Pragmatic definition: Time is what clocks measure
Physically, 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 said time is relative. By this 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 around the world in an airplane, then this clock runs slower than if it had stayed in place. It’s not slowing down because something slowed it down on the plane, but because time really does pass more slowly in fast-moving bodies—relative to a static vantage point.
In this sense, time is really what clocks measure. The unit of time, the second, is also only defined in this way. In this case, however, the “clock” is a cesium atom: A cesium atom “oscillates” around 9 trillion times per second and therefore physics simply says: We define the approximately 9.192 trillion times this oscillation period as a “second”. .
How can one describe the deeper nature of time?
The question is: does one have to think of time as something that always “flows” continuously? Physics knows today: 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 past and future that “is there” and then something “happens” in it. Einstein showed that this is not the case.
Both space and time are first spanned by matter and energy in the universe. At the presumed beginning of the universe, at the so-called Big Bang, these physical laws fail and it is completely unclear whether there was any time before the Big Bang or whether time as such only began to exist with the Big Bang.
The question “What was before?” would then no longer make any sense, because if there is no time, there is no “before” and “after”. Another phenomenon of time is that it may be “quantized” on a small scale. That is, metaphorically speaking, time does not run out evenly, but drips in tiny little portions of time that are much shorter than we can perceive.
Difference to space: You cannot move backwards in time
Another feature is the so-called arrow of time. In our mind, time only knows one direction. We cannot stop time, we cannot turn it back, it flows from the past to the future, separating cause and effect. The past is what happened and cannot be changed, the future is open. This is astonishing, because in classical physics – also with Einstein – time has no direction.
The laws of motion apply forwards and backwards. If I were filming a billiard ball rolling on the pool table, I could run the film backwards without anyone noticing. It’s different when I film a pane of glass that breaks – here I can see immediately when the film is running backwards: the shards are reassembled into a pane.
This is due to the so-called entropy: Events unfold in such a way that the world as a whole tends to become messier. And if it does become tidier somewhere – for example when we do the dishes – then that’s only possible because we create disorder elsewhere – in this case in the sewage system. It is this increase in disorder that gives physical direction to time. Conversely, this means: The energy of the universe must have been extremely “ordered” at the beginning – otherwise there would not be the possibility of becoming more and more disordered.
But if you think this through, it is conceivable that in many billions of years the universe will at some point assume a highly disordered state. That would be a kind of radiance. In this extremely disordered state, the arrow of time would dissolve, so to speak, the universe would be so monotonous that, at least on a small scale, the past no longer differs from the future; there are no causes and no effect.