(In case I have readers not in the fandom, the basic premise of The Sentinel is a genetic gift that enhances the five senses to an extraordinary/superhuman degree.)
15: Photon Detectors
Your eyes are very sensitive, able to detect just a few photons of light. If you take a look on a very clear night at the constellation of Andromeda, a little fuzzy patch of light is just visible with the naked eye. If you can make out that tiny blob, you are seeing as far as is humanly possible without technology. Andromeda is the nearest large galaxy to our own Milky Way. But "near" is a relative term in intergalactic space – the Andromeda galaxy is 2.5m light years away. When the photons of light that hit your eye began their journey, there were no human beings. We were yet to evolve. You are seeing an almost inconceivable distance and looking back in time through 2.5m years.
So, this falls into the realm of stuff I already knew. I remember reading a version of this in a children's book back when I was a kid. Um...Willard Price? Could have been. Anyway, I was aware that "how far can you see?" is a pretty meaningless question as long as there are visible stars out there.
I'm not sure if this has major implications for the Sentinel ability. Not unless we're suggesting a genetic mutation can somehow alter the speed of light, which I think would take suspension of disbelief a tad too far. But eyesight is about more than distance. It's about focus, and the ability to process what is seen. Take this in combination with 20:
20: Optical delusion
The picture of the world we "see" is artificial. Our brains don't produce an image the way a video camera works. Instead, the brain constructs a model of the world from the information provided by modules that measure light and shade, edges, curvature and so on. This makes it simple for the brain to paint out the blind spot, the area of your retina where the optic nerve joins, which has no sensors. It also compensates for the rapid jerky movements of our eyes called saccades, giving a false picture of steady vision.
But the downside of this process is that it makes our eyes easy to fool. TV, films and optical illusions work by misleading the brain about what the eye is seeing. This is also why the moon appears much larger than it is and seems to vary in size: the true optical size of the moon is similar to a hole created by a hole punch held at arm's length.
This actually applies to all the senses; everything about the way we experience the world is essentially a simulation created by the brain from various different inputs. But let's stick with sight.
The blind spot, for example. The reason we all have one is an evolutionary quirk: in effect, part of the mamalian eye is 'backwards', meaning that the optic nerve has to thread through a hole to get the visual signals to the brain. That hole is the blind spot. You don't notice it because you have two eyes: each compensates for the blind spot of the other. If you lose sight in one eye, you would become aware of having that blind spot, and some kinds of brain damage can also make it 'reappear'.
Does a Sentinel have a blind spot? It would theoretically be a fairly simple change - redesign the back of the eye so things are the right way around. If you were designing an eye on a drawing board you'd do it that way. The reality is that's a major physiological difference and probably couldn't happen...but what if it could? Does that have implications for the other ways the brain compensates for weaknesses in the sight?
Take Jim's eyesight in canon, for example. He essentially "switches on" a kind of "hawk mode", where everything blurs out except the one thing he's focussed on, the way a raptor sees its prey from high above. Jim does it by instinct, not usually by conscious control, and we are free to interpret the special effects differently. But that's how I see it. I'd guess that makes the whole optical illusion thing unlikely to work with Jim. Those "magic eye" pictures that were such a craze 20 years ago...I'll bet Jim could never "see" the image hidden in them. But then he should also have far better than average spacial judgement for the same reason - no optical illusion to make things "appear" closer or further away. Except the opposite is true in canon - Jim can't judge distances well when using his Sentinel eyesight.
16: Sensory Tally
Despite what you've probably been told, you have more than five senses. Here's a simple example. Put your hand a few centimetres away from a hot iron. None of your five senses can tell you the iron will burn you. Yet you can feel that the iron is hot from a distance and won't touch it. This is thanks to an extra sense – the heat sensors in your skin. Similarly we can detect pain or tell if we are upside down.
Another quick test. Close your eyes and touch your nose. You aren't using the big five to find it, but instead proprioception. This is the sense that detects where the parts of your body are with respect to each other. It's a meta-sense, combining your brain's knowledge of what your muscles are doing with a feel for the size and shape of your body. Without using your basic five senses, you can still guide a hand unerringly to touch your nose.
The first example is something I've always thought of as touch. What the article's author is doing is distinguishing between "touch" meaning you've got to have contact, and the perception of the same nerves before contact is made. Is it a fair distinction? I'm not sure, but if it is valid, what implications does this have for our Sentinel?
Do five enhanced senses become six? Or is the sixth the one the Sentinel is missing? The first is more logical and this does have potential. There's a thing new-age types do to "feel" a person's "aura". Without commenting on the validity, the actual experiment does work. Rub your hands together vigourously. Then hover your hands above another person's bare skin. Slowly move closer, but don't make contact. You will "feel" the other person before you touch.
Now let's enhance that. Theoretically it works by sensing heat (the scientific approach) or life (the new age version). Pick whichever, as suits your worldview. Wouldn't that mean that Jim would be able to sense someone, even with all his other senses blocked out somehow? Find his way around even if there was absolutely no light? Could that sense even be refined enough for him to distinguish one person from another?
The ability to tell if we are upside down - that's a function of gravity and the inner ear and it does go away in a weightless environment.
But the last example is even more interesting. "Meta-sense", meaning something the brain does with all the sensory input at its disposal... My thought is that this is actually the missing element in the Sentinel's ability. Because that would explain most of the disadvantages we see in Jim's ability in canon. Jim's "meta-sense" is NOT super-human. But the sensory input it's getting is so much greater than "normal", if he tries to do too much, the "meta-sense" can't take it. He zones. Or spikes. Or, in extreme circumstances, his brain shuts down one of his senses (blocks its input from his "meta-sense") in an attempt to process the rest. It explains why Jim's spacial awareness actually suffers when his Sentinel ability kicks in. It also suggests that an ability to control or enhance the "meta-sense" would create a superior Sentinel...a true super-human.