Delayed Choice Quantum Eraser Question

rkastner

Alex, I think the ability to send a signal superluminally wouldn't imply retrocausality as we've defined it here, because there is really no well-defined direction of the signal in a causal sense.

San K

Good point rkastner. May I add that:

The idler's state during collapse is not controllable/pre-determinable

rkastner

San K: indeed.

alexepascual

Even though you can't decide the particular value you'll get for the observable you are measuring on the idler photon, you can decide which observable to measure. Don't you think that that would affect the statistics on the other side? Even if this was possible, given the fact that both measurement events are space-like, this sould not imply retrocausality, but it would imply faster-than-light signaling. I know most people think that in Dopfer's experiment coincidence counting is the only way to get an interference pattern, but I am not convinced yet.

alexepascual

Ruth, If after making a measurement, or having some quantum state recorded in a stable way (decoherence or "collapse" in the language of CI) you have a definite state, then the set of all these outcomes in the past, present and future could be mapped on a diagram, constituting what could be called "a block model". Wouldn't it ? This is if we think that there is only one outcome after measurement, which all one-world interpretations (not only TI) propose.

alexepascual

I agree. It happens that I have read some other places (I don't remember if it was on this forum or some article) people saying the opposite. So I was interested in knowing your opinion.

rkastner

As far as I know, every time someone claims to have devised an experiment that could do this, there is a problem with it; e.g. a contribution has been disregarded as negligible that really isn't. You always need a coincidence count in order to see such statistics.

Regarding sending a signal into one's own past: in order to do this using FTL signalling you have to collaborate with someone in a different inertial frame, both parties using FTL signalling. Wiki has a pretty good discussion of this here: http://en.wikipedia.org/wiki/Minkowski_diagram (under 'speed of light as a limit'). The problem is that each observer just sends a FTL signal to the other observer based on their observation of an event, but they have have no way of knowing whether that event was a 'signal' or not.

rkastner

You can only put the determinate events on a ST diagram after the events have been actualized in spacetime. In my interpretation (PTI) the future really does not exist yet except as indeterminate possibilities and those are not spacetime entities. Since future events don't exist in spacetime you can't put them on a spacetime diagram. Similarly, any past situations that are not actualized (such as which path a photon took in a delayed choice experiment whose choice has not yet been made) cannot be put on a ST diagram either. This is an important issue and I discuss it in detail in Chapter 8 of my book.

alexepascual

Ruth, I think this would make a very interesting conversation. I will give you my point of view perhaps tomorrow because I have some personal problems to take care now. I'll also reply to your other post at that time. Thanks, --Alex--

alexepascual

Ruth, If you think that when a measurement is made and you get one of the possible outcomes, that represents an actualization and that is the only outcome that gets to be part of "reality" (if you don't have objections to using that word). We could imagine a universe composed of all those events. Even if there is no way for us to know what the events in the future will be, we know that each act of measurement will produce only one value for the observable being measured. In other words, we have two alternatives in representing the future 1) We look at "one history" or chain of events or 2) we can look at a branching scheme where all the possible outcomes of each measurement are included and each leads to different chain of events that in turn branch again when new "measurements" are made. I put "measurements" in quotes because actually we don't need a measurement carried by a physcisist in the lab but just an interaction between the quantum state with a macroscopic object that "collapses" the wave function. If you believe that a measurement produces only one eigenvalue, I don't see why we could not imagine a future that consists of all these unique autcomes, even if we can't predict now what the value will be.
We can also imagine that the weather this year will show some kind of definite evolution which we can't predict either. But we can imagine that at the end of the year we'll be able to put the actuall data in a diagram. We usually imagine a single future, not a series of possible ones. Of course if can consider the different alternatives and their probability in order to make some decision, such as what crop to plant now if that's what we do. But usually have the belief that the future will be one. I think that's the usual way to look at things. Of course there is the many-worlds interpretation where all possible paths of evolution count and are "real" and the consistent histories where we consider different histories but only one is part of reality. I'll appreciate your opinion on this and it would be nice to hear from Dr. Chinese.

alexepascual

I already expresed my agreement with this, but I would like to add that part of the difficulty here may reside in the fact that most people use the term causality without having a very clear definition. In classical physics, of course for something to be a "cause" it has to be in the past light cone of what is considered to be the "effect", but if someone talks about "retrocausality", then we are not considering that order in time as the main characteristic of causality. We usually think that between the cause and the effect there is some "mechanism" and usually we think in terms of locality or in other words "no action at a distance". But if we can't look at the "mechanism", then maybe we can find some useful definition of causality in terms of the type of correlation between cause and effect. This is something that I have not investigated by looking for articles on it, but I can imagine that it would also apply to analysis of causal connections in medicine, biology and other subjects. Probably one of the elements that we would consider is that the "effect" would show an increase in frequency when we use our "freedom of choice" or some quantum random number generator that we assume is not influenced by the "effect". Tweaking the "effect" by some means should not influence the statistics of the "cause". I know I have expressed this in a not very clear way. It is because I am pondering this as I write. Perhaps you or one of our friends in the forum can throw some more light on this.

rkastner

Causality is a very slippery notion indeed. I do address this in Chapter 7 of my book.

alexepascual

I won't be able for the moment to get a hold of your book. Maybe you could give us your point of view summarized in a few sentences.
Thanks,
--Alex--

rkastner

you can find it here: http://transactionalinterpretation.org/

Scroll down a bit

DrChinese

I liked the PowerPoint presentation, Ruth. Nice job of delineating some of the differences between TI and CI.

alexepascual

Thanks Ruth, I'll take a look at the material in your website and let you know if I have questions or comments on it.

rkastner

Thanks Dr. Chinese! :)