the laws of physics must evolve -- 4/26/23

26 Apr 2023

the laws of physics must evolve -- 4/26/23

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Today's selection -- from Time Reborn: From the Crisis in Physics to the Future of the Universe by Lee Smolin. The author argues that a comprehensive understanding of the universe will only be possible if we embrace the idea that the laws of physics can evolve through time:

"For cosmology to progress, physics must abandon the idea that laws are timeless and eternal and embrace instead the idea that they evolve in a real time. This transition is necessary so that we can arrive at a cosmological theory -- one that explains the choices of laws and initial conditions -- that is testable and even vulnerable to falsification by doable experiments. Having (I hope) made the case for this in principle, I'll demonstrate it … by comparing the ability of two theories, one timeless and one with evolving laws, to explain and predict observational results.

"The theory in which laws evolve is called cosmological natural selection, which I developed in the late 1980s and published in 1992. In that paper, I made a few predictions, which could have been falsified in the two decades since but have not been. This of course doesn't prove the theory is correct, but at least I showed that a theory of evolving laws can explain and predict real features of our world.

"For an example of a timeless theory, I will take a version of the multiverse scenario called eternal inflation, proposed in the 1980s by Alexander Vilenkin and Andrei Linde and widely studied since. Eternal inflation comes in different forms, reflecting the fact that some of its hypotheses are adjustable. To make my point, I've chosen one simple form that best fits the 'eternal' because it gives a timeless picture of the multiverse. There are other versions of inflationary multiverses in which time plays a more essential role, and to the extent that these involve a genuine notion of evolving laws they share some aspects of cosmological natural selection.

"One reason that cosmological scenarios with evolving laws succeed in making real predictions is that they don't rely on the anthropic principle -- which states that we can live only in a universe whose laws and initial conditions create a universe hospitable to life -- to connect the multiverse with the universe we observe. One of the tasks of this chapter is to refute the claim that the anthropic principle can play a role in making a theory predictive.

"Cosmological natural selection was the subject of my first book, The Life of the Cosmos, so I will describe it in just enough detail to make clear why evolution of the laws in time leads to a falsifiable explanation of them.

"The basic hypothesis of cosmological natural selection is that universes reproduce by the creation of new universes inside black holes. Our universe is thus a descendant of another universe, born in one of its black holes, and every black hole in our universe is the seed of a new universe. This is a scenario within which we can apply the principles of natural selection.

"The mechanism of natural selection I use is based on the methods of population biology that serve to explain how some parameters governing a system can be selected that make it more complex than it would otherwise be. Applying natural selection to a system to explain its complexity requires the following:

A space for parameters that vary among a population. In biology, these parameters are the genes. In physics, they are the constants of the Standard Model, including the masses of the various elementary particles and the strengths of the basic forces. These parameters form a kind of configuration space for the laws of nature -- a space called the landscape of theories (a term borrowed from population biology, where the space of genes is called the fitness landscape).
A mechanism of reproduction. I adopt an old idea proposed to me by my postdoctoral mentor, Bryce DeWitt, which is that black holes lead to the births of new universes. This is a consequence of the hypothesis that quantum gravity does away with the singularities where time begins and ends -- a hypothesis for which there is good theoretical evidence. Our universe has a lot of black holes, at least a billion billion of them, which suggests a very large population of progeny. We can suppose that our universe is itself part of a line of descent stretching far into the past.
Variation. Natural selection works in part because genes mutate or recombine at random during reproduction, so that the genomes of offspring differ from that of either parent. Analogously, we can hypothesize that each time a new universe is created there is a small random change in the parameters of the laws. Thus we can mark on the landscape the point corresponding to the values of the parameters for that universe. The result is a vast and growing collection of points on the landscape, representing variations in the parameters of the laws across the multiverse.
Differences in fitness. In population biology, the fitness of an individual is a measure of its reproductive success -- that is, how many offspring it produces who thrive long enough to have children of their own. The fitness of a universe is then a measure of how many black holes it spawns. The number turns out to depend sensitively on the parameters. It's not easy to make a black hole; therefore many parameters lead to universes that have no black holes at all. A few parameters lead to universes that have lots of black holes. These universes occupy a very small region of the parameter space. We will assume that these highly fertile regions in the parameter space are islands surrounded by regions of much lower fertility.
Typicality. We also assume that our own universe is a typical member of the population of universes, as that population is after many generations. Thus we can predict that any properties shared by most universes are properties of our own.

"The power of natural selection as a methodology is such that strong conclusions can be drawn from these minimal assumptions. The basic consequence is that after many generations most universes have parameters within the highly fertile regions. It follows that if we change the parameters of a typical universe, the result will most likely be a universe that forms many fewer black holes. Since our universe is typical, this must be true of our universe as well.

"This is a prediction that can be checked indirectly. We already know that many ways of changing the parameters of the Standard Model result in universes without the long-lived stars needed to produce carbon and oxygen. And, remarkably, carbon and oxygen are necessary to cool the gas clouds in which the massive stars that give rise to black holes are formed. Other ways to change the parameters weaken the supernovas that not only lead to black holes but inject energy into the interstellar medium -- energy that drives the collapse of the clouds, thus forming new massive stars. We already know of at least eight ways to slightly change the parameters of the Standard Model that would lead to universes with fewer black holes.

