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Post by StuartG on Sept 29, 2010 14:41:17 GMT 1
A visual aid for discussion upload.wikimedia.org/wikipedia/commons/f/f1/EM_spectrum.svg...Blue Shift. It seems to work 'locally'. An object moving away from the observer is seen to change in em frequency. A spectrum of its light displays certain bands [like bar-codes] that signify its make up. These bands denote an element [say] but the em frequency discerned is lower than would be expected for the substance. This apparent shift in frequency has been dubbed red shift, this also works for an object moving towards the observer, blue shift depending on a lower or higher perceived frequency. Can this shift appear for frequencies outside the visible spectrum? Stuart |
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Post by abacus9900 on Sept 29, 2010 14:44:13 GMT 1
A visual aid for discussion upload.wikimedia.org/wikipedia/commons/f/f1/EM_spectrum.svg...Blue Shift. It seems to work 'locally'. An object moving away from the observer is seen to change in em frequency. A spectrum of its light displays certain bands [like bar-codes] that signify its make up. These bands denote an element [say] but the em frequency discerned is lower than would be expected for the substance. This apparent shift in frequency has been dubbed red shift, this also works for an object moving towards the observer, blue shift depending on a lower or higher perceived frequency. Can this shift appear for frequencies outside the visible spectrum? Stuart  |
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Post by abacus9900 on Sept 29, 2010 14:48:23 GMT 1
It's an effect when an object is moving towards an observer. The wavelengths get 'bunched-up' due to the fact they are being compacted towards you.
Radio waves and x-rays are subject to the same effect, yes, since they are all forms of EM radiation.
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Post by StuartG on Sept 29, 2010 15:46:00 GMT 1
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Post by speakertoanimals on Sept 29, 2010 16:01:15 GMT 1
We have a galaxy that has been observed with a spectroscopic red shift of z = 6.96
They looked at the Lyman alpha line from hydrogen, which with no shift is at 1216 angstroms in the ultraviolet region of the spectrum.
The observed line was at 9682 angstroms, which is in the infrared.
z is just (lambda_{observed}/lambda_{unshifted})-1, which gives the figure of z = 6.96
In relativity there are 3 causes of a Doppler shift. The easiest to see is the object approaching you emitting regular blips, but second blip hasn't got as far to travel as the first (it has got closer in the time between blips), so blip frequency (and frequency of anything else) increases.
Except in relativity, you also have to allow for the fact that the clocks for the emitter might not be running at the same rate as yours. So, moving clocks run slow, which gives a correction to the classical result, and means there is a Doppler effect even when the source is moving perpendicular to the line of sight.
Gravity can also give a red shift, and this has been measured over distance as small as the height of a large stairwell.
Finally, we have the cosmological redshift, which isn't due to distance galaxies receeding at speed, but instead due to photons passing through space that is expanding as they travel. If we look at the z value given earlier, for small velocities the Doppler effect is given by:
z = v/c, hence we sometimes see figures of v greater than c quoted.But this isn't a real relative velocity, just the velocity that would give the same shift according to the low-speed formula.
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Post by Progenitor A on Sept 29, 2010 16:03:28 GMT 1
It works for all waves (I think). The most common one is the police siren high-pitched as it approaches you, low as it moves away
Helicopters are also interesting. You willl hear a heavy thudding as a helicopter approaches you but the heavy thudding diminshes as it passes over and moves away. - a quite startling effect similar to doppler shift - as the helicopter approaches you the forward direction rotation of the baldes are added to its forward velocity, making them close to the speed of sound and creating small explosions - as it moves away, the rearward direction of rotation is subtracted from by the forward speed and the small explosions of air cease.
