Wikipedia:Reference desk/Archives/Science/2018 September 11

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September 11

Planets with natural satellites with natural satellites?

Are there any planets (or so-called "dwarf" planets) that have natural satellites with natural satellites? As far as I know, there aren't any. But I've never heard anyone definitively say that there are none that we know of. A Quest For Knowledge (talk) 01:10, 11 September 2018 (UTC)

see http://curious.astro.cornell.edu/about-us/44-our-solar-system/the-moon/general-questions/104-can-moons-have-moons-intermediate Graeme Bartlett (talk) 01:19, 11 September 2018 (UTC)

We don't know of any. Most moons around planets have Hill spheres too small to support significant satellites, and our Moon has the added problem of mascons making many lunar orbits unstable. However there may have been some in the past. Rhea (a moon of Saturn) has been speculated to have rings, and the equatorial bulge on Iapetus may have been formed by a de-orbiting subsatellite. (While Iapetus has a significant Hill sphere, more than any other moon because of its size combined with its distance from its parent planet, it was tidally locked to Saturn, and therefore ended up rotating slowly enough that its subsatellite would have suffered significant tidal deceleration. This process could have happened on some other moons, but only on Iapetus and perhaps Oberon would the process take long enough that the equatorial ridge wouldn't have been obscured by later impacts.) Double sharp (talk) 01:32, 11 September 2018 (UTC)

We don't find such systems in our own solar system, and we probably won't find them elsewhere, because they are unstable ...for reasonable and realistic mass and distance parameters. In astronomy we have a special appreciation for the statistical likelihood of observing an astronomically-unlikely event amidst an astronomically large sample-size; but at this time, our best observational methods still do not allow us to see such detail as a planet's satellite except for those planets that are inside our own solar-system. Even our own solar system is sparsely explored, and we have very limited observations of our outer planets. So, it's unlikely we'll see such a moon-with-satellite within, say, the next few generations of human lifetimes.

A standard homework problem for students of astrophysics is to calculate the number of orbits (years) until an n-body system - such as a sun/planet/moon/satellite system - becomes unstable. Here's an example from Cornell University's Astro 3303 class. The hard part is making sure your numerical method isn't more unstable than the physical system! The standard method is unfortunately not suitable - it is this homework-problem that tragically results in the loss of mathematical naïveté for many an up-and-coming computational physicist. Nimur (talk) 04:52, 11 September 2018 (UTC)

It may be thinkable on an Polar orbit where the "moon-moon" would keep an equal distance to the moon and the planet, but it would remain a delicate balance act to stay there for billions of years. Even more close to impossible wonder would be the history how it got there, but then, our earth is a real wonder too with all the different, needed ideal conditions to support life, met on a single planet. --Kharon (talk) 05:04, 12 September 2018 (UTC)

Not only would it need to be in a polar orbit, but it would need to be in a planet-synchronous orbit (analogous to a sun-synchronous orbit) in order to maintain a constant distance from the planet's surface. To maintain stability of this orbit the tendencies for the orbital parameters to evolve would need to cancel one another. Over the periods of time relevant to the evolution of the solar system such an orbit would be vanishingly unlikely to remain stable. Changes in the moon's equatorial bulge or radiation pressure due to solar output changes could be significant enough to gradually lead to destabilization and a reduction in orbital inclination. 114.124.144.185 (talk) 06:27, 12 September 2018 (UTC)

Polar orbits would tend to be unstable to the Lidov–Kozai mechanism, in which inclination is traded for eccentricity. Double sharp (talk) 06:30, 12 September 2018 (UTC)

Nereid may be a binary satellite of Neptune according to this paper. 114.124.144.185 (talk) 05:50, 12 September 2018 (UTC)

Your link is to the mythological entity, the relevant article is Nereid (moon). {The poster formerly known as 87.81.230.195} 2.122.60.253 (talk) —Preceding undated comment added 17:15, 12 September 2018 (UTC)

That paper is very old, though: it's pre-Voyager. So this may have been refuted by newer data. Double sharp (talk) 08:12, 12 September 2018 (UTC)

Update: maybe not Nereid, but it seems Saturn XXIV Kiviuq could be a binary or contact binary. Double sharp (talk) 14:30, 29 August 2021 (UTC)

Default human template

I have heard that the "default" human template is female, and in the (perhaps artificial) absence of testosterone during development a female body type will result, irrespective of even the sex-determining chromosomes.

