Why Time Travel Kills Free Will (Part 1)

Plane Wave AnimationA while back I was asked about potential consequences of time travel. After a bit of thought, I replied, “Well, if time travel is possible, it almost certainly means that free will doesn’t exist.” It’s an assertion that has often been met with confusion and skepticism, so I have repeatedly promised that I would write out a full explanation of how I came to that conclusion — and here it is! However, because I’m assuming that anyone reading this does not have a background in physics or chemistry, I’ll also include a “brief” explanation of some basic quantum mechanics as well. This is entirely too much for a single blog post; therefore, without further ado, here is my five-post series on…

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

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

One of the most intriguing discoveries in physics during the twentieth century was the ability of electrons to behave as either waves or particles — the so-called “wave-particle duality” that underlies much of the confusion and consternation regarding quantum mechanics. To understand why this is such a huge shift in thinking, however, first, we need to understand waves and how they interact with each other.

What is a wave?

Basically, waves are the oscillations or vibrations that occur within a material or at the interface between two different materials. What most people think of when they think of waves — swells on the ocean, or waves crashing onto shore — are the second type of wave. However, we are more interested in the first type of waves. (Examples of these include sound waves, earthquake waves, and electromagnetic waves.) There are a ton of demonstration videos regarding waves and wave behavior online; for us, however, the main point is the oscillation aspect: waves involve something moving back and forth repeatedly. What that “something” is doesn’t really matter; all we really care about is how waves behave. In particular, we’re interested in what happens when two waves meet (or when a wave meets itself) — a process called superposition or interference. (For our purposes, those words mean the same thing.)

Wave Interference

Remember, each wave is actually due to “something” (doesn’t matter what) moving back and forth. So when we draw a picture of a wave, like


Red wave


what we’re really drawing is how the “something” is shifted by the wave at a particular point in time. (Note, as well, that waves are dynamic entities — they’re always moving, like the picture at the start of this post — so these pictures are merely snapshots at one point in time, too.) A second wave drawing, like


Blue wave


similarly indicates how the “something” is shifted by this second wave. (I’ll refer to these as the red wave and the blue wave.) Because the wave is simply a shift of “something”, when two waves meet, they are both trying to shift the same “something”. For example, imagine you’re in a boat on the ocean, and two swells come along. If both of them are trying to move the water under your boat upward at the same time, the water (and you!) will rise higher than it would if only one wave or the other were passing through. Conversely, if one was trying to move the water upward, while the other was trying to move the water downward, the water would move less than it would from either individual wave. In other words, when two waves meet, their effects combine — sometimes in complex ways:

Image created by Yuta Aoki, 11/28/2006

Image created by Yuta Aoki, 11/28/2006

In the image above, the red wave (moving to the right) interferes with the green wave (moving to the left), and the result is the blue wave (that isn’t moving either direction — a standing wave). For our purposes, then, there are two primary types of wave interaction that we’ll focus on. The first is when the two waves add, and result in something larger than either one:

Pure Constructive Interference

Blue + Red = Purple

If the shift of the two waves is directly opposed, on the other hand, the two waves will partially cancel, and result in something that is smaller:

Partially Destructive Interference

Blue – Red = Purple

(Note that I’m specifically focusing on the cases where the two waves line up perfectly; at any other point in time we’ll get partial addition and partial subtraction, depending on how imperfectly lined up (aka “out of phase”) the two waves are.) But in this last case, where the waves cancel, something quite interesting happens when the waves are the same size but directly opposed — the two completely cancel each other out:

Pure Destructive Interference

Blue – Red = 0!

I mentioned two primary types of wave interaction for our focus: the first one, where the waves add up, is called (pure) constructive interference; the second, where the waves completely cancel each other out, is called (pure) destructive interference. (The one in the middle that we’re not using is partially destructive interference.) These two extreme cases lay the foundation for understanding the basics of quantum mechanics, and (surprisingly) for describing almost everything there is to know about the hydrogen atom. But we’ll start tackling that stuff next week, in part two.

1 Comment

  1. Avatar

    okay, I am reading this- slowly…. and i think I understand waves (ish).
    thanks! on to the next lesson.
    Thanks, Teman.


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