**In honour of the memory of Oskar Klein, the Organizing Committee of the Oskar Klein Memorial Lectures every year invites a distinguished researcher to give a Memorial Lecture and to receive the Klein medal.**

### The 2018 Oskar Klein Memorial Lecture

was given by

## Leonard Susskind (Stanford University)

with the title

### The Quantum Origins of Gravity

The lecture took place in the Oskar Klein Auditorium, AlbaNova, Stockholm on January 29, 2019.

**Abstract**: It is often said that general relativity and quantum mechanics are separate subjects that don’t fit together comfortably. There is a tension, even a contradiction between them—or so one often hears. I take exception to this view. I think that exactly the opposite is true. It may be too strong to say that gravity and quantum mechanics are exactly the same thing, but those of us who are paying attention, may already sense that the two are inseparable, and that neither makes sense without the other. Two things make me think so. The first is ER=EPR, the equivalence between quantum entanglement and spatial connectivity. In its strongest form ER=EPR holds not only for black holes but for any entangled systems—even empty space. One may say that the most basic property of space—its connectivity—is due to the most quantum property of quantum mechanics: entanglement.

*The second has to do with the dynamics of space, in particular its tendency to expand. One sees this in cosmology, but also behind the horizons of black holes. The expansion is thought to be connected with the tendency of quantum states to become increasingly computationally complex: a “second law of quantum complexity.” If one pushes these ideas to their logical limits, quantum entanglement of any kind implies the existence of hidden Einstein-Rosen bridges which have a strong tendency to grow, even in situations which one naively would think have nothing to do with gravity. To summarize this viewpoint in a short slogan: Wherever there is quantum mechanics, there is also gravity.*

Oskar Klein (1894-1977) was 23 years old when he came to Niels Bohr in Copenhagen. He stayed there until 1931 at which time he became professor in theoretical physics at Stockhom University where he stayed until his retirement in 1962. Klein had a broad interest in various fields of physics but is perhaps best known for the Klein-Gordon equation and the Klein-Nishina equation. His attempts to unify general relativity and electromagnetism by introducing a five-dimensional space-time dates back to 1926 and is today known as Kaluza-Klein theory.

For a biography of Oskar Klein, please read here.