Wolv1's questions about causality and particles

I think Ben will want to comment on this but to my understanding he was saying that virtual particles on their own violate causality. Now we can't measure virtual particles so whether or not they violate causality is irrelevant - what really matters is whether their effect causes a measurable violation of causality.

The only good question you can ask is the latter, so that is the one I have been answering - the effect of virtual particles does not violate causality. The bad question, which may or may not be the one you have been asking is whether a single virtual particle that exists for some time during an interaction of 2 real particles violates causality. The answer to that question may be that it does, or it may be that it doesn't - we do not, and will never know the answer.

 

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ya see that's what he was talking about that threw me off. So what is it that violate causality?

 

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Nothing that we can measure.

 

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The way I understand it, and the way I've explained to you many times in the past, is that individual virtual particles MAY violate causality (i.e. propogate outside of the light cone). But because one must sum over the effects of ALL virtual particles, the NET EFFECT is that causality is maintained.

If someone can correct me, I would welcome it. The two point correlation function falls off like some kind of exponential outside the light cone, as I recall---I don't have my copy of Peskin handy.

 

The first discussion of causality provided by P&S is on P13~15, where they show that relativistic 1 particle systems have the property you describe. However, this still means causality is violated. By the introduction of antiparticles, QFT solves this because their contributions cancel out, just as you/we've been discussing.

Wolv seems obsessed with asking questions on this. He's PM'd me many times, up to the point where I've ignored him because I've had better things to do (like sleep or pick my teeth...). He wants to details but doesn't want to learn the theory. Any subtlies which comes from knowing the theory he's not interested, he's just desperate for someone to say "Yes, they violate causality", then I imagine he'll cling to that statement like a fat kid to the last cupcake.

 

This is what annoys me. Causality at best has only been observed 100% operating on the scale of macroscopic objects. Causality could very well be broken at the subatomic scale, but for some reason, scientists crawl up in a ball at the thought of it, as if the macroscopic world and the quantum world should somehow share the same rules, which is ironic at best.

 

No, we have measured, to parts per million or billion or even close to a trillion, purely microscopic (beyond microscopic!) systems involving electrons, photons, quarks and gluons and if causality were broken on those scales, we'd see it.

Accelerators like LEP didn't average over a billion electrons, they were measuring results from a single electron hitting a single positron and the products of that.

I suggest you read up on the huge place causality has in particle physics and how it can have a big effect on phenomena before claiming that physicists curl up into a ball at the thought of it. Plenty of physicists go looking for it. After all, the best way to show your ideas are worth a damn is to try vigorously to disprove them by any and all means at your disposal.

 

no on the contrary,you got the wrong notion of me thinking im going to say "yes they violate causality".

 

so the violation is so small that we can't measure it?

 

No! There is no violation that we can measure.

 

No, there is NO measurable violtation. It's not that it's so small we cannot measure it (like the size of strings), the breaking of causality happens only on things we cannot measure AT ALL.

Pay attention!

 

so it's safe to say causality is never violated? you might want to check post's 7,8, and 9.prometheus,so virtual particle's on there own violate causality? this is what you told me.

 

It's possible my interpretation is way off - so wait until one of the physicists oks this!

To my knowledge, "virtual particle" is simply the name given to certain parts of a Feynman diagram. Now when I say "Feynman diagram", I refer to a schematic diagram that helps one understand the different components of a certain computation: a computation of some scattering amplitude. Now certain objects turn up in Feynman diagrams because of the type of the scattering experiment we're considering, so it makes sense that we make some sort of correspondence between those objects and the particles. In fact, we go a step further and just call those objects "particles". In the back of our mind though, we have to remember that this is our doing, and we've done it just to aid a computation. What we're calling "particles" are just terms that correspond to a big computation.

There are other objects that always seem to turn up in these diagrams, so it might be worth giving them a name. The objects look a bit like those that we now call "particle", but they don't correspond to the particles we spoke of before. So instead, we call them something else - "virtual particles". But again, we remind ourselves that this is our doing, and these things are simply objects in a schematic diagram that help us do certain mathematical computations.

I think why this kind of thing comes up a lot is because people learn about the intuitive use of Feynman diagrams before they're taught that the components of the diagram are just a schematic representation of some mathematical computation.

As I mentioned though, it might be worth waiting for one of the physics guys to pop along before you take my interpretation as gospel!

 

so we don't even know if virtual particle's violate causality? let alone if they exist.i read that virtual particle's are involved in antenas or something.

 

Virtual particles exist.

 

That's close to my view. Quantum Field Theories are about the relativistic quantum fields of point particles, but Feynman diagrams are about chopping up the whole of the quantum field into artificial pieces for approximate calculation. The virtual particles which appear are a useful but not fundamental way of looking at weakly interacting theories. Strongly interacting theories can't be approximated well by this method, which is another reason to suspect that virtual particles are not fundamentally the most coherent description of QFTs. For one, counting virtual particles is not (to my understanding) possible and that makes them seem even less "real."

Arguments for the realness of virtual particles (fermion magnetic momements and Casmir force) are indistinguishable from the general success of Quantum Field Theories which is the only context where virtual particles are really useful for computation.

My $0.02.

 

what is the longet a virtual particle can last?

 

so we don't even know if they violate causality or not? isn't that kind of contradicting it's self by saying "yes vitual particles violate causality" then saying " no, they don't violate".Prometheus said '' Virtual particles can indeed travel faster than light as well, although it is relatively straightforward to show that they do not affect causality. For causality to be affected a pile of real particles or waves must be sent back in time." and you say they do???

 

so virtual particle's don't precede there effect's? i have some links i posted on 7,8,9 might want to take a look at it.

 

so the effects of virtual particles violate causality?