[Tam Hunt sent me another lengthy interview, this time with Lee Smolin. Smolin is a faculty member at the Perimeter Institute for Theoretical Physics in Canada and adjunct professor at the University of Waterloo. He is one of the founders of loop quantum gravity. In the past decades, Smolin’s interests have drifted to the role of time in the laws of nature and the foundations of quantum mechanics.]
TH: You make some engaging and bold claims in your new book,
Einstein’s Unfinished Revolution, continuing a line of argument that you’ve been making over the course of the last couple of decades and a number of books. In your latest book, you argue essentially that we need to start from scratch in the foundations of physics, and this means coming up with new first principles as our starting point for re-building. Why do you think we need to start from first principles and then build a new system? What has brought us to this crisis point?
LS: The claim that there is a crisis, which I first made in my book,
Life of the Cosmos (1997), comes from the fact that it has been decades since a new theoretical hypothesis was put forward that was later confirmed by experiment. In particle physics, the last such advance was the standard model in the early 1970s; in cosmology, inflation in the early 1980s. Nor has there been a completely successful approach to quantum gravity or the problem of completing quantum mechanics.
I propose finding new fundamental principles that go deeper than the principles of general relativity and quantum mechanics. In some recent papers and the book, I make specific proposals for new principles.
TH: You have done substantial work yourself in quantum gravity (loop quantum gravity, in particular) and quantum theory (suggesting your own interpretation called the “real ensemble interpretation”), and yet in this new book you seem to be suggesting that you and everyone else in foundations of physics needs to return to the starting point and rebuild. Are you in a way repudiating your own work or simply acknowledging that no one, including you, has been able to come up with a compelling approach to quantum gravity or other outstanding foundations of physics problems?
LS: There are a handful of approaches to quantum gravity that I would call partly successful. These each achieve a number of successes, which suggest that they could plausibly be at least part of the story of how nature reconciles quantum physics with space, time and gravity. It is possible, for example that these partly successful approaches model different regimes or phases of quantum gravity phenomena. These partly successful approaches include loop quantum gravity, string theory, causal dynamical triangulations, causal sets, asymptotic safety. But I do not believe that any approach to date, including these, is fully successful. Each has stumbling blocks that after many years remain unsolved.
TH: You part ways with a number of other physicists in recent years who have railed against philosophy and philosophers of physics as being largely unhelpful for actual physics. You argue instead that philosophers have a lot to contribute to the foundations of physics problems that are your focus. Have you found philosophy helpful in pursuing your physics for most of your career or is this a more recent finding in your own work? Which philosophers, in particular, do you think can be helpful in this area of physics?
LS: I would first of all suggest we revive the old idea of a natural philosopher, which is a working scientist who is inspired and guided by the tradition of philosophy. An education and immersion in the philosophical tradition gives them access to the storehouse of ideas, positions and arguments that have been developed over the centuries to address the deepest questions, such as the nature of space and time.
Physicists who are natural philosophers have the advantage of being able to situate their work, and its successes and failures, within the long tradition of thought about the basic questions.
Most of the key figures who transformed physics through its history have been natural philosophers: Galileo, Newton, Leibniz, Descartes, Maxwell, Mach, Einstein, Bohr, Heisenberg, etc. In more recent years, David Finkelstein is an excellent example of a theoretical physicist who made important advances, such as being the first to untangle the geometry of a black hole, and recognize the concept of an event horizon, who was strongly influenced by the philosophical tradition. Like a number of us, he identified as a follower of Leibniz, who introduced the concepts of relational space and time.
The abstract of Finkelstein’s key 1958 paper on what were soon to be called black holes explicitly mentions the principle of sufficient reason, which is the central principle of Leibniz’s philosophy. None of the important developments of general relativity in the 1960s and 1970s, such as those by Penrose, Hawking, Newmann, Bondi, etc., would have been possible without that groundbreaking paper by Finkelstein.
I asked Finkelstein once why it was important to know philosophy to do physics, and he replied, “If you want to win the long jump, it helps to back up and get a running start.”’
