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Measurable quantites vs Base units

  1. Jul 9, 2014 #1
    So here's the familiar SI base units from NIST

    electric current
    thermodynamic temperature
    amount (mole)
    luminous intensity

    Something has been bugging me about this.
    For whatever reason I am thinking all quantities are calculated by just three on the list -
    Lengths, times, and amounts.
    There's something special about these three things. Are these the only units we can really measure? (or observe?)

    What I'm getting at is that you can't really measure something like mass. It's calculated from displacement of a spring or whatever method you prefer. The same goes for current, it's calculated from a count (amount) and time. temperature is calculated by the displacement of mercury, but do you see what I mean? the actual measuring (or observing) part of everything comes down to lengths, times, and amounts.

    Does anyone follow me? Does that seem strange? What are your thoughts on this?

    (I might be abusing the word measure here, maybe observe is a better choice?)
  2. jcsd
  3. Jul 9, 2014 #2


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    Please explain to me what you mean by amount?
  4. Jul 9, 2014 #3


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    You also need a definition of either charge or current.

    And time would be nice as well - otherwise you are stuck in a static world!

    And you will find that every measurement is made in terms of other "things" - there is always an intermediary. Thus the requirement for standards. The standards rely upon physical laws which relate the quantities.
  5. Jul 9, 2014 #4
    well yeah like you cannot explain current in amounts and time only
    for eg.
    1-CURRENT = θ/T



    THEN PROBABLY (by definition) YOU MUST SAY
    1C IS 'amount' of charge flowing when 1 ampere current flows for one second


  6. Jul 9, 2014 #5
    Can't current be 'measured' by the number of charge carriers which pass a point in a specific time?
    That's just an amount, and a time.
  7. Jul 9, 2014 #6
    Well, the current depends on "how much charge" each carrier is carrying as well as the number of carriers per unit time.
  8. Jul 9, 2014 #7


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    What I don't understand is, why are time and length different from "amounts"?

    They seem to me to be an "amount" as much as everything else is

    That reduces your question to "why is everything measured in amounts" which is just because of the mathematical ground that science rests on.

    Maybe I'm overlooking something here, but what's unique about time and length that they can't be considered "merely amounts"?
  9. Jul 9, 2014 #8
    I don't know. I guess length is really a matter of two positions. I see what you mean when you say something is some amount of inches. Maybe position is a better term to use than length for this.
    Time follows similarly, however I can't think of a word for a similar 'position in time'

    So that leaves us with position and 'position in time' are not amounts.
  10. Jul 9, 2014 #9


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    Position and date/time is relative to a coordinate system. Coordinate systems are arbitrary. What is not arbitrary is "distance" and "duration".
  11. Jul 9, 2014 #10
    This is true - there are two very special things about position in space and position in time, but it is not (as you assert in you first post) that they are the only things we can measure directly - in fact it is the opposite.

    They are the only things that CANNOT be measured exactly, and they are the only things whose measurement is dependent on the observer. They are therefore no good as bases for a system of units, and that is why we use the difference between two points in space and in time as our bases. The other quantities measured by SI units however can (in principle) be measured exactly and are identical for all observers and are therefore defined absolutely.
  12. Jul 9, 2014 #11


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    I would say time is an amount of a cycle.

    For example, 1 second is a certain amount of atmoic cycles (I don't know exactly how it's measured)

    It's like UltrafastPED said, every "thing" is described in terms of another "thing" (as evidenced by the very notion of units)
  13. Jul 9, 2014 #12


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    Even length is measured in terms of length in Euclid. You choose two points, this distance is your unit of distance; everything else is scaled up or down from this arbitrary unit.
  14. Jul 9, 2014 #13


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    Actually, you can define charge purely in terms of kinematical quantities. This is the approach taken by Gaussian units. One statcoulomb has dimensions of ##M^{1/2}L^{3/2}T^{-1}##

    To the OP, the required fundamental units is not actually entirely constrained by physics. It is mostly a matter of convention which is determined by the system of units. You should compare SI, Gaussian units, and geometrized units for some examples.
  15. Jul 9, 2014 #14
    This is quite the observation. I should've realized this sooner.
    I agree we can't determine some 'absolute' position or position in time, and so we use the differences between two positions and two positions in time.

    However, it seems to me that these two things which we can't measure exactly, are the only two observable properties (for lack of a better word) of things - and their differences being the only truly measurable things (observed and quantified).

    I think all the other "base units" are calculated (not measured) from combinations of differences of positions and times (measured) of things related to various objects, while the actual "base unit" chosen depends on what the thing and object is (and to an equal degree the specific combination of lengths and times).

    (I am thinking now that an amount can simply be reduced to a group of positions)

    Let's call this group of observable and quantifiable properties (length and time), 'basic units'.

    So mass for example, isn't a basic unit, because it isn't directly observable. The only way to determine mass is by measuring the basic units (in some specific way) and fitting an equation of motion to those measurements.

    Am I making any sense?
  16. Jul 9, 2014 #15


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    It actually depends on the system of units. In principle you can reduce everything to length. See my comments above in post 13, and compare SI, Gaussian, and geometrized units.
  17. Jul 9, 2014 #16
    ok i provide you a large charge carrier carrying charge of 1010101010 :tongue: electrons
    now define current vs one 'charge carrier' like single electron

    Your 'amounts are same' but current......
  18. Jul 10, 2014 #17
    I don't really think this is the case, because what is done with the measurements (lengths and times) or how they are quantified, is beside the point. However I did find geometrized units to be particularly odd. But that is a different discussion, so I made a new thread for my questions on them.

    However, I would like to know the logic behind reducing everything to length
    Last edited: Jul 10, 2014
  19. Jul 10, 2014 #18


    Staff: Mentor

    I don't see how you could possibly think it is beside the point. You have looked at the SI base units and correctly concluded that they are not the only possible set of base units. I have pointed out other systems with different sets of base units, thereby confirming your point.

    In what way is providing a few concrete examples of your point besides the point?
  20. Jul 11, 2014 #19


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    Yes it can, and it is likely that this is how the Ampere will be re-defined in a few years. The realization will then by some sort of electron pump that send electrons one at a time and the current will be I=e*f where e is the electron charge and f the pump frequency. e would then be defined to have an exact value. These devices already exist, but at the moment they are not quite accurate enough (they need to improve by about a factor of 5).

    However, this would not in any way change the fact that the Ampere is a base unit in the SI; only the definition(and realization) will change.
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