Wikipedia:Reference desk/Archives/Science/2013 October 26

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October 26

Elements in other stellar systems

Most of the exoplanets that have been found are gas giants. Elements up to iron are made by fusion in stars and heavier elements come from supernovae. (1) Is there an estimate of what percentage of stellar systems have elements above iron? (2) Is life possible without any elements heavier than iron (I assume it is)? Bubba73 ^{You talkin' to me?} 02:55, 26 October 2013 (UTC)

Life on Earth might not have been possible without iodine. Count Iblis (talk) 03:28, 26 October 2013 (UTC)

See also here. Count Iblis (talk) 03:34, 26 October 2013 (UTC)

See metallicity, and, for a general treatment, read the best hard science book ever written, The Elements: Their Origin, Abundance, and Distribution. μηδείς (talk) 03:54, 26 October 2013 (UTC)

Thanks. I don't see a breakdown on the percentages of the different types of populations of stars, though. Bubba73 ^{You talkin' to me?} 04:18, 26 October 2013 (UTC)

Population I stars (rich in heavy elements) vastly outnumber population II stars (poor in heavy elements). The visisble disk of any galaxy is made up of population I with population II found only in the halo. SpinningSpark 13:49, 26 October 2013 (UTC)

This implies that all galaxies are either old enough to have had one generation of stars die, or are made from earlier galaxies which did. Are there no young galaxies, recently coalesced from hydrogen ? StuRat (talk) 14:41, 27 October 2013 (UTC)

So far as I am aware you get "new" galaxies as the result of the collisions of old galaxies but these still consist of old stars. I am not an expert, but I have never heard of recent new galaxies from scratch. μηδείς (talk) 17:17, 27 October 2013 (UTC)

Quasars are probably first generation galaxies but are such vast distances away one is effectively looking into the far past when observing them. So yes, your surmise is fundamentally correct. SpinningSpark 19:09, 28 October 2013 (UTC)

What is the velocity of the particle at the origin?

Two positive charges q1 and q2, each having a charge of 2 microcoulomb, are placed on the +x axis and -x axis respectively, both having a distance of 2m from the origin and the distance between q1 and q2 is 4m. A third charge, q3, of magnitude -4 microcoulomb is placed on the +y axis at a distance of 4m from the origin. Suppose, q3 is dropped towards the origin, find the time and instantaneous velocity of the charge q3 when it reaches the origin? Scientist456 (talk) 10:14, 26 October 2013 (UTC)

Please do your own homework.

Welcome to Wikipedia. Your question appears to be a homework question. I apologize if this is a misinterpretation, but it is our aim here not to do people's homework for them, but to merely aid them in doing it themselves. Letting someone else do your homework does not help you learn nearly as much as doing it yourself. Please attempt to solve the problem or answer the question yourself first. If you need help with a specific part of your homework, feel free to tell us where you are stuck and ask for help. If you need help grasping the concept of a problem, by all means let us know. Red Act (talk) 11:42, 26 October 2013 (UTC)

Sorry, in hurry I forgot to mention that it's not a homework problem. It's a self made problem. The original question was just to find the net electrostatic force acting on q3. After handling this simple problem, I myself extended the problem, and then failed to compute it because the acceleration of q3 is not constant. Also, the law of conservation of energy doesn't seem to give any satisfactory result (this could be wrong). I even don't know the answer. Scientist456 (talk) 12:58, 26 October 2013 (UTC)

The question as it stands can't be answered as we don't know the mass of q3. Apart from this, assuming that q1 and q2 are fixed, and that the system is horizontal, what you need to do is work out an equation that relates the force on q3 in the y direction to its position on the y-axis, and integrate to get its position with respect to time. You say that q3 is "dropped" towards the origin - if the y-axis is vertical, you need to add the constant gravitational force on q3 to the equation. Tevildo (talk) 13:17, 26 October 2013 (UTC)

Let us take the masses of q1, q2, and q3 be m1, m2, and m3 respectively. You are free to use any other variables also. We'll also assume that there is no external force (like gravitation) acting on the system. And q3 is falling because of the attractive force of q1 and q3. The answer of this problem can also involve some variables. Scientist456 (talk) 13:51, 26 October 2013 (UTC)

In reality q3 is unlikely to pass through the origin at all. The slightest peturbation will cause it to accelerate towards either q1 or q2 thus completely missing the origin. SpinningSpark 13:34, 26 October 2013 (UTC)

Solving for q3's velocity at the origin is fairly easy, in that it doesn't require doing any integration. Just use the fact that q3's kinetic energy at the origin is equal to the change in the system's potential energy.

