Markus Hanke

Unfortunately I have yet to find good literature on this topic that is accessible to the general reader. Most papers about this spacetime out there are quite mathematical, including the one you quoted. If I can find anything, I’ll post it here on the forum. Generally speaking, the Vaidya solution is a generalisation of Schwarzschild in that spacetime isn’t assumed to be empty, so it is one of the simplest non-vacuum solutions to the field equations. It’s basically the kind of geometry you get for a spherically symmetric, non-charged, non-rotating body immersed in uniformly ingoing or outgoing matter or radiation. As a result, the parameter M in the metric now depends explicitly on time, so this is can be used as a toy model for a growing or evaporating black hole.

Well, it would be like being confined to a perfect sensory deprivation chamber that is somehow able to completely suppress all external inputs. All that remains than are internal inputs, ie thoughts, memories, dreams, etc etc. Most people will probably consider this the highest form of torture. I can’t really answer these questions, also because this is not my area of expertise. Out of all the current attempts to explain consciousness, Integrated Information Theory seems to make the most sense to me, in which case the precise nature of the physical substrate underlying consciousness really isn’t relevant. Neurons are an extremely efficient solution, but in principle at least a network of machines that work in similar ways should do the job just as well. And yes, if IIT holds any water at all, then there should be degrees of consciousness, depending on complexity and structure of the network in question. In the future there might be ways to experimentally test this, but right now I think it’s pretty much all conjecture.

Precisely! +1 No such observers physically exist. The closest you can come to it is to describe the situation only in terms of generally covariant quantities, ie quantities that do not depend on the choice of observer at all - meaning all physical observers must necessarily agree on them. That’s pretty much a God’s eye view on the situation. Mathematically, this means tensors (and spinors) of any rank will be used to describe the physics. For the situation at hand, the obvious choice would be the length of the in-falling particle’s world line (which is by definition equal to proper in-fall time) - which is finite and well defined, and as being a rank-0 tensor everyone agrees that it is finite and well defined. There’s also another issue - coordinate in-fall time diverging to infinity is valid only in Schwarzschild spacetime, which relies on a number of boundary conditions, most notably asymptotic flatness. In other words, to get Schwarzschild spacetime, you need to assume that the universe is everywhere completely empty so that spacetime far from the black hole is approximately flat. Obviously, in the real world, this only ever holds at most as a rough approximation - in reality, there will be stuff orbiting or falling into the black hole, and other gravitational sources at various distances. If you account for in-falling matter and/or radiation, but retain all other symmetries, then you end up with a different kind of spacetime, called Vaidya spacetime - and here even the coordinate in-fall time as measured by a distant observer is quite finite (though much longer than the proper in-fall time in the particle’s frame).

No, not really. I was just aiming at the question of how to define “conscious”, as opposed to a machine that merely appears to be conscious. The difference between these is surprisingly hard to define. As for the nature of consciousness itself, I would conjecture that it probably arises as a global property of a complex system such as the brain, so I would think it resides in the global connectome and signal timings of the brain, rather than the nature of the individual building blocks. So if you were to replace all the neurons with machines that process inputs and outputs in the same manner, and are connected in the same way, then that new machine brain should be just as conscious. But of course, that’s purely conjecture - perhaps I’m entirely wrong on this. Yes, I did mention that we need to provide sensory channels, or else the results will be unpredictable. It would also be an ethical issue - imagine finding yourself conscious as a disembodied brain with all sensory channels turned off? Not good.

Yes, but evolution continues to function, meaning those chimps will continue to evolve - what will they look like in another million years?

All very good points +1 Just in case it came across differently - I am not trying to be argumentative about any of this. The reality of the situation is that we are all speculating here; there is really no hard data available to us to privilege any of the possible solutions to the Fermi paradox over any other, never even mind the issue of us not even knowing the complete set of all possible solutions. I simply think that DF is a scenario that, based on what we do know, and based on certain mathematically considerations, cannot be readily dismissed - as unsatisfying and scary as it is. But truth be told, this is one issue where I would genuine love to be proven entirely wrong

There are no searches for the graviton taking place, for two fundamental reasons: 1. There is simply no physically reasonable detector that would be able to unambiguously detect individual gravitons, since their interaction cross section is so extremely small 2. The concept of ‘graviton’ comes from applying the tools of quantum field theory to General Relativity, in the same way as we do for the other forces. We already know that the resulting model cannot be renormalised - it contains infinities that cannot be removed, and thus it is impossible to extract physically meaningful predictions from such a theory. Gravity just doesn’t work the same way as the other interactions. It is therefore highly doubtful that the naive concept of ‘gravitons’ has any physical meaning at all.

