Dr. Andrew Newberg, a neuroradiologist at the University of Pennsylvania, is one of those researchers. Newberg says his research shows a marked increase in brain activity in the frontal regions of the brain. "At the same time," he adds, "the parts of the brain that monitor our sense of time and space became less active."
Newberg says this contributes to an individual's feeling of "losing that sense of self." The feeling, he said, is "attributed to God, for example. And then they feel that God is providing them that energy, that feeling."
But for Ellen Johnson, president of the American Atheists, science or no science, heaven is a myth.
"Heaven doesn't exist, hell doesn't exist. We weren't alive before we were born and we're not going to exist after we die. I'm not happy about the fact that that's the end of life, but I can accept that and make my life more fulfilling now, because this is the only chance I have," she tells Walters.
Walters also talks with people who feel certain of heaven's existence, apart from their faith, because they believe they've had a glimpse of it in near-death experiences.
A U.S. News & World Report from the late 1990s says as many as 18 million Americans believe they have had near-death experiences that gave them a glimpse of the afterlife.
Dianne Morrissey tells Walters she felt the "white light of God" when she was electrocuted. "My near-death experience changed everything about me. … There is not a single experience on Earth that could ever be as good as being dead," she said.
British psychologist Susan Blackmore has spent decades searching for a scientific explanation: "When the oxygen levels fall in the brain … you get massive over-activity in the brain. … I think there is a true transformation, but not because you've been to heaven."
Walters talks with California's first lady, Maria Shriver, whose early experiences with loss as a member of the Kennedy family prompted her to write a book about heaven for children. "I had, growing up, a lot of questions about these deaths that occurred in my family with no person to really talk to them about it," she tells Walters.
"My daughter, who was about 6 or 7 at the time, started asking me a lot of the same questions that I had had as a child, really basic questions: 'Why do you put somebody in a coffin? Where does she go now? Is she scared in the box? Can she breathe in the box?' And what was interesting, Barbara, was that she started answering the questions for herself. So I started writing down her answers," she said.
Walters also talks with Mitch Albom, author of "The Five People You Meet in Heaven," to get his personal take on the afterlife.
Albom tells Walters, "There's one thing I would say about heaven. If you believe that there's a heaven, your life here on Earth here is different. You may believe that you're gonna see your loved ones again. So the grief that you had after they're gone isn't as strong. You may believe that you'll have to answer for your actions. So the way you behave here on Earth is changed. So in a certain way, just believing in the idea of heaven is heavenly in and of itself," he said.