I’m curious to know generally what is the reasoning behind why it wouldn’t survive?
Thanks!. Unfortunately his site seems to be down. If you have his email you might want to inform him.
Oh, just checked again and it seems to be working now. 
First a philosophical point: a singularity is a point where the equations in your theory break down - this is usually an indication that there is something wrong with the theory that will be fixed in a more accurate theory.
In a theory of quantum gravity, we would expect that there are no singularities, as we would expect that a theory of quantum gravity will not break down anywhere (this is the reason why the theorem by Wall is surprising).
That said, you are looking at this the wrong way. It is not that there are reasons why the singularities predicted by the BGV theorem wouldn’t survive in quantum gravity, but instead, there is no proof that these singularities are still singularities in quantum gravity. Does that make sense?
The derivation of the BGV theorem is completely classical general relativity. One needs to provide proof that the singularities it predicts are singularities also in quantum gravity. This proof has not been given.
For example, what if in quantum gravity there is no spacetime at all (i.e. spacetime is an emergent phenomenon)? Then all of the equations used in the derivation of the BGV theorem is invalid in this regime.
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I’m not so sure that does make sense. It seems to me that it would be the other way around. If what we know from direct empirical verification of physical entities is that so far all had a beginning, and BGV strongly indicates a beginning through classical GR all the way down to the initial moment of planck time, it seems, unless there is quite a compelling reason, if there is such a reason, in quantum gravity theory to the contrary, it would be perfectly warranted to conclude, tentatively of course, that there was a beginning.
The derivation has nothing to do with:
That is a philosophical argument that you might want to use, but certainly not a physics argument that has a place in a physics theorem.
Again, I repeat: the BGV theorem cannot probe Planck time - that regime is quantum.
You seem to think that because the BGV theorem is valid in classical GR, then it is likely for it to be valid in quantum gravity. This is not true - there are many examples of classical theorems that becomes invalid in the quantum regime. The validity of classical theorem cannot be extrapolated to the quantum regime in general.
Just to be clear, if you ask me to bet, I would bet that the Universe do have a beginning. It’s just that this belief is not supported by the BGV theorem.
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Yes, maybe I wasn’t clear but I meant the initial moment of planck time would not be included in the BGV theorem.
OK. Thanks for clarifying that.
This I’m not clear on what you mean. If I were making an abductive argument to infer a beginning of the universe, are you saying I couldn’t use the BGV theorem as supporting evidence?
Or are you saying that it couldn’t be used as a physics argument that has a place in a physics theorem similar to the second comment in your previous post?
Then you cannot say anything about the beginning of the Universe with the BGV theorem, as the beginning of the Universe concerns ~Planck time.
Well this definitely is true:
But this statement seems to be true to me as well:
The BGV theorem is not evidence for the beginning of the Universe, as in general theorems in classical physics cannot be extended to the quantum regime. In fact, now that I think about it, it is hard to think of a theorem in classical physics that survives in the quantum regime. Here are some examples of classical theorems that fail in the quantum regime:
- Conservation of energy: fails in the quantum regime
- Conservation of momentum: fails in the quantum regime
- Conservation of number of particles: fails in the quantum regime
- Continuum of energy states for particles in bounded potentials: fails in the quantum regime
- etc
Most classical theorems become invalid in the quantum regime, so why would I think that the BGV theorem (which is classical) remains valid in the quantum regime?
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I guess my response to all of the above, outside of the constraints of methodological naturalism, is that BGV, which accounts for all of past physical reality except for the last moment of plank time, seems to seriously infer a beginning of physical reality.
The fact that there seems to be many philosophers and scientists who seem to agree with that inference, I think would suggest that it is a reasonable inference. The fact that all physical reality that is empirically confirmed has a beginning and a cause is further evidence to support that inference.
