nuclear fusion

Originally posted by Andrew Hamilton
Well, I can give you half of the answer to that: I don’t know WHY when equal quantities of matter and anti-matter meet, all the mass in both the matter and the anti-matter is converted to pure energy according to E = mc^2 but, given that all the mass is converted to energy and given the fact that, despite the small masses of these tiny particles, so ...[text shortened]... great a fireball that expands so rapidly that the shockwave from it would blow the Earth apart.

But why not calmly join together? Why this sudden suicide?
The key question is "Why".

Originally posted by FabianFnas
But why not calmly join together? Why this sudden suicide?
The key question is "Why".

Some times they do not annihilate, they can become bound together to create "exotic matter".

For example, an electron and a positron (the positron is the anti-matter equivalent of the electron) can be bound together in a mutual orbit to form Positronium.

In this system the bound atom is similar to the hydrogen atom, but because of the significantly reduced mass its energy levels (and thus obsorption frequencies etc...) are much lower.

It is worth noting that this is a two way process. matter and antimatter can annihilate together to produce a photon, but a photon can also decay into a matter-antimatter pair (though a 3rd particle must be involved so that momentum and energy can be conserved at the same time).

Originally posted by MattP
Some times they do not annihilate, they can become bound together to create "exotic matter".

For example, an electron and a positron (the positron is the anti-matter equivalent of the electron) can be bound together in a mutual orbit to form Positronium.

In this system the bound atom is similar to the hydrogen atom, but because of the significantly redu ...[text shortened]... 3rd particle must be involved so that momentum and energy can be conserved at the same time).

So if an anti and a regular come together, it's just a matter of the exact closure velocities and the exact angle of the path. Sometimes the kinetic energy is enough to overcome nuclear forces and they collide to make a gamma but sometimes a slightly deflected angle of closure and just the right kinetic energy will allow quantum forces to take over and 'orbits' to form, which I assume follows regular quantum jump laws and they
thus just spiral inward to collide once postitronium is formed.
I assume that anti protons and regular proton can form a quasi atom too?
If so, what is it called? Is there such a thing as an anti neutron? I would guess yes, and could they form a quasi atom too?
How stable is Positronium? Does it decay within a few microseconds? Could you make macro amounts of positronium? Assuming you had a lot of AM. Have any of its properties been worked out, like can if form chemical bonds with something else, like Positronium Oxide or some such? Positronium perchlorate🙂
Well I was right about the other atoms mixing with positronium, a hydrogen atom and positronium is called Positronium Hydride! here is a link:http://en.wikipedia.org/wiki/Positronium_hydride
I says the lifetime is about 500 picoseconds. That is an interesting question, what makes lifetimes so short in some particles and essentially infinite in others? What is it about the fundamental underpinnings of such 'atoms' that they decay in picoseconds or microseconds and what makes them all have the same decay time, like this hydride at half a nanosecond.

Originally posted by sonhouse
So if an anti and a regular come together, it's just a matter of the exact closure velocities and the exact angle of the path. Sometimes the kinetic energy is enough to overcome nuclear forces and they collide to make a gamma but sometimes a slightly deflected angle of closure and just the right kinetic energy will allow quantum forces to take over and 'orb ...[text shortened]... s and what makes them all have the same decay time, like this hydride at half a nanosecond.

Positronium is a short-lived particle, with a lifetime in the region of picoseconds (I think, I cant remember the exact timescale off the top of my head - it may even be as long as microseconds), anyway, it is a split second timescale.

However, it has been studied and its energy levels have been observed to match with predicted levels. The different spin states have also been observed, by means of observed the mean decay rate. Different spin total spin states allow "singlet" and "triplet" states as it can have a spin of either 0 or +1. Parallel spins decay at different rates to antiparalell spins.

You're questions about what governs the lifetimes of particle is very interesting indeed. It has to do with the forces involved. There are 4 fundamental forces in the universe; gravity, coloumb (charge), strong nuclear, and weak nuclear.

Different particles interact via different forces, or a combination of forces. For example, leptons do not feel the Strong nuclear force, they only interact via the Weak force and Gravity. Only charged paricles interact via coloumb forces etc...

We must make the distinction between fundamental particles and composite particles. The nuclei of particles are COMPOSITE, they are made of many fundamental particles. Fundamental particles (like protons, electrons) are not made of anything smaller (forget about quarks for this discription, and just assume that quarks are fundamental, it is not certain they are but that doesnt matter here).

