In this gem of a book, Smolin describes his quest for a theory of the universe, a cosmological theory that explains why the universe is necessarily large and complex and the way it is. The old Newtonian, clockwork model of the universe is a picture of a cold, inhospitable, essentially sterile place in which life is a fluke, a vastly improbable statistical fluctuation, and in which all is futile, heading inexorably to the inevitable Heat Death. Fortunately it is a false picture, based on a 19th century understanding of physics. Unfortunately, it is also the picture that many believe to be what physics is still telling us. Smolin overthrows the old, showing how the 20th century physical theories of general relativity and quantum mechanics lead to a picture of a vast, vibrant, complex self-organising universe, hospitable to life, growing, and exhibiting ever more variety.
He takes a philosophical approach to describing physical theories. He points out that two of the mainstays of classical physics, reductionism and atomism, are simply incompatible: reductionism works by describing the whole in terms of its parts, whereas the fundamental "atoms" have no parts. Instead, he takes Leibniz' ideas of relative descriptions, and the principle of sufficient reason, very seriously. He shows how general relativity has moved us away from a theory with absolute space and time (implying some absolute, externally-imposed frame of reference) to a description based on the relationships between things, how position, motion, acceleration are relative concepts (hence the name, "relativity"). And he shows how quantum mechanics has moved us away from a theory of individual things acting independently, to one of entangled things all dependent on each other. And the consequences of these two theories seem to point to a universe that is necessarily large and complex.
Smolin starts by showing that the universe we live in appears to be vastly improbable. In particular, because it contains stars. Stars are necessary for life: they synthesise the sufficient variety of chemical elements (and particularly carbon) needed to build sufficiently complex systems, and they provide the long-term thermodynamic gradients needed for stable far-from-equilibrium systems. A universe full of stars is necessarily quite complex (for example, such a universe needs to contain carbon, to cool the interstellar medium sufficiently that stars can be made). Yet the fundamental physical constants need to be tuned astonishingly precisely to allow a universe with stars, and hence with life. Change those constants only very slightly, and there are no stars, and hence no life.
Why do those constants have the very finely-tuned values they do? Coincidence? Smolin says not, and provides a mechanism for giving them the values they have. General relativity predicts singularities; but we don't know, by definition, what happens at a singularity. Quantum mechanics suggests that maybe there was no singularity at the Big Bang, that maybe there are no singularities inside black holes. What if, Smolin speculates, inside black holes, the "singularity" actually produces a whole new universe? And what if the laws of physics (ie the values of the fundamental constants) are slightly different from those in the "parent" universe? Then each new "singularity" (be it a black hole, or the Big Crunch at the end of the universe) produces a universe slightly different from its parent. A form of natural selection can act: the random walk through value space eventually finds universes where black holes are abundant, and such universe generate many more child universes, with similar values of the constants, and hence will come to dominate. Any universe chosen at random will tend to be one that generates many black holes. Such universes are necessarily rather complex, and so are also good for life. A universe where black holes are abundant must have stars (to turn into black holes) and carbon (to help make stars). This is not simply wild speculation. It is proper scientific theory and is testable/falsifiable: it makes predictions that any universe with different values of the fundamental constants has fewer black holes.
This idea explains why the universe we live in is hospitable to life, without having to invoke special pleading such as the anthropic principle. But we have to give up some "neatness" if this idea is correct. If our universe is a result of natural selection by random walk over the space of values of the fundamental constants, these values may not be "simple" -- there may be no reason for the lower order digits to have the precise values they do (a value very close, but differing in all later digits, might also be tuned sufficiently well). [Is there any connection with Chaitin's work on algorithmic information theory here? Might some of the novelty come from the massive amount of information in the infinitely precise, but essentially random, constants? Or are they actually not infinitely precise, given the practical limitations of measuring them with a finite universe?]
