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Of pancakes and particles

  1. Nov 5, 2004 #1
    I was thinking about relativity after a lecture the other day and I came across something, well, odd.

    Imagine an electron flying at near c. It would look like a pancake that doesn't much care for aerodynamics. How much it resembles a pancake would pop out of the Lorentz equations for its contraction as its speed nears c. So we can say that a particle that is by all means spherical when it is at rest would end up being contracted to zero length in the direction it's travelling should it ever reach c.

    The contraction, therefore, applies to elementary particles. But what makes people think it also applies to macroscopic objects? A macroscopic object is a bunch of microscopic objects. So let the contraction apply to tiny parts of you as you near the speed of light. I'm pretty sure it won't bother me if all the particles in my body take on a slightly more flattened shape. Why should my shape change because of this?

    And most importantly, is there experimental evidence for macroscopic objects flattening like this as they near relativistic speeds?
  2. jcsd
  3. Nov 5, 2004 #2


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    What macroscopic objects travelling at relativistic speeds are there? We have clouds of particles at such speeds, and they are flattened. The effect has to be allowed for in designing targets.
  4. Nov 6, 2004 #3
    Hm. What I mean is this. If there are a lot of particles in a row, going at c. Like this:


    Then they will be flattened individually:

    | | | | | |

    Yet, why should they get closer to one another as their shapes deform and thus contribute to the macroscopic deformation of whatever object they constitute?

    Furthermore, for such a deformation to occur as is predicted (and observed, I guess), all particles except the first one have to move faster to catch up with the first one, no?

    Where does the border between macroscopic bodies and separate microscopic particles go with respect to relativity?
    Last edited: Nov 6, 2004
  5. Nov 6, 2004 #4

    Doc Al

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    What makes you think the Lorentz transformations only apply to observations of elementary particles? The LT apply to all measurements of length and time between moving frames.

    The "flattening" of your body surely won't bother you, since you won't be able to detect it. Length contraction is only observable from a frame that sees you moving. To you, your shape doesn't change. The same reasoning that allows you to apply length contraction to the particles comprising your body would equally apply to the macroscopic dimensions of your body.
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