The Memory Machine by Cory Shaeffer

Back in 1991, 14 year old Jeffrey Archer had the ideal life. He had a close-knit group of 7 friends he'd grown up with, a neighborhood he loved, and a close relationship with Nina, a girl his age whom he adored. After falling off his bike and injuring his head one evening, Jeffrey began having a recurring dream in which an elderly woman was trying to contact him, seemingly from another dimension. After confiding in his friends about the dream, several of them revealed to Jeffrey that they were also having dreams about the same elderly woman. Their parents were quick to dismiss the dreams as coincidence, and eventually life went on as usual for Jeffrey and his friends.

30 years later, however, Jeffrey, who is now a physics professor at a Pittsburgh University, experiences a series of tragic events and his life begins to spiral out of control. Shockingly, the elderly woman starts reappearing in his dreams again. With nothing to lose and more curious than ever, Jeffrey befriends another professor at the University named Dr. Crenshaw, who claims to have invented a device that can record, analyze, and even alter memories. Dr. Crenshaw calls it "The Memory Machine." Their quest to discover the identity of the elderly woman unearths a series of clues to a decades-old unsolved murder which seemingly connects to a mysterious waterfall located in the neighborhood Jeffrey and his friends grew up in.

Eager to use the Memory Machine to correct the wrongs of their past, Jeffrey and Crenshaw successfully convince the elderly woman to join their efforts. With a time-altering device but no way to physically travel backwards in time, the trio realizes that they cannot complete their quest without help. With the key to solving the murder in their possession and the authorities desperately trying to get their hands on the Memory Machine, Jeffrey and Crenshaw attempt to slip the Memory Machine and the evidence through a time portal - to 14 year old Jeffrey and his friends back in 1991.