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Idiot's question: What is mass anyway?

  1. Sep 1, 2004 #1
    Don't get me wrong. I'm not here to ask for any standard textbook answer. This actually came from a teacher of mine who raised this interesting question.

    All physics textbooks start with the sentence "mass is a measure of matter". But that doesn't really make sense if you think it out clearly. So what kind of quantity is it actually measuring, anyway? I mean, for the same number of atoms, you can get different masses for different elements. Is it the same number of electrons and protons and neutrons and.....well, you get the picture. But no....we say that a electron is approximately ~1/1867(sorry if I got the wrong number) the mass of a proton. Where does THAT come from, anyway?

    Most people try to say that same masses weigh the same. However, that doesn't make sense either. Weight is actually gravitational force, which Newton proved to be based on mass(F=GM1M2/r^2). So, you can't use such a force to define mass. Actually, you can't even use force, because force is defined as the acceleration of mass. Using them would the same as using mass to define mass. And since I believe that most of us still remember that long winded argument over what a force is, I wouldn't suggest using force to define mass, which is another approach.

    Some people talk about Newton's first law as a definition of mass: "Some property of matter that opposes acceleration". That has a clear definition of what mass is, and brings out the idea of equality of mass, which is "two pieces of matter that have equal ability to oppose change in motion". But still, there is a critical piece missing in the picture: the quantitization of mass. What is equal mass? What is twice? What is half? If we don't first define force, we don't get anywhere, because different forces give rise to different accelerations....but how can we first have force if it depends on mass?

    Adding relativity into the picture makes this even more strange, because it kills the idea of "absolute mass" and brings in "relativistic mass". So, perhaps it would be better to stick with classical physics for the while.

    Time, mass and displacement have probably been the three most critical elements in physics. Time has already been an all time confusing puzzle. Displacement is now getting more and more unclear with the introduction of modern physics. Then comes mass, which doesn't seem to have a good definition at all. None of them had very firm and solid bases before we started working on them.

    Of course, we could have left all these questions unanswered and continue on with our exploration of "new stuff". But how can we be sure about what we do if we never really knew if we were right in the first place?

    What do you think?
  2. jcsd
  3. Sep 1, 2004 #2


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    I think all of the definitions you listed are in fact excellent definitions. Certainly, they define mass in terms of other physical quantities, but what else can you expect? All definitions define words in terms of other words.

    - Warren
  4. Sep 2, 2004 #3


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    Standardizing force does not require having already defined a unit of inertia (mass).

    the unit force can be defined prior to inertia. force measurement can be standardized using quantum devices and the atomic clock.

    see if you can find the NIST webpages about the "Electric Kilo" program
    In modern metrology, volts and amps can be standardized using quantum devices and the atomic clock.
    Force can be standardized by electrical measurment---in a device called the "watt balance"

    one can measure force without using any standardized quantity of inertia

    the kilogram block of metal is still respected but no longer strictly necessary.

    In principle, if not yet officially, one can use the force standard to determine the unit of inertia

    the NIST (nat'l inst of standards and techology, the old NBS) calls this
    their "Electric Kilo" program. it is pretty far along altho it has not officially replaced the old metal kilo.

    the upshot is that mass (measured for an object at rest) really is no different from its INERTIA at rest----which is determined by applying a known force.
    so the force unit is, in a certain sense, more fundamental
    (it didnt use to be that way but the field of metrology is gradually shifting because of things like the quantum volt and amp standards and the watt balance)
    I suspect the Einstein of 1905 would have liked that, since he thought of mass simply as "die Inertie".

    Next year, 2005, is the centennial of Einstein's first paper to mention
    E = mc2
    (but written differently)
    and in that paper he treats mass as the same thing as INERTIA

    The title of the paper, if you convert it to english, reads
    "Is the Inertia of a Body dependent on its Energy Content?"

    Ist die Inertie eines Koerpers....

    so he doesnt bother to even say "mass" he just calls the quantity inertia.

    In 1905 he wrote the formula m = E/c2

    (actually with a W for work rather than an E, but basically same formula just a little rearranged)

    "Relativistic" mass was not Einstein's idea, I think it was Dirac who introduced it. Anyway it has not driven out the ordinary idea of "inertia of a body at rest". To do Quantum Field Theory, for instance, you dont have to use the concept of "relativistic mass". that is generally the case (unless your textbook is one of those that still use it) and "relativistic mass" has gone somewhat out of fashion. there are alternative ways of handling the increase in total energy near the speed of light. Some still cherish the idea of "relativistic mass" so one should try not to offend them.
    Last edited: Sep 2, 2004
  5. Sep 3, 2004 #4
    Mass is a property of things which are material in nature (as opposed to etherial). It is evidenced by the phenomena of inertia, resistance and BOTH mutual attraction and repulsion (attraction in the form of gravity when the masses are apart and repulsion when they come together as a 'critical mass').

    Many think that only things which have mass 'exist'.


    If it were not for the 'existence' of space, the Universe would be in a state of critical mass with nowhere to explode.
  6. Sep 3, 2004 #5
    Mass is electromagnetic change. m = hv/cc. The only variable in the equation is electromagnetic change.

    Oops ! Sorry ! :smile:
  7. Sep 3, 2004 #6
    Quoting the conventional wisdom I am sure you are correct. But do you believe everything you read in the textbooks?

    Question EVERYTHING...then question the question.
  8. Sep 3, 2004 #7
    I try very hard not to "believe" anything. I only suspect things :smile: But I don't think I was quoting the conventional. That idea is probably just borderline acceptable (maybe not) in a well run physics forum such as this.

    Keep on chuggin !!

  9. Sep 4, 2004 #8
    mass isnt the only unit of measure we have trouble defining :yuck:

    temperature: "the average kinetic energy of an object"
    Say an object is sitting still on a table. it would have a kinetic energy of 0, except for the fact that the earth is spining. If the earth wasn't spinning then it would have a kinetic energy of 0, except for the fact that the earth is revolving...except for the fact that the galaxy is spinning... except for the fact that the galaxy is revolving... as far as we know there could be infinate points of reference.
    therefore since we cannot absolutely know temperature, we measure it reletively and it fits our purposes.

    time: you cannot use the words "time" or "rate" in the definition because those are redundant (therefore you can't say, "the pysical manifestation of rates" because r=(unit)/time). even if you could, according to the theory of relativity time moves slower for a person traveling at high velocities so again there are infinate points of referance.

    length: "the distance between two points" that is a redundant definition because a length is a distance. also, where would you define the end of an atom? at the end of the nucleus? atoms are mostly empty space, at the end of the electron? well the next atom doesnt position itself neatly next to the other... thats why we can only measure length reletive to something else.

    you get the idea, my head is spinning :rofl:
  10. Sep 4, 2004 #9


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    No, that's completely wrong. The "average kinetic energy of an object" refers to the kinetic energy of the motion of the molecules, atoms, etc. making up the object relative to its center of mass. It as nothing to do with the fact that it is "sitting still" or that the earth is spinning!
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