Continuity mistake: Near the beginning, as Ray has a vision of the baseball field, look at the difference of the angle of the sun. During the beginning of this sequence it's near noon. When Ray sees the field materialize the sun is obviously further west in the sky. Then, at the end of the sequence it is noon again.
Continuity mistake: The sun changes considerably when Doc is leaving to walk into the corn and the previous and following shots. When it cuts to Shoeless Joe, the sun is at his back. Back to Doc leaving, the sun is setting.
Continuity mistake: The clock at Fenway jumps two minutes in very little time from 10:30 to 10:32 in the space of two short scenes.
Continuity mistake: During the first practice, the sun changes from late afternoon initially to midday and back to late afternoon throughout the sequence.
Answer: It was actually his own voice (Kevin Costner) that sent him on the journey. At the end of the movie there's even the part where Ray is recalling the voices he heard and turned to Shoeless Joe Jackson and says, "It was you". At which point Joe Jackson turns around and says, "No Ray. It was you".