The Physics That Makes Interstellar Travel IMPOSSIBLE | Leonard Susskind

DESCRIPTION: It sounds impossible to say that interstellar travel is fundamentally forbidden. After all, rockets launch, probes cross the Solar System, and physics has never stopped us from going farther with better engineering. Yet modern physics suggests something deeply counterintuitive: the structure of spacetime itself may place absolute limits on how far, how fast, and how meaningfully we can travel between the stars. In this video, we explore what physicists really mean when they say that faster-than-light or practical interstellar travel may be impossible in principle—not merely difficult, but constrained by the deepest laws of nature. According to special and general relativity, spacetime is not a passive stage but a dynamical structure with built-in speed limits and geometric constraints. The speed of light is not a technological barrier; it is woven into the causal structure of the universe, defining what events can influence one another. Attempts to bypass this limit—through wormholes or warp geometries—run directly into severe energetic and stability constraints. Drawing on ideas associated with Leonard Susskind, this exploration examines relativistic causality, spacetime geometry, quantum field constraints, and the energy conditions required for exotic metrics. Without invoking equations, we consider why negative energy densities, horizon formation, and quantum instabilities make many speculative propulsion schemes physically suspect rather than merely futuristic. This does not mean that space exploration is futile or that relativistic travel within our galaxy is forbidden. The claim is subtler: the universe may allow travel, but not in ways that violate its causal architecture or compress cosmic distances into human timescales. This strange behavior reveals something profound about reality. The same principles that protect causality and preserve consistency across spacetime may also ensure that the cosmos remains vast, structured, and resistant to shortcuts. Limits are not engineering failures; they are features of the universe’s deep design. This video avoids equations and focuses on intuition. Through clear explanations and thought experiments, we explore whether the dream of interstellar travel collides with the fundamental architecture of spacetime itself. This is a calm, reflective documentary for viewers who want to understand one of the strangest consequences of modern physics. How this was made