Markus Hanke

Just want to throw in here (as an uninvolved reader) that you all seem to be tacitly equating religion with theism. But these are not the same things at all - not all religions are theistic in nature. I think it is important to distinguish these concepts more carefully.

You cannot ‘separate’ nearby gravitational wells neatly, because gravity is non-linear - spacetime in the Earth-Moon system is not simply the sum of ‘Earth gravity well’ + ‘Moon gravity well’, but something more complicated (though GR effects are quite small here). The same is true for all other planets in our solar system, since they are all subject to the gravity of the Sun. Of course, that goes without saying - the entirety of physics is about making models that describe aspects of the world around us. We don’t do philosophy or metaphysics or religion here, so everything that is being talked about in the physics section of this forum has to do with models. And whatever idea you have in mind about gravity is also a hypothesis about gravity, and not gravity itself. But here’s the point - GR has been extensively tested, and found to be in excellent agreement with experiment and observation. So we can regard the model as valid within its domain of applicability, and thus extract predictions from it. Now, if you take the ‘time’ out of spacetime, thus reducing the universe to three dimensions, then GR tells us very clearly what would happen so far as gravity is concerned. As such, if you claim that gravity still works in 3D in the exact manner as we can observe around us, then the onus will be on you to present a mathematically self-consistent model that shows this, and we shall be happy to take a look at it. Of course it isn’t. That’s been known for at least the past 50+ years! I don’t know why you use the word ‘admitting’, as if this is something that secretive and kept hidden. It isn’t. Alternative theories of classical gravity, as well as theories of quantum gravity, are probably the two most active areas of research in modern theoretical physics. Just search for some related terms of arXiv, and see yourself the amount of hits you get. So yes - the idea that the domain of applicability of GR might be limited even within the classical regime, and thus that it might only be a special case of something more general, is most definitely conceivable, and is being actively researched; there is a very large number of alternative models in existence. Do note though that when doing a direct comparison, none of these models - with the possible exceptions of relativistic MOND and Einstein-Cartan gravity - come even close to the versatility and success of GR. Also do note - and this is important - that the domain of applicability of a model being limited is not the same as that model being wrong. We still use Newtonian gravity extensively to model scenarios where relativistic effects can be neglected, so the model continues to be right within its domain of applicability, even though we have had GR for over a century. It’s just that this domain is limited, and we have a pretty good idea exactly where those limits are. The same will eventually also be true for GR, though at the moment we have only a very rough idea where the limits might be, and only some educated guesswork about what lies beyond these. Within the classical domain, it is very unlikely that any successful model of gravity will have a notion of time that is substantially different from that of Einsteinian spacetime, because any such theory will need to preserve local Lorentz invariance (and thus CPT invariance), which puts stringent bounds on what such models can look like. There are also more fundamental reasons based in topology that dictate why GR looks the way it does, so the notion of spacetime isn’t just a wild guess. The domain of quantum gravity is another matter entirely though - it is not just possible, but nearly certain, that we will have to fundamentally rethink our notions of both time and space to make quantised gravity work. Note that GR would be the classical limit of any such model. So then, why is the GR definition a problem for you? It works really well.

