Talk:Surface gravity

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)

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)

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)

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)

"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)

Of course one could use any definition one chose of "surface" for a given purpose, but the question still stands. What definition of "surface" is used in the table of planetary surface gravities? On Earth it's presumably standard sea level. On an airless body like the Moon it might be something like the mean ground level, a different definition. On a gas giant like Jupiter it must be a different definition again, and it can't just be "the top of the atmosphere" because the atmosphere doesn't have a distinct top. What definition of "surface" is used? — Preceding unsigned comment added by 108.36.196.84 (talk) 06:27, 18 October 2015 (UTC)
 * Pick a surface, any surface. It is a reasonable question, but the definition of surface gravity does not depend on the answer.  The definition is different for a rocky planet, a gas giant, a star like the sun, or a supergiant. Lithopsian (talk) 22:19, 17 May 2016 (UTC)

Hmm, according to http://coolcosmos.ipac.caltech.edu/ask/112-How-strong-is-the-gravity-on-Saturn- the surface gravity of a gas giant is measured at the 1 bar level, which seems convenient if you're interested in space colonization but otherwise arbitrary. — Preceding unsigned comment added by 108.36.82.245 (talk) 21:25, 17 May 2016 (UTC)

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)

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)

Edit Request
The Mass, Radius and Surface Gravity section says "g" is in units of Earth's g and then goes on to say it is equal to the more familiar Newton's Law formulation (g = GM/r²) with only the obscure statement "Without using Earth as a reference body..." to indicate that g is not g. Does using it as a reference body mean its mass and radius are used in the equations? If so, then what does it mean to not use it? The prose is obscure. The only people, imho, that will understand this 'explanation' for the change in units of r, m, etc. are people who are unlikely to need to read the section in the first place. I suggest that g should (obviously, imho) be used in only one way in each section, and preferably only one way in the entire article. In other words, pick another symbol for the "reduced" g, since it seems that is only briefly used. I suggest gamma, although using ge may be just as good.98.21.64.18 (talk) 13:23, 23 June 2017 (UTC)

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Simple algebraic expression
I added a simple algebraic expression
 * $$\kappa = \frac {c^4} {4GM} = \frac {c^2} {D_{BH}}$$

to the black hole surface gravity formula in SI units.

User:Tarl_N. argues that this derivation is non-obvious and that it would take a non-physics expert more than a minute or two to figure it out.

I disagree. This is a routine primary school and non-original calculation. Schwarzschild radius is $$ R_{BH} = \frac{2 G M_{BH}}{c^2}$$. Hence $$ M_{BH} = \frac{R_{BH} c^2}{2 G}$$. Thus a black hole surface gravity is $$g_{BH} = \frac{G M_{BH}}{R_{BH}^2} = \frac{G}{R_{BH}^2} \frac{R_{BH} c^2}{2 G} = \frac{c^2}{2 R_{BH}} = \frac{c^2}{D_{BH}}$$.

Does such a trivial and routine calculation violates WP:CALC, WP:OR or WP:SYNTH Wikipedia policies?

There is a simple formula for escape velocity. Assuming that it cannot exceed the speed of light you get the Schwarzschild radius of a black hole. This is primary school arithmetic. Guswen (talk) 22:18, 8 December 2021 (UTC)
 * The full discussion is here. There are several issues:


