Black Hole Information Paradox

  • By Greg Kestin
  • Posted 05.09.18
  • NOVA

Do black holes delete information forever? Or could information somehow escape them? In this episode of What the Physics?! find out how every possible answer to these questions breaks physics as we know it. 

Running Time: 03:56


Black Hole Information Paradox

Published May 10, 2018

Greg Kestin: What do you think would happen if a black hole swallowed the Earth?

Your house is gone. Your puppy's gone. Your mom's lasagna recipe—just gone, behind the horizon of the black hole. If that weren't bad enough, then radiation leaves the event horizon of the black hole, stealing the black hole's mass and shrinking it until—poof—it's gone.

So, what about humanity's legacy? It's one thing to say that it's just trapped behind the event horizon of the black hole. But it's another thing to suppose that the world is gone altogether, scrambled up in random particles that are leaving the event horizon of the black hole.

So here's the question: Is it truly gone forever? From the evaporated remains of the black hole, could you ever really reconstruct the knowledge of humanity? This is essentially the 40-year-old unsolved puzzle called the black hole information paradox.

Here are the three leading answers. Unfortunately, every single one of them breaks physics as we know it.

So, humanity's legacy—and your mom's lasagna recipe—are trapped behind the black hole's event horizon. And for them to escape the intense pull of the black hole, they would actually have to travel faster than the speed of light. In normal, everyday life, that can't happen; that's an Einstein no-no. Because if you could somehow surpass the speed of light, then your clock would actually start moving backwards and you could go back in time...possibly changing the past.

But maybe the physics in a black hole is special. If things could travel faster than the speed of light in there, then maybe information could escape somehow.

What if the Earth never actually fell into the black hole? What if there's some wall at the event horizon, and when the Earth hits the black hole, all its information goes splat onto that wall? And when the black hole evaporates from that event horizon, the Earth's information is on the outside. It's right there to evaporate with those particles that are leaving.

So the problem here is that in relativity, a black hole is nothing but a point in space, just a massive dot, surrounded by empty space. It's warped space, but empty space. So, having this random wall that soaks up information—well, that goes against everything we'd predict in relativity.

Okay, so what if all the information from humanity is simply, just gone? Here's the problem with information loss: When you send information in a phone call or a text or an email, what are you sending? You're sending energy. Energy and information are tied to each other. Losing information actually means losing energy. And if there's one rule that holds the universe together, it's conservation of energy. If you get rid of that, then things could just disappear without warning.

So, what would actually happen to our legacy if a black hole ate the Earth? Physicists are using the Event Horizon Telescope to look at the event horizon of the supermassive black hole at the center of our galaxy. Observations of gravitational lensing of light around the event horizon could tell us whether we have to accept backwards time travel, information loss, random walls that just steal information, or something completely new.

In the meantime, what do you think? If the Earth were swallowed by a black hole, would the information of humanity be lost forever? Or could you get that lasagna recipe back?

Also, you can learn much, much more about black holes in NOVA's two-hour special, "Black Hole Apocalypse." The link is in the description.



Host, Producer
Greg Kestin
Samia Bouzid
Greg Kestin
Peter Chang
Samia Bouzid
Greg Kestin
Scientific Consultants
Joe Polchinski
Netta Engelhardt
Steve Giddings
Editorial Input from
Julia Cort
Ari Daniel
Filming, Editing, and Animation
Greg Kestin
Samia Bouzid
Special thanks
Entire NOVA team
From the producers of PBS NOVA © WGBH Educational Foundation
Funding provided by FQXi
Music provided by APM
Sound effects from
Images courtesy of MEDIODESCOCIDO (Stewie Griffin) and Paul Anderson (Grumpy Cat)

Related Links