Are we gravitationally affected by where an object is now, or where it was when the light we see started its journey to us?

Because all indications are that gravity travels at the speed of light, we can only be affected by where a body is at the SAME time we see the light from it.

Take a look at the diagram above. It's called a Feynman Diagram and it shows two electrons exchanging a photon (wavy line) with time on the vertacle axis and space horizontal. The electrons start out at the bottom and are approaching each other, then the left hand electron emits a photon that is absorbed by the right hand electron. The emission causes the left hand electron to recoil, and the absorption causes the right hand electron to recoil in the opposite direction. The horizontal line that passes through A and B is the time when the left hand electron emitted its photon. Notice that the location of the right hand electron would be at point B. The right hand electron still has to move forward in time in order to be affected by the emitted photon. During this time, it is moving through space as though nothing is happening. This shows that because the force is not instantaneous, the righthand eledctron is only affected by what it is receiving from where the lefthand electron used to be, not where it is when the righthand electron is at point 'B'. For gravity the situation is identical because it too is a 'force' that travels no faster than the speed of light.

The light travel time from the Sun to the Earth is 8.5 minutes. When we look at the Sun NOW, we are seeing it where it was 8.5 minutes ago. It is also the position of the Sun NOW that determines the strength of the gravitational field that the Earth is feeling NOW. Where the sun actually is NOW has no affect upon us because this information is coded in the gravitational field which Earth will experience 8.5 minutes from now. This is why astronomers do not care where distant objects in the universe are RIGHT NOW. We only care where their light tells us they are NOW because it is this information that controls its affect upon the Earth and Sun.

Return to Dr. Odenwald's Gravity page at the Astronomy Cafe Blog.