Virtual particles

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kith

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Thanks for your extensive comment, muppet.

So you seem to agree, that there is no clear distinction between real and virtual particles.

I don't really get the bottom line from the second part of your text. Maybe, I'll come back to it, if I have some more basic knowledge. I'll put Taylor on my list, but a 500-pages on nonrelativistic scattering theory alone, may exceed my reading capabilities. ;)
 
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Thanks for your extensive comment, muppet.

So you seem to agree, that there is no clear distinction between real and virtual particles.

I don't really get the bottom line from the second part of your text. Maybe, I'll come back to it, if I have some more basic knowledge. I'll put Taylor on my list, but a 500-pages on nonrelativistic scattering theory alone, may exceed my reading capabilities. ;)
You mean the part about the leading order term in an expansion of the effective action in powers of hbar? You'll definitely come across that eventually. Read it as "quantum field theory reproduces classical field theory when Planck's constant goes to zero" :smile:

Happily, the discussion in Taylor I'm referring to is in chapter two; chapter one is mathematical preliminaries that I skipped and referred back to as and when needed. The full book is too much for my reading capabilities too- I picked it up as the treatment of scattering in the quantum field theory text by Peskin and Schroeder is based on 3 chapters of that book.

I suppose the best way to summarise my opinion on virtual particles is by analogy with ordinary NRQM.
If you have a simple harmonic oscillator potential, you expand your state in oscillators of different frequencies, and you'll be working with states that correspond exactly to the eigenstates of the system.
If you have a system "close to" a SHO, you can perturb around your idealised SHO; it's a good calculational scheme, and it gives you an intuition for the physics, but you're no longer talking about an exact correspondence.
If you have an arbitrary potential, you can still expand states in the SHO basis, as it's a perfectly good basis. But the calculations will be intractable and you'll have no understanding of the physical picture of the situation at all.

Free particles are the idealisation, analogous to the SHO. The virtual particles exchanged in scattering processes are exactly like perturbations around the SHO; they're an intutive way of understanding the result of poking a system you understand well, but you shouldn't do any rigorous philosophy treating them as entities in themselves. The confusion arises because what we think of as "states" and "potentials" in QFT are created using the same operators. But the "particle" states behave differently in the presence of interactions to our conception of what a particle should be, as they're no longer eigenstates of the Hamiltonian that actually describes the system. Any circumstance in which a state created by the action of a field behaves like we imagine a real particle should will be one in which we can neglect the effect of interactions upon the time evolution of that state. And situations exist where perturbation theory breaks down and the particle picture conveys almost no useful information, because we're no longer basing our approximation on a picture that describes the physics well.

As a totally unrelated aside, proofreading this post reminded me of an observation one of the permanent staff here in my university recently made to me:
I like the way you speak, in these closed sentences like something out of 'Yes Minister' that somehow eventually manage to end.
(For those unfamiliar with the reference: )
I hope it's not too verbose to be of some help.
 
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The opening query of this post is the claim that virtual particles 'flat-out don't exist'. Hawking is a highly imaginative theorist, but his exploitation of flat-out non-existent particles is hardly comparable to that of exploiting the aether, or phlogiston.

The non-observation, incidentally, is not a falsification of Hawking evaporation proper. We need a positive observation of a radiation-signature of some process antithetical to Hawking evaporation: if x is happening, y cannot be happening.
_______

On, the other hand, I may have the cart before the horse: Hawking radiation would be as much evidence of virtual particles as Hawking BH-evaporation.
So its safe to say virtual particles don't exist in physical reality?also wouldnt they violate causality since they can travel faster than light?
 
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I know virtual particles travel faster than light,and faster than light travel has to also deal with time travel backwards in time because from what i understand if a particle were to travel faster than light it will in one frame travel to the past,now do virtual particles travel backwards to the past from them traveling faster than light?
 

Vanadium 50

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I know virtual particles travel faster than light
Well, they don't.

