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Imagine a theoretical superhard steel cylinder pointed towards the moon , 1 cm in radius and 380,000 km long, traveling at 1km per hour towards the moon. Parallel to this cylinder, 1 meter away we have a magnetic cylinder of equal dimensions, not moving. If we pushed the magnetic cylinder "down" towards the moving steel cylinder, causing the steel cylinder to stop, would both the earth side and the moon side stop at the same time?

Would this pulse be a faster than light morse code machine?

Harish Chandra Rajpoot
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Diogo Beltran
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  • A key question for this is how did you push the magnetic cylinder? You'll find that distortion is a big deal along the length of 380,000km. It won't just move sideways as a rigid body. Thus, how you made it move sufficiently rigid to accomplish your goal will have a great effect on how far signals had to travel. – Cort Ammon Mar 01 '21 at 19:00
  • we are in the future. we have 3000 mini space ships pre-synchronized to push the magnetic rigidly cylinder down. concept of faster than message. the cylinders could be a rigid ring around the sun. – Diogo Beltran Mar 01 '21 at 19:25
  • Possible duplicate by OP: https://physics.stackexchange.com/q/617851/2451 – Qmechanic Mar 01 '21 at 19:52

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If you synchronize the pushes at the two ends then you can make the ends stop at the same time. Or you can make one step before the other, either way around.

Consider the following related idea. Suppose New Year's day is approaching and we arrange firework displays all around the world. Everyone has an accurate clock. We can arrange the start of the displays so that a sequence of rocket launch events happens in all the cities, one just before the next, and with careful preparation the sequence can proceed faster than the speed of light around the world! Or to have an example involving longer times, put the events on different planets. Then the timekeeping will not have to be so precise. It will be pretty easy to arrange for an event on Mars to start just a few seconds after an event on Earth for example. But you don't do it by sending signals from Earth to Mars just when the Earth event happens. Rather, you plan ahead and a day or so before you could tell the people on Mars to get ready and then do it at the agreed time.

The point is that by preparation one can make things at different places happen simultaneously, or one after another, or one before another, however we like. But the preparation signals---the ones used to set up the experiment---will all have to travel slower than light.

(If with the long rod you just stop one end, then an impulse will travel down the rod at the speed of sound and eventually the other end will stop, but that was not the experiment you had in mind.)

Andrew Steane
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  • thanks for the answer. even if we didnt, prepare, if we randomly decided to to move two 310,000 km long parallel rods together, we would get simultaneous morse pulse of the rods colliding in both sides. even if part of the message is instantaneous, isnt that part of the message faster than light communication? – Diogo Beltran Mar 01 '21 at 20:23
  • lets say. the simultaneous one pulse message was to simultaneously start two machines, light years apart, in different planets. couldnt faster than light messaging be theoretically podsible? – Diogo Beltran Mar 01 '21 at 20:55