Gordon Kane, director of the Michigan Center for Theoretical Physics at the University of Michigan at Ann Arbor, provides this answer.
Virtual particles are indeed real particles. Quantum theory predicts that every particle spends some time as a combination of other particles in all possible ways. These predictions are very well understood and tested.
Quantum mechanics allows, and indeed requires, temporary violations of conservation of energy, so one particle can become a pair of heavier particles (the so-called virtual particles), which quickly rejoin into the original particle as if they had never been there. If that were all that occurred we would still be confident that it was a real effect because it is an intrinsic part of quantum mechanics, which is extremely well tested, and is a complete and tightly woven theory--if any part of it were wrong the whole structure would collapse.
But while the virtual particles are briefly part of our world they can interact with other particles, and that leads to a number of tests of the quantum-mechanical predictions about virtual particles. The first test was understood in the late 1940s. In a hydrogen atom an electron and a proton are bound together by photons (the quanta of the electromagnetic field). Every photon will spend some time as a virtual electron plus its antiparticle, the virtual positron, since this is allowed by quantum mechanics as described above. The hydrogen atom has two energy levels that coincidentally seem to have the same energy. But when the atom is in one of those levels it interacts differently with the virtual electron and positron than when it is in the other, so their energies are shifted a tiny bit because of those interactions. That shift was measured by Willis Lamb and the Lamb shift was born, for which a Nobel Prize was eventually awarded.
Quarks are particles much like electrons, but different in that they also interact via the strong force. Two of the lighter quarks, the so-called "up" and "down" quarks, bind together to make up protons and neutrons. The "top" quark is the heaviest of the six types of quarks. In the early 1990s it had been predicted to exist but had not been directly seen in any experiment. At the LEP collider at the European particle physics laboratory CERN, millions of Z bosons--the particles that mediate neutral weak interactions--were produced and their mass was very accurately measured. The Standard Model of particle physics predicts the mass of the Z boson, but the measured value differed a little. This small difference could be explained in terms of the time the Z spent as a virtual top quark if such a top quark had a certain mass. When the top quark mass was directly measured a few years later at the Tevatron collider at Fermi National Accelerator Laboratory near Chicago, the value agreed with that obtained from the virtual particle analysis, providing a dramatic test of our understanding of virtual particles.
Another very good test some readers may want to look up, which we do not have space to describe here, is the Casimir effect, where forces between metal plates in empty space are modified by the presence of virtual particles.
Thus virtual particles are indeed real and have observable effects that physicists have devised ways of measuring. Their properties and consequences are well established and well understood consequences of quantum mechanics.
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21 Comments
Add CommentIf matter and antimatter anilate themselves after popping out, why can we not detect the energy that is given out?
Reply | Report Abuse | Link to thisThe theory that particles pulse in and out of existence is an odd theory. Could the particle be shrinking to a point where we cannot see it and then expanding again to a size that we can view? That would certainly create a push and pull throughout.
Reply | Report Abuse | Link to thisDon't be fooled by the statements in the article. Nearly all "particles" are artifacts of test equipment and the need of researchers to earn grants and awards for greater and greater complexities and discriptions in their theories. That is why "particles" are interchangable. Only Protons are real things, and atoms are made up of protons and their electron shells. Electrons and all other "particles" are interchangable as has been shown in many test experiments. An Electron shell is a charge field not an orbiting electron "particle".
Reply | Report Abuse | Link to thisA neutron is a hydrogen atom with an electron shell in a compressed condition, and is slightly larger then a proton and has slightly more mass then a hydrogen atom. This lead to the first totally invented none existent particle, the neutrino, to cover up the error in thought with more B.S. and earn the "inventor" greater standing with his peers. Much later test equipment gave hints of something that almost matched the theories discriptions so everyone had more bragging rights for finding the much sought particle and "proved" the theories.
Today the discription of a dependable test equipment artifact "particle"can earn lifetime fame and fortune.
The discription of the "electron" as a real particle was the first error of quantum machanics and everything else has been built on that.
