Alan Moore and Dave Gibbons invented the cast of Watchmen to evoke familiar superheroic themes that their audience would recognize. Everybody knows that all good superheroes have a secret weakness, so even the godlike Doctor Manhattan had one, too.
To put it in Superman terms: Tachyons are his kryptonite. And on Watchmen episode 8, we see the flaw put to use once again.
Tachyons are hypothetical particles that travel faster than the speed of light. The word itself was coined in 1967, plenty of time to have passed into the science fiction lexicon but to still have that frisson of hard science to them when Watchmen was being written.
Tachyons mess with what is arguably Doctor Manhattan’s most terrifying power: His ability to experience time as non-linear, which allows him to see the future. In the Watchmen graphic novel, Adrian Veidt/Ozymandias neutralizes Doctor Manhattan’s powers by enveloping his own Antarctic retreat in a shower of tachyons, preventing Manhattan from knowing that he would one day discover Ozymandias’ plan until he actually discovered it.
Doctor Manhattan experiences this as a fuzziness to his memory of the future. As he tells Laurie when he brings her to Mars to discuss whether he should save humanity, “I return to earth at some point in my future. There are streets full of corpses. The details are vague. [...] There’s some sort of static obscuring the future, preventing any clear impression. [...] Beyond that, events grow even sketchier: I am standing in deep snow ... I am killing someone. Their identity is uncertain.”
HOW DO TACHYONS WORK?
This is going to be a short explanation: Tachyons are theoretical science. Watchmen is a comic book. There’s no scientific explanation for why they do this to Doctor Manhattan, because there’s no scientific explanation for Doctor Manhattan.
SOME WEDDING RING
The seventh episode of HBO’s Watchmen, “A God Walks Into Abar,” establishes that the network of satellites and generators that created a tachyon shower in Adrien Veidt’s lair were not his first attempt to create a tachyon-based weapon to disable Doctor Manhattan. Veidt reveals a small box and asks Doctor Manhattan what is inside it. The unsurpriseable man is surprised: He doesn’t know.
That’s because the box contains a tachyon device, fashioned much like Manhattan’s hydrogen atom symbol, and his memory of its existence could not propagate backwards through time. Manhattan gave the device to Angela Abar so that she could use it to restart his memory, erasing all memory of his life, so that they could live happily as equals. In order to restore him to his full memory and powers, all she had to do was [checks hand] bash his skull open with a hammer.