Conflict between the Block Universe and Bell Tests?

[PeterDonis]

Wouldn't we say SR is not deterministic because my past light cone at any time is larger than it was at an "earlier time." So, according to SR, I have no way to know if I will be bombarded with X-rays in the next instant.

Your not knowing what you will observe in the next instant has nothing to do with whether what you will observe in the next instant is fully determined by initial conditions.

SR makes it impossible for any observer to predict the future, right?

Only because, to predict the future in a deterministic spacetime, you need initial data on an entire spacelike 3-surface, and the only data any observer can actually have is in their past light cone, which will never cover an entire spacelike 3-surface.

 

[msumm21]

Your not knowing what you will observe in the next instant has nothing to do with whether what you will observe in the next instant is fully determined by initial conditions.

OK, I was thinking that may be the definition of deterministic. However, if we reject the block universe, aren't we saying that there aren't real, fixed initial conditions? Not sure, still thinking...

 

[PeterDonis]

if we reject the block universe, aren't we saying that there aren't real, fixed initial conditions?

No. We're just pointing out that no observer can ever know a complete set of initial conditions for the entire universe. But any observer can know a complete set of initial conditions for what happens at his current present event, since the only set of initial conditions required for that event is what happens in its past light cone. He just doesn't know, at that event, a sufficient set of initial conditions to predict what happens in his future, since those conditions will include events that are outside his current past light cone.

 

[msumm21]

No. We're just ...

It seems to me that accepting a fixed, real time slice is accepting BU. Surely there's not one special time slice that's fixed and real, but others are not. Right?

 

[PeterDonis]

It seems to me that accepting a fixed, real time slice is accepting BU.

What is a "fixed, real time slice" and where did I talk about "accepting" one?

 

[RUTA]

There seems to be some confusion over the term “block universe.” Let me try to clarify what that means.

We all experience Now as unique from past experiences or experiences that are yet to happen (I won’t qualify all such statements with “I assume”). We tend to believe that we only exist (are only “real”) Now, i.e., my past self no longer exists and my future self has yet to exist. I assume there are other things that coexist with me at each Now, e.g., my twin brother who lives 600 miles away has a Now of his own and his Now coexists with my Now. When we turned 50 our 49 year-old selves no longer existed and our 51 year-old selves were yet to exist. We use this reasoning out of necessity nearly every day. If I want to call my brother I first ask myself, “What is my brother doing Now?“ because I don’t want to call if I believe he is sleeping or at work, for example. Not only is it perfectly reasonable to assume the validity of a shared set of Nows with other people, but it is absolutely necessary in dealing with objects at large distances. If I work for NASA and I’m in charge of a probe circling Jupiter 40 light-min away, I need to know where it is at any given Now, so I know where it will be 40 min from that Now when it receives my next orbital command. When I talk to engineers in Australia who will take over command of that spacecraft in 12 hours, we all share the same notion of our set of overlapping Nows, albeit with different time stamps for our different time zones. Of course, it could be that our probe has been struck by a meteor and it doesn’t exist where I think it does at some Now — I won’t know it has been struck for at least 40 min. But, that’s simply a result of the finite speed of light that carries information to me from the probe. That is, it’s a fact about my knowledge of the probe at some Now, not about whether or not the probe actually coexists with me (is “equally real”) at any Now. The probe doesn’t experience Now, but it’s meaningful to think about it as only existing (as only being “real”) at each Now. If everyone agrees on a shared collection of Nows, that would constitute a global “Now slice“ of spacetime. That is Presentism and that accords nicely with our everyday experience.

Unfortunately, that intuitive notion cannot hold if everyone measures the same speed of light regardless of their motion relative to the source, which is an empirical fact. That empirical fact is itself very counterintuitive. If we are at rest with respect to each other and I throw a ball away from me and towards you at 5 m/s relative to me, then you measure the speed of the ball to be 5 m/s relative to you. If I’m in a car moving at 30 m/s towards you and I throw a ball away from me and towards you at 5 m/s relative to me (and the car), then you measure the speed of the ball to be 35 m/s relative to you. That’s intuitive. But, if I do the same with a flashlight, you always measure the speed of the light from my flashlight to moving towards you at c. Even if I’m moving towards you at 0.5c, in which case intuition tells us you should measure the light from my flashlight to be moving towards you at 1.5c, you will still measure its speed to be c.

As a consequence of that fact, observers moving relative to each other will no longer agree on what events are simultaneous (are “equally real”), i.e., on what events occupy a “Now slice.” Thus, if everyone continues to attribute “existence” only to those events in spacetime that share their Now slice at any given moment of time in their existence, e.g., my attributing existence to the version of my twin brother that is the exact same age as me, then there is some frame of reference in which my Now coexists with a younger (or older) version of my brother. Thus, that special feeling of Now that I experience along my worldline in spacetime exists equally everywhere along my worldline as far as other observers are concerned. Here is 9-min video showing the relativity of Now slices (aka the relativity of simultaneity) that results from the light postulate. That is the block universe and it violates our everyday experience.

Of course, that doesn’t entail that our dynamical experience of time is ”an illusion” as the video implies. You can rather believe that the block universe is simply a tool for coordinating, explaining, and predicting our experiences. See Section 5 of this paper by Mermin, for example. That’s an “interpretative” aspect of block universe.

 

[Sunil]

The word "theory" is not correct here. These are interpretations of QM, not theories.

No. Pilot wave (or dBB) theory is a different theory. It gives quantum predictions only in particular states, namely in quantum equilibrium.

 

[PeterDonis]

Pilot wave (or dBB) theory is a different theory. It gives quantum predictions only in particular states, namely in quantum equilibrium.

The Bohmian interpretation includes the requirement of quantum equilibrium, which is why it is normally called an interpretation.

However, I agree that you can make a more general theory, different from standard QM, out of it by dropping the requirement of quantum equilibrium. Of course, this theory will then make all kinds of predictions that do not match experimental results.

 

[Kolmo]

Where is it actually proven that it gives the same results for all observables. I've seen the proofs that with quantum equilibrium we get the same results for single particle position measurements, but I've never seen the proof of the general case.

 

[Sunil]

That's done in Bohm's original paper.

Bohm, D. (1952). A suggested interpretation of the quantum theory in terms of "hidden" variables, Phys.Rev. 85(2), 166-193

And this is the main contribution of this paper. That the results for position measurements are fine was known already before by de Broglie 1927 or so. But de Broglie was unable to show that for non-position measurements this works fine too.

 

[Kolmo]

That's done in Bohm's original paper.

Bohm, D. (1952). A suggested interpretation of the quantum theory in terms of "hidden" variables, Phys.Rev. 85(2), 166-193

And this is the main contribution of this paper. That the results for position measurements are fine was known already before by de Broglie 1927 or so. But de Broglie was unable to show that for non-position measurements this works fine too.

I've read that before, but it seems to give some nice details about energy and scattering not a description for general operators. Also I've read a few criticisms of it, such as the inability to replicate multi-time correlations. Is there a presentation of the equivalence for general operators in some modern text?

 

[Sunil]

I've read that before, but it seems to give some nice details about energy and scattering not a description for general operators.

Hm. I read at page 180 "Let us now consider an observation designed to measure an arbitrary (hermitean) "observable" Q, associated with an electron."

 

[Demystifier]

Where is it actually proven that it gives the same results for all observables. I've seen the proofs that with quantum equilibrium we get the same results for single particle position measurements, but I've never seen the proof of the general case.

See e.g. the paper linked in my signature.