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#21
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neophyte question about hubble's law
In article , Nicolaas Vroom
writes: I have no problem with the statement one can interpret redshift as velocity. IMO the issue is how. The current point of view is that for values of z 1 one has to use the equation v = c*z (Also called the nonrelativistic equation for the Doppler shift) You don't have to use it, but you CAN use it and get the same result as a more detailed analysis. I have a problem with that equation. Suppose a galaxy at a far distance in the past is receding from us with a speed of 0.01c resulting in a value of z of 0.01. Light from that galaxy in an expanding universe is travelling towards us at a speed c and is stretched. OK. Suppose we receive it now. Is it not possible in principle that we measure a value of z=0.02 implying a speed of v=0.02*c ? Why should that happen? My point is what we measure is not the true speed of the source at the point of emission. This speed is much lower because the waves are stretched. Again, for the low redshifts at which one can use the Doppler formula, the change in speed between the time of emission and the time of absorption is negligible. Even if we measure a z=2 it does not mean that the source in the past was travelling at a speed higher than c. But it could be. The overall implication is that maybe there is no reason to use the relativistic equation for the Doppler shift. Right. A second implication in principle is that the true speed, of a galaxy with z=2 measured now here, could be zero over there. Possible. But now you are in the high-redshift regime, where you can't get a useful answer from the Doppler formula. A third implication is that the size of the Observable Universe is much smaller than 47 Gyr. See the posting by Hans Aberg. I don't think that anyone claims that the size of the Observable Universe is as large as 47 Gyr. |
#22
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neophyte question about hubble's law
Phillip Helbig---remove CLOTHES to reply wrote:
But it is this redshift-velocity interpretation that results in speeds exceeding c? Yes, but that's not a problem. But it would appear as though you arrive at two different, non-unified concepts of velocity: of relativity which excludes FLT for massive objects, and of universe expansion of unclear independent verification. See ..... @ARTICLE {EHarrison93a, AUTHOR = "Edward R. Harrison", TITLE = "The Redshift-Distance and Velocity-Distance Laws", JOURNAL = APJ, YEAR = "1993", VOLUME = "403", NUMBER = "1", PAGES = "28", MONTH = jan } This one is avilable he http://tinyurl.com/ygq7aq2 expanded might appear as broken: http://articles.adsabs.harvard.edu/c...&clas sic=YES It does not seem to discuss redshift-velocity formulas, though, only formulas depending on distance. And my guess there is no experimental verification of such a formula at high speeds. Suppose a particle at speed close to c emits a photon, what is the measured wavelength shift? I'm sure this happens all the time in particle accelerators which produce synchrotron radiation. So how are the redshift-velocity formulas used in astronomy derived from that? Hans |
#23
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neophyte question about hubble's law
On 22 Oct, 17:34, Stupendous_Man wrote:
If you actually read Hubble's work for yourself (here's a copy of his 1929 paper, for example) http://spiff.rit.edu/classes/phys240.../hub_1929.html you'll see that he discusses a relationship between distance and radial velocity. Note the title of the paper, for example: "A RELATION BETWEEN DISTANCE AND RADIAL VELOCITY AMONG EXTRA-GALACTIC NEBULAE" Hubble used several methods involving stars (including Cepheids and luminous blue stars) to estimate distances to other galaxies. He converted the shift in apparent wavelength of their spectra into radial velocities. It is true that he offered two explanations for the shift in wavelengths, one of which is motion (radial velocity) and the other some sort of scattering. I recommend that people who argue about the work of old-timey astronomers actually read those old-timey papers themselves, rather than reading an interpretation of those papers on someone's website. If you had fully read the web reference http://home.pacbell.net/skeptica/edwinhubble.html , you would have found there "To the best of my knowledge Hubble’s 1929 paper (3) is the only published paper where the reader is left with the view by Hubble, and now apparently universally adopted, that the linear law of redshifts applies only as a velocity-distance relation. It is no wonder that this is the paper that is usually cited by itself in astronomy textbooks." and also the quote from Hubble's book 'The Realm of the Nebulae': "Meanwhile, red-shifts may be expressed on a scale of velocities as a matter of convenience. They behave as velocity-shifts behave and they are very simply represented on the same familiar scale, regardless of the ultimate interpretation. The term “apparent velocity” may be used in carefully considered statements, and the adjective always implied where it is omitted in general usage. --pp. 122-123" As is well known, the interpretation of the redshift in terms of velocities was already suggested much earlier by Vesto Slipher, and even he considered other causes possible. I quote from from http://en.wikipedia.org/wiki/Vesto_Slipher : Slipher first reports on the making the first Doppler measurement on September 17, 1912 in The radial velocity of the Andromeda Nebula in the inaugural volume of the Lowell Observatory Bulletin, pp.2.56-2.57. In his report Slipher writes: "The magnitude of this velocity, which is the greatest hitherto observed, raises the question whether the velocity-like displacement might not be due to some other cause, but I believe we have at present no other interpretation for it." Thomas |
