1.2 Why Do We Study Condensed Matter Physics?
There are several very good answers to this question
1. Because it is the world around us
Almost all of the physical world that we see is in fact condensed matter. Questions such as
• why are metals shiny and why do they feel cold?
• why is glass transparent• why is water a fluid, and why does fluid feel wet?
• why is rubber soft and stretchy?
These questions are all in the domain of condensed matter physics. In fact almost every
question you might ask about the world around you, short of asking about the sun or stars,
is probably related to condensed matter physics in some way.
2. Because it is useful
Over the last century our command of condensed matter physics has enabled us humans to
do remarkable things. We have used our knowledge of physics to engineer new materials and
exploit their properties to change our world and our society completely. Perhaps the most
remarkable example is how our understanding of solid state physics enabled new inventions
exploiting semiconductor technology, which enabled the electronics industry, which enabled
computers, iPhones, and everything else we now take for granted.
3. Because it is deep
The questions that arise in condensed matter physics are as deep as those you might find
anywhere. In fact, many of the ideas that are now used in other fields of physics can trace
their origins to condensed matter physics.
A few examples for fun:
• The famous Higgs boson, which the LHC is searching for, is no different from a phe-
nomenon that occurs in superconductors (the domain of condensed matter physicists).
The Higgs mechanism, which gives mass to elementary particles is frequently called the
“Anderson-Higgs” mechanism, after the condensed matter physicist Phil Anderson (the
same guy who coined the term “condensed matter”) who described much of the same
physics before Peter Higgs, the high energy theorist.
• The ideas of the renormalization group (Nobel prize to Kenneth Wilson in 1982) was
developed simultaneously in both high-energy and condensed matter physics.
• The ideas of topological quantum field theories, while invented by string theorists as
theories of quantum gravity, have been discovered in the laboratory by condensed matter
physicists!
• In the last few years there has been a mass exodus of string theorists applying black-
hole physics (in N-dimensions!) to phase transitions in real materials. The very same
structures exist in the lab that are (maybe!) somewhere out in the cosmos!
That this type of physics is deep is not just my opinion. The Nobel committee agrees with
me. During this course we will discuss the work of no fewer than 50 Nobel laureates! (See
the index of scientists at the end of this set of notes).
4. Because reductionism doesn’t work
begin{rant} People frequently have the feeling that if you continually ask “what is it made
of” you learn more about something. This approach to knowledge is known as reductionism.
For example, asking what water is made of, someone may tell you it is made from molecules,
then molecules are made of atoms, atoms of electrons and protons, protons of quarks, and
quarks are made of who-knows-what. But none of this information tells you anything about
why water is wet, about why protons and neutrons bind to form nuclei, why the atoms
bind to form water, and so forth. Understanding physics inevitably involves understanding
how many objects all interact with each other. And this is where things get difficult very1.2. WHY DO WE STUDY CONDENSED MATTER PHYSICS? 3
quickly. We understand the Schroedinger equation extremely well for one particle, but the
Schroedinger equations for four or more particles, while in principle solvable, in practice are
never solved because they are too difficult — even for the world’s biggest computers. Physics
involves figuring out what to do then. How are we to understand how many quarks form
a nucleus, or how many electrons and protons form an atom if we cannot solve the many
particle Schroedinger equation?
Even more interesting is the possibility that we understand very well the microscopic theory
of a system, but then we discover that macroscopic properties emerge from the system that
we did not expect. My personal favorite example is when one puts together many electrons
(each with charge −e) one can sometimes find new particles emerging, each having one third
the charge of an electron!1 Reductionism would never uncover this — it misses the point
completely. end{rant}
5. Because it is a Laboratory
Condensed matter physics is perhaps the best laboratory we have for studying quantum
physics and statistical physics. Those of us who are fascinated by what quantum mechanics
and statistical mechanics can do often end up studying condensed matter physics which is
deeply grounded in both of these topics. Condensed matter is an infinitely varied playground
for physicists to test strange quantum and statistical effects.
I view this entire course as an extension of what you have already learned in quantum and
statistical physics. If you enjoyed those courses, you will likely enjoy this as well. If you did
not do well in those courses, you might want to go back and study them again because many
of the same ideas will arise here.
1Yes, this truly happens. The Nobel prize in 1998 was awarded to Dan Tsui, Horst Stormer and Bob Laughlin,