The Analogy In Physics: Help Me Collect Examples

Tommaso Dorigo

In a few weeks I will speak in Torino, Italy at Comunicare Fisica 2012, a conference devoted to the communication of Physics in the media, in schools, through web sites, etcetera. And I need your help !

Let me explain. I have submitted an abstract which reads as follows:

The analogy is a powerful tool to explain even apparently hard physics concepts. By substituting a complex system with one closer to one's experience, we allow the listener to construct a mental image which is the basis of a successful understanding of the properties or behaviour of the system being discussed. In this presentation the author will discuss his experience with offering successful and insuccessful analogies for hard-to-grasp particle physics concepts to an audience of laypersons through his personal blog, A Quantum Diaries Survivor .

As you see, the topic is rich and interesting. The question is, will I be able to say anything original on this topic ? It would be very nice if you would point me to some examples of texts containing analogies used to explain physics around the web, maybe commenting critically on the quality and success of the analogy used. Please use the comments thread below, for an open discussion...

Even if you cannot contribute with new examples, maybe you can provide your own comments on existing ones. Below I collected a summary of some of the analogies I have myself used in years of blogging practice, discussing the merits of each, and feedback they received.

1) In a post a couple of years ago I decided that a apparently tough physics effect, the weakness of weak interactions at low energy and their becoming "stronger" at higher energy, could be conveyed through a rather involved analogy with the diffusion of the scent of chocolate. Here is the gist of it:

The large mass of Wand Z particles is the reason why weak forces are called that way: the mass oft hese vector bosons is a hindrance to their ability to mediate long-range interactions, and a parameter which determines the interaction strength. To understand how a massive mediator may be less effective, and act more weakly than a mass-less one, compare a cup of hot chocolate and a raw chocolate bar: the hot vapour of the former disperses around many very small, light-weight particles, which you can easily smell from a distance; the latter can onlyr elease a few small specks of solid chocolate if you get very close and inhale powerfully. The specks are much more massive and less copious than the corpuscles evaporating from the cup, and are thus incapable of carrying the chocolate interaction far away; furthermore, even at small distance the chocolate smell one may experience from the bar is way less intense, because of the small rate at which the bar releases a speck of chocolate when you sniff it!

The behaviour of the smell of the cup and the bar when sniffed may be likened to the behaviour of electromagnetic and weak interactions at low energy: the former will appear much more intense. Now, however, let us imagine for the sake of arguing that we construct a computerized sniffer that analyzes the odour of solid as well as liquid materials. It works by taking the material under test, vaporizing it,and analyzing the absorption lines of the produced vapour. Such a device will find that cup and bar of chocolate have the same intensity of chocolate smell; that is to be expected, in fact, since the molecules of the two substances are the same. Likewise, electromagnetic and weak interactions become equally strong at very high energy, once the different mass of chocolate particles and solid specks -pardon, of photons and W/Z bosons- becomes irrelevant.

2) Then there is the famous explanation by Michelangelo Mangano of the "naturalness problem", i.e. why theorists consider innatural that the Higgs boson has a mass in the 100-GeV range, since there are a dozen different quantum corrections to that value which add or subtract amounts that are much larger. Here is the thing:

Radiative corrections to the Higgs mass amount to a sum of different terms whose value gets multiplied by the square of the energy at which the Standard Model breaks down. If that energy scale is as large as the Planck mass (the scale at which quantum gravity enters the game), then one has to hypothesize that the several correction terms cancel out to a part in 10^34 (a hundred billionths of a billionth of a billionth of a billionth), if one is to make the Higgs mass smaller than a lead brick.
To see how likely that is (and here is the part where you get to touch things with your hand), Michelangelo proposes you to do the following experiment: ask 10 friends to tell you a random number of their liking between -1 and +1, but make it a irrational number. Then, add the ten numbers. How likely is it that you come up with a number as small as 10^-32 ??

