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Question about movement

  1. Mar 11, 2006 #1
    Ok this is kind of a weird question...but:

    Assuming that something on earth that is not moving is still in reality moving because of the planet's rotation around the sun, and assuming also that even the sun which seems to be stationary is also moving about the galaxy, and so on and so forth on to galaxies and clusters, etc., couldn't it be said that nothing can ever be stationary?
    At some point in the logic, could it be said that in order for something to be really stationary it would have to be moving at the same speed and in the same direction as the expansion of space?
    I know this is a lot of assuming but just bear with me.
    So, what if the above was true, and what if something, by nature, could be made to simply stop "moving"? Depending on the relative speed of space's expantion, couldn't that thing be moving, relative to us on earth, at possibly very great speeds?
    Obviously this is a ton of conjecture, but what of it anyway?
  2. jcsd
  3. Mar 11, 2006 #2
    All motion is relative in the sense that something can only move with respect to something else. I.e. there is no one natural reference frame we can use to measure absolute movement.
  4. Mar 11, 2006 #3


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    Then let's make MORE conjectures.

    What if our physics doesn't care and looks the same in all inertial reference frame?

    What if our measurement of velocity only depends on the frame we measure with?

    What if every inertial frame is equally valid in describing every phenomena that we have encountered?

    What if Albert Einstein has already made such similar conjecture and called it Special Relativity?

    As da_willem already mentioned, we have made no detection of an "absolute" space or reference frame. This means that you cannot say something is "stationary" without invoking the reference frame in which that object is being observed.

  5. Mar 13, 2006 #4

    ok well I guess I shouldn't expect a concise answer if my question is so...not concise. I suspect that in order to answer the question you need a few assumptions, one of which might be that the universe is finite, but expanding...in which case there would be an inertial reference frame, only we wouldn't really know because there would still be an illusion of things moving and not moving, regardless of whether or not anything was in reality moving or not, with respect to our own planet/solar system/galaxy/cluster/etc.
    I'm no expert by an means but I seem to remember quite a lot of leaps in logic (dark matter) in the BBT just in order to make the universe possibly infinite, but that's beside the point. I only mention it in a feeble attempt to make my own ludicrous question seem more valid. Anyway thanks for trying.
  6. Mar 13, 2006 #5


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    The "expansion" of the universe that you have refered to is not as simple to fathom as you think. When cosmologists use such terms, it is in reference to the General Relativity scalling value. This "expansion" is an expansion of the spacetime manifold. It isn't an expansion into a preexisting space, the way a balloon expands as you add more gas. So it is the very frame that we use to designate our space and our time that is expanding.

    The key thing here is not to be tricked into thinking that the terminology that is used in astronomy and physics has the same connotation as that used in everyday language. That has always resulted in numerous errors of judgement.

  7. Mar 13, 2006 #6
    I find this to be very interesting. And I think that you are correct.
  8. Mar 14, 2006 #7
    I think I understand that point. What I don't understand is why cosmologists, who use such terms, seem to think that they can explain how spacetime can expand while not expanding into anything, yet they do not do so. Isn't this terminology dependant on the idea that expansion, by definition, would mean that space, as a non-perfect vacuum, containing some mass, could not "create" itself in a new environment (because that would defy the laws of thermodynamics), and as such could only expand if the "space", whether it be of another nature or whatever, was there to expand into? If not, then the definition of spacetime itself would constantly be changing, in which case calling it "spacetime" as a constant seems to be sort of a contradiction. But that is niether here nor there, so I have to assume that this is one of those "given"s that comes along with most of the cosmological theories. Which is fine.
    I am not questioning the theory of spacetime expanding, niether am I asking if you agree if it is finite or infinite or expanding or not or if you really understand said expansion. I'm only asking 1. IF it is assumed that spacetime is expanding, and 2. IF it is assumed that space is finite: isn't it true that nothing could be said to really be stationary unless it was moving in the same direction and at the same speed of the expansion of space, since the definition of spacetime changes to accomodate its expansion, and that this thing which is moving synchronously with spacetime could, relative to us, be moving at very great speeds depending on the speed of spacetime's expansion, even though by the only true definition under these assumptions, it is stationary?
    I know this sounds like I think I know what I'm talking about, but I truly am simply asking a question because I don't know. I'm trying to be as clear as possible so that hopefully you guys who know things can enlighten me. Thanks again.
  9. Mar 14, 2006 #8


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    I don't know anything about general relativity, so I'm not going to tackle this "expanding spacetime" point you have brought up. By the same token, I think it might be moot. Just to reiterate something that others have mentioned, but perhaps not emphasized enough. The short answer to your question, "can anything be said to really be stationary" is NO. Not if you mean stationary in some absolute sense, i.e. in some preferred rest frame. There are no preferred frames. Something can only be considered stationary as measured with respect to some inertial frame of reference, and if it is stationary with respect to one inertial frame, it will not be stationary with respect to others.
  10. Mar 14, 2006 #9
    Agreed. To my knowledge, science has not found a primordial static reference point.
    After the "beginning" everything is "moving"
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