Wikipedia:Reference desk/Archives/Science/2014 July 3

= July 3 =

Mach's Principle
I am wondering if there has been an experimental confirmation of Mach principle? I am wondering also it such an experiment makes sense? Well, one of the formulations of Mach principle is as follows: "Events in local inertial frame are dependent on mass distribution of distant stars."

A few questions: how distant is "distant?" Can it be defined in terms of the parsecs? Ten parsecs? One hundred? A kilo parsec or what? Where does the influence end?

Our Sun is located on the periphery of the Galaxy, off center, thus it can be surmised that the gravitational pull of the "distant stars" that determine local inertial frames should be one sided to an extent. Can it be verified?

Imagine a plank rotating around its center, sort of a carrying pole. The two shoulders of the plank must be absolutely symmetrical of course. While rotating they will generate centrifugal force. Provided the angular velocity is stable, can it be measured if the force is absolutely the same over the 360 degrees of circumference? Or the pull toward the center of our Galaxy is stronger than in other directions? Thanks, --AboutFace 22 (talk) 01:24, 3 July 2014 (UTC)


 * That plank length (pardon the pun) would have to be many light years long to have any measurable effect of being pulled more towards the galactic center when on the side closer to it. However, closer stars might have an effect on somewhat smaller scales, enough to noticeably perturb Oort Cloud objects, for example. StuRat (talk) 02:29, 3 July 2014 (UTC)

StuRat, I am at a loss. I cannot understand what you are talking about. What does Planck Length have to do with my question? --AboutFace 22 (talk) 02:39, 3 July 2014 (UTC)


 * That part was a pun, related to your example of a rotating plank. The serious part followed that. StuRat (talk) 03:10, 3 July 2014 (UTC)


 * Mach's principle by itself is really too vague to be testable. From the article, "... because the principle is so vague, many distinct statements can be (and have been) made which would qualify as a Mach principle, and some of these are false."  On the other hand, general relativity was to a certain extent inspired by Mach's principle, although it isn't 100% compatible with it, and general relativity certainly is precise enough to be testable; see Tests of general relativity.  Red Act (talk) 02:56, 3 July 2014 (UTC)

StuRat, thanks. I must have been tired last night, I did not notice the pun when I typed my question. I guess Planck and plank are separated by many convolutions in my brain. Anyhow, RedAct says Mach's Principle is too vague to be tested, so what? What about the centrifugal force? Is it caused by the whole mass of the Universe or not? --AboutFace 22 (talk) 13:39, 3 July 2014 (UTC)
 * The only way you could answer that question scientifically is by extracting a numerical prediction from Mach's principle that's different from a prediction of general relativity, but Mach's principle is too vague for that. At best you might "predict" something totally different from GR, in which case it's probably ruled out by existing data.
 * You can alternately ask "how Machian" general relativity or some rival theory is, philosophically. People have argued about this for the last century. On the one hand, in GR the gravitational field is spacetime and does completely determine inertial behavior, and massive objects are sources of the field, at least in the conventional terminology. On the other hand in the Kerr solution for a rotating massive object, the object rotates relative to the vacuum despite being the only object in the universe, which seems quite non-Machian.
 * Brans–Dicke gravitation may still be consistent with experiment, and I seem to recall that it was promoted as more Machian than GR by its creators. I'm not sure what they meant by that or whether anyone else agreed with them. -- BenRG (talk) 15:43, 3 July 2014 (UTC)


 * As others have pointed out, Mach's principle is more a suggestive idea than a definite predictive theory. This makes it hard to pin down.  One reading is that, according to Mach's principle, it's impossible for the matter in the universe to have an overall net rotation.  In the bucket experiment, this means that the surface of the water has to be flat when the bucket isn't rotating relative to distant galaxies.  This is hard to test, because we find that the matter in the universe doesn't have any overall net rotation, and our LCDM cosmology doesn't predict any such rotation.  So the question becomes whether such a net rotation can't exist in principle, or could exist but doesn't.  Since this is a question of hypotheticals, there's no way to test it empirically.  Of course, there are other readings of Mach's principles and other Machian ideas, which have been suggested above.  --Amble (talk) 20:17, 3 July 2014 (UTC)

Thank you everyone who contributed but the answers left me disappointed. I posed a specific question but the answers drifted into much criticized vagueness of Mach's Principle. Phrased differently my question is about a possibility of detecting the contribution the gravitational field 26,000 light years away makes to the curvature tensor here on earth if you like. With that rotating plank one can imagine one-directional dynamometers built into respective ends of the plank and while it is rotating the readings could be made. If the readings (and I am talking about the centrifugal force F here) are uniform and do not change at different angles then either the contribution is below threshold or nonexistent. I also assume that the experimenter took care of finding the most advantageous initial orientation of the plank which should roughly coincide with the Galactic Plane. What kind of numerical predictions are needed here? The idea of the net rotation of the Universe I don't even want to touch. I consider it absurd. Net rotation in respect to what? Is there a reference point "outside the known Universe" to effect such a measurement? Thanks, --AboutFace 22 (talk) 16:40, 4 July 2014 (UTC)


