Experiments Show Gravity Is Not an Emergent Phenomenon | Hacker News Reader

Experiments Show Gravity Is Not an Emergent Phenomenon

technologyreview.com

163 pointsraganwald38 comments

the paper is surprisingly readable (although i don't know enough to say whether it's correct or not) - http://arxiv.org/pdf/1108.4161v1

this is a pity - it would have been a very cool approach if true. the criticism seems quite general, unfortunately (it identifies some consequences that come from associating gravity with entropy and shows that they conflict with experimental results).

the paper / main link is a theoretical argument based on an earlier experiment using neutrons and gravity. that earlier work is critical because it ties together QM and gravity observationally (very hard to do because gravity is very weak compared to the other forces that are usually important in QM).

update: here's a better link than the slides below, which includes an explanation of the earlier experimental work - http://www.sciencedaily.com/releases/2011/04/110418083349.ht...

[old text: i've been poking around and this set of slides (slow to load) gives some background on the neutron / gravity observations - http://admin.triumf.ca/docs/seminars/Sem5145306260-71715-1.S... . there's a slide almost exactly half way through called "Experiment on QM and Gravity Using UCN" - that and the intro above are relevant, i think. the experiment involves increasing the height of a "passageway" until neutrons can get to the other side. the classical interpretation would expect the number to come through to vary smoothly with height, but in practice there's a minimum height, related to the QM wavefunction (which includes a term with gravity, since that is the force acting on the neutrons).]

ps this kind of work (ultra-cold neutrons + gravity) is just as interesting and important as the higgs particle work at the lhc. it's also cheaper and smarter. so it's depressing that what is reported in the popular press is so distorted (by scientists themselves, who need to drum up money and support for "big science") - we see a huge amount of coverage of the lhc and nothing on amazing work like this.

Thanks for sharing then.

As someone who is very interested in this but feels a little intimidated by arxiv, I'd be very interested in seeing more posts about the results.

(Either that or subscribe to a site that can summarize the results for an easier read, linking to the technical papers for further exploration. Being so interested in the discoveries of what we're made of, without taking time off to fully understand the math or dig through arxiv, I'd happily pay for such a physics news/education site.)

Also, that second link has such a good title. Lots of good, visual info too.

Hmm... Stackoverflow/Hacker-News for pure science ?

It seems to me like a really fantastic idea, but for some reason it has never happened. I've seen a few 'startup'/webapp projects that attempt this kind of idea, but none of them ever seem to gain enough traction. My theory is that it's some combination of funding cycles and the personalities of scientists.

As an ex-scientist, I'd add one more contention: the cranks. They take way too much time. Limiting the site to active researchers (at least for posting) would help, but even that wouldn't be enough. Not willing to name names in public, but every university has a That Guy. Sometimes - often, in fact - they've done truly revolutionary work in the past, but for whatever reason they've now lost it.

So much as HN common sentiment goes against this, it would need to be heavily moderated and have a reasonably high credentialist (qualifications or published work, preferably both) entry barrier. The FT's "Long Room" forum is a good model. Life is too short for anything else.

Are you aware of Reddit's AskScience?

+1 .. Reddit's ask science is my second favorite online forum after HN.

Probably just me being an idiot, but if AskScience were separated from Reddit, I'd join to contribute questions and basic answers. Duplicate questions aside, I think it's awesome and should be a definite bookmark for anyone curious about our world. Rest of Reddit is pretty trashy though.

I actually went through and removed the reddit homepage and most of the other default reddit stuff from my account, so now it just shows AskScience and a few other niche areas. It's awesome.

That's a good idea. I would still feel dirty though. ;)

Edit: Would definitely participate if there were an active equivalent based on the HN code (tweaked to have expertise tags, etc).

Also, it has great voting guidelines (HN lacks that IMO).

Tenured college professors often become cranks? I thought crankhood was reserved to those who had not achieved a legitimate doctorate.

What about string theory? It seems a specialty designed for cranks: you can't do experiments, it's "a piece of 21st-century physics that had fallen by accident into the 20th century." and that, as the joke states, requires 22nd-century mathematics to solve. Are string theorists cranks?

Arguably yes, though in practice they tend to lack numerous qualities cranks have.

"you can't do experiments"

That's really the one issue, and speaking as an experimentalist it is a big one.