However, it looks to me like also calculating the time when q3 crosses the origin would involve solving a differential equation that's difficult or perhaps impossible to solve analytically. Under the circumstances, some of the different approaches you could take to the problem are:

1) Solve the problem for a specific numerical value of m3, by using numerical integration.

2) At least gain an understanding of how the system behaves when q3 is started off close to the origin, instead of 4m away. When q3 is close to the origin, the system is approximately Hookean, i.e., q3's acceleration is approximately proportional to its distance from the origin, so in approximation you have a harmonic oscillator, for which the solutions are known.

3) Sometimes in real-world problems, you only need a rough estimate of a value, like sometimes just the right order of magnitude. There are a number of ways you can get an order-of-magnitude estimate for the time. For example, you could calculate the time as if the system was Hookean, but pretending the value for Hooke's constant was reasonably accurate for q3 all the way out to 4m away from the origin. Or you could calculate an approximate time as being the time it would take if the system was Hookean, q3 was released 4m from the origin, and q3 had the known (exact) final velocity. My guess is the second approximation would be more accurate, which could be confirmed or falsified by doing a numerical integration to get (very close to) the real answer. Red Act (talk) 02:17, 28 October 2013 (UTC)

Flexible-fuel vehicle: diesel + ethanol + gasoline

Can a single engine run on these fuels (mixed in the same tank, but without being adapted)? Is there any commercial car that runs on them? It's clear to me that ethanol/gasoline is not a huge achievement, but what would be necessary to adapt to have a diesel/gasoline engine? OsmanRF34 (talk) 14:36, 26 October 2013 (UTC)

I doubt if it's practical to do that, as diesel engines are quite different from gasoline engines. StuRat (talk) 14:46, 26 October 2013 (UTC)

You doubt it's practical for mass production or in general, including military vehicles? OsmanRF34 (talk) 14:52, 26 October 2013 (UTC)

Well, military vehicles have different reqs, where reliability is more important, and cost is less critical. So, if I were to design such a beast for the military, perhaps it could have a small gasoline engine on one end of the vehicle, to drive those two wheels, and a small diesel engine on the other end, to drive that pair of wheels (I'm assuming a 4 wheeled vehicle here). Either engine alone could drive the vehicle alone, or you could use both at once when more power or torque is needed. This would provide the max flexibility in case either engine is disabled or either fuel becomes unavailable. However, the extra weight from two engines would make it have poor fuel economy and limited cargo space. StuRat (talk) 14:46, 26 October 2013 (UTC)

There's no need to speculate. STANAG 1135 is the NATO document specifying interchangibility of fuels. The operating principle is that if a major ground war breaks out, the logistics should be straightforward: get any fuel to the vehicles that require it: Military Fuels and the Single Fuel Concept. Nearly all American and European military vehicles comply with this standard. Here's a publically available cheat-sheet: Aide Memoire on Fuels in NATO.

Around here they call it "JP-8" - other NATO nations call it "F-34"; ordinary people call it "kerosene" or "jet fuel," but it's a diesel-like goop that works in cars, trucks, utility vehicles, small airplanes, fighter jets, strategic bombers... it costs about $5.20 a gallon at my local airport, which is very close to what automotive gasoline costs. Nimur (talk) 15:16, 26 October 2013 (UTC)

That's fine for NATO, but smaller militaries around the world may need to scrounge for whatever fuel they can get, especially during a war. So, having such a vehicle could be beneficial. StuRat (talk) 15:19, 26 October 2013 (UTC)

If you're willing to trade off fuel efficiency, engine lifetime, and safety, you can put nearly any type of combustible fuel in a diesel engine, and it will usually operate. The principle of the internal combustion engine simply requires a combustible reaction. But, it isn't a great idea; engines that are sold commercially are carefully engineered for peak performance with a specific fuel. Their air intake valves are calibrated for the stoichiometry of a particular fuel-air ratio. Their cylinders are designed to withstand a peak pressure and temperature specific to one chemical reaction. Mechanical stresses, and timings between cylinders, are designed based on the combustion reaction. Interchangeability of fuel necessarily means that no specific optimization for fuel type can be made - which makes a less efficient engine: one that will mechanically destroy itself faster; one that emits worse nasty byproducts of incomplete combustion; one that is less thermodynamically efficient and therefore yields more carbon dioxide and pollutants for each unit of useful work. If you can live with all of those tradeoffs, you can pour any old goop into a diesel engine today.

