The 20th century produced an unprecedented avalanche of scientific discoveries — more than enough to make selecting a mere two dozen or so of the period’s greatest breakthroughs a daunting project. But that is the task Alan Lightman assigned himself for his new book, “The Discoveries” (Pantheon), a work that surveys exactly 22 crucial achievements in science from 1900 to 1972 and chronicles the lives of the men and women responsible for them.

The result is an intriguing mix of the famous and the unfamiliar. Some chapters in ‘“The Discoveries” assess legendary figures and ideas with mass-culture currency, like Albert Einstein’s theories of relativity and Werner Heisenberg’s uncertainty principle. Others discuss scientists almost unknown in their own time-and ours-such as Henrietta Leavitt, a Harvard astronomy researcher of the 1910s, whose work helped measure the size of the universe and establish the fact that it is expanding.

But Lightman, a trained physicist, prolific author (his works include novels, essays, and histories of science), and adjunct professor of the humanities at MIT, says he was less concerned with generating a standard greatest-hits checklist of discoveries than illuminating the thoughts of the scientists in question. ”I wanted to tell the human story of each discovery and not just the science itself,” explains Lightman.

Indeed, in the essays he devotes to each discovery, Lightman weaves in biographical sketches of the scientists while deftly unpacking the lines of thought in their breakthrough papers-25 of which are also reprinted in ”The Discoveries” (three discoveries involve two papers each). A central motif in the book, as in other of Lightman’s works, is that solving scientific problems is not just a matter of plugging in numbers or grinding through experiments, but requires bursts of creativity, which feed a larger ”exhilaration of discovery” scientists share with pioneers in other areas of endeavor. ”The same pattern often occurs in artistic discovery,” said Lightman, who-speaking of creative impulses-was in Chicago when we reached him, for the opening of a theater production of his 1994 historical novel, ”Einstein’s Dreams.”

IDEAS: Is ”The Discoveries” a formal list, implying these are the greatest discoveries of the century, or simply a representative sample of important advances in science?

LIGHTMAN: I see it as a representative sample. Probably anyone asked to choose the two dozen greatest discoveries of the 20th century would choose Einstein’s paper on relativity and Watson and Crick’s paper on DNA, but after a few like that, there would be a range of opinions.

IDEAS: If those were the easiest discoveries to pick, which ones were the hardest?

LIGHTMAN: One of the last choices I made was the work by Henrietta Leavitt, who discovered a way to measure the distance to the stars. I did not originally include her in my list. But I did include, from the beginning, work by the astronomer Edwin Hubble, who discovered that the universe is expanding. And almost everything he did depended critically on work that Leavitt did. Yet she had been given very little credit. So I decided I should include her as well.

IDEAS: The last discovery in the book is gene splicing, from 1972. Do we not know which more recent advances are important?

LIGHTMAN: I don’t think great discoveries stopped in 1973, but it is too soon to tell, in the same way you can’t really tell whether a book published three years ago is going to become great literature or not.

IDEAS: Einstein is represented twice in the book, for the quantum theory of light and the theory of special relativity, but some observers believe the theory of general relativity is his greatest achievement, and that’s not a separate entry. Why not?

LIGHTMAN: General relativity was certainly a great achievement. But general relativity depends on the earlier foundations of special relativity, which completely redefined our notions of time and space. And that’s a fundamental pillar of general relativity, so I thought that if I had to choose one of those two discoveries, I would choose the earlier and more foundational one.

IDEAS: One interesting feature of this book is the inclusion of the original papers-and the way you dissect how they were written.

LIGHTMAN: It’s very important to hear the voices of the scientists. There’s a mythology I wanted to challenge with this book, that science, unlike all other human activities, in an activity where only the bottom line matters. That it’s not how something is said, it’s just the final result, the final equation, or the experiment that can be duplicated. And because of that mythology, scientists rarely read the original literature in their fields. That’s a terrible loss. There is a great wealth of human information and personality and voice in the original papers. They are part of our cultural heritage, like novels and symphonies. The nonscientists may need a little help following their lines of thought, but that can be provided.

IDEAS: Which of these scientists have the most compelling authorial voices?

LIGHTMAN: The papers by Einstein have a very strong authorial voice. You really see Einstein wrestling with God, I think, trying to understand the fundamental secrets about the way the world works, and you have a very personal sense of him. The paper by Watson and Crick is understated, but they were very aware of the significance of their work, and the language is charged.

IDEAS: Thomas Kuhn’s idea of the ”paradigm shift” created enormous debate about patterns of discovery in science. Did this project give you new insights into the nature of discovery?

LIGHTMAN: There are about six or seven different categories of discovery. One kind is just an accident, like Alexander Fleming’s discovery of penicillin. But there is one common pattern despite all the differences. First you work hard on a problem and have what I call a prepared mind. You’ve done your homework, you’ve defined the problem. Then you get stuck. But getting stuck is a very important part of the process. It’s a good thing, not a bad thing. It catalyzes the creative imagination. There is a change in perspective, a shift in thinking, and you see the problem in a different way. That leads to discovery.

IDEAS: Is the rise of ”big science” eliminating those moments of discovery? Can individual scientists make breakthroughs in the manner they once did?

LIGHTMAN: It’s an excellent point. And I do believe that the trend toward big science, in which there are large teams of people working on scientific problems, diminishes the personal nature of discovery. But there is still a lot of science done by one or two people, especially in theoretical science. So it’s mainly experimental projects that we’re referring to here, in physics, biology, chemistry, and astronomy, that are changing the experience of doing science.

IDEAS: You chronicle scientists making breakthroughs while driving, walking in the snow, and even dreaming. Can we explain how and when these moments of discovery happen?

LIGHTMAN: You can never tell when a shift in perspective is going to come. You have to have a prepared mind and be open, but it can come at any moment.