Relation between condensed matter physics and QFT

In summary, there are two different perspectives on how Quantum field theory (QFT) relates to condensed matter physics. While condensed matter physics deals with the macroscopic properties of matter, QFT deals with the quantum states of particles. However, the concept of particles in QFT, such as photons, is viewed differently by high-energy physicists and condensed matter physicists. Additionally, QFT does consider temperature in systems with a large number of particles, but the disturbance from the vacuum is predictable compared to the unpredictable disturbances in macroscopic systems. Despite these differences, QFT is a useful method for understanding and analyzing quantum systems with many degrees of freedom, even if they appear as individual particles.
  • #1
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I am confused about the relation between condensed matter physics and Quantum field theory (QFT). Here are my "naive" questions.
1. The condensed matter physics deals with the macroscopic physical properties of matter. The number of atoms of the system is as large as Avogadro's number. But in QFT, although there are infinite virtual particles, there are only a few (two, three, four) real particles. How could these two be connected with each other?
2. In a macroscopic physical system, the physical quantity temperature is very important, because now the second law of thermodynamics will dictate the irreversible evolution of the system. While in QFT, there is no such thing as temperature. Even we use it for convenience, the concept temperature is vain/imaginary in QFT.
3. A macroscopic system is always disturbed by its environment. This unknowable, unpreditable and chaotic disturbance may have a serious impact on the quantum state of the system. While in QFT, the disturbance from the vacuum is also serious, but predictable, and under control.
So why could QFT methods be used in condensed matter physics?
 
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  • #2
The confusion stems the fact there are two different "philosophies" of QFT, one typically used by high-energy physicists (HEP) and the other typically used by condensed-matter physicists (CMP).

1. What HEP call "particle" in QFT, such as a photon, CMP call "quasiparticle" or collective excitation in QFT, such as a phonon. Just as a typical physical state contains only a couple of photons, similarly a typical physical state contains only a couple of phonons.

2. Temperature is relevant in QFT when the number of particles (e.g. photons) is large.

3. The vacuum (e.g. photon vacuum or phonon vacuum) is a very simple state, which is why its disturbance is predictable. The disturbance of more complicated environment states is more difficult to predict.

QFT is a method to deal with quantum systems with a large number of degrees of freedom. Even one photon or one phonon is a state that really originates from a large number of degrees of freedom that are more fundamental than the photon or phonon itself.
 

1. What is the main difference between condensed matter physics and QFT?

Condensed matter physics deals with the study of physical properties and behavior of matter in its solid and liquid states, while QFT is a theoretical framework for describing the behavior of subatomic particles and their interactions.

2. How do condensed matter physicists use QFT in their research?

Condensed matter physicists use QFT to study the behavior of particles in condensed matter systems. They apply concepts and techniques from QFT, such as renormalization and symmetry breaking, to understand the properties of materials at a microscopic level.

3. Can QFT be used to explain macroscopic phenomena in condensed matter systems?

Yes, QFT can be used to explain macroscopic phenomena in condensed matter systems. For example, the theory of superconductivity, which describes the flow of electricity without resistance in certain materials, is based on QFT principles.

4. What are some current research topics at the intersection of condensed matter physics and QFT?

Some current research topics include topological phases of matter, quantum field theory of strongly correlated systems, and quantum simulation of condensed matter systems using QFT techniques.

5. How does the study of QFT in condensed matter systems contribute to our understanding of fundamental physics?

The study of QFT in condensed matter systems allows for the exploration of new and exotic states of matter that have not been observed in traditional high energy physics experiments. This contributes to our understanding of the fundamental laws of nature and the behavior of matter at a fundamental level.

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