As de Grey sees it, there are seven types of cellular junk, the gerontological equivalent of the seven deadly sins. They include “cross links” that gum up the machinery and glue cells to one another and mitochondria that fail with age. Then there is junk within cells and junk in the spaces between cells, along with cells that no longer work but hang around and cells that die and poison cells around them. And then there are old cells that acquire dangerous mutations and give rise to cancer. Weiner’s strength as a writer is his ability to flesh out these complex theories without losing the reader. De Grey’s dream of conquering death may seem far-fetched and unreal, but Big Pharma is already at work on some of these ideas — the first cream that overcomes cross-links, which cause our skin to stiffen and wrinkle, will be a blockbuster.

Fortunately, “Long for This World” is not all about Aubrey de Grey. Weiner writes engagingly about other researchers and their work in the field: on so-called Methuselah mutants, creatures that live much longer than the rest of their species; genes like Sir2 (Silent Information Regulator 2), which may be responsible for some of the life-extending effects of extremely calorie-restricted diets; and related proteins, known as sirtuins. That work led to the discovery of resveratrol, a compound in the skin of grapes that can activate sirtuins and prolong a lab animal’s life.

But as Weiner points out, there is a big problem with immortality. Traditionally, we have viewed our lives as unfolding in stages: Shakespeare’s seven ages of man capture our progression from infant to schoolboy to lover to soldier to justice to clown, ending finally in “second childishness and mere oblivion; sans teeth, sans eyes, sans taste, sans everything.” Immortality could wind up being a terrible stasis. “A huge part of the action and the drama in the seven ages comes from the sense of an ending, the knowledge that all these ages must have an end,” Weiner writes. We might live forever in a state of unending boredom. And the technology might benefit the wrong people: “If biologists could have done for the dictators of the 20th century what they can now do for roundworms and flies — double their life span — then Mao Zedong might still be alive.”

As a young physician caught up in the early years of the H.I.V. epidemic, I was struck by my patients’ will to live, even as their quality of life became miserable and when loved ones and caregivers would urge the patient to let go. I thought it remarkable that patients never asked me to help end their lives (and found it strange that Dr. Kevorkian managed to encounter so many who did). My patients were dying young and felt cheated out of their best years. They did not want immortality, just the chance to live the life span that their peers could expect. What de Grey and other immortalists seem to have lost sight of is that simply living a full life span is a laudable goal. Partial success in extending life might simply extend the years of infirmity and suffering — something that to some degree is already happening in the West.