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?