Everything about Quantum totally explained
In
physics, a
quantum (plural:
quanta) is an indivisible
entity of a quantity that has the same units as the
Planck constant and is related to both
energy and
momentum of
elementary particles of
matter (called
fermions) and of
photons and other
bosons. The word comes from the
Latin "quantus," for "how much." Behind this, one finds the fundamental notion that a physical property may be "quantized", referred to as "
quantization". This means that the magnitude can take on only certain discrete
numerical values, rather than any value, at least within a range. There is a related term of
quantum number.
A
photon is often referred to as a "
light quantum." The energy of an
electron bound to an
atom (at rest) is said to be quantized, which results in the stability of atoms, and of
matter in general. But these terms can be a little misleading, because what is quantized is this
Planck's constant quantity whose units can be viewed as either energy times time or momentum times distance.
Usually referred to as quantum "mechanics," it's regarded by virtually every professional physicist as the most fundamental framework we've for understanding and describing nature, for the very practical reason that it works. It is "in the nature of things", not a more or less arbitrary human preference.
Development of quantum theory
Quantum theory, the branch of physics which is based on quantization, began in
1900 when
Max Planck published his theory explaining the
emission spectrum of
black bodies. In that paper Planck used the
Natural system of
units he invented the
previous year.
The consequences of the differences between
classical and
quantum mechanics quickly became obvious. But it wasn't until
1926, by the work of
Werner Heisenberg,
Erwin Schrödinger, and others, that
quantum mechanics became correctly formulated and understood mathematically. Despite tremendous experimental success, the philosophical interpretations of quantum theory are still widely debated.
Planck was reluctant to accept the new idea of quantization, as were many others. But, with no acceptable alternative, he continued to work with the idea, and found his efforts were well received. Eighteen years later, when he accepted the
Nobel Prize in Physics for his contributions, he called it "a few weeks of the most strenuous work" of his life. During those few weeks, he even had to discard much of his own theoretical work from the preceding years. Quantization turned out to be the only way to describe the new and detailed experiments which were just then being performed. He did this practically overnight, openly reporting his change of mind to his scientific colleagues, in the October, November, and December meetings of the
German Physical Society, in
Berlin, where the black body work was being intensely discussed. In this way, careful experimentalists (including
Friedrich Paschen,
O.R. Lummer,
Ernst Pringsheim,
Heinrich Rubens, and
F. Kurlbaum), and a reluctant theorist, ushered in a momentous scientific revolution.
The quantum black-body radiation formula
When a body is heated, it emits
radiant heat, a form of
electromagnetic radiation in the
infrared region of the
EM spectrum. All of this was well understood at the time, and of considerable practical importance. When the body becomes red-hot, the red
wavelength parts start to become visible. This had been studied over the previous years, as the instruments were being developed. However, most of the heat radiation remains infrared, until the body becomes as hot as the surface of the
Sun (about 6000 °C, where most of the light is green in color). This wasn't achievable in the laboratory at that time. What is more, measuring specific infrared wavelengths was only then becoming feasible, due to newly developed experimental techniques. Until then, most of the
electromagnetic spectrum wasn't measurable, and therefore blackbody emission hadn't been mapped out in detail.
The quantum black-body radiation formula, being the very first piece of quantum mechanics, appeared Sunday evening October 7, 1900, in a so-called back-of-the-envelope calculation by Planck. It was based on a report by
Rubens (visiting with his wife) of the very latest experimental findings in the infrared. Later that evening, Planck sent the formula on a postcard, which Rubens received the following morning. A couple of days later, he informed Planck that it worked perfectly. At first, it was just a fit to the data; only later did it turn out to enforce quantization.
This second step was only possible due to a certain amount of luck (or skill, even though Planck himself called it "a fortuitous guess at an interpolation formula"). It was during the course of polishing the mathematics of his formula that Planck stumbled upon the beginnings of Quantum Theory. Briefly stated, he'd two mathematical expressions:
- (i) from the previous work on the red parts of the spectrum, he'd x;
- (ii) now, from the new infrared data, he got x².
Combining these as
x(
a+
x), he still has
x, approximately, when
x is much smaller than
a (the red end of the spectrum); but now also
x² (again approximately) when
x is much larger than
a (in the infrared). The formula for the energy
E, in a single mode of radiation at frequency
λ, and temperature
T, can be written
»
This is (essentially) what is being compared with the experimental measurements. There are two parameters to determine from the data, written in the present form by the symbols used today:
h is the new
Planck's constant, and
k is
Boltzmann's constant. Both have now become fundamental in physics, but that was by no means the case at the time. The "elementary quantum of energy" is
hλ. But such a unit doesn't normally exist, and isn't required for quantization.
Beyond electromagnetic radiation
While quantization was first discovered in electromagnetic radiation, it describes a fundamental aspect of energy not just restricted to photons.
The birthday of quantum mechanics
From the experiments, Planck deduced the numerical values of
h and
k. Thus he could report, in the German Physical Society meeting on December 14, 1900, where quantization (of energy) was revealed for the first time, values of the
Avogadro-Loschmidt number, the number of real molecules in a
mole, and the unit of
electrical charge, which were more accurate than those known until then. This event has been referred to as "the birth of quantum mechanics".
Further Information
Get more info on 'Quantum'.
|
External Link Exchanges
Do you know how hard it is to get a link from a large encyclopaedia? Well we're different and will prove it. To get a link from us just add the following HTML to your site on a relevant page:
<a href="http://quantum.totallyexplained.com">Quantum Totally Explained</a>
Then simply click through this link from your web page. Our crawlers will verify your link, extract the title of your web page and instantly add a link back to it. If you like you can remove the words Totally Explained and embed the link in article text.
As long as your link remains in place, we'll keep our link to you right here. Please play fair - our crawlers are watching. Your site must be closely related to this one's topic. Any kind of spamming, dubious practises or removing the link will result in your link from us being dropped and, potentially, your whole site being banned. |
