Black holes are some of the most incredible objects in the Universe. There are places where so much mass has gathered in such a tiny volume that the individual matter particles cannot remain as they normally are, and instead collapse down to a singularity. Surrounding this singularity is a sphere-like region known as the event horizon, from inside which nothing can escape, even if it moves at the Universe's maximum speed: the speed of light.
While we know three separate
ways to form black holes, and have discovered evidence for thousands of them,
we've never imaged one directly. Despite all that we've discovered, we've never
seen a black hole's event horizon, or even confirmed that they truly had one.
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The locations of the radio dishes that are planned be part of the Event Horizon Telescope array. Image Credit: Event Horizon Telescope / University of Arizona |
Next year, that's all about
to change, as the first results from the Event Horizon Telescope will be
revealed, answering one of the longest-standing questions in astrophysics.
The idea of a black hole is
nothing new, as scientists have realized for centuries that as you gather more
mass into a given volume, you have to move at faster and faster speeds to
escape from the gravitational well that it creates. Since there's a maximum
speed that any signal can travel at — the speed of light — you'll reach a point
where anything from inside that region is trapped.
The matter inside will try
to support itself against gravitational collapse, but any force-carrying
particles it attempts to emit get bent towards the central singularity; there
is no way to exert an outward push. As a result, a singularity is inevitable,
surrounded by an event horizon. Anything that falls into the event horizon?
Also trapped; from inside
the event horizon, all paths lead towards the central singularity. Practically,
there are three mechanisms that we know of for creating real, astrophysical
black holes.
When a massive enough star
burns through its fuel and goes supernova, the central core can implode,
converting a substantial fragment of the pre-supernova star into a black hole. When
two neutron stars merge, if their combined post-merger mass is more than about
2.5-to-2.75 solar masses, it will result in the production of a black hole.
And if either a massive star
or a cloud of gas can undergo direct collapse, it, too, will produce a black
hole, where 100% of the initial mass goes into the final black hole.
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Artwork illustrating a simple black circle, perhaps with a ring around it, is an oversimplified picture of what an event horizon looks like. Image Credit: Victor de Schwanberg |
Over time, black holes can
continue to devour matter, growing in both mass and size commensurately. If you
double the mass of your black hole, its radius doubles as well. If you increase
it tenfold, the radius goes up by a factor of ten, also. This means that as you
go up in mass — as your black hole grows — its event horizon gets larger and
larger.
Since nothing can escape
from it, the event horizon should appear as a black "hole" in space,
blocking the light from all objects behind it, compounded by the gravitational
bending of light due to the predictions of General Relativity.
All told, we expect the event
horizon to appear, from our point of view, 250% as large as the mass
predictions would imply.
Taking all of this into
account, we can look at all the known black holes, including their masses and
how far away they are, and compute which one should appear the largest from
Earth. The winner? Sagittarius A*, the black hole at the center of our galaxy.
Its combined properties of
being "only" 27,000 light years distant while still reaching a
spectacularly large mass that's 4,000,000 times that of the Sun makes it #1.
Interestingly, the black hole that hits #2 is the central black hole of M87:
the largest galaxy in the Virgo cluster. Although it's over 6 billion solar
masses, it lies some 50-60 million light years away.
If you want to see an event
horizon, our own galactic center is the place to look. If you had a telescope
the size of Earth, and nothing in between us and the black hole to block the
light, you'd be able to see it, no problem.
Some wavelengths are
relatively transparent to the intervening galactic matter, so if you look at
long-wavelength light, like radio waves, you could potentially see the event
horizon itself. Now, we don't have a telescope the size of Earth, but we do
have an array of radio telescopes all across the globe, and the techniques of
combining this data to produce a single image.
The Event Horizon Telescope
brings the best of our current technology together, and should enable us to see
our very first black hole.
Instead of a single
telescope, 15-to-20 radio telescopes are arrayed across the globe, observing
the same target simultaneously. With up to 12,000 kilometers separating the
most distant telescopes, objects as small as 15 microarcseconds (μas) can be
resolved: the size of a fly on the Moon.
Given the mass and distance
of Sagittarius A*, we expect that to appear more than twice as large as that
figure: 37 μas. At radio frequencies, we should see lots of charged particles
accelerated by the black hole, but there should be a "void" where the
event horizon itself lies.
If we can combine the data
correctly, we should be able to construct a picture of a black hole for the
very first time. The telescopes comprising the Event Horizon Telescope took
their very first shot at observing Sagittarius A* simultaneously last year.
The data has been brought
together, and it's presently being prepared and analyzed. If everything
operates as designed, we'll have our first image in 2018. Will it appear as
General Relativity predicts? There are some incredible things to test:
Whether the black hole has
the right size as predicted by general relativity,
Whether the event horizon is
circular (as predicted), or oblate or prolate instead,
Whether the radio emissions
extend farther than we thought, or
Whether there are any other
deviations from the expected behavior.
Whatever we do (or don't)
wind up discovering, we're poised to make an incredible breakthrough simply by
constructing our first-ever image of a black hole. No longer will we need to
rely on simulations or artist's conceptions; we'll have our very first actual,
data-based picture to work with.
If
it's successful, it paves the way for even longer baseline studies; with an
array of radio telescopes in space, we could extend our reach from a single
black hole to many hundreds of them. If 2016 was the year of the gravitational
wave and 2017 was the year of the neutron star merger, then 2018 is set up to
be the year of the event horizon.
For any fan of astrophysics,
black holes, and General Relativity, we're living in the golden age. What was
once deemed "untestable" has suddenly become real.
This article was initially published on Forbes. You can read the article here.