Wikipedia:Reference desk/Archives/Science/2015 August 29

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August 29

Stellar sized black holes, occasional collision with supermassivemassive black holes

Hello, I've certainly read popular science accounts recently of two large (megasolar mass)black holes orbiting each other and expected to collide sometime in the distant future. It would seem to me more common, enough that it could be seen a few times in a human lifespan, to happen that stellar sized black holes would be in the accretion disk of a supermassive black hole at a galactic center, and from time to time the smaller one actually gets "swallowed." Have there been verified observations of this?--I presume there are not verified observations of such things yet, but maybe there are published "candidate" observations where what was seen is hypothesized to be a stellar mass black hole swallowed by a supermassive one? What type of phenomena might be expected to be seen by telescopes from such an event, which might be a fairly hohum event from the viewpoint of the supermassive black hole? (I've read stuff already about spacetime dislocations in the case of merging black holes of similar sizes.) I would also be interested in information about white dwarfs or neutron stars being swept into a supermassive black hole, since those are also stellar sized. thanks. Rich (talk) 04:29, 29 August 2015 (UTC)

This source would suggest that there are indeed several candidate binary black holes, but in all candidate pairs, both holes are of the supermassive variety. A few are mentioned in Binary black hole. I assume these are just much easier to detect. Someguy1221 (talk) 04:39, 29 August 2015 (UTC)

I've read that black hole mergers should lead to gravity waves - according to binary black hole they can also "kick" themselves right out of a galaxy by emitting gravity waves in one direction, which is quite surprising to me.

This may stray from the topic a bit, but I'm curious: are the event horizons of two black holes orbiting one another always at the same gravitational potential energy level? Wnt (talk) 11:01, 29 August 2015 (UTC)

Your question is a bit over my head, although I did just review gravitational potential energy. I'm interested if you could explain more of what your question means.Rich (talk) 19:44, 30 August 2015 (UTC)

Well, this started in a sidetrack at Wikipedia:Reference desk/Archives/Science/2014 September 1 that eventually went off track itself... electrical potential energy is typically considered to be arbitrary, with no knowable "zero potential" that is universal; electrical ground potential varies from place to place and hour to hour on Earth let alone off it. However, I think gravitational potential may be marked with a number of clear absolute points of reference that anyone in the cosmos might be able to agree on that surround black holes and other massive objects: the potential at the Schwartzschild radius at 2 GM/c^2, or the photon sphere at 3 GM/c^2; I think (not sure) the mass that can be released by objects descending to these points may be m/2 and m/3, where m is the mass of the object. But I want to check are they really the same potential energy? My feeling is that because the event horizons eventually merge together, and everything on the merged event horizon should be at the same potential energy, they definitely become the same; so the question is whether the potential energy at an event horizon can change due to the gravity fields of distant objects. Wnt (talk) 20:30, 30 August 2015 (UTC)

The gravitational potential energy of a small mass m is -G M m/r, so if we make r the Schwarzschild radius, we get E = -(1/2) m c^2, confirming Wnt in both points: the potential at the Schwarzschild radius is the same for all black holes, and it's half the mass-energy of m. I'm not sure if relativistic effects (GR in this case) could change that, though. 217.255.164.203 (talk) 11:25, 1 September 2015 (UTC)

Interesting, thanks to everyone.2601:681:4902:31B8:85D6:C4DA:397F:4283 (talk) 05:32, 2 September 2015 (UTC)

Vacuum energy

--2.187.70.127 (talk) 09:40, 29 August 2015 (UTC)

We have an article on vacuum energy. Someguy1221 (talk) 09:43, 29 August 2015 (UTC)

Some dumb questions about genetics

I've never taken a biology class but I've been self teaching myself the basics via books and papers by Dawkins, Trivers, Pinker, Dennett, etc. as well as relevant Wikipedia articles (several of which are excellent btw). But I have some nagging questions that I think are so basic its hard to find direct answers so I thought I would try here. BTW, I know a lot of computer science so that is the way I tend to try and understand this stuff as analogs to things I know from CS, which may be part of my confusion. Anyway here goes:

1) Difference between an Allele and a Gene. The Gene article says: "Genes can acquire mutations in their sequence, leading to different variants, known as alleles" This to me sounds like a Gene is equivalent to a property in object-oriented terms and the allele is equivalent to the possible values for that property. So if I was modelling a person as a class I might have a property called hairColor with the possible legal values {brown, blonde, red}. So hairColor would be (if I'm interpreting this correctly) the name of the gene and the possible alleles would be {brown, blonde, red}. Is that correct? BTW, I realize its seldom this simple and there is usually not a 1:1 correspondence between genotype and phenotype but ignoring that for these simple examples.

2) Difference between selection driven by mutations that change the genotype vs. those that just select alleles from existing phenotype. Sticking with my overly simple example above it seems to me that hairColor could be selected for in two ways. Suppose that the climate changes and people with blonde hair are much harder for predators to spot. Suppose it is so dramatic that it makes a major shift in the survivability of humans and that in a few generations all humans have blonde hair. Nothing has changed in the genotype but we still have selection and a change in the species as all humans end up with blonde hair. Suppose on the other hand that there is a mutation resulting in striped hair and that striped hair gives humans natural camaflauge similar to what has been hypothesized for Zebras. Eventually all humans have striped hair after a few generations. My question is, are there terms to distinguish between these two types of selection? It seems to me in the biology books I've been reading this distinction is usually ignored and the two cases are treated as both examples of "mutations" (e.g., the quote above from the Wikipedia article) but I would say its only in the second case where the genotype changes to allow striped hair that we have a mutation driving the selection where as in the first case there is no mutation. Are there any statistics on how often selection is of one type vs. another?

3) Can genes code for continuous (analog) values or only discrete digital ones? Suppose we have a gene for height. Is it possible that the specific value that gene encodes for individuals can vary as a real number (e.g., for normal humans a value from 5 feet to 7) or does the gene only code for discrete values like {short, medium, tall} and its the environment and other interacting factors that result in two people who both have the "tall gene" still being different in height?

Hope those questions make sense, sorry if they are kind of long winded, I realize I need to get into a class where I can talk to someone one on one but I've had some health issues that make that difficult so thought Wikipedia might be helpful in the mean time. --MadScientistX11 (talk) 15:03, 29 August 2015 (UTC)

On 1 you're correct. An allele is a particular sequence of DNA. A gene is a collective term for the position on DNA that usually has something like that sequence. Now these aren't actually defined in terms of computer science - you can have a gene duplication event where (I might say) "one allele has two copies of a gene in a head-to-tail arrangement", which is probably hella confusing. The point is though, organisms can't live with totally scrambled chromosomes (at least, not yet), so there's always someplace you can go back or forward on the DNA strand and find it's a copy of the sequence every other organism of that type has. And then you can define that section, whether it is a null allele with a DNA deletion or a gene duplication or some more complicated scramble, as an "allele", and say that one organism has that allele while the other has the boring allele with one copy of the gene you expect.

On 2 I am afraid I missed the distinction between your scenarios. I think you're looking for the difference between positive selection for new (or newly relevant) alleles that turn out to be better under a specific circumstance and neutral mutation, as opposed to negative selection that removes undesirable changes. There are even specific bad mutations that happen over and over again, like Fragile X, due to the underlying mechanical instability of DNA at, say, a trinucleotide repeat. There are specific statistical tests that can discern, rather approximately, between these modes of selection. Wnt (talk) 15:15, 29 August 2015 (UTC)

On 3 height seems to be affected by a number of genes - yes if one could lock babies away and raise them exactly the same one might just about be able to detect it is discrete but no-ones going to do that! There are also some characteristics which have a gene which can have a unit repeated which may be used for fine tuning as a number of them are implicated in mental diseases when they are outside the normal range - see Trinucleotide repeat disorder. Dmcq (talk) 17:43, 29 August 2015 (UTC)