FUNDAMENTAL PARTICLES:
The decay rate (and thus lifetime) of these particles is goverent by the number of possible decay paths they have and the strength of the forces involved in each path. Each of the 4 forces have an assocated strength, and a coefficent of reaction that has been experimentally determined. A particle with many decay paths (like a high energy electron, which can decay into any of the lepton pairs) will decay faster then one which has less decay paths. Each of the paths is weighted by the strength of the force that governs that particular interaction. The strength of the forces, in decreasing order is: Strong, Coloumb, Weak, Gravity. So decay paths involving Strong force have a higher probability of happening then ones what involve the Weak force.

COMPOSITE PARTICLES:
The life time of Nuclei is a bit more complicated. The nucleus can be modeled a bit like a water droplet, with terms analogous to surface tension and internal pressure, but there are some quantum terms in the equations which have no classical analogy. The stability of the nucleus is governed by its internal energy and the energy per nucleon (proton of neutron). Imagin there is a large nucleus. There may be a combination of two smaller nuclei that can be made from the large nucleu's nucleons. If the two smaller nuclei have a lower combined internal energy then it is energetically favourable for the large particle to decay. The rate of decay depends on the difference between the total energy of the possible states and the energy avilable to the particle to decay.

Sorry if this is a bit muddled, I hope it is helpful 🙂

Originally posted by MattP
Positronium is a short-lived particle, with a lifetime in the region of picoseconds (I think, I cant remember the exact timescale off the top of my head - it may even be as long as microseconds), anyway, it is a split second timescale.

However, it has been studied and its energy levels have been observed to match with predicted levels. The different spin s ...[text shortened]... lable to the particle to decay.

Sorry if this is a bit muddled, I hope it is helpful 🙂

That sort of makes sense. I would sure (and I am sure real theoretical physicists would too) like to be the fly on the shoulder of a positronium and follow its every move from the time it gets formed to the time it generates a gamma.
Are there intermediary paths then that lead to the ultimate decay. I guess so, it glides down an energy scale until presumably the electron and positron are no longer "orbiting" but actually collide, is that the ultimate end of these particles? They stop orbiting and just collide? So there is a temporary balance of forces that allow a kind of atomic like 'orbital' to take place but for some reason, it is losing energy every attosecond? Or are the energy decays themselves subject to quantum effects and they just jump from a higher level to a lower level and only have some certain previously laid out plan of a maximum # of jumps it can sustain before the final plunge into self-annihilation?

Originally posted by sonhouse
That sort of makes sense. I would sure (and I am sure real theoretical physicists would too) like to be the fly on the shoulder of a positronium and follow its every move from the time it gets formed to the time it generates a gamma.
Are there intermediary paths then that lead to the ultimate decay. I guess so, it glides down an energy scale until presumab plan of a maximum # of jumps it can sustain before the final plunge into self-annihilation?

The energy levels of positronium are quantised. This is true of the energy levels of all atoms/nuclei.

You need to careful when imagining two particles orbiting each other, as although this is a very intuitive way of picturing it, it is not an adequate description to fully understand the physics of the system.

The energy levels are quantised because of the boundary conditions of the system. Take hydrogen as a nice example. Imagine the wavefunction of the electron which "orbits" the proton. The wavefunction is quantised by the fact that it must be of a form which repeats every 2pi radians. Imaging a standing wave on a guitar string, if it carries a sine wave the wavelength must be such that the ends of the string do not move, so only certain discreet wavelengths are allowed. As wavelength sets the energy, only certain discreet energies are possible - the wavefunction of hydrogen is analogous to this but possessed 3D spherical geometry (Solved using Legendra polynomials with quantised values of angular momentum, spin etc... in the standard way).

Rather then think of the two particles (electron and positron) moving gradually towards each other (which would yield a continuum of energy levels) it is perhaps better to imagine the wavefunction of the particles interacting in quantised stages, just like the electron that orbits a proton in hydrogen.

An interesting question arises from all this to do with matter/antimatter ratio. Clearly the vast majority of particles in our universe are matter, BUT, it seems that matter is always produced in a pair with antimatter (to conserve all the appropriate quantum numbers). As particles are annihilated in matter/antimatter pairs also, it would seem that there should be an equal amount of matter and antimatter in the universe. This is not the case, and this poses a very very interesting question.