 * 1) That is not what they originally inserted and re-reverted attempting to force in. See here.
 * 2) The derivation is not obvious - at least not without specifying where the terms are coming from. That's what could be solved with a footnote.
 * 3) It's uncited. From the discussion, it's obvious that this has never been published somewhere. Thus this qualifies as WP:NOR or WP:SYNTH.
 * 4) As best I can tell, it's unnecessary for the article. It seems to be a cute factoid, but given that Guswen can't find someone who has published this somewhere else, why do we need this factoid in this article?
 * 5) Lastly, by describing the discussion in terms of "you can't understand primary school arithmetic!", I've become irritated.
 * Again, if Guswen disagrees with my conclusions above, they aer welcome to any of the three dispute resolution mechanisms I suggested. Simply taking it to another forum and starting the discussion over from scratch isn't generally the best way to do this. Regards, Tarl N. ( discuss ) 01:21, 9 December 2021 (UTC)
 * I am really sorry if I made you feel irritated. Honestly I did not mean to. (To my excuse kindly note that I never said anything like "you can't understand primary school arithmetic!").
 * I believe this cute factoid (that's a brilliant definition) may be insightful for further research. I argue that such a simple, obvious, non-original algebraic expression doesn't have to be published anywhere.
 * If I, inadvertently, violated some Wikipedia polices by taking the resolution to another forum and starting the discussion over from scratch, I do apologize. I am not a Wikipedia expert but I will draw conclusions from this misbehavior. Guswen (talk) 08:29, 9 December 2021 (UTC)
 * Can I restore this diameter expression? Guswen (talk) 09:20, 15 December 2021 (UTC)
 * It has been contested, reverted, and placed for discussion. You have not achieved consensus that your addition is an improvement (that is, nobody else has said it should go in). You can always try one of the three resolution mechanisms I mentioned if you don't like my answer. Tarl N. ( discuss ) 11:31, 15 December 2021 (UTC)
 * I seem to have tried one of the three resolution mechanisms you mentioned, and you raised an alert. Would you please try another one? (It remains a mystery to me why do we even discuss such obvious primary school arithmetic. Do we need a consensus to say that 2+2=4?) Guswen (talk) 22:58, 19 December 2021 (UTC)
 * There's no G in my obvious version and that's an improvement. Guswen (talk) 23:02, 19 December 2021 (UTC)
 * I suggested "possibly third opinion, request for comment, or dispute resolution noticeboard". You chose a 4th path (venue shopping), which is generally frowned upon. The dispute resolution methods are not appeals courts where you can infinitely try to get an appeal until success. They are a method of trying to resolve differences between editors. There is a difference between us, and the dispute resolution methods are attempts to find out if someone is being unreasonable. Pick a resolution method, noting that in general, people resolving disputes are not inclined to look favorably on being the 3rd or 4th attempt to resolve a dispute.
 * At this point, we are (per WP:BRD) in the 3rd phase of "discussion", and reached an impasse. That means as far as I'm concerned your change is unacceptable. It remains unacceptable, because it isn't sourced on anything published. Wikipedia exists to summarize and index other publications - NOT to come up with something new. Repeatedly calling it "primary school arithmetic" (to demonstrate how smart you are that such things are so immediately obvious they don't need to be stated), does not improve your credibility. It is far beyond WP:CALC (which envisions things like calculating a person's age based on birthdate). Tarl N. ( discuss ) 02:41, 20 December 2021 (UTC)
 * By the way, I just noticed you already tried to take it to WP:DRN on 8 December, and were turned away - . You missed something in the instructions: Ensure that you deliver a notice to each person you add to the case filing by leaving a notice on their user talk page. That didn't happen. The comment he made is also relevant, "Nothing about black holes is primary school level", which should give you a hint about the direction the discussion would have taken. Note that this doesn't preclude you from still taking it through one of the resolution mechanisms, but read the instructions more thoroughly next time. And consider his comment a preview of the result. Tarl N. ( discuss ) 18:57, 21 December 2021 (UTC)
 * Łorever. I just don't know why would one want pursue a struggle to hide such a simple truth. You won mate. Guswen (talk) 04:48, 22 December 2021 (UTC)
 * No. You didn't. Bring this issue under arbitration if you question this primary school arithmetic. Guswen (talk) 13:12, 5 March 2022 (UTC)
 * You've been informed what the appropriate conflict resolution paths are - simply waiting several months and trying to sneak it in isn't one of them. Tarl N. ( discuss ) 00:17, 7 March 2022 (UTC)