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Astronomers observe some galaxy

  1. Dec 17, 2004 #1
    Hi!

    Let's say that astronomers observe some galaxy or something several million lightyears away and then come to conclusions about that object. But since the light from there has been on the way for millions or even billions of years, then is it correct, that they are actually seeing the object as it was back then? So any conclusions they may draw from these observations apply to that object as it was a long time ago and not today?

    So my questions is: what effect, if any, does this have on science? For example when they try to fit together data from current lab experiments and observations of ancient evets, i.e. they sky must have changed since then and so any theories we apply to those events only describe those ancient things and not the current universe (in the cosmological sense)?

    - Kamataat
     
  2. jcsd
  3. Dec 17, 2004 #2

    Garth

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    Yes indeed we are seeing distant galaxies etc. 'back then', and this has to be taken into account when observing the very furthest galaxies, say on the Hubble Ultra Deep field plate.

    However if we ask, "What are they like 'now'?" we have to ask what do we mean by 'now'? Relativity theory measures the interval across space and time between the object itself and the observation of that object to be zero - as far as the photon is concerned it has made that vast journey instantaneously. So the 'back then' that we see is really the ' here and now'!

    Garth
     
  4. Dec 17, 2004 #3

    Chronos

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    Dang Garth, you could cause some innocent soul to blow out a brain fuse talking that way :biggrin:. Because of inflation in the early universe and subsequent expansion, we see distant galaxies as they appeared when the universe was younger. The scientific value of this is in the ability to compare the appearance of the universe in earlier times to its appearance in more recent times. This gives us clues how objects and structures emerged and evolved in the universe. Think of it as the cosmic equivalent of fossils. By studying ancient fossils we obtain clues how life emerged and evolved on earth.
     
  5. Dec 17, 2004 #4

    selfAdjoint

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    Only to the photon. The rest of us, not moving at c relative to the galaxies, see it as billions of years. And yes we are not moving at c relative to the galaxies, even the ones for which the distance is increasing faster than c. Space expansion is not speed.
     
  6. Dec 18, 2004 #5

    Garth

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    Chronos - SelfAdjoint - Of course!

    Garth
     
  7. Dec 19, 2004 #6
    ok, thanks

    - Kamataat
     
  8. Dec 21, 2004 #7

    Garth

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    Let me expand a bit.

    Use our own world line as a frame of reference, to foliate space-time into a series of time-like slices, then:

    If we ask what is here? (With no specification of when) The answer is to look in the immediate vicinity, at all those events whose space coordinates are 'displaced' from ours by only a 'small' amount. But they could be ages in the past or in the distant future, so we do not generally experience them.
    Small is arbitrarily small, tending to zero
    If we ask what is happening now? (With no specification of where) The answer is to look at all those events whose time coordinate is 'displaced' from ours by a small amount. This generates a surface of simultaneity, as defined by our own fame of reference, but it is actually unobservable and so we cannot in general experience these events.

    What we are experiencing is the here and now, which includes those events that are displaced across space and time by only a 'small' amount. However we also observe the rest of the universe along our back light cone, all the way out and back to the surface of last scattering of the CMB. All these observed events are separated from us by zero 'displacement' across space-time and could be said to be 'here and now' for us!

    (Using the SR metric)
    ds2 = dx2 + dy2 + dz2 - c2dt2 so along the light cone ds2 = 0

    We do experience them by seeing them!
    Just a thought
    Garth
     
    Last edited: Dec 21, 2004
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