Length contraction near the speed of light

  1. It is my understanding that an astronaut could get from earth to the far edge of the Milky Way within the length of a human lifetime if she were traveling close enough to the speed of light. From earth’s perspective, time would pass very slowly for her as she moved across the galaxy. From the spaceship’s perspective, the length of the galaxy would contract in her direction of motion with respect to the galaxy.

    Therefore, it seems to me that, from the astronaut’s perspective, the distance between earth and the far edge of the galaxy would be very short (comparatively speaking) in her direction of motion. But, of course, the number of stars in the Milky Way that she will pass on her journey will not change.

    How do all those stars fit in such a small space?
  2. jcsd
  3. A.T.

    A.T. 6,286
    Science Advisor
    Gold Member

    They are also contracted.
  4. Yes, but hasn't all of physics changed from the spaceship's perspective? How, for instance, do all the atoms of a contracted star fit within such a short distance without collapsing into a black hole?

    Does the astronaut have to teach her child a different kind of physics regarding the size of an electron orbit around a hydrogen atom, etc?
  5. Many laws of physics, such as the one that you hint at, are formulated for systems that are in rest; that allows for the simplest formulation. They should not be applied to moving systems, instead the moving system description must be transformed to that of a rest frame. This is to a lesser extent already the case in classical (Newtonian) mechanics: the orbits of planets in a moving solar system are not ellipses but spirals. And of course, this problem does not occur for the electron orbits of the spaceship's atoms.
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