Communication signals and the speed of light

Click For Summary

Discussion Overview

The discussion revolves around the implications of the speed of light as defined by the theory of general relativity, particularly in relation to hypothetical communication methods between distant celestial bodies. Participants explore the feasibility of signaling faster than light using a taut rope or rigid rod, examining the physical limitations of such materials.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant proposes that if a perfectly taut and non-deformable rope existed between two planets, it could allow for instantaneous signaling, thus violating the speed of light limitation.
  • Another participant counters that physical disturbances in materials propagate at the speed of sound, which is below the speed of light, and that even a rigid rod would not transmit a signal instantaneously due to the mass and elasticity involved.
  • Examples are provided, such as the propagation of sound in a train's air brakes, to illustrate the limitations of signal transmission in physical materials.
  • A later reply acknowledges the previous answer and expresses understanding of the limitations discussed.
  • Another participant notes that the concept of a perfectly rigid body is not feasible, reinforcing the argument against faster-than-light signaling.

Areas of Agreement / Disagreement

Participants generally agree on the limitations imposed by the speed of sound in materials, but there is a disagreement regarding the hypothetical scenario of faster-than-light signaling through a taut rope or rigid rod, with no consensus reached on the feasibility of such a scenario.

Contextual Notes

The discussion highlights assumptions about the properties of materials, such as rigidity and elasticity, and the dependence on definitions of speed and disturbance propagation, which remain unresolved.

auxengin
Messages
2
Reaction score
0
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 occurred 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?
 
Astronomy news on Phys.org
auxengin said:
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 occurred 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.
 
Understood your answer. Thanks for your time.
 
This is a reason a perfectly rigid body is not possible.
 

Similar threads

  • · Replies 148 ·
5
Replies
148
Views
13K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 42 ·
2
Replies
42
Views
3K
  • · Replies 12 ·
Replies
12
Views
3K
  • · Replies 24 ·
Replies
24
Views
4K
  • · Replies 2 ·
Replies
2
Views
3K
  • · Replies 21 ·
Replies
21
Views
5K
  • · Replies 7 ·
Replies
7
Views
3K
  • · Replies 5 ·
Replies
5
Views
2K
  • · Replies 5 ·
Replies
5
Views
2K