The axiom that what goes up, must come down doesn’t apply to most places in the universe, which are largely empty space. For most places in the universe, what goes up, just goes up. On Earth, the tendency of upwardly-mobile objects to reverse course in mid-flight and return to the surface is, to say the least, remarkable.
It’s even more remarkable if you go along for the ride.
If you launch in a rocket you will be pushed back into your seat as long as your rockets fire. But as soon as you cut the engines you will experience weightlessness as you arc around and fall back down again, following a similar path that a cannon ball fired up from the Earth’s surface would take. And remarkably, you will continue to experience weightlessness all the way down – even though an external observer will observe your rocket steadily accelerating as it falls.
Now consider a similar chain of events out in the microgravity of space. Fire your rocket engines and you’ll be pushed back into your seat – but as soon as you switch them off, the rocket ship will coast at a constant velocity and you’ll be floating in free fall within it – just like you do when plummeting to your accelerated doom back on Earth.
From your frame of reference – and let’s say you’re blind-folded – you would have some difficulty distinguishing between the experience of following a rocket-blast-initiated parabolic trajectory in a gravity field versus a rocket-blast-initiated straight line trajectory out in the microgravity of space. Well OK, you’ll notice something when you hit the ground in the former case – but you get the idea.
So there is good reason to be cautious about referring to the force of gravity. It’s not like an invisible elastic band that will pull you back down as soon as you shut off your engines. If you were blindfolded, with your engines shut off, it would seem as if you were just coasting along in a straight line – although an external observer in a different frame of reference would see your ship turn about and then accelerate down to the ground.
So how do we account for the acceleration that you the pilot can’t feel?
An improvement on the standard two dimensional rubber sheet analogy for curved space-time - although it still lacks the contribution of the all-important time dimension.
Without a blindfold, you the pilot might find the experience of falling in a gravity field a bit like progressing through a slow motion movie – where each frame you move through is running at a slightly slower rate than the last one and where the spatial dimensions of each frame progressively shrink. As you move frame by frame – each time taking with you the initial conditions of the previous frame, your initially constant velocity becomes faster and faster, relative to each successive frame you move through – even though from your perspective you are maintaining a constant velocity.
So – no force of gravity, it’s just geometry.