But that doesn’t answer the original question. How do you define “conscious”? It seems to me that there’s really no objective standard for this; it relies entirely on either self-reporting, or on behavioural analysis, neither of which are reliable indicators. Or let me put the question differently - suppose you build a machine the goal of which is to approach the anatomy and function of the human brain as closely as possible. The basic building blocks are miniaturised computers that process inputs and produce outputs in the same way as neurons do, ie as electrochemical potentials with the proper timing. You start with a single one of these - I think we can all agree that there’s no conscious experience here yet. Now you begin to add more and more of these computers, and connect them together at the same degree of network complexity as would be found with real-life neurons in the human brain. Further assume that along the way you provide sensory channels similar to those us humans have, but all based on miniature computers. Will this network ever become conscious? At what point does this network become “conscious”? And how can you tell that it has become conscious? Remember we will eventually have an exact replica of the human brain, except that, instead of biological neurons, it is made of computers.

Yes, that’s a good point. I should remind you though that the concept I described about relativistic projectiles was just my own idea of the simplest possible way to go about this. Obviously, if the target civilisation is spread out, then this would call for more sophisticated tactics. No, it’s just game theory. I kind of come from the opposite direction - I find the assumption that all advanced civilisations must necessarily be ethical and/or benevolent to be questionable. It is also a very dangerous assumption, should you get it wrong. Of course I would want the more benevolent scenario to be the case, but…well. I’m personally hoping you are right. Unfortunately, Bright Forest (which btw isn’t an official term, it’s just something I came up with in my last post) relies on these civilisations either being able to effectively communicate and thus come to an agreement that ensures peaceful coexistence; or on there being some kind of universal ethics that is somehow shared between all highly developed forms of life, even prior to contact, and which prevents someone acting like in the DF scenario. Effective communication is highly constrained by the laws of physics, as described in my last post, and also by the compatibility problem. Basically, if you aren’t reasonably close to one another, both in spatial as well as psycho-cultural terms, then meaningful communication will be extremely difficult. I find it exceedingly unlikely for this not to be the case, unless the galaxy is swarming with intelligent species in close proximity who have somehow all managed to work around the compatibility problem. As for ethics, there’s really nothing at all we can say, since we ourselves are the only available data point. So I can’t guess as to any probabilities here. It’s too late for this anyway. We’ve been unwittingly bleeding all manner of obviously artificial EM radiation out into the cosmos, so if there’s anyone within a radius of ~100LY or so who cares to monitor these EM bands using sensitive enough receivers, then they will already know that we are here. For better or worse, the stay silent option is no longer available to us.

It causes a clash only so long as you tacitly assume that there is only one concept of ‘time’ that is somehow equally valid for every observer anywhere in the universe. But GR tells us that this is not so - there is no global, universal time. There are only local clocks, meaning time is a purely local concept. And because time is local, you cannot use a distant, stationary clock to try and figure out what happens to an in-falling particle; you have to use a clock that is actually local (ie comoving) with that particle. That being said, it is important to remember that the distant stationary observer is still right about his own conclusions - but he is only right in his own frame of reference. For that distant observer, the particle really does never reach the horizon. Likewise, the observer attached to the in-falling particle is right about his conclusions, again in his own frame of reference; for him, the particle really does reach (and fall through) the horizon. So you have a situation where you got two observers who arrive at completely different conclusions, yet they are both right! This flies right in the face of everything we are used to, based on our own direct experience of what the world is like. But the trouble is that our experience is limited to a very specific domain - the classical, low-energy, low-velocity, non-relativistic, Euclidean domain. Within this domain, space and time have the same meaning everywhere, and can be neatly separated - so there is no distinction between “local” and “global” in that sense. But around a black hole, things are very different - time and space are inextricably intertwined, and measurements of time and distances are meaningful only within small local regions. You cannot project some specific observer’s notions of time and space someplace/sometime else, and expect to be able to tell what happens there locally. Needless to say, once the situation is analysed properly using the appropriate mathematical tools (which aren’t necessarily intuitive), there is no paradox, nor even a contradiction - this is all entirely self-consistent and logical, though counter-intuitive when viewed in light of our own Euclidean-based experience of the world.

