PDA

View Full Version : In what ways, if any, do photons differ from virtual photons?



AndrewJ
2009-Feb-16, 02:55 AM
How are photons that propagate radiation different from those that mediate interations between charged particles? If EM radiation is a perturbation in an electric field leading to a magnetic field leading to an electric field etc where do photons fit in?

I once asked my teacher how a charged particle "knew" that an another charge was within its field so that they could interact together. She said they "just did". This put me off science for my school years. I would be interested to know how virtual photons turn up to do the mediating and how they manage both attraction and repulsion.

gzhpcu
2009-Feb-16, 07:18 AM
How are photons that propagate radiation different from those that mediate interations between charged particles? If EM radiation is a perturbation in an electric field leading to a magnetic field leading to an electric field etc where do photons fit in?

Quantization: Energy, charge, spin, matter, etc. come in quantized amounts. Einstein discovered that light is quantized: the photon.
Force carriers — quantization of a force. The photons were found to propagate electromagnetism and are "real" particles.

Quantum Electro Dynamics describes charged particles (and their antiparticles) interacting with each other by the exchange of "virtual" photons. The magnitude of these interactions can be computed using perturbation theory; these rather complex formulas have a pictorial representation as Feynman diagrams.

The photons that are exchange particles are "virtual" particles. A real photon in a free state can be observed, but a virtual photon pops in and out of existence to fast for observation to be made. These photons are “virtual” because their existence violates the conservation of energy and momentum.

AndrewJ
2009-Feb-16, 07:12 PM
Quantum Electro Dynamics describes charged particles (and their antiparticles) interacting with each other by the exchange of "virtual" photons. The magnitude of these interactions can be computed using perturbation theory; these rather complex formulas have a pictorial representation as Feynman diagrams.

The photons that are exchange particles are "virtual" particles. A real photon in a free state can be observed, but a virtual photon pops in and out of existence to fast for observation to be made. These photons are “virtual” because their existence violates the conservation of energy and momentum.

So a photon is the massless component that defines the amount of energy being radiated or exchanged? Setting aside the wave/particle duality issue I still struggle to envisage where the photon exists in the model of EM radiation as alternating perturbed electric and magnetic fields.

Regarding virtual photons: suppose electron A strays close to electron B. How do the electrons "know" of the existance of the other such that they can exchange virtual photons and be repulsed? Does each electron carry a net-like field of virtual photons around with it? If the virtual photons arise spontaneously what prompts them to do so? If we replave elctron B with a proton how would the same virtual photons mediate an attractive force?

gzhpcu
2009-Feb-16, 09:08 PM
A charged particle is surrounded by an electromagnetic field in classical mechanics. The electromagnetic field exerts a force on all charged particles. The field extends to all of space. The field is either positively or negatively charged.

Quantum field theory "quantizes" the field. The electromagnetic force is seen as an exchange of virtual photons. Like charges repel and unlike charges attract because the field equations of Quantum Electrodynamics (The QFT theory for electromagnetism) say they do. As far as we can tell, there's no 'why' it has to be like that. It just turns out that if we use QED to predict how a system will behave, it mirrors what we measure to an extraordinary degree of accuracy.

Grey
2009-Feb-16, 09:55 PM
Regarding virtual photons: suppose electron A strays close to electron B. How do the electrons "know" of the existance of the other such that they can exchange virtual photons and be repulsed?They don't. But in a virtual particle model, any charged particle is constantly "creating" virtual particles, which can interact with anything nearby. Think of shining a light on a mirror that reflects it back. You don't need to know that the mirror is there for that to work, you just send out a beam of light, and if there happens to be something capable of reflecting it, it will.


Does each electron carry a net-like field of virtual photons around with it?More or less, yes.


If the virtual photons arise spontaneously what prompts them to do so?We have no idea. But in some sense, this isn't any different from anything else in science. Physics is really good at describing how something happens, but it's not so good at answering why something happens the way it does. You could say that charge is just the ability to produce virtual photons, but that really just pushes the question back a step, and you're essentially asking why the universe should be filled with particles that possess this attribute called charge, that interact in this certain way, and we really just don't have a way to answer that question.

