Hope springs eternal, but how about human life? Jonathan Weiner reports in Long for This World.

Aubrey de Grey, your star immortality theorist, envisions adding hundreds of years to the human lifespan. He also wants his head frozen if he dies. Is longevity research for eccentrics, or is he on to something?

Aubrey is at the far fringes, but there are more and more serious scientists interested in aging. The field has become respectable partly because of advances in the lab making mutant worm, fly, and even mouse Methuselahs, but how much that means for humans remains to be seen. Writing about the field is like writing about gravity before Newton.

Aging seems to involve the buildup of toxic junk in cells. Will we find a magic pill, or will a longer life require a huge re-engineering and reclamation project?

Hard to know. One interesting line of work involves coaching the immune system to get rid of gunk that may cause Alzheimer's or Parkinson's. The trick is to clear away gunk and none of our essential working parts. Extreme calorie restriction, unfortunately, seems to have a big payoff in extending the life span of animals. If we figure out what calorie restriction does, then maybe we can come up with a pill—there's a drug called rapamycin that's of interest—with the same effects. We could have our cake and eat it, too.

Some creatures seem to be immortal. Why aren't we?

A hydra, a simple pond organism, seems to live without aging: it's no more likely to die at three years old than at one year or one week...

I take it three years is extremely old for a hydra.

Right. A hydra that's lucky enough not to get gobbled up or dry up in a vanishing puddle can live years beyond that. So why can't we? One interesting argument is that it's because of our sophisticated nervous systems, which allow us to remember and learn. The hydra doesn't have much saved in its neurons, and it can afford to slough them off and make new ones. But our nerves are mortal; we can't replace them without losing our identities.

Do you want to live forever?

I'm not a disciple of the immortalists. If everybody is immortal, where's the room for the next generation, for children and grandchildren? I don't think I have any prospect of living forever; writing this book was my attempt to come to terms with that. But even though I've been thinking about the arguments against dramatic life extension, whenever I read a paper that raises my hopes, my hopes flare.