Reality Conditions

Thursday, April 13, 2006

Relational Quantum Mechanics

A very interesting paper on the interpretation of quantum mechanics has appeared yesterday:

Relational EPR
Matteo Smerlak, Carlo Rovelli

The article attempts to explain the EPR experiment using the relational interpretation of quantum mechanics championed by Rovelli (presented here and here). The main idea of the relational interpretation is that a quantum state is not an "absolute" description of a system, but only relative to a given observer, and that a same system may be described at the same time by many different states. For example, in the "Wigner’s Friend" version of the Schroedinger Cat paradox, one observer inside a box makes a measurement of a quantum system and sees a definite result, while for a second observer outside the box the whole system including the first observer is still in an indeterminate "superposition" state. The relational interpretation has a simple description of the situation: the state is collapsed relative to the first observer and superposed relative to the second observer. (In contrast, the better known "many worlds" interpretation would say that the "true" state is the superposed one and that the first observer’s impression is a kind of illusion produced by the "branching" of his consciousness. The relational interpretation is more "democratic"; none of the descriptions is privileged.) A key feature of the relational interpretation is that according to it any quantum system can be called an "observer"; counscious beings have no special status, and any interaction can be a "measurement". If system A is first in a superposition state Ia1> + Ia2> (with respect both to a system B and to an external system C) and then interacts only with system B, relative to C the global state may be now Ia1>Ib1> + Ia2>Ib2>, even though relative to B one of the options has “actualized” and the state is now Ia1>Ib1>.

I think this is a quite elegant solution to the measurement problem. It does not involve any change in the testable predictions of QM, unlike the models with a physical wavefunction collapse; it does not involve extra physical baggage like hidden variables models do, and it does not involve the extra ontological baggage of the many worlds interpretation. (From the point of view of the relational interpretation, the many worlds interpretation would seem to privilege as the only "true" state one which is not relative to any particular observer; God’s point of view, so to say. I think that the relationist should deny that there is any such state, like a "wavefunction of the universe". This should have implications for quantum cosmology.) Even more attractive, for me at least, is that the interpretation is not instrumentalistic: quantum mechanics is not merely a tool for calculating and predicting but a true description of how the world works; the description must be done from the "point of view" of some physical system, but there is no privileged choice for the reference system (much like the situation with reference frames in special relativity).

One criticism that comes to mind is that the relational interpretation is not very clear on which events count as "interactions" or "measurements". In the situation described above, in which system B "measures" system A and changes its state relative to it, at what time does this change occur exactly? From the external point of view no change has occurred: relative to an external observer the whole state has evolved continuously into a superposition. So there is a question that seems real enough, "Exactly when did B's state relative to A change?" but to which quantum mechanics provides no answer and no experiment will ever shed light on. This seems strange; if Rovelli has a good answer to this question, I am not aware of it.

As applied to the EPR experiments discussed in this paper, the apparent paradox in the usual formulation of quantum mechanics is that a measurement in system A (which "collapses" and changes its state) can produce an instantaneous change in the state of a system B entangled with A, even if B is spacelike separated from A and so no physical signal can carry the information of the collapse. The solution, using the relational interpretation, is explained in the article in the following way: If observer O makes a measurement on A, then the state of A relative to O changes, and ipso facto the state of B relative to O insofar as B and A are entangled, but this change in the state of B is not problematic as it only reflects the information available to O. The state of B relative to the observer O' who is measuring it remains uncollapsed when O measures A if O and O’'are spacelike separeted. But after O' measures B, when O and O’ meet to compare results (it is stressed that this comparison is also a physical interaction, involving a measurement), they are garanteed to find the correlations predicted by quantum mechanics. Relative to O, the measurement of O' (comparison of results with him) will always yield O' to be in the state of having measured the opposite spin to the one measured by O, if the original state was a singlet.

One noteworthy feature of the paper is that it quotes Wittgenstein! Propositions 1.1, 2.01, 2.011 y 2.0121 of the Tractatus. Not very common in your average physics paper! The reason for me remarking this is that I was fascinated by the Tractatus at one period of my life and organised a reading group for it with some friends. I'm sad to report it only lasted one meeting.

There is an ongoing discussion of this paper at Christine Dantas' blog.


  • I'm impressed with the quality of this almost same-day review of a very interesting paper. I tried to give adequate notice of the paper yesterday on physicsforum and did not do such a good job. I come here and find out just what needed to be said. For me this blog about the Smerlak Rovelli paper sets a mark for quality. thanks for doing that.

