Big Bangs and Boltzmann Brains

This past week, I watched a fascinating TED Talk from cosmologist Sean Carroll. The question he poses is one that has puzzled scientists, philosophers, and theologians for millennia: Why does our orderly universe exist at all? The second law of thermodynamics states that entropy (disorder) tends to increase over time. This is intuitive to most of us; it’s easy to break an egg, but difficult to put a broken egg back together, because there are far more arrangements in which the pieces of the egg can be shattered than unbroken. As a result, things tend toward more chaos and disorder. But why was the universe ever orderly to begin with?

Carroll does a great job demolishing the explanation which I’ve always favored, that our universe is just a fluke. Even though individual atoms move in hard-to-predict ways we can nevertheless predict the behavior of macroscopic objects. For example, the molecules in the air and ground are constantly jostling one another, moving in every possible direction. But we don’t expect them to spontaneously arrange themselves into a car, because the probability of the molecules simultaneously moving into the correct position is extremely unlikely. But over a long enough period of time (vastly longer than the age of the universe) it will eventually happen by pure chance.

This same argument could be scaled up to the universe itself. Quantum mechanics indicates that particles are constantly popping in and out of existence at random. If you wait for an unimaginably long time, all of the particles that make up our universe will spontaneously pop into existence out of the nothingness and assemble themselves in an orderly way, in a random quantum fluctuation. Many have speculated that such a fluctuation caused the Big Bang. The length of time we would have to wait for such an event to occur is called the Poincarre Recurrence Time, and is estimated to be on the order of 10^10^10^10^10^1.1 years. I’ve always been fond of this idea, and it seems easy to invoke the Anthropic Principle here: Conscious observers will only be able to marvel over their own existence during those rare periods of time when the universe is in a low-entropy state by pure chance. But as Carroll points out (using an argument originally made by Ludwig Boltzmann over a hundred years ago), that isn’t necessarily the case. It would be far easier for an individual brain to form from a quantum fluctuation, than an entire universe with hundreds of billions of galaxies. If our existence was a fluke of an infinitesimally unlikely quantum fluctuation, we would predict that we should be lonely Boltzmann Brains floating through empty space, rather than inhabitants of an enormously complex universe. Since we do indeed live in a complex universe, Carroll therefore concludes that our universe is not merely a random quantum fluctuation.

But if the Big Bang wasn’t merely a freak random occurrence, what caused it? Unfortunately, neither physicists nor philosophers have a solid answer for this question. Carroll suspects that there must be a multiverse, in which unknown laws of nature occasionally give rise to low-entropy universes such as our own. He cites the work of Lee Smolin, who proposed the idea of the fecund universe, in which a Big Bang occurs at the singularity of a black hole and a new universe is created. This would solve the Boltzmann Brain paradox, because universes would constantly be spawning new universes through the laws of nature, but randomly-occurring Boltzmann Brains would still be unfathomably rare.

Another possibility is that our understanding of entropy is incomplete; perhaps our universe is indeed a random quantum fluctuation, but for reasons we don’t understand it’s easier for entire universes which can eventually produce conscious observers to randomly fluctuate into existence, than it is for Boltzmann Brains to do so. Perhaps the laws that govern the creation of universes tend to give rise to things like our own universe rather than Boltzmann Brains. Perhaps our universe is merely a holographic representation of a two-dimensional universe with different laws of physics than those we observe, and therefore entropy doesn’t apply at all. Perhaps our world is a simulated reality, and the simulators find complex worlds more interesting than lone Boltzmann Brains. Or perhaps we are indeed Boltzmann Brains, and for some reason don’t realize it.

Whatever the reason we live in an orderly universe, our understanding of physics is slowly making it possible to weigh the different theories to see which are the most likely. As our understanding of cosmology continues to improve, perhaps we will one day be able to understand what (if anything) caused the Big Bang. If Carroll’s multiverse theory turns out to be correct, perhaps it will vindicate the Hindu concept of cyclical creation and destruction: Universes are born, die, and are eventually reborn in different forms.

Watch the short version of Sean Carroll’s talk here. If you’re interested in the full hour-long lecture, watch it here.

Rebuttal to the Simulation Hypothesis

According to Nick Bostrom’s simulation hypothesis, every universe’s inhabitants would be equally likely to be living in a simulation, even if they were running simulations of their own. This leads to the uncomfortable conclusion that our universe is much more likely to be one of a huge number of simulations, rather than the one parent universe.

