Physicists: Black holes don’t erase information

Excerpt from earthsky.org

Since 1975, when Hawking showed

that black holes evaporate from our universe, physicists have tried to

explain what happens to a black hole’s information.

What happens to the information that goes into a black hole? Is it

irretrievably lost? Does it gradually or suddenly leak out? Is it

stored somehow? Physicists have puzzled for decades over what they call

the information loss paradox in black holes. A new study by physicists at University at Buffalo – published in March, 2015 in the journal in Physical Review Letters – shows that information going into a black hole is not lost at all.

Instead, these researchers say, it’s possible for an observer

standing outside of a black hole to recover information about what lies

within.

Dejan Stojkovic, associate professor of physics at the University at

Buffalo, did the research with his student Anshul Saini as co-author.

Stojkovic said in a statement:

According to our work, information isn’t lost once it enters a black hole. It doesn’t just disappear.

What sort of information are we talking about? In principle, any

information drawn into a black hole has an unknown future, according to

modern physics. That information could include, for example, the

characteristics of the object that formed the black hole to begin with,

and characteristics of all matter and energy drawn inside.

Stojkovic says his research “marks a significant step” toward solving

the information loss paradox, a problem that has plagued physics for

almost 40 years, since Stephen Hawking first proposed that black holes

could radiate energy and evaporate over time, disappearing from the

universe and taking their information with them. 

Disappearing information is a problem for physicists because it’s a violation of quantum mechanics, which states that information must be conserved.

Stojkovic says that physicists – even those who believed information was not

lost in black holes – have struggled to show mathematically how the

information is preserved. He says his new paper presents explicit

calculations demonstrating how it can be preserved. His statement from

University at Buffalo explained:

In the 1970s, [Stephen] Hawking proposed that black holes

were capable of radiating particles, and that the energy lost through

this process would cause the black holes to shrink and eventually

disappear. Hawking further concluded that the particles emitted by a

black hole would provide no clues about what lay inside, meaning that

any information held within a black hole would be completely lost once

the entity evaporated.

Though Hawking later said he was wrong and that information could

escape from black holes, the subject of whether and how it’s possible to

recover information from a black hole has remained a topic of debate.

Stojkovic and Saini’s new paper helps to clarify the story.
Instead of looking only at the particles a black hole emits, the

study also takes into account the subtle interactions between the

particles. By doing so, the research finds that it is possible for an

observer standing outside of a black hole to recover information about

what lies within.
Interactions between particles can range from gravitational

attraction to the exchange of mediators like photons between particles.

Such “correlations” have long been known to exist, but many scientists

discounted them as unimportant in the past.

Stojkovic added:

These correlations were often ignored in related

calculations since they were thought to be small and not capable of

making a significant difference.
Our explicit calculations show that though the correlations start off

very small, they grow in time and become large enough to change the

outcome.

Bottom line: Since 1975, when Stephen Hawking and Jacob Bekenstein

showed that black holes should slowly radiate away energy and ultimately

disappear from the universe, physicists have tried to explain what

happens to information inside a black hole. Dejan Stojkovic and Anshul

Saini, both of University at Buffalo, just published a new study that

contains specific calculations showing that information within a black

hole is not lost.

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Physicists: Black holes don’t erase information