Disclaimer: I am not a scientist by any stretch of the imagination. I am, however, someone who thinks. As someone who thinks, time travel is one of the many subjects that fascinates me. Other (smarter) people have thought of the premise which I outline here, and it appears I could either be right or wrong. If you are scientist, a time-traveler, or just someone who also thinks, please comment with your thoughts. Actually, if you’re a time-traveler, you should probably be doing something more useful, like killing dictators or inventing rock and roll.
Time travel is a very complicated concept. Leaving aside the incredible feat of engineering and physics that time travel would necessarily entail, it could also open up a veritable door to hell when it comes to paradoxes. From killing your own grandfather to accidentally creating a dystopian future, time travel makes it quite easy to screw everything up with the slightest change to the timeline (otherwise called the butterfly effect). These paradoxes have been studied (and laughed at) for a very long time, but they mainly focus on logical and metaphysical inconsistencies. Right now, I want to talk about physics.
You may have heard of a little thing called the Law of Conservation. Since matter and energy are, in certain senses, interchangeable, this principle applies to both matter and energy. As you may know, the Law states that neither of these two things can be created or destroyed. All the matter and energy in the universe right now was present at the moment of the Big Bang. The same matter and energy, in some form, will still be present long after we are dead. Someday your pet iguana might become part of a flying car, and chances are you yourself are partly made of William Shakespeare’s atoms. Nothing is ever lost (except when it comes to black holes); everything just changes. If the Law of Conservation is global—that is, it applies everywhere all the time—it may be true (and this will be important later), that the universe has a finite volume. So what does this mean for time travel?
First, we have to talk about teleportation. It’s a pretty cool concept all by itself. Actually, when you think about it, it’s closely related to time travel. Let’s say I want to teleport an apple from one place to another. In theory, this experiment should work in the same fashion whether I’m moving the apple from Egypt to China or if I’m moving it from my living room to my kitchen. To make this scenario easier for my hypothetical self, I’ll only move it across my house. So I put the apple in one teleportation box at point A (my kitchen), and in a flash, it ends up in the box at point B (my living room). But wait, aren’t I destroyed and/or creating matter? Not really. Even though I am making the apple disappear and reappear, I am not violating the Law of Conservation. Although the apple seems to pop out of existence for a moment, it comes back almost instantaneously. I’m not really destroying or creating any matter or any energy. In order to teleportation to actually be possible, I’d have to take the apple apart atom by atom, change some of it into energy, move it across some space, then re-arrange it back into the same apple. This would be incredibly difficult, yes, but taking into account the laws of physics, it really is possible. When we get into the mechanics of time travel, however, things get much, much weirder.
Traditionally, there are three states of time: past, present, and future. If we delve into these states, we start to notice the similarities and differences between time travel and teleportation. With time travel, you are transporting an object from one place to another almost instantaneously, but the idea is that it all happens in the present. Sending something—an apple, your iguana, or yourself—into the future is basically teleportation, except that instead of moving that object across space, you are moving it across time. Space and time are pretty much the same thing, so it appears that time travel with a time machine should be possible. The only difference so far has been that we cannot travel faster than the speed of light. But with worm holes or an Alcubierre Drive, traveling across space and time faster than light may actually be possible. But again, this is all about either traveling in the present or the future. The question is: how could we travel into the past?
In order to really grasp time travel, first we must to understand the expansion of the universe. When people say that the universe is expanding, it’s easy to assume that the universe is like a giant box growing ever larger, and we’re all inside it, watching the walls grow ever-distant from us. This analogy is closer to the truth than some, but here’s a better one: Imagine that you have a partially filled balloon in one hand and a marker in the other. You make little dots all over the outside of the balloon with the marker (which represent matter) and begin blowing up the balloon. As the balloon expands, the distance between the dots increases. The balloon itself, the material it’s made of, represents the fabric of space and time. That is what is expanding.
I can guess what you’re thinking: But didn’t you just say the universe has a finite volume? Sort of. The universe really is expanding, but at any given moment, at any given “present,” the universe has a finite amount of matter and energy, and therefore a finite volume. But as time goes on, the balloon gets bigger and bigger; the surface area between the dots increases. This means we can potentially add “dots” (matter and/or energy) from nowhere by sending things into the future, with a little room to spare. So, by avoiding violating the Law of Conservation and by avoiding tampering with the timeline, traveling into the future is by far easier and safer than traveling into the past, but let’s push it a little further.
Have you ever put too much ice in your water and spilled it everywhere? As you probably know, it’s because the ice displaced the water, and the water had nowhere to go but out of the cup. If you’re paying attention, this usually isn’t a problem, because in order for your water to spill, the volume of the ice has to be greater than the volume of water in your glass, and common sense usually kicks in before this happens. As I said earlier, the universe has a finite volume at any given point in time. So really, the universe is a big cup, or maybe a really huge swimming pool, filled with stuff. As I said earlier, it may not be impossible to send something into the future, because the pool may be getting bigger (and would therefore allow more space for matter and energy to fit), but sending something into the Pool of the Past would be…problematic.
The past is, more or less, a point in time. For this case I’ll choose July 21, 1969 (the day of the moon landing). On July 21, 1969, the universe had a finite volume. In fact, it had a smaller volume than we have today. So, even if the universe expands in proportion to the amount of matter and energy in it, it was still smaller on that day than it is today. Just like the balloon, we cannot fit as many dots on a deflated balloon as we can on an inflated balloon. So, to really make this an accurate comparison, we have to place a seal over our Pool of the Past. This can’t be any old pool cover: it has to be indestructible, rigid, and airtight, because if any water escaped, it would violate the Law of Conservation. You are luckily outside of this pool, and you have a magical time machine or a teleportation device (remember, they’re almost the same thing), and you have the power to send something into the pool. Ah, but what to send? Well, a pool is very big and you want to notice the displacement of the water, so how about a grand piano? Big enough, right? Let’s make some history.
You start up the time machine or teleportation device or whatever, and it whizzes and buzzes and beeps, and finally…ding! like a microwave, it’s done. The piano has popped out of existence here, and popped back into existence there, inside the Pool of the Past. But wait a minute; because of the Law of Conservation, the pool has a finite volume and no water can ever, ever escape. So, how would you insert a piano into the water without displacing anything? That’s the question, isn’t it? How could we ever put that piano into that pool without something going horribly, horribly wrong? There are many scenarios that could come of this, most of them apocalyptic. The pool may explode (which means we’ve compromised the space-time continuum). The piano may just reject the time travel and stay in the present, or perhaps the atoms in the piano would attempt to convert to matter and energy back and forth so as to satisfy the Law of Conservation, possibly producing a big nuclear explosion. Who knows?
I can yet again guess what you’re thinking: The Pool of the Past isn’t exactly like our universe. Our universe has lots and lots of empty space. Instead of risking sending yourself back in time only to end up wedged inside a coffee table, why not put yourself into empty space? There’s plenty of room out there. The pool has no empty space for the piano to go, even on July 21, 1969.
Good thinking, but still wrong. For all the vast amounts of vacuous space that our universe has—more space than we could ever comprehend—the Law of Conservation still applies here. Even if we drained the pool a little bit, if that piano displaces too much water, we’d still have a complete disaster on our hands. While traveling into the future may be safer than traveling into the past, there’s no way to guarantee it’s completely safe and in-line with Conservation. So, before your future self has any ingenious ideas about going back in time, remember: you may just violate a law of physics along the way and completely annihilate the universe, which wouldn’t be very nice.