Dark matter / Dark energy is the primal building block of the hydrogen1 atom and everything else. All "quanta" are test equipment measurements of the same thing. That is why all quanta are interchangeable, one changing to another. Just different test results at different points on the test track. This has been known for at least 40 years that I know of, but there is not much academic profit in that.
Dark matter / Dark energy, two word descriptions of the same thing, Aether. Aether is supramagnetic, ( easily influenced by magnetic fields) has charge, ( an excess charge is negitive, a lack of charge is positive) and is in chaos (travels or has movement in 3 dimensions, causes the effects of inertia / mass and the transport of EMF (photonic)energy. Sorry no electrons,photons,neutrinos, mesons, glueons, gravitons and ons and ons. No need for Nobel prizes for yearly B.S. of greater complexities.
Reply | Report Abuse | Link to thisEnter Your Comment Here.
Reply | Report Abuse | Link to thisA lot of money has gone into understanding the atomic and sub-atomic world. It seems there are more and more elaborate explainations for what is the ulltimate particle. I thought the old adage of keep it simple should be applied. Occums razor says the simplest answer is probably the right one. I'm sure we will all be surprised when the real answer is found.
Reply | Report Abuse | Link to thisWhen it comes to dark energy and dark mater, it sounds like the old "ether" answer, when mysterious things needed an explaination people could grasp. It wouldn't be the first time misunderstanding results of experiments lead to the wrong conclusions. I guess I'll just have to watch the fireworks from the sidelines, I don't have a collider at home.
Back in the "good old days", 1955 - 1965, some of my peers built acelerators and colliders in their fathers garages. later they built larger machines on the government's dime, same ideas just larger toys.
Reply | Report Abuse | Link to thisIn my opinion they are searching for the constituents of matter in the manner of smashing marbles to see what they are made of. You get shards of different colors and size of glass to classify, but you don't learn that glass is made of metal oxides.
There has been a lot of information developed over the last 200 years, what is needed is to re-examine all the facts and repostualate what will fit all that is known.
The article above presents both virtual particles and quarks as real entities. As an independent thinker, I'm not convinced. All "particles" as far as I can fathom, are actually fields, probably magnetic in character and spinning or oscillating at some frequency. The electron, in my view, has no edge and therefore doesn't need to send out virtual photons to communicate with other charged particles. It is already in contact with them. However, the attributes of its own infinite structure could be modeled as virtual entities.
Reply | Report Abuse | Link to thisMagnetman is a lot closer to reality then most educated savants.
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Reply | Report Abuse | Link to thisUnfortunately, the article fails to give references to support
the claim that virtual particles are indeed real particles.
There is not a single verified measurement of a virtual particle.
Measured particles are always on-shell.
The tests mentioned are not tests of the reality of virtual
particles, but tests of QED and the standard model in general,
together will talk about the behavior of virtual particles that
may be a common fantasy of many physicists, but has no support at
all by experiments.
The Lamb shift and the Casimir effect can be calculated in a number
of different ways, not all requiring the concept of virtual particles.
How can it then be experimental proof of the latter?
Even when virtual particles are involved, their alleged properties
are very different in different approximation techniques for the same
quantum field theory. For example, the virtual particles in light
front calculations have nothing in common with the virtual particles
in the standard covariant presentations of perturbation theory.
On the Z-boson, the article writes, ''the value agreed with that
obtained from the virtual particle analysis, providing a dramatic
test of our understanding of virtual particles.''
This virtual particle analysis (when followed through - no
references are given) consists in perturbative calculations
that start with Feynman diagrams involving virtual particles.
But these are unmeasurable things with an infinite mass and
infinite interactions, both physically meaningless. They are turned
into predictions only by a perturbative renormalization process
that restores physics to the ill-conceived bare interpretation.
After renormalization, when everything is finite, there is not
the smallest trace left of virtual particles. Moreover, modern
nonperturbatives renormalization techniques via lattice gauge
theory or similarity flows give comparable predictions for quantum
field theories, without encountering virtual particles anywhere.
