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I'm confused by the famous wave-particle duality mystery:

When a particle is left unobserved, it acts like a wave and can explore all classically available particle trajectories simultaneously. By looking at it, you force it to decide on a single trajectory, like going through the left or right slit, or like Schrödinger's cat that ends up being either dead or alive; the wave-like characteristics are lost.

Are there theories that actually explain this behaviour?

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This isn't a "problem". This is how the universe actually behaves. It's unintuitive, but true. – Colin K Mar 30 '13 at 2:10
No, I'm not. Maybe if you asked a more specific question you would get the answer you want. Right now, you're simply stating a fact and saying it confuses you, and the title refers to it as a problem, as if it were some illusion to be explained away. It's not an illusion, or a problem, or something that can be explained in the context of everyday experience. It is how the world actually behaves. I know it's strange, but that doesn't make it untrue. – Colin K Mar 30 '13 at 2:16
You've edited the title, but it doesn't change the answer. The explanation is that the universe actually behaves in this strange way. If you want to know "why" you're going to be asking about religion or philosophy. – Colin K Mar 30 '13 at 2:20
Outlier, you should be interested in what are called "interpretations of quantum mechanics". They are the type of theories you're after. – Mitchell Porter Mar 30 '13 at 9:41
Edited and reopened, as per meta post linked to by Christoph above. – Manishearth Mar 30 '13 at 16:35
up vote 9 down vote accepted

The answer by JKL is sufficient but I want to address particularly the Why?.

I am so confused. Why does it act the way it does?

If one reads a bit about the history of science and physics in particular, it becomes clear that physics at the ultimate end does not answer the ultimate Why?. Physics posits laws and uses sophisticated mathematical tools to theorize from "axioms", get equations, and check against the data experimentally. It finds How, from "axioms" one ends with predictions for measurements that validate them.

The Why? questions addressed to the "axioms" has the only answer: Because .

When one is validating a theory, as for example Newton's gravitational theory, and one hits a disagreement with the data, new "axioms" and new theoretical tools are developed to explain the Why of the disagreement and the new theory validated for the regime of disagreement. Special and General Relativity are an example. The history of physics has other examples : Thermodynamics, developed theoretically to explain bulk behavior, statistical mechanics, out of classical mechanics emerge out of asking How and assuming "axioms" to contain the Why. Each theory with its own regime of validity.

Within a theory a question with a Why is answered by proofs of How finally hitting on the "axioms". Quantum mechanics is the last in the series of exploring the microcosm. The Why you are asking hits against the Because of its "axioms".

There are people who continue the exploration, trying new axiomatic theories of how a quantum mechanical theory can emerge from a smaller regime where we are back to classical concepts, and contain the Why in "axioms" for their new theory. They are not successful except with small models. The bulk of theoretical physicists either ignores their efforts or proves that their new theories would violate a basic validated law as, for example, Lorentz invariance. And that is the story of Why in Physics. Ultimately the answer is Because.

Edit: "axioms" in quotation marks , to include mathematial axioms and physics postulates. Physical theories start from strict axiomatic mathematical theories where everything is contained and rigorously self consistent and add postulates in order to connect physical observables to the mathematical variables/functions. Postulates are a necessary choice to turn a mathematical theory into a physics theory and in classical theories are referred as "laws". I call them "axioms" because they are the foundation stones of the theories, if one goes the whole theory goes.

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Are there theories that explain wave-particle duality?

Yes, several actually, but they are interpretations rather than new theories.

As anna v explained in her reply, at its most fundamental level the only answer a physical theory can give to the question Why? is Because!.

It's impossible to give a more meaningful definite answer until we've taken another step down the rabbit hole (assuming we've not yet arrived at the bottom, of course).

However, you're not the only one uncomfortable with the situation, and that's where interpretations of our physical models come in, and there's a whole bunch of them for quantum mechanics.