Yes I can. As I have already pointed out, the kind of gravity we observe around us requires a very specific concept of time, which is the Einsteinian one. You can’t do away with it, or else the whole thing will no longer work. No, I’m not attached to anything (in fact, modified and alternative theories of gravity are a special interest of mine). I’m simply considering what works within a given domain of applicability, and what doesn’t. If you look at the classical low-energy regime, then clearly GR works very well in that it provides an excellent description of the phenomenology of gravity as we see and measure it, within its domain of applicability. The experimental evidence for this from the past 100+ years is incontrovertible and overwhelming. As I have said multiple times, this requires a specific notion of time - if you do away with that notion, or replace it with something different, then the model can no longer function. That’s all there is to it. Of course not. How could you think that? We already know that GR has a limited domain of applicability - at a minimum it cannot incorporate any quantum effects, and it is possible that even within the classical regime some phenomena might require modifications to ordinary GR. The jury is still out on that one. But that’s not the point here at all. This thread deals with your claim that time is not required for classical gravity to exist in the way we see it around us, and that’s manifestly false. MOND is only one example - there is in a fact a large number of alternative theories of gravity out there (none of which does away with time, btw!). The problem is that once you consider all available data (not just some isolated phenomenon), then none of the other models provide nearly as good a fit as GR does. We already know that it won’t be the final theory of gravity, but it really is the best one we have at present. It’s not - no one said such a thing. There are many different ways to define a concept of “time”, both in physics and in philosophy. The point we are making here is that classical gravity as we observe it around us requires a specific concept of time, being the Einsteinian one. Quantum field theory - which is the most fundamental theory we currently have - requires a Minkowski space-time background in order to work; the entirety of the Standard Model is formulated against this background, so Einsteinian time is as fundamental to QFT as it is to GR. To be precise, the Lorentz symmetry we find in local patches of Minkowski spacetime implies the CPT invariance of the Standard Model Lagrangian, and vice versa. One example of a model where a different notion of time is used is ordinary quantum mechanics - which is a low-energy, low-velocity, non-relativistic approximation to QFT that works only for systems where particle numbers are conserved. Here, time is simply a free parameter that is used to describe the evolution of a given system, it is not an observable of the theory, ie it can’t be consistently written as a Hermitian operator. GR and SR are models that describe aspects of the universe - as such they answer mostly only the how questions, but not the why ones, in the same manner as a map describes the topography of some territory without providing an explanation of the geological processes that gave rise to that territory. That doesn’t make the map any less useful, if you’re trying to find your way from A to B. The invariance of the speed of light is equivalent to saying that all inertial observers experience the same laws of physics; that’s an empirical finding about the world we find ourselves in. We don’t know yet why this is so, but if you really think about it, you’ll realise very quickly that the absence of this symmetry would immediately lead to physical and logical paradoxes that cannot be resolved, so at a minimum it is a matter of logical consistency. As for the gravity wells, the answer is in my signature. There is a well defined relationship between local sources of energy-momentum, and certain aspects of local spacetime geometry, as described by the Einstein equations. Thus, the relevant aspects of gravity in the interior of things like planets, stars etc are directly given by the distribution of energy-momentum there. But because spacetime as a manifold is everywhere smooth and continuous, the exterior vacuum geometry must match up with the interior geometry in a way that guarantees smoothness and continuity at the boundary. This provides a boundary condition that - along with the asymptotic behaviour far away from the central body - determines the exterior geometry. In most ordinary cases the embedding diagram belonging to this exterior geometry will look roughly like the “gravity well” you are referring to. This wasn’t the point. The issue is whether - given a geometric theory of gravity, be that GR or some of its viable alternatives - you can have gravitational radiation without there being time. The answer is clearly no. Thus, the presence of gravitational radiation implies some level of physical reality for time. Spacetime is not a physical medium, so there is nothing there that “vibrates” - that’s the kind of unfortunate misconception that is spread in pop-sci media. In reality, local geometry within some region of spacetime is determined not just by local energy-momentum, but also by distant sources. These do not appear explicitly in the field equations, but have to be provided in the form of initial and boundary conditions when solving them (they are differential equations after all). If it so happens that there is a distant source that has some form of non-vanishing quadrupole or higher multipole moment, then the local geometry will not be invariant under time translations, meaning you get tidal components that are explicitly time-dependent, even though all your local sources are stationary. This is how you get the characteristic effects of gravitational radiation.

