According to classical physics, also known as Newtonian Physics, from the name of the famous English physicist Isaac Newton, everything that happens in the Universe can be predicted knowing the initial conditions of the system.
For example, we can know exactly the speed that a ball will have when it touches the ground as long as we know some factors a priori, for example the height from which it falls, if it is thrown (and therefore if it has a initial speed) or if it is simply dropped from a standstill, if the surface of the ball is smooth or rough and various other parameters.
In an ideal experiment, based on the degree of precision you want to achieve, some parameters can be neglected which act on the ball to make the problem simpler, albeit less precise: in the case of the ball, if it was made of a metal that was heavy enough and we threw it from the height of our shoulders, imparting a certain force in pulling it on the ground, we can say without incurring a significant error that a slight breeze will not significantly affect the speed with which it will touch the ground.
On the contrary, if we drop a feather from a cliff, the speed and direction of the wind would have consequences on the point where the feather will land on the ground.
In particular in the last case described, so many fluid-dynamic factors that cannot be controlled outside a laboratory come into play, that in fact the result will be affected by a error due to the difficulty, and in some cases the impossibility, of keep in consideration everything that acts and modifies the event: the parameters involved are so numerous that we need to talk about statistical forecast.
In this case the statistics it is closely linked to our ignorance, that is, the impossibility of knowing exactly the initial conditions of each part that makes up the system and how they act moment by moment on its evolution. It often happens that this ignorance is not attributable to physics, but to the lack of mathematical tools capable of solving certain types of very complicated calculations.
All in the hands of a demon
Pierre Simon Laplace, French physicist who lived between the eighteenth and nineteenth centuries, to save Newtonian determinism, introduced the concept of Laplace's demon.
This demon is the representation of the Universe itself, the only one capable of knowing everything from the past and, consequently, knowing exactly the present state of everything and its future evolution.
This demon, which is obviously only an allegorical representation, is able to know every little uncertainty and the way in which it fits into universal evolution following the note (albeit in a romantic rather than mathematical way) chaos theory.
To get practical, we could say that Laplace's demon knows by name and surname the butterfly whose flapping wings will create a hurricane on the other side of the world and knows exactly at what speed and in which direction it has flapped its wings, so as to to be able to predict the cities that will be hit by the hurricane, what speed the winds will have and how long it will last.
Obviously the famous butterfly effect is more a philosophical idea than a real physical reality, but it manages to present in a fairly simple form as a small event, albeit apparently insignificant, can create a significant deviation in the forecast.
In the case of weather forecasts, you can imagine how many butterflies in the world flap their wings at each moment, that is how many seemingly insignificant small events occur in that area between the highest layer of the atmosphere and the darkest depths of the oceans, and you will have the answer why you often go out in boots and an umbrella on sunny days or, on the contrary, go home soaked during what must have been a bright day of tanning and nature.
An unsolvable ignorance: quantum statistics
However in physics there is another statistics conceptually very different from the one just described and concerns the infinitely small, therefore it belongs to the world of quantum physics, which has nothing to do with ignorance but which is intrinsic to the way in which the world is built.
In quantum mechanics a wave function that describes the physical state of a quantum system, that is, it mathematically returns the possible results of a measurement on the system, when it is physically observed collapses, returning one of the possible results in an apparently random way.
Using the wave function, you can go back to chance that one of the possible measures can be obtained.
This collapse Statistical is diametrically opposed to Laplacian determinism as not even the famous demon would be able to predict what value the observable will return after the collapse of the wave function.
Some basic concepts are difficult to die, among them is determinism. The randomness and impossibility of responding to "what will happen"with exact numbers, they led to the birth of many interpretations that try to bring the intrinsic statistics into a"ignorance statistic". Einstein himself scolded supporters of the then nascent quantum physics, using the famous phrase"God doesn't play dice".
To save determinism also in the quantum field, one of the proposed theories is that of hidden variables.
There would be variables that, like the fluid dynamic environment in the case of the feather, are responsible for the evolution of a quantum system and for the measurement obtained by the collapse of the wave function. These variables, however, unlike the fluid dynamic environment of the classic example, are not known and cannot be known until the moment of the measurement itself.
This unsolvable ignorance is responsible for the randomness that permeates quantum mechanics. In this vision, the Laplace's demon knows at all times, not only the wave function, but also the hidden variables, having all the data needed to make predictions also on the quantum collapse.
The theory of many worlds
Another theory, formulated by the American physicist Hugh Everett and defined as many worlds theory, it does not add further variables for the justification of probabilistic theory, but immerses us in a universe worthy of the best science fiction writers.
According to Everett's theory, when a wave function is measured, a branch of "parallel universes" is created, a branch for every possible measure obtainable.
This means that there are an infinite number of parallel universes, as many as are possible for each wave function to collapse.
How does our probabilistic reality fit into this context? The answer to this question is linked to our existence, to the question, certainly not original, but of central importance: "Where am I?".
"I find myself"in every Universe that foresees my existence and, with the passing of the time line, each of the selves parallel it separates continuing its existence in one of the branches created.
Consciousness can only live one branch at a time, therefore, with each branch, the me and another will be created me different from me who will continue to live separately in the new branch.
When you finish reading this article, there will be a wave collapse in the Universe that will split you up, sending you into the various branches created. In the short time, the others you splits will perhaps continue to think about what they have just read, to wonder what problems theoretical physicists have. Probably, among all of you split, there will be someone who will not believe the words written in these pages, to someone else instead this article will turn on a light of curiosity, then go to Google to continue the search on the topic. The latter, perhaps, will become so passionate that he will decide to study physics.
In the future just after the branch, there are only many you parallels which are in front of the PC, all perfectly identical: each Universe split from this collapse will distinguish itself from the other only for a result assigned to a tiny particle.
However, this small difference can create a profound difference in the long term. Think of the Big Bang, that cluster of singularities that created the Universe. Think of how many wave decays occurred in the moments immediately following the explosion. Try to imagine how deeply different the parallel universes "children" of the other branches can be.
What will happen to me, the determinism inherent in this theory, is closely linked to the branch in which I as a consciousness find myself. This answer can only be given with certainty following a measurement, the measurement on the collapse that caused the splitting of the Universe.
Each of the branches separates in an incredibly short time, a fraction with 21 zeros of a second, in which, however quickly we can perform the measurement, there will still remain a time in which we will not be sure of the result and there will be an indeterminacy on the branch that we have taken.
The answer to this uncertainty can be revealed before the measurement itself only on a probabilistic level: the probability of obtaining a given result for a measurement corresponds to the probability of being in the branch of the Universe that foresees that result.
In other words, the result of each wave function is deterministic since it will return all the expected results, the uncertainty is which of the possible universes our eyes they will continue to watch.
These theories are as particular and abstruse as they are far from a possible resolution. After all, we are like birds trapped in a cage, desperately trying to know what is beyond the room they see, studying what ends up inside the cage itself.
Physics, especially the theoretical one, continually bypasses new limits which previously seemed impassable. Those limits, if looked at from a distance, seemed metaphysical but, as we got closer, they became more and more outlined, more and more knowable to be totally known.
Even now the research looks at what from a distance appears only metaphysics, but we must have confidence in the future … indeed, in all future parallels in which splits of us will end.
