Particle and antiparticle

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gptejms said:
Guys,I was referring to the loop in a Feynman diagram which is a virtual process(!)--all I said was that the short-lived electron-positron pair in such a scenario may be given the kind of interpretation I gave above.
You need to be careful with what you say here. Although i get the point you are making, it needs to be said that loops in QED and QCD correspond to self-energy terms. Indeed a short-lived electron positron pair can be created and "killed" shortly after. You do not need loops for that though since this process happens constantly in the vacuum, ie vacuum fluctuations of QFT. When enough energy is available this virtual pair can become real and "long-living".

Like i stated before : i see no reason what so ever to keep on making a difference between a positron and a hole since they are fundamentally seen exactly the same in QFT. Like in this post with them loops, i had to mention the concept of self energy, expressing the fact that a particle in QFT does NOT exist alone, although it can be considered to be independent of other particles.

regards
marlon
 

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gptejms said:
Guys,I was referring to the loop in a Feynman diagram which is a virtual process(!)--all I said was that the short-lived electron-positron pair in such a scenario may be given the kind of interpretation I gave above.
Is there a reason why you are going out of your way to pick one of the most EXOTIC, unlikely, and frankly, the strangest way to generate a positron? All the positrons that are used in accelerators, medical applications, etc., are NOT generated this way. Do you think arguing about the GENERAL properties of a positron, but using a very unusual and highly specific example, is a valid thing to do? I mean, if you really want to pick a highly specific situation, why not use the positron in a loop in a feynman dragram generated during a solar eclipse and initiated by an Amish girl who is pregnant via an immaculate conception? Why just stop half-way?

Zz.
 
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marlon said:
You need to be careful with what you say here. Although i get the point you are making, it needs to be said that loops in QED and QCD correspond to self-energy terms. Indeed a short-lived electron positron pair can be created and "killed" shortly after. You do not need loops for that though since this process happens constantly in the vacuum, ie vacuum fluctuations of QFT. When enough energy is available this virtual pair can become real and "long-living".
marlon
At least I got the idea across by bringing in the loop!I know this process constantly happens in vacuum and this is what I meant when I said 'virtual electron-positron pair' but somehow this was taken to conclude(by Zz) that I didn't know the difference between a virtual particle and an antiparticle!
 
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ZapperZ said:
Is there a reason why you are going out of your way to pick one of the most EXOTIC, unlikely, and frankly, the strangest way to generate a positron?
I am going out of my way just to get the simple idea across to you,science advisor.Let me make a few things clear--I got into this thread to put across what I guessed the originator of this thread 'might' have in his mind.I don't subscribe to the idea myself(that is why I said 'for the sake of argument').As for general positron or a virtual positron,it's obvious that the idea,within the constraints of time-energy uncertainty relation,applies to a virtual positron only.It's a kind of symmetry plus 'allowed for interpretation'(of course this is debatable) that exists for a (short-lived)virtual electron-positron pair.
 
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I started this thread in view of an article I read in Scientific American, Sept. 2004 issue. Two expt., ATHENA and ATRAP, at CERN are underway to test the spectroscopic properties of hydrogen and antihydrogen atom (see the SA issue). The experiments are conducted to test the CPT violation and consequently the Lorentz violation.

If we are so sure about the physics of particle-antiparticle then why do we need to test the CPT. Do we expect an atom to behave differently then an antiatom or matter as compared to antimatter (any comments ZZ)?

In a semiconductor, the position of a hole is below Fermi energy level whereas electron is above it. In contrast, the position of a Positron is below 0 i.e. -ve energy where as an electron has +ve energy. That is why electron-hole pair results in radiation of few eV, while electron-positron recombination results in a high energy radiation. Although the physics (explanation) of electron-hole looks similar to electron-positron, one really need to prove the similarities experimentally.

Is there any experimental proof where a large number of electrons and positrons are trapped and made to behave as they do in a semiconductor material. What I mean to say is an electron-positron recombination in vacuum results in emission of radiation following the Einstein's mass energy eqn. Can we experimentally prove that in a many body problem involving actual electrons and positrons, their recombination will be according to electron-hole recombination process.
 

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anuj said:
I started this thread in view of an article I read in Scientific American, Sept. 2004 issue. Two expt., ATHENA and ATRAP, at CERN are underway to test the spectroscopic properties of hydrogen and antihydrogen atom (see the SA issue). The experiments are conducted to test the CPT violation and consequently the Lorentz violation.

If we are so sure about the physics of particle-antiparticle then why do we need to test the CPT. Do we expect an atom to behave differently then an antiatom or matter as compared to antimatter (any comments ZZ)?

In a semiconductor, the position of a hole is below Fermi energy level whereas electron is above it. In contrast, the position of a Positron is below 0 i.e. -ve energy where as an electron has +ve energy. That is why electron-hole pair results in radiation of few eV, while electron-positron recombination results in a high energy radiation. Although the physics (explanation) of electron-hole looks similar to electron-positron, one really need to prove the similarities experimentally.