Since, as I understand it, we don’t even have a theory of quantum gravity due to the problems that arise when applying quantum field theory to the force of gravity, I would say it’s premature to suggest with any significant degree of confidence what is or isn’t possible at the quantum level in regards to a beginning of physical existence.
So from the perspective of an abductive argument, I would say it’s perfectly warranted to argue that the best explanation of the evidence we currently do understand is that physical existence had a beginning.
“Seriously” why? As of now, I can’t say that it “seriously infer a beginning of physical reality”. I can as non-rigorously say the fact that conservation of energy breaks down in the quantum regime seriously imply that the BGV theorem will also break down in the quantum regime.
Again, this is appeal to authority towards personal beliefs of people. This is non-rigorous. Show me the math.
That is my point. It is premature to suggest with any significant degree of confidence that the BGV theorem survives in the quantum regime. So don’t use it in these arguments.
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How confident can we be that conservation of energy breaks down in the quantum regime is true? There’s no way to empirically confirm that, is there?
What I’ve stated is that from all confirmed empirical evidence and from other evidence that we do understand, like BGV in the context of classical GR that says that in the past direction physical reality heads towards a boundary point, it is warranted to infer a beginning to the universe. Can you demonstrate to me from evidence that is empirically confirmed and well understood that what I’m inferring is unwarranted?
As close to 100% as is possible in the sciences. It’s baked into the uncertainty principle, which is at the heart of quantum mechanics. If it turns out that the conservation of energy is always preserved at the quantum scale, then all of quantum mechanics comes tumbling down. Any empirical confirmation of quantum theory (which is the most well tested theory of physics) is essentially a confirmation of this fact.
Remember, my entire point is not to say that the universe has no beginning, but that we do not know whether the universe has a beginning. It’s easy to support my argument: Because we don’t have a theory of quantum gravity, then it is premature to suggest that the BGV theorem survives in the quantum regime. Our knowledge of this limitation of contemporary physics is well understood and non-controversial. So your inferring, which requires that the BGV theorem survives in the quantum regime is unwarranted.
What about the other way? You now need to show me that, given that the BGV theorem is a classical theorem, and given that in general classical theorems are invalid in the quantum regime, that it is warranted to use it to infer the beginning of the universe, which is necessarily quantum.
Also note:
This “boundary point” is just a point where GR breaks down, nothing more. It says surprisingly little about the beginning of the real Universe, which can be either at that point or even beyond that point - if that can even be well-defined. For example, what happens if the Universe balloons on the other side of that boundary?
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I was about to answer this way, but was beat by the astrophysicist!
Even in GR the conservation of energy is not absolute: I.e. to start:
https://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/
- The BGV theorem applies to classical GR
- The universe at the beginning was not classical
- The BGV theorem is a useful guideline to where the classical description breaks down
- Thus the BGV theorem is not as a great of apologetic for the Christian worldview as some apologists might like to imagine
- And I would add in agreement with @PdotdQ, while Yahweh was the ultimate source of the universe… things like the BGV theorem aren’t part of this belief
For example, Dr. Hugh Ross dubiously (in light of this thread and my conclusion) uses this argument for Yahweh in particular. @PdotdQ, was I accurately understanding a Christian apologists/astrophsicsts use of the BGV theorem:
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Thank you for this post. I really like what you said here:
@Jim, to be clear, I am a Christian, just like you (and so is @pevaquark). I would like it if the BGV theorem supports the assertion that the Universe has a beginning, but it is simply not true. Let us defend Christianity with good science, not misunderstandings of scientific results.
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Correct me if I’m wrong, but from a laypersons perspective how I see it is that there are varying degrees of certainty in science. The empirical, i.e., observational and experimental, is objective and when confirmed is pretty much considered as fact and accepted without debate. The theoretical is subjective and can be and should be disputed until confirmed by direct observation of some kind, or it can be shown that almost all the evidence that can be available is, and that it is decidedly on the side of the theory.
