String Theory, Quantum Gravity and Black Holes (Or, Are We Holograms?)

World Science Festival・2 minutes read

Physicists Brian Green and Juan Maldacena discuss merging gravity and quantum mechanics through String Theory, highlighting the challenges and potential solutions in understanding the universe's origins, particularly in black holes. Despite the complexity and ongoing quest for definitive predictions, insights from String Theory have shaped research for over 20 years, offering hope for unraveling the essence of space-time and the profound connection between gravity and quantum field theory.

Insights

  • String Theory proposes vibrating filaments as the basis for all particles, aiming to reconcile the conflict between gravity and quantum mechanics by providing a mathematically rich approach to understanding the nature of reality.
  • The Holographic Principle in String Theory suggests that gravity operates fundamentally differently from other field theories, indicating that the true degrees of freedom in theories with gravity reside on boundary surfaces rather than within the volume itself, offering a unique perspective on gravitational physics and the nature of the universe.

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Recent questions

  • What is the goal of unifying gravity and quantum mechanics?

    The goal of unifying gravity and quantum mechanics is to create a single theory that explains the fundamental forces governing the universe. By merging these two theories, scientists aim to understand the origins of the universe and resolve the discrepancies between the macroscopic scale of gravity and the microscopic scale of quantum mechanics. This unified theory would provide a comprehensive framework to explain the behavior of particles, the nature of space-time, and the interactions between matter and energy.

  • How does String Theory propose to reconcile gravity and quantum mechanics?

    String Theory proposes that the fundamental building blocks of the universe are tiny vibrating filaments known as strings. These strings are believed to give rise to all particles and forces in the universe, offering a way to unify gravity and quantum mechanics. By viewing particles as different vibrational modes of strings, String Theory aims to resolve the contradictions between these two fundamental theories. This approach provides a mathematically rich framework that offers revolutionary insights into the nature of reality and the fundamental forces of the universe.

  • What is the holographic principle in String Theory?

    The holographic principle in String Theory suggests that gravity operates fundamentally differently from other field theories. It proposes that the true degrees of freedom in a theory with gravity reside on the boundary surface rather than within the volume itself. This concept implies that the information about a three-dimensional volume can be encoded on a two-dimensional surface, much like a hologram. By scaling degrees of freedom with area rather than volume, the holographic principle offers a unique perspective on the nature of gravity and space-time.

  • How does String Theory address the issue of black hole entropy?

    String Theory has made significant advancements in understanding black holes, particularly in calculating black hole entropy. By considering the microstates of black holes, String Theory has been able to provide a more detailed and accurate description of black hole entropy. This approach has shed light on the thermodynamic properties of black holes and their relationship to quantum mechanics, offering new insights into the behavior of these enigmatic cosmic objects.

  • What is the significance of the AdS/CFT correspondence in String Theory?

    The AdS/CFT correspondence in String Theory explores the idea that the true degrees of freedom in a theory with gravity reside on the boundary surface rather than within the volume itself. This correspondence suggests that placing a surface at the edge of the universe allows for a more explicit description of a theory without gravity. By linking theories without gravity at the boundary to theories with gravity in the interior, the AdS/CFT correspondence provides a powerful tool for understanding the nature of gravity and space-time in String Theory.

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Summary

00:00

Unifying Gravity and Quantum Mechanics: String Theory

  • Brian Green from Columbia University discusses the topic of unification in physics, aiming to merge gravity and quantum mechanics.
  • The conversation with physicist Juan Maldacena delves into the challenges of unifying these theories and the potential solution of String Theory.
  • String Theory is highlighted as a mathematically rich approach with revolutionary insights into the nature of reality.
  • Juan Maldacena, a prominent figure in string theory and quantum gravity, is introduced as the Carl Fineberg Professor at the Institute for Advanced Study.
  • The discussion touches on the importance of unifying gravity and quantum mechanics to understand the universe's origins.
  • Black holes are emphasized as a crucial domain where gravity and quantum mechanics intersect, especially in their interiors.
  • The conversation explores the significance of gravitational waves in confirming Einstein's theory and the recent direct images of black holes.
  • Einstein's skepticism towards black holes and gravitational waves is discussed, highlighting the evolution of understanding in physics.
  • The general theory of relativity is explained as viewing gravity as a consequence of the curvature of spacetime, leading to the expansion of the universe and the existence of black holes.
  • Einstein's discomfort with quantum mechanics is outlined, focusing on its probabilistic nature and the underlying quest for a more fundamental theory.

