Eckard
"You imagine that the past must cease immediately..."
Take St Paul's Cathedral (again). Now, due to weathering, a molecule of stone detatches from the bell tower. I hope there is no concept other than that the state which involved attachment preceded the state which involved detachment, and that they did not occur at the same time?
But, while quite detailed, this is not a conceptualisation of what we refer to as St Paul's Cathedral at its existential level. That is an incomprehensible configuration of elementary substances each in some particular state with respect to their own innate properties. The fact that we cannot comprehend it is irrelevant. As at any given point in time 'St Paul's Cathedral' must be in one, and one only, definitive physically existent state, otherwise it cannot exist. And that involves no form of change, because change indicates more than one such state. The immediately previous state in the sequence of existence of 'St Paul's Cathedral' must have ceased. Nothing can have more than one physically existent state at a time.
The problem here is our conceptualisation of physical existence. Understandably, we conceptualise it at a much higher level than what actually occurs. For the most part this does not matter in generating understanding, because we only want it at that level. But, we need to recognise, when it is appropriate to do so, that this conceptualisation is ontologically incorrect. Put simply, there is no such entity as St Paul's Cathedral, or indeed all the other 'its' we invoke, when considering how physical existence actually occurs. There is a highly complex sequence (system), which only gives the appearance of St Paul's Cathedral when conceptualised at a high level. That is, certain features at that level constitute it, but they are entirely superficial, in the context of physical existence.
Look at this another way. We do not touch it, so over time St Paul's Cathedral becomes a pile of stone, wood, timber. It no longer 'exists'. But how does that differ, physically, from any of the physically existent states which occurred, and we were content to designate as St Paul's Cathedral? The answer is because what constitutes it no longer has the superficial physical features that we deemed to be St Paul's Cathedral. Physically, logically, this pile of debris is just another configuration in the sequence! In other words, St Paul's Cathedral (and any other such 'it') was only ever a concept.
"In reality, there are traces to be found that memorize...."
Forget memories, etc, etc. The distinction must be drawn between physical existence and knowledge of physical existence. Physical existence occurs independently of the mechanisms whereby we are enabled to be aware of it, albeit within the confine of existence. We are part of physical existence. We cannot 'escape' it. Which addresses your first concern about what constitutes the closed system I refer to. We only have knowledge.
"In principle, your imagined sequence..."
Yes, physical existence must ultimately have a discrete state, as at any given point in time, otherwise it could not exist, let alone change. The trick to identify that, and not confuse it with one that appears to occur when conceptualised from a higher level, or with elementary substance, in itself.
"then tell me please its width and why it cannot be attributed to the past"
Its 'width', ie duration, will be equivalent to the duration taken for the fastest change in reality to occur. Duration being the common denominator unit in the measuring system known as timing. In other words, at such a level of differentiation, no form of change would occur, so what was occurring would be a physically existent state, ie what existed as at that point in time. Obviously, not every form of change occurs at that speed, so one could have exactly the same physically existent state in some sequences occurring for more than one point in time. I just use the word point to emphasise non-divisibility/singularity. But what it constitutes must relate to physical reality, not just a concept, indeed, the same applies to the concept of spatial position and dimension. Any form of change indicates difference, but difference, as such, does not exist, states do. It is just that when compared difference can be identified.
Paul