Physicists rewrite the fundamental law that leads to disorder
quantamagazine.org
quantamagazine.org
https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.128.080...
Figs 1 and 2 show the setup. Here's the conclusion:
> "This gives a clear proof of the compatibility of constructor-based irreversibility with unitary quantum theory, providing a frame for the emergence of thermodynamical irreversibility in quantum mechanics."
It seems like an extraordinary claim, but is very interesting. The theoretical framework (constructors) is a bit confusing. Hopefully this will be replicated.
I recall someone talking about something rather similar once, related to so-called chaos theory (now more generally called 'sensitive dependence on initial conditions'), which describes how deterministic systems can give rise mostly unpredictable phenomena (strange attractors etc.), in which indeterminate quantum states at the atomic level are the ultimate cause of the phenomenon.
The chaos can act as a magnifier of quantum fluctuations so that they can produce sizable effects in the world around us. But we know that that can happen often. - Murray Gell-Mann
I guess the idea is that if you have a collection of particles in a system that displays sensitive initial dependence, then any quantum jiggling gets rapidly amplified.
[edit] As you note this doesn't explain the chaos phenomena, because a stable dynamic system would squelch any quantum weirdness rather than amplify it.
it's the same phenomenon underlying the wisdom of crowds (and the pricing function in fair markets): each individual can be wildly incorrect in an observation, but averaging many (independent) observations together reveals stable insight.
Some nonlinear DEs exhibit chaos. That’s a purely mathematical property. Whether any particular DE is a useful model of a particular physical system is a matter for the imagination, and either backed up or refuted by experiment.
Sure, but differential equations describe physical systems, and there is a canonical way to derive a quantum differential equation from a classical one by quantifying the classical Lagrangian using the Path Integral formulation. Giving a fairly natural distinction between the types of equations
> Whether any particular DE is a useful model of a particular physical system is a matter for the imagination, and either backed up or refuted by experiment.
This doesn't make sense to me. The Navier-Stokes equations are known to describe the classical behavior of water and are experimentally confirmed to predict things like trajectory. Their effectiveness has nothing to do with my imagination. If I write x=x' for the position vector of atoms in a fluid that will completely fail to describe anything physical.
Note that there is no Largrangian for the NS equations, by the way.
> Note that there is no Lagrangian for the NS equations, by the way.
I don't know much about fluid dynamics, but I was under the impression that Bennett derives the Lagrangian form in the book Lagrangian Fluid Dynamics
So this is primarily an interesting mathematical restatement at the quantum level. I don't know how useful this is, not having much knowledge about existing quantum information theory/thermodynamics.
isn't that the wolfram thing? Simple patterns create complex unpredictable patterns? And our universe are just made up a few very simple rules and in combination it created this complex universe. Yeah most ideas has being around for a long time, it is just now we have the technology to verify some of these claims.
Chaos is unpredictable but for a different reason than quantum systems: Chaotic systems are unpredictable because we can never measure the real world perfectly (even ignoring quantum weirdness) and that lack of precision in measurement coupled with nonlinearity means our measurement errors amplify quickly. Nevertheless, chaos is theoretically deterministic. It's just not practically deterministic (except in the coarse-grained statistical sense of saying "the system's position in phase space is somewhere on a strange attractor path bounded by X.")
Quantum systems are not even deterministic in theory, let alone practice. And ironically quantum systems are fundamentally linear. So the spookier theory in a sense is based on more tractable math.
Chaotic orbits of whole moons are non-deterministic on quite short time horizons.
It's important to note that the parts of quantum mechanics that are linear are also fully deterministic - the Schrodinger equation predicts the evolution of the wave function of a system with exact precision (as do the more complex equations of QFT).
The part that introduces non-determinism is the non-linear adjustment of the Schrodinger equation that you have to perform after a measurement - the Born rule, also called wave-function collapse. Here the complex-valued wave function of a system randomly takes a single real value at the moment of measurement. It does go back to its deterministic linear evolution afterwards though, but you have to perform this non-linear adjustment.
This non-linear adjustment can be interpreted in many ways, but without it QM doesn't agree with even the most basic experiments, so one way or another it is a vital part of the theory, rendering it non-linear overall.
When qbits have to be read, we convert them from complex to real and that step is both nonlinear and nondeterministic.
