we can make it go. So, it’s only fitting that a massless particle defines the upper limit of speed in the universe.
Now, why does speed matter in our quest for time travel? And I’m going to go on a bit of a tangent right now, but I promise it’ll all converge to make sense soon enough.
As Einstein determined, time is relative. The faster you move (and the closer you are to the speed of light), the slower time ticks for you relative to a stationary or lower-speed observer. The catch is that you wouldn’t quite notice. To illustrate this, let’s say you were going on a trip to space on a super-fast spaceship. You and your friend who stays here on Earth both have the same exact watch and they are perfectly synchronized. When you’re back from your space trip and land on Earth, the time on your watch will be behind the time on your friend’s watch. You just saved some time.
“Pffft… a watch ticks a little slower when you go fast into space… so what? How does that prove any actual change in time has happened? I have to readjust my clock here on Earth every week because it runs slow. Does that mean I’m time travelling?”
—You, maybe.
Okay, perhaps that clock readjustment thing is a little personally motivated, but it’s never a bad idea to be skeptical of a major idea like this. But let me provide a real example that should get you that much closer to understanding time dilation and its effects.
Muons are a type of fundamental subatomic particle typically found in the nucleus of an atom. They are made when high-energy cosmic rays collide with the particles in Earth’s atmosphere. But the fun thing about muons is that they’re highly unstable and have an average lifespan of just about 2 microseconds (0.000002 seconds). Oh, and they can travel at 99.4% of the speed of light. That means in their 2 microseconds of life, they should be able to manage to travel about 600 metres from the atmosphere down. Impressive, but not very meaningful.
Or so it seems.
These muons are created about 10 kilometres above sea level and decay in two-millionths of a second, so really, they should get nowhere near the surface of the Earth. But what if I told you that approximately 10,000 muons touch every square meter of the earth’s surface every minute? That’s about one for every meter on the way down. In fact, they can travel up to 16 kilometres in their tiny lifespan of just 2 microseconds.
So, how does this happen? How does the particle travel over 10,000 metres in its lifetime when it should only go about 600 metres? The muon still lives for the same amount of time and travels at the same speed, so how does it cover so much more space? Time dilation. The sheer speed at which they travel makes their internal decay clocks tick slower relative to ours. We, as low-speed observers, only observe the muon to live for 2 microseconds. And the same muon (yes, we’re giving it a consciousness now) itself perceives its lifespan as 2 microseconds too. The difference is that relative to human time, the muon’s microseconds pass a whole lot slower than ours. It experiences time differently—slower. And you would know this to be a fact already if you could feel muons colliding with your body because I can assure you that’s happened many times in your life before.
Okay, cool. Muons can technically last longer because they move at ridiculously high speeds and therefore time is slower for them. But again, how does this relate back to the initial problem of getting us into the future? First of all, we have particle accelerators that can simulate this perceived increase in lifespan with particles of our choosing. So we’ve got technology that’s on the right track. Secondly, it’s important to note that a muon doesn’t (unfortunately) have a watch it can wear, so we can rule out that time dilation is just a watch malfunction. But most importantly, if you’re living in a situation where time is slowed relative to a stationary or low-speed observer, that form of time becomes your normal time. Your time is personal, remember. That means your biological systems also follow your relative time. You’ll see where I’m going with this very soon.
But first, let’s get back to a human example. And for fun’s sake, we’ll make it extreme. And for my sake, I’ll have genius Prof. Brian Cox explain this one*:
“Let’s say that we catapulted Jim in a rocket flying out into space at 99.94% the speed of light for 5 years according to his watch. Then, we tell Jim to turn around and come back. It takes another 5 years to get back to the Earth, so for him, the journey would take 10 years. But for us, with our watches ticking faster than Jim’s (because of the immense speed he is travelling at relative to us), 29 years would have passed.”
—Prof. Brian Cox, 2015.
And that’s where time travel starts to become apparent. Having only lived 10 years upon reentry to Earth, Jim has found himself 29 years in the future of the planet since his departure. He’s gotten the iPhone 43 today whereas he’s only lived long enough, relative to the other Earthlings, to be able to see the iPhone 24. And hey, he’s aged 19 years less than everyone on Earth too!
That’s time travel. Time travel into the future. The only issue with accepting this is that when we think of time travel, we think of being able to go back and forth in time without any major loss or repercussions and we think of it being instantaneous. Rather, if you want to time travel, you’re going to have to do a lot of waiting.
So when can I sign up?
We’ve established that time travel to the past is messy and largely impossible. We’ve also established that, theoretically, big-time time travel to the future is very much possible. In fact, we time travel a bit every day when our time is slowed riding in a car on the highway relative to a pedestrian, just that the factor by which time dilates is unnoticeable. Unfortunately, we don’t have the technology to experience an adventure like Jim’s yet, and there’s no guarantee we ever will. It would require an unfathomable amount of energy for us to send a human to space at 99.94% of the speed of light. But with all that mentioned, forward time travel is theoretically possible, and very much proven, just not in the way that we might have imagined when choosing our pick for the superpower we wanted as kids.
So, let’s revisit the question we started with: is time travel real?
The answer: time travel is(n’t) real.
*If you were interested in watching Prof. Brian Cox and Neil deGrasse Tyson talk about time dilation and time travel, here are two great videos that inspired whatever you read above:
