Hypothetical question about speed and distance across space

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Discussion Overview

The discussion revolves around hypothetical scenarios involving a pole that spans 200 million light years and its implications for speed and communication across vast distances in space. Participants explore concepts related to the speed of light, the speed of sound in materials, and the nature of communication through mechanical means.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant proposes a scenario where a pole anchored to Earth rotates, suggesting that its end would move faster than the speed of light, raising questions about the implications of such motion.
  • Another participant counters that an impossibly stiff rod is not feasible, noting that any motion would propagate at the speed of sound within the rod.
  • In response to the second question, a participant explains that communication via Morse code would be limited by the speed of light, indicating that it would take 200 years for a signal to travel across the pole.
  • A later reply seeks clarification on whether the propagation of movement along the pole would create a spiral effect and confirms that any movement would only propagate at the speed of sound.
  • Another participant acknowledges the misunderstanding regarding the Morse code machine, reiterating that movement would still propagate at the speed of sound, not instantaneously.

Areas of Agreement / Disagreement

Participants generally agree that the scenarios presented involve limitations based on physical principles, particularly regarding the speed of sound and light. However, there remains some uncertainty and exploration of the implications of the hypothetical scenarios without a consensus on the feasibility of the proposed ideas.

Contextual Notes

The discussion highlights limitations in understanding the propagation of motion and communication through hypothetical constructs, emphasizing the dependence on the properties of materials and the constraints of relativistic physics.

furix
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I have two hypothetical questions about breaking the speed of light that has been bugging me for quite a while.

Question 1:

Assume an impossibly sturdy and light pole is attached to the earth. Its length spans a distance of 200 million light years into space. Since it is anchored on earth, it rotates with the earth. Unbelievably, as the Earth spins, the end of this pole (or any part of it) does not hit any bodies in space.

Wouldn't the end of this pole be moving across the universe in a full 360 rotation in 24 hours? Moving at speeds way over the speed of light?

[PLAIN]http://img823.imageshack.us/img823/8533/question1k.jpg
(ignore the "tap like morse code, that is for question2)
http://img823.imageshack.us/img823/8533/question1k.jpg Question 2:

This same pole, is not attached on Earth and does not rotate with the earth. However, it can be pushed (remember, it is extremely sturdy and light). At the opposite end of this pole is a morse code machine. Couldn't we effectively communicate across 200 million light years instantaneously?

[PLAIN]http://img23.imageshack.us/img23/6382/question2c.jpg
http://img23.imageshack.us/img23/6382/question2c.jpg Does that make any sense?
 
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Welcome to PF. This is a common question. The answer is pretty simple: An impossibly stiff rod is impossible. In reality, Any motion will propagate along the rod at the speed of sound in the rod.
 
As stated above, welcome!

For the second question, remember that the electrons used to send the message are only traveling at the speed of light, so it will just take 200 years for the Morse code to reach the other end of the pole (assuming no dissipation in the signal).

Good thinking, though. These types of mind experiments were how the Theory of Relativity was developed in the first place. Glad you asked. :smile:
 
Thanks for the responses!

Did you mean speed of sound or light in regards to the answer to question 1? So you're saying that the rod at 200 million light years across is being whipped around the earth, slowly creating a spiral?

In regards to question 2, so by pushing on the rod, it would move along the rod at only the speed of light? So the end wouldn't move for 200 million years, but it's moving from the beginning like a ripple at the speed of light?

Thanks!
 
Oh, sorry, I misread that it was being used as a morse code machine. The movement would still only propagate at the speed of sound of the material.
 

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