Universe Today

The Planck constant – written as h (and also called Planck's constant) – is one of the fundamental physical constants (along with e, the charge on the electron, m_e, the electron's mass, c, the speed of light in a vacuum, and α, the fine structure constant).

It is named after Max Planck, who laid the foundations of quantum physics. It first appeared as the constant which relates the energy (of a photon, E) to its frequency (ν): E = hν. As sometimes happens in the history of physics, two people working more or less independently arrived at this equation; namely Planck (who derived it from his assumption that blackbody radiation is quantized) and Einstein (who derived it from his work on the photoelectric effect).

A closely related constant is the reduced Planck constant ℏ ("h-bar"), which is simply h divided by 2π (you can easily see the relationship … consider that frequency can be measured either in cycles per second or radians per second: there are 2π radians in one cycle!).

In SI units, h is 6.626 068 96(33) x 10^-34 J s (joule seconds) – this is the 2006 CODATA recommended value.

Planck's constant pops up all over the place in quantum theory; not only in blackbody radiation and the photoelectric effect, but also in atomic structure (the Bohr model), the uncertainty principle, and much, much more.

Just as the definition of the metre was changed to one based on c (a fundamental physical constant), so there is considerably work being done on changing the definition of the kilogram to one based on h (not least because the mass of a lump of platinum-iridium in Paris may not be constant!) … however, there are competing proposals.

More on Planck's constant: Planck's Constant and the Energy of a Photon (University of Colorado), Determining Planck's Constant with LEDs (Kansas State University), and Planck's constant (University of Oregon).

Have the Constants of Physics Remained Unchanged?, New Study Finds Fundamental Force Hasn't Changed Over Time, and How Advanced Can a Civilization Become are just three of the many Universe Today articles in which Planck's constant plays a key role.

Wave Particle Duality is an Astronomy Cast episode which highlights just one of many areas where Planck's constant rules.

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If none of the electromagnetic radiation that hits an object is reflected or transmitted – the object absorbs all of it – then it is a black body. However, unless such an object is at exactly 0 K, it will give off EM radiation, and that radiation is called blackbody radiation.

Because blackbody radiation depends only on the temperature of the black body doing the emitting, blackbody radiation is an example of thermal radiation (EM radiation due to the heat of the object, not that which is reflected or transmitted).

If you plot the intensity of blackbody radiation against wavelength (or frequency), you get a curve with a very characteristic shape – it has a long tail (on the low frequency side), a well-defined peak, and a sharp drop (on the high frequency side); no surprise to learn that it is called the blackbody spectrum!

The blackbody spectrum was intensely frustrating, yet also intriguing, to 19th century physicists … because they could not explain it. In a breakthrough that heralded the quantum revolution of the early 20th century, in 1901 Planck derived an equation – called Planck's law – for the blackbody spectrum, using an assumption about the quantization of energy; prior to this Wien had accurately described the short wavelength side (the steep drop), and what is now called the Rayleigh-Jeans law described the long wavelength side (the long tail). A great many books on the history of physics make the mistake of giving Planck credit for photons, the quanta of EM radiation; in fact, it was Einstein who first applied the idea of quantization to light (in the photoelectric effect, this is what he got his Nobel Prize for), and not until 1924 did Bose 'close the loop' by developing photon statistical mechanics (from which Planck's law can be derived, 'from first principles').

No physical object is a black body; apart from some residual reflection (nothing – except a black hole – is totally black), the structure and composition of real objects leave their marks on the emitted radiation, even at very low temperatures: atoms and molecules emit and absorb at preferred wavelengths (lines and bands, respectively); the electrons in metals behave as a Fermi gas; in semiconductors the band gap(s) produce characteristic distortions; … and irregularities on the surface – pits, ridges, etc – can give rise to interference effects.

All the more exciting, then, when something with a spectrum approximating a blackbody far more closely than anything seen – or made – on Earth was discovered … by astronomers; the cosmic microwave background (CMB). And why does it have so nearly a perfect blackbody spectrum? Another Guide To Space article explains why.

Want more? Black Body Radiation, by Michael Fowler of the University of Virginia, is part of a 200-level university course , MyBlackBodyApplet (the name says it all), another one (with cross-reference to stellar spectral types), and Blackbody Radiation & Wien's Law, from NASA's Imagine the Universe!

Blackbody radiation is mentioned in only a few Universe Today stories (other than those on the CMB!); an example: Spitzer Discovers Early Galaxy Forming Region.

Energy Levels and Spectra, an Astronomy Cast episode, puts blackbody radiation into a broader context.

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One of the newest telescopes in space, the Planck spacecraft, recently completed its "first light" survey which began on August 13. Astronomers say the initial data, gathered from Planck's vantage point at the L2 point in space, is excellent. Planck is studying the Cosmic Microwave Background, looking for variations in temperature that are about a million times smaller than one degree. This is comparable to measuring from Earth the body heat of a rabbit sitting on the Moon.
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As of August 13, 2009, the Planck mission is officially in business. It is now seeing light billions of years old, left over from the Big Bang. From its location in the L2 point, the spacecraft started collecting science data as part of the "First Light Survey" which is intended to check out all the systems. If all goes as planned, these observations will be the first of 15 or more months of data gathered from two full-sky scans.
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