A Question For The Lone Ranger

[ceptimus]

I googled and found one site that talks about the radiation from the Cobalt-60 with a magnetic field produced by a current in a coil nearby. It says that if the radiation streams towards the coil then the current is going round clockwise, and if it streams away from the coil the current is going anticlockwise. That seems clear enough and providing it's true I agree that this gives a good way of telling our alien friends what we mean by left and right.

[Crumb]

What happens if you turn the coil around?

[ceptimus]

I suppose the lump of cobolt is on the axis of the coil but off.to one end. You position the apparatus so that the cobalt is closer to you than the coil. Then if the radiation streams away from you, the current is clockwise. If you turn the coil over.the radiation would start.streaming towards you - but now you'd be looking at the coil.with an anticlockwise current.flow.

[lpetrich]

Originally Posted by Crumb

What happens if you turn the coil around?

I'm not sure what you mean here.

[Crumb]

Ah never mind, I think I got it.

[lpetrich]

The three main discrete elementary-particle symmetries are:

• C - charge - matter-antimatter reversal
• P - parity - space reversal
• T - time - time reversal

There is a theorem called the CPT theorem that states that all of them must be conserved together, even if not necessarily individually.

Gravity, electromagnetism, and the strong interaction all respect all three symmetries.

The weak interaction was discovered to violate C and P separately in the 1950's, though CP together was still conserved in what was observed. It was discovered to violate CP together in the 1960's, and that has an interesting consequence. T violation, meaning time asymmetry. Combined with baryon-number violation, that can potentially explain the matter-antimatter asymmetry of our Universe.

[But]

Originally Posted by ceptimus

Yes, I agree with that - but if we're trying to tell them what we mean by left and right, it's vital that the description of the parity-violating experiment doesn't include any reference to left/right or clockwise/anticlockwise either explicitly or hidden (e.g. in the convention for what is a positive angle in polar coordinates, which comes from our x axis increasing to the right).

I think it's the opposite - if we want them to understand what we mean by left and right, we have to include those terms in the description.

Based on what is known about the observable universe, we can safely assume they aren't made out of antimatter. Otherwise, let's imagine we send them a movie of the parity-violating process and they interpret the time coordinate backwards, mistake electrons for positrons and switch left and right. Everything makes perfect sense to all lifeforms involved until they send one of our exploring spaceships a bunch of MREs containing not L-glucose, which we can't digest, but L-antiglucose, and there is a huge explosion.

[ceptimus]

Yes, when you finally get to meet one of the aliens (in a perfect vacuum in space, so you're both wearing space suits) offer to shake hands and if she holds out her left hand then she's either forgotten our social customs or she's made of antimatter! If it's the latter then it would be a good idea NOT to actually grasp her hand!

I know you'd probably realize before this meeting as space isn't a perfect vacuum so an antimatter spaceship travelling through would leave some clues! - but it's a nice story!

[But]

That's the joy of thought experiments, you can gloss over the details.

[Kamilah Hauptmann]

Was this from one of your classes?

Professor: Make a music video over any subject we’ve covered and I’ll curve your overall grade.
... pic.twitter.com/uO6IBMl2CS

— Julien Turner (@JuicyJu11) November 3, 2017

[MonCapitan2002]

I have a new question for The Lone Ranger. Please see the image below. It is both linked and embedded.

Link.



Can a tree actually grow as big as the one in the background upon which a village was built around. I'm guessing no, but I could be wrong. Also, if trees could grow that big wouldn't it be a bad idea to build a village around a living one? It will still be growing, so the village would be destroyed by the tree's growth eventually.

[JoeP]

To the second point, it would not be hard to put flexible joints between the straight sections to accommodate the changing curve, and to lengthen bridge sections one slat at a time to accommodate the increasing circumference. They'd have plenty of time after all!

TLR's answer to the first point will probably involve hydrostatic tension and stuff like that.

[The Lone Ranger]

Is this supposed to be on Earth? On Earth, probably not; it might be possible on a planet with lower surface gravity than the Earth's.


The primary limitation to how tall a tree can grow is thought to be its ability to move water upward, against the force of gravity. On Earth, the general consensus seems to be that a tree could be no more than 120 - 150 meters tall, because it wouldn't be able to move water to tissues that are any higher above the ground.


So, how does a tree move water, considering that it has no active pumping mechanism?

Trees (and most other plants) have two kinds of tissues that transport materials. Phloem is living tissue that primarily transports sugars and other metabolic products. Xylem is not living at maturity, and consists of hollow tubes connected together; xylem is used to transport water and some water-soluble substances. The xylem and phloem are typically bundled together into vascular bundles.

