Recently, as a result of my most recent book, A Universe from Nothing, I participated in a wide-ranging and in-depth interview for The Atlantic on questions ranging from the nature of nothing to the best way to encourage people to learn about the fascinating new results in cosmology. The interview was based on the transcript of a recorded conversation and was hard hitting (and, from my point of view, the interviewer was impressive in his depth), but my friend Dan Dennett recently wrote to me to say that it has been interpreted (probably because it included some verbal off-the-cuff remarks, rather than carefully crafted written responses) by a number of his colleagues and readers as implying a blanket condemnation of philosophy as a discipline, something I had not intended.

Out of respect for Dan and those whom I may have unjustly offended, and because the relationship between physics and philosophy seems to be an area which has drawn some attention of late, I thought I would take the opportunity to write down, as coherently as possible, my own views on several of these issues, as a physicist and cosmologist. As I should also make clear (and as numerous individuals have not hesitated to comment upon already), I am not a philosopher, nor do I claim to be an expert on philosophy. Because of a lifetime of activity in the field of theoretical physics, ranging from particle physics to general relativity to astrophysics, I do claim however to have some expertise in the impact of philosophy on my own field. In any case, the level of my knowledge, and ignorance, will undoubtedly become clearer in what follows.

As both a general reader and as someone who is interested in ideas and culture, I have great respect for and have learned a great deal from a number of individuals who currently classify themselves as philosophers. Of course as a young person I read the classical philosophers, ranging from Plato to Descartes, but as an adult I have gained insights into the implications of brain functioning and developments in evolutionary psychology for understanding human behavior from colleagues such as Dan Dennett and Pat Churchland. I have been forced to re-examine my own attitudes towards various ethical issues, from the treatment of animals to euthanasia, by the cogent and thoughtful writing of Peter Singer. And reading the work of my friend A.C. Grayling has immeasurably heightened my understanding and appreciation of the human experience.

What I find common and so stimulating about the philosophical efforts of these intellectual colleagues is the way they thoughtfully reflect on human knowledge, amassed from empirical explorations in areas ranging from science to history, to clarify issues that are relevant to making decisions about how to function more effectively and happily as an individual, and as a member of a society.

As a practicing physicist however, the situation is somewhat different. There, I, and most of the colleagues with whom I have discussed this matter, have found that philosophical speculations about physics and the nature of science are not particularly useful, and have had little or no impact upon progress in my field. Even in several areas associated with what one can rightfully call the philosophy of science I have found the reflections of physicists to be more useful. For example, on the nature of science and the scientific method, I have found the insights offered by scientists who have chosen to write concretely about their experience and reflections, from Jacob Bronowski, to Richard Feynman, to Francis Crick, to Werner Heisenberg, Albert Einstein, and Sir James Jeans, to have provided me with a better practical guide than the work of even the most significant philosophical writers of whom I am aware, such as Karl Popper and Thomas Kuhn. I admit that this could primarily reflect of my own philosophical limitations, but I suspect this experience is more common than not among my scientific colleagues.

The one area of physics that has probably sparked the most 'philosophical' interest in recent times is the 'measurement' problem in quantum mechanics. How one moves from the remarkable and completely non-intuitive microscopic world where quantum mechanical indeterminacy reigns supreme and particles are doing many apparently inconsistent things at the same time, and are not localized in space or time, to the ordered classical world of our experience where baseballs and cannonballs have well-defined trajectories, is extremely subtle and complicated and the issues involved have probably not been resolved to the satisfaction of all practitioners in the field. And when one tries to apply the rules of quantum mechanics to an entire universe, in which a separation between observer and observed is not possible, the situation becomes even murkier.

However, even here, the most useful progress has been made, again in my experience, by physicists. The work of individuals such as Jim Hartle, and Murray Gell-Mann, Yakir Aharonov, Asher Peres, John Bell and others like them, who have done careful calculations associated with quantum measurement, has led to great progress in our appreciation of the subtle and confusing issues of translating an underlying quantum reality into the classical world we observe. There have been people who one can classify as philosophers who have contributed usefully to this discussion, such as Abner Shimony, but when they have, they have been essentially doing physics, and have published in physics journals (Shimony's work as a physicist is the work I am aware of). As far as the physical universe is concerned, mathematics and experiment, the tools of theoretical and experimental physics appear to be the only effective ways to address questions of principle.

Which brings me full circle to the question of nothing, and my own comments regarding the progress of philosophy in that regard. When it comes to the real operational issues that govern our understanding of physical reality, ontological definitions of classical philosophers are, in my opinion, sterile. Moreover, arguments based on authority, be it Aristotle, or Leibniz, are irrelevant. In science, there are no authorities, and appeal to quotes from brilliant scholars who lived before we knew the Earth orbited the Sun, or that space can be curved, or that dark matter or dark energy exist do not generally inform our current understanding of nature. Empirical explorations ultimately change our understanding of which questions are important and fruitful and which are not.

