The Twilight Saga definitely breaks away from traditional vampire descriptions, some of which are points of contention within the fanbase. The most obvious example is their sparkly skin. While classic vampires burn up in sunlight, Twilight vampires have skin that shines and sparkles. Although opinions on this will always clash, one fan theory has managed to explain the skin reaction with science.
Twilight vampires only sparkle when they stand in direct sunlight. If they're under shade or in indoor light, then it has no effect. Going off of this basis, a Reddit fan theory has narrowed down what aspects of sunlight could possibly be affecting their skin. As such, it needs to be something that "doesn't tend to exist inside" and "is fairly easily blocked by things like umbrellas or thick clouds."
Firstly, it's obvious that the sun has something to do with the reaction; it constantly radiates electromagnetic radiation, which in itself takes many forms, including microwaves, gamma-rays, radio waves and so on. So, the components of the sun's radiation narrow down all the possibilities of what could be affecting the vampire's skin.
Modern technology means that radio waves are all around, so they couldn't be the cause. Then, there are microwaves, which are much more sparse in day-to-day life. Their main source is through direct sunlight and appliances such as microwave cookers, making them a strong contender for the skin reaction. However, clouds can't fully block microwaves, and the vampires don't get sparkly skin under cloudy skies. Many of the remaining components, like x-rays and gamma radiation, are lost as they enter the Earth's atmosphere. Therefore, this leaves ultraviolet (UV) radiation, a radiation that is largely blocked by both cloud cover and buildings.
With UV radiation or microwaves as top contenders, the theorist suggests that "UV radiation is much more likely to be the culprit." Not only because it rarely passes through cloud cover but because the frequency of UV radiation is much closer to the frequency of visible light. This means that whatever is on the vampire's skin could alter the frequency of UV radiation and make it visible as a sparkle.
While delving much deeper into the specifics than most fans, this theory makes a convincing argument. The science checks out, although it ignores what unknown property of the skin could actually be reacting to the radiation and the possibility of some supernatural component unknown to humans. Still, it would mean that Twilight vampires have simply had a reaction to UV radiation all along, and all they would need to prevent it is to apply some strong sunscreen.