    BTW I noticed your comment at christine dantas, and then later something about your review of Relational EPR at "backreaction" the blog of Sabine Hossenfelder

    By Anonymous marcus, at 5:33 AM, April 14, 2006  

  • Thank you very much for your comment! I saw yesterday your generous notice at Physics Forums and was going to write an update today pointing to it.

    By the, way, I have realized now that the only criticism I make in the post of the relational interpretation is incorrect. I will write now a short post explaining where my mistake was.

    By Blogger Alejandro, at 1:09 PM, April 14, 2006  

  • A propos Sabine H., she had a question about Relational EPR which she posed on Physicsforums. Would you like to take a try at answering it? It may be that she just wants things clarified with more detail or there may be some essential unanswered question. In any case here is her post:

    ---quote from Sabine---
    Thanks, marcus. Excuse me for asking more dumb questions. I know about nothing about the topic, but I am too interested to be embarrased

    So, I have spatially separated observers, measuring the outcome of an experiment with an initially entangled state. Each observer measures some 'collapsed' state. Usually, the measurements have to be in some kind of relationship that has to be instantaneous, therefore the problem with locality.

    Now I say instead, well, there is no evidence for a non-local collapse as long as the observers haven't actually compared their measurements. So, I bounce back the information from one measurement to the other, to compare both. No non-locality in this, which is good. The comparison requires an interaction. Does this interaction process then make sure that the measurements fit together as parts of the entangled state? And if so, what is the difference to saying, that the 'collapse' propagates locally? I.e. imagine a continuum of observers whose measurements get compared. Or what did I miss with Rovelli's interpretation?

    When I reply to her I will suggest she check here for a possible response from you.

    By Anonymous marcus, at 1:28 AM, April 15, 2006  

  • here is the link to Sabine's question *in situ* at PF in case you wish to reply there:

    the same with context:

    Sabine has a confusing variety of names---Bee, Biene, Sabine, and Hossi---but otherwise I think she is admirably frank and straightforward. Hope you agree :-)

    By Anonymous marcus, at 1:50 AM, April 15, 2006  

  • Thanks for passing the question! It has forced me to rethink the whole paper with more care than I had before. I'm not sure if the following will be an answer to what Sabine is asking, but here's my current understanding of the situation:

    I think that Rovelli's interpretation of the EPR cannot be equated with a "collapse propageting locally". For Rovelli, the quantum state is just a book-keeping device for the information one "observer" (system) has about other system. So there is no physical collapse of the state to propagate. He says (in my notation) that when observer O measures system A and finds its state to be, say, spin up, then the state of B relative to him also changes, to spin down; but this is a harmless change in information only "like reading about China in the newspaper" and does not involve any nonlocality. My reading is that the key point is that B's state relative to O' is not changed by O's measurement of A. It is still in the superposed singlet state until O' measures it.

    This may seem paradoxical, beacuse O "knows already" what result O' "is going" to get. But this is telling the story from an omniscient point of view which the relational interpretation denies us. From the point of view of each observer there are no inconsistencies: for O the state "collapses" (is updated) when he measures A, and after that he can predict the result of his measurements of B and O' when they get back together. Symmetrically, for O' the state collapses when she measures B and this enables her to predict correctly her future measurement of O. And for an external observer which remains neutral, all the state remains superposed in a singlet (including the observers after they do their measurements, so O and O' become entangled) and this enables the external obrserver to predict correctly the correlations she will observe when measuring O and O' (getting the results from them) at the end of the experiment.

    Does this make sense? I am still not sure I am not missing something... what do you think?

    By Anonymous Alejandro, at 4:50 PM, April 15, 2006  

  • Hi Alejandro, thanks so much for your answer. I am still trying to get accostomed to this view, so I guess you have to be patient. Your explanation is very helpful, but I am afraid, I still don't completely understand it.

    I seems to me the relevant observer is the 'external observer' who measures the correlations. Which, no matter what interpretation, should be consistent with what experiment has shown us.

    Now I get from your explanation, that for this external observer, both O and O' seem to be entangled, even though they themselves have made their measurement. So, they have a different description about the status, but that does not bother us, because they have not been compared so far. This is only done through the external observer, who is in causal contact with both, and whose observations are in agreement with everthing QM dictates.

    But what happens after everybody has done their measurement and they have exchanged the outcome? Say, O has measured some list of data for OA=(1,1,1,-1,-1,1), O' has measured a different list O'B for B, NOT necessarily -OA. But of course it is OB=-OA and O'A=-O'B. Then the external observer comes, measures O and O' and finds the right elephant (Eq (5)). Does this measurement change either O or O' s data? Better not. If not, how can they all come to the same conclusion?

