# Do Neutron stars decay?

1. Jun 12, 2012

### iced199

After reading "The Five Ages of the Universe" by Greg Laughlin and Fred Adams, I wondered, if all matter composed of ordinary atoms (protons decay) decay, and black holes decay due to Hawking Radiation, do neutron stars decay in any way? They are composed entirely of neutrons that are kept stable by gravity. So is there a way that they eventually die along with the rest of the universe, or will they stand to become the supreme rulers of the distant future?

2. Jun 12, 2012

### IsometricPion

Using the decay processes given in Wikipedia- X and Y bosons one can determine that two decay paths are $n^0\rightarrow e^++\pi^-$ and $n^0\rightarrow \pi^0+\bar{\nu}_e$ (both via the Y boson). Assuming the masses of the X and Y bosons are similar, the bound neutron lifetime should be close to that of the proton.

Even if (for some strange reason) bound neutrons didn't have a new decay mechanism, the neutron rich matter in a neutron star is in equillibrium with a thin crust of ordinary matter. This means that as proton decay removes the ordinary matter from the crust it is replaced from below via ordinary neutron decay (since if the crust is removed the exposed neutrons are essentially free neutrons, in terms of their decay properties).

In any case, I would expect a neutron star to have a lifetime within an order of magnitude of a similar amount of ordinary matter.

3. Jun 13, 2012

### Xen Uno

I doubt that there is any decay once u get below the crust, assuming there are free neutrons (unlikely ... bound up in atoms). A neutron star is formed gravitationally ... and that gravity is so strong it would overwhelm any decay process. Surface atoms quickly get crushed to neutrons once covered with enough material. Once that happens, they are locked down ... forever (unless impacted by say .. an other neutron star). A neutron star is essentially immortal, as there is no evaporation mechanism or max lifetime of nuclear matter.

4. Jun 13, 2012

### Staff: Mentor

Can you give any reference for that?
A quick approximation gives something like ~70MeV gravitational potential per nucleon close to the surface. As the released energy is ~700MeV, gravitation should not stop the decay. In addition, n->pi0+neutrino and pi0->2gamma would give 2 massless and one nearly massless particle, which can escape the neutron star easily, even if they lose some energy. Alternatively, if the photons interact with the star, the heat can escape.

5. Jun 13, 2012

### phyzguy

Dare I point out that there is no evidence for proton decay, despite huge efforts expended in searching for it?

6. Jun 15, 2012

### qraal

Neutrons will decay via possible black-hole modes in the configuration of their constituent quarks - if a quark happened to be smaller than its event horizon it'd become a black-hole and evaporate by Hawking radiation. The probabilities are low and not well constrained by data so the decay life-time estimates are very large. Read a bit closer in Laughlin & Adams and you'll see their discussion of micro-black hole decay of nucleons.

The paper by Laughlin & Adams which became the popularization in the book is available online: A Dying Universe

Even if protons don't decay via these processes, the final decay is for the wave-functions of all the neutrons in a neutron star to super-impose and collapse inside their mutual event horizon. Freeman Dyson worked out the probability for this and it's very small, but still finite. Given enough time everything will decay: Time Without End

7. Jun 15, 2012

### scijeebus

How can a neutron decay when quarks can't be isolated? Or what would possibly cause them to spontaneously turn into a black hole given that matter in the universe is said to lose energy over time? Not even the hadron collider did it.
How does a wave-function changing make something a black hole? That doesn't make sense, a wave-function is a particle's probability. A black is an object that we can't test on of which has an escape velocity greater than light.
And if "everything" decays, what will the products decay into?

8. Jun 15, 2012

### IsometricPion

The idea is that the three quarks constituting a neutron are point particles the positions of which obey a probability distribution. Therefore, there is a very small probability that these quarks would happen to be close enough together to form a (single neutron mass) black hole. Such a black hole would have an extremely short lifetime (at least semi-classically).

The caveat being that this scenario involves length scales for which we have no data and for which we have good reason to believe that not all the relevant theories are valid (since the black hole would have a diameter smaller than the planck length).
The particles would continue decaying until some fundamental law prevented them from doing so (such as conservation of energy or charge). Assuming protons decay, the end products would probably be light (electromagnetic radiation) and neutrinos (assuming the charged particles eventually annihilate with their anti-particles and that there is nothing less massive than a neutrino into which it could decay).