
Entropic Gravity - rolph
https://en.wikipedia.org/wiki/Entropic_gravity
======
choeger
So far my absolute layman understanding of dark matter is: "Well, the
observations do not fit the theory. But _if_ there was this huge amount of
matter that we just cannot see, it would work out well." Whereas the
alternative theories boil down to: "Well, we do not know how gravity works
_over there_ , so maybe it just works like this." Whereas the usual answer is:
"No it cannot work like this, because [OBSERVATION/FORMAL ARGUMENT]."

Did I get this (roughly) right?

So from my perspective: Is there any prediction that would prove the existence
of dark matter? Preferably something that could be invoked repeatedly?

~~~
knzhou
Physicist here. It's roughly right, but it's kind of the picture we had 50
years ago. 50 years ago, dark matter and modified gravity were both reasonably
equally good hypothesis. Then we got a treasure trove of astrophysical,
galactic, and cosmological data from nearly a hundred distinct experiments.

The results of _all_ of these experiments do not fit with the naive theory.
And they each can be fit perfectly by adding in dark matter. But that's not
important; the crucial point is that all of them can be _simultaneously_ fit
perfectly by adding in the _same_ amount of dark matter. That's practically
the definition of what a good scientific theory should do. Put in a single
parameter and explain a hundred observed results.

Meanwhile, modified gravity theories have fared extremely poorly -- they were
originally designed specifically to fit galaxy rotation curves, and
accordingly it is very difficult to massage them into fitting anything else.
Usually if you get the rotation curves right, the astrophysics and cosmology
come out disastrously wrong. You could probably get it right if you added a
pile of ad hoc parameters, but that would be bad science.

Unfortunately every discussion of this subject ever just hyperfocuses on the
nearly 100 year old galaxy rotation curve observations... probably because
it's easy to understand.

> Is there any prediction that would prove the existence of dark matter?

If dark matter interacts in a non-gravitational way, and is present near the
Earth, then the smoking gun would be directly detecting it in an terrestrial
experiment. (People also work on indirect detection, by looking at possible
products of its decay or annihilation elsewhere in the galaxy, but this is
less definite because such products could be made by something else.) Of
course in all these cases, the specific kinds of predictions depend on what
you think dark matter is made of. Unfortunately, its good scientific
properties (i.e. fitting a lot of data with remarkably little input) also mean
that we have very little to go on here. Practically any kind of new "stuff"
that interacts weakly electromagnetically could work.

~~~
phkahler
I haven't seen any explanation for Dark Matter that explains its distribution.
It's always "hey, if there's some stuff with this distribution it will explain
this observation". An example would be a halo that fixes a rotation curve. If
it interacts with visible matter gravitationally then why doesn't it take on
the same distribution? That is never explained.

My other issue is that in discussions of rotation curves, I keep seeing
reference to Kepler, which simply shouldn't apply. Where can I see the math
behind the "expected" curve - I suspect an error.

~~~
Keysh
"My other issue is that in discussions of rotation curves, I keep seeing
reference to Kepler, which simply shouldn't apply. Where can I see the math
behind the "expected" curve"

"Keplerian" in this context is an approximate term. It refers to the fact that
most of a galaxy's visible mass is centrally concentrated, and so as you get
further and further away, with virtually all of the mass inside whatever
distance you're at, the rotation curve should converge on a true Keplerian
one, because the difference between the effect of the true mass distribution
and one where all the galaxy's (visible) mass is concentrated in a point at
the center gets smaller and smaller.

Actual published fits to galaxy rotation curves always use the measured
visible-mass distribution for a given galaxy to compute the non-dark-matter
curve. No one working in this field is confused about this.

