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View Full Version : So, is the apparent size of a black hole down to gravitational lensing?



WaxRubiks
2008-Sep-26, 09:04 PM
Yeah, perhaps all the matter that comprised a dying star end up falling, in a greatly time dilated manner, towards a central position, and due to gravitational lensing the whole thing looks much bigger.

So a) there would be no singularity and b) there would be event horizon.

How would this be for a black hole model?

how exactly would light be bent around a highly dense, collapsing piece of matter? Would there not be a lensing effect that would make it look bigger?

Please move to ATM, if necessary; although it is just a question, and is it really that ATM?

mugaliens
2008-Sep-27, 02:47 PM
This raises a good question: If one could park one's self just a smidgen above the event horizon, would the BH's invisible disc occupy just a smidgen less than one hemisphere?

Now back away to a save distance where the BH's invisible disc occupies just 1 steradian. What can be inferred about the BH's actual dimensions, then?

Lepton
2008-Sep-27, 03:16 PM
Does an invisible sphere appear larger due to gravitational lensing? Is that what your asking?

Hornblower
2008-Sep-27, 04:13 PM
Does an invisible sphere appear larger due to gravitational lensing? Is that what your asking?

It's not invisible. It will occult anything behind it, such as the Milky Way. I would expect the dark spot to be enlarged by gravitational lensing.

Lepton
2008-Sep-27, 04:29 PM
It's not invisible. It will occult anything behind it, such as the Milky Way. I would expect the dark spot to be enlarged by gravitational lensing.

To a degree. The objects "hidden" behind a black hole are less the closer you become to the black hole. In any event, the black hole itself is an invisible sphere unless it is radiating light.

pilgrim
2008-Sep-27, 04:43 PM
how exactly would light be bent around a highly dense, collapsing piece of matter? Would there not be a lensing effect that would make it look bigger?

As far as I've encountered gravitational lensing it only seemed to involve changing the apparent position of a star when seen in the field of view with a dense object in it, or causing multiple images of the star to be visible. So I am mostly just trying to make a logical guess...

In the gravitational lensing described, the gravity of the black hole would be the thing that would cause light from any luminous object behind it to bend. So in other words, the black hole itself would be acting as a lens for the background light. Therefore, I would say it would be any stars/objects "behind" the black hole that may have their apparent size changed, rather than the black hole itself, much like a lens itself does not appear bigger just because you're viewing something else through it. However, like I say, I know very little on the subject....

WaxRubiks
2008-Sep-27, 07:12 PM
I don't think the analogy with a glass lens is very good, because as you say a glass lens isn't magnified.

I'm just wondering what effect time dilation has on the apparent size of objects...and the matter collapsing in a black hole is experiencing considerable time dilation.

Lepton
2008-Sep-27, 09:03 PM
I don't think the analogy with a glass lens is very good, because as you say a glass lens isn't magnified.

I'm just wondering what effect time dilation has on the apparent size of objects...and the matter collapsing in a black hole is experiencing considerable time dilation.
Umm....Froggy my good friend, the matter coalescing into a black hole is NOT experiencing anything out of the ordinary. It's "internal" clock is experiencing time normally. It's time is dilated from our perspective.

ETA - Why should time dilation have an effect on the apparent size of a black hole? Could you provide a mathematical reason that it should?

astromark
2008-Sep-27, 09:31 PM
From our external view point the black hole will distort the passage of light from objects near to and behind it. As such a massive gravity force must. The actual size of the black hole is I think difficult to nail. As a measurement is of mass and gravity force. The visible component is not the black hole. It is all that material near to and orbiting it. There is much discussion regarding the state of matter in the actual black hole itself. This is a ongoing science and confirmation of some of these questions are the science of the Large Hadron Collider. This is a whole science that many greater minds are working to answer. I feel a lets gather more information and judge it accordingly would be the intelligent method don't you.?

Hornblower
2008-Sep-27, 11:34 PM
It's not invisible. It will occult anything behind it, such as the Milky Way. I would expect the dark spot to be enlarged by gravitational lensing.

I wrote in a bit of haste here. Objects directly behind the black hole would be visible as distorted images outside the silhouette as a result of gravitational lensing.

In an earlier thread a few months ago I found a link to a simulation of what a bright background would look like with a black hole in front of it. The presence of the black hole resulted in a sharp-edged black disk somewhat larger than the diameter of the event horizon, and the text explained why. I wish I could find that site now. Perhaps someone out there can help.

mugaliens
2008-Sep-28, 02:07 AM
To a degree. The objects "hidden" behind a black hole are less the closer you become to the black hole. In any event, the black hole itself is an invisible sphere unless it is radiating light.

Put your hands over your eyes. What do you see?

The room?

No???