A close friend converted a diesel engine to operate on food-waste oil (it's a common fashion statement among hip antiestablishment types around here; nobody mentioned to them that "flex fuel" was a military tactic developed under threat of nuclear war by men in suits at established think tanks). Her engine conversion was straightforward - a sturdier fuel pump and a better filter. The engine lasted for about four years before something died; by that time, the vehicle was already totalled and it wasn't worth the time or money to investigate. But I have always suspected she was pouring raw liquified food-waste in the fuel tank from time to time. Nimur (talk) 15:34, 26 October 2013 (UTC)

Er, Nimur, no you CAN'T put nearly any type of fuel in a diesel engine. If a fuel has a high enough octane rating so that it doesn't wildly ping in a spark ignition engine, it will be nearly, if not impossible, to start in a diesel engine. The whole idea of a fuel for a gasoline engine is to design the fuel so that ONLY the hot spark fires it off, and pressure and temperature do not cause detonation. In a diesel engine there is no spark; the fuel is set off by pressure and temperature. And even if you got a diesel engine to run on gasoline, it won't last long. In a very short time the injectors will be stuffed from lack of lubrication. Pistons will be damaged from delayed violent ignition. Air is not throttled in a diesel engine. Considerable heat is transfered to the fuel in common rail and especially in high pressure line unit injector enegines like the big Caterpillars. Fule is contiunally circulated between the injectors and the tank. It is normal to have fule cooolers in teh return line in engines over 20 litres. If you put gasoline in, it will vaporise. In effect, disel engines are run super lean, they are NOT operated stoichiometrically. All hyrocarbon fuels are suprisingly close in thermodynamic efficency (provide the engine runs cleanly) Last, but not last of your errors, gasoline engines produce small amounts of CO as the do run stoichiometrically and combustion isn't perfect. Diesel engines do not emit CO except in tiny tiny tiny amounts full power as there is always a considerable excess of air over fuel. 120.145.168.209 (talk) 15:45, 26 October 2013 (UTC)

I can confirm from my own accidental experiment that a diesel engine will not run on petrol (gasoline). As soon as the petrol gets through to the engine it cuts out. It cost me about £200 to have my tank drained and a new fuel filter installed, and then it was a couple of days before the petrol got all flushed through and the engine stopped acting up. Richerman (talk) 17:32, 26 October 2013 (UTC)

The military would love an engine that runs on anything, but what they have to settle for is running diesel engines on kerosene. A standard diesel engine will be hard to start when cold on kero, but that is fixed by fitting a larger starter motor that cranks the engine faster, giving higher cylinder temperature/pressure. 120.145.168.209 (talk) 15:57, 26 October 2013 (UTC)

For another approach, how about using a single, turbine engine. Those tend to be flexible on the fuel source. The fuel injection ports might need to be cleaned more often if you use certain fuels, though. StuRat (talk) 16:07, 26 October 2013 (UTC)

Good idea, except that gas turbines are really only good (good meaning competitive cost, reliable, and thermodymanically efficient) in large sizes. My only experience of them is turning out 20 to 100 megawatts on a mine site. Gas turnbines have dreadfull load step response. That usually isn't too much of a problem in a power station supplying 1000's of loads, but in a vehicle it's not good at all. While various organisations have tried to make it practical down to car sizes (~100 kiloWatt), for power outputs less that about 10 to 20 MW, you generally find diesel engines. Even if you have to install two sets, one for each of two fuels. 120.145.39.43 (talk) 16:33, 26 October 2013 (UTC)

Wikipedia:WHAAOE - see Multifuel which says; "One common use of this technology is in the drive of M35 2½-ton cargo truck military vehicles, so that they may run a wide range of alternative fuels such as gasoline or aviation (gas turbine) fuel. This is seen as desirable in a military setting as enemy action or unit isolation may limit the available fuel supply, and conversely enemy fuel sources, or civilian sources, may become available for usage. Currently a wide range of Russian military vehicles employ multifuel engines, such as the T-72 tank (multifuel diesel) and the T-80 (multifuel gas turbine)." The British Chieftain tank had an engine that could run on either petrol or diesel, but it performed poorly on either. See also Flexible-fuel vehicle. Alansplodge (talk) 21:20, 26 October 2013 (UTC)

A gas turbine could run on gasoline if need be, but the higher combustion temps mean that the burner cans will only last 30 hours or so before they need complete replacement. 24.23.196.85 (talk) 21:42, 26 October 2013 (UTC)

Couldn't the burner cans be redesigned to withstand those higher temps ? StuRat (talk) 02:18, 27 October 2013 (UTC)

