#369 Further Reflections on the Sean Carroll Debate

1.1 Classical spacetime began to exist

1.2 Quantum Gravity era began to exist

2.1 Universe began to exist in a low entropy condition

2.2 Quantum Gravity era began to exist

II. Teleological Argument: “The fine-tuning is not due to physical necessity or chance.”

1.0 Fine tuning is not due to Physical Necessity

2.0 Fine tuning is not due to Chance

As my red highlighting reveals, the Boltzmann Brain objection actually occurs at two points in my case: first, in the context of the kalam cosmological argument as an objection to Carroll’s attempt to escape the implication of the second law of thermodynamics that the universe began to exist (2.112); and second, in the context of the teleological argument as an objection to the multiverse defense of the alternative of chance as an explanation of the universe’s fine-tuning (2.1). This double appearance is highly unusual, since one would normally give independent arguments in support of one’s points—not to mention the fact that one’s listeners can easily become confused as to which point is under discussion! That I would make the Boltzmann Brain objection do double duty in my case is a measure of just how truly formidable a problem it is for views unlucky enough to confront it.

Boltzmann Brains and the Thermodynamic Evidence for the Beginning of the Universe

In my opening speech, after explaining how Carroll attempts to avert the beginning of the universe by means of his time-reversal model of the universe, I object:

People who have not read Carroll’s book From Eternity to Here will not realize the sense of desperation that attends Carroll’s effort to solve the problem of the initial low entropy of the universe, which points to its beginning. Throughout the book he is driven back and back, until finally, when “We almost seem to have run out of options,” caught “in a viselike grip of logic, with no remaining avenues to reconcile the evolution of entropy in our observable universe with the reversibility of the fundamental laws of physics” (p. 349), his back apparently to the wall, he comes up with his remarkable model as his only solution to the Boltzmann Brain problem (pp. 361-3). This model represents his best shot at avoiding the problem; if it doesn’t work, other solutions will be implicitly even more implausible.

Carroll recognizes,

Carroll makes no attempt to justify his optimism. So my challenge to Carroll is to give us some explanation of why, on his model, our observation of a low-entropy universe around us is veridical rather than illusory. I claim that his model doesn’t give any answer. The mother universe pictured in Figure 7 of the transcript is a so-called de Sitter space in which fluctuations are said to spawn baby universes, and each of the baby universes expands until it becomes a de Sitter space like the mother universe, spawning more babies. So why think that more of these fluctuations produce genuine low entropy universes rather than much more probable observers who are mere Boltzmann Brains having an illusion of a low entropy universe around them? Carroll might say, “But more and more baby universes with ordinary observers will continue to form!” Ah, but those observers are in other universes (remember: the babies pinch off to become separate spacetimes)! In this, and any, baby universe, the Boltzmann Brains will in the long run vastly dominate as ordinary observers become extinct. And even if it were true that inflationary patches spawning baby universes are more probable than fluctuations forming Boltzmann Brains (itself a moot point), that still fails to explain why we find ourselves surrounded by a low entropy condition.

Notice that I have now put the ball in Carroll’s court. Watch to see if he provides the requested explanation.

Boltzmann Brains and the Chance Explanation of the Fine-Tuning of the Universe

Later in my opening speech I return to the Boltzmann Brain problem when discussing the attempt to explain the fine-tuning of the universe as the result of chance. Proponents of the multiverse hypothesis (like Carroll) try, in effect, to multiply one’s probabilistic resources in order to increase the chances of the existence of finely tuned universes and then appeal to an observer self-selection effect to explain why we shouldn’t be surprised to observe that our universe is fine-tuned. Here is my objection:

Notice that I am willing, for the sake of argument, to concede the highly controversial assumption (i) that a very specific type of World Ensemble (or multiverse) exists. Rather I go after (ii), the observer self-selection effect. The fact that I bypass objections to (i) is a measure of just how vulnerable (ii) is. Multiverse defenders of the chance hypothesis gratuitously assume that either most of the observable universes in the World Ensemble are finely tuned or else that finely tuned universes, though fewer, are for some reason more probable than observable worlds that are not finely tuned. As I noted, not only is there no positive reason to think that either of these assumptions is true, but they appear to be false. This is hugely problematic for Carroll, who appeals to the multiverse to explain fine-tuning as due to chance.