As a scientist, the fascination normally associated with the classically phrased question "why is there something rather than nothing?", is really contained in a specific operational question. That question can be phrased as follows: How can a universe full of galaxies and stars, and planets and people, including philosophers, arise naturally from an initial condition in which none of these objects -- no particles, no space, and perhaps no time -- may have existed? Put more succinctly perhaps: Why is there 'stuff', instead of empty space? Why is there space at all? There may be other ontological questions one can imagine but I think these are the 'miracles' of creation that are so non-intuitive and remarkable, and they are also the 'miracles' that physics has provided new insights about, and spurred by amazing discoveries, has changed the playing field of our knowledge. That we can even have plausible answers to these questions is worth celebrating and sharing more broadly.

In this regard, there is a class of philosophers, some theologically inspired, who object to the very fact that scientists might presume to address any version of this fundamental ontological issue. Recently one review of my book by such a philosopher, which I think motivated the questions in the Atlantic interview, argued not only that one particular version of the nothing described by modern physics was not relevant. Even more surprisingly, this author claimed with apparent authority (surprising because the author apparently has some background in physics) something that is simply wrong: that the laws of physics can never dynamically determine which particles and fields exist and whether space itself exists, or more generally what the nature of existence might be. But that is precisely what is possible in the context of modern quantum field theory in curved spacetime, where a phenomenon called 'spontaneous symmetry breaking' can determine dynamically which forces manifest themselves on large scales and which particles exist as stable states, and whether space itself can grow exponentially or not. Within the context of quantum gravity the same is presumably true for which sorts of universes can appear and persist. Within the context of string theory, a similar phenomenon might ultimately determine (indeed if the theory is ever to become predictive, it must determine) why universes might spontaneously arise with 4 large spacetime dimensions and not 5 or 6. One cannot tell from the review if the author actually read the book (since no mention of the relevant cosmology is made) or simply misunderstood it.

Theologians and both Christian and Muslim apologists have unfortunately since picked up on the ill-conceived claims of that review to argue that physics can therefore never really address the most profound 'theological' questions regarding our existence. (To be fair, I regret sometimes lumping all philosophers in with theologians because theology, aside from those parts that involve true historical or linguistic scholarship, is not credible field of modern scholarship.) It may be true that we can never fully resolved the infinite regression of 'why questions' that result whenever one assumes, a priori, that our universe must have some pre-ordained purpose. Or, to frame things in a more theological fashion: 'Why is our Universe necessary rather than contingent?'.

One answer to this latter question can come from physics. If all possibilities -- all universes with all laws -- can arise dynamically, and if anything that is not forbidden must arise, then this implies that both nothing and something must both exist, and we will of necessity find ourselves amidst something. A universe like ours is, in this context, guaranteed to arise dynamically, and we are here because we could not ask the question if our universe weren't here. It is in this sense that I argued that the seemingly profound question of why there is something rather than nothing might be actually no more profound than asking why some flowers are red or some are blue. I was surprised that this very claim was turned around by the reviewer as if it somehow invalidated this possible physical resolution of the something versus nothing conundrum.

Instead, sticking firm to the classical ontological definition of nothing as "the absence of anything" -- whatever this means -- so essential to theological, and some subset of philosophical intransigence, strikes me as essentially sterile, backward, useless and annoying. If "something" is a physical quantity, to be determined by experiment, then so is 'nothing'. It may be that even an eternal multiverse in which all universes and laws of nature arise dynamically will still leave open some 'why' questions, and therefore never fully satisfy theologians and some philosophers. But focusing on that issue and ignoring the remarkable progress we can make toward answering perhaps the most miraculous aspect of the something from nothing question -- understanding why there is 'stuff' and not empty space, why there is space at all, and how both stuff and space and even the forces we measure could arise from no stuff and no space -- is, in my opinion, impotent, and useless. It was in that sense -- the classical ontological claim about the nature of some abstract nothing, compared to the physical insights about this subject that have developed -- that I made the provocative, and perhaps inappropriately broad statement that this sort of philosophical speculation has not led to any progress over the centuries.

What I tried to do in my writing on this subject is carefully attempt to define precisely what scientists operationally mean by nothing, and to differentiate between what we know, and what is merely plausible, and what we might be able to probe in the future, and what we cannot. The rest is, to me, just noise.

So, to those philosophers I may have unjustly offended by seemingly blanket statements about the field, I apologize. I value your intelligent conversation and the insights of anyone who thinks carefully about our universe and who is willing to guide their thinking based on the evidence of reality. To those who wish to impose their definition of reality abstractly, independent of emerging empirical knowledge and the changing questions that go with it, and call that either philosophy or theology, I would say this: Please go on talking to each other, and let the rest of us get on with the goal of learning more about nature.