A Personal Perspective on Black Holes
The history of black holes is a short one in the timeline of astronomy. The term “black hole” first appeared in print in 1964 by the science journalist Ann Ewing. Three years later a student used the term at a lecture by John Wheeler on “completely collapsed gravitational objects” Wheeler liked the sound of it and adopted it for its “advertising value” and it quickly caught on. Although Wheeler was a great advocate of black holes the name may have done them a great disservice. Black holes need better PR. Hawking said “Black holes are not as black as they are painted”.
Einstein “Black holes are where God divided by zero”
The press both scientific and public is full of the terrors of black holes.
Surprisingly the idea of black holes dates back to 1784. The English clergyman John Michell worked out that a supermassive body could create enough gravity to stop light escaping from it. OK his assumptions about the nature of light was wrong, but his big idea was correct and he should get more credit for it. Einstein and Hawking got lots of stuff wrong too. It’s the nature of science that if you dare to step beyond the standard model you’re going to need a bullet proof vest or a stubborn mind to withstand the slings and arrows.
Of course the theoretical work towards black holes started with Einstein in 1915 with General Relativity. By the 1960’s the theoretical case for neutron stars and black holes was well established, but the first evidence only came with the discovery of pulsars in 1967. Since 1971, with the detection of the accretion disc of Cygnus X-1 black hole science has grown enormously.
This essay, this personal perspective owes much to the fact that it was 1971 when I gave up physics at university; it was - boring! I missed my chance to study black holes then. I’m on catch up now in my retirement.
What has amazed me is the speed at which our knowledge of black holes has grown in only a few decades.
From 1974 it took 16 years for Stephen Hawking to concede the bet with Kip Thorne that Cygnus X-1 was indeed a black hole but 30 years later black holes are everywhere. We have black holes of every size, from a few solar masses to mind blowing multi-billion stellar mass monsters. Suddenly we have millions or is it billions of stellar mass black holes in our galaxy alone. Every galaxy - almost - has a supermassive black hole at its centre.
Given that we can only detect them indirectly through their gravitational affect this is probably only the tip of the iceberg. And now we wait with baited breath to see the first direct image of Sagittarius A* from the Event Horizon Telescope.
So what of the future?
Where are black holes taking us?
The future of our universe is reasonably well mapped out. At its simplest we’re all going to end up in a black hole. As the universe expands under the misunderstood force of Dark Energy each cluster of galaxies will disappear into the cosmos. Within each cluster, all the little stars who don't get to be black holes, all the black holes, big and small, will eventually, under the irresistible pull of gravity, end up in a black hole bigger than anything we have detected. The long run is that everything ends up in an impossibly big black hole.
It may take a while, maybe 10 quintillion years, but black holes are the future. All matter will, under the irresistible force of gravity be drawn into black holes.
But what about the past?
Possibly the great physicist today not to get the Nobel Prize, Sir Roger Penrose, has just published yet another paper this month.(1) Not bad at 87.
He proposes, not for the first time, that there have been other ‘universes’ before the Big Bang. These universes were, like ours, full of black holes and he has been trying to find the evidence to support the theory. Whether they have detected his “Hawking Points” in the CMB remains in doubt. His assumption of massive black holes degenerating from aeons raises a very valid question. Black holes may have been present in the the past as well as the future. Our universe may not be the first, nor the last.
It came as a great shock to science that it could only explain 95% of the universe. Actually, we can only explain 5% of the observable universe! There is a lot of black and dark stuff which has eluded scientific explanation for more than 20 years. If only Mr Hawking was around this would be stuff wagers are made of.
Whether it's Penrose’s theory or not, we need to know what is out there beyond our visible bit of the universe. It is more probable than not, that there is much more out there beyond our vision. My bet is that there is a cosmos full of black holes. We can see the galaxies and clusters which have already left our local universe. They are already out there 40 or 50 billion light years away. Others are out there beyond them. Penrose’s assumption that the cosmos is and was full of ‘black matter’ deserves attention. If not by detecting Hawking Points in the CMB, maybe the new science of gravitational wave detection will allow us to ‘see’ into the cosmos.
If black holes do predate our universe? If our universe really does sit within a much bigger cosmos? We should be looking for them. Who would not want to be the first person to detect an event from beyond our observable universe?!
Another ‘Mission Impossible’
What about the structure of black holes themselves? Most of the theory and all of the observation is about the event horizon. What about the core? What happens inside? The usual answer is “we can never see what’s inside a black hole etc.” We need to ask the question though. The core of black holes is where it all happens. The event horizon is but the veil of gravity.
The closest we can get to seeing that core is to look at neutron stars. The little brothers of stellar mass black holes. We see a change of state under the massive force of gravity as a neutron star is born. What further change of state will occur when a stellar mass black hole forms? Now scale that up to supermassive black holes. What about those cosmic scale black holes which will form with masses of galaxy clusters? When all the standard forces have been quashed and only gravity remains, what what kind of matter remains. The secrets of Dark Matter may lie within the cores of such black holes.
Does anyone else share my frustration with the standard answer? We can never see inside a black hole; we can never see what’s beyond the observable universe; what happened before the Big Bang? Even my hero Hawking wrote, "Events before the Big Bang are simply not defined, because there's no way one could measure what happened at them. Since events before the Big Bang have no observational consequences, one may as well cut them out of the theory, and say that time began at the Big Bang."
Well it’s not good enough! Give a child a wrapped box for Xmas, and with its natural inquisitiveness it will weigh it, shake it, probe it. It can’t see what is inside, but it will find a way around that problem.
We need to know what is out there; was out there. The inner secrets of black holes may lead us to understand Dark Matter; how the Big Bang really started; why gravity is so different from the other 3 forces.
From my perspective, black holes are the key to understanding the universe.
Our eyes will always be attracted to the light; the bright shining stars and galaxies, and the rainbow of radiation they produce. The answers we seek are hidden in the black and dark places. Our theories should not ignore these. The challenge is to find the tools to explore them.
The future of the cosmos is all about black holes and gravity. Gravity - the 'weak’ force - which controls the structure of the universe.
Remember, the universe has always been much bigger than we thought.
References
An, Meissner and Penrose. 6th Aug 2018 arXiv:1808.01740