[D66] Locality and Its Discontents: A Philosophical Essay on the Rift Between General Relativity and Quantum Physics

[René Oudeweg]

[even opfrissen waar de theoretische natuurkunde ook al weer was 
gebleven...]



    Locality and Its Discontents: A Philosophical Essay on the Rift
    Between General Relativity and Quantum Physics


      1. Introduction: A Fracture at the Heart of Modern Physics

Modern physics rests on two intellectual pillars of extraordinary 
success: *general relativity* and *quantum theory*. Each has reshaped 
not only scientific practice but also our deepest philosophical 
intuitions about space, time, causality, and reality itself. Yet these 
theories appear to stand in conceptual tension—perhaps even 
contradiction—over the principle of *locality*.

General relativity is a theory of spacetime and gravitation grounded in 
local causation: physical influences propagate continuously through 
spacetime at finite speeds, ultimately limited by the speed of light. 
Quantum theory, by contrast, seems to allow — and in some 
interpretations demands — *non-local correlations*, most strikingly 
manifested in quantum entanglement.

This essay examines this dispute not merely as a technical 
inconsistency, but as a philosophical conflict about what the world is 
like. The disagreement is not simply about equations; it is about 
whether reality is fundamentally local, separable, and continuous, or 
whether it is irreducibly holistic and relational.

------------------------------------------------------------------------


      2. Locality in General Relativity: The Geometry of Causation

In *general relativity*, developed by *Albert Einstein*, gravity is not 
a force transmitted through space but the curvature of spacetime itself. 
Matter tells spacetime how to curve, and spacetime tells matter how to 
move. Crucially, this interaction is *local*: what happens at a point in 
spacetime depends only on its immediate neighborhood.

Einstein’s equations are *local differential equations*. They encode a 
vision of the universe in which causal influence propagates smoothly, 
respecting light cones and prohibiting instantaneous action at a 
distance. This structure underwrites relativity’s commitment to 
causality and its compatibility with special relativity.

Einstein’s philosophical commitments here were explicit. He famously wrote:

    “The field at one point of space has no immediate influence on the
    field at another point of space.”

This locality principle was not merely a mathematical convenience; it 
was, for Einstein, a metaphysical requirement for intelligibility. 
Without locality, physics risked collapsing into what he regarded as 
“spooky” and unscientific correlations.

------------------------------------------------------------------------


      3. Quantum Non-Locality: Entanglement and the EPR Challenge

Quantum mechanics, however, seems to violate this ideal. The most 
dramatic challenge emerged in the *EPR paper* of 1935, authored by 
*Albert Einstein*, *Boris Podolsky*, and *Nathan Rosen*. The authors 
argued that quantum mechanics was incomplete because it allowed distant 
systems to exhibit perfect correlations without any mediating local 
interaction.

Einstein derisively referred to this as /“spukhafte 
Fernwirkung”/—/spooky action at a distance/.

Quantum entanglement appears to permit two particles, separated by vast 
distances, to behave as a single system. A measurement performed here 
seems instantaneously correlated with an outcome there. Importantly, 
this correlation does not transmit usable information faster than light, 
but it nonetheless resists any explanation in terms of local hidden 
variables.

Einstein’s concern was not primarily about faster-than-light signaling; 
it was about *separability*—the idea that spatially distinct systems 
possess their own independent real states.

------------------------------------------------------------------------


      4. Bohr’s Reply: Complementarity and the Limits of Classical Intuition

Einstein’s great opponent in this debate was *Niels Bohr*, who rejected 
the demand that quantum mechanics conform to classical intuitions of 
locality and realism.

Bohr’s response to the EPR argument was subtle and famously opaque. He 
insisted that quantum phenomena cannot be understood apart from the 
experimental contexts in which they are measured. According to Bohr:

    “There is no quantum world. There is only an abstract quantum
    mechanical description.”

