Wikipedia:Reference desk/Archives/Science/2014 January 10

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

January 10

Ships

Apparently, and I have no reason not to believe this, if two large ships travel parallel alongside each other they experience a force which will tend to draw them together. I have heard that this is due to the increased flow speed of the water causing a pressure drop in this region (analogous to an aerofoil). However I have also heard that the effect is due to the ships edges acting as the plates in the Casimir effect and restricting the wavelength of waves which can be sustained by constructive interference too a small range of values, and that this imbalance towards the spectrum of waves on either side of the ships results in a net force pushing the ships together. Is it known if either or both of these explanations is correct? — Preceding unsigned comment added by 86.134.72.50 (talk) 00:43, 10 January 2014 (UTC)

Interesting question! I don't know, but for convenience here's Casimir effect. My ignorant guess is that appealing to quantum field theory for an explanation of behavior of ocean liners is at best metaphorical. But there is apparently a water wave analogue of the Casimir effect, and our article contains this nice video [1]. SemanticMantis (talk) 00:52, 10 January 2014 (UTC)

The reason the ships are drawn together is best explained through the rather mundane classical physics explanations like Bernoulli's principle and the Venturi effect. Invoking a quantum mechanical effect like the Casimir effect (again, unless it is being done as an analogy, between the two superficially similar, though unrelated, phenomena) is unnecessary. The reason the ships are drawn together is is a Venturi effect situation (which is a special condition of the Bernoulli principle). As water passes through the constricted space between the two ships, the fluid pressure between the ships drops and the ships are "sucked" together. --Jayron32 02:57, 10 January 2014 (UTC)

Yes, it's definitely Bernoulli's principle in action. The extent to which the Casimir effect operates only happens when objects are the width of a few atoms apart. It has nothing to do with waves because the effect happens in dead calm seas. There is nothing complicated or exotic going on here. You can see Bernoulli's principle in action just by holding two sheets of paper about an inch apart and gently blowing your breath between them - you'll see that they are drawn together (which is probably the exact opposite of what you would naively expect). In that case, the air is moving and the paper is still - where in the case of the ships, the ships are moving and the water is still - but it's the exact same deal. SteveBaker (talk) 06:07, 10 January 2014 (UTC)

Thanks for your answers, but I think you have been too quick to dismiss the alternative possibility. Maybe I didn't explain well. So to be clear, the effect I described is analogous to the Casimir effect, I was not supposing it was 'the' Casimir effect. Two straight edges do enforce quantisation conditions on the oscillatory modes permissible in the water between the ships. The Casimir effect usually applies to the quantisation condition enforced on the EM field, whereas here it is on the water surface. I was never under the impression that it was an electromagnetic force pushing the ships together, but that doesn't preclude a hydrodynamic effect of analogous origin. — Preceding unsigned comment added by 128.40.61.82 (talk) 11:33, 10 January 2014 (UTC)

It seems reasonable to suspect that they're analogous, but I think that they aren't, because the Casimir force is purely quantum (loop-level). If whatever is drawing the ships together has an analogue in quantum electrodynamics, it ought to show up at the tree level. For the same reason I think that the "water wave analogue of the Casimir effect" linked earlier is not really an analogue, the AJP article notwithstanding. The random water waves are analogous to an EM radiation bath, not to vacuum fluctuations, and the effect that pushes the plates together in the tub is more like LeSage gravity. I may be missing something, though. -- BenRG (talk) 12:46, 10 January 2014 (UTC)

An interesting possibility! I will read more!

I think you're doing the experts here a disservice by suggesting that we dismissed your hypothesis without thought. We didn't. The idea that water waves are exerting some kind of force on the boats is clearly busted with the simple observation that this effect happens in completely calm water. Also, any effect that depends on the energy of very small waves to accelerate two gigantic thousand ton ships together is a non-starter from a conservation of energy perspective - and you'd find that in calmer water, the effect would be greatly diminished compared to in high wave conditions...which it's not. Furthermore - we know that the bernoulli effect must operate here - so even if you were right, the contribution from your analogous-to-casimir effect would be utterly negligible compared to the forces due to the venturii effect. This is definitely the bernoulli principle - people have known that this is the cause of the effect that you describe for at least a couple of hundred years. The "Where is the energy coming from?" question means that your hypothesis is busted without us having to understand the details of what you're trying to explain. SteveBaker (talk) 14:19, 10 January 2014 (UTC)

I thanked you for your input, and chose to further explain my point. I am sorry if you are offended as none was meant. We are all volunteers here and no one of us anonymous users has any more claim to be an expert than any other. No matter how prolific their contribution to the reference desk, and no matter wether they choose to register or not.

