Don't Be Deceived: There Is No Such Thing As A Photo Of A "Black Hole"

The much circulated image above - purported to be of a black hole in the center of the galaxy M87- is nothing of the sort.

As usual, the mainstream media - in an effort to grab clicks or reads- has misrepresented the recently circulated image claimed to be of a "black hole" in the Virgo galaxy M87.   Of course, this is impossible. One cannot get a photo or other image of a black hole itself.  That is invisible - or better "un-seeable" - at any wavelength, including radio..   What one has in the image, is not the black hole per se, but its event horizon which is somewhat different.

TO fix ideas, the image in question was captured by a global network of 8 radio telescopes, appropriately called the "Event Horizon Telescope" or  EHT.  The effective aperture essentially formed one giant radio telescope "dish" stretching from Spain in the north to Antarctica in the south.  The mass associated with the M87 black hole  is the equivalent of 6.5 billion Suns.  That implies its event horizon would have quite a significant scale.

What is this event horizon? It is basically an "infinite redshift" surface from which no signal, no light or radiation of any kind, can escape. The key cut off occurs at what's called the Schwarzschild radius or:

R(s) = 2GM/c²

where G is the Newtonian gravitational constant, c is the speed of light in vacuo, and M is the gravitating mass.   In this case - and for a "Schwarzschild solution" all observers will agree that the surface r = R(s) is an "event horizon".  It defines the boundary or edge at which all outgoing signals are cut off so they cannot have any external manifestation - whether as radio waves or optical ones.  

A good algebraic way to show how signals are cut off  at the r = R(s) Schwarzschild event horizon has been given by Ohanian and Ruffini ('Gravitation & Spacetime', p. 445).  Thus, one considers a radio signal propagating in the radial or r direction. Then the velocity of this signal, say with respect to r, t (radius, time) coordinates can be expressed:

dr/ dt =  + {1  -   r/ R(s)}

Now note what happens when the signal approaches the critical boundary, i.e. r = R(s).  Then we see:

dr/ dt =   + {1  -   r/ R(s)}   =   + {1  -   R(s) / R(s)} =  1 - 1   =  0

The graph shown in the same text (op. cit, p. 446, Fig. 8.3) can also help to illuminate the situation vis-a-vis interior and exterior to the black hole.   We see that in the exterior of the black hole (r > R(s))  the axis of the light cones is parallel to the time (t) axis.  But in the interior of the black hole (r < R(s)) the axis of the light cones is parallel to the r- axis.  The reversal is a direct result of r being a "time-like coordinate" and t space -like.   The existence of an event horizon at r = R(s) is "obvious from the inspection of the light cones in Fig. 8.3."  as noted by the authors. (Ibid.)  Hence, any kind of signal must travel in a spacetime direction that lies within a light cone. And "since the light cones in the black hole region are oriented toward r = 0 any signal in this region is unavoidably pulled toward decreasing values of r."

Thus, all such signals can never leave the black hole, hence we cannot obtain them from outside . This is no matter how many radio telescopes we assemble in a global network, as in the case of the EHT.

Bottom line, the EHT captured exactly what its title designated: the event horizon of a monster black hole in galaxy M87.  But not the black hole itself! Think of what we are seeing as an impenetrable 'cosmic curtain' impeding our accessing the actual black hole.