"It AIN'T so much the things we don't know that get us into trouble. It's the things we know that just ain't so."
 

tiny-tim

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I know virtual particles travel faster than light …
no, https://www.physicsforums.com/library.php?do=view_item&itemid=287" in the coordinate-space representation (of a Feynman diagram) are on-mass-shell, so virtual photons travel at the speed of light, while virtual electrons travel at any and every speed slower than light

however, in the momentum-space representation, the virtual particles are both on- and off-mass-shell: both virtual photons and virtual electrons travel at all possible speeds

(of course, this is all just maths … virtual particles aren't real … the clue's in the name! :wink:)
and faster than light travel has to also deal with time travel backwards in time because from what i understand if a particle were to travel faster than light it will in one frame travel to the past
no, travelling is always forward in time

but a faster than light object travelling forward in time from A to B in one frame may be travelling forward in time from B to A in another frame
now do virtual particles travel backwards to the past from them traveling faster than light?
virtual particles don't actually exist, they don't have an A or B to travel between

i suspect you're thinking of the rule that an anti-particle (real or virtual) travelling forward in time can be thought of as a particle travelling backward in time :smile:
 
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... a faster than light object travelling forward in time from A to B in one frame may be travelling forward in time from B to A in another frame
So whether an object is moving from A to B or from B to A depends solely on the observational frame of reference? I don't think so.
 
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On the contrary one could argue that nonvirtual particles do not exist. Every particle is virtual since it is always en route from one interaction to the next.
Beautifully put. We put a lot of trust in 'propagation', don't we? And sometimes even seem to ignore indeterminacy (HUP), as being a principle of its own, instead referring to 'virtual particles' as if they were limited by 'time' to be unmeasurable. I used to like the idea of 'virtual particles' but?

I think I like indeterminacy more.
 

tiny-tim

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So whether an object is moving from A to B or from B to A depends solely on the observational frame of reference? I don't think so.
think again! :wink:

if we regard something as moving north at speed v faster than light, then an observer moving at speed slower than light but faster than c2/v will regard the same thing as moving south at a speed faster than light …

do the maths, and you'll agree :smile:
 
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no, https://www.physicsforums.com/library.php?do=view_item&itemid=287" in the coordinate-space representation (of a Feynman diagram) are on-mass-shell, so virtual photons travel at the speed of light, while virtual electrons travel at any and every speed slower than light

however, in the momentum-space representation, the virtual particles are both on- and off-mass-shell: both virtual photons and virtual electrons travel at all possible speeds

(of course, this is all just maths … virtual particles aren't real … the clue's in the name! :wink:)


no, travelling is always forward in time

but a faster than light object travelling forward in time from A to B in one frame may be travelling forward in time from B to A in another frame


virtual particles don't actually exist, they don't have an A or B to travel between

i suspect you're thinking of the rule that an anti-particle (real or virtual) travelling forward in time can be thought of as a particle travelling backward in time :smile:
Doesn't relativity say that if matter or a particle were to travel faster than light it will in one frame of reference travel backwards in time? i also found this article maybe i misread it that says virtual particles travel faster than light. http://www.desy.de/user/projects/Physics/Quantum/virtual_particles.html
 
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tiny-tim

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Doesn't relativity say that if matter or a particle were to travel faster than light it will in one frame of reference travel backwards in time?
i don't really understand the concept of travelling backwards in time …

if an observer sees something moving, he sees it moving forward in time …

that's the way we see things …

why would an observer think that anything was travelling backward in time? :confused:

but yes, one observer may see an object travelling forward in time faster than light from A to B while another observer may see it travelling forward in time from B to A :smile:

(in other words: causality is not invariant :redface:)​
i also found this article maybe i misread it that says virtual particles travel faster than light. http://www.desy.de/user/projects/Physics/Quantum/virtual_particles.html
this usenet physics FAQ on https://www.physicsforums.com/library.php?do=view_item&itemid=287" is very misleading (it doesn't even consider whether they are simply a mathematical device) …

even the introductory section "What are virtual particles?" fails to answer its own question: after an initially promising overview, it (appropriately!) creates virtual particles out of nowhere without defining or describing them :redface:

as to faster-than-light, it says …
… the virtual photon's plane wave is seemingly created everywhere in space at once, and destroyed all at once. Therefore, the interaction can happen no matter how far the interacting particles are from each other.​
… from which it somehow gets to …
… the virtual photon can go from one interacting particle to the other faster than light!​

i'm not certain i understand what the first section means, and i am certain that the second section doesn't follow from it :redface:

as meoremuk :smile: says …
In my opinion, virtual particles are "made up" concepts.… Usually, the S-matrix is calculated from the Hamiltonian by using perturbation theory. These calculations involve a large number of rather complicated integrals.

In 1949 Feynman invented an ingenious technique of representing these integrals by diagrams. Each line and vertex in the diagram corresponded to a certain factor in the integrand. This technique enormously simplified manipulations with integrals of the perturbation theory.

