In this picture light(the green solid line) is leaving a star, but due to gravity bending the path(exaggerated in picture)does not the point of origin of the light appear along the green dotted line, for the observer of that light path?

And if so, if this were to apply to all light leaving that star, would not the star appear bigger for the distant observer, than for the close up observer?



(I asked this in "science and technology" but got no response, but I would like to know what the mainstream view on this is)

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Er, light doesn't leave stars obliquely, does it? I thought it left perpendicular to the surface.

You seem to be suggesting that a star bends its own light, which doesn't make sense. I thought an object could only bend the light coming from another object.

You see the sun as a disc covering a circular area of the sky. If it only radiated light perpendicular to its surface, what would that imply about its shape?

The surface does emit preferentially in the normal direction, exhibiting limb darkening as a result (http://en.wikipedia.org/wiki/File:Mercury_transit_2.jpg), but it emits quite a bit in other directions as well.


Light is light. It will follow the same path through space regardless of the object it originated from.

The diagram seems accurate. And yes, at a further distance, you will be able to see further around the horizon of the star. Note that there are parts that never become visible, light from those portions never being deflected enough to reach you, and that the effect is very weak for any but the most dense of objects. It might be visible in the intensity curve of pulsars...the "spot" would be visible slightly longer than otherwise.

This does not seem like gravitational lensing to me. Gravitational lensing occurs when the emitted light of an object behind another object is bent resulting in multiple images, for example.

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“He who asks a question is a fool for five minutes; he who does not ask a question remains a fool forever”
Chinese proverb
"All you need in this life is ignorance and confidence - and then success is sure." - Mark Twain.

yeah, I suppose not really lensing, which is why I put the stuff about light being bent.

But lenses do distort what is seen, an this is what is going on, i think.


But what I want to know is, does the object(the Sun/star in this case) appear bigger?

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I don't know the mainstream view, but my view, strongly shaped by
someone here on BAUT (you, maybe??) a few months ago, is that the
light is indeed lensed in such a way as to make the source appear larger.
Prior to my epiphany, I had guessed exactly the opposite: that any
object falling into a black hole would be gravitationally lensed so as to
appear smaller than its distance would suggest it should appear. I was
told this was wrong, for the reason you describe, and after drawing a
couple of diagrams like yours, saw that the lensing should produce a
magnification.

I said in a post within the last few months that my guess was that
light coming from just above the event horizon of a non-rotating black
hole would be lensed to make it appear to be twice as large-- that is,
the size of the photon sphere.

The light bending from the Sun has been observed and measured, not
just predicted by theory, but I've never actually seen it referred to as
magnifying. I think it should be referred to that way.

-- Jeff, in Minneapolis

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http://www.FreeMars.org/jeff/

"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"

"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves

plus, if a star appeared bigger, it would also maintain the measured speed of light at its surface, and compensate for the time dilation at the surface.



ETA: also it should make planets like Mercury appear to have a greater orbital radius from the star; I don't know whether that would be measurable, or not.

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When light from a background object is bent by the gravitational field in the foreground, you are calling it gravitational lensing. When the light is emitted from the foreground object (within the "lens", so to speak), it seems to you that the subsequent bending is not gravitational lensing.

To me, that is quibbling with words.

Does not seem like quibbling with words to me:

source: http://en.wikipedia.org/wiki/Gravitational_lensing

Otherwise, every stellar object would be seen as gravitational lensing.

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“He who asks a question is a fool for five minutes; he who does not ask a question remains a fool forever”
Chinese proverb
"All you need in this life is ignorance and confidence - and then success is sure." - Mark Twain.

ok, I shouldn't have used the word 'lensing', not as the term is used at the moment.

But the light does seem to be bending so that objects, like the Sun are magnified, are they, or are they not?

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There is no reason not to call it gravitational lensing. That's obviously
what it is.

Saying that it isn't gravitational lensing because the light comes from
the lensing mass is like saying the Moon doesn't rotate because it
revolves about the Earth, and nobody would be that silly!

-- Jeff, in Minneapolis

__________________
http://www.FreeMars.org/jeff/

"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"

"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves

Sorry to disagree, but that is not the way NASA sees it:

http://imagine.gsfc.nasa.gov/docs/fe...grav_lens.html

If it were as you say it is, why would they even introduce the intermediate galaxy?

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“He who asks a question is a fool for five minutes; he who does not ask a question remains a fool forever”
Chinese proverb
"All you need in this life is ignorance and confidence - and then success is sure." - Mark Twain.

The intermediate galaxy does the lensing. Something has to do it.
Why would you call it something different just because the source
of the light is also the mass that does the lensing?

Look up 'Shapiro delay' for possible info. I just scanned quickly
through the Wikipedia article
http://en.wikipedia.org/wiki/Time_delay_of_light and don't see any
reference to gravitational lensing by that name, but see the quote
of Einstein in the article.

-- Jeff, in Minneapolis

__________________
http://www.FreeMars.org/jeff/

"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"

"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves

Simply because I am using the mainstream definition of gravitational lensing. Look it up in Wikipedia.

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“He who asks a question is a fool for five minutes; he who does not ask a question remains a fool forever”
Chinese proverb
"All you need in this life is ignorance and confidence - and then success is sure." - Mark Twain.

Wikipedia: http://en.wikipedia.org/wiki/Gravitational_lensing

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“He who asks a question is a fool for five minutes; he who does not ask a question remains a fool forever”
Chinese proverb
"All you need in this life is ignorance and confidence - and then success is sure." - Mark Twain.

Can you find the term for when the light from a not very distant
source is "bent" around a massive object?

