# How invisible are black holes?

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#### el_naso

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I´m well aware of how bhs bend light in a way so that we can´t see them, but, wouldn´t there be a point, close enough to the bh where most of the light from behind it wouldn´t reach us? I mean, if the bh is so big (it really doesn´t need to be big for this) that it covers most of our sight, would we see it?<br /><br />Also, the point i´m refeering to is hipotetically outside the event horizon as the bh would be obviously seen at that point.<br /><br />thoughts?

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#### dragon04

##### Guest
Think of it this way. From outside the event horizon of a BH, light is so red shifted (moving away from you), that you can't see it.<br /><br />And the hard part is trying to imagine that there is no getting"behind" a BH. You can look at a 2 or 3 dimensional view of one, but that is only from your specific point of view.<br /><br />If you imagine a BH as a "shpere", it will look the same at any of the infinite points from which you can observe it.<br /><br />Let me make a really bad analogy. Go into your bathroom and flush your toilet. Watch it. As the water drains, you see a whirlpool. Look at the middle of the whirlpool. You see a hole in the water.<br /><br />Now imagine a toilet that drains in every imaginable point you can view it from. You will never see any water in the exact center of it. <br /><br />It's hard to get your brain around. A black hole just isn't a hole like we can think of. A hole to us is some round deep thing we are looking straight "down" at. From one point of view.<br /><br />A black hole is that very same hole that looks identical no matter how you look at it. you see that hole the same way whether you are standing over it, laying on the ground looking at it sideways, or from the tunnel you dug to get below it.<br /><br />That directly implies a 4th physical dimension. Or a 5th dimension including time.<br /><br /> <div class="Discussion_UserSignature"> <em>"2012.. Year of the Dragon!! Get on the Dragon Wagon!".</em> </div>

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#### newtonian

##### Guest
El_Naso - Black holes are not transparent.<br /><br />I do not know if gravitational lensing could make a black hole invisible through optical illusion.

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##### Guest
Ah you're saying being close enough to the BH the entire view would be black, so you couldn't see the size of the BH or even it's existence.<br />So how far away is it necessary to be able to work out it's size by discounting it's gravitational effects.

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#### el_naso

##### Guest
So, Newtonian (i´m not very keen on astrophisics) bhs CAN be seen? I mean, with a background of many galaxies that irradiate lots of light, would you see the bh itself? cause as far as I know you could only see how it´s gravity bends light arround it, and therefore, the resulting gravitational lensing<br /><br />also, I am well aware of the "shape" of a bh in a 3d space. pretty hard to imagine but it helps seing it in 2d, but it´s not the point anyways.<br /><br />Let me reformulate the question, the light that passes too close to a bh falls inside. The light that passes a little farther is trapped in orbit arround the bh. A little farther and light can escape, but it´s bended considerably. We can see this light being bended. So far so good.<br /><br />Now, again let´s get closer to the bh. A beam of light pointed at us so that it is lined up with the BH would be sucked in and we wouldn´t see it, but as the light propagates in all directions (this time not a beam) all we would be able to see is the light that passes far enough from the bh, magnified, if i´m not wrong by it.<br /><br />So the question would be whether or not there is a point (apart from the event horizon) where we wouldn´t see this light.<br /><br />(Btw, my english is not yet perfec or anithyng so maybe I don´t know how to express myself sometimes <img src="/images/icons/tongue.gif" />)

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#### thespeculator

##### Guest
I am by no means an expert but I think the way you put it sounds plausible. I think that once you get close to it, you will begin to to see it like an eclipse because all the bent light will be meshing with all the relatively straight light around it. The closer you get to it, the more the eclipse-like scene will fill your veiw.

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#### newtonian

##### Guest
El_Naso - I do not know the degree of gravitational lensing, and bending of light, observed due to black holes.<br /><br />Hopefully another poster will enlighten us.<br /><br />However, note that black holes often radiate very powerful jets which are not only visible but also dangerous!

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#### Saiph

##### Guest
This site may help clear things up for y'all:<br /><br />http://antwrp.gsfc.nasa.gov/htmltest/rjn_bht.html <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

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#### newtonian

##### Guest
Saiph - Thank you for the link!<br /><br />Have we seen the first Einstein ring for any black holes?<br /><br />Two excerpts from<br /><br />http://antwrp.gsfc.nasa.gov/htmltest/gifcity/nslens_effects.html<br /><br />“The more times the photon must circle the neutron star or black hole before reaching the observer, the more precise the direction of its emission must have been emitted to attain this trajectory, the less likely any photon will take this trajectory, the "dimmer" the Einstein ring. For this reason the higher order Einstein rings will usually carry little light when compared to the lower order Einstein rings. In fact, the relative brightness of each Einstein ring decreases exponentially. [10] <br />The first Einstein ring can be seen not only in a high gravity environment, but also in a low gravity environment quite a distance from much larger objects, such as normal stars, galaxies, and clusters of galaxies. In fact, complete first Einstein rings have actually been seen for radio galaxies. [22] A good review of extragalactic measurements of gravitational lens effects is given by Blandford and Narayan. [23]” <br />-----------<br /><br />“If the radius of the lens is small enough so that the lens exhibits a photon sphere, an infinite number of images can be seen of the source, no matter its location. One image of the source comes to the observer relatively undeflected. This image is between the zeroth and first Einstein rings and will be referred to as the primary image. A second image comes around the opposite limb of the lens from the first image, and therefore will appear to the observer 180 degrees around the face of the lens from the first image. This secondary image will always be located between the first and second Einstein rings. A third image comes around the same limb as the first image and is seen even closer to the apparent position of the photon sphere. This ima

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#### CalliArcale

##### Guest
I think Einstein Rings have only been observed around supermassive galaxies so far, although it is presumed that there are some really monster black holes at the heart of those effects.<br /><br />I don't think you can see a black hole itself. No matter how close you get, you'll still be seeing its effects, not the hole itself. They are not believed to have actual diameters. A really big black hole warps space so much that it's meaningless to try to measure its volume, although you can measure the volume of its event horizon. (The event horizon is not the surface of the actual object. It's the altitude at which escape velocity would be exactly light speed. So a circular orbit at this altitude is doomed; an object in such an orbit is doomed.)<br /><br />When astronomers talk about the size of a black hole, they are not talking about its diameter or volume, unless they are talking about the volume described by the event horizon. They're talking about the object's mass. And that can only be determined gravitationally. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

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#### nexium

##### Guest
Up to perhaps a trillion times the radius of the event horizon, the jet and accreation disk typically are quite bright in Xray and/or gamma, but a black hole can have negligible accreation disk and jet.<br />Assuming negligible accreation disk, the gravity of the black hole will produce detectable lensing effects up to perhaps a billion times the radius of the event horizon.<br />We likely can not survive even one second being close enough to a black hole to notice a black spot in the sky due to the black hole blocking our view of the stars behind it. At uncomfortable tide difference distances(and farther) the lensing likely fills the black spot completely. Am I over simplifing? Neil

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#### newtonian

##### Guest
Calli: Agreed, and thank you.

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#### alokmohan

##### Guest
Black hole cant be visible.

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