3) Another quite famous analogy is used in many physics books to explain the concept of hidden symmetry and the Higgs mechanism: the fact that a physical system can be asymmetric in its lowest-energy state, even if the laws of physics are totally symmetric. A common analogy is that of a plastic stick with its tip on a table, held vertical at the other tip with your finger. Here we have one relevant force - gravity - which determines one preferential direction of space, but with respect to which the two other directions (those along which our table surface develops) are invariant. If you apply a downward vertical pressure on the tip with your finger the stick may resist for a while, and then bend along a random direction among the infinity of possibilities; this infinity is described by a continuous angle on the plane perpendicular to the gravity axis. The bending of the stick makes the system asymmetric -there is a preferential direction in the space orthogonal to the gravity axis- but the laws of physics have remained symmetric: the symmetry is "hidden" by the stick having chosen one of the possible infinity of "minimum energy" states.

A different analogy to explain hidden symmetry, and probably a more powerful one, is the one of a ferromagnet: if you lived within a magnetized ferromagnet you would believe that there is a preferential direction of space, the one of the magnetization vector. For sure the physics experiments in electrodynamics you can do will depend on the orientation of your apparata with respect to that axis. The existence of a preferential direction is however accidental to your making experiments within the ferromagnet: if you stepped outside there would be none. Moreover, if the ferromagnet temperature were raised above a critical point, the magnetization would be lost. Here we come to terms with the similarity of our Universe "freezing up" and losing its symmetry when the vacuum chose one among the possible infinite configurations: the piece of iron also can, returning below the critical temperature, develop domains with non-null magnetization.

I must say I am not sure whether the above analogy is very useful: the person who should understand hidden symmetry is asked to consider a quite complex alternative physical system, and its nuisances, in order to grasp the essence of a quite general concept. I tend to believe that the plastic stick offers a more intuitive explanation, despite the lack of a bonus track explaining the concept of "freezing" of our Universe.

4) One other common analogy in HEP is that of the proton, which is likened to a garbage bag when one has to explain particle collisions and their outcome. I have used this analogy quite often, and I find it allows me to explain several different concepts in turn. For instance, I can explain that collisions may be soft or hard depending on whether one tin can within one of the two colliding bags hits another tin can in the other or if instead it is toilet paper which enters in a collision course with it. Also, I can explain what is "missing transverse energy" -you do not expect that all the garbage emitted in the collision will fly out in the same direction, with nothing on the other side; or even "b-tagging", when I search for evidence that the broken glass produced in the collision is caused by a shattered bottle of whiskey. I am not sure how far one can bring this analogy, but for sure it has some definite merits.

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Well, enough with my own examples. I will be grateful for other ones you may suggest, but please don't forget to add your own take on them !

Comments

When I was a phd student, the professor teaching us the Faddeev-Popov method to fix the gauge in the functional integral for a gauge theory was very fond of comparing the underlying geometry with that of a pizza pan. I understood the Faddeev-Popov trik then, when I studied it from a textbook, but to this day I have no idea how the whole thing could even remotely resemble a pizza pan. The moral of this story is that analogies maybe useful, rarely illuminating, but even the best ones only go so far. Cheers, and good luck with your talk.

Anonymous (not verified) | 09/23/12 | 17:23 PM

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The large mass of Wand Z particles is the reason why weak forces are called that way: the mass oft hese vector bosons is a hindrance to their ability to mediate long-range interactions,

Tommaso: W,Z massive and short-range; gluons massless and short range. Why/how?

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Amir D. Aczel

Amir D. Aczel | 09/23/12 | 18:02 PM

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The effect of mass on a particle such as the photon is partly analogous to the effect of a fog. If you are in a fog which reduces visibility to about 10 metres, then the fog has effectively given the photon a mass of about ℏ / (10 metres x c), which is about 2 x 10^-8 eV/c², or about 3 x 10^-44 kg. The W boson mass of about 80 GeV/c² means that this particle propagates in a "fog" where the visibility is down to about ℏc / (80 GeV), which is about 3 x 10^-18 metres. This "fog" is due to the non-zero value of the Higgs field in the vacuum. The Z boson interacts slightly more strongly with the vacuum Higgs field than the W boson, so the visibility distance for the Z boson is slightly shorter still. However the photon and the gluons do not interact directly with the Higgs field, so the Higgs "fog" is transparent to them, and they have no mass.