The ability of the Chieftain Tank engine to run on gasoline or diesel fuel is an interesting quirk. The Leyland L60 engine used is an opposed piston (two pistons "chooking" against each other in each cylinder) design, so you can alter the compression by altering the timing between pistons. Run them in-phase and get 15:1 compression so it will run on diesel fuel as a compression ignition engine. Run one piston about 30 degrees behind and get 8:1 compression, fit sparkplugs & an air throttle or carburettor and you can then run it as a spark ignition gasoline engine. It is reported that the conversion from diesel operation to gasoline and vice versa took several hours each time. However its legendary poor performance and attrocious reliability probably was just due to it being a product of British Leyland, a company run by idiots and continually propped up by equally stupid governments deliberately giving them orders to keep them going. I worked for a while for a Govt department. We got a fleet of Morris Mariners, a small 4-cyl car, on just this prop-up basis. They were the most dreadful rubbish imaginable. Built by Leyland, the engines came from one plant with the flywheel ring gear machined to whatever non-metric tooth size they thought was good. The starter motors they fitted were specified for engines from a different plant, and machined to a different metric tooth design. The result was a horrible grinding noise every time yout started a Mariner, and after about 8 to 12 months you needed a new starter motor and ring gear. For some reason they put the (little) clutch pedal about where it should be, a little brake pedal centred closer to the clutch than is normal, and the accelerator pedal about where the brake should be. Which meant that in an emergency and you intended to jump hard on clutch (left foot) and brake (right foot), what you actually did was jump hard on clutch and perhaps brake (left foot) and accelerator (right foot). Just about the most dangerous car ever produced! Fortunately, the overall wear and breakdown rate was such that the lot were scrapped within a year - by a department that normally kept its cars for 10 years or 150,000 km, whichever first. 120.145.39.43 (talk) 02:14, 27 October 2013 (UTC)

Morris Marina, incidentally. My father had one in the orange "safety colour" as illustrated in our article. Tevildo (talk) 10:02, 27 October 2013 (UTC)

In those days people wanted to buy British and would put up with almost anything to achieve that. Availability was terrible due to inefficiency and strikes. I know people who waited a year for delivery of a new vehicle. The government could not allow the company to fail no matter what because of the disasterous effect this would have on British industry and unemployment. SpinningSpark 10:25, 27 October 2013 (UTC)

Your right, I can't spell. We drive on the left side of the road, so the driver sits on the right hand side in the car. Do you remember that for some inexplicable reason the windscreen wipers on half of them were set up as though the driver sat on the left? I can remember driving the thing leaning toward the centre of the car in order to see in the rain. Perhaps Leyland planned to export to left hand drive countries, realised that such countries would never accept such rubbish, but had already made or purchased a large number of windscreen wiper linkages for left hand drive and didn't want to waste them. Or perhaps the wrong ones just got delivered to the factory at the wrong time. 120.145.39.43 (talk) 12:07, 27 October 2013 (UTC)

From the article http://en.wikipedia.org/wiki/M35_2%C2%BD-ton_cargo_truck :Multifuel engines are designed to operate reliably on a wide variety of fuels, to include diesel fuel, jet fuel, kerosene, heating oil or gasoline.194.105.120.70 (talk) 10:40, 29 October 2013 (UTC)

It does say that. But it is NOT right. It is totally misleading. Not only for the reasons given by several folk above, I refer you to the Operators's Technical Manual, US Army/Airforce publication TM9-2320-361-10, page 0002-00-13 & 14 Permissable Fuels. It makes it clear the "mutifuel" engine is primarily a diesel engine. Permissible fuels for continuous operation are listed. They are all various grades of military and commercial diesel fuels. First choice alternative fuels are listed with some limitations in cold weather starting. They are all various sorts of kerosene, eg aviation kerosene (certain types only), except for a couple more commerial diesel fuel grades, and mixes of diesel and kerosene/aviation kerosene. 2nd choice alternative fuels are listed. These do include gasoline, but require it to be a low octance (<85) not sold anymore, and require it to be mixed with up to 30% diesel. It says failure to add sufficient diesel to obtain smooth running will result in piston damage. I'm not surprised. With light loads and cold running, smooth running might require significantly more than 30% diesel fuel. The more gasoline in the mix, the more delayed will be ignition, and the more violent the detonation, until ignition is so delayed it won't even run. Finally, it says, in an emergency (as in the enemy is about to overrun you), you can run it for a short while on heating oil, which is not too dissimilar to diesel. Note that this truck has been supplied with a variety of diferent engine. If yours has the gasoline engine option, you can only run it on gasoline. If yours has the Caterpillar engine, you can only run it of modern commerical diesel fuel. 120.145.39.43 (talk) 15:46, 29 October 2013 (UTC)

