America's Best

Who says there are no Einsteins anymore? When it comes to science and medicine, the U.S. is blessed with galaxies of brilliant researchers who are the envy of the world

(TIME) -- Cells and souls and science and promises. How does a politician balance such volatile substances? George W. Bush tried as he pondered the research spearheaded by one of America's pioneering scientists. Biologist James Thomson's wizardry with embryonic stem cells had not only raised hopes for a medical panacea but also set off the national debate on whether that potential public good provided the moral justification for the infusion of massive amounts of public money. Already, Thomson's own personal balancing act — juggling scientific imperative and ethical caution, technical brilliance and moral quandary — had made him one of our choices for Time's list of America's Best in science and medicine, the second of our series on Americans at the top of their fields.

A century ago, the list would have been a short one, limited to names like Einstein and Pasteur and the Curies. Science was then a cottage industry practiced by a small group of men (and a few women) working mostly in isolation. Today, the scientific universe consists of interconnected microcosms of expertise. Apart from Thomson, our list of America's Best includes pioneers in a wide range of fields. Although few of these areas are as controversial as stem-cell research, they are all just as important to the way we live our daily lives. Among them: Lonnie Thompson, a climatologist who scales mountaintops to better understand global warming; Elizabeth Spelke, a developmental psychologist who has shown that babies are smarter than we thought.

Naming the best is a lot tougher now than it would have been 100 years ago. Science and medicine are enormous enterprises, requiring billions of dollars to support tens of thousands of researchers in universities, government labs and industry. Dozens or even hundreds of Ph.D.s might labor together to tackle a single problem — finding an elusive particle, say, or deconstructing a genome. On a project like that, it is hard to single out one researcher. Thus you will find very few household names among scientists today.

There is a reason for that. The questions scientists are tackling now are a lot narrower than those that were being asked 100 years ago. As John Horgan pointed out in his controversial 1997 best seller The End of Science, we've already made most of the fundamental discoveries — that the blueprint for most living things is carried in a molecule called DNA; that the universe began with a Big Bang; that atoms are made of protons, electrons and neutrons; that evolution proceeds by natural selection. Though today's problems are less sweeping, they are no less important. The diseases scientists are trying to cure still cause human misery and death; the answers they are seeking still stem from the central questions of human existence: Where did we come from? Where are we going?

So to select the best in science and medicine today, we focused on the most exciting fields of research and then looked for the men and women who are doing the most cutting-edge work within those fields. Along the way we learned that at least one thing hasn't changed: what it takes to be great in science or medicine. Although few researchers work alone anymore, and most have to spend at least part of their day worrying about how to keep large quantities of cash flowing into their labs, the greatest breakthroughs still come from brilliant individuals with a passion for understanding the world and the ability to concentrate obsessively on a problem until they have solved it. Here are 18 at the forefront of asking the crucial questions and finding the breathtaking answers.