
Does Dark Matter Affect the Motion of the Solar System? - GrantS
http://scienceblogs.com/startswithabang/2013/07/03/does-dark-matter-affect-the-motion-of-the-solar-system/
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cozzyd
On the other hand, the motion of the solar system is essentially why we may be
able to measure dark matter at Earth. If dark matter is made up of WIMPs, then
our ability to observe its direct (non-gravitational) effects depends heavily
on the velocity distribution function of dark matter at Earth.

Because the non-dark matter component of the galaxy formed a disk while the
dark matter halo will likely have remained roughly spherical, we expect that
Earth is moving through the dark matter halo at O(230 km/s). This has a few
important impliciations. First, it increases the expected kinetic energy of
WIMPs in our frame, thus resulting in more energetic WIMP-nucleon
interactions, making direct detection (which essentially consists of having a
bunch of material and waiting for an unexplained interaction within it)
significantly more feasible. Secondly, it leads to an annual modulation effect
(as reportedly, controversially. observed by DAMA or COGENT) related to the
Earth's motion around the sun. Thirdly, it means that recoils from
interactions with WIMPs will have a preferred direction away from the
direction of the Earth's propagation through the galaxy. Because no potential
terrestrial background can have this signature, a number of groups, including
the one I work with, are trying to develop detectors sensitive to the
direction of low-energy recoils in order to potentially unambiguously detect
WIMPs.

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karmakaze
I have a question--please let me know if I'm way off base here. Gravity
distorts space and we postulate the existence of dark matter from the
distortion of space without corresponding mass/matter. Why must space without
matter/mass be non-distorted to begin with?

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tjradcliffe
> Why must space without matter/mass be non-distorted to begin with?

It doesn't have to be. De Sitter came up with solutions to the equations of GR
that were free of ordinary matter but included a cosmological constant that is
the effect of an "inherent" curvature of space-time:
[https://en.wikipedia.org/wiki/De_Sitter_universe](https://en.wikipedia.org/wiki/De_Sitter_universe)

However, the kind of curvature you get from a cosmological constant is not
sufficient to explain everything that is observed, so even though modern
cosmologies typically have a "dark energy" term (which is another name for a
cosmological constant) they require an extra matter term (dark matter) to
describe the universe we see.

~~~
karmakaze
Great responses--different observations and types of curvature to explain. I
realize now that my question is inherently flawed--where space 'is' distorted
and we don't find ordinary matter we still need some explanation matterlike or
otherwise. I also found out about MOND and TeVeS "which try to account for the
anomalous observations without requiring additional matter" although minor,
does probe in a different direction altogether.

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omgitstom
Interesting article, I was hoping that dark matter would prove to be
observable in our solar system.

I'm more curious about dark energy though. Hopefully in my lifetime, we can
figure out what dark energy is. I'm curious what dark energy does to time
since it accelerates the expansion of space. The next golden age of science
resides in our understanding of the unknown matter and energy that we are
currently observing.

~~~
meric
Layman here, I can't help but notice the similarities of how 19th century
scientists describe an aether with magical properties to help plug holes in
their theories to dark matter.

1\. "Dark matter is a hypothetical kind of matter that cannot be seen with
telescopes but accounts for most of the matter in the Universe."

2\. "It is hypothesized to be matter that does not react to light."

3\. "Astrophysicists hypothesized dark matter because of discrepancies between
the mass of large astronomical objects determined from their gravitational
effects and the mass calculated from the observable matter (stars, gas, and
dust) that they can be seen to contain."

4\. "dark matter — that exerts a well-understood gravitational force but
neither absorbs nor emits light, and doesn’t collide (as far as we can tell)
with protons, neutrons or electrons. So while normal matter (in pink, below)
slows down and can even stick together when it runs into other normal matter,
dark matter (in blue, below) just passes right through both itself and all
other forms of matter."

