How does a very long object in space act with time

In summary, the conversation discusses the concept of using a long rigid object to mechanically send signals and achieve faster than light communication. However, a closer analysis reveals that any mechanical disturbance will propagate at a finite speed, known as the speed of sound in that material. This is due to the fact that all materials are held together by electromagnetic forces at the molecular level, and any disturbance must propagate at the speed of light. Therefore, it is not possible for any material to have a speed of sound faster than the speed of light. This is a commonly asked question and has been addressed in a new FAQ post on the forum.
  • #1
d3nd3
2
0
If i put a 100 light year long string in space in a straight line and i put a astronaut at each end. Then the astronaut at one end took hold of the string and pulled it, would the person who is 100 light years away at the other end of the string see immediately the string move away from him?

Just i find it difficult to favom that it can possibly be instantaneous , would it be like every molecule inside the string sends a flow of energy through it continuously to the next nearest one, stretching it and pulling it closer to that initial position. And these tiny forces between the molecules are tiny forces which do not occur instantaneously?

Thank you for any input, its all much appreciated

d3nd3
 
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  • #2
This is probably in the "FAQ" since we get it every few weeks. No the other end of the string (or 'rigid' bar) does not move instantaneously. For one thing the speed of light restriction would prevent that. But in fact it would move much slower than that- the string will stretch (or bar compress) as the force moves through it. You are talking about a stress wave moving through the material and such waves move at the speed of sound in the material.
 
  • #3
sorry about that.
 
  • #4
HallsofIvy said:
This is probably in the "FAQ" since we get it every few weeks.
Actually, it is not in the FAQ. This is a pretty big omission. I will propose a FAQ and get some feedback.
 
  • #5
Here is my proposed FAQ:

One common proposal for achieving faster than light communication is to use a long rigid object and mechanically send signals to the other end by pushing, pulling, or tapping it. The fundamental idea is that when one end is moved the other end is disturbed instantaneously, however, a closer analysis of the motion of an extended object shows that any mechanical disturbance will propagate at a finite speed.

This speed is called the speed of sound in that material, high stiffness materials like metal have a very high speed of sound and low stiffness materials like jello or air have a very low speed of sound. When you push on something made of jello, you can easily see that the disturbance propagates at a finite speed. When you push on something like metal, it is not so easy to see visually, but the disturbance still propagates at the finite speed of sound in the metal.

The speed of sound in diamond is about 12000 m/s which is about 25 thousand times slower than the speed of light (299792458 m/s). But what about some hypothetical "unobtainium"? Why couldn't unobtainium's speed of sound be faster than the speed of light? The answer is that all materials, even unobtainium, are held together by electromagnetic forces at the molecular level. When one molecule moves then the change in its electromagnetic field propagates to its neighboring molecule at the speed of light. So even in principle it is not possible for any material to have a speed of sound faster than the speed of light.

d3nd3, can you give me some feedback? Do you feel that is a full and complete answer?
 
  • #6
DaleSpam said:
Here is my proposed FAQ:

This should be posted for the other SAs as well however I think it's very good. The one thing I would say it needs is a simple examples (bolded are my additions)

DaleSpam said:
One common proposal for achieving faster than light communication is to use a long rigid object and mechanically send signals to the other end by pushing, pulling, or tapping it. For instance; a hypothetical string linking two people several lightyears away. The fundamental idea is that when one end is moved the other end is disturbed instantaneously, however, a closer analysis of the motion of an extended object shows that any mechanical disturbance will propagate at a finite speed.

This speed is called the speed of sound in that material, high stiffness materials like metal have a very high speed of sound and low stiffness materials like jello or air have a very low speed of sound. When you push on something made of jello, you can easily see that the disturbance propagates at a finite speed. When you push on something like metal, it is not so easy to see visually, but the disturbance still propagates at the finite speed of sound in the metal.

The speed of sound in diamond is about 12000 m/s which is about 25 thousand times slower than the speed of light (299792458 m/s). But what about some hypothetical "unobtainium"? Why couldn't unobtainium's speed of sound be faster than the speed of light? The answer is that all materials, even unobtainium, are held together by electromagnetic forces at the molecular level. When one molecule moves then the change in its electromagnetic field propagates to its neighboring molecule at the speed of light. So even in principle it is not possible for any material to have a speed of sound faster than the speed of light.
 
  • #7
Thanks, I have posted it to the SA's with your suggested edit.
 
  • #8
Here is the final version of the FAQ published:
https://www.physicsforums.com/showthread.php?p=3537287#post3537287
 
Last edited by a moderator:

Related to How does a very long object in space act with time

1. How does the length of an object in space change with time?

The length of an object in space does not necessarily change with time. However, the perceived length of the object may change due to the effects of relativity. According to the theory of relativity, time and space are interwoven and can be affected by factors such as gravity and velocity. This can cause an object to appear shorter or longer to an observer depending on their relative position and motion.

2. Does time pass differently for a very long object in space?

Yes, time can pass differently for a very long object in space compared to a shorter object. This is again due to the theory of relativity, which states that time can be affected by factors such as gravity and velocity. An object that is longer may experience time dilation, where time appears to pass slower for the object compared to an observer on Earth.

3. How does the shape of a very long object in space change over time?

The shape of an object in space will generally remain constant over time unless it is affected by outside forces such as collisions or gravitational pulls from other objects. However, the perceived shape of the object may change for an observer due to the effects of relativity. As mentioned before, time and space are interwoven, so changes in time can also affect the perceived shape of an object.

4. Can a very long object in space experience time travel?

According to current scientific understanding, time travel is not possible. However, the effects of relativity on a very long object in space can cause time dilation, which means time may appear to pass differently for the object compared to an observer on Earth. This can give the illusion of time travel, but it is not true time travel as the object is still moving forward in time.

5. How does the speed of a very long object in space affect time?

The speed of a very long object in space can affect time in a phenomenon known as time dilation. The faster an object moves, the slower time appears to pass for the object. This is due to the time and space being interwoven and the effects of relativity. As the object approaches the speed of light, time dilation becomes more pronounced, and time may appear to almost stand still for the object.

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