« September 2006 | Main

October 3, 2006

How does a mass create gravity?

Every mass—be it an apple or the sun—distorts space and time, explains Physics and Astronomy professor Serge Rudaz. Gravity, he says, is the manifestation of that distortion.

It’s taken centuries for scientists to fit together the pieces of the gravity puzzle, Rudaz says. In the 17th Century Galileo Galilei put the first piece in place when he discovered that, if you take air resistance out of the equation, all objects fall at the same rate.

Some 60 years later, scientist Sir Isaac Newton contributed the next vital piece of the puzzle. Newton thought that whatever kind of physical law would be used to describe the motion of a falling apple, that the same physical law ought to be used to describe the motion of the moon and the planets. He further proposed that if you have a large mass such as the Earth, then it acts on any other mass in such a way that there is a force between the two of them that’s proportional to the product of their masses and inversely proportional to the square of their distance.

Newton’s methods of calculating the effects of gravity have been reliably used for centuries, most recently enabling scientists to launch the first space probes and plan the moon landings. But one important piece of Newton’s theories couldn’t stand the test of time.

“Newton saw gravity as an instantaneous influence,? Rudaz says. “If you have a mass, it instantaneously affects the motion of other masses.?

Early in the 20th century Albert Einstein saw this as a flaw in Newton’s theory and inserted a new piece in the gravitational puzzle—time. A basic ingredient of Einstein’s previous great work, the Special Theory of Relativity, is that the speed of light is a universal speed limit so that nothing in the universe happens instantaneously; everything takes time. With this, and building on the earlier insights of Galileo and Newton, Einstein concluded that clocks tick at differing rates and the lengths of meter sticks will differ depending on whether you are on the Earth, on a satellite orbiting the Earth, or elsewhere in the Universe. Einstein’s new view enabled scientists to map gravitational forces using a curved space/time grid.

One way to visualize it, Rudaz says, is to picture a large rubber mat painted with a grid, with a clock at each point of the grid and with the rule that objects traveling along that grid move at a constant speed in a straight line. Now imagine you place a large mass, like a bowling ball, on the mat. Near the mass, the mat will be indented and the grid distorted. Objects continue across the grid as before, but close to the mass the grid lines will be stretched. An object traversing the grid far away from the mass will not be noticeably affected by the distortion, but one traveling close to the mass will be seen to accelerate because, generally speaking, it will traverse the same number of gridlines in the same amount of time. However, due to the distortion, the gridlines are actually farther apart.

To draw a complete picture, Rudaz says, one must also take into account the gravitational effect on time, which passes slower close to the mass than farther away.

While Newton’s formulation is accurate enough to plan space missions across the Solar System, Einstein’s refinements are required not only to explain a number of very precisely measured astronomical phenomena, but are also at the heart of our modern understanding of the history of the universe. More surprisingly, perhaps, there is an example of everyday technology that serves to confirm Einstein’s views: The distortion of space and time in the Earth’s neighborhood that is prescribed by his theory of gravitation (known as the General Theory of Relativity) must be taken into account to accurately implement the GPS global positioning system.

“According to Einstein, gravity is a manifestation of the variable geometry of space-time, that is, how the lengths of meter sticks and the rates at which clocks keep time are affected by the presence of a mass,? Rudaz says. “Newton could not have thought of it this way, and he would have been very surprised: For him, time was absolute and, well, meter sticks would have been meter sticks!?

Serge Rudaz is a professor and director of undergraduate studies in the School of Physics and Astronomy.