Can an Electric Current Travel Instantly Across a 3000000km Long Cable?

[salor]

Hi, all.
There's a thought flashing through my mind yesterday.
Sorry for my lack of knowledge. The thought must be something wrong.
Can anyone correct it? Thanks
If there is a 3000000km long power cable, and the cable is pulled to 3000000km away from the earth.
A power source connected on one side A of the cable on the Earth keeps outputting the electric current, and the switch of side B on the cable (away from Earth 3000000km) is close. And there's a "current detector" on the cable away from side A by 1km. Does the detector detect the current?
At a specific time, if the switch of side B starts to open, does the current start to go from A simultaneously?
If so, people can open and close switch of side B to let people on side A know Morse code .
If the cable is 1LY long, will transmitting message to 1LY away instantly comes true?

 

[ZapperZ]

This is a very common question. See this thread, for example:

https://www.physicsforums.com/showthread.php?t=592826

I'd like to find someone to volunteer to write a FAQ on this.

Zz.