I suppose this goes to what one consider to be “Real”. Maybe you would be interested in reading Gordon Kane’s opinion of the matter here.
Before going off too far off tangent, @dga471, I wanted to know what you think of the charge that the BGV theorem is not evidence for the beginning of the Universe. I think the clearest formulation of it is @pevaquark’s:
Fair enough, although I wonder if it may be a matter of different scientists’ preferences. But as I understand it, Bayesian inferences are not fundamentally different from other types of inferences, just a way of putting a figure on an inference to provide a bit more clarity, and possibly a more practical way to be able to update once new info is at hand.
But whatever the type of inference, as far as I can tell, they’re all based on background knowledge and evidence. So in order to defend a position, the background knowledge, which would include any assumptions, and evidence should be made available. I don’t know if it’s just me, but I still don’t know whether or not that’s been done yet.
I have to say I’m still not convinced about the BGV not being useful in arguing for a beginning. It certainly isn’t evidence against it. And to point out that there are both philosophers and scientists, including Vilenkin himself, who seem to think it infers a beginning is not, as I understand it, appealing to authority, simply pointing out that as experts their opinions need to be considered as to whether or not they add any additional weight to the validity of the inference.
And considering the amount of uncertainty surrounding the quantum realm I would say using it to argue against the inference is weak at best considering the weight of the evidence in favor of the inference, i.e., all empirically confirmed physical entities with beginnings, expansion of the universe, 2nd law of thermodynamics, and BGV, i.e., all of history involving physical existence heading towards a singularity only possibly excluding the first moment of planck time, not to mention the philosophical reasons for a beginning.
I don’t know how else to convince you. This is the time when you have to answer the question:
Give me something rigorous to sink my teeth into, not just claims like the BGV theorem “strongly indicates” or “seriously infer” a beginning. You need to explain why “strongly indicates”? Why "seriously infer? What are your priors? How do you update your probabilities? If you do this in a convincing enough manner, this is a serious topic that can be published in a physics journal, so I am really looking forward to it.
Also, this:
Is definitely an appeal to authority. Saying that an expert’s opinion needs to be considered is the definition of appeal to authority. This is not really good reasoning. I can also cite many other experts that disagree with your experts.
Note that I do not claim that the “uncertainty” of quantum gravity means that the BGV theorem does not survive in the quantum regime. I argue that this very uncertainty is the reason that it is premature to say that the BGV theorem survives in the quantum regime.
I think this is the fundamental difference in our reasoning. I’m starting from what is known and arguing from that position. You’re starting from what isn’t known and arguing from that position. So it seems like the old Aristotelian vs Plato debate. Are ideas primary and experience secondary? Or is it the other way around? I think until we agree on that we’ll have to agree to disagree. 
Unfortunately I disagree even with that. I start with what are known:
These four things are knowns, and I argue from there.
Jim, I think what @PdotdQ is saying, is that using the BGV theorem to conclude anything about the beginning of the universe is not correct, as you are misapplying the theory. It is no different than saying, “Because many people in America drive gas guzzling cars, it must be the case that many people all over the world drive gas guzzling cars!” Such an inference is illegitimate because what is true in America might not be true elsewhere in the world.
So, based on similar reasons as what you, @pevaquark, Aron Wall and others have pointed out, I agree that using the BGV theorem alone is not a good argument for arguing that the Universe had a beginning. This could be the impression left by many Christian apologists including Craig himself in some of his debates. However, Craig’s advocacy of the Kalam Cosmological Argument (KCA) goes back much further than the BGV theorem, and his contemporary version of it goes far wider than simply citing the BGV theorem. I think Matthew is not representing it fully. It is certainly not a naive argument for the existence of God.
The most recent version of Craig’s KCA is (I think) the essay in The Blackwell Companion to Natural Theology, coauthored with James Sinclair. Here, they marshall a wide set of arguments to argue for the finitude of the past. The first two are philosophical arguments: 1) Arguing that the existence an actual infinite is impossible, and 2) Arguing that the formation of an actual infinite through successive addition is impossible. I think these philosophical arguments have been developed by Craig since the 80s.
