Can a virtual particle create another virtual particle?

I can see the answer being no for large particles, but what about strings? What stops them from expanding to make up the universe as we know it?
 

arivero

Gold Member
3,284
51
Forget virtual particles, think (and google for) off-shell and on-shell particles. That will help you to fix some of your ideas on it. Something was very wrong about explaining virtual particles in the nglish-speaking world time ago, and a lot of people get it wrong. Perhaps it was some kind of TV program in the nineties, perhaps it is just the semantic meanings of "virtual" in English.

So, if you look at ir with the right angle, virtual particles can create anything, real of virtual, and it only depends of which part of its energy is unaccounted and which part of its energy is real. The point about virtuality is that they incur in an energy debt E over the real energy available, and this phenomena can not be sustained beyond a time t=h/E, and then it can not reach a distance beyond hc/E.
 
1,283
0
Forget virtual particles, think (and google for) off-shell and on-shell particles. That will help you to fix some of your ideas on it. Something was very wrong about explaining virtual particles in the nglish-speaking world time ago, and a lot of people get it wrong. Perhaps it was some kind of TV program in the nineties, perhaps it is just the semantic meanings of "virtual" in English.

So, if you look at ir with the right angle, virtual particles can create anything, real of virtual, and it only depends of which part of its energy is unaccounted and which part of its energy is real. The point about virtuality is that they incur in an energy debt E over the real energy available, and this phenomena can not be sustained beyond a time t=h/E, and then it can not reach a distance beyond hc/E.
Well said. Think of it as borrowing energy and giving it back, but in a really short time.

arivero, is the equation really that simple and elegant, it exists for a time proportional to the planck length divided by energy?

Energy of what specifically, and is the time uncertain?
 
Thank's arivero, thats a good point, I didn't even consider virtual particles having real parts. I'm still bothered by this though. Wouldn't that mean that any particles propogated from the original particle only travel a finite distance. That being the case, it makes me wonder how gravity obeys the conservation of energy. We know that mass "absorbs" higgs, which means they can't be emitted by mass, and if they were emmited by virtual particles which propogated from mass, then gravity would be like a cloud hovering over an object. From this logic, you would have to say that higgs are created by fields in space. The rate they are created equal to the rate they are being "absorbed" since gravity is a constant. However the deepest points in space would be required to create the most higgs to keep the gravitational constant. The contradiction lies in that the deepest points in space are supposed to have the least amount of energy to do this.
 

arivero

Gold Member
3,284
51
Well said.
...
Energy of what specifically, and is the time uncertain?
incerto tempore ferme incertisque locis spatio

Energy, that of the relativistic equation, E^2=sqrt(m^2c^4+p^2c^2), where m is the rest mass of the particle and p its momentum.
 

Bill_K

Science Advisor
Insights Author
4,155
194
I'll see if I can explain it without resorting to Latin. :smile:
Think of it as borrowing energy and giving it back, but in a really short time.
Please do not think of it that way. The idea that the universe maintains a "loan desk" which one can borrow from and later pay back is a very colorful concept but far from the truth.

Energy is conserved exactly at all times, even by virtual particles. When arivero mentioned an "energy deficit", what he meant was this: A free particle "on the mass shell" must obey the relativistic equation E2 = p2c2 + m2c4. A virtual particle "off the mass shell" is not so constrained. Its values of E and p may be anything. However the farther it departs from the mass shell relationship, the shorter its existence.
 
5,598
39
Why forget virtual particles?
A basic question: Has any virtual particle ever been detected?

I skimmed Wikipedia here....
http://en.wikipedia.org/wiki/Virtual_particles

and it confirmed a few things I thought I knew:


If a single particle is detected, then the consequences of its existence are prolonged to such a degree that it cannot be virtual......The amplitude that a virtual particle exists interferes with the amplitude for its non-existence; whereas for a real particle the cases of existence and non-existence cease to be coherent with each other and do not interfere any more.....In the quantum field theory view, "real particles" are viewed as being detectable excitations of underlying quantum fields......
and some I don't understand:


In the quantum field theory view, "real particles" are viewed as being detectable excitations of underlying quantum fields. As such, virtual particles are also excitations of the underlying fields, but are detectable only as forces but not particles. They are "temporary" in the sense that they appear in calculations, but are not detected as single particles.
I never heard of any virtual particle being 'detected'.....that is observed.