? “The more gasoline in the mix, the more delayed will be ignition” If one adds gasoline to the fuel of an oil compression engine ( commonly now called a diesel ) ignition will advance, until so much is burn before TDC that the engine stops – not the other way around as you put it. The low octane petrol you refer to will make the situation even worse. Its called pre-detonation (- which isn't really detonation at all). --Aspro (talk) 22:12, 29 October 2013 (UTC)

No! What I said was correct. Adding gasoline does delay ignition. Consult any good book on diesel combustion. Your assertion that gasoline will burn up before TDC is nonsense. Possibly your error comes from a common misconception that low octane gasoline burns faster. (All gasolines burn at very close to the same rate. In fact all engine-usable liquid hydrocarbon fuels burn at similar rates, set mainly by the ratio of hydrogen to carbon)

You need to understand that, for all fuels containing hydrogen, combustion occurs in two distinct phases - 1) and initial delay period in which some chain reaction precursor products slowly build up;, and 2) the main phase, where a vigorous chain reaction occurs. There is little evolution of heat (heat may even be absorbed) or increase in pressure occurs during the first phase. All the excitement occurs in the second phase.

Gasoline fuels are designed so that pressure & temperature, as far as possible, does not cause detonation, and ignition comes only from the electric spark at the spark plug. Gasoline fuels are given an octane rating. Put simply, the octane rating is the fuel's resistance to pressure-temperature detonation. When the sparkplug is fired, there is a delay period in which not a lot happens. After this delay period (which occurs with all fuels containing hydrogen, and is due to the fact that combustion involves a multitude of chamical steps in a cahin reaction), somewhat rapid combustion occurs, proceeding outwards from the point of ignition. If a gasoline engine is run on a fuel that is a little too low in octane rating, what happens is that the pressure increase during the main combustion phase causes detonation of the fuel/air mix somewhere outside the main flame that is spreading out from the pint of ignition. This second point of ignition occurs under much higher temperature and prssure than the first (spark) ignition, and causes an extremely rapid combustiion, delivering a shock load to the pistion, causing the caharacteristic "ping" or "knock" noise. This is what happens in a gasoline engine, where the intended ignition is timed by the spark, but second and perhaps other pressure ignitions occur.

Diesel engines work differently. Diesel fuels are given a cetane rating. Put simply, the cetane rating is the ease of ignition with pressure/temperature. A fuel with a high cetane rating has a low octance rating, and vice versa. Ignition in a diesel engine is timed by the start of fuel injection into the cylinder. A diesel engine draws in only air, fuel injection comes later in the cycle. Well before injection starts, compression by the piston rising up raises the cylinder air pressure and temperature so that any fuel present would ignite. When fuel injection starts, at first not a lot happens, due to the delay inherent in igniting any fuel that contains hydrogen (due as I said to the fact that combustion is a multi-step chain reaction). After this short delay, combustion runs in earnest, determined partly by chemical parameters and partly by the rate at which fuel continues to be injected. If a fuel (such as one like gasoline) with a low cetane rating is used, the delay period becomes excessive. The main combustion does not start until a lot of fuel has been injected, and pressure/temperature is very high, so combustion is more rapid than normal. If the fuel cetane rating is really too low, the delay can be so excessive that detonation occurs well after the piston is coming down again.and the piston gets a shock load.

I'm sorry that this has been rather long, but from it you can see that both intended ignition and unwanted detonation processses are completely different in a gasoline engine as compared to a diesel engine. And why a high octane fuel is in consequence desirable in a gasoline engine, but a high cetane fuel (which means a very low octane rating) is desirable in a diesel engine.

You have refereed to a term "pre-detonation". The correct term is pre-ignition. This is a problem pecular to gasoline engines and arises because the fuel is drawn in with the combustion air. This means that a hot spot, eg a bit of carbon on the top of the piston, or an excessively hot exhaust valve, causes ignition before the timed spark. It causes engines to continue running at a low rpm after you turn the ignition off. It is a totally different problem to detonation, caused in gasoline by a fuel of insufficient octane rating and mainly a problem under load. Pre-ignition cannot occur in a diesel engine, because the fuel is not drawn in with the air. It is injected later when it is desired that combustion should begin (allowing for the ignition delay).

