I have long admired Leonard Susskind's contributions to science, but I was puzzled by his discussion of my and Peter Woit's books (Features, August 25). After more than 30 years of investigation, string theory still has no cogent formulation, nor does it seem able to make any experimental predictions by which it might be tested.

What is remarkable, given his tone, is that Susskind's comments are to a large extent in agreement with the conclusions of my book, The Trouble with Physics . What we disagree about is what to do about the situation of a once-promising theory that makes no predictions. He advises that we continue to put most of our intellectual resources into string theory. I propose that the full range of ideas and hypotheses about fundamental physics be investigated and that young scientists be encouraged to formulate new theories that may succeed more quickly.

Susskind also appears to agree with this, at least in principle. But his embrace of intellectual diversity seems to fail as soon as it comes to research outside of string theory. He characterises the approaches to quantum gravity that are seen by many experts as promising alternatives to string theory, including loop quantum gravity and doubly special relativity, as "sunken ships." In reality, these are active research programmes that have succeeded at crucial tests that have so far evaded string theory.

In the most shocking assertion of his essay, he says: "An accelerator powerful enough to study these tiny so-called Planckian objects would have to be as large as the entire galaxy." But since the late 1990s those working on quantum gravity have understood that we have an accelerator much larger than a galaxy - the whole universe. Experiments such as Auger and Glast detect photons and cosmic rays coming from across the universe, By studying their spectra we can probe at the Planck scale and shorter. By doing so, we are already testing predictions of some of the alternatives to string theory such as doubly special relativity.

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Susskind says unkind things about my status as a scientist and he implies, wrongly, that the most active alternatives to string theory are just my own failed ideas. This ignores the achievements of the hundreds of physicists and mathematicians who work on approaches to fundamental physics outside of string theory.

What is sad about this is that there is a genuine crisis, and polemics and name-calling will not help. My response was to write a carefully argued book motivated by respect for string theory and those colleagues who dedicated themselves to it. The arguments in my book could be right or wrong, but they are arguments based on evidence and logic. They are presented in good faith, in the hope they might be useful to thoughtful people working on the frontiers of fundamental physics. Those who care about the future of physics might surprise themselves by finding a lot they agree with in it.

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Lee Smolin
Perimeter Institute for Theoretical Physics, Canada