This is closer to my limited unerstanding and seems inconsistent with the prior quote:

Thus, in mathematical terms, they never appear as indices to the scattering matrix, which is to say, they never appear as the observable inputs and outputs of the physical process being modelled. In this sense, virtual particles are an artifact of perturbation theory, and do not appear in a non-perturbative treatment
Is this correct:

There are many observable physical phenomena resulting from interactions involving virtual particles.
Do they really mean "physical phenomanea which in theory result from virtual particles"?
 
5,598
39
Regarding strings: Since string theory is based on perturbation theory, you'd expect virtual "particles" to pop up and you'd be right.


A virtual string pair, a string and anti string pair, would have exactly opposite vibrational patterns.

Brian Greene in THE ELEGRANT UNIVERSE pages 289-295 describes:

...quantum jitters can cause momentary virtual string eruptions to occur any number of times producing a sequence of virtual string pairs....The size of the string coupling constant describes how strongly the quantum jitters of the initial loop and the two virtual loops into which it splits are coupled to one another....the larger the string coupling constant the more likely it is that quantum jitters will cause an initial string to split apart.....the smaller the constant the less likely it is for such virtual strings to erupt momentarily into existance.....
but so far, he writes, no one has been ablke to calculate those coupling constants in the five main string theories...as of 1999.
 
Thanks naty1. I'd been pondering these things for a while, and I knew I couldn't be the only one thinking them.

Is it the case that if these coupling constants are large, and that strings abruptly split apart, that they defy the conservation of energy? Is it also possible that other particles would have this capability?

If so, then physicist would be wrong to state that vacuum energy obeys the conservation of energy, and that with sufficient accelleration it would be possible to create overunity. By varying the acceleration, you would be able to determine the coupling constants from larger particles to smaller particles.
 
1,283
0
I'll see if I can explain it without resorting to Latin. :smile:

Please do not think of it that way. The idea that the universe maintains a "loan desk" which one can borrow from and later pay back is a very colorful concept but far from the truth.

Energy is conserved exactly at all times, even by virtual particles. When arivero mentioned an "energy deficit", what he meant was this: A free particle "on the mass shell" must obey the relativistic equation E2 = p2c2 + m2c4. A virtual particle "off the mass shell" is not so constrained. Its values of E and p may be anything. However the farther it departs from the mass shell relationship, the shorter its existence.
I actually apologize because I personally quiver when terms get thrown out like that, its almost an insult to physics and mathematics. Thanks for explaining it to me, I should stay away from those documentaries! Luckily, I'll get to all those neat information as I pursue my PhD. :)
 
5,598
39
....and that strings abruptly split apart, that they defy the conservation of energy?
There is nothing in string theory that "defys conservation of energy.....Bill K's explanation

Energy is conserved exactly at all times, even by virtual particles.
is as far as I know correct from a classical viewpoint....from a quantum viewpoint I think
things are a bit more blurred. Anything that destroyed Noether's Theorem validity would be a MAJOR upset to our understanding of the universe.

Check here:
http://en.wikipedia.org/wiki/Conservation_of_energy#Noether.27s_theorem

I do not know the mathematical interpretations involved but in my own layman's terms time at the Planck scale is unlikely continuous...so it seems time and distance details are "lost" in quantum foam...essentially how virtual particles can be produced even for brief 'instantaneous' moments remains a quantum mystery...an untested theory.

However, the source of mass, time, energy, gravity, "real" particles, etc also is also unknown so I don't see a special mystery about the origins of virtual particles being unclear.
 
351
4
Why forget virtual particles?
A basic question: Has any virtual particle ever been detected?

I never heard of any virtual particle being 'detected'.....that is observed.

