I find Crowther’s presentation very confused - upside down indeed.
But anyway, the essential point about superconductivity is that QM gets restored as having an effect at low temperature.
Electrons themselves emerge as the universe cools enough for them to pop out of the quantum foam as classical point particles with individual momenta or incoherent kinetic energy. They are fermionic and disentangled. So in EFT fashion, symmetries are broken and new universalised properties are established. Electrons become a universal thing described by their own emergent EFT - as in classical electrodynamics.
But cool down their world even further - and make it the world of a confined metalic lattice, another pretty arbitrary and non-universal or non-holonomic constraint - and you get a recovery of a quantum possibility. You can get point particles behaving like collective phonons with bosonic statistics. The asymmetry, or broken chiral symmetry, of being a fermion can be reversed by teaming up as "Cooper pairs".
So a general rule of the quantum level - Pauli's exclusion principle - becomes again explanatory of what is going on at a "super-classical" level. Electrons could be melted back to pre-fermionic matter - effectively - by travelling back in time to the white heat of the Big Bang. And they can also lose their fermionic identity at the end of time, when things get so cool that pairs of electrons can find it easy to entangle as symmetry-increasing combos. They may still have some individual kinetic momenta, but that is now so weak it can't combat the stronger urge to unit as a way to meet the energetic constraints imposed by a motion through a metallic lattice.
This all makes BCS a bad example of a "tower of EFTs". It kind of combines two different things at once. And that is confusing.
Let's step back a bit.
The key thing about EFTs is they are about a collective mode of organisation being spread out absolutely everywhere and so becoming a “law”. It is about a phase transition that occurs when a limit is "crossed".
For example, in a world of gaseous H2O, it is the law that the molecules behave everywhere and at all times like a liquid when the temperature and density crosses a critical threshold. The collective behaviour - due to weak van der Waals forces - overcomes the individual kinetic energies of the molecules. And so a new state of matter arises - one that itself has new relevant properties, like being a reasonably universal solvent, and having an unusual latent heat capacity, that are very relevant to explaining chemistry and even life itself.
So emergence is all about a collective mode of organisation becoming a universalised property. And it is "a property" - something that justifies "more as different" - from the point of view of the further complexity of which it becomes the generalised platform.
In a weakly emergent way - a supervenient way - water is just water. It is nothing more interesting than a form of organisation where an integrative force has taken over from a previously differentiating one. I mean water doesn't even seem an unpredictable surprise if you know that the same H2O molecules have both a kinetic energy and a van der Waals attraction as properties. We ought to be able to calculate "wateriness" from first principles knowledge of what was already lurking in the background - a suppressed integrative urge.
But then water, once it forms, becomes a generalised substrate that reveals it itself to have unpredictable properties - like being just right as a medium for complex organic chemistry and life. Higher level theories discover these properties in water. It takes further levels of context to give water these new measurable qualities. The emergence is essentially semiotic.
This gets to one of my discomforts with Crowther's presentation (which I only skimmed through, I admit). As a hierarchy of emergence, it doesn't start from the simple and move towards the complex - the natural or systems way of thinking about emergence. Instead it confuses things by trying to stick to the emergence of the simple from the simple - the "simple" meaning whatever is cosmically and universally the most simple condition given the time since the Big Bang.
Now this is a useful view. But it depends on accepting the reality of a thermodynamic direction to time. And of course, standard reductionist or mechanical physical models deal in laws that are fundamentally reversible or time-symmetric. They thus hardwire in a presumption of linearity. Going forwards and going backwards are two views of the same thing. There just is no room for the non-linearities that are these abrupt phase transitions, these sudden changes in state, when a collective mode spreads exponentially across a system to become a new universalised property.
Mechanical models that wire in time-symmetry cannot see emergence for this reason. Which is why collective modes of behaviour seem so spooky or epiphenomenal - not properly physical, causal and real.
In the big view, physics is working towards a directional understanding of a temporal cosmos. The Universe begins in a "pure" quantum state at the Big Bang. And it is headed towards a "pure" inverse of that state at the Heat Death. Everything will become quantum again - an undifferentiated sea of fluctuations - but the other way round. All that was as hot as possible will be as cold as possible. All that was as small as possible will be as large as possible.
