Wikipedia:Reference desk/Archives/Science/2017 March 21

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March 21

Spacecraft travelling at 99 percent the speed of light or greater

How could a spaceship possibly travel at 99 percent the speed of light or more? Do you need a special engine or special fuel?Uncle dan is home (talk) 00:43, 21 March 2017 (UTC)

At the level of practical engineering, no one knows how to do it. That in itself doesn't mean it can't be done.

At the level of speculation, see Bussard ramjet. They probably can't be made to work, but statements like that have a poor track record.

At the level of argumentativeness, just pick a reference frame in which the Earth is going 0.99 c, and then use the Earth. --Trovatore (talk) 00:56, 21 March 2017 (UTC)

(edit conflict) Well the easiest way is to just measure your velocity relative to something on the furthest edge of the visible universe... but I'm guessing you wanted a real answer. We have nice articles at Spacecraft propulsion and interstellar travel. The problem with most spacecraft propulsion technologies getting you to near the speed of light is that to go say twice as fast, you need to carry way more than twice as much fuel, since you have to get the fuel to make that extra speed bost, as well as even more fuel to get the extra fuel to that point. As a result, the fuel cost goes up exponentially, and that's not even taking into account relativistic effects (see Tsiolkovsky rocket equation). So a conventional chemical rocket or even an ion engine are probably no-goes if 99% c is your goal. An antimatter rocket may be able to reach near the speed of light, and the speed of a laser propulsion system is not limited at all by how much fuel you can carry. Someguy1221 (talk) 00:57, 21 March 2017 (UTC)

But where are you going to get that much antimatter? And for laser propulsion, doesn't a laser beam spread out, making its intensity drop the farther the sail gets away? Bubba73 ^{You talkin' to me?} 02:59, 21 March 2017 (UTC)

In the case of antimatter, you run into the first problem that trovatore lists. Regarding laser propulsion, it's not without problems! Simply that fuel is not one of them (well sort of. If you are using a laser-ablative propulsion system, then you still have fuel, but you're only carrying the inert propellant and not the power source). This is why our articles link to sources giving a "top speed" if you will. Any given system has a distance or speed beyond which it doesn't work, for one reason or another. Someguy1221 (talk) 03:51, 21 March 2017 (UTC)

In theory any engine and fuel that produces acceleration long enough would achieve something close to that speed eventually. In reality tho this seems rather pointless, because the only usefull task would be to travel somewhere and for that you would have to decelerate more or less the same amount eventually. So as it doesnt make much sense to reach the highest possible "top speed" in travels today it would not make sense for space exploration alike. Any aproach would mainly focus on economical problems to solve. So your question seems rather pointless. --Kharon (talk) 04:08, 21 March 2017 (UTC)

It's not pointless because this is not a simple engineering problem. Even with a non-relativistic rocket equation, you will find that ordinary chemical rockets simply can't do this in the real universe. Go and play with a calculator for the rocket equation here, for instance, and plug in values for ISP and final mass associated with conventionally propelled spacecraft. You'll find that for even a liquid oxygen-hydrogen engine such as those on the space shuttle, fuel requirements rapidly exceed the mass of the observable universe before you get close to the speed of light. So you can actually place a hard limit on how fast a given type of engine could ever propel a spacecraft of a given size, regardless of even unforeseen advances in engineering. Someguy1221 (talk) 00:02, 24 March 2017 (UTC)

Given a hypothetical engine capable of constant acceleration changing at mid-course when $v=0.99c$ to deceleration, by the Equations of motion often referred to as the "SUVAT" equations, where "SUVAT" is an acronym from the variables: s = displacement, u = initial velocity, v = final velocity, a = acceleration, t = time:

From $v=u+at$

Duration of the trip $2t=1.98c/a$

From $s=ut+{\tfrac {1}{2}}at^{2}$

Distance travelled $s=1.9602ac^{2}$.

However the "SUVAT" equations that are defined in a Euclidean space in classical mechanics prove inadequate when the constant $c=299792458$ metres per second is plugged in. Blooteuth (talk) 12:22, 21 March 2017 (UTC)

AFAIK, those particular equations are adequate and precise with regard to determining acceleration, velocity and distance traveled by any ship with respect to clocks and rods at rest with our Earth-based reference frame. The ships' relativistic masses will increase though. Modocc (talk) 16:26, 24 March 2017 (UTC)

