Wikipedia:Reference desk/Archives/Science/2015 July 31

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July 31

What do the galaxies, galaxy clusters and galaxy superclusters revolve around? Do they change place?

The Moon revolves around the Earth, the Earth revolves around the Sun, the Sun revolves around the supermassive black hole at the center of the Milky Way.

Now a question will arise. What does the Milky Way revolve around? Is it that, like binary star system or triple star system, the Milky Way and other nearby galaxies revolve around themselves?

Do the galaxy groups also revolve around themselves? Do the galaxy superclusters also revolve around themselves?

But even if the galaxies, galaxy groups and galaxy superclusters revolve around themselves, this means they are fixed at a particular location in the Universe, they are not moving away from that place. The Sun is changing place within the Milky Way because it is revolving around the galactic center. But the galaxies, galaxy groups and galaxy superclusters are not revolving around something at the center of the Universe. So they are in a fixed place, right? --IEditEncyclopedia (talk) 02:30, 31 July 2015 (UTC)

The thing is, the moon doesn't technically orbit "the earth". The moon and the earth both orbit their common Barycenter. As far as galaxies go, they are definitely not "stationary", the discovery of this being one of the greatest discoveries of modern cosmology. , Vespine (talk) 04:22, 31 July 2015 (UTC)

It's all an illusion. Everything moves in a straight line. Mass and energy warp space to create the illusion. The 2D example is to travel on a great circle on the surface of the earth. You travel due east and eventually "orbit" to your origin but that doesn't change the straight line motion. The 2D plane you travel in is warped around a 3D sphere. Gravity warps 3D into at least 4D to appear to orbit. --DHeyward (talk) 04:40, 31 July 2015 (UTC)

... and there is no known unique centre of the universe: Earth is the centre, along with every other point in spacetime. See Dark flow for a possible non-rotational drift. Dbfirs 12:50, 31 July 2015 (UTC)

The Sun orbits the center of the Milky Way and there is a super-massive black hole there. But note that while the Galactic Center#Supermassive black hole weighs in at an impressive 4.31 million solar masses, it's still tiny compared to Milky Way#Size and mass with estimates around 8.5×10¹¹ solar massess, 200000 times the black hole, and a lot of that inside the Sun's orbit. It's not the black hole that makes the Sun orbit. PrimeHunter (talk) 22:15, 31 July 2015 (UTC)

And it's nothing compared to some of the Dwarf galaxies. At some point when we realize what drove the "Big Bang" and what changes the rate of the universe expansion rate it will be as revolutionary as relativity and quantum mechanics. Personally, I'm skeptical of trying to fit cosmology to existing theories just as QM and Relativity left Newtonian mechanics and Maxwell in the dust. There's another event horizon that needs explanation. --DHeyward (talk) 04:19, 1 August 2015 (UTC)

The OP is advised to read about general relativity to understand some of the comments above. In addition, in the general n-body problem, the movement of objects is chaotic. In that sense there is not a single center of rotation because the orbits are, at a certain level, not regular enough for that concept to be defined. Note that even in an inertial reference frame the barycenter of a large number of objects will still move. The size of the universe must be finite if a "barycenter" for "everything" is to be defined. Indeed, it is nominally possible to define one for the observable universe, but it's impractically hard to compute that.--Jasper Deng (talk) 09:18, 2 August 2015 (UTC)

Of course that raises an interesting problem: if you could define the barycenter of the observable universe based on the distribution and motion of all of the masses of the observable universe, but then you found that the actual barycenter, based on other reliable observations, could be shown to be elsewhere, you would now have a way to learn more about what is beyond the edges of what you can observe. The basic principle (on a much smaller scale) allowed Alexis Bouvard to deduce the existence of Neptune. An interesting problem. --Jayron32 01:18, 3 August 2015 (UTC)

Gravity waves and gravity itself only travel at a speed of c. If some puckish alien with a well-aimed cue ball sinks our Sun into the corner pocket, the Earth will still revolve around "where it was" ... for a little while. (though simultaneity in relativity is a complex concept, and in a certain near light speed frame of reference there is virtually no time delay) That means that unobservable stars don't get a vote on the barycenter, unless and until their light arrives. Wnt (talk) 15:50, 4 August 2015 (UTC)

One can still define a center of mass for objects any distance away, though this notion of finite speed of gravity does really complicate the notion of a barycenter more than I thought.--Jasper Deng (talk) 17:44, 4 August 2015 (UTC)

Sustainable harvesting of wild parrots for the pet trade

I was just reading our Common hill myna article, where it says that people sometimes encourage wild myna birds to breed near where they live and then take a sustainable number of chicks to hand-raise for the pet trade, while leaving the adults alone. Why don't people do that with parrots more often? --146.198.142.19 (talk) 21:50, 31 July 2015 (UTC)

Telegraph Hill, San Francisco is also famous for it's feral parrots. Seems like some could be taken from there, too. StuRat (talk) 22:03, 31 July 2015 (UTC)

Because not exploiting the resource to its fullest extent means you're giving up some short-term gain. Harvesting of wild animals is a textbook example of a resource where there are significant externalities. Every person has an incentive to take as many animals as possible, even though it means in the future the animal population will go extinct, because the benefits are immediate and accrue exclusively to the people who are harvesting, while the costs are delayed and are spread among everyone who could eventually benefit from the resource. Any animals you don't harvest are available for a competitor to harvest. See also: overfishing, cap and trade, global warming. This kind of thing is a big deal in economics, with lots of attention given to designing regulatory and incentive schemes to make producers bear the costs of externalities. --108.38.204.15 (talk) 23:01, 31 July 2015 (UTC)

See degradation of the commons. Yes, laws would need to be put in place to limit "harvesting", although if they are an invasive species, environmentalists might actually be in favor of total removal. StuRat (talk) 23:19, 31 July 2015 (UTC)