However, is this status of "default" of any medical significance?--Leon (talk) 10:38, 11 September 2018 (UTC)

This "default" is due to the XY sex determination system used in mammals. The Y chromosome has relatively few active genes, mostly serving as a signal to activate the male pattern rather than the female. There are many more genes on the X chromosome. The different sizes (and, thus, different genetic contents) means that any gene form (allele) present on the X chromosome will be expressed, regardless of questions of dominant or recessive alleles. This makes certain genetic disorders (red-green colorblindness, hemophilia, etc) more common in males than females. --Khajidha (talk) 13:41, 11 September 2018 (UTC)

To elaborate for pedagogical value, only around half of humans have a Y chromosome, so obviously it can't contain any genes necessary for non-biological males. Conversely, the X chromosome behaves more or less like any other chromosome; most of its genes have nothing to do with biological sex or reproduction, since, as noted, there is no "trigger" to make a human fetus develop as female. The sex chromosomes evolved from a regular pair of autosomes; since only males inherit a Y chromosome, this means there's a massive evolutionary pressure to delete genes from said chromosome, which is what has caused it to become so small. In some mammals it consists of only the single TDF/SRY gene, which is the gene that triggers male development. --47.146.63.87 (talk) 19:28, 11 September 2018 (UTC)

I don't follow the evolutionary pressure bit. Why is there this evolutionary pressure?--Leon (talk) 19:35, 11 September 2018 (UTC)

Y chromosome § Origins and evolution: any alleles on the Y chromosome have a ~50% chance of hitting a dead end: if the possessor of the chromosome has only female offspring, the chromosome dies with them. This is why there's a single Y chromosomal Adam we can trace: the Y chromosome alleles from all other human males living at the same time have died out. Additionally, if a Y chromosome allele were essential for all members of the species—not just males—it would be lethal for any female offspring of the man carrying that allele, since they wouldn't have it. So, any genes that were on the X/Y chromosome's ancestral autosomes that are beneficial to the female phenotype tend to get removed from the Y, leaving only the copy on the X, which is inherited by both sexes. --47.146.63.87 (talk) 21:56, 11 September 2018 (UTC)

Yes, many if not most intersex conditions are the consequence of the interplay between components of the sex-determination system. For example, in androgen insensitivity syndrome, a fetus with one or more Y chromosomes will nevertheless develop a female phenotype, often with infertility. In this condition, the Y chromosome is present and functioning, but androgen receptors are defective, meaning they don't respond normally to testosterone. The gonads are triggered by TDF to begin developing as testes and secreting testosterone, but androgen receptors don't respond to it, so to varying degrees the body continues to develop along the "default" female pathway. --47.146.63.87 (talk) 19:28, 11 September 2018 (UTC)

All very interesting. Thank you!

A follow up question: I've just read something on X-inactivation, and noted the tortoiseshell cat as a typical example of how the differences between the two X chromosomes that females have can have an interesting effect. Is there anything as visually obvious as this that happens in humans?--Leon (talk) 21:10, 12 September 2018 (UTC)

As far as I am aware, no. X-inactivation doesn't mention anything. --47.146.63.87 (talk) 06:14, 15 September 2018 (UTC)

There can be phenotypic variation, but nothing so striking as that. Double sharp (talk) 08:12, 15 September 2018 (UTC)

Is there a material that it's 100% non electrical conductive?

