Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: pinballed on 03/10/2012 05:45:31

Hi.
I understood from a previous answer that virtual photons are a mathematical tool/description, but has no real existence.
Is it the same for all virtual particles, or do other virtual particles exist in reality?
Thanks,
Magnus

As far as I know it's the same for all virtual particles. So the explanation of Hawking radiation which uses the concept of virtual particles it's just a simplistic description even if a valid mathematical tool. If I remember, infact, even in this case it's possible to describe the effect without using the virtual particles paradigm (but the mathematical description is much more complex).

That's highly irritating :(.........so we are writing books, making tv shows, spreading an explanation that we know is not true,
just for the sake of having an intuitive explanation that mathematically would predict/solve an observation we know...but otherwise have nothing with reality to do.
I've been trying to bend my head around virtual particles, borrowing time, etc.....
But very much thanks for your answer.

But keep in mind that all particles are virtual in a sense. They all move from a source to ultimately interacting with something else and can be described by the same mathematical tools as the shortlived virtual particles we usually talk about.
What we usually call virtual particles are those that aren't caught by a detector in their short life span.

JP, if we allow virtual particles a timespan they shouldn't be called virtual, then they would be 'real' to me. If we by real mean that it contain those things we actually measure by our ruler and wristwatch. Then the 'foam' or 'virtuality' of space also should become a measurable phenomena with 'paths', as in some sort of 'recoil' and subsequent annihilation. One of the reasons why I find indeterminism' so releasing :)
And this virtuality seem to assume some sort of 'fabric', which I'm not that particular towards either. Indeterminism make no such claims, as I think of it. You could though make a argument in where what we see as a 'fabric of space' from some other point of view will not exist, making this observer dependent too, and that might be acceptable.

Yor_on, here's how I understand real/virtual and why it's a blurry line between them. Virtual particles are interactions between two events in spacetime. Real particles occur when one of those endpoints is far enough from the other so you can approximately ignore it and pretend the particle flies off freely. In reality if you actually included the absorption endpoints of all particles, they'd all be virtual, but we can ignore those endpoints in a lot of cases and call them real.
In practice this is important because some interactions happen on such a short scale that when we compare them to our detectors, we can never write the interactions as going off to infinity, so our detectors can never catch these particles.
Anyway, I don't know much more than the basics of this, so I'll refer you off to this FAQ on virtual particles by an expert :)
" . . . since all particle interactions occur over a finite time interval, in a sense all particles are virtual to some extent." http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html

Heh, I can, in a similar way, refer you too another guy explaining in a mathematical manner why virtual particles mathematically is a limited definition, making several assumptions by 'sleight of hand', that is if I now can find it? Been some time since I looked that one up :)
But I like the way you referred to 'paths' as events, defined as having 'paths' through/due to our local clock and ruler measuring, that is if I read you right? That's the way I see it too, but that doesn't make them virtual to me. 'Reality' is defined through your measurements, as long as we're talking scientific formalism, and also according to how I interpret it naturally:) And if it wasn't, then all scientific statements and propositions belongs to the realm where 'unicorns wander freely' as they only would be one of many belief systems, even if grounded on 'logic'. We all need a place to stand on, to turn the world.
Give me some time and I will find it JP, I think you will enjoy it.

Hey,
Don't follow all what you write.....but one essence is that there are a lot of phenomenon;
interactions between particles (with "virtual particles"), electric force of an electron (that is influenced by a cloud of "virtual particles"), and many more.
They are solved mathematically, and then this solutions can be built on different "explanations" although presently noone has been able in experiments to verify any of
the explanations (sofar at least). All the explanations has pros and cons, are mostly theoretically possible, and the future will show us the answer.
If I'm totally wrong  bring it easy on me  otherwise my already bad selfesteem in this subject will suffer more :)
Magnus

Well :) Magnus, the point is that i personally prefer indeterminism to 'virtual particles'. But it seems most physics prefer it the other way, although if we have some pure mathematicians here I hope them to know what I mean. To me it has to do with how we grow up perceiving our universe. When we later read and experiment we try to fit those results into what we already have learnt about 'forces' 'action and reaction' and causality chains macroscopically. It's my opinion anyway, and as I put a very large relevance to Einsteins definitions I find him differing, for example when describing motion, from what we've seen it before relativity. Then a uniform motion and a acceleration described the same thing, namely 'motion'. But if you check up on relativity you will see that in Einsteins definitions a constant uniform acceleration is equivalent to a gravity, whereas a uniform motion is equivalent to being 'at rest'.
==
All of this as a locally perceived description naturally. Comparing between two objects moving uniformly relative each other that means that you are free to define any of them as being 'at rest', relative the other. And if you now will want to define a 'potential energy' to any of them you then must start by define either, which one is 'really really' moving, so also defining a preferred frame of reference, which does not exist in relativity. Or you have to define a 'system' containing both objects, uniformly moving relative each other, and define a 'potential energy' as a result of that system.
That's definitly not the same concept as we had before, and I think Einstein correct.

There is the possibility though, philosophically seen, to define locality as the grounding principle from which we build, and in that motto create a 'preferred frame of reference'. But if you do so you then also have to recognize that my preferred frame, when comparing it with yours, will differ. To make such an idea fit relativity you will find yourself asking what 'degrees of freedom' really means, and also where the 'connections' between different frames of reference, as in making it possible for them to communicate, exist. Because they do communicate, and locally my definition of a time and a length will be yours too, as long as we're 'at rest' etc. But thinking that way 'motion' becomes a suspect subject to me :) although we all have access to, and see, it daily.

As a non scientist, I found this an interesting explanations of virtual particles.
http://profmattstrassler.com/articlesandposts/particlephysicsbasics/virtualparticleswhatarethey/

Yeah, that was a nice one Bill. And if you look at it from indeterminism then probability is what defines your 'virtual particle' as I think. You don't need something 'real' as in particles, although you do need some 'perturbation' creating a outcome, from where you observing can define a interaction as having took place. I'm still looking after the mathematical definition I used to have defining 'virtual particles' as suspect beings, the problem is that this definition was saved about three computers ago :) as a guess and although I expect to have it somewhere, and remember it as a eye opener, it will take me time to track it down. But it was good, at least as I read it.

As a non scientist, I found this an interesting explanations of virtual particles.
http://profmattstrassler.com/articlesandposts/particlephysicsbasics/virtualparticleswhatarethey/
Very nice find, Bill!

Yes, a great answer from Profmatt; but what is the 3D electromagnetic stuff that the field is made up of?
CliveS

A very nice link in more ways than one Bill, found my own 'old link' that i wanted to show in the comments there. http://www.mat.univie.ac.at/~neum/physfaq/topics/virtual
As for 3D EM stuff Ac :) read the comments closely and you will find descriptions of the different 'fields' assumed to exist.

Hey,
Don't follow all what you write.....but one essence is that there are a lot of phenomenon;
interactions between particles (with "virtual particles"), electric force of an electron (that is influenced by a cloud of "virtual particles"), and many more.
They are solved mathematically, and then this solutions can be built on different "explanations" although presently noone has been able in experiments to verify any of
the explanations (sofar at least). All the explanations has pros and cons, are mostly theoretically possible, and the future will show us the answer.
If I'm totally wrong  bring it easy on me  otherwise my already bad selfesteem in this subject will suffer more :)
Maybe, but the explanation which makes use of virtual particles it's not totally equivalent to the other which don't, for the following reason:
a free real particle with mass m, momentum p and energy E, obeys the equation
E^{2} = (cp)^{2} + (mc^{2})^{2}
A virtual particle doesn't.

