[Serious] Is the Past Real?

[polymath257]

Yes, I do agree that parsing out what constitutes the present subjectively from within the universe has value.  I am curious about time at the cosmic level. Does the universe, as a whole have multiple states. If space-time is entirely within the universe, then it would seem it could not have multiple states.

This reminds me a lot of questions about whether the physical universe is finite, whether it had a beginning, and so on.  What lies beyond the last star?  And what lies beyond that?  These questions seem to be provoked by intuitions which were built for a problem space that has linear, inductively predictable properties, i.e. the macroscopic world around us.  But we realize from both quantum mechanics and general relativity that these intuitions break down when we attempt to generalize these intuitions to other scales or questions.  I suspect that questions of time and the geometry of the universe are likewise, that the paradoxes and puzzles are an artifact of our minds being built for answering different questions than these.  Kant argues that our thought presupposes three-dimensional space and linear time, that we can't think outside those assumptions because they structure our thoughts.  Wittgenstein has said that whereof one cannot speak, thereof one must be silent, but I'm not sure he's entirely correct about such things.  If quantum mechanics and relativity are any indication, some of these questions may require a new way of speaking, of thinking things that cannot be thought.  Feinman has said that if you think you understand quantum mechanics, you don't understand quantum mechanics.  Likewise, the reality and ontology of time may be something that we can't properly understand, but with the aid of mathematics and empirical tools, we can develop models of it which can be reasoned about and tested.

And I would say that the mathematical models *are* our understanding. Too often there is an assumption that we must use classical ideas for our understanding (particles with definite properties at all times, etc). And, if you hold to this requirement, you won't be able to understand QM .

But we can *and do* have models without 3D space or linear time and people *do* understand those models and can use them to predict what happens in the real world. When the old intuitions break down, we formulate new intuitions. And yes, our minds can learn to think outside the old constraints and with the new models.

But let's face it. Feynman understood QM. He was central in the formulation of quantum field theories, after all. These are testable, mathematical models of the universe that agree with actual experimental results. We know how to use this model to predict the results of future experiments and in all cases those predictions have been correct. it is even possible to develop intuitions about what the correct results will be without actually doing the calculations.

What else, precisely, is required for 'understanding'?