Hypothetical question about speed and distance across space

AI Thread Summary
The discussion revolves around two hypothetical scenarios involving a pole extending 200 million light years into space. In the first scenario, it is questioned whether the end of the pole, rotating with Earth, would exceed the speed of light. The consensus is that an impossibly stiff rod is not feasible, and any motion would propagate at the speed of sound in the material, not faster than light. The second scenario addresses communication via a Morse code machine at the pole's end, concluding that signals would also only travel at the speed of light, taking 200 million years to reach the other end. These thought experiments highlight fundamental principles of physics, particularly relating to the Theory of Relativity.
furix
Messages
2
Reaction score
0
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?
 
Last edited by a moderator:
Physics news on Phys.org
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 propogate at the speed of sound of the material.
 

Thread 'Average velocity as a weighted mean'

Hello everyone, Consider the problem in which a car is told to travel at 30 km/h for L kilometers and then at 60 km/h for another L kilometers. Next, you are asked to determine the average speed. My question is: although we know that the average speed in this case is the harmonic mean of the two speeds, is it also possible to state that the average speed over this 2L-kilometer stretch can be obtained as a weighted average of the two speeds? Best regards, DaTario
View full post »

Thread '1:1 versus 2:1 Mechanical Advantage debate'

The rope is tied into the person (the load of 200 pounds) and the rope goes up from the person to a fixed pulley and back down to his hands. He hauls the rope to suspend himself in the air. What is the mechanical advantage of the system? The person will indeed only have to lift half of his body weight (roughly 100 pounds) because he now lessened the load by that same amount. This APPEARS to be a 2:1 because he can hold himself with half the force, but my question is: is that mechanical...
View full post »

Thread 'Newton's first law?'

Some physics textbook writer told me that Newton's first law applies only on bodies that feel no interactions at all. He said that if a body is on rest or moves in constant velocity, there is no external force acting on it. But I have heard another form of the law that says the net force acting on a body must be zero. This means there is interactions involved after all. So which one is correct?
View full post »

Similar threads

Space elevator wondering about torque
Replies
2
Views
2K
Coriolis effect, Conservation of Angular Momentium, Planes
Replies
24
Views
5K
A 1-Way Speed of Light
Replies
42
Views
649
Train traveling at speed of E field question
Replies
10
Views
2K
I How to Measure Speed of Light & Is It Constant?
Replies
13
Views
3K
Acceleration, distance and speed
Replies
29
Views
5K
I Perceived Speed of Objects in Space
Replies
5
Views
2K
How Do You Calculate Acceleration During Variable Rotational Speed?
Replies
25
Views
8K
I Question about the limits of space contraction near light speed
Replies
10
Views
1K
B Does Force Travel Faster Than Light?
Replies
8
Views
3K

Hot Threads

Recent Insights

Back
Top