Daniel Harlow on What Quantum Gravity Teaches Us About Quantum Mechanics | Mindscape 349
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Summary
Takeaways
- ❖Fundamental physics has seen slow progress since the 1970s, with quantum gravity remaining an elusive goal.
- ❖Gravity's universality, unlike other forces, offers hope for general arguments even without a complete theory of quantum gravity.
- ❖Black holes are 'easier' to study in quantum gravity than cosmology because an external observer can be posited for black holes, but not for the entire universe.
- ❖The black hole information paradox is being resolved by accepting that locality is violated, meaning spacetime is emergent and not fundamental.
- ❖Applying quantum gravity insights to a closed universe suggests it has only one possible state, implying zero fundamental degrees of freedom.
- ❖Daniel Harlow proposes that quantum mechanics itself is an emergent, approximate theory, valid only when an external, classical observer is assumed, a condition not met in cosmology.
Insights
1Stagnation in Fundamental Physics
Despite the collective effort of many physicists, progress in fundamental physics has been significantly slower since the 1970s compared to the rapid advancements seen in the early 20th century (e.g., Einstein's 10 years to General Relativity). New earth-shattering theories beyond the Standard Model, especially those incorporating quantum gravity, have not materialized, leading to frustration and questioning of current approaches.
Sean Carroll highlights the lack of 'earthshattering new theories' since the 1970s/80s, contrasting it with Einstein's rapid progress on General Relativity.
2Gravity's Universality as a Guide
Unlike other fundamental forces where particles interact in varied ways, gravity affects everything universally due to the equivalence principle. This universality allows physicists to make robust arguments and learn general features of quantum gravity even from 'unrealistic' toy models (e.g., wrong dimensions, cosmological constants), as these gravitational aspects tend to remain consistent across different theoretical constructs.
Daniel Harlow explains that 'everybody basically feels gravity the same way,' leading to 'generality of arguments about gravity' and 'inevitability to some of the features of gravity.'
3Black Holes are 'Easier' than Cosmology for Quantum Gravity
Studying quantum gravity effects in black holes is conceptually simpler than in cosmology. For black holes, an 'external observer' can be posited (e.g., sitting outside, dropping things in), allowing for conventional experiments and observations of phenomena like Hawking radiation. In cosmology, however, the observer is always part of the system, making it unclear how to apply standard quantum mechanics, which traditionally assumes an external classical apparatus.
Harlow states that 'black holes are easier than cosmology' because 'for black holes, you can sit outside... In cosmology, you're always part of the system.'
4Black Hole Information Paradox Resolution: Emergent Spacetime and Locality Violation
The black hole information paradox, which questioned whether information is lost during black hole evaporation, is being resolved by sacrificing the principle of locality. The prevailing view is that spacetime is emergent, meaning the notion of distinct locations is only an approximation. Information is preserved (unitarity) and black holes have finite degrees of freedom, but this requires exponentially complicated, non-local interactions to detect.
Harlow explains Hawking's paradox (cannot have finite degrees of freedom, unitarity, and locality) and the modern resolution: 'you have to give up option three which is locality' and 'spacetime is emergent.'
5The 'One State' Universe Problem in Quantum Cosmology
Applying the mathematical tools and insights gained from resolving the black hole information paradox to a closed universe (one with no spatial boundary) leads to a shocking result: the universe appears to have only 'one state' or zero fundamental degrees of freedom. This contradicts the observed richness and complexity of our universe and the expectation of infinitely many degrees of freedom in a classical or standard quantum field theory context.
Harlow describes using the path integral to count degrees of freedom for the whole universe, and the 'answer is always the same... and the answer is zero.'
6Quantum Mechanics as an Approximate Theory for Observers
To reconcile the 'one state' universe with observed reality, Daniel Harlow proposes that quantum mechanics, as we know it, is an approximate theory that requires an external, classical observer. In a closed universe where the observer is part of the system, quantum mechanics needs to be reformulated. This new theory would explicitly treat the observer as classical through a 'decohering channel,' making scientific predictions inherently approximate, with errors related to the observer's entropy.
