Wikipedia:Reference desk/Archives/Science/2015 July 17

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July 17

What happens when the inputs to a phototransistor is shorter than its response time?

Please see this drawing[1]. The first frame is my (admittedly poor) understanding of a phototransistor's response time. Please correct me if there's any mistake.

My question is, what happens when the duration of the input is shorter than the phototransistor's response time? Is there still a delayed response, or is there no response at all? (The latter case being like your car, if you don't turn the key long enough, it doesn't start at all)

Assuming there's still a response, what happens when the input is shorter than even the rise time? Would there still be a cut-off triangular-shaped rise? What would the response waveform look like?

Lastly, is there a minimal input duration below which the phototransistor makes no response at all? My other car is a cadr (talk) 06:36, 17 July 2015 (UTC)

I think you are correct in assuming that you will get a smaller output. You may be able to see this in a frequency response chart. At high frequencies the output will be reduced. Graeme Bartlett (talk) 08:33, 17 July 2015 (UTC)

Electromagnetic radiation vs particle radiation

The quantum of electromagnetic radiation is photon. Then why is electromagnetic radiation not considered to be a particle radiation? --IEditEncyclopedia (talk) 06:46, 17 July 2015 (UTC)

It is, you use the theory that is most useful to you. Graeme Bartlett (talk) 08:16, 17 July 2015 (UTC)

In our article Radiation, three types are described in the lead paragraph: electromagnetic radiation, particle radiation and acoustic radiation. EM radiation is not included in particle radiation. --IEditEncyclopedia (talk) 08:37, 17 July 2015 (UTC)

My reading of that lede is that what are being give are examples, which are neither exhaustive nor necessarily mutually exclusive. Notice that examples given of particle radiation include β radiation which comprises electrons, but electrons can also be interpreted as waves. These phenomena are neither "pure" particles nor "pure" waves, but something else that appear to be either, depending on how you look at/detect them: see Wave-particle duality. {The poster formerly known as 87.81.230.195} 212.95.237.92 (talk) 12:57, 17 July 2015 (UTC)

half life

Why is half life in first order reaction independent of initial concentration but not other order reaction? — Preceding unsigned comment added by 36.252.1.178 (talk) 08:28, 17 July 2015 (UTC)

In first-order reactions the rate of reaction is proportional to the concentration, and the rate of change of the concentration is the same as rate of reaction. You can see from the formula that the left over is proportional to the initial amount. This results in an exponential decay of the initial product. SO for half life look at the t for e^-kt=1/2. With other orders of reaction the relative rate will change on concentration. You can reverse the formula so C=C_i×1/2 after a half life for each C_i. Take the derivative of this. You will get rate of change of C is then proportional to C_i and not C_i² or any other power. Graeme Bartlett (talk) 08:39, 17 July 2015 (UTC)

In case of first order reaction, if initial concentration is $a$then after first half life time it will become 1/2a and after same time it will become 1/2 *1/2a and so on.

Then what is the similar physical interpration for other case (for those dependent on initial concentration)? Does it change continously for a same rxn with time or not? 36.252.1.178 (talk) 09:04, 17 July 2015 (UTC)

for second order the reaction rate is proportional to the square of concentration, for example sulfur monoxide reacting to form disulfur dioxide. Then the rate will drop off much more rapidly. But for zeroth order the rate is constant no matter what the initial concentration (within limits). Graeme Bartlett (talk) 12:53, 17 July 2015 (UTC)

Supermassive black holes

I was looking at the supermassive black holes article and I see no mention of the contribution of dark matter to their size. Has anyone estimated how much WIMPs would contribute to them, would it just be in proportion to the amount of dark matter in the galaxy or quite different? Dmcq (talk) 09:42, 17 July 2015 (UTC)

It is normally thought that black holes form almost entirely from barionic matter. Ruslik_Zero 19:26, 17 July 2015 (UTC)

Thanks I'd have thought after getting large a black hole would grow by the amount proportional to the density of matter in the neighbourhood times average speed by the area of the black hole. And there's lots more dark matter and it probably goes a bit faster if it forms a halo. I guess though the density of dark matter near the center of the galaxy must be low compared to the visible matter there which would explain why only the visible matter matters. Dmcq (talk) 21:00, 17 July 2015 (UTC)

