Netta Englehardt: The Woman Escaping the Black Hole Paradox

 

Netta Englehardt is a 32-year-old theoretical physicist. She’s a professor at MIT and is making waves for her work in resolving a decades old paradox, relating to Black Holes and Quantum Mechanics. In the 1970s Stephen Hawking came up with the Black Hole Information Paradox, and physicists have been working since then to make sense of the problem.

Hawking found that Black Holes don’t continue gobbling up matter forever. Black Holes have a temperature; large ones are very cold, but small ones a little warmer, and they all radiate energy. Eventually they totally evaporate and disappear. Just to be clear, big Black Holes can be 1/10^-14 degree Kelvin (almost absolute zero), and one the size of a Coronavirus would be room temperature. And eventually is a long time – many billions of years, so don’t hold your breath. None of this is paradoxical.

The paradox comes because Hawking’s calculations showed that matter evaporating from a Black Hole has lost the information about the matter that went into the Black Hole. That’s a problem for Quantum Mechanics, which holds that all processes must be reversible. A little hard to understand because it’s not intuitive to us, living in a world where we can’t perceive quantum scale events. But it’s a fundamental principle of quantum mechanics, called Unitarity. If all the information about the Black Hole matter has disappeared when the Black Hole has fully evaporated, then it’s not possible for the process to be reversed.

For over forty years physicists have been looking for an error in Hawking’s equations, trying to find another equation which shows that the information is not lost. The focus of all of this is on calculating the entropy of the Black Hole. As the Black Hole swallows particles, the information about those is particles is lost to the world outside of the Black Hole, and the entropy of the Black Hole increases. Hawking’s equations showed entropy continuing to increase as the Black Hole evaporates. That’s the paradox. To preserve Unitarity, the entropy has to decrease as the Black Hole evaporates. 

Englehardt aged 26 delivering a graduate lecture

The world of theoretical physics is very excited because Englehardt and her collaborators have found a different way of measuring the entropy of the Black Hole, and this measurement shows exactly what is needed – entropy increases as the size of the Black Hole increases, and then decreases as it evaporates. How do they do it? It’s so far beyond my understanding that I won’t pretend to be able to explain it, but it has something to do with the area of the event horizon. That’s the boundary beyond which a particle gets sucked into the Black Hole. And it involves an “extremal surface” where space-time is bent enormously by the forces of gravity in the incredibly dense Black Hole.

There’s a wonderful symmetry to Englehardt being a crucial contributor to solving Hawking’s paradox. Englehardt’s family emigrated from Jerusalem to Boston, when she was nine. “As a child, I loved reading,” she says. “But I didn’t speak any English. When we moved to the US, we had only taken a handful of Hebrew-language books with us. So I read everything in the house, until there was just the one last book left. It was ‘A Brief History of Time’ by Stephen Hawking”. And the rest was history. 

The Breakthrough Prize Foundation awarded Engelhardt the 2021 New Horizons in Physics Prize, an award of $100,000 that recognizes promising junior researchers who have produced important work in their field.