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    kzul's Avatar
    kzul Posts: 2, Reputation: 1
    New Member

    Nov 5, 2007, 12:19 PM
    Why do all the galaxies rotate?
    The law of conservation of angular momentum is one of the major concept in classical physics, :) and the universe is assumed to born from the Big Bang.:rolleyes: As the angular momentum of a point is meaningless, why do all the galaxies rotate?:mad: where does the initial angular momentum comes from?:confused: :confused:
    NeedKarma's Avatar
    NeedKarma Posts: 10,635, Reputation: 1706
    Uber Member

    Nov 5, 2007, 12:23 PM
    Perhaps you may want to brush up on the concept of gravity and it's effect of matter.
    asterisk_man's Avatar
    asterisk_man Posts: 476, Reputation: 32
    Full Member

    Nov 5, 2007, 12:54 PM
    I think the question is a good one and it's deeper than "why do galaxies rotate". If conservation of angular momentum must hold then it can not be created or destroyed and so it can't be generated by gravity unless the gravity were to induce an opposite angular momentum in the two bodies which are interacting.
    ebaines's Avatar
    ebaines Posts: 12,130, Reputation: 1307

    Nov 5, 2007, 02:47 PM
    You seem to assume that of galaxies rotate the universe must have an angular momentum, but that's not necessarily true. It may be that the angular momentum of all galaxies added up is zero. It's not hard to devise a mechanism that has zero angular momentum initially but that ejects two objects each of equal mass but spinning in opposite directions - such a system has zero angular momentum even though it has spinning parts. I don't know whether anyone has ever attempted to measure the total angular momentum of the universe, but I doubt it. Maybe some day astronomers will be able to determine the total angular momentum of the universe,and maybe it is indeed 0.

    The other thought I had is that your premise that the angular momentum of a point is meaningless may not be correct. Black holes are known to have angular momentum, but are considered to be points.
    kzul's Avatar
    kzul Posts: 2, Reputation: 1
    New Member

    Nov 7, 2007, 12:28 PM
    Thank you for your interest and answers.
    As much as I know the direction of the rotation of earth around itself and around the sun is the same for the northern semi sphere of the whole system. It is same for the Moon and other planets in solar system. Moreover the Sun's rotation around itself and around the center of the Milky way and the rotational direction of the local group which includes billions of galaxies including the Milky way is the same as well.

    This makes me a little bit confused :( it seams that all the systems in the universe are rotating in the same direction which creates huge angular momentum. The question is that Why and how?
    ebaines's Avatar
    ebaines Posts: 12,130, Reputation: 1307

    Nov 7, 2007, 12:58 PM
    You are correct that the angular momentum of the sun, the planets in their orbits, and the moons in their orbits are all roughly aligned. One exception to the rule is the rotation of Uranus, which actually rotates against its orbit. But in general you are correct with regards to our solar system. This is one of the major clues that drives theories of how the solar system formed - the concept being that the giant cloud of dust and gas that collapsed into the solar system must have started with some angular momentum, which was conserved as the sun, planets and moons formed. The same thing may very well have happened on a much larger scale with the formation of the Milky Way galaxy - all the star systems are rotating about the center of the galaxy in essentially the same direction. However, the ecliptic plane of the solar system is not well aligned with the plane of the Milky Way. We see this visually at night - if you live in a dark location and can see the Milky Way you will see it is not aligned on the ecliptic (unlike the planets). So the solar system is actually quite tilted with respect to the Milky Way. At larger scales things get even more confused. Looking at galaxies, we see that their alignment is essentially random - some are edge on to us, others are face on, and most are at some angle in between. Hence the vectors of angular momentum for the galaxies point in all sorts of random directions.
    Chuy949's Avatar
    Chuy949 Posts: 1, Reputation: 1
    New Member

    Dec 18, 2007, 11:12 AM
    who are you pepole
    Enowil's Avatar
    Enowil Posts: 1, Reputation: 1
    New Member

    Apr 13, 2012, 11:49 PM
    Galaxies rotate because they, like everything in space, bend space and therefore are continuously falling into the warped space.
    sean_s's Avatar
    sean_s Posts: 103, Reputation: 1
    Junior Member

    Apr 17, 2012, 12:56 PM
    the present model of universe prohibits a total angular momentum . We don't have a preferred direction of universe, which means, no angular momentum unfortunately, as angular momentum will lead to rotation axis, and a direction of the same :s, therefore, without an empirical study, we assume that the rotation of stuff are randomly distributed with mean zero
    TechSupport's Avatar
    TechSupport Posts: 43, Reputation: 5
    Junior Member

    Oct 18, 2012, 06:25 AM
    Things rotate in space because they are not points. We TREAT them as points mathematically to make the math easier, but they are not points. As such, there are at least two separate points on a planet, galaxy, or even mote of interstellar dust that are not at the same location at the same time. Therefore, there is rotation, since every other point in the galaxy that has mass in it is attracted to it.

    This means that as two points of mass approach each other, they will change their trajectories. If they become gravitationally bound into a speck of dust or a sun or a galaxy, then those continuously changing, but bound trajectories will create an elliptical motion about a common, imaginary point called the center of the system.

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