In physical terms, time is what clocks measure. This may sound unsatisfactory, but this definition corresponds not only to our everyday understanding, but also to Einstein’s theory of relativity. Einstein, after all, said that time is relative, and by that he meant that clocks moving through space at different speeds tick at different speeds.
This has also been proven: If you let a high-precision clock fly once around the earth in an airplane, then this clock goes slower than if it had stayed in place. And it does not go slower because it would have been braked in the airplane by something, but because the time really passes slower in fast moving bodies, relative to a static observation point.
So time in this sense is really what clocks measure. The unit of time, the second, is also defined only in this way. Except that the “clock” in this case is a cesium atom: A cesium atom “oscillates” about 9 trillion times a second, and therefore physics simply says: We define the approximately 9.192 trillion times this period of oscillation as a “second”.
This is the pragmatic definition – but how can we describe the deeper essence of time?
The question is: Should we think of time as something that is always “flowing”? Contemporary physics knows this: on the one hand, time – like space – is not something that simply exists independently of everything else. We often think of time as a grid between the past and the future that is “there” and then something “happens” in it. Einstein has shown that this is not the case.
Both space and time are created by matter and energy in the universe. At the supposed beginning of the universe, at the so-called big bang, these physical laws fail and it is not at all clear whether there was a time before the big bang or whether the time as such began to exist only with the big bang.
Also the question “What was before?” would not make sense any more, because if there is no time, there is also no “before” and “after”. Another phenomenon of time is that it is possibly “quantized” on a small scale. That is, metaphorically speaking, time does not flow evenly, but breaks down into small periods of time, which are of course much shorter than we can perceive.
One fundamental difference between time and space is that you can’t move backwards in time, right?
That is another characteristic: the so-called arrow of time. In our perception, time has only one direction. We can’t stop time, we can’t turn it back, it flows from the past into the future and separates cause and effect. The past is what has happened and cannot be changed, the future is up for grabs. This is amazing because in classical physics – and even for Einstein – time has no direction.
The laws of motion apply forward as well as backward. If I film a billiard ball as it rolls on the billiard table, I could also run the film backwards without it being noticeable. It’s different when I’m filming a sheet of glass breaking – here I can see immediately when the film is running backwards, when the pieces of glass reassemble to form the sheet. This is due to so-called entropy: events develop in such a way that the world as a whole tends to become more disorderly.
And if it does become more orderly somewhere – for example when we do the dishes – then this is only possible because we create disorder elsewhere – in this case in the wastewater. It is this increase in disorder that physically gives time a direction. Conversely, this means, the energy of the universe must have been extremely “ordered” at the beginning – otherwise the possibility of things getting more and more disordered wouldn’t exist at all.