This partial analogy between the effects of a mass and a fog applies properly only to the way the force resulting from emission and absorption of the massive or massless particle decreases with distance. The reduction of visibility due to a fog is more directly analogous to giving the photon a finite lifetime, e.g. about 10 metres/c in the above example, and ℏ divided by that lifetime is more directly analogous to the quantum mechanical width of the particle, which is the intrinsic quantum mechanical uncertainty in the particle's rest energy mc² due to its finite lifetime. A particle's rest energy is not normally smaller than its width, but can be far larger than its width, so while the reduction of visibility due to a fog corresponds to an effective finite lifetime for the photons in the fog, and a corresponding effective photon mass whose order of magnitude is roughly as above, the absence of a fog, allowing us to see quasars 12.9 billion light years away, does not imply any corresponding upper bound on the photon's mass.

The difference between photons and gluons is that photons do not carry a non-zero charge of any type, so they never emit or absorb any particles, whereas gluons can emit and absorb other gluons. Each gluon carries both a charge, which can be one of three types, analogous to red, green and blue, and an anti-charge, which is one of the three opposite types, analogous to cyan, magenta, and yellow. A gluon can have both a charge and the corresponding anti-charge, e.g. red and cyan, without them cancelling, but equal amounts of the three colors or the three anti-colors combine to white, i.e. no color, and the result of this is that only two of the red + cyan, green + magenta, and blue + yellow gluons are independent, so there are only 3² - 1 = 8 independent types of gluon. The interactions among the gluons preserve the total amount of each of the three types of charge, for example a red + magenta gluon can emit a blue + magenta gluon and simultaneously turn into a red + yellow gluon, since yellow is the anti-color of blue.

A quark carries one of the three color charges, and an anti-quark carries one of the three anti-colors. Colored particles interact so strongly with one another, by emission and absorption of cascades of gluons, that all isolated particles are colorless: they contain an equal amount of red, green, and blue. If two colored particles are knocked sideways out of the protons that contain them due to a head-on collision between them in the Large Hadron Collider, each of them emits a cascade of other colored particles behind them as they fly out, and the colored particles in these cascades rapidly assemble themselves into the colorless baryons and mesons seen in the detectors. The directions of motion of the colored particles that flew out first are detected as the central directions of the "jets" of baryons and mesons seen in the detectors.

The effective strength of the color charges increases slowly with increasing distance due to effects of higher order corrections, and reaches a critical value when the typical distance between adjacent colored particles is around 10^-15 metres, which is thus the typical size of a colorless particle made of quarks, antiquarks, and gluons.

Chris Austin | 09/25/12 | 07:15 AM

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Thanks so much, Chris!! That was very useful. Amir

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Amir D. Aczel

Amir D. Aczel | 09/25/12 | 11:10 AM

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Hi Tomasso - I enjoyed the Quantum Diaries discussions. The analogy is a useful tool to explain physics, but it could be misleading if one takes it too literally. Take the plum pudding model of the expanding universe. Take it but do not gobble it up before understanding it. OK - in my essay for the FQXi Foundational Questions essay contest the whole of section II (with a nice illustration) is entitled THE STATE OF PHYSICS IMAGINED AS A BUILDING. It is an analogy of the current state of mismatched physical theories (Quantum Mechanics not in harmony with General Relativity, etc.) seen as a rambling disjointed structure built on several foundations. Sections of the building cannot communicate with others, and the foundations (Time, Matter, Space and Radiation) need to be consolidated into one. Download the pdf of the essay from here:
http://fqxi.org/community/forum/topic/1323

Vladimir Tamari (not verified) | 09/23/12 | 18:40 PM

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Vladimir Tamari:
Are you serious?? Your drawings in the fqxi article are certainly pretty, but to argue that only the present "exists," without past or future, agrees with no accepted physical theory. And the reference to Poincare is misleading--the others are to your own "work." While Hawking and Mlodinow argue that the history of the universe is given to quantum fluctuations, it is only so because in the early universe, size made it a quantum object. And even then, they never say that the past "doesn't exist." To do physics requires some basic understanding of theory!

Anonymous (not verified) | 09/23/12 | 21:59 PM

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Dear Anonymous thanks - but actually yes I am serious. That time as a dimension may not exist is the subject of many recent papers and some books by serious physicists such as Julian Barbour. In fact Barbour won a previous fqxi essay contest on the nature of Time: http://fqxi.org/community/essay/winners/2008.1
Physics is a vast evolving subject, and only goes forward when current ideas are challenged, whether by a Hawking or a non-academic nobodies.

