• Kenosha Kid
    3.2k
    I had planned to write this follow-up to this thread, and drafted some of it, before I knew Roger Penrose was nominated for the Nobel Prize for Physics. There's some overlap, and I had cited some of his work, which is fortuitous.

    In the above thread, I laid out a defence of determinism against Copenhagenists based on the following:

    • most fundamental processes are reversible
    • the universe of special relativity should properly be thought of as a 4D space
    • relativistic wave equations have both retarded and advanced solutions
    • the proper boundary conditions for a particle are its creation and annihilation
    • only the conjugations of retarded and advanced wavefunctions are (quite literally) real (the transactional interpretation of QM)
    • while the trajectories of a single body treated this way are numerous, the rest of the universe will generally act to eliminate them (self-consistent histories)

    The above really only applies to the electromagnetic universe. There are some phenomena, such as some nuclear decays, that are irreversible. These I won't treat here. I don't actually have any clue about how I'd go about that, and I'm not particularly averse to the universe being only mostly time-symmetric, which it is in the standard model.

    Thermodynamics and cosmology are more interesting because we associate an arrow of time to them: entropy increases with time; the universe expands with time. Penrose is one of many theorists who have concluded that they are the same arrow of time. A steady state universe in thermal equilibrium everywhere would have neither arrow; if that universe suddenly expanded, it would have both arrows aligned.

    Just a quick point on thermodynamics specifically: all of the laws of thermodynamics are derivable from standard quantum mechanics. (Indeed, this is how we learned statistical mechanics at my university.) And all of standard QM deal with reversible processes as described in my previous thread (creation, annihilation, and movement). So the fundamental mechanics involved in thermodynamics have no such arrow of time, rather it is the starting conditions of systems that drives entropy increase.

    The ultimate ordered starting condition is the singularity at the big bang, wherein all matter now in the universe was contained in a single point. As the universe expanded (the cosmological arrow of time), the distribution of matter expanded with it (the thermodynamic arrow of time).

    There is a similar but backwards event in the universe: the black hole. A black hole is also a singularity, but all of matter is drawn in toward it rather than expands away from it when viewed in the normal way. The big question of black hole thermodynamics is: what happens to entropy? Bringing distributed matter to a single point is an ordering process: it decreases entropy! However, any entropy-increasing byproduct of this ordering process also cannot escape the black hole. Some options:

    • black holes reduce the amount of matter in the universe, therefore increase the number of possible configurations of matter in the universe by increasing the volume to density ratio;
    • black holes retain the matter they order, but increase the total entropy of the universe as it does so;
    • black holes pay the entropy back in the future;
    • black holes reverse the arrow of time.

    The first would breach a lot of conservation laws. Hawking put forward the idea that black holes obey conservation laws over multiverses, not individual universes. I put this in for completeness.

    Hawking radiation and the holographic principle might implement the second. As the mass of the black hole increases, the event horizon gets larger, and the amount it radiates also increases. The event horizon is considered a measure of the entropy of the black hole, while radiating energy through work is also an entropy-increasing process. However, the amount of Hawking radiation expected from even a supermassive black hole (SMBH) is extremely small: it is estimated it would take years for the largest (super-duper-massive?) black holes (SDMBH) to radiate to nothing. Further, entropy is effectively being redefined here: it is the sum of standard entropy plus this additional event-horizon--dependent term that is increasing with time, which isn't compelling cookery.

    Penrose proposes the third. He is an apostle of the conformal cyclic cosmology in which every aeon of a universe that begins with a big bang ends with a SDMBH which then radiates into a new aeon. I btw am not, but the mad bastard has a Nobel prize now and I haven't got mine yet. I'm no cosmologist (shoulda opened with that really) and won't claim to completely understand it, but as far as I can tell it does not locally increase classical entropy either. Instead, entropy is increased as the SDMBH Hawking radiates in the form of photons and, due to the fact that the proper time of photons is zero, no matter how long this takes this energy is essentially focused onto a single future event: the next big bang, at which point there's a massive entropy increase.

    10701_2018_162_Fig2_HTML.png

    Penrose.png

    I'm going to argue for the fourth. On the face of it, entropy decreases near a black hole, and simply finding something that increases, multiplying it by stuff that yields the right units (which is what Hawking did) and adding it to entropy isn't going to change that.