Smolin moves on to discuss the consequences of GR and QM in some detail. He explains how GR gradually helped break the ideas of absolute space and time (it took some time, but GR broke free from coordinate systems approaches with the geometrical approach described by Misner, Thorne and Wheeler). He also explains how QM gradually helped break the idea of it being legitimate to talk about single isolated particles. (Again, this did not happen immediately, because early QM tackled descriptions of single particles. It wasn't until multi-particle systems were tackled that quantum entanglement was discovered and fully appreciated.) In order for these theories to make sense, the universe must be sufficiently complex (for example, if every thing is relative, the universe must be complex enough, have enough variety, that the things in it can be distinguished from each other purely in terms of their relationships with each other, not in terms of some non-existent absolute position).
This all leads on to some very deep stuff. In particular, Smolin points out that GR and QM are at best only partial cosmological theories, and so more work is needed. He speculates on what a full cosmological theory might be. And on the way, he packs in numerous little gems (such as the Bekenstein bound: a black hole has an entropy proportional to its surface area; no region of space can have more information than can the largest black hole that can occupy that space; so the maximum amount of information in a region of space grows as the surface area, and not the volume, of that space.) No review could possibly hope to cover the breadth and depth of the topics Smolin explores (for example: his explanation of how Leibniz' philosophy and gauge theories are linked, and his description of the process of continual star production in the spiral arms of galaxies, are a joy to read).
Smolin is careful to distinguish between accepted physical theory, theories that are still currently under development, and more speculative ideas. Some of the more interesting points are the more speculative ones! Nevertheless, this beautifully written and fascinating account, by a leading quantum gravity researcher, is of a universe vastly different, and vastly more interesting, than the one we are used to reading about in more "popular" accounts of physics.
Smolin sets out to explain three different routes to a theory of quantum gravity, and how they might all be leading to the same place, and does so brilliantly. The three roads are black hole thermodynamics, loop quantum gravity, and string theory.
He starts with a description of cosmological logic -- that we can only know about events in our past light cones, that different observers have different light cones (so know different things), and these light cones are growing (so we continually know more). Putting together a logic that allows reasoning under these conditions results in a very different world from that painted by an all-knowing Platonic style of logic. But this less absolute logic isn't some wooly NewAge "everything is relative, and so everything is true" idea -- if observers have overlapping light cones, they will agree on what they can deduce about the shared region. It transpires that there is relationship between this kind of logic and something the mathematicians have already come up with: a very hairy branch of Category Theory known as Topos Theory. (Now I think I understand why Kauffman makes a passing reference to Category Theory in his Investigations -- given he has worked with Smolin, he could be referring to these ideas.)
Smolin also discusses in detail the ideas of "background independent" theories -- ones where there is no framework of absolute time and space for particles to move against, but rather ones where space and time themselves are integral, evolving, changing parts of the cosmos, and in which there are no static things, only dynamic processes. He touched on this to some degree in his earlier marvelous book, The Life of the Cosmos, but goes into more detail and explanation here. And it is all explained so clearly -- I was particularly enthralled with the description of the relativity principle link between the well-known radiation emitted at a black hole's event horizon, and the weird Unruh radiation seen by an accelerating observer in empty space.
Loop quantum gravity is a theory about how space and time are constructed, but the resulting universe doesn't appear to have much else in it. String theory, on the other hand, has particles, but it is a background dependent theory. Work is beginning on how these might be two components of the final theory. Black hole thermodynamics links everything together, and seems to be providing a clue about the relationship between thermodynamic entropy and information. We end up with the weirdness of the Bekenstein bound (again, touched on in TLotC, but gone into in more depth here) and the holographic principle.
Interwoven with all this hard science are some great little vignettes -- Smolin is one of the key players in the loop quantum gravity strand. These serve to enliven and enrich the scientific ideas, and illuminate the scientific process, rather than detracting from them with arbitrary personal details, as happens all too often in the less well written popular science accounts. (It is interesting that scientist popularisers write about science, whereas journalist popularisers tend to write about the people. My interests lie with the science -- I can get people stories anywhere.)
