Say I throw a pebble into a pond. The pebble makes a splash and ripples travel outwards from the point of impact. I know from plain old experience that no matter how many rocks I throw, in no way will this sequence of events ever change. All else equal, it will always be: thrown pebble, splash, ripples. In that order. This is known as “cause and effect” or “causality.”
But what about the opposite of causality? Is it possible for the reverse to be true, where effects precede causes? Well… no. Not that we know of.
So far, there is no experimentally-verifiable evidence that definitively contradicts causality. But the public is mighty confused on the subject. And understandably! The entire issue is riddled with nuance, esoteric mathematics, extreme technological hurdles, and rampant misinformation. I myself found the research required to write this premise to be, by far, the most demanding of them all. The question of whether the Universe is deterministic is not an easy one, believe me.
Causality vs Determinism
Firstly, we need to address the confusion between causality and determinism: They’re not the same. When we say “causality,” we are referring to the direction of time itself; cause then effect. When we say “determinism,” although there are many philosophical flavors, we’re generally referring to our ability to determine effects from causes. Causality is an absolute constant of our Universe, where determinism is referring to our ability to comprehend that absolute. One is a force of nature. The other is about understanding and predicting that force of nature. Make sense? The difference is important.
When you hear phrases like “the Universe is deterministic/indeterministic,” those folks (assuming they’re using the terms correctly) are not actually making a statement about whether the Universe itself is causal, they’re instead referring to what it is possible to know. Indeed, the argument has never really been whether the Universe is causal. It is. In fact, all of modern science, by definition of seeking to understand the causes of the effects we observe, necessarily rests upon a foundation of causality. Rather, the debate has instead focused upon the limitations of our ability to predict what happens next – to determine.
Before quantum mechanics (QM) arrived in the 20th century, there were centuries of back-and-forth about whether the Universe is ultimately deterministic, meaning it could be predicted perfectly given complete information and sufficient intelligence, or indeterministic, meaning it couldn’t. QM finally settled the issue: The Universe, although causal, is in fact indeterministic.
But that statement requires some serious unpacking, ergo the length of this premise.
QM revealed that there are actual, experimentally-verifiable laws of nature that place hard limits on our ability to perfectly determine the future sequence of events in any system, even if we know absolutely everything about its current state. And that sounds crazy! Even if you knew literally everything about something, you couldn’t perfectly predict what will happen next? Believe it or not, it’s true. Many heavily-scrutinized public experiments conclusively illustrate that the Universe is fundamentally uncertain.
Of course, this entire concept is pretty wild! Fundamental uncertainty? How can the next thing that happens not be predictable if you know everything about its present state?
The idea that the Universe is fundamentally indeterministic and uncertain threw a lot of people off (and still does to this day). Even Albert Einstein was wrong on this one: Einstein strongly believed the Universe to be deterministic. He believed there was simply something wrong with QM that hadn’t been discovered yet (known as “hidden variable theory”). But that wasn’t the case (as shown in “Bell’s Inequality”). In fact, the opposite happened! Today, quantum theories represent some of the most exquisitely accurate and mathematically intricate theories we have. Many modern technologies could only have been designed using the principles of QM; entire industries are only possible because of QM.
But how was Einstein wrong? How is the Universe indeterministic? These are big questions. To answer them, we’ll need to pop the hood of the Universe and take a much closer look at it’s engine – physics.
A Brief Look at Modern Physics
The first thing to know about physical theories is they’re all about reductionism, which is the ancient principle of reducing the Universe into the most basic and fundamental components possible. “Elementary particles” represent these most basic components. Elementary particles have no substructure and, as far as we know, cannot be measurably reduced any further. Since QM focuses on these particles, it can be said that QM is about exploring the tiniest stuff in the Universe. And this science is what you’ll want to grasp if you seek to answer whether the Universe is deterministic on the most fundamental level.
But physics is hard, especially areas like QM and relativity. So to kick off the learning process, I’ve created a timeline below to illustrate the context and milestones leading up to modern physics, followed by primers on relativity and quantum theory.
Don’t worry too much if the below sounds like jargon. Modern physics is very complex and there’s quite a bit of messy overlap. It will all come together in time, just keep reading! You can come back to this timeline later – think of it as a reference.
