Particle and antiparticle


by anuj
Tags: antiparticle, particle
anuj
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Oct18-04, 04:00 AM
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How much do we know about particles and antiparticles? Can we say that the two class of particles follow all the known laws of physics. i.e. no deviation or violation of known laws.
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ZapperZ
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Oct18-04, 06:43 AM
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Quote Quote by anuj
How much do we know about particles and antiparticles? Can we say that the two class of particles follow all the known laws of physics. i.e. no deviation or violation of known laws.
To what degree do you want to "know" about them? We know them well enough that we even put our lives at their mercy. A PET scan is a prime example. Or try any medical accelerators.

When we start using something to do something else, then this should immediately tells you that we know quite a bit already about its properties. When there is an application of an idea or principle, then that idea or principle is already well-known, well-understood, and well-tested.

Zz.
marlon
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Oct18-04, 06:53 AM
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Quote Quote by anuj
How much do we know about particles and antiparticles? Can we say that the two class of particles follow all the known laws of physics. i.e. no deviation or violation of known laws.
We know them very well. Anti-particles appear in relativistic quantummechanics and in quantum field theory. Just look at the "first and second" quantization procedure of the Dirac-field for example. They are the particles that correspond to the negative energy-solutions of the Dirac-fermions (i mean the quanta of the anti-commuting Dirac field). Anti-particles are also incorporated into the Feynmann-propagator in QFT as particles with positive energy that propagate backwards in time.

regards
marlon

mathman
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Oct18-04, 04:09 PM
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Particle and antiparticle


There is an extremely small difference in properties between particles and anti-particles. At the big bang, particles and anti-particles were born in equal number, but because of this diffference, there ended up a slight excess of particles, which make up the planets, stars, etc. Although physicists have discovered some of these differences, the final result (particle excess) is not fully understood.
anuj
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Oct18-04, 11:13 PM
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Truely speaking, we understand the physics (QM in general and relativity in particular) of positive energy particles moving forward in time. We do understand the negative energy particles or the particles moving backward in time as antiparticles to some extent.

For example, Can we say for sure with experimental support that the antiparticles do not violate laws of electrodynamics, heavenly bodies made up of antiparticles will follow newtons gravity laws, Physical meaning of moving backward in time (not the mathematical formulation).
ZapperZ
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Oct19-04, 06:26 AM
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Quote Quote by anuj
Truely speaking, we understand the physics (QM in general and relativity in particular) of positive energy particles moving forward in time. We do understand the negative energy particles or the particles moving backward in time as antiparticles to some extent.

For example, Can we say for sure with experimental support that the antiparticles do not violate laws of electrodynamics, heavenly bodies made up of antiparticles will follow newtons gravity laws, Physical meaning of moving backward in time (not the mathematical formulation).
If antiparticles "violate" the laws of electrodynamics, your modern electronics would NOT work. In condensed matter physics, the "holes" in your p-type semiconductors (part of your pn junction) are the antiparticles of electrons. The physics that describe electron-hole creation is IDENTICAL to the physics of electron-positron pair.

If antiparticles "violate" the laws of electrodynamics, ALL the particle detectors stationed at those particle colliders at CERN, Fermilab, KEK, DESY, SLAC, etc. would have been giving us bogus results.

If antiparticles "violate" the laws of electrodynamics, PET scans would be useless.

.. do I need to go on?

Zz.
Adrian Baker
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Oct19-04, 03:15 PM
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Quote Quote by mathman
There is an extremely small difference in properties between particles and anti-particles. At the big bang, particles and anti-particles were born in equal number, but because of this diffference, there ended up a slight excess of particles, which make up the planets, stars, etc. Although physicists have discovered some of these differences, the final result (particle excess) is not fully understood.
Are there any good resources on this on the 'net that you know of?
Thanks
jtolliver
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Oct19-04, 04:06 PM
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Quote Quote by Adrian Baker
Are there any good resources on this on the 'net that you know of?
Thanks
Try searching the web for baryogenesis.
mathman
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Oct19-04, 04:12 PM
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Are there any good resources on this on the 'net that you know of?
Thanks
Look for articles by Michael Turner on arXiv. For starters, astro-ph/0202005, 0202007, 0202008. (These are all more or less the same).
anuj
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Oct20-04, 02:05 AM
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Quote Quote by ZapperZ
If antiparticles "violate" the laws of electrodynamics, your modern electronics would NOT work. In condensed matter physics, the "holes" in your p-type semiconductors (part of your pn junction) are the antiparticles of electrons. The physics that describe electron-hole creation is IDENTICAL to the physics of electron-positron pair.

