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Length Contraction

  1. Oct 31, 2005 #1

    Just one quick question. What happens to the molecular structure of an object if it moves close to the speed of light? Does the high speed affect the atoms, protons, neutrons, electrons, molecular structures? Does the density/thickness of the object increase? Or do the protons and electrons contract as well?

    Also, if the mass increases because of the high speed, how does it affect the molecular structure? How can it have bigger mass then? Does it have more atoms and molecules then?

    Last edited: Oct 31, 2005
  2. jcsd
  3. Oct 31, 2005 #2


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    You've missed the point of what relativity is telling you. E = mc² says that the mass of an object is a measure of how much energy (both mass-energy AND kinetic energy) it contains.

    It is sufficient to increasing the kinetic energy of the molecules composing an object to see its mass increase.
  4. Oct 31, 2005 #3
    Nothing. Consider the object to be at rest and the observer moving.
  5. Oct 31, 2005 #4
    Length contraction is a consequence of describing the coordinates of a body in a frame of reference in which it is moving close to light speed. It says nothing about the body itself, except the fact that it is moving very, very fast in that frame. Such a frame can be chosen such that you are moving close to lightspeed, without having to accelerate you to such speeds. In that frame, you will seem to undergo length contraction in the direction you are moving.
  6. Oct 31, 2005 #5

    Physics Monkey

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    Consider a frame where you are at rest with your lab equipment and you have accelerated a molecule to some velocity.

    More atoms? No, the number of charges is fixed, accelerating the molecule won't change the number of atoms.

    More mass? No, mass in the modern terminology means rest mass (or more generally, center of mass energy).

    More energy? Yes, the molecule speeds up and so it has more kinetic energy.

    Does the shape distort? Yes. The molecule is bound by electromagnetic interactions and these interactions change when you set a molecule in motion (the response of the atoms also changes). All this is captured elegantly in Einstein's length contraction formula (provided you use it correctly). See for instance Bell's discussion in "How to Teach Special Relativtiy" in his book Speakable and Unspeakable in Quantum Mechanics.

    In a frame moving along with the molecule, nothing happens to the molecule, but the world around suffers the same effects I mentioned above.
    Last edited: Oct 31, 2005
  7. Oct 31, 2005 #6
    Thanks for the answers. This has helped.
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