At the age of 25, Albert Einstein sent three articles to the magazine Annalen der Physik. “Each of the three deserved the Nobel, and more”, writes Carlo Rovelli in Reality is not what it appears to us (Raffaello Cortina Publisher, 2014), volume from which we derive the scientific notions contained in this edition of "Accadde oggi".
But let's get to the anniversary: on November 21, 1905, exactly 114 years ago, the most famous of the three articles was published, the one introducing the theory of special relativity o special relativity (not to be confused with the general relativity, which we have already talked about and which saw the light only a few years later).
Special relativity is one of the conceptual pillars on which our current knowledge of the cosmos is based, but unfortunately it is also very complex and requires a patient apprenticeship, so we say it clearly: we do not claim to provide exhaustive explanations.
We will therefore limit ourselves to speaking in general terms of the concept of "extended present”, introduced precisely by Einstein's theory. Rovelli explains it this way: "Between the past and the future of each event (...) there is an intermediate zone, an extended present of that event, an area that is neither past nor future”. The duration of the extended present depends on the space: the more the distance from the event increases, the more the duration of the extended present lengthens.
If we take ourselves as a reference, at a distance of a couple of meters the duration of the extended present is a few nanoseconds, but on the Moon it reaches a few seconds and on Mars a quarter of an hour. It means that on the red planet "there are events that have already happened at this precise moment - continues Rovelli - events that have yet to happen, but also a quarter of an hour of events during which facts that for us are neither past nor future".
All clear? If the answer is no, there is little to worry about: to have a clear perception of what the extended present is we should have the ability to distinguish very short intervals of time such as nanoseconds (to be clear: there are as many nanoseconds in a second as there are seconds in 30 years).
However, we can draw one conclusion: if time varies as a function of space, it means that there is no absolute simultaneity, that is, there is no "now" valid at the same time for the whole universe. Our "now" only applies here. And this is how time and space merge for the first time into a single concept: spacetime.
