### Clue: QED! (7,15)
### Answer: QUANTUM ELECTRODYNAMICS 
 ### Explanation: Definition: QED
 Cryptic defn:  Arithmetically, "3 + 20″ indeed = 23" and, on the other hand, the scientific study of  the interaction between **LIGHT**(answer to 3down) and(+) **MATTER**(answer to 20down) is the **FIELD**(answer to 23down) of QED.  Hence, "quod erat demonstrandum";that is what had to be demonstrated.
Answer: The scientific field abbreviated to QED.