Is gravity not actually a force? Forcing theory to meet experiments
arstechnica.com
arstechnica.com
> "Say, Pop, I noticed something. When I pull the wagon,
> the ball rolls to the back of the wagon. And when I'm
> pulling it along and I suddenly stop, the ball rolls
> to the front of the wagon. Why is that?
>
> "That, nobody knows," he said. "The general principle is
> that things which are moving tend to keep on moving, and
> things which are standing still tend to stand still, unless
> you push them hard. This tendency is called 'inertia,' but
> nobody knows why it's true." Now, that's a deep
> understanding. He didn't just give me the name.
And what does it mean to ask if "gravity" is not actually a "force?" The idea of a "force" is just an abstraction we use to describe our observations. How is it a deep question to ask whether we give gravity the label of "force" or not?
Knowing the name of something does not help you understand it.
The idea of a force is that it's a prime mover - it's not caused by something else. The practical implications in the case of gravity are zero, but maybe we'll figure out something cool due to the implications someday.
The question is not "is gravity a force?". The question is "What is gravity?"
An accepted answer, what I was taught in school, and what I would answer if you asked me is "it's a force". And labeling something as "a force" has huge implications for people that know what that label means.
If a four year old asked me "What is gravity?", I'd know better than to give him a label, because he wouldn't know what a force is from school. Obviously.
If it turns out gravity is not really a force -- if modelling gravity as a force breaks down at some point -- that's a good fundamental result. It is a falsification of the theory "Gravity is a force". (So the line of thought goes: "What is gravity?", "It is a force!", "Is gravity really a force?", "Perhaps not!")
How is this not helping us attaining a deep understanding? How is this not advancing science? How is this any different from any science?
It definitely isn't. The word "force" implies a Newtonian model for physics. Under general relativity, gravity isn't seen as a force but a consequence of curved space-time.
The title was probably chosen by an editor without deep knowledge of the subject. It doesn't accurately describe the article's content either.
To clarify, I wasn't actually trying to say anything about physics, just about science. So it would have been better phrased as a hypothetical; If we assume that gravity is currently best modeled as a force, why is it valuable (nay, essential) to question whether or not it actually is a force? I hope I answered that hypothetical properly :)
But why does the presence of matter cause spacetime to curve?
If you want to argue that gravity isn't a force, I don't think it's enough to say "it's a consequence of curved spacetime". I think you have to answer the question of why it curves. I'm not aware of an answer to that question. If there isn't one, as I suspect, then I would say, there's your force of gravity: you're just describing it as acting on spacetime itself rather than on the objects in that spacetime.
The really interesting bit about black holes and universes encoding information on the surface is this: The Chandrasekhar limit radius for a mass equal to the weight of the universe, once you figure that out, look up the best known estimate for the radius of the universe.
I've never noticed the different to have any implications though, and not everyone even agrees with it, since gravity does have some elements of a real force.
See: http://www.av8n.com/physics/fictitious-force.htm for a pretty complete discussion.
Ignoring hawking radiation of course...
... unless the cosmic microwave background is the equivalent of the hawking radiation given off by black holes (note: for the picky, before you object, consider that hawking radiation consists of when one part of a short lived mutually annihilating pair of particles falls into the event horizon and the other falls out (hence gets radiated away) and thus from the point of view of those inside the black hole it would appear as those there was a constant rain of particles falling in... which I'm equating here to the microwave background noise), which would imply that our universe is shrinking...
...unless it implies that our universe is getting bigger, maybe if our universe is a black hole with hawking 'radiation' then on the other side of the 'universal event horizon' things work a bit differently and it is matter which is more likely to fall in, not anti-matter...
... which would explain why baryonic matter in this universe has an overwhelming tendency towards matter instead of anti-matter, which is one of those really big still outstanding questions in science...
In QG we've got two things: (i) general principle such as unitarity and the holographic principle, and (ii) specific models such as loop quantum gravity, string theory models, supergravity, etc.
Results based on type (ii) information are always going to be iffy because the detailed models are all about what happens on the Planck scale or inside a black hole so experimental confirmation is going to be difficult -- it may be hard or impossible to prove or disprove any one of these models.
Type (i) results, however, will hold no matter what is correct on the type (ii) level. Because we're not assuming a huge amount of "superstructure", the results are much more intellectually convincing than type (ii) results -- and with more certainty on the theoretical front, there's more "gravity" in the confrontation of theory and experiment...
(We accomplish very little by experimentally disproving any particular type (ii) model because we've got 100 others to take its place)
Certainly all of QM can be done in terms of information, and I think Seth Lloyd is working on this right now. Cool to see a new take on gravity though.
