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I Is the Transactional Interpretation deterministic?

  1. Aug 4, 2016 #1
    Hi,

    This is a question about the Transactional Interpretation of quantum physics proposed by John G. Cramer.

    As you surely know this interpretation proposes that interactions only happen when the emitter has some sort of 'confirmation of receipt' by an absorber. In a sketch way we might say that the emitter emits a retarded 'offer wave' towards any potential absorbers in the future. Potential absorbers in the future send advanced 'receipt offer waves' into the past. Based on criteria which yield the same results as Schrodinger's wavefunction the emitter 'chooses' which will be its actual absorber and the interaction becomes actual.

    In this scenario, at any given time it is still impossible to predict with accuracy what the outcome will be. To the eyes of any 'normal' observer this looks like non-deterministic physics. But knowing the future outcome in advance makes it deterministic in practice. The 'knowledge' of the future absorption by the emitter is the ingredient which makes the outcome what it will be.

    When the emitter behaves as it does because it already knows the future outcome in advance, we can say that the future outcome is deterministic, it is based on the present knowledge (even if that knowledge can never be obtained by humans, but only by the emitter entities).

    Considering this, should the Transactional Interpretation be considered an undeterministic or a deterministic theory / interpretation?
     
  2. jcsd
  3. Aug 8, 2016 #2
    bump!

    No answers at all?

    TX!
     
  4. Aug 8, 2016 #3
    Sorry that I didn't see this question earlier.
    TI is definitely an indeterministic interpretation. However this is only consistent if you let go of the block world ontology and allow a 'growing universe' picture. For the latest details, please see: http://arxiv.org/abs/1608.00660 , especially Part II.
    Best wishes,
    RK
     
  5. Aug 9, 2016 #4
    Thank you!
     
  6. Aug 14, 2016 #5
    I was trying to make sense of this by reading the suggested reference: https://arxiv.org/pdf/gr-qc/0703098.pdf . The topical point seems to be summarized by this sentence:
    I hope I've understood the following correctly: the "growing" happens as described by the causal ordering of the elements of the set, as new ones are added. My question is how close is the relationship between this and the usual notion of time. Apart from giving up the notion of distant simultaneity, is it what we usually describe as "(space?)time" that is "growing in chunks" with this process? Or is it just the content of what we usually describe as "spacetime" that is changing and becoming more definite?

    I've also read remarks on the cosmological expansion in the references but I couldn't understand much, I wonder if this is related with the above question.
     
  7. Aug 14, 2016 #6
    It is the 'spacetime fabric' itself that is growing. The set of events is that fabric.
    For an interesting discussion of the nature of 'time' in the picture, see http://arxiv.org/abs/1405.3492
     
  8. Aug 15, 2016 #7
    Thanks. Don't want to indulge in this but, is there a distinction between spacetime bits and their contents (particles...), as there usually is? Or not? I can't tell from these articles.
     
  9. Aug 15, 2016 #8
    Good question. In the causal set picture, there is no such distinction. Spacetime IS the structured set of events. This is actually consistent with Einstein's thinking as well.
    Yes, many people think of spacetime as a background 'container' that is filled with events (particles doing stuff). That's not the way Einstein saw it and IMHO that picture leads to misconceptions and dead ends in making progress with the connection between quantum theory and gravity. As Einstein pointed out, without matter/energy there is no spacetime. PTI, together with the causal set picture, accounts for spacetime emergence from the quantum level. Gravity is not an additional 'quantum field' that needs quantizing. It is simply the structure of the emergent spacetime.
     
  10. Aug 15, 2016 #9
    I didn't know that Einstein thought in this way. If it's not too much of a hassle, could you point me towards this? A quick googling gives me entirely contradicting answers. I've seen discussions on the empty spacetime, especially regarding the cosmological constant (if it's nonzero, then there is pressure/dark energy even when there's nothing within it). But then again this important experiment's official site says: "space and space-time can and do not exist apart from the matter and energy that creates the gravitational field. This is not speculation, but sound observation." I don't understand from what it follows.

    Anyway, I figured though that the distinction would look rather artificial for a discrete spacetime, since to imagine discrete boxes containing particles is kind of redundant. But, at this point, are there "null events" in the causal set picture? That is, where nothing (much?) happens, kind of like the equivalent of a vacuum region.

