Thread: Interaction between real particles and virtual particles

Hello everyone,

I was wondering, how do real particles interact with virtual particles created by the vacuum.

For instance, wouldn't a electron travelling through space regularily interact and annihilate with a virtual positron from a pair of virtual positron-electron created by the vacuum? If that were to happen I guess the remaining virtual electron would become real and would sort of replace the original electron... I don't know.

How can a particle exist for billions of years without annihilating with a virtual particle?

Nick.

Virtual particles don't last long enough to interact.

Originally Posted by Nic321

Hello everyone,

I was wondering, how do real particles interact with virtual particles created by the vacuum.

For instance, wouldn't a electron travelling through space regularily interact and annihilate with a virtual positron from a pair of virtual positron-electron created by the vacuum? If that were to happen I guess the remaining virtual electron would become real and would sort of replace the original electron... I don't know.

How can a particle exist for billions of years without annihilating with a virtual particle?

Nick.

Hello this kind of things are indeed happening and even measurable. When free electron is traveling through space a number ofprocesses can happen. Of course electron may propagate throu vacuum without any interaction and this is the most preferred process. However it can also emit virtual photon and reabsorb it. This is called electron self-energy (self-interaction). Amplitude for this process is lower than for free propagating electron but it has to be acconted for. Even higher order interactions can happen eg. emiting virtual photon which creates virtual e-p pair which anihilates creating virtual photon again which is reabsorbed by real electron. These are higher order interactions (4. in electric charge) so they are considerably dampened but when you account for all these interactions and calculate full electron propagator you will find out that these selfinteractions through vacuum have effect on electrons mass. Therefore you need to distinguish between physical mass (mass of electron that we measure 511keV) and so called bare mass which would be the mass of electron without any vacuum interactions (something we know nothing about). If you are interested you may look at this Mod-05 Lec-36 Electron Selfenergy Video Lecture, IIT Madras . You may ignore the math just focus on basic principles of vacuum interactions.

Originally Posted by Nic321

Hello everyone,

I was wondering, how do real particles interact with virtual particles created by the vacuum.

For instance, wouldn't a electron travelling through space regularily interact and annihilate with a virtual positron from a pair of virtual positron-electron created by the vacuum? If that were to happen I guess the remaining virtual electron would become real and would sort of replace the original electron... I don't know.

How can a particle exist for billions of years without annihilating with a virtual particle?

Nick.

Hi Nick

It is probably best to view the situation quantum mechanically rather than classically as virtual particles and their interactions with particles are a QM phenomenon. In QM, the only interpretation that treats particles as 'real' in any classical sense is the de Broglie-Bohm interpretation where continuous particle trajectories are guided by a pilot wave. Most other interpretations do not consider a particle to be a well defined 'thing' which classical mechanics describes. You may be familiar with the notion that fundamental particles in Quantum Mechanics can be described either as a wave or as a particle (wave-particle duality). This is attributed to the QM description that particles travel as waves and interact as particles.

Particles are better defined as excitations of quantum fields or waves of probability. What is represented classically as particles are defined in QM by a wavefunction which provides a probability distribution of the likelihood of finding a 'classical' particle in a given time and place. We are actually dealing with probability superpositions of quantum states as opposed to durable 'things' described as billiard balls. Have a look here for a better description.

In regards to virtual particles, these essentially are transient field fluctuations which classically might be described as particles that can only exist for a instant of time allowed by the Heisenberg Uncertainty Principle. More about virtual particles here. :-))

Originally Posted by Gere

Originally Posted by Nic321

Hello everyone,

I was wondering, how do real particles interact with virtual particles created by the vacuum.

For instance, wouldn't a electron travelling through space regularily interact and annihilate with a virtual positron from a pair of virtual positron-electron created by the vacuum? If that were to happen I guess the remaining virtual electron would become real and would sort of replace the original electron... I don't know.

How can a particle exist for billions of years without annihilating with a virtual particle?

Nick.

Hello this kind of things are indeed happening and even measurable. When free electron is traveling through space a number ofprocesses can happen. Of course electron may propagate throu vacuum without any interaction and this is the most preferred process. However it can also emit virtual photon and reabsorb it. This is called electron self-energy (self-interaction). Amplitude for this process is lower than for free propagating electron but it has to be acconted for. Even higher order interactions can happen eg. emiting virtual photon which creates virtual e-p pair which anihilates creating virtual photon again which is reabsorbed by real electron. These are higher order interactions (4. in electric charge) so they are considerably dampened but when you account for all these interactions and calculate full electron propagator you will find out that these selfinteractions through vacuum have effect on electrons mass. Therefore you need to distinguish between physical mass (mass of electron that we measure 511keV) and so called bare mass which would be the mass of electron without any vacuum interactions (something we know nothing about). If you are interested you may look at this Mod-05 Lec-36 Electron Selfenergy Video Lecture, IIT Madras . You may ignore the math just focus on basic principles of vacuum interactions.

Hi Gere, I watched the video but it's really complicated. I don't even know what the professor is trying to calculate.

In any case, I am not sure about the physical process of this electron self-energy. The energy of the photon has to come from somewhere. Do you mean that part of the kinetic energy of the electron is used to create a photon, which increases the mass of the electron and slows it down. And when the electron reabsorbs the photon, its mass decreases again and it accelerate ?

Wow, I didn't know that a virtual particle could split in several virtual particles.

Originally Posted by Implicate Order

Hi Nick

It is probably best to view the situation quantum mechanically rather than classically as virtual particles and their interactions with particles are a QM phenomenon. In QM, the only interpretation that treats particles as 'real' in any classical sense is the de Broglie-Bohm interpretation where continuous particle trajectories are guided by a pilot wave. Most other interpretations do not consider a particle to be a well defined 'thing' which classical mechanics describes. You may be familiar with the notion that fundamental particles in Quantum Mechanics can be described either as a wave or as a particle (wave-particle duality). This is attributed to the QM description that particles travel as waves and interact as particles.

Particles are better defined as excitations of quantum fields or waves of probability. What is represented classically as particles are defined in QM by a wavefunction which provides a probability distribution of the likelihood of finding a 'classical' particle in a given time and place. We are actually dealing with probability superpositions of quantum states as opposed to durable 'things' described as billiard balls. Have a look here for a better description.

In regards to virtual particles, these essentially are transient field fluctuations which classically might be described as particles that can only exist for a instant of time allowed by the Heisenberg Uncertainty Principle. More about virtual particles here. :-))

Hi Implicate order,

I am sorry, I read your post several times but I still don't get where you are getting at. I understand what you say, but how does it explain for instance that an electron will not annihilate with a virtual positron? When a pair of "virtual" e-p is created by the vacuum, they annihilate with each other, and when a "real" electron collides with a "real" positron, they annihilate too.

The fact that they are considered "real" or "virtual" depending on the interpretation we use doesn't change the fact that they do annihilate.

Originally Posted by AlexG

Virtual particles don't last long enough to interact.

Hi AlexG. Well, that could be the answer, but how many particles does the vacuum create, wouldn't it happen sooner or later?

You must realize that virtual particles are no particles at all. Transition amplitude between two states of field (eg. electron in point A and electron in point B) is given by something called S matrix. Using boundary conditions and S matrix you may calculate this amplitude and its square will give you probability of such transition. In most field theories (electrodynamics) this S matrix problem is solved using perturbation theory which takes into account only processes with highest amplitude (hence s-matrix expansion of x-order). Using something called Wick expansion you can rewrite S matrix in given order into set of terms which all represent some physical process (Compton scattering, Moller scattering, electron selfenergy, vacuum polarization...). In these terms there will appear functions called propagators. Propagator itself is a function which gives you probability of propagating particle from point A to point B. Since these same functions appear in expansion of S matrix a guy named Feynman introduced pictographic representation of terms which arise in S matrix expansion. Therefore he began calling them virtual particles. Therefore what we call virtual particles are in fact just mathematical functions in S matrix expansion which have nothing to do with reality.

Originally Posted by Nic321

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Hi Implicate order,

I am sorry, I read your post several times but I still don't get where you are getting at. I understand what you say, but how does it explain for instance that an electron will not annihilate with a virtual positron? When a pair of "virtual" e-p is created by the vacuum, they annihilate with each other, and when a "real" electron collides with a "real" positron, they annihilate too.

The fact that they are considered "real" or "virtual" depending on the interpretation we use doesn't change the fact that they do annihilate.