I've written about the logistics of this exact problem before (somewhere in the reference desk archives), for passenger ships and probes to attain ninety-nine percent the speed of light without any new or novel rocket engines, fuels or a leap in our present know-how, it's simply a matter of mass production and assembly of mass driver elements and supply ships. For example, we already have mass produced billions of road vehicles and powered millions of structures year-round on extensive electric grids as well as having produced millions of miles of massive roads. So it's conceivable that we could build and operate super long mass drivers (SLMDs) that are millions of kilometers long (more or less than one astronomical unit) or so. That is what it takes to accelerate to near c with the present coil gun accelerations that are on the order of 5 to 50 kgee (one kgee or kGee is a thousand g's of acceleration). Solar-powered these would accelerate and decelerate probes and supply ships that carry fuel, food, shielding and other supplies toward passenger ships that are in transit toward the stars and to other colonies with SLMDs that receive these; each passenger ship would have the capacity for repetitive and rapid in-flight docking to receive these supplies from the supply ships. Ordinary rocket engines could then easily sustain the thrust needed for every passenger ship's acceleration of one g for artificial gravity and with refueling these engines can operate for the year duration that is needed for the passenger ships to attain ninety-nine percent the speed of light. At that speed, relativistic mass is about fifty times their rest masses (rest mass)/sqr(1-(.99)^2) and since the passenger ships do not carry most of their trip's fuel, supplies and shielding (which will eventually need replacing at such speeds) the engine sizes and thrust requirements are completely manageable. So the answer to the question is yes our rockets could attain such speeds with today's know-how. The most challenging part of actual colonization though is not getting ships and probes to attain those speeds, but stopping them at their chosen destinations since fuel and other supplies cannot be sent long distances at the low velocities needed for docking. The supply ships' speeds from their initial departure would far exceed the passenger ship's speed as they stopped, making docking infeasible due to the enormous g forces that would be involved. This was not a problem with accelerating a passenger ship because the supply ships needed only go fast enough at constant speed to catch up to the passenger ships for their speeds to match. Perhaps an alien population would be kind enough to build similar super long mass drivers which can make the necessary deliveries near their homes... Thus the solution to decelerating small ships is to build additional SLMDs that are placed within the half light-year or so of the stopping distance short of each destination such that these can decelerate supplies that are sent from any star-based SLMDs such as ours. These destination stationed SLMDs can then send supplies to the passenger ships along their year-long decelerations. The strategic placement of these additional SLMDs requires that they function as megaships... by harnessing the shipments of fuels and reaction masses that are taken on-board to accelerate/decelerate. More than one SLMD station would need to be located at each destination, each to be placed closer and closer to the destination to compensate for the increased lead time for sending the deliveries at slower velocities since they must travel slower than the passenger ships with which they eventually dock. For explorers and colonists returning to Earth we would need to establish SLMD stations here to help with decelerating their smaller craft. In short, mass drivers can serve as the workhorses of establishing and serving colonies throughout the Milky Way and possibly elsewhere.--Modocc (talk) 16:35, 24 March 2017 (UTC)

What's wrong with my chemistry calculation?

[1] 2.40g/56g*82g=3.51g Na₂CO₃ 69.22.242.15 (talk) 01:54, 21 March 2017 (UTC)

{resolved} Never mind. I used the wrong value for sodium. Sorry. 69.22.242.15 (talk) 01:54, 21 March 2017 (UTC)

A real chemistry question

[2] The last two lines there are my work. What did I do wrong? Thank you. 69.22.242.15 (talk) 02:51, 21 March 2017 (UTC)

I think you need to balance the chemical equation first. See gallium arsenide for details. You may be assuming a 1:1 ratio, because it appears that way, even though the gallium arsenide article suggests that it's not really 1:1. 50.4.236.254 (talk) 03:32, 21 March 2017 (UTC)

Gallium_arsenide#Preparation_and_chemistry shows GaAs molecules as the products. 69.22.242.15 (talk) 03:59, 21 March 2017 (UTC)

Yes. But this chemical formula most closely matches your situation. 4 Ga + As
₄ → 4 GaAs or 2 Ga + As
₂ → 2 GaAs With this balanced equation in hand, try recalculating everything and see what you get. 50.4.236.254 (talk) 04:16, 21 March 2017 (UTC)

I appreciate your help, but don't understand the difference. The only thing that would affect the calculations is the relative molar or mass amounts of each reactant. If you double or quadruple each reactant, you're still ending up with the same relative amounts. Also, when the book shows a reaction like that it doesn't assume any need to change it unless explicitly stated. 69.22.242.15 (talk) 10:22, 21 March 2017 (UTC)

Given the apparent level of the question (this is a high-school or freshman-college general-chemistry course), I agree that the intracacies of figuring out the formula of an intermetallic product are probably well out-of-scope. OP, you can probably keep yourself from getting confused and from making easy mistakes in more complicated problems by following your book's example to use "g/mol" for atomic or molar masses rather than "g" for actual measured amounts of substance. For example:

X mol *	Y	g	= X*Y g
		mol

has correct algebra of the units as well as correct arithmetic of the numbers, rather than having to remember what your "g" represents in different contexts). See Dimensional analysis, and especially its "The factor-label method for converting units" section, for more details. DMacks (talk) 12:55, 21 March 2017 (UTC)

Your method is correct. The given options are all wrong. - Lindert (talk) 12:37, 21 March 2017 (UTC)

I agree. I'm pretty sure I have this textbook around my office...will look for errata and solution-manual to see if there are any further details later today. DMacks (talk) 13:22, 21 March 2017 (UTC)

It seems easier for me to start from scratch than to parse everything above, so my apologies if I repeat.

