Difference between a point particle and an extended particle

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Physics textbooks and educational websites still contain information saying that elementary particles are point particles. This is not entirely correct. Modern physics theories such as String Theory start with the axiom that all physical entities have spatial extension however small they may be. Even in SM (standard model of particle physics) the elementary particles have effective sizes even though for mathematical reasons they are considered as point particles. You may reject String Theory because it is not proven experimentally yet but the (experimentally proven) SM uses the concept of “virtual” particles surrounding the “real” particle creating an effective size for the “real” particle. The sea of virtual particle/antiparticle pairs surrounding the “real” particle is  known as “quantum vacuum.”

You can read the Fermilab info page on the point particle concept which says

“If you magnify an extended particle, it will look bigger. A point-like particle will not change in size, but the more closely you look at it, the stronger the field surrounding it becomes.”

“A point particle has no size, but it does have a field around it. The field gets stronger the closer you get to the particle. This field interacts with the particles in the quantum foam of empty space and orients them. In this manner, the point particle has influence in an extended way.”

The “quantum foam of empty space” mentioned above is actually the “quantum vacuum” which is explained in this web page as follows

quantum_vacuumThe spontaneous creation and annihilation of “virtual” charged particle/antiparticle pairs around a “real” charged particle is sometimes referred to as “vacuum polarization.” Here’s a nice explanation by Prof. Amanda W. Peet:

“Consider a lone electron, denoted by the dark blue disc in the cartoon picture below. The truth is that this lone electron is actually not alone. The electron sits in a vacuum: a sea of virtual particle-antiparticle pairs making fleeting appearances as allowed by Heisenberg Uncertainty. For the sake of expositional clarity, let us pretend for the moment that there are only electrons and positrons, to distill the issue to its purest form. Then our real electron (dark blue) is surrounded by virtual electron-positron pairs. Virtual electrons are denoted in the following cartoon by light blue discs, while virtual positrons a.k.a. anti-electrons are denoted by light yellow discs. (Note: colour is used in the cartoon just to keep track of which particle is which; it does not carry any physical significance.)”

“Using the rules for electrostatic repulsion/attraction, the virtual electron is repelled by the real electron while the virtual positron is attracted by it. So virtual pairs in the vacuum are oriented so that positive charges in the virtual pairs cluster nearer the real electron than the compensating negative charges in the virtual pairs. This creates charge separation and the effect is that the virtual dipoles screen some of the charge of the real electron.”

“The screening effect is very mild at low energies, because the virtual e-e+ pairs have a very fleeting lifespan. But the deeper you can dig nearer the real electron, the more of the real electron’s charge you can feel because less of it is screened from you. And because digging deeper requires more momentum for the probe (c.f. de Broglie wavelength from last week), this is why the electromagnetic force grows stronger at higher energies. This phenomenon is called running of the coupling.”

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About Suresh Emre

I have worked as a physicist at the Fermi National Accelerator Laboratory and the Superconducting Super Collider Laboratory. I am a volunteer for the Renaissance Universal movement. My main goal is to inspire the reader to engage in Self-discovery and expansion of consciousness.
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