Friday, April 12, 2019

The Black Hole Madness

Today I'd like to discuss the recent black hole image making the rounds on Media, because the supposed "news" sites and channels are, once again, butchering the truth.  Yes, this is an astounding scientific achievement, but journalists are just not getting it right.

First, the picture is NOT of a black hole.  Sorry to disappoint everyone, rather it is the shadow of a black hole and the light around the shadow is not a disk of light, but the light being warped by the warping of 3+1 dimensional spacetime.  This light isn't visible light that you can see with your eyeballs, but invisible light in the radio end of the spectrum.  I'll try and provide a better explanation.

So the black hole exerts such a powerful gravitational field that is literally curves space and time.  I understand that it is a difficult thing to visualize and is typically shown as the black hole causing a bend in a rummer sheet.  The analogy does, kind of, get the point across, however, it's still a two dimensional representation.  Readers need to extend the idea to our three spatial dimensions.  Length, width and height are all warped toward the black hole's event horizon. 

The event horizon is the area near the black hole where nothing that crosses is can ever make it out.  An observer outside the event horizon would probably see no light for a distance outside the horizon as all light would be heading toward the horizon and not outward toward the observer's eyes and instruments.  Beyond that space some light would be flung away from the area outside the black hole.

Ok, got that?  I hope so...

Now about the light.  It is light in the sense that we are seeing detected photons, but the wavelength is outside our visible range and into the radio part of the spectrum.  Why not look at the visible light?  Well there is material around the black hole being accelerated and ripped apart as it migrates toward the event horizon.  As this material accelerates it heats up generating all sorts of light from many different wavelengths, visible light included.   This material is transparent in the radio part of the spectrum, so radio waves can pass through some of it.  If we looked in the visible part of the spectrum we would simply see a bright, amorphous blotch which wouldn't really tell us much.  Looking in the radio part of the spectrum shows us a great deal.

Around the event horizon, material is heading in all directions.  Some circles around, some passes in front and some spirals inward.  We cannot see the radio light directly in front, because it is all heading toward the event horizon or spinning/spiraling around it.  This means that the matter and generated photons are not heading toward us, hence the blackness at the center of the image.  The doughnut of radio light we do see is coming from behind the region of the black hole. 

Very massive objects all warp spacetime and light from behind that passes close enough can get kind of sling-shotted around these objects which is a phenomenon known as gravitational lensing.  What we see in the image is pretty much just that.  Light generated behind the black hole is lensed by a region of severely warped spacetime near the event horizon.  Since the light comes from behind what we are seeing is a shadow of the area around the black hole.  The area of blackness is about 3 times the diameter of the black hole.  Ok, so big deal.  Is that it?  Actually there's more.

If you examine the image you may notice that it is dimmer near the top and brighter at the bottom.  This difference in brightness is due to doppler effect where the stuff at the top is moving away from us and the stuff at the bottom is moving toward us.  All this information fits very well with theory and contributes to confirming some of our understanding of how the universe works.

Finally, this image really does not convey the amount of effort involved in receiving this bit of confirmation.  Sure, it's not enough to move from theory to fact in out understanding, but the sheer amount of effort needs to be acknowledged.  It took a lot of very smart and dedicated people, something like 200-ish, and cooperation from 20 countries not to mention a lot of radio telescopes to get this data.  I am personally amazed at this effort and look forward to what the data can tell us once fully analyzed and the experiment repeated.  Exciting times!

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