Wikipedia:Reference desk/Archives/Science/2019 October 31

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October 31

Can neutrinos collide with each other?

If so, how often would this happen? Would collision be more common among extremely low energy neutrinos?Rich (talk) 07:28, 31 October 2019 (UTC)

Apparently so. --Jayron32 11:37, 31 October 2019 (UTC)

thanksRich (talk) 12:34, 3 November 2019 (UTC)

It’s less common at low energies, like neutrino-electron interactions. The interaction cross section increases a lot when the center-of-mass energy gets closer to the mass of the weak gauge bosons (W boson, Z boson). Icek~enwiki (talk) 08:33, 2 November 2019 (UTC)

But at higher energies the neutrinos are moving faster, doesnt thet mean less time to exchange the boson? I thought if the neutrinos were moving quite slowly(which might be unusual but seems possible)then there would be more time to interact.Rich (talk) 12:34, 3 November 2019 (UTC)

More important than this time of being close to each other is the energy available - or in a slighty different view, the frequency matching. Like all particles, the neutrinos have a wave nature, with the wavelength inversely propertional to the momentum and the frequency inversely proportional to the total energy. In this sense, the W and Z boson have a large minimal frequency due to their large rest mass-energy.

If we don't have the required energy to create @real@ W or Z bosons, the W or Z field will fall off exponentially with distance instead of propagating like a wave.

For details on how to calculate such things, see quantum field theory and Feynman diagram.

Icek~enwiki (talk) 17:50, 4 November 2019 (UTC)

Destructive interference and conservation of energy

Two waves that move in opposite directions meet and for a single moment cancel each other. When dealing with, say, water or sound waves, at the moment when two waves interfere and cancel each other, energy is preserved in the medium momentum. The water are briefly flat, but the water molecules are in motion. Where to the "missing" energy when electromagnetic waves meet goes? -IlanMa

The energy is to be found where the waves reinforce each other. The fields add together, and the energy/power is proportional to the square of the field. So you might think that with two waves you get four times the power, but that only compensates for the destructive interference. Graeme Bartlett (talk) 21:10, 31 October 2019 (UTC)

If one of the waves has positive amplitude and the other negative, there will be no reinforcing. Also, conservation of energy has to be satisfied for each point in time, and not be an average over an interval. The answer must be something else. אילן שמעוני (talk) 03:54, 1 November 2019 (UTC)

If you could launch two waves from the same point with opposite "amplitude" then they would cancel each other out. What would happen is that energy would go from one source which had current and voltage in-phase, to the other source that had the current opposite to the voltage. With alternating waves they will often be averaged over time to measure the power. But if you want instantaneous in time then you will get a sinusoidal power transferred. The energy will move from one point to another at the speed of light, and where there is reinforcement, the energy will travel there, and away from the cancellation area. Graeme Bartlett (talk) 10:58, 1 November 2019 (UTC)

Not necessarily. Take the most "primitive" way to create electromagnetic wave - simply to shift the position of a statically-charged object. Take 2 such objects, and shift them perpendicular to the line between them, but in opposite directions. You'll get an two electromagnetic waves propagating, with opposing amplitudes. The wave form will be whatever you desire (determined by changing the velocity during the shift). Anyway, I still think that satisfying energy conservation over time is not enough. It must be conserved for any given time, down to Plank time. I may be missing something obvious, but so far I see no answer to this conundrum. It's not that I believe that Mr. Anonymous found a breech in physics, but I still can't see any direction towards an answer. אילן שמעוני (talk) 11:19, 1 November 2019 (UTC)

Moving a charge to excite an electromagnetic wave requires adding work to the otherwise-closed system. In this scenario, energy is conserved only when you correctly account for the added work. Nimur (talk) 14:23, 1 November 2019 (UTC)

In the case of two moving charges moving in opposite directions close to each other, energy has to be put in when they approach each other, and it is extracted when they separate. So they will bounce backwards and forwards like a spring, and no energy will be radiated, it will just come in and out of the system via the "antenna". Graeme Bartlett (talk) 21:28, 1 November 2019 (UTC)

