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'If You Can't Explain It to Your Grandmother…'

  1. Aug 12, 2014 #1
    Author: ZapperZ
    Originally posted on Jan25-12

    "... then you haven't understood it."

    We all have heard variations of that. In fact, we also often get crackpots who DEMAND that we evaluate their ideas and give them comments in "simple, ordinary language", and that our failure to make them understand physics on such terms implies that WE don't understand the physics (as if they do).

    The idea here is that if you understood something, then you should be able to explain it in its simplest form to someone who has no background in a certain knowledge to be able to understand it. I've heard this told to physicists, and told by many people who should know better than repeating something unverified.

    And that's the whole problem. The statement is mentioned and repeated AS IF IT HAS BEEN VERIFIED TO BE TRUE! So when it is uttered, most of us tend to get defensive and try to deflect it, rather than confront it directly and ask to show proof that the statement is true.

    The problem here is that people confuse two things: (i) having knowledge and (ii) the ability to convey that knowledge in simple, understandable form. These two are NOT mutually inclusive! If it is, then someone needs to go out and proclaim that a physicist such as Dirac "doesn't understand quantum mechanics". Anyone who has read his biography could easily see that he wasn't much of an "explainer", and certainly not to the general public. So, who here in his/her right mind would like to proclaim that Dirac doesn't understand quantum mechanics? Anyone? How come this clear and obvious falsification to that statement was never brought up? I have plenty more examples from where that came from. Yet, people still continue to utter that statement, as if the understanding of complex physics ideas is an entitlement.

    Most of us who have been in physics for any considerable period of time have met people who we KNOW for a fact to be experts in certain areas, and yet, they suck at explaining what they do to us, much less, to someone who isn't familiar with the subject matter. This may happen for a variety of reasons: (i) lack of pedagogical skills (ii) laziness in figuring out how to present something at the level that the audience can understand (iii) or simply a complete ignorance of the fact that the audience is clueless to what he/she is saying. Being able to present something in understandable form is not a skill that comes with knowledge. It requires quite a bit of thought, a consideration to the level of knowledge of the audience, and a lot of consideration on how to present something that is in touch with what the audience already know. This takes effort, and this is something not everyone realizes. Thus, you get brilliant scientists who could have a lot of trouble explaining something that a grandmother can understand. It has no reflection on his knowledge of the subject matter.

    A few years ago, while explaining what we do to a group of general public visitors to our facility, one of our distinguished, senior theorist happened to be listening to my spiel. The next day, he walked into my office and told me that, after years of listening to my boss, and the previous boss of the group explained what the group's project is all about, yesterday was the first time he actually understood what we were doing! I was of course flattered, but also a bit shocked, considering that our group has been headed by people who are among the world's leading experts in this area! So to say that these people did not understand what they're doing is utterly false, because I KNOW for a fact that they do. And by that same token, just because I somehow have a bit of a skill in explaining such a thing, does that mean that I've understood it, and understood it more than they do? Nope, and certainly not to the same level as these experts that I look up to.

    A lot of crap gets thrown out nowadays and accepted as "fact". The notion that the ability to explain things in simple is somehow commensurate with one's mastery of the subject is patently FALSE. It carries as much validity as claiming that if you can't sing very well the songs you wrote, then you're not a very good songwriter. One has nothing to do with the other!


    Author: ZapperZ
    Originally posted on Jan25-12
  2. jcsd
  3. Aug 12, 2014 #2

    Simon Bridge

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    Hmmm, I think homilies should be examined as homilies, not as propositional statements.

    ... the statement is distinguishing between having knowledge and having understanding.
    There is a further confusion between explaining something and being convincing or satisfactory in that explanation.

    The statement is vague - it does not say what an explanation entails in this situation - if we require that the grandmother in question should end up with the same understanding, then the "explanation" in question could take some time.
    However, it is describing a rough test of understanding, like a rule-of-thumb:
    You know you must understand your subject when you have successfully explained it to someone else ... but failing to explain it does not mean that you are unable to do so. i.e. as a test of understanding it does not work the other way.

    I've seen people able to carry out quite complex calculations correctly but were unable to explain what they did or why they chose that particular method. The homilie is basically the opposite of the one that goes "Those who can, do; those who can't, teach."

    Consider: how would we tell if a student has understood a subject they have been taught?
  4. Sep 7, 2015 #3
    I attended a lecture with two colleagues and this particular lecture required a great deal of technical preparation. The consensus among us (all with at least BS degrees in physics, math, or engineering) was that none of us understood the subject after the first two or three minutes in the lecture. This is not entirely uncommon if you are reviewing a presentation outside your expertise. Later, a superior came in and told us all (without our even asking) that (s)he understood every word of the presentation and we should all strive to make our presentations as lucid at this one. This was peculiar, considering this superior had trouble solving a quadratic equation, and would come to one of us to help with a question on child's homework.

    I suspect the superior was impressed that the material presented was well-paced. Proper English was used throughout the presentation. I strongly suspect the superiors grasp of the material was no better than ours. The lecturer speaks English clearly, the superior understands English, therefore the superior understands the instructor (syllogism).

    I explain physics to my dog all the time. How much of it she understands is open to question. I will be quite surprised if I see my dog using a pulley to catch the treed cat someday. Simon Bridge asks us to consider how to tell if the student understood the subject that they have been taught. I agree. It is probably a tad too pedantic to subject our grandparents to a test on the material.
  5. Sep 9, 2015 #4
    And your point is...?

    Or are you just letting off steam?
  6. Sep 9, 2015 #5


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    I was once asked : "Can you explain what you do to your mother"? I answered, angrily : " I don't do _anything_ to my mother" .

    Still, maybe as an in-between from Greg and Simon ( Simon said) , maybe the standard is that the rule of thumb should be that with relatively -little work one should be able to explain it so that someone who is smart-enough and interested enough can get a good idea --non -technically -- of the subject.

    EDIT: There may be issues of circularity here. The person may have just _defined_ understanding something in this way.
    Last edited: Sep 10, 2015
  7. Sep 10, 2015 #6
    Just letting off steam Hornbeim. But my point is I do think it is a badly phrased homily. I do believe their is value in being able to put a concept or theory into language. This probably did not come out in my rant. However, the effectiveness of the explanation may not be appreciated. Thanks for listening. I will try not to get as preachy in following forums.
  8. Sep 10, 2015 #7


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    But that may be because mathematics itself is circular? Bourbaki gives the example of needing to know what "the same symbol" at two different places on the page means before reading a mathematics text. Since presumably there are no identical symbols, this is a matter of psychology. Hence psychology comes before mathematics, but presumably mathematics is a part of the language of physics which presumably describes psychology.
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