There are some incredibly deep ideas here, explained brilliantly. All in all, this is a marvellous book. It contrasts nicely with TLotC, complementing that one's grand scope with some really fundamental hard science, told in a refreshing and comprehensible manner.
Smolin lays out some problems he has with modern physics. He doesn't mean problematic results; he is talking about problems with the current process of doing physics that have led to an unprecedented stalling of progress over the last 30 years. When he says "physics", he means fundamental physics of gravity, quantum mechanics, and particle physics, and the attempts to come up with a unified theory, based on 30 years of research in string theory. We can see this focus from the start: in chapter 1, he identifies the five "great unsolved problems of theoretical physics":
He identifies these as the fundamental problems facing modern theoretical physics, and the fact that no progress has been made on them in the 30 years that string theory has held sway. It's not so much that string theory has failed to make progress however; it's the fact that the unprecedented dominance of one school of thought appears to have stifled any progress on alternative approaches that might bear fruit. Smolin himself is a theoretical physicist, a quantum gravity researcher, who has also worked deeply on string theory, so is in an ideal situation of knowing the field from outside, and from within. This gives his critique a lot of credibility.
He starts off with an historical overview of the attempts over the last century or more to unify the various forces of nature, and how this has led to the "standard model" of particle physics, and how gravity somehow never seems to fit. Actually, that last point is not quite true:
Unfortunately, the gravity that popped out was Newtonian gravity, not General Relativity. So that's no good then. (But I am greatly intrigued by the fact that it was Newtonian gravity that popped out. Coincidence? Surely not. So why? Maybe it is "obvious"? Because it is a good approximation? Because the same assumptions underlying Newtonian gravity are there in the approach? Or some other reason? Smolin does not go into this further.)
Despite not answering every single one of my questions(!), there is lots of lovely stuff in here, and very nicely explained. I particularly like the discussion of why physicists like to unify forces (or other general concepts), and what doing so lets you do that you could not do before. It's not intellectual thumb-twiddling: it serves an important and powerful purpose. Smolin works his way from early unifications to some very hairy string theory, keeping a beautifully clear style throughout. Being a physicist, he explains things using other, simpler physical analogies, such as this description of emergent particles:
However, rather than trying to do this for everything, when the going gets too tough, he declines to attempt a bogus explanation, and simply says it's too hard to explain in the space. That's fair enough: this is a pop science book, not a graduate text, after all. Even so, he manages to portray the beauty and excitement of the subject.
After the background explanation to help us understand what string theory explains, or fails to explain, he evaluates its success. The point to remember here is that string theory is about 30 years old now, and never before in theoretical physics has a single approach been employed, to the almost total exclusion of all other approaches, for so long with so little to show for it. Is it fair to evaluate it so soon? Well:
Quite damning. But surely it has given us some partial knowledge? But no. Smolin describes that string theory is actually a whole Vast family of theories. Some of these are known in detail -- they don't work. Some, those that are supposed to give the right answers, are merely conjectured to exist:
It's as if Newton had come up with a whole family of theories of gravity, with all those that could be worked out predicting square or triangular orbits, and with some others merely conjectured to exist, but if they did, they might give ellipses, and all being mediated by invisible angels that were moving much too fast for us ever to be able to see.
So why all the effort in string theory? Is it because there just are no alternatives? Smolin says no. And it's not just loop quantum gravity notions. He points out several observational anomalies, and several interesting and potentially promising alternative approaches. Of course, they might be completely wrong -- but shouldn't they at least be investigated? (He's not here talking about obviously barking "theories" by fringe lunatics; he's talking about suggestions by serious, but non-mainstream, scientists.)
