The Many-Worlds Interpretation Explained

By GrabBag | CrackpotPhysicist | 10 Oct 2020


Everything in the universe is defined by the small fundamental stuff that composes it, so scientists are very interested in how small stuff works. Unfortunately, things get really weird at the subatomic scale. I covered this in greater detail in my last article about the Copenhagen interpretation, which you should definitely check out if you're not familiar with particle-wave duality.

So, quantum mechanics tells us that things act like waves sometimes, and particles other times. Experimental evidence shows us that waves always act like particles when a measurement is taken. The Copenhagen interpretation tells us that measurements cannot be taken without first interacting with a quantum system, thus causing the waves to "collapse" into particles. There are problems with this ideology however, because things interact at the quantum level all the time and still maintain "coherence," which is a fancy term to describe a system of waves. That interpretation doesn't formally define the difference between a normal quantum interaction versus a "measurement," and this is the crux of the problem. 

So what exactly is a wave? Well, let's say you're listening to a baseball game on the radio. You can't see exactly what's happening, but you will hear the announcer's commentary after events occur. You happen to know a LOT about baseball (I don't!) and you calculate that the batter has a 10% chance of getting a home run, 70% chance of getting first or second base, and a 20% chance of striking out. Right now the batter is a wave: he's a set of probabilities. Then the announcer says it's a home run; now the batter is a particle - he has definite physical characteristics. So the argument could be made that things are always particles, and the wave function just represents our ignorance; by making an observation we're really just discovering more information about the system, and thus the wave function is no longer necessary. This is called a quantum informational interpretation of quantum mechanics, but it's easy to prove wrong with a simple experiment.

There is no more simple and insightful experiment in all of quantum mechanics than the double-slit experiment. Here's the setup: you have an emitter, two slits, and a detector. The emitter emits photons, the photons can travel through one of the two slits, then get detected at a specific point on the detector. Well, if photons acted like particles then the detector would detect two discrete lines of collisions.

Double slit with particles

Except, when this experiment is performed in real life, you get an interference pattern:

An interference pattern when waves overlap.

An interference pattern forms when the crests and troughs (highs and lows) of two waves overlap each other.

This interference pattern emerges even if particles are emitted one at a time. You can see a real example of this experiment here:

Remember what we said before: the quantum wave is a wave of possibility. And the different possibilities of a particle interact with each other, causing an interference pattern. So a wave isn't just a result of our ignorance.

The wave defines a physical reality about particles: that all possible timelines of a particle exist simultaneously. This is the many-worlds interpretation of quantum mechanics. Instead of saying that waves "collapse" into particles, the wave actually loses coherence, meaning all the different possibilities of the particle become separate universes and all continue to exist simultaneously. This branching of realities is a continuous motion, as natural as water flowing down a river.

The authors of quantum decoherence don't think their theory explains the measurement problem, but here's the crazy part: it does! This is where we need a formal definition of consciousness. Luckily, it's a very easy thing to do! Consciousness is the experience of matter and energy as it travels along a specific timeline. I think we can all agree on this, yes? What are you made of? Matter and energy. What do you do? You travel forwards in time, along a specific path.

Anything you observe will necessarily be a specific reality, not a collection of independent realities. When you observe something, you will only ever see a single outcome, but this is just one perspective. You also exist in other realities and observe different outcomes; but these different realities are decohered and no longer share information.

So then you could ask the question: "why do I experience this specific timeline and not the others? Isn't this the same problem as before?" Actually, it's not. Because all the different possibilities of yourself could ask the same question - it's meaningless. It's like asking, "why is the human race on Earth? Why this specific planet?" or "why was I born to these specific parents?" The real question is: why not? After all, you're experiencing anything at all right now because you're alive. How many sperm and how many eggs are there? What are the odds that you were born? Think about all the matter and energy in the universe that's not "alive" right now: it's not asking "why." You're asking the question because you can.

Let's try asking different questions. Can I choose which timeline I'm on? Can I interact with other timelines? If a photon can experience coherence, then can I? These are some very interesting questions that I will try to cover in future articles. Thanks for reading!

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