No, just the opposite - the entire scenario rests on the assumption that the laws of physics as we know them (specifically Special Relativity) cannot be circumvented, irrespective of your level of technological development. If it was possible to communicate FTL over large distances, then the parameters of the game will change fundamentally - you could then talk to the other races, observe them in real-time, ascertain their intentions, negotiate, and come to an agreement as to continued peaceful co-existence. Game theoretically, this will then become the most rational course of action. You could call this the “Bright Forest” scenario perhaps - it’s a situation where you can see the others, observe and study them, and find some way to coexist. The problem is that, given our current knowledge of physics, this scenario is ruled out on fundamental grounds. It’s not invasion, it’s annihilation. That’s not the same thing at all, because the latter is done from a distance. This is in fact disturbingly easy - all you need to do is fire a small, dense projectile (say for example a chunk of dense metal the size of an aircraft carrier) at just the right high-relativistic speed at the target planet. The speed must be just right - too slow and it won’t have the necessary oomph, too fast and it will likely punch right through the target and out the other side. But if it’s done right, all the kinetic energy should become free on impact. With catastrophic results. Of course this will still take time (and some accurate and reliable maths) to execute, given the distances involved, but it’s doable. All you need is enough energy, and some mechanism to sufficiently accelerate and aim your projectile, which shouldn’t be too difficult for an advanced civilisation. Yes. But game theoretically, in a scenario like DF, it is still the best and most rational among all realistically available options. What are the alternatives? You could just do your level best to ensure you remain invisible and undetectable to everyone else (hide); or you can simply not do anything and hope that the others won’t attack you due to ethical considerations (hope). Or you can take a gamble, and broadcast a message of the type “DON’T ATTACK WE ARE PEACEFUL” in all directions, and hope that anyone who picks it up will believe you. How credible such a message would be, given the state of current affairs here on Earth, is another question; and sending such a message would be like lighting a beacon, since everyone will know exactly where you are located. So it’s basically attack, hide, hope, or come out of hiding and show yourself. None of these are pretty, but it can be shown that pre-emptive attack will maximise your chances of survival in a game like this.

I think you are using a different definition of ‘time’. In physics, not only can it pass, but it must do so by definition - time in this particular context is simply what clocks measure, and this is the definition I am using here. As such, everything always ages into the future, always and inevitably. That is precisely what “time passes” refers to in this context - the ageing of a physical system into the future, as measured by a comoving clock in its own reference frame. Thus, the body (as a physical system) is no longer in the same state when your subject awakens as it was when they fell unconscious. Time “passing” means that the co-moving clock has advanced. If you are using a different definition of time than physics does, then that’s fine (I know there are a few other possibilities), but you need to be explicit about which one it is you are using, because these concepts are not interchangeable. Of course. No one would claim such a thing. Your birthday is in the past, isn’t it? It hasn’t “moved through time” with you. The point though is that physical systems evolve (age) into the future irrespective of whether they are conscious or not, in the sense that a clock comoving with that system will inevitably advance. Yes, but this is useless for the purpose of doing physics, because there’s no physical instrument that can measure this in a repeatable and objective way. It isn’t even reliable for the observer himself, because the sense of subjective time is just a model constructed by the brain (as is the entirety of the “flow of experience”), and as such it can get distorted or fail in all manner of ways, eg through disturbances in brain chemistry, or as the result of particular types of brain injury (dyschronometria). The same is true for the sense of space, btw. I don’t know what this means, because physical time isn’t a ‘thing’ or a valid point of reference relative to which anything could flow; it’s just part of a map of events that we call ‘spacetime’. Are you claiming that consciousness is an ontological entity separate from spacetime and the particles/fields that live on it?