At some point in the universe's history there must have been a period where the symmetry was broken, thus allowing an imbalance in the matter/antimatter ratio.

Also, certain relatively recently discovered phenomenon have shown that parity and other conservation laws to be violated! This has lead to the idea of "cabbibo processes", where the eigenstates of particles are different for the different forces! So the flavour states of quarks (up, down, etc...) are NOT what is seen by the Strong force, it instead sees different eigenstates which are linear combinations of the flavour eigenstates. Similar experiments involving the decay of Kaon particles has shown a slight preference for matter to be produced over antimatter.

I do not know very much about these developments, but they are certainly very interesting none the less!

Originally posted by MattP
The energy levels of positronium are quantised. This is true of the energy levels of all atoms/nuclei.

You need to careful when imagining two particles orbiting each other, as although this is a very intuitive way of picturing it, it is not an adequate description to fully understand the physics of the system.

The energy levels are quantised because of t ...[text shortened]... very much about these developments, but they are certainly very interesting none the less!

Yes, I know they are not in orbits, which is why I said 'orbits' instead of orbits. I think of it in terms of standing waves (I am a ham so have to deal with high standing wave ratios in antenna's) So it's not that big a stretch to think of an electron as just a circular or spherical standing wave structure. Does the wave structure precess around the nucleus or is it just a wave that literally stands still?
I wonder if you can take a simple atom like hydrogen, one proton, one electron and feel somehow the fields associated with the wave?
Like maybe at O and 180 degrees its mainly magnetic and at 90 and 270 exhibiting an electrical nature. Is that possible?

Originally posted by sonhouse
Yes, I know they are not in orbits, which is why I said 'orbits' instead of orbits. I think of it in terms of standing waves (I am a ham so have to deal with high standing wave ratios in antenna's) So it's not that big a stretch to think of an electron as just a circular or spherical standing wave structure. Does the wave structure precess around the nucleu ...[text shortened]... ees its mainly magnetic and at 90 and 270 exhibiting an electrical nature. Is that possible?

The standing waves are generally time dependant. so theydo precess around the nucleus. However, you can always transform into a frame of reference in which the wave is stationary.

You can indeed feel the fields associated with the wave. The electron has a magnetic dipole moment (just like a loop of current carrying wire), and a well defined electric field. These properties can be exploted to gain useful information from a sample. For example, the mangetic moment of protons are used exstensivly in MRI imaging.

Originally posted by MattP
The standing waves are generally time dependant. so theydo precess around the nucleus. However, you can always transform into a frame of reference in which the wave is stationary.

You can indeed feel the fields associated with the wave. The electron has a magnetic dipole moment (just like a loop of current carrying wire), and a well defined electric field. ...[text shortened]... from a sample. For example, the mangetic moment of protons are used exstensivly in MRI imaging.

So the waves are electromagnetic in nature? An electron is an EM wave?
or is there more to it than that? If it was just an EM, then it would seem to me to have to be a photon but photons are massless so that does not scan!
That brought me to another question: I was under the impression that the only force that could bend EM's was gravity. Now that I think about the electron wave nature spread around a nucleus, I wonder if the strong force would also bend the direction of travel like gravity? You know, if we could somehow get a raw strong force field, would it cause EM's to bend like photons do around the sun or black hole?

Originally posted by sonhouse
So the waves are electromagnetic in nature? An electron is an EM wave?
or is there more to it than that? If it was just an EM, then it would seem to me to have to be a photon but photons are massless so that does not scan!
That brought me to another question: I was under the impression that the only force that could bend EM's was gravity. Now that I thin ...[text shortened]... strong force field, would it cause EM's to bend like photons do around the sun or black hole?

Very interesting questions 🙂

An electron is not simply an EM wave, I think my previous explanation was a bit muddled and may have confused things. An electron has charge, so if the electron is moving there is a moving charge, which is a current. This current can generate a magnetic field, which can generate an electric field.

A nice way to imagine this is with a simply classical analogy. Imagine the electron orbiting the nucleus (although this is not strictly true). This can be thought of as charge moving in a loop around the nucleus. The current can be calculated from the angular frequency of the electron's orbit, as an angular frequency of 1 orbit a second would mean a charge of 1e passes past any point on the ring per second, so it is trivial to calculate the current if we know the orbital frequencies (which are fairly easy to derive). This orbital motion is not steady state, so results in a changing magnetic moment and hence electromagnetic wave due to Maxwell's equations in the standard way.