    Thanks.
     
  11. Aug 15, 2016 #10
    Regarding 'dark energy', Sorkin showed that if spacetime is discrete you get the cosmological constant from that discreteness alone. So having a nonzero cosmological constant can't be taken as an argument or evidence for the claim that there is a background spacetime independent of matter/energy.

    Einstein struggled with this issue of what is called 'spacetime substantivalism' (taking spacetime as a container or substance that exists independently of matter or events) for many years. He eventually decided that this view was not tenable. The argument against spacetime substantivalism is discussed in depth here: http://plato.stanford.edu/entries/spacetime-holearg/
    I also recall reading a brief quote from Einstein to the effect that there would be no spacetime without matter, but I'm traveling right now so can't refer you directly to it. It may have been in Carlo Rovelli's book Quantum Gravity, which presents a relational (non-substantival) view of spacetime as well.
    However here is another quote from Einstein in which he states that spacetime is discrete rather than continuous:

    One can give good reasons why reality cannot at all be represented by a continuous field. From the quantum phenomena it appears to follow with certainty that a finite system of finite energy can be completely described by a finite set of numbers (quantum numbers). … This … must lead to a purely algebraic theory for the description of reality. But no one knows how to obtain the basis to such a theory.” (from the final appendix of his book The Meaning of Relativity 5th Ed).
    It seems to me that the causal set approach is a likely candidate for such a theory . See Knuth and Bahreyni for a rigorous mathematical formulation of a theory of discrete spacetime emergence (they refer to theirs as a poset; it is distinct approach from that of Sorkin et al): http://arxiv.org/abs/1209.0881
     
  12. Oct 25, 2016 #11
    A common reason for disregarding TI is the 'Maudlin Challenge', presented as an alleged fatal difficulty.
    However, that problem has now been completely nullified in the light of the relativistic version of TI. See: https://arxiv.org/abs/1610.04609
     
  13. Oct 26, 2016 #12
    Since you need to know the future to predict the present, I would say it is predeterministic, but not deterministic.
     
  14. Oct 26, 2016 #13
    In the relativistic TI (previously known as possibilist TI or PTI), the future is not actualized; it is a set of possibilities. This is not a block world, it is a growing universe picture, and the 'future' does not exist yet. So there's no need to 'know the future' in TI. It's genuinely indeterministic. The transactional process takes place among quantum possibilities that are not spacetime objects/events while the 'back and forth' of offers and confirmations are going on. It's only upon actualization of one of the possible transactions that the corresponding spacetime events emerge. For details, see https://arxiv.org/abs/1411.2072
     
  15. Oct 28, 2016 #14
    Thanks guys. But the set of those future possibilities, even if they are not spacetime objects/events, must be defined at least in some abstract realm. They must depend on the set of the actual spacetime events (for example the possibility that our electron will meet a proton in its future depends on the existence of some proton traveling in that direction towards that intersection in the future of both particles. If there is no actual proton traveling towards that intersection point, the possibility of our electron meeting a proton there is zero so there is no confirmation wave for that kind of event).

    So we say that the confirmation waves do not represent any 'events with any physical existence', they exist only as an abstract set of possibilities the future is allowed to take. I do not see then which crucial difference this makes to, say, the Copenhagen interpretation where the wavefunction is also an abstract entity forming the set of possibilities of which only one will eventually collapse as actual reality.
     
  16. Oct 28, 2016 #15

    PeterDonis

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    Whether or not there is an "actual proton" that could interact in the future with the electron is itself not determined in advance; it gets gradually established by other offer wave/confirmation wave transactions. (And the same for the electron's trajectory.) So there don't have to be any pre-existing spacetime events for the electron and proton to exchange offers/confirmations.
     
  17. Oct 31, 2016 #16
    Thanks Peter, that is correct. It's also important to keep in mind that protons are not little particles traveling around on well-defined spacetime trajectories. They are bound states of offer waves with have well-defined amplitudes for interacting with other bound states or field excitations, just as in standard scattering theory. Offers and confirmations are characterized by their momenta, not their spacetime coordinates--they don't possess spacetime coordinates, and that's why one has to integrate over all of spacetime to get correct amplitudes.
     
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