When I was in high school, there was a girl in the cafeteria who stared at me. Every day. My friends and I would sit at a table at one end of the cafeteria, laughing and joking and eating our lunch, and I would get a creepy feeling that somebody was watching me. Sometimes I would turn around and catch this girl, sitting clear across the cafeteria, eating alone and staring at me. Other times I would point furtively, in the direction the creepy feeling was coming from, and ask my friends if somebody over there was staring at me. They would look in the direction I was pointing and announce that yes, she was staring at me again.
When we would catch her staring, she wouldn't look away, get embarrassed, or deny it. She would just smile, wave and continue staring.
I eventually got over the creepiness. She was a nice kid, and in time we became friends. But I always wondered why I could feel her staring at me, and why I could tell where she was sitting. I think I've finally got it figured out. Here is the scientific reason for why you can feel someone staring at you. It's rather long and involved, and admittedly anticlimactic, but hey, somebody's got to do it.
The Electromagnetic SpectrumFirst, you need to know a few things about the electromagnetic spectrum — the "rainbow."
Everything in the universe radiates energy. You're most familiar with the energy known as "visible light" — you know, ROY G. BIV, the colors of the rainbow. But not all light is visible. If you go past violet light, you find ultraviolet light, then X-rays, and then gamma radiation. On the other end of the rainbow, beyond red light, you have infrared light, then microwaves, and finally radio waves.
You have to get comfortable with the words "radiation" and "light" being synonymous. Normally we think of "light" as the good stuff we can see, that makes our world bright and warm and happy, and "radiation" as the dangerous stuff we can't see, that makes our world dangerous and sunburned and radioactive. In science, all electromagnetic radiation, from gamma rays through visible light and all the way down to radio waves, is called "radiation" — or, in the vernacular, "light." All of it. Scientists use the two terms interchangeably.
Light, or radiation, has different colors, or "wavelengths". Each wavelength has a different amount of energy associated with it. The higher the energy, the more stuff the light can shine through. For example, X-rays have enough energy to shine right through a human body, although they don't shine as well through teeth or bone as through soft tissue. That's why doctors and dentists find them useful. Gamma rays have so much energy that they can only be blocked by several inches of lead, or several feet of concrete. Microwaves have so little energy that they can be contained in a metal box the size of, well, a microwave oven.
What does all this have to do with that girl in the cafeteria? We're getting there. First, let's see what you can do with light.
Absorption, reflection, and radiationAll material things in the world absorb light. The fact that they absorb different wavelengths of light is what makes the world so colorful. Consider four automobiles: a white car, a red car, a green car, and a black one. The red car absorbs all the light from the sun, except for the red light. It re-radiates, or reflects, the red light away from itself. Your eyes see the red light and perceive the car as red. Likewise, the green car absorbs all but the green light and reflects the green light, which your eyes see. The white car absorbs very little light, reflecting most of it back to your eyes. The black car absorbs most of the sun's light, in every wavelength, reflecting just enough light for you to see the lines of its body.
What do things do with the energy that they absorb? They have to do something with it; it can't just disappear. They turn it into another form of energy. Solar cells take visible light and turn it into electrical energy. Plants use visible and ultraviolet light to power the photosynthesis engines in their cells. (In fact, plants use most of the visible light. The only wavelengths they can't use are yellow and green, so they reflect those colors, which is why we see grass and trees as green.)
Objects also emit, or radiate, their own energy. The sun radiates a mixture of wavelengths which, when combined, appear to our eyes as white light. A hot stove radiates energy in the red and infrared wavelengths. Your eyes can see the red, and your body can feel the heat, or the infrared energy, radiating from the stove top.
Some objects absorb light at one wavelength and re-radiate it at a different wavelength, usually (or always) a lower-energy wavelength. Fluorescent paints or dyes absorb ultraviolet light and re-radiate the energy as various colors of visible light.
Most objects, however, absorb the energy from light, turn it into heat energy, and radiate it as infrared light. This is why a black dog lying in the sun gets so toasty warm. And, going back to our example of the cars, this is the reason why a black car gets hotter sitting in the summer sun than a white car. The white car reflects most of the sun's energy, while the black car soaks it all up, turning it into heat and raising the interior's temperature to unhealthy levels.
The fact that everything radiates energy is why infrared goggles and cameras can "see in the dark." They're not looking for visible light, but for the infrared light that everything radiates.
Now let's talk about eyeballs.
Black bodiesThe wavelength of light emitted by an object depends on the amount of energy in the object, and the amount of thermal energy stored in an object is determined by its temperature. So theoretically, everything at a given temperature emits the same radiation.
Physicists like to talk about an imaginary object that absorbs all the energy that hits it. Like the black car in our example, which absorbs most visible light, physicists imagine this object painted a black that even absorbs the invisible radiation below red and beyond violet. They call this imaginary object a "black body."
The U. S. Air Force has a few aircraft that are pretty good at absorbing radiation and not reflecting any back, and not radiating very much of their own, either. These Stealth aircraft are painted a dull black on the outside, and they're made from materials that even absorb the radio and microwave energy of radar beams. They have special air channels to cool down the hot exhaust gases of their jet engines. They're practically invisible in the night sky — they are the physicists' "black bodies."
The Holy Grail for these physicists is an "ideal black body" — one that would absorb all wavelengths or energies of electromagnetic radiation that shine upon it, and not reflect or radiate any energy at all. If such an object existed, it would be the purest black you've ever seen. It would have a surface that was a deep, soft black — deeper and softer than black velvet, blacker than the soot on the inside of a chimney.
The problem is that any object you can make — say a black velvet brick, dipped in soot — will still reflect just enough light that you can see its edges, discern its shape. The "ideal black body" would be so perfectly black that, sitting on your kitchen table, it would look like an odd-shaped hole in the fabric of the universe.