Thanks! Very useful. I think on 2 there are just terms I'm not familiar with yet and once I take a class it will be clearer. One thing I find interesting is how much I just inevitably try to fit things into what I already know, for example I had a confusion earlier because I thought that genes could only work at "compile time" rather than at "run time"... of course neither expression really even makes sense for human beings since we aren't programs but I just find for me at least I inevitably start thinking of those kinds of analogies as I read something on a new topic. Anyway, thanks again. --MadScientistX11 (talk) 18:06, 29 August 2015 (UTC)

Your metaphor isn't all that far off ... after all, DNA presents a sequence of information that is converted into RNA, which you might call a sort of "object code", complete with compile-time switches (alternate splicing), which is then converted into the protein which is a sort of "executable file". The caveat being that as you say, they're real molecules, so sometimes catalytic RNAs have direct activity, or regulatory ncRNAs do something without being translated; indeed, sometimes the DNA itself has important characteristics like origin of replication, telomere, etc. But the sense that there are a lot of huge pieces of software being taken for granted in the process, like ribosomes and spliceosomes and RNA polymerases, each with their own sort of set of options which can lead to 'interesting' results, that's real. Wnt (talk) 18:19, 29 August 2015 (UTC)

I think our OP's failed analogy is that DNA would be responsible for the construction of the animal (or plant/whatever) - but once it's built ("compiled"), the way it performs would be determined solely by that initial construction phase. This is clearly NOT the case (and therefore the analogy is a poor one) because DNA is the blueprint for a whole bunch of proteins and enzymes that are continuously regenerated as they are used. So DNA most certainly continues to have a profound effect on living things long after their "construction" phase is complete. Perhaps a better way to consider it is that DNA is the software - and it runs both during the initialization phase and inside the main loop. There isn't really an analogy for "compiling" - it's machine code, and we don't have the high level sources (and you know what a pain THAT can be!)...DNA is also exceedingly badly written code - it's modified continually by (in effect) random number generators and continual copy-pasting between programs - plus the occasional moving of a few lines of code from one place to another.

From a programmer's perspective, it's truly remarkable that any living things survive at all - but when you see some of the horrible 'hacks' that have been done on the way from the primordial cell to the present day...a software engineer just has to shudder and look away!

One of my favorite "go-to" examples is the Recurrent laryngeal nerve. Our vocal chords are evolved from the gills of fish - and in a fish there is a nerve that runs from the brain to the left gill and another to the right. As it happens, the fishes' heart lies somewhere between the brain and the gills and both nerves have to take a slight diversion to avoid it. However, as we evolved, the two gills became our vocal chords and in the gradual evolution of the larynx and the narrowing of the neck and the movement of the heart down into our chests, on of the two nerves became entangled around an artery. Hence one of the two nerves that controls our voices goes directly from brain to larynx - and the other goes down the neck, around the heart and back UP the neck to the larynx. This is only mildly inconvenient for most animals - but the poor giraffe has one side of the larynx connected via an 18" nerve and the other one by a 20 foot nerve. The resulting timing differences down the nerve fibres make it almost impossible to synchronise the poor animals vocalizations - and (unsurprisingly) don't make a whole lot of fancy noises.

That's such a classic example of really terrible programming - where an old piece of code really, really needs to be rewritten...you just want to dive in there and refactor that tiny little subroutine to fix it so that the poor giraffes can communicate better. There are many, MANY other examples of that in nature. IMHO, it's the best reason to deny the existence of an intelligent designer...nobody in his right mind would have deliberately designed the giraffe that way...but, hey, the guy only had seven days to write the whole damned thing - I suppose some bugs and inefficiencies are inevitable!

