Theories and speculations

Seven questions we should be asking about the probabilistic nature of quantum physics

Quantum physics has a terrible reputation for being difficult, if not impossible to understand. This reputation sometimes appears to be actively promoted by physicists. Feynman’s observation that “if you think you understand quantum physics you don’t understand quantum physics” has itself confused and deterred a whole generation of thinkers and critics who do not ‘do the math’. Brian Cox’s 60 second definition gives unusual and welcome clarity. Quantum physics, he says, “is the calculation of the probability of the whereabouts of sub-atomic particles”.

If we accept this as a working definition, we can deduce a number of questions we should be asking about some of the odder things that have become popularly accepted as ‘quantum truths’. And each of these questions appears to have a simpler answer than we might have been led to believe.

  1. Does everything have a wave nature?

It is generally accepted that ‘everything’ in the universe is made of the tiny particles Brian Cox refers to, and therefore we might conclude that everything is intrinsically probabilistic. It is also accepted widely that probability has a wave form – we ‘know’ this, as the likelihood of events rising and falling. But isn’t it the case that it is not things themselves that therefore have a wave nature but the probability of their position, cause and context? Surely it is the probabilistic nature of all events that therefore imparts a wave nature to everything? This would be a very important distinction, because the term ‘wave’ leads us to imagine a physical body of movement. Isn’t it the case that, in quantum physics, waves are always waves of probability and have no physical appearance at all?

2. Does the slit experiment show the wave motion of particles?

This question is closely related to the first. The slit experiment shows exactly what Brian Cox describes, the probability of the whereabouts of particles. So surely the characteristic bands we detect simply show us where the particles are most likely to be detected, and where they are least likely to be detected? If so, the wave motion is a ‘motion’ of this probability, and the experiment does not therefore show us wave movement of the particles themselves? The pattern shows us that the probability of their detection varies in a regular way as it rises and falls. The temptation is to ask, yes, but why are some positions more or less probable? But as probability is the means by which we understand – and view – them, the question is a circular one. The only answer we can give is “because they are”. So it seems that the characteristic bands do not therefore show us that particles ‘move in waves’. In fact, when the movement of the particles is measured – something which is extremely difficult due to their size – it appears that particles move in straight lines. The wave motion is then said to be collapsed. But surely what is in fact collapsed is the probability of their position, since it is now measured?

3. Is it really surprising that quantum energy always come in discrete amounts?

Once again, this is something that is often regarded as weird. To give an example, it is said that the total energy in a light field is always a whole integer multiple of a fundamental unit of energy. Isn’t the answer to this simple? It lies in the word ‘fundamental’. If we are dealing with the smallest possible particles, with the smallest possible, ‘fundamental’ amount of energy, it would make no sense to imagine 0.345 of it. Or multiply it by π. If we are dealing with a fundamental unit wouldn’t it be meaningless (and impossible) to calculate a fraction of it?

4. Does it matter if the wavefunction is ontological or epistemic?

Because everything is made of particles, and the position of particles is probabilistic, and because probability has a wave nature, it is said that the description of a quantum system takes the form of a wavefunction. This statement itself raise a lot of questions. For example, what is ‘a quantum system’ if ‘everything’ is probabilistic? (I will come back to this). However, the most common debate about the wavefunction is whether it is ontological (a real, physical thing ) or epistemic (simply an indication of our knowledge, or lack of knowledge, about the underlying state of an object). But if we know everything is fundamentally probabilistic, surely it must follow that neither of these views are true, or that both are true, since there is no difference between an ontological and epistemic view if both are subject to probability? An object may or may not be real, and we may or may not have knowledge about it, and in either case the probability must be calculated before any other statement can be meaningful?

5. Is entanglement mysterious?

Entangled objects are related by interaction which determines the direction of spin of electrons, and each determines the other. Is it therefore a surprise that to understand the spin of one is also at that instant to understand the spin of the other?

6. Does quantum physics really only apply to very small things?

This is probably the biggest question of all. Clearly, for Brian Cox, quantum physics is a set of calculations relating to very small particles. Probability replaces observation because it must. But there are many things that cannot be observed. Quantum physics deals with things that are not observed directly by calculating probability, but why wouldn’t that apply to the whole of the world we imagine, which is not directly observed? The introduction of the idea that anything you can’t see (including events that are temporally ‘out of sight’ in the past or the future) can only be known or understood as a probability resolves many issues that have preoccupied thinkers (such as the ontological vs epistemic debate, for example). Equally, we can see that much of the world we live in is probabilistic, and can only be calculated for anything that is not singular, that is for everything that exists in dependence on cause or context as quanta. These ‘quanta’ might relate to a group of objects, or a group of temporal moments  (ie the journey of an object across a period of time). We constantly calculate the probability of such quanta to negotiate what we perceive to be reality, so why can’t the object which we contemplate be any object, rather than just than the tiniest particles in the universe (from which according to quantum physics all objects are in any case made)? Another way of framing this question would be to say that even in calculating the whereabouts of the tiniest things we talk about granularity and quanta in order for a probabilistic calculation to make sense, and if that calculation works for quanta at a tiny scale surely it must work for quanta at any scale? So it might easily be applied to the ‘existence’ of a table over a period of 100 years, or even to the existence of the moon over a period of several million years? It would seem that the granularity just changes in scale, and surely if it does so the wavelength is not diminished by that expanded scale but hugely expanded in direct relationship with the quanta under consideration?

7. Is the cat both alive and dead?

These six questions all seems to have clear and reasonably obvious answers, and they all spring from the principle that our observation of the fundament ‘units’ of the universe is probabilistic. So to the question of the cat. It is now an easy question to answer. If we follow logic, surely we have to say that the only meaningful consideration of these two states is to calculate the probability of each?

There is also a footnote to this question, however. Carlo Rovelli suggests that we should also consider the cat’s point of view, in which case the uncertainty vanishes whether the box is opened or not. But this takes us back to Brian Cox’s definition of quantum physics, which assume precisely that we cannot do this. Quantum theory is useful because humans are not gods. We cannot get into the heads of cats, unless we do so in fiction. Then, we can superpose the truth according to the cat. Human imagination is a superpower. But human observation is not. If we turn into an empty street, we can imagine a car coming towards us, but if we don’t see it coming we will die. Between these two extremities, we calculate probabilities.

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