I Beg To Differ

At some point in my undergraduate career I read a debate between Einstein and the philosopher Hans Reichenbach. Reichenbach argued that truth seekers of good faith couldn't agree to disagree if they were careful to define their terms. Einstein wasn't buying and I remember thinking that Reichenbach's argument was preposterous. Four plus decades have perhaps muddled my memories, but I think HR's point was that logical opposites could not coexist.

The problem is that our ideas about reality are not based in some commonly agreed axiom system but on our individual and only partially articulable conceptions of the nature of the world. We tend to disagree about both foundational issues and the criteria for consideration of evidence that bears on such questions. The theory of relativity was thought to present a profound challenge to philosophy because it challenges our common sense notions of the nature of space and time. Quantum mechanics turns out to be even more difficult, challenging our common sense notions of the real and the unreal.

The string theorist Leonard Susskind has proposed something he calls black hole complementarity to resolve certain problems in quantum gravity. Consider two narrative versions of what happens to a spaceship falling into a very large black hole. If we apply the laws of general relativity and quantum mechanics (in the semi-classical approximation) to the spaceship, we may calculate that the passengers perceive falling through the horizon as uneventful. The outside observer, on the other hand, sees something much different - as the spaceship approaches the event horizon, it encounters a hellish blast of Hawking radiation which tears the ship to pieces before it reaches that horizon. So which narrative should we believe? Well, there is one narrative for those on the ship, and one for those on the outside, and never shall the twain meet, for their world lines have become causally disconnected.

In one sense, this is just another, slightly more pointed version of the famous double slit experiment.

[One reason this post looks half baked is because it is. It was a partial draft that got published by mistake. I will try to finish it when I get time.]