Have you ever felt fear and anxiety from standing in front of a large audience and giving a speech? Or how about having to get up in class and talk to other students? While it’s normal in situations like these to wish for the ground to swallow you up, some scientists have suggested a slightly different remedy for anxiety – invisibility.
Invisibility has long featured in myths and fiction, but several advances in material sciences have demonstrated that the cloaking of large (living) objects – just like how the invisibility cloak works in Harry Potter – is becoming a realistic prospect. In the field of material sciences, the general concept of invisibility cloaking is actually fairly simple.
Theoretically, all that’s needed is a material that guides visible light (or another wave, like EM waves or heat flux) around an object, and anything within the gap it leaves will be rendered “invisible” to someone standing at the light source:
Light moves around the object (or person) as though it isn’t there. Image: Trevor Johnston/trevorjohnston.com
In practice, this is hard to achieve, as most naturally occurring materials reflect light, cast shadows and produce a reflection. However, hi-tech and exotic materials called “metamaterials” have made light bending possible. (Although latest research suggest that ordinary lenses can do just the trick!)
H G Wells, a man ahead of his time, wrote the The Invisible Man in 1897. The novel is about a protagonist who invents a method to change the human body’s refractory index to that of the air, rendering it invisible. (The twist comes when he performs the method on himself and can’t reverse it – but that’s beside the point.) The refractive index is the ratio between how light passes through a vacuum, and how it passes through any other medium; it’s the reason a spoon will look bent when placed in a glass of water. If water has a negative refractive index, the spoon would look as though as was bending back on itself instead.
In a recent paper in Scientific Reports, graduate students Arvid Guterstam and Zakaryah Abdulkarim and their advisor Henrik Ehrsson, a neuroscience professor at the Karolinska Institutet in Stockholm, said they believe invisibility cloaking of the human body is a thing of the future, and believe it’s high time we delve into what it feels like to be invisible.
To do this, Guterstam, Abdulkarim, and Ehrsson used virtual reality. In one of their experiments, 23 people were provided with a set of head-mounted displays (HMD), and were asked to look at their feet. The experimenter – Abdulkarim – stroked their arms, legs, and torso with a paintbrush with one hand, and at the same time, made identical motions with a second paintbrush with the other hand, on an invisible body or a mannequin. A pair of downward-facing cameras that were either mounted on a tripod or on the head of a mannequin sent a real-time video feed to the participants HMDs, giving them the sensation of being invisible, or making a body swap with a mannequin:
Study co-author Zakaryah Abdulkarim (middle) creates the invisible body illusion on a participant (left) wearing a set of head-mounted displays connected to a pair of cameras. Photo: Staffan Larsson
Here’s the surprise: after finishing with the paintbrush, each participant slowly lifted their gaze through their HMDs to find that they were being watched by a scornful-looking audience (consisting of 11 scientists instructed to stare at the participant). Quite creepy, and perhaps enough to through most people off – however, on a 100-point scale, participants reported their stress level as about 25 per cent lower, on average, when in a state of invisibility, and about a third less than in the mannequin version. The state of invisibility also lowered heart rates by a few beats per minute, suggesting that stress is intertwined with physiology.
The researchers write: “Our results demonstrate that healthy individuals can experience the illusion of owning an invisible full body.” They suggest their results could spur on better a design for virtual-reality based therapies for social anxiety, and may also help give neuroscientists gain new insight into phantom limb illusions.