Wikipedia:Reference desk/Archives/Science/2017 November 24

= November 24 =

Can ʻOumuamua be a starship?
ʻOumuamua 117.200.195.57 (talk) 00:05, 24 November 2017 (UTC)
 * Please see Fermi paradox. It would be very unlikely, considering the distance in time and space separating plausibe intelligent life in the universe.  Assuming that the object was not of natural origin, imagine the time required for the probe to reach us and the time required for its acquired data to be available to its owner (even at light speed).  — Paleo  Neonate  – 01:23, 24 November 2017 (UTC)
 * Besides, we took a spectrum of it and it's made of rock -- not a very likely material for a spaceship! 2601:646:8E01:7E0B:1984:C5C9:D8FC:B368 (talk) 02:55, 24 November 2017 (UTC)
 * Rock might make a pretty good material for such a ship. ←Baseball Bugs What's up, Doc? carrots→ 03:36, 24 November 2017 (UTC)
 * The spectrum of 30 Rock (not to be confused with 30 Rock) has a lot of rock but that doesn't mean it's a rock. Sometimes there's rock inside though. For instance, each time the Saturday Night Live musical guest is rock. Sagittarian Milky Way (talk) 04:49, 24 November 2017 (UTC)
 * We don't know that it's "made of rock." We only know that its surface reflects a light spectrum similar to particular types of rocky asteroids, but a civilization capable of making an interstellar spaceship could trivially easily paint it to simulate such a spectrum :-). Also, as Bugs suggests, making a large space habitat or craft by hollowing out an existing asteroid has long been a staple in scientific speculation. {The poster formerly known as 87.81.230.195} 90.208.173.186 (talk) 09:48, 24 November 2017 (UTC)
 * This isn't really an answerable question since we can't falsify starship status. We can say that, so far, it hasn't accelerated, fired phasors, exhibited surface artwork, turned Jupiter into a pumpkin etc.  We can also say that as starships go, so far it appears to be a damn slow one.  (The "20,000 years to leave the Solar System" figure compares quite unfavorably with Voyager 2 and Voyager 1).  But a starship is defined by intent, and intent is defined by consciousness, and we don't have an accepted theory of qualia and free will, which means that if an asteroid belt spits out an asteroid after a series of gravitational and physical interactions, we can't actually say that the multi-body interactions involved didn't plan that asteroid to come out and leap between systems; we can't prove that stars and oceans and ten thousand other things don't have some measure of intent to what they do.  So for now, the less philosophical people call it a rock, and a handful of more philosophical people call it a rock but imply all sorts of wonderful things that might mean but almost surely doesn't mean. ;) Wnt (talk) 03:36, 24 November 2017 (UTC)
 * The OP asked "can" it be, so the answer must be "yes". Then you get into probabilities, and the probability seems to be very low. ←Baseball Bugs What's up, Doc? carrots→ 04:16, 24 November 2017 (UTC)
 * As usual xkcd hits it on the head. 196.213.35.146 (talk) 06:31, 24 November 2017 (UTC)


 * There is as much evidence that this is a space ship as there is that this hacked the election. Perhaps a special counsel is called for?"It is not your responsibility to refute someone’s arbitrary assertion—to try to find or imagine arguments that will show that his assertion is false. It is a fundamental error on your part even to try to do this. The rational procedure in regard to an arbitrary assertion is to dismiss it out of hand, merely identifying it as arbitrary, and as such inadmissible and undiscussable."-Leonard Peikoff, The Philosophy of Objectivism lecture series, Lecture 6.
 * μηδείς (talk) 22:39, 24 November 2017 (UTC)


 * The shape is reminiscent of the famous book Rendezvous With Rama, but both the scale and rotation make it wrong for that. Reality doesn't quite match fiction. (yet) ApLundell (talk) 23:01, 25 November 2017 (UTC)
 * I have that book, and the paperback edition's dimensions are not in a 5:1:9 ratio. Or do you mean the hardcover? μηδείς (talk) 23:07, 25 November 2017 (UTC)
 * Where are you getting 5:1:9? The article says that ʻOumuamua' is very roughly 1:1:6.  (Which is still different than the 1:3 of the fictional Rama, but in my defense, Rama was usually illustrated as narrower than that.) ApLundell (talk) 20:23, 26 November 2017 (UTC)
 * I don't know where that figure came from (is it a starship?) but it reminds me of the 1:4:9 of TMA-1 and suchlike from Clarke (not Rama though). Wnt (talk) 21:54, 26 November 2017 (UTC)
 * You referred to the shape of the book, ApLundell, not the spaceship. As for the 5:1:9 ratio, I did see it somewhere, either in another publication or an earlier version of that article.  I noted it seemed strange, given that the artist's rendering resembles the spaceship in Clarke's book. μηδείς (talk) 16:50, 28 November 2017 (UTC)

