This article was originally published at The Conversation. The publication contributed the article to Space.com’s Expert Voices: Op-Ed & Insights.

Philosophers have debated the nature of “nothing” for thousands of years, but what has modern science got to say about it? In an interview with The Conversation, Martin Rees, Astronomer Royal and Emeritus Professor of Cosmology and Astrophysics at the University of Cambridge, explains that when physicists talk about nothing, they mean empty space (vacuum). This may sound straightforward, but experiments show that empty space isn’t really empty – there’s a mysterious energy latent in it which can tell us something about the fate of the universe.

Rees was interviewed for The Conversation’s Anthill podcast on Nothing. This Q&A is based on an edited transcript of that interview.