In War Times

Sam Dance is a young enlisted soldier in 1941 when his older brother, Keenan, is killed at Pearl Harbor. Afterward, Sam promises that he will do anything he can to stop the war.
During his training, Sam begins to show that he has a knack for science and engineering, so he is picked by his superiors from the daily grunt work of twenty-mile marches to study subjects such as code breaking, electronics, and physics, a science that is growing ever more important to the war effort. While studying, Sam is seduced by a mysterious female physicist teaching one of his courses and given her plans for a device that will end the war, perhaps even end the human predilection for war forever.
After his training, Sam is sent to high-pressure areas throughout Europe to solve both theoretical and practical problems for the Allies. He's even sent to the Pacific on the eve of the first atom bomb. When he has free time, Sam plays jazz and uses the chaotic nature of the music to help him reconstruct the strange device.
It takes more than a year, but the device is built and started. And then something happens, but Sam cannot initially discern what it might be, other than that the device melts into a solid block of metallic substance.
It's only much later that he discovers that the device worked, but in a way that he could never have imagined - he finds that he and the others exposed to the device can move among versions of reality and back in time, and possibly change history.