The third argument is the scientific one. Here, Craig & Sinclair’s argument is quite comprehensive, starting from the history of Big Bang theory. (Perhaps it would be interesting for you to read this complete article, as someone trained in GR and cosmology.) Before the 1920s, most physicists assumed that the universe was eternal and static. After Big Bang theory caught on, this no longer was the case - we can say that the universe is about 13 billion years old, and it included a singularity in the past, as C&S argue, due to the Hawking-Penrose theorems. C&S then talk about the exceptions to the H-P theorems, including eternal inflation, for which BVG is relevant. However, they also talk (with considerable technical detail) about quantum gravity models, including Pre-Big Bang inflation (Veneziano & Gasperini), ekpyrotic/cycling (Steinhardt & Turok), loop quantum gravity models, and Hartle-Hawking. For each of these models they argue that a beginning is required.
After C&S establish that a beginning is required, and that things which begin to exist have to be caused, they also talk about what kind of causal agent could have caused that. They argue that the First Cause must be beginningless, changeless, immaterial, timeless, spaceless, personal and enormously powerful. It is not merely saying “God did it”.
What do I think about all of these arguments? I think that C&S have done almost as much as is possible with current cosmological knowledge to argue that the universe must have had a beginning. BVG is spoken of a lot, but it is far from being the only thing they utilize. From an experimentalist point of view, none of these quantum gravity models have experimental corroboration yet, so I think that general scientific caution and skepticism is warranted. However, I think making a philosophical argument for the existence of God based on these comprehensive arguments is legitimate, even if I think they are not very strong arguments because we have very little knowledge about quantum gravity. (Assuming what C&S say about the science is correct. I am not sufficiently trained in GR & cosmology to judge that.)
Here by the way is a publicly available reply by Sinclair which basically summarizes the C&S paper:
As you know, Dr. Craig and I coauthored an essay on the kalam argument for the Blackwell Companion to Natural Theology . The price of this volume is steep, and the number of folks who have seen it is limited. Even for those who have, the essay can be somewhat daunting. This may have been the issue for “Arizona Atheist” (mere speculation here). It should be apparent to someone who read the piece that my contribution is an exhaustive discussion of two singularity theorems (Hawking-Penrose and Borde-Vilenkin-Guth) and all the exceptions to the theorems. This latter comment is important; let me repeat it. The piece exhaustively covers all the exceptions to the theorems.
Nowhere (in our essay) do we employ a supposed quote from Alex Vilenkin to the effect that his theorem (by itself) demonstrates a beginning to the universe and merely rely on it as some type of argument-from-authority. That said, there is a particularly famous (or infamous) comment that Vilenkin made in his 2006 book Many Worlds in One to the effect that his theorem DOES prove that the universe has a beginning. Dr Craig sometimes mentions this in his public talks (it doesn’t appear in our essay). But in certain settings where brevity is necessary, or one does not wish to lose the listener, one cannot engage in a million caveats. The full case should and has been made in rigorous works of scholarship.
Thank you for this great review. I am now curious about what the C&S essay talks about. We agree that:
But like I said, I would love it if there is actually a physics argument for the beginning of time. However, from the short reply by Sinclair that you posted, I think I will be left wanting. It seems that he requires stuff like the A-theory of time, which is not liked by relativists like myself. I am also dubious that they covered all of the exceptions to the singularity theorems - given that generalizations to the HP theorems are still an active area of research.
And even with all that, there is still the issue that no one knows what exactly goes on in regimes where quantum gravity is important. My bet is that time is emergent and is not even a good variable at really high energies, so all of this discussion is moot.
I too share your concerns that Craig’s insistence on the A-theory could be more problematic to his argument than the cosmological discussion. But again, this is also an area where Craig has done considerable work, writing at least four dedicated academic books to the philosophy of time. Edward Feser also defends the A-theory extensively in his latest book on philosophy of science, Aristotle’s Revenge, which I’m currently reading through (but haven’t got to that section yet). I think that book would also be something I’d be interested to see you read and discuss.
