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  • Physicists tell us that the universe is expanding, but does that mean we expand with it? I mean,

  • if so, wouldn't no one notice because everything would scale up at the same relative size and

  • it would seem like nothing had changed?

  • Well, the truth is, the universe isn't expanding. Space is. Of course, everything else in the

  • universe is "in" space, and ignoring the influence of any other forces, when the space between

  • things expands, those things will appear to be carried farther apart. But there are other

  • forces in the universe, and their strengths aren't directly affected when space expands.

  • For example, gravity and electromagnetism are able to hold atoms and solar systems together

  • because these forces are too strong for the slow expansion of space to overcome.

  • Think of it like this - if you and your cat are standing on a piece of ground that starts

  • to stretch horizontally between you, the ground will pull you and your cat away from each

  • other. But if the leash is strong enough - in particular, if it's stronger than the friction

  • force between your feet and the ground - it'll hold you and your cat together as space expands

  • beneath you.

  • In the real universe, it's the strength of the electromagnetic force that determines

  • the effective sizes of particles, atoms, and molecules so that even if the space they're

  • in expands, the molecules don't. Electromagnetism is basically the leash that holds atoms and

  • molecules together - and gravity is the leash that holds solar systems and galaxies together.

  • Only over super super long distances is gravity's attraction weak enough and the cumulative

  • expansion of all that space big enough to cause galaxies and galactic clusters to actually

  • grow more distant from one another over time - we know this because the light from distant

  • galaxies gets red-shifted as it's stretched by the expansion of the space it's traveling

  • through. Gravity certainly still pulls distant objects towards each other, but space expands

  • faster. Basically, the cat is off the leash.

Physicists tell us that the universe is expanding, but does that mean we expand with it? I mean,

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B2 space expands leash universe gravity expansion

Do We Expand With The Universe?

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    Halu Hsieh posted on 2013/08/26
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