What burst of light? How do you know this? Who has ever seen this? How has it been recorded?
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I knew an embryologist who assured me that this "burst of light" is unmistakable and occurs at the moment of conception.
He said it was caught on film many years ago and since then has been docuмented many times.
At that time the cause of the light wasn't known, but I haven't heard if any new discovery has been made lately.
Perhaps the source of the light is still a mystery, or maybe there has been further fruitful research.