Return to Home Page QUESTIONS AND ANSWERS Organized by Topics Select Topic Air Conditioners Airplanes Audio Amplifiers Automobiles Balloons Balls, Birdies, and Frisbees Bicycles Bouncing Balls Cameras Centrifuges and Roller Coasters Clocks Clothing and Insulation Compact Disc Players Computers Electric Motors Electric Power Distribution Electric Power Generation Electronic Air Cleaners Elevators Falling Balls Flashlights Fluorescent Lamps Incandescent Light Bulbs Knives and Steel Lasers Magnetically Levitated Trains Medical Imaging and Radiation Microwave Ovens Nuclear Reactors Nuclear Weapons Plastics Radio Ramps Rockets Seesaws Spring Scales Sunlight Tape Recorders Telescopes and Microscopes Television The Sea and Surfing Thermometers and Thermostats Vacuum Cleaners Violins and Pipe Organs Water Distribution Water Faucets Water, Steam, and Ice Wheels Windows and Glass Wood Stoves Xerographic Copiers Other Topics All Questions & Answers Ask a Question

 Question 1436

 If light has no mass, then how can it be affected by gravity? What property of light is gravitational force acting on? — DM
At low speeds, mass and energy appear to be separate quantities. Mass is the measure of inertia and can be determined by shaking an object. Energy is the measure of how much work an object can do and can be determined by letting it do that work. Conveniently enough, the object's weight—the force gravity exerts on it—is exactly proportional to its mass, which is why people carelessly interchange the words "mass" and "weight," even though they mean different things.

But when something is moving at speeds approaching the speed of light, mass and kinetic energy no longer separate so easily. In fact, the relativistic equations of motion are more complicated than those describing slow objects and the way in which gravity affects fast objects is more complicated than simply giving them "weight."

Overall, you can view the bending of light by gravity in one of two ways. First, you can view it approximately as gravity affecting not on mass, but also energy so that light falls because its energy gives it something equivalent to a "weight." Second, you can view it more accurately as the bending of light as caused by a change in the shape of space and time around a gravitating object. Space is curved, so that light doesn't travel straight as it moves past gravitating objects—it follows the curves of space itself. The second or Einsteinian view, which correctly predicts twice as much bending of light as the first or Newtonian view, is a little disconcerting. That's why it took some time for the theory of general relativity to be widely accepted. (Thanks to DP for pointing out the factor of two.)