The problem inspired a famous physics bet and a "black hole war."
Science enthusiasts around the world are mourning the loss of Stephen Hawking, the renowned physicist and beloved public figure, who passed away on Wednesday at the age of 76. Hawking spent his life exploring the universe’s deepest mysteries and advancing sophisticated frameworks to explain its most elusive phenomena, such as black holes, alternate universes, and the tenuous future of life on Earth and elsewhere.
The motor neurone disease ALS confined him to a wheelchair and necessitated that he communicate through an electronic speech aid, but Hawking’s own immense passion for the cosmos—along with his cheeky sense of humor—shone through in his many popular science books, public appearances, and mountains of academic research.
In the midst of heartfelt tributes from his friends, colleagues, and fans, the scientific community continues to build on Hawking’s prolific academic contributions, which cement his legacy as one of the most forward-thinking cosmologists in a field that has no lack of trippy cosmic visions.
One major question that Hawking identified, but did not live to see resolved, is the black hole information paradox. This heady problem dates back to Hawking’s 1974 prediction that black holes aren’t completely black—they “radiate,” losing small amounts of mass over time. This theoretical evaporation of black holes, known as Hawking radiation, may sound benign enough, but it challenges the laws of the universe as we know them.
At the crux of the paradox is a classic clash between quantum mechanics and Einstein’s theory of general relativity. Quantum mechanics is built on the assumption that the universe’s physical “information,” which means properties of elemental matter like mass, spin, or configuration, cannot be permanently destroyed. While physical forms might change—a piece of coal can be compressed into a diamond, for instance—information about cosmic junk is never deleted.
But the blackbody radiation that Hawking proposed would be theoretically scrubbed of this information, according to general relativity, so Hawking suggested that information entrapped in a black hole might be permanently erased. This theory erupted in a “black hole war” between Hawking and fellow physicist Leonard Susskind—as well as a famous bet between Hawking, Kip Thorne, and John Preskill. Hawking eventually conceded that information is probably not destroyed in a black hole, though the paradox itself has still not been resolved.
If that all sounds somewhat mindbending, try watching the below animated explainer which does a thorough job of visually illustrating these complicated concepts.
Hawking remained intrigued by this problem, and was publishing new research about it as recently as 2016. Likewise, other scientists have proposed a kaleidoscopic range of ideas and experiments to resolve or contextualize the paradox. One of the most discussed ideas, for instance, is that the information trapped by black holes might be reflected out in a mirrorlike process, and encoded in two-dimensional form onto the surface of event horizons.
Just last week, University of Rochester physicist Sreenath Kizhakkumpurath Manikandan presented a new view, developed with his advisor, physics professor Andrew N. Jordan, at a meeting of the American Physical Society. Over email, Manikandan and Jordan explained to me that it is not easy to study Hawking radiation in the cosmic wilds—that would require getting close to a black hole—so scientists must use analogous systems to simulate what might be occurring in these extreme environments.
“It is important to note that Hawking radiation from black holes has not been experimentally detected yet,” they told me, “but a similar mathematical approach to what Prof. Stephen Hawking took in his seminal paper can be applied to a variety of physical systems such as accelerating mirrors, and these analogies give us evidence that the kind of particle production does indeed occur the way Prof. Hawking has predicted.”
In their research, Manikandan and Jordan demonstrated that superconductors provide a useful analogy for understanding the information paradox. The key is Andreev reflection, an effect that has been identified in superconductors: When an electron jumps from a metal to a superconductor, it buddies up with another electron to form a team called a Cooper pair, but its absence from the metal substance is also recorded by a reflected hole that conserves the real electron’s information. This superconductor analogy sheds light on some of the black hole theories advanced by previous studies, notably one by Patrick Hayden and John Preskill, and another by Gary T. Horowitz and Juan Maldacena.
This research presents new possibilities concerning how information in black holes is stored. Manikandan and Jordan told me, for instance, that information could theoretically “traverse a wormhole (also known as an Einstein Rosen bridge) and exit to another universe.” The team hopes to follow this thread in the laboratory, using superconductors.
“We believe that the black hole/superconductor analogy we proposed allows us to test the quantum theoretical predictions made in the context of black holes using experiments that can be performed using superconductors, a possibility that has not yet been explored,” the researchers said, though they cautioned that black holes and superconductors are not perfectly analogous by any means, so studying them only paints part of the picture.
The University of Rochester team is far from the only group chasing new and innovative leads to solve the information paradox, which remains one of the most enduring dilemmas that Hawking pursued. In fact, the problem’s intractability partly explains why Hawking never received a Nobel Prize—that prestigious award is given to experimentally verified discoveries. Perhaps some day, we will find hard proof that demystifies this long-standing problem. For now, however, it’s enough to appreciate the brilliant and imaginative man who first proposed it.
“While pioneering what could only be described as one of the most challenging problems in theoretical physics of his time, [Hawking] also put in a conscious effort to communicate his ideas with others including the public, and inspired generations of students to pursue a career in physics,” Manikandan and Jordan told me.
“This would be a legacy Prof. Stephen Hawking will own,” they said, “and continue to inspire future generations as one [of] the most beautiful minds ever known to mankind.”
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