# Gravity - NY Times Article on Newly-Postulated Definition **URL:** \ **Category:** Factual Questions\ **Created:** [July 20, 2010, 6:34pm UTC](https://boards.straightdope.com/t/gravity-ny-times-article-on-newly-postulated-definition/547378 "2010-07-20T18:34:09Z")\ **Posts on this page:** 9\ **Page:** 2
**Author:** ![WordMan](https://avatars.discourse-cdn.com/v4/letter/w/e8c25b/32.png) [@WordMan](https://boards.straightdope.com/u/WordMan)\ **Post date:** [July 22, 2010, 6:42pm UTC](https://boards.straightdope.com/t/gravity-ny-times-article-on-newly-postulated-definition/547378/21 "2010-07-22T18:42:36Z")
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! ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [July 22, 2010, 9:37pm UTC](https://boards.straightdope.com/t/gravity-ny-times-article-on-newly-postulated-definition/547378/22 "2010-07-22T21:37:22Z")
> [@](#): > > Holographic principle. Imagine that physical space is not continuous but is instead a 3D grid of points. These points would be very close together, with the grid spacing perhaps near the Planck length, 10[sup]-35[/sup] m 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](http://en.wikipedia.org/wiki/GEO_600), but don’t hold your breath). ---
**Author:** ![Hari\_Seldon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hari_seldon/32/5173_2.png) [@Hari\_Seldon](https://boards.straightdope.com/u/Hari_Seldon)\ **Post date:** [July 23, 2010, 2:11am UTC](https://boards.straightdope.com/t/gravity-ny-times-article-on-newly-postulated-definition/547378/23 "2010-07-23T02:11:40Z")
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. ---
**Author:** ![WordMan](https://avatars.discourse-cdn.com/v4/letter/w/e8c25b/32.png) [@WordMan](https://boards.straightdope.com/u/WordMan)\ **Post date:** [July 23, 2010, 11:25am UTC](https://boards.straightdope.com/t/gravity-ny-times-article-on-newly-postulated-definition/547378/24 "2010-07-23T11:25:22Z")
> [@Pasta](#): > > _Holographic principle_. Imagine that physical space is not continuous but is instead a 3D grid of points. These points would be very close together, with the grid spacing perhaps near the Planck length, 10[sup]-35[/sup] m. Each grid point contains some information about what’s going on there. Naively, one would expect that the amount of information you could contain in a volume is equal to the number of grid points in the volume times the information per grid point. However, through some thermodynamic arguments that I’ll skip here, it can be shown that the information is limited to what could be contained on the boundary surrounding the volume – a much smaller amount. In other words, a 2D surface surrounding (say) the universe could have imprinted on it all the information needed to describe the goings-on in the 3D volume said surface surrounds. This is the holographic principle (so named because a hologram is a 2D film that contains all the information needed to see a 3D image). 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… ---
**Author:** ![Wendell\_Wagner](https://avatars.discourse-cdn.com/v4/letter/w/8491ac/32.png) [@Wendell\_Wagner](https://boards.straightdope.com/u/Wendell_Wagner)\ **Post date:** [July 23, 2010, 3:26pm UTC](https://boards.straightdope.com/t/gravity-ny-times-article-on-newly-postulated-definition/547378/25 "2010-07-23T15:26:10Z")
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: > **[Stationary-action principle](https://en.wikipedia.org/wiki/Principle_of_least_action)** > > The stationary-action principle – also known as the principle of least action – is a variational principle that, when applied to the action of a mechanical system, yields the equations of motion for that system. The principle states that the trajectories (i.e. the solutions of the equations of motion) are stationary points of the system's action functional. > The principle can be used to derive Newtonian, Lagrangian and Hamiltonian equations of motion, and even general relativity, as well as clas... 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. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [July 23, 2010, 4:12pm UTC](https://boards.straightdope.com/t/gravity-ny-times-article-on-newly-postulated-definition/547378/26 "2010-07-23T16:12:07Z")
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. ---
**Author:** ![Pasta](https://avatars.discourse-cdn.com/v4/letter/p/ecccb3/32.png) [@Pasta](https://boards.straightdope.com/u/Pasta)\ **Post date:** [July 23, 2010, 5:18pm UTC](https://boards.straightdope.com/t/gravity-ny-times-article-on-newly-postulated-definition/547378/27 "2010-07-23T17:18:45Z")
> [@WordMan](#): > > …so “where the action is” is on the 2D boundary surrounding the 3D space, no? Yes, although it is worth noting that what that action _is_ isn’t really addressed. (See below.) > [@](#): > > Also, if that is a boundary - between what? Our universe is the internal space; what is on the other side? 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: 1. A suitably symmetric source mass _M_ is surrounded by a holographic screen. Its energy (_M_) is allocated to the bits on the screen, endowing them with temperature. 2. A change in temperature is associated with a change in entropy. 3. A test mass _m_ approaching a holographic screen changes the entropy on the screen. 4. An accelerated object experiences a temperature. 5. Put ideas (1) through (4) in a blender, and… 6. Conclude that entropic changes on the boundary surface are the root cause of the acceleration of the test mass toward the test mass within. He points out that no single ingredient is novel, but the new look at the cause-and-effect of the situation could be meaningful: > [@Erik Verlinde](#): > > These equations do not just come out by accident. It had to work, partly for dimensional reasons, and also because the laws of Newton have been ingredients in the steps that lead to black hole thermodynamics and the holographic principle. In a sense we have reversed these arguments. But the logic is clearly different, and sheds new light on the origin of gravity: it is an entropic force! That is the main statement, which is new and has not been made before. If true, this should have profound consequences. ---
**Author:** ![Half\_Man\_Half\_Wit](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/half_man_half_wit/32/21766_2.png) [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)\ **Post date:** [July 23, 2010, 5:35pm UTC](https://boards.straightdope.com/t/gravity-ny-times-article-on-newly-postulated-definition/547378/28 "2010-07-23T17:35:31Z")
**Pasta** ’s summary is excellent, but the whole thing is discussed a bit more in-depth over at The Hammock Physicist, starting [here](http://www.science20.com/hammock_physicist/it_bit_case_gravity) (an accessible explanation of the concept behind entropic gravity can be found [here](http://www.science20.com/hammock_physicist/it_bit_entropic_gravity_pedestrians)). 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](http://www.science20.com/hammock_physicist/it_bit_how_get_rid_dark_energy) Hammock Physicist post, and [here](http://arxiv.org/abs/1002.4278)’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](http://arxiv.org/abs/1003.1528) --, 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](http://pirsa.org/10050022/), where one can also find related talks by [Ted Jacobson](http://pirsa.org/06090001/) and [Thanu Padmanabhan](http://pirsa.org/10060000/), 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](http://arxiv.org/abs/1005.3537). ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [July 23, 2010, 7:41pm UTC](https://boards.straightdope.com/t/gravity-ny-times-article-on-newly-postulated-definition/547378/29 "2010-07-23T19:41:00Z")
> [@](#): > > 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. [Previous page](https://boards.straightdope.com/t/gravity-ny-times-article-on-newly-postulated-definition/547378.md?page=1)