16:27

Unifying Quantum Mechanics and General Relativity

  • Quantum mechanics was recognized by Einstein and Podolsky as a thought experiment.
  • Devices now exist that make this thought experiment a reality.
  • Quantum mechanics and general relativity have been tested to high accuracy.
  • The issue arises when trying to combine quantum mechanics and general relativity due to different scales and objects.
  • Gravity is considered a field theory with fluctuations in spacetime at smaller distances.
  • At very short distances, quantum mechanics and gravity struggle to work together.
  • The theoretical problem arises when extrapolating equations to the beginning of time or inside black holes.
  • The goal is to create a single theory combining general relativity and quantum mechanics.
  • String theory proposes vibrating filaments as the basis for all particles, resolving the tension between gravity and quantum mechanics.
  • String theory originated from attempts to understand strong interactions in the late 60s, based on real objects observed in particle accelerators.

32:14

"String Theory: Exploring Dimensions and Multiverse"

  • In strength theory, equations are more constraining than in general relativity, limiting the dimensionality of SpaceTime.
  • Initially, only a few ways to curl up extra dimensions were known, with the goal of finding more to explain unexplained aspects of particle physics.
  • Over time, more possibilities for extra dimensions were discovered, leading to challenges in finding unique solutions.
  • Multiverse theory is considered to explain the vast possibilities in string theory, offering potential explanations for phenomena like dark energy.
  • Calculating probabilities for different universes in a Multiverse is currently beyond our understanding in string theory.
  • String theory has not yet provided definitive predictions, particularly in determining the shapes of extra dimensions or particle outcomes.
  • Remarkable progress has been made in understanding black holes through string theory, particularly in calculating black hole entropy.
  • String theory's ability to calculate black hole entropy through microstates has been a significant advancement in the field.
  • The holographic principle in string theory suggests that gravity operates fundamentally differently from other field theories, with degrees of freedom scaling with area rather than volume.
  • The AdS/CFT correspondence in string theory further explores the idea that the true degrees of freedom in a theory with gravity reside on the boundary surface rather than within the volume itself.

48:11

Holography theory simulates gravity in quantum systems.

  • Holography concept makes abstract ideas concrete by placing surfaces at different distances from black holes or the universe edge.
  • The theory suggests that placing a surface at the edge of the universe allows for a more explicit description of a theory without gravity.
  • In universes with negative curvature, gravitational force pushes everything towards the interior, creating self-contained systems.
  • The theory at the boundary of these surfaces resembles particle physics theories, lacking gravity.
  • The theory at the boundary accurately describes gravitational physics in the interior without gravity.
  • The conjecture proposes that theories without gravity at the boundary can mirror theories with gravity in the interior.
  • Thousands of follow-up studies support the conjecture, indicating widespread belief in its validity.
  • The possibility of artificially creating strongly interacting quantum systems in labs to simulate emergent gravity universes is exciting.
  • Estimates suggest that with thousands of qubits, it may be possible to simulate gravity in these quantum systems.
  • Loop quantum gravity, while an alternative approach, lacks the development and clarity of string theory, unable to reproduce fundamental aspects like Lorentz symmetry or gravitational wave scattering.

01:05:06

"String theory, supergravity, and quantum connections"

  • String theory and supergravity were developed simultaneously.
  • Initially, string theory was considered non-normalizable.
  • Later, it was realized that 11-dimensional supergravity was related to string theory.
  • Various ideas have been incorporated into modern theoretical understanding.
  • Loop quantum gravity and tensor networks are being explored for potential connections.
  • The firewall paradox emerged from black hole and quantum entanglement studies.
  • The paradox involves information preservation in black holes and quantum mechanics.
  • New formulas for black hole entropy have been crucial in understanding quantum information.
  • Quantum entanglement may create geometric connections in black hole interiors.
  • Einstein's work on wormholes and entanglement has influenced current research in quantum gravity.

01:22:16

Advancements in Understanding Black Holes and String Theory

  • Progress has been made in understanding previously deemed impossible concepts, particularly in the realm of black holes, with advancements in entropy formulas offering hope for further developments.
  • The field of string theory remains dynamic and ambitious, leading to the emergence of new ideas and mathematical formulas, aiming for a comprehensive understanding of theories related to gravity, quantum mechanics, and the nature of space-time.
  • Despite the ongoing quest for definitive testable predictions in string theory, insights into black holes, space-time, and the profound connection between gravity and quantum field theory on bounding surfaces have shaped research for over 20 years, indicating a potential key to unraveling the true essence of space-time, gravity, and matter.
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