~~~
phkahler
There is zero validity to treating galactic mass as a point mass. That is
exactly one of the mistakes I suspect keeps being made. At best it is a
misapplication of the divergence theorem. Disks dont behave like spheres and
rings dont behave like uniform shells. Proximity matters.

~~~
Keysh
It generally works a bit better if you say something like, "Hmm... it seems
like this approach would be wrong, for this reason that just occurred to me.
Am I missing something? Or: How do people in the field actually do it, so as
to avoid this error?"

If, on the other hand, you assume they must all be stupider than you are and
say things like "That is exactly one of the mistakes I suspect keeps being
made", then you're basically saying, "I'll bet none of the hundreds or
thousands of people working in this field for decades have ever thought of
this one point that just occurred to me!" The latter is, shall we say, rather
unlikely.

(In point of fact, Newton's shell theorem generalizes to the case of
axisymmetric, flattened spheroids with homeoidal density distributions, a
result that was derived by Laplace and others in the 18th and 19th Centuries.
Which means that disks _do_ behave somewhat like spheres.)

To get a sense of what's really involved, you could look at something like
Brandt's 1960 paper, and then some of the papers that cite it (including some
of the classic early papers by Vera Rubin and collaborators), to get an idea
of how much more sophisticated than an simple Keplerian rotation curve:

[https://ui.adsabs.harvard.edu/abs/1960ApJ...131..293B/abstra...](https://ui.adsabs.harvard.edu/abs/1960ApJ...131..293B/abstract)

------
parasense
A very bad analogy is air pressure. Because air pressure is really not a force
in of itself, it's just the presence of air at certain measures, but we
observe the effects of air pressure as we move through the atmosphere. That
drag coefficient changes depending on air pressure, and influences the
aerodynamics..... you get the idea. In orbit around earth the drag of upper
atmosphere is enough to decay orbits, even though it's so minute it's
practically a near vacuum.

That is something like gravity's inverse square law, or put another ways we
have been obsessing on the measured effects of gravity, but not truely
understanding.... like we used to not understand air pressure centuries ago.

Anyways, it goes without saying air pressure emerges from lots of air
concentrated in a space, just like gravity emerges from the presence of mass
in a concentrated space.

Gravity is the air of the universe.

~~~
cainxinth
_Fry:_ Usually on the show, they came up with a complicated plan, then
explained it with a simple analogy.

 _Leela:_ Hmmm... If we can re-route engine power through the primary weapons
and configure them to Melllvar's frequency, that should overload his electro-
quantum structure.

 _Bender:_ Like putting too much air in a balloon!

 _Fry:_ Of course! It's all so simple!

------
wwarner
Quite happy to see entropic gravity discussed here. One thing I really like
about EG is that it ought to be very testable. For example, there are a few
observations of very diffuse galaxies that exhibit very little if any dark
matter behavoir. These represent a direct challenge to EG, as gravity ought to
emerge from any concentration of mass, whereas under DM we can say there
simply aren't any DM particles there (of course _why_ this is the case is
still very important).

Similarly, with EG there should be testable pedictions at smaller scales.
Microscopically under EG, there should be no gravity ever between fundamental
particles, no matter how massive, for the same reason that a single atom
cannot be warm.

------
kingbirdy
I get the premise (gravity becomes linear at large distance), but I don't
understand any of the terms used in the article, and their respective pages
didn't do anything to help. Could someone give an ELI5 of entropic forces,
anti de-Sitter space, and the rest of the opening paragraph?

~~~
sklivvz1971
Basically, from the "derivation" in the page:

1) Two point-masses at a distance subtend a sphere with the radius equal to
the distance

2) That sphere has a radius and a maximum capacity for holding information (in
bits) depending on the area and the Planck constant

3) If we treat the bits as particles in a gas, we can derive the temperature
of that gas based on the mass of one of the two point-masses and Einstein's
equivalence

4) Assuming that the temperature is due to Unruh effect, we can calculate the
acceleration that would cause it

5) Lo and behold, this is the same acceleration due to Newton's law of
gravitation

------
dr_dshiv
Gravity is weird. On the face of it, it seems to directly contradict the
second law of thermodynamics, that overall entropy in the universe must
increase.

Entropy generally involves "spreading out." It eliminates gradients, it
doesn't fill them up. It is weird that there is a way to increase overall
entropy by bringing diffuse parts together.

Despite the term "entropic gravity", I still don't understand how this or
other versions of gravity do not violate the second law. Does anyone here?

~~~
username90
Entropy is not just about position, but about temperature as well. Objects
with higher temperature have higher entropy since you have more options with
how velocities are set, hence entropy increases as things gets pulled together
by gravity and energy gets spread out among the particles, at least with a
classical interpretation of gravity.

~~~
vanderZwan
> _Objects with higher temperature have higher entropy_

That surely must be a bit too oversimplified, since that would imply that the
beginning of the observable universe had more entropy than the current one
(given that it was way hotter than the present)

~~~
hvidgaard
We know that early universe was low entropy - that is the explanation for the
2nd law of thermodynamics. That is known as the past hypothesis. The laws of
physics dictate that the most likely state to come from and go to, is higher
entropy because there are vastly more high entropy states than low. That
directly violates the 2nd law, but is not unreasonable. Imagine 2 boxes of gas
with 10 gas molecules in side A and B. They are separated by a wall with a
single small hole in it. Every now and then a gas molecule with go from one
side to the other, A to B, without one going the other way. We just went from
a high entropy state to a lower entropy state in a closed system.

But, coming from a high entropy state does not fit with the world we see, so
we're pretty sure that the past hypothesis is true.

