Talk:General relativity

Improper reference

To Ancheta Wis (talk · contribs): You added a 'reference' to the General relativity#History section which says "See course notes by John Archibald Wheeler (1962), Geometrodynamics, followed by a 1964 book by Wheeler & three of his students, followed by a 1966 book on Spacetime Physics, etc. Wheeler & Ford 1998, p. 253 stated "In earlier years [before 1956], I had feared that a student trained [to Ph.D. level] in general relativity might have trouble finding a job...".". This is not a single reliable secondary source. Rather it appears to me to be a violation of WP:SYN. Since you re-added it after I commented it out, I must ask you to justify why it is not a violation of policy or else revert yourself. JRSpriggs (talk) 09:58, 24 August 2011 (UTC)

Can any one take a tea-break, and proceed with the same velocity, while traveling on the geodesics?

(i)As I understand, according to The General Theory of Relativity, Geodesic is a 'straight' line in the "curved space-time"; and motion of the planets are inertial-motions along the geodesics. Now, while traveling in straight-line on the flat surface one can take a tea-break and then proceed further. Is it possible to take a tea-break while traveling along the geodesic, and then proceed further with the same or different speed? (ii) Inertial motion of an object can be at any speed; is it possible for the planets to travel at any speed along their orbits? Hasmukh K.Tank.123.201.22.165 (talk) 14:37, 6 September 2011 (UTC)

(i)I understand taking a break as waiting some time in one given point in space.Here time is one of the coordinates of the space where the geodesic is being followed, so taking a break makes no sense since there isn't a second time coordinate with respect to which to take a break. (ii)Speed is defined as variation of space with respect to time but time here is one more coordinate of space so the inertial motion also fixes the speed. Now different geodesics will of course lead different speeds, but they will also lead to different trajectories so the planets would not be able to follow the same orbits at different speeds. Dauto (talk) 23:01, 6 September 2011 (UTC)

(ii) Inertial motion can be at any speed; whereas planets are compelled to travel at a specified speed predicted by Newtonian mechanics. I am not able to understand what Einstein wants to convey when he says: "Planetary orbits are just inertial-motion of planets in the 'curved space-time'. Moreover, can any one travel at any desired speed in time? Time always flows at its own speed, so time-axis is very much different from the space-axis; and so, in the so-called space-time-continuum, all axis are not equivalent. Hasmukh K. Tank123.201.22.176 (talk) 17:42, 7 October 2011 (UTC)

Planets are not compelled to travel at only a specified speed. If a planet has an encounter with another celestial body, its speed will change and as a result the path it follows through space will change as well.

In relativity, time is treated as a spatial dimension. So it is best to parameterize the trajectory of a particle with a fifth coordinate — it could be same as: the proper time, the time dimension in this or another coordinate system, a spatial dimension in some coordinate system, or some other convenient parameter. A point particle in free fall obeys the equation

$\frac{d p_\mu}{d t} = \Gamma^{\lambda}_{\mu \nu} p_{\lambda} \frac{d x^\nu}{dt} \,$

where $x^0 \,$ is the time coordinate and $t \,$ is the parameter (fifth coordinate) chosen for this particle's trajectory, $\Gamma^{\lambda}_{\mu \nu} \,$ is the Christoffel symbol (which is the gravitational force field), and $p_\mu \,$ is the linear momentum 4-vector given by

$p_{\alpha} = m \, g_{\alpha \beta} \, \frac{d x^{\beta}}{d \tau} \,$

where $m \,$ is the mass and $\tau \,$ is the proper time measured along the particle's trajectory. JRSpriggs (talk) 20:21, 7 October 2011 (UTC)

Implications of the OPERA Experiments Findings

Should a section, or note, be made regarding the faster than light neutrinos detected during the OPERA experiments and the potential implications they have in regards to General Relativity? — Preceding unsigned comment added by 216.123.241.196 (talk) 22:19, 20 November 2011 (UTC)

No. (mostly because there are none.)TR 22:44, 20 November 2011 (UTC)

'group=' okay?

Would it be objectionable to separate footnotes that are discussion notes, from footnotes that are citation refs, using 'group='? (The current Notes section intersperses both, of course.) Why? (I hope no one asks that!) Cheers, Ihardlythinkso (talk) 02:28, 30 December 2011 (UTC)

Vandalism

Somehow the code:

< div style="position: fixed; top: 0px; left: 0px; z-index: 99"><a href="/wiki/File:Autofellatio5.jpg" title="File:Autofellatio5.jpg"><img alt="Transparent.gif" src="//upload.wikimedia.org/wikipedia/commons/c/ce/Transparent.gif" width="1000000" height="1000000" /></a>

got inserted into the page. Firefox reads it and links the entire page to the pretty picture. IE has no issue with this. I can't find it in the source to remove it.

108.12.27.87 (talk) 04:22, 25 January 2012 (UTC)

It was vandalism on hidden, transcluded templates, which was reverted, but might still show up in some cached pages. Materialscientist (talk) 05:18, 25 January 2012 (UTC)

Gravity Wave

The Gravity Wave section mentions a decrease of 10^-21. However, they do not use units and I have not the necessary knowledge to correct it myself. Bayushikazemi (talk) 17:49, 20 February 2012 (UTC)

It means an increase or decrease of at most 10⁻²¹ meters for each meter length of the affected rod (or other delimited interval of distance). Since the meters in the numerator cancel the meters in the denominator, they do not need to be mentioned. Indeed one could use any other unit of distance (e.g. angstrom, inch, astronomical unit, light year, parsec) instead. JRSpriggs (talk) 06:09, 21 February 2012 (UTC)