Hmmm... And your hands aren't even invisible!

Thanks, Hornblower, for putting it so well. Simple elegance wins again.

WaxRubiks
2008-Sep-28, 04:16 AM
ETA - Why should time dilation have an effect on the apparent size of a black hole? Could you provide a mathematical reason that it should?


I started a thread about it here

http://www.bautforum.com/questions-answers/78858-do-objects-appear-get-smaller-they-approach-black-holes-event-horizon.html

basically I was asking whether, due to time dilation, and due to the speed of light being a constant in any reference frame, whether objects appear larger in order to compensate.

I imagined a desk with a lightbulb on the right side and a mirror, to reflect the lightbulb on the left, flling towards a black hole, like this
http://img396.imageshack.us/img396/6471/desk3tq6.jpg

if the time between the light bulb flashing and the reflection of the light bulbs flash increases, and you know that light still travels at 3*108m/s then if the time delay had increased then maybe the Universe would make it look like the light between the bulb and the mirror had traveled further, thus making the desk look longer(and higher also).

I calculated that this effect should make the Sun look around a couple of kilometers wider than it actually looked close up, due to a 2 in 1million part time dilation. Based upon the idea of, the factor of time dilation being proportionate to apparent length increase.

Lepton
2008-Sep-28, 04:30 AM
I started a thread about it here

http://www.bautforum.com/questions-answers/78858-do-objects-appear-get-smaller-they-approach-black-holes-event-horizon.html

basically I was asking whether, due to time dilation, and due to the speed of light being a constant in any reference frame, whether objects appear larger in order to compensate.

I imagined a desk with a lightbulb on the right side and a mirror, to reflect the lightbulb on the left, flling towards a black hole, like this
http://img396.imageshack.us/img396/6471/desk3tq6.jpg

if the time between the light bulb flashing and the reflection of the light bulbs flash increases, and you know that light still travels at 3*108m/s then if the time delay had increased then maybe the Universe would make it look like the light between the bulb and the mirror had traveled further, thus making the desk look longer(and higher also).

I calculated that this effect should make the Sun look around a couple of kilometers wider than it actually looked close up, due to a 2 in 1million part time dilation. Based upon the idea of, the factor of time dilation being proportionate to apparent length increase.

I remember that thread as well as your claim. However, the calculations you made were absent or you forgot to post them. So can you please answer the question with a mathematical reason that time dilation should have an effect on the apparent size of a black hole.

WaxRubiks
2008-Sep-28, 04:52 AM
I remember that thread as well as your claim. However, the calculations you made were absent or you forgot to post them. So can you please answer the question with a mathematical reason that time dilation should have an effect on the apparent size of a black hole.

well that's a bit difficult, as with the idea I'm proposing, only the matter falling towards the event horizon would be relevant for the apparent size of the black hole, and as that is experiencing varying degrees of time dilation, I'm not sure how I would calculate the apparent size.

With my idea, a black hole would appear to just keep growing, I think, as matter fell towards the event horizon, unless Hawking radiation compensated.

But if an apparent difference could be observed for the Sun, then


At the Sun's surface, time should flow more slowly by just 2 parts in a million than far from the Sun

http://scratchpad.wikia.com/wiki/CAT:time_dilation

with, Sun Mean diameter 1.392×109meters

1.392×109meters-1000,000/1000,002*1.392×109meters=2784meters

So the Sun should look 2784 meters wider, for a distant observer, than as for an observer close to the Sun.

And if this was applied to a lump of collapsing star matter, then you might get a much bigger difference in apparent sizes.


ETA: and those calculations are based upon the idea that apparent size is proportionately increase in proportion to the time dilation factor, to preserve the apparent speed of light.

Lepton
2008-Sep-28, 04:57 AM
with, Sun Mean diameter 1.392×109meters

1.392×109meters-1000,000/1000,002*1.392×109meters=2784meters.

Explain the math. I understand the 1.392 x 10109 but not the factor you are multiplying it by. Explain that please.

WaxRubiks
2008-Sep-28, 05:03 AM
Explain the math. I understand the 1.392 x 10109 but not the factor you are multiplying it by. Explain that please.


well, if time is dilated by a factor of two then the length of an object may appear twice as long(to preserve the apparent speed of light)

so if the Sun is observed as having a diameter of x, and the time dilation factor is 2 parts per million, then I was assuming that it was observed from a distance, thus dividing by 1000,002 and multiplying by 1000,000 should give you the size that the Sun would appear close up.

Lepton
2008-Sep-28, 05:07 AM
well, if time is dilated by a factor of two Where does this assumption come from and why is it justified?

, and the time dilation factor is 2 parts per million.
Where does this assumption come from and why is it justified?

