Information transmission

1. Nov 24, 2004

PBRMEASAP

Here's a scenario I thought of a few days ago. I know there must be an easy explanation, but it hasn't come to me ...

Bob and Alice (no Eve this time) are separated from each other by about a million miles, and they would like to communicate with each other without the five second delay of radio waves. Fortunately, there lies between them a million-mile-long pole. After talking back and forth over radio, they agree to communicate by poking each other in Morse code with the long pole. Are they communicating with no five-second delay? Is this a problem?

I wouldn't be surprised if this is a well-known relativity-style "puzzle". If someone could explain what's wrong with the reasoning, I'd appreciate it.

thanks,
JP

2. Nov 24, 2004

pervect

Staff Emeritus
The signal will travel down the bar at the speed of sound in the material, which will be far less than the speed of light.

It's a standard puzzle, the resolution is that there are no rigid bodies in relativity, a rigid body would have an infinite speed of sound, while the actual speed of sound is less than 'c'.

3. Nov 24, 2004

PBRMEASAP

That makes sense. Thank you!

JP