I was also extremely puzzled by the dismissal of probabilistic or statistical models as being fuzzy or something. Probability has always been the domain of possibility -- this is basically the Jaynesian-style account of probability. It's always seemed to me to be the case that at some level statistics is about selective loss of information -- that is, as you go from descriptions at the particle level to group-level statistical accounts, you summarize (in a coding sense) the ensemble of particles' behavior to shed information required to maintain complete information about the ensemble. To make an analogy, if you wanted to know people's heights, at some point storing information about 1 million person's heights would become burdensome, so you'd replace it with a single number (e.g., the mean) instead.
That's not to say what's being discussed in the article isn't interesting, but it just seems like a different level of analysis or something? There's also different ways of representing amount of information in the quantum realm vs classical realm, so there's that, but it's also the case that a chamber of gas will generally involve a lot of molecules in many classical cases.
I also had the sense that there was probably something more fundamental that you could invoke that would integrate a lot of this, at the level of logic or computational theory or something. But maybe at this level the difference is sort of moot.
Perhaps you're just saying that probability is useful in some practical ways? Deutsch agrees (see video below). But if that's not all you're saying:
If the "domain of possibility" (which I interpret to refer to quantum mechanics and thermodynamics) can be described without the whole concept of probability, why not eliminate it? Why eliminate it, though? As Deutsch says "We don't need a reason: Physics likes to do without things"? [1]
You may say constructor theory adds new fundamental concepts. I don't know constructor theory so I don't know whether that's true. In cany case, what's not to like about probability concepts? In what way are they "fuzzy or something", as you put it? Deutsch points out here [2] (few minutes later in the same talk as the other link) a sense in which probability statements don't describe physical facts: for example "drawing to an inside straight is very risky" is logically consistent with any sequence of cards in a poker game. I think that's the "fuzziness" they want to eliminate from Physics. He then goes on to deal with some objections to that idea such as "in the long run, probability statements describe physical facts".
If the game of Physics is describing physical reality, and we don't need that specific kind of fuzziness, success would seem to me an advance in Physics.
[1] https://invidious.namazso.eu/watch?v=wfzSE4Hoxbc&t=13m12s [2] https://invidious.namazso.eu/watch?v=wfzSE4Hoxbc&t=14m18s
Could someone who knows the topic tell us (and by “us” I mean layman like me) if there is something to keep from this article? It brings so much context that after reading it I still have no idea what this “rewrite of fundamental law” is about.
There has always been something unsatisfactory about the second law of thermodynamics. Unlike many other laws in physics, which precisely state both what kinds of structures exist in reality and how they behave, the second law has a built-in lack of precision. Specifically, there are multiple statements of what the second law says, but they all suffer from referencing some concept which is inherently fuzzy at some scale. For example, they may refer to "heat" as part of the law, but the physical definition of "heat" is fuzzy at the microscale -- it only has a clear meaning for "large systems".
However, using a new physio-mathematical formalism called "constructor theory" the researchers have now found a way to state the second law of thermodynamics which is precise at all scales.
"A scientific explanation is a statement of what is there in reality, and how it behaves and how that accounts for the explicanda." (emphasis added) [1]
Deutsch argues strongly against the view of physics which says that physical explanations are just about predictions of behaviors and not about what is really out there. See also his 1997 book The Fabric of Reality.
[1] https://www.sciencedirect.com/science/article/pii/S135521981...
Sure, you can say stuff about "what is there in reality", but I don't see any way one can verify such statements.
Everything humans have access to is mediated through their perceptions. Everything that we sense "outside" of us is to us a perception.
This includes the results of all the experiments ever done, all our interaction with tools and scientific instruments, and all our observations. They're all perceptions.
Our thoughts and feelings are also perceptions. The results of our deductions and inductions are perceptions.
Humans seem to be hermetically sealed from the "outside world" (if there is one) by our perceptions, and I don't see any way out of that... not through science, not through philosophy, not through religion, not through anything.
"Mach, Einstein, and the Search for Reality" https://dash.harvard.edu/bitstream/handle/1/37902464/2002383...
In short, late in the 19th century and early 20th century, the physics philosophy of Ernst Mach held great sway. Mach's viewpoint was the very viewpoint that you describe: the only things we know about are our own perceptions and proper formulations of physical theories consists at the fundamental level of descriptions of these things.
