Thursday, 9 July 2009

Quantum Mechanics

Quantum Mechanics:
The field of quantum mechanics concerns the description of phenomenon on small
scales where classical physics breaks down. The biggest difference between the
classical and microscopic realm, is that the quantum world can be not be perceived
directly, but rather through the use of instruments. And a key assumption to quantum
physics is that quantum mechanical principles must reduce to Newtonian principles at
the macroscopic level (there is a continuity between quantum and Newtonian
Quantum mechanics uses the philosophical problem of wave/particle duality to
provide an elegant explanation to quantized orbits around the atom. Consider what a
wave looks like around an orbit, as shown below.

Only certain wavelengths of an electron matter wave will `fit' into an orbit. If the
wavelength is longer or shorter, then the ends do not connect. Thus, de Broglie matter
waves explain the Bohr atom such that on certain orbits can exist to match the natural
wavelength of the electron. If an electron is in some sense a wave, then in order to fit
into an orbit around a nucleus, the size of the orbit must correspond to a whole number
of wavelengths.
Notice also that this means the electron does not exist at one single spot in its orbit, it
has a wave nature and exists at all places in the allowed orbit (the uncertainity
principle). Thus, a physicist speaks of allowed orbits and allowed transitions to
produce particular photons (that make up the fingerprint pattern of spectral lines).
Quantum mechanics was capable of bringing order to the uncertainty of the
microscopic world by treatment of the wave function with new mathematics. Key to
this idea was the fact that relative probabilities of different possible states are still
determined by laws. Thus, there is a difference between the role of chance in quantum
mechanics and the unrestricted chaos of a lawless Universe.
The quantum description of reality is objective (weak form) in the sense that everyone
armed with a quantum physics education can do the same experiments and come to the
same conclusions. Strong objectivity, as in classical physics, requires that the picture
of the world yielded by the sum total of all experimental results to be not just a picture
or model, but identical with the objective world, something that exists outside of us
and prior to any measurement we might have of it. Quantum physics does not have this
characteristic due to its built-in indeterminacy.
For centuries, scientists have gotten used to the idea that something like strong
objectivity is the foundation of knowledge. So much so that we have come to believe
that it is an essential part of the scientific method and that without this most solid kind
of objectivity science would be pointless and arbitrary. However, quantum physics
denies that there is any such thing as a true and unambiguous reality at the bottom of
everything. Reality is what you measure it to be, and no more. No matter how
uncomfortable science is with this viewpoint, quantum physics is extremely accurate
and is the foundation of modern physics (perhaps then an objective view of reality is
not essential to the conduct of physics). And concepts, such as cause and effect,
survive only as a consequence of the collective behavior of large quantum systems.
A combination of quantum mechanics and relativity allows us to examine subatomic
processes in a new light. Symmetry is very important to physical theories. For
example, conservation of momemtum is required for symmetry in time. Thus, the
existence of a type of `opposite' matter was hypothesized soon after the development
of quantum physics. `Opposite' matter is called antimatter. Particles of antimatter has
the same mass and characteristics of regular matter, but opposite in charge. When
matter and antimatter come in contact they are both instantaneously converted into
pure energy, in the form of photons.

Antimatter is produced all the time by the collision of high energy photons, a process
called pair production, where an electron and its antimatter twin (the positron) are
created from energy (E=mc2).
One of the surprising results of quantum physics is that if a physical event is not
specifically forbidden by a quantum rule, than it can and will happen. While this may
strange, it is a direct result of the uncertainty principle. Things that are strict laws in
the macroscopic world, such as the conversation of mass and energy, can be broken in
the quantum world with the caveat that they can only broken for very small intervals
of time (less than a Planck time). The violation of conservation laws led to the one of
the greatest breakthroughs of the early 20th century, the understanding of radioactivity
decay (fission) and the source of the power in stars (fusion).
Nuclear fission is the breakdown of large atomic nuclei into smaller elements. This can
happen spontaneously (radioactive decay) or induced by the collision with a free
neutron. Spontaneously fission is due to the fact that the wave function of a large
nuclei is 'fuzzier' than the wave function of a small particle like the alpha particle. The
uncertainty principle states that, sometimes, an alpha particle (2 protons and 2
neutrons) can tunnel outside the nucleus and escape.

Induced fission occurs when a free neutron strikes a nucleus and deforms it. Under
classical physics, the nucleus would just reform. However, under quantum physics
there is a finite probability that the deformed nucleus will tunnel into two new nuclei
and release some neutrons in the process, to produce a chain reaction.
Fusion is the production of heavier elements by the fusing of lighter elements. The
process requires high temperatures in order to produce sufficiently high velocities for
the two light elements to overcome each others electrostatic barriers.

Even for the high temperatures in the center of a star, fusion requires the quantum
tunneling of a neutron or proton to overcome the repulsive electrostatic forces of an
atomic nuclei. Notice that both fission and fusion release energy by converting some
of the nuclear mass into gamma-rays, this is the famous formulation by Einstein that
Although it deals with probabilities and uncertainties, the quantum mechanics has been
spectacularly successful in explaining otherwise inaccessible atomic phenomena and
in meeting every experimental test. Its predictions are the most precise and the best
checked of any in physics; some of them have been tested and found accurate to better
than one part per billion.
This is the holistic nature of the quantum world, with the behavior of individual
particles being shaped into a pattern by something that cannot be explained in terms of
the Newtonian reductionist paradigm. Newtonian physics is reductionistic, quantum
physics is holistic.
Where a reductionist believes that any whole can be broken down or analyzed into its
separate parts and the relationships between them, the holist maintains that the whole
is primary and often greater than the sum of its parts. Nothing can be wholly reduced
to the sum of its parts.

The atom theory of the Greeks viewed the Universe as consists of indestructible atoms.
Change is a rearrangement of these atoms. An earlier holism of Parmenides argued
that at some primary level the world is a changeless unity, indivisible and wholly
The highest development of quantum theory returns to the philosophy of Parmenides
by describing all of existence as an excitation of the underlying quantum vacuum, like
ripples on a universal pond. The substratum of all is the quantum vacuum, similar to
Buddhist idea of permanent identity.

Quantum reality is a bizarre world of both/and, whereas macroscopic world is ruled by
either/or. The most outstanding problem in modern physics is to explain how the
both/and is converted to either/or during the act of observation.
Note that since there are most probable positions and energy associated with the wave
function, then there is some reductionism available for the observer. The truth is
somewhere between Newton and Parmenides.

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