Communication signals and the speed of light

[auxengin]

As per the theory of general relativity nothing can travell faster than the speed of light. Therefore we would always be in the past light cone of a distant celestial body, say planet X in another solar system, that is 100 light years away. A minimum of 100 years would have to lapse before an event that occured on that planet would have any effect on us. However in a hypothetical situation, if there existed a perfectly taut and non deformable (cannot be stretched) chord or rope between that planet and ours, a person on planet X would be able to signal us immediately by just pulling on the chord, thus sending a signal faster than light. Is this possible?

[Astronuc]

As per the theory of general relativity nothing can travell faster than the speed of light. Therefore we would always be in the past light cone of a distant celestial body, say planet X in another solar system, that is 100 light years away. A minimum of 100 years would have to lapse before an event that occured on that planet would have any effect on us. However in a hypothetical situation, if there existed a perfectly taut and non deformable (cannot be stretched) chord or rope between that planet and ours, a person on planet X would be able to signal us immediately by just pulling on the chord, thus sending a signal faster than light. Is this possible? No - because a physical disturbance in a material would propagate at the speed of sound, which is well below light. In addition, a string has elasticity, and a small displacement would be 'distributed' over a long distance so it would have essentially no effect several hundred meters from the origin.

Now consider a rigid rod, which is not purely rigid, but simply has a large elastic constant, or in otherwise, is very stiff. The disturbance still must propagate at or below acoustic velocities, and of course, the disturbance will be dispersed (attenutated) as it propagates. The rigid body has mass which will resist propagation of a disturbance in the mass.

One can observe air brakes on a long train. Activation of the air brakes propagates from the front to the rear, pretty much at the speed of sound in the gas. Also, when the locomotives start pulling, one will hear the propagation of the sound of couplers tensioning from front to rear of the train.

[auxengin]

Understood your answer. Thanks for your time.

[Matterwave]

This is a reason a perfectly rigid body is not possible.