The diagrams looked so nice that many people (including Feynman) started to use them to "explain" in layman terms what occurs in scattering events. Lines were interpreted as virtual particles that "move" between vertices, etc. etc. These explanations became so "sticky" that many people now believe that at large magnification a collision of electrons really looks like a web of virtual particles jumping back and forth. In my opinion, these beliefs have nothing to do with reality.
 
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In case it hasn't been posted before, http://profmattstrassler.com/articles-and-posts/particle-physics-basics/virtual-particles-what-are-they/" [Broken] is quite informative on the subject of virtual particles.
 
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i don't really understand the concept of travelling backwards in time …

if an observer sees something moving, he sees it moving forward in time …

that's the way we see things …

why would an observer think that anything was travelling backward in time? :confused:

but yes, one observer may see an object travelling forward in time faster than light from A to B while another observer may see it travelling forward in time from B to A :smile:

(in other words: causality is not invariant :redface:)​


this usenet physics FAQ on https://www.physicsforums.com/library.php?do=view_item&itemid=287" is very misleading (it doesn't even consider whether they are simply a mathematical device) …

even the introductory section "What are virtual particles?" fails to answer its own question: after an initially promising overview, it (appropriately!) creates virtual particles out of nowhere without defining or describing them :redface:

as to faster-than-light, it says …
… the virtual photon's plane wave is seemingly created everywhere in space at once, and destroyed all at once. Therefore, the interaction can happen no matter how far the interacting particles are from each other.​
… from which it somehow gets to …
… the virtual photon can go from one interacting particle to the other faster than light!​

i'm not certain i understand what the first section means, and i am certain that the second section doesn't follow from it :redface:

as meoremuk :smile: says …
So virtual particles travel faster than light but don't exist?
 
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Vanadium 50

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Byron, a number of people have noticed you keep posting the same question over and over and over and over and over....

Do you not understand the replies? If not, it would help us to know what you don't understand, or at least what your background in physics is.
 
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think again! :wink:

if we regard something as moving north at speed v faster than light, then an observer moving at speed slower than light but faster than c2/v will regard the same thing as moving south at a speed faster than light …

do the maths, and you'll agree :smile:
I've been trying really hard to imagine this, but it seems I'm running out of brainpower.
I'm sure the maths are right, but can you help me imagine how this works ?

And if one observer sees the object moving north and another sees it moving south, then there should exist a third observer who sees the object as stationary, right ? But I still can't imagine it.
 
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Byron, a number of people have noticed you keep posting the same question over and over and over and over and over....

Do you not understand the replies? If not, it would help us to know what you don't understand, or at least what your background in physics is.
I don't understand how virtual particles don't travel backwards in time when they travel faster than light.
 

tiny-tim

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I'm sure the maths are right, but can you help me imagine how this works ?
See http://en.wikipedia.org/wiki/Velocity-addition_formula" [Broken] :wink:
And if one observer sees the object moving north and another sees it moving south, then there should exist a third observer who sees the object as stationary, right ?
No, to see it as stationary, an observer would need to have the same velocity, v, ie also faster than light, wouldn't he? :wink:

But there would exist a third observer (with speed c2/v, of course, slower than light) who sees the object as moving infinitely fast! :smile:
So virtual particles travel faster than light …
byron, that's not true … read my https://www.physicsforums.com/showpost.php?p=3584038&postcount=256" again :redface:
 
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ive been reading on this forum that virtual particles flat out dont exist?then why is it said they exist for a certain amount of time?
Virtual particles are not detected as rest of particles. However, virtual particles appear in the equations of QFT and some people speak about their existence in this narrow sense.

In rigor, QFT does not say that «they exist for a certain amount of time», because «spacetime» in QFT has only a formal meaning.
 

DevilsAvocado

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think again! :wink:

if we regard something as moving north at speed v faster than light, then an observer moving at speed slower than light but faster than c2/v will regard the same thing as moving south at a speed faster than light …
This is probably one of the most astonishing claims I’ve seen in a while... are you sure this is not a 'mix-up' with RoS...??

do the maths, and you'll agree :smile:
Eh well, maybe, but let’s skip the advanced math for awhile, and use our common sense and pictures to begin with:
  • We stand on the South Pole together with Alice and her brand new superluminal spaceship.

  • Alice has decided to set a new speed record, traveling to the North Pole.

