To what extent can invisibility be achieved within this century?
Stephen Liu
From H.G.Wells ’ Invisible Man to Harry Potter’s invisibility cloak, invisibility has always been a technology touted by literary fiction. Invisibility, the art of obscuring objects to visible light, is being pursued by many researchers. First, the viability of invisibility must be established, which requires a theoretical and conceptual proof. Then we must then establish the obstacles that currently hinder the development of invisibility technology and determine whether these obstacles can be overcome within this century. The extent of the development and technique used can also help us determine the potential applications of invisibility technology on society. Whenever advanced technology is discussed, the military is often at the forefront. The desirability of invisibility for the military has spawned decades of research into stealth technology. While stealth is not entirely the same as invisibility, both rely upon reducing the reflected electromagnetic radiation. Although much current stealth technology is classified, the basic principle is to reduce the radar cross section (RCS – a measure of an object’s ability to reflect radar) of an object. The Salisbury Screen (Fig.1) was one of the first technologies to be implemented and relies upon wave interference. However, this technique is restricted since it only obscures specific frequencies of radar waves 1 - even multi-layered screens only cover a very small bandwidth of wavelengths at the expense of thickness, which can impede the aerodynamics of the system. 2 Meanwhile, anechoic technology can be used to minimize reflected radar waves via attenuation. The amount of wave reflections with the surface is maximized, which reduces the strength of the reflected wave (Fig.2). This can be further helped by using radio absorbent material (RAM) such as carbonyl iron powder. This substance is so effective at absorbing radar waves (Fig.3) that it is still implemented today despite being first used over four decades ago. However, it is important to note that stealth technology only reduces the reflection of radar waves rather than eliminating the reflections. Thus, stealth technology is not equitable invisibility technology. Therefore, invisibility technology cannot use techniques implemented in stealth technology. New techniques must be developed to create genuine invisibility.
Figure 1: Salisbury Screen, where I is incident wave, λ is wavelength of I, n 0 is the air, n 1 is a dielectric and n 2 is a metallic surface. When wave I strikes dielectric n 1 , the wave is split in two (R 1 and R 2 ). R 1 is reflected at the dielectric, whilst R 2 travels through n 1 and is reflected by n 2 . Since n 1 has a thickness of λ/4, R 2 travels λ/2 more than R 1 . Hence, R 2 will have a phase difference of π with R 1 , which results in destructive interference between R 1 and R 2 . This reduces the amplitude of reflected waves. 3
1 Vinoy, KJ and JHA, RM (1995) ‘Trends in radar absorbing materials technology’, Sadhana , 20(5), p817. 2 Gaylor, KL (1989) ‘Radar Absorbing Materials – Mechanisms and Materials’, p13, ResearchGate (Accessed: 6 July 2020).
3 James Carron (2018) Quick Intro to Radar Absorptive Materials. 19 April. Available at: https://www.youtube.com/watch?v=plGRZQngND8 (Accessed: 6 July 2020): 00:38.
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