What is back-reaction?

The following is a posting to the QUANTUM-D list from Rhett Savage entitled "What is back-reaction?" (February 19th, 1996)

This message presents portions of a conversation had by Stanley Jeffers and Jack Sarfatti, with my comments strewn throughout... Also discussed is a paper of Henry Stapp's. This is a follow up to Sarfatti's posting "Is Consciousness a Violation of Quantum Mechanics?" which is at:


Sarfatti wrote that people such as Josephson, Stapp, Penrose and others have suggested changes in quantum theory which allow for the possibility of "intent" or the like to bias quantum outcomes, but that all these authors operate using the Copenhagen picture in which there really is a "collapse" of the wavefunction. Jack advocates a Bohmian picture in which both wave and particle are always real and there is no collapse. So how does mind enter the world? It must have been here from the start. Jack describes an explicit dualism in which both mind and matter exist...

"In accord with Chalmer's idea, I posit that the wavefunction is intrinsically 'mental' capable of qualia."

...and he suggests equating the guiding wave in Bohmian mechanics with the mental aspect of the universe, generally: the particles are "matter," "mind" the pilot-wave.

That might be uninteresting except for the next step: the "mental" aspect of the universe can be upgraded to life and consciousness by self-organization. This happens when a physical system uses its own nonlocality in its organization. In this case a feedback loop is created, as follows: the system configures itself so as to set up its own Bohmian pilot wave, which in turn directly affects its physical configuration which then affects its nonlocal pilot wave which affects the configuration, etc...

Normally in quantum mechanics this "back-action" is not taken into account. The wave guides the particles but the back-action of the particle onto the wave is not systematically calculated - of course, the back-reaction is physically real: the movement of the particle determines the initial conditions of the next round of calculation. But there is no systematic way to characterize such feedback. One reason that this works in practice is that for systems that are not selforganizing the back-action may not exert any systematic effect.

This is an interesting way to utilize nonlocality despite Eberhard's proof that point-to-point signaling by the quantum connection is not in the cards! (If a physical system occupied a dynamical stability based on such a feedback loop then it would be a "nonlocal" physical system, without superluminal signals.)

Questions of consciousness aside, consideration of "back-action" as a dynamical fact nourishes a suspicion that linear quantum theory is fundamentally an approximation...

On 1/28, Stanley Jeffers wrote:

> I would like to offer the following comments on Jack Sarfatti's
> recent post (Jan 25,1996) of his abstract "Is Consciousness a
> Violation of Quantum Mechanics?"
> > > Bohm showed that the Schroedinger equation and the Born
> > probability interpretation of orthodox quantum mechanichs
> > depend upon the approximation that there is a new kind of
> > "organic" or "wholistic" non-local and context-dependant
> > "quantum force" that the wave function exerts on matter
> > in addition to the electro-weak, strong and gravitational
> > forces...
> There is no such approximation involved in Bohm's analysis. Starting
> with a real wave given by psi=R exp(iS/h), the Schroedinger equation
> separates into two equations, one of which is a continuity equation in
> R squared. The other looks, for all the world, like a Hamilton-Jacobi
> equation but includes a quantum potential term which is independent of
> the amplitude of psi and distance. The analysis is exact and does not
> depend on any approximations.

In an ensuing discussion Jack vigorously clarified his intent...

Sarfatti: "The Schrodinger equation implies a sourceless continuity equation. That is the absence of backaction. I can cite the specific places where Bohm introduces the idea. Backaction would be a source term on the right-hand side of the continuity equation which depends on the actual trajectory of the physical system."

Jeffers: "I repeat that there are no approximations involved in Bohm's analysis of the Schroedinger equation.

The exact analysis is given as appendix A in Chapter 3 of Peter Holland's excellent book, The Quantum Theory of Motion."

Sarfatti:"Well you asked for it. First look at section 9 in Bohm's first hv paper in the Wheeler Zurek book on Quantum Measurement (Princeton).

Second, look at the way Bohm uses "inhomogeneities" in section 4 of that same paper. That term is same as what I mean by "back-action".

Third, p 30 of Undivided Universe asserts "the Schrodinger equation for the quantum field does not have sources, nor does it have any other way by which the field could be directly affected by the condition of the particles....."

In other words the Schrodinger equation is based upon the assumption of zero back-action! Stanley is dead wrong and should apologize if he is a gentleman."

Being a gentleman among other things, Stanley did apologize (after a fashion)...

Jeffers: "I now have a clearer sense of Jack's point. His initial claim was not that Bohm made any approximations in his analysis but that something is missing from the Schroedinger equation which could represent an interaction between particle and wave..."

Sarfatti: "Yes, that is correct."

Jeffers: "...(And yet) there are compelling reasons to assign zero to this 'missing' term since it is strongly contradicted by the experimental evidence. To wit, if there were an extra term it could not simply be a constant but would be a function of space and time. Then on substituting Bohm's psi, one would get an additional term appearing in the quantum potential. However, without this term the pattern of trajectories computed for particles reproduces the intensity distribution in the double slit experiment. The addition of any other term would ruin this agreement with experimental data. Therefore this 'additional' term is zero.

If I initially misunderstood Jack's position, herewith an apology."

Jack's response:

"The case that you make for the absence of a new term is correct for nonliving but not for living matter.

The whole point for living matter is that it violates the statistical predictions of orthodox quantum mechanics. This idea was first suggested by Brian Josephson and it is a feature of Henry Stapp's model in Phys Rev A July 1994 p.18.