The entire life cycle (from creation to decay) of elementary particles such as the muon can be visually observed in cloud chambers. You could also do something similar with flipping of spin directions, if one was to combine a cloud chamber with some suitable version of a Stern-Gerlach setup. The very existence of gravitational radiation depends on the reality of time (in the GR sense), so any type of gravitational wave detector is in effect an instrument that demonstrates the existence of “time” in a rather direct way. If you want to make things absolutely airtight you can combine gravitational radiation with movement-less clocks - so you could for example have a spatially extended sample of many decaying elementary particles, and observe the local variations in mean lifetimes of these as they are exposed to a gravitational radiation field. The effect would be exceedingly small, and way outside of what we can detect with current technology, but you get the general idea - not only will the mean lifetimes be effected even though these particles lack internal mechanisms, but all kinds of particles/clocks will be effected exactly equally, irrespective of their kind and internal make-up (if any). This demonstrates rather neatly that time exists independent of specific mechanisms.

I tend to agree. The “tick rate” of a clock is an observer-dependent quantity, so this type of question really does very little to demonstrate the underlying physics. A more illuminating kind of scenario would consider the total elapsed time on clocks, which is equal to the geometric lengths of their world lines, and thus a quantity that all observers agree on. Of course, one would have to connect the same set of events to make a meaningful comparison between world lines, which we don’t have in the OP’s scenario. As for the problem at hand, clock A on Earth is not inertial, but under constant acceleration; the same goes for distant B, unless it is far enough away that we can roughly consider it inertial. So you have two distant non-inertial frames, and one inertial frame in radial free fall. All of this is embedded in a curved spacetime, so one must also consider how to actually define simultaneity here, otherwise comparing distant clocks is meaningless.

The concept of Riemann curvature as it is used in ordinary GR applies in any dimension equal to or greater than 2. That means that yes, you can have “curved” 2D and 3D spacetimes as well. The big difference is the level of complexity - in 2D, the Riemann tensor has exactly one independent component, so it is simply a scalar; in 3D, it has 6 components, and can be shown to equal the tank-2 Ricci tensor. Hence these situation have a lot fewer degrees of freedom than we see in our 4D world. Geometrically speaking, in 2D you have only scalar curvature, so the only thing that can happen is that the area of a 2D surface differs as compared to the same situation in a flat spacetime. It is very simple. In 3D, you get a new kind of curvature which is Ricci curvature, which means that volumes may differ as compared to the same situation in flat spacetime. In 4D and above then you have, in addition to scalar and Ricci curvature, also Weyl curvature, which introduces (relative) tidal forces and shear between neighbouring geodesics, meaning it (roughly speaking) distorts shapes as compared to the flat spacetime situation. So yes, it is in fact possible and meaningful to talk about GR in three dimensions. However, since such as theory contains only scalar and Ricci curvature, but no Weyl curvature, the resulting phenomenology is very different from what we actually see in the real world - at a minimum there would be no gravitational radiation, no gravity at all in vacuum, and gravity in the interior of bodies would behave quite differently.

My post referred to gravity as we experience it (in the real world around us). Yes, you would also have some version of gravity in a universe without the GR concept of time (see my last post), but it would be very different from what we actually see in our universe.

The point wasn’t the decay process itself - rather, my point was that a well-defined period of time elapses between the creation of the muon and its decay, and that there is no “movement” of any constituent mechanism happening during that time, since it is an elementary particle. In the case of muons - nothing, because they are elementary particles without any “internal” mechanisms. Do you mean in the path integral formulation? If so, then no, the trajectories involved here are not in physical spacetime, but in the quantum system’s phase space; the path integral is a functional that returns a probability amplitude.

To sum up, “true reality” is clearly subjective, and meaningful only relative to a given observer: I’ll get my coat.

It is easy to find examples of processes that involve evolution of states, but no “movement” of any kind. Consider muons - they are elementary particles with a mean lifetime on the order of \(10^{-6}s\). As being elementary, they have no internal structure or “moving parts” of any kind; there’s nothing there that “moves” or even “changes” at all during their lifetime. And yet, they decay after a statistically well defined amount of time. So here you have got an example of an interval of time without “movement”; it’s an evolution of states without motion of constituent parts.

The universe begs to differ. Without time (in the sense that it is used in physics), gravity as we experience it could not exist - more specifically, there would be no gravity at all outside massive bodies, and gravity in the interior of mass-energy distributions would be very different, meaning there wouldn’t be stars and planets as we observe them. Physics makes models that describe aspects of the universe around us, like a map; it doesn’t do ontological claims. So you are confusing the map with the territory. As such, no physicist ever claimed time or spacetime to be a “thing” in the sense you use it here; and neither are any of the other elements of our models, such as space, energy, momentum, charge, spin, mass, acceleration etc etc. They are simply descriptions of certain aspects of the world. This does not mean, however, that time as it is used in physics isn’t necessary concept - it very much is, because if you were to take it out of our models, then you end up with something that is no longer a valid description of the world around us. And no, replacing time with some nebulous notion of “movement” will not fix this. General Relativity works exceedingly well as a model of gravity - thus calling it a “fallacy” is completely meaningless. Once again, you are confusing the map with the territory.

The other problem is that, even if the environment of the planet is only slightly different than that of the home world, evolution will take its very own cause as soon as you got it going - so after a sufficiently long time, you’ll end up with something that doesn’t resemble your home world very much.

Completely agree, I’ve got the same experience.

Yes, the vagueness is inherent in the languages themselves, not just an artefact of the writing system. But as I mentioned, you can make things more precise by adding extra elements to the sentence, if that’s required - e.g. 我去 adds “I” (as opposed to you, they etc) as a subject who performs the act of going someplace, yet still without any indication of tense or modality. But the tendency is always towards ‘minimalism’ in a sense - you don’t explicitly say what is already known from context, unless you strictly need to.

Then you either misunderstood me, or perhaps it is my own fault and I gave off the wrong vibes here. This thread started off on the topic of UAPs, and how these may or may not be of extraterrestrial origin. We then spoke about how probable and improbable it is that such civilisations exists, followed by some general comments on their possible motivations for coming here to visit us (or not). I then stated that I am personally partial towards the Dark Forest scenario, and this was meant as a possible solution to the Fermi paradox, since, in the absence of further data, it is one of the few conjectures that has at least some scientific grounding (ref game theory). Several aspects of DF were then discussed in more detail, including how pre-emptive strikes across interstellar distances could be practically implemented - my aim here was simply to show that this cannot be ruled out on technological grounds. Please take careful note of the highlighted bit above. The entire discussion was about DF as a solution to the Fermi paradox, which, in my opinion, is a valid discussion topic on a science forum. I also stated that I am partial to this particular solution, and I meant this in the context of this solution as opposed to other possible solutions of the paradox. I did not imply that, on a personal level, I somehow advocate violence, mass destruction or genocide. Under no circumstances have I ever personally condoned, nor will I ever condone, any such acts - and quite frankly, I am astonished that someone would have understood my posts here as being indicative of my advocating genocide. So, just to make this perfectly clear: on an ethical and personal level, I find the DF scenario to be abhorrent and frightening, and would gladly rule it out as a possibility if I could; this, however, is not a valid argument against this scenario, since a) we can’t know what kind of - if any - ethical systems alien civilisations would abide by, and b) in a scenario such as DF, acting ethically might seriously compromise your prospects of survival, so even a moral civilisation may still choose to prioritise survival over all other considerations. In such contexts we need to be able to have difficult conversations at times, even if they go against our sensitivities; and more importantly, we need to resist our natural tendency to assume that alien species must necessarily be subject to similar psycho-ethical motivators as ourselves. What I advocate in our present state of ignorance is caution, since we have no way of knowing who or what is out there, and how they might relate to us; hence I find it unwise to loudly proclaim our presence and location through sheer carelessness, which is unfortunately what we are doing right now (and I don’t just mean radio silence, which is only a small part of this). If a direct encounter of some kind does happen (inadvertently or otherwise), then I would advocate every possible effort at peaceful communication - but this may prove extremely difficult, due to the constraints placed upon us by the laws of physics, and also the compatibility problem. Note again that all of this is purely conjecture. We’ve talked about this earlier on this thread. By not advertising your presence, or at least not your exact location. There really isn’t any other effective defence, since you can’t know what precise form such an attack would take. This has already been discussed earlier. This has also been discussed already. Population growth is not one of the three basic assumptions in DF. It’s too late for this, since we’ve been leaking EM signals for the past century or so. On the other hand, as someone has pointed out quite rightly, it would take some extraordinarily sensitive equipment to detect these at distances larger than a few LY. Well, I would think that they are either too far away, that they don’t use electromagnetism as the basis for their technology, or that they are just far enough away so that their signals are already here, but they are too faint to be distinguishable from the radio background. Or, perhaps, there just simply isn’t anyone out there who’s on par with or ahead of us. It is at least conceivable that we could, in fact, be the most technologically advanced civilisation in this galaxy - which is another possible solution to the Fermi paradox.

In Thai and Chinese, as well as some other Asian languages, verbs are not inflected at all, and pronouns are used only if absolutely necessary to avoid misunderstanding, ie if the information is not already implied by the context. So for example, in Chinese, 去 might mean “to go”, “I go”, “we went”, “you’ll go” etc, depending on context. These languages thus simply express the idea of “going from here to someplace else”, and leave the rest up to context. You can, of course, add specifics if you want to, but unlike in many European languages, those are optional. In Thai in particular, there’s quite some emphasis on not being specific, unless strictly necessary.

Yes, indeed. The ‘Self’ is an idea (which is one type of mental construct)…and as such it is manifestly real. But what the idea refers to…not so much. One can also say that the Self is a process, rather than a thing. Every moment of experience is made self-referential. We don’t generally experience objectively as in “there is pain”, “there is a visual impression”, “there is happiness”, but like “I am in pain”, “I see”, “I am happy”. It’s an on-going process of selfing.

I know very little about numerical GR, but from what I do know, this type of simulation is prohibitive expense in computational terms. It a shame that a model like GR, which is conceptually so simple, is so hard to actually solve! Yes, very true. A lot of investigative work still needs to be done here.

I don’t know an awful lot about philosophy, so I’m not qualified to comment on any “rights” or “wrongs”. It just seems obvious that in terms of phenomenology, all that is ever directly experienced is the presence of interconnected thoughts, thus we can safely conclude that there is a thinking process going on. The existence of an “I”-agent that is somehow “doing the thinking” is only inferred, but never empirically observed. Thus it seems suspect to state its presence as if it was an empirical fact. But that’s purely my own two cents about this topic.

I came across this by chance the other day, and felt that this is a particularly good explanation of the meaning of “metric” in the context of GR, so I thought I’d share it here for whoever is interested: Some nice visualisations in there. Also, the other GR-related videos on this particular channel are quite good. No particular point for discussion, I’m simply sharing this for all those who wish to learn more about this subject without having to dive too deeply into the abstract maths. Particular emphasis on comments from time stamp 19:40 onwards (on coordinate vs proper measurements), as that’s were all those many common misconceptions arise. EDIT: Apologies, I’m just after realising that there’s an open thread for posting YouTube science content: Moderators, feel free to merge my comment there, if necessary.

The extra effects here are highly non-linear, so there wouldn’t be any straightforward correlation - what it comes down to is that for some systems, it is in fact necessary to use exact solutions to the full Einstein equations, rather than linearised approximations, even though such systems are not ordinarily considered generally relativistic. To put it differently, within such systems, the error introduced by treating gravity as a linear perturbation of flat spacetime is much larger than is naively expected, due to additional non-linear effects in the full non-perturbative EFE. The big question is what characteristics must be present within a given system for this to be the case, and that question is still very much open. It appears that it being rotational may contribute to it, but it may also be down to other things. Within non-linear systems, it is very difficult to tell how much error margin you introduce (relative to the full non-perturbative solution) by linearising it locally, and then cutting off higher-order terms in the expansion to make it computationally accessible. Which is essentially what is happening here, since complex systems such as galaxies can only ever be simulated using simplifying approximations.

Except that it should be “There are thoughts, thus there is thinking”. The “I” is not part of the phenomenology here.

