Light year long rod to beat speed of light

This is an interesting argument (wrong of course) on how one can beat the speed if light by having a rod that is one light year long. 

The argument goes like this. 

Lets say there is a rod that is one light year in length i.e. the length of the rod is equal to the distance traveled by light in one year. Lets put two persons A and B on either end holding the rod. Person A pushes the rod and Person B instantaneously feels the nudge in his hand but he can only 'see' Person A pushing the rod after an year. Yay, the speed of light is beaten. 
Two men holding a light year long stick to see if they can beat light speed
The above argument looks convincing based on our real world experience where pushing one end of an object instantaneously moves the other end. 

However, in reality the other end of the object doesn't move instantaneously but moves after some time. When an object is pushed at one end, a series of compression waves are created that propagate through the object until it reaches the other end and the speed at which these waves travel is equal to the speed of sound in the material the object is made of. It is these compression waves that convey the information that the object has been pushed and thus move the entire object. 

In our hypothetical example, lets say the rod is made of steel. Speed of sound in steel is 6100 m/s and the speed of light is 299792458 m/s. So the pressure wave (that travels at the speed of sound in steel) would take 49146 times more duration to reach the other end. Thus in our example, Person B would feel the nudge after nearly 49000 years. So much for their attempt to beat the speed of light. 

In fact, the slow propagation of the the compression waves can be seen easily if the right material is chosen. The video below shows slinky drops at more than 300 fps and one can clearly see that the other end doesn't move until the compression wave reaches it. Same happens in rigid objects but at a much higher speed. 




Did the simple science for kids in this post make you a little smarter?
If you have any ideas / questions you want to learn about, please leave a note in the comments section. 
Would be glad to know if there are any inaccuracies in the article.

8 comments:

  1. So if you had a sufficiently rigid object, the speed of the propagation of the pressure wave could be higher than the speed of light?

    I don't see how it would break any physical laws. After all, it's information, not mass, that travels above the speed of light.

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    1. According to Einstein's relativistic predictions, nothing (not even information) can go faster than light. Speed of light is a universal speed limit everything obeys.

      So, the law basically precludes the possibility of a 'sufficiently rigid object' existing. If you do find one, you have just proven Einstein wrong and made time travel feasible. Such a discovery would have huge implications but quite unlikely.

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    2. Cant darkness beat the speed of light since its literally everywere earth space etc.. its as it was just there then light

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    3. Darkness is nothing at all. So it has no speed. If you are looking at it as if it is something then it is more like the evacuation of something, such as light, so darkness would fill a room after light has left, which would be at the speed of light; so darkness would appear to move at the speed of light, when actually, light is leaving at the speed of light.

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    4. When an object is pushed at one end, a series of compression waves are created that propagate through the object until it reaches the other end and the speed at which these waves travel is equal to the speed of sound in the material the object is made of.

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    5. If information can't travel faster than the speed of light than how does quantum entanglement work? If two particles are entangled then they are supposed to respond instantaneously when the state of one particle changes.

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  2. Replies
    1. It is the distance light travels in an year. About 10 trillion kilometers (9.4607 × 10^12 kilometers to be precise)

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