sethoflagos

I've been watching your comments on feedback with increasing interest, and you raise important points a) because there is an awful lot of misunderstanding mixed in amongst the loose terminology, and b) because it gets really complicated really quickly. Loosely, feedback occurs whenever a process output is fed back into the input thereby modifying the subsequent output. However, there are some major provisos here, particularly with regard to causal links. 'Feedback' cannot as of current scientific concensus refer to the transfer of anything back to an earlier point in time. It therefore is not a transfer from an output back into the input that created that output. It is a transfer from an output phase into a subsequent input phase. Some examples may help explain: This one is curious since mathematicians tend to irk engineers by insisting that pendulum type system really are controlled by feedback, because... reasons. I think the reasoning goes that since the equations of motion for an ideal, frictionless pendulum are time-reversible, they do not contradict the assertion that the last maximum displacement was a consequence of the next rather than vice versa. ie mathematically they are indistinguishable from a process controlled by negative feedback. Maybe that's a simplification, but at least we both agree that this system does not feature feedback. Compare with a father pushing a child on a swing. Hopefully, the father monitors the vertical displacement on the forward swing, compares that against some recommended maximum enjoyment criterion and adjusts his push at the top of the backswing accordingly. If we assign a phase angle 0 to the top of the backswing, and a phase angle pi radians to the top of the forward swing, it is clear that the input is being modified with an antiphase addition from the output - classic negative feedback. Arguably so, I think. Consider the following reaction. 2H2S + SO2 -> 3S + 2H2O If the reactants are initially dry, the reaction does not proceed. But add a small squirt of water to get it started and the reaction rate will rapidly accelerate via positive feedback. The output product phase is 'fed back' (at least partially) into the input reactant phase by eg. turbulent mixing and the reaction becomes self-sustaining, limited only by the continuing supply of reactants.

At best, GH seems to say no more than stable is stable, unstable is unstable. What is there to disagree? At worst, it implies (at least for some adherents) that stable is good, unstable is bad. This strikes me as more of a political idea than a scientific one. Looking back over the last half billion years or so, the Cambrian, later Devonian, late Carboniferous, the Triassic and early Cenozoic were all periods of great biotic instability. These amount to a substantial time percentage of the total. But far from being negative, each of these episodes led to major diversifications and advances of life forms, far more so than the 'good' periods of relative stability. Arguably, stability tends towards a gradual decline as the reduced need to adapt quickly often leads to the loss of the power to adapt quickly. Evolutionary potential tends to be remain concentrated in the small, generalist and opportunistic forms who often lead the recovery of life after an extinction event. If you want a more political allegory, try the History of the Decline and Fall of the Roman Empire by Gibbons. Similar ideas.

I'm sorry, but I'm having difficulty following your reasoning here. Surely, for each stage of a simple lossless cycle, the output moreorless by definition eventually returns as the input - a positive feedback loop of unity gain. What am I missing here? Without feedback playing a substantial part in the carbon cycle, it's hard to see how tipping points could even be a thing, positive or negative. Again, I seem to be missing the point you're trying to make. Perhaps it's age catching up with me. Geological weathering, irrespective of the local climate, requires an external energy input to maintain itself indefinitely, doesn't it? Does not the overwhelming majority of that energy come ultimately from the sun? A clue might be in the large amount of sulphuric acid in the Venusian atmosphere. Carbonates and strong acids tend not to coexist for very long.

The carbon in the methane feedstock gets oxidised to CO2. The hydrogen product derived from the methane feedstock ultimately produces H2O. How is this significantly different to simply combusting methane in a single (and far less costly) process? By what Byzantine mental process are we to understand your use of the term 'green'?

