# Prominent physicist Maldacena gives Ehrenfest Colloquium

On November 21, theoretical physicist Juan Maldacena gives the Ehrenfest Colloquium. Maldacena is known worldwide as the inventor of AdS/CFT correspondence, which might be key to a theory of quantum gravity. Maldacena is winner of the prestigious Dirac Prize and the Breakthrough Prize in Fundamental Physics. The Ehrenfest Colloquia series has been running since 1912.

One of the biggest problems in Physics is that we don’t understand how gravity works at the smallest scale. This so-called quantum gravity must combine two theories that don’t match at first sight: theory of relativity and quantum mechanics. The former states that every mass exerts gravity by curving the space around it. One of the predictions of the latter is that particles—with mass—continuously emerge from nothing and then quickly disappear again. The theory of relativity predicts that all these particles combined induce a curvature of the Universe up to 10^{118} times higher than observed. To be clear, that is a one with 118 zeros. The Universe would immediately collapse.

In the late 1990s, Juan Maldacena discovered a surprising mathematical relationship that translates quantum gravity problems so they can be solved with quantum mechanics, which scientists understand better. At the same time, it provided scientists with a key to expand quantum mechanics using knowledge from the theory of relativity, and vice versa.

## Bet

Maldacena's pioneering discovery led, among other things, to Stephen Hawking losing his famous bet on black hole memory loss. One of Hawking's greatest discoveries is that black holes secretly emit a small amount of radiation, contrary to the classical idea that nothing can escape from a black hole. If a book falls into a black hole, its *energy* won’t be lost for eternity. Black holes can eventually evaporate and thus give back all the energy that has been swallowed up.

At first, Hawking thought the *information* would actually be lost. If a black hole evaporates, you will never be able to find out which books had stored, he said. This goes against our laws of nature, which state that if you would for example burn a book, you could in principle still find out what information it contained, by tracing each particle from the heat radiation, smoke and ash. Information is always retained. Hawking closed a bet with colleague John Preskill against this statement.

## Conservation of information

With Maldacena's discovery you can almost literally translate black hole radiation in terms of burning a book. This means that it is indeed possible to reconstruct the information of every object that has ever fallen into a black hole from its radiation. Information is never lost, and therefore Hawking’s bet is. He acknowledged this and bought Preskill a baseball encyclopedia. The outcome means that conservation of information—an essential aspect of quantum mechanics—and black holes—a crucial component of the theory of relativity— are compatible.

The origin of Maldacena's discovery lies in an abstract mathematical theory—string theory. In reality, we can only verify its solution in theoretically constructed black holes. However, it seems that the way in which conservation of information works here, is generally applicable. Maldacena has therefore found a possible key to a theory of quantum gravity that also applies to real black holes, the Big Bang and the world around us.

## Ehrenfest Colloquium

*After each **Ehrenfest colloquium**, the speaker traditionally signs the famous signature wall, which was originally part of the Ehrenfest house and can now be found in the Oort building at Leiden University. The list of previous speakers includes Einstein, Bohr, Pauli, De Sitter, Planck, 't Hooft, Englert and Thorne. The colloquium is free of charge. If there are no seats left or if you can’t make it to Leiden, you can watch the **livestream**.*

*You can find more information on string theory and AdS/CFT correspondence on the **QuantumUniverse** website of the Delta Institute for Theoretical Physics.*