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Comparison Between Two Isolated Systems

  1. Aug 31, 2008 #1
    If we think of two isolated systems, A and B, existing side by side, the proportion of the energy between those systems should always stay the same.

    Here is a thought, that I would like some comments on:

    System A contains a planet, and the advanced civilisation on this planet, have created a box with matter and antimatter ready to annihilate. Some electronic in the box will carry out this annihilation automatically, let’s say when the strength of the surrounding gravity field decreases below a certain limit.
    Now, they are pushing this box out in the universe with the exact escape velocity of this planet.

    The situation mentioned above, also happens in system B. The two systems are completely identical, except that the planet in system B has more mass, yet the same size.

    So it is more “expensive” for the civilisation in system B, to push the box out in space, because of the higher escape velocity of this planet. It takes more energy.

    Sometime in a very far future, the boxes will reach the decreased value of gravity that makes the matter and antimatter annihilate.

    The two boxes will explode at exactly the same very low strength of gravity, and at very low speeds.

    So, (that portion if not all) of the energy that is being converted from mass to electromagnetic radiation when the matter and antimatter annihilates, must be the same in the two systems.

    How can the proportion between the energy in these two systems still be the same, as the civilisation in system B did spend more energy, pushing the box out in space?

    Where and how in the two systems do we now see a change, that reflects and equalizes the two civilisations different use of energy, so that the proportion of the energy between system A and B will stay the same?
  2. jcsd
  3. Aug 31, 2008 #2

    Doc Al

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    Staff: Mentor

    OK, this is the energy contained in the boxes, not the energy of "the system".
    Who said they were the same? Where does the energy to impart KE to the boxes come from? And when KE is given to the box, does that energy leave "the system"?
  4. Aug 31, 2008 #3
    The boxes is not isolated systems themselves, so the energy contained in the boxes are part of the energy of the systems.

    When the experiment starts there is a difference between the energy in system A and B, because the planet in system B has more mass. That difference should always be the same.

    The energy of KE is not leaving. What appears to be leaving, (and is probably not), is the extra energy the civilisation in system B used, to carry out this experiment.
  5. Aug 31, 2008 #4

    Doc Al

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    Staff: Mentor

    What makes you think it changes?
    That energy isn't lost, just transformed into increased gravitational PE.
  6. Aug 31, 2008 #5
    Increased gravitational PE between the planet and the box, yes.
    When the box is gone, or part of it, what happens to the PE?

    Who can now benefit from the PE?
  7. Aug 31, 2008 #6

    Doc Al

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    Staff: Mentor

    Since the box is "infinitely" far from the planet, you don't have to worry about PE any more.
    I suspect the folks back on the planet are quite happy to have given up energy (on the planet) to send that box sailing away before it exploded.

    But interesting question about how the radiation created by the annihilation would be affected by the gravity of the planet.
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