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Starts With A Bang

Throwback Thursday: Quantum Reality

The nature of our Universe defies our intuition. That just might be the greatest thing of all.

“I asked the Zebra,
are you black with white stripes?
Or white with black stripes?
And the zebra asked me,
Are you good with bad habits?
Or are you bad with good habits?
Are you noisy with quiet times?
Or are you quiet with noisy times?
Are you happy with some sad days?
Or are you sad with some happy days?
Are you neat with some sloppy ways?
Or are you sloppy with some neat ways?
And on and on and on and on and on and on he went.
I’ll never ask a zebra about stripes…again.” –
Shel Silverstein

When it comes to the classical world — the world on a macroscopic scale — we all feel comfortable using the word reality. While we may quibble over the finer, technical points of the definition of the word, you and I know reality when we see it.

Image credit: NASA / Apollo 17.

There are all sorts of properties we assign to real objects: they have energy, they exist at certain points in space and moments in time, they have certain properties of motion, and are measurable and quantifiable in a variety of other ways.

This ranges from microbes here on Earth to the largest structures in the Universe: all of these are quantifiable as real in terms of energy, position, time, and momentum, among other properties.

Image credit: Adam Block / Mount Lemmon SkyCenter / University of Arizona.

But if we head into the quantum realm, down to scales so small that our classical laws and pictures break down, we discover that things are drastically different, and that reality no longer conforms to our expectations.

Image credit: Wikimedia commons user Reyk.

You might think of an atom the same way you think of a planet orbiting the Sun: an electron moving in orbit around the center-of-mass of the electron/nucleus system. But whereas if you knew a planet’s orbital properties and the mass of the star it was orbiting, you’d be able to know with certainty where that planet was and how it was moving (i.e., its position and momentum), the quantum world is a little different.

Okay, a lot different. Because you can no longer predict the position of that electron — only the probabilities of finding the electron in a certain position — as time goes on.

Image credit: Hideomi Nihira / U. Rochester, based on Z. Dacic Gaeta and C. R. Stroud, Jr.

If you’re like most people, this is going to trouble you. It is so ingrained in us — by our own experience — that objects are real, particles are real, and that these real things have definitive properties, that we instinctively start asking questions like, “Okay, where is that particle, really, when we’re not looking at it?”

And we assume that this question makes sense. We assume that there is a real position for this real particle at every moment in time, and a real momentum, and a real amount of energy assigned to it. We assume that it’s our knowledge that’s somehow limited, and so we struggle to fit this troubling observation in with our picture of what reality is.

Image credit: Science / AAAS.

It’s no wonder that quantum mechanics has a number of different interpretations behind it: we’re trying to understand reality, and yet the things that we’re observing are completely unlike what we experience as reality! Some people, and this point of view is quite sympathetic, view this as a tremendous problem. After all, there’s no consensus as to which interpretation is the “right” one, or even the best one!

Image credit: Maximilian Schlosshauer, Johannes Kofler, & Anton Zeilinger.

There are some interpretations that are demonstrably wrong: the idea that physics is local (things can only affect things they interact with), real (as opposed to complex, or partially imaginary), and deterministic cannot all be simultaneously true. So you might ask which ones are true, and I wouldn’t blame you for asking.

The problem is, not only are multiple interpretations equally valid, but none of them tell you anything more or less than any of the others! And there are plenty of valid ones; here’s a brief summary.

Image credit: Wikipedia’s comparison of interpretations of quantum mechanics.

Rather than go through what the different interpretations are, I prefer to look at it in these terms:

  • We have a new set of physical laws that describe the Universe on the quantum level: how things exist, how they evolve in time, how they interact with one another.
  • These laws allow us to predict the probability distribution of certain outcomes that we can measure, but not what the outcome of a given measurement is going to be.
  • There is an intrinsic amount of uncertainty that is always preserved, and so it is impossible to know certain properties of a system — in tandem — to an arbitrary accuracy.

That is what reality is. Different interpretations may look at them differently (maybe a wavefunction is collapsing, maybe an operator is evolving, maybe a selection is being made from an ensemble of possible outcomes), but that does not change what reality is. And it is not reality’s fault that it is unintuitive to our minds and our experiences.

