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blue shift and speed of light

 
 
bevinp
 
Reply Tue 4 Mar, 2014 03:34 am
Special relativity states the speed of light is the same for all observers. But astronomers talk of the doppler effect (ie blue shift) to determine relative speed between us on earth and a distant star. If we on earth see the speed of light being the same as that from a distant star, how can doppler shift occur?

Putting my question another way, imagine a piece of titanium (or other metal) burning in space and thus giving off a specific colour (frequency of light), and that it is moving away from us at a relativistic speed. Why don't we see the same colour given that we perceive the same speed of light as that emanating from the receding flame?
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Type: Question • Score: 5 • Views: 2,705 • Replies: 43
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rosborne979
 
  1  
Reply Tue 4 Mar, 2014 05:32 am
@bevinp,
Because the color of light is associated with its wavelength, not its speed.
Brandon9000
 
  1  
Reply Tue 4 Mar, 2014 06:02 am
Don't try to disprove a century of physics, especially without any education in physics. It's dumb.
0 Replies
 
dalehileman
 
  -1  
Reply Tue 4 Mar, 2014 12:07 pm
@bevinp,
Bev that's really a good q, one I've long pondered

http://able2know.org/topic/209488-1

I've also wondered Bev about other relativistic phenomena: time dilation, Lorenz contraction, mass increase, etc. Of course all these apparent shifts are necessary to satisfy certain mathematical necessity but still Intuition remains puzzled

Indeed why should the clock seem to squeeze into a disk, run slower, get heavier just because it's moving; and yes, how come we nonetheless experience Doppler

After mulling it over for several decades I speculated that all these effects could easily be explained by a novel way of looking at time-at-a-distance

http://able2know.org/topic/187876-1

In short, it suggests that they only seem to happen to a moving object because we "underestimate" its velocity. Though my speculation has acquired little support, the ease with which it explains those relativistic effects to the complete satisfaction of the Intuition suggests there might still remain something as yet uncomprehended, perhaps presently inaccessible owing to dualistic tendencies of our language, abstract, philosophical to be sure, as yet unexamined, the approach to which might lead to new ways of looking at the Universe

....but thanks Bev for this opportunity to shoot off my ancient mouth
0 Replies
 
Miss L Toad
 
  0  
Reply Tue 4 Mar, 2014 11:46 pm
@bevinp,
I feel the space between us is expanding while we blue.

http://en.wikipedia.org/wiki/Blueshift

It makes me blush.
0 Replies
 
dalehileman
 
  0  
Reply Wed 5 Mar, 2014 12:01 pm
@rosborne979,
Ros, Bran, Miss Toad, I'd have to agree with Bev that the Doppler effect is puzzling to the Intuition. If for instance you imagine a burst of light as a physical object, then if it always passes you at the same speed, the distance between successive waves should remain fixed
rosborne979
 
  1  
Reply Wed 5 Mar, 2014 12:43 pm
@dalehileman,
dalehileman wrote:
Ros, Bran, Miss Toad, I'd have to agree with Bev that the Doppler effect is puzzling to the Intuition. If for instance you imagine a burst of light as a physical object, then if it always passes you at the same speed, the distance between successive waves should remain fixed

Do you find the sound of a train whistle approaching you and then receding to be puzzling to the intuition as well?
dalehileman
 
  0  
Reply Wed 5 Mar, 2014 01:10 pm
@rosborne979,
Quote:
Do you find the sound of a train whistle approaching you and then receding to be puzzling to the intuition as well?
No Ros, since it doesn't always pass at the same speed
rosborne979
 
  1  
Reply Wed 5 Mar, 2014 01:31 pm
@dalehileman,
dalehileman wrote:
Quote:
Do you find the sound of a train whistle approaching you and then receding to be puzzling to the intuition as well?
No Ros, since it doesn't always pass at the same speed

And yet the speed of sound is always the same (at least approximately given temperature and pressure of the atmosphere).
dalehileman
 
  0  
Reply Wed 5 Mar, 2014 02:30 pm
@rosborne979,
Quote:
And yet the speed of sound is always the same
Yes but it doesn't always pass at the same speed relative to the observer

Exactly what's puzzling Bev. She's asking in effect, if a burst of sound waves always passed at the same speed its frequency would always be the same; so then why isn't this also true of light
rosborne979
 
  1  
Reply Wed 5 Mar, 2014 02:47 pm
@dalehileman,
dalehileman wrote:
She's asking in effect, if a burst of sound waves always passed at the same speed its frequency would always be the same

It's frequency is not always the same. That's exactly why there is a doppler effect. I think you need to understand the difference between speed and frequency. You might also want to look up wavelength while you're at it Wink
Romeo Fabulini
 
