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Quantum Mechanics Is a Hell of a Way To Run a Universe

So why does the universe use it to make the rules? A new theory may have an answer.

by Michael Byrne
May 15 2013, 5:55pm

When you get to the very bottom of the mysteries of the universe, it might be tempting to stop early. Quantum physics is weird and difficult enough without having to explain not just what it is--"I think I can safely say that nobody understands quantum mechanics," Richard Feynman famously said--but why it is as well. Why did nature write rules for the very, very small world of particles differently than for relatively big things, like bedbugs and human beings? So, an electron or photon can be in more than one place at once, or even interact with another particle instantaneously across space-time, yet here we are in the macro world tied to the speed of light and definite positions. What the hell kind of deal is that?

A gut reaction might be that it seems totally unreasonable for the universe to be otherwise and still deliver life and, eventually, us. This is called the anthropic principle and it kind of just means that everything is like it is because it had to be to deliver us to observe it. Which is not a very good answer to any "why?" and it also happens to be way too general–sort of a bedrock "why?" and less a "why?" that specifically explains particles being in two places at once. Or, more accurately, why it has to be so that a particle can be in two places at once.

Here is one answer proposed by researchers in the new issue of Nature Communications. It's not a particularly easy answer, but it sums up well. First note that answers or reasons in science come in the form of principles, fundamental rules that all the other rules and junk come from. So what's being proposed is a new principle governing the universe and making quantum mechanics so.

A principle of 'accepting the facts' implies that a quantum bit (typically pictured as a 'Bloch ball') can look like a sphere but not like a polyhedron. Polyhedral bits have been linked to theories of discrete spacetime. By ruling out various alternative theories of nature, the principle may help to explain why the world is quantum/Timothy Yeo / CQT, National University of Singapore

It's basically a formalised version of "accepting the facts," according to the University of Singapore's Stephanie Wehne. It goes like this: the only way to get information from something (a system, properly) is to disturb it. Which actually sounds just like a restatement of what we always say about quantum mechanics: you can't observe a particle without changing it. This is the property that lies at the heart of quantum encryption–an eavesdropper can't look at a message without changing it and alerting the sender. The above principle is different, however, in that it demands the reverse. That is, if you get no information from an observation, there is no way you could have observed it.

Now that is actually something to let bounce around your skull a bit: if a thing is entirely the same before and after the observation, there was no observation. Or at least there was no useful, or new, observation. You could still convince yourself otherwise, however: say, in the case of Schrodinger's cat, existing in a superposition of alive and dead until someone opens the box and makes a determination either way. If you opened the box and found a dead cat, closed the box, and opened the box again to find the same dead cat, you didn't actually observe the cat because the cat has already been disturbed (by the first opening of the box), according to the new principle. On the second opening, you have gained no new information. Same dead cat.

Here is how co-author Corsin Pfister explained it in an email: "The principle that we suggest could, roughly speaking, be rephrased as follows: Every measurement for which you know the outcome in advance can be performed in a way such that it does not lead to disturbance. This is what we assume in our paper. From this assumption, we derive that a physical state space cannot be discrete in the sense that it only has finitely many states. This impossibility of discreteness is a consequence of the principle, not the principle itself."

As to how this actually might be tested in the future, Pfister was kind of enough to provide an explanation:

Scientists try to explain things that seem counterintuitive at first as logical consequences of things that they do find reasonable. The latter is what is called a "postulate." A postulate is, by its very definition, something for which no further justification is provided. It is a starting point of logical reasoning. On a theoretical level, you can accept it, or reject it. If you accept it, then you will also accept its consequences, because they can be derived from the postulate by nothing but logical reasoning. Our principle is a postulate, and as such, it can be accepted by theoretical physicists--or not. The importance of our work is to show the strong consequences of this innocent-looking principle.

Let me give you some motivation for why this principle is reasonable. First of all, our everyday life experience is completely in accordance with the principle. This is because everyday life is governed by classical physics, which perfectly respects our postulate. Quantum theory also satisfies the principle. Even more, it satisfies it in a much stronger form: If you know the outcome of a measurement with high probability, then you can perform the measurement such that it causes only little disturbance; the higher the probability, the lower the disturbance. Summarized, the "reasonable" theories of nature that we have respect the principle. 

Experimental physicists, however, don't just accept physical assumptions, they test them. When it comes to testing our postulate, there are two things to be mentioned. First, our principle is universal in its character, meaning that it doesn't rely on a particular physical system or quantity, but instead applies to all physical systems. Thus, falsification of our postulate is not restricted to a particular experiment.

I'm going to go think about this for a while on a rock with weed, and I suggest you do the same. Maybe ponder how your relationship to the world, or your ability to have a relationship to the world, is dependent on being able to exchange new information with it. That's less "well, duh" than it seems.

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