Proponents of string theory generally tend to be quite reasonable in discussion however (recognizing the experimental verification issues for example), and don't practice the many logical fallacies and whatnot that cranks seem to love. You don't hear them arguing about how "big [somebody]" is keeping them down for example.

And yeah, once someone reaches tenure they pretty much have full reign, including being able to be a crank. There isn't all that much people can do about it at that point. Being formerly sensible isn't fullproof protection against crank-ism, some people just... snap (or move on to other topics they were never scientific about in the first place).

I had no idea. But I guess Isaac Newton's later religious ideas were the product of a wondrous intellect gone astray.

Nobel laureates have been known to crank out. No-one is safe, seriously.

How does this prove that gravity is not an emergant phenomena?

Doesn't this only claim that gravity and entropy don't have some type of a first order (direct) relationship?

Rant:

I never understood how you can explain gravity as 1) a bending of space-time caused by mass and then 2) pretend it's a force transmited by "gravitons".

If 1) is true then it's an effect (phenomena), not a cause (force). If 2) is true than it's a cause (force), not an effect (phenomena).

This article does not claim to "disprove that gravity is not an emergant phenomena" it claims to prove that gravity is not an emergant phenomena

Fixed the double negative.

A phenomena is the summation of several causes. Even if gravity is a phenomena, there are still the causes for it to look for. And if it's not, well then we still have to find the single cause. We don't even have that.

What strikes me as odd is that entropy dictates things eventually break away from each other while gravity pulls things closer. The two are at odds with each other. And that entropy is a phenomena itself.

Phenomena is plural of Phenomenon.

No, this paper claims that introducing gravity as an emergent phenomenon in Verlinde's framework breaks quantum mechanics. He shows that Verlinde's proposal changes the Hamiltonian. However, the solutions to the standard Schrodinger equation have been measured in experiments with ultra-cold neutrons and shown to agree (ergo, disagreeing with Verlinde's theory).

Regarding your "rant": either way, it's still a force. Gravity still causes things to move. How that force is explained is clearly different in different theories. General Relativity suggests (1). Quantum Gravity theories suggest (2). Newton didn't really explain how it was mediated.

You could apply the same rant to QED: Either electrodynamics is caused by the bending of a vector potential by electric charge or it's a force transmitted by photons. The two statements are equivalent. But with gravity, we're not sure if quantum part is true.

The same way you can explain electromagnetism as a field throughout space-time, and then pretend that it's a force transmitted by photons. Feynman once famously claimed that no one really understood quantum mechanics, if that makes you feel any better; I'm not sure that's strictly true, but perhaps we've simply defined understanding down a bit :-)

Usual disclaimer about the pending detection of the graviton, etc, etc.

Your last sentence draws an arbitrary distinction between a "force" and a "phenomenon" which does not appear to be borne out in reality, and if I may say so, is at the heart of your trouble understanding why both 1) and 2) are correct.

Pictures 1 and 2 are equivalent. See chapter 18 in Misner, Thorne, Wheeler. We can't distinguish a universe where space is bent by mass from one with flat space-time that has a massless spin-2 particle (a graviton). The flat space-time in the latter case would be unobservable in principle.

Feynman has expressed his opinion on this in one of the Messenger lectures on the Character of Physical Law that he gave at Cornell.

This is an example of the wide range of beautiful ways of describing nature. When people say that nature must have causality, you can use Newton's law; or if they say that nature must be stated in terms of a minimum principle, you talk about it this last way; or if they insist that nature must have a local field - sure, you can do that. The question is: which one is right? If these various alternatives are not exactly equivalent mathematically, if for certain ones there will be different consequences than for others, then all we have to do is to experiment to find out which way nature actually chooses to do it. People may come along and argue philosophically that they like one better than another; but we have learned from such experience that all philosophical intuitions about what nature is going to do fail. One just has to work out all the possibilities, and try all the alternatives. But in the particular case I am talking about the theories are exactly equivalent. Mathematically each of the three different formulations, Newton's law, the local field method and the minimum principle, gives exactly the same consequences. What do we do then? You will read in all the books that we cannot decide scientifically on one or the other. That is true. They are equivalent scientifically. It is impossible to make a decision, because there is no experimental way to distinguish between them if all the consequences are the same. But psychologically they are very different in two ways. First, philosophically you like them or do not like them; and training is the only way to beat that disease. Second, psychologically they are different because they are completely unequivalent when you are trying to guess new laws.