Harlow argues that 'quantum mechanics is an emergent theory in the limit where you have this external observer' and that in cosmology, 'we need to develop a theory that lets us do physics that way... that theory won't quite be quantum mechanics.'
Bottom Line
The universe, if closed, might fundamentally exist in only one quantum state, implying zero degrees of freedom.
This challenges our most basic understanding of reality and information. If true, the perceived richness and evolution of our universe must arise from an emergent description, not from fundamental quantum states.
Develop a new foundational theory of physics that can reconcile a single-state universe with the emergence of complex, classical reality and the subjective experience of observers. This could involve a radical redefinition of quantum mechanics itself, where the observer's classicality is an axiom.
Science itself, as a concept, might be inherently approximate, with a fundamental lower bound on precision tied to the entropy of the observer.
This suggests that there are intrinsic limits to how accurately we can describe reality, not just technological ones. It implies that our 'objective' scientific measurements are always filtered through the lens of a classical, decohered observer, and perfect precision is a theoretical ideal not achievable in a self-contained universe.
Explore the implications of 'approximate science' for epistemology and the philosophy of physics. Can we quantify these limits and design experiments to probe them? This could lead to a new understanding of the relationship between consciousness, observation, and the physical laws of the universe.
Key Concepts
Spacetime as Emergent
The idea that spacetime, and thus locality, is not a fundamental property of reality but rather an approximate description that arises from more fundamental, non-local quantum degrees of freedom. This concept is crucial for resolving the black hole information paradox, where information is preserved by violating locality in ways that are exponentially difficult to detect.
The Oracle at Delphi (Gravitational Path Integral)
The gravitational path integral is a mathematical tool that 'knows' certain truths about quantum gravity (e.g., black hole entropy, unitarity) without explicitly performing state counting or providing a canonical formalism. It offers answers, but its implications require careful interpretation, much like an oracle whose pronouncements are profound but ambiguous.
Science as Approximate (Observer-Dependent)
The radical notion that scientific knowledge and the laws of physics are inherently approximate, with a fundamental lower bound on error determined by the entropy of the observer (e.g., e to the minus the entropy of the observer). This approximation arises because, in a closed quantum cosmological system, the observer cannot be arbitrarily large, slow, or external, thus limiting the precision with which 'objective' measurements can be made.
Lessons
- Question fundamental assumptions: Even established theories like quantum mechanics might be incomplete or approximate when applied to extreme cosmological scenarios.
- Embrace counter-intuitive results: Seemingly 'crazy' theoretical outcomes (like a universe with zero degrees of freedom) can be crucial signposts for new physics and force a re-evaluation of foundational principles.
- Consider the observer's role: The act of observation and the nature of the observer might be more central to the formulation of fundamental physical laws than traditionally assumed, especially in self-contained systems like the universe.
Notable Moments
Maxwell's analysis of Saturn's rings as a 'useless problem' that validated classical mechanics.
This historical anecdote illustrates that seemingly abstract or unobservable problems (like black hole information loss or quantum cosmology) can be crucial for testing the validity of fundamental physical laws. Solving such 'useless' problems can reveal profound truths about the universe, even if experimental verification takes centuries.
Quotes
"Very few earthshattering new theories have come along in fundamental physics since then. We've had a couple of discoveries... But still, we don't have the answer. And this is decades later."
"Everybody basically feels gravity the same way... And that leads to this generality of arguments about gravity."
"In cosmology, you're always part of the system. You know, it's not like you're on the outside looking in trying to see how it reacts."
"The view that many of us have converged on sort of gradually over the last 20 years is that you have to give up option three which is locality. And the slogan that we use for that is that we say that spacetime is emergent."
"If holography says that that's where the fundamental degrees of freedom live, that if there's no spatial boundary, there's no fundamental degrees of freedom."
"I claim that science as a concept is approximate with a lower bound on the error given by e to the minus the entropy of the observer."
"I think that quantum mechanics has an additional axiom that's not the Schrodinger equation which tells you that there's a physical inner product. And the reason it's physical is because of the Bourne rule which is there in the list of axioms."
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