As I understand it, if virtual particles do not recombine within the plank time they become 'real'.

This is proposed to happen in Hawking Radiation, where one virtual particle crosses the black hole's event horizon and the other does not, where it robs some energy from the black hole.

Lawrence Krauss also proposes in his talk 'A Universe from Nothing' that from nothing (meaning the QED Foam) with a net energy of zero, a universe can be born, presumably (if simplistically) 'creating' matter by 'borrowing' energy from the Foam.

These are both just proposed, but are there any other proposed (or demonstrated) mechanisms by which virtual particles 'become real'? Say extremely unlikely 'borrowings', or two close simultaneous events that repelled the particles before recombination, or small 'universe birth' events within our universe?

If it is indeed possible, to maintain the conservation of energy would there be some sort of 'negative energy' field left in the wake of the two created particles?

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    $\begingroup$ Virtual particles have no special relationship to the Planck time. The typical time associated with a virtual particle is the time after which it has to "return" the borrowed energy - the energy $\Delta E$ by which it deviates from the right energy dictated by the dispersion relations - and this $\Delta t$ is of order $\hbar /\Delta E$ or $\hbar / mc^2$. ... The pair creation making virtual particles real is a property of black holes and all event horizons. There's nothing "similar" but different we know. If you meant something specific what you would count as similar, explain what you mean. $\endgroup$ – Luboš Motl Jun 25 '13 at 6:09
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    $\begingroup$ I did include Krauss's proposed 'universe from nothing' as well, but really what I'm asking is; does the transition from 'virtual' particles to 'real' particles in a situation where there isn't an obvious energy source to rob like a black hole (also note that Hawking Radiation is a proposed mechanism, and may not exist). What I really want to get at is: if the particles DID become real, would they leave some sort of 'negative energy well' in the QED Foam where they formed? Could such a thing exist? $\endgroup$ – Ehryk Jun 25 '13 at 6:15
  • $\begingroup$ @Ehryk : See a nice explanation by Professor Steve Carlip $\endgroup$ – Trimok Jun 25 '13 at 9:09
  • $\begingroup$ Related: physics.stackexchange.com/questions/30597/… $\endgroup$ – John Rennie Jun 25 '13 at 9:42

This question involves the concept of "virtual particle" which was discussed a few days ago here. In a nutshell, a particle is virtual when it is a connecting line in a Feynman diagram between two vertices. It has all the quantum numbers of its name ( photon, electron, etc.) but not the mass, which is the measure of the four vector describing it. In that sense energy is not conserved with a virtual particle.

A virtual particle may continue out of the vertex and become "real", but a search for corresponding Feynman diagrams for Hawking radiation does not give any clear ones.

I found this heuristic one:


These loops are handwaved as coming "from the strong gravitational field in the hole .

This is the Feynman diagram of how pair creation is in the lab

pair creation

It is necessary to have a second interaction because the created pair has invariant mass while the photon has zero mass.

One can adapt the left diagram to the heuristic loops of the Hawking plot above.

As the unification of gravity with the other three forces is not done yet, the diagrams are a guess that is missing an incoming "photon, Z0, gluon .." and an outgoing "photon, Z0, gluon..." ( do not now if gravitons can make a pair directly, were they unified with the rest) as the real particle in the feynman graphs. We do not know whether this will really work, and thus it is heuristic. I did find a publication that states the problem of feynman graphs and gravity for Hawking radiation.Anyway lets call the particle a generic "graviton".

If we assume that the graphs work ,then a "graviton" creates an e+e- pair which recombines within the horizon to the same "graviton". For certain conditions at the horizon one of the pair interacts with another "graviton" which supplies the second vertex, and falls into the hole while the other continues free. In this sense virtual is the e+ of the picture, whose second vertex is a "graviton" in the hole while the electron goes on as real picking up the energy from the potential of the hole. Thus black holes evaporate eventually in this scenario. No need for negative energies.

  • $\begingroup$ Would this summation be accurate: "In an instance in which one member of pair production is taken out of the negative energy well, the well converts itself into providing mass of the remaining particle"? It seems a bit bizarre to me that it somehow 'chose' which particle to provide mass to, why couldn't it provide mass to the in-falling particle instead? $\endgroup$ – Ehryk Jun 25 '13 at 7:01
  • $\begingroup$ Well, the falling particle keeps on being virtual, by its interaction with the "graviton" within the hole. After that vertex, within the hole, it might become real or not depending if it keeps having interactions, i.e. vertices. It is a many body complicated problem, but the energetics should be simple, by energy and momentum conservation for the outgoing real. $\endgroup$ – anna v Jun 25 '13 at 7:09
  • $\begingroup$ But then, if the in-falling particle is/becomes real, wouldn't that just add to the energy of the black hole? Wouldn't it need to have negative energy, or negative mass, to 'rob' from the black hole and balance the conservation of energy? (and lead to the disintegration of the black hole?) $\endgroup$ – Ehryk Jun 25 '13 at 7:15
  • $\begingroup$ No. It does not work like tennis balls. As long as it is virtual there is no mass to talk about, and within the hole everything should essentially be virtual. The overall energy balance comes from the outgoing real one, which takes up energy and diminishes the total four vector of the black hole by leaving. $\endgroup$ – anna v Jun 25 '13 at 7:19
  • $\begingroup$ Okay, so you have a real particle, with mass, leaving. By what mechanism does the virtual particle 'steal' energy or mass from the black hole by falling into it? Does it annihilate a particle without a release of energy? The outgoing particle can't balance its own creation energy, or even if it did how would that affect the black hole whatsoever? $\endgroup$ – Ehryk Jun 25 '13 at 7:26

Yes, the Unruh-effect: accelerating detector in vacuum detect photons. But you could argue due to the equivalence principle its the same as Hawking radiation (although experimentally this would be quite different)


Vacuum fluctuations are the cause of the Casimir Effect, which has been demonstrated to be real. In it, two metal plates that are very close together undergo a force caused by the different ways in which virtual particles interact with the space between the plates, compared to outside them. If this effect is real, as it seems to be, then virtual particles certainly "exist" and hence Hawking Radiation and "A Universe from Nothing" become possible.

  • $\begingroup$ There are other explanations for the Casimir effect that also exist, as the Wikipedia article you linked will tell you. It therefore seems to be inadequate evidence for virtual particles. $\endgroup$ – A-B-B Feb 11 '14 at 2:39

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