Thursday, March 06, 2014

On the Craig vs. Carroll Debate

Hi, folks. 

A number of you have emailed me about the Craig vs. Carroll debate, so I have decided to relent and link to it. Here you go. Thanks to Justin Schieber for the pointer to the link. Other relevant links include Carroll's post on his thoughts after the debate, Jeff Lowder's post on it, and Craig's pre-debate remarks.

Since I find most interesting the stuff on the Borde-Guth-Vilenkin theorem, I thought I'd post a quote from Carroll on Craig's appeal to it:
The second premise of the Kalam argument is that the universe began to exist. Which may even be true! But we certainly don’t know, or even have strong reasons to think one way or the other. Craig thinks we do have a strong reason, the Borde-Guth-Vilenkin theorem. So I explained what every physicist who has thought about the issue understands: that the real world is governed by quantum mechanics, and the BGV theorem assumes a classical spacetime, so it says nothing definitive about what actually happens in the universe; it is only a guideline to when our classical description breaks down. Indeed, I quoted a stronger theorem, the “Quantum Eternity Theorem” (QET) — under conventional quantum mechanics, any universe with a non-zero energy and a time-independent Hamiltonian will necessarily last forever toward both the past and the future. For convenience I quoted my own paper as a reference, although I’m surely not the first to figure it out; it’s a fairly trivial result once you think about it. (The Hartle-Hawking model is not eternal to the past, which is fine because they imagine a universe with zero energy. In that situation time is an approximation rather than fundamental in any case — that’s the “problem of time” in quantum gravity.)  
Sadly, Craig never responded to my point about the QET. Instead, he emphasized another “theorem” in a paper by Aron Wall. This is a great paper, well worth reading — but it doesn’t say what Craig wants it to say, which I was only able to check after the debate. Wall (like BGV) proves theorems that apply to semiclassical gravity (classical spacetime with propagating quantum fields — see comment from Aron below), and then speculates “the results may hold in full quantum gravity” and “there is a reasonable possibility that the Penrose singularity theorem can be proven even in the context of full quantum gravity.” As good as the paper is, proving a theorem in the semiclassical case and then opining that it is probably extendable to the full quantum gravity case does not actually represent a “theorem” about the quantum case. And in fact I think it’s highly unlikely to be extendable in the sense Craig wants it to be, since the QET says that’s impossible (unless the universe has zero energy or a time-dependent Hamiltonian, in which case it’s easy to avoid eternity). But I had never seen Wall’s paper before, and Craig didn’t give a precise statement of the purported theorem, only the above quote about “reasonable possibility”; as a result I didn’t know the range of applicability of the “theorem” or its assumptions, so chose not to talk about it rather than making guesses. That was probably a strategic mistake on my part. 
While I’m lingering over my mistakes, I made a related one, when Craig emphasized a recent paper by Anthony Aguirre and John Kehayias. They examined the “emergent universe” scenario of George Ellis and Roy Maartens, in which the universe is in a quasi-static pre-Big-Bang state infinitely far into the past. Aguirre and Kehayias showed that such behavior is unstable; you can’t last in a quasi-static state for half of eternity and then start evolving. Personally, I didn’t think this was worth talking about; I completely agree that it’s unstable, I never promoted or defended that particular model, and I just didn’t see the relevance. But he kept bringing it up. Only after the debate did it dawn on me that he takes the specific behavior of that model as representative of any model that has a quantum-gravity regime (the easiest way out of the “beginning” supposedly predicted by the BGV theorem). That’s completely false. Most models with a quantum-gravity phase are nothing like the emergent universe; typically the quantum part of the evolution is temporary, and is surrounded on both sides by classical spacetime. But that’s so false that I didn’t even pick up that WLC was presuming it, so I never responded.

17 comments:

Mike Almeida said...

How does it matter to Craig's position that the universe did not have a beginning that we can date? That's how he runs the Kalam, but it doesn't seem required for the argument. Let the universe be coeternal with God and caused by God, if it is insisted that it had no beginning and will have no end. It still has a cause. Nothing new in the suggestion that coeternal things might be dependent.

exapologist said...

Hi Mike,

Whether the universe had a beginning is certainly relevant to the kalam argument's soundness, although (as you say) no originating divine cause of the universe is compatible with a sustaining divine cause.

Best,
EA

Mike Almeida said...

Right, so the Kalam need not assume that the universe began to exist. Either the universe began to exist or it has always existed. In either case, it has a cause. The alternative is urge that it is a necessarily existing object, which I don't think anyone wants to claim. I'm part of the universe; I'm pretty sure I'm not necessarily existing (pace Williamson, Linsky and Zalta)

exapologist said...

