There’s been something a minor discussion of Rupert Sheldrake’s ideas going on in the comments, so it was on my mind when I sat down with a beer in a sunny pub garden today, lit a cigarette, and started vaguely thinking about The Sense of Being Stared At.
There are two main reasons for the conventional dismissal of the sense of being stared at. First, it is classified as ‘paranormal’. It is normal in the sense that most people have experienced it for themselves. But it falls foul of the general taboo against psychic phenomena. For generations, educated people have dismissed it as a superstition.
Second, it conflicts with the orthodox scientific theory of vision, first published in 1604 by Johannes Kepler (1571–1630), best known for his discoveries in astronomy. Kepler’s was an ‘intromission’ theory, according to which light came into the eyes, but nothing went out of them. Vision was not in the outer world where it seemed to be, but inside the head…
On Kepler’s theory, the sense of being stared at ought not to exist. Hence it has generally been ignored by scientifically educated people, or dismissed as a superstition.
I’d thought about it before. But this afternoon I suddenly realised that we always think about eyes solely as recipients of external light, with the light going from outside the eye to inside the eye and onto the retina. Never mind Kepler, we don’t think of light going the other way, with the eye acting like a cine projector, only because we don’t have any light source inside our eyes to shine out onto the world outside.
Except that we do have such a light source inside our eyes.
It’s just that it’s not a visible light source. It’s an infra-red light source. And this infra-red light source is the warm retina at the back of our eyes. This is radiating infra-red light back out through the eye’s lens onto the outside world. Assuming the retina is at the same 37°C temperature as the body tissue surrounding it, this warm retina will be radiating heat out onto the cooler outside world (about 17° C in the pub garden). So while our eyes are receiving visible light from the outside world, they should also be emitting IR light onto the outside world. But it won’t be very much.
In fact, we can estimate roughly how much power our eyes are emitting by using the Stefan-Boltzmann equation, q = σT4A, where σ is the Stefan-Boltzmann constant 5.6703 10-8 W/m2K4, T is the temperature difference between the eye and its surroundings ( 20° K), and A is the area of the eye retina. The human eye has a diameter of 24 mm, and the area of the retina is given as 1,094 square mm or 0.001094 m2. So power q = 9.92 10-6 watts. And given two eyes, 19.8 10-6 watts, or 19.8 microwatts.
Could anyone possibly sense something as small as 20 microwatts?
Well, if (and it’s a very big if) those 20 microwatts can be focused on one single point on somebody’s skin for a period of several seconds or minutes while they are being stared at, there will be a very small localised temperature gain. If the single point was a single human cell, for example a fibroblast cell with a volume of 2000 μm3, a density of 1000 kg/m3 (water), and a specific heat of 4.183 kJ/kg °K (water), it will rise 1.25°C after half a minute of receiving a 20 microwatt heat input.
And maybe somebody could feel that, as a burning or pricking sensation or something.
That’s the radiative physics of it, to the best of my knowledge. And there’s nothing ‘paranormal’ about it.
It might be possible to devise an empirical physical experiment to check this. One experiment might be to have a frozen droplet of water (or a temperature sensor) surrounded by 10 or 20 people who keep their eyes fixed on it for 10 minutes, and see whether its temperature rises. Another experiment might be to stare into an IR camera, although I somehow doubt they can register microwatts.
Rather more speculatively, it might be wondered whether retinal temperatures rise when someone is staring very intently at something, and their retinas are working harder, just as body temperatures rise when muscular work is being done. If so, retinal IR emissions would be correspondingly high. Maybe there’s something to the lyrics of Lana del Rey’s Blue Jeans?
Blue jeans, white shirt.
Walked into the room, you know you made my eyes burn
Even more speculatively, when rock stars (like Lana del Rey) are ‘bathed in adulation’ in front of thousands of rapt, adoring fans, maybe they’re getting a storm blast of infra-red from their massed fans’ eyes.
Cats’ eyes have a reflective layer – the tapetum lucidum – behind the retina, which means that any incoming light that passes through the retina is reflected back through the retina a second time, effectively doubling its intensity. But this same reflective layer would act to double the intensity of infra-red light being radiated by the eyes. Might cats be able to hunt in complete pitch darkness, using their eyes as searchlights?
Anyway, the microwatt power levels of infra-red eye emissions look as though they’re probably far too low for anyone to ever notice them. But given that dogs can detect odours at 1 part in one trillion, maybe some animals can detect radiant heat at microwatt levels when they’re being watched, and be able to tell from what direction. For them it would be like standing in sunshine.