The question is: how much gallium reacts with 5.50 grams of arsenic to form gallium arsenide (GaAs)? (Subtract that from 4 grams actually used to get an unused remainder)

For this, we multiply the 5.5 grams of arsenic by several conversion factors equal to 1. These are:

• 1 mol arsenic / 74.921595 grams arsenic
• 1 mol gallium consumed / 1 mol arsenic consumed, according to the formula they gave.
• 69.723 g gallium / 1 mol gallium

To recap, 5.5 g As * (1 mol As / 74.92 g As) * (1 mol Ga / 1 mol As) * (69.72 g Ga / 1 mol Ga) = 5.12 g of gallium consumed. Ooops, that's too much... Rather than retyping the above starting from gallium, let's scale down the reaction by a factor of 4/5.12 so we know 4 g of gallium are used, and multiply the 5.5 grams arsenic used by the same, for 4.30 grams of arsenic, with 1.20 grams left over -- as you say. If I assume an error somewhere it can't be smaller than that they put a 2 for a 5 in option d. Wnt (talk) 00:03, 23 March 2017 (UTC)

Is trophic level based on what keeps an organism alive instead of what the organism can eat?

Humans can drink urine or eat feces, if they want to. There doesn't seem to be anything stopping them. Since they can eat feces and drink urine, regardless of nutrition or toxicity, does that mean that their trophic level includes coprophagia? Or is trophic level based on what keeps an organism alive? 50.4.236.254 (talk) 05:27, 21 March 2017 (UTC)

Defining a trophic level is ultimately somewhat arbitrary. Humans are frequently referred to as apex predators, but then there are vegans. Even apex predators in nature will eat carrion when hungry. And many decomposers will behave as facultative parasites and pathogens when the opportunity arises. From my own observation, occupancy of a level seems to be defined as what an organism mostly does. Someguy1221 (talk) 07:02, 21 March 2017 (UTC)

The trophic level of an organism is the position it occupies in a food chain. Blooteuth (talk) 11:29, 21 March 2017 (UTC)

• The usefulness of the concept of trophic level is ecological; systems with an odd number of levels are more productive. The concept has nothing to do with coprophiliac fetishes. μηδείς (talk) 00:28, 22 March 2017 (UTC)

Why no antibiotic resistance against cathelicidin?

Cathelicidin is used by the immune system to destroy pathogens, apparently there is no antibiotic resistance problem here. Why not? Count Iblis (talk) 05:43, 21 March 2017 (UTC)

It is a problem. Both pubmed and google scholar list many papers studying cathelicidin resistance (though some of the papers that show up in such a search are about resistance of cathelicidin to proteases). Someguy1221 (talk) 06:57, 21 March 2017 (UTC)

See Antimicrobial_peptides#Bacterial_resistance. Ruslik_Zero 20:33, 21 March 2017 (UTC)

Thanks! Count Iblis (talk) 22:52, 21 March 2017 (UTC)

Anthocyanins

Are there any anthocyanins or anthocyanidins which change color at pH of about 5? 2601:646:8E01:7E0B:F88D:DE34:7772:8E5B (talk) 23:59, 21 March 2017 (UTC)

Extracting Anthocyanin from household plants, especially red cabbage, to form a crude pH indicator is a popular introductory chemistry demonstration. You can judge its appearance at pH=5 in the 3rd tube from the left here. Anthocyanidin, its sugar-less counterpart is also pH sensitive but I don't find an illustration. A really sensitive pH indicator at pH=5 is Resorcin Blue, you might also consider Ethyl Red 2-(p-Dimethylaminophenylazo) pyridine found in this table. Blooteuth (talk) 15:04, 22 March 2017 (UTC)

Sorry, these won't do -- I need a non-toxic pH indicator with transition at pH 5 or thereabouts (doesn't have to be exactly 5, but does absolutely have to be non-toxic). 2601:646:8E01:7E0B:FD58:9940:89EE:DFD5 (talk) 06:11, 23 March 2017 (UTC)

So, the 3rd tube from the left is at pH 5, right? That would be a good enough color change for my purposes -- but I have to ask a couple of additional questions: (1) What is the pH in the 2nd and 4th tube from the left? (never mind -- it says in the caption, which I missed at first) (2) Which of the anthocyanins is this? 2601:646:8E01:7E0B:FD58:9940:89EE:DFD5 (talk) 06:15, 23 March 2017 (UTC)

2) It would be a 2-(3,4-Dihydroxyphenyl) chromenylium-3,5,7-triol or Cyanidin derivative. Graeme Bartlett (talk) 10:31, 24 March 2017 (UTC)

You're saying that this is cyanidin -- did I understand you correctly? 2601:646:8E01:7E0B:D027:23D3:3787:6187 (talk) 11:53, 24 March 2017 (UTC)