Basically in order to fall into the black hole, matter of any type has to lose angular momentum by interactions with other matter -- otherwise it will just orbit around the black hole indefinitely. But the ways that dark matter interacts with itself or with normal matter are essentially not understood at all. So it is impossible to predict the rate at which it will fall into the black hole, even within orders of magnitude. Looie496 (talk) 13:45, 18 July 2015 (UTC)

I thought those orbits also caused the organised orbits as in the disc of the galaxy. In that case if the dark matter has very littler interaction it should just be in a blob rather than form nice orbits, so instead of orbiting the black hole nicely it would just approach it at random perturbed by the gravitation as it goes out to the halo and back. But then again it looks from the articles that spiral galaxies are younger than the elliptical ones so they must start off going in nice orbits and later get mixed up which is rather strange. It's not what I expected. Dmcq (talk) 14:48, 18 July 2015 (UTC)

Black holes aren't "made out of" matter at all. They possess a singularity that has mass, but has no volume. [[2]] Malamockq (talk) 16:56, 21 July 2015 (UTC)

Where does Supermassive black hole say that such an object has 0 volume? ←Baseball Bugs ^{What's up, Doc?} carrots→ 17:04, 21 July 2015 (UTC)

Why doesn't the B2 Stealth Bomber need a rudder to control yaw?

How does it do vertical stabilization/control with no rudder? 20.137.7.64 (talk) 15:29, 17 July 2015 (UTC)

There is a general discussion of the problems of yaw control in flying wing aircraft at flying wing#yaw control. Not sure which specific solution is used in the B-2. Gandalf61 (talk) 15:45, 17 July 2015 (UTC)

The B2 has a drag rudder and a very complex computer. 209.149.113.45 (talk) 15:46, 17 July 2015 (UTC)

The aircraft is actively controlled for stability. Here's an Air Force website that explains: "The outboard wing trailing edge has split (drag) rudders which provide yaw control. Three sets of elevons (outboard, mid, inboard) provide pitch and roll control."

Nimur (talk) 16:13, 17 July 2015 (UTC)

Here's a document produced by a senior aerodynamicist at BAE, available on DTIC, Stability and Control of UCAV Configurations. It's not about B-2; but it expounds several options for unconventional yaw control on blended body and flying wing aircraft. Among the options: thrust vectoring, reaction control systems, symmetric split drag control surfaces (as used on the B-2 drag rudder). Here's an aerodynamic engineering lecture note from Virginia Tech on the topic of B-2 aerodynamic control specifically. Its author calls out numbers for CG and CP as well as details of the control surface.

Nearly all technical information about B-2 is confidential - even though the aircraft is a few decades old - so a lot of what you will find is analysis by way of reverse-engineering; and little snippets of facts that are officially released to the public. It is difficult to verify the accuracy of such details. Nimur (talk) 16:25, 17 July 2015 (UTC)

I feel obliged to remind the non-aviators: bank turns the aircraft - not yaw! It took a while to find a good reference to clarify this point using only illustrations and non-technical language, and finally: here is Banking Turn, from the NASA Glenn Research Center's Beginner's Guide to Aeronautics.

This is one of those things that's a little bit unintuitive to the non-pilot: you don't (normally) turn the aircraft using the rudder. You control yaw using the rudder, but you turn by rolling into a bank and holding attitude. This causes aerodynamic lift generated by the wings to act as a centripetal force, pulling the aircraft into the turn. In coordinated flight, the pilot also controls yaw to ensure passenger comfort and aircraft stability. For the B-2, where yaw control is marginal, passenger-comfort and aircraft stability are also marginal.

Furthermore - B-2 aircraft operate out of only specific military airfields, where they won't have to contend with serious crosswinds. This eschews the need for very effective cross control. We probably cannot find exact numbers for the maximum demonstrated crosswind capability of the B-2 - but it's probably lower than the published procedures for the U-2 - which can safely land with crosswind component in excess of 15 knots! (This is to say nothing of a conventional landing gear aircraft like the Citabria, where rudder authority means everything: the manufacturer advertises the 7GCAA with a maximum demonstrated crosswind component of 17 kts!)

