Gambini & Pullin on measuring time in quantum physics

In summary: The paper starts with reviewing one of the old problems with reconciling General Relativity with quantum physics: General Relativity has no absolute time, no universal clocks, only relative distances between events in spacetime; but evolution in quantum physics is formulated specifically in terms of an absolute time variable, and if you try to reformulate the theory relatively you lose the ability to compute things.They focus on a specific 1983 proposal for relativizing quantum physics, which they call "Page–Wootters" (this sounds to me like a very respectable sports bar) where instead of thinking in terms of a universal t you pick some specific physical quantity which you define as your "clock"; you then formulate all other observables in terms of "
  • #36
Thomas Larsson said:
the observer's physical properties can not be ignored.

If you are talking about extremely obese observers, this possibly makes sense. But for us normal-size people this looks like an irrelevant speculation about "100th decimal position".
 
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  • #37
I now read both the Undecidability paper and the montevideo paper.

I found the papers to not contains the details I was hoping for. Perhaps the other related papers contain more, or more is on it's way.

If I interpret them right then...

- What I like with their reasoning is that that they tend to relax the determinisim in QM, and that the unitary evolution is more like an expectation; this is exactly how I like to see it as well.

- Their view also contains a new view of physical law, where laws correspond to observers expectations, rather than hard forcing constraints acting from a gods perspective.

However they do not present a reconstruction of the interactions in a deeper informational way - which is what I personally expect to come along with this. I also expect an informational description of the evolutionary process.

A key problem they attacki is the problem of wether an observer can decide wether ANOTHER observers has performed a measurement or not. To me it's clear that this can never be decided 100%. In that respect I agree. The question I ask, is how to act, given that you don't know. IE. what is the logic of guessing, and playing this undecidable game?

I was hoping for more development there but it seems they are not yet there?

Or is this in another paper?

/Fredrik
 
  • #38
Meopemuk,

The purpose of a Gedanken experiment is not that it be realistic (then you could do the real experiment instead), but to explore the limits of existing theories, and find the cracks where a better theory is needed. That is why I find it exciting that QFT assumes incompatible limits for the observer's heavy and inert masses.

Note also that major progress in the past has been connected to a more observer-dependent view of nature:
* SR tells us that the separation of spacetime into space and time is observer-dependent.
* QM tells us that position and momentum cannot both be observed sharply.

I suspect that QG requires us to take observer dependence to its logical conclusion: that the observer has quantum dynamics and affects the system just as much as the system affects the observer. After all, that is how all physical observers behave. To turn this philosophy into a concrete formalism will of course take a lot of effort. My attempts in this direction can be found on hep-th.
 
  • #39
I'll jump right to a particular point here

Thomas Larsson said:
I suspect that QG requires us to take observer dependence to its logical conclusion: that the observer has quantum dynamics and affects the system just as much as the system affects the observer. After all, that is how all physical observers behave. To turn this philosophy into a concrete formalism will of course take a lot of effort.

I think I fully share this vision. But from various readings on this, I have distinguished at leat a couple of quite different ways to handle observer dependence. I'm curious on Thomas view here:

1) Either you invent a theory, that represents a birds view, that "explains" how different observer views relate, and how different observers/systems interact. This is a form of realism still, a sort of realist view of physical law, that still acknowledges the observer dependence.

In this view, physical law, is a fact of nature, in a realist sense. Ie. the rules for relating different systems, are system independent. This is a mixed relational view, with a realist view of physical law.

Those who think like this considers some mathematical reality that exists independent of observation; but in a way that does explain how different real observations related. And this mathematical reality is accumulating truths about nature, as we learn about it.

2) Or you take the observers independence to also apply to the knowledge, not only of physical degrees of freedom such as position, momentum, charge etc in your environment, but also to physical law, or the regularity that is de facto realized by nature. In this view, the entire notion of physical law, becomes not elements of mathematical reality or some realist birds view, but is itself constrained by the relational complexity and capacity of physical systems.

I adhere to this second view, and from what I can see case 1 is more common. Smolin seems to be one of a few that seriously considers 2.

Current physics as in standard models etc, are definitely (1). The question is if the solutions to current fundamental problems are possible without leaving the view of law represented by (1)?

I don't think so.

This is why I was originally attracted to Rovellis reasoning. But the more I read the more did I see that he is stuck in (1).

As I see it, smoling is not, he is open for (2).

No doubt (2) is more strange, and weird, but also more consistent from the point of view of reasoning, and the philosophy of science.

This is why I think that the observer dependence is much deeper than the HUP applied to the observer.

If this even makes sense to anyone, I'm curious to hear Thomas comment on my decomposition into two principal ways here?

/Fredrik
 

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