"Cosmological natural selection thus offers a genuine explanation for why the parameters of the Standard Model appear to be tuned for a universe that is filled with long-lived stars that over time have enriched the universe with carbon, oxygen, and other elements needed for the chemical complexity our universe is blessed with. The parameters whose values are thus to a greater or lesser extent explained include the masses of the proton, neutron, electron, and electron neutrino, and the strengths of the four forces. There's a bonus: While the explanation involves maximizing the production of black holes, a consequence is to make the universe hospitable to life.

"Moreover, the hypothesis of cosmological natural selection makes several genuine predictions, which are falsifiable by currently doable observations. One is that the most massive neutron stars cannot be heavier than a certain limit. The idea here is that a supernova leaves behind the exploded star's central region. This core will collapse to either a neutron star or a black hole. Which of the two is produced depends on how much mass the core has; a neutron star can exist only if its mass is below a certain critical value. If cosmological natural selection is right, that critical value should be tuned as low as possible, because the lower it is, the more black holes are made.

"It turns out that there are several possibilities for what neutron stars are made of. One possibility is just neutrons, in which case the critical mass would be rather high, between 2.5 and 2.9 times the mass of the sun. But another possibility is that a neutron star's center contains exotic particles called kaons. This would lower the critical mass compared with the neutrons-only model. The extent of that lowering, though, depends on the details of theoretical modeling; the various models give a critical mass somewhere between 1.6 and 2 times the solar mass.

"If cosmological natural selection is right, we would expect that nature has taken advantage of the possibility of making kaons in the center of neutron stars to lower the critical mass. This could have been accomplished, it turns out, by tuning the mass of the kaon to be light enough; this can be done without affecting the rates of star formation by tuning the mass of the strange quark. When cosmological natural selection was first proposed, the heaviest neutron stars known had masses of less than 1.5 times that of the sun. But recently a neutron star has been observed that has a mass just under twice that of the sun. This would refute cosmological natural selection if the mass of the kaon-neutron stars is at the lower end of the theoretical range, but the theory just manages to fit if the right answer is the upper theoretical estimate, which is also twice the mass of the sun.

"However, there is a less accurately measured neutron star whose mass is estimated to be as much as two and a half times that of the sun. If that finding holds up under more precise measurements, cosmological natural selection will be falsified.

"Another prediction comes from thinking about a surprising feature of the early universe, which is its extreme regularity. The distribution of matter in the early universe is known, from observations of the CMB, to have varied only slightly from place to place. Why was this? Why did the universe not begin with large variations in density? If there were large variations in density, the highly dense regions would have collapsed right away to black holes. If the variations in density were large enough, these so-called primordial black holes would have filled the early universe, leading to a world with many more black holes than our own. This seems to falsify the prediction of cosmological natural selection, which is that there be no way to make a small change in the parameters of the laws of physics to make a universe with more black holes than our own.

"Cosmologists describe the variations in the density of matter by a parameter called the scale of density fluctuations. This is not a parameter of the Standard Model of Particle Physics, but there are models of the early universe that do have adjustable parameters that can increase the density fluctuations, and it's fair to ask whether these are incompatible with cosmological natural selection. In most versions of inflation, there is a parameter that can be increased to raise the level of density fluctuations and thus flood the universe with primordial black holes. But in some of the simplest inflation models, raising this parameter shrinks the universe by sharply limiting the time over which the universe can inflate. The result is a much smaller universe, which, though filled with primordial black holes, has overall many fewer black holes than our own. This means that cosmological natural selection is compatible only with a simple theory of inflation that cannot overproduce primordial black holes. If evidence is found that inflation happened in a way requiring a more complex theory, cosmological natural selection would be ruled out. That there be no such evidence is hence a prediction of cosmological natural selection.

"Of course the right theory of the very early universe may not be inflation, but this example serves to show that cosmological natural selection is vulnerable to disproof by any discovery of a mechanism acting in the early universe that might have produced many primordial black holes.

"Cosmological natural selection is inconceivable outside the context in which time is real. One reason is that all that need be claimed is that our universe has only a relative fitness advantage over universes differing by small changes in the parameters. This is a very weak condition. We needn't assume that the parameters of our universe are the largest possible; there very well might be other parameter choices leading to an even more fertile universe. All the scenario predicts is that they can't be reached by making a small change from the present values.

"Thus the population of universes may be diverse, consisting of a variety of species, each relatively fertile compared with those that are slightly different. The mix of kinds of universes will continually change over time, as new ways to be fertile are discovered by trial and error. This is the way biology works. There are no maximally fit species that persist forever; rather, every era in the history of life is characterized by a different mix of species, all relatively fit. Life never reaches an equilibrium, or ideal state; it is ever evolving. Similarly, whatever laws are typical in the population of universes will change in time, as the population evolves. Were there a final state -- in which, once reached, the mix of universes would stay the same -- time would cease to matter, and we could say that a timeless equilibrium had been reached. But the natural-selection scenario does not assume or imply that. Time is always present in the scenario of cosmological natural selection.

"Moreover, the scenario requires that time be universal as well as real. The population of universes evolves rapidly, growing each time each universe makes a black hole. If we are to deduce predictions from the theory, it must establish how many universes have such-and-such properties at each moment of time. This time must be meaningful not only throughout each universe but across the whole population. So we need a notion of time that gives us a picture of simultaneity within each universe and across that population."