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Brady Yoon
2004-Jun-25, 08:25 PM
In many of the models predicting the future of our universe, it always says protons will decay in some 10^50+ years later. I have several questions about this.

1. If protons were to decay now, what would happen to life, the Earth, and the universe in general.

2. Why do they decay and how was this predicted?

3. When exactly will this happen?

swansont
2004-Jun-25, 11:51 PM
Particle decay is a statistical/probablistic event. A sample of particles with a lifetime of one year will have 1/e particles left after one year; they won't all decay at the one year point.

Gullible Jones
2004-Jun-26, 02:27 AM
If all the protons were to decay now, we'd all cease to exist pretty quickly.

elgarak
2004-Jun-26, 02:16 PM
In many of the models predicting the future of our universe, it always says protons will decay in some 10^50+ years later. I have several questions about this.

1. If protons were to decay now, what would happen to life, the Earth, and the universe in general.

2. Why do they decay and how was this predicted?

3. When exactly will this happen?

1. All matter as we know it would cease to exist.

2. The proton is NOT an elemtary particle, but is build up out of three quarks. Therefore, it has SOME probability to fall apart. However, this probability is very small. For all practical purposes the proton is stable. The half-life (the time after which half the protons in a given amount of them are gone) is something like 10^33 years. Remember, the age of the universe is something around 10^10 years!

3. Given the probabilistic nature of elementary particles, ther's no answer to that questeion. Put some protons in a bucket, and 10^33 years later half of them are gone. Some go now, some later. As far as I know, no one has yet observed a proton decay experimentally. Especially some neutrino observation experiments are most likely to observe them, and the proton decay is some possible background event they have to consider. But so far, no event has been observed that has to be considered a proton decay. The 10^33 year value is purely theoretical. The time you have to wait to observe one proton decay, or the time between two proton decay can be used to calculate an experimental value, which, IIRC, is much longer than the theoretical 10^33 years, considered the time people have set up an experiment usable to observe the proton decay (like the neutrino observatories) without detecting of one.

stu
2004-Jun-27, 05:00 AM
Not that this is at all relavent (well, it sort of is), but I read a book once (called something like the Five Ages of Time or the Five Ages of the Universe) and it discusses this in an interesting way. Imagine you're some form of intelligent life living in the universe in 10^50-some-odd years. The universe has expanded so much that most surving matter is streatched so thin that the closest particle to another is light years away. It takes a (by our standards) an enormously long time to execute a single thought, but this doesn't matter because on the time-scales that we're in, the universe would still have a long way to go (I think this was assuming a Big Freeze cosmology).

And you, as this increasingly abstract intelligence, wonder about those particles with lifetimes only a fraction of the age of your universe, and by all your instrumentation, they'd be impossible to measure or even really prove to have existed.

I'm going to stop this increasingly rambling post, but it's something to think about!

Tobin Dax
2004-Jun-27, 06:05 AM
Not that this is at all relavent (well, it sort of is), but I read a book once (called something like the Five Ages of Time or the Five Ages of the Universe) and it discusses this in an interesting way. Imagine you're some form of intelligent life living in the universe in 10^50-some-odd years. The universe has expanded so much that most surving matter is streatched so thin that the closest particle to another is light years away. It takes a (by our standards) an enormously long time to execute a single thought, but this doesn't matter because on the time-scales that we're in, the universe would still have a long way to go (I think this was assuming a Big Freeze cosmology).

That's not necessarily true, though, as I understand it. Galaxies are basically "island gravity wells," if you'll allow me to tweak the old description. So, unless they evaporate (can they?), galaxies will stay relativity intact. That means that if you are a lifeform (as we know them, anyway), the nearest particle will be quite close, but the nearest galaxy will be *very* far away.

Excelsior
2004-Jun-27, 11:29 AM
Has proton decay ever been actualy observed ?

wedgebert
2004-Jun-27, 02:28 PM
I don't think so, the half-life is so long that it would take an amazing amount of luck to catch a single proton decaying.

stu
2004-Jun-27, 05:16 PM
Not that this is at all relavent (well, it sort of is), but I read a book once (called something like the Five Ages of Time or the Five Ages of the Universe) and it discusses this in an interesting way. Imagine you're some form of intelligent life living in the universe in 10^50-some-odd years. The universe has expanded so much that most surving matter is streatched so thin that the closest particle to another is light years away. It takes a (by our standards) an enormously long time to execute a single thought, but this doesn't matter because on the time-scales that we're in, the universe would still have a long way to go (I think this was assuming a Big Freeze cosmology).

That's not necessarily true, though, as I understand it. Galaxies are basically "island gravity wells," if you'll allow me to tweak the old description. So, unless they evaporate (can they?), galaxies will stay relativity intact. That means that if you are a lifeform (as we know them, anyway), the nearest particle will be quite close, but the nearest galaxy will be *very* far away.

Without the book in front of me, I can't really be sure of this, but I'm pretty sure that by this time, all that will be left will be black holes and the stray elementary particle. Remember that if protons have decayed, stellar remnants would no longer exist.

stu
2004-Jun-27, 05:19 PM
Has proton decay ever been actualy observed ?


I don't think so, the half-life is so long that it would take an amazing amount of luck to catch a single proton decaying.

