Star Trek: Aldea and the Physics of Invisibility

Star Trek the Next Generation (1987-1994) (Picture)
Season 1, Episode 16
When the Bough Breaks

At the start of episode 16, the Enterprise stumbles upon the mythical planet Aldea, which has been said to possess a cloaking device and other advanced technology. The cloaking device is intriguing and I am going to explore the possibility of the technology ever existing.


What does it mean to see something, anyway?

As a very basic explanation, the human eye and brain "see" electromagnetic radiation in the visible range of the electromagnetic spectrum. All electromagnetic waves are called photons and these photons can have different frequencies (and therefore energies) depending on where they fall in the electromagnetic spectrum.

A diagram of the  electromagnetic spectrum (Source)

The above diagram shows the electromagnetic spectrum. The range of frequencies humans can see are shown as the visible range. In order to make an invisibility cloaking device that tricks humans the photons in this range need to be bent around the planet Aldea.

How do you bend Electromagnetic Waves (Photons)?

The bending of electromagnetic waves is called refraction. In Star Trek the cloaking device the planet used was described as an electromagnetic shield, a complicated light refracting mechanism.

So far, so good. Star Trek is right in saying that an invisibility device would need to refract electromagnetic waves around the planet to make it appear as though it is not there.

A common way of bending electromagnetic waves is by using a waveguide. Waveguides internally reflect the electromagnetic waves to get them from point A to point B.

 A diagram of a waveguide (optical fiber in this case). (Source)

Note in the above diagram that the incoming waves (from the left) bounce around within the waveguide and do not exit until they reach the other end. This is what a waveguide does. By reflecting the waves completely inside they are able to pass them from one end to another. Waveguides do not need to be straight, like in the diagram: they can bend and curve, as well.

By utilizing some form of waveguide it is possible to bend waves. Depending on the material used it can be done with any wavelength of electromagnetic radiation, as far as I know.

Ok, but how could the Aldeans cloak an entire planet?

This I do not know. There has been research done to try to make an invisibility cloak. What I have found suggests that there is now a suit that can bend microwaves (long wavelength electromagnetic radiation - near left side of the above spectrum) around a person. There is a short video explaining this here.

 A look at how an invisibility cloak might work. (Source)

This method would make a person invisible if we saw things in the microwave part of the spectrum. Since we see things in the visual, or optical, part of the electromagnetic spectrum, this does invisibility cloak would not make us invisible to one another.

Using this method the Aldeans would have to surround their entire planet in a physical material, which seems impractical. This cannot be how they solved the problem, given that they are able to turn it on and off whenever they want. 

In the episode it was proposed that an electromagnetic field was responsible for bending the light around the planet.

Other Issues...

In addition to cloaking electromagnetic radiation in the visible range, the Aldeans would have had to reflect other wavelengths of the spectrum as well. I am sure there are species in Star Trek that do not see purely in the visible range. On earth, for example, bees are well known to see a different range of electromagnetic frequencies. 

This shows the different frequencies that humans and bees see. (Source)

Bees see into the ultraviolet part of the spectrum and see much less of the lower energy optical wavelengths we associate with the color red.

Sorry for the tangent about bees - what I wanted to bring up is the fact that different species see different parts of the spectrum, so the cloak used by the Aldeans would have to refract a very large range of frequencies (maybe all of them). This is well beyond current technological capabilities.