wex@milano.UUCP (01/31/86)
One of my coworkers has posed me the following problem, and my knowledge of
physics is too weak to answer him. Please *MAIL* replies to me, as I am not
a regular reader of this group. If people are interested, I will post the
concensus answer (if there is one).
Imagine that you and I have devised a communication scheme whereby we can
measure the movement of the end of a rod, and decode these movements to get
the meaning of the message.
Now imagine constructing a rod in space that is 1 light-year long. When I
move my end of the rod (which we assume is rigid, and unaffected by gravity
over the span of 1 light-year), your end moves too. And the movement of the
end conveys information.
But doesn't this violate relativity (which I take to say that information
cannot be propagated faster than the speed of light)?
What is the answer to this gedanken problem? Is it simply some property of
the rod? If the rod is truly inflexible and inelastic, then both ends ought
to move at (essentially) the same time.
--
Alan Wexelblat
ARPA: WEX@MCC.ARPA
UUCP: {ihnp4, seismo, harvard, gatech, pyramid}!ut-sally!im4u!milano!wex
"What a long, strange trip it's been"rimey@ernie.berkeley.edu.BERKELEY.EDU (Ken &) (02/02/86)
In article <703@milano.UUCP> wex@milano.UUCP writes: > >Now imagine constructing a rod in space that is 1 light-year long. When I >move my end of the rod (which we assume is rigid, and unaffected by gravity >over the span of 1 light-year), your end moves too. And the movement of the >end conveys information. > >But doesn't this violate relativity (which I take to say that information >cannot be propagated faster than the speed of light)? >-- >Alan Wexelblat Yes, it violates relativity. Build rods out of whatever material you like; none will be THAT rigid. When you move one end of a 1 light-year long rod, the other will not move the slightest bit until at least a year later. Being less abstract, you can picture the rod's atoms pushing each other with electric force (an overly simple picture), and recall that electromagnetic fields propagate at only the speed of light. Or you can observe that if the induced motion of the rod is small you are really talking about the propagation of a sound wave down the rod. Even in the stiffest metal, the speed of sound is much less than the speed of light. If the induced motion is larger, it will propagate faster than sound, but the speed of light is still an upper limit. Ken Rimey