StarTalk Cosmic Queries: Arrow of Time, Black Holes, Dark Matter, and Quantum Mysteries with Sean Carroll

Overview

In this StarTalk Cosmic Queries edition, host Neil deGrasse Tyson chats with Sean Carroll about the arrow of time, the evolution of entropy since the Big Bang, and how the early universe set the stage for a universe with a preferred temporal direction. The conversation broadens to the role of fields in physics, the history from Faraday to Maxwell, and how electromagnetic phenomena underpin so much of our everyday tech.

They also dive into black holes and Hawking radiation, explain how observers influence perceptions near an event horizon, and discuss whether dark matter is a particle or a sign of modified gravity. The episode folds in quantum foundations, including the delayed-choice quantum eraser and the many-worlds interpretation, and ends with reflections on the limits of theory, the possibility of a theory of everything, and the future of Sean Carroll's trilogy on The Biggest Ideas in the Universe.

Introduction and the Arrow of Time

The episode opens with a consideration of the arrow of time, asserting that the early universe began in an unusually organized, low-entropy state and has become more disordered over 14 billion years. The discussion frames time as a cosmology question rather than a purely philosophical one, noting that Newtonian physics does not distinguish past from future, which motivates deeper questions about initial conditions of the cosmos.

Fields, Electromagnetism, and the History of Physics

Sean Carroll emphasizes the central role of electromagnetic fields, tracing the conceptual arc from Faraday’s lines of force to Maxwell’s equations, which mathematically describe electric and magnetic fields that fill space. The dialogue revisits Newtonian gravity’s mystery about action at a distance and underscores how field concepts emerged to explain diverse phenomena from heat to radio waves.

Hawking Radiation, Black Holes, and Observers

The conversation turns to Hawking radiation and the apparent paradox of what an observer experiences when crossing the event horizon. Carroll explains that, from the falling observer’s frame, crossing the horizon can appear uneventful, while distant observers see radiation. The discussion highlights the complexity of reconciling different viewpoints within general relativity and quantum field theory.

Dark Matter and Gravity

The hosts examine whether dark matter is a particle or a signal of modified gravity. They discuss observational evidence for dark matter, the range of possible masses, and the possibility that gravity itself could be altered, without discarding additional dark matter components. The distinction between dark matter and dark gravity is clarified with careful caveats about current data.

Quantum Foundations: Copenhagen vs Many-Worlds

The episode delves into how quantum mechanics is interpreted, contrasting the Copenhagen view with the many-worlds hypothesis. The discussion covers why multiple worlds might be considered real or merely a mathematical framework for predicting outcomes, and how this relates to measurements, entanglement, and the Schrödinger equation.

Notorious Delayed Choice Quantum Eraser

Sean and Neil discuss the delayed-choice quantum eraser experiment, explaining how entanglement allows choices about information to occur after particle detection without implying retrocausal influence. Carroll even references a blog post he wrote that demystifies the experiment for a broad audience.

Cosmology, Isotropy, and the Rotation of the Universe

The guests explore creative ideas like a spinning universe and how isotropy constrains cosmological models. They assess whether a rotating cosmos could address the Hubble tension or relate to dark energy, while noting that current observations favor a largely isotropic, simple cosmology with a cosmological constant.

Everything, Simulation, and the Theory of Everything

The conversation circles back to the possibility of a single theory of everything, acknowledging the philosophical and practical questions about whether such a description could ever exist or be accessible. They discuss the limits of human intellect, the reliability of physical laws across extreme conditions, and the value of examining thought experiments like horizons in simulations or the Truman Show analogy.

Evidence from the Early Universe and the Road Ahead

Carroll highlights Big Bang nucleosynthesis and the cosmic microwave background as remarkable tests of our physics, illustrating how extrapolating known laws to extreme conditions yields predictions that match observations. The conversation closes with Sean outlining his trilogy, including Quanta and Fields and Complexity and Emergence, and with encouragement to keep looking up at the cosmos.