an autodidact meets a dilettante…

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New Scientist 3244 August 24 2019

Canto: Being dilettantes and autodidacts, we engage endlessly in educational reading, bootless or otherwise, so I thought we might take the effort to talk/write about, and expand on, what we’re learning from the texts we’ve perused, rather than providing ‘content hints’ as before.

Jacinta: Well of course science mags cover a wide range of topics at very various depths, so we’re going to limit ourselves to the ‘cover story’, if there is one. 

Canto: So today’s topic comes from a New Scientist that’s been hanging around for a while, from a year ago, but since quantum theory is more or less eternally incomprehensible, that shouldn’t matter too much. 

Jacinta: Yes I’ve heard of Lee Smolin, and in fact we can listen to many of his online interviews and lectures via youtube, and he’s described as a ‘realist’ in the field, which doesn’t mean much to me at present, but neither of us know much about quantum mechanics, in spite of having read numerous articles on the topic. 

Canto: You probably have to ‘do the math’, as the Yanks weirdly say.  

Jacinta: Well we won’t be doing much of that. The cover story is titled ‘Beyond weird’, and Smolin’s idea is that we need to move beyond quantum weirdness to something more coherent and unifying. He describes current quantum mechanical theory as comprised of two different laws:

The first… describes quantum objects as wave-like entities embodied in a mathematical construction known as a wave function. These objects evolve smoothly in time, exploring alternative realities in ‘superpositions’ in which they aren’t restricted to being in any one place at any one time. That, to any intuitive understanding of how the world works, is distinctly odd. The second law applies only under special circumstances called measurements, in which a quantum object interacts with a much larger, macroscopic system – you or me observing it, for example. This law says that a single measurement outcome manifests itself. The alternative realities that the wave function says existed up to that point suddenly dissolve.

Canto: So both of these laws – and of course I’m in no position to doubt or to verify their mathematical exactitude or explanatory power – make little sense from a ‘common-sense’ or ‘realist’ perspective, in which objects must always be objects and waves waves, and, if objects, they must be in a particular place at a particular time, regardless of anything observed. So it seems perfectly cromulent to me that Einstein and no doubt many others found something incomplete about quantum theory, in spite, again, of its apparently vast explanatory power. Like it was an intellectual placeholder for something more real or coherent.

Jacinta: Well Smolin seems to be one of those dissatisfied physicists, – he mentions de Broglie and Schrödinger as others – pointing out that the two laws are in apparent contradiction, with the second law unable to be derived from the first. The theory also ‘seems to’ violate the principle of locality, in which forces are dependent on distance. Quantum entanglement does away with that principle. So Smolin sees a way out by trying to incorporate gravity into the quantum world, or at least trying to connect the general theory of relativity and quantum theory into a seamless whole, as their current incompatibility constitutes a major problem. General relativity presents ‘a smooth, malleable space-time’, while quantum theory suggests ‘discrete chunks, or quanta, of space or space-time’.  String theory and loop quantum gravity are some of the attempts to bridge this divide, but these are currently untestable theories. Also, apparently general relativity is compatible with our perception of the flow of time, whereas quantum theory is more problematic, an issue which, I think, Gerard ‘t Hooft attempts to address in his essay ‘Time, the Arrow of Time, and Quantum Mechanics‘ . 

Canto: Yes, he feels that time, with its arrow pointing eternally forward, with no need for or possibility of reversibility, must be an essential element of a grand physical theory.

Jacinta: Maybe. He’s saying I think, that any explanation of our world, any theory, is arrow-of-time dependent, as it necessarily involves preceding causes and antecedent consequences. But let’s just stick to Smolin’s article. He argues that both relativity and quantum theory have issues with the conceptualisation of time. And there are problems, such as dark matter and dark energy, which don’t easily fit within the standard model. So he feels we need to go back to first principles, ‘in terms of events and the relationships between them’. So, according to these principles, space is an emergent property of a network of causal relationships through time.

Canto: Well to keep more strictly to Smolin’s description, he has five hypotheses. One – the history of the universe consists of events and relations between them. Two – that time, as a process of present causes and future consequences, is fundamental. Three – that time is irreversible, cause can’t go backwards and ‘happened’ events can’t unhappen. Four – that space emerges from this cause-consequence chain. Five – that energy and momentum are fundamental, and conserved in causal processes. 

Jacinta: Good, and this is an ‘energetic causal set model’ of the universe, as he and others describe it, to which he’s added a sixth hypothesis, derived from ‘t Hooft, which says that ‘when two-dimensional surfaces are defined in the emerging geometry of space-time, their area gives the maximum rate by which information can flow through them’.

Canto: Now that sounds horribly mathematical. I do note that area = space and rate = time, and so this hypothesis somehow marries space-time with information flow?