I think the experiments have been set up to try to measure this, but I don't think they've actually done it yet. From what I remember, the experiment would involve having gagillions of protons and detectors set to measure the decay products. Assuming the half-life is 10^30 years, the setup would be to have well over 10^30 protons, and then statistically they should measure 1 decay per year. This info might be out of date, though.

milli360
2004-Jun-27, 07:15 PM
stu:
Assuming the half-life is 10^30 years, the setup would be to have well over 10^30 protons, and then statistically they should measure 1 decay per year.
I'm not sure of the proton decay info, but the gram molecular weight of water is ten grams, which would contain 6.022 x 10^23 molecules (Avogadro's number). Each molecule would have ten protons (two in the hydrogens, 8 in oxygen), so every gram has 6.022 x 10^23 protons. To get 10^30 protons, we'd need 10^30/6.022x10^23, or 1.6x10^6 grams of water. That's 1600 kilograms, or 1600 liters (around 400 gallons). My bathtub could hold 400 liters, easily.

ToSeek
2004-Jun-27, 08:24 PM
Apparently the current lower boundary of the proton half-life is 10^35 years. (http://en.wikipedia.org/wiki/Proton_decay) So we need a very big bathtub.

milli360
2004-Jun-27, 08:28 PM
Apparently the current lower boundary of the proton half-life is 10^35 years. (http://en.wikipedia.org/wiki/Proton_decay) So we need a very big bathtub.
I think that's been established because I haven't found any in my bathtub for the past 86 years. I just need to soak longer.

swansont
2004-Jun-27, 08:46 PM
I'm not sure of the proton decay info, but the gram molecular weight of water is ten grams, which would contain 6.022 x 10^23 molecules (Avogadro's number). Each molecule would have ten protons (two in the hydrogens, 8 in oxygen), so every gram has 6.022 x 10^23 protons. To get 10^30 protons, we'd need 10^30/6.022x10^23, or 1.6x10^6 grams of water. That's 1600 kilograms, or 1600 liters (around 400 gallons). My bathtub could hold 400 liters, easily.

MW of water is 18 grams (O-16; you forgot the neutrons)

Tobin Dax
2004-Jun-27, 08:56 PM
If you're putting constraints on the proton lifetime/halflife using water, wouldn't the neutrino experiments with "big bathtubs," like SuperKamiokande, put a constraint on this?

Actually, as I think about it, I wonder if that would work. Have we seen heavy water beta-decay into helium-2 or two H_1's? If that happens, that should be observed, since the neutron's mean lifetime is about 15 minutes, IIRC. If neutrons don't decay in this situation, would protons?

milli360
2004-Jun-27, 08:59 PM
MW of water is 18 grams (O-16; you forgot the neutrons)
D'oh!

Brendan
2004-Jun-27, 10:10 PM
That's not necessarily true, though, as I understand it. Galaxies are basically "island gravity wells," if you'll allow me to tweak the old description. So, unless they evaporate (can they?), galaxies will stay relativity intact. That means that if you are a lifeform (as we know them, anyway), the nearest particle will be quite close, but the nearest galaxy will be *very* far away.

Remember that the book was about very long time spans. It described how galaxies could evaporate because of close encounters between stars. They are rare, but over long times, their effects add up. The lighter ones would gain orbital energy from the heavier ones. The lightest ones get ejected from the galaxy, taking away energy. This makes the galaxy smaller since the remaining stars lost orbital energy to the ejected ones. The galaxy get smaller and denser, increasing the rate of close encounters and ejections. The ones that do not get ejected fall into the supermassive black hole in the galaxy's center. So stars either get ejected or swallowed up.

Brendan

ToSeek
2004-Jun-27, 10:37 PM
Apparently the current lower boundary of the proton half-life is 10^35 years. (http://en.wikipedia.org/wiki/Proton_decay) So we need a very big bathtub.
I think that's been established because I haven't found any in my bathtub for the past 86 years. I just need to soak longer.

When my wife asks me why I'm taking so long, I now have a good reason to give her!

stu
2004-Jun-27, 10:40 PM
That's not necessarily true, though, as I understand it. Galaxies are basically "island gravity wells," if you'll allow me to tweak the old description. So, unless they evaporate (can they?), galaxies will stay relativity intact. That means that if you are a lifeform (as we know them, anyway), the nearest particle will be quite close, but the nearest galaxy will be *very* far away.

Remember that the book was about very long time spans. It described how galaxies could evaporate because of close encounters between stars. They are rare, but over long times, their effects add up. The lighter ones would gain orbital energy from the heavier ones. The lightest ones get ejected from the galaxy, taking away energy. This makes the galaxy smaller since the remaining stars lost orbital energy to the ejected ones. The galaxy get smaller and denser, increasing the rate of close encounters and ejections. The ones that do not get ejected fall into the supermassive black hole in the galaxy's center. So stars either get ejected or swallowed up.

Brendan

So you've read the book, too? Maybe you could check what I said and make sure that there's no glaring mistake?

Eta C
2004-Jun-28, 01:52 PM
In fact, most of the current neutrino detction experiments were originally conceived of as proton decay experiments. They were put down in mine shafts, tunnels, etc to shield them against cosmic ray background. The neutrino events were originally background. As time went on, none of them found proton decay events, and it became clear that the decay mechanisms predicted by various grand unified theories were not occuring.

The current Particle Data Group (http://www-pdg.lbl.gov/2004/tables/bxxx.pdf) lower limits on the mean life of the proton are on the order of 10^30 years. As others have pointed out, this is about 20 orders of magnitude greater than the lifetime of the universe.

Even though the proton decay experiments didn't pan out as such, they have had much more impact as cosmic ray neutrino observatories. First came the dections of neutrinos from SN1987A. And much more importantly, they provided the recent resolution of the "solar neutrino deficit" and the confirmation that neutrinos have mass. The latter is truely an indicator of physics beyond the standard model.