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Matter & Mass

  1. Aug 27, 2003 #1

    I had an interesting conversation with an individual who stated that matter cannot be converted into energy, rather mass can be converted into energy and vice-versa.

    1) What is the difference between matter and mass?

    2) Also, why can't matter be converted into energy?

  2. jcsd
  3. Aug 27, 2003 #2


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    Matter is a slightly archaic word for something with mass, as in the conservation of matter (which must be paired with the conservation of energy to still hold true.

    Mass can be converted back and forth between energy, so therefore so can matter.

    Of course relativistic mass is conserved as it's a function of the energy of an object in that reference frame.
  4. Aug 27, 2003 #3


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    Hello Whitestar, first a testimonial for jcsd, he gives clear correct answers consistently. But I'm going to confuse things by saying that in different branches of physics they use the word "matter" differently.

    In General Relativity papers what they mean by matter is anything that isnt the gravitational field. So LIGHT is included in the matter category. When a guy says "OK that is how we describe the gravitational field, now let us introduce some matter" what he may introduce as an example of matter is some radio waves.

    I know, this is bad news. People use words differently. But other people will talk as if light and matter are two separate categories.

    Also people differ about their definition of "mass"-----majority usage is that light has zero mass, an object can have mass only if it can be at rest and its mass is, by definition, its inertia when it is at rest.

    But an active and dedicated minority likes to use the concept "relativistic mass" for mass.

    Its not worth arguing about---but be alert to differences in how people use words.

    What can we say for sure?

    That "mass" originally meant BREAD DOUGH (Latin "massa")
    and "matter" originally meant WOOD (Latin "matera")
    That since Newton the primitive meaning of mass has been inertia----a force-per-unit-acceleration quantity that tells how resistant an object is to being pushed around or accelerated this way and that by forces. If an object has a big inertia then it takes a big force to make it accelerate one meter per second per second, or one foot per second per second (whichever units).
    Ultimately this is the root meaning of mass in post-1700 physics
    and all the various definitions link back to inertia in some way.

  5. Aug 27, 2003 #4

    Ah, thanks for the clearup! :)

  6. Aug 27, 2003 #5


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    OOPS, well if my preceding post cleared things up, then this one will make it murky again! This all has to do partly with language, which is never as streamlined as one would want.

    In the metric system they have a unit kilogram for inertia

    and a unit "mole" for "quantity of material" which is
    essentially a number of particles

    It is an interesting distinction. If you have Avogadro's number of iron-56 atoms that is always one mole of iron-56. Lets say
    it is made into a small iron cube.

    The MASS of that chunk of iron, that batch of atoms, will change with temperature. the inertia of a chunk of material increases with temperature.

    The difference is so slight that it's not ordinarily measurable but it is generally believed in.

    So a mole is a mole is a mole but it can have different amounts of mass.

    I think there must be some subtle difference between matter and mass.

    In the sun-----four hydrogen nucleuses consisting of 12 quarks get together and form a helium nucleus, which has less mass.
    But there are still 12 quarks and they are still packaged in 4 nucleons (2 protons and 2 neutrons). No particles have been destroyed---they've simply been REARRANGED so they weigh less.

    This releases energy which eventually you see as light.

    There are rules governing the annihilation of particles. Matter in the sense of numbers of particles CAN be annihilated if you follow the rules (like for example balancing the numbers of particles and antiparticles). But such a radical thing as annihilating particles does not happen as much as just rearranging particles to have less mass (basically what is going on in the sun).

    You asked:
    1 what's the difference between matter and mass?

    I am not quite sure but I think the key thing is "How are they measured?" Mass is based on measuring inertia. Matter is measured basically by counting.

    2 why cant you convert matter into energy?

    You CAN but you have to follow certain number-conservation rules. You can make an electron and a positron annihilate and give light----so instead of 2 particles you now have some electromagnetic waves carrying energy. But it is not simple and it is not what is going on in the sun.

    In the sun it is just a simple case of MASS being converted into energy---the particles when they are rearranged have less inertia

    Part of the complexity here is just a "matter" of words---human language and its historical roots: always amazingly complicated. Nature is simple by comparison, I think.
    Last edited: Aug 27, 2003
  7. Sep 12, 2003 #6
    Re: Re: Matter & Mass

    But if you were to convert someone into energy and reconvert him or her energy back into matter, would it still be the same person with the same personality, i.e. would he or she survive the procedure considering that matter and energy are both interconvertible, or would it be a replica?

  8. Sep 12, 2003 #7


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    Re: Re: Re: Matter & Mass

    everymorning when you wake up you are a replica of
    who you were yesterday

    you arent made of exactly the same atoms you were yesterday

    a lot of cells have died a lot of nitrogen has been flushed out in urine
    a lot of carbon from dead cells has been burned up and breathed out in CO2
    a lot of skin has sloughed off
    a lot of replacement stuff has been replicated and put in place and is operation

    identity (self) is independent of material

    you are a replica made of different material but somehow

    (I dont remember what the turnover rate is, when I saw the calculation some time back it surprised me how rapid it is)

    but somehow----do we really understand how----this replica remains the
    same you

    I see no advantage to annihilating the material of a body and converting it to energy----that seems pointless.
    What one wants to do is to extract the INFORMATION and then
    create an exact replica from off-the-shelf chemicals.

    converting matter to energy and energy to matter would be unnecessary, inefficient, time-consuming and on the whole a bit "trek"-ish.
    the identity of the person must reside in the information and not in the material---so what happens to the material seems irrelevant

    but producing an exact replica with precise information obtained non-destructively seems like a good thing to strive for----albeit not feasible by present means----and would presumably yield an individual with the same memories and personality
    Last edited: Sep 13, 2003
  9. Sep 13, 2003 #8


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    This, of course, depends on the particular definition of "matter" that you're using. Einstein defined "matter" as follows. From "The Foundation of the General Theory of Relativity," Albert Einstein, Annalen der Physik, 49, 1916
    This implies that matter is the same as mass in Einstein's use of the term "matter." This also follows from Einstein's use of the term "mass" in the same article
    However mass-energy as used above is a bit different than energy in the normal sense since Einstein is refering to a closed system and all energy is taken into account. If the system is not a closed system then potential energy becomes energy which is not part of the mass. I.e. A system of two charges has a total mass which depends on their mutual potential energy. However if you have one charged particle which is in an EM field then the energy of that particle contains potential energy and that energy is not part of the mass-energy.

    Energy is a quantity which is constant (for the most part anyway). For example: Consider a charged particle moving in an EM field. The energy of the particle is

    E = K + E_o + V


    E = total energy
    K = kinetic energy
    E_o = rest energy
    V = q*Phi = (charge)x(Coulomb Potential) = potential energy

    If V is constant and non-zero then so is E. However if V is not zero then mass is not conserved since m = m_o/sqrt[1-(v/c)^2] {m_o = rest mass} and since the field does work on the particle increasing its speed and thus increasing m.

    If you have two particles, one being the anti-particle of the other and each having a non-zero rest mass then they can annihilate and produce photons (at least 2) which have zero rest mass. However the energy will be the same and so will the total mass. As Einstein defined "matter" then the amount of matter will have remained unchanged too. The invariant mass will have remained unchanged too. However the sum of the rest masses will have changed. Thus the form of the energy changed and that is what is meant by mass being converted to energy.

    Under a different definition of mass you'll get a different answer. For example: See http://www.geocities.com/physics_world/stp/pg_246.htm

  10. Sep 13, 2003 #9
    Re: Re: Matter & Mass

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    Please let me know what you think.

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