So, I’m under the weather for a few days, and what happens? A prominent scientist puts forth a bold new idea, and the media promptly get it wrong. Horribly, horribly wrong.
Stephen Hawking, holder of the Lucasian Chair of Mathematics at Cambridge, author of the best-seller A Brief History of Time, discoverer of Hawking radiation, and general pop-science superstar, put out a paper on the Cornell University ArXive preprint server (a paper, mind you, which has not yet been peer-reviewed) proposing a possible resolution of the AMPS firewall paradox. (“Huh?” I hear you cry. More on that in a moment.)
What happened next was quite predictable. The press (and much of the blogosphere) promptly picked up on the story (this involving the legendary Stephen Hawking, after all) and, quite unsurprisingly, botched it. Headlines breathlessly proclaimed “Stephen Hawking: ‘There are no black holes’.” (That headline, by the way, was lifted directly from Nature, of all places.) Bloggers blogged. FaceBookers FaceBooked. Twitter was atwitter. The pronouncement even inspired a bit of political satire on the Borowitz Report that, Poe’s Law being what it is, many took to be an actual story.
But Hawking wasn’t claiming that black holes don’t exist. The supermassive black hole designated Sagittarius A* known to be lurking at the center of our Milky Way galaxy didn’t evaporate overnight in a puff of Hawking radiation to comport with what Hawking had allegedly proposed. What Hawking instead was proposing was a mathematically subtle redefinition of one of the key attributes of a black hole, the event horizon, replacing it with a concept he calls the “apparent horizon.”
To understand what prompted this, it is necessary to go back to the 70’s, to the start of the Black Hole War. (No, that isn’t the title of a cheesy sci-fi flick, but it is the title of an excellent and quite accessible book by Leonard Susskind about the subject.) At a private meeting of scientific luminaries, Hawking presented an argument which seemed to show unequivocally that an inescapable consequence of general relativity is that information is destroyed by black holes. This did not sit well with Leonard Susskind (one of the pioneers of string theory) and other specialists in the field of quantum theory, since this would violate conservation of information.
Well, there isn’t REALLY a Law of Conservation of Information, but there is a quantum mechanical requirement something called “unitarity” be preserved, which basically means that the probabilities of all possible outcomes of an event have to add up to one. Hawking’s result runs afoul of this. Not for the first time, and certainly not for the last time, quantum theory and general relativity were butting heads, giving contradictory results.
This problem resulted in a blossoming of a particularly specialized sub-discipline of theoretical physics known as black hole thermodynamics. (When one is working on the bleeding edge of theoretical physics, “thermodynamics” and “information theory” are pretty much synonymous, for reasons I won’t even begin to try delving into here. That is topic worthy of a whole series of posts in of itself.)
Fast forward to 2012, when Ahmed Almheiri, Donald Marolf, Joseph Polchinski, and James Sully put out a controversial paper, “Black Holes” Complementarity or Firewalls?” This paper (generically referred to as “AMPS” for short) posited that event horizons might not be as usually described in general relativity (where an inbound observer would notice only that they couldn’t get out), but rather are sheathed in a “firewall” which consumes incoming matter via the Hawking radiation mechanism prior to it even crossing the event horizon.
Rather than rehashing the details of the Great Firewall Debate and attempts to resolve it, I shall here refer you to some relevant articles:
- Black Hole Firewalls Confound Theoretical Physicists | Simons Foundation
- Is Alice burning? The black hole firewall controversy | Quantum Frontiers
- Guest Post: Joe Polchinski on Black Holes, Complementarity, and Firewalls : Cosmic Variance
- [1207.5192] Complementarity Is Not Enough
- [1207.3123] Black Holes: Complementarity or Firewalls?
- [1208.3445] Singularities, Firewalls, and Complementarity
- Phys. Rev. Lett. 110, 101301 (2013): Better Late than Never: Information Retrieval from Black Holes
- [1306.0533v2] Cool horizons for entangled black holes
- Black Holes Won’t Incinerate You, After All – Starts With A Bang
Which brings us to Hawking’s proposal. He replaces the concept of the event horizon with something a bit – mushier, something not as clearly defined such that the need for the firewall is sidestepped. What’s more, Hawking’s “apparent horizon” is temporary, eventually disappearing later in the lifecycle of the black hole, allowing an opportunity to sidestep the information paradox that prompted all of this.