Why Time Travel Kills Free Will (Part 2)

Image by Quatar, 7/2007

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 the beginning of last week’s post, I pointed out that “wave-particle duality” — the ability for electrons to behave as either waves or particles — was one of the most intriguing discoveries in physics during the twentieth century. This discovery motivated (at least in part) the exploration and development of quantum mechanics, a physical theory that was (and still is) incredibly successful — not just at explaining a wide variety of previously known (and sometimes quite puzzling) phenomena, but also at predicting an equally wide variety of new (and sometimes quite puzzling) behaviors. (I am not exaggerating when I say that quantum mechanics basically predicts and explains most of the field of chemistry.) It’s an absolutely fascinating field, and sometimes produces incredibly surprising results. The two that are important to our discussion — Young’s double-slit experiment and the atomic structure of the hydrogen atom — are direct consequences of the wave nature of electrons, and will draw directly upon last week’s discussion of waves and superposition. (Fair warning, for those who either haven’t read it or are a bit fuzzy on how waves interact!)

Young’s Double-Slit Experiment

In 1801 this guy named Thomas Young came up with the idea of shining light at a pair of slits. (There’s a long back story on why he thought to do this; needless to say, wave-particle duality isn’t as new as most physicists would have you believe.) His argument was basically that light was a wave, and because of this, the light wave coming out of one slit should interfere with the light wave coming out of the other slit. In some places the two waves would experience constructive interference, making the light brighter; in other places they would experience destructive interference, canceling out the light altogether. In other words, if the light coming out of one slit looks like this

Image created by Lookangmany, 10/14/2011, with thanks/help from Fu-Kwun Hwang and Francisco Esquembre

Image created by Lookangmany, 10/14/2011, with thanks/help from Fu-Kwun Hwang and Francisco Esquembre

then the light coming out of two side by side slits looks like this:

Image by Quatar, 7/2007

Image by Quatar, 7/2007

Note that the alternating bright and dark lines are the peaks and troughs of the wave diagrams from last week’s post; in both of these images, the high parts of the wave are lighter, and the low parts are darker. More importantly, notice how in the second picture the two waves form a set of bands that seem to radiate outward. The center band (along the center line of the image) ripples bright and then dark; this, then, is a line of pure constructive interference. On either side, however, are lines where there is no rippling at all; it appears that nothing is happening in these places. These are lines of pure destructive interference. The band pattern, then, is an alternating set of lines of constructive and destructive interference! Unfortunately, for many waves we can’t see what they’re doing in the open space; the only way for us to see this interaction pattern is once it interacts with a viewing device — such as the screen (or some other detection system) marked in grey on the far right side of the image above. This is the place where the light (for Young’s experiment) shows up.  Looking at the screen, then, we should see alternating bands of bright and dark lines — bright lines where the constructive interference band strikes the screen, and dark lines where the destructive interference band strikes the screen. On the image above, those points are marked out as red and black spots, respectively. Therefore, when something passing through two slits produces a set of alternating bands (called an interference pattern) like the one below…

Image created by Epzcaw, 7/23/2011

Image created by Epzcaw, 7/23/2011

…that something is wave-like in nature. If you’ll remember, last week I said “all we really care about is how waves behave”; this interference pattern is the direct marker of a wave source. This experiment can be (and has been) done with electrons instead of light and electron detectors instead of a screen; surprisingly, the same result occurs. (I say “surprisingly” because this was not the result that the physicists who did this experiment expected!) For a variety of reasons, physicists had trouble accepting that electrons could act like waves; this lead to a series of increasingly complex experiments that severely stressed our intuition about how things work at the smallest natural scales.

Things get weird.

(In the interest of keeping things brief, I’m simply going to describe the results of a series of experiments. If you’d like more information on each, please check out the Wikipedia page on the Double-Slit Experiment.)

Presented with an experimental result that didn’t make a whole lot of sense, physicists thought that maybe the electrons were exhibiting some kind of collective behavior. As a group, they acted like waves, but perhaps individually they would act normally. “Surely each and every single electron wasn’t acting like a wave and interfering with itself,” they thought. When they sent electrons through the slits one at a time, they saw a single dot appear on the screen. However, if they sent enough single electrons through, one at a time, the interference pattern reappeared. Evidently, a single electron acts like a wave; it can and does interfere with itself.

Physicists found this very hard to believe. Therefore, they cheated! By setting the experiment up in a specific way, they could peek and see which slit the electron was going through without blocking the electron’s path through the device. As before, they let the electrons through one at a time — and the interference pattern ceased to exist! Evidently, peeking served to destroy the wave-like nature of electrons. So the physicists decided to get crafty. To eliminate other possible influences, they set up the experiment so that they could wait to make the decision about whether or not to measure which slit the electron had gone through until well after the electron had gone through the slits. They would wait until the electrons had almost made it to the screen before peeking — and the interference pattern would vanish! Not only did peeking destroy the wave, it seemed to destroy it after the fact! Or, as Wikipedia describes it, “A variation of this experiment, delayed choice quantum eraser, allows the decision whether to measure or destroy the ‘which path’ information to be delayed until after the entangled particle partner (the one going through the slits) has either interfered with itself or not.[4] Doing so appears to have the bizarre effect of causing the outcome of an event after the event has already occurred.” As it turns out, one way to explain this result is that the electron ignores spatial distances when it comes to such decisions; quantum mechanical effects somehow ignore location.

This has been observed even more directly using another phenomenon in quantum mechanics — quantum entanglement. I won’t jump into this thorn bush; instead, I’ll simply say that a study from early 2013 tried to measure the speed at which changes in quantum properties propagated between physically separated entangled particles. At their best estimate, these entangled particles react to each other at a minimum of 10,000x the speed of light, and most likely, instantaneously. As the article states, “entanglement dynamics may operate external to time, or at least may ignore time as it ignores distance.” We’ll come back to this point later on.

Further study of quantum mechanics has demonstrated that these points are not simply flukes, but are part and parcel of how the world works at small scale. In fact, the wave-like nature of electrons can beautifully explain the structure of the hydrogen atom; that will be our stop next week.

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