Jacinta: Yes, it’s all threatening to move beyond our brains’ event horizon here. Smolin says that ‘in this picture’, and I’m not sure if he’s talking about the ‘picture’ derived from the sixth hypothesis or by all six taken together, but ‘in this picture, every event is distinguishable by the information available to it about its causal past’. This he calls the event’s sky, because the sky, or what we see (speaking about horizons) at any one instant, is what he calls ‘a view of its own causal past’. This has to do with the speed of light – we can’t see what we can’t see. And this sixth hypothesis, combined with the first law of thermodynamics, can apparently be used to derive the equations of general relativity, bringing gravity into the picture. 

Canto: I don’t get the laws of thermodynamics.

Jacinta: The first law is about energy used in a closed-system process, which can be transformed in that process but is always conserved. Anyway, we’ll try to quit before we get in too much deeper. We know that there’s a ‘measurement problem’, a problem of causality in quantum mechanics, in which it is said that a measurement, or observation, ‘collapses the wave function’ to define a particle’s specific place at a specific time. This is counter-intuitive, to put it mildly, and highly unsatisfactory to many physicists, because it seems to make a mockery of how we understand causality. It seems to be a long-standing impasse to the unification of the two major theories. So we’ve only described a fraction of what Smolin has to say here, and there’s also the problem of entanglement. In ‘classical physics’ proximity matters in a way that it doesn’t in quantum theory. Smolin describes, or mentions, a lot of work being done on ‘ensembles’ in an attempt to solve this measurement problem.

Canto: I think one of the issues that the ‘realists’ are concerned with, but perhaps deliberately not mentioned in Smolin’s piece, is the many worlds hypothesis, or the multiverse, embraced for example by Max Tegmark in Our mathematical universe. Neil Turok is another skeptic of this apparent solution to the causality impasse. 

Jacinta: Yes, I don’t think Smolin is an embracer of the multiverse, tantalising though it is in a sci-fi sort of way. Of course we don’t have the mathematical wherewithal to give an informed view one way or another, or to know whether mathematical wherewithal is what’s really needed. I’ve heard it said – possibly by Tegmark – that a multiverse fits so neatly with the mathematical equations that we need to accept it against our intuitions, which have been wrong in so much else. I don’t know… we’ll just have to watch with interest this intellectual battleground, and see if anything decisive crops up in what remains of our lifetimes.

Canto: Singular or plural…

Other references

https://www.frontiersin.org/articles/10.3389/fphy.2018.00081/full

The universe within, by Neil Turok, 2012

Our mathematical universe, by Max Tegmark 

https://en.wikipedia.org/wiki/First_law_of_thermodynamics

 

Written by stewart henderson

September 12, 2020 at 1:08 pm

How do trees transport water such long distances? Part 2: the mechanism remains a mystery (to me)

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and I still haven’t found what I’m looking for…

So scientists have learned a lot, though not everything, about water’s travels from soil to leaf in a plant or tree. It’s a fascinating story, and I’m keen to learn more. But the real mystery for me is about energy. As the excellent Nature article, upon which I’m mostly relying, points out, animals have a pump-based circulatory system to distribute nutrients, oxygen and so forth, but plants are another matter, or another form of organised matter.

I actually posed two questions in my last post. How do plants – and I think I should specify trees here, because the massive distance between the soil and their top leaves makes the problem more dramatic – move water such large distances, and how do they know they have to transport that water and how much water to transport?

So let’s look at the Nature Education explanation:

The bulk of water absorbed and transported through plants is moved by negative pressure generated by the evaporation of water from the leaves (i.e., transpiration) — this process is commonly referred to as the Cohesion-Tension (C-T) mechanism. This system is able to function because water is “cohesive” — it sticks to itself through forces generated by hydrogen bonding. These hydrogen bonds allow water columns in the plant to sustain substantial tension (up to 30 MPa when water is contained in the minute capillaries found in plants), and helps explain how water can be transported to tree canopies 100 m above the soil surface.

Notice how we’re again returning to the explanations questioned by Wohlleben – transpiration and capillary action. But we’re introduced to something new – the C-T mechanism. The thesis is that water’s cohesiveness through hydrogen bonding creates a tension (the tension that makes for capillary action) that enables water to be shifted up to 100 metres – all because of the minuteness of capillaries found in plants. And trees? Somehow, I just can’t see it. Perhaps the key is in the phrase ‘helps explain’.  There must surely be more to this. The thesis also mentions ‘negative pressure’ generated by transpiration. This is the signalling I wrote about before. Somehow the plant’s chemistry recognises that there’s an imbalance, and of course this happens in all living things, regardless whether they have a complex nervous system. So maybe there’s no need to worry about ‘knowing’. All living organisms respond to their ever-changing environment by altering their internal chemistry, by opening or closing barriers, by selectively adding or subtracting nutrients, and there are unknowns everywhere about precisely how they do that. It’s a kind of organised chemistry that seems like everyday magic from the outside, whether we’re focusing on a beech tree or our own intestines.

