# Is Earth's orbit around the sun affected by the ~8 minutes light delay?

Gravitational change occurs at the speed of light. As a consequence, we experience on Earth the gravitational attraction of the sun based on its position relative to us ~8 minutes ago. How does this delay affect the geometry of Earth's orbit compared with the classical Newtonian model?

Nordvedt postulated that a gravito-magnetic component makes it appear as if the interaction is instantaneous (i.e.) classical Newton with infinite speed of interaction. Was Nordvedt correct?

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You seem to have misunderstood some things about the work of Kenneth Nordtvedt (spelled that way, not Nordvedt). He is mainly known for pointing out that in some well-motivated alternative theories of gravity (i.e., not general relativity), the equivalence principle could be violated. Massive, self-gravitating bodies would have slightly anomalous ratios of inertial to gravitational mass. This is known as the Nordtvedt effect. The effect was searched for in lunar laser ranging experiments. The observations did not detect any Nordtvedt effect, and they placed an upper bound on it. This is discussed in section 3.6.1 of the review article Will 2006.

In any relativistic theory, an attraction or repulsion from an object at a distance $r$ is not toward the object's current location but toward the position of the object that would have been extrapolated from its state of motion at a time $r/c$ in the past. This is thoroughly verified by experiment and is not controversial. There is a discussion of this in Feynman, section II-26-1. In the sun's frame of reference, this extrapolation has no effect. In any other frame, the earth is seen to accelerate toward the position where the sun would have been extrapolated to be based on its position and motion 8 minutes ago.

Nordtvedt did not claim that gravitational interactions propagate instantaneously. General relativity's prediction that gravitational effects propagate at c has been accurately, although indirectly, verified by observations of the Hulse-Taylor binary pulsar. Solar-system tests are not capable of unambiguously testing this feature of GR; see Samuel 2003 and Will 2003.

It sounds like you've misinterpreted someone's explanation of the extrapolation idea. An equivalent way of stating the extrapolation idea is that if A's motion is inertial, then the force of A on B acts along the line defined by A's current instantaneous position, not its position retarded by $r/c$. This is not the same as saying that the interaction actually propagates instantaneously. None of this is a special, unorthodox theory proposed by Nordtvedt. Carlip 2011 gives a nice explanation. In the electromagnetic case, consistency is achieved because there is both an electric force and a magnetic one. In general relativity, you can talk about a similar type of gravitomagnetic force, and it plays a similar role in the analogous argument. Again, none of this is unorthodox or due to Nordtvedt.

Will 2006 - "The Confrontation between General Relativity and Experiment," http://www.livingreviews.org/lrr-2006-3

Feynman, The Feynman Lectures

Samuel 2003 - http://arxiv.org/abs/astro-ph/0304006

Will 2003 - http://arxiv.org/abs/astro-ph/0301145

Carlip 2011 - "Does Gravity Travel at the Speed of Light?," http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html

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