Reading Russell's ABC of Relativity, stuck in chapter 3 (about speed of sound)

Click For Summary

Discussion Overview

The discussion revolves around understanding the propagation of sound as described in chapter 3 of Russell's "ABC of Relativity." Participants explore the implications of sound traveling in a moving train relative to stationary air outside the train, and how this contrasts with the behavior of light.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about the difference in sound speed inside a moving train compared to stationary air outside, questioning the relative motion of air molecules.
  • Another participant clarifies that sound travels through air that is stationary relative to the ground, suggesting that the scenario changes if sound were to travel inside the train.
  • There is a repeated inquiry about why similar effects do not occur with light, indicating a curiosity about the fundamental differences between sound and light propagation.
  • Participants acknowledge the complexity of sound propagation, noting its ability to "go round corners" unlike light.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the differences between sound and light propagation, and there remains uncertainty regarding the implications of sound traveling in different reference frames.

Contextual Notes

Participants reference specific pages in the book and external links for further clarification, indicating a reliance on the text for understanding the concepts discussed.

Wattever
Messages
37
Reaction score
0
This is turning out to be difficult for me, and I haven't even reached the part where he explains the theory of relativity itself :(

"Let us suppose that the shot is fired from the guard's-van, and the echo comes from a screen on the engine. We will suppose the distance from the guard's-van to the engine to be the distance that sound can travel in a second (about one-fifth of a mile), and the speed of the train to be one-twelfth of the speed of sound (about sixty miles an hour). We now have an experiment which can be performed by the people in the train. If the train were at rest, the guard would hear the echo in two seconds; as it is, it will take two and 2/143 seconds"

Isn't the motion of air molecules (inside the train) relative to the moving train no different from motion of air molecules outside the train relative to earth? Why is there a difference in speed?

This got me confused about sound propagation in general, how can the compressions and rarefactions be http://bumper.gmi.edu/~drussell/Demos/waves/wavemotion.html" the place?!

I would really appreciate any help.
 
Last edited by a moderator:
Physics news on Phys.org
Wattever said:
Isn't the motion of air molecules (inside the train) relative to the moving train no different from motion of air molecules outside the train relative to earth? Why is there a difference in speed?

Hi Wattever! :smile:

I had a look at it at pages 22 to 23 at http://books.google.com/books?id=uF...ussell&client=safari#v=onepage&q=echo&f=false …

the screen is on the engine, so the sound is traveling through the air outside the train …

in other words, the air is stationary (relative to the ground), and the sound is traveling as if the train wasn't there.

(It would be different if the sound was traveling inside the train … then the time would be 2 seconds, however fast the train was moving. :wink:)
This got me confused about sound propagation in general, how can the compressions and rarefactions be http://bumper.gmi.edu/~drussell/Demos/waves/wavemotion.html" the place?!

Ah, it isn't neat …

that's why sound (unlike light!) goes round corners, and is generally fuzzy! :biggrin:
 
Last edited by a moderator:
Uhuh, thanks a lot! :D
 
OK I'm sorry, but why won't the same happen with light? Edit: That is the difference in time
 
Last edited:
Wattever said:
OK I'm sorry, but why won't the same happen with light? Edit: That is the difference in time

erm :redface: … that's what the book's about! :biggrin:
 
Oh, lol!
 
tiny-tim said:
… that's why sound (unlike light!) goes round corners...

But, um...
 

Similar threads

Graduate C as the speed of light and the speed of sound in Michelson-Morley.
  • · Replies 54 ·
2
Replies
54
Views
15K
Undergrad Problems with Einstein's 1920 "Relativity"
  • · Replies 58 ·
2
Replies
58
Views
8K
Graduate Is the Speed of Light Boundary Relative to a Fixed Point in the Universe?
  • · Replies 10 ·
Replies
10
Views
3K
Undergrad Some questions about Cosmic Inflation
  • · Replies 4 ·
Replies
4
Views
5K
Can Evolved State: Dichotomy Capture Your Imagination?
  • · Replies 1 ·
Replies
1
Views
4K
Do Movies Teach Us Realistic Life Skills?
  • · Replies 1 ·
Replies
1
Views
10K
At a crossroads (drop out? chase a dream? time out?)
  • · Replies 13 ·
Replies
13
Views
5K