Hi Mike,

Hmm. I guess I don't see how we can call it the kalam cosmological argument if we change its nature to that of the Leibnizian cosmological argument. I'm not really interested in discussing that point, though. I am interested in your larger point. Fwiw, I'm R&Ring a paper on Craig's recent defense of the Leibnizian cosmological argument, and I'm writing up a couple of papers on material causality (which have implications for the prospects for the Leibnizian version). Versions of them are in sketch form in several of my posts. Comments welcome!

All best,
EA

Mike Almeida said...

If that's too Leibnizian, there is another line open to Craig. Carroll thinks the universe came into existence from empty space (actually, not really empty, it has particles or virtual particles in it) in which random changes give rise to a universe. This is supposed to happen infinitely back to the past and into the future. Our universe will be "empty" in this sense too, as the expansion of the universe continues. And it will randomly give rise to another universe. Craig's Kalam works perfectly here; Craig will argue that it can't happen, given the actual infinities involved (it ought ot be observed that Carroll's objection to the Big bang is roughly that relativity predicts infinite density, infinite rate of expansion, etc. Carroll sounds like Craig on this.). Though in this case it is narrowly applied to Carroll's favorite cosmology.
I don't know Craig's position on our universe U coming into existence. I don't know whether he thinks it's a necessary truth that, if U exists, it came into existence. Or whether U might exist eternally, or what. I was, in the OP, trying to give him room to maneuver under the assumption that, possibly, U is eternal.
I'll try to send you some comments, if you send the papers. Can't guarantee it will be soon, though.

exapologist said...

Thanks, Mike!

I'm still not sure I follow your point. Is the idea that the universe might be eternal in the sense that part of its career consisted in a quiescent state? He seems to reject this possibility in at least his early writings on the argument (although Wes Morriston has critiqued him on this point).

Best,
EA

Mike Almeida said...

We're apparently working from different data sets. I've heard him endorse this as his preferred view. That's the only reason I mentioned it.

exapologist said...

Fascinating! If you can recall the source, I'd be grateful to learn of it. The early sources I had in mind are here and here.

Mike Almeida said...

Thanks for the links. I think we're talking past one another. I was saying that Carroll seems to endorse the quiescent view. I was urging that the Kalam argument works well against such a view. Are we saying the same thing?

exapologist said...

Hi Mike,

Now I'm even more confused. In the papers I linked to, Craig had in mind a universe with no events whatsoever occurring. Carroll doesn't endorse a quiescent universe hypothesis of that sort. Perhaps, then, you were instead alluding to your earlier point, viz., that Craig's a priori arguments against infinite traversals, if sound, would entail that any scientific account of the origin of our universe (and thereby Carroll's account as well) will require either an absolute beginning to the physical world or a state of absolute quiescence (at least the latter of which would require a miracle)?

Best,
EA

Mike Almeida said...

Well, we don't want you more confused! Maybe I misinterpreted Carroll's remarks in his interviews with Robert Kuhn. These are online. Look especially at his remarks on whether the universe had a beginning.

exapologist said...

Thanks for the pointer, Mike. I'll give it a look.

Best,
EA

exapologist said...

Hi Mike,

I just gave the video you mentioned a look. I can see how you might have thought that he said the universe arose from an absolutely quiescent state (viz. his remark about empty space), but near the end of the segment he says that empty space is not quiescent, but has positive temperature (and thus activity). He thus basically proposes the standard view of Krauss, Hawking, et al. about this, and not the sort of absolutely quiescent universe that Craig had in mind in his earlier writings.

Best,
EA

Mike Almeida said...

Right, I think I said as much in my initial post on this (above). He thinks the universe contains some particles or some virtual particles, etc. It is not strictly empty or strictly quiescent. But this is a smallish point for my main claim which is that the Kalam should work just as well against views on which we have an infinite series of universes.

exapologist said...

Ok, good. So it looks like your point is the one I attributed to you in my 9:40pm comment. I think you're right that if any of Craig's a priori arguments against the existence and traversability of actual infinites work, then they would work against views on which there is an infinite series of universes. For what it's worth, though I'm not persuaded that any of Craig's arguments in that vein work. For some reason, I thought you agreed with me about that. Perhaps I'm wrong, though.

In any case, I don't want to get too far from the main point of the original post, which was that Carroll seems to have called Craig's appeal to the BGV theorem into serious doubt. I'd be interested to hear your thoughts (and those of others as well) about that point.

Best,
EA

Mike Almeida said...

Right. I said there was another way to apply the Kalam, given Carroll's actual views. It touches upon the criticism in the OP in this way: whatever Carroll's criticism of Craig, the Kalam still applies to his views. I concede that it is not directly to the point of the OP. But when was the last time a thread was directly ot the point of the OP? :)

exapologist said...

;-)

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