**“The large scale structure of space-time”**by

**S.W. Hawking**and

**G.F.R. Ellis**.

The sublime Mediterranean sunshine, the water skiing lessons over the glittering waves, the entwined aromas of salt air and freshly caught fish sizzling on the grill — it all went away for an hour or so while I took a swim in Chapter 4 — The Physical Significance of Curvature.

This is an extremely sexy chapter, and not just because curves are sexy. What’s especially sexy about this chapter is the way it begins with the simple idea of the spacetime paths of massive and massless objects, and ends up laying out the basic mathematical conditions for spacetime singularities and time travel.

Now how does this happen? The key to all this is known as Raychaudhuri’s equation, discovered independently by Indian physicist **Amal Kumar Raychaudhuri** and Soviet physicist **Lev Davidovich Landau**. This fantastic equation, also known as the **focusing equation**, tells us when the spacetime curvature of a given gravitational system will force light cones to collapse and form spacetime singularities and when the curvature will keep them from converging, allowing conditions to develop where time travel is at least theoretically possible.

Time travel, water skiing and grilled fish make for quite a day at the beach.

Here’s to a beautiful man and to all of his beautiful books!