True, lightarrow, but the way I've understood it is that it's a continuum from virtual particles which can be off shell (violating that equation) to real particles which have negligible probability of being off shell. This seems very similar to the way in which small objects show quantumness, while large objects don't, even though all objects should obey the rules of quantum mechanics.
Or put another way, don't the same underlying equations govern virtual and real particles? The only difference being whether you can make certain approximations corresponding to "realness" in the real particle case?
http://en.wikipedia.org/wiki/On_shell_and_off_shell

Thank you very much for the explanations.
Thanks,
Magnus

True, lightarrow, but the way I've understood it is that it's a continuum from virtual particles which can be off shell (violating that equation) to real particles which have negligible probability of being off shell. This seems very similar to the way in which small objects show quantumness, while large objects don't, even though all objects should obey the rules of quantum mechanics.
Or put another way, don't the same underlying equations govern virtual and real particles? The only difference being whether you can make certain approximations corresponding to "realness" in the real particle case?
Unfortunately I'm not so expert in virtual particles to provide a definite answer [:)] I see the being "off shell" of a particle as I see the electron in the H atom being prevented to stay between two near orbitals; but maybe it's you which is correct.

True, lightarrow, but the way I've understood it is that it's a continuum from virtual particles which can be off shell (violating that equation) to real particles which have negligible probability of being off shell. This seems very similar to the way in which small objects show quantumness, while large objects don't, even though all objects should obey the rules of quantum mechanics.
Or put another way, don't the same underlying equations govern virtual and real particles? The only difference being whether you can make certain approximations corresponding to "realness" in the real particle case?
Unfortunately I'm not so expert in virtual particles to provide a definite answer [:)] I see the being "off shell" of a particle as I see the electron in the H atom being prevented to stay between two near orbitals; but maybe it's you which is correct.
That might be right, too. Orbitals extend way out beyond the edges shown in textbooksit's just a vanishingly small probability that the electron would occupy those points in space.

Feynman diagrams usually produce a large number of probable out comes for particle collision of which only a small amount actually occur. Several scientist working in collaboration at the Hadron Collider found that they could reduce the possible outcomes to only those that actually occurred by removing the virtual particles from the process. I think this will ultimately lead to revisions in Standard Theory which will allow the transfer of force through space without virtual particles. Easy reading: Loops, Trees and the Search for New Physics; Scientific American, page 38, May 2012. More detailed reading: Precise Predictions for w+4 Jet Production at the Large Hadron Collider. Physical Review Letters.

thanks for that one Scico.
"It turns out that the Feynman method is really an elaborate way for essentially bruteforcing a solution. If what you're treying to calculate, the metaphor goes, is the ways that a person could get from one given station on the London Underground to another, it calculates not only the valid methods of movement through the system, but also all of the impossible ones, throuhg the solid rock or earth beneath London. Most of the possibilities that end up being calculated end up canceling out with each other, or simplifying down, but you still need them for the intermediary Feynman calculations. The three gentlemen who wrote this article, however, have resurrected an old idea from the '60s, and developed it into a theory that they call the "unitarity" method
What it does, is it assumes that the total probability of every possibility will equal 100%, and it calculates things by breaking them down into smaller steps, essentially finding the probabilities that a person riding the London Underground will go through a given turnstyle next, rather than trying to calculate their entire route at once. Not only has this method been used to refine the LHC results recently, but they've also resurrected Supergravity theory with it. It was previously thought that Supergravity theory entailed too many everspiraling infinities in the quantum foam to possibly be correct, but it turns out that, upon refinement with the unitarity theory, these infinities cancel out. They are able to calculate particle collisions that would traditionally require the calculation of 10^30 terms by only calculating a few dozen terms. They have already won several bottles of wine as bets from other physicists that they won't be able to correctly calculate things with their theory are proven wrong time and time again. They conclude by commenting that they are on the verge of solving 7 gluon interaction, and that, if this works, supergravity may be seen as alive and well, and well on the road to finding a quantum theory of gravity. They comment also that gravitons, under their model, are predicted to interact simply like a pair of threegluon conglomerates." from
Advanced Science Study. (https://sites.google.com/site/advancedsciencestudy/summariesofscientificarticles/loopstreesandthesearchfornewphysics)

So what both methods presume is causality chains existing, although they have two ways to get there from probability. And that is the question.
What makes causality chains?

