Good lord - I get that, too. I gotta thank you physicist Dopers for the clear explanations.
Really fascinating. It does seem that given the topics being explored and postulated upon, we are nearing a breakthrough insight or two that either builds on or replaces some/all of our current Newtonian + Einsteinian models…
Thanks you guys!
Actually, one appealing thing about the holographic principle is that it predicts that the grid spacing should be significantly larger than the Planck length, and hence in principle detectable (and may even have already been detected, but don’t hold your breath).
A physicist should correct me if I am wrong, but here is the analogy I pictured when I read the article. Is pressure real. Toricelli would surely have said so. But now we understand that pressure is just an emergent phenomenon that arises as the result of 10^30 or so random motions of gas molecules in this room. So what Verlinde seemed to be saying is that gravity is a phenomenon that emerges from something more basic that exists at a more fundamental level that we cannot see any more then we can see the random motions of these molecules.
Processing this: so “where the action is” is on the 2D boundary surrounding the 3D space, no? Both in terms of what (you say “thermodynamically”) is going on in that boundary layer which emerges as gravity within the internal space. Also, if that is a boundary - between what? Our universe is the internal space; what is on the other side?
Sorry if I am misunderstanding…
The claim that gravity is merely an emergent property from some other principle (which in this case seems to be about maximizing disorder) is similar to the claim that one can derive a lot of physical laws from the Principle of Least Action:
So does is the Principle of Least Action really basic and the various physical laws derived from it mere consequences or are those physical laws really basic and the Principle of Least Action a mere consequence? I don’t know, and I don’t understand how anyone could know. With a lot of physical laws now, there are various ways of stating them in various mathematical formats, and there doesn’t seem to be any way of saying which the basic principles and which are merely accidental consequences.
Oh, I should also mention: I have no problem at all with the idea that gravity is an emergent phenomenon of something else, but I don’t really see how it could be from the Second Law, since gravity is time-symmetric. In fact, if this really is the holographic principle we’re talking about here, I wouldn’t expect the thing it emerges from to even have any name at all, since physics in general should be very different on the two-dimensional “real” space.
Yes, although it is worth noting that what that action is isn’t really addressed. (See below.)
The holographic principle in general doesn’t care what’s on the two sides. It’s just about information bookkeeping. So whatever is going on outside a universe-sized boundary isn’t relevant to what’s encoded on the boundary. However, in this article, the holographic idea is used for a more specific purpose: to separate two massive bodies and to address the information on the dividing surface.
The paper is fairly qualitative in reaching its main observation, which goes something like this:
He points out that no single ingredient is novel, but the new look at the cause-and-effect of the situation could be meaningful:
Pasta’s summary is excellent, but the whole thing is discussed a bit more in-depth over at The Hammock Physicist, starting here (an accessible explanation of the concept behind entropic gravity can be found here). An interesting result not mentioned here so far is that Verlinde’s arguments can be adapted to yield a possible explanation for the accelerated expansion of the universe; the basic argument can be found in this Hammock Physicist post, and here’s a paper by a group including Nobel laureate George Smoot on the matter (I just noticed that they’ve apparently also published an explanation of cosmic inflation within the same framework – here --, but I haven’t had a look at that one yet).
There’s also a talk by Verlinde on the matter at Perimeter Institute’s recorded seminar archive, where one can also find related talks by Ted Jacobson and Thanu Padmanabhan, who both advanced related ideas prior to Verlinde. These are rather technical, however.
EDIT: Right, I forgot, apparently Dark Matter is also being explained along some of the same lines.
Oh, that’s nothing new. Everyone has an explanation for dark matter. If even a tenth of the explanations for dark matter were correct, we’d have the opposite dark matter problem, that there isn’t as much as people are predicting. Personally, I’ve suspected for a while that dark matter isn’t a single phenomenon, but that there are a lot of different components contributing to it.
Now, if this new idea can account for the dark energy, and in particular account at least roughly for the strength it’s observed to have (our current best guesses about dark energy contain the most ridiculously ludicrously worst estimation in all of history), that is something.