  • Bob is flying his old "c2/v wreck" to inspect the event at the equator.

  • Alice takes off at superluminal speed in direction towards the North Pole.

  • Bob is dropping his jaw, because according to you; he will see Alice taking off at the North Pole, in direction towards the South Pole!
Now the question I have for you, my friend: – If Bob now travels with his old wreck to the South Pole to talk to Alice about what went wrong, where is Alice actually located, the North Pole or the South Pole?

...
 

Fredrik

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This is probably one of the most astonishing claims I’ve seen in a while... are you sure this is not a 'mix-up' with RoS...??
...
Eh well, maybe, but let’s skip the advanced math for awhile, and use our common sense and pictures to begin with:
I'll use pictures, but I won't use common sense. :biggrin: Think about it in terms of spacetime diagrams. Let's make north the positive x direction. Imagine a tachyon gun that's fired at event A with x coordinate 0. The beam moving at speed v>1 hits the target at event B with x coordinate L>0. Now imagine the simultaneity lines of an observer moving at speed u in the positive x direction. (It has to be in the positive x direction). The larger the u, the more the simultaneity lines get "tilted" toward the line x=t representing light speed. The slope of the simultaneity lines can get arbitrarily close to the slope of the line x=t. So clearly, if u is large enough, there will be a simultaneity line that is "below" A and "above" B, meaning that in that guy's (comoving inertial) coordinate system, B is the earlier event.

Since the time axis is drawn in the "up" direction in a spacetime diagram, the world line of an object with speed u have slope dt/dx=1/u. If that object is the spaceship containing the observer, then its speed is <1 and its simultaneity lines make the same angle with the x axis as its world line makes with the t axis. So the slope of the simultaneity lines is 1/u.

  • Bob is dropping his jaw, because according to you; he will see Alice taking off at the North Pole, in direction towards the South Pole!
Now the question I have for you, my friend: – If Bob now travels with his old wreck to the South Pole to talk to Alice about what went wrong, where is Alice actually located, the North Pole or the South Pole?
That's an interesting question. The simplest possibility is that Alice doesn't exist anymore. In the south pole's comoving inertial frame, Alice (the tachyon) is created at A and destroyed at B. In Bob's (original) comoving inertial frame, Alice is created at B and destroyed at A. So A is an emission event in some coordinate systems and an absorption event in some coordinate systems. The same goes for B of course. When Bob has landed at the south pole, there is no Alice, at no point in "space" as defined by his new comoving inertial system, which is of course the one comoving with the south pole.

Suppose that the people at the south pole create Alice at an earlier time, and have "her" bounce back and forth between two tachyon mirrors until one of the mirrors is removed at event A. Suppose also that the people at the north pole are equipped to "catch" Alice at event B, and then have her bounce back and forth between two tachyon mirrors. In that case, the answer to your question must be that Alice is at the north pole.

On the other hand, if that scenario is possible, then it must also be possible to set things up so that from the point of view of Bob's original comoving frame, Alice is kept bouncing back and forth between tachyon mirrors at the north pole for some time before event B, where she is released, and later caught at event A, where she ends up bouncing back and forth between tachyon mirrors at the south pole. If this is how things were set up, then the answer to your question must be that Alice is at the south pole.
 
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DevilsAvocado

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I'll use pictures, but I won't use common sense. :biggrin:
Phew, that’s a big relief!

(:biggrin:)

Think about it in terms of spacetime diagrams. Let's make north the positive x direction. Imagine a tachyon gun that's fired at event A with x coordinate 0. The beam moving at speed v>1 hits the target at event B with x coordinate L>0. Now imagine the simultaneity lines of an observer moving at speed u in the positive x direction. (It has to be in the positive x direction). The larger the u, the more the simultaneity lines get "tilted" toward the line x=t representing light speed. The slope of the simultaneity lines can get arbitrarily close to the slope of the line x=t. So clearly, if u is large enough, there will be a simultaneity line that is "below" A and "above" B, meaning that in that guy's (comoving inertial) coordinate system, B is the earlier event.