I think Yor_on's post sums it up well: they haven't thrown out Feynman's path integral method, but they've figured out ways to refine it and to make approximations to allow the computations to be done quickly and accurately.
I don't think the theory in itself will change anything about the way forces are transmitted, since it doesn't disagree with Feynman's predictions, but it might allow new techniques to be developed because it allows us to do previously intractable computations.

This one should be read too. It's about 'super gravity' becoming a outgrowth/principle of 'super symmetry'. The idea that everything has a 'partner' of sorts, for fermions as 'matter' becoming some 'boson' etc. So for each new particle found at LHC it then should mean that there is some balance, a 'opposite mirror' that must exist according to super symmetry, all as I interpret it. And if you combine (Einsteins) 'symmetries' with GR you get to 'super gravity' as a hypothesis. But what it is as a 'field' I'm not sure, it doesn't seem to fit our descriptions of 'forces' at least, as something resembling a EMfield? Then again, it's all seems observer dependent if you go by Einstein. Take a look at Viewpoint: Vanquishing infinity. (http://physics.aps.org/articles/v2/70)

To be perfectly frank, I don't recognize yor_on's claims about virtual particles at all.
He seems to be basically saying that virtual particles don't exist, but both electrostatic and magnetic fields basically consist of virtual particles; so it's not like virtual particles don't have massive realworld effects, because they definitely do.
Virtual particles 100% definitely do exist.

You can call it 'virtual particles' if you like Wolf. I think of it otherwise though. To me it has to do with a arrow, assuming a 'virtual particle' inside a arrow makes it real to me. Then it is a particle, not anything virtual. Assuming it outside a arrow is to me something else. Then i find 'particles' a bad choice of words, and think of it in terms of outcomes, governed by the probability of them becoming. What that probability then should be seen as is another matter :)

Does this help?
http://profmattstrassler.com/articlesandposts/particlephysicsbasics/virtualparticleswhatarethey/

I really like the way he thinks of 'virtual particles' Bill, as 'ripples in a field' more or less, if I get it right. But then you have relativity in where a EM field becomes observer dependent. What kind of 'fields' would this be, assuming a existent arrow? Observer dependent too? And would they then be the 'background' from which SpaceTime becomes? Could there be something from where 'fields' come to be? Or, is there no background at all? The fields creating it all?
Somehow it all feels as a question of hidden variables, our observables being what we can measure.

and the next step, if I presume something not measurable for us, is the question how we ever will be able to validate what we can't measure. Where do we draw the limits? Are 'photon paths' observables or are they assumptions? I prefer observables myself, when I want to make a assumption, but how far can I go from them? You can turn it around and make a presumption in where what we can measure is the tip of a iceberg, mostly hidden below water. Then you can from there question if what we measure should decide what we can't. I like the idea of symmetry breaks myself, and think of SpaceTime as one.
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If you think of SpaceTime as a symmetry break, then the arrow and other degrees of freedom we find is a expression of that 'wound', if I may. From that you also can wonder about how SpaceTime is seen from whatever 'place' it breaks free from. And there you have the question of what those degrees of freedom really mean, if they also are shown to observer dependent, which they are according to relativity..

Yor_on, I'm not sure what you're getting at exactly, but as long as you believe the Standard Model is correct, virtual particles are a part of the model by their very definition. They're manifested in the mathematics of the model and can often be used to simplify computations. Now, what we can debate without throwing out the Standard Model is whether or not "virtual particle" is the best term for the physical processes/mathematics they represent.

Just explaining why I personally doubt 'virtual particles' as being 'particles' :) But as you say, that has to do with what one may mean by using it. We have a geometry, that we measure. But if relativity is correct that geometry can be seen as 'shrinking/disappearing' coming close to lights speed.
Why? And what would that mean if locality, meaning yourself measuring locally, can be the only first hand description of a universe? If we believe in direct measurements being the correct approach to the universe then the theoretical counterpart, our conceptualization of the same into a 'indivisibly same' universe for us all together becomes a contradiction in terms. Because our direct measurements are always done locally, and even if we can translate them to each others descriptions, it doesn't change that my measurements will define my existence, as yours will you.
I like fields too, but I'm not sure what they are.

