Stephen William Hawking, the brilliant theoretical physicist, is about to experience weightlessness for a first period of 25 seconds today, although not in actual space, but in a simulated "free-fall" of a plane.

The 65-year-old Professor of Mathematics at the University of Cambridge is known for his contributions to the fields of cosmology and quantum gravity, especially in the context of black holes, and his popular works in which he discusses his own theories and cosmology in general, like A Brief History of Time.

He also has the paralyzing disease ALS, also known as Lou Gehrig's disease. He'll be the first person with a disability to fly on one of the flights offered by Zero Gravity Corp., a space tourism company. It has flown about 2,700 people on Florida-based flights since late 2004 and began offering flights in Las Vegas this week.

Unable to talk or move his hands and legs, Hawking can only make tiny facial expressions using the muscles around his eyes, eyebrows, cheek and mouth to communicate. Otherwise, he relies on a computer to talk for him in a synthesized voice. The computer is attached to his wheelchair and allows him to choose words on a computer screen through an infrared sensor on a headpiece that detects motion in his cheek.

He raises an eyebrow to signal "yes," and tenses his mouth to the side to indicate "no."

So, he will be assisted into a modified Boeing 727-200 which will take off from the Kennedy Space center and ascend steeply to an altitude of 30 thousand feet (around 10 km) before plunging into a controlled free fall.

As the plane reaches the apex of the parabolic arc, Hawking and all others aboard will be rendered weightless. He will float freely, away from his wheelchair, in the cabin of the airplane - liberated for approximately 30 seconds from the effects of a disease which has left him unable to move most of his body. A medical team and a team of handlers will be on board to assist him throughout the flight.

Normally, the plane conducts 10 to 15 plunges for its passengers who pay $3,750 for the ride, although that fee has been waived for Hawking.

"I am looking forward to experiencing weightlessness. It has been many decades since I have been out of my wheelchair," said Hawking.

Other plunges will be made only after the two doctors and three nurses who are accompanying him have made sure he is enjoying it.

He won't have his wheelchair and talking computer on the jet with him, although his assistant will bring a laptop and a card with the letters of the alphabet in case Hawking wants to communicate beyond facial expressions.