This is something mentioned in the Stephen Baxter novel Time. Proton decay giving an indication of the age of the universe. What can you tell me about this?

Some theories predict that protons will decay with a half-life of about 10³¹ years (from memory). Measuring proton decay rates won't tell you anything about the age of the universe, though.

Protons either have a finite mean lifetime, or are stable and never decay. The difference is a large one for particle physics; the stability (or lack thereof) is an input parameter -- it's something that can't be derived, and instead must be measured by experiment. So far, bigger and bigger experiments have pushed up the minimum mean lifetime to absurdly long lengths of time. Most particle physicists believe that the proton is indeed stable.

- Warren

So protons are made of quarks. Has there been any observed motion of quarks like an up or down leaving a proton due to some reaction or anything?

Originally posted by Adam
So protons are made of quarks. Has there been any observed motion of quarks like an up or down leaving a proton due to some reaction or anything?
The strong force is so strong that pulling apart two quarks requires an energy even larger than the rest mass of a new quark. In other words, when you pull two quarks apart, two new quarks materialize to accompany the two original quarks, and both pairs go about their business. The strong force essentially prevents any lone quarks from existing, but protons and other hadrons are routinely broken up. What you'll often see from proton-antiproton collision experiments are what are called "jets." They are hundreds of particles, all travelling in much the same direction, that emanate from the collision. The jets announce the direction in which a free quark was launched; the attendant particles are all materialized from the strong force.

In even higher-energy physics, you can smash hadrons together with huge temperatures and achieve a quark-gluon plasma (QGP). To get normal plasma, you heat up atoms until you separate the electrons and nuclei, and thus get an electron-nuclei plasma. To get quark-gluon plasma, you heat up hadrons enough that quarks and gluons (the force carriers of the strong force) are separated. Particle physics is on the verge of declaring QGP a reality, as several experiments have hinted that it has been accomplished.

- Warren

Hi Adam,

[reply cut]

Damn chroot, you beat me to it again ;)

Bye!

Crisp

So quarks just appear from nothing? That seems wrong. In fact it seems stupid. What about free quarks existing loose, and being drawn in by the "hole" when one from a proton is knocked out? Much like electron movements.

Originally posted by Adam
So quarks just appear from nothing? That seems wrong. In fact it seems stupid.
Why does it seem wrong or stupid? Sometimes, in particle physics, you must accept what the math and experiment tell you, even if it seems aesthetically unpleasant. You must also remember that the subatomic world is a place very much unlike the macroscopic world you percieve directly with your senses.

What about a radioactive nucleus undergoing beta decay? A neutron suddenly emits an electron and an antineutrino, and becomes a proton. Is this any less wrong or stupid?

Or when you shake an electric charge, and photons come pouring out. Is this any less wrong or stupid?
What about free quarks existing loose, and being drawn in by the "hole" when one from a proton is knocked out? Much like electron movements.
I'm not sure I follow.... you'll have to be much more specific.

- Warren

Or when you shake an electric charge, and photons come pouring out. Is this any less wrong or stupid?

Am I correct in thinking this is matter pouring out as energy? Matter/energy equivalence and all that? I mean, it doesn't just come from nowhere, as I understand it.


I'm not sure I follow.... you'll have to be much more specific.

I mean... With movements of charge, say through a metallic conductor, you have "holes" which draw free electrons. Not really, but I can see it that way. Could not there be unattached quark floating about, ready to be drawn into protons which are broken up?

Originally posted by Adam
Am I correct in thinking this is matter pouring out as energy? Matter/energy equivalence and all that? I mean, it doesn't just come from nowhere, as I understand it.
You shake an electric charge, and the electric and magnetic fields due to it are shaken similarly. The energy put into making the charge shake is converted through "radiation resistance" into an electromagnetic wave, which can be represented mathematically by two equivalent models: either field waves or spin-0 Bose particles called photons. The energy that went into shaking the charge comes out in the form of particles. The particles, if you will, materialize from the field.

In another process, a photon of adequate energy can lead to the spontaneous production of a particle and its antiparticle, a prospect that sounds "wrong and stupid," yet happens routinely and is modelled by elegant and relatively simple math. A powerful electromagnetic field can spontaneously give rise to new particles. The field's energy can be transformed into the rest mass of new particles.

The strong force operates analogously. If you shake a hadron (something which is made out of quarks) you get waves in the strong field, which show up as free gluons. The strong force is incredibly strong, and thus it takes incredible energy to get free gluons. The gluons are accordingly very very massive.

Now let's see why you can't have an isolated quark, but you can have an isolated electron.

In the limit of static electric charges, the force between a proton and electron is just the Coulomb force,

<B>F₁₂</B> = q₁ q₂ / (4 <font face="symbol">p e</font>₀ r₁₂² ) <b>e₁₂</b>

And the work done in pulling an electron and proton apart is just the integral of force over distance. We'll take the work necessary to separate the charges from the Bohr radius (ground state mean distance from the electron to the proton in a hydrogen atom, 0.0529 nanometers, 4 <font face="symbol">p e</font>₀ (h-bar) / m_e c <font face="symbol">a</font>) to an infinite distance, and get -13.6 eV. It takes 13.6 eV of energy to break apart the hydrogen atom into an unbound electron and proton. Compare 13.6 eV to the rest mass of the electron, m_ec², or about 0.511 MeV. That's right, the energy required to create a new electron is four orders of magnitude larger than the energy required to separate an electron and proton. Pulling apart an electron-proton pair does not create any new particles.

