Talk:Surface gravity

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Contents 1 Proposed preamble to provide `context for readers unfamiliar with the material' 2 Second sentence 3 Ideas for a more basic introduction 4 Objection to the "rewrite" 5 "Surface" 6 Need for a more accessable introduction 7 Article organization

[edit] Proposed preamble to provide `context for readers unfamiliar with the material'

"In relativity, the Newtonian concept of acceleration turns out not to be clear cut. For a black hole, which can only be truly treated relativistically, one cannot define a surface gravity as the acceleration experienced by a test body at the object's surface. This is because the acceleration of a test body at the event horizon of a black hole turns out to be infinite, when the acceleration is defined as the locally measured magnitude of the four dimensional relativistic acceleration.

Therefore, when one talks about the surface gravity of a black hole, one is defining a notion that behaves analogously to the Newtonian surface gravity, but is not the same thing. In fact, the surface gravity of a general black hole is not well defined. However, one can define the surface gravity for a black hole whose event horizon is a Killing horizon."

I'm not an expert on what constitutes a reasonable context for a reader unfamiliar with the material, so any comments would be welcome. I think it is long enough, but it could be made much longer. Maybe one would add a sentence or two explaining that the force required to be exerted at infinity is not the same as the force required to be exerted at the horizon.--Eujin16 06:03, 6 November 2007 (UTC)

One possible definition of the surface gravity of a black hole that is finite and non-zero is the total gravitional flux around the black hole at a distance where the gravity is Newtonian divided by the surface area of the black hole. I can't tell whether the definition given is equivalent to this. Karl (talk) 13:35, 12 February 2008 (UTC)

[edit] Second sentence

The second sentence of the article is "It is often expr the term (such as the definition given for black holes given below) are natural extensions of this concept." As a fluent English speaker, that sentence has zero meaning to me. Could someone who knows (or can at least guess) what it's trying to say add a verb? 71.201.66.22 05:27, 27 April 2007 (UTC)

[edit] Ideas for a more basic introduction

If I am correct, the current main paragraph starting with: "The surface gravity κ of a Killing horizon is the acceleration, as exerted at infinity, needed to keep an object at the horizon." is just another example. It should be put under "Surface gravity of a Killing horizon" or "Black hole surface gravity".

My stab at an into paragraph:

Surface gravity is a mathematical calculation of the physical acceleration needed to hold an object in a defined path. (maybe s/path/orbit). Methods from many diverse branches of mathematics are needed in the calculation of the surface gravity. The mathematics involved depend on the physical constraints of the object and what it is orbiting. Classic examples are planetary orbits and photon orbits around black holes. I'm not an expert in this field so didn't make the change to the main page. Jeff Carr 10:38, 21 January 2006 (UTC)

[edit] Objection to the "rewrite"

I have several objections to Yevgeny Kats's "rewrite" of the introduction:

• It is overly specific, defining surface gravity in a way that excludes several uses of the term, including the technical section that was originally present and still follows it
• It uses non-standard terminology ("stellar bodies") to further restrict the definition
• It doesn't actually explain what surface gravity is, or provide any conceptual context; it's akin to saying "your temperature is what a thermometer measures when you stick it under your arm"

I intent to revert to the previous version, and continue adding to it, but will wait a few days for discussion on this point in case anyone wishes to disagree.

Dear editor, thanks for giving me an opportunity to discuss my last edit. I didn't intend to modify the meaning or omit important ideas, but just to make the definition easier to read. As for your particular comments, I don't understand what exactly you are referring to in your first comment. Anyway, I'd suggest to keep my simple definition, and add the other uses in a separate paragraph. As for your second comment, I've never heard about surface gravity referring to anything else rather than stellar bodies, so my formulation does not restrict the definition, but gives the context. As for your third comment, gravity is discussed in its own article, so it doesn't need to be reproduced here again. Anyway, if at the end of the discussion you decide to restore the previous version, please note that I also did several miscellaneous corrections that need not be reverted. Yevgeny Kats 23:23, 17 June 2006 (UTC)

Instead of "stelar bodies", do you mean "astronomical bodies"? Bubba73 (talk), 00:00, 18 June 2006 (UTC)

I've merged our versions & material from the comments here, and added some additional material (e.g. practical applications) to indicate where there's more to be said on this topic. This should (I hope) clarify some of my objections above. I also retained Yevgeny's miscellaneous corrections. --MarkusQ 17:36, 18 June 2006 (UTC)

[edit] "Surface"

May sound stupid, but what's the definition of "surface" used here? A gas planet or a star (probably) doesn't have a "surface" in terms of how that term would be understood by the layman. --Squiggleslash 10:03, 3 March 2007 (UTC)

[edit] Need for a more accessable introduction

I did A-level physics and a university degree in Biology, I was surprised to find that a planet could be five times as massive as the Earth and yet only have 2.2 times the gravity. Looking now at the equation and the discussion page on Gliese 581 c, I think I now see. But this is a general-purpose enyclopedia: if Stephen Hawkins could describe his work with just one equation, why can't it be done here?

Since the composition of Gliese 581 c is a matter of speculation, the subject could be popularised by describing what the surface gravity would be given various compositions and volumes. I'm not qualified to do it, but someone out there must be. --GwydionM 16:24, 28 April 2007 (UTC)

Since no one took this up, I had a go.--GwydionM 11:33, 27 May 2007 (UTC)

[edit] Article organization

I have combined the series of English sentences that describe the formula g=Gm/r², the formula, and the example of its usage into one section. Having two sections dealing with the same topic is undesirable. Spacepotato 21:22, 29 May 2007 (UTC)