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Thread: Accuracy of the "Wiki".

  1. #1 Accuracy of the "Wiki". 
    Your Mama! GiantEvil's Avatar
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    I notice that "The Wiki" is a commonly used source in this forum. I use it frequently myself, but... Sometimes it is inaccurate. I will give examples;
    Mass-energy equivalence;
    The formula is dimensionally consistent and does not depend on any specific system of measurement units.
    The real formula is this:
    I see specific units.

    Uncertainty principle;

    The real formula is,
    Planck's constant is different than, half of Planck's reduced constant.

    We should all be careful of who we buy our candy from. 8)


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  3. #2  
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    Yea, it is a good source for raw data though.


    Of all the wonders in the universe, none is likely more fascinating and complicated than human nature.

    "Two things are infinite: the universe and human stupidity; and I'm not sure about the universe."

    "Great spirits have always found violent opposition from mediocrities. The latter cannot understand it when a man does not thoughtlessly submit to hereditary prejudices but honestly and courageously uses his intelligence"

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    A Wikipedia article is no better than the person who writes it, which could be anybody, including you. You should sign up and edit the article if it's wrong.
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  5. #4 Re: Accuracy of the "Wiki". 
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    Quote Originally Posted by GiantEvil
    I notice that "The Wiki" is a commonly used source in this forum. I use it frequently myself, but... Sometimes it is inaccurate. I will give examples;
    Mass-energy equivalence;
    The formula is dimensionally consistent and does not depend on any specific system of measurement units.
    The real formula is this:
    I see specific units.

    Uncertainty principle;

    The real formula is,
    Planck's constant is different than, half of Planck's reduced constant.

    We should all be careful of who we buy our candy from. 8)
    Accuracy of Wiki varies from very good to very bad. It is a useful source for some things, but one should not believe something just because one sees it in Wiki. A much more definitive, but less comprehensive, source is Scholarpedia, which is similar to Wiki in some aspects, but is written only by invited experts and is peer-reviewed prior to release. The authors in Scholarpedia are serious experts, Fields Medalists, Nobel Laureates, and original authors of seminal research. The authors in Wikipedia are whoever signs up and has the last word on the article, and since some authors have their own agendas, the information in Wiki can be suspect.

    I have seen statements in Wiki that are just plain wrong.

    What is very useful in Wiki articles are links to the refereed literature and scholarly works by recognized experts.

    I am not sure what your equation for energy is supposed to mean, as I have not checked it in detail. It appears to be some tensor version of E=mc^2. In any case E=mc^2 is correct so long as one interprets "m" as relativistic mass, which some people do not like to do. In that case you get E^=m^2c^4 + pc^2

    the important thing is not the symbology but knowing what the symbols mean and the ultimate equivalence of mass and energy.
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  6. #5 Re: Accuracy of the "Wiki". 
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    Quote Originally Posted by GiantEvil
    I notice that "The Wiki" is a commonly used source in this forum. I use it frequently myself, but... Sometimes it is inaccurate. I will give examples;
    Mass-energy equivalence;
    The formula is dimensionally consistent and does not depend on any specific system of measurement units.
    The real formula is this:
    I see specific units.

    Uncertainty principle;

    The real formula is,
    Planck's constant is different than, half of Planck's reduced constant.

    We should all be careful of who we buy our candy from. 8)
    In both cases, the Wikipedia formula is the one that looks right to me.
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  7. #6  
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    The subscript to the "E" indicates the value is expressed in Joule seconds. The subscript on the "M" indicates the value is expressed in kilograms. The subscript on the "" indicates the value is expressed in meter's per second.
    If we tried to use Einstein's equation with electron volts for E, and penny weight for M, and miles per hour for c, it would give erroneous results. Although photons have no mass (rest mass technically), they have an energy. And through this permutation of Einstein's equation, E/c^2=M, we can assign the photon a mass. And now light ray's can be deflected by gravity. The energy for a photon of a given wavelength is determined by . Or, that is, the energy of the photon is equal to the product of it's wavelength and Planck's constant.
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  8. #7  
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    Quote Originally Posted by GiantEvil

    The subscript to the "E" indicates the value is expressed in Joule seconds. The subscript on the "M" indicates the value is expressed in kilograms. The subscript on the "" indicates the value is expressed in meter's per second.
    If we tried to use Einstein's equation with electron volts for E, and penny weight for M, and miles per hour for c, it would give erroneous results. Although photons have no mass (rest mass technically), they have an energy. And through this permutation of Einstein's equation, E/c^2=M, we can assign the photon a mass. And now light ray's can be deflected by gravity. The energy for a photon of a given wavelength is determined by . Or, that is, the energy of the photon is equal to the product of it's wavelength and Planck's constant.
    You don't have to specify the units. mc^2 has nuits of mass *length^2/time^2 which are the units of energy. You simply plug in mass multiply by the speed of light and out comes energy. Now if you want the units to be Joules, then mass should be in kilograms and the speed of light in meters per second. If you want ergs, then mass is in grams and the speed of light is in centimeters per second. If you want foot-pounds then mass should be in slugs and the speed of light in feet per second.