In other fields, we can recognize people like Richard Dawkins, Daniel Dennett, Lynn Margulis, Steve Gould, Carl Sagan, etc. as natural philosophers. They write books that argue the central issues in evolutionary theory, with the hope of changing each other’s minds. But we the lay public are able to read over their shoulders, and so have front row seats to the debates.
There are also working now a number of excellent philosophers of physics, who contribute in important ways to the progress of physics. One example of these is a group, centred originally at Oxford, of philosophers who have been doing the leading work on attempting to make sense of the Many Worlds formulation of quantum mechanics. This work involves extremely subtle issues such as the meaning of probability. These thinkers include Simon Saunders, David Wallace, Wayne Mhyrvold; and there are equally good philosophers who are skeptical of this work, such as David Albert and Tim Maudlin.
It used to be the case, half a century ago, that philosophers, such as Hilary Putnam, who opined about physics, felt qualified to do so with a bare knowledge of the principles of special relativity and single particle quantum mechanics. In that atmosphere my teacher Abner Shimony, who had two Ph.D’s – one in physics and one in philosophy – stood out, as did a few others who could talk in detail about quantum field theory and renormalization, such as Paul Feyerabend. Now the professional standard among philosophers of physics requires a mastery of Ph.D level physics, as well as the ability to write and argue with the rigour that philosophy demands. Indeed, a number of the people I just mentioned have Ph.D’s in physics.
TH: One of your suggested hypotheses, the next step you take after stating your first principles, is an acknowledgment that time is fundamental, real and irreversible, effectively goring one of the sacred cows of modern physics. You made your case for this approach in your book
Time Reborn and I'm curious if you've seen a softening over the last few years in terms of physicists and philosophers beginning to be more open to the idea that the passage of time is truly fundamental? Also, why wouldn't this hypothesis be instead a first principle, if time is indeed fundamental?
LS: In my experience, there have always been physicists and philosophers open to these ideas, even if there is no consensus among those who have carefully thought the issues through.
When I thought carefully about how to state a candidate set of basic principles, it became clear that it was useful to separate principles from hypotheses about nature. Principles such as sufficient reason and the identity of the indiscernible can be realized in formulations of physics in which time is either fundamental or secondary and emergent. Hence those principles are prior to the choice of a fundamental or emergent time. So I think it clarifies the logic of the situation to call the latter choice a hypothesis rather than a principle.
TH: How does viewing time as irreversible and fundamental mesh with your principle of background independence? Doesn’t a preferred spacetime foliation, which would provide an irreversible and fundamental time, provide a background?
LS: Background independence is an aspect of the two principles of Leibniz I just referred to: 1) sufficient reason (PSR) and 2) the identity of the indiscernible (PII). Hence it is deeper than the choice of whether time is fundamental or emergent. Indeed, there are theories which rest on both hypotheses about time (fundamental or emergent). Julian Barbour, for example, is a relationalist who develops background-independent theories in which time is emergent. I am also a relationalist, but I make background-independent models of physics in which time and its passage are fundamental.
Viewing time as fundamental and irreversible doesn’t necessarily imply a preferred foliation; by the latter you mean a foliation of a pre-existing spacetime, specified kinematically in advance of the dynamical evolution. In our energetic causal set models there does arise a notion of the present, but this is determined dynamically by the evolution of the model and so is consistent with what we mean by background independence.
The point is that the solutions to background-independent theories can have preferred frames, so long as they are generated by solving the dynamics. This is, for example, the case with cosmological solutions to general relativity.
TH: You and many other physicists have focused for many years on finding a theory of quantum gravity, effectively unifying quantum mechanics and general relativity. In describing your preferred approach to achieving a theory of quantum gravity worthy of the name you describe why you think quantum mechanics is incomplete and why general relativity is in some key ways likely wrong. Let’s look first at quantum mechanics, which you describe as “wrong” and “incomplete.” Why is the Copenhagen (still perhaps the most popular version of quantum theory) school of quantum mechanics wrong and incomplete?
LS: Copenhagen is incomplete because it is based on an arbitrarily chosen division of the world into a classical realm and a quantum realm. This reflects our practice as experimenters, and corresponds to nothing in nature. This means it is an operational approach which conflicts with the expectations that physics should offer a complete description of individual phenomena, with no reference to our existence, knowledge or measurements.