Now let's look at the strong interaction. It's horribly messy to deal with the math on a forum like this, so we'll use words instead. The strong interaction is 137 times stronger than the electromagnetic interaction (i.e. the fine structure constant <font face="symbol">a</font> is 1/137) and does not fall off as the square of the distance as the electromagnetic force does -- in fact, the force gets stronger the larger the separation. This means that the work done in pulling two strongly-interacting partcles apart from quark distances (~10^-15 m) to infinity is much, much, much higher -- about 1 GeV per fermi for lengths less than a few millimeters, or 1 GeV per 10^-15 m. This means that when the quarks are pulled apart by only a small fraction of a fermi, there is already enough energy in the field to create two new quarks!

In both cases, you have to put in energy to pull the particles apart. Electromagnetism, with its "weak" coupling constant and inverse-square law, only puts up a fight to the tune of 13.6 eV for the separation of a proton-electron pair, which is nowhere near enough energy to create a shiny new electron. The strong force (more accurately, the colour force) that binds quarks together is entirely different, and is so strong that pulling two quarks apart requires as much energy as creating a whole slew of new quarks, pions, and other particles. These are all observed together as "jets" from proton-proton collisions. Each jet marks the path of one of expelled quarks.
I mean... With movements of charge, say through a metallic conductor, you have "holes" which draw free electrons. Not really, but I can see it that way. Could not there be unattached quark floating about, ready to be drawn into protons which are broken up?
In a metallic conductor, you effectively have a first-order ideal electron gas. There are no "holes."

According to the math of the colour force, there is no way for an isolated quark to exist. The energy in its own colour field is so strong as to be able to create new quarks. It makes its own friends, so to speak.

- Warren

Originally posted by chroot

in fact, the force gets stronger the larger the separation.
Now that is just odd.

But thanks for the lengthy explanation.

Originally posted by Adam
Now that is just odd.

But thanks for the lengthy explanation.
What makes you think it's odd? :bugeye:

Any exponent is equally ad hoc. There's nothing special at all about "2."

- Warren

I'm just trying to think of a possible mechanism by which particles would have a stronger attraction the further apart they are. Not the names of the force, but how it works. It just seems very odd.

Originally posted by Adam
I'm just trying to think of a possible mechanism by which particles would have a stronger attraction the further apart they are. Not the names of the force, but how it works. It just seems very odd.
If you mean, "how the math works," just imagine the Coulomb force with an exponent of "1" on the r₁₂ rather than the -2.

- Warren

No, not the math. I'm wondering about the physical mechanism by which it can increase in strength with distance. The makeup of quarks and the forces involved, things like that.

Originally posted by Adam
No, not the math. I'm wondering about the physical mechanism by which it can increase in strength with distance. The makeup of quarks and the forces involved, things like that.
Unfortunately the colour force is enormously more complicated than the electromagnetic force. In fact, the colour force is just about as complicated as it could possibly be -- it depends on as many things as it possibly could. It depends on three charges, not two -- it depends on spin -- it depends on direction of relative motion -- it depends on distance in a very complex way. The math is not at all simple.

- Warren

Well, never mind. I'll finish reading up on it all first.

Originally posted by chroot
Protons either have a finite mean lifetime, or are stable and never decay. The difference is a large one for particle physics; the stability (or lack thereof) is an input parameter -- it's something that can't be derived, and instead must be measured by experiment.

Are you sure about that?

this is the situation as i understand it:

the standard model predicts a stable proton. it is the lightest baryon, and there no allowed decay modes that preserve the quark numbers as required by the standard model, so it must be stable.

Supersymmetry, and other extensions to the standard model (string theory? i dunno), predict that quarks can decay into leptons by exchanging very heavy particles. this is not allowed in the standard model. because the energies required for this exchange are so high, it would only happen at room temperature once every 10^31 years, or whatever.

so looking for proton decay is looking for experimental confirmation of supersymmetry. so far, none has been observed, by proton decay or otherwise. still many physicists believe there must be some physics we do not yet know beyond the standard model.

Originally posted by lethe
Are you sure about that?

this is the situation as i understand it:
lethe, I'm not sure how what I said is different than what you said -- that the decay (or lack thereof) is an experimental quantity that cannot be derived from first principles. We need to look for it to know which of several competing theories are correct (or incorrect).

- Warren

oh i see what you mean. i was thinking "of course the decay of the proton can be derived from first principles, but it depends on what you want to use as your first principles: the standard model, or supersymmetry". but since we cannot decide from first principles which theory is correct, we need experimental input to decide which is correct. there are no first principles which will tell us which theory is the correct one.