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The Perplexing Physics of Imaging a Black Hole

abril 10, 2019
The Perplexing Physics of Imaging a Black Hole
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This is a big day for astronomy (and all humans, really). The first image of a black hole has been freed. It was created using the Event Horizon Telescope– A collaboration of radio telescopes around the world. This image shows the material around a very massive black hole in the center of a galaxy distant 55 million light years away.

Yes, there is a ton of cool physics here, involving the crazy gravitational things that occur in extreme cases like a black hole. But that's not what I want to watch. Instead, I would like to review some of the more fundamental physics issues related to this image.

Is it difficult to see a black hole because it's black?

No, yes, yes. It is true that the black holes are black. Normally, we see things like stars and objects, because they indicate that they go to our telescopes (or directly in our eyes) and that we detect them. Black holes are black. They do not emit visible light (because of the effects of senseless gravity), so you can not see them.

But that's not the big deal with a black hole. If we had one in our solar system, you could see it. You could see the warp of space due to its presence and you could see the stuff orbiting the black hole. If you saw the movie Interstellar, you might have an idea of ​​what a black hole would look like. This visualization of a black hole was created with the help of astrophysicist Kip Thorne.

The black hole is so difficult to see because it is tiny. OK, it's not so small in the sense of an ant. It's tiny in the sense that a human is tiny seen from afar. To visualize something, one must consider its size, but also its distance. The best term to use is the angular size. If you turn your head in a circle, you will get a 360 degree angular view (but not without turning your body). If you hold your thumb at arm's length, it corresponds to about 1/2 degree of angular size. It's about the same angular size as the moon – that's why you can cover the moon with your thumb.

So, what about the size of these things around the black hole? Yes, it's huge. But it is also about 55 million light years away. That means it is so far that light (traveling at 3 x 108 meters per second) would take 55 million years to get there. It's super far. But really, it's angular size. The black hole (at least the part you can see) would have angular size of about 40 microarcseconds.

What's a microarsecond? Well, a circle is divided into degrees (for ancient reasons). Each degree can be divided into 60 minutes of arc and each minute corresponds to 60 seconds of arc. Then, if you break that second into a million, you get a microphonesecond of arc. Remember how the moon has an angular size of 0.5 degrees (seen from the Earth)? This means that the angular size of the moon is 45 million times that of the black hole. The black hole is angular.

Wait, it's getting worse. Due to diffraction, we can not see small angular things. When the light goes through an opening (like a telescope or the pupil of your eye), the light diffracts. It bends in such a way as to disturb the rest of the light that goes through the opening. In the case of the eye (with visible light), it means that humans can solve objects with an angular size of about 1 minute arc.

This means that something is angular (I will continue to use this phrase) because a black hole is quite difficult to solve for an image.

How do you overcome the diffraction limit?

Well It is very difficult to see the little angular things. How do we see what is going on around a black hole? The angular resolution of a telescope actually depends on two things: the size of the aperture and the wavelength of the light. The use of smaller wavelengths (ultraviolet or X-rays, for example) gives better resolution. But in this case, the telescope uses a wavelength of light in the order of a millimeter. This is a fairly large wavelength compared to visible light, which is in the 500 nanometer range. So it's bad.

This means that the only way to exceed this diffraction limit is to build a larger telescope. That's exactly what the Event Horizon telescope does. This is basically a telescope the size of the Earth. It's crazy but true. By taking data from multiple radio telescopes in different parts of the world, you can combine them into a single GIANT telescope. It's tricky, but that's what it does. Even with that, there are some problems. With the help of a handful of telescopes, the EHT group uses analytical techniques to determine the most likely image from the data collected. But this will allow them to get the image of something very small, like something around a black hole.

Is this a real photo of a black hole?

If you look through a telescope and see Jupiter, you actually see Jupiter. Side note: If you have not already done so, you should definitely do it. It's great. The sunlight is reflected on the surface of Jupiter, then crosses the telescope and enters your eyes. Boom Jupiter C is real.

That's not what's going on here with this black hole. The image you see is not even in the visible range. This is a radio image using the wavelengths of light in the radio region. So what is the difference between radio waves and visible light? Really, it's just the wavelength that is different.

Light and radio waves are electromagnetic waves. They are a propagation of a changing electric field with a changing magnetic field (at the same time). These waves travel at the speed of light because they are light. However, since radio and visible light have different wavelengths, they interact differently with matter. If you turn on your radio inside your home, you may receive a signal from a nearby radio station. These radio waves cross your walls. The visible light, on the other side, does NOT go through the walls.

This applies to images. If you have visible light from an object, you can see it with your eye and you can save this image on a movie (yes, it's old school) or with a digital detector (a camera CCD). This image can then be displayed on a computer screen, allowing you to see what it looks like. This is what happens when you view a visible image of the moon.

For objects around the black hole, it is not an image in visible light. This is a radio picture. Each pixel in the image you see represents a particular wavelength of a radio wave. When you see the orange parts of the image, you get a false color with a wavelength of about 1 millimeter. The same thing happens if you want to "see" an infrared or ultraviolet image. We have to convert these wavelengths into something we see.

So, this black hole image is not a normal photograph. This is not something you can see if you look through a telescope, but it's still really great.

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