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pooria
2007-Sep-20, 08:20 AM
Hi ;

In the Quantum Theory it is believed that the electromagnetic force is carried by particles called photons.
We know that there is no diffrence in the nature of photons but we see that particles of like charge repel each other and particles of unlike charge attract each other. How is it possible to have two kinds of electromagnetic force by the same photons ?

Ken G
2007-Sep-20, 09:50 AM
This is a very good question indeed, and comes under the heading of "quantum electrodynamics". I don't actually do QED, so to answer questions as tough as yours I have to look to those who do, like John Baez. Here's his answer:
http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html
You may find it hard to follow, but this is the upshot as I see it. You are really asking two questions:
1) how can exchange of virtual particles ever give you attraction?
2) how can that exchange know about the sign of the charges, to be able to tell when to give attraction?

1) The answer to the first question is easier: virtual particles cling to existence on the coattails of the uncertainty principle, meaning that their energy can only be said to "exist" for a short enough time that it is not certainly distinguishable from zero. That's also why you cannot measure a virtual particle: a measurement is what is in place after a long time, and after a long time virtual particles must have zero energy-- no virtual particle at all. A ramification of existing due to the uncertainty principle means that you don't really know where a virtual particle is or which direction its going, so even once you specify that it is emitted from particle A and absorbed by particle B, there is still no requirement that it "moves along the direction from A to B". Hence, it doesn't matter on which side of B you find A, A can still emit a virtual particle that impinges on B from the opposite direction! And that's what you need for attraction.

2) This is a lot trickier-- why do unlike charges attract and like charges repel? According to (1), this requires that if the charges are alike, the virtual particle B gets will come from the side of A, and if they are unlike, it will come from the opposite side of A. How do we "tell" the virtual particle about the charges? The way we do that has to do with the sign of the probability amplitudes.

As soon as someone uses the word "amplitude" it means they are really talking about interference, which means they are adding up all the different ways that something could happen, in the form of an amplitude, then squaring the result to get a probability. Here the "something that could happen" is either there is no virtual photon interchanged, or there is one virtual photon interchanged (or more, but this is the simplest version). If we tell the virtual particles the sign of the charges by giving the virtual particle wave function a sign to match, roughly speaking, we haven't done anything too violent because a wave function is also an amplitude and so it's square is what is "real", and we can give the amplitude any sign that works. But before you square amplitudes, you have to add them over all the things that can happen, and that's where the sign matters-- that's where interference shows its face.

The upshot of a pretty involved calculation is then, if the charges are alike, then the wave function for one virtual particle to be exchanged interferes with the wave function for no virtual particle to be exchanged, in such a way that having particle B stay where it was gets destructively interfered, having it move toward A gets destructively interfered, but having it move away from A gets constructively interfered. The last two are interchanged if the charges are unlike.

So putting (1) and (2) together to answer your question, one must remember that the bottom line in wave mechanics is that what happens is what doesn't get destructively interfered. The virtual particle from A has an uncertainty in its location and its direction of motion, so if the charges are unlike, what doesn't get destructively interfered is that the virtual particle from A comes from the opposite side of B and pushes it toward A when its momentum is absorbed after a long enough time that the virtual particle can't exist any more (but its momentum leaves its trace because A is missing that momentum and so momentum is conserved). If the charges are alike, then what doesn't get interfered is that the virtual particle from A came from the side of A and pushed B away.

mugaliens
2007-Sep-21, 01:13 PM
Hi ;

In the Quantum Theory it is believed that the electromagnetic force is carried by particles called photons.
We know that there is no diffrence in the nature of photons but we see that particles of like charge repel each other and particles of unlike charge attract each other. How is it possible to have two kinds of electromagnetic force by the same photons ?

Photons (http://en.wikipedia.org/wiki/Photons)are the only particles which are identical in their matter and antimatter states. In other words, there are no matter/antimatter photons.

I find that very interesting, particularly with respect to antimatter (http://en.wikipedia.org/wiki/Antimatter).

But I was more intrigued by this quote from Wikipedia:


Antihelium
The Balloon-borne Experiment with Superconducting Spectrometer (BESS) is searching for larger antinuclei, in particular antihelium, that are very unlikely to be produced by collisions. (One of the current experiments, under assumptions of current theory, would take 15 billion years on average to encounter a single antihelium atom made that way.

Ok. Let me get this straight... There's a publically-funded experiment in place trying to detect antihelium which, on average, would take 15 billion years, about five times as long as the Earth has even existed, to encounter a single molecule...

Why am I not surprised that Congress funded this?

Why are we still electing idiots to Congress?

Ah, yes - simply another of the many unsolved mysteries of the universe...

Sock puppet
2007-Sep-24, 12:05 PM
Antihelium
The Balloon-borne Experiment with Superconducting Spectrometer (BESS) is searching for larger antinuclei, in particular antihelium, that are very unlikely to be produced by collisions. (One of the current experiments, under assumptions of current theory, would take 15 billion years on average to encounter a single antihelium atom made that way.)

Ok. Let me get this straight... There's a publically-funded experiment in place trying to detect antihelium which, on average, would take 15 billion years, about five times as long as the Earth has even existed, to encounter a single molecule...

Why am I not surprised that Congress funded this?

Why are we still electing idiots to Congress?

Ah, yes - simply another of the many unsolved mysteries of the universe...
(bolding in wiki quote mine)

Well, it might be worth doing to see if you find something you don't expect. Also, as the bolded part says, it is expected to take 15 billion years to encounter an antihelium nucleus produced by collisions. Maybe they have another mechanism for antihelium production, which would be supported by a detection (if collisions are insuficient to produce them).

NEOWatcher
2007-Sep-24, 01:03 PM
...Ok. Let me get this straight... There's a publically-funded experiment in place trying to detect antihelium which...
I thought this looked familiar (http://www.bautforum.com/off-topic-babbling/65007-antihelium.html).

Being that it was the exact same post, posted at nearly the same time, before anyone had a chance to respond, tells me that this really irritated you.

OldGuySythe
2007-Sep-24, 01:54 PM
Maybe someone in this group can set me straight on this: I have always thought that "Virtual" particle was nothing more than an "imaginary particle" that has been assigned certain attributes/characteristics that define it's interactions in our physical world as we currently understand it. More and more, I am beginning to think that people actually believe that a photon/graviton/whatever is an actual physical thing. Where do "real" physicist stand on this?

Ken G
2007-Sep-24, 02:17 PM
I have always thought that "Virtual" particle was nothing more than an "imaginary particle" that has been assigned certain attributes/characteristics that define it's interactions in our physical world as we currently understand it. That is a pretty good definition of any particle. What distinguishes a virtual particle is that it cannot be observed to exist because it doesn't conserve energy, but its influences are seen. In many case, the same particle (say, a photon) can be virtual in some situations (like when it is "mediating a force") or real in other situations (like as you see this).
More and more, I am beginning to think that people actually believe that a photon/graviton/whatever is an actual physical thing. Where do "real" physicist stand on this?
I would say that "real" physicists should keep their "beliefs" separate from their physics-- the latter is merely a prescription for making predictions using our current best model. But as humans, we do tend to believe in what works.

mugaliens
2007-Sep-24, 09:36 PM
I thought this looked familiar (http://www.bautforum.com/off-topic-babbling/65007-antihelium.html).

Being that it was the exact same post, posted at nearly the same time, before anyone had a chance to respond, tells me that this really irritated you.

Sadly, yes. I'm not sure why waste irritates me so much. Fact of life, I suppose.