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E=MC2 is it so impressive?

  1. Dec 21, 2008 #1
    Einstein said that matter was just a special form of energy then later came up with the equation of Energy(in ergs)=Mass(in grams) * C(speed of light in cm)^2 ... well ... what would the equation be if the standard unit of energy measurement were C(speed of light in cm)^2 ergs? ... seems to me it would be E = M ... well wasn't that what he really said to begin with? =o
  2. jcsd
  3. Dec 21, 2008 #2
    yes he meant special as if not normal. not normal meaning that the mass had some unconventional properties when reaching the velocity of speed of light thus relating to energy. which is entirely different from E=M.
  4. Dec 21, 2008 #3


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    It's not the c that's cool, it's the E=m part. It's quite common to use units in which c=1.
  5. Dec 21, 2008 #4
    by moving the c2 over to the left side of the equation you imply the 'speed of light part of his equation'
  6. Dec 21, 2008 #5
    E=M tells you that anything that has energy has a mass and visa-versa. For example, a moving photon has a mass.
  7. Dec 21, 2008 #6


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    it's not like the c^2 just goes away. It becomes 1, but the units are still there.

    It's not E = M as in (energy = mass).

    it's still E = MC^2, but C^2 isn't just 1 it's 1 (velocity units)^2

    E = M*C^2
    E = M*(velocity units)^2
  8. Dec 21, 2008 #7
    Velocity has no units :P Remember, time and position are just two dimensions, so their units cancel out when you are talking about speed. Pretty much it's E=M.
  9. Dec 21, 2008 #8

    I'm pretty sure velocity has units...
  10. Dec 21, 2008 #9


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    are you trolling me?

    position and time do not cancel each other. You have E = M (change in position/change in time)^2
  11. Dec 21, 2008 #10


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    It doesn't in relativistic units, which are commonly used for problems of this sort.
  12. Dec 21, 2008 #11


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    but is it a matter of convenience (I.e. you ignore the units since the value is constant and always 1)?


    kg*(m/s)^2 != kg

    do they?


    from http://www.sparknotes.com/physics/specialrelativity/dynamics/terms/term_6.html [Broken]

    so the units don't actually cancel, you just ignore them for convenience. You put them back when you're done with calculations to make the statement physically true.
    Last edited by a moderator: May 3, 2017
  13. Dec 21, 2008 #12
    I don't get it. Doesn't relativity theory put time and space on an equal footing? If so, then isn't velocity (space / time) a pure number?
  14. Dec 21, 2008 #13
    No, I am not trolling you. And please bear with me if you think that my understanding of relativity is inferior to yours. What I learned from GR is that time and space are both just dimensions. Just like you can't say that units of one space coordinate and another are different, you cannot say that units of time and space are different, so velocity is nothing but a slope - a number with no unit.
  15. Dec 21, 2008 #14


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    I don't really think that your understanding is inferior. I was thinking special relativity, actually. And my only exposure to it is in electromagnetism. We never canceled different units there.

    We did have terms like sqrt(1 + (v/c)^2) so that velocity is canceled with velocity.

    It would be nice to have a relativity expert answer this. Perhaps I'm more ignorant than I assume, but it's really hard for me to justify that so far from what I'm looking up.
  16. Dec 21, 2008 #15


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    Actually, it is normally 'c * dt' which is used as the dimension (with c=1), not just dt. This is done exactly so that units work out.
  17. Dec 22, 2008 #16
    Simple question here:

    If i took C^2 (in cm) which i believe is approximately 898755178736817640000, and therefore decided that my unit of measurement should be 898755178736817640000 ergs as my standard unit ... then my E would be 898755178736817640000 ergs = 1 gm of matter ... correct?
  18. Dec 22, 2008 #17

    Length has units. We typically use meters.

    Times has units. We typically use seconds.

    Using the speed of light as a conversion factor, we could define one in terms of the other. In fact, sometimes we use the unit of a lightsecond to mean 1 second of length.

    We could say that velocity is then measured without units (an object's velocity is then it's ratio to the speed of light). Acceleration would be in hertz. Force would be in kilogram hertz.

    But that's not the canonical way to do it, and people won't understand you unless you go to lengths to make it clear you are treating them as equal. Even though meters and seconds are isomorphic, we treat them as if they were distinct. A nickle is five pennies, but that doesn't mean a nickle is made out copper. Similarly, a meter is c seconds, but it isn't a measurement of time.

    Additionally, spacial measurement is a vector, and must be multiplied by a unit-vector. Time is scalar.
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