Expert: Jesse Martinez - 1/14/2010

Question
If the sun suddenly disappeared, would it take 8 minutes to notice the gravitational effect?

Same question. If an object with mass suddenly changes position would the motion take d/c= time to be noticed?

Answer
Hello Carl,

This is a good question. The reference to the speed c, is the key. All electromagnetic and gravitational effects are limited to c, 186,000 miles per second. In the case of the sun's sudden demise, the gravitational and visual shock would coincide. There are some theories that say that gravitational effects are instantaneous as Newtonian physics holds. In this case, the world would start crumbling eight minutes before we saw the cause. Intuitively, I don't think this is what would happen. I think gravitational effects travel at the speed c. Notice that like you, I have not referred to c as the "speed of light".

Although 186,000 miles per second is the speed of light, c is actually a factor for converting units of time to the units of space in Lorenz transformations which describe time dilation, the speed at which the effects of massless particles, like photons and gravitons are felt and the other relationships between time, space and relative speed.

Now the second part of your question, while on first examination seems to be a "ditto" ... I am thinking it is not that simple. If the movement is at the speed c, the effect would not be noticed until the moving object falls below that speed. But since an object with mass cannot travel at c, the effect would be noticed in a manner described by the Lorenz transformations. The closer to c that the speed approaches, the greater the lag of the effects.

The other option I see in you second question is: If the object is suddenly located at a different distance instantaneously from another object, what would happen? In this case there would be two reactions, one caused by the disappearance of the mass delayed by d(original)/c and the other by the reappearance at the different point. That reaction would be delayed by the time d(new)/c. E.g., if the two objects were one light second apart, and one of them skipped out an extra light second instantaneously, there would be two reactions on each object, one at one second after the event and another two seconds after the event. I hope this clarifies the question. If you need more followup, just ask, but I think you will get it.