The Doppler Effect: Existed Before Railroads?

Article By : Bill Schweber

Of course the effect did exist before the railroads. However, it wasn’t yet defined by the physics of the day, nor was it even noticed before the railroads...

The headline is somewhat of a trick question: of course the effect did exist before the railroads. However, it wasn’t yet defined by the physics of the day, nor was it even noticed before the railroads. After all, there were no steady sources of sound moving on a straight line to listen to, to be perceived with a rising pitch as it approached and falling one as it departed.

This may seem like a trivial discussion, but it’s not. An article in the current issue of Physics Today, “The fall and rise of the Doppler effect” discusses the history of the uncovering acknowledgment of the Doppler effect, named after 19th-centry physicist Christian Doppler who proposed it and worked out the equations. It has serious and important implications across a wide scale of measurements from advanced blood-flow assessment, to ultrasound imaging, radar, to sophisticated characterizations of the galaxy, Figure 1. It so well known that even many non-technical people are at least aware of the term even if not too clear on what it means.

Doppler effect

Figure 1:The well-known Doppler effect applies (with some differences) to acoustic and electromagnetic waves and is an essential principle for many systems spanning biomedical research to communications to astronomy.
(Source: Elsevier Science Direct)

The Physics Today article is fascinating. It details the difficulties which Doppler had in getting his concept accepted, and how it was both dismissed and even ridiculed by other leading scientists and physicists at the time; some of them sort-of agreed but felt it applied to acoustic waves and not electromagnetic ones. One inconclusive test involved placing a musical band on a train moving at a constant speed, but that didn’t work out for various reasons. I won’t retell the other details of the story of how it came to be fully accepted, as you can read the well-written story yourself.

Doppler’s travails are yet another demonstration of the difficulties in proving a postulated scientific principle before there is any evidence for it, as compared to providing an explanation of something already seen, and then seeing if that proposal is credible.

Consider some examples from who else but Albert Einstein. He was awarded the Nobel Prize in 1921 not for his theory of general or even special relativity, but instead for his explanation of the photoelectric effect, Figure 2. This clearly observed phenomenon defied explanation by conventional, pre-quantum physics, with many unsatisfactory explanations.

photoelectric effect

Figure 2:The contradictions and confusion when trying to understand the clearly observed photoelectric effect led Einstein to propose a radically new understanding of electromagnetic waves and particles.
(Source: The Physics Hypertextbook)

Even when he presented his radical theory, many leading physicists were skeptical (as they should be), and it was not fully accepted until precise experiments by Robert Millikan using Einstein’s approach returned the same value for Planck’s constant (h) as had already been determined by unrelated, orthogonal experiments. (Ironically, Millikan did not support Einstein’s theory but he still executed his experiment with great care, and he received the Nobel Prize in 1923.)

It’s important to be cautious with these new theories even if they seem complete, as it’s entirely possible to present one which appears solid but is also wrong. Think of the Aristotelian, Earth-centric view of the universe which was able to predict with accuracy the motions of the planets, moon, eclipses, stars and more, until the Copernican solar-centric view overtook it.

Similarly, Einstein employed relativity to explain an extremely tiny but distinct aberration in the precession of the perihelion of Mercury, which had been observed for centuries, Figure 3 (and how did they make such precise measurements their “crude” telescopes and clocks?). Previous attempts to understand this discrepancy, including the minute effects of gravity from nearby bodes or perhaps even unseen ones, were unsatisfactory. Still, an explanation that “works” is not necessarily correct.

the orbit of Mercury

Figure 3:A tiny yet inexplicable aberration in the precession of the orbit of Mercury was finally understood by application, again by Einstein, of his general theory of relativity.
(Source: Cornell University)

In fact, the most difficult and stringent test of a theory is its ability to predict something which no one has even imaged before. That’s why Einstein’s prediction that gravity would bend light was so dramatic at bringing over skeptics. His thesis was verified during a 1919 eclipse when the apparent position of stars was shifted slightly as their light passed near the Sun on its way to Earth.

In the case of the Doppler effect, a large part of the problem was that the theory predicting something that could not be tested at that time is a tough sell. Doppler‘s theory was not only rejected by other members of the establishment – many of whom did important work which we still rely today – but he died before it was fully accepted.

Have you ever made a supposition about what is happening and why with respect to your design and debug, then have it rejected – yet were able to demonstrate later that your explanation was correct? Did you make predictions about as-yet unseen effects, and then have to set up experiments and tests to observe them to validate your ideas?

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