If I’m not mistaken deductive inferences are based on direct evidence and are objective in nature. Inductive inferences are based on an overwhelming amount of indirect evidence in favor of a claim that, though subjective, leaves very little, if any, room for doubt about its truth.
Abductive inferences are based on limited evidence where there is still a substantial amount of information that isn’t available either because a way has yet to be devised for further exploration, or due to it being beyond human limitations, which leaves considerable room for debate as to the best explanation of the limited evidence, and is therefore the most subjective type of reasoning.
Now when you say “as close to 100% as is possible in the sciences,” I find that too ambiguous of a statement to deal with. My limited understanding is that the uncertainty principal and the observation of oscillation patterns of energy fields are involved in positing the temporary violation of the law of conservation of energy.
What I would like to know first is, what other evidence is there besides a principal, i.e., confirmed mathematical formula, and oscillation patterns of energy fields? And correct me if I’m wrong, but I’m assuming besides indirectly, e.g., through oscillation patterns, there is no direct observation of an actual occurrence of the proposed violation, but that it is wholly theoretical at this point in time.
(This may be getting a bit off topic?)
@Jim, it is getting off topic. Regardless, I think you have the wrong view of how scientists operate. Either that or we are using very different languages, so much so that we are talking past each other.
You seem to think that scientists rank the certainty of their theories based on whether they used deductive, inductive, or abductive inference. In reality, what scientists do is use Bayesian inference, which is a mathematically rigorous way to determine the probability of a theory being true given the evidence at hand. The probability that is outputted from Bayesian inference is what I refer to when I say stuff like “as close to 100% as is possible in the sciences”.
Second, the idea of “direct” evidence is somewhat ill-defined in physics. I did not appreciate this fact, but @dga471 and others corrected me in this thread, and I became convinced that this is the case.
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I largely agree but would point out Bayesian reasoning has severe limits. I more understand science as a type of hypothesis testing process.
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I assume that you’re referring to virtual particles here? Since conservation of energy does hold in “standard” (non-relativistic) QM if we’re talking about a time-independent Hamiltonian, see e.g. this review here. Even in the case of virtual particles, this is one way people make sense of this, although it sounds a bit hand-wavy:
In many decays and annihilations, a particle decays into a very high-energy force-carrier particle, which almost immediately decays into low-energy particle. These high-energy, short-lived particles are virtual particles.
The conservation of energy seems to be violated by the apparent existence of these very energetic particles for a very short time. However, according to the above principle, if the time of a process is exceedingly short, then the uncertainty in energy can be very large. Thus, due to the Heisenberg Uncertainty principle, these high-energy force-carrier particles may exist if they are short lived. In a sense, they escape reality’s notice [emphasis mine].
The bottom line is that energy is conserved. The energy of the initial decaying particle and the final decay products is equal. The virtual particles exist for such a short time that they can never be observed.
Correct. But I want to add:
It’s true that at the end energy is conserved, but this statement is not true for short timescale. And also they said:
But, while it’s true that we cannot observe virtual particles like we do other particles, the reality of their existence is corroborated by their effects on other observables, such as the Lamb Shift.
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Yes, we can calculate the Lamb shift by summing the relevant Feynman diagrams. But it’s not clear to me whether physicists regard virtual particles in these diagrams as “real” in the same respect as the observables. One could say that they are just a calculational tool. Famously, before Feynman diagrams became popular, Oppenheimer and others tried to calculate the Lamb shift with traditional perturbation theory techniques and found them to be divergent (as detailed in Chapter 4 of Schwartz’s QFT textbook). However, Feynman’s method was able to get over this, essentially by canceling infinities with other infinities. Even today, this aspect of QFT (renormalization) makes people uneasy. Most people don’t think much about it as QFT can give such accurate predictions. But it seems to me that one could regard this as a placeholder, heuristic theory for a more fundamental one. Which is why I’m not sure if we should regard the virtual particles as existing in the same way as the actual experimental observables.
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