Physics

On the Asymptotic Causal Structure in Gravitational EFTs

Bruno Bucciotti, Paolo Creminelli, Alessandro Longo, Warin Patrick McBlain, Enrico Trincherini

Featured May 20, 2026

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Simply

In gravity, how fast light travels depends on the number of dimensions: in more than four dimensions, new gravity effects can make light go super-fast far away, but in exactly four dimensions, light always follows the same path as normal gravity, no matter what new effects are added.

In depth

This paper reveals a fundamental dichotomy in how causality is preserved in gravitational effective field theories (EFTs) depending on spacetime dimension. In dimensions

D > 4

, higher-derivative operators can induce genuine asymptotic superluminality, meaning signals can arrive earlier than in General Relativity, which constrains the EFT's ultraviolet cut-off. Conversely, in

D = 4

, a logarithmic infrared divergence ensures that prompt causal curves always remain far from the black hole, making the asymptotic causal structure universally identical to Schwarzschild and insensitive to these higher-derivative corrections.

Key Takeaways

1
In spacetime dimensions $D > 4$ , gravitational EFTs with higher-derivative operators can exhibit asymptotic superluminality, where signals propagate faster than allowed by the background metric's asymptotic structure, leading to strong constraints on the EFT's UV cut-off.
2
In $D = 4$ spacetimes, the asymptotic causal structure of gravitational EFTs is universally Schwarzschild-like. A logarithmic infrared divergence prevents prompt null curves from probing near-horizon regions, making them insensitive to higher-derivative corrections.
3
While asymptotic superluminality is absent in $D = 4$ , local causality constraints still apply: the effective metric can develop hyperbolicity-breaking regions near black holes, imposing a local cut-off bound on the EFT.

Conceptual Flow

HIGH LEVEL

Methodology: Comparing Signal Paths

To check if new gravity effects make light go super-fast, the study compares how long it takes for light to travel along different paths around a black hole.

Black Hole

Light Path 1

Light Path 2

Compare Travel Times

Fastest Path

Results: Dimension-Dependent Causality

The study found that in higher dimensions, new gravity effects can make light travel faster than normal, but in four dimensions, light always travels at the normal speed, even with new effects.

Spacetime Dimensions

Check Light Speed

D > 4: Faster Light Possible

D = 4: Normal Light Speed