Physcs
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Perpetual motion machine inventors do have principles. Unfortunately the physical principles they assume are often ones not obeyed by nature.
The Museum of Unworkable Devices Physics Gallery
What causes gravity
Unfortunately, my next statement may confuse rather than enlighten as the concept of dark energy has not yet been clearly defined and I may be guilty of hijacking the term.
Universe Today — Space and astronomy news
SpaceX Dragon approaches the ISS on COTS 2 test flight slated for May 19 liftoff and docking on Day 4.