 Given that the speed of light in a vacuum will equal the light's frequency times its wavelength, all we need to do is to use a laser to create monochromatic light and measure its frequency and wavelength. To fully understand how this is done, we need to take a little deeper look at the laser that creates the light. The name laser is an acronym for light amplification by stimulated emission of radiation. The key is stimulated emission. You may recall from the How Small Is It segment on the atom that electrons exist in discrete energy levels around the nucleus. When a photon with an energy E hits an electron in a shell around a nucleus that has a higher shell it can reach with this same exact energy, the photon's entire energy is transferred to the electron instantaneously. This jumps the electron to the higher energy level and the photon is eliminated. When the electron drops from this excited state back to a lower energy level, a photon with the exact difference between the energy levels is emitted in a random direction. In 1916, Einstein theorized a photon with its electromagnetic field and the right energy could stimulate an electron to drop to a lower energy level and emit a photon with the exact same energy, trajectory, polarization, and phase as the incident photon. Where there was one photon, there are now two. They will travel off in random directions. Probabilities have it that every once in a while a trajectory will be parallel to the tube they are in. Lasers have one mirror that reflects 100% of the light that reaches it and the other reflects 99% and lets 1% through. So these photons will travel to a mirror and be reflected. As they pass back through the tube they will stimulate the production of additional duplicate photons as we just described for as long as the pump keeps energizing electrons to higher energy levels. In short order, there will be trillions of identical photons leaving through the 99% mirror. This is the laser light. By the 1970s, the development of methane stabilized helium neon lasers with high energy spectral stability and accurate cesium clocks made measurements to within an error of plus or minus 1 meter per second possible. To measure the frequency, a technique called infrared frequency synthesis was used. It is the same idea we see with sound wave beats. We start with a known frequency, mix it with a higher unknown frequency, and count the beats created by the combination. Here we see a 400 cycle per second tone mixed with a higher frequency that produces a 5 cycles per second beat. This gives us the frequency of the second wave at 405 cycles per second. Using an iterative process, this can measure still higher sound frequencies. For light frequencies we use an oscillator with a known frequency and mix it with a higher frequency using a mixer diode to count the beats. We iteratively get to the frequency of the laser light's output. They found that the laser produces light with a frequency of 88.376181627 trillion cycles per second. To measure the wavelength, we use an interferometer. First we set up equal path lengths for the light by getting the maximum constructive interference. Then, as we shorten one of the paths slowly to get, say, 10,000 friend shifts, the wavelength of the light is the distance that we had to use to get this many shifts divided by twice the number of fringe shifts. The results showed that the wavelength is 3.392231376 micrometers. This measured wavelength, along with the measured frequency, gives us the speed of light at 299,792,456 meters per second. As of 1983, the speed of light in a vacuum is defined to have an exact fixed value when given in standard units. In fact, the meter has been defined by international agreement as the distance traveled by light in a vacuum during a time interval of 1 over 299,792,458 seconds. This makes the speed of light fixed. In the next segment on special relativity, we'll cover why this value also represents the fastest that anything can travel.