Describing the electromagnetic force as an exchange of ghostly virtual particles that can't be directly observed seems really weird. The reason that we do consider it a good model is that the predictions you can make from it match extraordinarily well with the results you get when you do experiments. The predictions of this model are accurate to about 1 part in 100 million.


If we replave elctron B with a proton how would the same virtual photons mediate an attractive force?One of the weird properties of virtual particles (as opposed to real ones) is that they need not always have positive energy and momentum. I've often heard the exchange of virtual particles visualized as two people on skateboards throwing a basketball back and forth. That works really well for a repulsive force. Well, since a virtual particle can have negative momentum, you can imagine that each time you catch a basketball, it pulls you forward rather than pushing it away. That can't happen with a real particle, but it works fine with a virtual particle (you have to work out the math in a little more detail to see why it works out that like charges are repelled and opposite charges are attracted, but it really does work out).

So, are virtual particles "real" or are they a convenient fiction that works well to describe things to 8 decimal places, but don't "really" exist. It's probably not as easy to draw a distinction between those two cases as you might think. :)

AndrewJ
2009-Feb-16, 11:24 PM
A charged particle is surrounded by an electromagnetic field in classical mechanics. The electromagnetic field exerts a force on all charged particles. The field extends to all of space. The field is either positively or negatively charged.


Does each electron carry a net-like field of virtual photons around with it?

More or less, yes.

Ah, so the electric and magnetic fields are rooted in/emanate from their source particle the charge of which determines how their field interacts with other charged particles. I appreciate that classical physics cannot answer "what is an electric field made of" but I feel the scales slipping from my eyes somewhat.

I don't want to push my luck by asking too many questions but one that occurs would be: if charge is the ability to produce virtual photons (which must have energy even if they don't have mass) does their creation "bleed" the source particle of its charge or its energy in general? If, as I assume, not is this becuase virtual photons are, after all, not "real"?


One of the weird properties of virtual particles (as opposed to real ones) is that they need not always have positive energy and momentum. I've often heard the exchange of virtual particles visualized as two people on skateboards throwing a basketball back and forth. That works really well for a repulsive force. Well, since a virtual particle can have negative momentum, you can imagine that each time you catch a basketball, it pulls you forward rather than pushing it away.

I've read that the virtual photon's capacity to both repell and attract was due to its ability, in some sense, to go both forward and backward in time. Any validity in this?

Smoke Ring
2009-Feb-16, 11:35 PM
Andrew did you like the smoke and mirrors explanations?

Jeff Root
2009-Feb-16, 11:37 PM
One of the weird properties of virtual particles (as opposed to real ones)
is that they need not always have positive energy and momentum. I've
often heard the exchange of virtual particles visualized as two people on
skateboards throwing a basketball back and forth. That works really
well for a repulsive force. Well, since a virtual particle can have negative
momentum, you can imagine that each time you catch a basketball, it
pulls you forward rather than pushing it away. That can't happen with a
real particle, but it works fine with a virtual particle
So, what is the difference between a real (physical) photon and a
virtual photon? The original question of the thread. What about a
virtual photon allows it to either push or pull on a charged particle,
while physical photons can only push? And how is a virtual photon
emitted by a negatively-charged particle different from a virtual
photon emitted by a positively-charged particle? And is a physical
photon emitted by a negatively-charged particle also different from
a photon emitted by a positively-charged particle?

-- Jeff, in Minneapolis

gzhpcu
2009-Feb-17, 05:43 AM
So, what is the difference between a real (physical) photon and a
virtual photon? The original question of the thread. What about a
virtual photon allows it to either push or pull on a charged particle,
while physical photons can only push?
The crucial point is that the electromagnetic force is expressed as an exchange of virtual particles, not real ones. And virtual particles can have any momentum — even negative, as Grey mentioned. So by throwing a negative-momentum particle from one body to another, you actually draw them together, A negative-momentum particle travels backwards in time.

Jeff Root
2009-Feb-17, 07:42 AM
So, what is the difference between a real (physical) photon and a
virtual photon? The original question of the thread. What about a
virtual photon allows it to either push or pull on a charged particle,
while physical photons can only push?
The crucial point is that the electromagnetic force is expressed as
an exchange of virtual particles, not real ones.
Okay. So, what is the difference between a real (physical) photon
and a virtual photon?

