In the 19th century some scientists thought that if we could know, at any given time, the position and velocity of every atom, we could in principle calculate the future. They believed the past determines the present, and the present determines the future in all details. The universe was like clockwork. There could be no “free will,” and bad behavior had some excuse. Hmm.
Today we know many reasons why that is wrong, apart from its practical impossibility. Some things (not only people) are much too complex, but even a single electron behaves in unpredictable ways.
The sun will rise tomorrow morning, of that I am certain. Wait, no, the sun doesn’t do anything, it’s the Earth spinning round once every 24 hours. A law of physics called the “conservation of angular momentum” guarantees that, and laws of physics cannot be broken.
Knowing the laws of motion and gravity enables us to send spaceships to explore the moons of Jupiter and Saturn, thanks to powerful computers. Closer to home, the path of a baseball and the motion of the bat that hits it determine where it will go. But the spaceship may get hit by an asteroid, or a sudden gust of wind may deflect the baseball. In many sports a championship may be determined by a this-way-or-that bounce of a ball. Determinism – and predicting the future – has limits, even in science. Check out chaos theory.
Do you want to know if it will be raining on your wedding next April? Will we have a white Christmas? You can find out the probability, based on past experience, but even our best computers cannot calculate the weather accurately weeks in advance, at least not in Illinois. It’s just too complex. Climate is more predictable; unlike weather, climate is an average over large areas, and over years rather than days.
Eighty years ago, physicists were in shock when they discovered that even the very simplest subatomic particles such as electrons and photons behave in unpredictable ways. Even if you knew everything possible about their position and motion, it’s impossible to know where they will be a second later. In this weird quantum world, also the decay of a radioactive element happens randomly; one can give only its probability in a time interval. Einstein was very skeptical about this, being convinced that “He (the Old One) does not play dice.” Ninety years later we have very convincing evidence that for once, Einstein was mistaken.