I would try it out with small boats in a bath tub (not too small, surface tension might pull them together). The venturi principle sounds nice, but the water doesn't necessarily have to go faster: if you push a funnel through water, the water outside the funnel will move faster relative to the funnel than the water inside. Depending on the shape of the ships and the depth of the water, they could be pushing most of the water between them forward, in which case the water level would be higher than on the outsides and the pressure would push them apart. Ssscienccce (talk) 07:25, 10 January 2014 (UTC)

Watch out with the Bernoulli principle btw, google gives 464000 hits for Bernoulli misconceptions (our article on Bernoulli lists some of them). The static port of an airplane is connected to the altimeter, which would give strange results if air pressure was determined by air speed. Coandă effect in particular is often mistaken for Bernoulli. Ssscienccce (talk) 12:24, 10 January 2014 (UTC)

I've always thought it was simply due to waves and/or wind. They hit one ship with full force, while the other is somewhat protected from them by the first ship, so doesn't move as much. If I am correct, the only time they wouldn't be drawn together by waves or wind is if both happen to move along the direction of the parallel hulls. StuRat (talk) 17:52, 10 January 2014 (UTC)

I"m not denying that other forces could be being applied to the ships - only that even in the absence of wind, waves, tides, currents, nearby black holes and the breath of angels - two ships moving close together on roughly parallel courses will be drawn together by hydrodynamic forces according to bernoulli's principle...and that in practical seamanship, this is the dominant force in most situations. SteveBaker (talk) 18:06, 10 January 2014 (UTC)

I agree that whatever happens will be because of the Bernoulli principle, I agree it's likely they will be pulled together in normal circumstances, because low pressure (a trough) on the section between bow and stern seems to be a characteristic of efficient hull design. I don't think it will happen with "true" (unrealistic) V-shaped hulls (flat bottom, vertical sides), nor will it happen when speed is large enough to create long bow waves interacting and creating higher pressure (higher water level) between the ships.

I made a mistake earlier, talking about the speed of the water relative to the funnel: that is exactly one of those misconceptions: Bernoulli's equation is constant along a streamline only, so it's the change in velocity of the water that matters, not the velocity relative to other objects. That's why you can't say in general that higher velocity equals lower pressure. Ssscienccce (talk) 14:52, 11 January 2014 (UTC)

Thanks everyone. I am convinced that the bernoulli effect is the dominant force for ships which are moving together. The question then I suppose is does there remain a significant force for stationary ships (i.e. co-moving with the flow), as a casimir like force would remain but the bernoulli force would be zero. Does anyone (possibly with maritime experience) know if this is such a thing? An obvious example would be do moored ships tend to bump into the harbour wall rather than pull away?

• I don't think calm water would actually disprove a Casimir effect, because a macroscopic Casimir effect would have to do with the ships blocking the waves that could occur, or be said to be occurring at very short ranges and vanishing. They'd exist in calm water. :) Wnt (talk) 22:34, 10 January 2014 (UTC)

Moored ships would be subject to tidal water flow - or river flow. You'd need dead stationary water to eliminate the venturii force...with tall enough waves to have a decent amount of energy and sufficiently little wind not to completely overwhelm whatever effect you think you'd see...that's a tall order!

Small waves would carry VERY little energy - our article wave energy points out that the energy that can be derived from waves is proportional to the square of their height. So a 1mm high ripple carries one millionth the energy of a one meter high wave. To accelerate a million kilograms of ship against the drag of the water (in the least hydrodynamic direction of motion for it's hull) requires energy at least somewhat comparable to what the ship's engines can produce...and tiny little waves simply don't possess that energy - so it's not going to happen...no matter what weird and wonderful casimir-like effect you might dream up.

Sometimes, even a clever idea is a complete non-starter - and this is one of them!

SteveBaker (talk) 01:47, 11 January 2014 (UTC)

A couple comments:

1) You would need some mighty calm water to only have 1 mm waves. I suppose that would also imply virtually no wind, but there could still be underwater currents.

