The principle of causality is rational principle, of the thought. It is stated in two fundamental points:
- any phenomenon has a cause
- under the same conditions, the same cause is followed same effect.
As opposed to what suggests the common direction, the law of causality is not confirmed by the experiment, but it is a principle not refuted, for example in chemistry, like physique at the macroscopic level.
The complex systems do not allow the so simple checking of it.
- when they include/understand many loops of feedback: one can extremely well request the same service from somebody without having each time the same response of his share!
- when they call upon a very great number of factors, even without feedback: if the pulling of the Lotto gave each time the same result, the thing could (see )
But everywhere where these systems are analysable in terms of macroscopic and chemical physical components, its validity does not lend to a calling into question. Indeed, if the components are determined and that no other influence plays, then the compound cannot be unspecified without .
Problems at the borders
At the levels of size concerned with quantum mechanics, the validity of the principle is more difficult to establish, because experiments seemingly identical give sometimes different results (although according to probabilities', they, completely strict). One a long time allotted these differences to the existence of hidden variables that one would identify one day. Inequalities of Bell showed which were the conditions to fill so that these hidden variables exist.
An experiment undertaken to the laboratory of Orsay by Alain Aspect following a problem arising from Einstein, Podolski and Rosen under the name of paradox EPR showed that these inequalities were not satisfied. This experiment was reproduced since in other laboratories and with various scales of distance and is not disputed today. Its consequences are as follows:
- The inequalities of Bell not being satisfied, there are not hidden physical variables
- That implies that to explain the experiment of Aspect (conforms by elsewhere to the models existing of quantum mechanics), it is necessary to give up is with locality, that is to say with causality.
It may be that the consideration of spaces to more than four dimensions (the 5 theories of the existing supercordes utilize some either 10, or 11) mitigates the non-existence of hidden variables: what differentiates the particles would not be a physical characteristic, but considerations of position on nonobservable dimensions. This difference would explain their difference in behavior, as it would account for the characteristics of wave mechanics.
Note: assumption of Everett
(a little the framework of causality overflows; in making an article with share?)
Hugh Everett cannot, like Einstein, to admit that nature behaves in a random way, or more exactly than the lifting of the risk by one is shown also little exempts symmetry (why nature it answered of this way rather than of the other, since nothing did distinguish them? Let us not see not in the remainder of the nature which any action counterbalances some share by an opposed action, suggesting one to some extent neutrality of the universe ?) In , it presents a theory according to which it is not necessary to wonder whether the modifications observed relate to nature itself, but to stick to the facts: all that we can affirm, it is that the observer subsequently note to the experiment a result. That does not give according to him the right to exclude, for this reason of neutrality, the existence of a symmetrical observer from which the result of experiment is different! The quantum model becomes about it for the blow much simpler to include/understand, failing to be simpler to calculate.
Its owner of thesis John Wheeler does not take this idea literally, but presents nevertheless the work of Everett in time thatassumption of thought clever. It will remain thereafter associated not only in the name of Everett, but that of Wheeler itself, because this one will be let little by little convince by the idea of his/her former collaborator (Everett having left the university once its finished thesis).
This assumption bears in English the name of many-worlds or many-minds (see David Deutsch).
In the standard model resulting from and for the moment allowed (2004), the origin of space and time is it Big-bang. It is not necessary thus to wonder whether this one has one cause within the framework of this model. It "is", quite simply.
On the other hand, that does not prevent from wondering whether it has one reason, which is different from a cause since not utilizing time (see ); bus after very if a big-bang arrives, that indicates well that the natural laws authorize of it the existence in one way or another, and in this case just as easily of other possible universes (see ).
Certain physicists like Gabriele Veneziano (author of the first theory of the cords) work with models in which the big bangs would constitute physical objects like the others, result for example of a collision of branes.
The two approaches remain for the moment of the field of the speculative one.
" under the same conditions "a problem poses of significance, because if one twice does what seems to be the same experiment, something necessarily differs from it
- maybe in term of space (the experiment is made elsewhere)
- maybe in time (it is made at another time)