Nimur (talk) 18:18, 17 July 2015 (UTC)

What does "authority" mean here? Is it part of aviation controlled vocabulary, or are you just using it in a fairly normal metaphorical sense to indicate the large effect of the rudder in the control system? SemanticMantis (talk) 22:38, 17 July 2015 (UTC)

In the context of steering a vehicle, or controling a vehicle, the word authority is used qualitatively to describe the responsiveness of the vehicle to the actions taken by the person steering or controling the vehicle. For example, if a yachtsman applies a large force to move the tiller through its maximum deflection but the yacht changes direction only slowly, it is reasonable to say this type of yacht does not provide its helmsman with adequate directional authority. Conversely, if a small force and a small deflection of the tiller cause the yacht to make a prompt change of direction at a satisfactory rate, it is reasonable to say this type of yacht provides its helmsman with excellent directional authority. Dolphin (t) 07:20, 18 July 2015 (UTC)

This phraseology - "rudder authority," "control authority," and variants - is very common. It appears in both the Airplane Flying Handbook and in the Pilot's Handbook of Aeronautical Knowledge, among many other great resources. I'm not certain what you mean by "controlled vocabulary"; a different resource (the Pilot/Controller Glossary) standardizes terminology for use between airmen and air traffic controllers, and still another resource (the AIM) standardizes radio phraseology; but "rudder authority" isn't in these resources - it's sort of a different scope.

So, although "control authority" shows up in a lot of training manuals, none of the major book resources actually define it! I can provide my ("non-authoritative") definition: "authority" means that when you operate the controls, the aircraft actually responds. In plain English: when you hit the stick, the airplane pitches and rolls; when you kick the rudder pedal, the airplane yaws.

Maybe the surprising thing is that not all aircraft have great control authority in all operating conditions. For example, at low speeds in some aircraft, you might find poor aileron effectiveness: pushing the stick left and right may not actually roll the airplane very much at all. The ailerons, for example, need lots of fast moving air flowing over the wing in order to exert a strong torque and cause a roll. Intuitively, you expect that when you manipulate the control, the aircraft should just move - but that's not how aerodynamics works!

This intuitively-simple behavior requires many things:

• The aerodynamic forces must not be too powerful for the pilot (or the hydraulic system, or whatever else moves the control surface). In plain English: if the wind pushes the control surface so hard, forcing it back to equilibrium with such great pressure that the pilot can't operate it, that constitutes poor authority. This plagued the Mitsubishi Zero, and it recently killed a Mustang pilot (and many spectators) at the 2011 Reno Air Race - a trim tab became damaged, and the stick pushed back against the pilot's manual input with hundreds, perhaps thousands, of pounds force. No human pilot - conscious or otherwise - could have pushed the stick with enough force to control the aircraft.
• It also means that the deflection of the control surface is within its maximum limitations. In plain English: if the stick is all the way to the right and the plane is not rolling as much as you need, that constitutes poor authority - and defective aircraft design!
• It means that aerodynamically, a deflection of the control surface actually causes the aircraft to maneuver as expected. The magnitude of the effect should be commensurate to the pilot's input. The actual amount of control authority depends on all the engineering details for any specific airframe and the mechanism of its control surface actuation (the size and position of the control surfaces; the mechanical advantage of any cables, push-rods, or hydraulics; the operation of any electric motors, aerodynamic spades; the effects of any software and electronic interlocution; or whatever else is designed into the aircraft). It depends on messy details of fluid dynamics, laminar flow, slipstream effect, and so on.

I checked another one of my books - Conventional Gear, expecting to find an excellent definition... but to my surprise, author David Robson does not appear to ever use the phrase "rudder authority." Instead, he always uses the term "power" - which I find a little bit erroneous (power has a specific meaning in physics). For example, in his section on flight dynamics, he writes: on the design of aircraft forIncreased Rudder and Elevator Power:

"...Since the taildragger is designed to land at the stalling angle of attack, there must be enough elevator power at minimum airspeed to still pitch the nose up. Thus the taildragger has more elevator power than the trike. Similarly, at this minimum airspeed it must have sufficient rudder to provide positive directional control. This control power is invaluable, but can cause departure from controlled flight or overstress if misused at low or high speed respectively."

In this case, he refers to "power" where I would say "authority;" nonetheless, he accurately describes that tailwheel aircraft tend to have more of it than their trike counterparts.

Bringing it all back to the original question: the B-2 has poor rudder authority compared to some hypothetical, differently-designed flying wing with a large vertical stabilizer and rudder; but that's okay, because B-2 is operated in conditions where this doesn't affect safe and effective flight.

