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Expanding and Accelerating Space in Layman's Terms

  1. Oct 18, 2007 #1


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    Expanding and accelerating space in layman’s terms.

    First, Einstein:
    You and I buy two really nice watches. They are accurate to within a billionth of a second per year, and will calculate to the trillionth of a second. You and I are standing next to each other as we synchronize our new watches. Now, I run away from you for 20 feet and then stop. We check our watches. What is this, they are no longer synchronized! As we move away from each other, our frames run at slightly different rates. This is space-time.

    Expanding space:
    Now you and I find the “zero frame” somewhere in the universe that runs at the same speed as the universe. I think it is actually called the “privileged frame”. Anyway, we enter this frame and find it to be a cube with internal dimensions of 20ft X 20ft X 20ft. I stand with my back against one wall and you stand with your back against the opposite wall. Again we synchronize our watches to each others, and to the frame. We stand there perfectly still for a year. We check our watches again and we find they are still perfectly synchronized to each other, and to the frame. We measure the distance between us and find it is 50ft. What is this? You and I have not moved in relation to each other, or to the frame (our watches prove this), and yet the distance between us has increased! Somehow space has just “welled up” between us.
    What did the frame expand into? The wall that I have my back against has not moved at all. The wall that you have your back against has not moved at all. The frame did not expand into anything!

    You and I decide to stand perfectly still for another year (even though I am tired of your complaints about bathroom breaks by now). At the end of this second year we check our watches. They are still synchronized with each other, and with the frame. We check the distance between us and find it to be 1000ft! What is this? The rate of expansion has increased. It is as if the more space there is between us to begin with, the more opportunity for new space to “well up”. Empty space begets more empty space.
    If we decide to stand there for a few hundred more years, we will be “moving” away from each other at speeds greater that C, maybe several times greater, yet our watches are still synchronized! Neither of us has moved!

    Please comment. Thank you.
  2. jcsd
  3. Oct 19, 2007 #2


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    In the beginning, the universe expanded FTL. After resting for about 5 billion years, it decided to do it again. BTW, your watch thing is . . . incorrect.
  4. Oct 19, 2007 #3
    Just to expand on the previous post the inflationary period was caused, or was supposedly caused, by the decoupling of the electro weak force from the other forces which released a whole heap of energy which went into spacial expansion at a rapid rate (faster than the speed of light). This more or less solves the problem of having objects 13.7b LY in one direction and 13.7b LY in the other direction but they cant see each other.

    This...doesnt make sense. Im not sure the universe has a speed moreover a central point or central reference frame. In fact im pretty sure the whole point of SR is to say there is no absolute reference frame only relative ones.

    Expanding space is a problem for some people because they dont understand whats actually happening. If two galaxies have a large cosmological redshift this is not because they are flying apart at amazing speeds (although it seems that way, i think) instead more space is being 'inserted' between them so the distance is increased and thus you have the illusion of relativistic velocity when in fact IN SPACE there has been no movement and so relativistic effects dont apply and relativity holds (ie expansion can go faster than light but we cannot). Acceleration of this would require large amounts of energy and is thought to be fueled by so called dark energy which is proving elusive.
  5. Oct 19, 2007 #4


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    With a username like Chronos, I am going to have to get your input about the watch part being wrong. Please elaborate if you don't mind. Thank you.

    If space just appears between two objects, you still get time dilation (or red shifting)?
    Last edited: Oct 19, 2007
  6. Oct 19, 2007 #5
    You get cosmological redshift as the photon is stretched in transit rather than during its 'creation' but you do not get time dilation (i dont think).
  7. Oct 19, 2007 #6


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    But how do you check each others watches? You are far enough apart that there is a measurable light-speed delay between you. You must take that into account in the simultaneity of your checking.

    The gravtiational attraction - even between two small bodies suxh as yourselves - easily overwhelms the expansion of space. Before the year is out, the two of you find yourslves gently bumping into each other in the middle.

    This is not incidental. It is critical to understanding why galaxies are not torn apart by the expansion of trhe universe.

    Again, how do you check? This is not trivial.

    You may bat this around, refining and modifying the thought experiment, but ultimately, you will find that our measurements of time and space are not independent of the space-time in which we conduct them.
    Last edited: Oct 19, 2007
  8. Oct 19, 2007 #7


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    There is no 'privileged frame' in either GR or SR. On the other hand in the Friedman-Lemaitre-Robertson-Walker (FLRW) solution of GR, which describes a Universe that is isotropic (looks from each point the same in all directions) and homogeneous (the same conditions everywhere) you do have a set of privileged observers known as 'co-moving' observers. This is not a 'frame' (which has a specific meaning that is not in accordance to your use of it above). This set of observers are 'at rest' in the co-ordinates of the solution and 'move apart' solely due to the overall expansion of the Universe. In practical terms these observers would see no CMB dipole anisotropy, which is simpler terms means the relic microwave radiation in the Universe looks the same in all directions. For the Earth this is not true due to local motions and hence we see the CMB on one side of the sky slightly blue shifted and the other slightly red shifted.

    But this is not the point I'd like to make. The big misconception that you are perpetuating here is that the expansion of space causes anything at all to happen. Expansion of space is an effect rather than a cause. To be specific, if two test observers sit at rest with respect to each other in an expanding universe they do not subsequently drift apart at the rate of 'the expansion of space'. They will only do so if they also have initial velocities with respect to each other that match the recession velocities of the co-moving material at the distance they are apart. Put simply, they will only move apart if they did so to begin with. In fact in an expanding but decelerating universe observers at rest with respect to each other initially will move towards each other, rather than away.

    The expansion of space is a description of the properties of a Universe in which everything is moving apart. It is not a casual force that actually does anything in and of itself.

    For more detailed info see Expanding Space: the Root of all Evil?

    One last point is that your assertion that the test observers in this and the accelerating case will continue to have syncronised watches is incorrect. As noted above you can't sweep the issue of how you would verify the syncronisation under the carpet. In fact, the further apart the two observers get, the greater the redshift between them. Redshift can be thought of as a time-dilation effect, since it is a change in the observed frequency of light. If one observer sees another's clock ticking at a different rate then clearly the frequency (cycles per second) of something they both observe will be different, hence redshift of light.

    For instance a particular type of Supernovae (exploding star), known as Type 1A have properties that are consistent between each supernovae. The two most important of these is the peak brightness (the brightest that they get during the explosion) and the length of time the explosion takes. What we see however is that the length of the explosion gets longer with the observed redshift of the Supernovae by precisely the amount you would calculate based on the time dilation interpretation I described above. The explosion takes the same time to anyone at the supernovaes location, but the clock of this observer will be different when viewed by a distant observer and hence the explosion will be seen to take a different amount of time.
    Last edited: Oct 19, 2007
  9. Oct 20, 2007 #8


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    In the beginning, all watches read zero. Two observers sitting at the middle of this maelstrom noticed their watches were perfectly sychronized. They subsequently noted they could not communicate instantaneously and their watches were no longer reliable. I do not find that surprising. The time component of the universe broke their watches.
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