Things get increasingly peculiar from here on out. Relativity forced us to reexamine our basic assumptions of time and space, but quantum mechanics is absolutely absurd. It would appear at the quantum level the universe acts in a way so obscure and contradictory to the familiar that even physicists have trouble making sense of it.

In 1923, Prince Lous de Broglie reasoned, roughly speaking, that Einstein, through special relativity, equated matter with energy, Planck related energy with waves, therefore matter may have wave-like properties as well. De Broglie’s theory was soon verified experimentally by the double-slit experiment. Rather than using a wall, however, a phosphorescent screen was used to track the end-locations of electrons. Even if a string of single electrons were fired they somehow found a way to interfere with each other, creating the interference pattern akin to waves. This was a remarkable discovery. Physicists were forced to conclude that matter exhibited wave-like properties in conjunction with particle properties more commonly associated with matter.

There is another interesting phenomenon associated with this experiment. What if you wanted to ‘look’ and see which slit an electron went through? One way to do this would be to shine light on the electron to see it – that is bounce photons off of it. The problem with this approach is that by doing this, you have modified the experiment. On macroscopic scales the energy carried by a photon is negligible, but when dealing with something as miniscule as an electron the energy carried by a photon could severely alter the trajectory of the electron. When this is done to the double-slit experiment, the resulting pattern on the phosphorescent screen is no longer an interference pattern, but it is a pattern we would expect if electrons only had particle properties. It’s as if the electrons know they are being watched and respond accordingly!

The logical question that follows from these realizations is how do these conclusions coincide with real world experiences. When de Broglie mathematically determined the wavelength of matter waves, he found that they are proportional to Planck’s constant. Since the constant is so small, the wavelength of matter waves are also consequently small.

The next question I suspect most people would have is what exactly is matter a wave of. The answer proposed by Max Born in 1926 is perhaps the most shocking conclusion of modern science. Born suggested that the electron wave is actually a probability wave of where the electron will be found. In other words, at the fundamental level, the universe intrinsically has a seed of randomness. This has interesting consequences on the philosophy of determinism, which I intend to discuss in the next section.

This means that if an experiment involving a fundamental particle of the universe such as an electron were repeated multiple times in an identical manner the results would vary with each experiment. Erwin Schrodinger defined an equation for the probability wave. Though quantum experiments can’t be reproduced identically, the probability wave created by iterated experiments can be mathematically modeled, tested and reproduced. The probability wave has been tested and reproduced with high-fidelity making Born and Schrodinger’s counterintuitive suggestions a valid scientific theory and a seemingly accurate description of the quantum world.

Following WWII, Feynman took quantum theory in a new direction. Richard Feynman’s perspective did not change the mathematics behind quantum mechanics but he provided a new way to analyze the probabilistic nature of matter. Feynman theorized that electrons did not travel through only one slit in the double-slit experiment, but through both slits simultaneously. Not only that, the electron traversed every possible trajectory it could take. It went smoothly through the left slit. It went zig-zagged through the right slit. It took a trip to the Andromeda galaxy and came back. Feynman assigned a number to each of these paths. When he averaged them out he found the exact same result as the probability wave of Born and Schrodinger. This drastic change of perspective alleviated the quantum world from the compulsory probability wave but proposed something perhaps more bizarre.

Once again, it is important to answer why these effects are not noticed on a macroscopic scale. Feynman showed for all particles larger than atoms his method of assigning numbers to paths allows all paths except one to cancel each other out, What we’re left with is the path that us macroscopic sentients are familiar with.