1.a. What is Quantum Mechanics

Learning Outcomes

During the course of this lesson, you will:

  • Learn the most important features of quantum physics
  • Understand the differences between classical and quantum physics
  • Get an overview of the most interesting (and puzzling) quantum phenomena and paradoxes, such as:
  • Heisenberg’s uncertainty principle, Schrödinger’s cat, the wave-particle duality, and quantum entanglement

1. Introduction

An understanding of Quantum Mechanics is potentially relevant to meditators and contemplative practitioners who are interested in gaining a deeper insight into the nature of reality. One important point is that, at the microscopic quantum level, the solidity and stability of our experience of the world seems to break down, and interactions begin to play a more prominent role than individual entities (such as material particles). Similarly, in deep meditation, it is possible to experience reality as deeply interconnected, rather than separate from our observations of it, and the “self” is experienced as fluid rather than stable and unchanging.

Physicist Richard Feynman famously said: “If you think you understand Quantum Mechanics, then you don’t understand Quantum Mechanics!”. This is because matter and energy behave in a highly counterintuitive way at the microscopic level, even though this is what makes up the world we experience at the everyday level. In this lesson, some of the key differences between classical physics (which describes macroscopic, everyday objects very accurately) and quantum physics (which describes the behaviour of microscopic entities, such as atoms, electrons, and quarks) are discussed and explained.

2. Watch

3. Read

Part 1 – What is the Difference Between Classical and Quantum Physics?

The Uncertainty Principle

In classical physics, both the position and momentum of an object can in principle be known simultaneously with complete accuracy. If we know both the initial position of an object and its momentum (i.e., its mass and velocity in a particular direction), then we can predict its trajectory and exactly where it will be at a future point in time.

But at the quantum level, this is not possible, even in principle: the more accurately we know either the velocity or momentum of a particle, the less accurately we will know the other of these quantities. Or we might know roughly where a particle is and roughly how fast it is moving in a particular direction, but not know either of these quantities with any degree of certainty.

So, at the quantum level, particles do not have a defined trajectory, only a “probabilistic” one. There are only likelihoods that they exist with certain properties at a certain time and place. This uncertainty isn’t caused by limitations in our measurement tools but is an intrinsic property of any quantum system. Quantum particles simply don’t exist in the definite way we’re used to with everyday objects.

Waves and Particles

In classical physics, if particles pass through one slit in a screen, they will form one line on a background screen, corresponding to the shape and size of the slit they’ve passed through. Similarly, if particles pass through two slits in a screen, then they will form two lines on a background screen, corresponding to the shape and size of those two slits. However, in quantum physics, when particles pass through two slits in a screen, they form an “interference” patten on a background screen. Such a pattern is characteristic of waves because the “peak” of one wave cancels out the “trough” of another wave but amplifies the “peak” of another. So, it is as if a particle passes through both slits at the same time, with two states of that particle being simultaneously “superimposed” on each other like colliding waves. Therefore, at the quantum level, entities can and do exhibit the properties of both particles and waves at the same time.

Even stranger is what happens when a measurement is made next to either slit to “detect” which slit the particle “actually” passes through. When this happens, the result of the experiment changes! An interference pattern no longer appears on the background screen, but rather two lines again appear, just as in the case of classical physics. One of the two “superimposed” wave states has collapsed. This shows that the observations made of a quantum system seem to be integral to how it behaves.

Schrödinger’s Cat

Schrödinger, one of the fathers of quantum mechanics, devised a provocative thought experiment to highlight the paradoxical nature of quantum behaviour. He imagined a radioactive atom which either decays or doesn’t decay. Since atoms are quantum systems, these two possible states aren’t exclusive; they exist simultaneously as two superimposed states, which coexist until an observation causes one or the other possibility to “collapse”, thereby resolving the ambiguity.

In the context of quantum mechanics, this description agrees with the results of experiments on atoms and subatomic particles. But what would happen if we were to couple a quantum system to a classical everyday system? In order to illustrate the paradoxical consequences of this line of reasoning, Schrödinger described the following imaginary experiment. He imagined that the radiation emitted by a decaying atom was set up to hit a detector. This detector would trigger the release of a hammer which would in turn break a glass bottle containing lethal poison. All of this takes place inside an enclosed box with a cat inside it.

The paradoxical result is that, at the classical level, the cat inside the box must be both alive and dead at the same time. This remains the case until someone opens the box and “collapses” the quantum system into one state or other. At this point, they would either find a dead cat or a living cat inside the box. However, according to a traditional interpretation of quantum mechanics, until then the system exists in both states simultaneously. This imaginary experiment reveals how difficult it is to apply the logic of quantum systems to the classical world we perceive through our senses.

Part 2 – Other Quantum Paradoxes

The Measurement Problem (Wigner’s Paradox)

Wigner formulated a more nuanced version of Schrödinger’s thought experiment. He imagined that a quantum experiment takes place in a laboratory in which a quantum state of “heads” or “tails” is resolved by observation. However, from the perspective of another experimenter, who is waiting outside the laboratory door, the outcome of the experiment is unknown and so, for them, the quantum system still exists in two different superimposed states. This presents a seeming paradox concerning whether the quantum state is really one of “heads” or “tails”: different observers would perceive two different realities at the same time, one where the quantum state is either heads or tails, and another where the two quantum states are still superimposed (“heads” and “tails”!).

Entanglement (Einstein-Podolsky-Rosen Paradox)

There are different kinds of properties that a quantum particle can have but, to keep the explanation simple, let’s imagine that a quantum particle can be either “red” or “blue” with 50% probability. Now, it is possible for quantum particles to become correlated or “entangled” such that the properties of one automatically define the properties of its entangled “twin”. For example, when one particle has the property “red”, the other will always have the property “blue”, and vice versa.

Experiments have shown that it doesn’t matter how far apart two such entangled particles are for them to instantaneously affect each other like this, even across astronomical distances. Einstein called this effect “spooky action at a distance”. His theory of relativity maintains that no particle or information can travel faster than the speed of light, but entanglement effects are instantaneous. This is an example of “nonlocality” and is another important feature of quantum behaviour.

Lecture Notes

We recommend reading the lecture notes before you start watching the content. This will help you to start contemplating some of the topics before you begin to watch the lecture.

Download Module 1 Lecture Notes