sethoflagos

I'm struggling a little to recollect beer mediated musings from 30-odd years ago, so please bear with me. Am I correct in understanding your last point as CPT symmetry reversal is not a physically realisable phase change in contrast to say electroweak symmetry breaking or recombination?

Probably the wrong word to choose. 'Pointlike' may be better. Suggesting that an instantaneous 'now' of zero duration doesn't exist. So a unit of Planck time, say, wouldn't have any clearly definable start or endpoint.

More of a dual-universe speculation. Perhaps one way out of the t=0 conundrum is to drop the idea of absolute time at these scales in favour of a sequence of time intervals, one of which happens to span t=0. A form of quantisation if you will. The surface of that cell should have no associated infinities, but half of the boundary surface is time-reversed and that (the point you raise) would need to be addressed. It's probably complete tosh, but the idea of two universes being spawned in opposite time directions has a pleasing symmetry to me.

How would we know? From what I understand of CPT symmetry (not much), an antimatter biased big bang expanding in the reverse time direction (from our perspective) would behave no differently to the universe we're in now, I think.

Not what the OP was asking. I don't live in the US. Sub-saharan diets typically don't include added sugar except for rare treats so your issue is less of an issue for the rest of us.

You're right, it's a question of degree. I seem to remember seeing the LD50 of carrots set at 40 kg. A bit much to eat at one sitting perhaps, but a caution to be wary of some extreme carrot concentrate. But the previously mentioned botulinum toxin is at an entirely different level. And cumulative poisons with no known positive biological function like lead and arsenic really have no clearly definable upper safe limit.

Sugar is simply not in the same league. It just happens to be something many of us choose to consume to great excess despite knowing that it will harm us. I managed to wean myself off sweets in childhood to the extent that even artificial sweeteners can make me feel quite nauseous. Yet the moment my weight drops below 70 kg (bout of malaria) I experience an intense craving for sweet tea. Quite a shock the first time it happened. But I learned that when my body really needs some glucose fast, it will tell me in no uncertain terms. So I don't buy that sugar is intrinsically 'deadly': quite the opposite. The real problem lies elsewhere.

I've downloaded it and will be very interested to see how it performs. I don't actually need much help in identifying a diederik cuckoo visually, but if the app picks up the die-die-diederik call I'll be impressed. I'll be even more impressed if it spots the very convincing local kestrel call the diederik makes when mobbed by the species it parasitises.

Fine distinction. I suspect such nuances would be lost on his target demographic. Maybe he was simply pointing out that sensationalist headlines can distort our perspectives. Some people need to be told that. He does undeniably important work in science communication so I don't really see the purpose of this thread. So what if he and his audience don't have much time for old white philosophers: it isn't as if an adult film starlet had spanked him with a rolled up copy of Scientific American or something.

Isn't this another way of phrasing Tyson's concluding sentence What point am I missing?

Just to be clear, I'm leaving discussion of resonance effects etc to others. They're two separate topics. In context, a shock wave signifies a pressure discontinuity - a step change in pressure level rather than the piecewise continuous waveforms of normal sounds. It indicates that something somewhere has moved faster than the speed of sound in air. It is characteristic of brittle fracture. No, that isn't it. When paper is wet, the fibre-fibre bonds are almost totally replaced by fibre-water bonds and water-water bonds. When torn it is the weak water-water bonds that dislocate leaving the fibres pretty well intact. Here there is no brittle fracture and hence no shock waves. The pieces come apart through viscous shear and what little energy is released is in the form of heat rather than sound. This is the opposite end of the spectrum to brittle fracture. Between these two extremes are various degrees of 'ductility' of which two common types are plastic deformation and viscoelastic deformation. They are a rather diverse bunch of mechanisms and are each separate topics in their own right. I'm not familiar with the product. I'd guess it could be somewhere in the viscoelastic regime but specialist adhesives would be more up @exchemist's street than mine.

Okay... Small steps. As the moisture content of paper rises from bone dry to something oto 30% by weight, the 'tearing' sound gradually diminishes to practically zero. Why do you think that is?

Define 'kind of'. What makes you think there is a vacuum under the tape? What do you understand by the term 'shock'? If it's 'silent', then what sound are you asking about?

Fracture mechanics can become very involved very quickly, but if we limit our scope to simple brittle fracture... Both fracture propagation speed and elastic recoil shock of a fractured solid are functions of the speed of sound in that material and typically many times the speed of sound in air. Therefore a space opens up faster than that information can be passed to the surrounding air resulting in a substantial vacuum. Eventually the pressure wave information gets passed on and the air rushes in at its own sonic speed to fill the void. When the void is filled the air it is brought to a halt with extreme rapidity and releases it's kinetic energy primarily as acoustic shock waves (variously known as 'surge' or 'water hammer'). This is the process that produces the loud 'crack' of a bullwhip. Now rather than one big whip imagine several thousand cellulose fibre 'whips' doing the same thing per second at a microscopic scale. That's the sound (at least a major component of it) of paper tearing.

