Lộc Lương

Once upon a time, when humans came down from living on trees to the ground (either on purpose or by supernatural will), they were curious about everything around them. They invented fire, developed civilizations, made wars, and built tons of beautiful wonders, and so much more. Human observed and look at nature and have some forms of explanation for how thing works. For example, Aristotle believed that all objects naturally come to rest; and they only move when a force is applied.

At some point, they began to ask deeper questions about nature and about where they lived. For a long time, they believed they lived on a flat surface and at the center of the universe. This idea survived for centuries—until some tough men resisted: “No, we are not at the center, and the Earth is not flat.” And people put them in jail for saying that.

Nicolaus Copernicus said that the Sun is at the center of the solar system. Galileo then laid the groundwork for the laws of motion, which were later developed by Isaac Newton. When Newton discovered gravity and established the three laws of motion, they became fundamental to many achievements and tools we use today. Later, he invented calculus (independently with Leibniz), which, in my opinion, is one of the most useful inventions in mathematics, perhaps the best tool ever from the toolbox of mathematics to study physics as well as other engineering applications.

For a long time, people believed that if we have a set of rules and we know the initial conidtions, then we can predict what a physical system will do in the future. So the universe is like a big mechanical clock, everything just follow the rules and nothing is really random. Inside this clockwork machine, everything is predetermined by precise physical laws, and it works pretty well. People used this idea to build the foundation for fluid and solid mechanics and many other fields too. They wrote down equations that can predict how fluids and solids behave, just by assuming matter is smooth and continuous (and these equations are still everywhere today).

Then, a very peculiar guy looked at the strangeness of electromagnetism and postulated that the speed of light is fixed and is the maximum. Nobody really understood his relativity at the time, and he later worked out general relativity after years of working alone.

But as humans looked deeper into the worlds of very small things, then some weird things started to show up. Experiments showed that matter at very small scales does not really behave like a smooth continuum anymore. For instance, matter is made of even smaller particles like electrons, protons, and neutrons, or even smaller sub-atomic particles. And particle sometimes behave like waves and waves sometimes behave like particles.

Max Planck came up with the idea that energy is quantized by solving the blackbody radiation problem. And then that same peculiar guy, Albert Einstein, showed that light itself can behave like particles. But light also behaves like waves as shown in the double slit experiment. Niels Bohr proposed a new model of the atom that could explain the observation data very well. Later, Heisenberg showed that we cannot simultaneously know both the exact position and momentum of a particle with arbitrary precision, and Erwin Schrödinger came up with a wave equation to describe a quantum state of a physical system, and many others (e.g., Dirac, Feynman) continued to added more to the understanding of this microscopic world.

In this new picture, the world is not deterministic anymore like we used to think. Laplace once imagined a “demon” that knows where every particle is and every force acting on it, so it could see the whole future and the whole past. But even this demon would be stuck here. Even if we know the wavefunction of a system perfectly, we still cannot say for sure what will happen. We can only talk about probability, like the chance of finding an electron somewhere around the nucleus (this idea came from Born). Even Schrödinger didn’t fully get what his own wave equation meant. It is normal that physicists usually write the equation first and figure out what it means later. Later, Bohr’s idea that measuring a particle “collapses” its wave function, together with Heisenberg’s uncertainty principle, became what we call the Copenhagen interpretation.

The clock isn’t broken. Newton’s world still works great for the things we can see. But under that smooth surface is a quantum world, where everything is just chance, and things aren’t really decided until you measure them (and I still don’t understand any of this).