# Relationship between gravity and the speed of light

### Speed of gravity - Wikipedia

Gravity and the Speed of Light It refers to the relationship between the amount of matter in the Universe and the gravitational pull that all. So in the presence of gravity the speed of light becomes relative (variable Finally, there is no difference between the effects of g-forces experienced from these. It simply is the maximum speed that light and gravity travel at. . the only special speed, since it describes the relation between space and time.

### Speed of Light in Gravity

The consequence of this is that static fields either electric or gravitational always point directly to the actual position of the bodies that they are connected to, without any delay that is due to any "signal" traveling or propagating from the charge, over a distance to an observer. This remains true if the charged bodies and their observers are made to "move" or notby simply changing reference frames.

This fact sometimes causes confusion about the "speed" of such static fields, which sometimes appear to change infinitely quickly when the changes in the field are mere artifacts of the motion of the observer, or of observation. In such cases, nothing actually changes infinitely quickly, save the point of view of an observer of the field.

For example, when an observer begins to move with respect to a static field that already extends over light years, it appears as though "immediately" the entire field, along with its source, has begun moving at the speed of the observer.

This, of course, includes the extended parts of the field. However, this "change" in the apparent behavior of the field source, along with its distant field, does not represent any sort of propagation that is faster than light.

## Does Gravity Travel at the Speed of Light?

Newtonian gravitation[ edit ] Isaac Newton 's formulation of a gravitational force law requires that each particle with mass respond instantaneously to every other particle with mass irrespective of the distance between them.

In modern terms, Newtonian gravitation is described by the Poisson equationaccording to which, when the mass distribution of a system changes, its gravitational field instantaneously adjusts.

Therefore, the theory assumes the speed of gravity to be infinite. This assumption was adequate to account for all phenomena with the observational accuracy of that time. It was not until the 19th century that an anomaly in astronomical observations which could not be reconciled with the Newtonian gravitational model of instantaneous action was noted: Based on Newton's force law he considered a model in which the gravitational field is defined as a radiation field or fluid.

Changes in the motion of the attracting body are transmitted by some sort of waves. This velocity was used by many in the 19th century to criticize any model based on a finite speed of gravity, like electrical or mechanical explanations of gravitation.

If a charged particle is moving at a constant velocity, it exerts a force that points toward its present position, not its retarded position, even though electromagnetic interactions certainly move at the speed of light.

### Why Does Gravity Travel at the Speed of Light? - D-brief

Here, as in general relativity, subtleties in the nature of the interaction "conspire" to disguise the effect of propagation delay. It should be emphasized that in both electromagnetism and general relativity, this effect is not put in ad hoc but comes out of the equations.

Also, the cancellation is nearly exact only for constant velocities. If a charged particle or a gravitating mass suddenly accelerates, the change in the electric or gravitational field propagates outward at the speed of light. Since this point can be confusing, it's worth exploring a little further, in a slightly more technical manner. Consider two bodies—call them A and B—held in orbit by either electrical or gravitational attraction.

As long as the force on A points directly towards B and vice versa, a stable orbit is possible.

If the force on A points instead towards the retarded propagation-time-delayed position of B, on the other hand, the effect is to add a new component of force in the direction of A's motion, causing instability of the orbit. This instability, in turn, leads to a change in the mechanical angular momentum of the A-B system. But total angular momentum is conserved, so this change can only occur if some of the angular momentum of the A-B system is carried away by electromagnetic or gravitational radiation.

• Relative Speed of Light
• Speed of gravity

Now, in electrodynamics, a charge moving at a constant velocity does not radiate. Technically, the lowest-order radiation is dipole radiation, and the radiated power depends on the second time derivative of the electric dipole moment; two time derivatives give acceleration.

So, to the extent that A's motion can be approximated as motion at a constant velocity, A cannot lose angular momentum. For the theory to be consistent, there must therefore be compensating terms that partially cancel the instability of the orbit caused by retardation. This is exactly what happens; a calculation shows that the force on A points not towards B's retarded position, but towards B's "linearly extrapolated" retarded position. It depends on where I am and where you are it depends on locations.

So in the presence of gravity the speed of light becomes relative variable depending on the reference frame of the observer. This does not mean that photons accelerate or decelerate; this is just gravity causing clocks to run slower and rulers to shrink.

Recalling the very famous second postulate of Special Relativity by Einstein From Newton's second law: The other factor besides acceleration is gravity. Einstein emphasized in his paper in For example, an observer outside gravitational fields measures the speed of light locally in his location at At the same time an observer freefalling into that black hole zero-g measures the speed of light locally in his location at If he tries to resist his freefall into that black hole by firing his rockets for example he will not measure the speed of light locally anymore at Again when he looks towards the black hole he sees the speed of light there much slower; when he looks away from the black hole he sees the speed of light there much faster.

In any case, freefalling or not, he will never see the speed of light outside gravitational fields at Finally, there is no difference between the effects of g-forces experienced from these rockets and the effects of g-forces experienced when standing on planets, stars In the presence of gravity the speed of light becomes relative.

To see the steps how Einstein theorized that the measured speed of light in a gravitational field is actually not a constant but rather a variable depending upon the reference frame of the observer: