May 23, 2011
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.