From a logical standpoint, this argument makes sense to me. But I’m always eager to poke holes in philosophical arguments, so here’s my best rebuttal as devil’s advocate. It doesn’t directly attack the logic of Bostrom’s philosophy; rather, it creates a probabilistic argument that we are NOT a simulation.

Let’s assume that simulations can be “turned off” by the parent universe at any time. Perhaps the inhabitants decide that the simulation is no longer needed for whatever reason, or perhaps the simulation is accidentally destroyed, or perhaps they are in a simulation themselves which is turned off by their parent universe. If this is the case, it would break the simulation chain. If Simulation A was the parent of Simulation B, which was the parent of the Simulation C, which was the parent of Simulation D, the inhabitants of Simulation A would be able to break this chain and destroy all of the simulations in this chain by turning off Simulation B.

The simulation hypothesis concludes that we are in a simulation in all probability, and that every universe is equally likely to be a simulation. This means that the universe that begat ours is also probably a simulation, as is the universe that begat our parent universe. If this is the case, it would be very likely that our own universe is merely one node in a huge chain of parent universes.

But this creates an interesting question. If any of those universes could break the chain at any time by turning off their simulation, the probability that not a single one of them would do so must be extraordinarily low. This strongly suggests that we are not in a simulation.

There are a couple of responses to this argument which I can foresee, so allow me to preemptively address them. Some might invoke the Anthropic Principle. It doesn’t make sense to marvel at the unlikelihood of our own existence, they will reason, because if our universe had been turned off we wouldn’t be here to speculate about it. In my opinion, this is a flawed application of the Anthropic Principle because there is another plausible explanation for our existence: Our simulated universe hasn’t been “turned off” by any of its parents because they don’t exist. We are the original universe.

So we have two possible explanations for our own universe. Either we are in a simulation, and are here due to the infinitesimally unlikely whims of an unimaginably vast chain of parent universes…or we are simply not a simulation. If these are the two possibilities, the latter seems much more likely from this probabilistic standpoint. It also has the advantage of surviving Ockham’s Razor.

What do you think? Is my probabilistic argument for our actual reality as strong as Bostrom’s argument for our simulated reality? What flaws do you see in my logic?

Resolving Fermi's Paradox

If intelligent life exists elsewhere in the universe, where is everyone? Enrico Fermi’s 1950 question has elicited many theories in the sixty years since. If advanced life was as commonplace as Frank Drake or Carl Sagan believed, it seems unlikely that not a single extraterrestrial civilization would have made itself known to us.

Some, like futurist Ray Kurzweil, believe that this means that we are alone in the universe: the product of a series of astronomically unlikely occurrences. Kurzweil reasons that any advanced civilization would use all the matter of its own planet for computing, then radiate outward from their home world. The fact that no one seems to have done this already is supposedly evidence that we are alone.

I think this is a remarkably anthropocentric view of the universe. Both Fermi’s Paradox and Kurzweil’s reasoning employ the same faulty assumption that extraterrestrial life would act the same way that humans might. How can we possibly guess the goals of an extraterrestrial civilization when we don’t even know how our own civilization will act in the future? Why should we assume that extraterrestrials will be expansionists, desiring to conquer (or even communicate with) the rest of the universe? This is a remarkably human trait to project onto other potential civilizations. It’s entirely possible that extraterrestrial life does exist, but is simply not interested in interacting with us, content with not venturing far from their home worlds. Any civilization capable of interstellar travel or communication will almost certainly have access to anything it wants at home, and perhaps would have no particular reason to be interested in a pale blue dot populated by bipedal apes.

There is another, darker explanation for the lack of contact with extraterrestrials. If advanced civilizations tend to eliminate any inferior civilizations with which they come in contact, then we will have no evidence of them until immediately before our extinction, if at all. The fact that we have not yet been eliminated, then, simply means that no other advanced civilizations are aware of our presence yet. We can employ the Anthropic Principle here: We would not be here to speculate about Fermi’s Paradox if we had been discovered by another civilization.

These are the explanations for Fermi’s Paradox which seem the most plausible to me. The least plausible explanation is the one Kurzweil suggests: That we are alone. Even if advanced life developed in only one in a billion solar systems, there would be trillions of civilizations in the vastness of our universe. Finding them, however, may be much more difficult. Perhaps a truly intelligent civilization would leave no evidence of its existence at all.