Imagine bring in a boat drifting down a river. You can't determine any motion internally only by external reference as the river & you move at the same speed. Similarly, free-falling with gravity you can't feel it so it must be going at the same speed as you. Try going faster or slower & gravity drags on you just as a river does.<br />In a space ship sling-shotting around a giant planet you would feel absolutely nothing, despite increasing or reducing your speed & even changing direction! Similarly aiming directly at the Sun you could not know your speed except by external reference.<br />How can this be? Well it must be because the speed of gravity is a second constant of the universe & is zero. However we measure it the answer is always the same just as light.<br />The problems now are: we can't make any measurement as nothing I know measures zero; our maths, of which I've complained for most of my life, can't deal with zero, except as a trick.<br />The universal constant of gravity was measured here in the UK by use of a plumb-bob deflected by a mountain. This leads us to think it is contained within mass but can't be true else a black hole would be just as invisible to gravity as light, since neither can escape. Therefore it appears to be a property of space which mass, somehow, is able to dent. Just as space seems to keep the speed of light C constant does it keep the speed of gravity G? constant.