Fields are things that exist and have energy at every point in spacetime. As spacetime geometry changes, fields warp along with it. They don't have to be quantum and by thinking of them as quantum you're making things harder.
Would you agree that we know how light and electromagnetic forces classically exist and travel around the universe? Those are due to fields. We can model gravitational lensing and red/blue shifts as due to the geometry of spacetime. If we take that classical field and lower the energy so much that we need to worry about quantum effects, we still have a field. It just has slightly different rules, but it still lives over spacetime and bends along with spacetime.

A very nice description JP, and I see how you think there. As for radiation (light if you like) propagating I'm not sure how it works. Whatever way it express itself as 'propagating' it still seems the simplest explanation to assume it moving as some corpuscles or excitation from 'a to b'. But I can't ignore the 'warping' of SpaceTime, because according to locality it's no 'warping' at all. It's just as true measuring as it will be for someone not having that 'speed' finding another 'warping'.
If it was so that no matter your uniform motion, you and SpaceTime would express itself the same relative some observers then we would have a common nominator from where we might define a warping. But as far as I understand it isn't so, with different uniform motions you will find different time dilations & LorentzFitzGerald contractions. Without that there is no warping anywhere, because a warping actually assumes that common nominator from where you can measure one. But I would love to see one defining what a warping should be.

what this reasoning is trying to lead to is the question what is more 'real'? If you have three observers, describing and measuring the same field. Each one finding it to be different, is it still the same 'field' for them? In a way you can argue that it is, conceptually and logically finding rules for how it change with the observers.
In another, if we discuss it as your 'local truth', yours is not 'the same' as the other observers, and the difference you find will define your universe. Accepting that locality is the definition of what can be 'true', which i find, it becomes very hard to argue for a 'indivisibly same universe' for us all. And that will, if we look at SpaceTime as 'fields', be just as true there. Your measurement is not mine, and your field is not mine either, even though we can translate them to each other. Could you state that as all languages can be translated into each other, they all must be the 'indivisibly same'? I don't think so myself. A translation is a expression of it existing some principle, aka logic, making it possible to communicate/translate changes, as perceived locally, in a logically understandable way for all.
And without that principle all communication would fall. But I don't see it as a guarantee for anything, more than there is a logic to SpaceTime.
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One may see it as two versions of Relativity. One is a 'conceptual SpaceTime' in where we try to find principles guaranteeing that what I see is translatable to what you see, and so find a 'indivisibly same' although strictly conceptual 'reality'. The other goes out from who and where you are, using local measurements. That 'reality' is in fact the one you will live, and die, in. And your measurements there must define the limits of what you can do, whilst living.
So which one is truer? The conceptual or the one you live in?
The one you live in.
And it's deeper than that. What do 'you' think your life is? A conceptual exercise, or a reality. According to relativity it can only be real for you locally, as soon as you incooperate a 'whole indivisibly SpaceTime for all' you have moved from measuring locally to 'translating' observer dependencies conceptually. All of that would be good if it wasn't so that what you measure locally is your 'destiny', it will be what defines you until you die.

But you're right JP :)
I do have a tendency to put it all to quantum (energy), including relativity. And that's also what 'locality' implies to me. That locality should be what 'reality' can be traced too, and there I prefer it to be Planck scale that defines when something becomes 'real', including ones arrow.
This one is really cool. Observation of the Dynamical Casimir Effect in a Superconducting Circuit. (http://arxiv.org/abs/1105.4714) If one like one can call it a 'field of energy' with 'virtual particles' becoming 'real' as measurable inside a arrow I suppose. But if one does, one also must admit that this 'field' will be observer dependent, and so always locally defined.

