From what I understand of how antimatter is currently created, it involves firing a laser at a suitably dense target (gold foil I believe?) and collecting the positrons or anti-protons or whatever from particles absorbing gamma rays from the resulting plasma.
So, a regular particle somehow gets “turned into” (quoted since I have no clue how that works) into a regular particle and its opposite (or the energy is turned into antimatter, whatever).
What I was wondering was whether it is theoretically possible to change a fundamental particle like an electron’s properties so as to turn it into a positron. It doesn’t seem like conservation of energy is being violated. After all, it still has to collide with a regular electron to release energy, right?
What it would be is a really really efficient way of converting all of the matter in a particle into energy.
Granted, we must have no clue how to do this, or we’d be zipping about the solar system by annihilating beer bottles - but is there any actual physics reason why a particles properties can’t be changed? Does talking about inverting a proton’s charge and magnetic moment even make sense?
Basically, there is a balance to these things. You do a sort of arithmetic, with the total number of antiparticles subtracted from the total number of particles. You will end up with the same sum before and after any interaction.
A gamma ray can give rise to a positron, but only if it simultaneously creates an electron. You start with no particles at all (For the purposes of this sort of thing, a photon doesn’t count as a particle. In reality, it’s sort of like its own antiparticle.) By the same token, a neutron can decay into a proton, an electron, and an antineutrino. In this case, you start with a particle, and end with two particles and an antiparticle. Add them up, and you’ve got one particle at both ends of the decay.
While the products don’t have to be all the same variety of particle, you must end up with the same total number of partiles on both sides of an interaction. So anything that creates antimatter will not create just antimatter. It always ends up with some matter in the mix.
So the universe requires that the sum of all particles and antiparticles be in balance? How so? I know I’ve skimmed speculations on the imbalance in the current universe’s supply of matter and antimatter, but now it seems like you’re suggesting there has to be a particle for every antiparticle. Are there half the galaxies out there antimatter (come to think of it, this doesn’t seem likely or one would expect more dramatic fireworks in the skies as galaxies collide :))? And how does a gamma ray give rise to a particle/antiparticle pair? I thought it did it through collision with a regular particle?
No, I’m not suggesting that at all. I’m just saying that in any particle interaction in the present universe, you have to balance. So, in order to convert an electron into a positron, you would need to create two particles (1 particle==>2 particles + 1 antiparticle).
As far as why the universe came out of the Big Bang composed almost exclusively of matter and with almost no antimatter, that is one of the BIG QUESTIONS. This is one of the things that a true unified field theory would hope to explain, but I’m not sure if the current theories do or not.
Hopefully, Chronos will show up soon to add to my wholy inadequate knowledge of the current state of the science.
Sorry, I neglected to answer this. The answer, frustratingly enough, is: It just does. As we all know, energy and matter are really just different forms of each other. So, a gamma ray of sufficient energy could just as well be an electron-positron pair. If you add up the rest mass and the kinetic energy of the pair, they are the same as the energy of the original photon. As someone once said (Bohr?): “Anything not prohibited by the laws of physics is mandatory.”
Most often, the pair only travels for a tiny distance before annihilating each other, creating a gamma ray.
Oooh. I know.
Ok. We somehow change a proton into an anti-proton. Immediately according to this principle you stated exists, a proton pops up to balance it. We let the two annihilate, and the original proton is converted completely to energy.
How bout that, is that legal?
Come to think of it.
“As we all know, energy and matter are really just different forms of each other.”
and
'As someone once said (Bohr?): “Anything not prohibited by the laws of physics is mandatory.” ’
Forget twiddling protons, shouldn’t we be able to get them to spontaneously convert into energy?
Or are particles a more stable state then energy or something like that?
Actually, a photon must interact with something (another photon, an electron, etc.) to produce a pair. One way to realize this is by conservation of momentum. For any pair of massive particles, there is a zero-momentum frame, a frame of reference in which the momenta cancel out. But, a single photon has a non-zero momentum in all frames.
Another way to think about this is that for any photon there is a reference frame in which the photon’s ‘energy’ is too low to create a pair. It must interact with something.
Yeah, in general, matter is most stable. In the quantum foam, particles and energy change into each other constantly, but it’s not that easy to get it to yield pure energy.
Protons, in fact, seem to be just about the most stable particles around. When last I’d heard, we still haven’t observed one decaying, though the theories say that they should. Just not very often.
The issue here is conservation laws. Everyone’s heard of conservation of energy and momentum, and conservation of charge and angular momentum are covered in most physics courses, but there’s a host of other quantities in particle physics that are also conserved, such as lepton number (actually three separate quantities, electron number, mu number, and tau number, each of which is conserved independently) and baryon number, plus several, like strangeness, that are usually but not always conserved. For any reaction, the conserved quantities must match on the left and right side of the equation. For instance, if you try to turn an electron into a positron, e[sup]-[/sup] --> e[sup]+[/sup], you violate both conservation of electron number (1 on the left, -1 on the right) and charge (-1 on the left, 1 on the right). OK, what if we start with a proton, instead, to fix the problem with charge: p[sup]+[/sup] --> e[sup]+[/sup] ? Now we’ve got problems with both electron number (0 on the left, and -1 on the right) and baryon number (1 on the left, 0 on the right). We can fix the baryon number problem by also putting a neutron on the right, which has baryon number 1 and charge zero, and we can fix the electron number problem by also putting a neutrino on the right, with electron number 1 and charge 0. The reaction is then p[sup]+[/sup] --> n + e[sup]+[/sup] + v, with both sides having a charge of 1, baryon number of 1, and electron number of zero. This is the process called inverse beta decay, and given a source of energy, really does occur, so it is possible to get antiparticles from particles.
One final note: The only absolute conservation laws are energy/momentum, angular momentum, charge, and magnetic charge (if there is such a thing). All other conservation laws can be broken by a black hole, and it’s possible that they can be broken in other extremely rare reactions predicted by the Grand Unified Theories.
By the way, Saltire, protons aren’t the most stable particles known, they’re just the most stable non-elementary particles. Electrons, photons, and probably neutrinos (elementary particles all) are more stable than protons.
curious, how can a photon have momentum? momentum, i believe, is defined as mass times velocity, and since photons travel with velocity c, they must have no mass, giving them zero momentum, no? or is the problem that i’m using a newtonian formula to describa a relativistic phenomena?
Yup, I was pretty sure I was stating that imprecisely. I should have left it out entirely. Of course, I knew I could count on you to straighten it out. Thanks.
Yes, it is relativistic. I don’t have a derivation handy, but I believe p = E/c, or p = h(nu)/c
where E is the energy of the photon, (nu) is the frequency of the photon, h is Planck’s constant, c is the speed of light.
IIRC, radiation pressure, it is what drives the solar winds, is caused by the momentum of photons.
I think there it explains why you want to make antiprotons instead of antielectrons, even though the latter are easier to make: e+'s decay into gamma’s which cannot be directed to produce thrust, whereas p-'s decay into some charged pions that can be directed with a magnetic field for a few meters/nanoseconds until they decay.