Nimur (talk) 08:10, 18 July 2015 (UTC)

Thanks, I hadn't heard the usage. I guess it's a little less formal than being part of a controlled vocabulary, but a little more specialized than a regular metaphor. Perhaps term of art covers it. SemanticMantis (talk) 14:59, 18 July 2015 (UTC)

Big bang(s)?

Since our Big Bang is supposed to have happened out of nothing in a zero spacetime complete non-entity, what is the probability of other big bangs having occurred or going to occur in completely separate unlinked non-entities? Also, can non entities ever get connected to each other before or after they have had their bangs?? Also, when we've already had one big bang, is there anything to stop another big bang occurring in our existing universe?--81.147.170.83 (talk) 19:11, 17 July 2015 (UTC)

You will probably enjoy reading about the Multiverse. Probability is ontologically weird in this context, it's not clear to me that any of the Probability_interpretations would make sense.

Ultimate_fate_of_the_universe outlines some possibilities of some things that are sort of like Big Bangs, but not. The Cyclic_model suggests that we could have a Big Crunch followed by another Big Bang. SemanticMantis (talk) 19:37, 17 July 2015 (UTC)

There are times when you just have to admit you don't know... even if you're a physicist. It is still up in the air what cosmic inflation really means, whether it is a model that makes any testable predictions. Anything before that seems to be pure speculation (I welcome anyone to differ!) So what happened before the Big Bang, multiple Big Bangs and all that... we don't know. Also, AFAIK, since there were (apparently) more and more collisions and interactions at higher and higher energies the closer you look back toward the Big Bang, there is in a sense an infinite amount of "history" in that finite interval as measured by our definition of time (also feel free to correct me there; I'm speaking out of my depth). Wnt (talk) 23:38, 17 July 2015 (UTC)

"In the beginning God created the heaven(s) and the earth." Replace "God" with "the forces of nature" and "the heavens and the earth" with "the universe", and the bottom line is that the writers of the Bible knew as much as the physicists know (and vice versa). ←Baseball Bugs ^{What's up, Doc?} carrots→ 03:52, 18 July 2015 (UTC)

I say this without any intent to demean religious thought: your argument is very faulty. First, it is not fair to selectively replace phrases at whim: you are changing semantic meaning of the phrase. If one may randomly replace words, one can construct any new semantic meaning at all. Are you really suggesting that we are free to substitute any words of the English language with any other words that we wish? Shall we replace "Baseball" with "Eleven"; "Bugs" with "wallabies", and suggest that you are equivalent to a mob of marsupials? This is not how written language facilitates communication.

Secondly, the bottom line is that physicists make quantitative and testable predictions that are frequently correct. Each part of this explanation is an element of an important distinction between science and faith-based philosophy:

• quantitative: when a physicist says that gravity pulls objects together, this statement is qualified with a precise and accurate numerical representation about how much force is exerted. Religious prognostications about forces of nature rarely make accurate and precise statements; in particular, Genesis uses inaccurate and imprecise measurement of time to describe how long Creation required. You may argue that some religious writings were metaphorical, or must be interpreted non-literally; but that's exactly my point: Isaac Newton did not write metaphorically when he calculated the second derivative of position with respect to time. Physicists use formal mathematics and quantitative analysis to whittle the core concepts into ...
• testability, by way of controlled experiment, which makes each scientific statement falsifiable. Again, physicists do not speak metaphorically when they state that objects attract each other: they prescribe a means to easily determine truth or falseness of the premise.
• frequently correct: physicists do not claim infallibility. Instead, we are confident in our methodology; it is well-designed and when exercised correctly, it leads us to empirically-validated truth. When we are wrong, we adjust our assumptions, rescind our erroneous claims, and strive towards greater correctness in the future.

Isaac Asimov wrote an impassioned article, The Relativity of Wrong, (1989); excerpts are published online. Here is one republished copy hosted by Tufts University in Massachusetts. If you have not already read this article, please read it. While you are at it, why don't you also read Genesis? The first chapter doesn't take very long at all, and I find it to be more instructive to read such source-material than to read any of the voluminous second-hand commentaries on the topic of religion and science. A free, free copy is hosted at Project Gutenberg; there are many variants: each different English translation usually reflects subtle variances that make more sense in the context of some theological or political history. When you are done reading your favorite translation of Genesis 1, read Asimov's article again, and reconsider whether you still believe that the writers of the Bible "knew as much as the physicists know."

