Wikipedia:Reference desk/Archives/Science/2016 March 26

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March 26

Information ~ Energy. Maxwell's demon. Szilard

Question	Remark
Szilard says that measurement of 1 particle speed (is speed more than average = yes/no question = 1 bit) is accompanied by entropy increase of $k\log 2$. Is this entropy increase of Universe or what? E.g. demon converts work to heat and makes maesurement: $W=Q$; Then he tries to convert back heat to work : $Q=W'+kT\ln 2,W'<W$; How much work can demon obtain using information about speed? Can it be equal to $kT\ln 2$?	[ http://www.weizmann.ac.il/complex/tlusty/courses/InfoInBio/Papers/Szilard1929.pdf ]
As we know, $k\ln 2$ is lower boundary for entropy increase during measurement. If entropy increase is more than $k\ln 2$, can we obtain entropy decrease via information more than $k\ln 2$?	Entropy in thermodynamics and information theory Maxwell's demon

37.53.37.94 (talk) 16:22, 26 March 2016 (UTC)

SI status of the litre

Is the litre somehow not an official SI unit? I've always taken it to be a core part of the SI, but the intro to tonne says:

Although not part of the SI per se, the tonne is "accepted for use with" SI units and prefixes by the International Committee for Weights and Measures, along with several other units like the bar, litre and day.

Did sommeone just misunderstand something about the litre, or is it really not an SI-official unit? Nyttend (talk) 17:45, 26 March 2016 (UTC)

I think some wires got crossed: litres (or liters - the exact same unit, spelled in American English), are an SI unit, but they are not an SI base unit. The distinction is purely a matter of the difference between an SI derived unit and an SI base unit. Nimur (talk) 18:10, 26 March 2016 (UTC)

On the other hand, after consultation with a few references linked from our numerous articles, such as this NIST Special Publication SP-330 The International System of Units, it seems that many authoritative sources consider the liter to be "outside" the SI system! I guess this really depends on who you ask - and here in the USA, the National Institute of Standards and Technology is the end-all and be-all authority for standardization of units of measurement - so I guess I will respectfully defer to their opinion! Nimur (talk) 18:18, 26 March 2016 (UTC)

More important, the document that Nimur linked to is the standard that defines what is and is not an SI unit. (It's the US edition of it, so it uses some different spellings than other countries, but the spelling of units is not standardized and anyway the document includes footnotes on such issues.) The liter is indeed not an SI unit; you find this table 6 on page 32 (37th page of the PDF). --69.159.61.172 (talk) 18:48, 26 March 2016 (UTC)

See Non-SI units mentioned in the SI. AllBestFaith (talk) 18:58, 26 March 2016 (UTC)

It's roughly the metric analog to the "quart". So, being actually useful, it's shunned by the standards gods. ←Baseball Bugs ^{What's up, Doc?} carrots→ 19:46, 26 March 2016 (UTC)

The main problem with the litre is that the value (not merely the definition) of the unit has changed over the years, which makes it undesirable for use as a scientific unit. Tevildo (talk) 20:26, 26 March 2016 (UTC)

It seems to work fine for sodapop sales. ←Baseball Bugs ^{What's up, Doc?} carrots→ 20:35, 26 March 2016 (UTC)

The idea of SI is to cut the number of 'base units' to the barest minimum - and to standardize the meaning of them to physically realizable things. Then other units are derived from them for convenience. This is in contrast to previous systems where you'd have units for everything, all independently defined and therefore requiring a bunch of ugly constants to be introduced all over the place. So, a "liter" is nothing more than a convenient shorthand for "a thousandth of a meter-cubed". It's just a word with a meaning - not a fundamental unit like the meter, the kilogram and the second. SI does recognize a bunch of convenience units (called "derived units") so, for example, the "watt" is just shorthand for "one joule per second" and a "joule" is just short hand for "a force of one newton moved through one meter" and a "newton" is just shorthand for "the force to accelerate a kilogram at one meter per second squared". So although the watt, the joule and the newton are all very handy units - they're all ultimately are described in terms of the "base" units - the meter, the kilogram and the second.

The liter is one tier down from the watt, the joule and the newton in that it's not "officially" a derived unit either. But it doesn't matter. Just think of it as a word with a well-defined meaning - but it's not a "base" unit because it can be defined more simply in terms of the meter and it's not a 'derived unit' because...well...it's just not.

This really matters very little to anyone unless they happen to be in the business of writing papers about unit standardization! The thing you might need to care about it that there is no "international standard liter"...as there is with the kilogram (which is currently defined as the mass of a lump of platinum–iridium alloy kept in a vault in a laboratory just outside Paris). Instead, the liter is a thousandth of a meter cubed - and a meter is the distance traveled by a beam of light in 1/299,792,458th of a second and a second is the amount of time some particular caesium atom takes to emit 9,192,631,770 vibrations under some specific circumstance.

SteveBaker (talk) 01:58, 28 March 2016 (UTC)

The real basis of the metric system was a small fraction of the (presumed) distance from the equator to the north pole along the Greenwich Meridian. Those crazy numbers you listed were conversions away from the original definition. ←Baseball Bugs ^{What's up, Doc?} carrots→ 02:23, 28 March 2016 (UTC)

Presynaptic cell must be neuron or it can be also miocyte for example?

93.126.95.68 (talk) 22:31, 26 March 2016 (UTC)

Well, to call something a synapse it usually has to have an axon ... however, as that article explains, you can (occasionally) have dendro- and somato- synapses as well, i.e. from the dendrites and cell body. That article also explains that traditionally only neuron on neuron contacts are called synapses and neuron-on-something-else contacts are called "junctions". If you take the loosest possible definition, possibly you could argue cardiac myocytes make synapses with each other, but I would say that's an error because the intercalated discs that connect them use gap junctions to make direct contact. What makes a synapse a synapse is that one cell releases a transmitter that is interpreted by another in an intelligent way, rather than acting as a simple short circuit hardwiring the two together. There may be something that comes closer to what you want but I'm not thinking of it. Wnt (talk) 23:11, 26 March 2016 (UTC)

Thank you for your comment. When you open Campbell biology book (10th edition, p.1062) you see that postsynaptic can be a muscle, a neuron or a gland cell: "Each branched end of an axon transmits information to another cell at a junction called a synapse. The part of each axon branch that forms this specialized junction is a synaptic terminal. At most synapses, chemical messengers called neurotransmitters pass information from the transmitting neuron to the receiving cell. In describing a synapse, we refer to the transmitting neuron as the presynaptic cell and the neuron, muscle, or gland cell that receives the signal as the postsynaptic cell." 93.126.95.68 (talk) 01:26, 27 March 2016 (UTC)

Nerves (usually via axons) can go to a lot of funny things ... like melanocytes. [1] But your question above asked for a non-neuron as the presynaptic cell, which is harder to think of. Wnt (talk) 01:59, 27 March 2016 (UTC)

There are electrical synapses, so chemical neurotransmission is not an essential criterion for the definition of a synapse. The use of the term differs between scientists: in the oldest use, the term synapse was reserved for the contact between two neurons (Sherrington, with the term 'junction' (such as the neuromuscular junction used for the contact from neurones to other tissues), but even from the mid-20th century (eg. Bernard Katz wrote a book called "Nerve, Muscle and Synapse") the use was looser - however, the presynaptic component is always a neuron. Communication between other nearby cell types is called paracrine signalling. Klbrain (talk) 22:58, 28 March 2016 (UTC)