Register to reply

Can I send a signal faster than light by pushing a rigid rod?

by DaleSpam
Tags: faster, light, rigid, signal
Share this thread:
DaleSpam
#1
Oct3-11, 02:07 PM
Mentor
P: 17,543
Can I send a signal faster than light by pushing a rigid rod?

One common proposal for achieving faster than light communication is to use a long perfectly rigid object and mechanically send signals to the other end by pushing, pulling, or tapping it. For instance; a hypothetical rigid rod linking two people several lightyears away. The fundamental idea is that when one end is moved the other end is disturbed instantaneously.

However, there is no such thing as a perfectly rigid rod: a mechanical disturbance at one end of any material can only move through the material at a finite speed. This speed is called the speed of sound in that material.

High stiffness materials like metal have a very high speed of sound and low stiffness materials like jello or air have a very low speed of sound. When you push on something made of jello, you can easily see that the disturbance propagates at a finite speed. When you push on something like metal, it is not so easy to see visually, but the disturbance still propagates at the finite speed of sound in the metal. (see e.g. http://www.physicsforums.com/showthr...55#post4414855)

The speed of sound in diamond is about 12000 m/s which is about 25 thousand times slower than the speed of light (299792458 m/s). But what about some hypothetical "unobtainium"? Why couldn't unobtainium's speed of sound be faster than the speed of light? The answer is that all materials, even unobtainium, are held together by electromagnetic forces at the molecular level. When one molecule moves then the change in its electromagnetic field propagates to its neighboring molecule at the speed of light. So even in principle it is not possible for any material to have a speed of sound faster than the speed of light.

The following forum members have contributed to this FAQ:
DaleSpam
Ryan_m_b
DrGreg
tiny-tim
with additional review and discussion by several others
Phys.Org News Partner Science news on Phys.org
'Office life' of bacteria may be their weak spot
Lunar explorers will walk at higher speeds than thought
Philips introduces BlueTouch, PulseRelief control for pain relief

Register to reply

Related Discussions
2cars moving slightly less than speed of light look faster than light to each other? General Physics 3
How to send a faster-than-light signal (spot Paradox)?! Special & General Relativity 59
Speed of light Schmeed of light: things can travel faster Special & General Relativity 13
Cheating a faster than light signal. Quantum Physics 19
Faster than light signal General Physics 5