-- Jeff, in Minneapolis

gzhpcu
2009-Feb-17, 07:48 AM
Okay. So, what is the difference between a real (physical) photon
and a virtual photon?

-- Jeff, in Minneapolis

A real photon in a free state can be observed, but a virtual photon pops in and out of existence to fast for observation to be made.

Jeff Root
2009-Feb-17, 08:56 AM
A real photon in a free state can be observed, but a virtual photon
pops in and out of existence to fast for observation to be made.
Okay, so, what is the difference between a real (physical) photon
that can hang around to be observed, and a virtual photon which
pops in and out of existence too fast for observation? Why don't
they both pop in and out of existence too fast to be observed?
Why don't they both hang around long enough to be observed?
What is the difference between them that causes them to have
different behaviors instead of identical behaviors?

-- Jeff, in Minneapolis

BigDon
2009-Feb-17, 03:00 PM
Andrew did you like the smoke and mirrors explanations?

Hey, Smokey, since you showed up here you've been nothing but a fight looking for somewhere to happen. I think your homeschooled, country boy, self would probably be happier over at this website.

http://www.godlikeproductions.com/forum1/02/17/09/pg1

Your welcome.

gzhpcu
2009-Feb-17, 03:24 PM
Okay, so, what is the difference between a real (physical) photon
that can hang around to be observed, and a virtual photon which
pops in and out of existence too fast for observation? Why don't
they both pop in and out of existence too fast to be observed?
Why don't they both hang around long enough to be observed?
What is the difference between them that causes them to have
different behaviors instead of identical behaviors?

-- Jeff, in Minneapolis
A virtual particle exists for a limited time and space, as allowed by the Heisenberg Uncertainty Principle. It "borrows" energy and gives it back before it is detected. A real photon is a quanta of light.

In the case of electromagnetism, the force, the particles affected exchange "virtual photons" which carry the transferred momentum, as is shown in Feynman diagrams.



The most obvious problem with a simple, classical picture of virtual particles is that this sort of behavior can't possibly result in attractive forces. If I throw a ball at you, the recoil pushes me back; when you catch the ball, you are pushed away from me. How can this attract us to each other? The answer lies in Heisenberg's uncertainty principle.

Suppose that we are trying to calculate the probability (or, actually, the probability amplitude) that some amount of momentum, p, gets transferred between a couple of particles that are fairly well- localized. The uncertainty principle says that definite momentum is associated with a huge uncertainty in position. A virtual particle with momentum p corresponds to a plane wave filling all of space, with no definite position at all. It doesn't matter which way the momentum points; that just determines how the wavefronts are oriented. Since the wave is everywhere, the photon can be created by one particle and absorbed by the other, no matter where they are. If the momentum transferred by the wave points in the direction from the receiving particle to the emitting one, the effect is that of an attractive force.

The moral is that the lines in a Feynman diagram are not to be interpreted literally as the paths of classical particles. Usually, in fact, this interpretation applies to an even lesser extent than in my example, since in most Feynman diagrams the incoming and outgoing particles are not very well localized; they're supposed to be plane waves too.

source:http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html

Smoke Ring
2009-Feb-17, 03:46 PM
BigDon, your right and I apologize. I grew up while the standard model got most of it's legs and I bought into it. It just seems to me that SM has had leaks through out it's development and someone would design a peg to stop the leak. Feynman once expressed that exact sentiment. Einstein didn't believe it. But most of the worlds brilliant people do buy into it. So you are right who am I to question these people. I think the BB was radiation but that answer fries the standard model. The SM's answer looks like smoke mirrors so if my opinion is a sin I'm sorry.

Grey
2009-Feb-17, 04:18 PM
I don't want to push my luck by asking too many questions but one that occurs would be: if charge is the ability to produce virtual photons (which must have energy even if they don't have mass) does their creation "bleed" the source particle of its charge or its energy in general? If, as I assume, not is this becuase virtual photons are, after all, not "real"?Feel free to ask all the questions you'd like. Someone will do their best to answer them. That's what the board is for!

