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Is space contraction uniform?

  1. Jan 5, 2016 #1
    Hello, from time to time I got question in physics for which I can't readily find answers. I hope I can get answers here. Here is my current one: Usually textbooks state that space contracts in direction parallel to direction of motion. It sounds strange, because time changes uniformly, and as I understand it space should contract in the same way in orthogonal directions too. - otherwise (to my understanding) some lows of physics will be different.
    Example: in relation to earth all planets are spheres , but for an object moving from sun to earth, if planet will "contract" only in one direction it will became elliptical. But planets are spheres because lows of physics determine their spherical form , so IMO space should contract uniformely.
    Please help
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  3. Jan 5, 2016 #2


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    If you got something like Lorentz contraction in the direction perpendicular to the motion it would be rather surprising. It would mean that me moving in one direction would start other things moving perpendicular to me as they "width contracted" with the motion. I'm not sure I could see an explanation for that.
  4. Jan 5, 2016 #3


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    Space does not contract as much as different observers have different notions of what "space" is. Note that, like time dilation, length contraction is symmetrical.

    That planets are spheres is not a law of nature, it is a result of how gravity affects an object at rest. This will work differently in a frame where the planet is moving, but the underlying laws are still the same. Note that your chosen example is a bad one, gravity is not included in special relativity and you will need general relativity to make sense of it. You cannot simply try to apply Newtonian gravity - it is not Lorentz invariant.
  5. Jan 5, 2016 #4


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    That would violate the first postulate (equivalence of inertial frames). Say a very fast spaceship flies through a hoop. It just fits through the hole in the frame of the hoop. But in the frame of the ship, the hole would be of smaller diameter, while the ship would be wider, so it wouldn't fit through. Both frames must agree on whether the ship fits through the hole or crashes into the hoop.

    All the force fields, which lead to a spherical shape in the rest frame of the planet, are contracted in other frames. So there is no contradiction between the flatten shape and what these forces result in.
    Last edited: Jan 5, 2016
  6. Jan 5, 2016 #5


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    The laws of physics do not predict a spherical shape for all planets. In order for a planet to be spherical the laws of physics require that it be made of a fluid, in equilibrium, non rotating, sufficiently large, and free from tidal forces. If it is rotating it will be ellipsoidal. If it is moving fast it will be ellipsoidal. If it is under tidal forces then it can be a multipole shape. If it is not in equilibrium or if it is solid then it can have much more complicated shapes.

    The laws of physics do not require spherical planets. It is just that you are unfamiliar with this particular law.
  7. Jan 7, 2016 #6
    Ok, that helps.
    I got some understanding of how planets evolve and I brought this example of planets in order to illustrate a contradiction which I was unable to explain. Of course planets are spherical not because it is a law of nature, but because under conditions you mentioned they will evolve into spheres. And that brought me to a conclusion that if I notice that a planet comes closer to me fast and is elliptical, so that's because I am moving and that means that I process of planet evolution works differently in my frame of reference, which can't be.
  8. Jan 7, 2016 #7


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    Depends on what you mean by "works differently". All the particles that form the planet and their fields are length contracted in your frame, so they will evolve into a contracted planet.
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