Here is an extended version of my latest piece for Nature News. There are of course more details about lots of the things discussed here in my book Invisible.
Experiments that give subjects the illusion of having an invisible body might reveal how we respond to the ensuing temptations.
We’ve all had those moments of social embarrassment: you’ve just said or done the dumbest thing, everyone is looking at you, and you wish you could just – well, vanish. But what if you really could? Would that help?
Apparently it would. Using a virtual-reality headset and a calculated confusion of the senses, neuroscientists at the Karolinska Institute in Stockholm, Sweden, have been able to give people the illusion that their body is invisible . The subjects feel someone stroking their body with a brush, but when they look down all they see is the brush moving through thin air. This is enough, they testify, to give them the sensation that their entire body cannot be seen. It feels, many say, not only invisible but hollow – a weird dematerialization of their physical person.
And according to both the testaments of the subjects and objective measurement of their physiological response to stress as revealed by their heartbeat rate, this sensation of invisibility reduces their anxiety in social settings, for example if they can see an audience of “serious-looking strangers” seated and watching them.
Why would you want to know what it’s like to have an invisible body? One potential reason is that the technique could be used to treat social anxiety disorder, in which people are acutely susceptible to stress in situations such as having to deliver a presentation or perform before an audience. This is a very common disorder, affecting an estimated one in ten people at some point during their life. They sweat, shake, blush and hear their heart thumping away. Aside from prescribing anti-anxiety medication, this condition is generally treated with cognitive behavioural therapy, which attempts to condition the subjects by degrees to stay calm in a socially stressful setting.
Virtual reality has already shown its value in such treatment. But what if, say the Stockholm team, led by Henrik Ehrsson, you could “give” a patient an invisible body and then gradually make them more visible in stages? The illusion that they create is, after all, a matter of degree: subjects said that they experienced varying depths of visibility, depending on the conditions.
The illusion is also related to studies of how a sense of “body ownership” is triggered by visual and tactile stimuli. In 1998, psychologists Matthew Botvinick and Jonathan Cohen showed that when people see a rubber hand being brushed at the same time as their own hand, out of sight, experiences the same sensation, they feel that the rubber hand is part of their body . Ehrsson and colleagues recently showed that this “rubber hand” illusion can even be invoked for an “invisible hand”, when subjects see the sensation that they feel applied to empty space . That’s what inspired them to carry out the present study, and the results could also cast light on the “phantom body” illusion experienced by some people with paralysis due to spinal-cord injuries, in which they feel they have a body that is out of alignment with their real one.
So, plenty of potential clinical applications. But the research touches on something far deeper, for notions of personal invisibility feature in many of our myths, legends and stories, from Plato’s telling of the myth of Gyges in his Republic to H. G. Wells’ Invisible Man, J. R. R. Tolkien’s The Lord of the Rings and Harry Potter. Invisibility there has other connotations. Gyges is a shepherd of Lydia who discovers a ring of invisibility by chance, and uses it to seduce the queen, kill the king, and make himself ruler. The moral, says Plato’s narrator Glaucon, is that
“No man can be imagined to be of such an iron nature that he would stand fast in justice. No man would keep his hands off what was not his own when he could safely take what he liked out of the market, or go into houses and lie with anyone at his pleasure, or kill or release from prison whom he would, and in all respects be like a God among men.”
Wells seems to have intended his novel to be an updating of the Gyges story, demonstrating the corrupting temptations of invisibility and its severance of personal responsibility. He took great pains to make his invisibility scientifically plausible by the standards of the time, and interestingly he anticipated that an inability to see one’s limbs would confuse the coordination: his invisible man initially can barely walk. Ehrsson’s coauthor Arvid Guterstam says this is something they can now test, but he doesn’t anticipate such an effect. “We are already very used to moving our limbs without directly looking at them, when they are occluded or in the dark”, he says, “so guiding our limbs through space without direct visual feedback shouldn’t be a major issue.” We wouldn’t, it seems, become like David McCallum’s Invisible Man in the 1970s television serial of that name, forever blundering into potted plants – an exigency forced on him, the series producer Robert O’Neill admitted, by the challenge of convincing an audience that an invisible person was actually there.
But would invisibility also confuse our morals? This is where the new work could get really interesting, because the researchers want to examine this “Gyges effect”. “We are planning to expose participants to a number of moral dilemmas under the illusion that they are invisible”, says Guterstam, “and compare their responses to a context in which they perceive having a normal physical body.” I anticipate the worst here, not least because the Gyges effect seems already to operate in internet trolling .
One thing is for sure: we should take with a big pinch of salt the authors’ suggestion that there is “the emerging prospect of invisibility cloaking of an entire human body being made possible by modern materials science”. This alludes to recent work on so-called metamaterial invisibility cloaks [5,6]. But not only is that work very far from achieving full cloaking in the visible spectrum (let alone in a wearable suit), but there is good reason to suspect that it will stay that way for the foreseeable future. The hiding of a cat and fish that Ehrsson and colleagues mention  sounds impressive but is in the end a kind of high-tech version of the Victorian stage magician’s trick of using mirrors (and probably smoke) to make half a woman’s body vanish.
There are other technologies afoot for making “invisibility cloaks” for humans, generally involving a kind of adaptive camouflage in which the background is either projected onto highly reflective clothing  or captured with “onboard” cameras and displayed on wearable LED screens . The first offers a compromised and static illusion – if you want to appear transparent while you give your presentation, you’d do just as well to stand in front of the projector. The second is another speculative and remote (albeit fun) idea that is in any event undermined by the laws of optics . So fear not – no one is going to become a real-life Gyges any time soon.
1. Guterstam, A., Abdulkarim, Z. & Ehrsson, H. H., Sci. Rep. 5, 9831 (2015). (here)
2. Botvinick, M. & Cohen, J., Nature 391, 756
3. Guterstam, A., Gentile, G. & Ehrsson, H. H. J. Cogn. Neurosci. 25, 1078–1099 (2013).
4. Hardaker, C. Guardian 3 August 2013. (here)
5. Schurig, D., Mock, J. J., Justice, B. J., Cummer, S. A., Pendry, J. B., Starr, A. F. & Smith, D. R., Science 314, 977-980 (2006).
6. Leonhardt U.&. Philbin, T. G., Geometry and Light: The Science of Invisibility. Dover, Mineola, 2010.
7. Chen, H. et al., preprint http://www.arxiv.org/abs/1306.1780 (2013).
8. Tachi, S. Proc. 5th Virtual Reality Int. Conf. (VRIC2003), 69/1-69/9 (Laval Virtual, France, 2003). (here)
9. Zambonelli, F. & Mamei, M. Pervasive Computing 1(4), 62-70 (2002) (here).
10. See comments by M. Hebert in ref. 8.