Saturday, 5 March 2016

book review: Time Reborn

Lee Smolin.
Time Reborn: from the crisis in physics to the future of the universe.
Penguin. 2013

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:
[pxiv] The future does not yet exist and is therefore open- We can reasonably infer some predictions, but we cannot predict the future completely. Indeed, the future can produce phenomena that are genuinely novel, in the sense that no knowledge of the past could have anticipated them.
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:
[pxxiii] All the major theories of physics are about parts of the universe—a radio, a ball in flight, a biological cell, the Earth, a galaxy. When we describe a part of the universe, we leave ourselves and our measuring tools outside the system. We leave out our role in selecting or preparing the system we study. We leave out the references that serve to establish where the system is. Most crucially for our concern with the nature of time, we leave out the clocks by which we measure change in the system.
This is what Smolin dubs the traditional Newtonian paradigm of doing “physics in a box”. It rests on some underlying assumptions:
[p44] We should be aware that this powerful method is based on some powerful assumptions. The first is that the configuration space is timeless. It’s assumed that some method can give the whole set of possible configurations ahead of time—that is, before we watch the actual evolution of the system. The possible configurations do not evolve, they simply are. A second assumption is that the forces, and hence the laws the system is subject to are timeless. They don’t change in time, and they also presumably can be specified ahead of the actual study of the system.
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.
[p97] The universe is an entity different in kind from any of its parts. Nor is it simply the sum of its parts. In physics, all properties of objects in the universe are understood in terms of relationships or interactions with other objects. But the universe is the sum of all those relations and, as such, cannot have properties defined by relations to another, similar entity.
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.
[p104] The challenge we face when extending science to a theory of the whole universe is that there can be no static part, because everything in the universe changes, and there is nothing outside of it—nothing that can serve as a background against which to measure the motion of the rest.
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.
[p121] The notion of timeless laws also violates the relational principle that nothing in the universe acts without being acted on. If you choose to except the laws of nature from this principle, seeing them as something outside the universe, you put them outside the realm of rational explanation. To make laws explicable, we must consider them as much a part of the world as the particles they act on. This brings them into the purview of change and causality. They become explicable only when they participate in the dance of change and mutual influence that makes the world a whole.
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:
[pp147-8] These two features of quantum systems let us replace the postulation of timeless laws with the hypothesis that a principle of precedence acts in nature to ensure that the future resembles the past. This principle is sufficient to uphold determinism where its needed but implies that nature, when faced with new properties, can evolve new laws to apply to them.
    Here’s a simple illustration of the operation of the principle of precedence in quantum physics: Consider a quantum process in which a system is prepared and then measured, and assume that this process has occurred many times in the past. This gives you a collection of past outcomes of the measurement: X many times the system said yes to a question, and Y many times it said no. The outcome of any future instance of that process is then picked randomly from the collection of the outcomes of past cases. Now suppose that there’s no precedent, because this system has been prepared with a definite value of a genuinely novel property. Then the outcome of the measurement will be free, in the sense that it is not determined by anything in the past.
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.
[p257] How can we get rid of the conceptual structure of a divided and hierarchical world separating the natural and artificial? To escape this conceptual trap, we need to eliminate the idea that anything is, or can be, timeless. We need to see everything in nature, including ourselves and our technologies, as time-bound and part of a larger, ever evolving system. A world without time is a world with a fixed set of possibilities that cannot be transcended. If, on the other hand, time is real and everything is subject to it, then there is no fixed set of possibilities and no obstacle to the invention of genuinely novel ideas and solutions to problem.
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.

For all my book reviews, see my main website.

No comments:

Post a Comment