The Nature of Conscious Experience


Why Phonons?


In simple terms, maybe because our everyday world of warm, rattling about, bumpable into, medium-sized things is the world of phonons.

Most people have never heard of phonons. Of those that have some might paint them as ‘notional mathematical conveniences’. However, when I first came to the conclusion that whatever ‘has’ a human experience is likely to be a phononic mode in a cell membrane I was heartened by a UCL colleague who assured me that phonons are absolutely real. She is an authority on diamonds and since you cannot have diamonds without phonons and phonons are essentially weightless, the twinkle in her eye suggested that one might regard phonons as the most valuable things in the universe, weight-for-weight.

Without a formal training in physics it is difficult to get a grasp of what a phonon really is. As indicated above, even with such a training, not everyone comes to grasp the idea exactly the same way. One option is to give up. The other is to try to find a way of thinking about phonons that allows one to bypass matrix algebra and come to some conclusions about their everyday implications without committing serious errors of interpretation. I think this is worth a try.

Modern physics indicates that our universe is populated by ‘packets’ of energy. Most of us are familiar with packets called electrons, quarks and photons. Some of these have rest mass, but some, like photons, do not. What is less well known is that there are other packets of energy, which make a relatively small contribution to the total, but are directly relevant to our daily lives because we live those lives in the context of relatively tiny energy changes. Our brains receive most of their input from the world from photons. However, even our knowledge of the world via photons is totally dependent on other inputs like sounds and bumping into things. To a first approximation this is the world of phononic modes. The acoustic waves that give us the sense of sound are themselves phononic modes. The objects we bump into are also occupied by phononic modes.

The terminology here is a little confusing but as I understand it not too opaque. We are used to packets of energy being ‘single particles’ like electrons in the sense that when the packet interacts you cannot have some of the energy going off in one direction and some in another. Unlike electrons, the ‘packets’ of energy that obey Bose statistics, are modes or ‘ways of being’ of energy that, when they interact, may do so as if they are made up of several subunits, often called quanta, such as photons or phonons. So a phononic mode is so-called because it can notionally be divided into a whole number of phonon subunits, but there are reasons for thinking that these subunits do not have any real separate existence within the mode. This may be one reason why phonons are sometimes regarded as notional, but they are no different from photons in this particular respect.

The reason for calling these packets modes is that they all take the form of a dynamic oscillation, hence the term ‘wavefunction’. They might be considered ripples in spacetime. However, for familiar particles like an electron, this ripple takes a mathematical form completely incompatible with any enviseageable ripple. The oscillation is complex, in the sense that it can only be described using ‘imaginary’ numbers that include multiples of the square root of minus one. It is not an oscillation in the space we are familiar with. Nothing in our measurable world is a multiple of the square root of minus one. Another way of putting that is to say that these cannot be ‘classical’ oscillations.

As I understand it, phononic modes are arguably rather different in this respect, for reasons related to the fact that their phonon subunits have a ‘spin’ value of zero. This turns out to mean that at least the aspects of phononic modes that are of practical significance tend to be describable purely with real numbers. They behave like real waves, like the vibration of a violin string. They are in this sense ‘classical’. However, I do not think that this should make us think they do not belong to quantum field theory, since the real description arises from exactly the same general schema of that theory that gives rise to complex values for electrons. One sometimes sees the suggestion that phonons are not really true ‘quantum-type’ particles but I find it hard to see the justification for that. One thing is for certain - the energy is real.

Phononic modes come in a wide range of forms but the traditional acoustic modes are the easiest to understand. It is fairly easy to see that everything we can reasonably call a ‘separate object’ will have ways of oscillating, or vibrating, in real space and so every object comes with a new family of phononic modes. These modes inhabit the whole object in an indivisible way. You cannot have a half-vibration. They are in a sense what justifies us treating things as individual objects but they are not made up of the atoms involved, they are additional energy packets on top of the atoms.

The reason why I think it is of interest to consider the phononic modes associated with structures at the biological level in the context of experience is that these modes should take part in fundamental, or immediate, interactions without any reference to the involvement of individual atoms. This gets around William James's concern that any interaction that was mediated by an endless chain of intervening atoms would be too 'second hand' to constitute something that might be experienced as a whole. When an electrical potential influences an acoustic vibration, as I experienced as if by magic as a child listening to what was then called a ‘crystal set’, it does so immediately, with no intervening steps related to one atom or another. Photon energy is exchanged for phonon energy with nothing in between. The crystal modes in the set are only set up to ‘experience’ rapid back and forth reversals of electrical potential along their entire length. A cell membrane would, by the same argument, ‘experience’ all the detailed fluctuations of potential generated by opening of ion channels all along the length of the receiving dendrites. It might well be argued that so far we have little evidence that cells respond to very detailed patterns of input. Nevertheless, at least we might have a situation where something does have a very detailed pattern impinging on it, which it might reasonably be thought to ‘experience’.