This is a theory made by Schrodinger that he did not test but thought about.

The thought experiment took place inside a box with a bomb that only killed the cat if the radioactive transmitter emitted waves telling it to. In this case, the transmitter only sends out waves when it detects a radioactive decay in the box so Dr S put in a radioactive cell with a 50-50 chance of decaying. After putting in the radioactive cell, Dr S closed the lid of the box. Inside the box, the radioactive cell either decayed or didn’t decay and because there was a 50-50 chance the cat was neither dead or alive!!!