## Mashy 2 years ago Speed of light doubt In this documentary http://www.youtube.com/watch?v=PxW0jBcUqZQ about at 28th minute or so.. they mention saying that the speed of light as seen by someone stationary is independent of the motion of the source.. (unlike speed of the ball thrown by a man which depends on whether the man is moving or not).. what i say is.. isn't that true FOR EVERY WAVE??? i mean consider sound.. a jet travelling at mach 2 or 3.. creating sound.. the sound travels at its own speed.. hence jets can travel faster than sound.. its a not a speciality of light! :O :O

1. masumanwar

from einstein special theory of relativity speed of light is the highest as 3*10^8 m/sec

2. Mashy

that totally doesn't answer what my question is :-/

3. asnaseer

it is only the speed of electro-magnetic waves that is 3 x 10^8 m/s^2 other waves have different speeds depending on various factors - e.g. sound waves travel faster in solids

4. Mashy

I KNOW THAT... SEE MY QUESTION.. ALL WAVES behave the same... their SPEED (regardless of how much it is) is independent of the SOURCE.. !! thats what I am saying!!

5. asnaseer

Electro-magnetic wave are special in that their measured relative speed does not change regardless of who measures them. i.e. even if you were travelling at Mach 2 when you measured the speed of light (relative to you) - you would always get the same answer 3 x 10^8. However, the relative speed of sound will be measured as a different number.

6. Mashy

I DON"T CARE ABOUT THE RELATIVE speed... please look at my question!!.. i know C is a cosmological constant doesn't depend about the frame of refrence.. all i said is that the speed of ANY WAVE is independent of the SPEED OF THE SOURCE..!!

7. asnaseer

your question says "...the speed of light as seen by someone stationary is independent of the motion of the source..." I believe the "motion of the source" here refers not to the actual "source" but the relative speed. i.e if you were stationary and someone else was travelling in a very fast train and they generated a sound wave, then your measurement of the speed of THAT wave as it passed you would be different compared to if you measured the same wave while the train was stationary.

8. Mashy

no.. the speed would be the same..!!!

9. Mashy

that is how you derive the formula of doppler shift!!.. check doppler shift derivation!!.. the speed of sound will ONLY depend on the atmospheric factors. and not on the motion of source.. however.. the frequency would change. !

10. asnaseer

yes, frequency would change leading to the doppler effect. but also your measurement of the speed of the wave (if it was generated by a source that was travelling past you) I believe would be different. maybe I am wrong or have misunderstood your question? I'll let someone else answer this for you.

11. Mashy

no sir.. it wouldn't be different... and yes.. now you are on the right track( i mean the right arguement :D).. I assure you.. the speed of sound as measured by me if i m stationary would be the same.. regardless of whether the source is travelling at near the speed of light!!!...

12. asnaseer

just to clarify - someone measuring the speed of sound inside the train would get the same value for the speed since it is carrying the "medium" with it. However, I am not certain now of what the effect on an outside observer would be - since the sound source is generated inside the train which is carrying the "medium" (i.e. air) with it, but the "medium" in which you are is the air outside the train - and I believe that is not carried with the train. interesting question though - I will need to investigate this further. :)

13. Mashy

oh wait.. now i understand YOUR point.. I wasn't talking about something that is CARRYING the medium along with it :D.. i mean think abou tit.. planes can go faster than speed of sound.. how ?? i mean if when plane was going at mach 2.. the sound's speed would add up cause the medium is going along with them!.. maybe inside the plane.. if the guys spoke to each other.. the sound speed is now twice.. but outside the plane it still travels at the same speed.. and its the OUTSIDE sound that matters... does it make sense ? :D

14. asnaseer

yes - that is what I was trying to say originally :)

15. Mashy

ok.. ok .. but i think its usually the OUTSIDE sound that matters right??.. and what happens when the sound that is inside goes outside?

16. asnaseer

that is the interesting thought that you just raised in my head! :D

17. asnaseer

I will need to do some research on it...

18. Mashy

i think the sound will come back to its normal speed.. i mean think about it.. sound RELATIVE to air is always travelling at the same speed.. lets keep all other atmospheric factors constant.. so when it goes into the outside air which is not moving.. it should come back to its normal mach1 ... if you were running on a threadmil.. and then suddenly you jumped and started running on the ground.. your running speed remains the same.. hence you now start going forward :D

19. asnaseer

hmmm- but you would get a sudden "jolt" as you jumped off and hit the ground - so would that slow you down - even momentarily before you eventually picked up speed again?

20. Mashy

yes.. cause of your inertia right?? but does sound have inertia?.. sound is basically waving of air molecules right.. its like.. every atom just goes and hits another atom.. so that wouldn't provide that 'jolt' you were talking about!.. dunno for sure though.. m just a high school physics teacher :P never the less interesting.

21. asnaseer

I getting more and more swayed towards your argument - you do put a reasonable case forward. :)

22. asnaseer

I found an very interesting discussion on this here: http://www.physicsforums.com/archive/index.php/t-620108.html

23. Mashy

thanks .. :D.. but its always nice to keep an open mind... even though when things seem so ridiculously obvious, i always try to keep an open mind.. maybe something is missing !!.. so please do the research you were saying.. :)

24. asnaseer

will do! :)

25. Mashy

oki thanks for your time! .. i ll look into that article!

26. asnaseer

yw :)

27. Mashy

the argument there is exactly like i argued BEFORE you talking about the medium being carried away.. no one brought up the point there!.. you should post there :D

28. asnaseer

maybe - let me formulate my thoughts a bit better first. :)

29. Vincent-Lyon.Fr

This is simply true for all waves, when the observer shares the same frame of reference with the medium. What is important in relativity,is that the speed of light is the same measured by the person who is moving wrt ground and sends the signal , as measured by the person on the ground.