If all space-time already 'exists' then

[DarkMatterHol]

...is the impression that "God plays dice with the universe" an illusion?

If all time in the universe is already defined, then, is the apparent randomness a result of the nature of the experiments we can perform?

Does a 'sum of histories', and quantum probability... 'square' with the concept that all time and space exists already? If so how?

Is there a test or thought experiment that could reveal the answer?
If there is no test, then what does it matter, and should we dispose of the notion altogether?

OK, that's several question already, but I'm not sure i could have made my query clear otherwise. thanks.

 

[PeterDonis]

If all time in the universe is already defined

Why do you think this is the case?

 

[jcsd]

I would actually bring up an interesting point: there is actually a massive element of unpredictability in the framework of general relativity. For any given set of initial conditions (though in some cases even the concept of initial conditions can become tricky), there are actually an infinite number of spacetimes compatible with those initial conditions. Only by imposing stringent conditions on the spacetime can we actually make sure that we get a set of initial conditions that determine a unique set of final conditions.

 

[DarkMatterHol]

Why do you think this is the case?

Well, this is proposed in models like the space-time 'loaf', is it not?
If not then maybe I'm misunderstanding just what IS being proposed in such a model.

 

[WannabeNewton]

If all time in the universe is already defined

It isn't. Nature doesn't provide us with a global time function that partitions the space-time of the universe into "past", "present", and "future" throughout the universe; in other words there is no god given notion of global time that defines "now" throughout the universe. We can only do this relative to a family of observers disseminated throughout the universe that satisfy certain very specific properties.

 

[Chestermiller]

Well, this is proposed in models like the space-time 'loaf', is it not?
If not then maybe I'm misunderstanding just what IS being proposed in such a model.

This is only a model (sometimes called Block Universe, or Block Time). There is not proof yet that this corresponds to physical reality, although there have been many physicists, past and present, who have accepted this model. The model is consistent with all observations of special relativity. However, there may be other models consistent with special relativity that are equally valid.

 

[DarkMatterHol]

The model is consistent with all observations of special relativity.

I see. The model does appear to be irreconcilable with 'quantum uncertainty' model though, does it not?
If it does, wouldn't a theory of everything have to rule out 'block time' or rule out 'uncertainty' as we understand these concepts today?

 

[Chestermiller]

I see. The model does appear to be irreconcilable with 'quantum uncertainty' model though, does it not?
If it does, wouldn't a theory of everything have to rule out 'block time' or rule out 'uncertainty' as we understand these concepts today?

In my opinion, yes, but, knowing PF, I'm sure that others here will differ with this view.

 

[PeterDonis]

Well, this is proposed in models like the space-time 'loaf', is it not?

Yes, but I would suggest looking at previous threads on PF on that topic before reading too much into that. Try these for a start:

https://www.physicsforums.com/showthread.php?t=676547&highlight=spacetime+loaf

https://www.physicsforums.com/showthread.php?t=675194&highlight=spacetime+loaf

If not then maybe I'm misunderstanding just what IS being proposed in such a model.

No, you're not, but you do need to realize that this is an *interpretation* of SR; it's really philosophy, not physics.

 

[PeterDonis]

The model does appear to be irreconcilable with 'quantum uncertainty' model though, does it not?

Yes, it is; but as I've pointed out in other threads (including the ones I linked to just now), it's also irreconcilable with ordinary *classical* uncertainty. In order to construct a fully deterministic classical model of the entire universe, you have to have a set of initial data on an entire spacelike slice. We do not have such a data set and never will: the best we can do is the set of data in our past light cone, which only covers a finite portion of any spacelike slice. That data is not sufficient to deterministically predict what will happen at any event that is spacelike separated from us; we can extrapolate from the data in our past light cone to say what *might* happen at such events, but any such prediction basically assumes that nothing of any interest happens outside our past light cone, which is a very extravagant assumption.

This point is glossed over in thought experiments because in thought experiments, we *make up* the data: we just declare by fiat that the events we put into our model are the only events of interest. Of course if you do that, you are basically declaring the initial data on an entire spacelike slice, so of course you can construct a deterministic model. But in the real world, we can't do that, and the argument for the "block universe" based on relativity of simultaneity requires us to assume that we can; otherwise the argument does not go through.

To put it another way, the argument for the block universe based on relativity of simultaneity requires us to assign physical reality to "simultaneous spaces": but that amounts to assuming the conclusion! We don't directly observe simultaneous spaces; we only observe what's in our past light cone. We *construct* simultaneous spaces in our model in order to help us understand what's going on; but they are not required to make predictions and no physical observable depends on them being physically real. The only reason to believe they are physically real is that the "block universe" view says they are; but of course that means you can't use the reality of simultaneous spaces as an argument for the "block universe" view, because you'd be arguing in a circle.

I should clarify, btw, that all the classical theories we have--Newtonian mechanics, Maxwell electrodynamics, special and general relativity--*are* deterministic in the sense I gave above; so if any of these theories were actually true of our world (which they aren't because of quantum mechanics), our world would in fact be deterministic. But that still wouldn't make Greene's argument for the "block universe" based on relativity of simultaneity valid; it would just mean he happened to hit on the right answer, but for the wrong reason.

 

[llynne]

Space exists, time happens. It is the verb to space's noun. Even if we were to take a slice of time and analyse it as if it had happened all at once we would see the verbiness of time. Objects moved relative to one another. Temperatures changed. Generation and degeneration take place.
Even for an entity outside of time,viewing it all at once it is not determininistic simply already done.

 

[Mordred]

...is the impression that "God plays dice with the universe" an illusion?

If all time in the universe is already defined, then, is the apparent randomness a result of the nature of the experiments we can perform?

Does a 'sum of histories', and quantum probability... 'square' with the concept that all time and space exists already? If so how?

Is there a test or thought experiment that could reveal the answer?
If there is no test, then what does it matter, and should we dispose of the notion altogether?

OK, that's several question already, but I'm not sure i could have made my query clear otherwise. thanks.

some of your questions above are already answered, but this article may assist you in understanding time in the universe and some of the problems involved in defining a physical basis for describing time.

"On the physical basis of cosmic time"

http://arxiv.org/abs/0805.1947

 

[ZapperZ]

On that note, this thread is done.

Zz.