The Physics of Superheroes: Spectacular Second Edition by James Kakalios Page B

accentuates the updraft behind the departing train, pulling loose newspapers and litter in its wake. Lacking a confined space, the Flash can generate a low-pressure region that can slow the descent of falling people, cars, or giant bombs, or as shown in fig. 10, or help detain and levitate a crook by using a vortex created by running in a circle.
    Returning to the topic of the speed of sound in air, whenever the Flash runs faster than a velocity of 1,100 feet/sec (or 750 mph), his communication with others must become visual only. The Flash would not be able to hear anyone standing behind him or even at his sides, as he would outrace the sound waves trying to reach him. Of course, for anyone standing in front of him, the Flash outracing the sound waves would not be a problem, but there would still be a barrier to communication. Even when he can hear someone talking to him, the speech will have a high and tinny quality as heard by the Flash.

    Fig. 10. When the Flash runs at high velocity in a circle, he leaves a low-pressure region in his wake, which makes it easy to bring Toughy Boraz (yes, that’s actually his name) and his stolen loot to police headquarters. From Flash # 117.
    What we term “sound waves” are variations in density consisting of alternating regions of expansion and compression. The “wavelength” of a sound wave is the distance between adjacent compressed (or expanded) regions in the medium (whether it be air, water, or a solid), which is related to the pitch that we hear. The pitch, or frequency, measures the number of complete wave cycles that pass a given point per second. Long wavelengths have low pitches (think of the deep tones from a bass violin, where the length of the strings is related to the wavelength of the sounds they can produce) while shorter wavelengths are heard as higher pitches. As the Flash runs, even if he does not outrace the sound wave, his high-speed motion affects the pitch that he hears. Let’s say he runs toward someone who is yelling a warning to him. The sound waves have some wavelength, which marks the average distance between adjacent compressed or expanded regions. If the Flash were standing still when these alternating density regions reach him, the tone he would hear would be determined by the wavelength originating from the speaker. But as the Flash runs, one region of compressed air reaches him, and as he is running toward the speaker, the next region of compressed air reaches his eardrum sooner than it would if he were standing still. The Flash thus hears a smaller wavelength and hence a higher frequency due to the fact that he is running toward the source of the sound. The faster he runs, the greater this shift in the wavelength and frequency of the detected sound.
    This phenomenon is known as the Doppler effect, and if one knows the wavelength of a stationary source of waves, and measures the wavelength of the detected waves with a moving detector, one can determine the speed of the detector. Alternatively, if one sends out a wave of a known wavelength and it bounces off a stationary target, it should return with the exact same wavelength. If the target is moving toward the source, the reflected wave will have a shorter wavelength, while if the target is moving away from the source, the detected wavelength will be longer. Doppler radar, as often used in predicting weather, involves detection of this wavelength shift, which enables meteorologists to calculate the wind velocity in an approaching storm front.
    This is also the basic premise underlying radar guns, which use radio waves of a known wavelength. From the shift in wavelength of the reflected wave to the incident wave, they can determine the velocity of the object (such as a thrown baseball or a speeding automobile) that reflected the waves. The faster the target is moving, the greater the wavelength shift, and the higher the pitch of the detected wave. If the Flash were to run at 500 mph toward someone