Time Travel Isn't Possible…Or Is It?

# Time Travel Isn’t Possible…Or Is It?

According to special relativity, the three dimensions of space and the single dimension of time are woven together like a fabric. It’s difficult to think of them as separate entities; they’re all part of the same thing: space-time. We can’t think about motion in space without thinking about motion in time, and vice versa. Left-right, up-down, back-forth, and past-future are all considered equal.

And yet, time does seem a little different. We have complete freedom of movement in space, but we cannot escape our future. Time appears to have a “arrow,” but spatial dimensions seem to be ambidextrous. Given the unity of time and space, the obvious question is: Is time travel possible in any shape? In what circumstances? At all?

### Into the future: Sure

Oddly enough, the answer is yes! We cannot avoid our futures, but we can manage how fast we move through time. This is due to another relativity lesson: not all clocks are the same.

The speed at which you move through space impacts your speed through time. In the succinct phrase: moving clocks run slow.

If you could build a large enough rocket (don’t ask how; that’s an engineering problem) to provide a constant acceleration of 1g (9.8 meters per second per second; the same acceleration provided by the Earth’s gravity at its surface), you could travel to the center of the Milky Way galaxy — a healthy 20,000 light-years away — in just a couple decades of your personal time.

You could pause for a few hours, have a picnic near Sagittarius A* (the black hole at the core of the galaxy), and then re-board your rocket to come back to earth.

By the time you return, you’ll be eligible for retirement benefits, if the institution that offers such benefits still survives, because while you only traveled for a few decades according to the clock aboard your ship, around 40,000 years will have passed on Earth.

### Closing the loop

While time is relative, it always flows in the same direction for everyone. To question if we can go backwards is the domain of general relativity (GR) – this is the mathematical language we use to understand not only gravity, but the entire relationship between space-time and motion.

In GR, we ask a little more technical question: Is there any arrangement of matter and energy (the stuff that warps space-time) that allows closed time-like curves, or CTCs, to exist? I realize it’s jargon, but it’s a great term to use at parties. “Curve” refers to a path, “time-like” refers to never moving faster than the speed of light, and “closed” refers to it return to its starting point — in other words, its own past.

### The possibilities are finite

There are around a quarter of known space-time configurations that allow CTCs, or time travel into the past. For example, Kurt Gödel (of Gödel’s Incompleteness Theorem fame) discovered that if the universe’s expansion is accelerating (which it is) and the universe is also rotating, CTCs would be allowed and we could travel into our past on a whim.

As far as I can tell, Gödel used this solution to suggest to Albert Einstein that maybe GR wasn’t all it was made up to be — after all, shouldn’t any self-respecting natural-world theory avoid such an obviously absurd solution?

However, Gödel’s criticism was moot because all observations show that the universe is not rotating, hence that solution does not apply to our universe, and time travel into the past is prohibited.

Ah! But what if we were to construct an infinitely long massive cylinder and set it spinning on its axis near the speed of light. It would pull on space-time around it, and certain pathways would wind up in their own past. We’re lucky that there aren’t any infinitely long massive cylinders in the universe, or we’d have to worry.

Wait, I’ve got one: You could waltz right through the wormhole throat and end up in the past if you created a wormhole (a shortcut between two distant locations in space-time) and sent one end racing out near the speed of light and bringing it back. What exactly is it? Wormholes require “negative mass” to exist, but does negative mass exist in the universe? Oh, well.

### Into the past: Nope

It’s the same story every time (pardon the too-hard-to-resist pun). For every scenario we concoct in general relativity to allow CTCs and time travel into our own past, nature finds a way to confound our plans and rule out the scenario.

What exactly is going on? In theory, general relativity allows for time travel into the past, however it appears to be ruled out in every case. It seems like something funny is afoot, that there ought to be some fundamental rule to disallow time travel. However, there isn’t one. We are unable to identify any particle interaction at the subatomic level that definitely prevents CTC formation.

The inevitable progression of time from the past to the future resembles another indomitable law of nature: entropy. That is the iron law of thermodynamics, which asserts that closed systems go from ordered to disordered. (This law explains why an egg will never just unscramble itself if left alone long enough.) Is time related to entropy? Perhaps, but that’s a subject for another article….