Again, the ball has been put squarely in his court: in order to defend (ii), he needs to show that most observable worlds are finely tuned or that for some reason finely tuned observable worlds are more probable worlds. What will he say?

Carroll’s Response to the Boltzmann Brain Problem

Well, look at his response in his opening speech. He chooses to pass over the Boltzmann Brain problem in the context of the cosmological argument, commenting, “Finally, he makes a big deal about Boltzmann Brains. I’m going to talk about that a little bit later.”1 When we come to his discussion in the context of the teleological argument, the Boltzmann Brain problem comes up in his defense of the multiverse hypothesis (his fourth objection to the teleological argument). Here is what he has to say:

Does this reply answer the challenges I posed in my opening speech? Has Carroll returned service? Clearly not! His reply amounts to mere hand-waving, with a promissory note attached. He just asserts that there are viable multiverse models in which Boltzmann Brains do not dominate observable worlds. No specifics or documentation is given. Notice that he makes no attempt to show that Boltzmann Brains would not dominate in observable worlds according to his model, which is his best shot at solving the low entropy problem. Instead, he appeals to unspecified models which are not even described, much less shown to be plausible.

I’m not sure of the contents of his unpublished paper, but as an illustration of the problems Carroll faces, consider one suggestion he has made in his blog. There Carroll advises that we “need to destroy the universe (or at least the state it’s currently in) to save ourselves from the invasion of the Boltzmann Brains.”2 (That’s pretty strong medicine!) So he speculates that maybe our universe is hung up in a false vacuum state, and before Boltzmann Brains can form, the universe will quantum tunnel to a zero- or negative-energy vacuum state in which Boltzmann Brains cannot arise.

Unfortunately, Carroll requires a good deal of ad hoc physics to ensure that the lower energy vacuum state will be a zero- or negative-energy vacuum. He gives three options: we must either (i) say that the consensus about the mass of the top quark is wrong (which he admits is unlikely) or (ii) appeal to a new physics (an alternative he rejects when it comes to his own calculations) or (iii) adopt a minority view about the probability measure governing worlds. All of these are ad hoc and implausible moves.

Moreover, it’s not enough to destroy our universe. To avoid the Boltzmann Brain problem Carroll has to say that all or most of the baby universes are hung up in this higher vacuum state. Given their independence of one another, that is highly improbable. Even if ours is, why think that the others are? As Robin Collins pointed out in discussions prior to the debate, “one would have to hypothesize a special multiverse that varied the other constants, but was such that it did not vary the [relevant] masses from universe to universe. That is very ad hoc and implausible.”3

Rather than destroy the universe, isn’t a simpler way of avoiding the Boltzmann Brain problem to say that the universe began relatively recently in a low entropy state? This is not the God-of-the-gaps caricature that Carroll criticizes. One avoids the Boltzmann Brain problem by holding that the universe began to exist relatively recently in an initial low entropy condition—precisely what most cosmologists think. God doesn’t even enter the picture at this point.

Counter-Response to Carroll

In my rebuttal speech, in reviewing the kalam cosmological argument, I pointed out Carroll’s failure to defend his model against the Boltzmann Brain objection:

Remember that his model represents his best effort to come to grips with the initial low entropy condition of the universe; that is its entire raison d’être. But now he declines to defend it.

When it comes to the teleological argument, I sought to advance the discussion by exposing a further weakness of the multiverse explanation regarding Boltzmann Brains:

This is a really devastating objection, which I owe, as noted, to discussions with Robin Collins. The point is really quite obvious. Suppose Carroll could somehow show that in finely tuned universes (like ours) ordinary observers will dominate, perhaps due to the ad hoc physics appealed to above. That still wouldn’t rescue the multiverse hypothesis from the Boltzmann Brain problem! For what about all the worlds that are not finely tuned? Because these universes have no ordinary observers at all and are incomprehensibly more plenteous than finely tuned universes, the multiverse as a whole will be dominated by Boltzmann Brains. Carroll’s confident assurances that there are viable multiverse models that escape the Boltzmann Brain problem do nothing to show how to solve this problem.

Carroll’s Counter-Response

So when we come to Carroll’s rebuttal speech, we’re still waiting to see how he will meet the challenges I posed both to his specific model and to the World Ensemble hypothesis in general. With respect to the kalam cosmological argument, again he backs away from a defense of his model against the Boltzmann Brain objection: “On a more specific level we talked about my model. Again, I’m not trying to defend my model; I’m the first one to say that it has problems.” He claims that its problems are not the ones I mention; but we’re still waiting to hear how it solves the Boltzmann Brain problem. If his model can’t solve that problem, then it loses its raison d’être, since it fails as a plausible explanation of the initial low entropy condition of the universe.