The distances are relevant for the original problem, being that galactic rotation curves flatten out at large radii, ie far from the galactic center. This is not what we’d expect based on ordinary Newtonian gravity, hence the ‘traditional’ need for Dark Matter. The deviation between weak-field GR and Newtonian gravity seems to arise specifically for rotational systems, at least that’s the kind of systems almost all of these papers investigate. This is to say that within such systems, one cannot naively equate the weak-field regime to simple Newtonian gravity - the difference between the two (according to some of the above papers) amounts to as much as 30%. IOW, there are circumstances were you can get very significant non-linear GR effects, even though the situation only deals with weak fields. Traditionally such systems have been treated as Newtonian, on the assumption that any GR effects would be minute and thus negligible; but now it might turn out that this was a fundamental error on our part. However, we need to be cautious here, because Dark Matter is also observationally relevant in systems without any significant rotation, for example the Bullet Cluster. So unless it can be shown that such weak-field non-linear effects also occur in more general systems, at least under some circumstances, then this whole thing may still not provide a good explanation for Dark Matter. But it’s an interesting line of research that’s well worth pursuing further. But it’s mathematically very challenging.

Interesting paper from back in July (how did I miss this?), co-authored by Giorgio Immirzi, one of the foremost experts on GR and quantum gravity: https://arxiv.org/abs/2207.04279 This is just the latest paper within an increasingly large body of work that indicates that ‘dark matter’ as a separate phenomenon may be entirely superfluous. The basic idea here is that, under certain specific circumstances, even in the weak-field and low velocity regime, there may be non-negligible GR effects that aren’t found in Newtonian gravity. Hence, sometimes Newtonian gravity is not a valid approximation to GR in the weak field domain - which is the very assumption from which the idea of dark matter arises in the first place. What’s more, it turns out that under certain circumstances not even weak-field approximations of GR (such as GEM for example) are valid - specifically, this appears to be the case for rotating systems. In other words, within this paradigm, dark matter would neither be some new exotic form of particulate matter, nor is it a modification of the laws of gravity; it’s quite simply an artefact of us not having applied standard GR correctly, because the assumption “weak field”=“Newtonian” does not always hold. Small selection of other papers along the same lines: https://arxiv.org/abs/2207.09736 https://link.springer.com/article/10.1140/epjc/s10052-021-08967-3 https://www.worldscientific.com/doi/abs/10.1142/S0218271808012140 https://arxiv.org/abs/astro-ph/0610370 https://academic.oup.com/mnras/article-abstract/496/2/2107/5850386?redirectedFrom=fulltext https://arxiv.org/abs/1509.09288 https://arxiv.org/abs/2112.04116 https://arxiv.org/abs/2109.13515 https://arxiv.org/abs/2102.11282

When I look at the history of even just the 20th century, I cannot help but disagree. But that’s really not the point of this thread. That’s true, but unfortunately we cannot speculate on what kind of ethical standards - if any - an alien civilisation may adhere to. It’s easier to speculate about technology. I mentioned it as one of the many possible solutions to the Fermi paradox, so I think Dark Forest is perfectly entitled to appear on a science forum, especially since it is a direct consequence of game theory. The original impetus for this was the idea that an alien species being advanced and civilised must necessarily imply them being benevolent and moral - while I very much hope for this to be so, there is really no evidence that such an implication necessarily holds true. Please remember that Dark Forest is not my idea - it’s the mathematical result of some basic assumptions, plus game theory. We can’t just dismiss this only because we don’t like it on moral grounds. You cannot know this. This discussion wasn’t about us and our survival specifically; it was sparked by the Fermi paradox, and the question as to the probability of us being visited by alien spacecraft (UFOs and all that). I never suggested that we should attack anyone - the question as to what should be done if we ever did detect an alien civilisation is a different conversation. I simply suggest that it might be unwise to advertise our presence and location too loudly, until we have a better idea what the consequences of such an act might be. I do not think that is unreasonable or paranoid.