Quantum equations suggest the big bang never happened

[magneticnorth]

Uh oh, my bad , expiated was the wrong word . I "sinned " ! would you believe I meant "explained " . Trust me fellas if I meant to be religious I would have used promulgated

 

[phinds]

Uh oh, my bad , expiated was the wrong word . I "sinned " ! would you believe I meant "explained " . Trust me fellas if I meant to be religious I would have used promulgated

I figured as much ... I was just raggin' you

 

[martinbn]

The term "singularity" means "the place where our model breaks down and we don't know what was going on".

I disagree. Singularity means that in the space-time there are incomplete geodesics (or some variation of that).

It has never, in the context of the big bang, meant a "point in space"

This "point in space" interpretation of "singularity" in reference to the big bang singularity is pop science woo woo.

I completely agree.

 

[magneticnorth]

I just looked up the term Singularity , in the Stanford Encyclopedia [of philosophy ?] The term itself is arbitrary . There are at least 5 different hypotheses on what a Singularity is [ or should be ] ,and those depend on what it is being applied to , which also varies . All having nothing to do with the Big Bang what are youmacalit . I would guess that until there is something more than hypothetical explanations ,the answer to what a singularity is , or do they exist beyond theoretical mathematical equations is , yes, no , yes .

 

[martinbn]

For the question what a singularity is, I recommend this paper.

http://www.itp.uni-hannover.de/~giulini/papers/Geroch_Singularity_AnnPhys1968.pdf

 

[Ken G]

That is indeed an interesting paper, and it makes us wonder if the mathematical structure that is GR is completely consistent with the intuitive notion of a singularity. But if we are to remain in the realm of the intuitive concept, we can note that in all cases so far in the history of physics, every intuitive notion of a singularity has always flagged an incompleteness in the theory. So that may be true in GR as well, regardless of the problem of giving a mathematical meaning to the concept. Some regard it is a bug in any theory that exhibits (in principle) singular behavior in the intuitive sense of "a measurable going to infinity", but I would say that flagging when it will break down is a great feature of any theory, on the grounds that a great theory motivates new observations-- it gives you signposts of where to look for the next great discovery.

 

[martinbn]

My disagreement was with the statement about the break down of the theory. The theory may have to be modified, for example quantized, but as it stands as a theory (as a piece of mathematics) it doesn't break down. There is nothing in the theory that mathematically doesn't make sense, in fact it is completely rigorous. Having strange, from our point of view, properties is not a reason to say that it breaks down. Yet I often here the phrase about the break down of the model. Strangely enough I've never heard anything like that about quantum field theory, even though it is mathematically quite problematic!

 

[Ken G]

Then the issue is in what it means for a scientific theory to "break down." To me, that has nothing to do with mathematics. Newton's theory is perfectly sound mathematics-- yet it "breaks down" when speeds approach c. So to me, breaking down does not mean there are mathematical problems, it means there are physical problems.

 

[Ghost117]

Remember also that Newton was criticized early on, before calculus was made rigorous by mathematicians. And it took Maxwell's mathematics for Faraday's field concept to get accepted... And people thought Dirac was crazy with the liberties he took mathematically etc.

So basically, if something doesn't "break down" mathematically early on, the physicist probably isn't doing his job right lol.

 

[martinbn]

Then the issue is in what it means for a scientific theory to "break down." To me, that has nothing to do with mathematics. Newton's theory is perfectly sound mathematics-- yet it "breaks down" when speeds approach c. So to me, breaking down does not mean there are mathematical problems, it means there are physical problems.

I agree, but there is a differenece when it comes to general relativity. In the case of Newton's theory there are observations and experiments that show the theory "breaks down" when speeds approach c. With singularities in GR there is just the maths and it is perfectly fine. So why the statement that it breaks down!

 

[atyy]

I agree, but there is a differenece when it comes to general relativity. In the case of Newton's theory there are observations and experiments that show the theory "breaks down" when speeds approach c. With singularities in GR there is just the maths and it is perfectly fine. So why the statement that it breaks down!

I don't think classical GR breaks down, only quantum GR.

 

[Ken G]

I agree, but there is a differenece when it comes to general relativity. In the case of Newton's theory there are observations and experiments that show the theory "breaks down" when speeds approach c. With singularities in GR there is just the maths and it is perfectly fine. So why the statement that it breaks down!

