# Comment on "Measurements without probabilities in the final state proposal"

## TL;DR

This paper defends the final state proposal for black hole unitarity by demonstrating that relevant probabilities can be consistently defined using the ABL rule, countering claims that it is inherently inconsistent.

## Contribution

It shows that the ABL rule allows for defining probabilities in the final state proposal, challenging previous assertions of inconsistency.

## Key findings

- Probabilities can be defined using the ABL rule in the final state proposal.
- The analysis by Bousso and Stanford does not conclusively rule out the final state proposal.
- The final state proposal remains a viable alternative to the firewall hypothesis.

## Abstract

The final state proposal [G.T. Horowitz and J.M. Maldacena, J. High Energy Phys. 2004(2), 8 (2004)] is an attempt to relax the apparent tension between string theory and semiclassical arguments regarding the unitarity of black hole evaporation. The authors of [R. Bousso and D. Stanford, Phys. Rev. D 89, 044038 (2014)] analyze thought experiments where an infalling observer first verifies the entanglement between early and late Hawking modes and then verifies the interior purification of the same Hawking particle. They claim that "probabilities for outcomes of these measurements are not defined" and therefore suggest that "the final state proposal does not offer a consistent alternative to the firewall hypothesis." We show, in contrast, that one may define all the relevant probabilities based on the so-called ABL rule [Y. Aharonov, P.G. Bergmann, and J.L. Lebowitz, Phys. Rev. 134, 1410 (1964)], which is better suited for this task than the decoherence functional. We thus assert that the analysis of Bousso and Stanford cannot yet rule out the final state proposal.