It sounds to me like a kind of Aether 2.0.

~~~
tjradcliffe
In the sense that dark matter is a theoretical entity that is invoked to
explain a particular set of astronomical observations, this is a fair
comparison. Late 19th century critics of aether might have compared it to
caloric (a theoretical entity that explained a particular set of thermodynamic
observations.) Caloric turned out not to exist.

By the same token, we might compare dark matter to neutrinos, which were a
theoretical entity invoked to explain a particular set of observations on
radioactivity (the shape of the beta spectrum.) Neutrinos turned out to exist.

As such, while the comparison to aether is superficially apt, it is not
something we can draw any conclusions from, because the comparison to the
neutrino is equally apt.

Science is the discipline of publicly testing ideas by systematic observation,
controlled experiment and Bayesian inference. It is not the discipline of
testing ideas by making analogies to other ideas. There is a reason for this:
making analogies to other ideas has consistently proven to be almost
completely useless for creating knowledge of reality, while the discipline of
science has been wildly successful.

Nor are the properties of caloric, aether, neutrinos or dark matter "magical".
They are merely the ones required of an entity that is able to explain our
observations in each instance. In the case of caloric it turned out to have
self-contradictory properties, when the full deductive closure of the theory
was teased out. In the case of aether it turned out to have properties that
made predictions that were false. In the case of neutrinos the required
properties made predictions that were true.

In the case of dark matter: we don't know yet, and the only way we will ever
know is if we continue on with our program of systematic observation,
controlled experiment and Bayesian inference. There is no other way to know.

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meric
Your first two sentences is what I was trying to describe.

I may be addressing a straw man here, but anyways;

I would contest this that analogy is _completely_ useless for creating
knowledge of reality.

Analogy is a tool to translate facts into thoughts another person can
understand. Physicists describe Dark matter as an analogy to matter with
exceptions to make it easier for others to understand what they're looking
for. "It is like matter, except in these ways". If it works as a tool of human
communication, and creating knowledge of reality requires human communication,
then it is a tool that can be potentially useful.

Humans, unlike machines, rely on human interaction to cooperate. It is one
thing to say the scientific method is very useful in describing the underlying
reality in which we all exist, and yet something else altogether to show
disdain to all other kinds of thinking. The scientific method is a tool at
humanity disposal, to believe human thinking and human knowledge can be
reduced to Science with a capital "S" is to mock it.

~~~
bobwaycott
I believe the parent was intentionally particular in its choice of words on
this point, and perhaps you've glossed over that intentionality, as well as
how it differs from your idea here.

The original:

>> " _... making analogies to other ideas has consistently proven to be almost
completely useless for creating knowledge of reality ..._ "

vs your statement:

>> " _Analogy is a tool to translate facts into thoughts another person can
understand ... If it works as a tool of human communication, and creating
knowledge of reality requires human communication, then it is a tool that can
be potentially useful._ "

The parent is talking about _creating_ knowledge of reality by systematic
observation, controlled experimentation, and Bayesian inference--and then
repeating that process based on the results and predictions of the process to
gradually create knowledge of reality previously unknown. However, you're
pointing to _communicating_ knowledge of reality _in a more understandable and
relatable format_.

I do not think the parent was showing any "disdain to all other kinds of
thinking"\--because analogies are not _another kind of thinking_ , but are a
_tool for communication_.

Analogies cannot be tested and judged on their predictive capabilities. You
cannot infer new knowledge of reality from analogies. The most rigorous test
one can subject analogies to is determining whether they are, in fact,
homologous. You can use analogies to [usually poorly] communicate knowledge of
reality to those who do not already understand it, relating unknown knowledge
to something already known, familiar, and understood. This can help a person
grasp this knowledge where s/he previously did not. However, analogies do not
allow for systematic study, testing, experimenting, predicting, etc.--all the
things the scientific method allows. One cannot make Bayesian inferences from
analogies--how exactly would such a thing work? The most one can do with
analogies is use them to lubricate a conversation when one finds it necessary
to help someone lacking the requisite verbal or technical understanding join
the discussion.

If you'll grant that we can obtain both verbal and technical understanding of
any given piece of knowledge, analogies help obtain the former, but can do
nothing for the latter. That is, analogies help increase verbal understanding
by facilitating easier communication, but they do nothing for arriving at
technical understanding or discovering knowledge in the first place.