A typical vascular bundle, containing both living phloem and hollow, non-living xylem.



Vascular bundles visible in a stem of celery.

The thing is, phloem can actively transport many substances. That is, it can expend energy (in the form of Adenosine Triphosphate or ATP molecules) to actively move substances from one part of the plant to another, even against a concentration gradient or against the force of gravity.

Xylem, being non-living tissue, can't do that. Besides, there are no known mechanisms by which living organisms can actively transport water. So, plants (since they lack pumping hearts) can only passively transport water. That, more than anything else, limits how tall a tree can grow.

So, how does a tree transport water upward? This is called transpiration, and it primarily relies on the fact that water molecules are hydrophilic and are thus attracted to other hydrophilic molecules. The attraction between water molecules is cohesion; the attraction between water molecules and hydrophilic molecules other than water is adhesion.

Most organic molecules are hydrophilic, and so water molecules tend to "stick" to the walls of xylem because of adhesion.

The key to this is that there is a continuous column of water in the xylem, from the leaves at the top of the tree to the roots in the soil.


At the top of the tree, water evaporates from the leaves. As a water molecule evaporates from the surface of the leaf, it pulls (through cohesion) an adjacent water molecule upward. That one, in turn, pulls the one below it upward. And that one pulls the one below it upward, and so on. Since there's a continuous column of water in the xylem, this continues all the way down to the roots; as water moves upward in the roots, cohesion pulls water out of the soil and into the roots.

Capillary action also plays a role in the movement of water in the xylem. Because the xylem tubes are hydrophilic, water molecules are attracted to the walls of the tubes through adhesion. This causes the water to "climb" the tubes. Even so, capillary action is not terribly important in trees.

Water (or other hydrophilic liquids) will tend to "climb" a tube if the tube is made of a hydrophilic substance. This is capillary action.



Other factors being equal, the more narrow the tube, the higher the water inside will move through capillary action. This is because the narrower the tube is, the greater the percentage of the water inside is in contact with the tube walls. Xylem can be as small as 10 µm in diameter, and so in small plants, capillary action alone is sufficient to move water from the roots to all parts of the plant.

So, long story short: a tree's ability to move water upward is mostly (though not entirely) due to the forces of adhesion and cohesion.

It's worth noting that this works only so long as there is an unbroken column of water in the xylem. In trees, the tissue that contains the xylem and phloem is a thin layer just beneath the bark. So cutting into the wood beneath the bark will interrupt the flow of water and nutrients past that point. If you cut all the way around the trunk of a tree, this is called girdling the tree, and the tree will be unable to transport water or nutrients above that point. Consequently, everything above the portion of the tree that has been girdled will die. People sometimes "prune" trees by girdling branches; any branches that have been girdled will die and eventually fall off.



If a tree grows tall enough, the weight of a column of water in its xylem will be enough that adhesion and cohesion are incapable of drawing water any higher against the force of gravity. On Earth, as mentioned, it's generally thought that this limits a tree to 120 - 150 meters or so.

A planet with lower surface gravity should be able to support taller trees, of course.

[ceptimus]

It's surprising to me that a pure vacuum can only raise water about ten meters in a man made pipe, whereas a bunch of leaves sucking on the tiny xylem "pipes" inside a tree can raise water fifteen times higher.

Obviously it's impossible to create a greater suction than a pure vacuum, so there's something complicated and clever happening inside those xylem.

[Kael]

The trick is that you can technically have negative pressure, not just the zero of a pure vacuum.

[ceptimus]

I did a little internet search and it seems that no one is exactly sure how it works, but the theory is that liquids in thin pipes can behave like a piece of string or wire and so, in effect, can exhibit negative pressure. The thin column of liquid acts in just the same way that a wire under tension does - so it doesn't rely on atmospheric pressure and can "suck harder than a vacuum."

A complicating factor for liquids is that they usually boil at low pressure - so cold water will start to boil (while still remaining cold) when the pressure gets down to anywhere near vacuum - let alone "pressures" much lower than vacuum. Again the theory is that when constrained in tiny pipes the water is prevented from boiling by some vaguely understood mechanism.

It's a shame that engineers haven't been able to replicate this sort of behavior of water in plumbing systems. It would make it much easier to pump water out of mines because the pump could be installed up at ground level for easy access and maintenance. At present if we want to raise water more than about twenty feet, we have to put the pump down at the bottom and use very strong pipes that can withstand the high pressure necessary to push the water uphill. As usual, nature has designed a far more elegant solution.