TH: Your objections just stated (what’s known generally as the “measurement problem”) seem to me, even as an obvious non-expert in this area, to be fairly apparent and accurate objections to Copenhagen. If that’s the case, why is Copenhagen still with us today? Why was it ever considered a serious theory?
LS: I don’t think there are many proponents of the Copenhagen view among people working in quantum foundations, or who have otherwise thought about the issues carefully. I don’t think there are many enthusiastic followers of Bohr left alive.
Meanwhile, what most physicists who are not specialists in quantum foundations practice and teach is a very pragmatic, operational set of rules, which suffices because it closely parallels the practice of actual experimenters. They can get on with the physics without having to take a stand on realism.
What Bohr had in mind was a much more radical rejection of realism and its replacement by a view of the world in which nature and us co-create phenomena. My sense is that most living physicists haven’t read Bohr’s actual writings. There are of course some exceptions, like Chris Fuch’s QBism, which is, to the extent that I understand it, an even more radical view. Even if I disagree, I very much admire Chris for the clarity of his thinking and his insistence on taking his view to its logical conclusions. But, in the end, as a realist who sees the necessity of completing quantum mechanics by the discovery of new physics, the intellectual contortions of anti-realists are, however elegant, no help for my projects.
TH: Could this be a good example of why philosophical training could actually be helpful for physicists?
LS: I would agree, in some cases it could be helpful for some physicists to study philosophy, especially if they are interested in discovering deeper foundational laws. But I would never say anyone should study philosophy, because it can be very challenging reading, and if someone is not inclined to think “philosophically” they are unlikely to get much from the effort. But I would say that if someone is receptive to the care and depth of the writing, it can open doors to new ideas and to a highly critical style of thinking, which could greatly aid someone’s research.
The point I would like to make here is rather different. As I discussed in my earlier books, there are different periods in the development of science during which different kinds of problems present themselves. These require different strategies, different educations and perhaps even different styles of research to move forward.
There are pragmatic periods where the laws needed to understand a wide range of phenomena are in place and the opportunities of greatly advancing our understanding of diverse physical phenomena dominate. These kinds of periods require a more pragmatic approach, which ignores whatever foundational issues may be present (and indeed, there are always foundational issues lurking in the background), and focuses on developing better tools to work out the implications of the laws as they stand.
Then there are (to follow Kuhn) revolutionary periods in science, when the foundations are in question and the priority is to discover and express new laws.
The kinds of people and the kinds of education needed to succeed are different in these two kinds of periods. Pragmatic times require pragmatic scientists, and philosophy is unlikely to be important. But foundational periods require foundational people, many of whom will, as in past foundational periods, find inspiration from philosophy.
Of course, what I just said is an oversimplification. At all times, science needs a diverse mix of research styles. We always need pragmatic people who are very good at the technical side of science. And we always need at least a few foundational thinkers. But the optimal balance is different in different periods.
The early part of the 20th Century, through around 1930, was a foundational period. That was followed by a pragmatic period during which the foundational issues were ignored and many applications of the quantum mechanics were developed.
Since the late 1970s, physics has been again in a foundational period, facing deep questions in elementary particle physics, cosmology, quantum foundations and quantum gravity. The pragmatic methods which got us to that point no longer suffice; during such a period we need more foundational thinkers and we need to pay more attention to them.
TH: Turning to general relativity, you also don’t mince your words and you describe the notion of reversible time, thought to be at the core of general relativity, as “wrong.” What does general relativity look like with irreversible and fundamental time?
LS: We posed exactly this question: can we invent an extension of general relativity in which time evolution is asymmetric under a transformation that reverses a measure of time. We found two ways to do this.
TH: You touched on consciousness as a physical phenomenon and a necessary ingredient in our physics in your book, Time Reborn (as have many other physicists over the last century, of course). You spend less time on consciousness in your new book — stating “Let us tiptoe past the hard question of consciousness to simpler questions” — but I’m curious if you’ve considered including as a first principle the notion that consciousness is a fundamental aspect of nature (or not) in your ruminations on these deep topics?
