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Ridiculous explanation of 'string theory'

  1. Dec 11, 2007 #1
    This isn't so much a question specifically as a point that came up in my mechanics class... so sorry if I'm posting it in the wrong place!

    In a basic mechanics class; we were finding the frictional force on a box by thinking about someone moving it by pulling an attached string. Someone in the class asked if where the string was attached made any difference to the movement; so my teacher went off on a rant about this being the difference between maths and physics- that maths uses perfect models and physicists want to deal with what actually happens (fair enough). He then went on to say that 'in maths we model the box as a dimensionless point, or 'particle'.. physicists deal with the fact it isn't by replacing it with a string' and then gave an extremely basic approximation of what a string is supposed to be. Now I'm sure most people on here know more about string theory than I do; but is that not the most ridiculous explanation of why it is needed that you ever heard??
  2. jcsd
  3. Dec 11, 2007 #2
    Well this isn't really string theory I don't think. String theory deals with very very very small objects (A box is much too big), but it's still an interesting question.

    As to the original question the student asked, yes it does matter because if the sting is placed somewhere other then at the center of mass there could potentially be a torque on the object.
  4. Dec 12, 2007 #3

    Gib Z

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    That statement your teacher made could be corrected as such;

    In previous theories Physicists modeled fundamental particles (such as electrons or an Up quark) as point particles, but in String Theory they replace the point particle with a string as a string model could have different properties relating to their harmonic frequencies, so perhaps we could show that all particles and energy are made from the same things (strings) and that they are just vibrating at different frequencies, and even nicer, interactions between particles and energy transfers are really just frequency changes on the strings. As an aside, it also abolishes the point-particle model with a finite non-zero size model.

    There were just a few corrections =]
  5. Dec 12, 2007 #4
    I see what he's trying to say, but he's got it a bit arse-about-face. It's also not directly anything to do with String Theory (with a big S), I think you've got the wrong idea.

    What he's getting at is that in classical mechanics it usually doesn't matter what the geometry of your objects is, unless it's part of the problem. A mass on the end of a string hanging in midair is exactly equivalent to being attached to a point particle of the same mass, or a point particle exerting some other force, or even an entirely disembodied tension acting on the string itself - from the point of view of the string. Likewise, in the simple model, a string isn't a piece of string but a line of constant length that transmits a force from one object to another.

    Essentially he's talking about the nature of the models you use.
  6. Dec 12, 2007 #5
    Yes, that was my point. Although I see how the corrected version of the statement makes sense; I just thought it was particularly pointless to bring string theory into somewhere where it is neither relevant or necessary.
  7. Dec 12, 2007 #6
    I'm not sure that your teacher was trying to point to String Theory, but might have happened to use the word "string" (as in a rope) and "theory" next to each other.

    A string attached to an object has absolutely no correlation to a "string" in String Theory.
  8. Dec 13, 2007 #7
    Is Brian Greene a member here?
  9. Dec 13, 2007 #8

    Gib Z

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    Haha why would that be so?
  10. Dec 13, 2007 #9
    I'm aware of this =P no really. And he honestly did try to tell us it was to do with String Theory- he explained replacing particles with one-dimensional oscillating strings.. I can't remember his exact wording but it was definetely String Theory capitalised and all. However in the same lesson he managed to imply the Quantum Physics and String Theory were one and the same ("Physicists have a problem that their equations don't match up, String Theory doesn't match mechanics".. I *think* that refers to quantum physics and special/general (not certain of the difference!) relativity. I don't think physics is his strongest subject.
  11. Dec 13, 2007 #10


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    I have no idea why your teacher is telling you these sorts of things; I presume it is a mathematics class on mechanics?

    The important thing to take away from his comments (I think) is his point on modelling; the one that Sojourner makes. In this sort of problem we don't care what the shape of an object is, but take the ideal model that the object is a point particle. Whether this is physically possible or not is irrelevant-- we do it to make the equations solvable.
  12. Dec 13, 2007 #11
    Yeah its nice and simple M1 (a levels I'm talking). I'm not too impressed with his mechanics teaching (as you may have guessed).. I spend half the time correcting him =]
    Yeah the point particle thing makes sense, thanks.
  13. Dec 13, 2007 #12


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    I guessed as much. The problem is that A level maths teachers will have a degree in mathematics, and presumably a PGCE, but won't necessarily have studied much applied mathematics. The good thing is that M1 and M2 aren't really very difficult; and if you're taking physics as well, then there's such an overlap that hopefully it won't matter too much if your mechanics teacher isn't that good! Then, of course, there's PF if you have any questions :smile:
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