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 !