124.178.150.165 (talk) 00:17, 30 October 2013 (UTC)

You said “Consult any good book on diesel combustion.” Here is the google book search: [1] Please find such a book so the I might learn something that you don’t think I know.--Aspro (talk) 01:04, 31 October 2013 (UTC)

physics (T-equation is the the real solution of time dependent Schrodinger wave equation)

Is solution of T-equation is the the real solution of time dependent Schrodinger wave equation?Then ,what is the solution of time independent Schrodinger wave equation? — Preceding unsigned comment added by Titunsam (talk • contribs) 15:01, 26 October 2013 (UTC)

Given a solution of the time dependent equation, you can extract the energy eigenvalues and eigenfunctions by performing a Fourier transform w.r.t. time. If the inner product of the initial state with any of the eigenfucntions does not vanish, you will find all the energy eigenvalues and eigenstates this way. Count Iblis (talk) 15:14, 26 October 2013 (UTC)

Hunting oscillation

Hunting oscillation states "Today, most high-speed trains use steel wheels, as a result of this research" suggesting that steel wheels reduce hunting. This raises two questions for me: what were wheelsets made from previously? And why would steel wheels reduce hunting anyway? Or am I reading it wrong? That statement seems to have been in the article for a long time, so I'm assuming it's correct.--Shantavira|^{feed me} 15:57, 26 October 2013 (UTC)

Did some of the earliest trains just use iron ? As long as oil was kept on them, that might work. StuRat (talk) 16:13, 26 October 2013 (UTC)

Not pure iron, as it wouldn't last 5 minutes. The traditional material was cast iron, quite a diffrent material, strong, rigid, and hard wearing. And no, you don't ever put oil on the wheels. Traction is limitted in railway working as it is. Sicne the 1950's, railway wheels with rubber tyres and other strange things have been tried. 120.145.39.43 (talk) 16:39, 26 October 2013 (UTC)

But without a coating of oil, cast iron will rust. StuRat (talk) 18:15, 26 October 2013 (UTC)

True, but friction between the wheel and the rail continually removed the rust (which also formed on the rail, as seen on rarely used tracks, such as on discontinued routes and in run-down shunting yards). Obviously this gradually reduced the diameter of the wheel, for which reason wheels usually had an outer iron/steel tyre, which was replaced as necessary. {The poster formerly known as 87.81.230.195} 90.213.83.178 (talk) 20:55, 26 October 2013 (UTC)

That explains how rust is kept clear on the edge which touches the rail, but how about the rest of the cast iron wheel ? StuRat (talk) 03:11, 28 October 2013 (UTC)

On locomotives (and tenders), the outer faces at least of the wheels were usually painted, the colour used (black or otherwise) being specified as part of their livery. (Railways in the UK sometimes had one livery for express passenger locomotives, another for ordinary/mixed traffic locos, and still another for pure goods locomotives, with sometimes also special liveries for prestige or dedicated service locos.) It would seem logical that the inner faces of the wheels on locos, coaches and wagons were also painted, as protection from rust was/is the primary and decoration only the secondary purpose of paint, but I don't have definite knowledge of that detail. {The poster formerly known as 87.81.230.195} 212.95.237.92 (talk) 14:35, 28 October 2013 (UTC)

Incidentally, although the running surfaces of rail tyres aren't lubricated, the flanges are, to prevent excessive wear and friction on curves. The only place we seem to cover this at the moment is at Rail speed limits in the United States#Curves, but a search on "flange lubricator" will pick up some other articles. Tevildo (talk) 21:06, 26 October 2013 (UTC)

The article might be trying to say that steel wheels could be used, rather than maglev or other wheeless system but it's not very clear. SpinningSpark 16:49, 26 October 2013 (UTC)

Seconded. The relevant text is: "This behaviour [hunting] limited trains to operate at speeds of about 225 km/h (140 mph) or less and led to a number of research projects in the 1960s using hovertrains and maglev systems to avoid it and reach higher speeds. But after empirical studies by the British Rail Research Division in the 1960s, remedial measures, particularly in the design of suspension systems, have been introduced permitting speeds exceeding 290 km/h (180 mph). Today, most high-speed trains use steel wheels, as a result of this research." In other words, the research concluded that high speeds are possible with steel wheels, it's not necessary to use maglev. Tevildo (talk) 17:12, 26 October 2013 (UTC)

I've clarified the article text a little. Tevildo (talk) 17:22, 26 October 2013 (UTC)