The C-T mechanism is only new to me I should add. It can actually be traced back to 1727 and a book by Stephen Hales, in which he pointed out that without what he called perspiration the water in a plant would stagnate, and that it was also required to allow for the capillary movement of water, because ‘the sap-vessels are so curiously adapted by their exceeding fineness, to raise [water] to great heights, in a reciprocal proportion to their very minute diameters’. But this ‘reciprocal proportion’, according to Wohlleben, as quoted in the last post, can only account for a maximum of 3 feet of upward force in ‘even the narrowest of vessels’.

The water transport system, referred to in the last post as the water potential difference or gradient, also has another name, the Soil Plant Atmosphere Continuum (SPAC). I also mentioned something about an ‘apoplastic pathway’. Water enters the tree by the roots, which are divided and subdivided much like branches and twigs above-ground, with the thinnest examples being the fine root hairs. Water enters through the semi-permeable cell walls by osmosis. Cell-to-cell osmosis carries the water deeper into the root system, and thence into an apoplastic pathway. According to this video, this pathway provides an uninterrupted flow of water (no cell wall barriers) which allows a mass flow ‘due to the adhesive and cohesive properties of water’. This is the cohesion-tension theory again. Apparently, due to evaporation, a tension is created in the apoplast’s continuous stream, leading to this ‘mass flow’.

This makes absolutely no sense to me. What I’m so far discovering is that it’s pretty hard to start from scratch as an amateur/dilettante and get my head around all this stuff, and in my reading and video-watching I’ve yet to find a straightforward answer to the how of long distance, fast transport of water in plants/trees – there probably isn’t one.

I’ll try again after a diet of videos – so far I’ve found a large number of videos in Indian English, and their accents defeat me, I’m sad to say. No transcripts available. Meanwhile, I’ve compiled a little glossary (from various sources) to help myself…

apoplast – within plants, the space outside the plasma membrane within which material can diffuse freely. It is interrupted by the Casparian strip in roots, by air spaces between plant cells and by the plant cuticle.

Casparian stripa band of cell wall material deposited in the radial and transverse walls of the endodermis, which is chemically different from the rest of the cell wall – the cell wall being made of lignin and without suberin – whereas the Casparian strip is made of suberin and sometimes lignin.

cortical cells – in plants, cells of the cortex, the outer layer of the stem or root of a plant, bounded on either side by the epidermis (outer) and the endodermis (inner).

exudation – An exudate is a fluid emitted by an organism through pores or a wound, a process known as exuding.

guttation – water loss, when water or sap collects (at times of low evaporation, dawn & dusk), at tips of grass, herbs (not to be confused with dew, caused by condensation).

hydrostatic pressure – the pressure exerted by a fluid at equilibrium at a given point within the fluid, due to the force of gravity. This increases in proportion to depth measured from the surface because of the increasing weight of fluid exerting downward force from above.

lignin – a class of complex organic polymers that form important structural materials in the support tissues of vascular plants and some algae. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity and do not rot easily.

osmosis – the movement of water from an area of high to low concentration through a semi-permeable membrane. ‘Pumps’ in the cell membrane transport the specific ions into the cell which means water moves in by osmosis thus maintaining hydrostatic pressure.

phloem – the living tissue that transports the soluble organic compounds made during photosynthesis and known as photosynthates, in particular the sugar sucrose, to parts of the plant where needed. This transport process is called translocation.

plasmodesmata – narrow threads of cytoplasm that pass through the cell walls of adjacent plant cells and allow communication between them.

root pressure – the transverse osmotic pressure within the cells of a root system that causes sap to rise through a plant stem to the leaves. Root pressure occurs in the xylem of some vascular plants when the soil moisture level is high either at night or when transpiration is low during the day

sap – a fluid transported in xylem cells (vessel elements or tracheids) or phloem sieve tube elements of a plant. These cells transport water and nutrients throughout the plant.

suberin – an inert impermeable waxy substance present in the cell walls of corky tissues. Its main function is as a barrier to movement of water and solutes.

symplast – the network of cytoplasm of all cells interconnected by plasmodesmata. The movement of water occurs from one cell to another through plasmodesmata

tracheid – a type of water-conducting cell in the xylem which lacks perforations in the cell wall.

vascular (plants) – also known as tracheophytes and also higher plants, form a large group of plants (over 300,000 accepted known species) that are defined as those land plants that have lignified tissues (the xylem) for conducting water and minerals throughout the plant.

xylem – one of the two types of transport tissue in vascular plants, phloem being the other. The basic function of xylem is to transport water from roots to shoots and leaves, but it also transports some nutrients.

 

On the Trump’s downfall. What a memo. One wonders if the DoJ is running out of patience with the wannabe dictator and his imbecilities, which may bring things to a head sooner rather than later. But those in the know say that Mueller is always thorough and unlikely to be distracted, so I shouldn’t project my own impatience onto him. Dog give me strength to suffer the horrorshow for a while longer.

 

Written by stewart henderson

February 5, 2018 at 3:48 pm

Posted in biology, botany

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