From a merely physical point of view, time is the fourth dimension. To specify the position of an object we need to give the three spatial coordinates and the momentum. If we meet a friend, we not only tell him where, but also when. Now, if for centuries it had been believed that time was something absolute, identical for all observers in the universe, in 1905 everything changed. That year, Albert Einstein discovered new and surprising properties of time. In the first place, that it is not something totally detached from space but rather forms part of what he called the space-time continuum . And secondly, he showed that time depends on the speed of the observer, it is relative .
Does time have size? No, this is not a stupid question. In 1980 the physicist P. Caldirola proposed that there could be a minimum indivisible unit of time , an idea that was later refined by the Nobel laureate in Physics DT Lee and by a brilliant and highly imaginative physicist named David Finkelstein. Just as atoms cannot have a proton and a half, time must be made up of discrete quantities. They have even given it a name: the chronon , 10-24 seconds. In this way we can imagine time as an infinite row of marbles of equal size, the chronons. Can there be something between two of those marbles? Some have speculated about the existence of a Shadow-Universe , whose existence is intertwined with our own. Let’s say that between chronon and chronon of our universe there is a chronon of that shadow-universe to which we will never be able to travel and not even know if it exists. It is a region that is closed to us.
And not only that. In 1966 Frank Russell Stannard suggested in the prestigious journal Nature – not very seriously – that two worlds in which time passes in different directions could occupy the same volume of space-time, interspersed but without influencing each other , as if on a board two games of checkers will be played simultaneously, one on the black squares and the other on the white ones. Stannard named this other world ‘Faustian’ in homage to Goethe’s character who was allowed by Mephistopheles to travel back in time.
Be that as it may, one thing is clear: the passage of time indicates a direction in the evolution of things and is fixed in the idea of irreversibility: things happen in one direction of time, but not in the other. Thus, we remember our past but not our future: it is the psychological arrow of time , the one that we define from our memories. In the same way we always see how a glass breaks, but not how it is put back together from its pieces. This is the thermodynamic arrow of time and is linked to the concept of entropy, the disorder of a system. According to the Second Law of Thermodynamics, the entropy of the Universe increases with time. That means that we can order the universe in a temporal sequence simply by measuring its global entropy.
The expansion of the Universe also marks a temporal direction, the cosmological arrow, which allows us to distinguish a past moment of the universe from a future one. Another arrow of time is the radiative one, which is deduced from the fact that the light moves away from the source that originates it: the light leaves the bulbs and does not go towards them, despite the fact that there is no law that prohibits the light back to the bulbs.
Finally, at the subatomic level we can also distinguish a direction in the flow of time in the disintegration of the K0 particle or neutral kaon . The K0 has a permanent identity crisis, as it keeps oscillating between itself and its antiparticle. To make it more complicated, this oscillation can take two forms, as two mixtures of K0 and anti-K0, called mixture 1, K1, and mixture 2, or K2. K1 usually decays into an even number of other particles and K2 into an odd number. Now, in 1964 Val Fitch and James Cronin discovered that K2 also did it in an even number 1% of the time. What made this discovery of paramount importance was that it implied that the kaon has, to put it simply, an intrinsic sense of past-future, differentiating between the two time directions.
But do you know what is truly mysterious? That all these arrows of time, which have no relation to each other, point in the same direction. And nobody knows why.