We all know we live in three-dimensional space. But what does it mean when people talk about four dimensions? Is it just a bigger kind of space? Is it "space-time," the popular idea which emerged from ...

The original version of this story appeared in Quanta Magazine. When it comes to understanding the fabric of the universe, most of what scientists think exists is consigned to a dark, murky domain.

In one of the most memorable interviews of my career thus far, I was at my desk, head in my hands, talking on the phone to a physicist about extra dimensions. I was trying to understand what it means ...