Vladimir Tamari (not verified) | 09/24/12 | 07:16 AM

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Doesn't look to me that Barbour says what you say. He reiterates at the end of his essay that he is not negating the existence of time. That time is a dimension was proven by Einstein, and few sane individuals will ever deny the validity of special relativity. It takes a minimum of mathematical understanding to appreciate what Einstein has done, and "non-academic nobodies," as you call them, if they have no such preparation in mathematics and physics, should realize that if they pronounce their views in a public domain, they are fair game to be ridiculed.

Anonymous (not verified) | 09/24/12 | 10:58 AM

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OK , here is the entire abstract of Barbour's winning fqxi essay "A review of some basic facts of classical dynamics shows that time, or precisely duration, is redundant as a fundamental concept. Duration and the behaviour of clocks emerge from a timeless law that governs change." Mathematics can point to physical truth but it is by no means its only source as Einstein himself knew (It was Minkowski who formulated the spacetime continuum, and Grossman who helped Einstein formulate up his gravity=acceleration idea). Open your mind a bit and you may one day learn something new.

Vladimir Tamari (not verified) | 09/24/12 | 19:54 PM

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I like the analogy of the Higgs field as a molasse that slows down everything traveling through it. Although this analogy doesn't catch the different slowness of different particles.
Still about the Higgs field, there is the well-known analogy of a politician passing through a crowd of journalists. This is nice because it is also easy to include in the analogy the different masses (different importance of different politicians) and the Higgs self-interaction (a rumor spreading through the crowd).

Andrea Giammanco (not verified) | 09/24/12 | 00:21 AM

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Andrea,

I consider those analogies less than helpfull since they implicitly convey the notion of absolute space.
Or can you really figure a relativistically invariant molasse ;-)

Anonymous (not verified) | 09/24/12 | 01:46 AM

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Sure, but no analogy can be a 1-to-1 model of the real thing.
By the way, your criticism doesn't apply to the second analogy, because one can easily figure out relativistic politicians ;)

Andrea Giammanco (not verified) | 09/24/12 | 01:52 AM

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I used to explain "the Higgs is not just another particle" by the following analogy which is similar to the molasse analogy (and of course not relativistically invariant): Consider pollen on a large polished plane where it can move freely. These are the tiny masslell particle providing little resistance against an accelerating force. Now consider pollen swimming on the surface of a pond (At that time I had this conversation there was a pond with some visible pollen on its surface). The water (which we call "Higgs field") interacts with the pollen and particles resist more when there is an accelerating force, thus effectivly giving mass to the particles. Now consider what happens when we collide a few of these pollen on the surface of the pond at very high energy: The fundamentally new thing is that we will make the water splash and it's exactly these oscillations that we observed. Hence we know that there's an Higgs field filling space, interacting with the "usual" particles in such a way that they look massive.

I know this analogy has several weaknesses and probably can't be carried further. But it helped me convey the message why the Higgs is special.

Robert (not verified) | 09/24/12 | 02:42 AM

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There is the somewhat famous article "The Pooltable Analogy to Axion Physics" by Sikivie (http://arxiv.org/abs/hep-ph/9506229). It is often recommended, but I find it a very poor analogy that actually explains very little to the novice reader. I'd file it under the failed-analogy category.

DanielGr (not verified) | 09/24/12 | 06:13 AM

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Frankly I worry that analogies are a substitute for making algebraic sense of what is going on, even within physics, never mind explaining it to an audience of those outside of physics. Isospin is a kind of analogy with Spin. But the real question is why each generation has a neutrino as well as a charged lepton, and then we wonder if right handed neutrinos exist at all, or whether neutrinos are massless, and why one wants raising and lowering operators. If bosons also have a generation structure then if you have neutrinos among the leptons, you should also have 2 bosons - a photon and a Z, and a question of why bosons act like operators and fermions like objects, whence one wants W to transform neutrinos and electron, but not transform a Z into a photon. The Standard Model works great, but why does it take the form it does ? Why are there two different ways to look at Pauli matrices ? What forces the existence of 3 generations of fermions ? Feynman said that a muon is just like a heavy electron - an analogy, but is it a really good analogy, or is there a deeper algebraic reason why they are different ? So, is one making analogies about analogies ?

