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Dear Emily Adnam

I appreciate your skeptical view of the nature of evidence, and how we must be cautious in this regard. However I do believe you are giving much too much credence to the Boltzmann Brain argument.

You talk about the histories stored in our memories, but there is much more to history than that: it is for example also stored in the geological record, as well as in the...

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Dear Emily Adnam

I appreciate your skeptical view of the nature of evidence, and how we must be cautious in this regard. However I do believe you are giving much too much credence to the Boltzmann Brain argument.

You talk about the histories stored in our memories, but there is much more to history than that: it is for example also stored in the geological record, as well as in the radiation reaching us from the end of the hot big bang era nearly 14 billion years ago. You state "it is vastly more likely that the essay you are currently reading was produced by a random convergence of molecules than that at some time in the past another conscious person existed and wrote the essay." Do you really believe that? This depends on a whole series of extremely unlikely assumptions: inter alia equilibrium that lasted for almost an eternity of time, when there is no evidence this ever happened; the assumption that complex structures such as the brain can emerge from random fluctuations, when we've never even seen an amoeba emerge in this way, let alone a fly or a frog. It's a fun speculation, but I can't see how one can take it as very likely. The most that we can realistically expect to emerge from fluctuations are particles, or conceivably atoms.

I fully agree with your criticism of the Everett interpretation, but then you kind of recant from that criticism. Why not assume there is indeed a collapse mechanism, we just don't know how it works? You say "the Everett interpretation might still, after all, be true." Well yes, but it might be false; the collapse hypothesis might be true. I can't see why that statement carries the day.

It is good to have the reminder "transcendental considerations are frequently

an important part of our scientific practice, but also they have their limits: even when we choose to rule out certain possibilities for practical reasons, we should view our avoidance of them not as rigid prohibitions but as useful heuristic guides for the present." Wise advice. This kind of meta analysis is very welcome: it is so often missing in physics writing.

George Ellis

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Dear Emily,

You write exceptionally well. You give a balanced and mature analysis that reveals a strong grasp of the issues you address, without being carried away by any particular argument. I have a few thoughts for you to consider.

1. Of course you are correct that classical microdynamics is time-symmetric, but we know beyond reasonable doubt that classical statistical...

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Dear Emily,

You write exceptionally well. You give a balanced and mature analysis that reveals a strong grasp of the issues you address, without being carried away by any particular argument. I have a few thoughts for you to consider.

1. Of course you are correct that classical microdynamics is time-symmetric, but we know beyond reasonable doubt that classical statistical mechanics is not fundamental. A general mechanism that produces time-like asymmetry across a broad range of “fundamental” physical theories is asymmetry in configuration space. My own favorite version is causal configuration space, as described in my essay:

On the Foundational Assumptions of Modern Physics

The idea is that different possible universes are related to each other in ways that make time-like asymmetry inevitable. I say "possible" here because I don’t believe one has to be a committed Everettian to make use of configuration spaces and Feynman’s sum over histories method. I also explain in the essay precisely what I mean by "time-like" in this context.

Julian Barbour’s essay in this contest mentions a different type of configuration-space asymmetry, shape space asymmetry, which is relevant under different assumptions. He can explain his approach better than I can.

2. As you point out, there is a self-referential difficulty associated with doubting one’s own memory; you mention this by remarking that your essay is “more likely” of random origin than produced by a conscious person. To your credit, the essay itself is strong evidence against this supposition, but more seriously, I believe that the pragmatic assumption you mentioned is necessary, if only as a last resort. There is no incompatibility between pragmatism and idealism in this regard, unless one is certain that one can never do better than the pragmatic assumption. You can continue to do science and seek better foundations at the same time.

3. On the subject of decoherence, I will mention that Jorge Pullin and Rodolfo Gambini have an essay in this contest that attempts to refine the decoherence approach to the measurement problem. I will also repeat that ascribing some degree of reality to the various histories in Feynman’s sum doesn’t necessarily imply full-blown Everettianism; in particular the relationship between observers and the configuration space admits several possible interpretations.

4. Einstein’s objection to nonlocality need not have been totally wrongheaded, even if it was misapplied in the case of quantum theory. In particular, it relies on assumptions about the structure of spacetime. I discuss this point in my essay, as well.

Thanks for the great read! Take care,

Ben Dribus

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Hello Emily,

Thank you for your reply. I agree that if we have to question evidence or assumptions, it's often better to questions assumptions.

I just reread your essay - to me you show the limitations of certain theories very well, by showing what happens when they're applied outside their domains of validity. One reductio ad absurdam you set out is that according to statistical...

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Hello Emily,

Thank you for your reply. I agree that if we have to question evidence or assumptions, it's often better to questions assumptions.

I just reread your essay - to me you show the limitations of certain theories very well, by showing what happens when they're applied outside their domains of validity. One reductio ad absurdam you set out is that according to statistical mechanics your essay was more likely to have arisen by chance than be written. I think one more positive side of your work is that it might contribute to defining the boundaries, as we probably need a more exact understanding of what that sort of physics can and can't describe. To me, having shown these limitations to that sort of physics, it's a pity if you then take the theories as being actually applicable in those domains, as you sometimes seem to.

I'm trying to understand your idea about memory - you say:

"For although the claim that we are at a local minimum of entropy is inconsistent with the history stored in our memories, it is not inconsistent with the existence of those memories - they, together with the order we perceive around us, could have been created by a spontaneous fluctuation rather than by the events they apparently report."

So a random fluctuation might have caused all of our memories, and the order we percieve around us, to come into existence? I just don't understand how so much order, and consistent order - our memories tend to agree on things - could arise from a random fluctuation, whatever one thinks about the way in which memories are stored. Wouldn't a random fluctuation be likely to create something very much more... random? Douglas Adams once described a planet covered entirely with luxery hotels and casinos, that had all been 'carved out of the rock by the natural processes of wind and erosion'. To me, your idea looks a bit like that. Surely the order we find around us is more likely to have appeared in the kind of way we think it did, but with gaps in our knowledge about it.

And where you talk about applying probabilities to the history of the universe - to do that you tend to need to know everything. It seems to me that because we don't, we're not in a position to do that.

I think what I may be seeing underneath your essay is a different version of something I find in many places nowadays - the implicit assumption that we now have all the pieces of the jigsaw in front of us, and only need to arrange them correctly. No-one would actually say that, but people nevertheless think and write as if it were the case. Many people are 'shuffling the principles' at present, making basic principles that were thought to be fundamental become emergent, and vice versa. I've had a discussion with Ben Dribus about this - personally, I think rearranging what we have will not be enough. In my essay, I remind people that there must be missing pieces, and that we need to allow for them, and try to guess - from the clues we do have - what the clues we don't have might look like.

And this principle of 'allowing for unknowns' would be very helpful in your discussion about QM and the Everett interpretation. It seems likely to me, and to many, that the real interpretation of QM is something different from all of our 5 or 6 present alternatives, all of which have their own problems. If so, feeding in what we have now and assuming it's everything will only give nonsense out, which is I think what you do, and what you get. I know you intend to show that many of these avenues of thought simply don't work, and I think you're right, you show that very well - to me only your suggestion about why they don't work is wrong.

Anyway, best wishes, Jonathan

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