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Traditionally, the idea has been that an experiment can either confirm or refute a theory. An experimental result yields positive evidence if it supports the theory, while a result that contradicts the hypothesis is negative evidence.

In the 20th century, a notion arose that the key to a theory — the thing that makes it scientific — is whether it can in some way be shown to be false. This principle of falsifiability can be controversial when applied to string theory, which theoretically explores energy levels that can’t at present (or possibly ever) be directly explored experimentally. Some claim that because string theory currently fails the test of falsifiability, it’s somehow not “real science.” (Check out Chapter 18 for more on this idea.)

The focus on this falsifiability is traced back to philosopher Karl Popper’s 1934 book, The Logic of Scientific Discovery. He was opposed to the reductionist and inductive methods that Francis Bacon had popularized three centuries earlier. In a time that was characterized by the rise of modern physics, it appeared that the old rules no longer applied.

Popper reasoned that the principles of physics arose not merely by viewing little chunks of information, but by creating theories that were tested and repeatedly failed to be proved false. Observation alone could not have led to these insights, he claimed, if they’d never been put in positions to be proven false. In the most extreme form, this emphasis on falsifiability states that scientific theories don’t tell you anything definite about the world, but are only the best guesses about the future based on past experience.

For example, if you predict that the sun will rise every morning, you can test your prediction by looking out the window every morning for 50 days. If the sun is there every day, you have not proved that the sun will be there on the 51st day. After you actually observe it on the 51st day, you’ll know that your prediction worked out again, but you haven’t proved anything about the 52nd day, the 53rd day, and so on.

No matter how good a scientific prediction is, if you can run a test that shows that it’s false, you have to throw out the idea (or at least modify your theory to explain the new data). This led the 19th century biologist Thomas Henry Huxley to define the great tragedy of science as “the slaying of a beautiful hypothesis by an ugly fact.”

To Popper, falsifiability was far from tragic, but was instead the brilliance of science. The defining component of a scientific theory, the thing that separates it from mere speculation, is that it makes a falsifiable claim.

Of course, it might not be easy to falsify a scientific claim. The claim may be a prediction at the forefront of current scientific knowledge, perhaps requiring additional technological progress before the experiments that would prove the claim false can be run. This is where things can get tricky for string theory, where much of the benefit is the ability to skirt along the edge of currently testable knowledge.

It’s also worth noting that mathematics as a whole doesn’t really follow this idea of falsifiability. Instead, it invents some more or less reasonable starting points (axioms) and, through a series of deductions, provides theorems that establish precise relations between mathematical objects. Although math itself is not falsifiable, it cannot be denied that it is incredibly useful to understanding nature, and we’d be hard-pressed to do without it when building a house or flying a plane.

Popper’s claim is sometimes controversial, especially when it’s being used by one scientist (or philosopher) to discredit an entire field of science. Many still believe that reductionism and inductive reasoning can lead to the creation of meaningful theoretical frameworks that represent reality as it is, even if there’s no claim that’s expressly falsifiable. The central element of this belief is the idea of confirmation — direct positive evidence for a theory, rather than just a lack of negative evidence against it.

String theorist Leonard Susskind and physicist Lee Smolin amicably clashed over exactly this point online in 2004 (you can view the debate at www.edge.org/3rd_culture/smolin_susskind04/smolin_susskind.html). To support the idea of confirmation, Susskind lists several theories that have been denounced as unfalsifiable: behaviorism in psychology along with quark models and inflationary theory in physics. Scientists initially believed that certain traits in these areas couldn’t be examined, though methods were later developed that allowed them to be tested.

There’s a difference between being unable to falsify a theory in practice and being unable to falsify it in principle.

It may seem as if the debate over confirmation and falsifiability is academic. That’s probably true, but some critics of string theory frame the debate around this theory as a battle over the very meaning of physics. Many string theory critics believe that it’s inherently unfalsifiable, while some string theorists believe a mechanism to test (and falsify) the prediction of string theory may someday be found.