Like a customised self-gravitating body ~100+ km diameter built from carefully redirected small asteroids to put it into a path of ejection from the solar system? Internal thermal energy may be a viable long term energy source (or nuclear). Deep subterranean accommodation caverns should give reasonable protection from small collisions. Not sure I'd pick the lifestyle choice myself...

RIP Dickey Betts

I can buy this. Nitrogen fixation is energetically expensive, so the evolution of BNF would not be favoured unless abiotic processes failed to satisfy the biological demand for fixed nitrogen. However, the claimed unavailability of molybdenum simply does not square with an early appearance of Mo based BNF. The two are mutually contradictory. The reference quoted for this low Mo condition is "Proterozoic ocean chemistry and evolution: a bioinorganic bridge?", Anbor & Knoll, Science 297 (2002). I've not seen the full paper, but the abstract describes the limited availability of (by inference) molybdenum under the Proterozoic (543 - 2,500 mya) conditions of oxic surface waters and anoxic depths. This is one and a half billion years after the period we are discussing and a very different chemical redox environment. Our present interest is in the availability of Mo3+ and Mo4+ in the anoxic surface waters of the early Archaean around 4 billion years ago, not Mo6+ in oxic waters of a couple of billion years later. Having now invested a bit of time in this, I'm not seeing any relevant evidence for the claim of low Mo availabilty during the period of interest.

Me neither. Unaltered sediments from those far distant times are exceedingly rare. Examples from just the right place and right moment are likely long gone. And yet life definitely started therefore there was sufficient fixed nitrogen around. So both the 'no molybdenum' and 'no fixed nitrogen' claims are belied by the undisputable evidence of our existence. It seems that somebody somewhere is extrapolating a small and questionable data set way beyond its scope of applicability.

Far be it from me to question this, yet the paper itself offers strong evidence of the early appearance of Mo based BNF which implies that Mo was definitely available to life in the oceans. Something of a paradox. Molybdenum is not a rare element, especially around hydrothermal sites. Perhaps its low apparent oceanic concentration at that time was a reflection of an intense biological demand that kept it locked up in biomass. Just a thought.

Bear in mind that for its first 2 billion years, the earth was a temperate water world with much reduced continents, and a mildly reducing atmosphere of N2 and CO2 perhaps with minor CO and CH4. Sediments from that period (unlike later deposits) generally feature pyrite, pitchblende and siderite indicative of reducing conditions throughout the ocean basins. Levels of volcanic activity were considerably higher than today, including forms like kimberlite pipes that recycled material from very deep within the earth's mantle, including large amounts of the heavier transition metals. However, it is possible that modern plate tectonics didn't really get going until after the major chemosynthesis and retinal based photosynthesis processes had developed so this happened in a somewhat different geological setting than we see today. Thermal vents must surely have played a part in the development of life - too good an opportunity to miss, but with many of the key micronutrients widely distributed in maybe more soluble reduced forms throughout the oceans, there seems less need for all the ingredients for abiogenesis to be concentrated at a single source location. Splitting up the task between a diverse range of mineralised environments, that may or may not have been closely spaced massively improves the odds in favour.

Who are the primary producers in the thriving communities around deep ocean hydrothermal vents? Not much photosynthesis going on down there.

The air bubbles reduce the overall density of the fluid mixture in the tube. Ignoring complications like friction etc., hydrostatic equilibrium demands that fluid surface height (measured from the base of the tube) multiplied by density is the same both inside and outside the tube. In principle, half the density, double the height. Though there are always losses in practice.

You might find this article on Prandtl number interesting and relevant. https://www.sciencedirect.com/topics/engineering/prandtl-number Edit: actually don't. Some of the numbers are out by several orders of magnitude (!) But the Wikipedia page on Prandtl number is okay for the first few paragraphs.

Add to this that gas thermal conductivity is directly proportional to CV favouring selection of the low specific heat monatomic gases, and that argon is relatively cheaply available from air liquidation facilities.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2907586/ states that 'possibly endosymbiotic' rod-like methanogenic prokaryotes were seen in close proximity to the hydrogenosomes during scanning electron microscopy. If so then perhaps the circle is closed at least in part by the reasonably exothermic (and biological carbon fixing) of CO2 reduction by H2.