Twitter got me thinking. It has a terrible habit of doing that. Specifically, this time we were talking about being invisible and abusing the advantages. One of my tweeps mentioned being invisible in the Caribbean- and I began to wonder about it. I asked if tanning/skin cancer would be a concern, and she said it would be, since UV light is also invisible.
At this point, I was intrigued. Invisibility is great, but how could it work? I came up with a few scenarios.
First, UV light is the primary source of tanning. But, how do we gather heat from the sun? It has been demonstrated that the most energetic wavelengths of light, on Earth at any rate, also correspond nicely with the visible spectrum (particularly green, hence chlorophyll). The sun "just happens" to emit light most strongly in this spectrum. But what does that mean for the Invisibles? Heat is a byproduct of kinetic movement, in that molecules, and especially their electrons, get agitated to the point that they move around faster, jostling back and forth. In a normal, visible human, light rays bombard our skin and clothing and smack into millions of molecules on our skins, speeding things up and warming us.
So, would the invisibles be cold all the time, unable to draw heat from the light that was hitting them, only able to get warmth via ambient heat? Or would we be too hot, since ultraviolet rays could somehow pierce through our skin and shine directly on, well, everything inside, agitating a lot more than our surface area? Neither of these answers has implications that pass muster. Being opaque to UV radiation seems absurd if we're going to be transparent to everything else, since right now we're opaque to most everything else as well. If they were cold all the time, that makes (very slightly) more sense. X-Rays pass through us, and we don't notice any significant heat difference because they are passing through us, not colliding with our skin or, since we'd be completely transparent, organs (well, at least not colliding with us in any significant number- I'm sure the occasional atom will absorb a random photon here and there, but not on the scale that something opaque does at every minute of every day).
But then we have the problem of what would we be made out of? There are lots of reasons that we aren't invisible (for one thing, materials that are simply aren't conducive to life), but how could we be? Water makes up most of our cells, but it's not invisible, either. For something to be invisible, it would need to refract the light in precisely the same way as the ambient air. And water simply can't do that. So the rest of our materials would have to optically correct for the failure of our water. For one thing, we do tan, and melanin would have to go. Sorry, bones, calcium is no good- perhaps we can restructure the molecules so that they are largely silicon and oxygen, (out with you, calcium!) and never mind the pesky hemoglobin (a very complicated molecule in its own right) that is bright red and performs the minor task of transporting oxygen to the rest of our bodies... and I think you can see how this explanation rapidly degenerates into one more extravagant excuse than the last.
Well, maybe one way, but I don't particularly buy this one either: the Invisible Man tackled this issue by simply putting the protagonist into another dimension, at least partially, which neatly sidesteps all of the aforementioned problems (and puts the explanation, and any bothersome details, neatly out of the grasp of most non-physicists, including myself).
So, screw all that. There's no way, without some kind of cloaking device, we'd be invisible, and that's not particularly tenable either, for many of the same reasons, including one more that applies to all three.
Everything else aside, I don't buy it simply because our eyes function by capturing light, and if they can't because light goes through them, then I'm invisible(Yay!) and blind(Crap!). In the case of the alternate dimension approach, I would think that our ability to see would be based in whatever dimension our eyes were in, for the same reason. Of course, on that front, I could be completely uninformed. About all I know about multiple dimensions is that, mathematically, they have been proven to exist. And if something is mathematically proven, that's about as certain as we can be of anything at all.
In that case, though, good news. I am invisible- to anyone not in the room with me right now. At least, dimensionally distant.
Verdict? Metaphorical invisibility sucks, but at least you still have 5 senses going for you.