One of the leading theories about the early universe and how it lead to the
world we know today, is inflation. In that theory "stuff" is basically created
out of nothing, meaning that the early universe was small, hot, and with
(relatively) few particles in a specific configuration. The inflation and
reheating dramatically increased the entropy. In other words - there was much
less stuff, and it needed to be in a particular configuration to expand into
the universe as we know it. That is low entropy.

Note: I am not a theoretical physicist, so the above is my layman
understanding of the topic.

------
Sir_Cmpwn
Would like to draw the reader's attention to the criticism section:

[https://en.wikipedia.org/wiki/Entropic_gravity#Criticism_and...](https://en.wikipedia.org/wiki/Entropic_gravity#Criticism_and_experimental_tests)

And a relevant xkcd:

[https://www.xkcd.com/1758/](https://www.xkcd.com/1758/)

~~~
ColanR
It looks like the criticism boils down to, 'its really hard to test, so let's
look at other theories for now'. Anything I missed?

~~~
Sir_Cmpwn
That interpretation of the criticism text is at best wishful thinking and at
worst willful ignorance. Take off the rose colored glasses and read it again -
there are several fundamental problems brought up.

~~~
ColanR
I don't have any rose colored glasses. I read the section, found it tough to
parse, stated what I understood, and asked for input. You have no reason to
get rude.

------
Beldin
From a comment deleted as i was writing this:

 _> 3) this predicts missing masses in specific places_

Not quite... It predicts that either

1\. the observations were incomplete (so, not enough data was fed into the
model, hence the model's predictions are off), or

2\. The model itself is wrong.

Dark matter is about option 1, modified Newtonian dynamics is about option 2.

And is great that both are being investigated. Imagine dark matter had been
proposed in 1900: the deviations in Mercury's orbit (from Newton's model of
gravity) could have been attributed to dark matter. No need for general
relativity...

Hence, both aspects should be researched: improving our observational
capabilities to detect the oossible unknowns (including putting limits on
their mass etc); and attempting new models that address these issues directly.

~~~
renox
>Imagine dark matter had been proposed in 1900: the deviations in Mercury's
orbit (from Newton's model of gravity) could have been attributed to dark
matter. No need for general relativity...

You should have a look [here]([https://io9.gizmodo.com/the-200-year-old-
mystery-of-mercurys...](https://io9.gizmodo.com/the-200-year-old-mystery-of-
mercurys-orbit-solved-1458642219)): TLDR: they looked for a 'Vulcain' planet
which could cause Mercury's precession, but they didn't found one.

And for the dark matter, you're right that both aspects should be researched,
and both ARE researched but dark matter currently fits better the experimental
data as already said. Of course until we know exactly what is dark matter,
there will be room for other theories..

------
Double_a_92
Can somebody explain to me how my "internal" imagination of gravity is wrong?
For some reason I started imagining it as a "consequence" of different space
densities. I.e. if some particle randomly moves a bit, it tends to
automatically move towards where there is "more" space to be in. Imagine a
piece of paper with many small dots on, symbolising places where a particle
could be. If a particle randomly jumps to neighbouring dots wouldn't it
eventually get trapped in areas where there are a lot of dots? (Also somehow
the presence of particles increases the amount of dots?)

~~~
chousuke
I think that intuition would imply that objects could randomly escape even
black holes, even if the sequence of random jumps required would be very
unlikely.

Also it would mean that matter somehow affects the distribution of those dots.
If the transition between dots is a quantum event, it sort of sounds like what
imagined quantum gravity could look like, but I have no idea how you'd
translate the thought experiment into actual maths.

------
dilawar
It's hard to tell apart physics and SciFi these days. Or may be it's just me.

------
Bootwizard
Does this entropy of gravity theory lend itself to the possibility of modeling
the expansion of the universe?

------
m4r35n357
[https://en.wikipedia.org/wiki/Entropic_gravity#Criticism_and...](https://en.wikipedia.org/wiki/Entropic_gravity#Criticism_and_experimental_tests)

~~~
yiyus
The theory has its problems, some of them are serious problems.

I attended a talk by Verlinde and he is well aware of them, but I liked his
answer. He basically said that even if this theory was not a valid alternative
to dark matter and was eventually proven wrong, some of their results looked
very interesting and could develop into different theories, so he thought that
it was worth to keep going further with this line of research.

Nevertheless, you could feel that he still had some hope they could fix these
problems in some way. But it looked to me like this feeling was more a
personal desire than real scientific expectations.

~~~
cryptonector
Why would you like such an answer? If proven wrong, then it's wrong -- why
continue studying it?

A better answer is that dark matter is itself not free of problems either, so
maybe some effort should be devoted to not-dark-matter theories. The entropic
gravity theory space costs very little to fund by comparison to dark matter,
so it's a reasonable thing to continue funding some of it for now.

