What if the distribution of particles is Lorentz-invariant ?
The the argument of more impact at the front of Earth
does not imply. Vacuum fluctuations do have indeed
such a distribution (I remember an old Sci.Am. article
about this.) Gravity would then be caused by matter
influencing the...
It is not clear if time travel is indeed possible.
The most plausible idea for a time machine
was proposed by Frank Tipler. A simple explanation
of it is http://yolanda3.dynalias.org/wbpage/wpage.html