Imagine a world with elevators that rose independently for thousands of feet into the air. These elevators are solar powered and free to use for anyone. Instead of floor stops the elevators have off ramps that lead away from the elevators in all directions. Some lead to other elevators and some back to the ground but there are so many of them that it is easy to get to where ever you want to go.
The elevators are scattered around the countryside and close enough together that it is possible to start at one elevator, ride up it and take a ramp that goes to another elevator but is sloped so that all you have to do is roll down the ramp with nothing but gravity to propel you. By using successive elevators and ramps, it is possible to travel for as long as you want on no fuel.
OK, that’s a good start for a science fiction story. But let’s add a couple of details to make it more interesting. First, the elevators and ramps are invisible. They work the same but you can’t see them. If you are on one and wander off because you can’t see it, no problem there are enough (invisible) ramps that you won’t fall and you can even take the ramp back onto the elevator, if you can find it.
And one more thing, the elevators don’t start at the ground level. They start at some height above the ground. You can’t tell how high, because they are invisible.
So, how to get to the elevators? One way might be to climb up on to some high place and find a ramp. This (invisible) ramp might take you to an elevator or it might not. Sometimes you might see someone else in an invisible elevator going up so you can take the ramp that goes there. If not, you just have to guess which ramp goes to the (invisible) elevator.
Another way of course is to use some sort of powered machine to take you up to the elevator and drop you off there. This might take wandering around till you ran into an elevator, but it would be more efficient than just rolling down random ramps hoping to get lucky.
All that is fine and the makings for a good game or sport, but it’s not really useful as a way to travel. It’s not practical, for one thing, the elevators have two other characteristics, they are not fixed in place and they are not even always there. As mentioned, they are solar powered so if there is no solar energy, they don’t run and how many there are and how fast or powerful they are; is solely (pun intended) dependent on the solar energy available. Too, they tend to move around since they are not fixed to the ground.
Now, even though this may sound like an imaginary world, it isn’t. It is the real world we live in. And while the conditions make this elevator / ramp system impractical as transportation for us humans, it is in fact the very system used by thousands of flying creatures. The astute reader will by now have understood that I am describing thermal soaring as done by soaring birds and humans in various flying machines, humans for sport and birds as a way of life.
It is often speculated that birds have the capability to “see” the thermal elevators. Whether or not they can, it is clear (again pun) that we humans cannot. For us to get to a point where soaring becomes a practical mode of travel rather than a leisure time activity one thing only is needed, the ability to see the thermals. This is a fairly simple technological problem and the solution is at hand.