    I.e. I guess my problem is, that I agree that they all have the same correlation (in the sense that there is no violation of spin-conservation in either data-set), but does their notion of +/- 1 agree with everybody elses?

    By Blogger Bee, at 5:32 PM, April 16, 2006  

  • Hi Sabine! I think that the problem in your question is the assumption that there is a moment when "everybody has done their measurement and they have exchanged the outcome". This would be a moment where all the quantum weirdness has gone away and the situation has "stabilized" to a classical outcome where there is a unique and public "reality". I think that Rovelli's point is that all the observers being quantum systems, you always have to describe the situation and ask the questions from the point of view of one of them. There is no neutral standpoint from which to make these comparisons.

    If you do the experiment from either the point of view of O, of O', or of the external observer, you always get a set of answers (after interacting with and meaasuring all the other observers) that are a) self-consistent, and b) in agreement with the predictions of QM. And this is all there is to say.

    I thank you for this question because thinking on it and rereading the relevant parts of the paper have made me realize better how radical Rovelli's view is -to the point of flirting with solipsism. He denies (beginning of 4.3) that each observer has a solipsistic reality disconnected from the others. But at the end of this section, after the already famous remark "everybody sees the same elephant" he precises the meaning of this as "everybody hears everybody else stating that they see the same elephant he sees". I think this last sentence is the real summary of the paper, not the other one. I can see a consistency between my observations and the reports of other observers' observations; there is no level of objectivity beyond this one from which to ask whether there "really is" or not such a consistency.

    But isn't this a kind of solipsism at the end? I'm not sure. Why not say that the other observers whom I see agreeing with me are just as real as I am and that our agreement is proof that we are seeing a common reality?

    By Anonymous Alejandro, at 3:20 PM, April 17, 2006  

  • Hi Alejandro, thanks for the explanation. Also those at the PF were very helpful. I think I finally get the point. Each observer has its story to tell and when they compare stories, they always hear the same story, there is no disagreement, and no need for a non-local collapse. I believe that this is probably a doable interpretation, but I admit I don't really like it.

    I guess I don't know how to get a connection to the classical world. E.g. A agrees with B to kill the cat when he measures 1 for alpha. He measures alpha, he kills the cat. B has also measured 1 for beta, concludes that alpha is -1, and confirms this when he is later in causal contact with A.

    Exchanging info with A's measurment agree's with B's world, ala Rovelli. But the cat is still dead. What does B conclude? That A is a cat-hater?

    Best, B.

    By Blogger Bee, at 11:01 PM, April 17, 2006  

  • >>>But the cat is still dead. What does B conclude? That A is a cat-hater?

    First, I'd say that if they agreed to set up an experiment in the way you describe, it means they are both cat-haters! ;-)

    On a more seerious vein the answer is no, because for B the cat is alive. Otherwise his measurements wouldn't be consistent with each other.

    But you are higlighting a problem for the relational interpretation that is stressed by "vanesch" at the Physics Forums thread. If the cat is alive in "the world according to B" and dead in "the world according to A", then aren't we in something very similar to the many worlds interpretation? Only without a global universal quantum state, it seems. But the troublesome branching of reality of MWI is still with us. We have "many observers" and a reality acording to each of them, instead of "many worlds". Is this real progress?

    This is the problem with interpretations of quantum mechanics... the more you get to understand them, the less sense they seem to make.

    By Anonymous Alejandro, at 1:54 AM, April 18, 2006  

  • Hi Alejandro,

    you are confirming my worst expectations. The world is full of cat haters!

    But seriously, I think I prefere non-local collapse.



    By Blogger Bee, at 7:18 PM, April 18, 2006  

  • good site

    By Anonymous language, at 5:12 AM, November 30, 2006  

  • There is no sustainable theory to Quantum mechanics. It is a mish-mash of guessalogical mumbo-jumbo.

    It is this way because Newton defines POWER ..NOT ..ENERGY.

    Kinetic Energy is the product of Mass x velocity^2 ......... IN A CIRCLE and NOT STRAIGHT LINE POWER of Mv^2/2.