"Real world" length contraction

Please explain the physics of how physical objects "shrink" and how the distances between objects in space would contract as a result of being viewed and measured from various relativistic frames of reference as per special relativity's length contraction. Thanks. — Preceding unsigned comment added by 63.155.141.178 (talk) 18:32, 26 October 2013 (UTC)

You were right to put "shrink" in quotes because physical objects retain their proper dimensions in their own reference frame. It is only the observations from a moving reference frame that appear to show a "shrinkage". The mathematics are given in the article Length contraction. Dbfirs 19:29, 26 October 2013 (UTC)

They don't "appear to show" a shrinkage. The shrinkage is real. Dauto (talk) 14:01, 27 October 2013 (UTC)

Yes, but only in the "observed length". The so-called "shrunk" moving object measures its own length as unchanged and observes the observer as having "shrunk". I suppose it depends on what you mean by "real". Scientist disagree on the "reality" of the "shrinkage". Dbfirs 16:49, 27 October 2013 (UTC)

Um, no, there is no current "scientific dispute" surrounding length contraction. The length simply depends on the frame of reference from which the measurement is made. Sebastian Garth (talk) 17:31, 27 October 2013 (UTC)

Yes, I thought that was what I was saying, except I call it the "observed length" to distinguish this from the length measured in the object's own reference frame. Dbfirs 22:57, 27 October 2013 (UTC)

I agree that there is no scientific debate about what this means. But it's most definitely "real" in the sense of the Ladder paradox.

A 20 foot ladder moving at sufficient speed with respect to some observer will "shrink" to under 10 feet in length and (very briefly!) fit comfortably inside a 10 foot long garage with all of the doors shut. It doesn't just appear shorter in the sense of an optical illusion...for a few nanoseconds, it really, truly does fit inside a space that's smaller than it would occupy if at rest next to the building. The part that hurts your head (and why it's called a "paradox") is that from the point of view of the ladder, it's still 20' long - but the garage has shrunk to a mere 5' long. It's a paradox that's been resolved - so technically it's not a paradox anymore...but it hurts your head to think about it. I strongly recommend that article. SteveBaker (talk) 03:56, 29 October 2013 (UTC)

Yes, I'd read that interesting article. You say "for a few nanoseconds, it really, truly does fit inside" but I wonder ... for a few nanoseconds of whose time? Are we not back to relativity of simultaneity? Dbfirs 08:01, 30 October 2013 (UTC)

This question (along with a few other unanswered questions) is why relativity was not initially accepted by the establishment, but that this question goes unanswered has earned Einstein his reputation as it is. Its easy to measure contractions, but the rub is how to interpret these. Its one thing to say that local accelerated materials contract, but another when contractions are not local. Will the width of the Earth change simply because I've boarded a flight? If my flight accelerates, its simply and obviously false to claim this Earth or the universe itself actually contracted as a result. The conclusion is that these are apparent contractions, with the universe's true dimensions being unchanged due to local changes in measurement. Why would it contract (or appear to) because of my flight's actions? As the OP has astutely asked, how do these contractions occur? The simple and most obvious answer is, nonlocal objects don't really, so the only explanation that is needed is to what causes the measurement of these apparent contractions. I believe I've discovered the why, really, but its original research. In fact, more generally speaking, local changes to our measurement standards(clocks and rulers) do indeed change when we accelerate, which is why the Lorentz transformations have been required. Thus the correct answer to this question is to give an explanation as to precisely why and how our clocks and rods undergo change when accelerated. --Modocc (talk) 19:06, 27 October 2013 (UTC)

Again, no there's no dispute that the contractions are real contractions. They are as real as real gets for physical measurements and that's that. No need to postulate any funky optical illusion or complicated philosophical argument or anything. The distance between to points in space simply isn't independent of the observer's point of view just as the width of my body depends on whether you're looking at me from front view or profile. Dauto (talk) 21:41, 27 October 2013 (UTC)

Sure the measurement changes... so the distances are seen to be unstable. Spherical one moment, but flattened the next. That is your assertion that these changes in distances are always somehow real, but still without an adequate explanation as to why that this is the case. Of course, with classical physics, the distances between points were assumed to not change (although plots of over time would differ due to motion). I assume the OP is asking what is the relativistic explanation for why the observed distances change with relative motion (for instance, why perfectly spherical simultaneous implosions will appear flattened and not simultaneous). -Modocc (talk) 22:14, 27 October 2013 (UTC)

