When do we stop digging? Conditions on a fundamental theory of physics by Karen Crowther

Gary Hansen

Dear Karen,

A belated answer to your bottom line question; 'What is the interpretation of the question that could be answered by "Yes"?'

My answer is that when the question is read as a literal statement of fact requiring confirmation, either the word 'What' is fundamental or it is not. Hence the alternative 'one word' answer is either 'Yes' or 'No'.

Since you didn't respond on my essay page, I am wondering whether you rated my essay. There is something 'going on' on the closing day for the acceptance of essays that suggests that some authors are drastically low-ranking the essays of others in the expectation that they may benefit as a result. If you didn't rank my essay, could you kindly do so, hopefully to return my rank to its former 'high tide' position of 6.8?

Again, you carry my best wishes as you move forward into the sunshine, the final phase of the contest, and far beyond.

Gary.

Terry Bollinger

Karen, Christi,

I can ask even if it's awkward for either of you: Does this downvoting appear to be gender related? You both have very good essays, so it's strange to see both of you hit by this.

My observation to any FQXi admins reading this: Please at least consider whether there has been some bias here. All I can say is that the high quality of these two essays has not changed, so a sudden downswing in the last couple of days to me just seems wrong.

Sincerely,

Terry Bollinger (63 year old male codger, since my first name is ambiguous... :)

Sylvia Wenmackers

Dear Karen,

I found your essay very clear! Unlike some others, it came exactly as announced in the abstract. :) I think your text is very accessible and relevant for, e.g., Master students of physics. (I would definitely have enjoyed reading such a survey at that point, to add some global perspective to more detailed courses of QFT, etc.)

Three more detailed comments on section 4:

- On p. 6 it appears that you assume the "problem of missing physics" (as Wilson calls it; i.e., the existence of gaps between theories) is only temporary. (In my own essay I have embraced the patchwork view of physics, as I think it is here to stay.) I am not sure whether rejecting patchwork is necessary for embracing the goal of physics (which you discuss at the bottom of p. 7): searching for a unique, unified, ... theory may well be the goal of physics, but I don't think it is inconsistent to admit at the same time that it is an unattainable one.

- "No weirdness" is a tricky requirement - as you may well be aware of -, since what we find weird or not strongly depends on our training and background knowledge.

- I particularly appreciate how you managed to escape Kantian worries by keeping us focused about what physics is (and isn't) about. So, I fully agree with your comment at the bottom of p. 7: indeed, physics isn't in the business of finding out what are things-in-themselves.

Best wishes,

Sylvia - Seek Fundamentality, and Distrust It.

Karen Crowther

Dear Sylvia,

Thanks very much!

Yes, you raise some good points here, and I agree. Regarding the "patchwork": it may well be that we continue to work in this way, building up a patchwork, ultimately aiming at a unified, unique theory, but also recognising that in the end we may have a set of theories. Actually, this is similar to comments in the paper on UV completion (with Niels Linnemann) -- that a UV complete theory may be an ultimate goal, and heuristically useful, but we can still use UV incomplete theories, and at the end of the day, that may be all we have (though of course currently in QG, the non-renormalisable theory we have is unsatisfactory since it breaks down at the Planck scale, which is what we want to describe). Really, I think this whole list represents an unattainable goal that nevertheless drives us!

Regarding "no weirdness" -- yes, it's a tricky one. I certainly wouldn't want to reduce it to what particular individuals, or particular research programs find weird, but some rough sort of disciplinary consensus. When a theory is unsettling enough that many researchers forge ahead for something deeper, and this research is recognised as legitimate by mainstream physics (or, at least, not entirely misguided)... that's the gauge of "weirdness" I mean. But I need to think about this more... Please let me know if you have any more thoughts on it!

Best,

Karen

Karen Crowther

Dear Alyssa,

Thank you for your comments. Sorry for my delayed response.

My idea in this essay was to provide the apparently necessary conditions on a fundamental theory of physics - not just a 'currently fundamental' physical theory. GR and QFT (or, really, some particular QFTs, like QCD) are both currently fundamental, and yet physicists are still searching for a deeper theory, for reasons that I present in the essay. And this is what led to me eventually realising that the distinction between fundamental and final needed to be dropped, due to the arguments regarding unification and uniqueness.

Regarding weirdness: the idea was motivated by the fact that so many physicists remain unconvinced that the framework can be complete, or correct, due to the measurement problem. Many people (physicists and philosophers alike) believe that a more fundamental theory (perhaps QG) must provide a solution to the measurement problem, and thus absolve this 'weirdness'. So, the lesson I took is that some degree of weirdness may be acceptable, but there may be a point where the theory is weird enough that it won't be accepted by mainstream physics as a fundamental theory, and people will seek to go deeper. (That said, I am not fully convinced by this condition, because the measurement problem could plausibly be "external" to theory... So I will think about this more).

Best,

Karen

Karen Crowther

Dear Armin,

Thanks very much! (And sorry for the delayed response).

You raise some very good points here, and offer a new perspective for viewing my essay. Yes, as you appreciate, I was taking the particular perspective of current mainstream high-energy physics, and trying to discover and articulate the conditions it apparently puts on a fundamental theory (while leaving open the possibility that these conditions change in the future). But, as you note, these conditions are revealing of the nature of the discipline itself at this point in time, beyond its conception of fundamentality. So, yes, I would certainly like to take up this idea in future work, and to better understand what it is about these conditions that makes them key to current physics -- and what may be modified in future physics.

Your project sounds like it could be a very difficult one! I'm curious what you mean, so I'll come have a look...

Best,

Karen

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