Talk:Gravitational constant

From the article:

Two important features of General Relativity are the curvature of spacetime and the distribution of mass-energy which causes it. How are the two related? To get from an energy density to a curvature quantity multiply the energy density by G/c⁴. This is the reciprocal of the Planck force, which mediates between energy density and curvature.

I've never heard the term 'Planck force'. Googling for it finds refrences on planck.com, and various other sources of 'alternative physics[?]' ideas, but not elsewhere...

See http://www.google.com/search?hl=en&q=%22planck+force%22

-- The Anome

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Planck's force is just the unit of force in Planck units. It's not a force but rather a unit of measure of force which happens to be one in God's units. Restored the paragraph. Someone needs to copyedit for reciprocal.

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Okay, I can accept that the system of Planck units defines a 'natural' unit of force. But I still can't Google a reference to "Planck force" or "Planck's force" on anything other than sites expressing 'alternative' opinions. Please cite some references from mainstream physics justifying your restoration of the paragraph, and I'll be happy. -- The Anome

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Moved from talk:Alternative physics[?]:

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Moved here since this is meta-discussion

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Alternative physics is apparently a term that can easily get misapplied in ignorance to mainstream physics concepts as this recent (Feb. 10) Talk comment shows:

"I've never heard the term 'Planck force'. Googling for it finds refrences on planck.com, and various other sources of 'alternative physics' ideas, but not elsewhere..."

The Planck quantities (Planck mass, Planck time, etc.) are mainstream physics and the Planck force c⁴/G is simply the force belonging to that set. It is the force which gives unit acceleration to the Planck mass. Once quantities of mass, time and length are defined other types of quantity derive from them in a standard way: as the metric unit of force (newton) derives from the kilogram, second, and meter.

The commenter branded 'Planck force' as an 'alternative physics' idea after a Google search failed to turn up reference which the commenter considered satisfactory. There are academic sites, particularly those concerned with String Theory which refer to the force. I would imagine the Caltech one that Barry Madur manages has some references to Planck force as the basic tension in strings. His may simply describe it as "10⁴⁰ tons" which is the order of magnitude size and an informal handle some physicists use. The lesson seems to be not to apply epithets hastily based on one unsatisfactory Google search, or assume that because you personally haven't heard of something it must be 'alternative'.

The Talk comment was regarding an entry on the gravitational constant and was accompanied by a bold-face warning headline that essentially said "danger! alternative physics!" It looked like a case of trashing or defacing. Please consider whether Wiki etiquette discourages this.

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I don't see a problem here. Googling works as an alarm system for detecting possibly dodgy material. It's remarkably effective. But it's not infallible - it just makes me aware that the material being discussed is either

• not mainstream or
• highly technical.

I don't deny that the Planck system of units defines a force, or that the name for it would be the "Planck force", just like the Planck energy, Planck time, or Planck distance.

Look, I can define a Planck pressure as the natural unit of pressure in Planck units, but it does not mean that it is a mainstream concept (although I just found that planck.com talks about it).

I'd just like some cites to convince me that the concepts you cite don't just appear in off-beat sources. Certainly, the earlier versions of this article contained some wacky stuff that has now been removed. Certainly, some of the ideas of mainstream physics are fairly wacky-looking. My physics training is some twenty years old, so I can no longer spot the good from the bad at a glance. Some of the other contributors here are up-to-date physicists and mathematicians, who may be able to help.

I can only find one legitimate-looking reference to "planck force" on Google: http://www.eps.org/aps/meet/APR00/baps/abs/S8440009 , out of 48 matches on "planck force", most of which are from idiosyncratic alternative physics sites. Searching for "planck's force" produces even poorer results.

This is compared to

• "planck energy" which yields 109,000 Google hits
• "string theory" which yields 535,000 Google hits

You imagine that Barry Madur's Caltech site contains references to the Planck Force. I can't find a Barry Madur via Google. (Do you mean Professor Barry Mazur at Harvard, or Barry Simon at Caltech?, or maybe John Schwarz' caltech website at http://www.theory.caltech.edu/people/jhs/strings/ ?) Convince me - find a cite, quote the URL here. Again, my inability to find the site you quote worries me.

A quick search through the http://arxiv.org/ physics preprint archive fails to find the term in titles or abstracts of preprints.

Please help me, I'd love to believe that this term is in common use in mainstream physics, and that the details here are entirely correct. I guess I'll just have to do the research.

-- The Anome

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I have removed the following from the article:

Within this system of units, the unit of force is Planck's force which in conventional units is c⁴ G^-1. In general relativity, this quantity is equal to the energy density of a region divided by the energy density curvature.

The first statement is certainly true. But it's only relevant here in the context of the second statement.

All the cites I have for Einstein's equation have some constant of proportionality. Try http://math.ucr.edu/home/baez/einstein/node3 and http://www.theory.caltech.edu/people/patricia/test/Einstein35 for cites. The ratio between curvature and energy density certaily have the units of force: in Planck units the ratio is a near-unity value. But the implication the second statement in the removed para does not appear to me to be strictly true. I think the mention of the 'Planck force' idea in this article is a red herring. Einstein's equation comes out neatly in Planck units, as does much of physics, and that's very interesting. But that does not mean that the Planck unit of force has any special significance in this context.

Again: I am not an expert, I could be wrong. But I think the presence of the para cited needs justifying.

-- The Anome

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I know the value of G isn't known as well as, say, the speed of light or Planck's constant, but surely it's known to better than two places? Cite? (And if so, please clarify whether that means the first two after the decimal, or of the whole thing.) -- John Owens 10:38 Apr 6, 2003 (UTC) --

From the Institute of Physics 2002-2003 diary:

gravitational constant G: 6.673(10)x10^-11 m³kg^-1s^-2. ie, 6.673x10¹¹ plus or minus 0.010¹¹. The second decimal place is **probably** 7.

Also, searching on the Web of Science gives a hit for Planck scale physics and Newton's ultimate object conjecture by Winterberg F, printed in ZEITSCHRIFT FUR NATURFORSCHUNG SECTION A-A JOURNAL OF PHYSICAL SCIENCES 52 (1-2): 183-209 JAN-FEB 1997. This features the planck force and has been cited four times (admittedly, only by himself), including one being printed in PHYSICS TODAY.

Dragon Dave[?]

Because the position of the planets are known far more accurately than the conversion factors to SI units, calculations in celestial mechanics are carried out using the units of solar mass, astronomical units, and years rather than the standard SI units.

It is not clear to me what is known accurately and what is not. Are masses accurately known in solar masses but not in kg? The various kinds of years seem to be accurately known in seconds, unlike suggested above. - Patrick 12:55 May 1, 2003 (UTC)

The point is that if you do it in astronomical units, you don't have to know the mass of the sun, since by definition, the mass of the sun is 1. If you do it in SI units, you have to give the mass of the sun in kg, which is known only to about 5 sig figs due to uncertainties in G. Put another way GM is known very accurately to eight or nine sig figs, but because G is not known accurately, then M is not known accurately. Roadrunner

Thanks. I edited the article accordingly. It would mean that also the mass of the sun can have an error of 1 to 700, which is 3, not 5 sign. digits. - Patrick 16:41 May 1, 2003 (UTC)