Is a "Cramer-like" Interpretation of QM falsifiable?

[septimus]

Disclaimer: My ignorance of physics has no bounds. I like to think my intuition isn’t too bad, and would like to have an expert address my questions in a way I can grasp.

Googling, I do see some discussion of “retrocausality” e.g.

Questions re Cramer's Transactional Interpretation?

but I’m hoping for a Layman’s Summary … but a different kind of summary than I’ve gotten so far.

@Half_Man_Half_Wit

septimus, perhaps the discussion of your idea would be something for a separate thread? Personally, I don’t see where you differ from Cramer’s transactional interpretation, and as with that interpretation, I’m not sure I really see the need. As leahcim notes, interpretational problems of EPR experiments mainly focus on how to save causality in the presence of apparently faster than light ‘spooky actions’ (which I consider neither spooky nor ‘actions’ in most senses of the word). To just throw out the notion of causality does seem like cheating somehow. And of course, the problems you get are those you get with any retrocausal proposal, such as paradoxical influences on the past, causal loops (i.e. A caused B retrocaused A), and of course, there’s also the problem of how our experience of a unidirectional arrow of time comes about; the usual argument of ‘entropy was lower in the past’ doesn’t work, since for every point in time, if causal influences propagate in both time directions symmetrically, one should expect that entropy is higher both towards the past and towards the future, as the dynamics that generate entropy increase work in both directions equally well.

OK. New thread.

As I understand Cramer, he starts from two ideas:

1. An event A[sub]Monday[/sub] –> B[sub]Tuesday[/sub] can be viewed instead, as B –> A with retrocausality.
2. The event A <–> B is a “transaction” in which a “retarded wave” from A and an “advanced wave” from B somehow “find each other.”

I think (1) is valid, but am agnostic on whether (2) is the best way to model the transaction.

The beauty of (1) is that much (or all) the Quantum Mystery associated with EPR, GHZ, etc. disappears.
That mystery is replaced with a different type of paradox/mystery, but one which my intuition finds much more acceptable.

So, is (1) (augmented with Cramer’s 2 if that is necessary) even a consistent interpretation of Quantum Theory? Set aside metaphysical objections like Grandfather Paradox which will not sway me.

Answering Half Man Half Wit:
“I’m not sure I really see the need”

• Mainly it makes QT much less paradoxical (to my intuition). To be more interesting, it would have to make falsifiable predictions…

“if causal influences propagate in both time directions symmetrically, one should expect that entropy is higher both towards the past and towards the future, as the dynamics that generate entropy increase work in both directions equally well.”

• suppose the Universe has boundary conditions imposed, e.g. that entropy is increasing along one time sense. Would that “time’s arrow” imply a “radiation arrow” in which, from our time sense, we observe far more retarded waves than advanced waves?

I asked the question previously, and got an answer from Stranger:

@Stranger_On_A_Train

@septimus

The collections of waveforms we observe share the same causality direction as us, but that’s because those are the only collections we can observe ! Simple example: we observe stars, and know they share our causality direction. But suppose there was a time-reversed star in range of our telescopes. We wouldn’t be able to detect its light, which in our time sense would appear as uncorrelated photons which, moreover, if intercepted for detection, wouldn’t have existed in the first place!

Suppose there was an area of the universe filled tusk to arsehole with invisible giant pink elephants? What would that prove? The universe–our world–is all and only those areas we can observer or infer by observing consistent phenomenon. Nobody outside of an episode of Dr. Who has ever seen a “time-reversed star” or anything of the sort, and we have no evidence by which to infer that such a thing could even exist.

@septimus

Question for astrophysicists: How would a retrograde-causality star appear? As “dark matter”?

Disclaimer: I understand very little about retrograde causality but don’t think it can be readily dismissed. I hope a physicist with clearest understanding will start a new thread on the topic.

This is not a sensible question. I don’t know where this notion of “retrograde causality” comes from, but it is not to be found anywhere in scientific literature, and no actual physicist has a “clearest understanding” of it. There are ways within the framework of relativity to formulate closed time-like curves which have endpoints that terminate before the start in the time direction, but that isn’t a violation of (local) causality, as the system following that path is always experiencing time as advancing, and there are physical reasons to believe that such paths, like roller coasters, have to return to their beginning point before one can exit.

Here and in another response, Stranger implies that relativity itself has a “time arrow.” ?

I postulate neither retrograde-causality stars nor pink elephants. I’m sincerely curious how a retrograde star would appear to us.

(Finally, and feel free to nominate me for Crackpot of the Year, I’ve always been confused that QT causality is described by complex numbers, i.e. 2 real scalars, and have wondered whether there might be a way somehow to relate these two real coordinates to the two time directions of cause-effect. I realize this suggestion is probably hilariously stupid, so respond to this only in BBQ Pit.)

