0 Introduction
Lots of people regard the multiverse as a strange, aberrant hypothesis, proposed only by those who have been reading too much science fiction. Theists often point to atheists using the multiverse theory to explain fine-tuning as an absurd consequence of atheism, indicative of the desperate lengths atheists will go to reject God. I think this is a mistake—we have quite overwhelming evidence for the multiverse. There is, of course, evidence from physics for the multiverse—both the many worlds theory and string theory predict a vast ensemble of universes. But I won’t review it here because I’m not a physicist. Instead, I’ll provide two largely philosophical arguments for a multiverse and explain why most of the criticisms of the multiverse are silly.
1 Fine-tuning
The constants of the universe are finely tuned. This means that if you tweaked the constants—which are the strengths of the values of the laws of physics—slightly, no life, no star, and no complex structure of any sort would have formed. This is a startling coincidence—our laws fall in a hugely narrow region of the possibility space that happens to be the only region that could have agents like us. How narrow? Well, as the Stanford Encyclopedia of Philosophy says:
The cosmological constant characterizes the energy density ρVof the vacuum. On theoretical grounds, outlined in Section 5 of this article, one would expect it to be larger than its actual value by an immense number of magnitudes. (Depending on the specific assumptions made, the discrepancy is between 10^50 and 10^123.) However, only values of ρV a few order of magnitude larger than the actual value are compatible with the formation of galaxies (Weinberg 1987; Barnes 2012: sect. 4.6; Schellekens 2013: sect. 3). This constraint is relaxed if one considers universes with different baryon-to-photon ratios and different values of the number Q (discussed below), which quantifies density fluctuations in the early universe (Adams 2019: sect. 4.2)
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The initial entropy of the universe must have been exceedingly low. According to Penrose, universes “resembling the one in which we live” (2004: 343) populate only one part in 10^10^123 of the available phase space volume.
Thus, the consensus among experts in the field is that there are about a dozen finely-tuned features of the universe, each of which falls in an extremely narrow range that happens to be required for life. Given how narrow the range is, it calls out for an explanation. What could the explanation be? Various people have given various answers.
There’s no explanation. If it hadn’t been finely tuned, we wouldn’t be here, so it had to be finely tuned. The problem with this is that even if your existence requires X that doesn’t mean X had to happen. The fact that you exist gives you evidence that your parents didn’t use effective contraception during all their sex acts, because if they’d done that, it’d be less likely they’d have had you.
There’s no explanation. Lots of events are super improbable, like a coin coming up tails, heads, heads, tails, tails, heads, heads, heads, tails, heads, tails, tails. Still because it had to turn out some way, you shouldn’t assume there’s some other explanation. The problem with this response is that we should look for the best explanation of some event. There’s no good explanation of why a coin would be rigged to get that particular sequence (it’s no likelier than any other sequence of 12 coin flips, so any theory that explains it will be just as improbable as the event in question). But when there is a good explanation of some improbable event, we take that explanation to be very likely. For example, because getting a bunch of royal flushes in poker is unlikely but well explained if a person is cheating, if a person gets a bunch of royal flushes we think they’re cheating. So then if there’s any theory that explains fine-tuning that’s much likelier than an event with 1/10^10^123 probability then that theory should get a massive boost from fine-tuning.
There might be more fundamental laws that require finely tuned constants. There might be some more fundamental force that requires that the universe started in a low entropy state. The problem with this explanation is that it’s equally likely that some more fundamental law would result in any sequence of states. Therefore, the theory that there’s a fundamental law that fixes the laws to be finely tuned has an absurdly low likelihood, because it’s just as unlikely as any of the other 10^10^123 theories that would fix the constants to be in some range. It would be like explaining why a person gets a bunch of royal flushes by appealing to a more fundamental law that requires they get royal flushes—why that law as opposed to some other law? Because there could be a more fundamental law that necessitates any sequence of initial conditions and constants, it’s ridiculously unlikely that it would result in the narrow slice of them that happens to be finely tuned.
There’s a multiverse. On this view, we find ourselves in a life-permitting universe because there are a bunch of universes and some of them will be life-permitting. We find ourselves in one of those because those are the only ones we could find ourselves in, by definition. This is a pretty good explanation. (People like Goff think this commits the inverse gambler’s fallacy but are wrong).
God exists and finely tunes the constants. This is also a pretty good theory. But God would obviously create a vast multiverse. It’s good to create a happy person and God has no limits on how many people he could make. So why stop at one universe? He wouldn’t stop—he’d create a vast multiverse given the value it could bring about. But that means that only two good explanations of fine-tuning both entail a multiverse. Thus, there’s probably a multiverse.
2 Anthropics
Suppose that a coin was flipped yesterday. If it came up heads, one person was created in a white room. If it came up tails, a million people were all created in white rooms. You wake up in a white room. What odds should you give to tails vs heads?
I think the answer is that heads is a million times less likely than tails. Because tails means a million times as many people will exist, it’s a million times likelier that you’d exist. This might seem a bit weird, but it’s supported by extremely powerful arguments that I’ve given here, here, here, here, here, here, here, here, and here.
Suppose you grant this broad picture, that more people existing is likelier. Well then of course you should believe in a multiverse. A multiverse means that there will be more people! If a theory that predicts a million times as many people is a million times better, all else equal, a theory positing an infinite multiverse is infinitely better than one positing just a finite multiverse.
So, therefore, given that you exist, you should think that more people exist. But given that more people exist, you should think there’s almost definitely a multiverse.
3 The completely terrible arguments against the multiverse
A bunch of popular science writers have written articles denouncing the multiverse. Their arguments are completely lame. Jamie Carter, for instance, asserts:
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