-- Jeff, in Minneapolis

__________________
http://www.FreeMars.org/jeff/

"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"

"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves

Gravitational lensing, whereby the term "distant" is relative... What I have never heard is the term used in conjunction with the light emitted from a massive object.

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______________________________________________
“He who asks a question is a fool for five minutes; he who does not ask a question remains a fool forever”
Chinese proverb
"All you need in this life is ignorance and confidence - and then success is sure." - Mark Twain.

Call it aberration, if you must. It's lensing, just a far less visible form. It seems obvious to me that the common "distance source, intermediate mass" illustration is simply to more clearly illustrate the analogy to an optical lens. The distortion of an image of an object by its own gravity is the exact same effect, and it is absurd to claim that calling it by the same name is inaccurate.

I agree that while the distortion is a lensing effect, it is not what is generally understood when using the term.
Just show me one link to a scientific paper which describes this occurence by that name, and I will stand corrected...

__________________
______________________________________________
“He who asks a question is a fool for five minutes; he who does not ask a question remains a fool forever”
Chinese proverb
"All you need in this life is ignorance and confidence - and then success is sure." - Mark Twain.

I think the parallel between lensing and rotation is pretty good. You don't
want to call gravitational lensing by a mass gravitational lensing if the light
being lensed comes from that same mass, and the other guy doesn't want
to call rotation rotation if the thing rotating is also revolving around some
other object or point in space.

-- Jeff, in Minneapolis

__________________
http://www.FreeMars.org/jeff/

"I find astronomy very interesting, but I wouldn't if I thought we
were just going to sit here and look." -- "Van Rijn"

"The other planets? Well, they just happen to be there, but the
point of rockets is to explore them!" -- Kai Yeves

Guess I shouldn't have brought the issue up. We can keep our separate definitions. Not really worth arguing about terminology.

__________________
______________________________________________
“He who asks a question is a fool for five minutes; he who does not ask a question remains a fool forever”
Chinese proverb
"All you need in this life is ignorance and confidence - and then success is sure." - Mark Twain.

Isn't the space around the sun like a gravitational well? (the old rubber sheet and ball bearing analogy). If so, why would radiating light change directions? Seems to me it would still be going straight (up the well and away...).

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______________________________________________
“He who asks a question is a fool for five minutes; he who does not ask a question remains a fool forever”
Chinese proverb
"All you need in this life is ignorance and confidence - and then success is sure." - Mark Twain.

The picture in the opening post shows it perfectly well. A radial ray will remain straight, with a small redshift as a result of the gravity. The slanted one in the illustration bends for the same reason that one from a background star bends. The gravitational field does not care where the light came from. If it is not perfectly radial with respect to the Sun's center, it will bend.

Try rolling a ball bearing through a depression on a non-radial trajectory. Is it going in the same direction on exit as it was on entry?

Another approach to the same result: consider a point source (eg distant star) geometrically close to but outside the closer star's cone of occlusion. Gravity causes the photons on this trajectory to fall into the closer star, so it is in fact occluded. What does the observer looking along this line of sight if not the remote object? the occluding star of course, so the occluding star must subtend a greater angle than implied by geometry.

Gravitational magnification, as special case of gravitational lensing. Seems logical to me.

Hornblower, tmb,
Right you are...

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______________________________________________
“He who asks a question is a fool for five minutes; he who does not ask a question remains a fool forever”
Chinese proverb
"All you need in this life is ignorance and confidence - and then success is sure." - Mark Twain.

Anybody care to calculate the size of the effect? Use the Sun for an example. I'm guessing it is very small.

I was trying to resist the temptation, but those are the magic words.
Well, not really, since I had already started working on it some. Here's where I attempted it a while back, if anyone cares to wade through the mess. I put most of my earlier calculations in ATM when I wasn't so much asking a question but wanted help and verification when working something out. One might start toward the end and work their way backwards or something. I had a couple of mistakes approximating in the beginning. In any case, I'll try to find a simpler method that I can post here for just the magnification.

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Let's put together the pieces of The Grand Puzzle . (website)

"Let's define another operator, Sz, which we won't pay any attention to."
"This transformation will automatically make zero equal zero."
"It may be true that zero equals zero -- and that is certainly an equality -- but I don't want to go into the details at this time."

Well, my attempt:
I had the idea that magnification is proportional the time dilation, so that light appears to still be c, at the surface, and if so, the Sun's surface has a time dilation of 2 parts per million and so should look 2784 meters wider, if that idea is true.

Also I read a similar idea that wave length to the viewer will be the same length, in appearance, at the emitter, and so an object will be, I suppose, magnified in proportion to the increase in wave length. Which I think might give the same answer.


I would like to know what the answer is, if anyone else has another method.

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The deflection of light passing near the Sun is a well known prediction of general relativity and measured at a solar eclipse in 1917 or so. It is about 1,8 seconds.

Now, imagine a light ray radiated by remote star that just grazes Sun´s photosphere and another ray with exact same (grazing) direction radiated by the Sun itself. The path before touching the solar surface must be symmetric to that after touching, so that the ray must bend 0,9 seconds before touching the Sun, and another 0,9 seconds after touching. The visible edge and horizon of Sun must be 0,9 seconds behind the geometric edge.

Now, precisely where does the deflection occur? If it occurred at the surface, the ray might bend 0,9 seconds while following the surface, then escape the surface in a straight line from the geometric edge. In this case the Sun would not be magnified at all. On the other hand, if the rays from the horizon were to travel straight and tangent an an expanding cone for some distance from the Sun and only then bend those 0,9 degrees some distance away, the Sun could be magnified to an arbitrarily large size.

How far from the Sun do the rays bend?