SteveBaker (talk) 02:28, 30 August 2015 (UTC)

1) Yes, you have it right, and Wnt's explanation is spot-on. But be aware many outlets mistakenly use "gene" when they should say "allele" - e.g. "John has a gene for blue eyes" :-/ SemanticMantis (talk) 21:01, 30 August 2015 (UTC)

Just want to say thanks again for all the great answers. I really liked the discussion about compile time vs. run time because for one thing it validates that my initial very high level grasp of what a gene is wasn't that far off but also the comment about how genes can still play a role in the living organism is what I've found as I delve deeper into some specifics. Also, the very last comment about how some people often write "gene" when they should write "allele" I think that might be one of the most useful ones. Again, its kind of what I've come to think on my own but I wasn't completely sure if I was missing something or if people were just being a bit loose with their terminology which I think now IS often the case. --MadScientistX11 (talk) 17:54, 31 August 2015 (UTC)

voice announcement of circuitbreaker

circuit breaking voice announcement by circuit breaker for project — Preceding unsigned comment added by 223.227.225.223 (talk) 15:03, 29 August 2015 (UTC)

This sort of project will need a microcontroller. It needs a way of getting the state of the circuit breaker (closed, tripped, or switched off - this will need some external electronics), a way of finding out when the breaker trips (polling or interrupt-driven?), and a way of sending out the voice announcement (a separate device, or on-board? Loudspeaker, telephone, or Internet?). Tevildo (talk) 08:51, 30 August 2015 (UTC)

No it doesn't. It needs a relay across the live side of the CB that switches on whatever signal is needed. An engineer does for a dollar what you've just done for 10. Greglocock (talk) 20:31, 30 August 2015 (UTC)

Well, the relay is one possible way of detecting if the breaker has tripped. (Can it distinguish between a trip and a deliberate switch-off? No. Is it a cheap solution if you have 100 circuit breakers to monitor? Probably not. Does the OP need to distinguish between a trip and a switch-off? Perhaps, we'll need more details, bearing in mind we can't do their homework for them.) However, the major part of the OP's project is generating the voice announcement and communicating it to the relevant people, which will need something more sophisticated. Tevildo (talk) 22:27, 30 August 2015 (UTC)

You get plug-in devices that beep very loudly when their power is removed. They are normally used by electricians to quickly determine which breaker controls the circuit they are about to work on. LongHairedFop (talk) 11:02, 30 August 2015 (UTC)

Fuel consumption of front vehicle during tailgating

During tailgating, the vehicle in the back is saving energy, however, is the vehicle in the front spending more? --Scicurious (talk) 15:26, 29 August 2015 (UTC)

The vehicle in front also expends less energy, because the tailgating vehicle will reduce its aerodynamic drag.--Shantavira|^{feed me} 16:49, 29 August 2015 (UTC)

See drafting (aerodynamics). --65.94.50.17 (talk) 19:13, 29 August 2015 (UTC)

The space in front of a moving vehicle is at higher pressure than the ambient air - and the space behind it is at lower pressure than the ambient air. The net drag force on the vehicle is dependant on the difference in pressure between the front and the back. But if someone is tailgating sufficiently closely, then the air between the two vehicles is just moving along at the same speed that they are and so long as the distance between them remains constant is more or less at ambient pressure. So the front vehicle saves fuel because there isn't as much pressure differential from front to back, and the tailgating vehicle gains for the exact same reason. The details of the relative sizes and aerodynamics of the two vehicles matters - as does things like turbulence and the Bernoulli effect - anything involving non-laminar flow is tough to do the physics on...but you would expect there to be at least some savings for both vehicles.

SteveBaker (talk) 02:07, 30 August 2015 (UTC)

Many vehicles where designed as moving containers or boxes benefit from aerodynamic drafting, a German translation in word by word is the "wind shaddow". Ostraciidae, called boxfishes, inspired a better desing to the automotive industry. Many customs use a much wider windshield than tail windows. The area of plain tail windows is gone. Aerodynamic drafting is a violation of safety restrictions, even a trailer is not close enough. Articulated buses use gaiters for several reasons. --Hans Haase (有问题吗) 10:07, 30 August 2015 (UTC)

