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Virtual Particles

uncertainty short principle energy

Virtual particles are subatomic particles that form out of "nothing" (vacuum fields conceptually analogous to lines of force between magnetic poles) for extremely short periods of time and then disappear again. Such particles permeate space, mediate particle decay, and mediate the exchange of the fundamental forces (electromagnetic, weak, strong, and—in accord with quantum theory—gravititational forces). Virtual particles are real and have measurable effects, but the same uncertainty principle that allows them to come into existence dictates that they cannot be directly observed.

Heisenberg's uncertainty principle, which explains the virtual particle phenomenon, is most commonly stated as follows: It is impossible to exactly and simultaneously measure both the momentum and position of a particle. There is always an uncertainty in momentum and an uncertainty in position. More importantly, these two uncertainties cannot be reduced to zero together.

One consequence of Heisenberg's uncertainty principle is that the energy and duration of a particle are also characterized by complementary uncertainties. There is always, at every point in space and time, even in a perfect vacuum, an uncertainty in energy and an uncertainty in duration, and these two complementary uncertainties cannot be reduced to zero simultaneously.

The meaning of Heisenberg's uncertainty principle is that "something" can arise from "nothing" if the "something" returns to the "nothing" after a very short time—an interval too short in which to be observed. These micro-violations of energy conservation are not only allowed to happen, they do, and so "empty" space is seething with particle-antiparticle pairs that come into being and then annihilate each other again after a very short interval. Although these particles cannot be observed individually, their existence can be demonstrated.

Normally, a metal plate experiences a storm of fleeting impacts from virtual particles on both of its surfaces; this "vacuum pressure" is equal on both sides of the plate, and so cancels out. If, however, two parallel metal plates are too closely spaced to allow the formation of relatively large virtual particles between them, the vacuum pressure between the plates is less than that on their outer surfaces, and they experience a net force pushing them together. This force is termed the Casimir effect after Dutch physicist Hendrik Casimir (1909–2000), who predicted its existence in 1948, and was experimentally measured in 1997.

The Casimir effect is only one manifestation of the reality of virtual particles. Virtual particles also mediate the exchange of all forces between particles. For example, when an electron experiences electrical repulsion from another electron (electrons are negatively charged, and like charges repel), it is actually exchanging virtual photons with that other electron. Higher-energy virtual photons are only allowed by the uncertainty principle to exist for shorter periods of time, as shown by the uncertainty equation, and thus cannot travel as far as lower-energy virtual photons; this explains why the electric force is stronger at short distances. (In fact, all the basic forces—electric, strong, weak, and gravitational—diminish with distance for this reason. Gravity, however, has not been satisfactorily integrated with the equations that describe the other three forces.)

A third role for virtual particles is in decay mediation. When an unstable subatomic particle decays (i.e., breaks down into two or more other subatomic particles), it does so by first taking the form of a virtual particle. The virtual particle then completes the decay process. In some cases, the intermediate virtual particle has more mass than the initial particle or the final set of decay products; this does not violate the conservation of mass because the intermediate particle is virtual, that is, exists for such a short period of time that it falls within the uncertainty bounds prescribed for the system's energy by the Heisenberg uncertainty principle.

This list of phenomena does not describe all the properties of virtual particles, but does indicate their prevalence.

See also Quantum mechanics.


Resources

Books

Barnett, R. Michael, Henry Mühry, and Helen R. Quinn. The Charm of Strange Quarks. New York: Springer-Verlag, 2000.

Ne'eman, Yuval, and Yoram Kirsh. The Particle Hunters. Cambridge, UK: Cambridge University Press, 1996.

Other

Lambrecht, Astrid. "The Casimir Effect: A Force From Nothing." PhysicsWeb. September 2002 [cited February 14, 2003]. <http://physicsweb.org/article/world/15/9/6>.


Larry Gilman

Virtual Reality - Historical Overview, Cultural Implications, Bibliography [next]

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over 3 years ago

The Inverse-Square Law, Gauss' Law, etc, do not require virtual particles in order to work. Ergo, forces diminishing with distance can occur without virtual particles.

The same goes for the Casmir Effect, as there are alternative explanations for the Casmir effect that do not require virtual particles.

Also, the genesis of Heisenberg's Uncertainty Principle has nothing to do with virtual particles; virtual particles originate from mathematical steps necessary to complete particle decay calculations. The HUP simply states that there are properties in quantum systems that are related in such a way that the more you know (measure) about one properly the less you can know (measure) about the other.

Virtual particles are like virtual apples: just in your head! We can add 3 apples and 4 apples to get 7 apples and we can also say that the 4 apples are in a way really 6 apples and 2 virtual apples where the 2 virtual apples cancel out 2 of the 6 real apples and you are left with only 4 real apples...yara yara yara.

Finally, space is not empty: it contains itself or space which is something or "some thing." Therefore, there is no case where something is coming from nothing or "no thing." True nothingness or no thing is a meaningless contradiction and therefore does not exist.

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over 3 years ago

Why do you think these particles just appear. By what you discribe the particles are there and there movement is blocked by the plates. You can not see them to know but yet you choose to believe what you can not see. Sounds just like a person who believes in God...

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over 1 year ago

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over 1 year ago

The Samsung Galaxy S4 manages to be more than just a very slim device with an amazing 5-inch 1080p display and impressive chips, it also packs a whopping 2600mAh battery. But how long can it last in real life? GSM Arena has done some extensive testing of the S4 battery and the results came in impressive.

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over 1 year ago

great