1. Aug 18, 2015

danihel

Hi, i heard that as the universe will expand and the energy will grow sparse, in the end the matter itself will turn into radiation that will loose energy.
So i wanted to ask, can a proton decay? and is it the same phenomenon as radioactive decay? Does it mean that even hydrogen (with no protons) has it's theoretical half-life?

2. Aug 18, 2015

Orodruin

Staff Emeritus
In the Standard Model, the proton does not decay. However, there are many theories beyond the SM in which it does. The experimental lower limit on the proton lifetime is of the order of $10^{29}$ years, i.e., several orders of magnitude longer than the age of the Universe.

If the proton decays, it is most certainly not through the same phenomenon as radioactive decays, which occur via the weak interaction.

It should be noted that the above concerns a free proton. If the proton is part of a nucleus, it may be possible for it to decay to a neutron through beta+ decay due to the different binding energies of protons and neutrons in the nucleus in question.

3. Aug 18, 2015

danihel

If i understood right, the weak interaction force carriers are not involved when free proton decays. So would it be just some quantum probabilistic nature of any matter particle turning into energy and vice versa?

Sorry i'm very curious about these things but i lack serious education in physics.

4. Aug 18, 2015

Orodruin

Staff Emeritus
Not really, particles do not "turn into energy", energy is a property of matter and not a thing of its own. The decay of a free proton would require the existence of physics and theorised particles which we so far have no experimental knowkedge of.

5. Aug 18, 2015

danihel

Thanks, what i meant by "turning in to energy" is turning into EM radiation. I think i heard someone referring to photons as pure energy, i don't know why (maybe because they have no mass and move at the speed limit, although i don't know if other particles that have these properties would be considered as energy).

So within the standard model there is no mechanism by which a completely isolated matter particle would turn into radiation or something else?

6. Aug 18, 2015

Orodruin

Staff Emeritus
Whoever you heard saying that is someone you should not listen to when it comes to physics. Mass is also a form of energy and with that definition, everything would be "pure energy", which would therefore be a completely vacuous statement.

Also, what happens when particles decay is not that they turn into EM radiation. In most cases you cannot talk about an intermediate state and EM radiation is also a classical concept, while particle decays must be handled in quantum theory.
What do you mean by a "matter particle" here? Free neutrons most certainly decay.

7. Aug 18, 2015

danihel

Well to explain my stupidity: I knew that radioactive decay is the process of conversion of particles into other particles mediated by the weak interaction, of which I have the vaguest understanding of all the forces. All I think I know is that the electromagnetic versus strong forces cause an energy state of an atom to become unstable which increases the probability of W and Z bosons to appear and cause this conversion. I also thought that half-life refers specifically to the measure of the probability of this phenomenon to occur.

And now when you replied that proton might have half-life but not caused by weak interaction, I am more puzzled by what that could mean.

Right now I’m afraid I have too big holes in my understanding to get any hang of this.

8. Aug 18, 2015

Staff: Mentor

The proton would decay via other interactions (other virtual intermediate particles, not W) - some processes that are not part of the standard model.
Not all decays happen via the weak interactions, there are also decays via the strong interaction and some via the electromagnetic interaction.

A proton cannot decay to radiation only. Its electric charge has to be conserved, so the decay products will include a positron.

There are good reasons to expect that protons can decay. The universe probably started with equal amounts of matter and antimatter, but now we see baryons (here: protons and neutrons) everywhere but nearly no antibaryons. Some unknown process in the early universe caused an asymmetry, and the same process should make proton decays possible.

9. Aug 18, 2015

Orodruin

Staff Emeritus
You should revisit this source of knowledge. Only some radioactive decays, the different types of beta decay, are mediated by weak interactions. Alpha decays are more related to alpha particles (essentially helium nuclei) tunneling out from the barrier caused by the interplay of strong and electromagnetic interactions, while gamma radiation is mainly due to state conversions within the nucleus.

Half-life is defined independent from the interaction which causes the process. It is the average time it will take for half of a collection of particles to undergo the decay process. The decay in itself may be caused by electromagnetic, weak, strong interactions, or a new interaction we have not yet discovered in the case of hypothetical decays.