Dark Energy Tested on a Tabletop | Hacker News Reader

Dark Energy Tested on a Tabletop

quantamagazine.org

21 pointsdigital559 comments

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.... We really need an inclusive term for the whole swath of people from across the political spectrum who reject science.

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?

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

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.

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?

Scientific journals don't always contain articles, or papers.

My understanding is being published on arxiv.org does not mean it is currently in a scientific journal. Being in a scientific journal does not mean it is on arxiv.org.

But, being published on arxiv.org is usually a precursor to the journal publication, passing peer review and the journal 's standards.

An author can choose to post their paper on the arxiv or not, as they see fit (policies of the journal they wish to publish in aside). The papers on the arxiv are not peer reviewed, but many are of high quality. The idea is for researchers to have a faster mechanism than standard journals to share results.

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

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.

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