Mach had a significant impact on many physicists' understanding of what they were doing and also had an impact on the development of logical positivism in the field of philosophy.
Einstein accepted Mach's philosophy initially. But gradually he came to see it as untenable given his own work in physics. The article linked to above describes how and why his viewpoint changed over time.
However, if I gave you the impression that I was a Machian, this is a mistake.
According to this paper, Mach equated reality with sensory experience (ie. the experiences afforded us through the five senses: seeing, hearing, touch, smell, and taste.. and perhaps some others like proprioception), and that speculation about a world "outside" and independent of such sensations was unwarranted.
This is not my position, as I include in "perception", the perception of thoughts, theories, conclusions, intuitions, feeling and everything else considered to be "internal" -- which includes everything Einstein or any other physicist could ever base their own theories on.
Also, I don't object to speculation as to an "external", "objective" world, as Mach does. Science can come up with all sorts of interesting and possibly useful things through such speculation. I'm happy to read of such conjectures, and if some people want to engage in it more power to them.
However, do I not see any way we could ever verify whether such speculation right or wrong (or, to put it another way, whether such speculation corresponded to the way the world really was), because neither mathematical simplicity or the elegant fitting in to other, larger theories (as Einstein seems to have favored) means that the speculation has anything to do with the "external" world as it is (or even that there is an external world).
Since we seem to be limited to our own experience, the belief that an "external" world exists and that we have any (direct or indirect) way to access it seems to be nothing but faith.
The scientific method is about predictions and testability of such predictions, therefore it implies behavior. Nothing more is needed for a scientific theory. To test for existence within the scientific method would require to test for non existence too, a clear paradox, which is the reason why ontological statements ("X exists") are meta-physical in nature.
How far do you take that stance?
If a biologist says a particular bacterium exists, are they doing philosophy, not science?
Edited to add another interesting case: creation "science" organizations like Answers in Genesis say that the theory of evolution is a philosophy not science since it describes non-observables in the past (events in the past are inherently not observable now). When a paleontologist says a particular animal once existed in the past, would you say they are they doing philosophy not science?
When a biologist says that a bacterium exists, they are making a descriptive statement, not an ontological one. To say that a bacterium exists is just a convenient proxy to refer to a specific aggregate as a whole. A bacterium is always reducible to its constituents, therefore has no ontological status nor existence in and of itself: in other words, if the molecules which the bacterium is made of would relinquish their structural integrity, the bacterium would cease to "exist" too.
Therefore I would agree with you that it's totally legitimate for a scientist to claim that a bacterium exists, in so far as we understand this claim as a descriptive proxy. A shorthand, essentially, to cluster together specific configurations of the underlying, supposedly more real, "stuff". And this is needed, if we want to make any progress. We have to communicate, effectively and succinctly. Science is precisely about _descriptions_ and predictions of behavior. Same goes with the paleontologist example.
It's a completely different matter instead to claim ontological status, that is, to say that something exists in and of itself, irreducible to something else, as a building block of the universe. This is a philosophical stance per-force, since the scientific method cannot follow us there. It's meta-physics by definition, and there are different rules at play there. Not Popper's falsifiability, but empirical adequacy, internal consistency, explanatory power, etc.
So when scientists claim that the ontological primitive is, e.g., the quantum field, or "energy" or anything really, they are stepping soundly and firmly into philosophy. Which is totally fine to do. We just need to be aware of this, and start to apply different rules to evaluate the validity of the claims.
Edits are typos.
However, note that in my original post I didn't claim that physical laws describe what exists at an ontologically fundamental level, just what exists in reality. And by that I mean "exists" in the non-subjective way that, say, hydrogen atoms exist. I wouldn't mean to imply that hydrogen atoms are irreducible.
Does Deutsch and his research group make claims to know what is ontologically irreducible? Not as far as I know, but I am open to correction!
[1] https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.128.080...
In the scientific sense, it is not enough to show that your system agrees with experiments to say it is true, you also have to show that the alternatives do not agree with experiments. Constructor theory is metaphysical because you'd get the same analysis whether you used it, ad-hoc intuitive thinking (in practice, people do not need instruction on universal principles like cause and effect), or some other alternative. All of the non-intuitive weight in that paper is borne by the QM stuff that exists independently of constructor theory, and in contrast, if you changed that the answers would start coming out wrong.