He starts off with some observations about a length scale defined by the cosmological constant (itself a relatively recent observation, currently "explained" by "dark energy"). This length scale is enormous: comparable to the size of the observable universe. We can use a standard trick in physics to convert this to a different kind of unit, by combining with fundamental constants like the speed of light c. (For example, the Planck length is defined by a combination of the fundamental constants h, G, and c.) R/c is roughly the age of the universe: that is not surprising. But c2/R is an acceleration, a tiny acceleration, and its value is potentially interesting:
And the possible Pioneer trajectory anomalies involve a similar acceleration, too. Smolin notes that such observations potentially indicate something completely unexpected by theorists. (And given all the earlier discussion of adding dimensions to get unified theories, I'm amused to note that force decreasing proportional to the distance, rather than distance squared, is what you would expect in a world with only two spatial dimensions.)
Smolin continues with a whistle-stop survey of non-string approaches, including twistors:
He also speculates where there may be a fundamental problem underlying our current conceptions, that old bugaboo, time:
Of course, Smolin is interested in quantum gravity, covered very well in his earlier book, Three Roads to Quantum Gravity. Again, he makes the point that a unified theory should be background-independent in order to incorporate gravity. Maybe starting with background-dependent theories (such as string theory) and then hoping to patch it up later just can't work. (After all, Newtonian gravity pops out of Nordstrom's work with EM, yet the move from Newtonian gravity to General Relativity isn't a fix-up, it's a complete paradigm shift. So maybe even if string theory were to "pop out" of some background-dependent theory, it could turn out to be to the "real" theory as Newtonian gravity is to GR. Not good.)
But it's not just a case of building in the "right" background. Maybe, for a truly fundamental theory of space-time, the space (and time?) should emerge from, not be built into, the theory:
So this is a great description of the current status of fundamental theoretic physics. And, as has been mentioned before, there's a problem: progress has stalled. But why? That is Smolin's topic for the final part of his book. He stops looking at just the science, and looks at how science is done, the social processes involved. He has a nice take on the philosophy of the scientific method: it's not one simple approach like Popperianism, but rather a collection of approaches that have been developed and honed. And these approaches are domain-dependent: what works well in theoretical physics need not work as well in biology, for example, because the properties of the two domains are quite different: the kinds of regularities the approach exploits are different in different domains, so could benefit from different approaches.
These scientific approaches do not exploit only the structure of nature. Just as importantly, they have been carefully honed to exploit and compensate for our peculiarities, particularly our ability to fool ourselves.
So, science requires "both rebellion and respect". Smolin's contention is that currently, the rebellion side of the coin is being stifled in theoretical physics. He cites several reasons, but the main one appears to be the ultra-conservative hiring policies in (US) universities. Rebels don't join big established teams researching conventional (here, string theoretic) approaches; rebels have difficulty getting published; so rebels have difficulty getting grants; so rebels don't get tenure. (Smolin points out how, ironically, some of the founder members of string theory were themselves rebels ploughing a lonely furrow, suffering from precisely this conservatism, until string theory took off and they became respected mainstream "overnight".) Smolin makes some eminently sensible suggestions for how to overcome this problem, for how to ensure that a certain proportion (it does not have to be large) of these rebels, or "seers", can get appointed and given the opportunity to flourish. But how to get anyone in a position to do anything about it to listen? Well, the Perimeter Institute for Theoretical Physics in Ontario, where Smolin is now a faculty member, seems to have some of the right ideas. So there is one place that sanity reigns, hopefully.
This is a great book, with clearly-written fascinating science, and thought-provoking discussions on the way science is (or ought to be) done. Although focussed on fundamental physics, some of the latter discussion is much more widely applicable. Recommended.
In Time Reborn physicist Lee Smolin calls for a major revolution in scientific thought, insisting we embrace the reality of time. Not only is time real, he argues, it is the most fundamental feature of reality. Time Reborn explains how the true nature of time shapes us, our world and the foundations of our universe.
Physics has a curious relationship with time. Most laws are time-reversible; famous ones that aren’t, like the Second Law of Thermodynamics, are approximate and emergent from underlying reversibility; in relativity a universal time cannot be defined consistently, and instead provides us with a static space-time. It’s almost as if physics doesn’t believe time exists.
