SEATED AT THE head of a table for 12 with a view of San Francisco's soaring skyline, Peter Thiel was deep in conversation with his guests, eclectic scientists whose research was considered radical, even heretical.

It was 2004, and Thiel had recently made a tidy fortune selling PayPal, which he co-founded, to eBay. He had spent what he wanted on himself — a posh penthouse suite at the Four Seasons Hotel and a silver Ferrari — and was now soliciting ideas to do good with his money.

Among the guests was Cynthia Kenyon, a molecular biologist and biogerontologist who had garnered attention for doubling the life span of a roundworm by disabling a single gene. Aubrey de Grey, a British computer scientist turned theoretician who prophesied that medical advances would stop aging. And Larry Page, co-founder of an internet-search darling called Google that had big ideas to improve health through the terabytes of data it was collecting.

The chatter at the dinner party meandered from the value of chocolate in one's diet to the merits of uploading people's memories to a computer versus cryofreezing their bodies. Yet the focus kept returning to one subject: Was death an inevitability — or a solvable problem?

A number of guests were skeptical about achieving immortality. But could science and technology help us live longer — to, say, 150 years? Now that, they agreed, was a worthy goal. Within a few months, Thiel had written checks to Kenyon and de Grey to accelerate their work. Since then he has doled out millions to other researchers with what he calls "breakout" ideas that defy conventional wisdom.

"If you think you can only do very little and be very incremental, then you'll work only on very incremental things. It's self-fulfilling," said Thiel, who is 47 and estimated to be worth $2.2 billion. "It's those who have an optimism about what can be done who will shape the future."

He and the tech titans who founded Google, Facebook, eBay, Napster, and Netscape are using their billions to rewrite the nation's science agenda and transform biomedical research. Their objective is to use the tools of technology — the software programs, algorithms, and big data they used in creating an information revolution — to understand and upgrade what they consider to be the most complicated piece of machinery in existence: the human body.

The entrepreneurs are driven by a certitude that rebuilding, regenerating, and reprogramming patients' organs, limbs, cells, and DNA will enable people to live longer and better. The work they are funding includes hunting for the secrets of living organisms with insanely long lives; engineering microscopic nanobots that can fix your body from the inside out; figuring out how to reprogram the DNA you were born with; and exploring ways to digitize your brain based on the theory that your mind could live long after your body expires.

Their confidence in technological wizardry and their own ideas may lead them to underestimate the downsides and even dangers of the work they are funding, say some science philosophers, historians, and economists. Their research in stem cells, neuroscience, genetically modified organisms, and viruses, for example, tinkers with nature in big ways that could easily go awry — and operates in a largely unregulated space. Laurie Zoloth, a bioethicist at Northwestern University, worries that some of the billionaires' obsession with longevity may be driven as much by hubris as by a desire to do public good.

"It's incredibly exciting and wonderful to be part of a species that dreams in a big way," she said. "But I also want to be part of a species that takes care of the poor and the dying, and I'm worried that our attention is being drawn away to a glittery future world that is fantasy and not the world we live in."

FOR MANY OF the tech entrepreneur-philanthropists, interest in medical science is personal. Sean Parker, 34, the Napster co-founder, suffers from life-threatening food allergies and has family members with autoimmune disorders. He has donated millions to finding a cure for allergies and to new cancer therapies.

Google's Sergey Brin, 41, has proposed a new kind of science that starts with masses of DNA and a community of people with certain genes. Brin has a mutation of the LRRK2 gene that is associated with a higher risk of Parkinson's disease, and has said he thinks the new approach could be "transformational." He has donated $150 million to the effort.

Several of the Silicon Valley billionaires married women with backgrounds in science or medicine, and those wives direct the philanthropy.

Anne Wojcicki, Brin's estranged wife, who studied biology and previously worked as a health-care consultant, founded her own personalized genetics startup, 23andMe, and is co-head of the couple's foundation. Priscilla Chan, a pediatric resident at the University of California at San Francisco, with her husband, Facebook's Mark Zuckerberg, 30, donated $75 million to San Francisco General Hospital, where 70 percent of the patients are underinsured or uninsured. The two couples also teamed up with others to create the Breakthrough Prize for scientists who make discoveries that extend human life. Its $3 million payouts — given to six scientists each year — dwarf similar awards, including the Nobel Prizes, currently about $925,000.

Pam Omidyar, a biologist and former research assistant in an immunology lab, co-founded the Omidyar Network with her husband, eBay's Pierre Omidyar, who became a billionaire at 31. They have donated millions to research about resiliency — the traits that help people bounce back from illness or other adversity.

And Page, who is now 42 and chief executive of Google, has made the biggest bet on longevity yet, founding Calico, short for California Life Co., a secretive anti-aging research center, with an investment of up to $750 million from Google.