Joel Rice (not verified) | 09/24/12 | 08:21 AM

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In dense aether model the existence of three dimensions follows from geometry of hypersphere packing - these hypersphere have most compact packing (as expressed with surface-radius ratio) just for 3D spheres. The existence of three particle generations can be modeled with nested density fluctuations inside of dense particle gas, like the supercrtical fluid - under certain circumstances such a fluid will form the density fluctuations composed of another density fluctuations recursively - but the number of these levels is limited to three just with geometry of particle packing. In essence, when the composite aggregates are composed of too many levels, then the distant members aren't distinguishable from noise at each level of packing. From this insight the number of particle generation follows.

http://www.aetherwavetheory.info/images/physics/aether/supercritical2.gif

Zephir (not verified) | 09/24/12 | 09:54 AM

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In dense aether wave theory (AWT) the space-time is modeled with water surface and the Higgs field is the Brownian noise. Because the transverse wave disperse in similar way at the both ends of distance scale into longitudinal ones, you can expect the analogy of power spectrum of CMBR in Higgs boson spectrum - this is one of predictions of this analogy. http://www.aetherwavetheory.info/images/physics/aether/cmbr_higgs.gif

Many other analogies exist here: the photons are Russels soliton, the neutrinos are Falaco solitons, the red shift is the result of scattering of ripples at the water surface, but the dark matter and dark energy can be illustrated with it too.. Just ask me for some physical concept or artifact - and I can try to find the AWT analogy for you...

Zephir (not verified) | 09/24/12 | 09:48 AM

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As someone working in sales, allow me to suggest an improvement to your colliding trash bags. You want to impress on people that protons are far from being the simple round balls depicted in school books. Protons are like people in that they have rich insides which are not easily seen. Most proton-proton encounters are uneventful as when people slightly repel each other or gently bump into each other on a busy street or in a train station. To plumb what is inside them, one has to substantially increase the interaction levels between them, and then you can see entirely new things develop, although exactly what comes out is very dependent on each person's (or proton's) “orientation” and “direction” before the interaction.

Energy is like money. The more you have the freer you can be, within reason. If money is tight, then you are going to be limited and bounded in what you can do. With more money, one has the potential to do entirely new things. As the available funds increases without limit, one becomes asymptotically free, like a jet setter, popping in and out of one exotic setting after another … or having a Ferrari in one's garage, ready to burst out at any time.

The force binding quarks is a Zen-like Chinese finger love trap. The harder you try to pull the quarks (or lovers) apart, the greater the resistance.

blue- green | 09/24/12 | 12:46 PM

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The problem with the energy is like money analogy is that while it can inform a physics discussion it leaves people with a lot of deeply wrongheaded ideas about what money is in the world of economics. Unlike energy, money is not in fact conserved (nor is the money supply centrally controlled in a direct sense) and analogies like this really screw up the already difficult problem of explainingwhat is really happening that in a way that people can really understand.

A more fruitful analogy with money is that it is a emergent phenomena that will arise in the most unexpected places even where you would think that conditions are highly unfavorable, something that many emergent pheomena in physics share.

ohwilleke (not verified) | 09/26/12 | 12:15 PM

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You are confusing money with paper money. To be more rigorous, one can put everything on a gold standard or better, on a gallon or litre of petro for which people pay directly most every day. N dollars = N quarts of gasoline or anything else that can be exchanged for N dollars. Money is convertible as are the energies with which we are familiar. Money does not grow on trees. It does not "emerge" unless you are thinking about printed money.

blue- green | 09/26/12 | 12:58 PM

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Feynman's analogy of a quanta as a particle with an internal stopwatch whose arrow directions are summed up vector style from QED is a good one.

The rubber band analogy to explain the strong force action of gluons is a common and useful one.

ohwilleke (not verified) | 09/24/12 | 12:36 PM

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My favourite is the 4). It's easy to grasp and allows a lot of exploits.
As for symmetry, I don't have problem with magnets. Nearly everybody I know has played with them as a child and the assymetry appears more deterministic with them than with a plastic stick.

Now a short list of my personal observations as a consumer of analogies:

1) The analogy must stem from the background of the audience because not every analogy works with everyone. To pick the proper analogy, you must be sure that the source concept is well understood by the audience, otherwise the analogy fires back - the things will appear more complicated than direct explanation.

