What is time? What is space? by Carlo Rovelli

Author:Carlo Rovelli [Rovelli, Carlo]
Language: eng
Format: epub, mobi
Publisher: Di Renzo Editore
Published: 0101-01-01T00:00:00+00:00

Time does not exist

Until now I have spoken only about space. The moment has come to speak about time.

Special relativity

Some ten years before he discovered the theory of general relativity, Einstein had already understood that space and time are not separate entities, but rather two aspects of the same entity. This discovery is called special relativity. More precisely the discovery of Einstein is as follows.

We are used to thinking that two events (the arrival of Columbus in America and the death of John Lennon, for example) are always ordered in time, that is, one happens before and the other happens later. We are used to thinking that time is something universal, and so it makes sense to ask what is happening precisely now in some part of the universe. Einstein understood that this is not the way nature works.

The most visual way to illustrate the relativity of time is the effect called “twin paradox”. Two twins travel with great velocity far away from each other, and when they meet again they have different ages. One is older, one is younger. This is called a paradox, but it is not a paradox; it is simply the consequence of the way the world is made. The only paradoxical aspect of the situation is that we are not used to observing these phenomena. Thus they seem strange to us. But it is true: precise experiments have been performed (not with twins but with identical and very precise clocks put on board fast airplanes) and it has been verified repeatedly and with great precision that the world is made just as Einstein had understood: the two identical clocks exhibit different times when they meet again.

The point is that when two events happen in positions sufficiently far away from one another, it makes no sense to say which of the two happens first. It makes no sense to ask what is happening precisely now, on the Andromeda galaxy, for example. Time does not flow in the same way everywhere. We have our time and the Andromeda galaxy has its own time, and in general the two times are only loosely related.

The only thing one can do is to exchange signals. But signals will need six millions of years to go back and forth between Andromeda and here. Imagine an extraterrestrial who sends us a signal from Andromeda, a certain day, the first day of the year 1,000,000 in Andromeda Calendar. We receive this signal today and reply immediately. The extraterrestrial will receive our reply a certain day, say the last day of the year 7,000,000 in Andromeda calendar. We can say that the day at which the extraterrestrial has sent his signal (his 1,000,000) comes before today and the moment at which he receives the reply (his 7,000,000) comes after today. But between these two dates, there is are six million years. Which date on Andromeda is today? There is no answer. There does not exist on Andromeda a particular day that corresponds to “precisely today”.

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