Hi Nic

Sorry about any confusion. In your original post you were referring to the notion of "a particle existing for billions of years" and I was just wanting to emphasise that in QM, particles are not 'eternal'. Particles appear and disappear in every process and other particles appear in their place. The laws of motion governing particle interactions DO NOT PRESERVE the particle's identities because these laws are expressed in terms of fields. All conserved quantities of 'fundamental partciles' correspond to field symmetries. Hence one may say that the real fundamental entities are not the particles themselves but the field properties which are transferred from one place to another in the only way that is compatible with the Poincare symmetry of spacetime. It is more accurate to view this not as travelling particles but travelling 'bundles of field qualities'. As Gere has stressed things are a bit more complex in the world of QM than just isolated particles travelling in a void. :-))

Originally Posted by Implicate Order

Hi Nic

Sorry about any confusion. In your original post you were referring to the notion of "a particle existing for billions of years" and I was just wanting to emphasise that in QM, particles are not 'eternal'. Particles appear and disappear in every process and other particles appear in their place. The laws of motion governing particle interactions DO NOT PRESERVE the particle's identities because these laws are expressed in terms of fields. All conserved quantities of 'fundamental partciles' correspond to field symmetries. Hence one may say that the real fundamental entities are not the particles themselves but the field properties which are transferred from one place to another in the only way that is compatible with the Poincare symmetry of spacetime. It is more accurate to view this not as travelling particles but travelling 'bundles of field qualities'. As Gere has stressed things are a bit more complex in the world of QM than just isolated particles travelling in a void. :-))

Well I understand what you explain, but isn't it true that apart from this, the vacuum creates pairs of virtual particles ( or transient fluctuations of the field if you prefer ).

I am a bit confused because I have read that the vacuum creates "virtual particles" thanks to the uncertainty principle if the fluctuation has enough energy to create them. For instance to create an e-p pair the fluctuation needs to have at least 2xme.c^2 of energy, and the surplus will go to the kinetic energy. Is that correct?

How likely are those transient fluctuations to happen? Can we quantify their frequency?

Originally Posted by Gere

You must realize that virtual particles are no particles at all. Transition amplitude between two states of field (eg. electron in point A and electron in point B) is given by something called S matrix. Using boundary conditions and S matrix you may calculate this amplitude and its square will give you probability of such transition. In most field theories (electrodynamics) this S matrix problem is solved using perturbation theory which takes into account only processes with highest amplitude (hence s-matrix expansion of x-order). Using something called Wick expansion you can rewrite S matrix in given order into set of terms which all represent some physical process (Compton scattering, Moller scattering, electron selfenergy, vacuum polarization...). In these terms there will appear functions called propagators. Propagator itself is a function which gives you probability of propagating particle from point A to point B. Since these same functions appear in expansion of S matrix a guy named Feynman introduced pictographic representation of terms which arise in S matrix expansion. Therefore he began calling them virtual particles. Therefore what we call virtual particles are in fact just mathematical functions in S matrix expansion which have nothing to do with reality.

Thanks Gere, I understand a bit what you say, it is quite complicated. I understand that they are not really particles.

I found this article:

quantum field theory - Are W & Z bosons virtual or not? - Physics Stack Exchange

There are a lot of comments from physicists who don't seem to all agree.

I am trying to make some sense of this, not easy...

Originally Posted by Nic321

Well I understand what you explain, but isn't it true that apart from this, the vacuum creates pairs of virtual particles ( or transient fluctuations of the field if you prefer ).

I am a bit confused because I have read that the vacuum creates "virtual particles" thanks to the uncertainty principle if the fluctuation has enough energy to create them. For instance to create an e-p pair the fluctuation needs to have at least 2xme.c^2 of energy, and the surplus will go to the kinetic energy. Is that correct?

How likely are those transient fluctuations to happen? Can we quantify their frequency?

Yes Nic, the vacuum does create virtual particles all the time. You cannot really talk in terms of the frequency of transient fluctuations as they are ubiquitous and you need to consider all combinations and types of interaction.

Consider the mutual repulsion of two electrons. In its most basic form you will have read about, this interaction is represented by the exchange of a single virtual photon. The reality however is that this interaction proceeds by the exchange of any number of virtual photons and the probabilities of these alternatives needs to be considered. Furthermore you must also acount for the probability that the virtual photon will turn into an electron-positron pair only to revert again back into a photon.

