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 along the kind of parabolic curve that would be followed by a cannon ball fired up from the Earth’s surface, you will be pushed back into your seat as your rockets fire. But as soon as you cut the engines you will arc around and fall back down again, experiencing weightlessness all the way – even though an external observer will note your rocket decelerating up to the top of the curve and then accelerating on the way back down.

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 trajectory out in the microgravity of space. Well OK, you’ll notice something when you hit the ground – 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.

One should also be cautious about the concept of gravitational energy. It is said that an object gains potential energy with elevation, which then converts into kinetic energy when it falls. However, this implies that object is somehow drawing energy from the gravitational field. Really the only energy in this scenario is the energy used to lift the object.

So how can we account for the acceleration?

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.

Imagine an unfortunate rocket ship captain has dropped off to sleep with his autopilot set to coast at a constant velocity from point A to point B – but suddenly an atmosphere-less planet C interposes itself between A and B. Unaware of anything untoward, the captain happily snoozes on until the collision results in his death.

A collision with a low mass titanium wall hastily erected between points A and B would kill him just as effectively, if his trajectory was halted just as dramatically. What’s different about the planetary collision is that an external observer will see the ship more obviously accelerate as it approaches the massive planet’s surface.

If the captain had stayed awake he might have found it 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 spatial dimensions 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 (relatively) faster and faster.

So – no force, no energy. Just geometry.

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