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JuanFlorencio
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So, do you think, Pound Coolish, that science has nothing to do with logic? Is science for you a kind of illogical thinking, full of absurdities?
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inocente
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And you!
I moved this to the front for simplicity (hopefully). The big bang theory predicts an initial singularity, in which all mass and energy in the universe was compressed to an infinitely dense point. Now momentum is mass * velocity, and since there is no space in the singularity, it must be zero since nothing has anywhere it can move. In the extreme conditions of zero space our equations don’t work, we cannot make any predictions, and that’s what’s meant by a breakdown in the physics.
The argument is that since we can’t make any empirical observations of the singularity and our equations don’t work, we have no basis on which to assume physical causality.
It doesn’t. The argument is that in physical terms we are effectively talking of two different worlds, since in the singularity the physics must be different in the absence of any spacial dimension. So we can say that our world is caused by the singularity, but we cannot state with any level of certainty that the singularity had a cause (unless we could prove that causality is a logical necessity in all possible worlds).
btw I’m no expert but although the argument seems to follow from the theory, it highlights something very wrong with the big bang theory itself - the theory fails to make predictions in the infinitely small point of the singularity, and infinities are in any case anathema in physics (presumably the theory’s failure to incorporate quantum mechanics is the root problem).
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JuanFlorencio
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Let me take this first, because any way it seems to be related to the core of the thread’s topic: which prerogatives does logic have when we talk about “the singularity” of the Big Bang?
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Charlemagne_III
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The mystery of creation baffles and will continue to baffle forever.
The notion, if anyone holds it, that science will eventually answer all questions is simply absurd.
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inocente
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I’m no expert in GR, but a gravitational singularity is zero space, infinite density, infinite gravity, so none of the laws of normal spacetime apply and determinism breaks down. There can’t be any differences between places as there are no places. There can’t be any events since nothing can move. Not sure what logic could find to work on in such a situation.
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Charlemagne_III
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God knows if you don’t!
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JuanFlorencio
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Every day that passes I feel more and more interested on the mathematical understanding of these theories. I can understand when it is said that a given variable tends to infinity or to zero (or to another value) if another variable tends to zero, or to another value, or when it grows without limit; but for the moment I attribute no meaning to expressions like “zero space”, “infinite gravity”, “infinite density” or others like these.
Concerning logic: you have suggested that we would need to prove the logical necessity of causality in order to affirm that the singularity has a cause. But I understand by logical necessity one that is derived from the application of our logic (the logic that everybody might come to know, if he wishes). So, when you say “Not sure what logic could find to work on in such a situation”, I ask you: to start with, might it be one that includes the principle of non-contradiction?
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Bradski
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I’ve just been re-reading The Goldilocks Enigma by Paul Davies. Lots of mind spinning stuff in there. In relation to events occurring at a very short time after the Big Bang he mentions Planck time and length. Anything shorter than either of these two measurements (and they are VERY short) and we’re in areas where it is understood that all current theories break down.
As regards the cause of the BB, he makes a very good point. An effect needs a cause and the one proceeds the other in time. If time did not exist before the BB, then causality is rendered meaningless.
He also talks about the probability of the difference between space and time becoming blurred at certain points. You really need to lie down for a while in a darkened room after reading some of this stuff.
And maybe the most telling quote in the book isn’t his. It’s by David Hume (quoting Philo):
"If we survey a ship, what an exalted idea must we form of the ingenuity of the carpenter who framed so complicated, useful, and beautiful a machine? And what surprise must we feel, when we find him a stupid mechanic, who imitated others, and copied an art, which, through a long succession of ages, after multiplied trials, mistakes, corrections, deliberations, and controversies, had been gradually improving?
Many worlds might have been botched and bungled, throughout an eternity, ere this system was struck out; much labour lost, many fruitless trials made; and a slow, but continued improvement carried on during infinite ages in the art of world-making. In such subjects, who can determine, where the truth; nay, who can conjecture where the probability lies, amidst a great number of hypotheses which may be proposed, and a still greater which may be imagined?"
So much for omnipotence…
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thinkandmull
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Stephen Hawkings idea of material duality, that the gravity and even the matter in the universe equals zero, is even further from reality than simply saying that the world came from nothing by nothing
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Charlemagne_III
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Mechanics are not necessarily stupid. They see how to put things together, and how to repair them when they break.
The long trail of mechanics who generation after generation succeeded in contributing something to the beautiful ship that at last emerges are to be congratulated for their work, not vilified as dunces. So what if they are copiers of other people’s works. The are at least smart enough to know how to copy. Some people can’t even do that.