WaxRubiks
2008-Sep-28, 05:32 AM
Where does this assumption come from and why is it justified?


well, the idea is to preserve the apparent speed of light.

Say if you were using the light bulb and the mirror to measure the speed of light, and all you knew was the length of time between lightbulb flash, and mirror reflected flash, then the apparent(measurable from a distance) length of the desk would be very important in calculating the speed of light, would it not?


And if the speed of light always measures as having the same speed, then it must be the desk which looks bigger.


Sometimes I think this makes sense, and sometimes I'm not so sure.....

Lepton
2008-Sep-28, 05:35 AM
well, the idea is to preserve the apparent speed of light.

That didn't answer my question to you. I asked you in your other thread and I will ask you here again. Please keep ATM ideas out of this section so it can remain an actual educational resource.

WaxRubiks
2008-Sep-28, 05:52 AM
I don't know that it is that ATM.

What is the time delay between lightbulb flash, and mirror reflected flash?
What is the speed of the light traveling between lightbulb and mirror, as measured by a distant observer?

Lepton
2008-Sep-28, 06:00 AM
I don't know that it is that ATM.

What is the time delay between lightbulb flash, and mirror reflected flash?
What is the speed of the light traveling between lightbulb and mirror, as measured by a distant observer?

This question isn't but your math assumptions with no justification and your "desk falling into a black hole" most certainly are. It is those I was referring to in case you were unclear about it.

WaxRubiks
2008-Sep-28, 06:07 AM
well it was just a thought experiment; and people seem reluctant to give answers to the questions about bulb-flash mirror-flash time delays and the speed of light between the bulb and the mirror.

I wouldn't mind answers to these questions.

Lepton
2008-Sep-28, 06:12 AM
well it was just a thought experiment; and people seem reluctant to give answers to the questions about bulb-flash mirror-flash time delays and the speed of light between the bulb and the mirror.

I wouldn't mind answers to these questions.

Then ask the questions in a straight forward manner without the ATM baggage in a thread meant for those questions and no other ideas. I am sure people will answer but it is kind of senseless to ask a question about the speed of light when you already know it is a constant and you just want to use the answer to promote more ATM nonsense.

WaxRubiks
2008-Sep-28, 06:43 AM
yeah, well I did ask the question, but didn't quite get the answers.

I'll ask again.


What is the time delay between lightbulb flash, and mirror reflected flash?
What is the speed of the light traveling between lightbulb and mirror, as measured by a distant observer?

I think maybe I should try to keep the baggage out of a Q&A thread, but it can be difficult if people start asking questions. You yourself asked me to back up what I said, to give context to the OP question; and so I did, but maybe I should start a new thread, and just link back to this one.

Will do, at some point.

astromark
2008-Sep-28, 06:08 PM
well, the idea is to preserve the apparent speed of light.

Say if you were using the light bulb and the mirror to measure the speed of light, and all you knew was the length of time between light bulb flash, and mirror reflected flash, then the apparent(measurable from a distance) length of the desk would be very important in calculating the speed of light, would it not?


And if the speed of light always measures as having the same speed, then it must be the desk which looks bigger.


Sometimes I think this makes sense, and sometimes I'm not so sure.....

----- Lets just say that a pulsed laser beam was pointed at a mirror on the moon.
That it is measured by very accurate equipment. The reflected image is observed to have taken just 1.25 seconds. The velocity of that light stream is the same in each direction. Light does not slow after a change in direction. The velocity of light through a vacuum is constant. At C.-----
Is that what you asked ?

WaxRubiks
2008-Sep-28, 07:29 PM
----- Lets just say that a pulsed laser beam was pointed at a mirror on the moon.
That it is measured by very accurate equipment. The reflected image is observed to have taken just 1.25 seconds. The velocity of that light stream is the same in each direction. Light does not slow after a change in direction. The velocity of light through a vacuum is constant. At C.-----
Is that what you asked ?


well it is more about the speed of light between a free falling emitter, and the mirror, in the same frame of reference as the emitter(light bulb).

and the bulb and the mirror are in a different frame of reference from the observer.

Sam5
2008-Sep-28, 09:11 PM
well it is more about the speed of light between a free falling emitter, and the mirror, in the same frame of reference as the emitter(light bulb).

and the bulb and the mirror are in a different frame of reference from the observer.

Have you read Einstein’s “gravitational redshift” theory of 1911? And have you read these later papers of his:

Einstein, 1912, a limit to the “constancy” postulate:
http://i26.tinypic.com/2wgc1ft.jpg

Einstein, 1913, abandons “constancy”:
http://i32.tinypic.com/so0ifa.jpg

Einstein, 1914, “constancy” limited to regions with no differences in gravitational potential:
http://i38.tinypic.com/25upl46.jpg

All three of these papers are in The Collected Papers of Albert Einstein, Volume 4, paperback, English translation:
http://press.princeton.edu/titles/5837.html

WaxRubiks
2008-Sep-28, 11:45 PM
I'm guessing that he saw the problem with time dilation and the speed of light...