    The whole truth is a self driving resonant power torque of GRAVITATIONAL EVERYTHING and the idiot proof facts are at

    By Anonymous Alan Cresswell, at 8:12 AM, February 18, 2007  

  • I can resolve the EPR paradox

    By Anonymous Anonymous, at 12:20 PM, April 08, 2008  

  • I suggest that the common unease with taking quantum mechanics as a fundamental description of nature (the “measurement problem”) could derive from the use of an incorrect notion, as the unease with the Lorentz transformations before Einstein derived from the notion of observer-independent time. I suggest that this incorrect notion that generates the unease with quantum mechanics is the notion of “observer-independent state” of a system, or “observer-independent values of physical quantities.” I reformulate the problem of the “interpretation of quantum mechanics” as the problem of deriving the formalism from a set of simple physical postulates. I consider a reformulation of quantum mechanics in terms of information theory. All systems are assumed to be equivalent, there is no observer-observed distinction, and the theory describes only the information that systems have about each other; nevertheless, the theory is complete.

    By Anonymous Converter, at 10:33 AM, December 30, 2009  

  • Thank you for the great story.

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    By Anonymous Raphaël CONFIANT, at 6:07 PM, April 07, 2010  

  • Definición de energía inteligente:

    La energía en estado primigenio, y la inteligencia; son inseparables. Nada existe sin una conciencia que posibilite, un orden y proceso ilimitado por un tiempo absoluto; de la energía primigenia y su trasformación.

    La conciencia es la línea que marca los ejes y procesos, de la energía primigenia; trasformándola en las demás energías. Estos ejes y procesos no son la inteligencia única, es la ecuación simultanea, y; *dualidad circular de la conciencia de un solo Ser.

    El Ser es ambas cosas simultáneamente, único y múltiple; y lo que no es ninguna de estas dos, la sustancia energética y material proyectada por el mismo.

    Como Ser único: trasforma y proyecta su energía, dirigida por la inteligencia primigenia: consciente de sí misma ésta percibe su subdivisión. Sin dejar de existir primigeniamente, percibe aspectos y realidades limitadas y cíclicas: en el resto de seres, realidades físicas y materiales.

    Como ser múltiple, e inteligencias simultaneas: bajo la ecuación y *dualidad circular en las que se subdivide: percibe su estado total y múltiple simultáneos. Las realidades creadas por los seres múltiples, tergiversan lo ilimitado y absoluto: en lo que "no lo es".
    Se crea un eje de separación entre: lo que "es" y lo que "no es". Ver: *conciencia del Ser.

    Decir que algo no es el Ser, siendo todas las cosas; es una tergiversación y olvido ante la única y múltiple experiencia. Está existe en todos los seres, que simultáneamente experimentan; bajo la conciencia del Ser. Sin la conciencia e inteligencia, la energía no existe; no es nada. Tanto lo energético y consciente, como lo material y físico "sin conciencia": provienen de la proyección mental del Ser: todo está echo de la misma sustancia.

    El Ser es la energía que ni se crea ni destruye, se trasforma así misma para experimentar su magnificencia: al ser simultaneo en su experiencia; y las múltiples realidades de el resto de seres. El Ser es todo: los seres, "no seres"; y el mismo.

    El Universo es el movimiento proyectado por el Ser. Él es el centro en el que todo órbita, trasformándose, y manifestándose en todo; y todos los seres que se realizan. "... El Ser es como un astro que irradia, y posibilita la causa primera; de todos los efectos que marcan los procesos naturales ..."

    La condición natural de la energía es existir, y siempre ha existido. Antes del Universo tomar forma y objeto, existe una inteligencia primigenia: como consecuencia natural de la energía, de no ser así el Universo carecería de objeto.

    * Dualidad Circular: Es la unión de los opuestos: en un solo punto. (La *conciencia del Ser) Todo lo que "es" y "no es", proviene de la conciencia del ser, que en concepto es centro de todo, pero se experimenta en la dualidad circular de su conciencia.

    Entonces hay un YO, en ti mismo, que conoce todas nuestras realidades, y qué aprendiendo de ellas, las utiliza para construir únicamente la posibilidad más ventajosa para sí mismo. Él es perfecto en ese aspecto, y su realidad absoluta. A veces mentalmente elegimos la posibilidad más ventajosa, sin preocuparnos del resultado especifico en nuestra realidad, aprendemos y experimentamos “el YO superior”. Esté es el máximo grado de conciencia, o Ser, que utiliza la energía para ventaja propia; y la hace extensible a todos los demás seres que forman parte de él mismo. Si hay un Ser, que conoce TODAS éstas posibilidades; y su perfecta disposición para aplicar sus leyes simultaneas desde su propia consciencia y marcadas por el mismo, éste Ser sería Dios.

