The consistent histories approach to quantum cosmology

[lowgrav]

I'm currently reading "Three roads to Quantum Gravity" by Lee Smolin. In the book he describes attending a presentation by Fay Dowker and James Hartle about the consistent histories approach to quantum cosmology. He describes that, "there were worlds that were classical now that were arbitrarily mixed up superpositions of classical at any point in the past", and that they concluded that, "if the consistent-histories interpretation is correct, we have no right to deduce from the existence of fossils right now that dinosaurs roamed the planet a hundred million years ago". This really confused me. I don't understand the how the quote about dinosaurs relates to the topic in the way he describes it. Why do we have no right to deduce that from fossils because of this theory? I am by no means a very intelligent person. I recently started trying to teach myself more. I was just wondering if someone could explain this to me in a relatively simple way. Thanks.

 

[Fredrik]

I don't have a good answer for you. I read that same comment years ago, and thought about finding out what it means. I got as far as finding out that it has something to do with gr-qc/9412067 by Dowker and Kent. I downloaded it then, but I still haven't studied it. It doesn't mention fossils or dinosaurs, but it says this:

Similarly, if Omnès' criterion for true propositions is adopted — and no other inferential rule has been suggested in the literature to date — the formalism almost certainly allows no ordinary quasiclassical deductions to be made unambiguously. It seems clear, practically speaking, that we cannot deduce, from the tides, from our perception of moonlight, or from any quasiclassical event on earth, that the moon is in a quasiclassical orbit. And, as we have argued, although it is hard to find a rigorous demonstration, this does also seem likely to be a problem in principle: a quasiclassical description of events on Earth should be consistent with an infinite number of pictures, in nearly all of which the moon does not behave quasiclassically. Since the most natural collection of actual facts appears to be our own experiences, and since the same inferential problem then arises for statements about our fellow creatures, the formalism also appears to lead to personal solipsism.

 

[Naty1]

This is a bit too philosophical for me, but I took that statement as a none too precise analogy during a discussion that was quite possibly a smidgen over my head... that

... the world contains many different equally consistent histories, each of which can be brought into being by the right set of questions.

pg 45...

Loosely, don't go looking for dinosaur fossils and you won't find any.

On the other hand, note that on that pgs 44-45 he discusses how different prominent people interpreted Dowker's work differently. They don't agree on exactly what the work means so why should we understand it...or the analogy he provides.

I made a note in my copy when reading that section:

"This sounds like Feynman's sum over path histories.."

[which I am positive I don't fully understand either!]

Wikipedia:

The path integral formulation of quantum mechanics is a description of quantum theory which generalizes the action principle of classical mechanics. It replaces the classical notion of a single, unique trajectory for a system with a sum, or functional integral, over an infinity of possible trajectories to compute a quantum amplitude.

instead of 'trajectory'...insert the word 'history'...or 'actual path'...in other words, which quantum state, or state, 'really' was there?? Different questions yield different results. They were all there!

Fredrik: thanks for the link to the paper...I'll have to give it a try!

PS: It's a book I liked..I have it highlighted to the very end...so don't give up...!

 

[Naty1]

As I feared, it has some really weird, apparently not yet well understood stuff...and is way too long [87 pages] for someone my age...From the abstract:

...We examine critically Gell-Mann and Hartle's interpretation of the formalism, and in particular their discussions of communication, prediction and retrodiction, and conclude that their explanation of the apparent persistence of quasiclassicality relies on assumptions about an as yet unknown theory of experience. Our overall conclusion is that the consistent histories approach illustrates the need to supplement quantum mechanics by some selection principle in order to produce a fundamental theory capable of unconditional predictions.

I don't feel a quick review enables me to appropriately summarize their wide ranging conclusions section...but they do mention the path integral formalism...and the double slit experiment...and that section is only about four pages...in which they mention fundamental open issues in quantum mechanics mathematics...

But this quote from another author address the authors concerns:

“. . . it must be granted that several of the interpretative comments the quoted authors
make of their theories stand quite at odds with the main conclusions reached here. Indeed,
while these authors do not actually say their theories are realistically interpretable, they
somehow give at various places the impression that they mean just precisely that. Such
a somewhat disquieting state of affairs seems to indicate that we physicists still have
efforts to make before we succeed in imparting to the words we use (and especially to the
nonoperationally defined ones) a strictness of meaning comparable with the strictness of
our mathematical manipulations."

We have many such examples of 'interpretation discussions' about quantum mechanics...what does it mean...in these forums.

 

[AlexCaledin]

...we have no right to deduce from the existence of fossils right now that dinosaurs roamed the planet a hundred million years ago ...Why do we have no right to deduce that...?

Why, of course we have no right to deduce that - because there was no one to observe those supposed dinosaurs; there were only quantum amplitudes for their existence. According to the most standard QM, one should think like this: when the very first observation occurred, things became real, including dinosaur fossils; before that, there was nothing material except some primordial "wavefunction" of uncertain temporal length...

 

[Nugatory]

This thread has been quiescent for almost three years, so it's unlikely that lowgrav is still around to read your reply.