What are virtual particles in quantum mechanics?

Virtual particles are temporary particles that exist briefly during particle interactions in quantum field theory.

In the realm of quantum mechanics, virtual particles are a fascinating and somewhat perplexing concept. They are not particles in the traditional sense, but rather fluctuations in a field that occur for a very short time. These particles are 'virtual' because they violate the energy-time version of the Heisenberg Uncertainty Principle, which states that the more precisely one property (like position) is known, the less precisely the other (like momentum) can be known.

Virtual particles are integral to the process of particle interactions in quantum field theory. They are often visualised using Feynman diagrams, which are graphical representations of the mathematical expressions describing the behaviour of subatomic particles. In these diagrams, virtual particles are represented as internal lines.

For example, consider the interaction between two electrons. According to classical physics, the electrons repel each other by exchanging a photon, the carrier of the electromagnetic force. In quantum field theory, this interaction is described by a Feynman diagram in which the two electrons exchange a virtual photon. This virtual photon is not directly observable, but its effects can be measured in the form of the electromagnetic force between the electrons.

Virtual particles also play a crucial role in the phenomenon of quantum tunnelling, where a particle can pass through a potential barrier that it would not be able to surmount according to classical physics. This is possible because the Heisenberg Uncertainty Principle allows for temporary violations of energy conservation, which can be thought of as the creation of a pair of virtual particles – one with positive energy and one with negative energy. The positive-energy particle can then tunnel through the barrier.

In summary, virtual particles are a fundamental concept in quantum mechanics, providing a framework for understanding particle interactions and quantum phenomena that defy classical physics. They may be fleeting and unobservable, but their effects are very real and have been confirmed by numerous experiments.