Monday, May 07, 2007

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.

3 comments:

  1. You certainly got me thinking with that post...here is my main point of contention.

    Just because nature CAN do something doesn't mean that it WILL. While being invisible would be a huge advantage in preditor / prey context, it might not be so good for finding a mate or might take too much energy for it to be worthwhile...Maybe there are no invisible flies because the trade-off isn't so good and nature knows it?

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  2. Interesting point. Of course, nature makes plenty of blind things that can mate. Don't flies and lots of other insects find mates primarily through pheromones anyway?

    Although nature doesn't use metamaterials to make things invisible, lots of creatures are essentially invisible in certain situations. Stick insects manage to find each other somehow, but I'll be darned if I can find one in a bush.

    You're absolutely right that it may be a question of trade offs. At the very least, this suggests to me that the current theoretical designs for metamaterials must be off base when it comes to invisibility. They would be at least as easy and cheap for nature to make as photonic crystals, which are very common in nature.

    So perhaps invisibility is possible, but the researchers at Purdue and other places working on invisibility haven't proposed any schemes that are prohibitively expensive or energy draining enough to discourage evolution. An invisibility cloak that required the power output of a typical city would do the trick. (Of course, that would discourage us a bit as well.)

    I hope I'm wrong because invisibility would be so cool and useful. I'll keep following the literature, but I'm very skeptical for now.

    -Buzz

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  3. Maybe it just hasn't happened yet. Multi-celled animals aren't that old. Half a billion years may seem like a long time, but it isn't eternity.

    My current favorite "what might evolve next" idea is radio telepathy. How might it evolve? Well, some sharks, and platypuses can sense prey's nervous system's electrical impulses. The platypuses have that long snout, presumably so that the detector is farther from the animal's own brain. But it could turn into an antenna. And, it's already attached to a pretty sophisticated brain.

    I can just see the future platypus ID folks arguing that Evolution could not produce such a complicated organ, because no less than complete organ would have a survival advantage.

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