When two waves traveling in opposite directions meet and interfere as you describe, the result is called a standing wave. With water or sound waves, for a single moment the displacements cancel out, and the velocities add. A little later, the displacements add, and the velocities cancel out. With electromagnetic waves, something very similar happens when you include both halves of the word electro-magnetic. If at a given moment you make the electric fields cancel, the magnetic fields will add, and vice versa. --Amble (talk) 05:33, 1 November 2019 (UTC)

You seem to assume that the oscillation in the magnetic field in 180 degrees phase shift from the electric field. I see no reason to assume that. The 1st diagram in Electromagnetic radiation, for example, illustrates perfect same-phase on both fields. In such case your suggestion fails. Anonymous already stated that the situation with in-medium waves is known to him (I didn't know up to this point, btw) and drawing mysterious hints. Last but not least, Standing wave is a totally different phenomenon - I asked what happens in a single pulse, which fits the original question, and will not produce standing wave. A knowledgeable answer is required. אילן שמעוני (talk) 07:10, 1 November 2019 (UTC)

Why not review Poynting's theorem in your favorite book on electrodynamics? In Griffiths' Electrodynamics, this is section 8.1.2 (Work) and 9.2.3, (Energy and Momentum in Electromagnetic Waves). To put it bluntly, the math is quite difficult, but it is well understood by knowledgeable physicists - it just takes some effort to study it. Nimur (talk) 13:54, 1 November 2019 (UTC)

In the standing wave, an equivalent wave is travelling in the opposite direction as the forward wave, and the electric field is flipped compared to the forward travelling wave if there is a short circuit at the end. If there is an open circuit then the magnetic filed will flip 180° at the termination. You can use a right hand rule to work out the direction of travel given a magnetic and electric field. If one is switched 180°, the the direction of travel will also switch 180°. In this case the energy is quadrupled where the waves reinforce, and zeroed where they cancel. The average is double because you have two waves, one forward and one reverse. Graeme Bartlett (talk) 10:37, 1 November 2019 (UTC)

Take note that electricity flow through a conductor, unlike photons in vacuum, has a medium. It's just the same as the water/sound/spring - the medium movement (the electrons in the conductor) take care for energy preservation, so trying to sort this puzzle through electrical signals thought experiment is unlikely to provide an answer.::::EDIT: I never thought such simple trivial question would give me such pain. אילן שמעוני (talk) 11:26, 1 November 2019 (UTC)

...Have you ever worked a problem in mathematical physics before? Simple questions often have alarmingly complicated implications. Part of formal education in physics is the repeated re-working of difficult standard-form equations in trigonometry, analytic geometry, and calculus, so that we can rapidly reduce to a previously-solved and easily-recognized standard form. After a few years and a couple thousand iterations solving for sinusoid coefficients, you too can follow along with the abbreviated shorthand notation that summarily executes the same equation-solving methods, and you start to focus only on the nontrivial parts. Nobody knowledgeable says that physics is easy. Nimur (talk) 14:15, 1 November 2019 (UTC)

Two similar electromagnetic pulses of the same amplitude travel in opposite directions towards each other. If their E-fields are aligned, then their H-fields must be opposite (because the direction of propagation is E×H). The energy density at any point in space is (εE²⋅+μH²)/2. For each pulse, half its energy is in the E-field and half in the H-field (because |E|/|H|=Z₀=√(μ/ε)). When the pulses meet, the E-fields add and the H-fields cancel. The total energy in the E-field doubles (because the square of the sum is twice the sum of the squares). The total energy in the H-field is reduced to zero. The total energy in the E- and H-fields together is therefore conserved. catslash (talk) 14:04, 1 November 2019 (UTC)

For a pulse travelling along a conducting wire, the energy is not in the movement of the electrons (or not much is). The energy is still mainly in the electric and magnetic fields. The mobility of the electrons (imperfectly) prevents the fields from entering the conductor (because the electrons move to cancel the field). Consequently, the fields carrying the energy slide along the outside of the wire like a bead. catslash (talk) 14:28, 1 November 2019 (UTC)

