Expert: Steve Nelson - 4/29/2009

Question
QUESTION: Hello,


I have a question about projectile motion in outer space.

Since space  is virtually a perfect vaccuum, according to the first law of motion, an object traveling in space would continue in a straight line forever with undiminished speed since there is no friction. But is that true? Wouldn't the object create gravity waves that would carry away some of its energy in the form of gravitational radiation? And wouldn't that  process eventually make it come to a complete stop? Lets assume it is a massive object like a planet or star.

ANSWER: No, once you set it and its gravitational field in motion you're done accelerating the object and you've put all the energy you need to into its gravitational field reconfiguration (which will spread out at the speed of light from the object like a *single* ripple on a pond, but it won't just keep generating random waves).  At that point it should stay in the same state of motion forever unless it interacts with the gravitational field of another object or objects.

---------- FOLLOW-UP ----------

QUESTION: What if the object's motion was orbital? like the earth around the sun.

Answer
Yes, because of the orbital motion, they would radiate away ridiculously tiny amounts of energy.  It's something which is actually measurable in pulsars, and the subject of a nobel prize in physics.  http://nobelprize.org/nobel_prizes/physics/laureates/1993/press.html