Smolin is having none of that. For him, time is the fundamental property of the universe, whatever else may emerge. We are not flies caught in the amber of a static space-time; time itself is real:
How can he say this, when all the physical theories seem to point in the other direction? His argument is that those theories are local, and cannot be simply extended to apply to the entire universe. Those theories assume that crucial parts of the process must be outside the region they describe:
This is what Smolin dubs the traditional Newtonian paradigm of doing “physics in a box”. It rests on some underlying assumptions:
If all the possible states of the system are predefined, and the laws under which the system evolves are predefined, then time does seem to be nothing more than an accounting variable: which of those states the laws say the system is currently occupying. What if the possible states of the entire universe aren’t predefined, because its laws aren’t predefined?
Smolin argues that this Newtonian paradigm, powerful as it is, cannot be extended to provide a theory of the entire universe.
It is not a simple task to make a truly universal theory: one that doesn’t just apply to every part of the universe, but that applies to the whole universe at once.
He also argues that our current theories are approximations: physicists pretend that the system inside their box is an isolated system, unaffected by the rest of the universe, and they go to a lot of experimental effort to make that approximation as good as possible. Good approximations make effective theories, but they are only as good as their assumptions (energy ranges, for example). These approximations inevitably break down whenever a theory is extended to encompass the entirety of the universe.
So the timeless nature of isolated, local, approximate theories cannot be taken to imply that the universe itself is timeless.
Having argued that the laws cannot be extended naively to imply a timeless universe, Smolin also argues that there is no reason to assume that the laws themselves are timeless.
Smolin explicitly links this view with his proposal for an evolutionary universe, where a new universe is born in each black hole, with its laws of physics being a mutation of its parent’s laws, as explained in his earlier work, The Life of the Cosmos. Smolin is a Leibniz fan: as well as following Leibniz’ relational view, he uses the Principle of Sufficient Reason: that everything must have a reason or cause, to show that the laws must also have a cause, an explanation. I wonder: do random mutations to the laws of physics obey this principle? (In passing: I was amused to discover that Smolin was introduced to Leibniz’ ideas by Barbour, but has come to rather different conclusions.)This mutational view does not mean that Smolin thinks the laws, despite being changeable by mutation, are set at the beginning of the universe, and fixed thereafter. He gives an example of how a quantum system might be free to choose a result in a situation for which there is no precedent:
Smolin suggests that this principle of precedence could be subject to experimentation, by preparing some genuinely novel quantum states, and measuring them. I’m not sure of the scope of the system’s freedom, however. What about all those more advanced alien races who have already done these experiments? Do those set precedents? Also, the second time a measurement is done, there is only a single precedent from which to select randomly; this seems to imply determinism.
I like his idea of explicable evolving laws; although I still wonder, does a random choice fit with the principle of sufficient reason? And I must admit, I’m not sure why these “principles”, of sufficient reason, of precedence, of whatnot, are allowed to be timeless and universal, when nothing else is. He mentions the need for meta-laws, laws to say how the laws change, but doesn’t go into this as deeply as I wanted. Are the meta-laws timeless? If so, why? If not, what governs their change? I didn’t get the answers here: Smolin refers his book with philosopher Unger, The Singular Universe and the Reality of Time; maybe the answers will be there. For the time being, I have a few new ideas for student projects: growing cellular automata or graphs with rules that depend on configurations, and only deciding on the rule when a new configuration is seen.
Smolin finishes up with more social concerns. He explains that our notion of the fundamental laws of nature as being timeless leads to a damaging distinction between the timeless natural (hence good and right being changeless) and the ephemeral artificial (hence bad and wrong being change). Rather, everything changes and evolves, and we should embrace that fact.
This is a clearly written and thought-provoking book. It makes plain some issues with physics, and its thesis, about time and change, opens up some fascinating possibilities. Well worth the read.