Yes it does - he ages into the future regardless of whether he is aware of that or not. Even if the subject in question is the only observer, all he needs to do is measure the ratio of naturally occurring radioisotopes in his very own body before and after the period of unconsciousness, and it will be obvious to him that objective time has passed while he was ‘out’. The human body is essentially a giant clock in that sense, and this is an easy experiment to perform. Also, if he was out for long enough (eg in a long-term coma after serious head trauma), the fact will be rather obvious to him from the deterioration of muscle and bone when he reawakens, even without isotope measurements. From a physics point of view, all particles trace out world lines in spacetime, meaning they always age into the future, even if they remain static and stationary with respect to some reference point. PS. It is quite unwise to post your email address publicly on the Internet for anyone to read and any bot to harvest, unless you want your inbox to get inundated with spam.

How do you define “consciousness”?

Yes, absolutely. That is why you need to remain invisible, undetectable, and silent yourself, so all anybody ever sees is at most shadows moving in the dark. If done right, your opponents won’t know where to direct that second shot at. It’s called ‘Dark Forest’ for precisely this reason - it’s a game of wiping out all potential threats, ideally before they even become aware of your own existence, in order to ensure your own survival. Like being lost in a deep dark forest, surrounded by silent and unseen predators. In this scenario, the resolution to the question of whether or not aliens exist might one day appear in the form of a small but massive projectile coming at us at high-relativistic speeds, and we’ll never even know who took the shot or where it came from. It’s pretty terrifying, and I sincerely hope this is not how things actually are out there. But unfortunately it is a rational scenario backed by game theory and fully compatible with the Fermi paradox, so one cannot simply dismiss it.

You probably mean gravitational waves - gravity waves are a phenomenon in fluid dynamics, and have no relation to black holes. The answer to your question is threefold: 1. Gravitational radiation during BH mergers does not originate only at the event horizon, but results from the quadrupole moment of the binary system as a whole. Any kind of binary system - irrespective of what kind of objects it is comprised of - will be a source of such radiation. It is, in that sense, a global phenomenon of such spacetimes, and its source cannot be localised to any one particular point or region, including the event horizon. That being said, the geometry of the horizons reflect the geometry of all the rest of this spacetime (in very complex ways), so observing the wave forms of the radiation field allows you to extrapolate what happens at the horizon during the merger. This whole process is really a global one, and doesn’t just happen at the horizon. 2. The diverging in-fall time you are referring to applies to Schwarzschild black holes, but the spacetime in a binary system of in-spiralling BHs is not of the Schwarzschild type, not even approximately. Figuring out the precise in-fall time of a test particle from far away into one of these BHs is a highly non-trivial task, that can only be done numerically, but my guess is that it wouldn’t be infinite at all (one of the necessarily prerequisites for an infinite in-fall time is asymptotic flatness, which does not hold in this type of spacetime); it would also explicitly depend on where and when the test particle begins its free fall. 3. Even for Schwarzschild BHs, the infinite in-fall time applies only to test particles moving on time-like or null geodesics of the undisturbed background, ie it applies only to test particles whose own gravity can be neglected. This, however, is not the case for gravitational waves, which will couple to the background curvature in non-linear ways. To put it differently, a spacetime that contains gravitational radiation cannot have Schwarzschild geometry, and thus the infinite in-fall time does not necessarily follow. Even in cases where the gravitational waves are weak enough so that the background could still approximately be treated as Schwarzschild (which is not the case for a binary BH system!), the wave fronts wouldn’t propagate along the same trajectories as free-falling test particles, due to non-linear interactions with the background curvature. No it doesn’t. The length of the world line of a test particle free-falling from far away and crossing the horizon is finite and well defined; spacetime at and around the event horizon is smooth and regular, so time proceeds as normal there. The thing with this is that in curved spacetimes, there is an important difference between coordinate in-fall time and proper in-fall time. The coordinate in-fall time is what a distant observer will calculate and observe, based on his own instruments, which are not themselves located at the horizon; the numerical value of this will depend on which observer you choose. The proper in-fall time, on the other hand, is what is directly measured by a clock that is attached to the freely falling test particle itself; by definition, it equals the length of that particle’s world line through spacetime. For the case of a test particle freely falling into a Schwarzschild black hole, a far-away stationary observer will determine a coordinate in-fall time that diverges (goes to infinity). However, the proper in-fall time of that same test particle is finite and well defined, so the particle reaches the horizon in a finite amount of time as measured by its own clock, and continues falling through the horizon and into the singularity (also in a finite, well defined amount of time). Because of the way that proper time is defined mathematically, all observers agree on it. On the other hand though, coordinate time is always specific to a chosen observer, and not valid anywhere else. In curved spacetimes, time is a purely local phenomenon; a far-away observer does not share any concept of simultaneity with processes that happen at the (for him) distant horizon.