2) There must be a clear cut where the analogy ends to keep the focus on the explained concept instead of the source one. Common audience tends to examine the analogy, or its source concept, leaving your topic out of scope.

3) KISS (Keep It Short and Simple) applies especially to use of analogy and combines with the previous point - the analogy is just a patch for a hole (so you can pretend the hole is not there for a while), which must be filled with proper knowledge as soon as possible.

Good luck with your conference.

Peter Krahulik (not verified) | 09/24/12 | 16:32 PM

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Dear Peter,

your points are insightful and very useful. I indeed agree with all of them. Thank you for the help!

Cheers,
T.

Tommaso Dorigo | 09/25/12 | 02:07 AM

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Dear Tomasso,

You are welcome anytime :-)

P.

Peter Krahulik (not verified) | 09/26/12 | 14:46 PM

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Here is an analogy about the constancy of the speed of light, quoted from my essay
http://fqxi.org/community/forum/topic/1323:
"There is an ingenious but weird logic to Einstein's assumption
of a constant speed of light – as if a fruit vendor decided to sell each apple at a single
price, offering larger or smaller fruits of better or poorer quality, according to supply
and demand, but always at that one price per apple! Later however Einstein declared
that (c) has to “vary with position” when light curves in a gravitational field."

Vladimir Tamari (not verified) | 09/24/12 | 20:00 PM

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Thanks to all of you who responded, either with your favourite example, or with suggestions.
I am going to come up with some draft and then submit it here for your criticism, so that you
get to see the result of this.

Best,
Tommaso

Tommaso Dorigo | 09/25/12 | 02:08 AM

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Hi Tommaso,

Here's one that I've been thinking about for a while, and I didn't know whether it has already been made. Googling pointed me to an article by B.B.Mandelbrot that seems to fit

users.math.yale.edu/mandelbrot/web_pdfs/howLongIsTheCoastOfBritain.pdf

The idea is that measuring and calculating say the strength of the electromagnetic coupling constant is a bit like measuring and calculating the length of a shoreline. The corresponding theories would be QED on one side and the model of the shoreline as a fractal on the other.

The fractal shoreline theory (FST :) predicts an infinite length of any piece of the shoreline if you simply try to integrate it by some prescription. However, you can calculate from the FST what a measuring apparatus with a finite resolution, e.g. a wheeled odometer with a certain diameter of wheel, will give as an answer. The FST will give you different answers as a function of the assumed resolution of your odometer, and the dependence will typically be logarithmic.

You can then check the theory by tracing a map of the shoreline with differently sized odometer wheels. This is very close to what we do when measuring coupling constants at colliders at different energies and comparing them to the predicted values at different renormalization scales.

(There is even the notion of a cutoff, because you know that the fractal shoreline theory will break down at length scales much below a kilometer or so at which the tide will create flat beaches, where the edge of the water is not even well defined due to fluctuations and the fact that sand is grainy anyways.)

The problem is communicating what a fractal is, so there is the danger of introducing something as complicated as the original idea. However, I think that many laypersons with an interest in science will at one point or another have encountered fractals, so the analogy might still be helpful.

Alex (not verified) | 09/27/12 | 03:52 AM

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This is indeed very nice Alex ! I was not aware of this analogy. I must say it is above the head of the typical "layperson" in that not only mentions fractals, but also integrating lengths. I believe it is useful for undergraduates in Physics though.

Thanks for the help,
T.

Tommaso Dorigo | 09/27/12 | 10:14 AM

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But it is true that a layperson who attends (without being forced) an event like that one is likely a layperson who is curious about science and therefore the probability that she/he heard about fractals is pretty high.
If you are afraid that "integrating lengths" is above the head of the laypersons in the audience, try with "total length" and they will get the message. (Indeed, this is just a perfect example of how the concept is simple, and the layperson is cut off of the discussion just because of language.)

Andrea Giammanco (not verified) | 10/01/12 | 05:36 AM

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Indeed, Andrea, one can do a much better job than I did above when actually using the analogy in an explanation. I think it has some potential though if one puts some more effort in expanding on it and ridding it of physics/maths slang. RG really is an important - but to the public as of now completely unknown and opaque - topic, and I have yet to come across a simpler analogy that captures the phenomenon adequately.