Is there any experimental proof where a large number of electrons and positrons are trapped and made to behave as they do in a semiconductor material. What I mean to say is an electron-positron recombination in vacuum results in emission of radiation following the Einstein's mass energy eqn. Can we experimentally prove that in a many body problem involving actual electrons and positrons, their recombination will be according to electron-hole recombination process.
What you are asking here now is a COMPLETELY different issue. It would have been clearer what your intentions were had you put your question within this context, rather than just asking a blanket question if we know anything about particles and antiparticles.

Let's narrow this down to a important issues:

1. We KNOW very well how particles and antiparticles behave according to our physical laws.

2. We however do NOT know why there are more matter than antimatter in this universe of ours.

That last part is one of the reason why there are ongoing studies on certain rare decay events, such as the Kaons, that exhibit the so-called CP violation (I even mentioned this in one of my postings in this thread!). There have been several theories that link CP-violation with the apparent asymmetry between the amount of matter and antimatter. It is why people want to make antihydrogen, to see if there are more exotic properties that can be gathered.

But again, if you have missed it, let me emphasize that these are VERY rare events. And the fact that we CAN detect such things and know when a CP violating event occurs implies that we know how these particles (and antiparticles) should behave. So asking if we know anything about them makes it rather strange. We HAVE to know something about them to know what properties to measure!

There is a misconception here that seems to think that just because we continue to study on something, that we know nothing about it. We know enough to know what we're looking for. However, we don't know everything (and in my opinion, never will). The work of physicists has always been "OK, we know that works here. Let's see what happens there!" It has always been the expansion of our current boundary of knowledge. Antihydrogen has not been studied before - so we study them and try to figure out if they are any different. As physicists, we won't be happy JUST having some theory or someone tells us that they should behave the same way - we want to TEST them ourselves and make sure!

But this is waaaay different than making a claim that our laws do not work on these things.

Zz.
 
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anuj said:
Is there any experimental proof where a large number of electrons and positrons are trapped and made to behave as they do in a semiconductor material. What I mean to say is an electron-positron recombination in vacuum results in emission of radiation following the Einstein's mass energy eqn. Can we experimentally prove that in a many body problem involving actual electrons and positrons, their recombination will be according to electron-hole recombination process.
Thanks ZZ. My question is answered to some extent. The last question regarding electron-hole and electron-positron still remains to be answered fully. I hope you can cite reference to some already performed expts to prove the positron-hole equivalance.
 

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anuj said:
Thanks ZZ. My question is answered to some extent. The last question regarding electron-hole and electron-positron still remains to be answered fully. I hope you can cite reference to some already performed expts to prove the positron-hole equivalance.
Look in a technique called inverse photoemission.

Zz.
 
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ZapperZ said:
Look in a technique called inverse photoemission.

Zz.
The inverse photoemission technique is used to study the unoccupied electronic states in solids. This is done using the ultraviolet or soft X-rays. Which basically means study of electrons-holes. What I am asking is the experimental proof for a large number of electrons-positrons made to behave as though they are in a semiconductor material showing similar characteristics as electrons-holes. An inverse photoemission technique can probably be useful here provided a gamma ray (>400kev) is used to generate an electron hole pair. Any experiment performed in that direction.
 

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anuj said:
The inverse photoemission technique is used to study the unoccupied electronic states in solids. This is done using the ultraviolet or soft X-rays. Which basically means study of electrons-holes. What I am asking is the experimental proof for a large number of electrons-positrons made to behave as though they are in a semiconductor material showing similar characteristics as electrons-holes. An inverse photoemission technique can probably be useful here provided a gamma ray (>400kev) is used to generate an electron hole pair. Any experiment performed in that direction.
Er... Huh? HUH?

What are you trying to get at?

Electron-positron pairs are not as easily made as electron-hole pairs. They require a lot more energy and intrinsicly at a disadvantage because they require this creation near a massive nucleus for momentum conservation. Electron-hole pairs require very little energy, and since they are already surrounded by matter, have absolutely no problem with momentum conservaton.

Furthermore, once they are created, then the environment they are in dictates what gymnastics they can do! Free particles have their own set of boundaries, particles in matter have another set! There are no reasons to expect they each should mimic each other's properties!

You also have an error in your understanding of what an "inverse photoemission" is. This is NOT a photoemission process and it does NOT generate electron-hole pairs as a product.

Zz.
 
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marlon said:
So basically a propagator expresses the probability that a particle with positive energy created in x' on t' will propagate to place x on time t. This is valid for t > t'. If t < t', then the propagator expresses the probability that a particle with negative energy created in x' on t' will propagate BACKWARDS in time to x on some time t. It is the SUM of both these probability-amplitudes that is relativistically invariant !!!
Can you write down the propagator(math. expression) for Dirac field and then explain each term
 

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