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Mass energy equivalence

  1. Sep 19, 2012 #1
    Mass energy equivalence question

    Hi I'm just wondering how energy is equivalence to the mass if E=mc ^2. I don't understand why you must times it by the speed of the light^2.

    And if the mass is proportional to the gravity, is it right to consider gravity as energy?
    Last edited: Sep 19, 2012
  2. jcsd
  3. Sep 19, 2012 #2


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    The speed of light is a constant with dimensions, so it can be regarded as merely a conversion factor from one set of units to another.
    It's like saying your weight in pounds and your weight in Newtons. They are equivalent, but they have different units so you need a conversion factor to convert between them.

    As for why the conversion factor happens to be the speed of light... well that's just how it happens to be. Informally speaking, the speed of light is the natural conversion constant between units of length and units of time, because light in vacuum moves at the maximum speed possible.

    I think you can regard gravity waves as energy, but not gravity.
  4. Sep 19, 2012 #3
    Re: Mass energy equivalence question

    I see one reason c2 can come into the equation E=mc2 is origin of all matters is electromagnetic waves. Appearance c2 in energy makes no sense and has no explanation whatsoever. It is like saying speed of a car depends on the lightnings in the sky.
  5. Sep 19, 2012 #4
    Re: Mass energy equivalence question

    I don't know where you are getting this from, Khashishi's explantion explains it quite well.
  6. Sep 20, 2012 #5
    That's seems about as far as current science may be able to take us.

    But we do understand that early in our universe radiation...electromagnetic energy....was all...together with space and time. As things cooled, particles emerged so it seems logical to me there should be some connection between mass and energy. When science better understands the exact relationships be tween mass, energy, space, time, 'c', those essentials that came from the origin of our universe, we may be able to provide a firmer connection.

    Perhaps unifying quantum mechanics and general relativity, so we can quantify the big bang and black hole singularities, will give us some of the necessary insights...via quantum gravity. I'd like to know why there is space and time and a speed of light.
  7. Sep 20, 2012 #6


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    The "c" in that equation is not from electromagnetism. While photons happen to travel at that speed, this applies to all massless particles. Photons are just the most important type.

    If you combine quantum mechanics and special relativity, you get that rest-energy as result. It depends on the universal speed limit (given by special relativity), and you do not need any electromagnetism in the derivation.
  8. Sep 20, 2012 #7


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  9. Sep 20, 2012 #8
    Re: Mass energy equivalence question

    I think the OP meant how this equation is derived, more specifically how c^2 was derived.
  10. Sep 20, 2012 #9


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    Energy is the ability to perform work. Gravity is a fundamental force. Forces cause things to happen, it is through the fundamental forces that everything interacts.
  11. Sep 20, 2012 #10
    If something is massless why its speed would be limited to 'c' only?
    It can have unlimited speed, unless some one has already shown otherwise I'm not aware of.

    This has been my thinking lately. That E=mc2 is a simple conversion factor.

    What if the conditions in early universe under which matters (mass) were formed were totally lost and are not reproducable. Which means our MATHs will not work in that early universe.
  12. Sep 20, 2012 #11


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    Special relativity. While there are mathematical solutions for velocities > c, they require particles with an imaginary mass (imaginary like a square root of -1) and if they can interact with other particles, they violate causality (you could modify your own past). In addition, if they lose energy, they get faster.
    Those solutions are called tachyons, and they are usually considered as unphysical.

    Right. If you use the same units for space and time, the equation is simply E=m.

    Up to now, no variations of physical laws with time were observed, and they would be really problematic in terms of the fundamental theories. Experiments today produce results which are consistent with the predictions for the early universe.
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