Yes, that’s a good and valid point. However, one must remember that we ourselves don’t hesitate to annihilate large groups of sentient individuals belonging to other species, if our own interests are threatened. Consider - just as a random example - spraying large swathes of agricultural land with insecticides, which is still common practice in many places. Such acts lead to the death of millions of insects, and we don’t bat an eyelid. Why? Because very many people do not consider insects to be worthy objects of moral concern, since we regard them as primitive, expendable, undeveloped, unintelligent, and a direct threat to our own interests. If they annoy or threaten us, we simply annihilate them. The crucial factor in the DF scenario is incompleteness of information. If two civilisations are, say, 10000LY apart (not unreasonable if there are only a handful per galaxy), and assuming the laws of relativity as we know them cannot be circumvented in some way, then there simply isn’t any workable method for these civilisations to talk to each other in any meaningful sense. As a consequence, neither one of them can truly know what the intentions of the other one are, and (more importantly) what their current level of development and technology capability is, since the latest available information about them will be 10000 years old. You cannot extrapolate how a civilisation might develop over such long periods of time, especially not based on limited data. So you are pretty much left completely in the dark - you have no way of knowing what they are up to, what their intentions are, and how they think about you. Of course you can assume that they are moral beings, but that’s a huge gamble to take, and if you’re wrong then that’s the last mistake you’ll ever make. So that’s the basic conundrum - the speed of light is very slow when taken to even just galactic scales.

True. And personally I’m partial to the third option - that we simply aren’t going to see any alien civilisations any time soon. But we may well see more primitive forms of extraterrestrial life in the near future, perhaps even within our own solar system. I’d also like to point out that, in a Dark Forest scenario, a civilisation acting in the most rational way within the confines of that mathematical game (ie eliminating other civilisations) does not imply malevolence on their part. They simply do what they need to in order to maximise their own evolutionary potential in what is a scenario with few other options, given the constraints imposed by the laws of physics. Even an otherwise benevolent and ethical civilisation may find it necessary to take such drastic steps. Also, just because we place a high ethical value on life (do we??) does not necessarily mean that others share this concept.

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No. Dark Forest would suggest that in the first instance, civilisations remain silent. When you have a substantial number of intelligent races all confined into a galaxy where the total amount of available resources remains constant while civilisations continue to evolve and expand, 2-way communications are subject to the laws of relativity, and all these races are motivated by a basic desire to ensure survival of their own species, then you have what is a called a sequential and incomplete information game. You can now apply the tools of game theory to this scenario, and the result is that the most advantageous and rational course of action (relative to the basic motivation of survival) for each civilisation is to eliminate all known competitors as soon as they are discovered. Conversely, in order to avoid getting eliminated yourself by others, you remain silent and undetectable. So this isn’t about conquering others, but about annihilating them from the game - which can be done easily and effectively from a distance, once you know where they are. In fact, you would want to do this so long as they are still largely planet-bound. The resulting situation would be a galaxy that is potentially full of advanced races, but appears empty and devoid of intelligent life, since these races use their technologies to ensure that their presence remains undetectable by all others. Within such a state of affairs, a newcomer to the scene such as ourselves, who is unaware of the situation and is loudly advertising our presence through various acts of carelessness, will eventually find themselves getting annihilated. This is a pretty bleak scenario, and the question then remains whether such civilisations are subject to ethical considerations that are strong enough to override following the most rational course of action as described above. An interesting conundrum.