~~~
yiyus
> Why would you like such an answer? If proven wrong, then it's wrong -- why
> continue studying it?

Because things are not black or white.

It is never clear what we get from the advancement in physics theories, but
many times we get more profit from the journey than from the final results. If
Einstein equations can be derived from thermodynamic laws, this may have some
very deep meaning, even if we still need dark matter to make cosmology models
work.

At this point, we have no perfect theory, you could say that all they have
been proven wrong in some way or another, and yet, many of them are useful.

I do not know enough about physics to know if this is the most practical way
to spend funding, but from the personal viewpoint of Verlinde, I think it
makes sense to keep working on his theory until a totally unsolvable problem
is found. But this is only my impression after attending a layman talk, I am
far from being expert.

------
PavlikPaja
I like to think of gravity as an anti entropic force.

Logically the maximum entropy possible would be with each particle placed
randomly with random momentum, but gravity makes that impossible on cosmic
scales, as such a system would soon collapse because of gravity pulling the
particles together.

We could see gravity as an entropic limiting force, with entropy increasing
statistical forces increasing entropy on the microscopic scale being
contradicted by entropy limiting gravitational effects on the super-
macrostopic scale.

------
r34
In my opinion black holes emit space just like stars emit light. That is why
galaxies are getting further and further from each other - because more and
more space is emitted. Stars settle around black holes just like planet settle
around stars - due to some balanced equilibrium.

Space is a substance - I can't imagine it any different due to existence of
gravitational waves. If something vibrates it's substantial.

~~~
danielsju6
In the wise words of my college physics professor, "Great, now use your grown-
up words (formal logic)."

~~~
r34
In the wise words of many people, it's good to keep your inner child alive
(and happy!).

My logic skills are good enough to predict down-voting of my little story ;)
Anyway, I think logically it is quite coherent.

~~~
chousuke
The problem with having "opinions" about reality is that reality doesn't care
what your opinion is, at all. If you actually formalized your idea that black
holes create space, I'm fairly certain you'd find out that it's not a good
description of reality.

It might make for some decent sci-fi, but don't confuse a cool story with
understanding.

~~~
r34
I started my opinion with "In my opinion", so it should be rather clear.

I would rather claim that due to epistemological reasons we cannot
"understand" reality. And i think that "The problem with having "opinions"
about reality is that reality doesn't care what your opinion is, at all." \-
is a great example of opinion (in my opinion) :)

~~~
chousuke
I have some fundamental disagreements with that sort of thinking. I think it's
dangerous to allow people to hide non-subjective claims behind the "it's just
my opinion" veil. It's intellectually dishonest, even if you don't mean it
that way.

Speaking of opinions in the context of reality implies that all (or at least
most) alternatives are somehow equally valid, which is plainly not true. A
justified belief is better than an unjustified one.

You have opinions about food. You may have opinions about what scientific
hypotheses are well-justified and which aren't. You can't have opinions about
how reality _is_.

You made a claim that black holes emit space. To justify it, you would have to
come up with a model that incorporates this mechanism and then see if that
model agrees with known facts before it has any practical value. If you have
no such model, please at least go a bit further and explore the implications
of your thought experiment.

As for epistemological reasons why understanding reality is impossible, that's
just meaningless wordplay. For any practical definition of "understanding
reality", it should be self-evident why opinions are irrelevant: reality is
not going to just change itself to accommodate your thoughts.

~~~
xaedes
So nobody should state ideas without full formal analysis? rude

~~~
chousuke
I'm trying to advocate for more intellectual honesty and clarity. It can
certainly be a lot of fun to just take a thought and run with it, and while
the exercise can lead to useful insights, people are also prone to seeing
patterns where none exist and stating their own ideas as if they were fact.

I think it's important to pay attention to how you communicate your ideas,
because it's _very_ easy to make completely baseless conjecture sound
convincing to yourself, let alone other people, if you're not careful.

~~~
xaedes
That may be true in general, but it surely does not apply to a post starting
with "in my opinion" and you even proceed to nag on that phrase alone.

Ironic, how does that fit with "advocating intellectual honesty and clarity"?

~~~
chousuke
"In my opinion" is not a magic phrase that you can use to say whatever you
want. Opinions are subjective evaluations of things for which there is no
justifiable objective answer, such as personal preferences and judgements.

Saying that the sky is green is not a valid opinion, because the sky is not
green, and it can be shown not to be so. Some things are not a matter of
opinion.