With respect to the teleological argument, Carroll again resorts to handwaving rather than give us the specifics:

Carroll is quite right that I did not respond to his point that multiverse hypotheses do not predict that everything will happen. That is simply because I have never labored under that misimpression and it plays no role in my argument. Quite the contrary, different universes in the World Ensemble will be more or less conducive to embodied, conscious observers based on the degree of fine-tuning which their fundamental constants and quantities exhibit. And the objection is that no explanation has been given why the World Ensemble, considered as a whole, will be not be dominated by universes featuring Boltzmann Brains rather than respectable observers like us. You can avoid this problem by adopting a special measure of probability favoring those relatively rare universes with ordinary observers, but then you owe us some explanation for this move.

Lest we lose the forest for the trees, recall that what is at stake here is the appeal to an observer self-selection effect in order to justify why we observe a low-entropy, finely tuned universe around us. For this appeal to work, the chances have to be such that a random observer will likely be an ordinary observer rather than a Boltzmann Brain. But Carroll has never shown that. Carroll’s point that Boltzmann Brains are forms of life only underlines Robin Collins’ claim that what is at stake in the fine-tuning argument is not the frequency of life-permitting worlds but worlds permitting embodied, conscious agents. Some explanation is needed why this latter sort of world exists, and here appeal a self-selection effect is, as I explained, vacuous.

Closing Statements

So it seems to me that I am quite justified when, in my closing statement, with reference to the Boltzmann Brain problem, I complain,

That sums it up rather nicely. Perhaps Carroll can answer these problems; but in the context of this debate he has not done so. He has resorted to handwaving rather than evidence and has just asked us to trust him.

In his final speech Carroll says with respect to his model, which he declined to defend, “[Craig] says that there should be more Boltzmann Brains than ordinary observers. I again explain why that’s not true because it’s a model dependent statement. In this particular model it turns out to be easier to make a universe than to make a brain. That’s a selling point of the model.” Really? Then that ought to have been defended. In his book Carroll says at most that it’s “plausible.” Even that is being optimistic. Carroll needs to respond specifically and substantively to explain how his model will avert the objections I lodged against it and why his model will, in fact, yield more ordinary observers than Boltzmann Brains.

Summing Up

Obviously much more was said in the debate concerning the evidence of thermodynamics for the beginning of the universe and concerning the hypothesis of chance as an explanation of fine-tuning. But I hope that by focusing on the Boltzmann Brain objection, I have, at least, illustrated how to assess the argumentation in a debate. You isolate an issue and then pursue that thread through the sequence of speeches (and through the answers to questions afterwards), seeing how the response and counter-response proceeds.

When we do that exercise with respect to the Boltzmann Brain objection, it becomes apparent how weak Carroll’s response is. With respect to the thermodynamic evidence for the second premiss of the kalam cosmological argument, Carroll’s attempt to avert the beginning of the universe by means of his time-reversal model fails to provide a convincing answer to the Boltzmann Brain problem. There are two reasons for this. First, since de Sitter space spawns Boltzmann Brains as well as baby universes and each baby universe becomes a de Sitter space itself, all the ordinary observers in any universe will eventually die out and Boltzmann Brains will dominate in the long run. Second, even if in finely tuned universes like ours, ordinary observers dominate, the fact remains that in the worlds which are not fine-tuned, Boltzmann Brains will vastly dominate, so that the World Ensemble will be dominated by universes whose observers, if any, are Boltzmann Brains. This result strongly disconfirms Carroll’s model as an attempt to undercut the evidence of thermodynamics for the beginning of the universe.

With respect to the attempt to rescue the alternative of chance as an explanation of the fine-tuning of the universe by appeal to the World Ensemble hypothesis, Carroll merely asks us to trust him that there are viable models that can avoid the Boltzmann Brain problem. Personally, I have no confidence in Carroll’s assurances because he said something similar with respect to viable models of a beginningless universe, and when one gets down to specifics all of those models turn out to be very problematic and implausible. But never mind; the more important point is that in a debate context assertion is no substitute for argument. Carroll needs to gives us the specific evidence for his claims if his assertions are to have any weight