Posts Tagged ‘metric’

Get well soon Stephen Hawking

Me in Woody Creek

Me in Woody Creek

There’s a photograph buried in my closet that was taken in the old days of analog photography and has never been digitized and hopefully never shall be. It shows a much younger me reclining on the sand at Club Med in Marbella, topless, as is the norm in such places, holding in front of me a copy of “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!

Is quantum gravity an oxymoron?

Quantum mechanics forbids a quantum system from being both knowable and objectifiable. But quantum mechanics and quantum field theory assume that the spacetime metric is both knowable and objectifiable. If the metric is not knowable or not objectifiable, then it’s impossible to define a quantum theory precisely. In that sense it seems like the term “quantum gravity” is oxymoronic. String theory demands that the graviton exist, but so far it hasn’t enlightened us on the ultimate resolution of this apparent oxymoron.