Graduate Is the Fractional Quantum Hall Effect an emergent property?

[Cato]

TL;DR

I have heard that the Fractional Quantum Hall Effect is a true emergent property. I have never believed that emergent properties exist. Is this an example of one?

I do not think that true emergent properties -- as defined by behavior of matter that cannot be reduced to fundamental physical law -- exist. Yet I have been told that the Fractional Quantum Hall Effect is an example of an emergent property. What is the consensus?

 

[PeterDonis]

true emergent properties -- as defined by behavior of matter that cannot be reduced to fundamental physical law

This is not the common definition of "emergent properties" in physics. I think most physicists would say that "emergent properties" are properties that we have equations for, but we do not know how to derive those equations from the equations describing the fundamental laws. That in no way means such properties are not governed by the fundamental laws. It just means we don't understand how (yet).

I have been told that the Fractional Quantum Hall Effect is an example of an emergent property.

Where? And was whoever told you using the same definition of "emergent property" that you are using? Or were they using a definition more like the one I gave above?

 

[Demystifier]

TL;DR Summary: I have heard that the Fractional Quantum Hall Effect is a true emergent property. I have never believed that emergent properties exist. Is this an example of one?

I do not think that true emergent properties -- as defined by behavior of matter that cannot be reduced to fundamental physical law -- exist.

One should distinguish between strong emergence and weak emergence. Strong emergence is that something cannot be reduced to fundamental laws in principle. Weak emergence is that something cannot easily be reduced to fundamental laws in practice. In physics, by emergence, one usually means weak emergence. In particular, FQHE is weakly emergent, not strongly emergent.

Is there anything which is strongly emergent? It's hard to tell, some suspect that consciousness might be strongly emergent, but that's another topic.

 

[vanhees71]

TL;DR Summary: I have heard that the Fractional Quantum Hall Effect is a true emergent property. I have never believed that emergent properties exist. Is this an example of one?

I do not think that true emergent properties -- as defined by behavior of matter that cannot be reduced to fundamental physical law -- exist. Yet I have been told that the Fractional Quantum Hall Effect is an example of an emergent property. What is the consensus?

Quantum many-body theory is full of "emergent properties". This is of course a buzz word. What's usually meant by this are collective excitations of a many-body system. These can often be described as "quasi particles", i.e., with a formalism of many-body quantum-field theory where these excitations can be approximately described in a similar way as "particle states" are described. These "quasi particles" need not be in any way related to true/elementary particles though. E.g., there are quasi particles with a "fractional charge", although of course there are no elementary particles with charges different from integer multiples of the elementary charge, $e$.

 

[vanhees71]

One should distinguish between strong emergence and weak emergence. Strong emergence is that something cannot be reduced to fundamental laws in principle. Weak emergence is that something cannot easily be reduced to fundamental laws in practice. In physics, by emergence, one usually means weak emergence. In particular, FQHE is weakly emergent, not strongly emergent.

Is there anything which is strongly emergent? It's hard to tell, some suspect that consciousness might be strongly emergent, but that's another topic.

By your definition something is "strongly emerent" simply, if it cannot be understood by contemporarily known physical theories. I think all of condensed-matter physics is pretty far from that, although there are phenomena which cannot be explained entirely "from first principles".