I read a book a few years back about sub-atomic particles that was written for the lay person (I wish I could remember title and author). In this book it explained that electrons were fundamental particles (i.e. not comprised of sub-particles) and their anti-particle was the positron. When a positron and an electon meet they instantly disintegrate into pure energy in the form of a Gamma ray. This did not surprise me since I had heard about matter/antimatter reactions by watching Star Trek. What I found to be incredible was that scientists had observed Gamma rays of sufficiently high frequency (i.e. energy)spontaneously becoming an electron/positron pair (travelling in opposite directions so that they did not instantly recombine). How is this possible? How can energy become matter? Is all matter just Gamma rays traveling in circles? I can understand how a pile of gun powder can ignite into a flash of light energy and hot gases (N2, CO2) but energy and hot gases can not spontaneously become a pile of gun powder. It violates one of the laws of thermodynamics. My faith in the fundamental laws of the universe has been shaken. I’ve tried to find answers in other texts but they were all over my head, and searching the web yielded nothing. Can anyone out there help me?
I’m sure you’ve seen the equation E=mc[sup]2[/sup]. This says, that energy and mass are eqivalent. So, they can turn into each other. In certain reactions, like matter-antimatter anihilation, it goes one way, but there’s nothing in the physics that keeps it from going the other way.
When it comes down to it, very little of your common, everyday, earthly phenomena seem to involve matter-energy or energy-matter conversions. Which is why there were laws of conservation for matter and energy for a long time before Einstein replaced them with the Law of Conservation of Mass-Energy. So it’s no surprise that this concept doesn’t appeal to your sense of how things work. But in our universe, if the physics allows it to happen, it will happen in some cases.
I think it was Neils Bohr (I’m too lazy to look it up) who said, “That which is not prohibited is mandatory.”
So basiclly your saying that Gamma rays convert into matter for the same reason a dog licks his balls…because it can. Somehow I find this answer unfulfilling, but I appreciate the effort.
I believe that SarumanRex (great name, btw) was primarily concerned with the second law of thermodynamics, and what seems to be a decrease in entropy of the system. One caveat right off the bat: Entropy isn’t well-defined for sufficiently simple systems. If you’ve got a videotape of a cue ball breaking a pool layout, it’s going to look awfully odd if you play it in reverse, because entropy is clearly increasing in the forward process, but decreasing in the reverse. If, however, you just have one ball bouncing around (assuming friction is small), it’ll look pretty much normal in reverse, because the system is too simple to have well-defined entropy. Similarly with the pair-formation: It’s awfully simple, so it’s reversible.
It does still involve some entropy, however. In general, massless particles like photons are more entropic that particles with mass, like electrons. Eventually, provided that it all doesn’t go crunch first, all of the Universe will just be a bunch of photons, gravitons, and possibly neutrinoes flying around. Pair production, which increases the number of massive particles, and decreases the number of massless ones, is very unlikely to occur unless there’s something else interacting with the system which can increase its entropy to compensate.
Chronos is correct - thermodynamics only applies to large systems - it’s essentially a statistical statement. There is no physical law that prevents all the air in your room from collecting in the upper left corner at some time in the future, it is just very improbable, hence the ‘law’ of thermodynamics is just a really really bulletproof law of averages.
I get in trouble with my wife all the time because I say something is impossible and she says it could theoretically happen. My response is that if I have to wait Avogadro’s number of universe lifespans to see it happen, it is sufficiently impossible, but she’s technically right.
Your question did give me pause, though, as it occured to me that a single photon producing a pair by itself in a pure vaccuum would cause some strange problems with frames of reference. I’ve asked for input and surmised a solution (it must interact with some other particle, even if very weakly) in
another thread.
If I’m correct, the photon then is not really doing it by itself but is in fact passing near an electron or something causing it to produce a pair. You could think of this as a photon balanced on a sharp edge, and when it passes near the instigator particle it wobbles just enough to fall over the edge and produce a pair, but if no other particle came near it it would never produce a pair.
to answer the original question;
can a gamma ray spontaneously turn into an electron-positron pair?
the answer is no.
energetically this is allowable, but if i remember my physics correctly, momentum cannot be conserved in such a reaction. the annihilation of an electron-positron pair necessarilly produces two gamma rays, and a single gamma cannot produce the pair by it’s lonesome.
But if it has somewhere to dump the extra momentum, it can in fact produce the pair. i once did some design/modeling work on a gamma ray Cerenkov detector; the basic design had the gammas passing through a lead plate and converting to an electron-positron pair. but the lead was necessary as that allowed the momentum to be balanced and the reaction could take place.
Thinking about it, i am sure i remmber correctly; here is a simple argument to back the claim: In the Center of Mass reference frame for the e-,p+ pair, the system momentum (if the system consists only of the e- and p+) is zero by definition. but the momentum of a single gamma can NOT be zero!. therefore, the conversion of a single gamma to an e-,p+ pair cannot take place.
-luckie
I fully support a “One China” policy.
We should support Taiwan if and when they decide to reclaim the mainland!
If I understand the physics correctly, an electron and a positron annihilate to produce two gamma rays which depart in opposite directions and have a characteristic energy (512 KeV, I think).
Like all reactions, there’s nothing that says it has to go one way. So if two photons of that specific energy meet (I think they have to be head-on and probably have to be either in-phase or 180[sup]o[/sup] out-of-phase), they will make an electron and positron.
Note that electrons and positrons, being of opposite charge, attract each other, and any positron released among ordinary matter is sure to be annihilated. Whereas two photons don’t attract and so will meet only by chance. Thus it’s much more likely that the annihilation reaction will happen than the reverse.
Thanks for all the info, you folks really helped clear things up for me. Do you have any input on my other question as to whether or not all fundamental particles (such as electrons) are just photons somehow traveling in circles? It seems that this would be a topic of great interest to physicists.
bump
The Standard theory of quantum mechanics does not involve fundamental particles being photons traveling in circles. This would be a decidedly nonstandard theory.
Fundamental particles fall into two groups: the leptons and the quarks. In addition, there are carrier particles for the forces.
The leptons consist of the electron, muon, tau lepton, and the three kinds of neutrinos and their anti-particles.
There are six kinds of quarks: up, down, charm, strange, top and bottom (and of course their anti-particles). Protons and neutrons are made up of three quarks each. Mesons are made of two quarks each.
Carrier particles are the photon, gluon, W and Z bosons, and the graviton. Respectively, they intermediate the electromagnetic, strong nuclear, weak nuclear (both W and Z), and gravitation forces. We haven’t actually detected the graviton yet.
All this is explained in more detail here.
When I posted the above message, I forgot about string theory. Your “photons in a loop” idea may be a distorted version of string theory.
String theory basically says that all those “fundamental” particles area really just different manifestations of the same object: a minuscule loop of “string”. The same kind of string for all types of objects. What distinguishes them is the vibration mode of the string. That is, if it vibrates in one mode, it’s an electron, in another a strange quark, etc. What exactly this string is, I’m not sure, but from what they say, the quantum mechanic Standard model can be derived from string theory.
The only problem with string theory is that it’s all theoretical. We haven’t any experimental data that either supports it or disproves it.
Anyway, here is a site describing it.
Thanx for the links they were very helpful. String theory does seem to offer some explanations for the internal workings of fundamental particles. I’ve ordered a book on the subject called “The Elegant Universe” and I’m hoping it will explain it all.