1

In this question it is explained that neutrons in nucleus don't decay because the next state would not be lower in energy than the previous.

How come neutrons in a nucleus don't decay?

But it doesn't say what causes the neutron to know that the next stage is not lower in energy?

Community
  • 1
zoran404
  • 371

1 Answers1

3

hdhondt is right. A neutron doesn't need to "know" that another state isn't lower in energy to stay in the state is it at. It stays in the state of lowest energy because it is the state of lowest energy (just like a ball at the bottom of a pit).

Furthering hdhondt's ball example: the ball in the pit moves to the bottom (where the gravitational potential energy is the lowest) and, assuming all its kinetic energy dissipates, will stay there. It stays there not because it explored all its possible energy states (all the locations in the pit) and therefore knows which one has the lowest energy but because (again, assuming the kinetic energy dissipates) gravity will cause it to come to rest at the place of lowest potential. I give this as an intuitive picture we've all had experience with. In a nucleus, there are different forces at work but the picture is basically the same. It's not that the neutron knows its in the lowest energy state, but that eventually the neutron will end up in the lowest energy state, just like the ball in the bottom of the pit.

NeutronStar
  • 5,404
  • 5
  • 36
  • 64
  • When the ball reaches the bottom it starts pushing the bottom but the bottom doesn't let it go down. My question is what is the "bottom" for the neutron? – zoran404 Jul 22 '13 at 12:45
  • I don't know enough about particle physics to give the specific answer you are looking for. Perhaps you should edit your question so future viewers will know how to answer. – NeutronStar Jul 22 '13 at 18:50
  • I though the question was very clear. what do you mean? – zoran404 Jul 22 '13 at 20:23
  • It was asked why a neutron knows it is in the lowest energy state, not what that lowest energy state is. – NeutronStar Jul 22 '13 at 21:32
  • And that is exactly what I want to find out. "why?" – zoran404 Jul 22 '13 at 21:44
  • 1
    @zoran404 The reason we 'know' that the neutron is in the lowest energy state (in the example in the link that you provided) is because making it go into any other state would require pumping in more energy than you get back. The principle of minimum energy tells us that systems try and minimize their energy. If decay can only happen if more energy is put in than is released then the system has gained energy overall. Since the nucleus cannot go from a low energy state to a higher one on its own, neutron decay is not allowed in these cases. – Siraj R Khan Jul 24 '13 at 10:35
  • @zoran404 So: If you start off with a particular nucleus and you imagine that neutron decay occurs you'll end up with a different nucleus. If the nucleus gives away energy overall, you know it's allowed (we know the original nucleus wasn't at the 'bottom', and we know that we've made it one step closer to the bottom). If the process takes up energy overall, (if the nucleus you end up with has more energy than the starting one) then you know that the original nucleus was closer to the bottom than the end nucleus, and this isn't allowed to happen on its own. – Siraj R Khan Jul 24 '13 at 10:40
  • So your answer is "it would require pumping in more energy", which makes something clearer to me but doesn't exactly answer my question. I think I should edit my question. – zoran404 Jul 24 '13 at 12:04
  • Actually I'll just ask a different question. – zoran404 Jul 24 '13 at 12:08
  • Note: It's not just that energy has to be put into the system. It's that more energy has to be put into the system than the amount of energy released by the system, resulting in a net increase in the energy of the system. If this happens you haven't moved the nucleus to a more stable (lower energy) state, you've moved it to a less stable (higher energy) state. – Siraj R Khan Jul 24 '13 at 12:36