    This means that, near a black hole: 1) the distance between matter decreases, and 2) entropy decreases, which looks like a time-reversed big bang.

    Indeed, early cosmological models for the big bang used black hole models with a minus sign: a white hole. White-hole--like (WHL) big bangs haven't been much in favour for a while, although seem to be making a bit of a comeback, and part of the reason is good ol' empiricism. Black holes exert tidal forces which make their mark on the matter around them. The cosmic microwave background (CMB) on the other hand is quite homogeneous (although Penrose believes he has found remnants of older WHL big bangs in the CMB).

    Standard models of the big bang also differ from white holes in their event horizon. From outside the event horizon of a black hole, all worldlines terminate at that event horizon (time slows to a standstill), while inside the event horizon all worldlines converge without every meeting (time goes on forever).

    sk-6e18589671fe8a938f6c592059ccb7a7.jpeg

    This means that, for a white hole, outside the event horizon, all worldlines originate at the event horizon while, inside, all worldlines have origins infinitely far in the past. The standard cosmological model is quite different, in which all worldlines originate from a common event a finite distance back in our past. The event horizon of the standard big bang model also gets smaller as it contains a denser distribution of mass, unlike a black hole whose event horizon increases with density.

    VvffCyKNYY1Yi5cF8
    (Image from https://www.sciencedirect.com/science/article/pii/S1631070515001310 )
    (edited for traceability)

    The lack of evidence of tidal forces from a WHL bang is based on the assumption that the universe isn't much larger than the observable universe, and therefore if there was any large-scale structure to the CMB, we would have seen it.

    SyAztm2DS2VKCb2KczvNC7-970-80.jpg.webp

    A perfectly straightforward counterargument, then, is that this is not necessarily the case. In fact, there is some debate about whether large-scale anomalies to the homogeneity have already been found (see above). There are no periodic features like this, but if we were looking at only a small portion of the sky, we wouldn't expect to see any. Annoyingly, the only way to know is to wait until the observable universe gets larger, and our past light cone encompasses more of the entire universe from the past.

    A fairly recent paper makes the case that, not only should white holes exist in nature, we have already seen them.

    A super-duper massive white hole (SDMWH) model of the big bang was by Hellaby in 1987, based on the now-outdated Tolman model describing the now-outdated big crunch terminus. Here, the universe began with a chain of white holes exploding independently and merging to create a single universe. Roger Penrose's latest paper suggests that the afterglow of multiple previous big bangs is evident to 99.98% certainty in the CMB as six hot spots with characteristic angles; in his model, these are each a big bang from a previous aeon, but it's not difficult to see that they might also fit Hellaby's model. There is also a nice time-reversal here of Penrose's idea that the universe will end with one or more SDMBH black holes.

    As I said, though, I don't buy Penrose's view of the universe. General relativity is in itself time-reversible, and one expects that to manifest itself in nature. Putting all of the pieces together:

    • the cosmological arrow of time is given locally by whether space expands or contracts, i.e. whether it is dominated by big-bang-- or black-hole--like objects;
    • the thermodynamic arrow of time is reversible, such that entropy will decrease in the vicinity of a black hole;
    • black holes and their time-reversals, white holes, exist, likewise supermassive black and white holes exist;
    • the big bang was a super-duper-massive white hole;
    • worldlines start at the event horizon of white hole and terminate at the event horizon of a black hole;
    • this loosely describes the large-scale structure of the universe.

    sk-56945587f9f7bacaecaf5868ba6f2eab.jpeg

    The above gives a flavour of what a truly reversible universe might look like, and the question must be: is it consistent with empirical evidence. From our point of view (and the only points of view possible in this local part of the graph are those with the same arrow of time as us), the singularity behind us (and to our right) is our big bang: space and time expand from this singularity, matter forms, stars form, black holes form, SMBHs form, galaxies form, SDMBHs form. The latter, a la Penrose, gobble up most of everything in their local region of the universe: we do not need a single SDMBH at the end of time as Penrose does, which is a bit of an ask from the universe.

