# What is time, really ?

- Alexis Rigaud, CEO & CTO
- Metrology
- November 16, 2022

We know the famous reflection of St Augustine on the nature of Time in Book XI of The Confessions (397) :

What then is time? If no one asks me, I know what it is. If I wish to explain it to him who asks, I do not know.

## The mysterious flow of time

Indeed, thanks to Einstein and the theory of relativity, we know that the time is built in the very fabric of space-time. Here is a list of weird physical phenonemas that we don’t normally experience on a day-to-day basis :

### Relative velocity time dilation

Time dilation phenomenon can be seen when we compare two perfectly synchronized clock, and we make them travel with different of speed : for instance, one clock stay on the ground, whereas the other clock travels embedded in a plane. After some time, we compare again the two clocks. The clock traveling at higher velocity (compared to the other one) will see the time flowing slower ! The two clock are now out of sync. This phenomena has to be taken into account for GNSS system to work properly.

### Gravitational time dilation

We can make the same experience, with two clocks at the same altitude on earth. Now we elevate one clock compared to the other one. The higher one will experience less gravitational potential well, thus a slower time compared to the one lower ! This also needs to be taken into account into GNSS system.

### Shapiro time delay

When light travel, it will follow the spacetime geodesic, that is curved by mass. So light passing in the vicinity of massive object will experience a delay, called the Shapiro time delay.

### The block Universe

If we take the Einstein relativities theory serioulsly, there is now definition of “present time”. Our past is not vanishing to nothingness while we stay in the present voyaging to the future. The past, our past has a real existence, it still “exists”, althought is inaccessible.

### Closed timelike curve

Some solutions to GR (General Relativity) equations implies that closed time-like curve exists. At least, in the theoretical framework of GR, time travel to the past is possible in suche cases.

## (Temporary) conclusion

What is time really ? Einstein himself answered this question with the self-reflective definition :

Time is what is measured by clocks

Pulsars timing is a fantastic opportunity to build a clock that **measure time in a new way**, along with atomic clocks and its successors, optical-lattice clocks.