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Apologize in advance if this is the wrong group. I often watch Netflix shows about blackholes explained in a nutshell or laymen’s terms.

I had a recent thought/scenario that may relate to some advanced concept that I am unaware of.

Hypothetical Scenario (sorry if it is wonky’ish)

Planet A and Planet B do not rotate on an axis and have unobstructed view of each other. Planet A and Planet B are 10 million light years apart.

A carpenter on planet B wants to see if he can fix problem on planet A. He comes up with a plan. A screwdriver will be forged that will grow to cover the distance between planet A and planet B. The screwdriver is forged superfast and expands at the speed of light and will reach planet A in 10 million light years. The carpenter attached a note at the end of the screwdriver that instructed the observer on planet A to attach the end of the screwdriver to a screw 10 minutes after receiving the note. The carpenter on planet B takes a nap and set the alarm for 10 million years. The alarm awakens the carpenter just as the screwdriver end and note reached planet A. He then waits for 10 minutes and turns the screwdriver to the right to tighten the bolt that the observer has placed on a screw on planet A.

Question: Will the carpenter on planet B and the observer on planet A see the screwdriver turn at the same time?

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    $\begingroup$ The screwdriver is made of massive materials, and thus cannot move at the speed of light. The twisting will be noted to move down the screwdriver at most at the speed of light, and actually a lot slower because it should be closer to speed of (second) sound of the material. Instead, you should worry about how you synchronise the clocks on planets A and B so that "see [...] at the same time" even has a meaning at all. Scenarios of this kind is treated as standard teaching on SR goes. $\endgroup$
    – naturallyInconsistent
    Commented Dec 7, 2023 at 14:58
  • $\begingroup$ @naturallyInconsistent - Thanks for the reply. Syncing of clocks is a more logical and probable concept for sure. What is SR? $\endgroup$
    – Ross Bush
    Commented Dec 7, 2023 at 15:12
  • $\begingroup$ Special Theory of Relativity $\endgroup$
    – naturallyInconsistent
    Commented Dec 7, 2023 at 15:15
  • $\begingroup$ Ah, ok gotcha. Thanks. $\endgroup$
    – Ross Bush
    Commented Dec 7, 2023 at 15:21

2 Answers 2

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Will the carpenter on planet B and the observer on planet A see the screwdriver turn at the same time?

No. There is no such thing as a rigid material in relativity, although there is a type of motion called Born-rigid motion. When the carpenter turns the screw the torsion from that twisting will propagate as a shear wave along the length of the screw at the speed of sound in the material of the screw.

The screwdriver is forged superfast and expands at the speed of light

This should be at most a little less than the speed of light

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I think you made a wrong assumption that the screwdriver is perfectly rigid in a sense that the twisting motion is spreading with infinite velocity. But remember that the screwdriver is made of a lot of atoms. If you start to turn the screwdriver on one end you displace some atoms at your end. Then it takes some time until the atoms further down the screwdriver feel the force of the displaced atoms at your end. This spreading velocity is always lower than the speed of light so if you start twisting the screwdriver it will take more than 10 million years until it reaches the other end.

Also you need to be careful with what you mean when you say "at the same time". Simultaneity depends on the frame of reference because there is no absolute time.

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  • $\begingroup$ You mean there is no universal "right now"? $\endgroup$
    – Ross Bush
    Commented Dec 7, 2023 at 15:28
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    $\begingroup$ In a sense that what appears to be simultaneous in one frame of reference (i.e. at the same time) doesn't need to appear simultaneous in another frame of reference. $\endgroup$
    – Sharanor
    Commented Dec 7, 2023 at 15:44

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