As pointed out in the slashdot comments for the same article, this article has not been peer reviewed, "(Submitted on 21 Aug 2011)", so any conclusiveness implied by the headline is false / misleading.

Moreover, it's not really an "experimental" result; it's an argument that this theory is inconsistent with certain known facts. The known facts were previously derived from experiment, true, but the way the headline is written makes it sound like someone devised an experiment specifically to test gravity-as-emergence, and the results are now in, which is not what happened.

That isn't a bad thing. Consider person A, who says, "Light is a particle, as demonstrated by phenomena B, C, and D." It's perfectly cogent for person E to raise his hand and ask, "But what about phenomena such as interference, refraction, and so on?" E's argument isn't an experimental result, but that's ok, because A's conjecture is required to explain all existing observations before devising falsifiable experiments to conduct.

This criticism was first raised by Motl, and is dismissed by Koelman here: http://www.science20.com/comments/47263/Re_It_Bit_Whole_Sheb...

I am curious if there could be any theoretical reasoning why gravity can't be emergent, rather than experimental..

Could someone explain, what is meant exactly by `emergent'? What is the antonyme?

For example. I mean can't we assume all standard quantum model implies all quantum phenomena are emergent?

A reason that is theoretical must be backed by experiments. You cannot have one without the other without entering the religious realm. And you cannot have a completely mathematical explanation of reality because your axioms must have experimental backing.

As I understand it, emergent means the behaviour of a collection of entities that comes about due to their interactions in a way that is not predictable from a full theory of its constituents.

The opposite of an emergent theory would be a reductionist theory.

The new paper purports to deal with those criticisms. To my limited understanding it appears to be an argument against how the Hamiltonian is currently formulated in Verlinde's theory than a proof against entropic gravity.

So, a classical theoretical model tailored to exactly match known results is validated. And a brand-spankin-new theoretical model based on a different approach is invalidated.

This means all assumptions of the classical model are correct and all those of the new model are incorrect? Doesn't sound right to me.

These tests are validating / invalidating the predictive power of specific models, not testing their underlying assumptions. Issues of supervenience will not be worked out for certain until we have models of which we are more confident.

Was it suggested that all of the assumptions of the classical model are correct?

The motion gravity causes is explainable by the warping of 3D space that it causes. The real mystery to me is WHY matter warps space. It follows logically that if a lot of matter in 1 place warps space a lot, a little matter somewhere else warps space a little. So, at some very elemental level whats going on in an atom warps our universe.

Isn't the fact that entropy increases over time itself just a result of a priori statistical laws and the condition of zero entropy at the Big Bang 13 billion years ago, rather than a physical law of the universe?

Koelman's response is now up: http://www.science20.com/hammock_physicist/entropic_gravity_...

Okay -- maybe this is a stupid question, and I'm a college dropout (and never took a physics class), but here's a stab at plain logic:

Isn't there a law of thermodynamics that says that entropy increases in a system over time? And doesn't general relativity state that time is a function of gravity or acceleration or relative mass?

So wouldn't increasing gravity/acceleration/mass, i.e. relatively increasing the passage of time, also relatively increase entropy?

And if that's true, then wouldn't the equation work both ways -- such that an increase in entropy led to an increase in gravity?

Again, sorry if that's crazy, but I'd like to hear anyone's opinion on it...

> Isn't there a law of thermodynamics that says that entropy increases in a system over time?

Not exactly. The Second Law of Thermodynamics says that all processes cause the entropy of a system to increase. But entropy doesn't naturally increase over time without any processes happening.

> And doesn't general relativity state that time is a function of gravity or acceleration or relative mass?

The apparent passage of time ("proper time" in a particular reference frame) is influenced by gravity and acceleration.

> So wouldn't increasing gravity/acceleration/mass, i.e. relatively increasing the passage of time, also relatively increase entropy?

What actually happens is that gravity and acceleration decrease the passage of time (as compared to a reference frame not influenced by the gravity/acceleration.)

So if there are two otherwise identical experiments, the one with more gravity/acceleration will experience less time passage, and accumulate less entropy. But since increases in entropy aren't a direct result of the passage of time, it doesn't follow that increases in entropy lead to more passage of time, or any effect on gravity.

Hopefully that makes sense and is mostly correct, it's been a while since I studied this.

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