Indeed. I can see how homeostasis manifests at an ecosystem level: populations that consume a vital resource faster than it can be replenished will see birth rates decline and mortality increase; likewise, genetically homogeneous over-dense populations are likely to be thinned out by disease. These are classic negative feedback loops that have an intrinsically stabilising effect on all ecosystems. Extending this idea to the far more complex biochemical processes of an individual seems problematic as @exchemist has suggested. There's just too much going on in there for such a simplistic mantra to help understand the fine detail. Midway between these two levels, the potential impact of a game-changing single point mutation in the genome of a single individual is worth considering. Even a modest increase in the reproductive success of that individuals descendants creates a self-sustaining positive feedback that can completely overturn the status quo in relatively few generations. Entire new phyla have arisen through such events. Evolution is blind to future consequences. It operates strictly in the present and cares not a jot for long-term stability. Weathering is driven mainly by insolation; plate tectonics is predominantly driven by the gradual loss of the planet's internal heat. Neither is steady-state in any sense other than their usually extremely slow evolution towards a final lifeless thermodynamic equilibrium. Hence: Yes * 2

Please be aware that we are not talking about thermodynamic equilibrium here as that is incompatible with life processes (which depend on the existence of various gradients). Some of us (me included) have used the term 'equilibrium' when what we actually intend is better described as 'steady state'. This supports @joigus comment on solar influx and the consequent gradients of temperature, pressure, concentration, density etc that drive most of the Earth's surface processes, both organic and inorganic.

Somewhat. I find it quite 'Little England-ish'. Rather like Lord of the Rings, it has a grain of optimistic truth here and there. @exchemist is correct. Processes may feature negative feedback promoting stability, positive feedback (eg Agent Smith-like autocatalysis) promoting instability, or quite often a combination of the two. A stable global state is dominated by negative feedback loops at it's heart. A large enough disturbance my drive the system towards the boundary where positive feedback loops are more influential. Ultimately, the latter take over and drive the system towards a different equilibrium state. One perhaps where biochemistry can no longer occur rendering all consideration of photosynthesis etc. moot. Post scriptum. I meant to mention Le Chatalier's principle. It colours much of my thoughts on this regarding the local primacy of negative feedback.

There are many potential states of relative long-term global stability embedded as islands within a sea of relative instability: During the Cryogenian epoch ('Iceball Earth') the planet was near fully ice covered for hundreds of millions of years whereas it was pretty well ice free for most of the Mesozoic. Hopefully we're a very long way from a tipping point towards a Venus-like climate, but that state would be remarkably stable. Much depends on the relative movements of the continental masses and, in very recent times, human activity. Both are pretty random phenomena.

Aren't air-conditioners, heat pumps and refrigerators all fundamentally the same thing from a basic physics standpoint? The differences seem only to be on PoV focus: which bit connects to ambient and whether the desired product comes from the heat source (cooling) or heat sink (heating) Perhaps not relevant to a domestic application, but industrially it is common practice to 'daisy chain' refrigeration cycles. For instance a methane cycle can extract heat at -175 0C exhausting to the cold end of an ethylene cycle at -100 0C which in turn exhausts to the cold end of a propane cycle at -15 0C which exhausts to ambient. This temperature can be (and often is) extended with say a cryogenic nitrogen cycle at the cold end. While the final exhaust is simply regarded as 'waste heat', each additional cycle of the chain significantly reduces overall efficiency. But what if we added additional high temperature cycles to the upper end? Domestic heating has been already discussed, and this recovers not just the thermal energy extracted in the low temperature cycles, but also the electrical energy consumed in compression. This turns around the economic balance appreciably since the thermal exhaust now has value comparable to that of combustion heating, and easily beats electrical resistance heating. Water/steam heat pump cycles could achieve temperatures of ~550 0C before metallurgy becomes a real issue, and low cost thermal energy at these temperatures could substitute at an industrial scale for fossil fuels in globally important endothermic reactions like nitrogen fixation, catalytic cracking, drying, etc. etc. but perhaps that's a topic for a different thread. The key point is that if value is created from both ends of a refigeration cycle, then the economics is drastically improved. Scale it up to provide hot water/space heating for a group of apartment blocks or similar densely populated area, then the challenge is less economic, more political (in certain less community oriented jurisdictions).

