A Life of Ease in our Invisible Submarine

Wouldn’t it be neat if you could roam around wherever you wanted without anyone knowing you were there?  People have thought so for a long time, and have spent a lot of time and effort researching and developing stealth technology, particularly for military forces.  But as you can imagine, it’s hard to make yourself completely undetectable, especially when you’re riding around in something as big as, say, a submarine.

When an object – a boat or an animal, for instance – moves through the water, the water around it gets pushed around in all directions and flows all around the object disturbing it.  This turbulence creates a wake, a chaotic pattern of ripples and waves and churning water which trails being the moving object.  If you look out into the bay and see a line of swirling, churning water on the surface, you know that a boat recently passed by.  In the same way, a submarine leaves behind a wake underwater that reveals the sub’s presence.  Military forces would love it if there were a way to negate this wake effect, to create a submarine that doesn’t leave a trail.

This ship leaves a wake behind it.

Well, Drs. Yaroslav Urzhumov and David Smith at Duke University say there is a way!  They say that by surrounding a submarine with a specially-designed covering, they can manipulate the water around the sub in such a way as to cancel out all wake effects.  The idea is fairly simple, though their model is quite complex: the reason wake exists is that the water behind the sub is moving differently than it was before the sub passed by; if the sub had never passed, the water would still be flowing smoothly and calmly.  The idea behind this “mesh shell” is that it would be made of a water-permeable material and full of pumps and channels to redirect water around the sub.  Water would flow into the shell, be channeled smoothly around the submarine, and released back into the ocean flowing just as calmly and smoothly as if the sub had never been there.  Someone floating in the sea nearby wouldn’t feel any waves or hear any churning water – the sub could pass by like a shadow, silent and completely undetected!

Now, before we get too excited about this research, remember that it is only a model so far, and even the model works only for small objects moving slowly.  But making even a small object move undetected through the water is impressive.  It might be a while before this kind of technology can be used on a full-sized submarine, but the authors like the idea of using it for starters on small, robotic vessels, or to reduce drag on larger objects.

(Here’s a rarity: I can link you to the actual paper.  It’s available here, if you’re interested.  Thanks Cornell!)

Of course, being able to slip by unnoticed is pretty cool, but when the Navy is searching for submarines in the water, they don’t send swimmers out to feel for them, they use sonar.  Sonar is a detection system which sends out a series of sound waves, which travel invisibly through air or water, and bounce off of objects in their way.  The same way your voice bounces around a big cave and returns to your ears as an echo, underwater sonar bounces off of boats and submarines and returns to the sonar equipment, telling it what objects are present in the water (This is the same way bats and whales use echolocation).  Sonar doesn’t care whether or not your submarine is creating a wake in the water; as long as the sub is solid, sound will bounce right off of it and the sonar detectors will light up, alerting people of the sub’s presence. 

But there might be a way around this problem as well.  A team led by Nicholas Fang of the University of Illinois has developed a special device that warps sound waves.  It works in a way which is surprisingly similar to the mesh shell I described above.  This “acoustic cloak” surrounds an object, and when a sound wave hits it, the special material in the shield captures the sound waves, directs them around the object, and sends them on their way.  This way, the sound waves never actually hit the cloaked object, and it becomes invisible to sonar.

Above: Sound waves travel forward (red) until they hit an object and are reflected (green).
Below: The "acoustic cloak" directs sound waves around the object and sends them off in the same direction they were headed.  The "mesh shell" described above redirects water in much the same way.
(Figure by me!)

Currently, this sound shield is small enough to fit in the palm of your hand, and only works if the sound is coming from one of a few specific directions, so it’s not quite ready to cloak submarines, but it works!  These scientists think that soon it may be useful for medical applications, to shield certain objects from sonar-like systems that are used in medical machinery, for example. 

So someday soon, we should be able to shield submarines from sonar, and even stop them from making waves in the water, but the cherry on top of our stealth sundae would be to make a submarine completely invisible.  But surely that’s an engineering pipe dream, completely infeasible, right?

Not so!  Researchers have been successfully designing methods of making objects partially invisible for years.  The trick is manipulating light.  When light is produced, by the sun for example, it travels through the air (or space) as a wave.  The light waves hit an object, say a tree, and bounce off of it and into your eyes, where your brain translates the light into an image of a tree.  The different colors of the tree are a result of the different frequencies of light waves bouncing off of it.  A light wave with a frequency of 550THz would be interpreted by your brain as green, whereas a wave of 700THz you would see as red.  In fact, all forms of electromagnetic radiation – visible light, x-rays, microwaves, heat, ultraviolet – are different frequencies of light.

Earlier versions of “invisibility cloaks” functioned much like the “sound cloak” above.  They would direct light waves around an object and send them along.  Since the light never bounced off the object and into your eye, you could never see it!  Newer versions of similar technology are called “carpet cloaks.”  Imagine slipping a small item underneath a carpet.  This would leave a bump in the carpet, which you would see because light would bounce off the bump differently than it would off of the flat areas of the carpet.  These “carpet cloaks” are made of a special material that manipulates the direction light travels when it hits and reflects off.  Even though there is a bump in the carpet, light bounces off the material as though it were a flat surface, and so even if you were looking right at it, you would only see a flat carpet, completely unaware that something was hiding under it.

On the left: light (red arrow) reflects off of a flat surface.
On the right: the "cloak" placed over the bump causes the light to reflect as though off a flat surface

Until now, these cloaks have only worked for certain parts of the light spectrum – microwaves or infrared, for example, but a new version of carpet cloak works for all parts of the visible spectrum, that is, all the colors that humans can see.  For the first time, an invisibility cloak has been made that renders an object invisible in all kinds of visible light and from all angles.  Now that’s cool.

The catch: even the newest version of invisibility cloak only works on small objects.  Really small objects.  As in, one millionth of an inch wide.  So for now the cloak is only really good for hiding red blood cells or, I guess, a really tiny submarine (or the Magic School Bus in that one episode where Ralphie gets sick and the class goes inside his body to learn about the immune system).  Making bigger versions will take a lot of time and ingenuity on the part of the scientists, but a super-small invisibility cloak is still an invisibility cloak, and that’s quite an achievement.

More than the other two cloaking methods, the light-cloak has a long way to go before it’s useful for big, free-moving objects like a submarine, but the technology is up and coming, and being able to shield object from various forms of radiation has countless applications which can be developed along the way.  Just think, someday, when the costs of these methods are reduced, and the kinks are worked out, we may be able to send submarines through the ocean that don’t make any ripples, are undetectable by sonar, and invisible to the naked eye.  Even if you were floating right next to the submarine, you would never notice its presence, unless it crashed right into you – but who knows?  Maybe we’ll solve that problem someday too.