Other side of the black hole

Since the 1960s, astronomers have uncovered evidence that most galaxies contain so-called supermassive black holes at their cores. With masses between a million and a billion times that of the Sun, these leviathans first revealed their presence in so-called quasars – distant galaxies with cores so luminous the only plausible source of power is the intense gravity of black holes devouring matter.

A black hole is defined by its ‘event horizon’, the imaginary membrane that marks the point of no return for in-falling light and matter. If the Sun were to become a black hole – which is impossible since it is not massive enough – the event horizon would only be six kilometres across.

The biggest supermassive black holes are tens of billions of times more massive than the Sun and are in the hearts of all galaxies, though nobody knows why. These supermassive black holes have event horizons bigger than the Solar System.

If you crossed the event horizon and entered a black hole, space-time would be so distorted that time would become space, and space would become time. This is why you cannot avoid the monstrous infinite-density ‘singularity’ that lurks like a black widow spider at the heart of a black hole: it no longer exists across space but across time. It exists in the future, and you can no more avoid it than you can avoid tomorrow.

As for what exists on the other side of the singularity, some have speculated that this is a gateway to far-flung parts of our Universe or even other universes. The truth is that a singularity in a theory marks the breakdown of that theory, and the point at which it has nothing more sensible to say.

In order to truly understand what happens at the heart of a black hole and whether ‘What’s on the other side?’ is a meaningful question, we will need a better theory of gravity than Einstein’s – a ‘quantum’ theory of gravity. Finding one of these is one of the supreme challenges of science!

The evidence for supermassive black holes (SMBHs) at the centres of most (if not all) galaxies, is both direct and indirect. Observations of ‘quasars’ – the extremely bright cores of distant galaxies – shows that they’re releasing up to a trillion times the energy of a typical star, all within a volume no bigger than the Solar System.

The only mechanism that can explain this huge energy release is the conversion of gravitational energy into light by a SMBH. Any other kind of object, such as massive stars, simply cannot produce the observed amount of energy. Plus, it appears that stars of more than a few hundred solar masses are unlikely to form because radiation pressure prevents collapse of the ‘proto-stellar’ cloud. Even if extremely massive stars did form, their lifetimes would be too short (a few million years) to account for the number of quasars seen in the Universe.

Science Focus