Do Particles and Antiparticles Obey All Known Physical Laws?

[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.

 

[ZapperZ]

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]

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]

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]

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]

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]

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]

Are there any good resources on this on the 'net that you know of?
Thanks

Try searching the web for baryogenesis.

 

[mathman]

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]

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]

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]

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]

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]

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]

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)

 

[ZapperZ]

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.

 

[quantumdude]

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]

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.

 

[ZapperZ]

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.

But that is my whole point! If there is a particle going BACK in time, and our laws works the SAME way with such time reversal symmetry, then what's the problem? I NEVER made the equality of

time reversal symmetry = particle going back in time.

Zz.

 

[marlon]

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)

Absolutely NOT. Mass is always positive. This is the vision of QFT : the propagator contains terms like a * exp(-iwt+ikx) for t > t' and so on. they refer to a particle with positive energy moving forward in time. The propagator also contains terms like b' * exp(iwt-ikx) for t < t'. w is frequency and k is the wave vector. a is the creation-operator and b' is the annihilation operator.

The propagator is the SUM of these two contributions because otherwise it would not follow the relativistic invariance. So a creates a particle with positive energy w and momentum k and b' annihilates a particle with negative energy -w and negative momentum -k. To see this just write the second expression as exp(-i(-w)t)...

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 !

So in the end the creation of some particle with positive energy and momentum k is exactly the same as the annihilation of some anti-particle with opposite energy and momentum...

Besides making a difference between holes and positrons is useless because it depends on how you look at these "particles". In QFT they are exactly the same because both particles are members of a "surrounding" many body problem. The holes are surrounded by a crystal-lattice and the positrons are surrounded by a cloud of virtual particles and vacuum-fluctuations. Keep in mind that the idea of concepts like polarization insertions in QFT, is "stolen" from solid state physics. I am referring to the reduction of one many body problem to many one body problems by placing the many interactions into the particle's self energy and regarding it as being situated into a surrounding vacuum in stead of being surrounded by many other particles. This is how effective mass is "born" in solid state physics...


regards
marlon

 

[gptejms]

Zz,just for the sake of argument consider this:-in an electron-positron pair,a virtual positron could actually be an electron going back in time(or the other way round).Since the e-p pair is short-lived, this interpretation would be consistent with the time-energy uncertainty relation.

 

[vanesch]

Since the e-p pair is short-lived

But positrons by themselves can live quite a long time, you know. I remember that in the HERA ring, (where protons were collided onto positrons), they kept the positrons circling for 8 hours (and then they dumped the beam).

BTW, the reason why positrons were used (while the machine was originally designed to use electrons) was quite funny. At the startup of HERA, the lifetime of the electronbeam was quite short, and they found out that some ion pumps which kept the ring under a high vacuum lost positive ions, and these were attracted to the space charge of the electron beam, thus finishing IN the beam and steadily making the electrons get lost. So they decided to switch to positrons, and the problem was solved :-)

cheers,
Patrick.

 

[ZapperZ]

Zz,just for the sake of argument consider this:-in an electron-positron pair,a virtual positron could actually be an electron going back in time(or the other way round).Since the e-p pair is short-lived, this interpretation would be consistent with the time-energy uncertainty relation.

There is something seriously wrong with this scenario. In a pair production, NONE of the particles are "virtual"! The positron created in an electron-positron pair is certain as REAL as the electron! The positron can live as long as the electron if it doesn't bump into anything else!

There's something wrong with your understanding of what an antiparticle is, and what a "virtual" particle is.

Zz.

 

[ZapperZ]

But positrons by themselves can live quite a long time, you know. I remember that in the HERA ring, (where protons were collided onto positrons), they kept the positrons circling for 8 hours (and then they dumped the beam).

BTW, the reason why positrons were used (while the machine was originally designed to use electrons) was quite funny. At the startup of HERA, the lifetime of the electronbeam was quite short, and they found out that some ion pumps which kept the ring under a high vacuum lost positive ions, and these were attracted to the space charge of the electron beam, thus finishing IN the beam and steadily making the electrons get lost. So they decided to switch to positrons, and the problem was solved :-)

cheers,
Patrick.

And that's what I've been trying to convey too when I brought up the example at the APS here a few years ago when they were exclusively using positrons in their storage ring. If the positrons were to die off THAT quickly, the users on all those beamlines would have little useful synchrotron radiation to do any work!

Unfortunately, it is obvious that somehow that example simply slid off the tip of the tongue and was completely ignored.

Zz.

 

[gptejms]

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.

 

[marlon]

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

 

[ZapperZ]

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.

 

[gptejms]

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!

 

[gptejms]

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.

 

[anuj]

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.