Galaxy level objects at relativistic velocities?
We observe very far away galaxies moving away from us at relativistic velocities using Doppler red shift. Everything local, though, seems relatively well behaved within our solar system. It seems reasonable that the galaxy interactions required to create spiral galaxies would create objects that are relatively close with velocity/mass shifts that are not within normal solar system bounds. Have we ever observed large objects moving at larger relativistic speeds than normal objects? Do we know which objects created specific bands/arms in the galaxy and what their relative velocity is? --DHeyward (talk) 07:37, 24 November 2017 (UTC)
 * One "thing" that is macroscopic and travels at relativistic speed is the astrophysical jet. Other recently measured things that move close to the speed of light are neutron star merger and black hole mergers. A spinning neutron star may have a surface moving at several percent of the speed of light. PSR J1748-2446ad apparently has an equatorial speed of 0.24 times the speed of light. Nothing of this is galaxy sized. Graeme Bartlett (talk) 00:00, 25 November 2017 (UTC)

Optimal shape
A body, let's take steel, has a weight of 1 ton. It is dropped into water, let's say 10 km deep. What is the optimal shape for the fastest sinking/descending possible? Will the surface have to be smooth, dimpled or grooved? Every physics argument welcome.  GEEZER nil nisi bene 08:23, 24 November 2017 (UTC)


 * Rounded at the front, tapered to a point in the back, sort of lack a teardrop. See "streamlined" at right.  This the best way to reduce drag, which means your object will be able to fall faster.  Whether adding dimples would also help, I don't know.  Dimples help reduce drag on round objects, by reducing the wake turbulence, but the tapered shape already serves a similar purpose.  Dragons flight (talk) 08:52, 24 November 2017 (UTC)


 * The idea of different shapes that Dragons flight mentions is discussed article about drag coefficient. We're in three dimensions and with a fixed volume, so we need to consider whether the cross-section in the direction of motion should be circular (the object is a solid of revolution of the streamlined shape) or oval (like an airplane wing) and also the aspect ratio in terms of elongation in the direction of motion. Interestingly, our drag-coefficient article says a "flat plate" has a cd of 0.001–0.005, which is better than any of the thick bodies. I don't know if that means to be as elongated as possible and/or to be more oval cross-section is better. DMacks (talk) 09:47, 24 November 2017 (UTC)


 * Probably it should be built with some sort of tailfins, as you see on most airplanes, air-dropped bombs, arrows, and fish. They provide stability of orientation so that the thing will keep moving lengthwise.  See Fletching; this talks about air, but I think the same sort of considerations apply in water. --69.159.60.147 (talk) 10:06, 24 November 2017 (UTC)
 * I doubt tail fins will be necessary for a solid, streamlined object, as long as its centre of gravity will be forward of its centre of lift it should be self stabilizing. The reason that the other things you mention do need stabilizers is that their centre of mass isn't forward of their centre of lift. Rmvandijk (talk) 11:13, 24 November 2017 (UTC)


 * It's a fixed weight, not a fixed volume. So general arguments about shape have already been given, but the proportions could well optimise to different proportions depending on the volume and internal dimensions needed. Is this "1 ton" a small solid block, a large hollow shell, or half a ton of shell and half a ton of "works" inside it, needing some minimum dimension? Andy Dingley (talk) 11:07, 24 November 2017 (UTC)
 * In the thought experiment a solid body.  GEEZER nil nisi bene 13:15, 24 November 2017 (UTC)


 * Infinitely long, infinitely thin. Then back off a little.  --DHeyward (talk) 15:15, 24 November 2017 (UTC)
 * For a fixed mass, extremely thin ends up being worse than a sphere of the same mass. A streamlined shape is already a sort of compromise between reducing cross-sectional area and avoiding creating too much skin drag by having a long shape.  Dragons flight (talk) 15:40, 24 November 2017 (UTC)


 * That chart is cool, but it raises the question, what shape has a drag coefficient of 1.0? μηδείς (talk) 22:28, 24 November 2017 (UTC)


 * A cube rotated through 13.3 degrees from parallel? Plasmic Physics (talk) 23:26, 24 November 2017 (UTC)


 * According to the article, a road bicycle plus cyclist in the touring position. Or a coffee filter face up. ;) Note that this ratio is defined as a dimensionless quality by putting together the components of a force kg m / s^2 out of a density, velocity squared, and area.  Now the universe is probably trying to scream something important at us with the 1.0 figure for some tremendously mathematically beautiful curve but if so the article doesn't cover it. Wnt (talk) 00:02, 25 November 2017 (UTC)


