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By Deutsch: Ute Kraus, Physikdidaktik Ute Kraus, Universität Hildesheim, Tempolimit Lichtgeschwindigkeit, (Milchstraßenpanorama im Hintergrund: Axel Mellinger) English: Ute Kraus, Physics education group Kraus, Universität Hildesheim, Space Time Travel, (background image of the milky way: Axel Mellinger) [CC BY-SA 2.0 de (http://creativecommons.org/licenses/by-sa/2.0/de/deed.en) or CC BY-SA 2.5 (http://creativecommons.org/licenses/by-sa/2.5)], via Wikimedia Commons
Here’s one concept for the look of a black hole. Image: Ute Kraus, [CC BY-SA 2.5] via Wikimedia Commons
Black holes may have gotten a bad rap. And wormholes just might be a realistic way to travel Star Trek-style after all.

Years ago, the traditional wisdom about those exotic cosmic phenomena was pretty forbidding: Once something fell into a black hole, it was gone for good. Not a trace of the information describing that thing could ever be recovered. This view gave rise to a famous saying from physicist John Wheeler: “Black holes have no hair.”

And wormholes? Sure, maybe you could theoretically create an extradimensional shortcut between two points in spacetime. But it would take loads of never-seen negative energy, and anything you sent through the wormhole would be blasted to bits by extreme tidal forces. Hence, movies ranging from “Contact” to “Star Trek” and “Interstellar” are far more fanciful than factual.

Two recently published studies run counter to those bits of traditional wisdom. They may shed new light on black holes – but don’t expect to rev up the wormhole time-travel machine anytime soon.

Black holes may have soft hair

One study, written by British physicist Stephen Hawking and two colleagues, focuses on the black hole information question. Physicists have seen the traditional wisdom about black holes as paradoxical, because it goes against the view that the universe has time-reversal symmetry.

In principle, you should be able to figure out what a previous state of a quantum system looked like by working backwards from the current state. That’s time symmetry. But if things are completely wiped off the table once they fall into a black hole, there’s no way to work backwards. It’s no big deal in everyday life, but for theorists like Hawking, that information loss is deeply unsettling.

Stephen Hawking
Stephen Hawking (via Hawking.co.uk)

Hawking suspected years ago that there was a way out of the paradox – a suspicion that led him in 2004 to pay off on a bet he made with fellow physicist John Preskill. Now Hawking’s research paper, which was posted in January to the ArXiv pre-print server and published this week in Physical Review Letters, lays out a detailed explanation.

Hawking and his colleagues, Malcolm Perry and Andrew Strominger, argue that the surface of a black hole could be covered with “soft hair” – a layer of zero-energy photons or gravitons that records the information about the things that fall in, just as the information about 3-D objects can be recorded by a two-dimensional hologram.

At least in principle, the things that fall into a black hole could be reconstructed. “If you feel you are trapped in a black hole, don’t give up,” Hawking was quoted as saying in a New York Times report about the study. “There is a way out.”

Problem solved? Not quite. Gary Horowitz, a physicist at the University of California at Santa Barbara, says the study doesn’t completely solve the information paradox. But it does “point to a promising research direction that might lead to its long-awaited solution,” he wrote in a commentary. For Strominger’s explanation of the concept, check out this Scientific American interview from January.

Black holes may not be dead ends

Meanwhile, other physicists suggest that black holes might contain a built-in way out, in the form of a wormhole. What’s more, an object going into the black hole and through the wormhole just might be able to hold itself together as it goes through, rather than getting blasted apart.

Diego Rubiera-Garcia
Diego Rubiera-Garcia (via IAstro.pt)

“What we did was to reconsider a fundamental question on the relation between the gravity and the underlying structure of spacetime,” Diego Rubiera-Garcia of Portugal’s Instituto de Astrofisica e Ciencias do Espaco said in a news release. “In practical terms, we dropped one assumption that holds in general relativity, but there is no a priori reason for it to hold in extensions of this theory.”

In a study published last year in Physical Review D, Rubiera-Garcia and his colleagues – Gonzalo Olmo and Antonio Sanchez-Puente – laid out a theoretical description of a black holes that has a spherical wormhole at its center. They followed up with a study in Classical and Quantum Gravity, in which they argue that the objects falling into a black hole would be able to maintain their structural integrity.

“Each particle of the observer follows a geodesic line determined by the gravitational field,” Rubiera-Garcia said. “Each geodesic feels a slightly different gravitational force, but the [physical and chemical] interactions among the constituents of the body could nonetheless sustain the body.”

The scientists say that should hold true whether the body in question was a spacecraft or an astronaut. So who knows? Maybe falling into a time-bending black hole wouldn’t be as spooky as this scene from “Interstellar” makes it look:

https://www.youtube.com/watch?v=6VPEtUgNJj4

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