For Bohr, the mistake lay in assuming that physical properties exist 
independently of measurement. Entanglement did not signal non-local 
causation but rather the failure of classical categories—like separable 
systems and definite properties—to apply universally.

Philosophically, Bohr embraced a form of *anti-realist epistemology*: 
physics does not describe reality “as it is,” but rather what can be 
meaningfully said about nature under specific conditions.

------------------------------------------------------------------------


      5. Bell’s Theorem: The Death of Local Realism

The debate might have remained philosophical were it not for the 
decisive intervention of *John Bell* in 1964. Bell proved that no theory 
based on *local realism* could reproduce all the predictions of quantum 
mechanics.

Bell’s theorem transformed the issue from interpretation to experiment. 
Subsequent tests—most notably those by *Alain Aspect* and later 
loophole-free experiments in the 21st century—have overwhelmingly 
supported quantum mechanics.

Bell himself was clear-eyed about the consequences:

    “The idea that correlations can be explained by local causes is no
    longer tenable.”

What quantum theory seems to demand, then, is not merely non-locality, 
but the abandonment of a classical worldview in which the universe is 
composed of independently existing parts.

------------------------------------------------------------------------


      6. The Deep Conflict: Spacetime vs. Entanglement

Here the tension with general relativity becomes acute. General 
relativity presupposes a *local spacetime structure*; quantum theory, 
through entanglement, seems to transcend spacetime entirely. Entangled 
systems are not “connected” through space but are described by a single, 
non-factorizable quantum state.

Philosophically, this suggests a striking reversal: spacetime, once 
regarded as the fundamental arena of physical reality, may be emergent 
rather than fundamental. Some contemporary approaches to quantum 
gravity—such as holography and spacetime-from-entanglement 
proposals—explicitly entertain this possibility.

Yet these approaches also raise troubling questions. If spacetime 
emerges from entanglement, what becomes of causality? What replaces 
locality as the organizing principle of physics?

------------------------------------------------------------------------


      7. Interpretive Responses: Escape or Revolution?

Several strategies attempt to reconcile or evade the conflict:

  *

    *Hidden-variable theories*, such as Bohmian mechanics, accept
    non-locality explicitly.

  *

    *Many-worlds interpretations* deny collapse and reinterpret
    non-local correlations as branching structures.

  *

    *Relational and information-theoretic interpretations* downplay
    ontology in favor of operational consistency.

None, however, fully resolves the tension with general relativity. 
Either locality is abandoned, or spacetime itself is demoted from 
fundamental status.

------------------------------------------------------------------------


      8. Philosophical Reflections: What Must Give?

At its core, the dispute between locality and non-locality is a dispute 
about *metaphysical commitment*. Einstein sought a world that was 
intelligible, separable, and objective. Quantum theory, as it stands, 
offers a world that is holistic, context-dependent, and resistant to 
classical description.

The philosophical cost of quantum non-locality is high: it challenges 
our understanding of causation, individuality, and even reality itself. 
Yet the empirical success of quantum mechanics is undeniable.

Perhaps the deepest lesson is one of *epistemic humility*. As Bell once 
remarked:

    “We have an unromantic picture of the world that works, but we do
    not know how to think about it.”

------------------------------------------------------------------------


      9. Conclusion: An Unfinished Synthesis

The conflict between general relativity and quantum physics is not 
merely a technical problem awaiting better mathematics. It is a 
philosophical crisis that forces us to reconsider the foundations of 
explanation, causation, and ontology.

Whether the future belongs to a radically revised notion of spacetime, a 
deeper non-local theory, or an as-yet-unimagined synthesis remains 
unknown. What is clear is that locality—once the bedrock of physical 
reasoning—can no longer be taken for granted.

In this unresolved tension, physics reveals its most philosophical face: 
not as a catalogue of facts, but as an ongoing struggle to make sense of 
a reality that continually exceeds our conceptual grasp.

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.tuxtown.net/pipermail/d66/attachments/20251227/9f4c3548/attachment.html>