    However, they gobble up more than that which emerges from our big bang (see worldline above either neighbouring SDMBH). To understand how, we switch direction in time. Reversed, our big bang is a SDMBH, and each neighbouring SDMBH is now a SDMWH -- a big bang! Multiple big bangs, in fact, each completely independent of each other, each with worldlines that never meet except at the terminus of a SDMBH they share (our big bang).

    These big bangs form SDMBHs, one of which is our big bang, others having worldlines that never meet any worldline in our universe. One might think that these additional SDMBHs in the time-reversed frame will therefore be SDMWHs in our frame of reference, but this isn't really justifiable. Our arrow of time need not be parallel to theirs. Indeed, from Where We Are, it isn't necessary for the arrow of time heading toward a terminal SDMBH to be parallel to ours. From our frame, all worldlines terminate at the event horizon from all possible directions, thus a unique arrow of time is not given... Time stops at the event horizon, not the singularity, hence the worldlines of each singularity lie outside of time in our frame.

    What of Penrose's afterglows from previous big bangs? Consistent with general relativity, other black and white holes that are not large enough to destroy or create universes are possible. Black holes have been discovered, and perhaps Penrose's hot spots are previously undiscovered white holes.

    The number of terminals is open. Worldlines originate at one singularity and end at another; they are not affected by any other singularity. As such, one could have an infinite number of singularities. Large, small, or infinite, such an arrangement satisfies the condition for the universe to be much larger than the observable universe to allow for WHL big bangs.

    Don't take the above image too literally; it is merely an illustration of a principal, not an argument for a particular structure of the universe, or, as it effectively is, the multiverse. The real point of this post is to extend the principle of reversibility to cosmology and thermodynamics.

    Is this empirically justified? No, not as is. Recently, astronomers found six entire galaxies trapped by a SMBH. We can consider the time-reversal of this and ask if this is what we expect to see of a white hole, and the answer is obviously not. A white hole, for instance, still has mass and ought to attract matter from outside. But by definition nothing can go beyond its event horizon. And we certainly wouldn't expect a WHL big bang to just quickly spew out galaxies the way this black hole is apparently gobbling them up. One needs to address at least the fact that a true white hole must essentially be an anti-gravity well (or gravity barrier), the opposite to a black hole which is a gravity well.

    A more recent paper has argued for a spacetime metric that exhibits this reversal precisely, yielding a gravity that is repulsive for white hole scales and attractive thereafter. As a side note, this is an example of 5D general relativity, another of which is Kaluza Klein theory, my favourite mystery of unproven physical theory in which Maxwell's equations fall out for free, in addition to Einstein's GR equations. Alas the paper says nothing about this.

    Gravitational repulsion is an idea that comes up a lot in cosmology, and one empirical observation that may be explained by it is the predominance of matter over antimatter in the universe. This paper argues that this is due to matter and antimatter having opposite stress-energy tensors. After the big bang, which created matter and antimatter in equal measure, the gravitational repulsion between them forced them as far away from each other as possible, resulting in a local, observable universe dominated by one and not the other. In principle, it doesn't seem obvious that this is necessary. Our observations of galaxy clusters are electromagnetic, and galaxy clusters of antimatter would appear identical to galaxy clusters of matter. (For instance, an anti-hydrogen atom consisting of an anti-proton and a positron will have the same photon spectrum as a hydrogen atom.)

    Such a self-organisation would lead to net repulsion between matter and antimatter galaxy clusters, which would be a contributor, but not the sole contributor, of the current expansion of the universe (e.g. a dark energy candidate: full article here https://iopscience.iop.org/article/10.1209/0295-5075/94/20001/pdf).

    This potentially solves the asymmetry between observed black holes and our big bang. Smaller and super-massive black holes tend to form inside galaxies and galaxy clusters, i.e. they are made of matter or antimatter. Their time-reversed equivalents would not just be white holes but anti-gravity white holes which repel the kind of matter they are emitting from their event horizons. An antimatter white hole (a time-reversed matter black hole) would only be gravitationally attractive to matter and vice-versa. Any white hole that is attractive to matter such as constitutes our galaxy cluster would be an antimatter emitter. In equilibrium, such a hole would be its own anti-hole, similar to that described by Hawking: it would absorb only matter and emit only antimatter.

