Why Time Travel Kills Free Will (Part 4)

Image created by Terabass, 8/21/2014

Image created by Terabass, 8/21/2014

Why A Working Time Travel Machine Implies That There’s No Such Thing As Free Will

Part 1: Waves and Superposition
Part 2: Basic Quantum Mechanics: Young’s Double Slit Experiment
Part 3: Basic Quantum Mechanics: The Structure of Hydrogen
Part 4: Spacetime and Wormholes
Part 5: The Death of Free Will

At this point we’re almost ready to discuss time travel and free will; the only piece we’re missing is the time machine itself! Now, I’m not going to get into the specifics of actually building a time machine — after all, if I could do that, I’d be too busy conquering the world with my army of robot dinosaur ninja-pirates to explain how the machine works. Instead, I’d like to talk about some of the assumptions I’ll make about how the machine works, and why.

Assumption 1: The time machine is capable of moving backward in time.

Let’s face it — a time machine that only goes forward just isn’t that interesting! Besides, traveling forward in time is easy — you’re doing so right now, at a rate of one second per second. Traveling into the future faster than this is more difficult, but well within the bounds of currently understood physics. What generally captures the imagination, however, is backwards time travel — moving to a point earlier in time than when you started. (This includes, for example, traveling into the “future”, then returning to the “present”. From the point of view of those in the future, you’re traveling backward in time.)

Assumption 2: This machine transfers physical items.

Although there have some time machines that involve only mental travel (see TV Tropes for examples), most popular representations involve transferring physical objects through time — the travelers, their possessions, and often the time machine itself. As it turns out, the holographic principle may make this assumption unnecessary; even still, it’s easier to understand the point going forward if the time transfer involves physical objects.

As it turns out, there’s already a hypothetical method of time travel that embodies these two assumptions — an Eintein-Rosen bridge, or as they’re more colloquially known, a spacetime wormhole.  In short, wormholes are a proposed method of connecting two different regions of space with a tunnel.

Image created by Panzi

Image created by Panzi

For example, the image above depicts a simple analogy. Imagine that the universe is a sheet of flexible material. Light (and stuff) normally have to take the long path between any two points on the sheet. This path is represented by the red path in the image above. However, if the paper is folded over, then a short bridge (the wormhole) can be constructed to provide a shorter route — the green path.

“But,” you may be asking,” it looks the wormhole just makes a long trip shorter. That has nothing to do with backwards time travel!” As I said, there’s nothing in physics that currently prohibits traveling into the future faster than normal. If one end of the wormhole is taken on such a trip, then not only does it bridge space (e.g. going from “here” to “there”) it also bridges time, as it goes from “now” to “future”. And because the bridge is bi-directional — there’s nothing to stop you from going either direction through the wormhole — anything in the “future” time of the wormhole can travel back to “now” as well. Are there other possible conceptions for a time machine? Probably. However, any time machine that fits the two assumptions above will be functionally indistinguishable from one built out of an appropriate wormhole. For the sake of clarity and simplicity, then, we’ll stick with talking about wormholes.

There are two interesting things to note here. The first that the wormhole must connect the physical spacetime of the present to the physical spacetime of the past in order for a physical object to be transferred. Second, and perhaps more importantly, this physical connectivity creates the potential for loops1. (In the picture above, for example, could travel along the red path, fall into the wormhole, and end up back on the red path… only to fall back into the wormhole, and end up back on the red path… only to fall back into the wormhole, and end up back on the red path… I think you get the idea.) When combined with the quantum mechanics we’ve discussed over the previous three weeks — superposition, constructive/destructive interference, and instantaneous quantum entanglement — we run into some serious philosophical problems. We’ll tackle that next week, in the final installment.

1 As it turns out, this requires a change in the topology of the universe, which is a Big Deal©. Poking new holes in the fabric of space and time is highly nontrivial — it’s like an ant punching a hole through the center of a watermelon so that it doesn’t have to walk around the outside. Any ant with that much power is more likely to destroy the watermelon completely than create a safe and stable path through it.

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