http://www.swcp.com/~hswift/swc/Summer99/goswami9901.htm
The interpretational difficulties of quantum mechanics can be solved with the hypothesis (von Neumann, 1955; Wigner, 1962) that consciousness collapses the quantum wave function. The paradoxes raised against this hypothesis have now all been satisfactorily solved (Bass, 1971; Blood, 1993; Goswami, 1989, 1993; Stapp, 1993). There is, however, one question that continues to be raised: Is consciousness absolutely necessary for interpreting quantum mechanics? Can we find other alternatives to collapse and consciousness as the collapser?
Some of these alternatives propose to modify quantum mechanics in a major way (for example, nonlinear theories); others are not philosophically satisfactory (for example, decoherence theories); still others invoke other questionable physical theories in order to make sense of quantum mechanics (Cramers, 19; Penrose, 1994). But there are two theories, one due to David Bohm (19), and the other called the many worlds theory (Everett, 1957), that still attract a lot of adherents.
In this short paper, we will argue that Bohm's theory is better interpreted with collapse of the wave function (and therefore, consciousness brought into the arena). As for the many worlds theory, even the latest versions of this theory requires special treatment of the conscious observer in order to make sense, and is thus a dualist theory (readers can verify this following the same general argument as Squires (1987)). Some final comments are also made about the implication of this reinterpretation for Bohm's philosophy of implicate and explicate order.
The Reinterpretation of Bohm's Causal Interpretation
Bohm's basic idea is to represent the situation of quantum mechanics with a wave piloting a particle, an idea he took from de Broglie (19). However, whereas de Broglie envisioned quantum objects as a physical wave piloting a physical particle, Bohm's waves are not physical waves; instead, they satisfy the Schroedinger equation. In other words, they are wave of possibility given by the quantum wave function. By writing the wave function as a product of two quantities -- the amplitude (whose square gives us the probability of finding the object in a given region of space) and the phase, Bohm does recover Newton's equation for a particle with coordinate x (and velocity v) evolving in a trajectory. The trajectory then is claimed to represent the real world of Newtonian vintage. The wave guides the trajectory through the so-called quantum potential in addition to whatever other force-field the object is under. This quantum potential is non-local and the effect of it continues even in empty space, so, for example, Newton's first law that objects travel in straight lines in the absence of any external forces no longer applies.
In the case of the double-slit experiment, for example, Bohm's particle equation can show us curved trajectories of how a particle may be able to go through one slit and still arrive at classically forbidden places on a fluorescent plate. How does the particle know that the other slit is open and veer itself to the quantum mechanically allowed places? Through the nonlocal influence of the quantum potential, which acts as a source of "active information."
Because the particle equation has been derived from the Schroedinger equation with only a little bit of redefinition of momentum (so that both momentum and position of the particle are simultaneously definable), it is assumed that Bohm's theory is equivalent to quantum mechanics (although there are some subtle differences). Bohm and his collaborators think that this is a causal interpretation of quantum mechanics because a classical trajectory has been calculated. But this thinking is fallacious.
The classical equation in Bohm's theory is not, strictly speaking, a space-time equation because the quantum potential depends on the wave function, which has no space-time existence until it is collapsed. Thus the causal discontinuity of quantum mechanics still remains because without wave function collapse, without knowing where the particle ends up, Bohm's method cannot be applied to calculate the particle trajectory.
Through sheer sophistry, Bohm and his collaborators avoid dealing with the fundamental problem of quantum measurement: why only one of the possibilities become actual in a measurement while all others do not. As Henry Stapp (1989) has already pointed out, the measurement problem is "bypassed" by assuming that the quantum potential forces the particle into one channel, although the other channels of the wave function remain empty. Stapp also points out that only the probability is testable even in Bohm's theory (as in quantum mechanics), not the quantum potential. In Stapp's opinion, a theory such as Bohm's that does not add anything tangible to quantum mechanics, but only adds extra elements on the basis of classical intuition, is not worth much investigation.
However, over the years, Bohm's theory has enjoyed a certain popularity and should not be dismissed off hand. Like Stapp, we believe that the measurement problem is not solved by Bohm's interpretation of his mathematics, but suppose we interpret Bohm's equations without any classical prejudice. What then?
Suppose we agree, as logic dictates, that Bohm's calculations are pertinent only in the aftermath of the wave function collapse, only when we know where the particle has ended up in a given measurement. Bohm's method then enables us to calculate the entire trajectory leading to the point of collapse. Thus the collapse can be seen to entail not only the possibility wave collapsing to a particle at the point of collapse, but the collapse of the entire trajectory going backwards in time.
Notice that, in this view, discontinuity of collapse remains: out of all the quantum possibilities a unique actuality is discontinuously chosen (by our observation and in our experience). But now we can go back in time and reconstruct the pathway of events in space-time leading to the event of collapse. We cannot relive these past events in the present moment, but there may be (fossil) remnants of these events now that may enable us to verify the validity of such a reconstruction.
Bohm has always emphasized how beautiful his theory is for understanding and appreciating quantum nonlocality as the action of the quantum potential. Yet Stapp's criticism cannot be denied: the quantum potential is not an observable. But under the action of the quantum potential, the calculated trajectories of Bohm's theory have unusual characteristics. Could these unusual characteristics be observable?
One such unusual characteristic is faster-than-light propagation. In the phenomenon of quantum tunneling, the time taken by a quantum object while going through the tunnel can be measured, and such measurements are now revealing a compelling case of faster-than-light propagation (Chiao). This, in our opinion, proves the usefulness of Bohm's theory.
Is such faster-than-light propagation against the theory of relativity? Hardly. We still cannot directly observe the object in its faster-than-light condition, any attempt to observe will destroy the tunnel. Speed-of-light limit applies to trajectories that are directly observable; the trajectories of objects in tunneling a' la Bohm are unobservable, so no challenge of relativity theory is necessary.