Quite right - and in the even more generalized statement, it is only meaningful to calculate the energy of an electromagnetic wave when you correctly manage the bookkeeping for both fields, plus the energy in any coupled charges and currents. In this fashion, because of the coupled fields, an electromagnetic wave is entirely dissimilar to a simple acoustic wave or a transverse vibration on a string: an electromagnetic wave is a coupled system: its field amplitudes are described by a different wave function and different propagation equation; its energy is defined by a combination of two, coupled, energized fields, related by our great and powerful friends: the Maxwell's equations for electromagnetic dynamics. Nimur (talk) 14:33, 1 November 2019 (UTC)

user:Catslash, I want to see if I got this right, and I'll walk through each step seperately. I assume E-field is electrical, so the H-field is magnetic (usualy denoted as B)? אילן שמעוני (talk) 16:52, 1 November 2019 (UTC)

H and B are two different ways of quantifying a magnetic field. H is the magnetic field strength and quantifies the field by the current needed to sustain it (including displacement current). B is the magnetic flux density and quantifies the field by the force it produces on a moving charge. The two are related by B=μ₀H. Physicists tend to prefer B, calling it the magnetic field and regarding it as somehow more fundamental than H. My own field of endeavour is not mathematical physics but microwave and r.f. engineering; in this discipline B is almost never used. catslash (talk) 16:07, 2 November 2019 (UTC)

User:אילן שמעוני: I was responding to the original question, which posits two waves of equal amplitudes traveling in opposite directions. This is precisely a standing wave. The important thing is that you have to include the energy in both electric and magnetic fields, and it's not possible for both of them to cancel at the same time. This is similar to mechanical waves where you have to include the energy in both displacement and motion. No, I don't assume that the electric and magnetic fields are out of phase; it simply isn't relevant to the original question. --Amble (talk) 22:07, 1 November 2019 (UTC)

OK, still, if the phase of the magnetic and electric fields are identical, we still have a problem - they will both cancel out at the same moment. It seems User:Catslash answers that the fields can not be at the same phase, and must be shifted 180 degrees from each other. If that's true, it will solve the problem completely. It also means that the illustration at the beginning of Electromagnetic radiation is totally misleading. One thing at a time. אילן שמעוני (talk) 10:58, 2 November 2019 (UTC)

That 180° phase shift is happening when the waves are travelling in opposite directions. But perhaps they are starting from nearby points, as in an antenna array or diffraction grating. Then as you get far away the angle difference gets smaller and smaller and closer to zero. The waves then do cancel to close to zero in some directions and reinforce in others. Graeme Bartlett (talk) 11:54, 2 November 2019 (UTC)

User:אילן שמעוני No, it’s not just a question of phase, because the electric and magnetic fields are everywhere perpendicular to each other and perpendicular to the direction of travel (assuming we’re talking about free space). The two waves traveling in opposite directions have Poynting vectors pointing in opposite directions as well. This guarantees that, if the electric fields of the two waves cancel, the magnetic fields reinforce, and vice versa. If you try to make the two waves cancel in both the electric and magnetic field at the same time, they will necessarily have the same Poynting vector, and oops! now they’re traveling in the same direction instead of opposite directions, as specified in the original question. —Amble (talk) 14:38, 2 November 2019 (UTC)

Getting to the bottom line

The answers diverged into several branches. To get this right: Is the situation depicted in this illustration not possible and can never happen? If so I would love to dig a little further. If it is possible, I'm back to square 1. A definitive Yes/No will be much appreciated.

I'm tagging those brave men and women that tried to clarify this mystery: User:Graeme Bartlett, User:Nimur, User:Catslash, User:Amble

אילן שמעוני (talk) 09:24, 6 November 2019 (UTC)

The illustration is possible, but it does not depict two waves travelling in opposite directions. The Poynting vector (ExH) is in the same direction for both waves. They are thus both travelling in the same direction. It also confoundedly defines z in two different directions just to add to the confusion. SpinningSpark 12:53, 6 November 2019 (UTC)

There are arrows denoting the waves propagation directions, that is head to head. This is my own illustration. and I ask - Is this possible? If not, why? Reading about Poynting vector, it just states that it's "represents the directional energy flux (the energy transfer per unit area per unit time) of an electromagnetic field", with various ways to define it, a cross product of the electric and the magnetic fields. It deals with the energy flux (charge density can be very low, as in vacuum in space, so I figure that's negligible). It's been decades since I used such math stuff in college, and I was never too brilliant with it, still I can't see how it helps. It forces non-zero values for energy flux, but does not show how this comes to be true in the case I illustrated. אילן שמעוני (talk) 14:52, 6 November 2019 (UTC)

