Wikipedia:Reference desk/Archives/Science/2023 January 13

= January 13 =

liquid-state
hello, I read somewhere that the reason two back-to-back diodes don't work as a transistor isn't so much the geometry of the electrodes, but that the leads, plainly, don't pass minority carriers on which the transistor effect depends (very roughly speaking.) This got me thinking - electrolytes have ions and positive ions are basically holes. Does it mean two back-to-back diodes in which one of the leads (both anodes or both cathodes) wasn't metal but electrolyte, and the electrolyte was directly touching the semiconductor (P or N), could in principle work as a (bad) transistor? Aecho6Ee (talk) 15:31, 13 January 2023 (UTC)
 * Liquid state transistors exist, but instead use a low-melting alloy that is liquid at room temperature, rather than solutions: . -- Jayron 32 15:39, 13 January 2023 (UTC)
 * No. Thinking that a positive ion in an electrolyte is like an electron hole does not make it behave like an Electron hole in the rigid semiconductor crystal lattice of a junction transistor. Philvoids (talk) 23:40, 13 January 2023 (UTC)

A weightless universe
If I remember my high school physics correctly, the mass of an object is an inherent and unvarying characteristic, but its weight is a measure of its gravitational effect on some external object, which varies on the gravity of that object. This is why humans weigh less on the Moon than on Earth, for example.

Hence, since the universe can have no effect on any external object as by definition there are no objects external to the universe, the weight of the universe must be zero. Do I have this right? Or does this simply make no sense given that the possibility can never arise? Jack of Oz  [pleasantries]  22:25, 13 January 2023 (UTC)
 * I would say that, rather than being zero, the "weight" of the whole universe is meaningless. --Floquenbeam (talk) 22:28, 13 January 2023 (UTC)
 * The universe mass of ordinary matter calculated by m u = ΩB ρCRIT Vu where ΩB is universe baryonic matter density parameter, ρCRIT is universe critical mass density and Vu is volume of the observable universe equals 2 x 1056 grams according to WolframAlpha. The same source goes on to calculate what such a mass "would weigh on Earth" but that's just silly. Philvoids (talk) 23:18, 13 January 2023 (UTC)
 * Give me a weighing scale to put it on, and I shall weigh the universe. --Lambiam 23:37, 13 January 2023 (UTC)
 * Weight is the gravitational force acting on a body's mass, and the universe obviously has mass and it also has self-gravitation. I think I'll stare into the abyss now.  Sigh. Modocc (talk) 02:08, 14 January 2023 (UTC)
 * I doubt the universe is held together by self-gravity. An object in free fall is weightless. If the object (in our case the universe) is not in free fall, some force acting on it, other than the gravitational force, needs to impede its evolution in spacetime. Additionally, this unknown force needs to have a preferred direction, so that integration over the mass distribution of the universe does not cancel the resultant. --Lambiam 15:35, 14 January 2023 (UTC)
 * I doubt the universe is held together by self-gravity. I'm intrigued by that statement. Why the doubt? Gravity is the only "force" that acts on the scale of the universe. On the other hand, why would the universe need to be "held together"? In fact it doesn't seem to be as its expansion will be essentially exponential in the future (by way of gravity, incidentally). Actually, the level of intrigue depends on which part of the sentence is stressed, and maybe I've just repeated what you wanted to say. --Wrongfilter (talk) 16:24, 14 January 2023 (UTC)
 * Our article Self-gravitation defines this as follows "Self-gravity is the gravitational force exerted on a body, or group of bodies, by the body/bodies that allows it/them to be held together." So the claim made above that "the universe ... has self-gravitation" implies that this is what allows the universe to be held together; the universe does not fly apart thanks to its self-gravity . I questioned the correctness of this statement. While it appears to contradict the theory of the Big Rip, my issue with it is actually that it is meaningless to assert that the universe, not itself a body but a spacetime continuum of force fields, is "held together" by a force. --Lambiam 00:51, 15 January 2023 (UTC)
 * Planetary bodies are in free-fall about their stars and are still held together by self-gravitation (barring a supernova explosion), by exactly the same gravitational forces that gives them, or anything else, weight. To be clear, I defined weight above in accordance with our article on weight: "In science and engineering, the weight of an object is the force acting on the object due to gravity." Given this definition objects in free-fall are accelerated by their respective gravitational weights. Weightlessness in free-fall though requires defining weight more narrowly as "...the magnitude of the reaction force exerted on a body by mechanisms that counteract the effects of gravity...". No reaction force need be present with the term's broader definition. Modocc (talk) 16:27, 14 January 2023 (UTC)
 * Traditionally, weight was taught in textbooks as an operational concept. We know the weight of a body, not by computing the result of multiplying its mass by some acceleration, but by weighing it. Newton wrote: "innotescit semper per vim ipsi contrariam & æqualem, qua descensus corporis impediri potest ("[the weight of a body] is always known by the force opposite and equal to it by which the descent of the body can be prevented"). The nominal definition (mass times acceleration) is useful to clarify the conceptual distinction between mass and weight, but is IMO otherwise pointless. While seemingly having the advantage of being observer-independent (unlike the operational definition, which requires a preferred stationary framework), this advantage has been eliminated by Einstein's theory of general relativity. Reference [2]  in our article on weight, rather than supporting the definition given in the lead, discusses the historical and educational perspectives of these two definitions, both of which are found in science textbooks but get (incorrectly) mixed up in several, and argues (not being the first to do so) for adopting the operational definition as the standard one.  --Lambiam 00:30, 15 January 2023 (UTC)
 * Nevertheless gravity's collective presence is, of course, everywhere. Modocc (talk) 03:49, 15 January 2023 (UTC)
 * But see also Dark energy. {The poster formerly known as 87.81.230.195} 51.194.245.235 (talk) 04:54, 16 January 2023 (UTC)
 * The effect of dark energy is purely gravitational. In general relativity, pressure is a source of gravity, and DE is characterised by its negative pressure. There is no everyday analogue (that I know of), which is why DE appears to be so weird. --Wrongfilter (talk) 07:07, 16 January 2023 (UTC)
 * I have read recently (in New Scientist, so not really "fringe") that a few serious physicists theorise that antimatter (whose apparent absence from the observable universe is not satisfactorily explained) might have negative mass and exert a negative gravitational force. (Apparently the tiny amounts we are able to harness or synthesise (e.g. positrons, antiprotons, antihydrogen) have such small gravitational effects (whether +ive or –ive) that these cannot yet be measured.) If this were the case, perhaps the Big Bang's missing antimatter is hiding somewhere we haven't thought of and is the source of dark energy. {The poster formerly known as 87.81.230.105} 51.194.245.235 (talk) 15:51, 16 January 2023 (UTC)