I What is the Definition of Quantum Physics?

[vanhees71]

I find it very difficult to summarize it in a one-liner (or a couple of one-liners).

Sometimes a picture is worth a thousand words, and sometimes that picture also contains a lot of words.

Like this picture (from http://hyperphysics.phy-astr.gsu.edu/hbase/quacon.html):

View attachment 289930

As you may understand from this picture quantum physics is a pretty complicated and big topic.

The picture gets much more clear and consistent when you eliminate outdated precursors of modern quantum theory.

The shortest characterization is: Quantum Physics encompasses all of the known physics today except the gravitational interaction.

 

[Morbert]

Throwing my soundbite on the pile

"Quantum" is the generalising of the event algebra of a theory. A theory of mechanics of a system is made more general with a theory of quantum mechanics. A theory of statistical mechanics is made more general with a theory of quantum statistical mechanics. A field theory is made more general with a quantum field theory. A theory of gravity is made more general with a quantum theory of gravity. In each case, we move from a less general algebra to a more general algebra. More specifically, abelian algebras are generalised to non-abelian algebras. This is the primary distinction between classical and quantum theories.

This generalising is typically associated with particular scales and discretisations of phenomena. but is not a necessary condition. If a theory of finance modeled with some abelian event algebra is made more general with a non-abelian event algebra, you would have a theory of "quantum finance".

 

[vanhees71]

"Quantum economics" is a buzzword you already find here and there ;-)).

https://en.wikipedia.org/wiki/Quantum_economics

 

[CelHolo]

This generalising is typically associated with particular scales and discretisations of phenomena. but is not a necessary condition. If a theory of finance modeled with some abelian event algebra is made more general with a non-abelian event algebra, you would have a theory of "quantum finance".

A good example is quantum cognition used in studying survey statistics or other psychological data where the event algebra is generalised to a non-commutative one.

 

[A. Neumaier]

A good example is quantum cognition used in studying survey statistics or other psychological data where the event algebra is generalised to a non-commutative one.

Please give a key reference which explains why the noncommutative setting is useful.

 

[f todd baker]

My take is that what "quantum physics (QP)" is is mainly a semantics issue. The way I have sorted out this particular label for myself is that it is all the semi-classical stuff that came before the notion of a wave function arose; like black-body radiation, Bohr atom, Bohr-Sommerfeld atomic physics, Compton effect, photoelectric effect, deBroglie wave length, etc. Everything later I call quantum mechanics (QM). Once, for example, you start solving the Schrodinger equation you are doing QM, not QP. Again, that is just the way I compartmentalize. But the physics stands on its own, doesn't really need a label.

 

[vanhees71]

I think, today "old quantum theory", i.e., everything before the formulation of modern QT in three forms by Born, Jordan, and Heisenberg ("matrix mechanics" including field quantization of the em. field!), Schrödinger ("wave mechanics"), and Dirac ("transformation theory") is simply no longer part of quantum physics. You can forget about it completely (except that it is always good to know some of the historical development of our modern point of view).

Then I think usually one distinguishes quantum mechanics (QM) as the non-relativistic theory, which can be formulated in the "first-quantization formalism", as part of the general quantum theory (QT), which also includes quantum field theory. Relativistic QT cannot consistently formulated in the "first-quantization formalism" but most conveniently as a local relativistic quantum-field theory (although the old hole-theoretical formulation of QED by Dirac is in fact equivalent to modern QED but much less convenient).

 

[A. Neumaier]

My take is that what "quantum physics (QP)" is is mainly a semantics issue. The way I have sorted out this particular label for myself is that it is all the semi-classical stuff that came before the notion of a wave function arose; like black-body radiation, Bohr atom, Bohr-Sommerfeld atomic physics, Compton effect, photoelectric effect, deBroglie wave length, etc. Everything later I call quantum mechanics (QM). Once, for example, you start solving the Schrodinger equation you are doing QM, not QP. Again, that is just the way I compartmentalize. But the physics stands on its own, doesn't really need a label.

No. Quantum physics is the union of pre-1925 quantum physics, quantum mechanics, quantum field theory, quantum statistical mechanics, quantum optics, solid-state physics, etc., including all its approximation methods (in particular semiclassical models).

 

[f todd baker]

So, is this an official naming, like a Newton is the unit of force in the SI system, or is it what you like to call it? I can understand it if you want to call it everything which, in any way, involves the quantum ideas, but I would still contend that the naming is semantic, not really right or wrong.

 

[Mayhem]

Understanding what Quantum Mechanics is involves more than finding a soundbite. There are no prizes for describing quantum mechanics in thirty words or fewer.

I got one: Hard.

 

[A. Neumaier]

So, is this an official naming, like a Newton is the unit of force in the SI system, or is it what you like to call it? I can understand it if you want to call it everything which, in any way, involves the quantum ideas, but I would still contend that the naming is semantic, not really right or wrong.

It is what everyone can pick up by looking at how physicists actually use the term. I never heard the term used it in your ideosyncratic sense.

If you trust Wikipedia to give the standard usage you find:

Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles.  It is the foundation of all quantum physics including quantum chemistry, quantum field theory, quantum technology, and quantum information science.

According to this, quantum physics includes (not exclusively) quantum chemistry, quantum field theory, quantum technology, and quantum information science, and quantum mechanics is its foundation.

 

[Dale]

If you trust Wikipedia to give the standard usage you find:

I think "standard usage" is one place where Wikipedia excels.