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Space Expansion/Time Travel (#IMAN00B)

  1. Jul 27, 2011 #1
    So I'm just getting into physics to research for a short story idea I've had for a while, but I've got a few questions I'm having difficulty finding an answer to.

    As the universe expands, does space expand? Like, if someone took a marker and drew a small star on a balloon and then inflated it? The balloon would expand, and so would the star.... which brings me to my next question. If I'm correct and space is expanding, someone that wanted to time travel back to Roman times, would be going back to a time when A) the universe wasn't as spread out as it is today (the balloon is less inflated), so the earth would be in an entirely different place than it is now (not to mention a different place in it's orbit) B) Space itself would be more compact ... so even if they could get to the right place, they would be giants?...

    Clearly I'm a n00b, and really have no idea what terminology to use to even ask my question properly. So please don't be too hard on me for not making sense. If you could just direct me to a few n00b friendly books to read, I'd be eternally grateful.
  2. jcsd
  3. Jul 27, 2011 #2


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    Some time ago Earth was the same size as it is now, and the size of Earth orbit was the same (with regard to small change caused by loss of Sun mass due to emission of light and solar wind). Balloon got expanded, but your small star is made of hard plastic and glued to its surface. All 'small' distances are defined by some forces keeping particles on fixed distances. The distance between neutron and proton in deuterium nucleus results from strong nuclear force, not from Universe geometry. The sizes of atoms, molecules, then sizes of material objects (your foot and whole Earth) are defined by electromagnetic forces between molecules. Distance from Earth to Sun is defined by Kepler laws and conservation laws.

    You touched quite deep question: what 'distance' mean? What does it mean that Earth circumference is 40,000 km?
    In beginning of 19th century the answer was: that is how we define what 'metre' means (1/10,000 of distance from north pole to equator over the Paris meridian).
    In the beginning of 20th century the answer was: you may take a measuring rod from Sevre vault and lay it 10,000 times along the same path. Both the rod and Earth should be distorded the same by any changes to space curvature, so the measured distance must remain unchanged.
    This definition of 'metre' is closest to common meaning of distance. Natural units for people are those related to dimension of commonly found objects - e.g. their own feet. As all matter would be equally scaled, the dimensions of physical objects remain the same. Both Gulliver and Lilliputs were about 6 feet tall - their feet. If all dimension scale - there is no way to notice that except of comparison with something which is not affected by rescaling.

    Modern definition of 'metre' relates it to the distance light goes in some part of second, while the second is defined as some number of oscillations of Cesium atom. You may ask if such definition might have an effect. In order to do so, some physical 'constants' (like speed of light, electron charge, Planck's h, etc) would have to change along with Universe expansion. Such hypotheses were considered probable and lots of measurement had been done to test if they are really constant. No measureable changes had ever been detected - neither in short term (few years lasting) very precise measurements, nor when comparing billions years old geological and astronomical data with expectations made using modern values of those constants in the apropriate models. Billions years old rocks have such properties as chemistry (using modern values of physical constants) expect them to have.

    We believe all local experiments performed billion years ago would lead to the same results as they give now. The only difference is measured distance to far cosmic objects - where the distance is not related to forces/interactions between objects, but rather caused by inertia acting since beginning of the Universe. Small stars glued on balloon remains unchanged, just distance between stars increases. To make this metaphore closer to astronomic observations: don't paint/glue stars, but galaxies, or rather galaxy superclusters.
    Distances between stars within single galaxy are also defined by local interaction - Keplerian orbiting around galaxy centre.

    Romans had smaller feet than we have, and Lucy had even smaller, so the Earth was acually bigger then (if measured in feet) than now. But I won't attribute this to expansion of Universe :tongue2:
    Last edited: Jul 27, 2011
  4. Jul 27, 2011 #3


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    No. This is implicit in xts's response but I think the talk about scale difference could have made it a bit hard for you to get that.

    Time travel is pure fiction so if you're going to defy physics in that regard (as many SF writers have done very successfully) your audience is likely to give you some leeway in other areas but it is wise of you to try to get the basics right since one of the successful tenets of SF is to only ask your audience for a willing suspension of disbelief in ONE thing and you've chosen time travel as that thing. It would be good if nothing else in your story defied physics.
  5. Jul 27, 2011 #4


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    Fluxhavoc, I suggest hitting up Wikipedia and getting familiar with many of the articles on basic physics, quantum physics, and relativity. You don't need to worry about any of the math or equations really, just get a general understanding of the basics. It really helps!
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