Stephen Hawking and the Black Hole Firewall Phenomenon
Stephen Hawking recently published a summary of a talk that he gave back in August at a Kavli Institute of Theoretical Physics Rapid Response Workshop on the topic of black holes. Hawking is not saying that black holes do not exist, like the title of some articles claim, but that some characteristics of black holes may be different than what we previously thought.
Hawking discusses a proposed resolution to the information paradox, called a black hole firewall. This firewall is a region of high-energy radiation at the horizon of the black hole. He continues by describing a few scenarios that object the presence of firewalls:
- The location of the event horizon is not determined.
- The energy momentum tensor is regular. The energy momentum tensor describes the densities of energy and momentum in spacetime.
- Quantum gravity, an explanation for gravity according to the principles of quantum mechanics, is CPT invariant. This means that quantum gravity does not change under any charge (C), parity (P), or time (T) transformations.
Due to these objections, and a few others, Hawking states that event horizons and firewalls would not exist. An event horizon is the point where light can not escape from a black hole’s gravity. So if there are no event horizons, then what we define a black hole to be is not technically correct. But Hawking suggests that an apparent horizon, a horizon that is observer dependent, could exist in a long-term stable, but ultimately unstable, state.
During its existence, Hawking suggests that information could chaotically come back over the apparent horizon. He compares this to weather forecasting on Earth: we can’t predict the weather more than a few days in advanced.
Image by Andy Potts.