We used to say that wood or other materials such as plastic are not conductive for electricity, but based on what I understand everything is conductive but some of the materials are low conductive while others are highly conductive, and all materials have a measurement on a scale of the conductivity. Is it correct? Is there no an exception for material that is 100% non conductive? 93.126.116.89 (talk) 15:18, 11 September 2018 (UTC)

Some sort of Superinsulator? DMacks (talk) 15:22, 11 September 2018 (UTC)

How much theoretical physics do you want to use in evaluating your answer? Even if no electrons flow, an electric current may still occur: the displacement current. This is not even one of those weird quantum-mechanical, "only-there-in-the-statistical-sense" concepts! Displacement current is a real phenomenon and is a key component of classical electrodynamics. So even if you had a material that were somehow completely able to block the flow of charged particles, you still could not preclude the accumulation of electric charge outside of the boundaries of that material - in other words, even if you had a perfect insulator, electric current would still flow, and the stuff surrounding the insulator material would function as a capacitor. In yet other words, even if you magically prevent matter (electrons and ions) from moving, by building a super perfect insulation material, it is a law of nature that the photons will still get through and carry your very real, very physical electric current without the motion of any electron or other charged particles - we might say this is spooky action at a distance (although it travels only at a finite speed) - and we'd be in good company - that was the title of Chapter 23 in one of the most important physics books ever written!

This ingenius insight is attributed to James Clerk Maxwell, who corrected the faulty Ampère's law by generalizing it into a more correct conservation law - and it is this very same generalization from whence all of general relativity ultimately derives. It is no coincidence that this equation is the one which defines the speed of light, whether you consider its speed inside a material or through a totally empty vacuum.

One must work one's way up in this world: here's a few good books to get you started:

• Physics for Scientists and Engineers (Mosca & Tipler)
• Introduction to Electrodynamics (Griffiths)
• Classical Electrodynamics (Jackson)

...so you start with the advanced science-and-engineering stuff, and spend several years working your way up to the introductory material, and finally you read the really easy "classical" stuff. If you don't like those books, we also have a thorough list of textbooks in electromagnetism.

Nimur (talk) 16:31, 11 September 2018 (UTC)

• Over what distance? It's a (surprising) aspect that something isn't a perfect insulator immediately, but the current falls off exponentially with depth. So something that you think of as an insulator (or in a related case, something that's opaque to light) will still transmit or conduct it, if it's in a sufficiently thin layer. Andy Dingley (talk) 16:59, 11 September 2018 (UTC)

• That's technically correct that nothing is perfectly insulating, but it depends where you put the cutoff. Except in vacuum or maybe in a material at absolute zero, there is pretty much never zero free charge carriers, so any simple model (e.g. the Drude model) will tell you charges move (i.e. there is current) if there is an electric field. However, if you look at Electrical_resistivity_and_conductivity#Resistivity_and_conductivity_of_various_materials, you can see that the conductivity of metals is always above 10^6 S/m while that of common insulating materials is below 10^-6 (or 10^-4 if damp wood counts). The ratio between the two limits (worst conductor vs. worst insulator) is gigantic (10^10 to 10^12) - to put that in perspective, that is the same ratio as the height of Mount Everest to the width of a hair. If you were evaluating non-smoothness of Earth's surface, you would surely assume that lengths below the millimeter are outside your cutoff - you consider those as flat even if they are technically not perfectly flat. (This answer does not talk about semiconductors because my knowledge of those boils down to "here be quantum").)

You might also be interested in our article about electrical breakdown (whose most common manifestation is thunder inside "insulating" air). Tigraan^{Click here to contact me} 17:17, 11 September 2018 (UTC)

Gravity

What's the diff tween a gravity wave and a graviton? — Preceding unsigned comment added by 80.2.22.73 (talk) 23:35, 11 September 2018 (UTC)

A Graviton is a purely hypothetical part(icle) that some scientists believe exists. A gravity wave is actually more correctly a Time dilation-wave (also measured/noticed that way) that the mainstream of physical science decided to call "gravity wave", probalby because they assume such waves are caused when 2 black holes unite into one. --Kharon (talk) 04:09, 12 September 2018 (UTC)

A gravity wave is a wave in a fluid where the restoring force is gravity. This is the kind of wave that you see when wind blows on the ocean; if you're a meteorology nerd, this is also the kind of wave that causes certain high cloud formations. Here's a cool image of one from the NOAA Cool Image website at the Storm Prediction Center. Here's some more imagery from the MODIS satellite: Gravity Waves, from NASA Goddard. Oh, and here's a special issue of the peer-reviewed journal Atmosphere: Atmospheric Gravity Waves, the March 2017 issue, with just boatloads of science in it.

