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The Conversion of Matter & Energy

  1. Oct 6, 2003 #1
    In my post entitled, "To Beam Or Not To Beam?", https://www.physicsforums.com/showthread.php?s=&threadid=5152
    I discussed about how the transporter operates by converting matter into energy. What I had in mind is that once matter is converted into energy, that "energy" alone is occupying a bit of space without lurking in a mass or being used by material bodies to produce motion or heat. Could such a thing exists?


    Whitestar
     
    Last edited: Oct 6, 2003
  2. jcsd
  3. Oct 7, 2003 #2

    chroot

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    Mass and energy are directly interconvertible. You certainly can convert all of someone's mass into energy -- pure gamma radiation, for example -- and send it somewhere. Of course, you'll have a hell of a time sorting it out to build a copy of the person due to entropy and all, but hey -- that's your bag, not mine.

    Photons are the closest thing to "pure energy" you can get -- they have zero mass and travel at the speed of light.

    - Warren
     
    Last edited by a moderator: Oct 7, 2003
  4. Oct 7, 2003 #3
    Yeah Chroot nailed that with a ball-point hammer there.

    The fact that feeding energy into a body increases its mass suggests that the mass m0 of a body at rest, multiplied by c2, can be considered as a quantity of energy. The truth of this is best seen in interactions between elementary particles. For example, there is a particle called a positron which is exactly like an electron except that it has positive charge. If a positron and an electron collide at low speed (so there is very little kinetic energy) they both disappear in a flash of electromagnetic radiation. This can be detected and its energy measured. It turns out to be 2m0c^2 where m0 is the mass of the electron (and the positron).


    Thus particles can "vaporize" into pure energy, that is, electromagnetic radiation. The energy m0c² of a particle at rest is called its "rest energy". Note, however, that an electron can only be vaporized by meeting with a positron, and there are very few positrons around normally, for obvious reasons-they just don't get far. (Although occasionally it has been suggested that some galaxies may be antimatter!)

    An amusing "experiment" on the equivalence of mass and energy is the following: consider a closed box with a flashlight at one end and light-absorbing material at the other end. Imagine the box to be far out in space away from gravitational fields or any disturbances. Suppose the light flashes once, the flash travels down the box and is absorbed at the other end.


    Now it is known from Maxwell's theory of electromagnetic waves that a flash of light carrying energy E also carries momentum p = E/c. Thus, as the flash leaves the bulb and goes down the tube, the box recoils, like a gun, to conserve overall momentum. Suppose the whole apparatus has mass M and recoils at velocity v. Of course, v << c.


    A common misconception surrounding E = mc2 is that it entails that the entire rest-mass of a body can become energy. Strictly speaking, mass-energy equivalence only entails that a change in the rest-energy of a body is invariably accompanied by a corresponding change in the rest-mass of the body. For example, a body may lose a bit of its mass because it radiates a bit of energy. The stronger claim that a body may lose all of its rest-mass as it radiates energy is not a consequence of SR. However, this stronger claim is very well confirmed by experiments in atomic physics. Many particle-antiparticle collisions have been observed, such as collisions between electrons and positrons, where the entire mass of the particles is radiated away as energy in the form of light. Nevertheless, SR leaves open the possibility that a form of matter exists whose mass cannot become energy. This is significant because it emphasizes that mass-energy equivalence is not a consequence of a theory of matter; it is instead a direct consequence of changes to the structure of spacetime imposed by SR.
     
  5. Oct 15, 2003 #4

    So, what you're saying is that whenever you're converting a person into energy, you're basically destroying the life of that individual when the conversion into energy takes place. Correct?

    Whitestar
     
  6. Oct 15, 2003 #5

    chroot

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    You can be pretty damn sure of that... after all, do you know any lifeforms that exist as pure photons? I don't.

    - Warren
     
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