    On the other hand, our WHL big bang (SDMWH) is both a matter and antimatter emitter, just as Penrose's ultimate SDMBH at the end of time must be a matter and antimatter (and black-hole and anti--black-hole) absorber. This obeys the above anti-symmetry: a matter SDMBH would be attractive to matter and repulsive to antimatter but attractive to antimatter white holes (in the same way that an antimatter white hole would be repulsive to antimatter but attractive to matter) and vice versa. Ultimately, whatever is being emitted, there's something that will eat it.

    sk-c774e148a42f87febade49d9489d6498.jpeg

    Again, this is not a putative model of how things are so much as an illustration of what the theory says is possible with empirical data in mind. Nor is this anything like a representation of standard models. Cosmology is a problematic field with much dispute around the fundamentals and with notable figures (like Penrose) and notable institutions with open ears (such as phys.org) who are standing or entertaining ideas well outside of the mainstream. It feels like everything is still up for grabs, and so we should be wary of prematurely subscribing to a particular model when the gold standard is so rapidly varying. The news that the universe's expansion is accelerating came not that long ago, and ideas such as anti-gravity, white-hole--like big bangs, periodic or non-repeating boundary conditions, the relationships between arrows of time, matter and antimatter dominance, etc., etc. are still very much on the table in on-going research.

    My current inclination, as is probably obvious, is something like the above that both obeys and manifests the time-symmetry of our fundamental theories: general relativity and relativistic quantum mechanics. However, the odds of happening upon an accurate cosmological model, especially one with as many freedoms as above, is slight. It is more the underlying concepts that attract me, ones that do not rely on by-hand introduction of arrows of time into our universe when our best theoretical frameworks demand no such thing.
  • jgill
    3.5k
    This is the most sophisticated OP I've seen in the eleven months I've been here. It will be interesting to read the philosophical comments in reply. I'm curious about the images; where did they come from?

    I see analogous phenomena in dynamical systems in : attracting fixed points and repelling fixed points, then there are indifferent fixed points that may combine the features of the two. I wonder if there are cosmological similarities of the latter?
  • Kenosha Kid
    3.2k
    This is the most sophisticated OP I've seen in the eleven months I've been here.jgill

    Thanks! :) Although this is admittedly much more loosey-goosey than I'd like. I'm extremely worried that it might be read as a theory in itself, whereas it's meant more as an illustration that on-going, high-profile research in a field that is itself a tad loosey-goosey shows that there is a lot of obvious potential for massive paradigm shifts in some pretty fundamental areas. Some big names in cosmology have very different ideas about what kind of universe we live in, and those differences have a huge impact on the philosophical ramifications of cosmology. That's all I'm really trying to illustrate here.

    I'm curious about the images; where did they come from?jgill

    I should cite. The first comes from an older paper on Penrose's CCC model here https://link.springer.com/article/10.1007/s10701-018-0162-3 although I believe the image is older. The second is from Penrose's latest preprint on the same subject which identifies six hit spots in the CMB as earlier WHL big bangs: https://arxiv.org/pdf/1808.01740.pdf

    I made the third. Just seemed easier than tracking down a particular figure. The fourth was just a Google search that looked roughly right, i.e. had all worldlines converging to a point in finite time and had the right shape event horizon. I could maybe get a better one.

    The fifth image is from the Wilkinson Microwave Anisotropy Probe iirc, though I took it from PhysicsWorld:
    https://physicsworld.com/a/the-enduring-enigma-of-the-cosmic-cold-spot/

    The sixth and seventh images I drew myself for illustrative purposes. The insights are in the papers I link to, none of which had suitable images.