In short, electron wavefunction is not electromagnetic, the electron wavefunction gives the probability of finding the electron at any given point and time, so if averaged over time it effectively gives the charge distribution. This charge distribution can change in time giving rise to EM radiation (as long as the electron drops down an energy level to accommodate energy conservation).

The Strong nuclear force can not bend EM radiation, because photons do not couple to the Strong force (photons are bosons, which are force exchanging particles, in fact photons are the exchange particle for the coloumb force).

It should be noted that the strong force only has a range of ~10^-15m, which is smaller then a nucleus (about the size scale of individual nucleons). So the strong force is not responsible to the electron's attraction to the nucleus. It is the coloumb force which attracts the nucleus and electron together via charge interaction.

So unfortunately, whilst a Strong force field could possibly manipulate matter (if you devised a way to project it over a long range somehow hehe!), it could not have an effect on an EM waves emitted from the matter, as the Strong force does not effect photons.

Hey, thanks! I have read a lot of articles but haven't looked at them in that much detail. Learn something new every day, great!

Originally posted by whodey
I was talking with someone the other day about alternative energy sources and they mentioned that nuclear fusion is probably one of the few answers in terms of providing energy for the masses in a "safe" way. It has been done, however, it takes far more energy to produce such a reaction than it produces from the reaction and therefore, is not cost beneficial ...[text shortened]... downs etc. and/or are there any other potential energy sources out there that shows potential?

The trick to harnessing a stable long term fusion reaction is developing what is called a "magnetic bottle" or what is termed a"force field" chamber that contains the reaction indefinitely.We do not have the technology yet,but one day we will,i'm sure.Then seawater will be our main source of energy after the hydrogen in extracted.

Originally posted by KingofGloom
The trick to harnessing a stable long term fusion reaction is developing what is called a "magnetic bottle" or what is termed a"force field" chamber that contains the reaction indefinitely.We do not have the technology yet,but one day we will,i'm sure.Then seawater will be our main source of energy after the hydrogen in extracted.

I think this will happen far before we can imagine. No more energy crisis. No more CO2 emmitting. The recovery of atmosphere and nature.

Originally posted by FabianFnas
I think this will happen far before we can imagine. No more energy crisis. No more CO2 emmitting. The recovery of atmosphere and nature.

I think it won't happen for at least 25 years and it is looking more and more like we may be passing a point of no return in terms of climate change. There are at least three separate more or less viable approaches to fusion, now, the Z machine, magnetic confinement and inertial confinement (zapping a capsule with a terawatt or petawatt laser). None of these are exactly running way out in front technologically speaking, either one may win or all three may prove to be a dead end. At this point in time we just don't know whether ANYTHING can make economically sound fusion. I am sure the ITER will make energy, but it first has to be proven by building the thing which they are still in the design phase, no hardware has even been assembled yet so even THAT is years in the future and it only LOOKS like the projections say it will generate excess energy. Whether it actually does or not, the devil is in the details they say. It might turn out the radioactivity inherent in fusion may make it too expensive to keep replacing internal shielding because there will be a huge influx of neutrons into shielding material which complicates the real design decisions and all that has yet to be addressed. So for the short term, if manmade climate disaster is to be avoided, humans as a whole, EVERYONE on the planet, has to go on an energy diet to first, lower the CO2 and methane and other greenhouse gasses to hopefully preindustrial levels THEN hope fusion or solar or wind, renewable energy sources takes over for fossil fuels. If we don't do that, even if fusion works, it may come as too little too late.

Originally posted by sonhouse
I think it won't happen for at least 25 years and it is looking more and more like we may be passing a point of no return in terms of climate change. There are at least three separate more or less viable approaches to fusion, now, the Z machine, magnetic confinement and inertial confinement (zapping a capsule with a terawatt or petawatt laser). None of thes ...[text shortened]... fossil fuels. If we don't do that, even if fusion works, it may come as too little too late.

I'm not so pessimistic about it.

When governements feels that the point of no return is approaching, they will spend money. There is money floating around, sucked up by various meaningless wars. Release this vast amount of money, channel it into fusion research, and things will begin to move in the right direction.

I'm more optimistic about it.