LS: I am thinking slowly about the problems of qualia and consciousness, in the rough direction set out in the epilogue of Time Reborn. But I haven’t yet come to conclusions worth publishing. An early draft of Einstein’s Unfinished Revolution had an epilogue entirely devoted to these questions, but I decided it was premature to publish; it also would have distracted attention from the central themes of that book.
TH: David Bohm, one of the physicists you discuss with respect to alternative versions of quantum theory, delved deeply into philosophy and spirituality in relation to his work in physics, as you discuss briefly in your new book. Do you find Bohm’s more philosophical notions such as the Implicate Order (the metaphysical ground of being in which the “explicate” manifest world that we know in our normal every day life is enfolded, and thus “implicate”) helpful for physics?
LS: I am afraid I’ve not understood what Bohm was aiming for in his book on the implicate order, or his dialogues with Krishnamurti, but it is also true that I haven’t tried very hard. I think one can admire greatly the practical and psychological knowledge of Buddhism and related traditions, while remaining skeptical of their more metaphysical teachings.
TH: Bohm’s Implicate Order has much in common with physical notions such as the (nonluminiferous) ether, which has been revived in today’s physics by some heavyweights such as Nobel Prize winner Frank Wilczek (The Lightness of Being: Mass, Ether, and the Unification of Forces) as another term for the set of space-filling fields that underlie our reality. Do you take the idea of reviving some notion of the ether as a physical/metaphysical background at all seriously in your work?
LS: The important part of the idea of the ether was that it is a smooth, fundamental, physical substance, which had the property that vibrations and stresses within it reproduced the phenomena described by Maxwell’s field theory of electromagnetism. It was also important that there was a preferred frame of reference associated with being at rest with respect to this substance.
We no longer believe any part of this. The picture we now have is that any such substance is made of a large collection of atoms. Therefore the properties of any substance are emergent and derivative. I don’t think Frank Wilczek disagrees with this, I suspect he is just being metaphorical.
TH: He doesn’t seem to be metaphorical, writing in a 1999 article:“Quite undeservedly, the ether has acquired a bad name. There is a myth, repeated in many popular presentations and textbooks, that Albert Einstein swept it into the dustbin of history. The real story is more complicated and interesting. I argue here that the truth is more nearly the opposite: Einstein first purified, and then enthroned, the ether concept. As the 20th century has progressed, its role in fundamental physics has only expanded. At present, renamed and thinly disguised, it dominates the accepted laws of physics. And yet, there is serious reason to suspect it may not be the last word.” In his 2008 book mentioned above, he reframes the set of accepted physical fields as “the Grid” (which is “the primary world-stuff”) or ether. Sounds like you don’t find this re-framing very compelling?
LS: What is true is that quantum field theory (QFT) treats all propagating particles and fields as excitations of a (usually unique) vacuum state. This is analogized to the ether, but in my opinion it’s a bad analogy. One big difference is that the vacuum of a QFT is invariant under all the symmetries of nature, whereas the ether breaks many of them by defining a preferred state of at rest.
TH: You consider Bohm’s alternative quantum theory in some depth, and say that “it makes complete sense,” but after further discussion you consider it inadequate because it is generally considered to be incompatible with special relativity, among other problems.
LS: This is not the main reason I don’t think pilot wave theory describes nature.
Pilot wave theory is based on two equations. One, which is the same as in ordinary QM-the Schrödinger equation, propagates the wave-function, while the second-the guidance equation, guides the “particles.” The first can be made compatible with special relativity, while the second cannot. But when one adds an assumption about probabilities, the averages of the guided particles follow the waves and so agree with both ordinary QM and special relativity. In this way you can say that pilot wave theory is “weakly compatible” with special relativity, in the sense that, while there is a preferred sense of rest, it can’t be measured.
TH: If one considers time to be fundamental and irreversible, isn’t there a relativistic version of Bohmian mechanics readily available by adopting some version of Lorentzian or neo-Lorentzian relativity (which are background-dependent)?
LS: Maybe — you are describing research to be done.
TH: Last, how optimistic are you that your view, that today’s physics needs some really fundamental re-thinking, will catch on with the majority of today’s physicists in the next decade or so?
LS: I’m not but I wouldn’t expect any such call for a reconsideration of the basic principles would be popular until it has results which make it hard to avoid thinking about.