As far as wave-particle duality goes, here are the explanations given by various interpretations:

  • according to the statistical interpretation, quantum mechanis only describes ensembles and the question cannot be answered without an underlying new theory
  • according to the de Broglie-Bohm interpretation, there are both particles and waves, the former being guided by the latter
  • according to the transactional interpretation, we have waves going forwards and backwards in time, resulting in particle-like interactions by interference
  • according to the many worlds interpretation, we have particles going all possible ways in different worlds
  • according to the consistent histories interpretation (on which I should probably read up some more, so take this with a grain of salt), the particle or wave-like characteristics are just artifacts of a particular choice of history, which gain reality be decoherence

The Copenhagen interpretation is missing from this list by intention ;)

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This answer expands on the answer given by anna v

It is easy to become accustomed to a pattern of physics providing explanation after explanation.

For example, how is it that a gas has elastic properties under compression? If you compress it to half the volume, the pressure pretty much doubles. It's surprisingly similar to a spring, yet a gas isn't a spring, surely?

Physics offers a wonderful explanation: the molecules of the gas are moving fast, and collisions of molecules are perfectly elastic. The pressure of a gas against the walls of the vessel arises from elastic collisions of the molecules with the walls. If you half the volume the number of collisions pretty much doubles, hence the change of pressure.

What the physics did there: it moved the description of physics taking place to a deeper level. As it happens the physics on this deeper level (fast moving, elastically colliding molecules) is very readily visualized in things we know in the macroscopic world. We can readily visualize a population of bouncing balls.

In this particular case we are fortunate: the description at a deeper level consists of something that has a counterpart in physics we can see happening with our own eyes.

So yeah, it's natural to hope that a theory at quantum level can describe the physics taking place in terms of things we can visualize: particles, and waves. Well, in the case of quantum mechanics it turns out that what is expressed by the equations has no counterpart in the macroscopic world.

We make do with concepts we can visualize; particles and waves. In that sense the duality is introduced by the human thinking process; it's not inherent in the physics.

On more remark, again expanding on the anwser by anna v:
What is not in physics is exhaustive explanation. When a new theory replaces an existing theory the description of the physics taking place moves to a deeper level.

When newtonian mechanics was established it looked for more than 300 years that no deeper level existed. Then, in the course of development of relativistic physics, theory of motion moved to a yet deeper level. So we learned not to expect that we are at the most fundamental level. A deeper level of description may be possible, we don't know.

Physics is not in the business of looking for exhaustive explanation. For the duration of working at a particular level of description (which can last hundreds of years) the assumptions necessary for that level of description are taken as given.

For comparison: Newton assumed the existence of a gravitational force, acting over large distance, to explain the fact that the planets are orbiting the Sun. Newton's contemporaries were more demanding. For example, Descartes offered a hypothesis of vortices in space, with those vortices exerting an inward push on the planets. Descartes was determined to restrict himself to hypotheses with understandable elements only. He could visualize vortices, thus he ended up with vortices.

In retrospect we see that Descartes was just bogging himself down. Newton was more successfull because he operated on a need-to-assume basis.

Newton could not explain gravity itself, but he saw the strong explanatory strength it offered, and he moved ahead.

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"Well, in the case of quantum mechanics it turns out that what is expressed by the equations has no counterpart in the macroscopic world. We make do with concepts we can visualize; particles and waves. In that sense the duality is introduced by the human thinking process; it's not inherent in the physics." +1. The sooner a person stops trying to shoe-horn QM into classical thinking, the sooner they will 'get it.' – Andrew Gibson Mar 30 '13 at 13:14

You will probably find a satisfactory answer reading the experiment done by Shahriar Afshar, who claimes to have observed both, the wave and particle, properties of mater simultaneously. Here is the link,

You will find more links therein, as well as criticism of the interpretation of his experiment. The best way to settle scientific "conflicts" is by experiment. Debates and arguments have conceptual limitations.

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I actually agree with Motl on this one, unless Afshar was sending single photons, this is a classical interference experiment and there is nothing quantum about it. – daaxix Apr 11 '13 at 6:42
After reading the wikipedia article about this experiment, it is amazing to me how many professional physicists still seem to mix up probability interference and classical wave interference. – daaxix Apr 11 '13 at 6:43

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