gravity lens

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Gene243
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gravity lens

Post by Gene243 » Thu May 20, 2010 9:27 pm

Hi guys,
I'm new here and it looks like a newcomer can ask a question without getting slammed too hard. I have several but lets start with gravitational lensing. Einstien predicted that the stars that should be hidden by the sun would be visible at an eclips because the sun's mass would bend the light coming from those stars making them appear out of place when compared to photographs taken when the sun is not near the light path. We see a similar event every day on earth. The sun appears to rise early and and set late because our atmosphere bends the light coming from the sun. ( For a long time I wondered why the day and night times were not equal on the equinox dates.) This bending of light is explained by light bending toward the denser part of the medium it is travelling through. So. Wouldn't there be plenty of "atmosphere" around the sun to account for the changes in apparent star position? Coronal mass ejections happen all the time and the "solar wind" must be denser closer to the sun making a regular lens around the sun. Was this lens accounted for in proving the gravitational lens.

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Re: gravity lens

Post by Chris Peterson » Thu May 20, 2010 9:40 pm

Gene243 wrote:Hi guys,
I'm new here and it looks like a newcomer can ask a question without getting slammed too hard. I have several but lets start with gravitational lensing. Einstien predicted that the stars that should be hidden by the sun would be visible at an eclips because the sun's mass would bend the light coming from those stars making them appear out of place when compared to photographs taken when the sun is not near the light path. We see a similar event every day on earth. The sun appears to rise early and and set late because our atmosphere bends the light coming from the sun. ( For a long time I wondered why the day and night times were not equal on the equinox dates.) This bending of light is explained by light bending toward the denser part of the medium it is travelling through. So. Wouldn't there be plenty of "atmosphere" around the sun to account for the changes in apparent star position? Coronal mass ejections happen all the time and the "solar wind" must be denser closer to the sun making a regular lens around the sun. Was this lens accounted for in proving the gravitational lens.
The transparent atmosphere of the Sun is actually quite diffuse and also quite small compared with the rest of the Sun. So refractive bending is not a large effect, and to the extent it exists it can be compensated for.
Chris

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Re: gravity lens

Post by Gene243 » Fri May 21, 2010 2:07 pm

Thanks for your answer Chris, I'll let it soak in for a little while.
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Re: gravity lens

Post by Gene243 » Mon Jan 30, 2012 5:09 am

Could it not account for all of the red shift and aparent displacement of distant objects?

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Re: gravity lens

Post by Chris Peterson » Mon Jan 30, 2012 5:18 am

Gene243 wrote:Could it not account for all of the red shift and aparent displacement of distant objects?
Could not what account for these things?
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Re: gravity lens

Post by Gene243 » Mon Jan 30, 2012 5:40 am

The physical lens.

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Re: gravity lens

Post by Chris Peterson » Mon Jan 30, 2012 5:47 am

Gene243 wrote:The physical lens.
Sorry, I'm still not following you. What physical lens? What optical geometry are you describing?
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Re: gravity lens

Post by Gene243 » Mon Jan 30, 2012 5:57 am

The physical lens is the mass of the solar wind. It has mass ,can be measured, and causes some aparent shift in the position of stars photographed next to the eclipsed sun. I am asking if it is considered at all in these experiments.

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Re: gravity lens

Post by Chris Peterson » Mon Jan 30, 2012 6:04 am

Gene243 wrote:The physical lens is the mass of the solar wind. It has mass ,can be measured, and causes some aparent shift in the position of stars photographed next to the eclipsed sun. I am asking if it is considered at all in these experiments.
The solar wind has an unmeasurable effect on the position of stars near the Sun, and it has no effect at all on redshift.
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Re: gravity lens

Post by Gene243 » Mon Jan 30, 2012 6:23 am

Do you know if anybody is doing any research in this area? I think this is modern science's answer and must be incorect. I agree it is more likely that I misunderstand some physical law than I see the universe more corectly than everyone els. But isn't red shift just a chromatic aboration on an interstellar scaled lens.

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Re: gravity lens

Post by Markus Schwarz » Mon Jan 30, 2012 9:29 am

Gene243 wrote:Do you know if anybody is doing any research in this area? I think this is modern science's answer and must be incorect. I agree it is more likely that I misunderstand some physical law than I see the universe more corectly than everyone els. But isn't red shift just a chromatic aboration on an interstellar scaled lens.
You always have to keep in mind the various scales involved. Yes, the different parts of the sun's "atmosphere" do have a mass and density. Hence, they do deflect light because of their gravity and chromatic aberration. However, doing a quick search on Wikipedia gives that the density of the sun's chromosphere is about 1/10,000 than that of air (10^-4 kg/m^3 compared to 1kg/m^3 of air). Since the index of refraction depends on density (and frequency), its influence is negligible when it comes to position measurements, as Chris pointed out earlier.

