Why is unifying gravity with quantum mechanics so difficult?

The Standard Model of particle physics and general relativity work well individually but are inconsistent when applied together, especially in extreme conditions like the early universe or black hole centers. Applying quantum theory to gravity naively leads to problematic infinities.

What is Eric Weinstein's 'Geometric Unity' proposal addressing?

Geometric Unity aims to unify spacetime geometry with quantum field theory, suggesting that the space of metrics itself possesses a metric, which could resolve issues related to particle structure and the definition of particles in the absence of a fixed spacetime metric.

Why does Sabina Hossenfelder criticize the focus on 'beauty' in theoretical physics?

Hossenfelder argues that relying solely on mathematical beauty is unscientific, as it leads to cherry-picking successful examples (like Einstein) while ignoring the many who failed. She believes this approach has become dogmatic and detached from empirical feedback.

Is the scientific method flawed or non-existent according to the panelists?

The panelists debated the strict definition of the scientific method. Lee Smolin suggests science functions as a collective process of error detection guided by ethical principles, rather than a rigid method. Eric Weinstein believes the real method is more complex than the simplified explanation often given.

Can quantum gravity ever be tested experimentally?

While some believe testing quantum gravity requires impossible scales, Sabina Hossenfelder is hopeful, pointing to technological progress in creating quantum superpositions. She suggests that measuring gravitational fields of quantum systems might become feasible within the next few decades.

Why are there so many new theories of everything emerging now?

The resurgence is partly due to a sociological effect: physicists educated in the era of 'theory of everything' expectation, coupled with a credibility gap in established theories like string theory. This allows independent researchers to gain more attention online.

What advice do the guests have for aspiring physicists?

Sabina suggests focusing on the foundations of quantum mechanics. Eric advises exploring unasked questions, like the origin of specific particle generations or deeper aspects of locality and symmetry, and to look at physics before 1984.

Should physics prioritize large, expensive projects like the FCC?

Sabina argues against large colliders, believing resources are better invested in other experiments and education on resource limitations. Eric, while acknowledging the need for funds, believes the scientific community provides immense value and should not operate with a 'pauper's framework.'

Theory of Everything Controversies: Livestream

Q: What are the main approaches to quantum gravity?

Key approaches include String Theory, Loop Quantum Gravity, Causal Dynamical Triangulation, Asymptotically Safe Gravity, and Emergent Gravity, each with different strategies to reconcile quantum mechanics and gravity.

PBS Space Time

Education4 min read84 min video

Aug 10, 2020|379,805 views|5,280|1,639

lee smolin sabine hossenfelder eric weinstein theory of everything

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Key Moments

TL;DR

Physicists debate the 'Theory of Everything,' focusing on quantum gravity, historical failures, and the role of beauty in theory development.

Key Insights

A Theory of Everything (ToE) can mean unifying particle physics, unifying gravity with quantum theory, or completing quantum theory.

Current major approaches to quantum gravity include Loop Quantum Gravity, String Theory, Causal Dynamical Triangulation, Asymptotically Safe Gravity, and Emergent Gravity.

There's a critique of the physics community's over-reliance on 'beauty' (e.g., symmetry, simplicity) as a guide for theory development, leading to neglected areas and a lack of testable predictions.

String theory, while mathematically rich, has been criticized for failing to produce testable predictions and for evolving its promises over time without delivering.

The scientific method is debated, with some arguing for a community-based approach focused on error-finding rather than strict falsifiability, and others emphasizing the dangers of subjective aesthetic criteria.

A renewed interest in the foundations of quantum mechanics and the potential for experimental tests of quantum gravity in the coming decades is observed, despite historical skepticism.

DEFINING THE THEORY OF EVERYTHING

The concept of a 'Theory of Everything' (ToE) is multifaceted. For some, it signifies the unification of all elementary particles and their interactions into a single framework. For others, it primarily means reconciling general relativity (gravity) with quantum field theory. A third perspective suggests that quantum theory itself is incomplete, and resolving these foundational issues is key to a ToE. A provocative stance proposes that the focus should shift from explaining the 'why' of current laws to understanding the 'how' of their evolution, drawing an analogy to biological evolution.

CURRENT APPROACHES AND CHALLENGES

Several theoretical frameworks aim for a ToE. String theory postulates that fundamental constituents are strings and offers a path to quantum gravity and grand unification, though it has faced criticism for lacking testable predictions. Loop Quantum Gravity attempts to quantize spacetime. Other approaches include Causal Dynamical Triangulation, Asymptotically Safe Gravity (which some credit with predicting the Higgs mass), and Emergent Gravity, which views gravity as an emergent phenomenon. A core challenge remains the inconsistency between quantum theory and gravity, particularly in extreme conditions like the early universe or black hole interiors.

THE ROLE OF BEAUTY AND THE SCIENTIFIC METHOD

A significant debate revolves around the use of 'beauty'—often interpreted as symmetry and simplicity—as a guiding principle in physics. While luminaries like Einstein and Yang have cited beauty in their work, critics argue this aesthetic criterion can become a dogma, leading to theories detached from empirical verification, as exemplified by criticisms of string theory and supersymmetry. This raises questions about the scientific method itself, questioning whether strict falsifiability is always applicable and suggesting science may progress through community consensus and collective error correction rather than a single, rigid method.

CRITIQUES OF PAST AND PRESENT THEORIES

The discussion highlights historical failures and the persistence of certain research programs. String theory, despite its mathematical sophistication, is accused of repeatedly changing its promises and failing to deliver testable predictions. Some argue that the physics community has become overly focused on a narrow set of problems and aesthetic criteria, leading to a 'credibility gap.' This has allowed for a proliferation of new theories, as researchers feel less constrained by the established order. However, a lack of clear communication and substantive discussion around these new ideas, such as geometric unity, is also noted.

EVOLUTION OF LAWS AND THE NATURE OF REALITY

One perspective suggests that the ultimate laws of physics might not be fixed but evolve, similar to natural selection. This 'dynamical mechanism' for law evolution could provide explanations for current particle and force structures that might otherwise seem arbitrary. Furthermore, the resolution to quantum gravity might involve a deeper understanding of quantum foundations, potentially revealing spacetime itself as an emergent structure. This emergent nature of spacetime could redefine our understanding of locality and interconnectedness.

THE FUTURE: EXPERIMENTAL POSSIBILITIES AND DIVERSE IDEAS

Despite skepticism about testing quantum gravity, there's cautious optimism that technological advancements could enable such tests in the coming decades, bridging the gap between quantum phenomena and gravity measurements. There's a recognized need for greater diversity in theoretical approaches, moving beyond established, yet unproductive, programs. The discussion emphasizes the importance of asking new questions, encouraging 'irresponsible' exploration, and fostering a community that rigorously tests ideas against evidence, rather than relying on aesthetic appeals or authority.

Mentioned in This Episode

●Products

●Software & Apps

●Books

●Concepts

●People Referenced

Common Questions

A Theory of Everything can refer to a unified theory of elementary particles and interactions, a unification of gravity with quantum theory, or the completion of quantum theory itself. Different physicists prioritize these aspects differently.

Topics

Loop Quantum Gravity Foundations Of Physics Beauty In Physics Emergent Gravity Asymptotically Safe Gravity

Mentioned in this video

softwareThe Portal

Eric Weinstein's podcast where details of Geometric Unity can be found.

conceptE8

A large exceptional Lie group whose triality property was part of Garrett Lisi's initial work on particle generations.

conceptHiggs Boson Mass

Asymptotically safe gravity is credited with correctly predicting the value of the Higgs boson mass.

conceptOctonions

Mathematical structures related to ideas being pursued by Cole Fury.

personRosalind Franklin

Mentioned in the context of the discovery of DNA structure, illustrating how 'irresponsible' individuals sometimes get things right.

conceptRelativity

One of the great revolutions in physics mentioned as a foundational theory.

conceptCausal Dynamical Triangulation

An approach to quantum gravity that attempts to discretize space.

personCole Fury

Mentioned as someone working on interesting ideas related to octonion structures.

softwareInto the Impossible

Brian Keating's podcast, mentioned in the closing remarks.

personGarrett Lisi

Associated with a program that initially hoped to explain particle generations using E8 symmetry, but did not fully work out.

softwareThe Portal Podcast

Eric Weinstein's podcast, mentioned at the end.

conceptGeometric Unity

Eric Weinstein's proposal for a radical new idea in physics.

personPaul Feyerabend

Philosopher of science known for his work 'Against Method', influencing discussions on the scientific method.

bookLost in Math

Sabina Hossenfelder's book, available in paperback and hardcover.

bookLosing the Nobel Prize

Brian Keating's book, recommended in the closing remarks.

personJames Beach

Mentioned as a previous guest.

personSabina Hossenfelder

Theoretical physicist at the Frankfurt Institute for Advanced Studies, working in quantum gravity research, prolific writer, blogger, and YouTuber.

personStephen Weinberg

Originator of the idea of asymptotically safe gravity.

personStefan Alexander

Mentioned as a previous guest.

productFuture Circular Collider

A proposed large particle collider that Sabina argues is not a good investment compared to other experiments.

conceptAsymptotically Safe Gravity

An approach to quantum gravity that suggests gravity can be quantized similarly to other interactions.

personLisa Mullin

Physicist, founding member of Perimeter Institute, author, who has worked on loop quantum gravity, string theory, and foundations of quantum mechanics.

conceptEmergent Gravity

An approach suggesting gravity is an emergent force from microscopic constituents, pursued by Erik Verlinde.

conceptUnified Field Theory

An older project in physics that was, according to Eric Weinstein, replaced by the imperative to quantize gravity through a 'sleight of hand'.

personBrian Keating

Astrophysicist and friend of the host, co-hosting the series.

personErik Verlinde

Pursuer of the emergent gravity approach.

conceptMaxwell's Unification

Cited as a great example of unification in physics (of electromagnetic phenomena).