This is where I would take exception. However, first I think we might be talking past each other again. When I said I’m starting from what’s known, maybe confirmed would have been a better choice of words. Correct me if I’m wrong, but I have the impression that classical physics is not in dispute within objectively confirmed reality. By objective I mean what isn’t just at present theoretical, but already confirmed empirically.
Subjective reality is what I would refer to as the theoretical. It can be argued for, but it cannot, or has not yet been empirically confirmed. So what I mean by arguing from what isn’t known is arguing from a theoretical position, which, although may be true, is tentatively a subjective claim and might or might not be true, rather than something based on what is already objectively confirmed empirically in some manner. Does that make sense?
Can you clarify what you mean by taking exception to the statement that:
Do you disagree with this statement? Or do you say that this is not an “objectively confirmed” statement?
Classical physics is in dispute with “objectively confirmed” reality if what you mean by “objectively confirmed” is “empirical confirmation”. According to classical physics, an oven in your house will explode with arbitrarily high energy.
The latter.
Yes, objectively confirmed would be direct empirical confirmation.
Interesting. So if I understand correctly, there are some problems reconciling GTR with objectively confirmed reality that rely on solutions at the quantum level?
As we do not have access to any experiment in the quantum gravity regime, of course there is no “objectively confirmed reality” that disagrees with GTR in particular. But in general, classical physics has shown itself to be in dispute with what you call “objectively confirmed reality”. I only need the second statement to say that:
Is an “objectively confirmed” statement.
I think you would need to define how you’re using the term classical then. It seems to me that calling GTR classical in this case is not representative of what you seem to be referring to as “classical physics.”
Classical = not quantum, no quantization. This is not how I define “classical”, but how it is defined in physics. There is no question that GTR is a classical theory.
Perhaps you are confused with what I mean by “in general”? In mathematics and physics, “in general” means that “it is true for all”, in opposition with “in particular”, which is only “true for this particular case”.
So, the statement that GTR is a classical theory (which is a true statement based on its mathematical formalism), along with this “empirically confirmed” statement that:
Classical physics theorems cannot in general be extrapolated to the quantum regime
Means that we do not know whether GTR theorems like the BVG theorem can be extrapolated to the quantum regime, as I have been arguing with you the entire time.
Contrast this with your position, that the BGV theorem can say something about the quantum regime, which requires either that
GTR theorems in general can be extrapolated to the quantum regime
or the weaker statement:
The BVG theorem in particular can be extrapolated to the quantum regime
Neither of these two statements have been “objectively/empirically confirmed” at all!
Edit: Also, let me add that according to your “empirical confirmation” definition of “objectively confirmed”, the BVG theorem itself has not been objectively confirmed!
Well, I’m not sure that what you’re saying follows from my argument. What I’m arguing is that BGV infers a beginning, which I believe it does. Now whether or not it can be extrapolated to the quantum regime is unknown. So since it’s unknown, to say that BGV theorem infers a beginning based on what “is known” is perfectly warranted as far as I can tell. If you want to disagree based on what “isn’t known” that’s OK, but then, as far as I can tell, you’re taking the Platonic view of ideas as primary.
Good point. From what I read it is a deductive proof, so if that’s correct in that sense it is logically confirmed and could fall into objectively confirmed if that definition were to be broadened to include logically confirmed as well as empirical confirmed. Would that be an acceptable move in your opinion?
It doesn’t, as has been explained to you multiple times. It just shows that the equations of GTR breaks down in an inflating Universe. This is all you get if you do not want to extrapolate to the quantum regime.
Remember, when I said that the energy-time uncertainty principle in quantum mechanics is:
You do not like this, because
and asked for
The BVG theorem is in the exact same shoes. Worst, actually, as quantum theory is much better experimentally tested than GTR.
Regardless, your claim that
is false unless you broadened your definition of “objectively confirmed” to include “logically confirmed”. Let’s start there first. This opens a huge can of worms, as there are many theoretical and mathematical arguments for the incompleteness of GTR.
Edit: by the last paragraph, I specifically mean that with your new, broadened version of “objectively confirmed”:
I can answer with positive your previous question:
Before we go any further, since there are multiple ways of making inferences, what type of inference are you referring to?
Bayesian.