modified gravity?

Lubos suggests an explanation (or actually a replacement) for MOND, which sounds like entropic gravity to me (*). Did he not recently explain to us why such explanations have to be wrong?

The purpose of his proposal is to explain observations which suggest that gravity changes at low accelerations a < a₀ = 1.2x 10^-10 m/s² and it is based on the idea that a₀ could be the inverse size of the visible universe (times c).



(*) I should make it clear that Lubos never mentions 'entropic gravity' in his post, but how would a sentence like "The existence of this center on the hologram may be needed for the usual Kepler scaling laws to emerge." make any sense otherwise? See e.g. this paper on how MOND was derived previously from 'entropic gravity' using "a holographic principle".



added later: When I asked explicitly in a comment, he insisted that his proposal has nothing to do with "the crackpottery called entropic gravity". Alright, I will admit then that I do not understand what he is talking about and leave it to others to sort it out. Feel free to leave a comment to enlighten me. I will leave this post up as it is, because the links could be useful to others.

Perhaps I should make it clear that I am (still) not convinced 'entropic gravity and/or MOND make any sense.



added even later: While Lubos focuses directly on the quantity a₀ = c/T, with T being the age of the (visible) universe, I think it would be more natural to consider the energy E = h/T. The associated temperature is E/k, which is on the order of 10^-28 kelvin and comparable to the critical temperature used to derive MOND in the paper linked above; In fact plugging hk^-1/T into equ. (12) gives a₀ = c/T !



added several hours later: Obviously, there is a straightforward way to test this type of proposal. If one looks into the night sky one sees galaxies at different age T of the universe and the deviation from Newtonian dynamics should be stronger the further back in time one looks.
Perhaps there is already enough statistics of galaxy rotation curves to check this.



added much much later: As a counter point to all this speculation a paper [pdf] about an experiment which "finds good agreement with Newton’s second law at accelerations as small as 5 x 10^-14 m s^-2.

statistical theology

This post is part of a series about important theological issues [1, 2, 3, 4] and this time I want to emphasize the importance of Benford's law in statistical theology. In the recent paper "Law of the leading digits and the ideological struggle for numbers", it was used in religious demography research:

"We investigate the country-wise adherent distribution of seven major world religions i.e. Christianity, Islam, Buddhism, Hinduism, Sikhism, Judaism and Bhah'ism to see if the proportion of the leading digits conform to the Benford's law. We found that the adherent data on all the religions, except Christianity, excellently conform to the Benford's law."



What does He want to tell us with this exception?

lattice gravity

You may have noticed the link to quantum gravity on the left hand side of this blog, below the picture of the plastic bag; It leads to several blog posts about "lattice gravity" as well as posts about quantum gravity in general.



If you want to know more what "lattice gravity" is all about, you can browse some of the references provided here; Another good starting point would be Renate Loll's review (in particular sect. 3) and for even more motivation I recommend this paper.



One warning: It is very much possible, and actually quite likely, that these models have nothing to do with real (quantum) gravity. In fact there are several good arguments why a lattice approach can never work. In other words, it is very much possible that this will turn out to be a waste of time.

Just another reason it makes for a good topic on this blog...