Again - the answer is that the Schrodinger equation works for DEAD MATTER not for LIVE MATTER. Backaction is the essential signature of LIFE - all forms.

The Schrodinger equation only works for isolated systems between measurements (using Bohr's picture). Therefore, it is not applicable to any living system collective mode which is an open dissipative structure. The Schrodinger equation works OK for low level reduced density matrices in living matter but NOT for high complexity collective modes.

The external pumping is essential - living systems continually measure themselves. Nonunitarity is essential for life. The backaction is the nonunitary mechanism."

I think that Jack Sarfatti's original post attracted Stanley Jeffers' attention because Jack asserted that

"Bohm showed that... orthodox quantum mechanics depend upon the approximation that there is a new kind of 'organic' or 'wholistic' nonlocal and context-dependent 'quantum force' that the wave function exerts on matter."

That *is* a funny expression! Doesn't Jack mean that Bohm showed that if one translates the orthodox interpretation's mathematics into the quantum potential language (which was done precisely by Bohm, just as Stanley Jeffers said) then one can spot a formal characteristic of this "new kind" of potential: that it ignores feedback from movement of the sources? Physically one would not expect the feedback to be irrelevant. One might then say that Bohm "showed" that the orthodox interpretation is based on an approximation. (If valid, such a discovery would be a great accomplishment of Bohm's view!)

There is much that i find valuable in the suggestion of back-action, though i am still wondering if it is really a well-defined idea which will prove useful in developing new physics...

I read Henry Stapp's paper in Physical Review, curious to compare it with Jack Sarfatti's ideas. Stapp's purposes are different. In this paper, "Theoretical model of a purported empirical violation of the predictions of quantum theory," Stapp describes a model which is like quantum mechanics in many respects, but which differs in its capacity to accommodate certain causally anomalous phenomena (that have been claimed to have been empirically observed). He is thus giving a sort of "possibility proof" to show that certain kinds of nonlinear as well as atemporal coupling between observer and physical system would not pose an insuperable challenge for physical theory.

Stapp introduces a variation of a recent (1989) model of Weinberg's, in which the canonical equations are generalized leading to a theory similar to quantum mechanics but able to produce biased probabilities.

The anomalies he seeks to accommodate include subjects successfully influencing 'by intent' random events which had happened in the past and been recorded but never observed by humans.

To deal with this, one seemingly needs a theory which is 1) non- linear, 2) atemporal and 3) mind infused. The Weinberg model gives #1. The rest Stapp handles by working explicitly in a von Neumann, Pauli, Heisenberg picture, where reduction of the wave packet "is physically associated with the mental process of the observer."

This is very interesting:

"According to the interpretation of quantum theory adopted here, the mechanical recording of the detection of products of radioactive decay generates a separation of the physical world into a collection of superposed 'channels' or 'branches': the physical world, as represented by the wave function of the universe, divides into a superposition of channels, one for each of the possible recorded (but unobserved) results. Contrary to common sense the recorded but unobserved numbers remain in a state of superposed 'potentia,' to use the word of Heisenberg. Later when the human observer looks at the device, the state of his brain will separate into a super- position of channels corresponding to the various alternative macroscopic possibilities... Finally, when the *psychological* event of observation occurs, the state of the universe will be reduced by a projection onto those brain states that are singled out by the conscious experience of the observer."

With such a picture, the biasing of his model allows Stapp to give a logically coherent model of his anomalies: the biasing happens in the brain, based on the consciousness, and reaches out into the world by entanglement.

Consider how mind is embedded in Stapp's picture of the collapse of the wavefunction, and that he very explicitly makes use of this to construct his model of a generalized relationship between observer and world - in Bohm's picture where there is no collapse, how does one introduce mind into physics?

How does Bohm's theory handle the situations for which a wavefunction reduction model has seemed useful? For example, Penrose and Hameroff have recently discussed "autoreduction" among networks of cytoskeletal microtubules and their components as a fundamental process underlying consciousness - is there a natural way to look at this in the Bohmian picture (back-reacting or otherwise)?

Generally, why do we require and how can we find a place for mind in the Bohmian view? How does Bohm's theory differ in this respect from classical mechanics, for example?

Stanley Jeffers told Jack Sarfatti that his professed dualism had more in common with Bohr's thought than with Bohm's. Jack disagreed, maintaining that Bohm was a dualist with two kinds of entities, ie. material particles and guiding waves.

Stanley replied that Jack was misusing the term "dualism."


If as Bohm asserts the objective wave function and the material particle are both out there in the real world, where is the dualism?

Herewit a quote from Bohm, (p106, Dialogues with Scientists and Sages by Renee Weber, Routledge, Kegan and Paul, 1986)

"It has been commonly accepted, especially in the West, that the mental and physical are quite different but somehow are related but the theory of their relationship has never been satisfactorily developed. I suggest that they are not actually separated; that the mental and physical are two aspects, like the form and content of something which is only separable in thought, not in reality."

There is no dualism here.

Regardless, the technical questions about the physics of the brain, and of the mind, remain...


Stapp, H.P., "Theoretical model of a purported empirical violation of the predictions of quantum theory," Physical Review A (1994)

More detail about Jack Sarfatti's ideas can be found at: http://www.hia.com/hia/pcr/qmotion.html

This posting is a follow-up to:

1. http://www.teleport.com/~rhett/quantum-d/posts/sarf_1-25-96.html
2. http://www.teleport.com/~rhett/quantum-d/posts/jeffers_1-28-96.html

...and a hitherto unpublished email correspondence.

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