 * I think the answers fit the question you asked about Mach's principle. Your rephrased question seems to be about general relativity instead (since you mention the curvature tensor).
 * To get a very rough estimate of the locally detectable effect of the mass of the Milky Way, I'll treat it as a point mass of 1042 kg at a distance of 26,000 ly. Plugging that into the tidal acceleration formula 2GM/R³ gives ~10−30 gee/meter. I think it's safe to say that this is undetectable by experiments in the solar system since the tidal effect of the sun even at 100 AU is ~1010 times larger.
 * In GR it is possible for the universe as a whole to rotate in a detectable way. The rotation would have no center, but would have an angular velocity (magnitude and direction), which we could detect as an anisotropic Doppler shift of the cosmic microwave background. There is in fact a large-angle anisotropy in the CMBR which selects a preferred axis, but it's small enough that it can be blamed on cosmic variance. In fact cosmic variance implies that the visible universe must (with probability 1) have a nonzero net angular momentum. -- BenRG (talk) 17:55, 4 July 2014 (UTC)
 * Is it impertinent to say that in cosmology any argument about what is "most probable" is (most probably!) an anthropocentric fallacy? 84.209.89.214 (talk) 17:56, 6 July 2014 (UTC)
 * Not at all in my opinion. Cosmological arguments about what is 'most probable' are, in my opinion, at best meaningless. Martin Hogbin (talk) 12:00, 7 July 2014 (UTC)

BenRG, thank you for the calculations which I certainly could have never done myself. Using your numbers I am tempted to say that centrifugal forces on Earth do NOT have any contribution from the mass of the distant stars. It seems to indicate that the Mach's principle is false? Then what is the physical explanation of the centrifugal force? Thanks, --AboutFace 22 (talk) 18:45, 4 July 2014 (UTC)

That net angular momentum of the Universe is another fascinating topic I've tried to understand. I even posted about it before. Since you mentioned it here I want to ask you (1) if the net angular momentum is the sum of ALL angular momenta of rotating galaxies and other mass? And (2) if the individual angular momenta of visible large scale structures of the universe originated because of the net Universe angular momentum which must have appeared in the first super small fraction of the second after the Big Bang. When I posted on this topic before the meaning of the answers that came seemed to indicate that all local rotations are derived from unequal mass distribution or sound waves in the early Universe. --AboutFace 22 (talk) 19:10, 4 July 2014 (UTC)

Flush survivors
In general, which of the following will survive a flush down the toilet if healthy before the flush?


 * rats
 * goldfish
 * insects

(Before you animal lovers get up in arms, the question is hypothetical.)

—SeekingAnswers (reply) 05:23, 3 July 2014 (UTC)


 * Rats will drown, and goldfish would suffocate from toxic methane gas. Some insects might survive, though, but I'm not sure about that. 24.5.122.13 (talk) 05:31, 3 July 2014 (UTC)


 * Flushing is a brief, transient event. What would really matter is where the results of a flush end up.Sewerage systems vary a lot from place to place. Some would be not much different from open flowing water. Some would be far worse. HiLo48 (talk) 05:40, 3 July 2014 (UTC)


 * My thought was that a rat ought not to go gently into that good night. Fortunately, through the use of YouTube one can obtain objective data, with a certain risk of fraud.   seemed persuasive enough that the second flush is enough to put a mousy-looking "rat" down the hole - that I never expected.  On the other hand, sometimes the rat makes it the other way.   According to the Daily Mirror even a puppy can survive the trip, though I don't think it would do well in the sewers.  (With alligators YMMV) Wnt (talk) 05:58, 3 July 2014 (UTC)


 * I've seen videos of and heard reports of rats climbing out from toilet drain holes, and I've personally seen, on multiple occasions, insects doing the same, both of which lead me to believe that rats and insects will sometimes (on a semi-frequent basis, but not always) survive flushes. I'm looking for more insight into how they do so and under what conditions they will survive. —SeekingAnswers (reply) 02:17, 4 July 2014 (UTC)


 * Wouldn't the survival rate depend mainly on the configuration of the drainage system immediately after the U-bend? A gentle but free-draining gradient with some irregularities to hang on to should ensure that most mammals and insects would survive a brief immersion in (not too polluted) water.  For fish, the quality of the water lower down would be critical.  If flood-water drainage is mixing with the sewage then there would be a chance of survival for a time.    D b f i r s   07:07, 4 July 2014 (UTC)

Lost city of Ubar
I need some help finding the Lost city of Ubar, or more specifically some clarification on what SIR-C/X-SAR found and how Iram of the Pillars relates (or not). Please follow up here. Thanks! -- ke4roh (talk) 21:04, 3 July 2014 (UTC)


 * This paper says the shuttle data suggested a number of sites and that "these sites were targeted for a subsequent ground based expedition ... Most useful were the Landsat image data which revealed a network of tracks that converge at the modern day village of Shisr. Archaeological investigations of the ruins at Shisr indicate that it is most likely the site which inspired the Uhar legends."    Shisr (also transliterated as Shis'r or Shasar) is in the Dhofar Governorate of Oman, here - note that Google Maps labels it "Ubar". This archeology, and a discussion of whether it really is Ubar or anything like it, is discussed in at Atlantis of the Sands and following sections. A few photos of Shisr, again with things claimed to be Ubar, are at commons:Category:Shisr, Dhofar province, Oman. -- Finlay McWalterᚠTalk 21:51, 3 July 2014 (UTC)


 * Note, incidentally, that the co-author of that paper is NASA scientist Robert E. Crippen and not NASA astronaut Robert L. Crippen. -- Finlay McWalterᚠTalk 21:58, 3 July 2014 (UTC)