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Random Questions on General Physics

  1. Jun 14, 2014 #1
    I don't understand, if everything in this world is relative to something else, then cannot we essentially say that nothing exists independently? We say that the universe is considered to be the ultimate 'background'. However, if we say it is expanding, shouldn't it be expanding relative to something?
  2. jcsd
  3. Jun 14, 2014 #2


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    I'll answer your last question first. It is expanding relative to EVERYTHING.

    Secondly, the word "relative" is used simply to indicate that there are things define relative to some frame. But here, you are also missing several parts of physics in which things are NOT relative. The speed of light isn't relative because it is a constant all the time in vacuum. There are also convariant/invariant forms and quantities that do not change in value.

    But what is overlooked here is that even though things are relative, we and are fully aware how they are relative to. This is because we can easily transform our knowledge to whatever frame we want to be in (other than a frame at speed c). In other words, there is no ambiguity. A particle moving at v=0.9c can be easily studied without any vagueness, and I can cite its mass and no one will ask me "relative to what?" Look at the Particle Data Book if you don't believe me.

  4. Jun 14, 2014 #3


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    To elaborate on the last question, the expansion of the universe is measured and described as distances between all unbound objects increasing over time. On the scale of galaxy clusters and larger, the force of gravity is too weak to hold them together against expansion, so over time the distance between galaxy clusters and superclusters increases over time. Within galaxy clusters gravity is strong enough so that expansion does no take place. So as ZapperZ said, the expansion is relative to everything.
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