The Trouble with Invisibility (hint: nature says it can’t work)

There’s been lots of excitement in the past year or so over the possibility that physicists will soon develop a real-life version of Harry Potter’s invisibility cloak.

Check out this Google News search to get a taste of the coverage.

The technology relies on unusual manmade metamaterials, with intricate microscopic structures. In addition to cloaking devices, metamaterials are touted as potentially leading to superlenses that could provide distortion-free magnification far greater than any conventional lens.

The only problem with these wonderful applications for metamaterials is that I am pretty sure they can’t possibly work.

Why not? The short answer is that, as far as I know, there’s no such thing as an invisible fly.

You see, nature (which is to say evolution) is pretty good at making complex structures. The photonic crystals that make peacocks, coleoptera beetles, and Lycaenid butterflies so beautiful are at least as intricate as invisibility-cloak metamaterials would have to be (if they’re possible).

Some of my coworkers have argued that I’m being a bit narrow minded about this – after all, they say, insects didn’t evolve to make handguns, automobiles, telephones, or radio transmitters, yet those things are possible.

That’s true, but insect have ways of managing all the things that those devices do. They have evolved countless ways to move around, communicate, hunt, and defend themselves. Often those things look a lot like our inventions.

Some beetles fire jets of caustic liquids (natural handguns), electric eels can both stun prey and communicate with electrical signals that are detectable with hand held radios, and don’t even get me started on natural transportation solutions

It’s certainly true that many (if not most) living things rely on some form of invisibility from time to time. The patterns in the coats of jaguars and tigers can make them essentially invisible in the mottled shadows of the jungle. Chameleons and octopi control the flow of pigments in their skin to match their surroundings. Sticks insects, various leaf hoppers, some moths, and countless other insect can be invisible when nestled among the sticks and plants that they resemble.

So, invisibility is terribly important for predators and prey alike. Yet there are limits to most invisibility schemes in nature. A jaguar wouldn’t have much luck hunting penguins against the backdrop of the Antarctic, and a polar bear would never be able to hide it’s white coat on the savanna.

So here’s the problem in a nutshell:

1. True invisibility would give just about any living creature HUGE evolutionary advantages over it’s competitors.

2. Evolution is great at creating complex structures of the type that should theoretically yield the metamaterials necessary for invisibility cloaks.

3. And yet, there are no invisible flies, birds, jaguars, or anything else. (Despite that fact that an invisible jaguar would be really cool.)

Maybe I’m wrong. Perhaps we’re surrounded by invisible animals. I have a feeling, however, that we would know if there were any species of invisible predatory cats. And while mosquitoes are pretty stealthy, I’ve never been bitten by anything that wasn’t at least visible under a microscope.

Does Nature have some objection to metamaterials?

Perhaps. But I’m guessing that Nature hasn’t produced any invisible animals because nothing, including metamaterials, can make things invisible.

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