CC

# Discussion

Physics is defined as the science concerned with the nature and properties of matter and energy. A definition like this is limited because ultimately there is no way of defining matter and energy. What it comes down to is that we do not study mass and energy but just make a measurement of them and their interactions. Few of the general public know what electricity is but they all could measure the voltage of a battery with a suitable meter. When a physicist claims a knowledge of electricity, in reality, they have grasped an understanding of a model of electricity that helps fit the reading of the meter into a larger body of knowledge. It is the same with matter and energy, we make measurements and fit them to models - Newtons laws or general relativity as appropriate to the situation. Measurement is the key part. We can have physics without measurement but we are limited to writing observations down. We can fit these observations to models, but history would suggest that these are nearly always wrong. Only with measurement do we get the evidence to evaluate the adherence of a model to reality.

The question to answer is what is physics when there are no measurements or even observations? This is, of course, the region inside the event horizon of a black hole. We have no measurements and furthermore will never have any measurements. Speculation on what goes on there is interesting but it is not physics. Metaphysics perhaps. Maybe just mathematics. Mathematics has no problem in investigating domains without any physical reality - often to end up being useful. But this is not the whole story. I have used mathematical reasoning too - in particular, Birkhoff's theorem - extended into the interior, to reach my own conclusions. Nonetheless, there is still a sense in which this is valid. We may have no way of knowing what goes on inside the event horizon, but there is nothing wrong with using the model of a black hole with the outer layers removed. The exterior field of this would be measurable. All we can truly say is that a construction of a black hole as suggested would produce the same exterior field without also opening the door to unrealisable and paradoxical singularities. Clearly, there is an element of preference here. Do you prefer a universe without a singularity and all of the paradoxes that this entails, or not. I know that there are many who would opt for a Universe in which time travel, wormholes, etc. are at least a possibility. But a reality that is unobservable in principle, will never be of any real benefit.

Now consider building up a black hole from the centre, outwards. Take a small amount of matter and squeeze it down to its event horizon to create a mini black hole. Now add more matter, layer by layer, creating black holes as we go. We will create a black hole just as described here. Of course, there is no sense in wondering what goes on below the event horizon at any stage once a black hole is created. If there is no hope of knowing what goes on below the event horizon then how is it that we have deduced properties arising from this construction? Specifically, the fixing of the black hole's angular velocity at birth. The truth is that we never fully create a black hole as an attempt to squeeze matter down to its event horizon will always take an infinite time as measured by a distant observer. We could take a more mathematically sound basis here and talk of taking the limit as matter approaches the event horizon, but as an engineer, I will leave this for a mathematician to fret about.