 * God, with all the money wasted on looking for proton decay you'd think as a question d'honneur they'd have some platinum-iridium plate of spaghetti somewhere that defined this for metrification. μηδείς (talk) 00:40, 25 November 2017 (UTC)


 * Simply like a Torpedo. --Kharon (talk) 05:35, 25 November 2017 (UTC)


 * The torpedo article doesn't mention drag; am I missing your point? μηδείς (talk) 23:03, 25 November 2017 (UTC)


 * Hydrodynamics has yet to be linked. That article mentions "teardrop" shape→
 * —2606:A000:4C0C:E200:C11B:49D1:9CF3:5784 (talk) 19:07, 26 November 2017 (UTC)


 * Also, pay attention that the drag coefficient is a constant only for some range of (high) Reynolds numbers. For most day-to-day situations, it works, but it changes behavior at low or at very high Reynolds. This is (one of) the reason(s) golf balls are not smooth: it makes the high-Re drop in drag coefficient accessible. Tigraan Click here to contact me 14:31, 27 November 2017 (UTC)

Is there anything special about the speed of light?
Obviously there is something special about "the speed of light". However, is there anything significant about light travelling at this speed? Or is better to think of it as the important thing being that the universe has a maximum speed, and light is merely one of the things that can travel at that speed? Iapetus (talk) 15:50, 24 November 2017 (UTC)


 * From the first paragraph of Speed of light:
 * Though this speed is most commonly associated with light, it is in fact the speed at which all massless particles and changes of the associated fields travel in vacuum (including electromagnetic radiation and gravitational waves). Such particles and waves travel at $c$ regardless of the motion of the source or the inertial reference frame of the observer.
 * -- ToE 16:46, 24 November 2017 (UTC)
 * I think the OP is asking why c is what it is, not faster and not slower. I raised this question some months ago, but I don't recall the article name. But I think the answer (or at least the hypothesis) is connected with quantum physics. ←Baseball Bugs What's up, Doc? carrots→ 17:02, 24 November 2017 (UTC)


 * Maxwell's equations is probably worth a read. These are a set of descriptions which describe how a wave propagates through a medium with particular properties. But then this transforms the "Why is c, c?" question into "Why is the permittivity of free space, $ε_{0}$?" Andy Dingley (talk) 19:10, 24 November 2017 (UTC)


 * I think that's it's just an artefact of how we defined (past tense) our units of measurement when we first found the speed of light. You may be think the wrong way here, you shouldn't be thinking that light is going at 299,792,458 m s-1, but instead that the average walking speed is 4.7E-9 c. If you measure every speed as a proportional to the speed of light, you can make the speed of light anything you wish, even unity, of course you'll have to invent new units of measurent. Plasmic Physics (talk) 20:36, 24 November 2017 (UTC)
 * @Plasmic Physics: that's not on topic. The question was not about the unit of measurement.B8-tome (talk) 21:33, 24 November 2017 (UTC)


 * How else does one discuss the how the answer relates to the units used to measure without bringing units of measurement into it? Plasmic Physics (talk) 22:50, 24 November 2017 (UTC)
 * You can measure it in light years if you want. But no matter what units you use, it doesn't explain why light has that specific upper bound "c". Assuming that's what the OP wants to know. Maybe the OP could provide some clarification? ←Baseball Bugs What's up, Doc? carrots→ 22:59, 24 November 2017 (UTC)


 * I'd just ignore B8-tome's cavills, he's a ref-desk SPA who's only been active here since September, and has made not a single contribution to any other part of the project. That, and the bizarrely sexist comment below fits a certain profile that doesn't need feeding.  μηδείς (talk) 00:32, 25 November 2017 (UTC)


 * I'd ignore Medeis, unless s/he/it thinks Plasmic Physics peculiar way of thinking is a valuable contribution. B8-tome (talk) 20:14, 25 November 2017 (UTC)


 * And as predicted, B8-tome was indeffed for abuse on 29 Novemeber. μηδείς (talk) 17:46, 29 November 2017 (UTC)


 * Plasmic Physics is entirely correct here. What makes speed "fast" is the comparison to the speed of light!  In particular, non-relativistic things have a kinetic energy 1/2 mv^2, which can be compared to their rest mass energy of mc^2.  The relativistic formula is even more explicit - it is mc^2(gamma - 1) where gamma is 1/sqrt(1-v^2/c^2); see the article for more.  Note then that the total kinetic+rest mass energy is just mc^2 gamma.  So the very essence of what makes speed fast -- the amount of "whack" you experience when you crash into something, or the amount/efficiency of fuel reaction it took to get to that speed -- depends on a comparison of your speed to light speed.  If a crash is near light speed then there is a total transformation in the matter in a collision; if it is less, then increasingly minor details, like nuclear structure, chemical structure, even the non-covalent bonds that hold a person's cell membranes together, can all be preserved during a collision. Wnt (talk) 12:01, 26 November 2017 (UTC)
 * Maybe factually right, but does it answer the OP's question? ←Baseball Bugs What's up, Doc? carrots→ 14:31, 26 November 2017 (UTC)


