Can ‘truly’ new physics emerge in complex systems?

In this article, I will make the case for the 'logical possibility' of emergence of ‘truly’ new physics in a complex system. Historically, scientists have discovered new physics when they changed the energy scale of the systems they have been studying. So, should we expect an encounter with new physics as the complexity of the system in study changes?

To start this discussion, we should ask a question. Why do we discover new physics when we change our energy scale? For example, Newtonian physics worked very well. But when people observed particles at higher speeds, they observed, first of all, the existence of an upper speed limit that cannot be broken, among many other new phenomenons. At the same time, people found, for example, that as we cool metals to very low temperatures, their behaviour changes in a way that cannot be explained using classical physics, rather a new framework, called quantum mechanics, is required.

So, why do we discover new physics when we change our energy scale? I think the reason is that any theoretical model, let’s say Newtonian mechanics, is created to explain experimental observations made at some energy range. But there is no guarantee that as we move away from the energy scales that gave rise to the model, the model will still faithfully explain the experimental results. Newton, for example, could not guess that there was an upper speed limit for all objects.

Hence, new physics seems to emerge when we move away from the energy scales that motivated the present physical models.

What if the same thing happens as we move to systems with higher complexity? After all, what is the guarantee that a theoretical model, created for simple systems, will work as good for complex systems? Here, I am talking about the breakdown of reductionism. I am referring to something that some philosophers call ‘strong emergence’. That is, it is not just that the behaviour of complex systems cannot be efficiently studied from fundamental laws. The same cannot be done even in principle! That is, as more and more particles are studied, truly new physics may emerge.

For example, some bizarre things happen if we take quantum mechanics, a theory that was deduced from experiments done on a few small particles, and apply it to macroscopic objects. For example, if we apply quantum mechanics on a room with an observer doing a Stern-Gerlach experiment with a spin half particle, then we get a bizarre final state consisting of the observer in superposition of observing a spin up particle and the same observer observing the same particle spin down.

How should we interpret this final state derived using elementary quantum calculation? If we take this state seriously, we get a universe where everything that can happen, happens and is in quantum superposition. But we need not necessarily make this conclusion. My point is that quantum theory was developed for the microscopic world. It is not straightforward to use it to make assertions for more complicated systems, like macroscopic systems. New physics, that is, modification of quantum mechanics, may come about as the system gets more complicated. After all, all verifications of quantum theory have been done in limited complexity regimes, and our confidence in the model should decrease as we go away from the comfort zone of this regime.