The answer is that virtual photons do not bleed the source of it's energy. One way to think about this is through the Heisenberg uncertainty relation. In addition to the better known version about knowing position and momentum, there's a similar relationship between time and energy. Essentially, the limit on how precisely you can measure the energy of a system depends on how short a time scale you're looking at. Or, in other words, a particle with a certain energy can appear out of nowhere, as long as it disappears within a certain (very short) time period. The higher the energy, the shorter its lifetime. So you can think of virtual particles as "borrowing" this energy from the vacuum, so long as they "pay it back". That's one way of looking at the fact that the electromagnetic force gets smaller over distance. A high energy virtual photon can only last a short period of time and cover a short distance. A lower energy virtual particle can "exist" for a longer period of time.


I've read that the virtual photon's capacity to both repell and attract was due to its ability, in some sense, to go both forward and backward in time. Any validity in this?It's one way to think about it. If you're drawing a Feynman diagram of a virtual particle interaction, there is a time direction, but there's no real distinction between a given line representing a particle going forward in time or its opposite particle (and photons are their own opposite) going backward in time.

BigDon
2009-Feb-17, 04:19 PM
Okay, I apologize for being snappish myself.

But that thing you have to accept is that human intuition breaks down at both the sub-atomic and the extra-galactic scale.

Mainly because we are so adapted to living in the middle ground.

Grey
2009-Feb-17, 04:57 PM
Andrew did you like the smoke and mirrors explanations?Virtual particles are a smoke and mirrors explanation. :) The only reason we take the idea seriously at all is because it does such a phenomenally good job of predicting the results of experiments. It's one of the hallmarks of a scientific approach. It doesn't really matter to a scientist how weird an idea may sound, if the predictions from a theory match observation better than any other theory that anyone has come up with, we use that theory until something better comes along.

For example, we can show that if virtual particles "really" exist, then atomic energy levels would be slightly shifted from what they would otherwise be. Similarly, there would be a slight difference in the expected magnetic moment of the electron. When we look, we actually see exactly these differences, to a really remarkable level of precision. If you use how closely the experimental results match the theoretical model as your standard for how good a theory is, there is no theory that has ever been better than quantum electrodynamics.

chornedsnorkack
2009-Feb-17, 05:08 PM
Maybe a better idea about how virtual particles can attract is to think of nonquantized, Maxwell electromagnetic field.

A simple, Coulomb electrostatic field has no energy, no momentum. It can attract or it can repel, simply depending on the charge testing the electrostatic field.

If you look at magnetostatic fields, still no momentum, and the force can be in any direction.

However, consider a propagating Maxwell electromagnetic wave. It does carry energy, and it turns out to carry momentum as well. Which travels in the direction of the wave. This means that a Maxwell electrostatic wave can only repel a charge, no matter what its sign is.

Real particles come from quantization of propagating electromagnetic field waves. Virtual particles come from quantization of nonpropagating, massless electrostatic and magnetostatic fields.

AndrewJ
2009-Feb-17, 05:50 PM
Thanks for the time and effort you all put into answering my questions.

I suspect that explanations that invoke the Uncertainty Principle are probably a step beyond those which can be illustrated by analogy (i.e. eventually you have to work through the mathematics).

gzhpcu
2009-Feb-17, 08:37 PM
BigDon, your right and I apologize. I grew up while the standard model got most of it's legs and I bought into it. It just seems to me that SM has had leaks through out it's development and someone would design a peg to stop the leak. Feynman once expressed that exact sentiment. Einstein didn't believe it. But most of the worlds brilliant people do buy into it. So you are right who am I to question these people. I think the BB was radiation but that answer fries the standard model. The SM's answer looks like smoke mirrors so if my opinion is a sin I'm sorry.
Yet the Standard Model works pretty well, doesn't it? The important thing is that it makes very accurate predictions, forgetting what seems "logical".

In what area of testability do you find SM deficient?

What credentials do you have to question SM and the BB?

