Sir

As the originator of the term ‘Theory of Everything’ (TOE) to represent the ambition of string theory to describe all the elementary particles and their fundamental interactions (Nature 323, 595; 1986), I was dismayed by the tone of a recent review by my old friend George Ellis of The Quest for Unity (Nature 401, 527; 1999) by Etienne Klein and Marc Lachièze-Ray.

My definition of a TOE — a theory to “unify all the fundamental interactions” and “explain the number and couplings of all the elementary particles” — is not different from Ellis's “theory unifying all fundamental forces into one”, but I take issue with his (and your headline writer's) assertion that this “is … irrelevant to most physicists”.

In addition to the cosmologists cited by Ellis, the TOE ambition of particle physics is relevant to any astrophysicists interested in topics such as black holes, gravitational waves, active galactic nuclei, gamma-ray bursters, dark matter, structure formation, supernovae, high-energy cosmic rays and so on. Moreover, the theoretical tools of quantum field theory, conformal field theory, topological solitons and so on, discovered or developed by particle and string theorists, are central to much of condensed-matter theory, and even of interest to pure mathematicians; witness the Fields medal awarded some years ago to Edward Witten, even before the advent of Seiberg–Witten theory.

Like Ellis, I lament the fact that physics “is becoming ever more fragmented into disparate subjects”, but particle physicists are hardly to blame for the richness of the phenomena (relativity, quantum theory and so on) revealed by their predecessors in the quest for unification. Nobody can know what richness may be revealed by the TOE, and the fragmentation it may engender.

And, please, no more citations of the defunct Superconducting Super-Collider: its physics will be covered ably by the Large Hadron Collider (LHC) at a fraction of the cost. Indeed, the LHC, at $2 billion, is not so much more costly than the Spallation Neutron Source in the UnitedStates, at $1.3 billion, or the Japanese Hadron Facility, just to take two examples of current accelerator projects motivated in large part by condensed-matter physics. Where would these, and synchrotron light sources, be if would-be particle unifiers had not developed the necessary accelerator tools?

Finally, I agree with Ellis that “isolationism is a prerequisite for successful physics”, and with the authors of the book that it is a challenge for physics to “resist the temptation to project the false self-image of a religion able to reveal the ultimate truth”. However, I disagree with your headline that “there is no ultimate truth in grand unification”. There is some of the ultimate truth, though certainly not all.