"TimeTravel_0 : Altering gravity is not the hard part.
TimeTravel_0 : Detecting gravity is the hard part."
"TimeTravel_0 : A system had to be invented that would "hold" the machine to the Earth.
TimeTravel_0 : Its called VGL.
TimeTravel_0 : Its based on very sensitive clocks and gravity sensors.
TimeTravel_0 : It stops the time distortion machine if radical changes in gravity are detected."
https://metallicman.com/laoban4site/john-titor-and-detail-on-his-time-machine-part-3/
"The most important piece of Titor’s time machine was the VGL, or Variable Gravity Lock system. This was the system that kept the machine (and its passenger) fixed in place in its environment, so it wouldn’t end up floating out in empty space, or melding with any other objects, when it arrived in another time."
https://www.strangerdimensions.com/2013/02/04/john-titor-the-man-with-the-machine/
"The "energy receiver" (gravitational energy converter) had been built by Tesla himself"
http://waterpoweredcar.com/teslascar.html
"A gravitational wave is an invisible (yet incredibly fast) ripple in space. Gravitational waves travel at the speed of light (186,000 miles per second)."
https://spaceplace.nasa.gov/gravitational-waves/en/