What John Baez said at Edge 12 about the problem of time
http://www.edge.org/documents/archive/edge12.html
----quote from Baez at Edge 12, reviewing a 1997 paper by Kauffman and Smolin---
I won't try to define a "background-independent theory" here. It's easier to start with the main example of such a theory, namely general relativity. Normally we imagine running an experiment and timing it with a clock on the wall that ticks along merrily regardless of what's going on in the experiment. In general relativity it's known that this is not strictly true. This has to do with how gravity affects spacetime. Very roughly speaking, if you move an object, you change the gravitational field in the room and inevitably affect the rate of ticking of the clock. So if you run an experiment and ask "what did the voltage meter read when the clock said it was 5:30 pm?" you have to recognize that the experimental setup has affected not only the oscilloscope but also the clock. To put it rather floridly, you can't treat the spacetime measured by clocks and rulers as some sort of fixed grid upon which events play out while remainingly loftily unaffected by these events; instead, it interacts with them.
For most purposes these effects are small enough that we can either ignore them or treat them as small corrections. The fun starts when we try to figure out how to do physics while taking them seriously. In one approach, one decides to treat everything relationally. In particular, one treats clocks as just another part of the physical world with no privileged status: instead of asking what the voltage meter reads when the clock says it's 5:30, one could equally well ask "what did the clock say when the voltage meter read 217 V?".
Taking this viewpoint to the extreme, one can argue that the laws of physics don't describe how things change "as time passes": instead, they just express correlations between various observed quantities, like meter readings. This is what's meant by the "disappearance of time" in background-independent theories.
Julian Barbour is a strong proponent of this sort of purely relational view — though I'm surely oversimplifying his thoughts. Kauffman and Smolin seem to want a way out of this view. I'm not sure how clearly it comes through in their paper, but in conversations Smolin has made it clear that he wants to keep some notion of time in order to preserve the concept of NOVELTY. In the purely relational view, there is no fundamental notion of the passage of time; there is simply a fixed set of ways the world can be, and laws describing correlations, like: "if X holds, then Y holds with probability P". As Barbour noted, Smolin finds this idea "scary". Personally I don't find it scary, and I am also rather suspicious about pursuing a course of research to avoid some conclusion one finds scary — though I have no quarrel with striving to reach a conclusion one finds attractive.
Anyway, Kauffman and Smolin suggest the following way out: perhaps it doesn't really matter if there is a fixed set of ways the world can be, because we cannot tell what this set is! Here they are taking advantage of the work of Gödel, Turing and others. These folks showed that there are lots of mathematical...
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It doesn't really matter what Kauffman and Smolin said about the problem of time back in 1997. many resolutions have been tried over decades and theirs is just one attempt in a long struggle which is still going on. So I will stop quoting here. What I like is how Baez describes the problem, not what he says about this one attempt to solve it.
[edit: hey selfAdjoint! glad you're commenting too]