The weirdest thing with such a reasoning, accepting light (radiation) to be what binds those 'frames of reference' together (communicate changes). The weirdest, yep, absolutely strangest idea I get is that when we discuss 'multiverses' of any kind, we actually already have them, here and now. Each frame of reference can be seen as its own representation of a universe.
Also assuming some sort of 'observer'. If one accept that a observer is needed, and then accept this as an weird idea we get to the question what a observer would need to be, to make this one work. If so a consciousness isn't needed, as I think, at least not one inside a arrow, because those 'frames' exist even without us macroscopically. although if we go down in scale?
And I'm not that thrilled over multiverses myself :) It gets confoundedly complicated if we have both those and then assume that our 'frames of reference' also would represent something similar. Or maybe it makes it simpler? I don't know :)
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The point here is that when we go down in scale, consciousness might have something to do with what we will observe. But assuming that you measure a planets motion through space, not observing the moon, or not even knowing that there is a moon involved, you will still be able to 'reason forward' that moon, or at least something with mass. And that one we've already done in astronomy, finding things due to other suns motion through space. So the question if the moon is there even when you don't look I find to be moot. It's there, macroscopically at least. But from a quantum perspective? QM is seriously weiird :)

The weirdest, yep, absolutely strangest idea I get is that when we discuss 'multiverses' of any kind, we actually already have them, here and now. Each frame of reference can be seen as its own representation of a universe.
Just because frames are different representations of the universe doesn't mean they're different universes, though. It's still the same universe, and the representations are just different ways of looking at it.
The only reason you need a multiverse is if you think there are other universes, each with lots of reference frames. There's no need to assume multiverses in the Standard Model.

That's what I mean by the conceptual model of relativity JP. Assuming that 'reality' is best described conceptually you can get a 'indivisible universe' same for us all, expressed through Lorentz transformations. Assuming locality you instead get frames of reference, mediated through radiations constant, or 'lights speed in a vacuum' as we describe it. That's one reason why I'm still holding out on lights 'propagation', because in a thought up relativistic 'multi verse', described through lights propagation, and always relative your local frame of reference that 'propagation' becomes weird.

Eh, not that it isn't weird as it is already :) Disprove light as a constant and relativity should fall as I expect, but as it is it seems to fit the experiments we've done? But in a thought up mosaic of local frames it seems quite difficult to see how light would be thought to 'propagate' between them. In such a scenario you need some other mechanism for what we observe to be a propagation, and keeping light as a constant it must have to do with those local representations of our four dimensions, and then something more, as we do communicate between them. This one is really more fitting for 'new theories' though :) As I said, it's one of the strangest consequences I've imagined, if it now would be that way.
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My point here is that all local representations/observers each will see 'one undivided universe', consisting of all frames of reference, relative their local measurements. But they won't 'fit' together if somehow being able to take in all observers measurements simultaneously, except conceptually through logical transformations. And the 'conceptual point of view' can then, as I see it, just as easily be described as begetting the 'eyes of a God', and view one 'coherent universe' from it.
For each transformation you do relative a local description the difficulties to see them all (simultaneously) must rise. And how many representations can there be? A countless number, or a finite? In a way it very much reminds me of how to imagine looking at something through five dimensions, or more. Not a human way of thinking.

You could argue that there is no such thing as 'simultaneousness' as each frame present all other frames differently in four dimensions, when compared to each other. But then you also need to define how this would be possible, and as you do that find that the question above stays anyway. Because what you then comes to is still a mosaic, although now defined as each one having four dimensions slightly out of 'mode' with any other. And as each frame of reference will find unique definitions for all and any of the others frame of reference? Think about it.
I prefer the idea of 'time' as being somewhat of a 'fabric' ::)) And the arrow as directly connected to lights speed in vacuum, meaning locally 'same' for us all when superimposing frames of reference. But then my 'time' also must become a conceptual description, as there is no way of measuring time, the (local) arrow we can measure though. If we had no way of doing so all locally measured speeds, and so 'motion' itself, would be imaginary.
(which it may well be:)
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Frames of reference are quite fascinating, and confusing, to me. All of this depends on how seriously you take the idea of direct measurements being what science rest on, and relativity naturally. And if we go back to virtual particles, all of this makes any idea of virtual particles connecting/communicating changes in 'points' of SpaceTime observer dependent. It's not enough defining the arrow as 'observer dependent' as I see it. You have to do the same with all four of our 'degrees of freedom' to make it fit.