Flying Circus of Physics newsletter for Aug - Oct 2014
Data pubblicazione: 27-ago-2014
Flying Circus of Physics newsletter for Aug - Oct 2014
Invisibility
Can someone be invisible? I think the question began with the 1897 book by H. G. Wells The Invisible Man (in which a man is invisible but only in the reader’s imagination) and then continued with the 1933 movie of the same name (in which a man is invisible but only because movie trickery).
www.youtube.com/watch?v=pb3n0g2NenI
Wells and everyone who followed him with other stories of invisible people neatly sidestepped the fact that if a person is truly invisible, the person would not be able to see because the retina would not absorb any light. Besides, the person’s bodily materials would have to match the optical properties of air, and thus the person would need to consist of air. If the person were solid but transparent, then the body would distort our view of the background behind the person (like a lens). The distortion would be especially noticeable when the person moved. But of course, these details would have eliminated the plot laid out by Wells.
Nevertheless, not only did the idea catch fire (who would not want to be invisible at times?), but it became upgraded to the point where an entire starship could be cloaked in a 1994 episode of Star Trek The Next Generation. In fact, the cloaking device not only made the Enterprise ship invisible but also allowed it to pass through solid rock.
www.youtube.com/watch?v=M2Kp0JHhSqI
Of course, these days the most well-known example is the invisibility cloak of Harry Potter. The cloak magically allows us to see the background directly through the character and without any distortion.
www.youtube.com/watch?v=5YP3PWYEyBU
But in real life, can anything be cloaked or invisible? Well, here three possible examples to consider.
- A tank becomes invisible to infrared detectors and thus to thermally guided missiles: www.youtube.com/watch?v=KL_KdmSTSNA
- A cloak consisting of tiny retroreflecting beads conceals a man: www.youtube.com/watch?v=PD83dqSfC0Y
- A Mercedes is difficult to see: www.wimp.com/invisiblemercedes/
In the first example, the temperature of the panels on the tank are regulated to match the infrared emissions by the background. Thus an infrared detector cannot distinguish the tank from the background. However, I think this a case of camouflage rather than cloaking or invisibility. We are not seeing through the tank.
In the second example, the cloak can hide a man if the scene behind him is projected onto the front of the cloak, to give us the illusion of looking through him. A camera behind him records the background and sends the information to a projector. The image is then projected onto the front of the cloak. The retroreflectors in the cloak send the light back to us, and we seem to be seeing through the cloak and man. Here is the layout.
science.howstuffworks.com/invisibility-cloak7.htm
Again, this is camouflage.
The hidden Mercedes is also an example of camouflage. The background is recorded by a camera on the far side. The images are then sent to LED arrays on the near side so that the composite display mimics the background.
However, here are three examples where I think something is close to being invisible.
- A spider is mostly transparent except for the retinas, where material must absorb light. You can see the retinas move.
- www.wimp.com/transparentspider/
- A marine animal that is difficult to see, especially in water. The optical properties of the body almost match those of the surrounding water. So there is little distortion of light passing into and then out of the body. Also, there is very little scattering of the light within the body because the microscopic spacing within the structures is less than the wavelength of visible light.
- www.youtube.com/watch?v=uxqHoanGku8
- A fish with a transparent head or body.
- www.mnn.com/sites/default/files/gobies530.jpg
- www.youtube.com/watch?v=Zoygy-8PTtU
There is another way to make something invisible, a way that has been used in many carnival side shows. You make the background light travel around an object. Here is a simple but clever example using several mirrors.
www.youtube.com/watch?v=GAmWs6zfTj8
More ambitious methods using far more complicated techniques are now being studied. They each involve bending background light around an object so that we truly see the background and not the object. Many techniques involve newly fabricated materials called metamaterials, which consist of special periodic structures that can divert the light around an object. Another technique involves effectively hiding an object inside a material that so severely scatters the transmitted light that nether the object or its shadow can be distinguished.
phys.org/news/2014-06-optical-invisibility-cloak-diffusive-medium.html
Will there be a true invisibility cloak? Although some fake videos on YouTube suggest that the US military already have such cloaking, I think camouflage is the only possible way of hiding a person. However, I hope I am wrong because becoming invisible would certainly be cool, even highly desirable in certain embarrassing social situations.
If you would like to see my previous story about a transparent frog (not invisible, but largely transparent), click on the following link. Although I am not invisible and not even transparent like the frog, light actually passes fairly deep into my body, including through my skull to reach my brain. (Maybe that is why I can think better in bright Texas sunlight than in gloomy Cleveland winters.)
Cheers,
Jearl Walker