Did you ever hear about Buridan's donkey? It is one of those philosophical paradoxes that keep profound thinkers deeply engaged and gainfully employed for decades. The idea is that a donkey that is both hungry and thirsty is placed midway between a pile of hay and a pail of water. Being both parched and ravenous, this bewildered beast of burden should die of hunger and thirst because it is equidistant from both the food and water and shouldn't be able to decide if it should eat or drink first. Somehow, though, if someone managed to actually try this experiment, the donkey would certainly choose either the water first or the hay, depending if its hunger is of greater or lesser intensity than its thirst. Or, maybe the donkey would simply walk toward whatever it sees first. In reality there is always some subtle influence that perturbs any perfect balance.
The Exploratorium literally has billions of options of how to spend any given moment. That might seem a sublime state for the time-stuck and gravity burdened, of course. However, when we make a choice as to how to spend one minute, we forego innumerable other experiences we could have otherwise chosen. Like a solitary charged particle trapped in the center of a perfect circle of opposite charges, we should be suspended ghost -like within the infinitude of all space-time coordinates, unable to incline toward anything.
But, we're not suspended at all , because we exist in a virtual state. So, let'sjump off a cliff. Specifically, we have decided to jump off Verona Rupes, the highest known cliff (approximately 20 miles) in the solar system. Don't fret if you've never heard of it. Verona Rupes is not on Earth, but is instead on Miranda, a bitterly frigid ice moon of Uranus.
See? We are hardly moving now. One can ascribe this lethargic descent to Miranda's low surface gravity. Out here, the moon is drawing us down at an acceleration of 0.079 meters per second squared: (about 0.25 feet per second squared). If you were to leap off Earth's tallest cliff (on Mount Thor), you would plummet toward the ground at 9.8 meters per second squared (or, 32 feet per second per second.) You would rapidly crash to terra firma with a bone-pulverizing thud. Here, along Verona Rupes, we shall be trapped in a 12 minute free fall that will eventually send us careening into the unyielding ice at about 200 km/hour, a wallop that would prove fatal were we not virtual.
You have certainly noticed that even though our initial descent was almost comically slow, the acceleration is gradually, but inexorably, increasing our speed. Notice also that this rugged little ice moon lacks any atmosphere so we are descending vertically without any of that complex turbulence one might experience when plunging through Earth's fluid atmosphere. Falling through a fluid not only makes the free fall math a complicated mess, but it also impedes the descent. Nothing is impeding us as we waft gradually down toward the....
Ok, since now this would be the perfect time for a lot of cheap, tawdry poetic passages pertaining to scintillating ice fields bejeweled by the Sun's luster. Alas, since the point of this plummet is to discuss astronomy, we can dispense with the bombast and explain that Miranda does not resemble Earth's crystalline ice fields or glistening snow shrouds. Out here around the seventh planet the Sun is quite faint. Remember that Uranus is about 19 times farther from the Sun than Earth. Light intensity falls off with the square of the distance. (So, for instance, the Sun on a planet twice as far from the Sun as Earth would be a quarter as bright.) Uranus receives about four watts per square meter of energy from the Sun. Compare this meager portion to Earth's generous 1353 watts per square meter allotment. We are now moving noticeably faster into a gaping abyss of shadow.
Mind you, there are consolations! The stars above are exquisite: far brighter, sharper than they appear on Earth. Again, blame the layers of atmospheric gases that obscure the incoming star light. Out here I can watch the behemoth Orion and attendant Canis Major poised like giants over the distant ice peaks. So crisp are their stars they almost appear tangible: three dimensional sparks that cast pock mark reflections off the dark ice. Yes, we can see the same constellations Earth-watchers see because the stars are so much farther away from us than the planets. Earth's Orion is Uranus' Orion.
We've fallen far enough so that I must strain to look back and see the apex I abandoned.
Oh, yes, we certainly can see Uranus! It is not a brilliant blue orb as one finds when visiting those frauds at the planetarium. It is a mammoth sphere cast in a subtle tint, reminiscent of a midnight tree canopy rendered slightly luminous by a remote street light: just switch the color from yellow to azure. Miranda is locked in a synchronous rotation with Uranus so it always keeps its same side toward the planet, just as the moon does toward Earth. Therefore, on our icy little moon, an observer would either see the parent planet all the time or it would never appear at all.
Looming Gas Giant planet cast against a brilliant array of stars that look as though they could be plucked from the night hollows like phosphorescent berries. We are falling faster, ever faster toward the barely discernible base of this, the solar system's highest cliff. We can see the faint, but sharp, shadow we cast against the sheer cliff face. We know that within a few moments, we will snap out of existence only to reemerge somewhere else at some other time: astride the retreating Voyagers or cast into the star fields of Andromeda or wherever else the mood might move us...as of now we are about to crash onto Miranda in a cascade of dispersing virtual particles...What better way to embrace non-being.