Nancy Cartwright: Is It All a Big Lie? | DIE ZEIT

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Inhalt<br>Seite 1Is It All a Big Lie?

Seite 2»Reliability is a several-place relation«

Seite 3»It is about humility«

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DIE ZEIT: Physics and its laws lie—that is what you said more than 40 years ago, and it made you famous. Would you still call physics a liar today?

Nancy Cartwright: I would no longer write a book that accuses physics of lying in its very title. These are not good times for such a title; it can easily be misunderstood. Trust in the sciences has declined to an alarming degree. And in some areas, I can even understand why.

ZEIT: What do you mean?

Cartwright: I grew up in a small town in Pennsylvania, in what is now the Rust Belt. I know people there whose livelihood depends on whether the mines stay open. They cannot afford to believe that burning coal is bad for the environment. We all find our truth in our own surroundings, even if those surroundings are virtual. We derive it from the people around us, the ones we rely on. For many in the Rust Belt, the social system has not worked, and science is part of that system.

ZEIT: Where are the limits of your sympathy for that skepticism?

Cartwright: I understand the loss of trust, and even the way those who felt slighted cast their votes, though I think it morally wrong. But the fact that the Trump administration is shutting down medical research despite the fact that it knows better. That I do not understand. It strikes me as crazy. You need research to arrive at the kind of detailed, robust knowledge that is required for so much of what we rely on in modern life to work. And most of those who no longer trust science in general do, after all, take its trustworthiness for granted: They will still undergo laser surgery on their eyes. Today, I would write a book about how useful the sciences are. »On the Wonderful Art of Building a Laser«, that could be my working title.

ZEIT: But what has become of that liar of yours? Can physics now be relied on?

Cartwright: When I wrote my book in the early 1980s, philosophy was dominated by a very narrow idea of science that I wanted to change. It was confined to physics in its pure form: spare equations formulating general laws that everything obeys. An example: We assume that objects of mass m which experience a force F are set in motion with an acceleration a that we can calculate by dividing F by m.

ZEIT: Isaac Newton's second law of motion, the fundamental equation of mechanics.

Nancy Cartwright<br>The Philosopher: The 82-year-old American taught at the London School of Economics and Political Science. She is a leading figure in the philosophy of science. The Books: In How the Laws of Physics Lie (1983) she challenged the idea of a world governed seamlessly by the laws of physics. In 2022, she published The Tangle of Science, her plea for reliability.

Cartwright: Exactly. There is a great deal of evidence for the usefulness of this law. We use it to calculate the structural stability of our houses and the propulsion of the rockets we send to the moon. And yet, we are fudging things a bit when we pretend we have good evidence that the formula holds universally. Some 200 years after Newton's Philosophiæ Naturalis, Albert Einstein pointed out in the special theory of relativity that the equation of motion for objects traveling at close to the speed of light is far more complex, because mass itself increases with velocity. At about the same time, in the early 20th century, modern quantum mechanics emerged. It holds that the smallest, lightest particles also move quite differently. They do not even possess a precisely defined position or momentum. So Newton's law works only as an approximation, in a range between the large things and the small. But even with our most recent theories, the ones said to have superseded those old, »mistaken« ones, what we have above all are useful tools, not fundamental truths.

ZEIT: An illustration, perhaps?

Cartwright: The Austrian philosopher Otto Neurath illustrated this with the example of a 1,000-schilling note blowing in the wind across St. Stephen's Square. We cannot calculate where exactly it will land even with the best model. The approach taken by physics would be to set up an equation of motion as though that equation were universally true, and to try to infer the banknote's acceleration based on its mass and the force acting on it. But without writing down a defensible force function for the wind—and we cannot do that—it is mere faith to insist that the acceleration is determined by Newton's law. Perhaps things are simply more complicated than we think.

ZEIT: Before your time as a philosopher of science you studied mathematics and physics. When did you first suspect that the widely held...