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Quantifying Quantum Gravity Violations of Causality and the Equivalence Principle

Periodic Reporting for period 2 - QUANTIVIOL (Quantifying Quantum Gravity Violations of Causality and the Equivalence Principle)

Reporting period: 2019-03-01 to 2020-08-31

Black holes provide a unique battleground where fundamental principles of physics clash. Hawking calculated 45 years ago that black holes permanently destroy information that falls into them, violating the principle of unitarity. In the decades since Hawking's calculation, strong evidence has emerged that quantum gravity effects neglected by Hawking rescue the principle of unitarity, allowing information to escape form black holes.

However, the price of restoring unitarity is steep: we must give up at least one of two other cherished principles. Either black holes must violate causality, allowing signals to travel faster than the speed of light. Or, they must violate the equivalence principle, the foundation of Einstein's theory of general relativity, and burn up infalling observers at their event horizon.

In this project, we will quantify, for the first time, the extent of this conflict. How large must the violations of causality or the equivalence principle be in order to rescue unitarity? And can these quantum gravity effects be measured observationally?
We have made good progress so far towards achieving our objectives. We have quantified the entanglement between the quantum fields inside and outside the event horizon. We have also worked to better understand wormholes, which play a key role in understanding how the information escapes from black holes. While wormholes were confined to the realm of science fiction just 5 years ago, it turns out that they are ubiquitous in quantum gravity. My team has played an important role in understanding these wormholes.
We expect that we will be able to fully quantify the size of quantum gravity effects surrounding black holes by the end of this project. In particular, we expect to be able to quantify the violations of causality and the equivalence principle due to quantum gravity.