June 2, 2015

In the quest for knowledge about how our universe works, physicists have been trying to understand black holes for decades. In 2014, Michigan State University professor Chris Adami published a paper that arguably solved the paradox of classical information and black holes.

He discovered that if you throw classical information at a black hole, the information is copied and contained in the stimulated emission of radiation, but not in the Hawking radiation, named after physicist Stephen Hawking. This means information is preserved and the laws of physics remain intact.

Adami’s latest research on black holes and what happens to quantum information will once again dramatically change the way physicists study and think about these cosmic mysteries called black holes.

The breakthrough paper, appearing in the Journal of Physics A, mathematically confirms what physicists have debated for years: it is possible for black holes to copy quantum information, but the research insists that those clones can only be approximate.

“Think of a black hole as a copy machine. We now know that black holes can copy classical information, but the fate of quantum information has still been a mystery.” Adami said. “What we found mathematically is that black holes behave according to the known laws of physics and they do copy quantum information, but they cannot produce exact copies.”

Perfect copies of quantum information violate the laws of physics. But imperfect copies can be created, which in turn preserves other laws of physics.

Adami explained that copying quantum information is like painting a self-portrait. You can never really get a true self-portrait because as you move to start painting yourself, you are changing the original.

“In classical physics, it is possible to change the portrait, but not change yourself at the same time,” he said. “In quantum physics, the act of changing one thing means you’re changing the other.”

Previously it was thought that perhaps black holes could make perfect copies of quantum information after all, as long as nobody could check that such a feat had occurred (a theory called “black hole complementarity”). However, later work concluded that if black hole complementarity were correct, then inevitably the black holes would have to be surrounded by curtains of infinite energy: the so-called “firewalls.”

“Firewalls were the consequence of a flawed assumption,” Adami said. “The reason for assuming that black holes must copy quantum information has now disappeared, because imperfect copies are sufficient to safeguard all the laws of physics, and imperfect copying is perfectly allowed.”

What’s even more amazing is that most common black holes are close to the known limit of perfect cloning machines. In other words, black holes clone almost as well as possible.

“Black hole information has been a paradox in the physics community for nearly 40 years,” he said. “Mostly gravity theorists have been trying to find solutions, but they were using the wrong tools. To solve a paradox involving quantum information, we should be using quantum information theory. The theory of approximate quantum cloning was well developed already; it just needed to be applied to black holes.”

The study was co-authored by Greg Ver Steeg, University of Southern California, and is published in the Journal of Physics A.