On the other hand, interstellar dust can lead to a sizable reddening, and has to be taken into account when measuring cosmic redshifts. This is because interstellar dust clouds can be huge compared to the extension of the sun's outer layers. There are plenty of groups around the world doing research on it.

Similarly, the typical mass of a coronal mass ejection is about 10^12 kg, which is a trillionths of the mass of the Earth. Hence, it's save to neglect its influence on gravitational lensing.

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Re: gravity lens

Post by Chris Peterson » Mon Jan 30, 2012 2:57 pm

Markus Schwarz wrote:On the other hand, interstellar dust can lead to a sizable reddening, and has to be taken into account when measuring cosmic redshifts. This is because interstellar dust clouds can be huge compared to the extension of the sun's outer layers. There are plenty of groups around the world doing research on it.
Reddening and redshift are completely different things. Reddening is a filtering process, whereby the energy we receive at different wavelengths is altered, because long wavelengths are absorbed, and short ones scattered. Reddening doesn't change any wavelengths at all, however, so it doesn't affect redshift measurements.

Redshift is measured by identifying a set of spectral peaks of known wavelength, and then observing how far they are shifted from that wavelength. This doesn't change with reddening.

Reddening is taken into consideration for any photometric-type measurements, where absolute or relative intensities are important. It isn't taken into account when measuring redshifts.
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Re: gravity lens

Post by Gene243 » Mon Jan 30, 2012 5:13 pm

I'll need some time to absorb this again. Hopefully it won't be so long.
Thank you for the answers.
Gene

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Re: gravity lens

Post by Nereid » Fri Mar 02, 2012 1:49 pm

Another thing to keep in mind is that gravitational lensing is achromatic; the observed effect is exactly the same, whether you're observing it in the red part of the visual waveband or the blue ... or in UV, or IR, or x-rays, or radio! So, for example, the apparent position of a quasar - one that's 'radio loud' - is shifted exactly the same amount by the Sun's gravitational field when you observe it with an optical telescope or a radio telescope.

The more traditional lensing - think of light, with a magnifying glass - is chromatic; the amount a physical lens (whether it's made of glass, or air, or the rarified plasma of the solar wind, etc) deflects electromagnetic radiation depends very much on the wavelength of that 'light'. A good example of this is scintillation: the atmosphere makes stars twinkle, and you can sometimes see quite colourful twinkles in bright stars very low down in the sky (a dark, clear sky is best). If you observe the same stars (quasars are better) in the radio part of the spectrum, they don't scintillate like this. However, you can observe scintillation of distant radio sources ... caused by the interplanetary (and interstellar) medium; observing these sources with a space-based (optical) telescope (like the Hubble) you don't see this scintillation.

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Re: gravity lens

Post by Chris Peterson » Fri Mar 02, 2012 2:55 pm

Nereid wrote:Another thing to keep in mind is that gravitational lensing is achromatic...
That is a very interesting observation, which I had never really considered. As a person with a background in physical optics, it brings home to me that gravitational lensing isn't a refractive process- that is, it doesn't involve a change of direction of a wavefront because of movement in a medium with a different transmission velocity. Chromatic effects in refractive lenses stem from dispersion, the property of photons in a medium to travel at different speeds depending on their wavelength. But with gravitational lenses, there is no medium involved, so all the photons are traveling at c. The wavefront is distorted by the shape of spacetime, not by slowing in a medium.
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Re: gravity lens

Post by Nereid » Sun Mar 04, 2012 6:01 pm

IIRC, at least one of the microlensing searches (MACHO) uses/used the achromatic nature of gravitational lensing/deflection to select candidate events: they looked for stars which showed the same rise in brightness in their red band as in the blue (within their estimated uncertainties). While false positives are certain - if all there is is a single pair of datapoints - the light curve (apparent brightness as a function of time) of a microlensing event is very distinctive, so false positives are relatively easy to rule out (provided the event is not too faint, of course).

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Re: gravity lens

Post by Chris Peterson » Sun Mar 04, 2012 6:48 pm

Nereid wrote:IIRC, at least one of the microlensing searches (MACHO) uses/used the achromatic nature of gravitational lensing/deflection to select candidate events: they looked for stars which showed the same rise in brightness in their red band as in the blue (within their estimated uncertainties). While false positives are certain - if all there is is a single pair of datapoints - the light curve (apparent brightness as a function of time) of a microlensing event is very distinctive, so false positives are relatively easy to rule out (provided the event is not too faint, of course).
I hadn't heard of that approach, but it's clever and makes good sense. I love these sorts of elegant ideas in science.
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