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Post by Progenitor A on Sept 29, 2010 16:06:11 GMT 1
We have a galaxy that has been observed with a spectroscopic red shift of z = 6.96 They looked at the Lyman alpha line from hydrogen, which with no shift is at 1216 angstroms in the ultraviolet region of the spectrum. The observed line was at 9682 angstroms, which is in the infrared. z is just (lambda_{observed}/lambda_{unshifted})-1, which gives the figure of z = 6.96 In relativity there are 3 causes of a Doppler shift. The easiest to see is the object approaching you emitting regular blips, but second blip hasn't got as far to travel as the first (it has got closer in the time between blips), so blip frequency (and frequency of anything else) increases. Except in relativity, you also have to allow for the fact that the clocks for the emitter might not be running at the same rate as yours. So, moving clocks run slow, which gives a correction to the classical result, and means there is a Doppler effect even when the source is moving perpendicular to the line of sight. Gravity can also give a red shift, and this has been measured over distance as small as the height of a large stairwell. Finally, we have the cosmological redshift, which isn't due to distance galaxies receeding at speed, but instead due to photons passing through space that is expanding as they travel. If we look at the z value given earlier, for small velocities the Doppler effect is given by: z = v/c, hence we sometimes see figures of v greater than c quoted.But this isn't a real relative velocity, just the velocity that would give the same shift according to the low-speed formula. Goodness me! This normally quite useless article has attempted an explanation without the necessity to kick sand in the eyes and belittle the audience! |
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Post by speakertoanimals on Sept 29, 2010 16:08:35 GMT 1
Much of the Arp stuff has been replaced by subsequent observations -- such as redshift quantisation.
Anomalies are often reported early on, seems to be something weird going on, but a lot of these fade away when better observations and more observations come along.
I think the ridicule tends to be reserved for those that hang on to these alternative theories, even when the data has gone against them.
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Post by StuartG on Sept 30, 2010 22:04:35 GMT 1
StA... "spectroscopic red shift of z = 6.96" which seems to refer to the letter to 'Nature' www.nature.com/nature/journal/v443/n7108/abs/nature05104.html "(corresponding to just 750 Myr after the Big Bang)" and "clearly shows Lyman-alpha emission at 9,682 Å," Attempt was made to find a reference/explanation for the 'Lyman series'. Why is in the ultraviolet only, reference was made to 'hydrogen' and it was found that for 'normal hydrogen' [Normal hydrogen at room temperature contains 25% of the para form and 75% of the ortho form. periodic.lanl.gov/elements/1.html ] and the video www.youtube.com/watch?v=QI50GBUJ48s gives the figures, 410.28 nm purple, 434.16 violet, 486.26 turquoise, 656.43 red. but these are just the visible ones? or is this some other form of hydrogen ? The information about the ultra-violet values is comprehended from these en.wikipedia.org/wiki/Lyman_series#History and placing them on the 'spectrum' upload.wikimedia.org/wikipedia/commons/f/f1/EM_spectrum.svg The letter mentions the CMB after initial cooling (following the Big Bang) [Cosmological Genesis sounds better? less ambigious/misleading] but that is really a subject for another thread... Stuart |
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Post by speakertoanimals on Oct 1, 2010 12:49:27 GMT 1
This is utter rubbish, because what matters TO THE AIR, is the speed of the rotors relative to the air, which doesn't depend on whether you happen to be standing before or behind the helicopter at the time. It's the speed relative to the air that matters, NOT the speed relative to the observer.
So, you can have above speed of sound in forward flight for advancing rotors, but not for receding ones, and sonic booms are typically rather directional, but none of this has anything to do with whether the speed relative to you, just whether you are in front or behind, or even to the side.
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Post by StuartG on Oct 1, 2010 16:27:58 GMT 1
"Attempt was made to find a reference/explanation for the 'Lyman series'" Found the answer.. www.astrogeo.va.it/astronom/spettri/teoriaen.htm"When the temperature is close to zero K, all the hydrogen atoms have their electron at the lower level (n=1) and thus the absorbtion spectrum will contain only the Lyman series. When temperature grows, more and more electrons jump on the n=2 and higher orbits. Multiplicity of the n=1 state (1s) is 2 while for n=2 (2s+2p orbitals) multiplicity is 2+6=8. When the temperature reaches 10'000 K the Botzmann law says that 0,04% of the electrons are now on the n=2 orbit and in the spectrum will appear also the Balmer series." [The age appears to be translated from Italian] Is it correct that the spectrum shown in the video is the one for room temp? Stuart |
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Post by marchesarosa on Oct 2, 2010 22:31:40 GMT 1
I once thought you and your better half shared a board ID, nay, so variable seemed your contributions.
Consider the possibility that StA has a better half, too - the one who doesn't use quote boxes and doesn't say "piffle" and "tosh" and "rubbish".
Now salute nicely as a sign of appreciation for her treating us plebs like human beings!
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