Complexity and classicality are thus something that arise in the middle of it all. It is rather like a Benard cell.
What many don't tell you is that a Benard cell only appears briefly. You have to keep the oil in the pan at precisely the right "boiling" point to see a simple pattern of convection currents. Keep heating and the hexagons break up into turbulence.
So simple global order is what you get at the edge of chaos. The starting of a flip, the beginnings of an inversion, the middle of the change. It is the first big fluctuation that heralds the descent into that kind of fluctuation erupting fractally over all available physical scales.
Again, EFTs don't really capture this. They do describe fluctuations in the infinite limit. So they do describe the world that has gone to maximum fractal turbulence and now looks - if we could stand outside it - to be a flat and simple surface. A platform with universal properties that is now suitable for the next level of more complex hierarchical organisation, based on the further use of those properties.
So EFT thinking is good for accounting how the quantum realm becomes thermodynamically cohered at a certain physical scale and takes on the universal properties of the classical realm. And then in turn, biophysics is now investigating the nanoscale quasi-classical scale (of water as a solvent matrix) and showing how its hidden non-linearity is a resource of the semiotic properties that life needed to exist.
So you have the sweeping view of physics where all the quantumness of the cosmos can get brushed under the carpet when things get large and cold enough for electrons and protons to be modelled in terms of individual particles with particular momenta and forces.
The quasi-classical nature of the electron only becomes an issue again when electron behaviour has to be explained in the highly atypical scenario of a chilled metal lattice. Or when we roll forward to the end of time when quantum effects find a way to decay all matter back to cosmic radiation.
And then biology has to get interested in the particular and atypical scenario that is the quasi-classical scale of water. Life depends on the fact that it can insert itself into the zone of criticality, or the edge of chaos, where things are not generally stable but generally poised on the edge of instability. The whole point now is that the EFT outcome - the hitting of a universalised limit - hasn't happened. Instead, life is playing around at the point where it is in-between - like a Benard cell. The collective mode is only being expressed in a fractured or fractal fashion. It is a bit quantum and a bit classical. And so the emergence of a property is this very in-betweenness - this lability, this instability, this duality, indeed this liveliness.
So again, an important distinction that Crowther's presentation brushes over it seems.
There is EFT emergence - where a system has just gone to the limit and universalised a collective mode as a system-wide homogenous property. A phase transition has happened and there is no going back.
Well, reheating or depressurising the system can reverse its state. Water can re-evaporate. Particle accelerators can melt individual particles. But generally - if we are talking about the fundamental laws of a universe - then the direction of time itself locks in the transitions. EFTs seem the rule because they can safely presume symmetries have been broken, lines have been crossed, and there is no general way of ever going back. The general change is now dead and buried, safe to encode as an emergent law.
Then there is this other kind of emergence that is becoming now really important for understanding complexity. Instead of wanting a world that is fundamentally dead and stable, it needs a world poised on the brink of change. It needs instability - as that then becomes the something that it itself can exist to control. If the world is poised for bifurcation - ready to go either way towards begin fully a gas or fully a liquid - then there is room to insert "intelligence" or semiotic mechanism in that gap and mine it in useful fashion.
This was the dream of Maxwell's demon, by the way. And life - as negentropic or dissipative structure - can behave in that fashion, adding Benard cell like order that manages the transition from smooth flows to turbulent ones.
So yes, emergence is a tricky subject as Crowther says. But I am against the idea that it is essentially pluralistic. Let's not leap straight from the one to the many.
For me, a pan-semiotic metaphysics does bring all the strands of the story together. A triadic or hierarchical ontology makes complexity itself irreducible. And spontaneity or instability is part of that irreducible triad.
So between the one and the many is the three-ness of actual hierarchical causality - the true systems of holistic view. This is where it is all leading as you muddle through "more is different" and the move away from an essentially dead and timeless, reductionist or constructivist, physics.