    I see analogous phenomena in dynamical systems in CC: attracting fixed points and repelling fixed points, then there are indifferent fixed points that may combine the features of the two. I wonder if there are cosmological similarities of the latter?jgill

    Interesting, especially given the aforementioned complex nature of less sophisticated spacetime representations. Mathematics is wondrously self-similar.
  • Philosophim
    2.2k
    I'm extremely worried that it might be read as a theory in itself, whereas it's meant more as an illustration that on-going, high-profile research in a field that is itself a tad loosey-goosey shows that there is a lot of obvious potential for massive paradigm shifts in some pretty fundamental areas.Kenosha Kid

    Yes, you start off with the implication that you're going to discuss determinism, and by the end all I got were several different theories of big bang cosmology. =D

    The idea that black holes sucked up the universe, then exploded into a big bang again has been around for decades at least. I suppose what is exciting is the actual science to back up the theories? What is the major cosmology shift you see in all of this?
  • Kenosha Kid
    3.2k
    Yes, you start off with the implication that you're going to discuss determinism, and by the end all I got were several different theories of big bang cosmology. =DPhilosophim

    It's sort of a sequel thread. The previous thread established (or attempted to) the relationship between time-reversibility and determinism. I hadn't planned on reiterating the whole thing here, hence the link and the summary up top.

    The idea that black holes sucked up the universe, then exploded into a big bang again has been around for decades at least. I suppose what is exciting is the actual science to back up the theories? What is the major cosmology shift you see in all of this?Philosophim

    Yes, and I cite several papers from those decades in the OP. I don't predict a particular shift. Paradigm shifts in cosmology are very observation-driven. The next field-shattering observation may or may not invalidate any of the broad arguments cited here.

    However the six white hotspots are interesting, if the maths in the Penrose paper are good. Already Penrose is getting dung for interpreting them as old big bangs, and rightly so in my opinion, but they might turn out to be less momentous examples of white holes.

    I think generally the observations of low-frequency CMB features sound interesting. I drafted the OP quite a while ago and have since read more discussion that suggests these anisotropies are more prevalent than the OP suggests, which might be evidence of tidal forces in the very early universe.

    That aside, quite a lot of the ideas mentioned in the OP are probably untestable. We'll never know what's outside the observable universe, or whether a neighbouring galactic supercluster is predominantly matter or antimatter. So the only take-home is: the most obvious assumptions may be equally unjustifiable, and the ramifications of those assumptions shouldn't go by unquestioned.

    This is basically where the first thread started, with a discussion of how the idealised screen in the double-slit experiments, in lieu of actually being able to calculate the many-body wavefunction of the set-up, leads to the unquestioned acceptance of consequences for determinism that really ought to be scrutinised. The same goes here. The cosmological arrow of time relies on fundamentally or circumstantially irreversible processes taking place in the universe when the underlying theories have no privileged direction for time. The assumption of irreversible processes is largely a feature of our models, not our observations. The rest is an illustration of this principle.
  • EnPassant
    665
    Penrose is one of many theorists who have concluded that they are the same arrow of time.Kenosha Kid

    Just because both arrows point in the same direction does not mean they are the same. Time in the physical world has to do with mass and the speed of light. I don't see how heat flow can determine this.
  • Kenosha Kid
    3.2k
    Just because both arrows point in the same direction does not mean they are the same.EnPassant

    Penrose believes they are the same arrow, not aligned arrows, which is perhaps one of his least controversial views. However, if the two arrows are aligned in all circumstances, there's no criteria by which to differentiate them.
  • EnPassant
    665
    I just finished reading Carlo Rovelli's book The Order of Time which is an attempt to argue that time is a function of entropy. His arguments are very weak and confused.
  • Kenosha Kid
    3.2k
    I just finished reading Carlo Rovelli's book The Order of Time which is an attempt to argue that time is a function of entropy. His arguments are very weak and confused.EnPassant

    I might agree.
  • Wayfarer
    20.6k
    According to Penrose’s conformal cyclic cosmology, the universe goes through an infinite series of “aeons,” each of which starts with a phase resembling a big bang, then forming galactic structures as usual, then cooling down as stars die. In the end the only thing that’s left are evaporating black holes and thinly dispersed radiation. Penrose then conjectures a slight change to particle physics that allows him to attach the end of one aeon to the beginning of another, and everything starts anew with the next bang.

    This match between one aeon’s end and another’s beginning necessitates the introduction of a new field – the “erebon” – that makes up dark matter, and that decays throughout the coming aeon.
    — Sabine Hossenfelder

    I can’t comment on the science, but the idea of a cyclical cosmology has an ancient provenance; it’s reminiscent of Hindu cosmology which has always said the Universe expands and contracts over ‘aeons of kalpas’.

