marcus said:
I don't know that Chalnoth BELIEVES in a physical multiverse. If he did, that would not be a scientific belief, in my view. But I don't know his personal beliefs so let's leave the personal issue out.
I think belief is just the wrong way to think about this altogether. Either it's true or it isn't, and what we believe has no bearing whatsoever on the truth. All that we can do is gather evidence and from the evidence deduce what is likely and is not likely to be true.
So I deliberately choose not to say that I believe anything at all about the nature of the universe. There are merely degrees of confidence.
marcus said:
My point is simply that it is naive to assume that our present (inadequate) idea of physical law is a good place to stop and declare "Multiverse"
It's this sort of emotional objection to multiverse ideas that I dislike the most among scientists (as opposed to bad arguments from non-scientists, which can be far, far worse...). There are two things to point out:
1. Perhaps the question is a bad question altogether. To draw an analogy with another science, biology, for a long time scientists tried to answer the question, "What was the intention of this?" in regards to some feature of life. For example, what is the purpose of a bird's wing, or the red color of blood? For some things about life, these answers at least seem obvious. For others they aren't that obvious. And for still other things about life, the question is downright perplexing (e.g. what is the purpose of the appendix?). But what Darwin showed, and what has been confirmed by centuries of observation and experimentation, was that this was simply a bad question to ask in the first place: there is neither intention nor purpose to life, and we simply cannot understand how life acts until we first understand this.
If we do find that the answer to a particular question is indeed that it is a bad question in the first place, then we must accept that. And, in fact, recognizing that it was a bad question can open the door for a cornucopia of new possibilities for understanding better how the universe works.
2. It is often simply false that a multiverse idea is remotely similar to "giving up". Imagine, for example, the cosmological constant. Here is a very simple model for the cosmological constant which could be tested experimentally. Imagine that we have a theory which predicts that the cosmological constant can take on a huge number of values, but not every value. Let's say that the values within a factor of two of the measured density fraction, for example, are 0.332, 0.654, 0.736, 0.991, 1.02, 1.35. This hypothetical theory predicts it could be one of these, or some other number much larger or smaller, but it cannot ever be anything in between. If we go out and measure and find that the cosmological constant is 0.710, then we have falsified the theory. Alternatively, if we go out and find that it is precisely 0.736, then we gain confidence that this is, in fact, the right explanation.
So simply throwing out multiverse ideas before we even get started because you don't like them is being exceedingly premature. If this were merely a matter of getting annoyed at one particular multiverse proponent's arguments, then that's fine. There are many bad arguments thrown out for many different kinds of things. But I strenuously object to the tarring of an entire class of ideas with nasty epithets simply because you don't like them.Anyway, my personal take here is that it is extraordinarily likely, given what we know today about cosmology, quantum mechanics, and high-energy physics, that we live in a multiverse.
From cosmology, we can arrive at the conclusion that the whole universe is likely to be far, far larger than our observable part of it from a multitude of angles. For instance, grand unified theories predict the existence of heavy, stable magnetic monopoles which would, in the classical big bang theory, vastly outnumber the normal matter that we know and love. This is because in the big bang theory, heavier things precipitate earlier-on, and the earlier something is produced, the more of them are produced. But trivial observation demonstrates that magnetic monopoles are at least exceedingly rare in our observable universe, if they exist at all. A simple solution to this is inflation, where an exceedingly rapid expansion rate early-on, when these magnetic monopoles would have been produced, spreads the monopoles so far across space that we'd be lucky to have even a single one within our observable universe.
There are many other arguments that you can make, from different directions, but in the end it is very, very likely that the whole universe is much, much bigger than our observable portion of it. This leads to a multiverse of perhaps the simplest and most likely sort.
Then, from quantum mechanics, the wave function of quantum mechanics unambiguously predicts a multiverse of a multitude of outcomes from a single interaction. We can get rid of this multiverse if we simply assume it is not there, but at best this adds no predictive power to the theory. At worst it removes predictive power. So the quantum multiverse is extraordinarily likely. As a side comment, a number of cosmologists are looking into the idea that "the universe is big" multiverse and the quantum multiverse are one and the same thing.
From the high-energy physics side of things, we are finding that some aspects of high-energy physics that we experience are due to accidents in the past, known as spontaneous symmetry breaking events. If we combine these accidental events with the apparent fact that the universe is much, much larger than the part of it we can observe, we arrive at the conclusion that in parts of the universe far beyond our horizon, the laws of physics are likely to be rather different. We don't yet know exactly how different things can be, as we don't know all of the spontaneous symmetry breaking events that occurred. But from what we do know, it is very likely that they are different if we go far enough away.
This leads us to what is perhaps the most important aspect of the multiverse: that it is rather likely that there are some aspects of reality which are accidental rather than inevitable. Some people don't like this, and try to come up with rationalizations to avoid it. But I have yet to see anybody come up with a rational argument against it.
