In this scenario is faster then light speed possible?

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Discussion Overview

The discussion explores the concept of faster-than-light (FTL) movement through a hypothetical scenario involving an unbreakable rod connecting Earth to distant stars. Participants examine the implications of such a scenario within the framework of relativity, questioning the nature of rigidity and the behavior of objects as they approach light speed.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant describes a scenario where a person on Earth points an unbreakable rod at two stars, raising the question of what would happen if the rod's ends needed to move faster than light to maintain alignment.
  • Another participant asserts that perfectly rigid objects cannot exist in relativity, explaining that changes in motion propagate at the speed of sound in the material, which is always less than the speed of light.
  • A follow-up question considers the implications of removing the unbendable aspect of the rod, asking whether it would curve and if the far end would stretch and widen as it approached light speed.
  • One participant states that the rod would bend and reiterates that the scenario described is impossible.
  • A later contribution provides a general rule regarding FTL scenarios, emphasizing that massive objects cannot reach or exceed the speed of light and that any theoretical FTL travel would involve warping spacetime, with local velocities remaining sublight.

Areas of Agreement / Disagreement

Participants express disagreement regarding the feasibility of the scenario involving an unbreakable rod and the implications of approaching light speed. There is no consensus on the outcomes of the hypothetical situation, and multiple views on the nature of rigidity and FTL travel are presented.

Contextual Notes

The discussion hinges on the assumptions of rigidity and the behavior of materials under relativistic conditions, with unresolved questions about the nature of the rod's movement and the implications of relativistic effects.

smurfcatcher
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Take this scenario for instance. Now from the position of the Earth if you were to lookup into the night sky the distance between stars can seem like mere inches, but in actuality it could be 100’s of light years between the two. Now, for this scenario you are a person standing on the Earth looking up into the night sky at two stars. From your perspective the distance between these two starts is only a few inches. Yet, in actuality these stars are hundreds of light years away, and hundreds or light years apart from one another. Now, in your hand you hold an unbreakable and unbendable rod that stretches from Earth hundreds of light years all the way to one of these two stars. Now, you have this unbendable, unbreakable rod in your hand and you are pointing it at one of these two stars, and then you take the rod and quickly point it at the other star. Now, here on Earth your hand only moved a few inches in a few seconds from pointing at one star to pointing at the next. Yet, the end of the rod that was at the first star you were pointing at it would have to move faster then the speed of light to travel the hundreds of light years from the first star to the second star to keep up with the part of the rod in your hand. Now, remember this is an unbendable and unbreakable rod. So, what would happen?
 
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The simple answer is that perfectly rigid objects of the type you describe are impossible in relativity. Any change in velocity of one end of the rod will not influence the motion of the other end until a wave traveling at the speed of sound in the material has had time to move from one end to the other (and the speed of sound in any physically possible material is always less than the speed of light in a vacuum). See here for a little more info.
 
JesseM said:
The simple answer is that perfectly rigid objects of the type you describe are impossible in relativity. Any change in velocity of one end of the rod will not influence the motion of the other end until a wave traveling at the speed of sound in the material has had time to move from one end to the other (and the speed of sound in any physically possible material is always less than the speed of light in a vacuum). See here for a little more info.

Ok. So, if we were to remove the unbendable part out of this scenario what would happen? Would the rod simply curve from my hand through space until it hit the speed of light and stopped bending, and the rest of the rod would simply travel at the speed of light? Also, since as you progress closer and closer to the speed of light you begin to infinitely stretch. So, would the rod at the far end become infinatly wider then the section of the rod in your hand? Would the rod not only curve but also progressively widen as it curved?
 
smurfcatcher said:
Ok. So, if we were to remove the unbendable part out of this scenario what would happen? Would the rod simply curve from my hand through space until it hit the speed of light and stopped bending, and the rest of the rod would simply travel at the speed of light? Also, since as you progress closer and closer to the speed of light you begin to infinitely stretch. So, would the rod at the far end become infinatly wider then the section of the rod in your hand? Would the rod not only curve but also progressively widen as it curved?
Since both rods are rotating the changes in direction inside the rod will be propagated at the speed of sound.
 
The rod would bend. What you are describing is impossible.
 
Two threads on the same topic merged.
 
Here's a quick rule of thumb for all FTL questions.

In any scenario involving a massive object with ordinary (real, positive) mass, relative velocity at or above the speed of light is impossible.

The only theoretical means of traveling _apparently_ faster than light involve warping spacetime, but even then the local velocity is always sublight (and the energy requirements are far too massive to be practical, leaving aside causality paradoxes).
 

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