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#### DeepSummary

In this podcast episode, Sean Carroll interviews theoretical physicist Claudia de Rham about her work on modifying Einstein's theory of general relativity. They discuss the motivation for exploring alternatives to general relativity, particularly in the context of cosmology and the accelerated expansion of the universe. De Rham explains the challenges involved in giving the graviton, the hypothetical particle that mediates gravity, a small mass, which could potentially resolve the cosmological constant problem.

De Rham provides background on how general relativity is rooted in the concept of a massless, spin-2 particle (the graviton), and the constraints this imposes on any modifications. She describes the historical efforts to introduce a mass for the graviton, including the so-called "no-go theorems" that initially seemed to rule out such a possibility. However, de Rham and her collaborators found a way to evade these theorems and construct a theory of "massive gravity" that avoids issues like ghostly negative-energy modes.

While acknowledging the challenges and potential drawbacks of massive gravity, de Rham sees it as a promising avenue for addressing the cosmological constant problem. She outlines how observations of gravitational waves and the cosmic microwave background could potentially distinguish between general relativity and massive gravity. The episode highlights the ongoing effort to refine our understanding of gravity, even as Einstein's theory continues to be remarkably successful.

#### Key Episodes Takeaways

- Einstein's general theory of relativity has been remarkably successful in describing gravity across a wide range of phenomena, but is expected to break down at certain high energy scales and in regimes like the big bang or black hole singularities where quantum effects become important.
- The cosmological constant problem, related to the observed accelerated expansion of the universe, motivates exploring potential modifications or alternatives to general relativity.
- Giving the hypothetical graviton particle a small mass, leading to a theory of "massive gravity", is one such modification that could help resolve the cosmological constant problem.
- Early attempts to construct massive gravity theories faced significant obstacles related to ghostly negative-energy modes and so-called "no-go theorems" that seemed to rule out consistent theories.
- Claudia de Rham and collaborators have developed a framework for a four-dimensional massive gravity theory that avoids these previous issues, though constructing a fully non-linear and cosmologically viable theory remains an outstanding challenge.
- Potential observational signatures that could distinguish massive gravity from general relativity include modifications to the propagation of gravitational waves and imprints on the cosmic microwave background.
- While highly speculative, massive gravity exemplifies the ongoing effort by theoretical physicists to refine and potentially go beyond Einstein's successful but incomplete theory of general relativity.
- The constraints imposed by principles like quantum field theory and the observed absence of problematic modes guide and limit the possibilities for modifying gravity in theoretically consistent ways.

#### Top Episodes Quotes

- “Einstein had this wonderful theory of gravity that is done better than we ever had any right to expect. Not only does it explain things like the deflection of light and the precession of the perihelion of mercury, it also works for all these very, very far flung regions of the cosmos where we had no direct empirical evidence about when Einstein was doing his stuff.“ by Sean Carroll
- “Having said all that, of course we don't think that Einstein's theory is the final answer. General relativity, as we know, doesn't play well with quantum mechanics.“ by Sean Carroll
- “So with that, that really pushed us to understand much more what was going on with these no goes, and then to come up with a fully fledged four dimensional, not relying on extra dimension, four dimensional theory of massive gravity, which evades all of those problems related to these ghosts and to these instabilities.“ by Claudia de Rham
- “And so this is just to give you a little bit more intuitively, how the Higgs mechanism that can give a mass to fundamental particles, for instance, to the W and the Z boson, are related to the fact that it weakens some of the forces, it weakens the force mediated by this particle, in this case, the weak force.“ by Claudia de Rham

#### Chapter Details

#### Chapter 1: Introducing General Relativity and Gravitons

#### Chapter 2: The Challenge of Modifying General Relativity

#### Chapter 3: Overcoming No-Go Theorems for Massive Gravity

#### Chapter 4: Observational Tests and Implications of Massive Gravity

#### Chapter 5: The Cosmological Constant Problem and Massive Gravity

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#### Episode Information

Sean Carroll's Mindscape: Science, Society, Philosophy, Culture, Arts, and Ideas

Sean Carroll | Wondery

4/1/24

Einstein's theory of general relativity has been our best understanding of gravity for over a century, withstanding a variety of experimental challenges of ever-increasing precision. But we have to be open to the possibility that general relativity -- even at the classical level, aside from any questions of quantum gravity -- isn't the right theory of gravity. Such speculation is motivated by cosmology, where we have a good model of the universe but one with a number of loose ends. Claudia de Rham has been a leader in exploring how gravity could be modified in cosmologically interesting ways, and we discuss the current state of the art as well as future prospects.

Blog post with transcript: https://www.preposterousuniverse.com/podcast/2024/04/01/271-claudia-de-rham-on-modifying-general-relativity/

S*upport Mindscape on **Patreon**.*

Claudia de Rham received her Ph.D. in physics from the University of Cambridge. She is currently a professor of physics and deputy department head at Imperial College, London. She is a Simons Foundation Investigator, winner of the Blavatnik Award, and a member of the American Academy of Arts and Sciences. Her new book is *The Beauty of Falling: A Life in Pursuit of Gravity*.

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