The Revolution of Modern Physics
| Pre-20th Century | “Classical Physics” is mainstream: Particles are particles. Waves are waves. The Universe operates in an orderly, sensible, and predictably deterministic fashion. Physics is widely considered a near “complete” science. |
| 1801 | Thomas Young performs his “double slit experiment,” showing that light exhibits wave-like properties due to the “interference pattern” it generates. This experiment further corroborates James Maxwell’s classical electromagnetism. Significance: Light must be a wave. |
| 1900 | Max Planck derives the correct blackbody radiation spectrum (solving the “ultraviolet catastrophe”) by coming up with the radical idea that energy states are “quantized,” meaning they can only exist in discrete states and are not continuous. For example, they can only exist in an energy state of “1, 2, 3, etc.” but never in a state of “1.2, 2.4, 3.2, etc.” This means there is no “between” or fraction to these energy states; they are one or the other. Significance: The states of the quantum world must be discretely quantized. Prior to this discovery, it was thought these states were analog rather than digital; more like the gradient between two colors than a binary black and white relationship. This discovery is the seed of all quantum theory. |
| 1905 | Albert Einstein publishes his three paradigm-shattering papers: 1) An explanation of the photoelectric effect 2) The Special Theory of Relativity (STR) 3) An explanation of Brownian motion Significance: 1) Light must also be discretely quantized (wave-particle duality) 2) Time and space are relative to inertial reference frames 3) Brownian motion is actually the result of the random motion of atoms and molecules (incidentally proving they are real) |
| 1916 | Einstein later publishes a sophisticated paper outlining his General Theory of Relativity (GTR), which very accurately explains the relationship between space and gravity in all reference frames (not just “at rest” frames). Additionally, Einstein describes mass-energy equivalence with the famous E=mc^2 Significance: Einstein gave us a far more accurate theory of gravity, fully combined space and time into a single concept, and revealed the speed of light to be a fundamental constant of the Universe which, in-turn, revealed that mass and energy are actually equivalent. |
| 1924 | Louis Victor de Broglie publishes his PhD thesis hypothesizing electrons possess wave-like properties. A few years later, these “matter waves” were experimentally confirmed by shooting electrons (and eventually larger molecules) through Young’s experiment, revealing that all matter possesses wave-like properties and thus wave-particle duality. Significance: It’s not just light that possesses wave-particle duality, so does matter. |
| 1926 | Once we discovered wave-particle duality, the physics community needed to create math to deal with it. To do so, Erwin Schrödinger developed the differential “Schrödinger Equation,” which deterministically calculates a quantum system’s three dimensional wave function. In the same year, Max Born interpreted the wave function as a “probability amplitude,” which gave rise to the phrase “probabilistic physics.” Significance: Math now exists to formally describe the strange probabilistic behavior of the quantum world. “Quantum Mechanics” becomes a formal field in physics. Many “interpretations” of QM relating the math and experimental results to our broader reality will arise over the next century, yet even today no firm consensus exists on which is correct. |
| 1927 | Werner Heisenberg formulates the “uncertainty principle,” which states the more you know about a particle’s momentum, the less you know about its position, and vice versa. Significance: Classical physics, which states that position and momentum can be known at all times, is shown to break down in small enough realms. Heisenberg illustrates this with his uncertainty principle. The uncertainty is not a problem with our measuring instruments or experimental technique, it’s literally the way the Universe works: QM shows us that everything is some form of wave at the most fundamental level. As you go smaller and smaller, things become increasingly wave-like (wave) or pulse-like (particle). For something to be interpreted as a particle, you need high certainty of its location (but in so doing, you lose certainty about it’s momentum). For something to be interpreted as a wave, you need high certainty of its momentum (but in so doing, you lose certainty about it’s location). The two cannot be fully known simultaneously. |
| 1928 | Paul A.M. Dirac discovers the relativistic equation for the electron (introduces relativity to the Schrödinger Equation), which apparently requires the existence of positively charged electrons. Significance: Lots of physicists were trying to reconcile relativity with QM: Dirac tried combining E=mc^2 with Schrödinger’s Equation, which didn’t work at first. However, he took a leap that by introducing a sort of “negative” or “anti” particle (a positively charged electron, in this case called a “positron”), the math would become sensible. He was right! This led to the discovery of “antimatter” experimentally as we know it today. The existence of antimatter allowed for relativity and QM to be partially combined in a meaningful way, which is now known as “Quantum Field Theory” or QFT. |
Again, much of the above probably sounds like jargon, and that’s okay! We’re going to break this down into much easier bites in the next two primers. I encourage you to stay persistent, as these theories answer many profound questions about the Universe we live in.
Premise Four: The Universe, although causal, is fundamentally indeterministic.