Thrice Upon a Time

the long version

When Murdoch was summoned to his grandfather's isolated Scottish castle, he had no idea of the old man's latest discovery -- nor where it would lead him. Sir Charles, a genius in far-out physics, had found a flew in the law of conservation of energy; in any process, an incredibly tiny increment of energy escaped -- back through time! Using this "tau" radiation, he could send messages into the past. But Murdoch discovered records of messages he knew he had never sent. Were many futures possible? Could a message from Future X alter the past -- and thus wipe out Future X? But who would be foolish enough to send a message that could eliminate his own existence? Then disaster struck. An advanced fusion reactor threatened to destroy all Earth. Grimly, Murdoch sat down to send back the words that would destroy everything he had learned to love. (description from Goodreads)