Spectroscopy and Quantum Leap

October 20, 2008

In a recent article on the Big Bang, I mentioned the method of spectroscopy twice without explaining what it is or how it works. Although the art of spectroscopy was accurately used long before it’s mechanisms were fully explained, I think that it is both helpful and interesting to consider spectroscopy alongside its quantum mechanical workings.

One of the basic tenets of quantum physics is that an atom of any chemical element is only capable of particular levels of kinetic energy unique to that element. It is important to remember here that electrons, like photons, behave both like particles and like waves. We know that there is a direct relationship between an electron’s frequency and the amount of energy that it contains. Because of the wave nature of electrons, most paths that an electron could take around nucleus are forbidden, as they would cause the electron to catch up with itself and cancel itself out. This means that only specific frequencies/energy levels are allowed for an electron of any particular element.

A hydrogen atom, for instance may have -13.6 electron volts (eV), -3.4eV, -1.5eV, etc. An atom in normal circumstances has a tendency toward its lowest possible level of kinetic energy, known as its ground state. Any time an atom has more kinetic energy than its ground state energy, it is considered to be in an excited state. When an atom is in an excited state, it releases a precise amount of energy in the form of electromagnetic radiation in order to return to a more “comfortable”, lower kinetic energy state. This rapid change in energy is known as a quantum leap. Again, we know that there is a direct relationship between energy and frequency, so all this means that when an atom in an excited state returns to it’s ground state, it releases a photon of a very specific frequency, which our eyes interpret as color, allowing us to learn the chemical makeup of a substance by analyzing the light that it emits. You can test this for yourself by sprinkling table salt over a flame. You will see specs of yellow in the flame, a result of the chemical properties of sodium. This phenomenon is known as emission spectroscopy. When the light from a hot, low-density gas is shown through a prism, an emission line spectrum results (see figure 1). Each chemical element has it’s own “spectral fingerprint”. Neon atoms emit red light while mercury emits a bluer light, accounting for the color of light you expect to see from neon and mercury light fixtures.

Figure 1 - The emission spectrum of hydrogen.  Each line of color in the spectrum represents

Figure 1 - The emission spectrum of hydrogen.

Next we’ll consider absorption spectroscopy, which is the inverse of emission spectroscopy. When light with a complete spectrum (figure 2) is shone through a prism, absorption spectroscopy can be used to identify the chemical composition of the atoms in between the light source and the observer. The electrons of the atom will only absorb photons of the particular frequencies that will result in another allowed level of kinetic energy particular to that element. The result is a nearly continuous spectrum with dark lines representing the frequencies of light that were absorbed (figure 3). Note that the emission lines for hydrogen (figure 1) line up perfectly with the absorption lines for hydrogen (figure 3). Absorption spectroscopy allows us to learn the chemical makeup of the atmospheres of stars by analyzing the dark lines in the spectrum that they emit.

Figure 2 - A continuous spectrum

Figure 2 - A continuous spectrum

Figure 3 - The absorption spectrum of hydrogen

Figure 3 - The absorption spectrum of hydrogen

Finally, I’ll take a second to relate this all back to the Big Bang. What Hubble noticed when observing the absorption spectra from distant galaxies is that the easily identifiable spectral lines were shifted toward the red end of the spectrum as in figure 4. Hubble interpreted this as the Doppler effect, or the change in frequency and wavelength of a wave for an observer moving relative to the source of the waves. Hypothetically, had the spectral lines been shifted toward the blue end of the spectrum, scientists might have concluded that distant galaxies were moving closer to us.

Understanding quantum leap and spectroscopy in relation to galactic redshift is one of many things that allows me to appreciate the wealth of observational support for the Big Bang Theory of cosmology.

Figure 4 - Redshift of hydrogen

Figure 4 - Redshift of hydrogen


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