#24
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neophyte question about hubble's law
Hans Aberg wrote:
Phillip Helbig---remove CLOTHES to reply wrote: But it is this redshift-velocity interpretation that results in speeds exceeding c? Yes, but that's not a problem. But it would appear as though you arrive at two different, non-unified concepts of velocity: of relativity which excludes FLT for massive objects, and of universe expansion of unclear independent verification. Actually, I noticed after making the post that http://en.wikipedia.org/wiki/Redshif...nsion_of_space indicates it is a metric expansion of space. And my guess there is no experimental verification of such a formula at high speeds. Suppose a particle at speed close to c emits a photon, what is the measured wavelength shift? I'm sure this happens all the time in particle accelerators which produce synchrotron radiation. So how are the redshift-velocity formulas used in astronomy derived from that? And the link above also indicates different redshift formulas, for example, a gravitational one. Hans |
#25
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neophyte question about hubble's law
In article , Hans Aberg
writes: Phillip Helbig---remove CLOTHES to reply wrote: But it is this redshift-velocity interpretation that results in speeds exceeding c? Yes, but that's not a problem. But it would appear as though you arrive at two different, non-unified concepts of velocity: of relativity which excludes FLT for massive objects, Within the constraints of special relativity, i.e. Minkowski space-time. And my guess there is no experimental verification of such a formula at high speeds. Suppose a particle at speed close to c emits a photon, what is the measured wavelength shift? I'm sure this happens all the time in particle accelerators which produce synchrotron radiation. So how are the redshift-velocity formulas used in astronomy derived from that? There are two possibilities. One, imagine the expansion of space stretching the wavelength of light. This provides a quantitatively correct interpretation, and underlines the fact that, without further assumptions, the redshift tells us ONLY the ratio of the scale factor of the universe now to that at the time the light was emitted. Or, do the the full-scale GR derivation, which is too much for a usenet post. |
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neophyte question about hubble's law
On 25 Oct, 09:36, Thomas Smid wrote:
As is well known, the interpretation of the redshift in terms of velocities was already suggested much earlier by Vesto Slipher, and even he considered other causes possible. I quote from fromhttp://en.wikipedia.org/wiki/Vesto_Slipher: Slipher first reports on the making the first Doppler measurement on September 17, 1912 in The radial velocity of the Andromeda Nebula in the inaugural volume of the Lowell Observatory Bulletin, pp.2.56-2.57. In his report Slipher writes: "The magnitude of this velocity, which is the greatest hitherto observed, raises the question whether the velocity-like displacement might not be due to some other cause, but I believe we have at present no other interpretation for it." Thomas Just an add-on to this: in case of the Andromeda Nebula (and some other close galaxies), the line shift is of course actually a blue- shift, and thus is not representable by the Hubble law. But nonetheless, it seems that both Slipher and Hubble potentially questioned the interpretation of all these shifts in terms of radial velocities. Thomas |
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neophyte question about hubble's law
In article , Thomas Smid
writes: Just an add-on to this: in case of the Andromeda Nebula (and some other close galaxies), the line shift is of course actually a blue- shift, and thus is not representable by the Hubble law. But nonetheless, it seems that both Slipher and Hubble potentially questioned the interpretation of all these shifts in terms of radial velocities. Hubble did so, IIRC, due to a misunderstanding of the K-correction. Alan Sandage detailed this in his Saas-Fee lecture notes from the 1993 summer school. I have the proceedings, but haven't unpacked them since having recently moved house, but if there is interest I can post a short summary in a couple of weeks. Sandage, of course, was Hubble's assistant (at the same time that he was Baade's doctoral student). Ironically, when Hubble died he probably doubted that the expansion of the universe was real. (A note of caution to readers of historical literatu the modern definitions are often different than the ones used by authors in the first half of the 20th century. Caveat lector! (A similar situation exists with regard to 17th- and 18th-century physics literature.)) |
#28
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neophyte question about hubble's law
Phillip Helbig---remove CLOTHES to reply wrote:
But it is this redshift-velocity interpretation that results in speeds exceeding c? Yes, but that's not a problem. But it would appear as though you arrive at two different, non-unified concepts of velocity: of relativity which excludes FLT for massive objects, Within the constraints of special relativity, i.e. Minkowski space-time. As in the link of my other post (which probably did not appear before you made your post), it is called metric expansion of space. http://en.wikipedia.org/wiki/Redshift This is perfectly logical, but falls in the same category as for example MOND: one has incomplete information as to why something is happening, and adjust the questions so that known data fits. This is the problem of retrodiction without genuine prediction. And my guess there is no experimental verification of such a formula at high speeds. Suppose a particle at speed close to c emits a photon, what is the measured wavelength shift? I'm sure this happens all the time in particle accelerators which produce synchrotron radiation. So how are the redshift-velocity formulas used in astronomy derived from that? There are two possibilities. One, imagine the expansion of space stretching the wavelength of light. This provides a quantitatively correct interpretation, and underlines the fact that, without further assumptions, the redshift tells us ONLY the ratio of the scale factor of the universe now to that at the time the light was emitted. Or, do the the full-scale GR derivation, which is too much for a usenet post. Metric expansion is easy to understand from the mathematical point of view: just let it depend on time, as here http://en.wikipedia.org/wiki/FLRW_metric But then sort of the very physics principles change, because the metric is not a directly measurable quantity, but an intermediate used to explain connections between physically measurable quantities. I like the gravitational redshift more. If background radiation is produced by black hole, that would explain the heavy redshift there. I think such possibilities were in the past excluded because the GR predict the universe cannot be stable. But at that time, one did not know that only a small fraction of the mass is not visible. Hans |
#29
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neophyte question about hubble's law
In article , Hans Aberg
writes: As in the link of my other post (which probably did not appear before you made your post), it is called metric expansion of space. http://en.wikipedia.org/wiki/Redshift This is perfectly logical, but falls in the same category as for example MOND: one has incomplete information as to why something is happening, and adjust the questions so that known data fits. This is the problem of retrodiction without genuine prediction. Historically, it was exactly the opposite. The universe was believed to be static, Einstein saw that this was not what his theory predicted. So first the expanding space, and then, more than 10 years later, the observations to support it. (Einstein modified his theory---not just by introducing the cosmological constant, which someone else might have included from the outset, but by introducing it with a special, finely tuned value (the distinction is important)---to allow a static universe, but disowned this when observations confirmed his original prediction. I like the gravitational redshift more. If background radiation is produced by black hole, that would explain the heavy redshift there. It's not just the redshift. Cosmology, today, is a data-driven science. (This wasn't the case until quite recently.) There are a lot of data to explain. Any alternative theory has to explain the CMB observations in detail. |
#30
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neophyte question about hubble's law
Phillip Helbig---remove CLOTHES to reply wrote:
As in the link of my other post (which probably did not appear before you made your post), it is called metric expansion of space. http://en.wikipedia.org/wiki/Redshift This is perfectly logical, but falls in the same category as for example MOND: one has incomplete information as to why something is happening, and adjust the questions so that known data fits. This is the problem of retrodiction without genuine prediction. Historically, it was exactly the opposite. The universe was believed to be static, Einstein saw that this was not what his theory predicted. So first the expanding space, and then, more than 10 years later, the observations to support it. Did his analysis say that the universe must have a metric expansion? - I thought it just said that it could not be stable. I like the gravitational redshift more. If background radiation is produced by black holes, that would explain the heavy redshift there. It's not just the redshift. Cosmology, today, is a data-driven science. (This wasn't the case until quite recently.) There are a lot of data to explain. Any alternative theory has to explain the CMB observations in detail. The problem is that there is only retrofitting of data, and the theory seems designed so that it can't be refuted. For example, HE 1523-0901 is a 13.2 Gy old generation two star in the Milky Way and the BB universe 13.73 Gy. Suppose one would find a star older that this theoretical age, would the BB theory be judged wrong and scrapped? If not, what is the litmus test of this theory? Hans |
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