It's true that we can't point to an observation and say "GR is wrong there," but when a theory predicts there was an origin in a singularity, yet provides no physics of origination, nor any way to give physical credence to a concept of an infinite kinetic energy density, then we can say the theory is incomplete. We can even wonder how much of the problem traces to the cosmological principle, which is not really part of GR, it is part of making GR solvable. So what "breaks down" is really GR with the cosmological principle. Which raises an interesting question: what can be said about the singularity without adopting the cosmological principle?

 

[martinbn]

It's true that we can't point to an observation and say "GR is wrong there," but when a theory predicts there was an origin in a singularity, yet provides no physics of origination, nor any way to give physical credence to a concept of an infinite kinetic energy density, then we can say the theory is incomplete.

That's the whole point, there was no origin in a singularity. The thing, that's not even defined, and we want to call singularity is not part of space-time. In space-time everything is finte and perfectly well behaved.

We can even wonder how much of the problem traces to the cosmological principle, which is not really part of GR, it is part of making GR solvable. So what "breaks down" is really GR with the cosmological principle. Which raises an interesting question: what can be said about the singularity without adopting the cosmological principle?

The cosmological principle is not the problem, that's what the singularity theorems say. Under physically reasonable conditions Lorenzian manifolds are geodesically incomplete.

 

[Ken G]

That's the whole point, there was no origin in a singularity. The thing, that's not even defined, and we want to call singularity is not part of space-time. In space-time everything is finte and perfectly well behaved.

That's in the mathematics, where the singularity can be regarded as not in the spacetime. That was the point of that nice article you cited. But note that science is not just mathematical theories, it is taking mathematical theories to try to tell a story about what is happening, and how it could be predicted. So if the mathematics can avoid the singularity, the physics cannot-- there are frames that could be populated by hypothetical observers, as we always do in physics to say what is happening, and those frames will get unbounded observables like kinetic energy density and temperature. What's more, they only comprise a finite duration in proper time, with no accounting of t=0 itself. The mathematics can simply not extend to t=0, but in a physical description, that's an incompleteness. It is that incompleteness that causes people to make wholly unsubstantiated statements like "time itself began at t=0," or "time itself began with the Big Bang", and worse, to claim that this claim is part of the Big Bang model, when it certainly is not.

But I think we are basically agreeing here, because we are both saying, you from the mathematical perspective and me from the perspective of observational support and testing, that the t=0 instant is not in the model, is not tested by any observations, and no scientist or GR mathematician really has any basis for making any claims about it whatsoever, other than that it is a kind of flag or milestone worth noticing. In fact, I would say that milestone is the most important aspect of the Big Bang model, but it is not something that the Big Bang model actually models. The unfortunate part is that it is often one of only two things that appear in pop sci renditions of the Big Bang model: the origin point, and the ensuing expansion. The model is actually nothing about the former, and all about the latter, because only the latter is peppered with a vast array of observational evidence and "risky predictions" that proved true.

The cosmological principle is not the problem, that's what the singularity theorems say. Under physically reasonable conditions Lorenzian manifolds are geodesically incomplete.

OK, I was wondering about that. But let me stop you and ask, what do you mean by "physically reasonable conditions", and how do we know those are not the problem, rather than GR itself? Maybe the physically reasonable conditions are wrong, and since we have no observations to say those conditions hold at arbitrarily early times, we cannot then claim that a singularity is a prediction of GR, it is merely among the things we would like to test. I'm sure I don't need to remind you of all the "physically reasonable conditions" in the history of physics that turned out to not test out at all!

 

[PeterDonis]

I don't think classical GR breaks down, only quantum GR.

There's no such thing as "quantum GR". GR is a classical theory. When people talk about GR "breaking down" at singularities, they are talking about a classical theory, not a quantum theory.

 

[PeterDonis]

Under physically reasonable conditions Lorenzian manifolds are geodesically incomplete.

Yes, and the question is whether a geodesically incomplete manifold is physically reasonable. One of the main reasons for pursuing a quantum theory of gravity is that a lot of physicists think the answer to that is "no", and therefore, since the classical theory unavoidably implies geodesic incompleteness, the classical theory cannot be physically reasonable as it stands--more precisely, its domain of validity cannot extend arbitrarily close to a singularity.

 

[phinds]

Yes, and the question is whether a geodesically incomplete manifold is physically reasonable. One of the main reasons for pursuing a quantum theory of gravity is that a lot of physicists think the answer to that is "no", and therefore, since the classical theory unavoidably implies geodesic incompleteness, the classical theory cannot be physically reasonable as it stands--more precisely, its domain of validity cannot extend arbitrarily close to a singularity.

Which to me sounds a lot like "the theory breaks down ... "