Probability, Time and the Heat of Black Holes | Part C’

Probability, Time and the Heat of Black Holes | Part C’

Illusion or not, what explains the fact that for us time ‘runs’, ‘flows’, ‘passes’? The passage of time is obvious to us all: our thoughts and our speech exist in time; the very structure of our language requires time – a thing ‘is’ or ‘was’ or ‘will be’. It is possible to imagine a world without colours, without matter, even without space, but it’s difficult to imagine one without time. The German philosopher Martin Heidegger emphasized our ‘dwelling in time’. Is it possible that the flow of time which Heidegger treats as primal is absent from descriptions of the world?


As vivid as it may appear to us, our experience of the passage of time does not need to reflect a fundamental aspect of reality. But if it is not fundamental, where does it come from, our vivid experience of the passage of time?


I think that the answer lies in the intimate connection between time and heat. There is a detectable difference between the past and the future only when there is flow of heat. Heat is linked to probability; and probability in turn is linked to the fact that our interactions with the rest of the world do not register the fine details of reality.


The flow of time emerges thus from physics, but not in the context of an exact description of things as they are. It emerges, rather, in the context of statistics and of thermodynamics.
This may hold the key to the enigma of time. The ‘present’ does not exist in an objective sense any more than ‘here’ exists objectively, but the microscopic interactions within the world prompt the emergence of temporal phenomena within a system (for instance, ourselves) which only interacts through the medium of a myriad of variables.
Our memory and our consciousness are built on these statistical phenomena. For a hypothetically supersensible being there would be no ‘flowing’ of time: the universe would be a single block of past, present and future. But due to the limitations of our consciousness we only perceive a blurred vision of the world, and live in time. Borrowing words from my Italian editor, ‘what’s nonapparent is much vaster than what’s apparent’. From this limited, blurred focus we get our perception of the passage of time. Is that clear? No, it isn’t. There is so much still to be understood.


Time sits at the centre of the tangle of problems raised by the intersection of gravity, quantum mechanics and thermodynamics. A tangle of problems where we are still in the dark. If there is something which we are perhaps beginning to understand about quantum gravity, which combines two of the three pieces of the puzzle, we do not
yet have a theory capable of drawing together all three pieces of our fundamental knowledge of the world.
Using quantum mechanics Hawking successfully demonstrated that black holes are always ‘hot’. They emit heat like a stove. It’s the first concrete indication on the nature of ‘hot space’. No one has ever observed this heat because it is faint in the actual black holes which have been observed so far – but Hawking’s calculation is convincing, it has been repeated in different ways, and the reality of the heat of black holes is generally accepted.
The heat of black holes is a quantum effect upon an object, the black hole, which is gravitational in nature. It is the individual quanta of space, the elementary grains of space, the vibrating ‘molecules’ that heat the surface of black holes and generate black hole heat. This phenomenon involves all three sides of the problem: quantum mechanics, general relativity and thermal science.
The heat of black holes is like the Rosetta Stone of physics, written in a combination of three languages – Quantum, Gravitational and Thermodynamic – still awaiting decipherment in order to reveal the true nature of time.






Carlo Rovelli

Translated by Simon Carnell and Erica Segre





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