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JuanFlorencio
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Does he say how it is that all current theories break down at such small lengths and times? Is this anticipation just a consequence of the mathematical models, or does it have an experimental base?
If there was no “before the BB”…
It sounds like if the universe started, then it was uncaused; but once there was an expanding universe, everything must have a cause, and we look for it; or that it is nonsense if we are looking for a worldly cause of the Big Bang.
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hecd2
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Let me see if I can help here. First, let me caution that this is a fiendishly difficult topic, not just from the scientific point of view.
So, naively, if you run the expansion of the universe backwards you get to a time when all the mass-energy of the universe is contained in a mathematical point and this is called a singularity, because the density at such a point would be infinite. There are two ways of looking at this singularity:
- The density really is infinite (whatever that means), and because the conditions are so extreme, then none of our models of reality apply
- The space shrinking to a point idea is simplistic, and in reality the density, while unimaginably vast, was finite, because, for example, space is quantised and cannot be smaller than a Planck volume (4x10^-105 m^3). And the conditions are so extreme, that none of our models of reality apply.
So what about this idea that conditions get so extreme that none of our models apply. Well, if you remember James Clerk Maxwell famously unified the theories of electricity and magnetism to develop electromagnetic theory. In the 1960s, a good theory to unify electromagnetism and the weak force called electroweak theory was proposed by Glashow, Weinberg and Salam, and its predictions have been successfully tested by experiments in particle accelerators. Physicists think that the fundamental forces start out as a single force in the extreme conditions of the Big Bang and separate into individual forces by a process known as symmetry breaking as the Universe cools. In the same way that electricity and magnetism are unified in electromagnetic theory, and electromagentism and the weak force are unified in electroweak theory, so the strong force (for which we have a good standalone theory, quantum chromodynamics) and gravity (for which we have a good standalone theory, General Relativity) should also be able to be unified with electroweak theory.
The next to try to unify is the strong force, and theories that unify electroweak and the strong force (called Grand Unified Theories or GUTs) exist. The problem is that there are many of them and we don’t have an obvious way of testing their predictions and selecting between them because that requires energies on the GUT scale which is about 10^16 GeV (for reference the Large Hadron Collider operates at energies some 10^11 times below GUT energies).
Then, and closer to the Big Bang is a theory that unifies the favoured GUT theory (which we don’t have) with a theory of quantum gravity (which we don’t have - at the moment quantum theory and General Relativity work superbly in their domans but give grotesquely incompatible results under certain conditions - which tells us that one or both are incomplete). This unification is called a ToE (Theory of Everything) and while such theories exist we can’t currently distinguish between them because we can’t test them. Thew energies are even higher than GUTs.
Remember GUTs and ToEs are theories for finite but extreme conditions and we’re already in deep trouble.
Finally we get to the Big Bang itself and things get even more extreme - energies and densities are higher still and possibly infinite (whatever that means). The concepts of time and space become blurred together, various conservation laws break down, time possibly takes on a different or no meaning.
So that’s what people mean when they claim that the laws of physics break down as we approach and reach the Big Bang. Hope that helped. Perhaps in another post I’ll say what I think about the OP.
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thinkandmull
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Here is an interesting discussion I had:
physicsforums.com/threads/the-universe-adds-up-to-zero.826377/
The basic point is that Hawking’s theory does not take into account dark matter, which has some real properties.
Take the example of wind. What causes the wind to move? Whatever “pushes” the wind would have to be pushed by some else, and on the infinity unless there was God to create it. An infinity of motions would be an infinite past, an uncountable infinity if there ever was one. So it could never arise at a present, which is really regardless of what Stephen Hawking is saying. Shrinking the universe to a single point doesn’t any these questions and even as a possibility it has not definitely been proven.
physicsforums.com/threads/the-universe-adds-up-to-zero.826377/
The basic point is that Hawking’s theory does not take into account dark matter, which has some real properties.
Take the example of wind. What causes the wind to move? Whatever “pushes” the wind would have to be pushed by some else, and on the infinity unless there was God to create it. An infinity of motions would be an infinite past, an uncountable infinity if there ever was one. So it could never arise at a present, which is really regardless of what Stephen Hawking is saying. Shrinking the universe to a single point doesn’t any these questions and even as a possibility it has not definitely been proven.
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Bradski
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He says that there’s a rule of thumb that says that if a theory has no inbuilt unit of length then there is no way of knowing how far we can scale it down without it failing. If it has units, then they will fail at distances and times less than the Planck length and Planck time.
Hey, who wouldn’t know that…
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Bradski
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Now that I can understand. It’s all the trees waving their branches around.