'stressing out: I see no constriction that forces that opposing waves will both not have the same phase to E and B fields. In such case ALL factors of energy flux are zero at the moment of destructive interference. אילן שמעוני (talk) 15:08, 6 November 2019 (UTC)

One can not completely describe an electromagnetic wave by illustrating it in graphical form as you have done. It is not even clear what you have drawn - and truthfully I'm not interested in finding out: as an analogy, if I were to write a bunch of random squiggles and lines, and I told somebody it was Russian and I needed help to translate it... I mean, I might send the non-Russian-speakers in circles while they looked up resources and tried to help me correct my errors...

But a fluent Russian speaker would not be fooled by my nonsense. It is immediately obvious that my squiggles are not Russian; they are not even Cyrillic characters with a couple of errors; they are just squiggles. Reasonable people would not be able to meaningfully answer "yes/no" questions about the literary implications of these squiggles.

This is what your diagram looks like to a physicist. Creative, perhaps - but ... I'm not going to help you or anybody else decipher it, because it is essentially a grapical-form of nonsense.

One must properly write the wave equation, and then one can completely and correctly solve for the propagation (e.g. time evolution) and for the change in energy at each point in space at each moment in time; and then one can talk meaningfully about how wave interference moves - or does not move - the energy around.

Trying to solve a question of physics without using the tools of physics is like trying to build a house without wood, bricks, hammers, or nails. You're not even using the tools wrongly: you're just not using them - so how do you expect to fruitfully proceed?

Grab a good book on electromagnetic waves from your local library or school. If you like, we can recommend several great books at various levels of difficulty. If, as you say, you have not used these tools in decades and "can't see how it helps," then I think our discussion devolves in this manner: please accept on good-faith this argument from authority: very smart people work these questions of physics on a daily basis, and they all say that the study of electromagnetic waves requires a lot of math. If you want to work through problems about electromagnetic waves, you must learn this math, or you must let other people solve these problems without you. In a specific situation, we might be able to provide a simple summary of "how we fixed it," but there is no way to simplify the general answers. The reference desk isn't a suitable place for any of us to work your equations for you - what we can do is point you to encyclopedic reference material.

Nimur (talk) 15:46, 6 November 2019 (UTC)

The notion of electromagnetic wave as derived from charged objects displacements predated wave equations, and is still valid. If you wave a charged object you create an electromagnetic wave, this is straightforward and is beyond doubt. What isn't beyond doubt, not for me, is what is the form and phase of the accompanied B field wave.

I really appreciate the offort - no cynicism - and patience you put into this, but I don't think you are totally spot-on with the role of mathematics in physics, not at least as I've been taught some 30 years ago in collage. Mathematical considerations are of course essential, but they can not be used without a set of underlying theory and concepts. The complete physics package has to include both underlying concepts AND mathematical procedures. So I am looking for the concepts here, and expect the math to go hand in hand - so to speak - not to lead.

I recall a famous quote (by Feynman?) shut up and do the math, but this saying had a specific context - namely quantum mechanics - and here I think we're still in the realm of Maxwell's theory (from which, alas, I remember close to nothing, except that it can be summed to 4 differential equations, and that I had 82 in this course, to my relief).

However, if someone (I suspect that's you) was taught Maxwell's theory by means of close to pure math, it is more than like that it will be close to impossible to him/her to formulate explanations otherwise. To use your analogy, it is to describe something in a language that you are fluent with to someone who is fluent with another language.

