
Experiments Show Gravity Is Not an Emergent Phenomenon - raganwald
http://www.technologyreview.com/blog/arxiv/27102/?ref=rss
======
andrewcooke
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...](http://www.sciencedaily.com/releases/2011/04/110418083349.htm)

[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...](http://admin.triumf.ca/docs/seminars/Sem5145306260-71715-1.Sem20110312_lee.pdf)
. 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.

~~~
sandGorgon
_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_

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

~~~
Mizza
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.

~~~
adw
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.

~~~
giardini
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?

~~~
burgerbrain
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).

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

------
powertower
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).

~~~
sipior
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.

------
molecule
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.

~~~
_delirium
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.

~~~
raganwald
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.

------
beefman
This criticism was first raised by Motl, and is dismissed by Koelman here:
[http://www.science20.com/comments/47263/Re_It_Bit_Whole_Sheb...](http://www.science20.com/comments/47263/Re_It_Bit_Whole_Shebang)

~~~
diminish
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?

~~~
Dn_Ab
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.

------
noglorp
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.

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

------
dicroce
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.

------
selven
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?

------
beefman
Koelman's response is now up:
[http://www.science20.com/hammock_physicist/entropic_gravity_...](http://www.science20.com/hammock_physicist/entropic_gravity_getting_messy-82127)

------
noduerme
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...

~~~
elwin
> 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.