I am not proposing that this captures the essence of positivism, but one test that you can apply to check new ideas without going through the trouble of understanding them is to see whether they introduce any new conclusions that could not have been obtained under previous frameworks. If not, they may be metaphysical re-arrangements of existing ideas.
One of the points of the paper is that constructor theory makes such statements in a way that is compatible with other known laws. This is demonstrated via the experiment. So I'm afraid I do not understand your remark that "you could take the constructor theory language out and have the same paper". Taking the constructor theory language out would undermine a major point the authors are trying to make in the paper.
Yeah, but known laws are already compatible with themselves. You might make them more comprehensible by establishing a conceptual meta-framework that offers a consistent way to phrase statements about them, but that is clearly a philosophical task.
But I'll admit I don't understand constructor theory, and maybe it can hide any interesting discrepancies by overfitting.
I actually envisioned a similar thought experiment that could be done with a very complex quantum computer (say millions of qubits) to test whether the universe is actually keeping tabs of all these states or if it gives up at some point and produces irreversibility. But I guess if you'd formulate it like that the academic community wouldn't take you very seriously anymore. I'd wager Deutsch's actual motivation might be along this line, as he seemed very interested to find a way to test the many worlds interpretation against other interpretations of quantum mechanics.
See https://www.sciencedirect.com/science/article/pii/S135521981...
http://blog.rongarret.info/2019/07/the-trouble-with-many-wor...
The multiverse interpretation accounts for cases where the probabilities of each distinct observable outcome are equal according to the Born rule. But it cannot account for cases where the Born rule says there are non-equal probabilities for the possible outcomes.
This is a fairly common problem which critics of the multiverse interpretation raise. See [1] for another example where a critic raises that problem in more detail (with examples).
Deutsch and others have responded. In short, the reply is that the mathematics of the Schrodinger equation describes a density of universes in the continuum which is the multiverse, not a finite set of distinct, interacting single universes. This undercuts the critics' argument. See [2] for an example of this kind of response.
[1] https://arxiv.org/pdf/0810.2657.pdf [2] https://www.pnas.org/doi/pdf/10.1073/pnas.1324238111
Further, the author of that paper, Frank Tipler, is a notorious nutjob: https://en.wikipedia.org/wiki/Frank_J._Tipler.
I don't have time at the moment to review the paper in detail to locate any potential flaws, but I wanted to flag this for others. I think there is substantial reason to be skeptical.
I also agree there is no consensus that the problem of getting Born's rule from the Schrodinger equation has been solved to everyone's satisfaction.
Andrew Gelman calls it a "tabloid": https://statmodeling.stat.columbia.edu/2014/06/17/hurricanes....
I will check the paper out though.
[0] https://www.science.org/content/blog-post/no-more-prearrange...
Edit: I read the paper. It is borderline crank work. There are decent arguments for the Born rule in MWI but this is not one of them. (I like David Wallace's book, for example.)
One basic problem is that using symmetry arguments to justify the assignment of subjective probabilities (cf. page 4, right column) is going to fail when there isn't a symmetry available, for example for irrational probabilities.
A deeper issue is the implicit assumption that one can coarse-grain into discrete "effectively distinguishable universes" for the purpose of doing these counting arguments. There are significant conceptual difficulties doing this, and this claim is much stronger than the bare MWI. Wallace discusses this at length; see for instance the citations in the blog post you responded to.
> This undercuts the critics' argument.
No, it doesn't, for all of the reasons given in the blog post and the subsequent discussion. The Born rule is an observable fact of nature, and you cannot derive it from the Schroedinger equation, Duetsch and Wallace's claims to the contrary notwithstanding.
Edited to add my understanding: if the cardinality of the multiverse is uncountable, one has to talk about densities of branches. In that scheme, the problems which branch counting introduce do not arise.
It doesn't (see below) but even if it did...
> doesn't that undercut the argument in the blog post?
No. It's really very simple: the SE is linear. The BR is not. You can't derive a non-linear result from a linear equation.
The only thing that is even a tiny bit complicated about this is figuring out where the Deutsch-Wallace argument fails, because Deutsch and Wallace are not idiots and they hid their questionable assumption very well.
