Thursday, August 13, 2015

The unbearable persistence of Nothing

Have you ever heard about Aether?  I'm not talking about the chemical compound. You probably heard this word as a footnote in some Physics lecture about the massless medium through which the ancient believed lightwaves propagate in the vacuum. Do you remember now? Well, if your teacher talked about this, moments later he (or she) told you light does not need a medium to propagate, it is an oscillatory perturbation in the electromagnetic field which may travel even in the absence of matter.

This idea, in our present days, is no longer weird. We receive our entertainment from electromagnetic perturbations, we talk to the other side of the world using electromagnetic perturbations and everybody already felt their arm hairs being attracted by a surface under the action of an electrostatic field, certainly no medium is needed for this attraction to happen, but still, this must have caused headaches to the ancients: light travels through nothing, and nothing has properties (it can be electrically and magnetically polarized! at least, there are two physical constants, the vacuum permitivity, and the vacuum permeability).

We, Materials Scientists, are used to the nothing with properties. We call it "Vacancies". Although they are surely occupied by something (in conductors, the free electrons), vacancies are defects created by the absence of atoms in crystal lattice positions. As I always teach my students, vacancies don't exist, what exist are the atoms. Nevertheless, vacancies have properties, they interact with dislocations (another thing that does not exist, but has properties) , they are responsible for diffusion in a solid and, therefore, they have velocity. We use to talk about "vacancy wind" in diffusion (i.e. a wind or nothing, spooky, isn't it?).

Another example of nothing with properties in materials science we have the "holes" in the valence band of the electronic structure in a semiconductor (well, critics will say this belongs to condensed matter physics, but I think we can share responsibility). Here the situation is weirder (as always, when quantum mechanics gets involved). Holes (the absence of electrons) have mass, carry a charge (positive) and even diffract, as every particle does. A particle of nothing!

As a final example of this ode to nothing, we have dark matter (and its cousin, the dark energy). Here at least it is assumed it is something, we just don't know what it is. It is weird to think that only 4,9% of the universe we see is composed by our regular matter, the rest is dark matter and dark energy. As far as we know, dark matter particles could be traveling through our body without our knowledge. Creepy, isn't it.

So, hail to nothing, we couldn't live without it (literally)


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