@Hans Haase: I am having a hard time trying to decipher your message, not to speak about how it could relate to my question or other posts, if at all. --Scicurious (talk) 12:13, 30 August 2015 (UTC)

This is very normal. Hans talks like this on de.wikipedia as well. I firmly belive he is right an I hope to beable to understand his train of thought some time.--91.44.61.70 (talk) 21:50, 30 August 2015 (UTC)

Often Hans gives insightful and useful answers (when translated into standard English). Dbfirs 18:11, 1 September 2015 (UTC)

Thanks. The automotive industies first idea was the hatchback. Later, inspred by the boxfish, in this picture[1], see the roofs front and rear dimensions to understand what I mean. Even a trailer is not close enough, to benefit from aerodynamic drafting. Articulated buses to passenger trains with accessibe cars from each other use gaiters. --Hans Haase (有问题吗) 20:04, 1 September 2015 (UTC)

"Even a trailer is not close enough, to benefit from aerodynamic drafting."

Hans - you're guessing - and it's just not true. In 2007 the Mythbusters TV show (Big Rig Myths) actually tested the fuel consumption of a vehicle driven at a range of different distances behind a large truck moving at 55mph - they obtained the following results:

Controlled road test: fuel consumption
decrease from baseline at 55 mph (89 km/h):

100 ft (30 m)	50 ft (15 m)	20 ft (6 m)	10 ft (3 m)	2 ft (1 m)
11%	20%	27%	39%	28%

...clearly showing that the fuel economies that come from 'drafting' are highly significant even at an amazing 100 feet behind the truck. They also caution that driving too close to a big rig is incredibly dangerous. (100 feet sounds like a lot - but note that at 55mph, the official "stopping distance" is around 170 feet - or 300 feet if you include "reaction time"). The slight lessening of the fuel consumption benefits at 2 feet (eeek!) behind the truck were attributed to the difficulties of driving that precisely behind the truck without continually being on either brake or gas.

So, no - what you said was clearly untrue.

SteveBaker (talk) 01:58, 2 September 2015 (UTC)

Thanks for the table, but most passeger vehicles' shape design is not the first need for permanent trailer use. Even trailers are an other market. Your link remebered to a video to review driving an empty pickup with open of closed tail lid. I guess the results are caused by design of the cabines rear. --Hans Haase (有问题吗) 09:32, 2 September 2015 (UTC)

Obviously shape matters in overall fuel consumption - and the amount of savings due to drafting will also vary (possibly quite dramatically) with the shapes of the two vehicles bodies. But no practical shape is 100% drag-free, so there will always be savings due to drafting...although not always so large as in the Mythbusters results. But you said that there could be situations when there would be zero benefits - and that's just not true. SteveBaker (talk) 14:18, 2 September 2015 (UTC)

Boot camp and circuit

What is the difference between a boot camp class and a circuit class? They seem the same to me. Both work on power, strength over the whole body and cardio. 94.10.247.39 (talk) 18:30, 29 August 2015 (UTC)

I don't believe either has a particular formal definition (it's not like Spinning or Zumba, which have programmes prescribed by the companies that own those trademarks). A given gym or instructor might set a different programme for those two classes, but how they differentiate the two is up to them. If you're thinking about the offerings of a specific gym, you'd have to ask them. You might find bootcamp is early in the morning, is the more intense of the two, or simply is outdoors and contains more running. But you're quite right that they're often very similar classes. -- Finlay McWalterᚠTalk 18:43, 29 August 2015 (UTC)

Circuit training has a fairly specific meaning, but boot camp could mean just about any kind of tough training. StuRat (talk) 21:20, 30 August 2015 (UTC)

Fitness

Why is it that fitness drops significantly when you haven't trained for a while and also picks up quite quickly when you start again? Whereas people often say those who have never trained before & start have a much tougher time building up Fitness? 94.10.247.39 (talk) 19:16, 29 August 2015 (UTC)

You will find some relevant information at Cardiorespiratory fitness and Physical exercise. Dolphin (t) 07:10, 30 August 2015 (UTC)