This is closer to my limited unerstanding and seems inconsistent with the prior quote:

Is this correct:

Do they really mean "physical phenomanea which in theory result from virtual particles"?
They have not been detected, their existence as anything more real than, say, the GNP, which is nothing more than an ensemble of economic factors, has been ably disputed by skeptics, and the 'evidence' most readily proffered by the big-time boffins (who are afraid of coming on PF and facing the counter-arguments) is the Casimir effect, which has been derived without VPs.

So, to ask if VPs can spawn other VP's is interesting metaphysics at best. Another metaphysics is Hawking radiation, which has been derived by appeal to the conjectured VPs plus 'black holes' for which, once again, there has been no direct evidence as yet.

Quite the hat-trick then: two purported entities responsible for a second-order phenomenon of dubious existence.
 
They have not been detected, their existence as anything more real than, say, the GNP, which is nothing more than an ensemble of economic factors, has been ably disputed by skeptics, and the 'evidence' most readily proffered by the big-time boffins (who are afraid of coming on PF and facing the counter-arguments) is the Casimir effect, which has been derived without VPs.

So, to ask if VPs can spawn other VP's is interesting metaphysics at best. Another metaphysics is Hawking radiation, which has been derived by appeal to the conjectured VPs plus 'black holes' for which, once again, there has been no direct evidence as yet.

Quite the hat-trick then: two purported entities responsible for a second-order phenomenon of dubious existence.
I don't blame people not coming to physics forum for the simple fact that everything they say will be scrutinized by someone who thinks they're smart. Everytime I post something on here thats "unorthodox" it gets flagged becuase people don't understand it.

That aside, I believe there is very strong evidence for hawking radiation and vacuum energy. This article shows an experiment where particles accelerate around a nanotube. It shows that these particles go from 0.1 Kelvin to thousands of degrees Kelvin in less than a microsecond.” Since the unruh effect (hawking radiation) gives an increase in tempature proportional to acceleration, it seems logical to assume that the tempature increase is due to hawking radiation.

The article goes on to talk about how "Professor Lene Hau became famous worldwide in creating the first experiments able to stop light . His experiences also formed the basis for other scientists to capture the “nothing” , or at least what physicists call vacuum condensate."




http://www.neno-tech-views.com/?p=1536#more-1536
 

Vanadium 50

Staff Emeritus
Science Advisor
Education Advisor
23,243
5,585
That aside, I believe there is very strong evidence for hawking radiation and vacuum energy. This article shows an experiment where particles accelerate around a nanotube. It shows that these particles go from 0.1 Kelvin to thousands of degrees Kelvin in less than a microsecond.” Since the unruh effect (hawking radiation) gives an increase in tempature proportional to acceleration, it seems logical to assume that the tempature increase is due to hawking radiation.
Seems logical? It's certainly not logical.
(All cows are brown. That dog is brown. Therefore that dog is a cow.)

If you don't like the responses to unorthodox posts, maybe you should stop doing that. And take another look at the PF Rules if you want to better understand why.
 
Seems logical? It's certainly not logical.
(All cows are brown. That dog is brown. Therefore that dog is a cow.)

If you don't like the responses to unorthodox posts, maybe you should stop doing that. And take another look at the PF Rules if you want to better understand why.
Theres this thing in psychology where a child sees an insect and her dad tells her its a spider. So whenever the child sees an insect she calls it a spider. With the given information she is actually making a logical statement. For instance if her dad said that spider was an insect, and later she called all insects, insects, then she would be correct. The fualt lies in the understanding of the definition.

I know I'm not an expert, and whenever I make a statement where I say something "seems" to be stating a fact, I am merely stating there are common attributes and there is a possibility of that argument being true. Probability is the foundation of QM. My goal was to hear what others thought about that topic and not be scoffed.
 

Related Threads for: Can a virtual particle create another virtual particle?

  • Posted
Replies
3
Views
1K
Replies
7
Views
3K
Replies
2
Views
1K
Replies
23
Views
5K
  • Posted
Replies
8
Views
2K
  • Posted
Replies
3
Views
2K
  • Posted
Replies
6
Views
2K

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving

Hot Threads

Top