June 7, 2010 -- Celebrated physicist Stephen Hawking knows more about the universe than almost any other person ever to walk the planet, but some answers still escape even him.

When asked by ABC News' Diane Sawyer about the biggest mystery he'd like solved, he said, "I want to know why the universe exists, why there is something greater than nothing."

Hawking, who was honored last week at the World Science Festival in New York, is famous for probing the deepest questions of the cosmos.

Until he stepped down last fall, he held the post of Lucasian professor of mathematics at Cambridge University, a position once held by Sir Isaac Newton, the "father of physics" himself.

During Hawking's 30 years in the post, he gave his colleagues new ways to look at the universe. But he also gave the public, through his many books and occasional appearances, a way to connect with the often obscure world of theoretical physics.

'Brief History of Time' Is an International Best-Seller

"Stephen has had a tremendous impact on our understanding of the universe, particularly of warp space and warp time in the universe, black holes, the origin of the universe," said physicist Kip Thorn at last week's World Science Festival event honoring Hawking.

Hawking's most well-known book, "A Brief History of Time," sold more than 9 million copies and is an international best-seller. He's even made brief cameos in pop culture staples like "The Simpsons" and "Star Trek."

But exploring the origins of time inevitably leads to questions about the ultimate origins of everything and what, if anything, is behind it all.

"What could define God [is thinking of God] as the embodiment of the laws of nature. However, this is not what most people would think of that God," Hawking told Sawyer. "They made a human-like being with whom one can have a personal relationship. When you look at the vast size of the universe and how insignificant an accidental human life is in it, that seems most impossible."

When Sawyer asked if there was a way to reconcile religion and science, Hawking said, "There is a fundamental difference between religion, which is based on authority, [and] science, which is based on observation and reason. Science will win because it works."

Hawking: Alien Visit Would Be Like Columbus' Arrival in the Americas

Hawking, 68, was diagnosed with the motor neuron disorder ALS, or Lou Gehrig's disease, when he was 21. He was told that he likely would not live more than a few years.

But on his personal website he said that with "a cloud hanging over my future, I found, to my surprise, that I was enjoying life in the present more than before."

"I suddenly realized that there were a lot of worthwhile things I could do if I were reprieved," he said.

He went on to have a family (he has three children and one grandchild) and, though his condition progressed, he continued his research unabated with the help of a wheelchair and, later, an electronic speech synthesizer.

Though Hawking no longer holds his position with Cambridge University, he continues to hold the public's attention.

In April, he grabbed headlines after the premiere of the Discovery Channel series, "Into the Universe With Stephen Hawking."

Aliens Might Look to Conquer, Colonize, Hawking Said

In the program, he said it is likely that alien life exists, but a visit from extraterrestrials might be similar to Christopher Columbus' arrival in the Americas.

"If aliens visit us, the outcome would be much as when Columbus landed in America, which didn't turn out well for the Native Americans," he said. "We only have to look at ourselves to see how intelligent life might develop into something we wouldn't want to meet."

He also speculated that aliens' capabilities "would be only limited by how much power they could harness and control, and that could be far more than we might first imagine."

It might even be possible for aliens to harvest the energy from an entire star, he added.

"Such advanced aliens would perhaps become nomads, looking to conquer and colonize whatever planets they can reach," Hawking said.

Hawking to His Children: Look Up at the Stars

But though Hawking may think that looking for aliens is not a wise choice, his daughter Lucy, with whom he has written a children's book, told Sawyer that she disagrees.

"We have a fundamental disagreement on that one," she said good-naturedly. " Because I think if they're so smart that they could destroy us, then it doesn't matter if we look for them or not. Because if they're that clever, they know we're here. They can find us whether we look for them or not."

But Lucy said she and her father agree on many other things.

"We like the bustle, we like the excitement, we like the lights, we like the company," she said. "We like all the complicated things that human civilization has created."

As Hawking's children navigate the many complexities of human life, he told Sawyer that he's offered up three pieces of advice.

"One, remember to look up at the stars and not down at your feet. Two, never give up work. Work gives you meaning and purpose and life is empty without it," he said. "Three, if you are lucky enough to find love, remember it is there and don't throw it away."

If you want to learn more about the 5-day World Science Festival, click here.

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