
Dark Energy Tested on a Tabletop - digital55
https://www.quantamagazine.org/20150331-dark-energy-tested-on-a-tabletop/
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tjradcliffe
Sometimes non-physicists accuse physicists of ad hockery with regard to dark
matter and dark energy, and chameleon models are very useful in that case
because they show what ad hockery _really_ looks like:

"Chameleon models are not especially well motivated from the standpoint of
fundamental physics, admits Burrage, who began studying them in graduate
school, but since dark energy presents such a profound mystery, physicists are
willing to consider just about anything."

They are also something to bring up when your New Age [1] friends claim that
some weird effect "can't be tested by science" because it only happens when no
one is paying attention, or something. Chameleon models are specifically
designed to be "not there when there is a detection apparatus present", and
yet to be useful as an explanation at all they have to have _some_ interaction
with the rest of the world, so it turns out we can test them regardless.

This is a general property of things that are useful in explanation: if they
can explain something, they can cause something, and if they can cause
something, they can be investigated by systematic observation, controlled
experiment, and/or Bayesian inference.

[1] I just realized how dated the term "New Age" sounds. It turns out it went
out of fashion in the '90's:
[https://en.wikipedia.org/wiki/New_Age#Social_and_political_m...](https://en.wikipedia.org/wiki/New_Age#Social_and_political_movement).
We really need an inclusive term for the whole swath of people from across the
political spectrum who reject science.

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omgitstom
Really confused about what they are attempting to test under the tabletop
configuration.

It seems like their hypothesis needs to be tested in the middle of space away
from the gravitational force of the sun and earth. If your calculations are
off by a factor of 10^120, then making the test 1000 times more sensitive
isn't going to help either.

Wouldn't scientists have had evidence of this being the case with the pioneer
11/10 anomaly? There was a slight deceleration that couldn't be accounted for,
if dark energy was present in a vacuum wouldn't the pioneer probes (and the
voyager probes now) show an acceleration that matches initial calculations?

~~~
JoeAltmaier
Wasn't the Pioneer anomaly explained as light pressure from the infrared
coming from their own power supply?

~~~
omgitstom
It was, the thermal recoil force! It is either that, or back to the drawing
board for a 3rd attempt at calculating gravity.

Humans have sent a few satellites in deep space now, in a vacuum, away from
matter. Thinking of the chameleon effect, I would believe scientist should see
the presence of a force accelerating these craft that can not be explained
easily by gravity?

The anomaly should be acceleration, not deceleration.

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diminoten
Not really on topic: so arxiv.org is a "preprint" site for scientific papers
-- does that mean _all_ papers go to arxiv.org, and then only the ones that
pass peer-review actually end up in journals? Or are the papers on arxiv.org
considered "published" already, and are on the site for archival purposes?

~~~
privong
Almost anyone can post a paper to arXiv (I believe in practice it requires an
affiliation with a University or Insittute, or endorsement from someone who
has posted to arXiv previously). In that sense, posting to arXiv is not
considered "publishing" a paper, though, for all practical purposes, posting
to the arXiv is a good way to make people aware of your results or ideas.

Posting papers to arXiv is up to the discretion of the authors. Undoubtedly,
there are papers which are peer reviewed that do not have preprints posted to
the arXiv. Often people will post to arXiv as the papers are going through the
peer review process. This enables the community to provide feedback in
addition to the peer review process. Sometimes people do not post papers to
arXiv until they have already completed the peer review process, but before
the paper is actually published in a journal. And, less frequently, people
will post past manuscripts to the arXiv if they think the community would
benefit from having easier access to that piece of work (e.g., a chapter in a
book that has gone out of print, for which the author still retains
copyright).

I am speaking mostly from personal experience here, based on my interactions
with, and postings to the astrophysics portion of arXiv[0]. Norms may differ
in other fields using arXiv.

[0] [http://arxiv.org/archive/astro-ph](http://arxiv.org/archive/astro-ph)

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fargolime
Look up the relativistic rocket equations on the Usenet Physics FAQ. Calculate
in a spreadsheet what a crew member of a thrusting rocket would observe after
throwing a ball upward at close to the speed of light. Chart the path of the
ball's free fall. Use general relativity's equivalence principle to realize
the same _initial_ behavior must be observable to a person standing on the
ground. You've found "dark energy". A little logic from there shows that high-
redshift supernovae must accelerate away from us (the 1998 discovery that lead
to the idea of dark energy). Good luck finding anyone else who'll look at your
numbers.

