On April 10th 2019, the very first image of a black hole will be released to the
public and we're gonna preview it and understand this remarkable scientific
achievement starting now. Welcome back to Launch Pad, I'm Christian Ready your
friendly neighborhood astronomer . Chances are by now you've already seen the very
first image of a black hole. Well this video is designed to put that image into
proper context by understanding what was involved in making it. Now, I'm making
this video just a couple of days before the announcement, so I'm living in that
era of humanity where human beings have never really seen an image of a black
hole before - lucky you but I'll catch up don't worry. Anyway let's take a quick
look at how black holes have been depicted traditionally. When we think
about black hole images, they're usually surrounded by a disk of gas and dust
that is circling at very high speed. Some of its falling into the black hole, some
of it being accelerated out in bipolar jets. Now these illustrations are very
useful for understanding the geometry of the black hole and its accretion disk.
However, if we were to look upon this black hole and its accretion disk with
our own eyes, we would see something necessarily different. And that's because
black holes. like any massive object in the universe distort space-time around
it and light follows a path around this distorted or curved space-time. And so it
gives us a warped picture of the environment surrounding the black hole.
In the 1970s Jean-Pierre Luminet worked out what a black hole should look
like. He envisioned a black hole surrounded by an accretion disk and
using the technology available at the time - a photographic plate kind of
looking down on the black hole and it's disk from a slight angle. The light from
above the disk forms essentially the primary image, but light from underneath
the disk would also be deflected by the black hole and would be redirected
towards our photographic plate producing a secondary image. Lument then worked out
the paths that light would take around a rotating black hole. He then plotted each
photon on a piece of paper producing a virtual photograph; literally a graph of
every photon. The first thing you notice is that there's a really bright region
on the left-hand side. The black hole is rotating and so is the material in its
accretion disk. As that material is rotating toward us however it is moving
at nearly the speed of light. That means that the light gets stacked up and the
intensity of that light coming toward us is increased. This is an effect called
'Doppler beaming' or 'relativistic boosting', and it's the reason why the left side of
the disk appears to be so bright. Meanwhile on the right side of the disk
the material is rotating away from us at nearly light speed, stretching out the
wavelengths of light making them dimmer and dimmer so we get a very bright right
side and a ghostly, very difficult to see left side. Now exactly how this disk
appears to us is going to depend on our viewing angle. In the upper right-hand
corner of this simulation, we can see a single hot spot in the accretion disk as
we change our viewing angle. We can see how the black hole's gravity distorts
and brightens the hotspot as it rotates into our field of view. But all of this
is purely theoretical. In order to test relativity we need to
make an image of a black hole and if it looks anything like what we expect, then
that is pretty much the most stringent test of relativity we can come up with.
Fortunately our galaxy is home to a 4 million solar mass black hole. It's in
the direction of the constellation Sagittarius and so we call it
Sagittarius A-star or Sag A*. We can't see Sag A* at visible
wavelengths because of all the intervening gas and dust. But if we
switch to infrared we can make out individual stars as they orbit the black
hole. These observations allow us to make very precise measurements of Sag A star's
mass as well as its overall size. The object turns out to be about 4 million
solar masses and has a diameter of about 24 to 44 million kilometers. However, at
26,000 light-years away Sag A star would be just five point three millionth of an
arc second on the sky. Give you an idea just how tiny and angle
that is let's consider a soccer ball. It's about 22 centimeters in diameter
and our eyes can make out a soccer ball at a distance of about 3/4 of a
kilometer. Now obviously that depends on your eyesight but on average the human
eye can resolve down to about 60 seconds of arc. The Hubble Space Telescope can do
much better - it can see down to five one hundredths of a second of arc. Even so
that is about 10,000 times too low a resolution to image Sag A-star. In other
words, We need a bigger telescope. We can link multiple telescopes together using
a technique called interferometry. This however requires ultra precise timing so
that the light from each telescope converges at the same time to form a
single image and that's sets a practical limit on just how far apart
our telescopes can be. However at longer wavelengths the timing requirement is a
less stringent, so longer baselines are possible. However, the
longer the wavelength, the larger the telescope has to be in order to achieve
the desired resolution. Even microwaves which are about a millimeter across are
so gigantic of a wave that in order to see down to 53 micro arc seconds you
need a telescope the size of planet Earth. Interferometry at the planetary
scale requires very long baselines we even call this technique VLBI for Very
Long Baseline Interferometry. Interferometry uses a pair of telescopes
separated by some baseline due to the angle of the target and the curvature of
the Earth. Light is going to hit one receiver first; that means that the time
difference in the arrival of the signals has to be accounted for. In the case of
the Event Horizon Telescope, observations had to be conducted simultaneously and
synchronized using atomic clocks. The Event Horizon Telescope consists of
several telescopes around the globe - the Sub Millimeter Array and the James Clerk
Maxwell telescope at Mauna Kea, the Sub-Millimeter Telescope at Mount Graham,
Arizona, the Large Millimeter Telescope in Mexico, the IRAM 30-meter telescope
in Pico Vieta, Spain the Atacama Millimetre and Submillimetre
Array or ALMA in Chile and the South Pole Telescope at the South Pole station
in Antarctica. The 77-meter Greenland Telescope joined
EHT for a second round of observations. EHT can be thought of as a mirror the
size of Earth, but it's more accurate to describe it as fragments of a mirror
scattered around the Earth. That means each fragment can only form a piece of
the image. Luckily Earth's rotation allows those antennae to sweep out a
wider swath of the sky so it does help to fill in the rest of the image, but how
do you reconstruct a true image when all you have are image fragments? Well let's
imagine what we would see if we could only take images of a given subject. Well,
if we look at this image, we can start to make out some familiar shapes, patterns
colors, textures, and before long your brain has probably filled in that what
we're really looking at is an image of an apple. Now that's easy for us because
we know what an apple looks like. What exactly will Sag A star look like? Well
fortunately we can run some theoretical computations using supercomputers to
produce a virtual image. These are General Relativistic Magneto
Hydrodynamics simulations. In this image, we're looking at the black hole at about
a 45 degree angle. From that we can then take all the possible images that can be
derived from the image fragments and rank them from least likely to most
likely. The most likely image is the best representation of what it is we are
actually seeing. These observations are extremely difficult to make and in
December 2018 Tony Darnell and myself talked with Dr.
Geoffrey Bauer from EHT. I'll have a link to that conversation right here and in the
description below, so definitely check that out because it's a fascinating
conversation. And please join me in thanking my Patreon supporters for their
generous support in making this video possible. And if you'd like to talk more
about what we learned through EHT, make sure you subscribe and ring that
notification bell so that you're notified when that video comes out. Until
then, stay curious my friends.