[Kael]

Well, just rambling here but I imagine when you're dealing with tubes that tiny the number of water molecules at any given point is low enough that "boiling" isn't a very useful descriptor of their behavior. This is probably why the technique isn't useful for draining mines either – the volume of water that needs to be moved is just too much to do in a timely manner with some kind of "artificial xylem" network.

[The Lone Ranger]

It would be a rather slow way to move water, at least if it worked the same way as it does in plants. For example, a mature oak can weigh 50 tons or more -- and moves about 100 gallons of water per day.

[The Lone Ranger]

I should clarify that only a very small portion of a mature tree's tissues are devoted to moving water. Still, even a very large tree that weighs scores of tons typically transpires only about 100 gallons of water per day.

[ceptimus]

What happens when the leaves fall off? Do the ends of the xylem tubes have to be sealed up somehow to prevent the water falling back down the tube?

What happens if the xylem are injured somehow - by a branch breaking, or a woodpecker, or beetle, or forest fire, or drought, or whatever? If there is an air pocket somewhere in the xylem does the plant have a method of getting rid of the air and refilling the xylem with water so that the adhesion / cohesion process can get going again - or are those xylem rendered useless forever after?

[The Lone Ranger]

The xylem tubes have lateral perforations, so water can be routed around air bubbles or breaks, so long as they aren't too large. The water appears to effectively "flow" around breaks in the xylem. You have to almost completely girdle a branch or trunk before the plant loses the ability to transport enough water past that point to keep the "downstream" tissues alive.

Some diseases actually kill trees by causing breaks in the water columns. The fungal pathogens that cause Dutch Elm Disease, for example, release chemicals that reduce cohesion between water molecules, causing breaks in the water columns. Ultimately, this will kill the tree.


Trees typically seal off leaves at the base before the leaves are shed, but of course, leaves can be torn off by high winds, folivores, etc. So the plant must have some means of dealing with compromised water-transport tissues.

There's some evidence that trees will seal off xylem that has been damaged and has air bubbles in it, preventing the spread of air bubbles to adjacent tissues. More to the point, though, experiments with birch trees show that they, at least, can re-fill xylem that has air bubbles in it.

They appear to do this through root pressure. This involves the plant actively transporting mineral ions into nearby tissues, including still-functioning xylem. If the plant can build up a high-enough osmotic gradient, this can apparently create enough osmotic pressure to cause water to flow back into the compromised tissues.

[Ari]

The drawings of Xylem make me think of the tesla valve which resists flow in one direction.

[Dingfod]

Originally Posted by The Lone Ranger

I should clarify that only a very small portion of a mature tree's tissues are devoted to moving water. Still, even a very large tree that weighs scores of tons typically transpires only about 100 gallons of water per day.

The water-stealing bastards.

Originally Posted by The Lone Ranger

The xylem tubes have lateral perforations, so water can be routed around air bubbles or breaks, so long as they aren't too large. The water appears to effectively "flow" around breaks in the xylem. You have to almost completely girdle a branch or trunk before the plant loses the ability to transport enough water past that point to keep the "downstream" tissues alive.

Some diseases actually kill trees by causing breaks in the water columns. The fungal pathogens that cause Dutch Elm Disease, for example, release chemicals that reduce cohesion between water molecules, causing breaks in the water columns. Ultimately, this will kill the tree.

Sadly, it only took losing about 50% of its branches to kill a 50 foot tall burr oak tree.

[Watser?]

Originally Posted by The Lone Ranger

Yeah, that's definitely Procambarus clarkii (commonly called the Red Swamp Crayfish), which is distinctively different from most of the native European crayfish species, such as Astacus astacus.

Unfortunately, Europe's native crayfish species have been devastated by competition from (and diseases carried by) introduced American crayfish species, such as Procambarus clarkii and the Signal Crayfish (Pacifastacus leniusculus).

Procambarus clarkii, as your links note, is considered an invasive species in Europe; part of the reason that it does so well is because it's far more tolerant of low-oxygen and polluted waters than are most other crayfish. (And like many crayfish species, it can survive out of water for an extended time, so once they get into a waterway, they can quickly spread to nearby waterways by crawling across land.)


As the common name "River Lobster" implies, crayfish are indeed decapod crustaceans, and so are closely related to other decapods such as true lobsters and true crabs. Many people in the U.S. (particularly the Southeastern U.S.) consider crayfish to be very good eating, and I would certainly recommend that you round up and cook as many of the Procambarus as you can, if only to reduce their numbers and thus their impacts upon native species.

Apparently it reproduces by cloning itself

All-female mutant crayfish that clone themselves are taking over rivers and lakes around world | The Independent

[Kamilah Hauptmann]

I, for one, welcome our crayfish overladies.


Saturday Morning Breakfast Cereal - Warming