Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Grant on 28/05/2010 16:30:02
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Grant asked the Naked Scientists:
i am aware that the event horison of a black hole is not visible but if it were possible to see it,what would it look like?
The reason I ask is because when its traditionally described as a diagram it looks like a flat rubber sheet with a space time grid on it with the hole and singularity depicted as if a cannon ball were dropped into the centre creating the idea that the horizon is almost flat and two dimensional.
What I want to know is whether the horizon can be approached from any direction. and if that is the case why are most galaxies Catherine wheel shaped as if they are flat? How do super massive black holes create flat accretion discs when the force they exert is inward from all directions?
Thanks, Grant
What do you think?
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First of all, it's just not possible to give a meaningful answer to your very first question. There are two reasons why an Event horizon cannot be seen. The first is that there's nothing there to be seen; the EH isn't anything physical but just a point (or more correctly a surface) in space where certain numbers reach certain values. The second is that light cannot reach us from the EH and if it can't get from the EH back to us then we can't see it.
There are nearly always a couple of problems/issues with typical 'rubber-sheet' depictions of gravity.
The first is that it's not easy to explain or convey that the rubber sheet is just a two dimensional representation of something that's really three-dimensional, so the answer to another part of your question is yes, a Black Hole can be approached from any direction.
The second issue with most rubber-sheet representations is the type of grid that's typically overlaid upon them; usually it's a linear x-y type grid, which doesn't really show the proper shape of space-time around a BH, and as it's stretched downwards the grid lines stretch apart, implying that space is stretched when it's actually compressed - the lines should get closer together instead of further apart. A logarithmic polar grid would be better.
The image below, from wikipedia, is better in that it uses a polar grid, but the circular lines should get closer and closer together as they move inwards and downwards towards the mass at the bottom. Also note that the profile of the curve in this image is 'S' shaped, with the gradient of the slope slackening off as it approaches the bottom, which is incorrect; it should continue to get steeper and steeper until it reaches the surface of the body causing the deformation. As well as those inaccuracies, the spacing of the circular lines is haphazard and inconsistent. However, note that this is just an image of a non-BH gravity well, so that it has a finite bottom; with a BH the bottom is never actually reached and instead the sides just keep getting steeper and steeper as they get closer and closer to the EH. If you could actually reach the EH itself, the sides would have become vertical.
Another thing to remember about BH's is that apart from their existence and the effect they have upon their surroundings, all else we think about them is speculation. It is, admittedly well grounded speculation, but it is still speculation nonetheless.
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2Fd%2Fd9%2FGravityPotential.jpg&hash=87ae084476719a5872ce95c3223cd5d2)
Re the shape of galaxies; Afaik, the majority of the galaxies we've observed are ellipticals and not flat spirals. Elliptical galaxies range from being spherical to being fairly squashed and almost flat but they don't have the internal structures of typical spiral galaxies, such as the arms. As both elliptical and spiral galaxies are believed to contain super massive BH's it seem unlikely that the the shape of spiral galaxies is due to the BH at their center. Elliptical galaxies also appear to be much older than spiral galaxies, so if the central BH was responsible for the shape of spiral galaxies then older elliptical galaxies shouldn't exist as their central BH would have had even longer to turn the elliptical into a spiral.
A BH will create a flat accretion disk if it is rotating, which is actually believed to be the norm, or if all the matter it is accreting is coming from a more or less flat plane, as in a spiral galaxy where there's relatively little material above and below the BH. Actually, we don't really know for sure whether the super massive BHs believed to be at the center of most galaxies have formed accretion disks as we can't see them; it's possible that if matter is being accreted from all directions then an accretion halo may be formed instead, although don't forget that the rotation of the BH will still tend to concentrate accumulation of matter in the plane of rotation.
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The event horizon doesn't look like anything. You could cross over it without noticing (if the black hole were big enough).