Tensor
2008-Sep-29, 12:24 AM
I'm guessing that he saw the problem with time dilation and the speed of light...

Actually, he saw no such thing. What he was actually doing during the period covered by the papers Sam5 pointed you to, was working on the General Theory of relativity. All those papers are fits and false starts during the work on the General Theory. He never "abandoned" the constancy of the speed of light, in inertial frames of reference. Those papers can be misleading and you really need to look at the full General Theory of Relativity, as he presented it in November of 1915. Like here (http://en.wikipedia.org/wiki/General_relativity)or here (http://en.wikipedia.org/wiki/Mathematics_of_general_relativity)

Sam5 has tried before to push his own interpretation of Einstein's work during this period in the ATM section. You might want to do a search with relativity as the keyword and Sam5 as the author (or alternately, with Sam5 as the keyword and either Tensor, Normandy6644, Tassel, Fortis, Sean, Eroica, grey Kilopi or Celestial Mechanic as the author. Actually, don't bother, I found one of the threads (http://www.bautforum.com/general-science/8552-twin-paradox-definitive-proof-its-sr.html)Sam5 was involved in, in current breakdown it's 53 pages, it was 88 pages on the old Bad Astronomy board. I think that this was one of the threads that contributed to the 30 day limit in ATM. Or, for something more current, this thread (http://www.bautforum.com/against-mainstream/75007-instant-communications.html)where Publius tries to explain it.

astromark
2008-Sep-29, 05:56 AM
Frog march... Free falling emitters and from the frames of reference here and some place else.... Please talk English. I obviously do not know what you are on about. Light, whatever it is. Seems to behave as a radiating form of energy in the wavelength of the visible spectrum. It also can be tested to have behavior such as particles would have. It would seem also that it always travels through a vacuum at C. Regardless of other velocity factors. C. is it. Absolutely and always. Try slowing it down by passing it through gasses and yes you can, but you loose intensity. Stop it and its gone. In this universe light travels at light speed. There are no exceptions.
However this could be prov en to be wrong... why does that not surprise me. If you can explain in a simple manner please do so. I would advise a session of study with Stephen Hawkins a brief history of time. I have and condone all he says as compelling. As for quoting Albert and telling us he said this or that... No, those papers were works in progress. His well published relativity theorie is impossible to fault. Unlike any thing I have told you...:)

pilgrim
2008-Sep-29, 11:05 AM
I started a thread about it here

http://www.bautforum.com/questions-answers/78858-do-objects-appear-get-smaller-they-approach-black-holes-event-horizon.html

basically I was asking whether, due to time dilation, and due to the speed of light being a constant in any reference frame, whether objects appear larger in order to compensate.

I imagined a desk with a lightbulb on the right side and a mirror, to reflect the lightbulb on the left, flling towards a black hole, like this
http://img396.imageshack.us/img396/6471/desk3tq6.jpg

if the time between the light bulb flashing and the reflection of the light bulbs flash increases, and you know that light still travels at 3*108m/s then if the time delay had increased then maybe the Universe would make it look like the light between the bulb and the mirror had traveled further, thus making the desk look longer(and higher also).

I calculated that this effect should make the Sun look around a couple of kilometers wider than it actually looked close up, due to a 2 in 1million part time dilation. Based upon the idea of, the factor of time dilation being proportionate to apparent length increase.

The point I was trying to make is that, much like the size of a lens, the "size" of the black hole is unaffected. Namely, in the thought experiment you propose, I don't think it would be the time between the flash of the bulb and the flash from the corresponding reflection that would increase.

What I think the effect would be is, the bulb would flash less frequently, causing a reflection in the mirror less frequently, but with non-increasing time difference between the bulb flash and reflection flash. In other words, the cause (bulb flashing) and the effect (reflection in the morror) would be separated with the same amount of time, giving a constant size parameter. However, the cause and effect pair would appear less frequently.

Hornblower
2008-Sep-29, 03:06 PM
This has digressed into renewed discussion of a foreground object just in front of the black hole. What does this have to do with the apparent size of the BH itself?

WaxRubiks
2008-Sep-30, 01:39 AM
This has digressed into renewed discussion of a foreground object just in front of the black hole. What does this have to do with the apparent size of the BH itself?


well, the desk argument would apply to all the matter outside the event horizon..

So maybe without the apparent increase size(along the tangent of the event horizon) of the matter falling in, then maybe the event horizon would be small, maybe even non-existent, and all you would have was a ball of collapsing matter.