    Also, while I’m here - what’s your view of Sabine Hossenfelder? She has some pretty savage criticisms of current particle physics.

    And finally - I’ve noticed a few times your remark on the ‘creation and destruction’ of sub-atomic particles. This generally takes place also over cosmic time-scales doesn’t it? Like, many of the elements are created in stellar explosions, but then they last for cosmic periods of time after they’re created. Do I understand it right?
  • EnPassant
    665
    Allow me to ask you a question. For the sake of argument let's define time as 'a mathematical description of events in space'. Relativity is such a description that describes macroscopic events in ordinary physical space.
    But what about 'quantum time'? If the mathematics that describe change in the quantum world are different from the mathematics of change in the physical world then are there not two (space)times? Quantum time and physical time? Are the mathematics of quantum change sufficiently different from relativity to justify the idea that quantum particles live in a different spacetime?
  • Kenosha Kid
    3.2k
    I can’t comment on the science, but the idea of a cyclical cosmology has an ancient provenance; it’s reminiscent of Hindu cosmology which has always said the Universe expands and contracts over ‘aeons of kalpas’.Wayfarer

    I wonder if we're doomed to impose our own mythologies on everything we encounter...

    what’s your view of Sabine Hossenfelder? She has some pretty savage criticisms of current particle physics.Wayfarer

    I'll get back to you. She wasn't on my radar, thanks.

    And finally - I’ve noticed a few times your remark on the ‘creation and destruction’ of sub-atomic particles. This generally takes place also over cosmic time-scales doesn’t it? Like, many of the elements are created in stellar explosions, but then they last for cosmic periods of time after they’re created. Do I understand it right?Wayfarer

    It can occur at any time scales. Pairs of virtual particles (pairs that don't have enough energy to escape one another) can be created and annihilated in the smallest conceivable timescales; particles are created and destroyed very rapidly in particle colliders; a particle created soon after the big bang might go the lifetime of the universe without annihilating or might have done so almost immediately.
  • Metaphysician Undercover
    12.3k
    But what about 'quantum time'? If the mathematics that describe change in the quantum world are different from the mathematics of change in the physical world then are there not two (space)times? Quantum time and physical time? Are the mathematics of quantum change sufficiently different from relativity to justify the idea that quantum particles live in a different spacetime?EnPassant

    This is a problem which Kenosha Kid did not seem to want to acknowledge in the other thread. I presented it as a need for two distinct concepts of space. You present it as a need for two distinct concepts of spacetime. Kenosha seems to think that the problem can be resolved by allowing for time reversal, but that's naivety.
  • Kenosha Kid
    3.2k
    Allow me to ask you a question.EnPassant

    Shoot!

    Are the mathematics of quantum change sufficiently different from relativity to justify the idea that quantum particles live in a different spacetime?EnPassant

    The time of relativistic quantum mechanics is the same time as normal special relativity for a single object. But that does leave a lot of room for difference. For instance, we don't have a general relativistic quantum theory, so curved time is not understood quantum mechanically. There is also the concept of Planck time in quantum mechanics, which is the shortest interval of time that meaningful measurements can be made. This discretisation of time is counter to the continuous nature of time, position, and any other observable in classical field theories like relativity. Finally, in many-body classical physics, all objects have the same time axis in a given frame of reference, while in many-body quantum mechanics, every particle described by the wavefunction has its own time axis. This is one of the reasons why the ontological status of the wavefunction is doubted.
  • Wayfarer
    20.6k
    I wonder if we're doomed to impose our own mythologies on everything we encounter...Kenosha Kid

    It’s more that everything in nature goes through cycles of creation and destruction....why should the Universe be different? (Oh, and I don’t know if Indian cosmology is ‘my own’ ... actually Carl Sagan did a TV episode on that idea way back...)
  • Kenosha Kid
    3.2k
    It’s more that everything in nature goes through cycles of creation and destruction....why should the Universe be different? (Oh, and I don’t know if Indian cosmology is ‘my own’ ... actually Carl Sagan did a TV episode on that idea way back...)Wayfarer

    I didn't mean to suggest it was yours, rather by "our own" I'm referring to the human race as a whole. Cyclic universes have no real empirical basis; in fact, Penrose's CCC strikes me as an attempt to salvage the big bang/big crunch cycle that he entertained decades ago in the face of the recent-ish empirical evidence that the expansion of the universe is accelerating.

