Do galaxies have a negative mass defect?

In summary: In general, potential energy is simply the energy that a system has available to it. In the case of the fundamental forces, potential energy can be positive or negative. In the case of gravity, electromagnetism, and the strong force, potential energy is always negative. This is because when objects are moved closer together, the potential energy of the system increases. In the case of gravity, this is because the force between the objects becomes stronger. In the case of electromagnetism, this is because the force between the objects becomes stronger and longer-lasting. And in the case of the strong force, this is because the potential energy between the quarks becomes greater and greater the further apart they are
  • #1
kmarinas86
979
1
http://en.wikipedia.org/wiki/Binding_energy#Mass_Defect

Wikipedia:Binding Energy said:
Because a bound system is at a lower energy level than its unbound constituents, its mass must be less than the total mass of its unbound constituents.

But the opposite is true for galaxies! Quarks too! Their mass appears to be larger than their constituents as well...
 
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  • #2
In the case of galaxies, you have to consider the masses of all the constituents - including the dark matter. If you were to do that (which, of course isn't really possible until we know more about the nature of dark matter), you should find that the total mass of the galaxy is less than the masses of everything in it combined.

With quarks, however, things are a bit more complicated. The nature of the strong force between the quarks is that the farther apart they are moved, the greater their potential energy becomes without limit.

In gravitationally and electromagnetically bound systems, the potential energy is always negative and increases to 0 as objects are moved progressively farther away. A system being bound means, quite simply, that the particles do not have enough energy to the point where their separation is infinite. In other words, their total mechanical energy must be smaller than the potential energy of two particles an infinite distance apart. So, in gravity and E&M, bound systems must have negative mechanical energy. Since mass and energy are interchangable, negative mechanical energy means that the total energy (mass plus mechanical) is less than the energy just stored in mass.

By this discussion, we see that the existence of the mass defect requires that the potential energy be 0 at infinity. Going back to the strong force, I said above that it is a case where potential energy increases without bound. In other words, the potential energy from two quarks separated by an infinite distance is infinite. In other words, any system of quarks, no matter how much energy it has of any sort, is a bound system. So, because of the very different nature of this strong potential, the mass defect does not apply.
 
  • #3
Parlyne said:
In the case of galaxies, you have to consider the masses of all the constituents - including the dark matter. If you were to do that (which, of course isn't really possible until we know more about the nature of dark matter), you should find that the total mass of the galaxy is less than the masses of everything in it combined.

With quarks, however, things are a bit more complicated. The nature of the strong force between the quarks is that the farther apart they are moved, the greater their potential energy becomes without limit.

In gravitationally and electromagnetically bound systems, the potential energy is always negative and increases to 0 as objects are moved progressively farther away. A system being bound means, quite simply, that the particles do not have enough energy to the point where their separation is infinite. In other words, their total mechanical energy must be smaller than the potential energy of two particles an infinite distance apart. So, in gravity and E&M, bound systems must have negative mechanical energy. Since mass and energy are interchangable, negative mechanical energy means that the total energy (mass plus mechanical) is less than the energy just stored in mass.

By this discussion, we see that the existence of the mass defect requires that the potential energy be 0 at infinity. Going back to the strong force, I said above that it is a case where potential energy increases without bound. In other words, the potential energy from two quarks separated by an infinite distance is infinite. In other words, any system of quarks, no matter how much energy it has of any sort, is a bound system. So, because of the very different nature of this strong potential, the mass defect does not apply.

For the fundamental forces of gravity, electromagnetism, and the strong force, what exactly is it meant by a positive potential energy vs. negative potential energy? I thought negative potential energy represents attraction and positive potential energy (in the case of proton repulsion) represented repulsion. But apparently, when I look at the Cornell potential, http://72.14.203.104/search?q=cache...f+"cornell+potential&hl=en&gl=us&ct=clnk&cd=1 (with a negative sign for repulsion and positive sign for attraction). Why is it spoken like this?
 
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1. What is a negative mass defect?

A negative mass defect refers to the difference between the mass of a nucleus and the combined mass of its individual protons and neutrons. This difference can be positive, indicating that energy was released during the formation of the nucleus, or negative, indicating that energy was absorbed.

2. Can galaxies have a negative mass defect?

Yes, galaxies can have a negative mass defect. This occurs due to the presence of dark matter, which has a negative mass defect. Dark matter is believed to make up a significant portion of the total mass of a galaxy, but its exact nature is still not fully understood.

3. How is a negative mass defect measured in galaxies?

A negative mass defect in galaxies is measured through observations of galactic rotation curves. These curves show the relationship between the rotational speed of stars and gas in a galaxy and their distance from the galactic center. A negative mass defect can be inferred if the rotation curve does not follow the expected pattern based on the visible mass of the galaxy.

4. What are the implications of a negative mass defect in galaxies?

A negative mass defect in galaxies has significant implications for our understanding of the universe. It suggests the existence of dark matter, which is crucial for explaining the observed behavior of galaxies and the large-scale structure of the universe. It also challenges our current understanding of the laws of physics and the nature of matter.

5. Can a negative mass defect explain the expansion of the universe?

No, a negative mass defect alone cannot explain the expansion of the universe. While it does suggest the presence of dark matter, which is thought to contribute to the expansion, it is not the only factor at play. Other theories, such as dark energy, are also needed to fully explain the expansion of the universe.

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