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I have been reading about the mass defect in atomic nuclei recently and am trying to understand what it is that causes the defect.

To my understanding it is the loss of energy of the nucleus that causes an observed decrease in mass in the nucleus (using $E=mc^2$).

Therefore, when an object moves further from the Earth, since it gains gravitational potential energy, will it's observed mass increase?

Many thanks.

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    $\begingroup$ You are mixing or messing with things. What makes you to do the "therefore" step? Moreover the gravitational potential energy is not stored within an object but in the system configuration. $\endgroup$ – Alchimista Jan 22 '18 at 13:13
  • $\begingroup$ So what then is it that causes the mass of a nucleus to decrease when it forms from a given number of nucleons? It cannot simply be that the particles lose potential energy as they come together, as this would imply that it would be the same for all systems that have potential energy. What mechanism makes the mass of a nucleus smaller than would be predicted by the sum of it's components? $\endgroup$ – Benjamin Rogers-Newsome Jan 22 '18 at 15:05
  • $\begingroup$ It is like observing a box with gases inside. Higher T means the mass of the box increases . Nothing as such for the individual gas molecules. In your case it would be the mass of Earth plus nucleus to change, or better the gravitational field of the ensemble. Obviously I cannot be more precise without copying from books, else I would have answered instead of commenting . Let us wait for specialists. $\endgroup$ – Alchimista Jan 22 '18 at 15:20
  • $\begingroup$ So in the case of the nuclear mass defect is it the loss of potential energy that causes the loss in mass; that the individual nucleons do not lose mass but rather the loss in potential energy 'looks' like a loss of mass? $\endgroup$ – Benjamin Rogers-Newsome Jan 22 '18 at 15:23
  • $\begingroup$ Right. At least is what I always got. There is not such an additivity unless every forms of mass energy is considered $\endgroup$ – Alchimista Jan 22 '18 at 15:26
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Therefore, when an object moves further from the Earth, since it gains gravitational potential energy, will it's observed mass increase?

No, but it is possible to change your analogy in order to make it correct. The thing that's analogous to the nucleus is the system consisting of both the object and the earth, O+E.

If an external source of energy brings the object farther away from the earth, then the total energy of the O+E system is increased, and by $E=mc^2$ this is equivalent to an increase in the mass of the O+E system.

A real-world example of this process, although in reverse, is in the black hole mergers that we observe in gravitational wave events. The black holes transfer a bunch of energy into gravitational waves, which take energy away into the outside world. Typically the system loses about 5-10% of its mass through the merger.

Note that if the system does not exchange energy with the environment, then the system's total energy is conserved, and its mass stays the same.

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  • $\begingroup$ So does a loss in energy of a system always result in an accompanying loss in mass $\endgroup$ – Benjamin Rogers-Newsome Jan 24 '18 at 10:33
  • $\begingroup$ @BenjaminRogers-Newsome: Yes. $\endgroup$ – Ben Crowell Jan 24 '18 at 17:12
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Forget about E=m*c^2 for the system you desrcribe, because it is not a constant , it depends on velocity, whereas the mass of a nucleus is an invariant to Lorentz transformations.

I have been reading about the mass defect in atomic nuclei recently and am trying to understand what it is that causes the defect.

If you take a nucleus with a number of protons, $n_p$ and a number of neutrons , $n_n$ and weigh it, the mass of the Nucleus, $M_N$ < $n_p*m_p$ + $n_n*m_n$ . This is the mass defect, and is reflected the binding energy seen in this curve.

Therefore, when an object moves further from the Earth, since it gains gravitational potential energy, will it's observed mass increase?

This is a misunderstanding. A nucleus in deep space or in the sea will still have the same invariant mass. It is only under extreme conditions that the masses can be affected, as for example in this study with very strong magnetic fields. Gravitational fields are orders of magnitude weaker in comparison.

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