Length contraction near the speed of light

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Discussion Overview

The discussion centers on the concept of length contraction as it relates to an astronaut traveling near the speed of light, particularly in the context of the distances involved in traversing the Milky Way galaxy. Participants explore the implications of relativistic effects on both the astronaut's and the galaxy's perspectives, raising questions about the nature of physical laws in different frames of reference.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant suggests that an astronaut could travel from Earth to the far edge of the Milky Way within a human lifetime if traveling close to the speed of light, noting the effects of time dilation and length contraction.
  • Another participant asserts that the stars themselves would also be contracted from the astronaut's perspective, implying that the distances involved would appear shorter.
  • A question is raised regarding how the atoms of a contracted star can fit within a smaller distance without collapsing into a black hole, indicating a concern about the implications of relativistic physics on atomic structure.
  • Further elaboration is provided on the need to transform physical laws from a moving frame to a rest frame, suggesting that classical mechanics already shows differences in descriptions based on motion.

Areas of Agreement / Disagreement

Participants express uncertainty and raise questions about the implications of length contraction and the nature of physical laws in different frames of reference. There is no consensus on how these concepts reconcile with established physics.

Contextual Notes

The discussion highlights limitations in applying classical physics to relativistic scenarios, particularly regarding the behavior of atoms and the nature of physical laws in moving systems. There are unresolved questions about the transformation of physical descriptions between different frames.

chrispegg
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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?
 
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chrispegg said:
How do all those stars fit in such a small space?
They are also contracted.
 
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?
 
chrispegg said:
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?
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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