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Owing to the relative weakness of gravity, I would have assumed that the gravitational waves detected by LIGO couldn't expand / contract the nuclei of atoms (governed by the strong interaction) or atoms or molecules (held together by the electromagnetic interaction).

Is this right? Or are the waves in spacetime more powerful / fundamental than the strength of these interactions?

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Gravitational waves can affect both of those, but there are reasons why we essentially wouldn't notice:

  1. These are small structures to begin with, so the expansion is minuscule. Remember, we needed to make a kilometres-long path to measure any displacement in the first place.

  2. Even if they are expanded, the attractive forces would just make them shrink again. Spacetime is expanding everywhere, but galaxies aren't becoming larger because their gravity is holding them together.

Spacetime is very fundamental, so we expect it to affect everything. However, this is straying into the realm of QM, so the predictions of GR might not be appropriate at this level. But GR does predict that everything expands, because everything is in space, and space expands.

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  • $\begingroup$ Unless the strong force is stronger than the force expanding space? $\endgroup$ – Amphibio Mar 9 '16 at 21:07
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A gravitational wave (GW) would effect everything that it passes, including other gravitational waves that cross it.

Except some/hypothetical entity - This entity has to be completely rooted in itself, i.e. absolutely non-empty, meaning there is no space inside the entity. A GW would not change size of such entity but it can possibly change its position in space. Singularity may be such a rooted entity.

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