    I also had in mind Darwinist models of the universe's large-scale structure as described particularly by Hawking. It seems to me that already-attractive narratives flourish where experimental evidence is meagre. Empiricism is usually much more surprising than these narratives would permit.
  • EnPassant
    665
    I presented it as a need for two distinct concepts of space. You present it as a need for two distinct concepts of spacetime.Metaphysician Undercover

    I think it was Niels Bohr that said it is meaningless to say where a particle is before it is detected. But 'where' is referring to a position in physical spacetime. If the particle, before detection, is in quantum spacetime Bohr is answered with 'It is nowhere'. Nowhere in physical spacetime. It is 'elsewhere'.

    What is detection? We should have a rigorous definition of detection-

    Detection is where an event in quantum spacetime leaves a trace effect on physical spacetime.

    A trace effect can be a spot on a photographic plate. This trace effect is located in physical spacetime: you can point to it and say where it is. But where is/was the event that caused it? It is 'elsewhere' but nowhere in physical spacetime.

    The salient point here is that the trace effect is necessarily in physical spacetime. Since the detection apparatus is a physical object in physical spacetime it cannot be otherwise.

    So, what is happening here is that an n-dimensional event in quantum spacetime is projected onto the surface of a 4-dimensional physical spacetime. Say n = 10. This means 6 dimensions of information are lost because a 10 dimensional event is compressed into 4 dimensions.

    This is one of the reasons why the ontological status of the wavefunction is doubted.Kenosha Kid

    The idea that it is a real thing gives rise to all kinds of nonsense involving reality splitting into multiple universes etc. It seems to me to be a convenient mathematical device, nothing more.
  • Kenosha Kid
    3.2k
    The idea that it is a real thing gives rise to all kinds of nonsense involving reality splitting into multiple universes etc. It seems to me to be a convenient mathematical device, nothing more.EnPassant

    It cannot end there, I'm afraid. A mathematical device with an astounding ability to predict experimental outcomes, even at a statistical level, demands explanation.
  • EnPassant
    665
    A mathematical device with an astounding ability to predict experimental outcomes, even at a statistical level, demands explanation.Kenosha Kid

    I agree entirely. But compare it to ordinary statistics. You might argue that, statistically, 11 left-handed bachelors will enter a shop over a period of, say, 3 days. But are left-handed bachelors aligning their activities with your mathematical model? They are effectively acting in a random way.
  • Kenosha Kid
    3.2k
    They are effectively acting in a random way.EnPassant

    Randomness has its own distributions. If something is random, we can infer it, and ask why it is random and not regular as other things often are. QM predicts distributions, such as the banded pattern in the double slit experiment. Why this and not two Gaussians or a random distribution or something else? That is a question about why and how the mathematical wavefunction represents physical reality.

    Personally, if something diffracts and interferes, I call it a wave. That in itself doesn't make the particular wavefunction we calculate ontic, but it narrows the ontological gap. That said, none of this is meant to force a particular wavefunction ontology anyway. Whether you think that the wavefunction is merely epistemic, or if you think it's truly ontic, I think the arguments in these two threads may stand a little rewording but not much refactoring.
  • Metaphysician Undercover
    12.3k
    So, what is happening here is that an n-dimensional event in quantum spacetime is projected onto the surface of a 4-dimensional physical spacetime. Say n = 10. This means 6 dimensions of information are lost because a 10 dimensional event is compressed into 4 dimensions.EnPassant

    I'd prefer to refer to what you call the "n-dimensional" as non-dimensional. Dimensions are how we understand and represent physical space. It doesn't really make sense to try and understand the non-spatial type of event in terms of dimensions. However, since it is an event, it is temporal. But this means we need to divorce time from its status as the fourth dimension of space, allowing for non-dimensional activity, and perhaps consider that the existence of time is prior to the existence of space, as evidenced by your description of this event, causing an effect in dimensional space.
  • Wayfarer
    20.6k
    Cyclic universes have no real empirical basis...Kenosha Kid

    Neither do multiverses or the many-worlds interpretation. Both arise from discrepancies between observation and theory. I would have thought that when physics strays into metaphysics, then it also goes beyond empirical validation as a matter of definition. Quite a good article by Jim Baggott on all this here.