Laura Arrillaga-Andreessen, who is the wife of internet pioneer Marc Andreessen, and a well-known philanthropist, said that when many tech entrepreneurs look at the health-care system they see the "data of billions of people," collected through blood tests, online profiles, and fitness trackers.

"When that data can be accessed and mined and utilized for good in an instantaneous manner," she said, "that would be shattering in a positive way for the system as we know it."

SUCH "MOON SHOT" ideas are tantalizing, but some prominent ethicists and scientists have been troubled by the tech titans' unwavering conviction that conquering nature is desirable in the first place.

And there are few checks and balances on such initiatives. Once, two-thirds of scientific and medical research was funded by the federal government, beholden to the public good. Now, two-thirds is funded by private industry, a growing share by billionaires accountable to no one and impatient with the pace of innovation.

Zoloth, the Northwestern University bioethicist, said there is a reason why science often moves slowly. "Making scientific progress faster doesn't necessarily mean better — unless if you're an aging philanthropist and want an answer in your lifetime," she said. "Science is about an arc of knowledge, and it can take a long time to play out. Sometimes we won't know answers for generations."

Americans remain deeply ambivalent about using new medical treatments to live radically longer lives. In a 2013 survey conducted by the Pew Research Center, 51 percent said they believed treatments to slow, stop, or reverse aging would have a negative impact on society. Fifty-eight percent said treatments that would allow people to live decades longer would be "fundamentally unnatural."

Political theorist Francis Fukuyama, a senior fellow at Stanford and former member of the President's Council on Bioethics, argues that a large increase in human life spans would take away people's motivation for the adaptation necessary for survival. In that kind of world, social change comes to a standstill, he said; aging dictators could stay in power for centuries.

"I think that research into life extension is going to end up being a big social disaster," Fukuyama said in an interview. "Extending the average human life span is a great example of something that is individually desirable by almost everyone but collectively not a good thing. For evolutionary reasons, there is a good reason why we die when we do."

Although many scientists say they are grateful for the entrepreneurs' money and attention, some have been dismayed by what they see as Silicon Valley's superiority complex and insistence that the current methods used to fight disease are outdated and ineffective. At a medical conference in August 2012, for instance, Vinod Khosla, one of Silicon Valley's most revered venture capitalists, likened the practice of medicine to witchcraft. He argued that machines are better than the average doctor and that disruption in health care was more likely to be driven by those outside the industry than those in the profession.

The reaction from the medical community was swift. Columbia University–educated physician Bijan Salehizadeh tweeted that he was "nauseated" by "the anti-doctor rantings of the Silicon Valley tech crowd."

Some scientists also say they are concerned that private money — which can include seven-figure research grants and salaries that are two or three times what is offered in academia — distorts research priorities.

Preston Estep, director of gerontology for Harvard Medical School's Personal Genome Project, says some of the philanthropists are doing more harm than good by funding what he calls "pseudoscience" — approaches based more on emotional appeal than on solid research. Of some of the work that is being funded by the tech crowd, he said, "Nobody takes it seriously."

PETER THIEL IS the embodiment of Silicon Valley culture at its individualistic, impatient extreme. He believes death is the "great enemy" of humankind.

He said that in the past 25 years the pace of innovation in the biomedical realm has been demoralizing. "Nixon declared war on cancer in 1971, and there has been frustratingly slow progress," he said. "One-third of people ages 85 and older have Alzheimer's or dementia, and we're not even motivated to start a war on Alzheimer's."

Thiel's philanthropic investments in aging grew out of a series of late-night conversations with a friend, author Sonia Arrison. Her best-selling book 100 Plus lays out a future where living longer is the new norm.

For them, the possibility of a long life — perhaps to 150, nearly doubling the current average U.S. life expectancy — was exciting. Staying up late at night, the two would muse about ideas such as whether it was possible to bioengineer immune cells to recognize and kill cancer, or whether we could one day 3-D print human skin for burn victims — all sorts of different strategies to "repair people," as Arrison put it.

In the future they talked about, everyone would be like Harriette Thompson, the 91-year-old who broke records earlier this year by completing a marathon in 7 hours and 7 minutes.

It was Arrison who introduced Thiel to the scientists at the dinner-salon a decade ago. Since then, Thiel has funded such projects as a high-speed cooling technology for human organs, so they could be preserved indefinitely, and a way to grow bones using stem cells, to replace broken ones.

"I've always had this really strong sense that death was a terrible, terrible thing," he said. "I think that's somewhat unusual. Most people end up compartmentalizing, and they are in some weird mode of denial and acceptance about death, but they both have the result of making you very passive. I prefer to fight it."

Excerpted from a piece that originally appeared in The Washington Post. Reprinted with permission.