# (2) Scientific General Interest

(12)March 9, 2020

*Answered by: Jonathan Gorard*

## Do your models permit the possibility of time travel (i.e. the existence of closed timelike curves)?

The existence of closed timelike curves is forbidden by the requirement of causal invariance in our models (in much the same way as their existence is forbidden by the requirement of strong hyperbolicity in more conventional formulations of Hamiltonian general relativity). More specifically, a closed timelike curve manifests as a cycle in the multiway evolution graph, Read more

April 1, 2020

*Answered by: Stephen Wolfram*

## Is this a theory of everything?

It is intended to be an underlying theory of the whole universe, in perfect detail. If one could run the model long enough, then it is intended to reproduce everything about the universe, including the writing of this answer. However, the amount of computation required to do this would be immense—and the phenomenon of computational irreducibility implies that there cannot in general be shortcuts.

April 1, 2020

*Answered by: Stephen Wolfram*

## Are you saying that the universe is a cellular automaton?

No. Cellular automata are very useful models for many things, and provided the intuition that led to the development of this model. But cellular automata as such have rigid predefined notions of space and time, and a critical feature of our models is that space and time are instead dynamic and emergent. Read more

April 2, 2020

*Answered by: Stephen Wolfram*

## Could the universe just stop?

Yes, in principle the rules for the universe could simply no longer apply to any part of the spatial hypergraph. If this situation occurred, it would mean that time would no longer progress, and the universe would reach a final state, or fixed point, in effect giving the final result of the computation that corresponded to its evolution. Read more

April 3, 2020

*Answered by: Stephen Wolfram*

## Could there be more than one correct rule?

Yes, the concept of rule space relativity suggests that many rules can be equivalent, but each different rule will be appropriate for a different “observer”, or, more specifically, will be the rule suitable for an observer using a certain language to describe the universe. Our particular description language—based on our sensory experience, Read more

April 4, 2020

*Answered by: Stephen Wolfram*

## How will you know if you have the correct rule?

Computational irreducibility means that it may be irreducibly difficult to determine any particular consequence of a rule. However, there is reason to hope that certain properties will be identifiable. If the rule is simple, then it is to be expected that just getting a few specifics of our universe exactly correct will be sufficient to determine the particular rule. Read more

April 5, 2020

*Answered by: Stephen Wolfram*

## What does your model say about the simulation argument?

The model implies that there is a definite computational rule that determines every aspect of what happens in our universe. If the universe is to be considered a “simulation” this would suggest that the rule is being determined by something outside the system, and presumably in an “intentional” way. It is difficult enough to extend the notion of intentionality far beyond the specifics of what humans do, Read more

April 5, 2020

*Answered by: Stephen Wolfram*

## Are there other universes?

In our model, many possible rules yield causally disconnected regions of spacetime, often corresponding to disconnecting parts of the spatial hypergraph. In addition, there can be disconnected regions of branchial space, corresponding to causally disconnected branches of quantum evolution. These kinds of non-communicating regions are still operating according to the same underlying rule. Read more

April 5, 2020

*Answered by: Stephen Wolfram*

## How could your model be proved wrong?

Any particular rule could be proved wrong by disagreeing with observations, for example predicting particles that do not exist. But the overall framework of our models is something more general, and not as directly amenable to experimental falsification. Asking how to falsify our framework is similar to asking how one would prove that calculus could not be a model for physics. Read more

April 6, 2020

*Answered by: Stephen Wolfram*

## If the universe is computational, what computer is it running on?

None. It is just following certain rules that we can think of as computational. There is no underlying “substrate”. The universe is just doing what it does, and we are describing it in terms of computation. When we think about Newton’s laws describing the motion of the Earth using equations, we are also imagining the equations describe what the Earth is doing, Read more

April 7, 2020

*Answered by: Stephen Wolfram*

## Are you saying that the universe is a computer?

“Computational”, yes. Our model implies that the universe operates at the lowest level according to definite rules of the kind one could readily program on a computer. But when one says “is a computer” one often means that one imagines that something has been constructed for the purpose of being a computer. Read more

April 8, 2020

*Answered by: Stephen Wolfram*

## Does your theory make predictions?

Ultimately it should predict everything about the universe, although many of these predictions will be computational irreducibly difficult to work out in detail. However, even at this stage, there are a variety of surprisingly concrete directions for predictions. One issue is that we do not yet know the overall quantitative scale of the core phenomena (since we do not know for sure the elementary length for discrete distances in space). Read more