One universe is not enough. Learn about the Many-Worlds Interpretation of quantum mechanics in this exciting course taught by a renowned expert, Sean Carroll.
Episodes Season Season 1 · 0/24 0/24 watched · 2023 S01E01 Why Suppose There's More Than One World? 2023-11-24 32m
Fasten your seat belts and take off into the realm of multiple, maybe even infinite, worlds. Professor Carroll explains how quantum mechanics predicts the existence of a large number of universes parallel to our own. This far-out theory is one of the leading contenders for a rigorous formulation of quantum mechanics. Trace the history of, and motivation for, this idea. more
S01E02 The Classical Physics World That Never Was 2023-11-24
Investigate the classical picture of reality, which is how physicists thought the world worked before quantum mechanics. Codified by Isaac Newton, classical physics evolved into a nearly unified view based on particles and fields, and it included such revolutionary ideas as Einstein's theories of relativity. But starting in the early 20th century, scientists began to realize something was amiss. more
S01E03 Quantum Worlds Start With Waves and Particles 2023-11-24
The widely accepted system of classical physics began to unravel in 1900 when Max Planck proposed an idea that later became known as the quantum. Elaborated by Einstein, this theory held that light waves behave like particles. Later work by Louis de Broglie held that particles sometimes behave like waves. Discover how both ideas were amply confirmed and became tenets of quantum mechanics. more
S01E04 A Wave Function to Describe Particles 2023-11-24
Transition from the old quantum theory to full-fledged quantum mechanics with the mathematically elegant concept of the wave function, derived by Erwin Schrödinger in 1925. Professor Carroll guides you through the terms of the Schrödinger equation, which earned a Nobel Prize for Schrödinger and became the basis for wave mechanics-the theory that predicts how quantum systems behave. more
S01E05 Copenhagen Says the Wave Function Collapses 2023-11-24
Quantum mechanics was disquieting to anyone trained in classical physics. To dispel this unease, Niels Bohr and Werner Heisenberg devised the "Copenhagen Interpretation." Delve into the strengths and weaknesses of this influential view, which rejects speculation about what's "really happening." One reaction was Schrödinger's celebrated thought experiment involving a cat in mortal peril. more
S01E06 Is the Wave Function Real? 2023-11-24
Consider exactly what Heisenberg meant by his uncertainty principle, which is often misstated, even by physicists. Go deeper into wave-particle duality, studying the famous double-slit experiment, which shows light behaving simultaneously as a wave and a particle. Discover why a realist perspective on Schrödinger's wave function dissolves some of the key paradoxes of quantum mechanics. more
S01E07 Uncertainty in Action With Spin and Qubits 2023-11-24
Explore the fundamental quantum property of particles known as "spin," which can come in binary states, like the 0 and 1 bits in digital computing. For the purposes of quantum computing, spin can serve as a "qubit" to encode information at the subatomic level. Learn how spin makes the uncertainty principle much easier to understand and provides deep insights into the nature of the quantum world. more
S01E08 Quantum Entanglement and Action at a Distance 2023-11-24
Focus on Einstein's objection to a specific feature of quantum mechanics called entanglement, which he termed, "spooky action at a distance." When two particles are entangled, no matter how far apart they are, if you measure the property of one, you instantly know the corresponding property of the other. Einstein tried to use this feature to argue that quantum mechanics must be incomplete. more
S01E09 Entanglement Leads to Many Worlds 2023-11-24
Use concepts developed in the course so far to learn how physicist Hugh Everett arrived at a bold new approach to quantum mechanics. Called the Many-Worlds Interpretation, it holds that the wave function represents reality and evolves smoothly into multiple distinct worlds when a quantum measurement takes place. Contrast Everett's straightforward idea with the opaque Copenhagen Interpretation. more
S01E10 Decoherence Explains Branching Worlds 2023-11-24
Focus on decoherence, which does the same work in Many-Worlds as the collapse of the wave function in the Copenhagen Interpretation. Both explain what happens when a measurement is made, but in Many-Worlds the mechanism is more consistent with the underlying physics. Then, see how decoherence is the gateway to multiple branching worlds, which differ from the cosmological idea of the multiverse. more
S01E11 How Entanglement Powers Quantum Computers 2023-11-24
Many-Worlds theorist David Deutsch helped pioneer quantum computing, which he argues is an outgrowth on the Many-Worlds Interpretation. Investigate the principles behind quantum computing, comparing it to classical computing. Discover that the big difference is the architecture of logic gates. See how quantum computers can surmount this obstacle and excel at certain types of calculations. more
S01E12 Too Many Worlds! Five Objections Answered 2023-11-24
The Many-Worlds view seems to defy common sense. Why can't we see the other worlds? And don't they violate the laws of physics and other rules of nature? Professor Carroll answers five major objections, concerning the philosophical concept known as Occam's Razor, the problem of time asymmetry, the possibility of infinity, plus scruples about immortality and energy conservation. more