Since the time axis is drawn in the "up" direction in a spacetime diagram, the world line of an object with speed u have slope dt/dx=1/u. If that object is the spaceship containing the observer, then its speed is <1 and its simultaneity lines make the same angle with the x axis as its world line makes with the t axis. So the slope of the simultaneity lines is 1/u.
Phew2... I think the 'magic' word here is tachyon, a hypothetical subatomic particle... I have absolutely no problem with that, and we can easily use 'common sense' to understand that there is nothing strange at all seeing a superluminal tachyon arriving at the end point, before we see it leaving the starting point. A no-brainer! :approve:

Also we can thank "the old one" for that macroscopic objects like Alice and her new spaceship is not allowed to travel faster than light, according to Einstein... Bob would have to spend all his free time at the dentist, repairing his jaw... :smile:

If I may become a little bit 'picky'; I would like to add that we would probably be able to distinguish the "real Alice" from her "mirage-redshifted-image", and by this we would be able to conclude that she is actually moving in the direction from the South Pole to the North Pole... :rolleyes:

http://en.wikipedia.org/wiki/Tachyon" [Broken]

Because a http://en.wikipedia.org/wiki/Tachyon" [Broken] always moves faster than light, we cannot see it approaching. After a tachyon has passed nearby, we would be able to see two images of it, appearing and departing in opposite directions. The black line is the shock wave of Cherenkov radiation, shown only in one moment of time. This double image effect is most prominent for an observer located directly in the path of a superluminal object (in this example a sphere, shown in grey). The right hand bluish shape is the image formed by the blue-doppler shifted light arriving at the observer—who is located at the apex of the black Cherenkov lines—from the sphere as it approaches. The left-hand reddish image is formed from redshifted light that leaves the sphere after it passes the observer. Because the object arrives before the light, the observer sees nothing until the sphere starts to pass the observer, after which the image-as-seen-by-the-observer splits into two—one of the arriving sphere (to the right) and one of the departing sphere (to the left).

That's an interesting question. The simplest possibility is that Alice doesn't exist anymore.
I hear you; my brain has visited this "place" several times, reading thru this thread... :cry:

The serious (and perhaps simplest) answer is that Alice would cease to exist long before going > c. She have to accumulate infinite energy and would in this process acquire infinite mass = squashed in the heaviest 'black hole' of all times...

the answer to your question must be that Alice is at the north pole.
...
the answer to your question must be that Alice is at the south pole.
Now we’re talking! Thanks buddy!! :grumpy:

(:wink:)


P.S. If 'anyone' wishes to ask the question; "Does this mean that we can travel backwards in time?" the answer is as always NO.
 
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tiny-tim

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… let’s skip the advanced math for awhile…
it's not advanced, it's very elementary math :frown:see the link
…, and use our common sense …
ok, and your common sense should tell you that Alice cannot go faster than light

any argument based on the "common sense" concept of tachyonic Alice ageing is founded on nonsense, not common sense :redface:

(how would you apply the same "common sense" argument to a tachyonic particle without a concept of ageing? :confused:)
 

DevilsAvocado

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it's not advanced, it's very elementary math :frown:see the link


ok, and your common sense should tell you that Alice cannot go faster than light

any argument based on the "common sense" concept of tachyonic Alice ageing is founded on nonsense, not common sense :redface:

(how would you apply the same "common sense" argument to a tachyonic particle without a concept of ageing? :confused:)

With all due respect, I think you missed the main point:

where is Alice actually located, the North Pole or the South Pole?

Please feel free to exchange "Alice" to anything that pleases you; "object", "something", tachyon, etc. The important question here is; how one "FTL object" can be detected/observed at two different end points, depending on the frame of reference?

I have never heard anything like it, and I would appreciate it very much if you could (at least in principle) describe how this works (i.e. more than "do the maths").

If you still refers to "do the maths" and this http://en.wikipedia.org/wiki/Velocity-addition_formula#Special_theory_of_relativity", it looks to me that you may not have the complete answer for this very simple question.

(Don’t worry about the conversation between me & Fredrik, there are some 'silly jokes' there, just for fun nothing more.)
 
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tiny-tim

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where is Alice actually located, the North Pole or the South Pole?
Please feel free to exchange "Alice" to anything that pleases you; "object", "something", tachyon, etc. The important question here is; how one "FTL object" can be detected/observed at two different end points, depending on the frame of reference?
i'm sorry, i don't understand what the question is, nor what the difficulty is :redface:

can you spell it out, please? :confused:
 

tiny-tim

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hmm … how about i ask you a question instead …

i shine a laser beam at the the moon, so that the spot travels across the moon's surface faster than light (that's very easy! :smile:) …

i say it travels from A to B faster than light, another observer says it travels from B to A faster than light …

where's the difficulty? :confused:
 

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