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Although street lights can turn off by chance, such as high pressure sodium street lights cycling (turning on and off repeatedly) at the end of their life cycle, believers in street light interference tend to claim that it happens to them personally on a regular basis, more frequently than chance would explain. Some propose paranormal explanations for SLI, sometimes based on scientific terminology, such as the explanation that electrical impulses in their brain interfere with the workings of electric lights. Anecdotes about people's experiences of SLI have been reported by news sources.
SLI has never been demonstrated to occur in a scientific experiment, and those who claim to cause it have been found to be unable to reproduce the effect on demand; they give the explanation that the effect is not within their mental and physical control. Many times they do report, however, that it occurs with specific lamps and not just randomly shutting off street lights or electrical lamps in general.
Hilary Evans, an English author who writes about paranormal subjects, coined the word "SLIder" to refer to someone who causes this effect, in his book The SLI Effect. In this book on page 15 he explains that a wide variety of street lamp types show
|“||the effect is spontaneous and is apparently meaningless; it serves no practical purpose, nor does it seem to provide satisfaction for the individual or in any other way serve some kind of psychological purpose.||”|
In the preface of the book Evans says,
|“||SLI is an apparent phenomenon, based on claims by many people that they involuntarily, and usually spontaneously, cause street lamps to go out. Generally the effect is intermittent, infrequent and without an immediately discernible sequence of cause and effect. SLI deserves study because it gives the appearance of being an anomalous phenomenon in its own right. That is to say, it appears to be an effect which is not consistent with our current knowledge of how people interact with the physical world, and which occurs in specific circumstances.||”|
On page 16 he explains what seems most likely to be happening,
|“||Most commentators, confronted with the Street Light Interference phenomenon, look - and rightly - for a straightforward physical explanation. For example, when Robert McMorris of the Omaha World-Herald devoted two or three issues of his regular column to SLI reports in January 1990, he quoted Allen Klostermeyer, manufacturer's representative for Lighting Specialists Inc., who pointed out that when a sodium (amber) bulb nears the end of its useful life, it may go into an off-and-on sequence: ||”|
|“||This, it was suggested, is sufficient to explain the SLI effect; what happens is that the witness just happens to be passing such a lamp during its death-throes, and is led by the synchronicity to imagine that he is somehow responsible. But as the testimony shows, even if we allow the coincidence in place and time, this effect could account for only a small fraction of the reported cases. For one thing, other types of lamp are involved besides sodium lamps. Then again, only a small number of reports describe anything like an SL going off, then on, then off again. And what about when a witness extinguishes a whole batch of SLs: are we to conclude that the whole batch was purchased together, and so shared the same life-span, and such was the perfection of their manufacture, that they all reached their death-point simultaneously? Yet even if we allow that, there is still the fact that some SLIders extinguish a row of SLs in sequence, each one going out as the witnesses nears it: it is asking too much to suppose that a series of lamps would have been arranged in order of their life-span.||”|
The skeptical explanation to claims of SLI is to consider it an example of correlation not implying causation, or of confirmation bias: people are much more likely to notice when a nearby street light turns on or off than they are to notice a light turning on or off at a distance, or a street light in a steady state at any distance. This is compounded by a failure mode of street lights, known as "cycling", in which street lights of the high pressure sodium type turn off and on more frequently at the end of their life cycle. A high pressure sodium engineer at General Electric, quoted by Cecil Adams, summarizes SLI as "a combination of coincidence and wishful thinking". Massimo Polidoro notes in Skeptical Inquirer that "Paranormal phenomenon is the least likely possibility."
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