I understand it correlates with the Riemann zeta function of spin value (?). Some detail at https://mathoverflow.net/questions/414594/scattering-amplitudes-and-the-riemann-zeta-function

Then leadership of the free world must pass by default to the EU. The US is clearly unable to deliver leadership to match Scholtz and Macron.

Most of you seem to regard this as a US internal matter and of no one else's concern. But it was watched with interest in one bar in Lagos at least, by a bunch of not-much-younger- than-them expats of various nationalities. The consensus was that it amounted to a slanging match between the pub gobshite and the drunk in the corner who'd been there since yesterday. If that is the best that the USA can offer at this critical time then heaven help us all - which for an atheist doesn't give a great deal of hope.

Including Pongo, Pan, Gorilla and forebears? Just looking at late Homo erectus, adult brain sizes varied between 550 cc and 1250 cc depending on physical size and local environment due in major part to phenotypic plasticity - different populations adapting quickly to highly divergent habitats. Add to that the extreme rarity of good fossil crania and consequent large statistical uncertainty, you can imagine almost any trend pattern you like. But the data is just too scant and variable to justify it.

Villmoare et al (2022) have yet to see a serious challenge to their summary:

Aka: misogyny.

... is to accept that it isn't your prerogative to do the picking. Try relocating to somewhere where you stand out from the crowd and don't remind potential partners of an irritating sibling. Novelty provokes interest. After that, all you need to do is make them feel like their interest is worthwhile and reciprocated. Resist talking about yourself. It isn't rocket science.

As the OP specifically mentioned a tardigrade level of complexity, this would both push the timeline on by a couple of billion years and demand that those bio-friendly conditions persisted for this period without interruption by e.g. interaction with an active galactic centre (or indeed any galactic centre). The combined requirement for adequate metallicity and long duration stability may conceivably advance the timeline to a couple of billion years after the emergence of large, quiescent spiral galaxies: perhaps >50% of the current age of the universe. Ward & Brownlee and others have pursued these avenues through the Rare Earth Hypothesis. Not sure what you mean here. Something more advanced than tardigrades?

The temperature range 4000K to 60K corresponds to a period called the 'dark ages': after recombination but before the birth of the first generation III stars. The hydrogen + helium 'chemistry' of that era is not conducive to biology.

... or have both parties check whether a long pendulum suspended from a swivel precesses clockwise or anticlockwise. Difficult to explain how two points on a rigid plane could rotate in opposite directions without ripping apart.

Except the conflagration at Grenfell was due to burning of the exterior cladding so no escape route that way.

... so some say. I'm in no position to defend the concept at a deep technical level, however, others are and do so. Admitting the advanced wave solutions seems to offer neat solutions to some otherwise seemingly intractable phenomena such as EPR. Perhaps more pertinently, the apparent and 'unlikely' flatness of our universe just shouts 'negative feedback - asymptotic approach' to me. Okay, that sense implies some sort of retrocausal effect. It's a tough call, but I'd sooner trust to Feynman's gut feel than the dim designer of the OT.

Perhaps in the sense that the time evolution of the universe is 'just' thermodynamics. However, it's apparent preference for universes similar to ours, amenable to the evolution of life without appeal to either the anthropic principle or a 'creator', seems a valid enough point.

As I understand it (I may not), Feynman-Wheeler absorber theory postulates that every emitter is coupled to a future absorber. If this is the case, then it introduces an interesting aspect to the teleological argument: that there exists in the (possibly far distant) future, sufficient, and sufficiently diverse structures to provide all necessary absorbers. This suggests that the past is influenced by the consequences of its state and dynamics on its future evolution.

Just a poorly worded afterthought. I intended to say that in the absence of a ram pump or equivalent, the only driving force to hand is the velocity head of the source. On the other hand a ram pump could use a waste flow head drop of a metre, say, to generate surge pulses of up to 15 bar or so albeit for a much lower flowrate.

Yes, I'm well aware of how ram pumps work. But there was no ram pump in the system described, and therefore no means of channeling the input kinetic energy preferentially into the vertical output stream.