Let us consider the following scenario:
"B" is a brain in a vat. "Ann" is the mind associated with this brain. Ann perceives a 3D, virtual world that we can call "V." V is a phenomenal world that only she can "access" (i.e., that only she can perceive).(1)
Now let us consider the following two statements:
S1: The brain B is physically real; however, the world that Ann experiences (i.e., V) is not physically real.(2)
S2: The brain B is not physically real; however, the world that Ann experiences (i.e., V) is physically real.(3)
At first sight, statement S2 seems false. However, one can make the argument that S2 is true.
Quantum theory regards elementary particles as mathematical points (i.e., as entities that have no extension in space).(4) If we take this model of elementary particles seriously, we have to conclude that these particles are unreal. (As we know, physical entities that lack spatial extension are not real.)
A collection of point-particles is, by definition, unreal. Since Ann's brain B is a collection of point-particles, B is unreal.
In addition, we must remember that the brain is a quantum entity represented by a probability wave.(5) As we know, quantum entities are never entirely "real."(6) Since Ann's brain B is a quantum object, B is never fully real.
Since spacetime is a quantum entity, spacetime is physically unreal.(7)
In her world V, Ann can experience backward time travel. Ann can change past events in V in any way that she likes. No logical inconsistencies arise when she changes historical events because V is a subjective world. (Note: Only she can "access" and experience events that occur in V.)(8)
1. V is the subjective, phenomenal world that Ann perceives. She experiences herself as living inside and moving about in this virtual world.
2. In this commentary, I use the word "real" to mean: "having a concrete, PHYSICAL existence."
3. We can distinguish between the objective world (i.e., spacetime) and the subjective world (i.e., 3D, phenomenal space.)
According to John Smythies, the phenomenal world (PW) is more than a "mental image." Smythies claims that PW is a physical world (http://www.imprint.co.uk/pdf/smythies.pdf).
4. Some physicists may claim that elementary particles have spatial extension.
5. All physical objects (both microscopic and macroscopic) are quantum entities.
6. Quantum objects are not "real" in the sense that classical objects are real. Classical objects possess definite values for all their attributes. Quantum objects always possess some indefinite attributes.
7. In his book Other Worlds, Paul Davies questions the reality of the unobserved, objective world.
In the book, Davies considers both the "realist" and "Copenhagen" interpretations of superspace (http://deoxy.org/superspace.htm).
8. Note: The mind cannot directly "access" or perceive spacetime. The mind can only perceive its own phenomenal experiences.