    So E=mc^2 works just fine as is. Pre-speofying the units simply adds confusion.
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  9. #8  
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    Okay, E=Mc^2 is dimensionally consistent. I was hung up on SI as universal standard. If anyone has worked on a mid 90's American automobile, they understand the frustration of mixed system types. Some of the bolts are metric, some are standard, guess which one's? Also, I have many physics text's, that somewhere in the beginning, contain strong exhortation's to always specify unit's. Really, where would math and science be without procedural discipline?
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  10. #9  
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    Quote Originally Posted by GiantEvil
    Okay, E=Mc^2 is dimensionally consistent. I was hung up on SI as universal standard. If anyone has worked on a mid 90's American automobile, they understand the frustration of mixed system types. Some of the bolts are metric, some are standard, guess which one's? Also, I have many physics text's, that somewhere in the beginning, contain strong exhortation's to always specify unit's. Really, where would math and science be without procedural discipline?
    There is need to specify units only when you will have need to do an experiment and take specific measurements. Otherwise they tend to get in the way.

    Much of special relativity is done in units in which c= 1. That keeps the formulas simple and avoids a ploriferation of c's.

    If you know how to work with dimensions, there is no need to specify them before hand. You just plug them into the algebraic expression and out pops the correct units, so long as you know how each unit is defined.

    So, for instance, in with we have

    = = = =

    The exhortation to define units is useful in very elementary texts in order to remind students that the quantities involved do have units and that pure numbers do not describe them. In more advanced texts, in which students know this "instictively" there is less religious furvor on the matter. But you can never forget it, and in some units one finds different constants involved that are necessary to keep things consistent. This is particularly the case in electrodynamics, and you can find cases in which Gaussian and cgi units are used in the same, excellent, text. So you have to be on your toes.

    Mechanical engineers, being perverse and using English units, and in fact using the pound for both mass and force use rather than where is required in order that be in lbf (pounds force) and in lbm (pounds mass).
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  11. #10  
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    On page 19 of Benjamin Crowell's e-book, Newtonian Physics. There is a fascinating little blurb about the, Mars Climate Orbiter. It crashed because an engineer forgot to convert an English measurement into metric.
    Also, but don't quote me on this, 32.2 is the acceleration coefficient in feet, seconds, for 1G.
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  12. #11  
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    Mixing units and leaving them out altogether are two different things.

    Anyway, on topic, I'd say that for technical articles, Wikipedia's generally very good. Not perfect, but there's usually references to scholarly works cited at the bottom of the page for people that need more information. And of course, if you recognize something as wrong, fix it. You don't even have to sign in unless you want credit for your work. Plus, you can always check it against similar projects like Scholarpedia and Mathworld.
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  13. #12  
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    Quote Originally Posted by MagiMaster
    Mixing units and leaving them out altogether are two different things.

    Anyway, on topic, I'd say that for technical articles, Wikipedia's generally very good. Not perfect, but there's usually references to scholarly works cited at the bottom of the page for people that need more information. And of course, if you recognize something as wrong, fix it. You don't even have to sign in unless you want credit for your work. Plus, you can always check it against similar projects like Scholarpedia and Mathworld.
    The problem with "fix it" is that apparently quite a few people who don't know what they are doing go right ahead and fix it.

    I trust Scholarpedia. Mathworld less so, but more so than Wiki.

    Wiki is usually pretty good, but the errors can be pretty bad. The references are the most useful aspect of Wiki, so long as you stick to references from unimpeachable sources -- like original papers in peer-reviewed reputable journals.

    Note that is peer-reviewed + reputable. Not all 'peer-reviewed" journals are reputable. Some are organs of the lunatic fringe with lunatic referees to match.

    Also do not confuse "peer-reviewed + reputabble" with "correct", except in mathematics journals. What you get from peer-review is the assurance that the work is not insane and is of some interest to the community. There may still be some controversial and even wrong elements, depending on the subject matter and how much is really known and settled.

    Even in mathematics journals the occasional mistake creeps in. It more rare that in other disciplines for this to happen.
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  14. #13  
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    That's true, but on average technical articles seem to get by without suffering too much, at least as far as I've noticed. When the articles become blatantly wrong, people tend to step in a correct them. Or maybe I've just missed the worst ones.
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  15. #14  
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    Quote Originally Posted by MagiMaster
    That's true, but on average technical articles seem to get by without suffering too much, at least as far as I've noticed. When the articles become blatantly wrong, people tend to step in a correct them. Or maybe I've just missed the worst ones.
    Most seem pretty good. I have seen some that are just plain wrong.

    The ones that are just plain wrong tend not to be those discussing scientific or mathematical theories, since there are enough people knowledgeable in the area and the information is widely available in the literature, but rather in specialized technical articles in which their facts are wrong, and the facts are not widely published. I have no intention of correcting the ones that I found most glaring, and I am not sure that I could do so without violating ITAR or getting uncomfortably close to classified or proprietary information.
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  16. #15  
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    I suppose I don't usually look into those kind of articles. Most of the weirdly detailed stuff I've looked for seems to be too weirdly detailed and either doesn't exist or is only a tiny stub.
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