[Half_Man_Half_Wit]

First of all, if it’s a bona-fide interpretation of quantum mechanics, it’s not falsifiable – since interpretations of quantum mechanics necessarily agree with respect to the physics they predict; otherwise, they’d be alternative theories, which may or may not be empirically viable (like quantum mechanics, and all other scientific theories, itself). I’m not sure whether the transactional interpretation is an actual interpretation in this sense – the wiki article on Cramer notes that he is currently engaged in ‘experiments at the University of Washington to test retrocausality by using a version of the delayed choice quantum eraser without coincidence counting’, which would imply TI to be a theory different from QM; also, it’s claimed that if the experiment were successful, one could use it for instantaneous signal communication, which is generally thought to be impossible, both by QM and by special relativity.

As far as I understand it, the TI needs the transaction to work; it eliminates advanced waves having an influence ‘before’ a quantum process occurs, and thus, keeps the theory paradox-free.

Also, it does not allow re-interpretation of a causal A–>B process as a retrocausal one, as you seem to say with (1), but rather, models every quantum mechanical interaction as both composed of causal and retrocausal parts, which interfere to give a sort of ‘standing wave’ that is supposed to be ‘what really happens’. (It’s also a collapse theory in the sense that this standing wave is not unique, but typically a superposed state, which then randomly collapses to one unique reality – since I’m not a fan of collapses, this alone makes me regard it with some skepticism.)

As for what is more or less paradoxical, well, seems we get down to a matter of tastes here; personally, I don’t think there’s anything paradoxical about EPR and the like, but retrocausality lands us in a mess of paradoxes. Fundamentally, I don’t think that entanglement is any more mysterious than classical correlations: I give you an envelope, and keep one for myself, together with the information that I have put one slip of paper of a set of two, one of which is green, and one of which is red, into each envelope. Once you open your envelope, you learn the color of your paper – and instantaneously, also the color of mine; this is completely free of problems.

The difference in the quantum case is just that quantum objects can enter into superpositions, so that the color of the slip of paper is now not merely limited to red or green, but to red, green, and any linear combination thereof. This is what yields the apparently paradoxical effect that your observation ‘causes’ a ‘collapse’ of the wave function, which determines the state of my paper; it’s there that an apparent ‘spooky action’ takes place.

So what really needs to be explained is the capacity of quantum systems to enter into superpositions – and I don’t see how retrocausality is of any use there. Personally, my favorite explanation is Zeilinger’s: a quantum system contains a limited amount of information, one bit in the case of a qubit. So if one property of a qubit is known – say, its spin along the z-axis is up – there’s no information left to determine another property, say, its spin along the y-axis. Measurement of that property thus must yield perfect randomness – the qubit is in a superposition wrt this property. Combine this with a correlation between the state of two qubits, and you get the phenomenology of entanglement and EPR.

suppose the Universe has boundary conditions imposed, e.g. that entropy is increasing along one time sense. Would that “time’s arrow” imply a “radiation arrow” in which, from our time sense, we observe far more retarded waves than advanced waves?

I’m not sure what you mean here. How would one impose boundary conditions to ensure such an entropy increase? You can’t just start with a theory that implies entropy increase in both temporal directions, and then say, well, let’s just say that entropy always increases in only one direction for some reason…

Besides, this seems to be a symptom of all retrocausal theories: first, you include the violation of causality to account for some phenomenon; then, you have to jump through all kinds of hoops in order to hide the effects of retrocausality, i.e. make the theory look like one in which there is only ordinary causality.

This is also what occurs in the Wheeler-Feynman absorber theory of electrodynamics: you have to impose a condition such that advanced and retarded fields cancel each other, which amounts in the end to just re-constructing the usual theory of electrodynamics from one in which you first introduce, then cancel out, retrocausality.

I’m also not sure what you mean by causality being described by complex numbers in quantum mechanics – it’s true that the wave function is a complex quantity, but causality, in quantum mechanics, is described by the evolution of the wave function: how a prior state determines a later one. Of course, complex numbers also do play a role there, but originally, they just come from the fact that quantum systems show interference effects, and this can be modeled with complex numbers adequately – nothing to do with causality (well, maybe one could construct something out of the fact that if evolution forwards in time is modeled by a certain operator, its Hermitian conjugate, which involves complex conjugation, models evolution ‘backwards in time’ – but that’s more than a little contrived).

As for the ‘retro-star’, one must distinguish here between microscopic and macroscopic notions of causality. Microscopically, all physical laws are in fact time-reversal invariant – like any given shot in a game of billiards might occur just as well in a time-reversed sense. Only in aggregate does a direction of time emerge, i.e. the number of balls on the billiard table decreases, and the direction in which it decreases gives the direction of time. So, on the microphysical level, nothing would be different essentially. On the macrophysical level, there are arguments that regions with two different arrows of time, if brought into contact, eventually evolve to a connected region with one single arrow of time in a process of equilibration (typically, the system with the greater number of degrees of freedom ‘wins out’, and the smaller system eventually agrees with its time-direction).