2) Note that the drag on the ship's movement is also much less at lower speeds. Unlike an object sitting on land, there is no static coefficient of friction to overcome in order for movement to start. So, a low force applied to it doesn't mean it won't move at all, just that it will move more slowly. StuRat (talk) 11:00, 11 January 2014 (UTC)

It's the Venturi effect, per this paper [2] on the topic. -Modocc (talk) 23:43, 10 January 2014 (UTC)

I'm gently wondering if it might be possible to rewrite the 'venturi' effect in terms of the wave spectrum generated by each ship, and the cancellation and reinforcement that occurs in the gap between the ships. The OP might like to do some experiments with Michlet, a program that allows you to model the surface waves generated by ships. Greglocock (talk) 06:52, 13 January 2014 (UTC)

Thanks for your replies. Here is the AJP article mentioned by BenRG [3], and a Nature news article of the subject [4]. — Preceding unsigned comment added by 128.40.61.82 (talk • contribs) 07:01, 13 January 2014‎

I found an article[5] on modeling wave interactions between ships moving parallel in close proximity. It's behind a paywall though. -Modocc (talk) 15:15, 13 January 2014 (UTC)

Propagator

Where does the formula :$\left(H_{x}-i\hbar {\frac {\partial }{\partial t}}\right)K(x,t;x',t')=-i\hbar \delta (x-x')\delta (t-t')$ from the propagator article come from? 74.15.137.253 (talk) 00:54, 10 January 2014 (UTC)

Perhaps the Math desk would do better... I do want to see this answered, so I'll start, but someone more familiar with the topic should finish it.

The article says plainly enough that "the propagator gives the probability amplitude for a particle to travel from one spatial point at one time to another spatial point at a later time. It is a Green's function for the Schrödinger equation."

A Green's function, G(x, s), of a linear differential operator L = L(x) acting on distributions over a subset of the Euclidean space Rⁿ, at a point s, is any solution of

$L^{*}G(x,s)=\delta (x-s)$ (1)

where $\delta$ is the Dirac delta function and L* is the adjoint L.

Now we can see plainly enough that this has been applied twice, on x and t and x' and t'. The time-dependent Schroedinger equation is also recognizable:

Time-dependent Schrödinger equation (general)

$i\hbar {\frac {\partial }{\partial t}}\Psi ={\hat {H}}\Psi$

With both terms to one side. Now where we separate the passing geek from the true physics acolyte is when we try to work out the Hermitian adjoint of the Schroedinger equation... though I can search for it online and find [6] which describes the 'famous Von Neumann "master" equation for the density matrix. But I'd still be lying if I claimed I could check whether the equation you've asked about is right or wrong! Wnt (talk) 15:14, 10 January 2014 (UTC)

Film temperature drop

Why film temperature drop is not shown as linear? It is always shown as a curve.Brahmarishiraj (talk) 09:22, 10 January 2014 (UTC)

What kind of film are you talking about and in what context?

In general, objects that are cooling from some high temperature always show a non-linear curve. Newton's law of cooling says that the rate of temperature drop is proportional to the difference in temperature between the object and it's environment. So when the object is very hot, it loses heat quickly and as the temperature decreases, so does the rate at which the temperature falls. Plotted as a graph the result is a curve that asymptotes down towards the temperature of the object's surroundings. However, Newton's law of cooling is really more of a rule-of-thumb than a hard-and-fast physical law - so some objects exhibit quite different cooling rates. SteveBaker (talk) 17:59, 10 January 2014 (UTC)

The OP may be referring to Film temperature. Rojomoke (talk) 18:09, 10 January 2014 (UTC)

Maybe it's in the context of counterflow heat exchangers? One might expect those curves to be linear, but that would only happen in special cases, like when T_{H_in}-T_{H_out} = T_{C_out}-T_{C_in}, and even then it would likely only be linear over a limited temperature range. Ssscienccce (talk) 16:16, 11 January 2014 (UTC)

bosons z and w much heavier than protons

since they are the force carriers of forces that keep the nucleons together I'd expect their weight should affect the weight of the nucleus but not.. why? thanx --192.35.17.11 (talk) 17:11, 10 January 2014 (UTC)