So... why does the SE not describe a "density of universes"? It's because the SE does not describe universes at all! Deutsch and Wallace specifically acknowledge this, which makes their argument all that much more untenable. The SE describes the behavior of a function called "the wave function". The wave function is not a description of anything physical. The domain of the wave function is not physical space, it is configuration space. This is the reason entanglement seems weird to us, because we live in physical space and the wave function does not.
In order to get anything physical out of the wave function -- particles, universes, anything resembling our classical reality -- you have to add some additional assumption, and any additional assumption that you add will be in some sense isomorphic to the Born rule. There is no way around this. Trying to derive the BR from the SE is like trying to prove Euclid's fifth postulate.
This is circular: you're assuming that realism about the wave function is incorrect to argue against the (tragically misnamed) many-worlds interpretation, but the only substantive claim the MWI makes is that realism about the wave function is true.
No, I'm not assuming it. I simply observe that the SR by itself does not describe the outcomes of experiments. And I'm not even talking here about quantum experiments. The SR cannot by itself even account for the fact that we live in a three-dimensional space or that there is an arrow of time. And it certainly cannot account for the Born rule, which is also an experimentally observed fact.
> the only substantive claim the MWI makes is that realism about the wave function is true.
Yes, I know. The problem with this is that it is violently at odds with observations that every human makes every moment of their lives, and it does not account for the Born rule.
Other than that there's no problem with it.
No formalism describes the outcome of experiments by itself. There always needs to be an interpretation attached, and our choice of interpretation is always underdetermined by observation.
> The problem with this is that it is violently at odds with observations that every human makes every moment of their lives
Like all other interpretations of all other physical theories, MWI isn't and can't be at odds with our observations alone, because our observations alone aren't enough to imply any particular relationship between observations in general. And like all other interpretations, it is and must be at odds with competing interpretations, and in particular with the folk-physics model we're naturally inclined to use. But this means very little, because folk-physics is wrong, and this far from the paleolithic savanna, it's wrong in ways that matter. Intuition is evidence, but it's not particularly strong evidence, and it's certainly not proof.
> and it does not account for the Born rule.
Sure, it still needs a separate account of subjective probability. But so does everything else: it's just a little harder to notice in the classical regime, since the relevant probabilities are always either 1 or 0.
That is so wrong as to border on the ridiculous. The quantum mechanics formalism, i.e. the SE plus the BR together describe the outcomes of experiments. No interpretation is required. You just turn the crank on the math and predictions pop out. There is a reason that "shut up and calculate" is a thing.
The only reason people talk about interpretations at all is that the structure of the math is violently at odds with our intuitions and subjective experience, and this leaves some people feeling queasy and wanting an explanation. There's nothing wrong with that. But it has absolutely nothing to do with the power of the formalism to make predictions, which is completely independent of any interpretation. (And this is true of classical mechanics as well.)
This paper introduced constructor theory: Deutsch, D. Constructor theory. Synthese 190, 4331–4359 (2013). https://doi.org/10.1007/s11229-013-0279-z (Download preprint version: https://arxiv.org/abs/1210.7439)
Then this paper applied constructor theory to form a constructor theory of information: Deutsch David and Marletto Chiara 2015 Constructor theory of information Proc. R. Soc. A.4712014054020140540 (Download: https://royalsocietypublishing.org/doi/10.1098/rspa.2014.054...)
Then this paper applied the constructor theory of information to probability: Marletto Chiara 2016 Constructor theory of probabilityProc. R. Soc. A.4722015088320150883 http://doi.org/10.1098/rspa.2015.0883 (Download: https://royalsocietypublishing.org/doi/10.1098/rspa.2015.088...)
Finally, this paper applied the constructor theories of information and probability to thermodynamics: https://arxiv.org/pdf/1608.02625.pdf (not yet published in a journal, to my knowledge)
For what it's worth, the Deutsch "Constructor Theory" stuff has its own homepage at https://www.constructortheory.org/
There is also a Wikipedia page that has a list of references and external links.
https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.128.080...
"(2022) Emergence of Constructor-Based Irreversibility in Quantum Systems: Theory and Experiment, Marletto & Vedral"
That's related to this quote from the article:
> "But crucially, the theory also predicts that the row becomes even less able to do the reverse transformation from B2 to B1. The researchers have confirmed this prediction experimentally using photons for B and a fiber optic circuit to simulate a row of three qubits."