    As they have explored the metaphysical landscape of a mathematically-defined independent reality, the theorists have misappropriated and abused the word ‘discovery.’ So, they ‘discovered’ that elementary particles are strings or membranes. They ‘discovered’ that there must be a supersymmetry between different types of particle. They ‘discovered’ that the theory demands six extra spatial dimensions which must be compactified into a space so small we can never experience them. They ‘discovered’ that the five different types of superstring theory are subsumed in an over-arching structure called ‘M-theory.’ Then, because they ‘discovered’ that there are 10-to-the-power-500 different ways of compactifying the extra dimensions, each of these must describe a different type of universe in a multiverse of possibilities. Finally, they ‘discovered’ that the universe is the way it is because this is one of the few universes in the landscape of 10-to-the-power-500 different kinds that is compatible with our existence.

    I want you to be clear that these are not discoveries, at least in the sense of scientific discoveries. They are assumptions or conclusions that logically arise from the mathematics but for which there is absolutely no empirical evidence. It’s not really so surprising that the theory struggles to make any testable predictions. There is simply no way back to empirical reality from here.
    — Jim Baggott

    also https://platofootnote.wordpress.com/2018/01/22/peter-woit-vs-sean-carroll-string-theory-the-multiverse-and-popperazism/
  • Ciceronianus
    2.9k
    Those damn Copenhagenists should stay in Copenhagen, where they belong, and forget about the universe.
  • EnPassant
    665
    I'd prefer to refer to what you call the "n-dimensional" as non-dimensional.Metaphysician Undercover

    Whatever the case may be, if there are two mathematically different universes then detection is when a trace effect is left on the interface between the two universes. But quantum time must be in the equations somewhere.

    There is simply no way back to empirical reality from here. — Jim Baggott

    lol. The multiverse is quantum fiction for fantasists.
  • Metaphysician Undercover
    12.3k
    Whatever the case may be, if there are two mathematically different universes then detection is when a trace effect is left on the interface between the two universes. But quantum time must be in the equations somewhere.EnPassant

    The difficult challenge would be to establish the mathematical axioms required to relate the two distinct universes. How we apprehend free will becomes an important issue with much significance because if a free willing living being can make radical changes to the physical universe at a moment in time, this implies that the continuity of the physical world, as time passes, is not necessary. If this continuity, described by Newton's first law, is not necessary, then we must consider it possible that the entire physical universe is recreated at each moment of passing time, as free willing beings have the capacity to influence or interfere in this recreation.

    If the continuity of time is denied in this manner, it is necessary to assume quantum time as the base of the physical universe, producing the illusion of continuous temporal existence through a succession of discrete units. However, there is still the matter of in between the discrete moments, as the in between is when real change, free will acts, and real "becoming", changes which are inconsistent with the continuity of the discrete moments in time, occur. Because "becoming" is represented as what is between the states of existence, it has traditionally been portrayed as unintelligible. However, we could gain some insight into it if we could determine the real discrete moment of time, as the fundamental unit of physical existence, rather than the the proposed Planck time which is not based in an understanding of any real wave, pulsation, or vibration.
  • Kenosha Kid
    3.2k
    Neither do multiverses or the many-worlds interpretationWayfarer

    Allow me to correct my wording: cyclic universes are contrary to empirical evidence, requiring contortions like CCC to make them work.

    Those damn Copenhagenists should stay in Copenhagen, where they belong, and forget about the universe.Ciceronianus the White

    :rofl:
  • Kenosha Kid
    3.2k
    And we certainly wouldn't expect a WHL big bang to just quickly spew out galaxies the way this black hole is apparently gobbling them up.Kenosha Kid

    Or maybe we should: https://phys.org/news/2020-10-galaxies-infant-universe-surprisingly-mature.html
  • Kenosha Kid
    3.2k
    In this 1983 paper, the author of the transactional interpretation of QM directly addresses the boundary conditions of the big bang and black holes and their implications for that interpretation: http://faculty.washington.edu/jcramer/TI/The_Arrow_of_EM_Time.pdf
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