The Threshold Where Light Becomes a Projectile
Light strikes a metal plate, and electrons fly out. This is the photoelectric effect, and it works only if the light has enough energy to cross the threshold that holds electrons inside.
Quantum Debts: When a Photon Has Negative Presence
Niels Bohr insisted: you cannot simultaneously know which path a photon took and see interference. For a long time, this was considered a limitation of knowledge — we simply cannot obtain both types of information at once.
The Quantum Coin: Bohr & von Neumann's Solution
A quantum particle, an inhabitant of the elementary particle world, resembles a coin spinning in the air: neither heads nor tails, but a blend of possibilities. Yet when it lands on the table, the outcome becomes definite. What serves as the table in a measurement?