Talk:Dynamical friction

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Google[edit]

~~(William M. Connolley 10:03, 11 Jul 2004 (UTC)) No obvious evidence for the use of this term found via google. OTOH the everyday meaning of friction is found.~~

(William M. Connolley 12:35, 11 Jul 2004 (UTC)) Hmm, perhaps I'm wrong: http://citebase.eprints.org/cgi-bin/citations?id=oai:arXiv.org:astro-ph/0305052

I tried Google out of curiosity (since your claim seemed unbelivable) and got 13,400 links to articles mentioning dynamical friction (are you sure your spelling was right?). Here is a link to the first of them: http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1976ApJ...203...72T&db_key=AST

Jim 19:45, 11 Dec 2004 (UTC)

(William M. Connolley 19:57, 11 Dec 2004 (UTC)) I can't account for my first comment, which is clearly wrong. I think my second was intended to correct it.

Dynamical friction per se is fine with me. But I'm under the impression, that it can only effect "slowly" moving things and that the effect should be zero for something traveling at the speed of light. The increased gravity formed by masses being attracted to the moving mass-energy, has no chance to influence the mass-energy speeding away with c. --Pjacobi 10:46, September 4, 2005 (UTC)

The last part of the article as it was written was crackpot cosmology. I've removed it and clarified why it's unphysical. --TMB 14:11, 8 Dec 2005 (AEDT)

Is anyone going to be brave enough to put a derivation down for the full dynamical friction formula?

${\textbf {F}}_{dyn}=M{\frac {d{\textbf {v}}_{M}}{dt}}=-{\frac {4\pi ln\Lambda G^{2}\rho M^{2}}{\nu _{M}^{3}}}\left[erf(\nu /({\sqrt {2}}\sigma )-{\frac {{\sqrt {2}}\nu }{\sigma {\sqrt {\pi }}}}e^{-\nu ^{2}/2\sigma ^{2}}\right]{\textbf {v}}_{M}$

I would if I could understand it.

Zwicky and drag on photons[edit]

I think the whole section on Zwicky is invalid. As noted in the article, the effect of gravitational drag is generally to transfer momentum away from more massive bodies. Zwicky's proposal for a transfer of momentum away from photons seems particularly implausible.

In fact, the same year that Zwicky wrote his 1929 paper proposing gravitational drag as a way of obtaining cosmological redshift, an error in his analysis was pointed out by Arthur Stanley Eddington. Zwicky was quick in acknowledging the correction in a letter to Physical Review.

(Zwicky, F. (1929). "On the Possibilities of a Gravitational Drag of Light". Physical Review. 34 (12): 1623–1624.)

Zwicky did still continue to think that the method might be made to work, but as far as I know there has never been any analysis confirming gravitational drag on light over long distances in deep space, and because the relativistic mass of photons is so small the analogy that Zwicky attempted seems doomed to fail. Comments? —Duae Quartunciae (talk · cont) 05:49, 31 July 2007 (UTC)[reply]

Followup. I have been WP:BOLD, and submitted some revised content for this section. I am still inviting comment, of course. —Duae Quartunciae (talk · cont) 06:15, 31 July 2007 (UTC)[reply]

Conservation laws section[edit]

The initial sections of this article are very poorly written and confusing. The concept itself is pretty straightforward, the discussion of conservation laws does not need to be this opaque. I'm not sure how to fix it. —Duae Quartunciae (talk · cont) 07:03, 31 July 2007 (UTC)[reply]

Again, I have been WP:BOLD. I have taken the initial section to be a simple intuitive introduction, intended to help understand how the effect occurs without going into a lot of mathematical detail. I have thus stripped out subsections, and avoided the mention of the divergence formula, which does not actually add much to the topic, in my opinion. You can see a diff of the changes applied to the initial section here: [1]

Would other editors prefer me to restore subsection headings for the "wake" and the "conservation" argument? Let me know! —Duae Quartunciae (talk · cont) 07:52, 31 July 2007 (UTC)[reply]

Chandrasehkar formula[edit]

I have also rewritten the Chandrasehkar formula. Previously it had some bad formatting and mismatched brackets, and none of the variables were actually defined. I've added in the definitions, and formatted the functions Ln and erf using the LaTeX mbox form. The formula is basically from Binney and Tremaine, but I'd really appreciate someone double checking that I've described it all correctly. —Duae Quartunciae (talk · cont) 16:01, 31 July 2007 (UTC)[reply]