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Quantum Physics


Quantum Physics is the theory that advocates that the energy is not continuous, rather being concentrated in packs named "quanta". For instance, the electromagnetic energy is carried by photons (light particles). Each "quantum" (photon or any other boson) holds a quantity of energy that is proportional to its wave frequency.





In a first distinction, the particles can be divided in 2 types: bosons and fermions. The bosons are the field particles that can be featured as the ones with whole numbered spins:

    1. 0 (they look the same whatever the side from which they are observed);
    2. 1 (they only take the same aspect as we rotate them 360║);
    3. or 2 (they take the same aspect if we rotate them 180║).

The fermions are material particles (in most cases, massive ones) and have a spin of 1/2 (they take the same aspect only after being rotated 2 times 360║)



The fermions are subdivided in hadrons and leptons.

The hadrons are composed by quarks of different "flavours":

    1. up, electric charge: +2/3, mass: 310 GeV/c2;
    2. down, electric charge: -1/3, mass: 310 GeV/c2;
    3. strange, electric charge: -1/3, mass: 505 GeV/c2;
    4. charmed, electric charge: +2/3, mass: 1500 GeV/c2;
    5. beauty, electric charge: -1/3, mass: 5000 GeV/c2;
    6. truth, electric charge: +2/3, mass: >22500 GeV/c2.

Each "flavour" of a quark exists in 3 "colours": red, green and blue. Therefore, there are 6*3 = 18 varieties of quarks. Each quark variety is corresponded by an antiquark variety, with an opposite electric charge.

A quark can't exist isolated, because it would otherwise be "coloured" (red, blue or green). As the strong nuclear force keeps the particles always bound in white combinations ("colourless"), that means that quarks have to exist in combinations of 3 (blue + green + red = white) or 2 (blue quark + blue anti-quark = white, etc).

The triplets of quarks are called baryons and the pairs are called mesons. The mesons, consisting in combinations of particles and anti-particles (which tend to mutually annihilate), are unstable and give birth to electrons and other particles. The baryons include the protons and the neutrons that exist in atomic nuclei. A proton consists in a combination of 2 up quarks (2/3 + 2/3 = 4/3) and one down quark (-1/3). Therefore, the electric charge of the proton is 4/3 - 1/3 = 1. The neutron contains 2 down quarks (-1/3 -1/3 = -2/3) and 1 up quark (2/3). So, the electric charge of the neutron is -2/3 + 2/3 = 0. Particles can be formed from other flavours, but as they are heavier (they have a higher mass) they decay very quickly in neutrons and protons.


Above: neutron, below: proton (MoonRunner Design UK)


The leptons, which are elementary particles, include:

    1. the electron, electric charge: -1, mass: 0,511 GeV/c2;
    2. the muon, electric charge: -1, mass: 106,600 GeV/c2;
    3. the tau, electric charge: -1, mass: 1784,000 GeV/c2.

Each one of this particles is corresponded by a neutrino (also a lepton), each of them holding an electric charge = 0 and a very tiny mass. There is the electronic neutrino, the muonic neutrino and the tauonic neutrino.



The bosons, or field particles, carry each of the 4 fundamental forces under the form of energy packs:

    1. the graviton is responsible for the transportation of the gravity force, acts in an infinite range, has an intensity 10-40 times that of the strong nuclear force and doesn't have any mass (it moves at the speed of light). It has a spin of 2 and its existence has not been experimentally proved yet;
    2. the photon is responsible for the transportation of the electromagnetic force and also acts in an infinite range, has an intensity 10-2 times that of the strong nuclear force and doesn't have any mass either. It has a spin of 1;
    3. the intermediate bosons (the W+ and W-, endowed with electric charged and with a mass of 81 GeV/c2, and the Z0, with no electric charge and with a mass of 93 GeV/c2), are responsible for the transportation of the weak nuclear force, act in a range smaller than 10-16 mm and have an intensity 10-13 times that of the strong nuclear force. Since they have mass, they move at a velocity lower than the speed of light. These 3 particles have a spin of 1;
    4. the gluon is the responsible for the transportation of the strong nuclear force, acts in a range smaller than 10-13 mm and has no mass. It has a spin of 1.

Gravity, as it is analyzed in the page about relativity, is an exclusively attractive force whose intensity depends on the mass of the bodies and on the distance that separates them.

Electromagnetism acts on electrically charged particles, as in the case of the electrons or quarks. It may be attractive (between particles with electric charges of opposite signal) or repulsive (between particles with electric charges of identical signal).

The weak nuclear force is responsible for phenomena as radioactivity (which is the disintegration of heavy nuclei - as the uranium, the thorium or the actinium nuclei - into lighter nuclei as lead + helium nuclei + electrons + photons) or the decay of a neutron into a proton + electron + antineutrino. It's a force that acts on massive particles.