Most of the conundrums - like the ladder paradox and its many variants - are only conundrums if you don't follow the mathematics carefully to its conclusion. Frequently, new students of relativity try to apply intuitions that are invalid; those intuitions are invalid because they depend on simplifying assumptions that are mathematically unsound. Conundrums about material properties, for example, are easily resolved when you recognize that material properties are mediated by physical processes that are also subject to relativistic transformations. Conundrums about size and position are often due to the student's refusal to grok the relativity of simultaneity. Relative simultaneity is not a "minor side-effect" of the Lorentz transform; it's a fundamental underpinning of the full and consistent understanding of the mathematics that govern motion and space in our universe. Nimur (talk) 00:51, 28 October 2013 (UTC)

Students are not usually prone to unsound mathematics though, but they often do apply concepts such as simultaneity that are inconsistent with the relativistic paradigm. Max Born and others have made it clear that relativity of simultaneity is, without question, a mathematical consequence of Einstein's postulates for his paradigm to be self-consistent. I agree with this view of course, even though I am certain that his postulates and the paradigm are merely an unfortunate misinterpretation of sound measurements. -Modocc (talk) 01:42, 28 October 2013 (UTC)

Modocc nailed it above (debunking length contraction) with the Earth shrinkage example. How "real" is a measurement of Earth's diameter from a frame approaching in the direction of the axis at 86.6% of lightspeed and measuring the polar diameter to be 4000 miles? Now let the frame (future ship) turn around and approach at the same speed in the direction of the equatorial diameter. Now it is measured to be 4000 miles, while the polar diameter has returned to its proper length of just under 8000 miles. Keep changing directions and speeds of approach and, according to length contraction, the diameter will change to all different lengths in all different directions. This exposes the absurdity (falsehood) of SR's claim that all frames of reference are "equally valid." The same absurdity applies to shrinking distances, say between stars as measured by theoretical relativistic interstellar travelers. Then the distances between stars would vary with the velocity of all possible frames traveling between stars. How "scientific" is that... that all possible different observations create all possible different diameters of planets (in all different directions) and an infinite variety of distances, say between the Sun and Alpha Centauri, varying with the velocity of the observer? Length contraction advocates, please explain. — Preceding unsigned comment added by 63.155.141.178 (talk • contribs) 14:32, 29 October 2013‎

The mathematics of Lorentz contraction are firmly established; they are consistent with our theoretical framework for relativity; and they are consistent with very real observations, like the magnitude of muon flux observed at Earth's surface, or the magnetic deflection of high-voltage electron beams in a cathode ray tube. Anyone can reproduce those measurements; the required equipment is available in high-school and university physics classrooms around the world. Objects that travel at relativistic speeds relative to any other frame need to be considered subject to the Lorentz contraction, or your predicted values will not match what you measure. Our opinions about the implications of those measurements, or their relationship with any sort of objective reality, carry no weight in this sort of scientific discussion. Physics is concerned with observables. Nimur (talk) 21:36, 29 October 2013 (UTC)

Theorists are also keen on figuring out on how to best interpret the observables and therefore how to best represent them too, with the proper maths of course. -Modocc (talk) 01:58, 30 October 2013 (UTC)

Yes, the point is that the "observables" must be interpreted in a way that does not require physical objects and the distances between them to "morph" as their measured images morph due to relativistic effects. Earth's diameter does not change with all possible frames measuring it from different directions and velocities of travel relative to Earth,... nor does the distances between stars. Incoming muons, at higher velocities than lab muons decay more slowly ('live longer') and therefore have a further range of travel than expected of lab-accelerated muons. The atmosphere depth/thickness remains unchanged at around 1000km all around Earth, not contracted "for each muon." Observation (measurement from different frames) does not change objects or distances observed/measured. This is the basic fallacy of misinterpretation perpetuated by SR.

You are not a muon, and your motion relative to the Earth is not anywhere close to the speed of a muon; so you don't perceive or measure the contraction in the depth of the Earth atmosphere.

Your failure to perceive something doesn't make the effect any more or any less real. Do you also believe that X-ray radiation is not "real" simply because your human biological senses cannot detect it? Certain physical phenomena can only be observed by proxy. Nimur (talk) 22:44, 30 October 2013 (UTC)

No, I am not a muon and I am not traveling anywhere close to lightspeed relative to earth. (I'm in Earth's frame measuring its "proper diameter.") I do know that the frame "for a muon" ('measuring' the atmosphere) requires the Lorentz transformation to "get" the actual physical depth, as I described (Earth science) above. I must assume that you didn't pay much attention to my last two comments above, or do not understand the implications of various observations all creating their own "world," as if "the real world" didn't exist as independent of observation.