The experiment is rather complex, and would take a long time to analyze and understand in depth. They appear to be claiming that microscopic reversibility, a cornerstone of atomic-molecular kinetic theory, is being violated in their setup? Seems like a rather extraordinary claim but it passed peer review in a major journal so, worth digging into probably.
For those who want an introductory overview of the issue, this is a good one:
https://www.thoughtco.com/laws-of-thermodynamics-p3-2699420
> "Kinetic Theory & the Laws of Thermodynamics
The laws of thermodynamics do not particularly concern themselves with the specific how and why of heat transfer, which makes sense for laws that were formulated before the atomic theory was fully adopted. They deal with the sum total of energy and heat transitions within a system and do not take into account the specific nature of heat transference on the atomic or molecular level."
In practice the second law is not all that useful although it's philosophically important I suppose. For example, a redwood tree growing from a redwood seed plus simple inputs (CO2, H2O, photons, NH3 etc.) is clearly a local decrease in entropy, as opposed to the dispersal of those elements when a redwood tree burns or falls and is digested by fungi, but since solar fusion is creating a much greater increase in entropy, there are no second law violations. Similarly the creation of heavy elements in a supernova would seem to be a decrease in entropy but the release of all that energy to the surrounding universe ensures no second law violations take place. In practice it's very difficult to do all the accounting for even moderately complex systems.
An air conditioner (heat pump), for example, causes heat to flow opposite to the temperature gradient, pushing heat from your house into the outside, and increasing the temperature difference, thereby decreasing the entropy in your area. But you have to expend energy to do so, hence your electricity bill.
See https://math.stackexchange.com/questions/628720/why-heat-equ...
Metaphysics isn't necessary (or helpful) in figuring out how we got here.
Isn't metaphysics the only thing we have to figure out how we got here? Provable or not? Even science agrees that there are true statements that are not provable.
Ever since then the entropy of the universe has been increasing overall. We are about half way to the heat death of the universe from an entropy calculation point of view.
The universe has a collective state now, so I’m not seeing how scaling changes that.
I like to think of entropy as "the universe prefers to have the greatest degrees of freedom available to it and there is a cost to reducing degrees of freedom". For example, to make a perfect crystal you have to cool to absolute zero (and even then there can be residual entropy!). Effectively, entropy = the logarithm of the number of ways a thing can be ordered.
If a thing can only be ordered in one way, then its entropy is minimized.
That instant could roll to anything from its current position (and according to many worlds, did) — the phase space of that object is huge, far beyond anything we normally see. That single object has all the complexity of every possible universe stacked into an infinitesimal region!
We’re in a colder universe than it was at that compacted moment, which I’m having trouble aligning with your explanation about crystals: we’re seeing the cooling towards absolute zero as a superheated droplet spreads out and freezes.
Defects forming in the crystallization of iron doesn’t imply the cold, defect laden iron has more entropy than the liquid drop it cooled from.
Just before the Big Bang (again, if it occurred), there would have been a perfect 1:1 relationship. There was only one possible way to be, and everything was that way. Zero entropy.
It is basically a definition, not empirical. Entropy only increases; the universe has a definite and specific history; therefore the beginning of things must have been only possible exactly one way, the lowest-entropy way.
If this sounds weird and non-scientific, it is! Scientists don’t know if the Big Bang occurred. We have basically no evidence to support any theory of what things were like before inflation.
The issue I’m having is this:
There are a lot of states that initial singularity could have been in, leading to a whole diversity of universes.
How is it different than your smoke example?
At least, in the context of this universe. But that is the entire and only context available to us.
Whereas all sorts of different clouds of smoke exist, some probably even right this second just here on Earth.
But we know things take “multiple paths” — why does that not apply to the original singularity diverging into multiple universes?
> Whereas all sorts of different clouds of smoke exist, some probably even right this second just here on Earth.
But only a particular cloud exists — the one that comes out of your fire. You go from one log as it exists to one cloud as it exists… one state to one state.
We normally explain that as “there’s more potential ways to be a cloud than potential ways to be a log”, but that brings us back to my question…
Why do the potential other smoke clouds count as entropy in the case of smoke clouds but not potential other universes in the case of universes?
What these physicists are working on though is a math framework that might help simulate old state by being able to account for eons of time and vast distances traversed. So our intuition is as you say it’s just hard to accurately model what things would look like if such a reversing was to happen.