David Deutsch has drawn a line in the sand.
Well, actually, he has drawn a line between worlds.
It's not a secret that I'm a fan of the "
Many Worlds" interpretation of the "
Multiverse."
(I am also considering the "
simulation-argument" in the context of a multiverse scenario.)
Here's an extremely over-simplified explanation of the issues involved:
As
technology penetrates the material world at smaller and smaller scales, we discover the need for a 'realist' interpretation of quantum theory, to explain the nature of the real world that quantum theory corresponds to. It is no longer acceptable to merely view the quantum theory as a tool used to make accurate predictions.
As a result, the interpretation of quantum theory -- the explanation behind the predictions made by the theory -- requires taking the picture of the world that emerges from the theory seriously.
Three primary contenders for the "realistic interpretation" of quantum theory are:
[A]
Pilot wave interpretations, like
David Bohm's theory (and the original de Broglie interpretation) -- and recent speculative extensions, like
Anthony Valentini's research[B] The
Many Worlds interpretations, which originate with Everett's relative state interpretation
[C] Dynamical "collapse" theories, like
Sir Roger Penrose's idea of "objective reduction" (OR)
The pilot wave picture imagines real particles on real trajectories, which are guided by a new kind of information wave, that Bohm called the quantum potential. Bohm's theory requires two fundamental physical elements: real particles, and a real quantum information pilot wave.
The Many World's interpretation claims that
the interference seen in experiments, and predicted by the quantum formalism, is caused by "shadow particles" that exist in other universes (In Bohm's theory it is the pilot wave guiding the particle that results in the interference patterns seen in quantum experiments.)
Dynamical collapse theories invoke other effects at the boundary of our understanding of how quantum theory and Einstein's General Relativity behave at scales where both effects are important. For example, where a quantum experiment might result in Yes OR No, Penrose's objective reduction claims that nature makes a choice: subtle differences in the curvature of spacetime due to energy differences would force nature to make a choice of one OR the other. (In Bohm's theory the location of the particle determines one OR two, and the particle path
is guided by the pilot wave. In Many Worlds theory the world splits, as does the observer doing the experiment: thus, in one world the observer sees "yes," and in the other world he sees "no." What connects the two observers, now in different parallel universes, is their common history.)
If we ignore the dynamical collapse theories, we are left comparing the pilot wave theories with the Many Worlds.
And here is where things get interesting, because of two predictions of possible effects that distinguish the theories.
Anthony Valentini has shown that pilot wave theories exist that violate the fundamental rule of quantum probability, which is called the Born rule. Matter that violated the Born rule would allow superluminal communication: signals transmitted and received faster than the speed of light in a vacuum. The speed of light determines causality: In other words, the light speed limit is what allows for an event A to occur before an event B, which is fundamental to the everyday idea of cause and effect.
Many Worlds theory allows for self-consistent time machines: paths between different universes, because "
other times are special cases of different universes." In the Many Worlds theory, if you build a time machine today, and your grandson pops out of the machine from fifty years into the future -- and then destroys the machine, in order to prevent himself from entering the machine in the first place -- there is no paradox involved with his appearance in our world.
When the machine was built, it was connected to an alternative future, one of many worlds of possible futures where your grandson entered the machine. When he appeared in the present moment, and destroyed the machine, he broke that connection. He is still trapped in the present
moment, some fifty years in the past of the world that he left behind.
From the point of view of that alternative future world, your grandson entered the time machine, and disappeared. The time machine continues to exist in that alternative world, but the connection to the past is with the world that branched into universes where your grandson decided not to destroy the time machine.
So which of these ideas is correct?
There is no physical evidence for matter that violates the Born rule and allows for superluminal communication. There is no physical evidence for time machines.
I wrote to David Deutsch and asked if there was common ground between the realist interpretations, since he is on the record as stating his opinion that Bohm's theory is "Many Worlds in denial."
Deutsch responded, "With or without such loops, distributions deviating from the Born Rule are not compatible with Everett [many worlds] quantum theory. If such distributions were
observed,
they would refute Everett and strongly suggest that something like the Bohm theory can be made to work."
Sounds like David is reconsidering his original position that Bohmians are "Many Worlders in
Denial"?
I have requested further clarification from Dr. Deutsch.
Stay tuned!