Nimur (talk) 23:34, 18 July 2015 (UTC)

Physicists know nothing about what triggered the Big Bang, nor did the writers of Genesis. ←Baseball Bugs ^{What's up, Doc?} carrots→ 04:11, 19 July 2015 (UTC)

Well actually, the writers of Genesis "know" that everything began with a deliberate act of creation by God (who is implied to exist before time). Qualitatively, there is a difference between physicists who generally profess ignorance about how the universe began and theologians who profess knowledge that a divine intelligence deliberately created everything, Religion claims to have an answer. Whether or not that answer and the "proofs" supporting it are satisfying to any given person will vary, but it is a rather different condition than professing ignorance. Dragons flight (talk) 05:43, 19 July 2015 (UTC)

Actually, if you interpret each of the "days" of Genesis 1 as an age of varying length, it is, with one exception, a reasonably good description of what scientists today believe. The exception is that the fourth day, the creation of the Sun and Moon, is out of place, because the formation of the solar system should be the second of six ages of varying length. Robert McClenon (talk) 23:37, 18 July 2015 (UTC)

If you interpret "Robert McClenon" as "herd of platypuses," your user name is (with one exception) a reasonably good description of "Baseball Bugs." Regrettably, fluent speakers of the English language do not allow this type of unfettered dynamic readjustment of semantic meaning of words: your argument does not stand to reason. Nimur (talk) 23:49, 18 July 2015 (UTC)

Yes, because scientists agree that grass and fruit trees were created before the sun / moon / all animals. And that birds appeared before land animals. Not to mention that all animals were created as vegetarians. And so on. If you want to think there is a bit of truth in Genesis, you can think that, but suggesting that it is a "reasonably good description" of what scientists believe is both selling science short and giving much more credence to the Genesis creation myth than it deserves. Dragons flight (talk) 05:22, 19 July 2015 (UTC)

Complicated by the time line contradictions between the first and the second creation story in the first two chapters of Genesis. ←Baseball Bugs ^{What's up, Doc?} carrots→ 05:28, 19 July 2015 (UTC)

All we know is that the universe has expanded and cooled from a very uniform, dense plasma. That state might have resulted from inflation, in which case we have no idea what started the inflation or what the universe was like before, or it might have arisen in some other way, in which case we know even less. There's absolutely no evidence that the universe came into existence 13.7 billion years ago or at any otherearlier time. Though modern cosmology still uses the name "big bang cosmology", it doesn't have a big bang in it. -- BenRG (talk) 07:20, 18 July 2015 (UTC)

I hope some real physicists reply here and school you trolls. I'm sure the writers of the bible knew all about Particle Physics time dilation special relativity the photoelectric effect radiation periodic elements and so on etc etc etc.. evidence for the age of the universe redshift CMBR Void burn (talk) 17:48, 18 July 2015 (UTC)

@Void burn and BenRG: glancing at this, I realize BenRG's comment could be misinterpreted out of context. I think he would be first to say that the universe has been proven to be old, i.e. at least the 12.5 billion years that light has travelled since leaving the most distant known galaxy. [3] He's only saying (as I have) that we don't really know what happened at a very long time ago - whether the Big Bang extrapolation really goes back to the beginning, whether the history is a little shorter (like, seconds) due to cosmic inflation having more quickly expanded space, or whether it is a little or a lot longer because we know not what happened earlier. Wnt (talk) 21:44, 20 July 2015 (UTC)

I'm not certain but I think Void burn was replying to the people talking about the book of Genesis, not to me.

(Inflation makes the history of the universe longer than in the traditional big bang, even if it's brief. It's normally said that it has to have lasted for at least 60 e-folds (expanding by at least a factor of e⁶⁰) to explain observations. In this diagram, t = 0 is the traditional big bang and the time between 0 and t_R is 0.5 e-fold, so inflation can't have started later than around t = −119t_R < 0. Images that show otherwise, such as this one, are wrong.) -- BenRG (talk) 00:58, 21 July 2015 (UTC)

Good program on tv last night said that the big bang may have spawned many separate universes all in out own space! Also that these universes may collide :)--81.147.170.83 (talk) 15:15, 21 July 2015 (UTC)