Thought Experiments


Stephen Hawking Serves Up Scrambled Black Holes

Toast or spaghetti?

That’s the question that physicists have been trying to answer for the last year and a half. After agreeing for decades that anything—or anyone—unlucky enough to fall into a black hole would be ripped and stretched into spaghetti-like strands by the overwhelming gravity, theorists are now contending with the possibility that infalling matter is instead incinerated by a “toasty” wall of fire at the black hole’s horizon. Now, Stephen Hawking has proposed a radical solution: nixing one of the most infamous characteristics of a black hole, its event horizon, or point of no return.

Out of the firewall and into the frying pan? Credit: Flickr/Pheexies, under a Creative Commons license.

The original “spaghetti” scenario follows directly from Einstein’s theory of general relativity, which describes how gravity stretches the fabric of space and time. A black hole warps that fabric into a bottomless pit; if you get too close, you reach a point of no return called the horizon, where the slope becomes so steep that you can never climb back out. Inside, the gravity gets stronger and stronger until it tears you limb from limb.

The first hint that there was a flaw in this picture of a black hole came in 1975, when Stephen Hawking came upon a paradox. He realized that, over a very long time, a black hole will “evaporate”—that is, its mass and energy will gradually leak out as radiation, revealing nothing of what the black hole once contained. This was a shocking conclusion because it suggested that black holes destroy information, a fundamental violation of quantum mechanics, which insists that information be conserved.

How exactly does black hole evaporation imply that information is destroyed? Let’s say you are reading the last copy of “Romeo and Juliet,” and when you get to the end, grief overcomes you (sorry for the spoiler) and you throw the book into a black hole. After the book falls past the horizon, gravity shreds its pages, and finally it is violently compressed into the central point of the black hole. Then you wait as the black hole slowly evaporates by randomly shooting off particles from its glowing edges without any concern for Romeo or Juliet. As the black hole winks out of existence, only these random subatomic particles remain, floating in space. Where did the Montagues and Capulets go? They are lost forever. You could have thrown in “The Cat in The Hat” and the particles left after evaporation would be indistinguishable from the Shakespearian remnants.

Hawking realized that something had to give. Either quantum mechanics had to change to accommodate information loss, or Einstein’s theory of gravity was flawed.

Over the past 40 years theorists have battled in the “black hole wars,” trying to resolve this paradox. Two decades ago, most physicists declared a truce, agreeing to consider the inside and the outside of the black hole as separate spaces. If something falls into the black hole, it has gone to another realm, so just stop thinking about it and its fate, they counseled. This argument was largely accepted until July 2012, when UC Santa Barbara physicist Joseph Polchinski and his colleagues realized the paradox was even more puzzling.

Polchinski began with a similar thought experiment, but instead of Shakespeare, he imagined tossing entangled particles (particles that are quantum mechanically linked) toward a black hole. What happens, he asked, if one particle falls in the black hole and the other flies out into space? This creates a big problem: We can’t think of the two realms (inside and outside of the black hole) separately because they are tied together by the entangled particles.

Polchinski proposed a new solution that ripped apart Einstein’s idea of a black hole—literally. If there were something to prevent entanglement across the horizon, he thought, then there would be no problem. So he came up with something called a firewall: a wall of radiation at the black hole’s horizon that burns up anything that hits it. This wall is a tear in space-time that nothing can go through.

Is incineration finally the solution to the black hole information paradox? The father of the paradox, Stephen Hawking, recently put in his two cents (two pages, actually) in a very brief paper in which he argues against not just firewalls, but also event horizons as an ultimate point-of-no-return. This argument relies on quantum fluctuations in space-time that prevent a horizon from existing at a sharp boundary. He instead proposes a temporary “apparent horizon” that stores matter/energy (and information), chaotically scrambles it, and radiates it back out. This means that, as far as quantum mechanics is concerned, information is not lost; it is just extremely garbled. As Polchinski describes it, “It almost sounds like he is replacing the firewall with a chaos-wall!”

Are you skeptical? If so, you are in good company. Polchinski, for one, is hesitant, saying “It is not clear what [Hawking’s] picture is. There are no calculations.”

Steve Giddings, a theoretical physicist at the University of California, Santa Barbara, shares in this reluctance:

“The big question has been how information escapes a black hole, and what that tells us about faster-than-light signaling or a more serious breakdown of spacetime; the effects Hawking describes don’t appear sufficient to address this.”

Hawking’s new idea will need some flesh on its bones before we can truly embrace it, but if you don’t like spaghetti or toast, at least you have a third option now: scrambled black holes.

Go Deeper
Editor’s picks for further reading

arXiv: Modulated Hawking radiation and a nonviolent channel for information release
In this academic paper, Steve Giddings proposes a means by which information could escape from a black hole.

BackReaction: If it quacks like a black hole
Physicist and blogger Sabine Hossenfelder debunks myths about the implications of Hawking’s latest paper.

WIRED: A Brief History of Mind-Bending Ideas About Black Holes
WIRED’s Adam Mann delves into the history of black hole theory.

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Greg Kestin

    Greg Kestin holds a faculty position at Harvard University, where he conducts theoretical physics research, teaches, and produces educational online content. He earned his physics Ph.D. from Harvard, as a member of The Center for the Fundamental Laws of Nature, focusing on theoretical particle physics and quantum field theory. Over his career, he has also conducted research in nuclear physics, fusion energy, and gravitational wave physics. For over a decade he has been involved with innovative educational outreach endeavors, bringing science to both students and members of the public through his writings, videos, lectures, and multimedia.