    By Anonymous Anonymous, at 4:31 PM, May 07, 2011  


    El efecto inteligente, del Ser energético: es el desarrollo de otras inteligencias. Estas inteligencias son subyacentes a él mismo, siendo un proceso de autoexperimentación mutua.
    La energía inteligente se experimenta, desde un punto de referencia exterior a sí misma; y en diversos grados. Digamos que el concepto es: plantear en una misma y única; energía primigenia e inteligente que proyecta su imagen múltiple y externa a sí misma. Esta idéntica en base, y diferenciada en los diferentes grados y trasformaciones; extrae un límite sobre su yuxtaposición.

    ¿Qué imagino que debo ser? Es el concepto que da lugar a una amplia gamma de sucesos y creaciones. *leyes universales de la conciencia. Éstas leyes son observables, como un proceso de conocimiento, aplicación y sabiduría. Como causa primera, han de ser dispuestas por alguien qué; conoce, aplica y sabe.

    El Ser inteligente tiene un concepto total de su magnificencia, pero es en la proyección en la que imagina: "qué no es, él"; lo que crea las * leyes universales de la conciencia.

    El Ser energético imagino que no debía ser inerte, y creo esa proyección en la materia. Se crean así, los seres inertes. Marcando los límites entre la conciencia, y la ausencia de inteligencia. Creándose como dualidad circular, todos los seres orgánicos, marcando los límites entre la conciencia y la ausencia de inteligencia, que circunda la propia conciencia de cada ser consciente.

    La inteligencia del universo, se experimento en todos ellos, transformándose; mediante una encadenada evolución de la energía. Esta se proyecta interna y externamente: "al mismo tiempo que el hombre busca y experimenta "él ser energético", éste; ve a través de los ojos del ser humano. El objetivo de la trasformación energética es experimentar su estado original y observar su semejanza con todo.

    By Anonymous Anonymous, at 4:32 PM, May 07, 2011  


    Todos los procesos tienen una determinada causa, y crean un encuadro observable. En las leyes materiales se encuentra una propiedad intrínseca a ésta. Al observar mediante la lógica y el raciocinio, éstas propiedades, encontramos el efecto. Todo proceso, estado o tiempo de la materia, es una proyección de la conciencia. Comenzamos entonces a observar, la causa primera de todas las cosas, y; conocemos al ser que realmente somos.

    En todos los seres vivos, ésta proyección, está determinada por la conciencia; que forma su estructura. Ésta conciencia es interna y externa en el caso del hombre. Solo externa en apariencia, en los vegetales, porque el Ser inteligente en ellos; proyecta su desarrollo. Se puede decir que de forma interna qué: los vegetales poseen, una estructura de enlace; para el proceso evolutivo de la conciencia y el resto de seres. La conciencia existe interiormente en el vegetal, aunque "dormida". Esta conciencia se trasforma en el proceso evolutivo, pasando por los animales, y finalmente despierta en su desarrollo en el hombre.

    El ser humano la desarrolla hasta el momento de su muerte física. Todo organismo viviente que muera, libera su energía: (su ser).
    Ésta energía no se destruye, se trasforma, por lo tanto es un vínculo constante con el ser inteligente del cosmos. La conciencia y la energía son inseparables, no existe lo uno sin lo otro. Un cuerpo sin energía vital y sin conciencia es sinónimo de desmaterialización y trasformación. Exceptuando el caso en el qué el cuerpo sea conservado, por algún tipo de energía; dispuesta por la conciencia de la naturaleza o de otro ser.

    La vida de un ser orgánico no es posible, si carece de energía; y ésta no es posible si carece de conciencia o su influencia. Así que todas las energías, que son trasformaciones de una única, existen por influencia de ésta. Un hombre en estado vegetativo, continua teniendo energía vital, y está esta enlazada a su conciencia. Aunque su conciencia haya abandonado el cuerpo físico, sigue unida a éste. Desde otro plano de conciencia, experimenta su unión energética con el cuerpo material: hasta no liberar por completo de forma natural, o provocada; toda la energía vital de éste. " ... Tomando un objeto cualquiera con las manos, observamos en su tacto y forma, que: la energía causante de éste somos nosotros, y es idéntica a nosotros. Aunque ésta esta trasformada por la misma cosa ... " *La energía primigenia.

    *La energía primigenia es lo que siempre existió, antes de su proyección y trasformación en el universo conocido. Conociendo el universo, podemos descubrir: todo lo que ésta energía puede llegar a ser y no ser. Finalmente descubrimos que esta energía es el Ser.

    By Anonymous Anonymous, at 4:34 PM, May 07, 2011  

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