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The Curious Case of Street Lamp Interference

Notes on a Strange World

Massimo Polidoro

Skeptical Inquirer Volume 32.6, November / December 2008

It’s about midnight and you are heading home. Suddenly, the street lamp above turns off without reason, and you find yourself in the dark. It is natural to experience a chill. But what would you think if street lamps kept turning off when you passed them by?

It is something that many of us have experienced, at least once. Many don’t take notice, but others do and wonder if the cause of such interference lies inside them.

“The fact that so many witnesses are making claims which seem to involve a Street Light Interference (SLI for short), that they are doing so in apparent good faith, and doing so independently of one another and without awareness, that the effect may constitute a phenomenon in its own right, these circumstances encourage us to proceed on the basis that SLI, whatever its nature, does indeed occur.” These are the words of Hilary Evans, English author, fellow researcher and friend, who in 1993 founded “Project SLIDE”.

Says Evans: “Project SLIDE was created simply as a first step towards defining and assessing the apparent phenomenon. As its name implies, it sets out to be little more than an exchange of information between those who are interested.”

The idea is that there appears to be an effect that is not consistent with our current knowledge of how people interact with the physical world, and which occurs in specific circumstances.

Four explanations for SLI have been proposed.


“A primary question must be: does SLI occur at all, or are the alleged witnesses deluding themselves?” wonders Evans. “Until the phenomenon is scientifically tested, it is not possible to give a decisive answer to this question. However, SLI has not the ‘appeal’ of witchcraft or abductions: there is nothing like the same psychological pay-off. Individuals seeking to enhance their reputation for possessing special gifts will not find much to flatter themselves with in SLI. In short, it seems highly unlikely that all SLI experiences are delusory.”

Energy Fields

Some believe that what is causing street lamps to turn off could be some kind of “energy” emitted by the human body. Eyewitnesses report that the turning off of the lamp happened while they were tired, stressed, furious, or sad. Some others, however, think it might be some kind of static electricity produced by their body.

However, the only form of energy known to science produced by the human body comes via food and breathing and is then used by the body to walk and work. There are no other energies produced or emitted by the human body (except for body warmth, of course). Static electricity is not produced by the human body but by rubbing things, usually synthetic clothes, in a dry climate. It has nothing to do with one’s state of mind. Furthermore, the static electricity produced by a polyester jacket has no way of interacting with street lamps, usually high above street level.

Paranormal Phenomenon

Paranormal phenomenon is the least likely possibility. Science has never confirmed that the human mind can cause physical effects at a distance, which is what seems to be occurring in SLI. “However,” says Evans, “SLI does have one great advantage over most psychokinetic experiments: the subjects of the effect—the street lamps— are not easily manipulated.” The only problem is that experiments to test SLI are not easy to conduct since this appears to be a phenomenon that just happens at random and is not produced by one willing for it to happen.

Mechanical Effect

“The fact that a mechanical device is involved logically suggests that a mechanical explanation should be looked for,” says Evans. But what kind of explanation?

In order to answer this, we need first of all to determine what kind of lamps we are talking about. I asked Mario Bonomo, professor of illuminating engineering at the University of Milano, to illuminate me. “The most common ones, almost all over the world, are sodium vapor lamps. These are gas-discharge lamps that use sodium in an excited state to produce light.” There are two varieties of such lamps: low pressure and high pressure. Low pressure are those that produce the characteristic yellow light, while high pressure give a whiter light that allows colors to be recognized. Street lamps usually have low pressure bulbs.

“These bulbs take three to four minutes to light up and have a lifespan of 8,000 hours, two years approximately,” continues Bonomo. “When a bulb reaches the end of its life it shows a behavior that could explain SLI. Older lamps need a higher tension than the one they receive. This means that when they are turned on, the tension is sufficient. But when they reach their maximum luminosity, the tension required is more than what is received. This causes the lamp to turn off. Now, in order for it to light up again, the bulb needs to cool off first. And this takes a few minutes. After this, the process, known as cycling, starts again from the beginning until the bulb is substituted with a new one.”

This could explain the repeated turning on and off of the lamps. But how can we explain it when it is not just one lamp turning off but all the lamps on a street?

“There are two possible causes,” says Bonomo. “The first one is that the bulbs on that specific street are all the same age and, thus, they all get old at the same time, producing clustered but random on and off cycles. However, if street lamps in a specific street turn off all at once, then the problem lies in the central electric-control panel. There usually is one that controls all the lights in a specific block, or one every 200 square meters. A power failure or a short-circuit can cause all the lamps controlled by that panel to turn off.”

Finally, if the connection between the lamp and its socket is faulty and gets interrupted for some reason, even for a fraction of a second, the bulb turns off and then it will need a few minutes to turn on again. A contact, especially if already faulty, can be interrupted even by some minor vibration, like a kid kicking the lamp post, a large truck passing in the street, wind rocking the bulb, and so on.

The Power of Suggestion

This is all very interesting and could actually explain much of the SLI phenomenon. However, in order to understand fully what might be taking place here, it is important to consider the observer bias as well. Our mind is drawn by significant coincidences, and so we are much more likely to notice when a street light near us turns on or off than when a street light is in a steady state.

It could just be, then, that SLI is a mix of different natural factors. The normal behavior of bulbs getting older, observer bias, and maybe something else, like the fact that some specific lamps, such as those in gardens or on patios, have infrared sensors that can turn them on or off when something is moving within range. Other lamps are programmed with a timer that turns them on or off at specific hours. Some people realize this is the cause of such changes in the lights.

Evans, however, feels that there is something more in SLI and, thus, he says we should proceed as though the phenomenon exists. “For one very good reason, the fact [is] that a good many people are reporting the experience as though an actual phenomenon is involved. Certainly, people can be mistaken or deluded, and we must keep this possibility in mind. But that, too, is something that would have to be proved before we would accept it; and until such time as it is proved, it is right to respect the testimony of people who claim these experiences at first hand.”

The whole thing, however, could become really significant when the same person, at different times and with different lamps, over a consistent period of time, keeps on noticing anomalous behavior of street lamps. So far, however, nobody seems to have had this experience.

Further Reading

  1. You can download for free Hilary Evans’s booklet, The SLI Effect, Assap Publications, at

Massimo Polidoro

Massimo Polidoro's photo

Massimo Polidoro is an investigator of the paranormal, author, lecturer, and co-founder and head of CICAP, the Italian skeptics group. His website is at