If virtual particles were not just mathematical artifacts, the following
three questions would need a convincing answer:
- Why then do they behave so differently depending on the perturbation
scheme used?
- Why then is the mass of virtual electrons infinite?
This has never been observed.
- Why then do the values of all diagrams in QED involving virtual
particles come out infinite?
The only places where one can assign virtual particles some
physical meaning (though I think even this is misguided) are
- (i) in nonrelativistic quantum mechanics, where electromagnetic
fields are represented either by explicit Coulomb terms or by
external e/m potentials, and there are no photons,
- (ii) in a tree approximation to relativistic QFT; but this is
not enough for any of the great successes of QED.
Coulomb forces, Casimir forces, and the like (i.e., electromagnetic
fields) are real, the photons used to describe them in perturbation
theory cannot be.
Photons are transverse excitations of the e/m field, i.e.,
deviations from the mean value of the field. To consider the
Coulomb field, say, as being composed of virtual photons is a
superficial misunderstanding of quantum field theory.
None of the many very successful calculations comparing theory with
experiment depends on the reality of such virtual particles.
For more details, see Chapter A7 of the theoretical Physics FAQ at
http://www.mat.univie.ac.at/~neum/physfaq/physics-faq.html
I have a concern regarding virtual particles arising from the uncertainty principle in that there appears a certain base from which these particles arise. For example if electron/positron pairs can arise from the vacuum , each member having positive mass and energy, it would seem only fitting that an equal number of other pairs having negative mass and negative energy should be allowable by the same means. Though I would not venture to guess what negative mass and negative energy are, I should think it safe to assume that their actions on the whole would exactly counteract those of the positive variety. With virtual particles thus producing net zero effect (direct or otherwise), by Occam the theory disappears back from whence it came.
Reply | Report Abuse | Link to thisI have a concern regarding virtual particles arising from the uncertainty principle in that there appears a “certain” base from which these particles arise. For example if electron/positron pairs can arise from “the vacuum”, each member having positive mass and energy, it would seem only fitting that an equal number of other pairs having negative mass and negative energy should be allowable by the same means. Though I would not venture to guess what negative mass and negative energy are, I should think it safe to assume that their actions on the whole would exactly counteract those of the positive variety. With virtual particles thus producing net zero effect (direct or otherwise), by Occam the theory disappears back from whence it came.
Reply | Report Abuse | Link to thisThe question may have referred to the reality of vacuum fluctuations, in which all of space has the property that there is a non-zero expectation value of particle-antiparticle pairs popping into and out of existence. Both Feynman and Schwinger proved that creation destruction operators in QFT are a mathematical device, and that virtual particles always require sources. Hence empty space cannot support vacuum energy.
Reply | Report Abuse | Link to thisI was asked recently about the reality of VPs. Here's part of my answer:
Reply | Report Abuse | Link to thisThe question of their existence, in my "logical" view of science, misrepresents the type of questions science is capable of answering. Science takes our observations (the closest science gets to saying "this is true reality" is a repeatable observation) and develops models that can explain and predict the observations. Science does NOT have to say "those models truly describe actual reality!" In fact, if it does, it could very well be proven wrong. Newton's theory of gravity was replaced by Einstein's. Newton's theory is still used in many cases because its math gives us really close approximations and is much simpler to use. But observations tell us that Newton's physical description of gravity cannot be actual reality.
SO, what IF some new observation one day proves an effect that simply cannot be explained with virtual particles? We would be arrogant and foolish to think that can never happen. A new theory may arise that replaces the concept of VPs. If I vehemently say "virtual particles MUST, MUST, MUST exist and it is impossible to deny them!" and then tomorrow a new observation is made that VPs cannot account for, I would look like a fool! I'd be proven to be speaking nonsense and NOT scientifically about the subject. Since science should never discount the possibility of future discovery, science shouldn't make claims about the absolute undeniable reality of its models. The power of science is not in claiming that the unobservable mechanisms it's model's employ must reflect the absolute truth of reality. It's power is in claiming that, to the extent it's models work, they provide undeniable utility.