Spaceman Spiff
2009-Feb-18, 03:29 PM
BigDon, your right and I apologize. I grew up while the standard model got most of it's legs and I bought into it. It just seems to me that SM has had leaks through out it's development and someone would design a peg to stop the leak. Feynman once expressed that exact sentiment. Einstein didn't believe it. But most of the worlds brilliant people do buy into it. So you are right who am I to question these people. I think the BB was radiation but that answer fries the standard model. The SM's answer looks like smoke mirrors so if my opinion is a sin I'm sorry.

1. Science does not rely on the *opinions* of anyone, even those of some of its greatest practitioners. Data and a useful model to explain them, unifying as many data with the fewest number of assumptions, and the ability to predict new phenomena -- these are all that matter (in a nutshell).

Despite all of the successful predictions of the standard model of particles (SM) that have been borne out of the experiments (Grey's post above (http://www.bautforum.com/space-astronomy-questions-answers/84800-what-ways-if-any-do-photons-differ-virtual-photons.html#post1436594) lays out two of the most spectacular of them), the SM is known to be incomplete. Experimentalists at Fermilab and the LHC are in the business of trying to tease out an even more general explanation for the properties of matter and its behavior in different energy regimes.

2. re. what "you think" about the big bang being radiation. Out of curiosity: what does that mean, precisely, and how does it conflict with the standard BB model? Or are you referring to the standard model of particle physics? (Your explanations should be succinct or the moderator will tell you to post your non-standard models in the ATM.)

Jeff Root
2009-Feb-18, 04:57 PM
Okay, so, what is the difference between a real (physical) photon
that can hang around to be observed, and a virtual photon which
pops in and out of existence too fast for observation? Why don't
they both pop in and out of existence too fast to be observed?
Why don't they both hang around long enough to be observed?
What is the difference between them that causes them to have
different behaviors instead of identical behaviors?
A virtual particle exists for a limited time and space, as allowed by
the Heisenberg Uncertainty Principle. It "borrows" energy and gives
it back before it is detected. A real photon is a quanta of light.
This distinguishes the particles' origins (in a vague way), but
doesn't explain what difference there is between them that makes
them behave so differently. Just because a virtual particle acquires
its energy in a manner and from a source different from that of a
"real" (physical) photon doesn't necessarily mean that there will be
any difference between the resulting photons. What makes photons
created in one way have one set of behaviors and photons created
in a different way have a different set of behaviors?

-- Jeff, in Minneapolis

Jeff Root
2009-Feb-18, 05:21 PM
Real particles come from quantization of propagating electromagnetic
field waves. Virtual particles come from quantization of nonpropagating,
massless electrostatic and magnetostatic fields.
What do you mean by "nonpropagating"? If a balloon carrying an
electrostatic charge comes near me, I can see and feel the pull on
the hairs of my arm. Aren't virtual photons propagating between
the balloon and my arm?

-- Jeff, in Minneapolis

gzhpcu
2009-Feb-18, 06:38 PM
This distinguishes the particles' origins (in a vague way), but
doesn't explain what difference there is between them that makes
them behave so differently. Just because a virtual particle acquires
its energy in a manner and from a source different from that of a
"real" (physical) photon doesn't necessarily mean that there will be
any difference between the resulting photons. What makes photons
created in one way have one set of behaviors and photons created
in a different way have a different set of behaviors?

-- Jeff, in Minneapolis

Quantum field theory describes interactions (essentially forces) between real particles in terms of exchanges of virtual particles. Virtual particles are viewed as the quanta that describe fields of the basic force interactions, which cannot be described in terms of real particles, since quantum mechanics describes everything as particles.

Virtual particles are an artefact of perturbation theory, and do not appear in a nonperturbative treatment. As such, their objective existence as "particles" is questionable;

trinitree88
2009-Feb-18, 06:50 PM
The effect is much like Cinderella. A real date, with a real girl can last for a while, and might be repeated on another evening because she isn't under the constraints of transforming at midnight, along with her coach, driver and horses. But Cinderella does live under the Cinderella Uncertainty Principle, and must be back by midnight or she disappears....just like the virtual particle. Now which would you rather date?
psst, has anyone seen Cinder-Ali?:shifty::lol:

pete