Quantum Wave Behavior in Large Objects?

Is Dual nature of Matter actually proved or just a theory and are the objects (water,rubber ball, car, apple etc.) that we see all around us in day to day life exhibit dual nature in their natural state of existence (as perceived by the human eye) or only when they are broken down to molecular level? Thanks.
- Veeral (age 31)
Mumbai, India

Hi Veeral,

First of all, let's be clear on what we mean by "dual nature" or "wave-particle duality." This notion is fairly outdated; nowadays physicists just think of particles in terms of their wavefunctions, which really do behave like waves. The only "particle-like" behavior of particles is that they can't be detected in multiple places; a single photon wavepacket can only register one detection event on a camera.

That said, macroscopic, everyday objects look like localized in position; not at all like waves. As objects get larger, their associated de Broglie wavelengths shrink rapidly, so that a massive molecule just nanometers across has a de Broglie wavelength under 1 picometer across. So do we still have evidence for the wave behavior of such particles, or even larger, like Schrodinger's cat?

The wave nature of matter has been proven for many different types of particles, including photons, neutrons, electrons, and even large molecules composed of over 15,000 protons, neutrons, and electrons (reference: "Matter-wave interference of particles selected from a molecular library with masses exceeding 10,000 amu").

You can find a lot of articles if you search for "molecular interferometry" or something similar. (For example, http://scienceblogs.com/principles/2013/11/12/interference-with-10000-particle-particles-matter-wave-interference-with-particles-selected-from-a-molecular-library-with-masses-exceeding-10000-amu/.)

Such interference experiments were first done with small particles, like photons or electrons. Since then, researchers have increased the mass and complexity of the objects which were provably put into superposition states. In the near future, experimentalists hope to put mirrors consisting of ~10^14 atoms into superposition states. That's still a small mirror, but it's almost large enough to be seen by the naked eye! (Of course, you can't watch while the experiment is being done, since any light bouncing off the mirror will collapse the superposition state.)

Furthermore, while many physicists suspect that quantum theory should be true even on the macroscopic scale, they don't all agree on why we don't see objects all around us in superpositions states. (Again we have to be careful about what we mean: technically any state is a superposition of other states, since quantum theory is linear and has no special basis. But there are certain superpositions that we never see around us, like Here + There or Dead + Alive.) There are many ideas, but perhaps the most commonly accepted theory is that interactions with the environment cause states to look like they collapse to the classical states that we see around us. This theory makes predictions which disagree with alternative theories, like gravity-induced collapse or spontaneous localization. So, with any luck and increased technology, we'll be able to experimentally answer the question.

Depending on what experimental results tell us, there may be several possibilities. Quite possibly, there are fundamental restrictions on how large a state can be while maintaining its quantum coherence and "wave-nature." If there aren't such restrictions, then perhaps we can make large quantum systems which are so well isolated from the environment that they can exist in superpositions; so-called Schrodinger's cat states. 

Actual cats in superposition states are unthinkable even in the distant future because these states are highly unstable (they interact strongly with any realistic environment, and so decohere very rapidly). However, superposition states of viruses may be possible in a few decades... maybe.

Cheers!

David Schmid

(published on 11/12/2013)