# What the picture of the M87 black hole represents

The picture of the black hole at the centre of M87 is quite moving for anyone interested in physics. It’s 55 million light years away, about the diameter of our solar system, weighs about 6.5 billion times as much as our Sun and it’s rotating clockwise.

We’re gazing at something truly bizarre, something that currently defies a complete explanation.

To see why we first need to take a brief trot through some physics.

Two theories describe the universe’s four fundamental forces. Einstein’s General Theory of Relativity describes gravity and Quantum Mechanics describes the remaining three forces (electromagnetism, the strong nuclear force and the weak nuclear force).

Both theories are astonishingly successful, each having been tested to extreme accuracy and each having made hugely successful predictions many, many times. Both have proved themselves accurate to many digits to the right of the decimal point.

The General Theory of Relativity is a theory of the very large, describing how gravity shapes space. Quantum theory is a theory of the very small, describing how fundamental particles interact with each other.

The General Theory of Relativity is a *classical* theory, which sees its domain of operation as a continuous, infinitely divisible realm. It’s also deterministic and if a suitably omnipotent demon could know the current state of things *absolutely*, it could precisely predict how that state would evolve. It could predict the future, essentially.

A Quantum Theory on the other hand sees its domain of operation as being ‘chunked’ into bits rather than continuous, where we have a photon as an indivisible ‘unit’ of light for example. It’s also inherently random and even if our demon knew the current state of things absolutely, he could not predict with absolute precision how that state would evolve. Quantum theories also permit dualities that classical theories abhor, such as wave/particle complementarity where fundamental particles can be seen as either a wave or a particle depending on how you look at them. They can be both waves and particles yet also neither.

Classical theories are like ramps and quantum theories are like staircases, to use one woefully inadequate (but somehow still vaguely representative) analogy.

That’s just a general overview of course. It’s actually much more complicated than that and a lot of tedious mathematics surrounds it, so please forgive my simplistic description. But it’s probably enough to move us on.

The General Theory of Relativity is what we use to describe black holes. It’s all about gravity and things that are big. Unfortunately the theory gives scientists a headache at the centre of a black hole. Gravity becomes so strong that nothing can escape from a black hole (which makes it similar to the EU in many respects). Matter is compressed to infinite density, space becomes infinitely curved and the theory throws its hands up in despair and serves up something called a singularity to annoy physicists.

The thing is, in such a dense, compacted place like the centre of a black hole, small-scale quantum effects merge with large-scale gravitational effects but the two separate theories describing each of those things won’t talk to each other.

What we need is a quantum theory of gravity to get all the forces of nature speaking the same language. Attempts have been made to create such a theory — string theory, M-theory and loop quantum gravity to name just a few of the leading lights in this area — but none of these theories has been satisfactory to date.

There is something enticing about seeing a picture of a black hole. By its very nature we can’t see anything of the black hole itself of course, but we can see the swirling jets of energy it generates outside its event horizon. That’s all we’ll ever be able to see.

Studying those jets of energy will hopefully tell us more about the black hole itself but I think the picture is also an icon. It represents where we are in physics. It’s a milestone of sorts. We understand so much. We have two very successful theories to describe the universe, yet they’re still not sufficient to describe that image. Something has to give and new physics needs to emerge.