A gravitational wave is a relativistic phenomenon that has been theorized for a long time and only recently measured. These are very low-amplitude, self-propagating changes in the shape of the universe that occur when very very dense massive objects move. Ever since the theoretical math we call relativistic gravity was first studied - about a hundred years ago - we knew this should happen - but it was only 2016 when we first had instrumentation sensitive enough to measure this very tiny effect. For this achievement, and a lifetime of scientific contributions, the 2017 Nobel Prize in Physics was awarded to three scientists. Here is a video by some guy named Rainer Weiss hosted on Youtube, in which he just drones on and on about gravitational waves for like three quarters of an hour. If you can handle it, you'll glean little bits of wisdom: for example, gravitational waves don't just come from black holes - this other guy named Albert does a calculation for a tiny gravitational wave coming out of a passing train, but he said it was way too small to notice - praktisch verschwindenden.

A graviton is a mathematical description of the propagation of gravity as a localized, quantized amount of change. The math for this description does not work out very easily - so most scientists do not use this mathematical formulation. It is studied by certain communities of theoretical physicists. For a reputable source, here's a little bit of discussion about quantization limits on the LIGO - but there's more to worry about quantization noise, shot noise, photons, and phonons - these are far more practical concerns than any quantization limits of the gravitational wave itself. In fact, there's more gravitational noise from seismic vibrations - not because of the vibrations themselves (which are low frequency and can be easily measured and compensated), but because the compression waves cause mass density changes in the soil which then affect the conventional Newtonian gravity measured by the detector! The point is: no real scientist is measuring gravitational quantization; even if it did exist, it's way below the regime of what we can measure with today's experimental physics.

(It's almost as though we, mere denizens of the Wikipedia Reference Desk, can actually find free publications authored by professional experts who know about this stuff!)

Nimur (talk) 04:50, 12 September 2018 (UTC)

One central issue with all that is often overlooked tho. Its still unknown how gravity exactly works, as Nimur also circumscribe in his description of this graviton that may actually not exist in reality. Till now all science around gravity is actually better tagged as "phenomenology" than science. --Kharon (talk) 06:20, 12 September 2018 (UTC)

I'm sorry,... is it simply your own esoteric view that "all science around gravity" is not actually science, or can you cite a reference for that statement?

The Reference Desk is not a place to spout your own original pseudoscientific research. If you are not qualified to help respond to questions by finding encyclopedic-quality resources, you should not contribute answers to queries here.

Nimur (talk) 14:20, 12 September 2018 (UTC)

I think I see Kharon's point (and presume he's including phenomenology as science). However I'd also say that he was out of date. It's a fair description of the period from Newton to Einstein, but was invalidated by GR. Andy Dingley (talk) 15:13, 12 September 2018 (UTC)

GR describes the phenomenon of gravity but we don't have a theory of how it works (quantum gravity) the way we have a theory of electrodynamics. There's a ~ 10 minute youtube interview with Freeman Dyson[1] where he says there might be no gravitons or quantum gravity, and that gravity might instead be more like a temperature, a statistical property of bulk matter. I watched that recently and have actually been meaning to ask about the idea here. I see now that he also has a longer talk[2] so I guess I'll watch it when I get a chance. 173.228.123.166 (talk) 16:00, 12 September 2018 (UTC)

So you're broadly agreeing with my comments on Kharon's post, but seeing the "Till now" as much closer? I can go with that. Although even then, you're arguing 50-ish years vs. 100. Even post-GR theories have been around for some time now. Andy Dingley (talk) 19:36, 12 September 2018 (UTC)

Yes basically, afaict the "till now" extends to the present. GR is considered an unphysical theory because it's not quantized. And there are a bunch of approaches like superstrings that have too many loose ends to be considered theories the way QED is a theory. So at present, we do not have a physical theory of gravity. This is cool and hip, but it's still "out there". I don't understand much beyond the popular level, but the quantum gravity article describes some of the unresolved issues. 173.228.123.166 (talk) 21:05, 12 September 2018 (UTC)