Spacetime events from the inside out

TL;DR

The paper argues that spacetime events are best understood as quantum measurement outcomes, challenging the traditional separation between spacetime and gravity. By analyzing Bell tests with both multiple and single agents in time-like and gravity-influenced settings, it reveals how nonclassical causal structures can arise without signaling, hinting at retrocausal interpretations. It further develops an inside-out perspective with a toy Ringworld model where an agent learns geometric laws solely from internal measurements, illustrating how spacetime geometry could be inferred from within. Overall, the work advocates an agent-centric, measurement-based view of spacetime and gravity with potential implications for quantum gravity and the ontology of spacetime.

Abstract

We argue that special and general theories of relativity implicitly assume spacetime events correspond to quantum measurement outcomes. This leads to a change in how one should view the equivalence of spacetime and gravity. We describe a Bell test using time-like measurements that indicates a non classical causal structure that does not violate no-signaling. From this perspective, the violation of the Bell inequalities are already evidence for the non classical structure of flat spacetime as seen by an agent embedded in it. We argue that spacetime geometry can be learned by an embedded agent with internal actuators and sensors making internal measurements.

Figures (13)

• Figure 1: A two-party Bell experiment with entangled photons. A source at the origin produces pairs of entangled photons. The photons occupy oppositely directed spatial modes; one goes to detector-A and the other goes to detector-B. Both observers are space-lie separated. After each measurement, the setting and the outcome are sent, over classical channels, to a checker, observer-C, who stores the data for each trial and constructs the appropriate correlation function to check a Bell inequality.
• Figure 2: A Bell test with a single observer/agent. The agent generates an entangled photon pair, in a known state (indicated by a joint measurement result $s$) and each photon travels in opposite directions where it is reflected by a mirror at rest in the agent's rest frame. One photon is received back by the agent at the early time $t_1$ and the other photon is received back at the later time $t_2$. The dashed lines represent the world lines of the agent and the two mirrors. The variables $x,y$ represent the measurement settings and the variables $a,b$ represent the measurement outcomes as in a standard Bell test.
• Figure 3: A single agent Bell test with gravity.
• Figure 4: A single mass is in a superposition of two locations with respect to a fixed coordinate frame (the earth, say). A very sensitive clock is placed to one side. In a series of repeated trials with exactly the same conditions, the clock will experience two unequal redshifts, randomly choosing one or the other in each trial. As the clock is the only way we can infer the existence of a gravitational field in this setting, we conclude that the the gravitational field at the clock is fluctuating. It is not stationary.
• Figure 5: A scheme for using time-bin entanglement and a single agent Bell test to search for gravitational decoherence.