  1  
Reply Wed 5 Mar, 2014 04:10 pm
Doppler/red shift would be puzzling to the intuition if the speed of light was infinite, but it's not infinite, it travels at a set speed and is therefore influenced by all sorts of exterior effects as it travels through bendy stretchy space-time, no wonder the jury is still out
dalehileman
 
  0  
Reply Wed 5 Mar, 2014 05:59 pm
@rosborne979,
She's asking in effect, if a burst of sound waves always passed at the same speed its frequency would always be the same

Quote:
It's frequency is not always the same.
Of course not, and that's the reason for the "if". Given bursts of sound each containing a fixed number of cycles, if no matter how fast you went it passed you at the same speed its tone, pitch, frequency would be the same because you'd hear the same number of cycles each time

So of course Ros sound isn't a good example, doubtless the reason for your apparent confusion. But now you can see why at first I said, "If for instance you imagine a burst of light as a physical object…."

Say instead of bursts sound or light they're identical radiators, and the guy who launches them at you monitors your speed so each one always passes you 25 mph faster than you're going. Since wavelength or distance between adjacent sections of each radiator is fixed, with each encounter the same number of sections pass you at the same speed. Thus the frequency at which these sections pass is always the same no matter how fast you go

But this isn't true of light, and that's evidently the gist of Bev's puzzlement. Bev where are you
0 Replies
 
Romeo Fabulini
 
  1  
Reply Wed 5 Mar, 2014 09:05 pm
Here's a riddle/conundrum that i've never quite been able to get my head round-

Suppose you shone a powerful laser beam out into space at any star (say one on the right of this pic), and then swept it quickly across to point at any one on the left, would the tip of the beam "bend" in the same way as the jet of water from a hosepipe bends if you sweep the hose around?
Furthermore,would the tip of the laser beam have to be travelling faster than light as it sweeps across the sky from the first star to the second in the blink of an eye, considering the stars are billions of miles apart?

http://i53.photobucket.com/albums/g64/PoorOldSpike/sky-nite_zpse330f1b9.jpg~original
dalehileman
 
  1  
Reply Thu 6 Mar, 2014 10:59 am
@Romeo Fabulini,
Good q Romeo and my guess--yea only a guess--is that yes it would appear to "bend" although each photon would continue in a straight line
0 Replies
 
dalehileman
 
  1  
Reply Thu 6 Mar, 2014 11:19 am
@Romeo Fabulini,
Quote:
Doppler/red shift would be puzzling to the intuition if the speed of light was infinite,…..
Interesting Romeo that you should so speculate. In fact I had concluded just the opposite: that the finite, apparently constant, speed of light is puzzling because as Bev asks, "If we on earth see the speed of light being the same as that from a distant star, how can doppler shift occur? "

Thus she's asking, if a burst of light of the same frequency comes from two stars, one close by, hardly moving, and the other distant, receding at a huge velocity, if each burst contains the same number of cycles and passes at the same speed, then why don't we see 'em as the same color


Bev, where are you
I'm right, am I not, that's what you're asking
Romeo Fabulini
 
  1  
Reply Thu 6 Mar, 2014 12:46 pm
I suppose there are as-yet-unknown factors about the nature of light and its passage through space-time that need to be taken into consideration.
Incidentally here's another conundrum that's had me puzzled for a ling time-

When we look at a galaxy like Andromeda, it appears like this (below).
Andromeda is 260,000 light years diameter which means we're simultaneously seeing the rim furthest away from us and the rim nearest to us at the same time despite the 260,000-year time lag between the two rims.
Yet the galaxy appears as a perfectly normal spiral without any kind of distortion in the image!

http://i53.photobucket.com/albums/g64/PoorOldSpike/andromeda-galaxy_zps7ef5793e.jpg~original

http://space-facts.com/andromeda/
rosborne979
 
  1  
Reply Thu 6 Mar, 2014 01:30 pm
@Romeo Fabulini,
Galaxies don't change much in 260k years.
0 Replies
 
rosborne979
 
  1  
Reply Thu 6 Mar, 2014 01:33 pm
@dalehileman,
Most stars/objects don't recede from us because of velocity, but because of the expansion of the universe. The doppler red-shift we see because of expansion is different from the red-shift we would see from an object which had actual relativistic velocity. Even though both doppler shifts "look" the same, they are the result of two very different causes.
Romeo Fabulini
 
  1  
Reply Thu 6 Mar, 2014 01:40 pm
Quote:
Rosborne said: Galaxies don't change much in 260k years.

Ah but they spin, and that's where the conundrum comes in because we're seeing the spinning disk of Andromeda whose diameter is 260,000 light years, yet it appears normal despite the speed-of-light time lag from rim to rim
 

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