Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.

Holger Bech Nielsen, one of the founders of string theory, has apparently just played some sort of game between different potential dimensions for space-time and reached the conclusion that D4 wins in a natural way:


Could someone please try to explain the gist of this game and the broader reasoning behind it in laymans terms, and perhaps answer the following related questions: Does anyone pay notice to this developing research? Does the result have implications beyond the Standard Model, e.g. for string theory?

share|improve this question

1 Answer 1

up vote 3 down vote accepted

Nielsen identifies a quantity, a ratio of "Casimirs", which he thinks is maximized by the particular gauge symmetry groups and space-time symmetry groups that we see.

He has previously had the idea that some of the observed properties of physics are "random" or "accidental" - e.g. that there is some complicated deeper theory and the simple observed symmetries are emergent from it with high probability. On page 6 of this paper, he gives his intuitive justification for why his special quantity might be indicative of such an origin.

He wants to identify the groups that are "easiest to become good symmetries by accident", and (if I'm reading his prose correctly) the indication of this is to be that the matrix representation of a group element does not change much, as the group element is varied. (He proposes specific measures to quantify the amount of variation in the group element and its matrix representation.)

Although this is his motivation, he also adds that there might be some other reason why this ratio of Casimirs is maximized.

As for this work's reception... the calculation is not very strong evidence of anything (the symmetries and dimensions of observed physics are small and could be singled out by many other simple properties), and there isn't much idea of how the deeper theory works or how the emergence works. So it will get attention from people already interested in his "random dynamics" research program, and maybe from a few people working on "landscape" or "multiverse" theories.

share|improve this answer
Agree with the last paragraph, read the paper yesterday and it doesn't seem to interesting at all. He continually reverse engineers his game to get the correct result... –  Vibert Apr 25 '13 at 7:25

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.