Zz.
The physics of electron-hole may be identical to electron-positron but I suspect we can make a conclusion that a hole is nothing but a positron. If it is so, why do we need to make big expensive particle colliders to create antiparticles just to study their physical properties. Cant we do the same using some simple semiconductor devices.

Secondly, the negative energy should mean an imaginary mass that in turn should mean that two negative energy particles should repulse (newtons gravity, a week field as compared to electromagnetic field)

Is it right to consider the absence of -ve energy as +ve energy which is an antiparticle. i.e. the absence of an imaginary mass as a +ve mass particle.

Any reference for the comparison between hole and positron.
ZapperZ
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Oct20-04, 06:55 AM
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Quote Quote by anuj
The physics of electron-hole may be identical to electron-positron but I suspect we can make a conclusion that a hole is nothing but a positron. If it is so, why do we need to make big expensive particle colliders to create antiparticles just to study their physical properties. Cant we do the same using some simple semiconductor devices.
It is because you can't study "holes" outside of a many-body interaction of the material, whereas as you can with positrons. There are things you can study with positrons that you can't study with holes, and there are things you can study with holes that you can't study with positrons. Take your pick!

Secondly, the negative energy should mean an imaginary mass that in turn should mean that two negative energy particles should repulse (newtons gravity, a week field as compared to electromagnetic field)

Is it right to consider the absence of -ve energy as +ve energy which is an antiparticle. i.e. the absence of an imaginary mass as a +ve mass particle.

Any reference for the comparison between hole and positron.
Whaaaaa???

Look at the Dirac equation very carefully and see where the "negative" quantity is assigned, and see why you can't assign it simultaneously to a number of quantities!

You are deviating from your original question. Let's tackle ONE thing at a time and not let this meander aimlessly. Are you STILL questioning whether antiparticles (and particles too, if I recall) obeys the set of laws that we know of, inspite of the fact that we are already USING them in various applications?

Zz.
anuj
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Oct20-04, 07:25 AM
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Quote Quote by ZapperZ
It is because you can't study "holes" outside of a many-body interaction of the material, whereas as you can with positrons. There are things you can study with positrons that you can't study with holes, and there are things you can study with holes that you can't study with positrons.
Zz.
It is difficult to prevent a positron from interacting with an electron as the two quickly annihilates resulting in a photon of energy E following the Einstein's famous mass energy equation. How come, in a many body interaction the same positron finds it easy to stay alive (no anihilation).

Take the example of a semiconductor laser. The electron-hole recombination too results in emission of a photon but this time the energy of photon is equal to the quantum state of electron to hole transition. If a hole is same as positron, why the emission of light here is not according to Einstein's equation.
ZapperZ
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Oct20-04, 07:57 AM
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Quote Quote by anuj
It is difficult to prevent a positron from interacting with an electron as the two quickly annihilates resulting in a photon of energy E following the Einstein's famous mass energy equation. How come, in a many body interaction the same positron finds it easy to stay alive (no anihilation).

Take the example of a semiconductor laser. The electron-hole recombination too results in emission of a photon but this time the energy of photon is equal to the quantum state of electron to hole transition. If a hole is same as positron, why the emission of light here is not according to Einstein's equation.
1. Till a few years ago, the Advance Photon Source here at Argonne used POSITRONS in their storage rings to create the synchrotron radiation for use in all those studies. So your claim that they are difficult to be contained and maintain long enough before the get anhilated is wrong.

2. The creation of holes out of electron excitations above the Fermi energy is DIFFERENT than the presence of holes due to doping with acceptors! The creation of electron-hole pairs has the same-looking physics as the creation of electron-positron pairs! The doping of acceptors into an intrinsic semiconductor to turn it into a p-type extrinsic semiconductor is not the same! The holes have long lifetimes simply because your semiconductor is working at room temperatures (or even higher!). Go back to 0 K and they'll die off.

3. The "vacuum" state of solids is the Fermi level at 0K. The physics is identical as the vacuum state of QED. The mathematics that describe the physics are practically identical. But they REFER to different things. Your recombination process refers to energy states that CAN, in fact, be translated to the "effective mass" of the two particles! That is why this is a many-body system!


You still are avoiding answering my question...

Zz.
vanesch
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Oct20-04, 09:09 AM
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Quote Quote by anuj
The physics of electron-hole may be identical to electron-positron but I suspect we can make a conclusion that a hole is nothing but a positron.
I don't think anyone in his right mind thinks that the holes in semiconductors are positrons (and in fact, the "electrons" in semiconductors are also not really the electrons of elementary particle physics, but a result of the collective behaviour of those electrons). However, I know that the confusion exists, because I was confused at a certain point when I was a student, and suggested making a positron source by cold emission from a semiconductor

I think what Zapper is aiming at is that the QFT model is very similar, except that the gap for "elementary particle electrons and positrons" is 2x 511KeV, while the one for electrons and holes is a few eV.

cheers,
Patrick.
gptejms
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Oct21-04, 11:41 AM
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I guess what anuj is trying to ask when he asks if anti-particles violate physical laws is this---is an anti-particle really a particle (of opposite parity) going back in time?(CPT)
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Oct21-04, 02:27 PM
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Quote Quote by gptejms
I guess what anuj is trying to ask when he asks if anti-particles violate physical laws is this---is an anti-particle really a particle (of opposite parity) going back in time?(CPT)
The laws of physics work the same way either forward or backwards in time (let's not get CP violation into this). So why would it matter if this is a particle going forward or backwards in time?

Take note that there's nothing to prevent me from also equating a real particle moving forward in time with an antiparticle moving backwards in time.

But this is all moot and meaningless until someone can point to me the evidence that would lead to the original question of this thread in the first place. What IS the evidence that our laws of physics do NOT work on particles and antiparticles? I have tried to described as much as I can why we KNOW they work. It seems that the assertion that they MIGHT not work are simply based on speculation and "witch-hunting" without any kind of physical justification.

Zz.
Tom Mattson
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Oct21-04, 04:14 PM
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Quote Quote by anuj
If it is so, why do we need to make big expensive particle colliders to create antiparticles just to study their physical properties.
Ignoring for the moment "if it is so" (others have responded to that already), the answer to your question is that we don't build colliders to study the properties of positrons. We use e+e- collisions as a tool to investigate the physics of other particles that can only be created with extremely high CM energies.

Cant we do the same using some simple semiconductor devices.
No. The particle energies inside a semiconductor are not nearly high enough to meet the needs of modern experimental particle physics.
gptejms
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Oct21-04, 11:05 PM
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Quote Quote by ZapperZ
The laws of physics work the same way either forward or backwards in time (let's not get CP violation into this). So why would it matter if this is a particle going forward or backwards in time?

Take note that there's nothing to prevent me from also equating a real particle moving forward in time with an antiparticle moving backwards in time.

But this is all moot and meaningless until someone can point to me the evidence that would lead to the original question of this thread in the first place. What IS the evidence that our laws of physics do NOT work on particles and antiparticles? I have tried to described as much as I can why we KNOW they work. It seems that the assertion that they MIGHT not work are simply based on speculation and "witch-hunting" without any kind of physical justification.

Zz.
Zz,I was only trying to guess what anuj possibly meant by violation of physical laws by particles/anti-particles---onle he can clarify on that.But your statement 'how does it matter...' is a bit of an over-statement.To say that an equation has time reversal symmetry is not the same as saying that the particle actually goes back in time--so it does matter.


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