Hm. To be honest, I think to a neutral and external observer, the current state of humanity mightn’t look so appealing. There’s a lot of greed, hatred, delusion and ignorance. A very large percentage of our current population still lives in abject poverty. At any given time, there’ll be something like half a dozen armed conflicts in progress in different parts of the world. There are at present on the order of 13000 nuclear warheads available for launch (~90% of which in just two countries). Nationalism, militarism and various forms of religious and cultural extremism are rife everywhere. Even in developed nations with good education systems, the average person’s knowledge of scientific, philosophical, ethical, sociological and teleological concepts is at best rudimentary. Of course, our species has also achieved many positive things in the sciences, humanities, arts, etc etc - but it would seem to me that these sadly are not representative of our overall state of affairs, at least not at this point in time. Would I, as being an advanced civilisation, and based on what I am observing here on Earth right now, be inclined to share my knowledge and technology? I think probably not. A ultra-sharp kitchen knife is a really useful tool that enables you to cook a delicious meal - but you wouldn’t hand it to a toddler, would you?

That’s true, though it is not unreasonable to assume that at a minimum we share the basic motivation to ensure survival of our species. All known forms of life on Earth - irrespective of where and how they evolved - share this. Without such “motivation” (or more generally: biological mechanism), it is doubtful that any form of life would ever reach the stage of becoming a civilisation. I think such discussions serve no useful purpose, because we simply have no data available to us to base it on. It’s pretty much all conjecture. The only reason why I personally tend towards Dark Forest is that it is based on a minimal set of basic assumptions, along with the mathematical tools of game theory; it’s the most mathematical conclusion stemming from the smallest possible set of assumptions. I’m not saying that Dark Forest must be right; only that, among a vast array of guesswork and conjecture, it is presently the most scientific approach we have to this subject - though admittedly the outcome isn’t exactly pretty, and one might even say that it is somewhat scary. It would, however, resolve Fermi’s paradox quite neatly.

Yes. By “opposite” I simply meant the sign; the squared interval is the difference between the squared time part and the squared space part. The crucial bit is that whatever change occurs, it can only be such that the overall interval remains invariant.

https://www.quantamagazine.org/asymmetry-detected-in-the-distribution-of-galaxies-20221205/ https://arxiv.org/abs/2206.03625 Now this is very interesting! Despite the 7-sigma significance mentioned in the paper, this will of course require further corroboration. But if this turns out to be a real thing, then it would have far-reaching consequences. Definitely one to watch!

They do count as indications of some unusual event having taken place, but on their own they don’t allow us to conclude that said event was of extraterrestrial origin. As I said, the fact that there are unidentifiable objects and events out there isn’t in contention. No. As I have tried to explain, my assumption is that those photos which can be taken to be genuine (not photoshopped, not an intentional hoax) show unidentified objects. My problem is only that too many people jump to the conclusion “unidentified”=“extraterrestrial”, because this does not automatically follow. I agree with you, and at no point am I completely dismissing the possibility of us being visited by another civilisation. The probability for such a thing is not zero. I do maintain, however, that that possibility is extremely remote, even if intelligent civilisations are commonplace - which is why substantiating that claim requires some very hard evidence. Ultimately, to gain complete certainty that this is what is taking place, you’d need both one their crafts/probes to examine, as well as a specimen sample of alien biology. What we are seeing from all these reports really isn’t very consistent with how I’d expect an advanced technological race on an observational and scientific mission to behave - at a minimum they’d remain out of sight and detection, so as not to contaminate their observational data. Unless of course they don’t care for some reason, as in the scenario put forward in the fictional work Roadside Picnic by the Strugatsky brothers, but that’s even less probable. In fact, I see no reason why they would need to physically come here at all, since surely someone who has mastered interstellar travel would also have mastered observational techniques that enable them to collect the data that is of interest to them from a distance, as we ourselves are slowly starting to do with instruments such as the JWST. That’s a lot less resource intensive, which is likely an important consideration no matter how advanced you are. There are also important reasons to think that it might be a wise policy for you not to advertise your presence to anyone, as in Dark Forest Theory for example. I’m personally a bit partial to this.