The strong nuclear force is responsible for the cohesion of the quarks inside a proton or a neutron and of the protons and neutrons inside an atomic nucleus. Its messenger particle, the gluon, only interacts with itself or with quarks.


The Law of the Weakest

Gravity rules the Universe because its action range is infinite and it always exerts an attractive effect, in opposition to the electromagnetism, which also acts in an infinite range but has attractive or repulsive effects according to the circumstances. Although the intensity of the electromagnetic force is much higher than that of the gravity, the positive electric charges are counter-balanced by the negative ones in the big scales (for instance, even inside a simple hydrogen atom the positive charge of the proton is neutralized by the negative charge of the electron). Therefore, the big massive bodies tend to have a quite negligible electric charge.

Nevertheless, to demonstrate the innate weakness of the gravity force compared to the electromagnetic force, it's enough to look at an apple unfastening from the tree and falling over the floor. An apple is kept tied to the branch of the tree by the electromagnetic forces that bind the atoms of the apple to the atoms of the branch. It's necessary the gravity of the entire Earth to overcome the electric force inherent to the few atoms of a modest apple peduncle.


The Price of the Union

When 2 bodies are unified under the influence of one of these forces, the joint mass of both is reduced, as a fraction of that mass is released under the form of energy. For instance, an electron (negatively charged) united to a proton (positively charged) by the electromagnetic force exists in a lower energy condition than a freely moving electron. According to the Einstein's relativity, there is equivalence between mass and energy, so the excessive energy that was released when the electron was caught by the proton is also expressed as a loss of mass by these 2 bodies.



Principle of Uncertainty


One of the pillars of quantum physics is the Heisenberg's principle of uncertainty. According to this principle, to forecast the future position and velocity of a particle it is necessary to be able to measure its present position and velocity. To observe a particle it's necessary to target it with a light beam.

If the wavelength of the beam (photon) is too long, or in other words, less energetic, it will affect less the movement of the particle and it will be possible to know it's velocity with a higher accuracy. However, we can't determine the particle position with a higher precision than that represented by the distance between 2 consecutive wave peaks (or troughs). As the wavelength is long, that distance will be higher and, therefore, will also be higher the uncertainty about where is the particle positioned.

The opposite will happen if we target the particle with a shorter wavelength beam: is will provoke a bigger disruption over the particle's movement (implying a higher uncertainty about its velocity), but it will allow us to locate it with a higher accuracy.

Heisenberg demonstrated that the uncertainty about the position multiplied by the uncertainty about the velocity and still multiplied by the particle's mass can never be lower than a determined amount - the so-called constant of Planck.


Wavelengths and frequencies (MoonRunner Design UK)


Implications of the Principle of Uncertainty

The principle of uncertainty has deep implications in the way we perceive the world. It's impossible to make precise forecasts about future events, since it's not possible to accurately measure the present state of the Universe. The quantum physics allows several possible results of an observation, each one associated to a determined probability. Therefore, it informs us about the probabilities of each one of the possible futures of the world.


The Feynman's Sum of Stories

In the sequence of this idea, Richard Feynman advocated that a particle isn't connected to a single story or pathway in the space-time. Instead, it shall move between 2 given points following every possible pathway between them. The probability that a particle is found in a determined place is given by the sum of the waves associated to the stories containing that specific place.


The infinity of possible pathways, according to Feynman (MoonRunner Design UK)



The Atom


The Atom of Bohr

Even before Heisenberg formulated the principle of uncertainty (in 1926), the Danish physician Niels Bohr had defended, in 1913, that the electrons could only occupy specific orbits around the nucleus, the closest ones corresponding to lower energy levels and the farthest ones to higher energy levels. We understand better the idea if we think that the less energetic electrons are more easily and strongly dominated by the force that links them to the protons and, therefore, are less independent from them (they are hold by a shorter "strap").

So, an atom can only absorb or emit packs of energy (photons) with fixed and determined wavelengths, each one corresponding to the difference between the energy levels of 2 orbits. When an electron absorbs a determined amount of energy (a photon with a given wavelength), it moves to a farther orbit, and the opposite happens if it emits energy.


The Atom of Schr÷dinger

One of the main implications of the principle of uncertainty is precisely the way the atom works. The Schr÷dinger's model postulates that the orbits are similar to probability spheres or clouds, instead of looking like circumferences around the nucleus. It's not possible to exactly locate the electron that moves at a given orbit, but it's possible to calculate the probability of finding it in this or that position.


To the left: the atom of Bohr, to the right: the atom of Schr÷dinger (MoonRunner Design UK)



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