> Gravity's influences are all we can measure. We have tried
> to create gravity wave measuring devices, but so far the
> observations are unrepeatable.
> But Gravity's indirect observation is not the least bit
> surprising. MANY natural phenomena are observable only
> through their influences on other objects, including radio
> waves, magnetic fields, ultrasound, radioactivity,
> ultraviolet and infrared, neutrinos and apparently dark
> matter and dark energy.
that's the way physics works...
you can't measure something directly
if you measure the position or momentum of a particle, you're not measuring it directly, you hit the particle with another particle (tipically a photon) and from the particle you get back, you can get the observable of the other one, there's no way to measure any property of a particle/field (which is almost the same thing, particle are excitation of the vacuum state of the field) directly, it would require the existence of a privileged observer, which general relativity forbids (and this is an argument that can be used to physically disprove the existence of God as a privileged observer, otherwise he's not subject to physical laws, se we will never be able to interact with him)
and anyway, measuring a property of a particle requires interacting with it, otherwise its wavefunction would not collapse and you would not measure anything (the same concept of 'measuring' is based on wavefunction collapse, otherwise it's undefined, or we can say "un-thinkable")
every single quantity you can "measure" in physics is measured indirectly, otherwise the quantity you're measuring is not in a defined state (unless you measured it before and caused the wafefunction to collapse, in this case the same state would be held forever in the absence of interaction with external field or time evolution, a condition which is almost impossible to realize in an experiment, but can be approximated with extremely low temperatures, and that's why we're still far ahead from fully functional quantum computers)
well, that's just my contribution as a theoretical physicist to the discussion, there are words (like "measure", "position", "quantity", "effect") in physics which have quite a different meaning from their everyday usage, this is true expecially with modern physical theories