Also, it's neat to think that black holes are deterministic from their surface, though it's sort of required if you force thermo on it. After all, if the information's beyond the event horizon, it's not really in our universe, and thus violates thermo assumptions about closed systems. Sort of a case of mashing theory against the universe.
-Cd
I think this is really all we need to know.
>Kobakhidze uses Verlinde's approach to re-derive the wave equation that will describe the energy levels of neutrons falling in Earth's gravitational well. What he finds is that, in contrast to the more conventional equation, there are two extra terms now present. One would appear to account for the relativistic rest energy of the neutron; the other is a form of an extreme suppression of certain parts of the neutron's wavefunction. Both of these additions result in problems for Verlinde's theory.
>The first extra term, the relativistic rest energy, would manifest itself as a constant shift in the neutron's energy states (height above the bottom of the well)—this is not seen at all in the experiments. According to Kobakhidze, though, it is the second extra term that really throws a wrench into entropic gravity. The second term, if correct, would significantly change the dynamics of the experiment, essentially causing neutrons to fall through the small hole in the bottom of the potential well. There's no sign of this happening in any meaningful way in the experiment. Thus, Kobakhidze concludes, "we are driven to the conclusion that gravity is not an entropic force."
Ars regularly do this, discuss papers that are "hot off the press" and have not been fully peer reviewed yet. This is simply the reality of following the bleeding edge of science. They are always up front about it, I don't see a problem.
Peer review is a more important standard than you take it to be. For every announced-but-not-yet-peer-reviewed revolutionary discovery -- Wiles's proof of Fermat's Last Theorem comes to mind -- there are a massive number of P!=NP papers "awaiting peer review", every one of which is dead wrong, but each of which will excite some moron in the press to write about it.
The press has such an incredibly bad track record at this that my personal estimate, based on the performance of similar breathless "hot off the presses" articles I have read, is that without reading the article there is over a 95% probability that this paper won't pass peer review. Once I read the article, based on the fact that half of it is a sheepish admission that there are problems with the theorem, I think that this prior probability was more than confirmed.
With a probability this low, I see no reason to waste my time reading breathless articles like this.
This is how science works. Ideas are proposed, they are backed up, they are shot down. Over time, it would not surprise me to see Verlinde defend his work. Perhaps Kobakhidze's derivation and its extension to the microscopic case is incorrect. Perhaps Verlinde will revisit his original work to revise how microscopic cases should be handled. Whatever happens, science will move on; time, further arguments, and experiments will be the ultimate arbiter of which drastically different view of reality is correct.
I think this article has two purposes: explain a new, untested theory, and demonstrate how science works in practice. He links to, and discusses, two papers that refute the theory. You call that a "sheepish admission," but I think that was one of his goals.
"In this article, we are going to look at a manuscript that purports to overturn hundreds of years of accepted ideas about gravity, and use it as an illustration of how controversial ideas are dealt with in modern physics."
I don't think you can be any more up front than that. It's not a sheepish admission, it's the whole point of the article. Ars are trying to highlight exactly the problems you're talking about.
I think you're right -- on rereading I shouldn't be laying the smackdown on the Ars article: he does use the paper as an example of the scientific process at work. Nonetheless I think that by advertising a paper which hasn't been through peer review, he's promoting it. It'd have been a lot better if he told his story with an example which has already gone through the cycle of being proposed, shot down, and buried.
/serious-face
It’s true that the faithful followers of self-anointed prophets will believe without question the dogma of their prophet and perpetuate the dogma.
But such cults have always come to an end.
The Newtonian cult called physics will also end.
w t f
For instance, right now there is a satellite called SOHO at the Sun-Earth L_1 Lagrangian point. Why do you say that Newtonian gravity is a myth, when the existence of such an orbit violates Kepler's third law but is in perfect agreement with calculations according to the Newtonian model?
Postulating is al good fun but are you actually planning to do some physics with it? You're just saying Newton is evil and wrong but are not giving any reasons why his laws don't sufficicently confirm to reality. Neither are you explaining why your alternative fits reality better. Calling relativity an offshoot of Newtonian gravity is also not quite correct.
On the Verlinde article, I haven't read it. While it sounds interesting, it hinges on the definition of entropy. IMO, it might be a big circular argument.
But it is pseudo-science babble that I might find in an e-mail forward.
Not Hacker News.
(Hit my 365 days yesterday so I am officially "allowed" to complain.)
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"Anything that good hackers would find interesting. That includes more than hacking and startups. If you had to reduce it to a sentence, the answer might be: anything that gratifies one's intellectual curiosity."
I, for one, welcome posts of this kind