Судлаачид хүний тархины мэдээлэл боловсруулах онцлог нь гэрэлт цамхаг алсад хөдөлж буй мэт хуурмаг дүрслэлийг үүсгэдэг болохыг математик загварчлалаар тогтоожээ.
Далайчид болон аялагчдын дунд 89 жилийн турш яригдаж ирсэн, далайн мандал дээр гэрэл хөдөлж буй мэт харагддаг “хуурамч гэрэлт цамхаг”-ийн үзэгдлийг Мельбурнийн их сургуулийн судлаач Чарльз Кэмп болон түүний багийнхан судалж, тайлбарлалаа. Энэхүү үзэгдэл анх 1937 онд Францын Бриттани эргийн орчимд тэмдэглэгдсэн бөгөөд ажиглагчид гэрэлт цамхгийг яг хаана байрлаж байгааг мэдэж байсан ч гэрлийн эх үүсвэрийг далай дотор байна хэмээн эндүүрдэг байна.
Судлаачид “Open Mind” сэтгүүлд нийтлүүлсэн судалгаандаа Байесийн загварыг (Bayesian model) ашиглан хүний тархи хүлээлт болон нүдний харсан бодит мэдээллийг хэрхэн нэгтгэдгийг тайлбарлажээ. Харанхуй далайн мандал дээр баримжаа авах өөр биет байхгүй үед тархи нь аливаа зүйлийг ойрхон байдаг гэсэн туршлагадаа тулгуурлан таамаглал дэвшүүлдэг байна. Үүний улмаас гэрэлт цамхгийн цацраг туяаны алсын үзүүрийг дэндүү ойрхон гэж андуурснаар гэрлийн эх үүсвэр нь тэнгэрийн хаяанд, эсвэл далай дотор байгаа мэт хуурмаг дүр зураг үүсдэг аж.
Тус багийнхан виртуал бодит байдлын (VR) тусламжтайгаар хийсэн туршилтаараа энэхүү үзэгдэл тодорхой зайнаас эхлэн илэрдэг болохыг урьдчилан таамагласан байна. Жишээлбэл, 30 метр өндөртэй гэрэлт цамхгийн хувьд ажиглагч 67 метрээс илүү зайд байрлах үед л энэхүү хуурмаг үзэгдэл үүсэх магадлалтайг тогтоожээ. Гэсэн хэдий ч судлаачид энэхүү загвараа бодит гэрэлт цамхаг дээр албан ёсоор туршиж үзээгүй байгаа бөгөөд судалгааны үр дүн нь хүний тархи орчны мэдээлэл дутмаг үед хүлээлтэд тулгуурлан бодит бус дүр төрхийг бүтээдэг гэсэн дүгнэлтийг бататгаж байна.
Дэлгэрэнгүйг эх сурвалжаас харах
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The lighthouse seemed to be far out at sea, and sometimes it even appeared to move, like a light carried on a speeding boat. In reality, it was standing still on the island behind the man watching it. Charles Kemp, a researcher at the University of Melbourne, saw this on a seaside holiday at Lady Elliot Island, at the southern tip of Australia’s Great Barrier Reef, without ever having heard of the effect.
The island has two lighthouses: the original tower, built in 1873 and retired in 1995, and a newer light standing close by. Neither sits out on the water. Yet the phantom lighthouse illusion is so reliable there that the island’s tourist guide mentions it, and in Kemp’s experience the false source could appear hundreds of meters from the real one.
Kemp and two colleagues now have a detailed explanation, published in the journal Open Mind. The brain, they argue, badly misjudges how far away the distant end of a lighthouse beam is, and that single error is enough to flip the light’s apparent source to the opposite side of the sky.
A Puzzle First Reported Off Brittany in 1937
The illusion is far older than Kemp’s holiday. The earliest known report was published in 1937 by G. Colange and Y. Le Grand, who described it off the coast of Brittany in France. A letter about the same puzzle appeared in Nature a year later, and the cause has been argued over ever since.
Most accounts come from sailors, who sensibly keep more than a kilometer away from a lighthouse and the reef it guards. One writer on optics, M. Minnaert, called the effect a “most impressive sight.” Knowing the truth does not help: even observers who know exactly where the lighthouse stands cannot shake the impression that the light comes from the sea.
Why the Brain Gets the Distance Wrong
Kemp worked with Simon Cropper of the University of Melbourne and Weilun Xu of EPFL in Lausanne. They built a Bayesian model, a mathematical description of how the brain blends what it expects with what the eyes report. When the evidence is weak, expectation wins.
At night over open water, the evidence almost vanishes. There are no trees, buildings or waves to show how far away anything is. The brain falls back on a lifetime of experience in which the nearest object along any line of sight usually lies within a few hundred meters. A beam built to reach ships far out at sea breaks that rule badly.
The authors write that the illusion “arises from underestimating the distance of the remote parts of the beam.” As a beam rotates, its far end sweeps through an enormous arc and moves much faster than the part passing overhead. Because the brain places that far end too close, it reads it as moving slowly. And a beam whose far end moves slowly is exactly what you would see from a lighthouse sitting on the horizon.
The same mistake explains a second trick of the eye, the bent beam illusion. Lighthouse beams run parallel to the ground, yet people often see them curving down toward the earth or the sea.
The 67-Meter Test
The model makes a prediction that older explanations do not. One long-standing idea blamed perspective alone, the way parallel lines seem to meet in the distance. If that were right, people would see the phantom at almost any distance from the lighthouse.
The new account draws a line. For a light about 30 meters (100 feet) tall, the illusion should appear only once the observer stands more than about 67 meters (220 feet) away. Right at that point, people should be unsure whether the light is in front of them or behind them, and taller lights should push the threshold farther out.
The team rebuilt the scene in virtual reality, letting viewers watch simulated beams from different distances, and the results matched the predictions. The researchers say their own nights on Lady Elliot Island fit the model, though they have not yet reported formal trials beside a real lighthouse.
Writing in The Conversation, Cropper and Kemp put the lesson simply: what people see is a blend of “what they expect to see and the evidence provided by their senses.” On a dark sea, with almost no evidence to go on, the brain draws in a lighthouse that was never there.
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