Never. Impossible. Words we are learning to use with extreme caution in the language of science. As time goes by we become increasingly aware that much of what is impossible today is probably due to the lack of adequate technology and not because it violates some absolute law. What is impossible today might eventually become possible or even normal in the future.
What was impossible yesterday and what is impossible today
We are learning to distinguish between what is currently impossible and what is absolutely impossible. This should enable us to avoid drastic statements which might become a source of embarrassment. Such as those made in the past by some famous scientists and philosophers. For example in 1835 Auguste Comte, went overboard by making quite a peremptory statement. He said that we would never be able, in any way, to study the chemical composition of stars. As a positivist philosopher he was convinced that we could understand the nature of the matter which composed the stars if we could analyze it in a laboratory.
And to think that the basis for disproving Comte had already been established twenty years earlier by Fraunhofer's studies, which adopted Newton's method to decompose the sunlight using a prism and had noticed the absorption lines in the Sun's spectrum and ended up classifying more than 500 of them!
Fifteen years after Comte's incorrect assertion it became clear that the absorption lines were due to the presence of certain chemical elements in the Sun's atmosphere. Hydrogen, of course, but also helium, oxygen, sodium, iron and others. Today stellar spectroscopy allows us to know not only the chemical composition of stars but also their age and temperature. It enables us to study the rotation and, under certain circumstances, to determine whether they are characterized by the presence of a planetary system.
Another scientist (a physicist) who made peremptory assertions about the impossibility of certain events was William Thomson, better known as Lord Kelvin. In 1895, Thomson argued that it was impossible to make flying machines that were heavier than air. Eight years later, in 1903, the Wright Brothers successfully completed the first controlled flight with a pilot aboard an airplane with an engine, the Wright Flyer. Then, once airplanes became common some considered it impossible to "fly" out of the atmosphere and to undertake space travel.
In 1768 the chemist Lavoisier, who was involved in the analysis of samples of a rain of stones recorded in a French village, had excluded with absolute certainty that the stones could have come from the sky, citing as an incontrovertible reason that there were no stones in the sky. The effect of this declared impossibility was that for many years meteor showers were simply ignored thus delaying their study and the understanding of their extraterrestrial origin. This belief however changed at the turn of the nineteenth century - not without difficulty - thanks to the tenacity and the work of Ernst Chladni and Jean-Baptiste Biot. There are also contemporary examples.
Even today, in fact, what is possible can be labeled as being impossible! The chemists establishment did it for years to Dan Shechtman's results and his 1982 discovery of materials which showed ordered and aperiodic configurations with exotic symmetries: the quasi-crystals. However they made up for this by assigning him the Nobel Prize for chemistry in 2011 for his discovery which had been disputed for an exceptionally long time.
Thus common sense would have it that we should define as being impossible only what violates the so-called fundamental laws of Nature and to admit that everything else is possible, even if we know that we will have to wait for further technological developments to see the changes come about.
The possible quantum
As quantum mechanics has developed we have had the opportunity to review some of these laws, particularly by understanding that it is possible to observe phenomena at a microscopic level which is unknown at a macroscopic level. It is as if there are two realities: one daily "normal", in which many things remain impossible, and another one which is somewhat strange and extreme where the physical phenomena observed is bizarre and anti-intuitive. For example, a person who finds himself in a room with two doors can get out by choosing one rather than the other. Certainly not by passing through both at the same time! A particle, however, can go though both, as shown by the interference experiments conducted by sending electrons to a detector through a screen that has two openings.
What is emerging as particularly interesting is that the phenomenon is not confined only to the elementary particles. The wave-like behavior of matter (which explains the phenomena of interference) has in fact also been verified, by way of the double opening experiment, with complex molecules such as fullerenes (C60 and C70), the fluorinated fullerenes, composed of 60 atoms of carbon and 48 of fluorine, and also with objects at a mesoscopic scale (electric currents in nanocircuits).
So what is the limit of the applicability of quantum laws that seem to constantly redefine the concept of possible and impossible? Maybe it's what makes decoherence inevitable resulting from the system's interaction with the surrounding world and destroys the superposition of states that are typical of quantum systems. It is unclear, however, whether there is a limit beyond which the decoherence becomes inevitable. In other words, we do not know whether there is a fundamental law that categorically prevents a macroscopic system to maintain and show quantum behavior.
In recent years, several experiments of quantum teleportation were conducted with success (even if limited to carrying out transporting from one side of a laboratory to the other) of a single photon (but also of a nuclear spin or the quantum state of a trapped ion). Over time atoms too have been teleported and the distance has increased considerably going from a few meters to the distance between the opposite banks of the Danube in the region of Vienna and then between two Canary Islands. Currently the record is held by an experiment conducted in Italy with the Matera Laser Ranging Observatory which has had success between the Earth and a satellite in low orbit. The quantum teleportation based on the entanglement between the two entities used (photons or ions, for example) means that even though they are far from each other they are reciprocally and instantaneously affected by any change of the quantum state. As a result if you act on one you obtain an immediate effect on the other.
What is impossible?
At this point however we should be prudent in avoiding the risk of being contradicted by future generations. We should no longer consider teleportation as absolutely impossible; a phenomenon that seemed limited to science fiction. All he had to say was "Beam me up Scotty", and Captain Kirk would dematerialize from a certain location and reappear in the control room of the Enterprise.
Do we know any fundamental laws which cannot, now or ever, be violated?
My generation grew up believing in the energy conservation principal, in the Heisenberg uncertainty principal and the Pauli exclusion principal. And learning that the speed of light represents an absolute limit that a material particle can never reach (an infinite amount of energy would be needed), nor exceed. The recent neutrinos case has confirmed the "impossible". But if it were to be verified that some elementary particles can exceed the speed of light new possibilities, which are currently not taken into consideration, would open up.
So is there is something that is definitely impossible? For those of you that are curious I recommend reading an interesting book written by Michio Kaku and published a few years ago in Italy by Code editions: Physics of the Impossible.
Taken from: The stars - n°105, April 2012