According to a common horror trope, a werewolf can only be killed or incapacitated by the element silver. I'd like to know if science can explain why no other metallic element except silver is lethal to a werewolf?
No, because the premise is illogical and unscientific.
There is no scientific nor logical reason why anything would be immune to all forms of harm to begin with, much less anything invulnerable except to harm from a specific substance. This could not work under conventional physics. If we are dealing with magic then anything is possible because magic does not exist in our reality and is not subject to our rules.
Invulnerability is a fantastical concept.
This is an example of a common folklore convention known as a vulnerability, whereby a monster is immune to all forms of harm except from a specific substance even if that substance normally cannot cause the sort of harm depicted. It occurs quite commonly, for example:
- the Wicked Witch of the West is invulnerable to everything except being dissolved in water
- European fairies are invulnerable to weapons that are not forged of iron
- According to legend, the Beast of Gévaudan was killed by silver bullets.
- In the Brothers Grimm fairy tale The Two Brothers, a bullet-proof witch is killed by silver bullets.
- According to folk songs, the Bulgarian rebel leader Delyo was invulnerable except to a silver bullet.
Silver and werewolves were linked only recently.
The specific instance of werewolves being vulnerable to silver appears to originate from the 1935 movie Werewolf of London. Prior to that film, werewolves and silver were not linked in folklore nor the popular imagination.
However, the concept of werewolves being vulnerable to silver is recounted in the 1863 book Curiosities of Indo-European Tradition and Folk-lore by Walter Keating Kelly. There it is said that throwing iron or steel at a werewolf will cause it to turn back to a man and that werewolves which are otherwise invulnerable may be harmed with inherited silver and elder pith.
In the 1987 movie Monster Squad, a group of teenagers initially disbelieve that a werewolf is invulnerable to harm from non-silver sources. They test their hypothesis by using dynamite to explosively dismember the werewolf. The werewolf not only survives but slowly and gruesomely reassembles its body. It is finally killed by a silver bullet to the heart.
Silver has the highest electrical, thermal and reflectivity of any metal. I suggest one of these must be the reason.
Electrical - maybe werewolves have a higher level of electrical nerve function leading to their improved reactions and better senses. Silver short circuits this electrical activity away and causes werewolves nerves to fail.
Thermal - werewolves don't produce heat as well as humans due to them being partially dead and having more energy going into movement more efficiently. Silver removes their heat faster than they can create it. This kills werewolves.
Reflective - Still working on this one. Something to do with the moon being reflective maybe?
If you're looking for something scientific sounding to explain the the silver vulnerability. Start with your explanation of why they're invulnerable to everything else. Something unique to silver disrupts their vulnerability.
For instance the "invulnerability protein" reacts violently with silver creating toxic byproducts.