Alex (not verified) | 10/01/12 | 07:22 AM

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Here is the link to an analogy between electromagnetic waves and sound waves:
http://www.youtube.com/watch?v=LyRcmXId-a8
Not exactly particle physics, but seems to be in the same ballpark, given the wave-particle duality.

GM2012 (not verified) | 09/27/12 | 09:54 AM

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And here is another analogy I cooked up to explain Planck's loading theory of light quantum absorption and emission.
In this theory that denies a point-photon concept, waves from many sources are gradually absorbed by an atom until a threshold is reached. At that point a full quantum wave is released. In an essay Fix Physics! downloadable from this page http://fqxi.org/community/forum/topic/1323 (see p. 6, Q5) I likened Planck's loading theory to the Shishi Odoshi Japanese garden device made of bamboo balanced on a pivot. It fills up with water gradually from a steady stream until it is full, at which point it tips over and unloads all the water at one go. Here is a video of the bamboo device, originally invented to create a sound to chase away deer! http://www.youtube.com/watch?v=6KJlMYFklZc&feature=related

Vladimir Tamari (not verified) | 09/28/12 | 03:34 AM

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There's at least one nice book on analogies in physics (and other sciences):
Joel Levy's "A Bee in a Cathedral".
http://www.amazon.com/Bee-Cathedral-Other-Scientific-Analogies/dp/155407...

Levy has got other nice books. I liked the doomsday book.

plm (not verified) | 09/28/12 | 22:49 PM

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George Gamow's "Mr Tompkins in Wonderland", and his following books with Mr. Tompkins, are full of wonderful analogies to explain relativity, quantum mechanics, and they are great fun.

http://en.wikipedia.org/wiki/Mr_Tompkins

David Orban (not verified) | 09/30/12 | 18:14 PM

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Ciao David,

thanks for the good lead! I will check it out.
Cheers,
T.

Tommaso Dorigo | 10/01/12 | 01:56 AM

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Hi Tommaso and thanks for the "call for contributions"!
I personally feel that the effectiveness of an analogy resides in its resemblance to everyday life: the closer the analogy is to what you can regularly do and see, the easier to grasp the concept will be. If some details of the original idea have to be sacrified, for the sake of familariaty of the analogy, this is a price worth to be payed.
On Example 1: I like this. Beside chocolate, I think the speaker can easily find examples of something smelling weakly when raw, and intensively when cooked (barbecue? ;-)). On the other hand, introducing a device that "does spectroscopic analysis of vapours" brings a way apart from common experience: why not to point out simply that the raw and the cooked are actually the same material?
On Example 2: Hierarchy problem is impressive by itself, and I think there're many straightforward ways to present it. If you and me are asked to invent a 34-digit number, we write our own number and then mine and yours are the same, digit by digit... shouldn't it be the same? In my opinion, an average audience is not appealed by the "irrational numbers"!
On Example 3: Easy to understand... even if people don't press sticks vertically regularly. For sponteneous simmetry breaking, I like much more the analogy due to Salam. Guests sit on a round table, and there's a piece of bread for each guest, just aside to the dishes. Each guest can choose the bread on the right or the left side: but, if Salam picks the piece on his left, then everybody is forced to the same. The ferromagnetic analogy is for students in physics, not for common audience: I don't think that a layman, when confronted with magnetized iron, wonders how the hell its ferromagnetic domains are oriented, and what happened when warming the iron...
On Example 4: a bit "rude" example Tommaso, if I can say... personally I've never seen two garbage bins clashing one against the other, and I've the image of two too much inhomogenous containers with quite unpredictable behaviour. Maybe here one could invite the audience to build round bags with rigid spheres inside, make them roll and collide...
Concerning other analogies in your blog, one of them is sticked in my mind: when explaining a 2D cut in an analysis, you proposed to visualize the resulting phase-space as a sperical object inserted in a rectangular box: the box corresponded to uncorrelated cuts, the sphere was the results of correlated cuts. I found that idea extremely bright, meaning that it was perfectly able to go in the core of the thing in a very visual way.
Finally, I suggest to have a look to this page (only Italian unfortunately): http://www.gravita-zero.org/2012/07/spiega-higgs-in-140-caratteri-e-vinci.html. To me, this is an example of how effectively (or not) an analogy can reproduce (or not) the essence of a complex idea.
Apologize if something similar had already commented above

Leonardo Benucci (not verified) | 10/01/12 | 15:06 PM

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DearTommaso,

This is one of the greatest parts of discussing and teaching physics!!!

