The Effect of the Millennium

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‘It may […] be more than apt to call the Doppler Effect […] the effect of the millennium.’ – Anton Zeilinger, Wolf Prize in Physics recipient and President of the Austrian Academy of Sciences, July 2007.

Have you ever noticed how the pitch of a passing ambulance siren seems to change? This is the Doppler effect at work: the observed frequency of a wave depends on the relative motion of the source and detector.

 

Picture ripples on the surface of a pond after a pebble has been cast, where the circular wavefronts are equally separated. Since a wave’s velocity is determined by the medium through which it propagates, the wavefronts travel at a fixed speed. Now imagine a duck is swimming in the pond. The wavefronts created are no longer evenly spaced because the centre of each new wavefront is slightly displaced in the direction of travel. If it approaches an unmoving swan, from the swan’s perspective, the wavefronts will appear to cluster together. This scenario is analogous to any wave source approaching a detector: the effective wavelength is reduced. On the other hand, the wavefronts produced by a receding source appear to spread out, increasing the effective wavelength. The wave equation shows that frequency equals velocity divided by wavelength. Because velocity is unchanged, a longer effective wavelength results in a lower observed frequency and vice versa. This explains the perceived increase and subsequent drop in pitch of an ambulance siren as it passes by.

 

Formulated by the Austrian physicist Christian Doppler in 1842, the phenomenon has proven to be immensely influential in the modern world, with applications ranging from healthcare and meteorology to law enforcement, satellite communications and more. They all use the Doppler effect equation which links the apparent change in frequency with the velocity of the waves, source and observer.

 

Globally, the ‘weather’ is searched for on Google over 65 million times per month (according to the research tool Ahrefs). The Doppler effect is behind the weather forecasts we see on our phones, computers and televisions every day. In meteorology, pulse-Doppler radars interpret frequency data from radio waves reflected off clouds to determine their velocities. Meteorologists use this information to predict precipitation as well as wind speed and direction. Similar technology is implemented in radar guns which police use to track speeding motorists.

 

Doppler radars also measure the velocities of satellites and receivers, enhancing the accuracy of navigation and tracking systems. Most notably, in 2014, experts attempting to locate missing Malaysia Airlines Flight 370 relied on the Doppler effect to analyse satellite data and approximate the aircraft’s position. Offsets in the frequencies of signals between the aircraft, Inmarsat satellite and the ground station in Australia ultimately helped discover the final location of Flight MH370 – the southern Indian Ocean.

 

Moreover, the effect underpins Doppler ultrasonography, vital in diagnosing various potentially life-threatening conditions including blood clots and aneurysms. This medical imaging technique involves the transmission of ultrasound waves. Ultrasonic devices then measure the frequency shift of waves reflected off moving objects such as blood cells to calculate their velocities. This provides crucial life-saving information about the movement of tissues and bodily fluids.

 

Relativistic Doppler shifts are a type of redshift/blueshift caused by the Doppler effect from motion through space. Redshift and blueshift describe how the wavelength of electromagnetic radiation increases and decreases, respectively. They allow astronomers to determine the nature of star systems (multiple, binary or single) and measure the speed of galaxies. Hubble’s law relates the recessional velocity of a galaxy to its distance away from Earth. In 2016, the oldest and farthest known galaxy was detected by measuring spectroscopic redshift: GN-z11 is around 32 billion light-years away (present proper distance – accounts for the expansion of the universe) and existed just 400 million years after the Big Bang, which took place 13.8 billion years ago.

 

Almost two centuries after its discovery, the Doppler effect still affects society today with its many far-reaching applications. Recently, it has also been incorporated into robotics and driverless cars where the analysis of a constantly changing environment is key. Thus, with the rise of artificial intelligence and autonomous vehicles, the Doppler effect will likely continue to impact the world for many years to come.