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

## 20 Mar Probability, Time and the Heat of Black Holes | Part B’

At first glance, the idea that our ignorance implies something about the behaviour of the world seems irrational: the cold teaspoon heats up in hot tea and the balloon flies about when it is released regardless of what I know or don’t know. What does what we know or don’t know have to do with the laws that govern the world? The question is legitimate; the answer to it is subtle.

Teaspoon and balloon behave as they must, following the laws of physics in complete independence from what we know or don’t know about them. The predictability or unpredictability of their behaviour does not pertain to their precise condition; it pertains to the limited set of their properties with which we interact. This set of properties depends on our specific way of interacting with the teaspoon or the balloon. Probability does not refer to the evolution of matter in itself. It relates to the evolution ofthose specific quantities we interact with. Once again, the profoundly relational nature of the concepts we use to organize the world emerges.

The cold teaspoon heats up in hot tea because tea and spoon interact with us through a limited number of variables amongst the innumerable variables which characterize their microstate. The value of these variables is not sufficient to predict future behaviour exactly (witness the balloon), but is sufficient to predict with optimum probability that the spoon will heat up.

The cold teaspoon heats up in hot tea because tea and spoon interact with us through a limited number of variables amongst the innumerable variables which characterize their microstate. The value of these variables is not sufficient to predict future behaviour exactly (witness the balloon), but is sufficient to predict with optimum probability that the spoon will heat up.

Such issues lead us to the heart of the problem of time: what exactly is the flow of time?

The problem was already present in classical physics, and was highlighted in the nineteenth and twentieth centuries by philosophers – but it becomes a great deal more acute in modern physics. Physics describes the world by means of formulae that tell how things vary as a function of ‘time’. But we can write formulae that tell us how things vary in relation to their ‘position’, or how the taste of a risotto varies as a function of the ‘variable quantity of butter’. Time seems to ‘flow’, whereas the quantity of butter or location in space do not ‘flow’. Where does the difference come from?
Another way of posing the problem is to ask oneself: what is the ‘present’? We say that only the things of the present exist: the past no longer exists and the future doesn’t exist yet. But in physics there is nothing that corresponds to the notion of the ‘now’. Compare ‘now’ with ‘here’. ‘Here’ designates the place where a speaker is: for two different people ‘here’ points to two different places. Consequently ‘here’ is a word the meaning of which depends on where it is spoken. The technical term for this kind of utterance is ‘indexical’. ‘Now’ also points to the instant in which the word is uttered, and is also classed as ‘indexical’. But no one would dream of saying that things ‘here’ exist, whereas things which are not ‘here’ do not exist. So then why do we say that things that are ‘now’ exist and that everything else doesn’t? Is the present something which is objective in the world, that ‘flows’ and that makes things ‘exist’ one after the other, or is it only subjective, like ‘here’?
This may seem like an abstruse mental problem. But modern physics has made it into a burning issue, since special relativity has shown that the notion of the ‘present’ is also subjective. Physicists and philosophers have come to the conclusion that the idea of a present that is common to the whole universe is an illusion, and that the universal ‘flow’ of time is a generalization that doesn’t work. When his great Italian friend Michele Besso died, Einstein wrote a moving letter to Michele’s sister: ‘Michele has left this strange world a little before me. This means nothing. People like us, who believe in physics, know that the distinction made between past, present and future is nothing more than a persistent, stubborn illusion.’

SEVEN BRIEF LESSONS ON PHYSICS
Carlo Rovelli

Translated by Simon Carnell and Erica Segre

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• A person who never made a mistake never tried anything new .

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