# Temperature of black body without photon

Can the temperature of a black body be defined without using radiation? Energy spectrum of black body is a function of temperature so when we say a given blackbody is at temperature $$T$$ we measure the energy of radiation (ultimately photons) to assign $$T$$ using following kind of graph

In other words, is it possible to operationally define the temperature of a black body without going through the route of detecting photons?

• You can measure body temperature using a lot of different technologies, just choose some. Feb 28, 2020 at 12:47

## 1 Answer

Temperature is defined thermodynamically as the energy change as the entropy changes: $$T=\left( \frac{dU}{dS}\right)_{P,V},$$ entirely independently of radiation. To operationally measure a temperature of a body without measuring photons you will have to use a thermometer or something similar.