 * There may be a very small difference in the speed of light of different energies, see MAGIC (telescope). If this is true then there is 'the speed of light' which is a physical speed limit, and the speed of a photon of light in space which may be slightly slower because of some interaction with space. The evidence from black holes colliding shows gravitational waves travel at the same speed as light to the limits of measurement so far. Dmcq (talk) 22:53, 24 November 2017 (UTC)
 * Here's a video from PBS Space Time addressing just this topic. --47.157.122.192 (talk) 22:56, 25 November 2017 (UTC)
 * There are two important things to understand here. Only massless particles can travel at c (as a particle with any mass would require infinite energy to do so), and massless particles must travel at c (since a massless particle with any amount of momentum must travel at c, and a massless particle with no momentum has no energy). MChesterMC (talk) 09:45, 27 November 2017 (UTC)
 * Apologies, I've been away for a few days and couldn't check the replies. To clarify: I know that light isn't the only thing that travels at the speed of light.  What I'm interested in (and I realize this may be an impossible to answer, "chicken and egg"-type question) is: is there something special about light that means that however fast it goes, that becomes the maximum speed of anything in the universe?  Or is it that the maximum speed is controlled by something else, and light (and other zero-mass entities) just go as fast as they can given those restrictions?  Iapetus (talk) 12:18, 27 November 2017 (UTC)
 * You may be interested in exploring some of the various Variable speed of light hypotheses. Since the speed of light is a property of the universe, it may be variable depending on the properties of that universe.  See also Time-variation of fundamental constants, and Dirac large numbers hypothesis for more general information on these ideas.  They are not very well accepted ideas in Physics, but they are also not inherently disproven either. -- Jayron 32 12:45, 27 November 2017 (UTC)
 * Have you viewed the PBS Space Time video, The Speed of Light is NOT About Light, linked above by 47.157.122.192? It argues that c is the speed of causality, and that photons travel at that speed because they are massless.  Also note that the list of massless particle is short. -- ToE 14:08, 27 November 2017 (UTC)
 * Thanks. I'll have a look at all of those. Iapetus (talk) 16:15, 27 November 2017 (UTC)

Alcohol in a Royal Navy rum ration
Rum ration seems to give two versions of how much rum a Royal Navy sailor got in the late 1700's. Rum ration says  "The rum ration, or "tot", consisted of one-eighth of an imperial pint (70 mL) of rum at 95.5 proof (54.6% ABV), given out to every sailor at midday. Senior Ratings (Petty Officers and above) received their rum neat, whilst for Junior Ratings it was diluted with two parts of water to make three-eighths of an imperial pint (210 mL) of grog." An Imperial pint is apparently 20 fluid ounces (unlike the US 16 ounce pint), so the cited article says the issue was 2.5 ounces. (70 mL would be 2.4 fluid ounces). Later in the article it indicates a larger volume of rum, and says one half pint was added to one quart of water to make grog. Grog says " a half pint or "2 gills" of rum gradually replaced beer and brandy as the drink of choice." Gill (unit) says the measure is 5 ounces implying a daily ration of 10 ounces of rum. Black Tot Day says "In the 17th century, the daily drink ration for English sailors was a gallon of beer. Due to the difficulty in storing the large quantities of liquid that this required, in 1655 a half pint of rum was made equivalent and became preferred to beer." So how much 95.5 proof spirit in the total daily ration? Is the confusion due to the ration being given at two times in the day in some eras? Edison (talk) 23:03, 24 November 2017 (UTC)
 * One detail: 70 ml is closer to 2.5 Imperial fluid ounces than to 2.4. --69.159.60.147 (talk) 20:55, 25 November 2017 (UTC)


 * The Black Tot Day article talks about going from a "half pint" (cup?) of rum to a quarter pint and then to an eighth of a pint before abolishing it. That plus the dilution and splitting of rations makes for a confusing situation, even if practices at any given time were totally consistent.  My wild guess would be that the officers would have quaffed the first half of the conscript's rum ration themselves, then the second, log them as dispensed, and if any disagreement ensued, they'd flog the sailor half to death, keelhaul him, lose him to the sharks, report him in the log as mustered out, and put those funds to their own account also... but I'm not sure I am giving them enough credit in their art. ;) Wnt (talk) 23:55, 24 November 2017 (UTC)