astronomyblog: Quantum Vacuum Quantum vacuum w…

astronomyblog:

Quantum Vacuum

Quantum vacuum would be the space in which apparently nothing exists for any observer but contains a minimal amount of energy, mainly electromagnetic and gravitational fields and virtual particles (force particles) interacting with each other.

Previously, there was thought to be a physical entity called the absolute vacuum on which several scientists of the Middle Ages, including Blaise Pascal, have carried out various experiments to try to reaffirm this idea.

The absolute vacuum would be one in which nothing would exist, no chemical elements, fields and particles of force, etc. However, it was found that if such Absolute Vacuum really existed, it would contradict Werner Heisenberg’s famous Principle of Uncertainty, the postulate and major basis of Quantum Mechanics.

Quantum mechanics, generally and simpler, is the physics of probabilities, in which there is no certainty as to the position and velocity of a particle, but a measure of probabilities of finding the particle in a given position and at a given velocity.

image

The Principle of Uncertainty of the Physicist Werner Heisenberg, emphasizes mathematically this idea, in which it is impossible to simultaneously determine with infinite acuity the position and velocity of a particle. The more precisely the position is determined, the less speed is determined, and vice versa.

image

This is because when we need to determine the position and velocity of a particle, we need to focus light on it. This light has a certain frequency and consequently a given energy. Thus, the position and / or velocity of a particle are altered according to the frequency and energy of the light used to observe them. That is why the more you determine one thing, the more indeterminate another and vice versa. This depends on the frequency and energy of the light used for observation.

image

The typical example is of the two uncharged conductive plates in a vacuum, placed a few nanometers apart. In a classical description, the lack of an external field means that there is no field between the plates, and no force would be measured between them. When this field is instead studied using the quantum electrodynamic vacuum, it is seen that the plates do affect the virtual photons which constitute the field, and generate a net force – either an attraction or a repulsion depending on the specific arrangement of the two plates. Although the Casimir effect can be expressed in terms of virtual particles interacting with the objects, it is best described and more easily calculated in terms of the zero-point energy of a quantized field in the intervening space between the objects. This force has been measured and is a striking example of an effect captured formally by second quantization.

Contrary to what is commonly understood, the vacuum is full of potential particles, virtual matter pairs and antimatter, which are constantly being created and destroyed. They do not exist as observable entities, but they exert pressure on other particles (Casimir effect).

The creation of virtual particle pairs does not violate the law of conservation of mass / energy because they exist in very small time intervals, much smaller than Planck’s time (10 ^ -44s), so that they do not impact the laws macroscopic.

The quantum vacuum is the lowest state of energy known in the universe (rather than the absolute zero).

Visualizations of Quantum Chromodynamics

The animations above illustrate the typical four-dimensional structure of gluon-field configurations averaged over in describing the vacuum properties of QCD. The volume of the box is 2.4 by 2.4 by 3.6 fm, big enough to hold a couple of protons. Contrary to the concept of an empty vacuum, QCD induces chromo-electric and chromo-magnetic fields throughout space-time in its lowest energy state. After a few sweeps of smoothing the gluon field (50 sweeps of APE smearing), a lumpy structure reminiscent of a lava lamp is revealed. This is the QCD Lava Lamp. The action density (top) and the topological charge density (below) are displayed. The former is similar to an energy density while the latter is a measure of the winding of the gluon field lines in the QCD vacuum.

This animation shows the suppression of the QCD vacuum from the region between a quark-antiquark pair illustrated by the coloured spheres. The separation of the quarks varies from 0.125 fm to 2.25 fm, the latter being about 1.3 times the diameter of a proton. The surface plot illustrates the reduction of the vacuum action density in a plane passing through the centers of the quark-antiquark pair. The vector field illustrates the gradient of this reduction. The tube joining the two quarks reveals the positions in space where the vacuum action is maximally expelled and corresponds to the famous “flux tube” of QCD. As the separation between the quarks changes the tube gets longer but the diameter remains approximately constant. As it costs energy to expel the vacuum field fluctuations, a linear confinement potential is felt between quarks.

The manner in which QCD vacuum fluctuations are expelled from the interior region of a baryon like the proton is animated at above. The positions of the three quarks composing the proton are illustrated by the coloured spheres. The surface plot illustrates the reduction of the vacuum action density in a plane passing through the centers of the quarks. The vector field illustrates the gradient of this reduction. The positions in space where the vacuum action is maximally expelled from the interior of the proton are also illustrated by the tube-like structures, exposing the presence of flux tubes. A key point of interest is the distance at which the flux-tube formation occurs. The animation indicates that the transition to flux-tube formation occurs when the distance of the quarks from the centre of the triangle (< r >) is greater than 0.5 fm. Again, the diameter of the flux tubes remains approximately constant as the quarks move to large separations. As it costs energy to expel the vacuum field fluctuations, a linear confinement potential is felt between quarks in baryons as well as mesons.