Image credit: Jim Branson of UC San Diego.

If you look around, periodically, you’ll see all sorts of science articles that are published in a subfield of research known as “Foundations of Quantum Mechanics.” We all have our proclivities and preferences for what the most intuitive way to interpret the full suite of data that our inherently quantum Universe provides to us, and so long as your set of assumptions about the Universe allow you to predict results that are in agreement with the data, you’re welcome to whatever interpretation “feels” right to you.

But in the end, your interpretation doesn’t really matter. Which is why articles like this may be valid, but they’re not very interesting from a physics point of view.

Image credit: Science Daily via Texas Tech, at http://www.sciencedaily.com/releases/2014/11/141112131927.htm?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+sciencedaily+%28Latest+Science+News+–+ScienceDaily%29.

Look at that, it’s a new interpretation of quantum mechanics, because that was what we needed: another one with a myriad of untestable assumptions that give the same experimental predictions as all the others!

Unless you have a way to experimentally distinguish your “preferred” interpretation of our quantum reality from any other interpretations, what you assert doesn’t really matter. Parallel worlds? Fine, why not. An infinite number of probabilitistic histories? Sure, we can have that. A single, forever indeterminate quantum wavefunction for our Universe? Can’t see a problem there. What about a completely deterministic Universe filled with hidden, unseen, immeasurable, and nonlocal variables? We can have that, too! Or, you can simply have your original, Niels Bohr interpretation, which works just fine nearly a century after it was first proposed.

There is no difference in their predictions.

Image credit: Andrew M. Smith, Lucas A. Lane & Shuming Nie, via http://www.nature.com/ncomms/2014/140731/ncomms5506/full/ncomms5506.html.

Some people find this line of research interesting to think about, but I’m not one of them. Personally, I find “news” like this boring, in the sense that — at this point in time — speculations about the “fundamental nature” of the Universe shed absolutely no light on the Universe at all. Our particles and fields, and the interactions they experience with one another, simply are what they are. The only way to develop any sort of intuition for what’s going to happen in a given situation is… to figure out what’s going to happen in a variety of situations, until you begin to develop an intuition for it! In other words, the most lampooned quote of all time (when it comes to quantum mechanics),

“Shut up and calculate!” –David Mermin,

is actually the one-and-only thing you can actually do for yourself in order to better understand reality.

Image credit: http://library.thinkquest.org/19662/high/eng/exp-stern-gerlach.html.

In other words, it doesn’t matter how you arrive at the results, and there are many path there that are equally good. What matters is that, irrespective of how you interpret it (or even whether you interpret it), what you call “reality” at the end of the day matches what your theory predicts.

If you can do that, then your physical theory — or your favorite interpretation — is just as valid as any other. And if it doesn’t, you’re compelled to discard it. However, and you should consider this a warning, this is not without danger.

Image credit: Institute of Physics.

This carries with it the danger that you can make something as philosophically complex as you want to satisfy as fully as nature will allow whatever preconceptions you have about how reality should behave. If you demand locality, you can force it. If you demand realism, you can force it. If you demand determinism, you can force that, too. If you demand wavefunction collapse, you can make that happen.

And if you demand non-locality, or non-realism, or non-determinism, or wavefunctions that never collapse, you can force those just as easily. Even if you want an interpretation where information travels faster than light, you can make one up, and it still works! But it’s no more a mirror of “reality” than one where it doesn’t.

Image credit: Robert Austin and Lyman Page / Princeton University.

In the end, all that matters is that your method of calculating predictions aligns with what you’ve observed. And if you can get it right, then you’ll understand reality as well as anyone.

Travel the Universe with astrophysicist Ethan Siegel. Subscribers will get the newsletter every Saturday. All aboard!

So let other people be “embarrassed” for quantum mechanics, that we still don’t know which interpretation is the right one. Because the answer is they all are, the same way there’s no one uniquely right way to solve an equation like 162 ÷ 9. If you can let go of your classical notions of what an interpretation ought to be, you’ll have discovered something even better.

Image credit: Technical Services Group (TSG) at MIT’s Department of Physics.

You’ll understand the quantum reality of our Universe.


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