Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Can a virtual particle create another virtual particle?

  1. Nov 25, 2011 #1
    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?
  2. jcsd
  3. Nov 26, 2011 #2


    User Avatar
    Gold Member

    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.
  4. Nov 29, 2011 #3
    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?
  5. Nov 29, 2011 #4
    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.
  6. Nov 29, 2011 #5


    User Avatar
    Gold Member

    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.
  7. Nov 29, 2011 #6


    User Avatar
    Science Advisor

    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.
  8. Nov 29, 2011 #7
    Why forget virtual particles?
    A basic question: Has any virtual particle ever been detected?

    I skimmed Wikipedia here....

    and it confirmed a few things I thought I knew:

    and some I don't understand:

    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"?
  9. Nov 29, 2011 #8
    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:

    but so far, he writes, no one has been ablke to calculate those coupling constants in the five main string theories...as of 1999.
  10. Nov 29, 2011 #9
    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.
  11. Nov 30, 2011 #10
    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. :)
  12. Nov 30, 2011 #11
    There is nothing in string theory that "defys conservation of energy.....Bill K's explanation

    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:

    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.
  13. Dec 5, 2011 #12
    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.
  14. Dec 5, 2011 #13
    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."

  15. Dec 5, 2011 #14

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    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.
  16. Dec 5, 2011 #15
    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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook