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Philosophy of Time

Posted 5 years ago
POSTED BY: Barry Silverman
11 Replies
POSTED BY: Jarek Duda

Let me add a remark about CPT theorem and its possible applications - most physicists believe this symmetry is satisfied, it also contains time symmetry. So can we prove that causality only works past -> future? (as in Euler-Lagrange, in contrast to the least action principle).

For example laser causes excitation of target later - so shouldn't CPT analogue of laser cause deexcitation of target earlier?

While building "CPT analogue of laser" might seem extremely difficult, for free electron laser (FEL) it looks quite simple: CPT FEL

POSTED BY: Jarek Duda
Posted 2 years ago

I would say yes. Look up "rulial multiway graph" and you will see that there is a structure that does all rules on each step. Half of the rules would be to go "back" one step while the other half go "forward" a step, so not only is the present being generated by the rules, but the past and future are constantly being (re)generated.

POSTED BY: Jeff Yates
Posted 5 years ago
POSTED BY: Steve Paige
POSTED BY: Barry Silverman
Posted 5 years ago

Particular hypergraph states are always reference-frame dependent. One possible ("cosmological)" reference frame is updating the entire hypergraph at once at each step, but other choices are possible as well.

And we are not generally using the word "time" to describe evolution steps, we either call them steps or generations.

For any single branch's causal graph, any space-like slice through it (including horizontal slices in between layers corresponding to steps) would produce a unique space hypergraph. But, of course, multiway branching is possible, in which case you would not get a unique graph at any given step.

POSTED BY: Max Piskunov
Posted 5 years ago

I should clarify that the hypergraph, which is the state of the model, represents only space at a particular instance in time (in some reference frame). And rules operate only on that hypergraph.

From these rule applications, one can compute a causal graph, which represents spacetime. But rules don't operate on the causal graph directly, which is why the past is "protected".

Whether the past is "real", I think, is a philosophical question that our model does not answer, and it seems to me it fundamentally depends on what "real" means. To compute the future evolution of the hypergraph (on any of the multiway branches) past is not needed, so in that sense, it is not real. Whether the past can be reconstructed from the present would be rule-dependent. However, the causal (i.e., spacetime) graph can be computed from the evolution. So the past does exist in that sense.

POSTED BY: Max Piskunov

Thank you for your reply. I did not understand that the evolution of the graph is not somehow relate to time as observed from inside.

How then, does the evolution of the system by running the rule relate to "time" as perceived by an observer inside the system? How is the Arrow of Time represented, and how is the portion of the graph representing the past protected from further "evolution" by future events?

POSTED BY: Barry Silverman

If the universe is a deterministically evolving hypergraph - is the state of previous cycles preserved?

This is another way of asking (in this model) if the past is real? Is the future real? or is this model an example of Presentism rather than a Block Universe?

POSTED BY: Arsalan Lavang

An interesting and subtle question!

Of course, our model is only deterministic up to measurement. As detailed in my quantum mechanics paper (https://www.wolframphysics.org/technical-documents/) different choices of quantum observation frame, corresponding to different choices of measurement sequence, generally distinct causal graphs (in much the same way as different foliation choices in the causal graph generally yield distinct spatial hypergraphs). So I'd argue that our model implies a refinement of the block universe concept, in which it is not spacetime (i.e the causal graph), but rather branchtime (i.e. the multiway causal graph), that is the invariant structure containing all past, present and future events.

POSTED BY: Jonathan Gorard

I was listening to the working session on April 30th, and Stephen seemed to be saying that Time is special, and the idea of a block future universe (where the future is "calculated" in advance) can't happen because of computational irreducibility.

Did I misunderstand the discussion?

POSTED BY: Barry Silverman
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