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hecd2
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So to the OP. This is a tricky question and I’m going to declare a spoiler before I start: We don’t know.
Let’'s take the current Universe. From a time as short as we can imagine after the instant of the Big Bang itself (say a Planck time - 10^-43s) causality applies. We might not have empirically supported theories for the processes while the fundamental forces are unified and the densities, temperatures and energies of interactions were as high as they were, but I think physicists accept the metaphysical proposition that the principle of causality applied. One thing led to another even if we don’t know how the processes worked. In the current Uinverse the principle of causality seems to apply so far as we can see. It has wobbled a couple of times in the face of experiments such as the delayed choice quantum eraser, but most people (including me) think that causality is not violated by those experiments for reasons that I could go into if you want.
What about quantum events that seem not to have an immediate cause such as radioactive decay? Such things might not have proximate causes but they require the substrate of the universe and operate according to certain statistical rules.
So our metaphysics, built as it is on sense experience, dictates that the principle of causality applies through all time and space. But we can’t necesssarily extrapolate this conclusion back to the Big Bang itself. There are at least two possible alternatives:
The hypotheses in 2) can be past-eternal or temporally unbounded. A past-eternal model where our universe is caused by processes in some greater always-existing big-U Universe, or a model in which the universe appears out of a timeless state, essentially uncaused, does not seem to violate the metaphysical principle of causality in this universe.
The bottom line: I don’t think anyone can make the confident claim that the principle of causality applies to the start of our universe itself and, if it does, whether or not the cause can lie in some greater Universe which is past eternal or time unbounded. On the other hand, the opposing claim that the universe needs no cause is equally unwarranted, in my view. We don’t know, I don’t know how we could know, and we might never know. That doesn’t mean that it isn’t fun or instructive to build hypotheses for the larger pre-existing Universe, but it’s salutary to remember that these are little more than speculative toy models.
Let’'s take the current Universe. From a time as short as we can imagine after the instant of the Big Bang itself (say a Planck time - 10^-43s) causality applies. We might not have empirically supported theories for the processes while the fundamental forces are unified and the densities, temperatures and energies of interactions were as high as they were, but I think physicists accept the metaphysical proposition that the principle of causality applied. One thing led to another even if we don’t know how the processes worked. In the current Uinverse the principle of causality seems to apply so far as we can see. It has wobbled a couple of times in the face of experiments such as the delayed choice quantum eraser, but most people (including me) think that causality is not violated by those experiments for reasons that I could go into if you want.
What about quantum events that seem not to have an immediate cause such as radioactive decay? Such things might not have proximate causes but they require the substrate of the universe and operate according to certain statistical rules.
So our metaphysics, built as it is on sense experience, dictates that the principle of causality applies through all time and space. But we can’t necesssarily extrapolate this conclusion back to the Big Bang itself. There are at least two possible alternatives:
- If there is an actual singularity at the Big Bang then a) we can’t ever look beyond it and b) the conditions are such that not just the physics but the metaphysics absolutely breaks down because there is no such thing as time and therefore no such thing as causality.
The hypotheses in 2) can be past-eternal or temporally unbounded. A past-eternal model where our universe is caused by processes in some greater always-existing big-U Universe, or a model in which the universe appears out of a timeless state, essentially uncaused, does not seem to violate the metaphysical principle of causality in this universe.
The bottom line: I don’t think anyone can make the confident claim that the principle of causality applies to the start of our universe itself and, if it does, whether or not the cause can lie in some greater Universe which is past eternal or time unbounded. On the other hand, the opposing claim that the universe needs no cause is equally unwarranted, in my view. We don’t know, I don’t know how we could know, and we might never know. That doesn’t mean that it isn’t fun or instructive to build hypotheses for the larger pre-existing Universe, but it’s salutary to remember that these are little more than speculative toy models.
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JuanFlorencio
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Thank you, hecd2, you know the business!
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JuanFlorencio
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I would be interested on your description and interpretation of the experiments, hecd2!
As I see it, mathematical models do not represent causality, but are only representations of a regularity that might have been observed. The notion of causality is something we use to develop a physical interpretation of the model. However, I would like to have your insight on this too: What is your opinion, hecd2?
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hecd2
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Follow it up then. Recommendations: “A First Course in General Realtivity” by Bernard Schutz, “Special, General and Cosmological Relativity” by Wolfgang Rindler, “Gravitation and Inertia” by Ciufolini and Wheeler; and the bible of modern GR (an undergraduate elective or postgraduate text) is Misner, Thorne and Wheeler’s " Gravitation".
Nor does anyone else!
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JuanFlorencio
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