אילן שמעוני (talk) 16:46, 6 November 2019 (UTC)

It's not entirely clear what you mean by the diagram. However, I can speculate that you mean to ask "can an EM wave in free space travel to the right while its Poynting vector points to the left?". The answer to that question is No. --Amble (talk) 19:32, 6 November 2019 (UTC)

At first glance the diagram seems to be asking "Can I draw the same EM wave twice, once in right-handed coordinates and once in left-handed coordinates?". The answer to that question is Yes (although I don't think this is the question you intended to ask). --Amble (talk) 19:40, 6 November 2019 (UTC)

If I may speculate a little more, you may be assuming that you can choose some configuration of E and B, and then also freely choose ∂E/∂t and ∂B/∂t, so that you can set up "this wave traveling to the left" and "this wave traveling to the right". But Maxwell's equations don't let you do that. Once you have chosen the configuration of E and B, you can plug them in and calculate the unique solutions for ∂E/∂t and ∂B/∂t that tell you the dynamics (in this case, how the wave will propagate). (We're talking about a wave in free space, where ρ and J are zero). --Amble (talk) 22:04, 6 November 2019 (UTC)

Thanks, user:Amble! At long last it seems I have the answer, though I don't think you had to speculate - that indeed was my question.

On to a follow-up question: does that means that at the point the wave propagates from there's a discontinuity in ∂B/∂t? אילן שמעוני (talk) 09:34, 7 November 2019 (UTC)

Various types of divergences and discontinuities can happen when you have things like point charges, infinitely thin current-carrying wires, or currents that switch on instantaneously. Depending on the physical scenario you have in mind, some of these might be present, or there might not be any such thing as "the point the wave propagates from". Your source might be, for example, a ball of finite radius and uniform charge density throughout its volume, which oscillates up and down in a fixed sinusoidal pattern forever. This scenario has no divergences or discontinuities in the fields or their time derivatives. --Amble (talk) 19:27, 7 November 2019 (UTC)

Salmon foam

After I cook salmon sous-vide, there's a white foam on top that solidifies when it cools. What is this, and is it healthy to eat ? SinisterLefty (talk) 22:27, 31 October 2019 (UTC)

Probably fat and gelatin from the fish. Fats will liquefy as they reach their melting points and then solidify as they cool; think of melting butter. I sometimes notice some foamy white stuff when I make salmon, usually by grilling. --47.146.63.87 (talk) 23:28, 31 October 2019 (UTC)

I was thinking either fats or cholesterol, but then the question remains, are those good fats and cholesterol I should eat, or bad fats and cholesterol to avoid ? Gelatin would be fine to eat, I assume. SinisterLefty (talk) 02:13, 1 November 2019 (UTC)

That depends nearly entirely on your BMI status. If your weight is of no concern, there is close to no reason not to consume fat and cholesterol. There is only weak correlation between cholesterol consumption and blood cholesterol. Over 90% of the blood cholesterol is produced in your liver. Overweight, on the other hand, has been shown again and again to be a serious health risk and to lower life expectancy. — Preceding unsigned comment added by אילן שמעוני (talk • contribs) 03:51, 1 November 2019 (UTC)

I get the impression that everyone should consume as much good fat and good cholesterol as possible, while minimizing bad fat and bad cholesterol. Not an easy task, though, as they are often mixed together. SinisterLefty (talk) 06:09, 1 November 2019 (UTC)

There are many publications here's one that there is little to no effect of cholesterol consumption on cholesterol in blood. Sure, high LDL levels in the blood are lethal, but you will have them with or without cholesterol consumption as your liver is the culprit. Unless you suffer from another condition that makes cholesterol consumption bad for you, you can munch on cholesterol to your liking.

The most prevalent condition that makes fat consumption a very bad idea is obesity. Across all food sources, fat contains most calories, double than pure sugar. אילן שמעוני (talk) 07:19, 1 November 2019 (UTC)

But that ignores the satiety effect of eating fats. If eating the sugar leads to a sugar crash and then you eat more to compensate, for several cycles, you could well end up eating more calories than in fat, if it keeps you feeling full. SinisterLefty (talk) 08:27, 1 November 2019 (UTC)

True, true. אילן שמעוני (talk) 08:59, 1 November 2019 (UTC)

The fat from salmon is pinkish-orange in colour (WP:OR); the white stuff is probably protein leaching out of the meat. Cooks refer to this stuff as "scum" (second def, I guess). On one of his shows, chef Alton Brown compares it to spume due to its similar content and consistency. Matt Deres (talk) 00:12, 3 November 2019 (UTC)