S01E13 Testing the Many-Worlds Interpretation 2023-11-24
Address another objection to the Many-Worlds Interpretation: its testability. This refers to philosopher Karl Popper's famous falsifiability criterion, which discounts any theory that can't in principle be proven false. The proliferation of worlds that can't ever be observed might seem to qualify Many-Worlds as unfalsifiable, but Professor Carroll shows that it is testable where it counts. more
S01E14 Where Does Probability Come From? 2023-11-24
Yet another hurdle for Many-Worlds is the origin and nature of probability. The Copenhagen version of quantum mechanics is fundamentally probabilistic, rather than deterministic. This is a key feature in its success. By contrast, Many-Worlds is deterministic. We can derive an understanding of probability by thinking about where we are in the quantum wave function. more
S01E15 Quashing Worlds With Wave Function Collapse 2023-11-24
Given the mind-boggling implications of Many-Worlds, many physicists have sought plausible alternatives. In this lecture, consider the possibility of altering the Schrödinger equation-the jumping-off point for Many-Worlds. Investigate two proposals that try this tactic: GRW (named after its inventors, Ghirardi, Rimini, and Weber) and CSL (Continuous Spontaneous Localization) theory. more
S01E16 Blocking Worlds With Hidden Wave Variables 2023-11-24
Does the wave function tell the whole story? Explore the hidden variable theory, devised by Louis de Broglie and refined by David Bohm. According to this view, particles are guided by pilot waves constructed from the wave function. The "hidden variables" are the precise positions of particles, which are being guided by the pilot waves. Learn why some critics call the idea "Many-Worlds in denial." more
S01E17 Mind Before Matter in Quantum Theory 2023-11-24
Since quantum mechanics and consciousness are both mysterious, could they be connected in some way? Examine several arguments that relate quantum phenomena to the involvement of conscious observers. The Copenhagen Interpretation is particularly open to such speculations. Also, look at Quantum Bayesianism, or QBism, which sidesteps quantum paradoxes by dispensing with the idea of objective reality. more
S01E18 The Quantum Emergence of the World We See 2023-11-24
How does the structure of observed reality emerge from the wave function in Many-Worlds? In other words, where do the worlds come from? This question relates to the "preferred basis" problem that attempts to link the quantum realm to everyday macroscopic objects. See how Schrödinger's clever thought experiment involving a cat provides a conceptual tool for solving this puzzle. more
S01E19 The Challenge of Quantum Gravity 2023-11-24
Quantum theory accounts for a remarkable array of particles and forces-but not gravity. Learn why constructing a successful theory of quantum gravity has vexed physicists for nearly a century. Professor Carroll lays the groundwork for discussing the Many-Worlds perspective on gravity by focusing on two popular alternative theories: string theory and loop quantum gravity. more
S01E20 Space Emerges From Entanglement 2023-11-24
Starting with the basic ingredients of quantum theory-wave functions, Schrödinger's equation, and entanglement-and following the Many-Worlds approach, probe this question: What circumstances lead to emergent branches of the wave function that look like matter moving in curved spacetime-that is, in gravitational fields? Find that gravity may be a natural consequence of quantum mechanics. more
S01E21 The Quantum Emergence of Time 2023-11-24
As space might be an emergent property of quantum entanglement, could the same be true of time? Divide a wave function into subsystems and watch how the rest of the universe becomes entangled in a manner that can be interpreted as time passing. Along the way, learn the ideas behind the Wheeler-DeWitt equation, which helped define the "problem of time." more
S01E22 Free Will, Determinism, and Many-Worlds 2023-11-24
Get philosophical by probing a pair of profound questions that arise from the far-out implications of Many-Worlds. Are multiple branching worlds caused by our decisions? Is human free will possible, especially in light of the deterministic nature of Many-Worlds? Professor Carroll analyzes the way the macroscopic world of human thought and action interact with the quantum realm. more
S01E23 What Happens to Ethics Under Many-Worlds? 2023-11-24
A theory in which every moral act entails an immoral one taking place in a branching universe is rife with ethical quandaries. Now, consider whether you could be moral in each of the universes of a Many-Worlds scenario. One stumbling block is imagining that the version of you that took a branching path is actually you. You may share a past, but the two of you are really different people. more
S01E24 A Future Renaissance for Quantum Mechanics 2023-11-24
Many-Worlds and competing theories on the foundations of quantum mechanics may seem essential for our understanding of reality, but they were long ignored by no-nonsense practicing physicists. Close the course by witnessing how the tide is turning, as it becomes increasingly clear that the foundational issues are likely the key to unlocking the outstanding mysteries of the cosmos. more 