Now, if VPs DO exist as real, physical entities, their strangeness make it hard (perhaps impossible) for us to imagine their reality. How would the physical existence of VPs somehow account for every possible combination of virtual particle interactions between two "real" particles and then determine how those particles interact?
The simple answer to the question "are VPs real" is (once you understand what I've said above) "who cares!" If they are NOT real, the QM model that uses them still works to predict the things we want it to predict--considering them as "real" in a model is totally utilitarian and undeniably useful. If they ARE real, then we can't directly observe them to "prove" it; we can't use them for anything other than making predictions about their directly observable consequences!
More interesting is the theory that the existence of virtual particles is what keeps electrons from falling into the nucleus. This theory has received some confirmation with the discovery of the Lamb Shift.
Reply | Report Abuse | Link to thisOne can detect the radiation given off when particles annihilate. Great examples of this are lightning bolts and exposed potassium 40 (if I recall correctly). Potassium 40 in high concentration offers visible light when the positron encounters the electron. The Fermi satellite has detected such particle interactions via the gamma-ray outputs along earth's magnetic field lines.
Reply | Report Abuse | Link to thisMore interesting is the idea that the particle does not really exist. The particle itself could just be a "field" that is detected. That field could be energy trapped or rotating in a given state. This could explain why two anti particles unravel themselves into gamma radiation. In short, the particle is only experienced because we detect the effect of the field, not that it is anything other than energy in a given state occupying empty space.
Reply | Report Abuse | Link to thisI would consider the idea that the particle itself is just be a "field" that is detected. That field is energy trapped or rotating in a given state. This could explain why two anti particles unravel themselves into gamma radiation. The particles themselves then can be evaluated in terms of their total energy output (E=MC2). In short, the particle is only experienced because we detect the effect of the field, not that it is anything other than energy in a given state occupying empty space.
Reply | Report Abuse | Link to thisHypothesis: No single object can occupy both space and time at different locations simultaneously. If a particle diverges then they become two separate particles not one particle in two places with the same mass for each particle. The reconvergence of the two particles that then has a higher mass as implied by the author. Where has the extra mass come from?
Reply | Report Abuse | Link to thisThis article includes in its header the following: "Quantum theory predicts that every particle spends some time as a combination of other particles in all possible ways. These predictions are very well understood and tested. "
Reply | Report Abuse | Link to thisThis shows that the writer of the header is practically logically illiterate. He is probably a professor of physics. Quantum theory is expressed in terms of virtual adventures of actual particles, but such expressions are not predictions in a physically real sense; they are explanatory constructions. But it sounds more impressively "scientific" to say that such expressions are "predictions", so a person illiterate in logic, wanting to sound scientific and advertise quantum theory, will hardly hesitate to say so. It is true that the really physical predictions of quantum theory are experimentally tested and verified, but the explanatory constructions are not the really physical predictions. It is true that the explanatory constructions are well understood, but that does not make them really physical predictions.
I'm just an old neophyte, but without references I can't confirm your statements:
Reply | Report Abuse | Link to this"Both Feynman and Schwinger proved that creation destruction operators in QFT are a mathematical device, and that virtual particles always require sources. Hence empty space cannot support vacuum energy."
It seems that whatever they might have proved in the context of quantum field theory might not apply in other contexts.
The source of virtual particles might be vacuum energy. As I understand, the existence of energy is only inferred from the material effects it produces. In the absence of matter, vacuum energy may be largely undetected, except perhaps as the dissipating force that produces the unexplained physical metric expansion of spacetime.
It seems to me that if transient manifestations of virtual particles physically represent a rarely detected underling energy source (vacuum energy), they and potentially related phenomena may offer some substantiation. Please see:
http://en.wikipedia.org/wiki/Virtual_particle#Manifestations
It asserts that "There are many observable physical phenomena that arise in interactions involving virtual particles" and offers a dozen or so examples of "... field interactions which may be seen in terms of virtual particles ..."