When the virtual photon fluctuates into an electron-positron pair, its energy must be shared, part going into the electron and part to the positron, but there are actually an infinite number of different ways the energy can divide itself between the positron and the electron. The energy of either the eectron or positron can be pretty much anything as long as the other has the correct amount of energy to compensate and make the total add up to the virtual photon's energy. It is only through renormalisation that you can make sense of all this, otherwise you end up with infinities. Through renormalisation what you actually do is dress up a 'bare electron' with its associated virtual cloud (fleeting vacuum fluctuations) represented by other virtual photons and the virtual electron-positron pair.

Anyway, I will leave it to those with better knowledge. :-))

Originally Posted by Implicate Order

Originally Posted by Nic321

Well I understand what you explain, but isn't it true that apart from this, the vacuum creates pairs of virtual particles ( or transient fluctuations of the field if you prefer ).

I am a bit confused because I have read that the vacuum creates "virtual particles" thanks to the uncertainty principle if the fluctuation has enough energy to create them. For instance to create an e-p pair the fluctuation needs to have at least 2xme.c^2 of energy, and the surplus will go to the kinetic energy. Is that correct?

How likely are those transient fluctuations to happen? Can we quantify their frequency?

Yes Nic, the vacuum does create virtual particles all the time. You cannot really talk in terms of the frequency of transient fluctuations as they are ubiquitous and you need to consider all combinations and types of interaction.

Consider the mutual repulsion of two electrons. In its most basic form you will have read about, this interaction is represented by the exchange of a single virtual photon. The reality however is that this interaction proceeds by the exchange of any number of virtual photons and the probabilities of these alternatives needs to be considered. Furthermore you must also acount for the probability that the virtual photon will turn into an electron-positron pair only to revert again back into a photon.

When the virtual photon fluctuates into an electron-positron pair, its energy must be shared, part going into the electron and part to the positron, but there are actually an infinite number of different ways the energy can divide itself between the positron and the electron. The energy of either the eectron or positron can be pretty much anything as long as the other has the correct amount of energy to compensate and make the total add up to the virtual photon's energy. It is only through renormalisation that you can make sense of all this, otherwise you end up with infinities. Through renormalisation what you actually do is dress up a 'bare electron' with its associated virtual cloud (fleeting vacuum fluctuations) represented by other virtual photons and the virtual electron-positron pair.

Anyway, I will leave it to those with better knowledge. :-))

Thanks for the explanation Implicate order, I understand a bit better now.

Nick.

Originally Posted by Nic321

Hello everyone,

I was wondering, how do real particles interact with virtual particles created by the vacuum.

For instance, wouldn't a electron travelling through space regularily interact and annihilate with a virtual positron from a pair of virtual positron-electron created by the vacuum? If that were to happen I guess the remaining virtual electron would become real and would sort of replace the original electron... I don't know.

How can a particle exist for billions of years without annihilating with a virtual particle?

Nick.

Please bear with me, because I will be writing from what I think is intelligent thinking grounded on logic and facts.

Just tell me to stop already if anyone scientist or anyone at all gets so annoyed, that he or they take action and succeed in getting me banned forever as a [sic] religious crackpot; that happened to me on April 17, 2014 in the http://www.physicsforums.com.*


First, I will try to get my attention on something that is concurred on by particle physicists to be really existing and operating in objective reality, let us take the photon because I understand that the photon is the sub-atomic particle that is operating in the laser printer or laser copying machine.

So, let me just substitute the following strike-out words with the word photon.

Originally Posted by Nic321

I was wondering, how do real particles photons interact with virtual particles photons created by the vacuum.

For instance, wouldn't a electron photon travelling through space regularily interact and annihilate with a virtual positron photon from a pair of virtual positron-electron photon_p-photon_e created by the vacuum? If that were to happen I guess the remaining virtual electron photon would become real and would sort of replace the original electron photon... I don't know.

How can a particle photon exist for billions of years without annihilating with a virtual particle photon?

What I am trying to do is to transit from the realm of concepts in our mind to the realm of objective things in physical reality, where events take place and affect us and also we can in some circumstances control them.


Let me read your reactions to my thinking.

And I will try to learn how to think and write as not to get banned forever here.



Pacho


*My name there is yrreg.