For one, gluons keep the nucleus together, not Z's or W's. 74.15.137.253 (talk) 17:52, 10 January 2014 (UTC)

but protons interact thru weak interaction.. --80.180.36.134 (talk) 19:10, 10 January 2014 (UTC)

No, strong. Leakage from the forces binding their quarks together. — kwami (talk) 08:29, 11 January 2014 (UTC)

strong but also weak, otherwise weak interaction is incorrect..--79.24.145.75 (talk) 11:32, 11 January 2014 (UTC)

Protons (or rather their constituent quarks) do interact through the weak interaction, but not through any process which keeps the nucleus together in any substantial fashion. From the weak interaction article: "The weak interaction does not produce bound states (nor does it involve binding energy)". You may have been mislead by the third paragraph of that article, which states "The weak interaction is responsible for both the radioactive decay and nuclear fusion of subatomic particles." That's correct, but the weak interaction doesn't directly keep the particles together. Instead, in fusion reactions like the proton-proton chain you see processes where you have H + H => D. The weak interaction is involved in transmuting one of the protons into a neutron, forming the stable deuterium. If there was no weak interaction involved, you'd just have a highly unstable diproton, which would fission into two protons almost immediately if it wasn't for the beta decay pathway the weak interaction provides. Virtual weak bosons do theoretically contribute to the mass of the nucleus ("everything not forbidden is compulsory"), but because they're so heavy the probability that they exist is very low, so they don't contribute all that much, especially when compared to the strong interaction. -- 162.238.240.55 (talk) 00:25, 12 January 2014 (UTC)

I am interested in becoming a nuclear engineer

so far through life i have always done things just to get by i have never given 100% of myself to something. I am 25 and i am interested in becoming a nuclear engineer, however, i don't know if i am smart enough, i really want this but i have become discouraged. I will like to know what subjects should i be strong in?? and how do i get started? — Preceding unsigned comment added by 205.145.19.4 (talk) 18:56, 10 January 2014 (UTC)

Generally, to get a job as a nuclear engineer, you're going to need to get a bachelor's degree in nuclear engineering. Here's a web page from the Bureau of Labor Statistics called How to Become a Nuclear Engineer . According to ABET, there are 21 accredited Nuclear and Radiological Engineering programs in the United States. One of those programs is at the Georgia Institute of Technology, which might enable you to get in-state tuition rates. Here is a list of the classes you would need to pass in order to get a BS in Nuclear and Radiological Engineering from Georgia Tech. You can hover your mouse over each course in that list to get a description of the course. Red Act (talk) 19:33, 10 January 2014 (UTC)

Not to put too fine a point on it, but start by finding the energy in yourself to press the shift key to spell the word "I" correctly. Laziness and poor communications skills are unhelpful to your career in nuclear engineering. 88.112.50.121 (talk) 21:05, 10 January 2014 (UTC)

Why not become a Nuclear Safety Inspector instead? Seems a little easier to do. Clarityfiend (talk) 04:32, 12 January 2014 (UTC)

Why a 'nuclear engineer' might I ask? It may sound to you like a highly academic and prestigious career but from what I can gather, the are more nuclear engineers than jobs available for them. If you want to get a really good and valuable skill, why not look at be coming a certified Linux systems guy. Those jobs are expanding. You will be amazed then, that how many of your relatives, friends and neighbours appreciate your skills over those of a nuclear engineer.--Aspro (talk) 23:05, 10 January 2014 (UTC)

The Bureau of Labor Statistics says that "Job prospects are expected to be relatively favorable in this occupation", with a 9% increase in number of nuclear engineers needed in the next decade[7]. And here's an article entitled "The New Hot Job: Nuclear Engineering" in U.S. News & World Report a few years ago that says "…the [American Nuclear Society] estimates that 700 nuclear engineers need to graduate per year to support the potential demand. The organization currently expects only 249 new engineers to be available each year." Red Act (talk) 00:26, 11 January 2014 (UTC)

Well first, you'll need the mathematical ability and knowledge to get through an engineering degree (in my experience the maths is the hardest bit for most people with an engineering aptitude). You'll also need to be able to pay for that degree. You should be aware that most jobs in the nuclear industry are heavy on paperwork and procedures, and deliberately rather low in excitement.Greglocock (talk) 06:59, 13 January 2014 (UTC)