The complex search for black holes

The international group EHT (acronym for Event Horizon Telescope, Event Horizon Telescope) has released a "relevant discovery" in our galaxy.

In 2019, this international team obtained the first image of a black hole, specifically, the monster with a mass equivalent to 6,500 million suns that hides in the heart of the galaxy M87.

The observations had also included the black hole (perhaps

of the black holes) that occupies the center of the Milky Way.

It is known as Sagittarius A*, where the asterisk is the symbol used to name these mysterious celestial bodies.

Many galaxies are believed to harbor one or more at their core.

Compared to M87*, Sagitarius A* is small.

Just four million soles.

But it is also much closer, just 30,000 light years away instead of the 50 million light years that separate us from the first.

It's 2,000 times smaller and 2,000 times closer, so their apparent size from Earth is about the same.

Of course, it had never been photographed, but it had been possible to locate it exactly, by analyzing how the stars in the nucleus of our galaxy move.

Measuring their positions over several years has shown that a few dozen trace elliptical paths around an apparently empty point.

There's Sagittarius A*.

It seems that, given its proximity, it should be easier to study than M87*, but it is just the opposite.

Seen from Earth, Sagitarius A* is immersed in the dense swarm of stars in the galactic core, and its light must pass through the thick clouds of gas and dust in the spiral arms that make up the Milky Way.

The algorithms that "clean" the image have to discard much more "garbage" than, in the case of a more remote spiral, such as M87*.

Needless to say, despite their tremendous mass, these bodies are tiny.

Impossible to see with a conventional telescope.

It would take one the size of a planet.

In fact, what the Event Horizon Telescope observes are not photons of visible light, but radio waves.

Combining the signals from receivers thousands of kilometers apart, the result is similar to what a giant target of the same diameter would offer.

Of course, with the limitation that two fixed antennas only provide information in a single dimension.

It takes the combined work of several receivers for a long time to come up with a complete picture.

As the Earth rotates, some observatories get in position to see the black hole while others lose it.

Due to the distance that separates the antennas, the light reaches each one with very slight time lags that change minute by minute.

Taking advantage of these differences is the basis of a technique called "very long base interferometry".

For this, perfect synchronization is essential: the radio signals are recorded together with microsecond-by-microsecond clock pulses, essential for later being able to combine them properly.

They are not easy signs to detect, far from it.

About a billion times weaker than a television broadcast.

To turn on a simple 40-watt bulb with them in a flash of a couple of seconds, it would have to be collected during a time comparable to the existence of the Universe.

It is a slow task.

The image of M87* required four days of data collection at eight radio telescopes spread from Hawaii to Pico Veleta and from the Atacama Desert to the South Pole.

With such large interference bases, resolutions of 25 millionths of an arc second can be achieved.

More than enough ―for example― to count from Madrid the stars of the border in a one-cent coin located in Barcelona.

The amount of information collected in these experiments is enormous.

About five petabytes (a petabyte is a million gigabytes).

Unable to stream over the internet.

Hundreds of hard drives had to be physically shipped to the data collection center in order to combine all the observations and compose the image of the black hole.

A task that would take years.

So to speak, the signal received by an antenna at any given moment is like a note in a symphony.

As other antennas join with the signals picked up as the Earth rotates, the staff fills with new notes.

The complete piece, with all its nuances, will never be recomposed: it is too complex.

But, hopefully, the melody itself can be identified.

More information

The first image taken of Sagittarius A*, the black hole at the center of our galaxy

Several teams worked independently to recompose the photo of M87*, and presumably Sagitarius A* will do the same.

A very complicated job that involves really arcane techniques: "correlators", signal delay lines, phase adjustments, Gaussian distributions, integration times and endless mathematical operations mostly with complex numbers.

Sometimes all that appears is a simple white noise screen;

others, just a shadow.

And finally, as a result of the consensus of all the teams involved, the famous ring of light emerges around a dark disk, the image that fits the theoretical prediction of how what, by definition, was invisible should be seen.

Rafael Clemente

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