(See the end of this article for a cool update about that "odd-shaped hole in the fabric of the universe.")
One of the most useless things I learned in school, I thought, was how to make an ideal black body. Imagine you have a block of coal or charcoal — something really black. Now imagine that there's a bubble inside this block, a spherical cavity near the surface, like the bubbles in a block of Swiss cheese. Now, shave away at the surface of the block, one thin layer at a time, until you just barely break into the bubble. If you shine a flashlight beam — or better yet, a laser pointer — through that pinhole into the bubble, the light will bounce around on the curved inside walls until all of it is absorbed and none of it comes back out the hole. That cavity in the block has all the characteristics of an ideal black body.
When I was in college, I could think of few mental exercises as useless as that ideal black body. I could think of nothing in the world that approximated such a construction. Many years later, looking into the eyes of my newborn son, I found one.
Think of the human eye. It's a spherical shape. It has a pinhole opening in the front — an adjustable pinhole called the "pupil." The pupil admits light into the eye, where it bounces around until most of it is absorbed. The lens behind the pupil ensures that most of the light gets absorbed at the back of the eye, in a remarkable structure called the "retina." The cells of the retina absorb different wavelengths of light, converting the light's energy into electrochemical signals that are sent to the brain and help us perceive light, color, and shapes — red cars and green cars, black dogs and laser pointers.
The eye is not quite an ideal black body: its inner surface is not painted black. If you take a flash picture and the flash is too close to the lens of the camera, then the blood vessels on the inner surface of the eye will reflect red light back to the camera, resulting in the obnoxious "red-eye" effect that has plagued casual photographers since the days of Pocket Instamatic cameras.
Okay, now imagine a room where everything in the room is at the same temperature, so everything is emitting the same wavelength, and mostly the same intensity, of radiation. Through an infrared camera, you would see an even grey (or green) picture — well, not "even," but uniformly random.
Now, imagine that somebody in the room lights up a cigarette, or pulls out one of those hand-warmer packets you see at ski resorts. Using an IR camera, you will see a non-random, bright spot against the uniformly random background. If infrared light were visible light, the sudden appearance of this visible light would draw everyone's attention like a strobe or a flashlight beam.
(Time for a mental detour!
Question: Why does everybody turn in the direction of a flash, a sudden movement, or other changes in their field of vision?
Answer: I think it's an instinct that goes back to the days of cavemen, of predator and prey. The ability to notice a change in the wind, the sound of a snapping twig, or a moving shape among the trees, could mean the difference between life and death. Those who noticed subtle changes in their surroundings survived, and those who didn't notice, didn't survive. The instinct remains with us today, being useful on the basketball court, on the battlefield, and in the corporate boardroom.)
Now, imagine the same room as before, and this time somebody has a shoebox with a lid, the inside painted black, and a hole cut in one end. When the hole is turned to face the camera, the IR camera sees the hole as a non-random, black spot against the uniformly random background — just like the cigarette or the hand warmer, but dark instead of light. Why wouldn't everyone's attention be drawn to the sudden appearance of the black hole?
Well, what makes you think that it isn't?
Feeling is believingWe need to change the subject for a moment: let's talk about biology. Your skin has nerve endings just under the surface. These nerve endings give you the sense of touch. Some nerve endings respond to skin temperature: cold, warm and hot. Skin can be warmed or cooled by air, water, rocks, dogs — or light. (Well, warmed by light, not cooled by it. We already talked about that.)
In some creatures, these temperature-sensitive nerve endings are directional. Pit vipers, such as rattlesnakes, have organs in a pair of "pits" on their snout, which they use to "see" heat in three dimensions.
My theory is that the temperature-sensitive nerve endings in your skin are also somewhat directional. When camping in the woods on a cold night, you can close your eyes, hold up your arm, and turn your hand like a vane to find the source of warmth that is the campfire, even from a considerable distance. Try it sometime. You can also use your face, your neck or any other exposed skin to find the campfire.
Now, imagine that you're standing in the center of a ring of campfires, on a cold night, and there's a gap with no campfire. (You may not be able to really try this.) Your brain and all the thermal receptors in your skin will work together to find the direction of that gap in the ring — the absence of radiated heat — even with your eyes closed.
Okay, so what about that girl staring at you?Now you've got all the pieces to answer the question of why you can feel somebody staring at you. It's time to put all the pieces together.
You're sitting in a crowded restaurant. People are milling all around you, minding their own business, nobody looking directly at you. But then the thermal receptors on your ear, your cheek, or the back of your neck detect an anomaly in the uniformly random infrared background — a dark spot, a pinpoint absence of energy.
Two pinpoints, in fact. Someone all the way across the restaurant is staring at you. The pupils in her eyes are the focal points of two ideal black bodies, absorbing instead of emitting infrared radiation.
Your directional, sensory nerves communicate their readings to your brain, which subconsciously processes the anomaly, determines its location, and either sends your motor nerves the instinctive command to turn your head and glance in that direction, or sends the emotional center of your brain a creepy feeling, one that makes you ask your dining companion, "Is somebody staring at me from over there?" while you make furtive gestures over there. If you or your friend look, you may catch the person staring at you.
With any luck, she will flash a radiant white smile at you, which will warm you instantly — and all because two ideal black bodies in her head caused a low-energy anomaly in the random thermal background radiation, which your rattlesnake-like sense of touch detected, all the way across the room.
UPDATE, April 17, 2017: The Smithsonian Magazine has written about two black pigments that are so black that they reflect only a miniscule fraction of light. Objects painted with these pigments really do look like "odd-shaped holes in the fabric of the universe." The pigments are not without controversy in the artist world, ironically. Here are the links to read about vantablack and Black 2.0.
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