So I am really glad to be able to attend to thisdiscussion!

I have a couple of analogies I used duringdiscussions with colleagues. I hope you will find them useful:

1) I was asked, why can you not measure exactly the energy of a muon in aneutrino telescope unless it is fully contained?The answer was this : "assume that you have a camera watching a ten mile-longroad. Now let us assume that you have a car that passes through. It is a caryou are familiar with, so you know the amount of gasoline it burns per milewith respect to its speed. What you need to measure is how many liters ofgasoline are inside the car. If the car starts and stops within thereach of your camera, you know its range. You also know its gasolineconsumption. Therefore, by multiplying the two you find the amount of gasolinewithin the car. Now if the car enters at speed from behind and leaves theroad without stopping you will only know how much gasoline it spent, not howmuch it had. Same goes if it starts within reach and stops outside or viceversa. Now, if you know the gasoline consumption with respect to itsspeed, you might be able to make a crude calculation of its amount in the car,but with huge error.Same goes for muons in a neutrino telescope. Ifyou cannot have them start and stop within your effective volume, you can onlymake crude estimates of their energy. That's why one needs big arrays!"

2) In a discussion about the neutrinographyof sun made by Super Kamiokande, an astronomer friend of mine asked ourprofessor why it had such poor angular resolution. The answer was this"Think of a drunk man who starts walking. If he walks slowly, hisdeviations from the staright path will be more prominent. If he walks fastly,the deviations will be negligible compared to his rectilinear motion. Same goesfor the angular resolution. It improves with energy"

3)A student of mine could not understand why momentum and energy cannot commutein a bound particle. I told him to think of a free particle and make two setsof measurements. The first set, at t0 would be momentum and energy. Then att0+dt he would measure the same parameters . If he subtracted the two productsp*E at t0 and t0+dt he would find zero. If he did the same experimentwith an oscillator, the result would be different since total energy would beconstant at any moment, but momentum would change with respect to position,thus with respect to time. Therefore the (p*E)|to - (p*E)|to+dt would bedifferent from zero.

4) A discussion about zero pointenergy between a friend of mine and me led us to the followingexample. Since a particle is trapped within a potential, whydoes the lower energy state not correspond to E=0? The example was tothink of some person who wants to be as lazy as it gets. Therefore they shouldstay all day at bed and consume ~zero energy. But one has to eat, drink, go to thebathroom,socialize (perhaps). Therefore there is always a minimum of activity -even when one is in comma! This is the zero point energy one needs toconsume. Stated another way, if someone has pressingobligations ( a strong potential), he can never be actually free. He should tryand find some sort of equilibrium, by doing the least activity and reachingmaximum result. This minimum activity also corresponds to the zero point energy in analogy.

(Ok I think that 4 is too far fetched, but itseems legit!)

I hope I have been ofassistance.

Yours,Manos

P.S: I wouldlike to see the draft when you prepare it or/and the proceedings perhaps of theseminar if available. This part is most exciting to me .

Emmanuel Chaniotakis | 10/10/12 | 20:42 PM

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Dear Manos,

thank you for your examples, they are very good. I liked the one about muon range in particular. I do not think I am going to use them because my time is limited (20') and I already have an enormous amount of material - need to cut stuff instead than adding. But collecting such examples is always useful.
As for the slides: the talk is in Italian. I however will translate them for the blog, so keep watching.

Cheers,
T.

Tommaso Dorigo | 10/11/12 | 03:48 AM

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Dear Tommaso, thank you for your good words.
Personally, I speak italian rather fluently therefore there is no problem with that.
One way or another, despite the seminar, the topic is really interesting and I hope that we will keep it up!

Yours
Manos

Emmanuel Chaniotakis | 10/11/12 | 09:02 AM

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Εντάξει τότε! Αυτό σημάινει πως δεν πρέπει να το δημοσιέυσω στο ελληνικό μπλόγκ μου ;-)

Γεια,
Τ.

Tommaso Dorigo | 10/11/12 | 10:18 AM

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Caro Tommaso, d'accordo! Aspetto le tue novita con impatienzza!
Ciao,
Manos

Emmanuel Chaniotakis | 10/11/12 | 15:54 PM

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