What is the physics of time

What is time

From a physical point of view, time is what clocks measure. That may sound unsatisfactory, but this definition corresponds not only to our everyday understanding, but also to Einstein's theory of relativity. Einstein said that time is relative, and by that he meant that clocks that move through space at different speeds tick at different speeds. This has also been proven: If you let a high-precision clock fly around the earth in an airplane, this clock runs more slowly than if it had remained in place. And it does not go slower because something would have slowed it down in the airplane, but because time really passes more slowly in fast-moving bodies, relative to a static observation point. In this sense, time is really what clocks measure. The unit of time, the second, is also only defined in this way. Only that the "clock" in this case is a cesium atom: A cesium atom "vibrates" around 9 trillion times a second and therefore physics simply says: We define approximately 9.192 trillion times this period of oscillation as " Second".

That is the pragmatic definition - but how can you describe the deeper essence of time?

The question is: Do you 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. We often imagine time as a grid between the past and the future that “is there” and then something “happens” in it. Here, too, Einstein showed that this is not the case.

Both space and time are first created 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 a 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 sense, because if there is no time, there is also no “before” and “after”. Another phenomenon of time is that it may be “quantized” on a small scale. In other words, figuratively speaking, the time does not pass evenly, but rather it drips in tiny little time portiönchen, which are of course much shorter than we can perceive.

A fundamental difference to space is that you cannot move backwards in time?

This is another feature: the so-called time arrow. In our perception, time only knows one direction. We cannot stop or turn back time, 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 open. This is astonishing because in classical physics - even with Einstein - time has no direction.

The laws of motion apply forwards and backwards. If I was filming a pool ball rolling on the pool table, I could run the film backwards without it being noticed. It is different when I film a pane of glass that breaks - here I can immediately see when the film is running backwards, when the shards are reassembling to form a pane. This is due to what is known as entropy: Events develop in such a way that the world as a whole tends to become more messy.

And if it does get tidier somewhere - for example when we do the dishes - it only works because we create disorder elsewhere - in this case in the wastewater. It is this increase in disorder that physically directs time. Conversely, this means that the energy of the universe must have been extremely “ordered” at the beginning - otherwise there would be no possibility of becoming more and more disordered.

But if you think this through to the end, it is conceivable that the universe will eventually assume a maximally disordered state in many billions of years. That would then be a kind of pulp of rays; and in this maximally disordered state the arrow of time would, so to speak, dissolve, 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.

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