Considering some students' curiosity,
here is a layman's explanation of the concept of photons—or rather, quantum entanglement.
As long as you can read, you should be able to understand.
First, let's give an example.
Suppose two identical stationary disks in space, placed side by side, are blown apart by a person named Hao using a bomb.
So, they begin to rotate.
After they have flown quite far apart, we capture one of the disks and measure it.
We find that its angular velocity is w.
Then we can immediately know that the other disk's angular velocity must be -w.
Because according to the conservation of angular momentum, the sum of the angular momentum of the two disks must be zero, so their angular velocities must be opposite.
That is, w and -w cancel each other out.
Quantum entanglement is somewhat similar.
After a pair of entangled photons has flown far away in opposite directions, we capture one of these photons.
