Light Year Long Pole Spinning
Name: Peter R.
If I have a pole 1 light year long and rotate it at one end will the other
end spin at the same time or in 1 year?
What do you mean by "rotate" and "spin?" Do you mean the pole is spun along its own
longitudinal axis, or do you mean swung in a circle with you at the center and the far
end of the pole pointing away from you? I shall assume you mean the latter.
In that case, the far end of the pole would take the same time to make a full circle as
the end near you. Consider a CD that contains a scratch that radiates from the center to
the edge along the CD's radius. When the CD spins on its drive, both ends of the scratch
make one revolution in the same time interval. However, the linear velocity of the part
on the periphery is traveling faster that at the center because it must travel further
to complete one trip around the full 360 degrees.
If you have a pole 1 light year long, you have a very long pole, indeed. It is, in fact,
about 9.4 E15 m long. (The E15 means move the decimal place 15 places to the right, so
the length is 9,400,000,000,000,000 m. If your pole has a square cross section 10 cm on
a side (about 4 inches), it has a volume of 9.4 E13 m^3 (94,000,000,000,000 cubic meters). If it is made of iron, it will have a mass of about 7.4 E17 kg. You will find it difficult to make such a pole and not at all easy to push it around!
But to answer your question. Nothing, including information, can travel faster than the
speed of light, so you have to wait at least a year. In fact, the information that the
pole was twisted at one end cannot move down the pole faster than the speed of sound in
that medium. Since the speed of sound in iron is 5130 m/s, it will take about 58,500
years before the far end realizes the near end was twisted.
I do not recommend this as an extra credit experiment for your science course...
Best, Dick Plano, Professor of Physics emeritus, Rutgers University
Assuming that your pole is not a quantum mechanical pole, it would take 1 year minimum.
Depending upon the material your hypothetical pole is made from, a twist at one end will
propagate at a speed of the order of magnitude of the speed of sound (+/- 100) and that is
much slower than the speed of light. Even an idealized pole the atoms cannot move at a
speed exceeding the
speed of light, their mass would increase as their speed approached that of the speed of
The pole moves as one object. It takes a year for the light from the end other end of the
pole to reach your eyes. You don't see the motion until a year after it happens.
Dr. Ken Mellendorf
Illinois Central College
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Update: June 2012