A typical analogy explaining redshift involves a siren or a train whistle. So how do we know the same phenomenon works with light?

454 views

A typical analogy explaining redshift involves a siren or a train whistle. So how do we know the same phenomenon works with light?

In: 2

8 Answers

Anonymous 0 Comments

Both can be seen as a sinusoidal wave (type it in google images). If the source of that wave (acoustic wave for sound, electromagnetic wave for light) moves toward a certain direction while emitting the wave, the peaks of the wave are going to be closer to each other (the distance between them, called wavelength, gets lower). Thus, to whoever receives that wave, it will be as if it was a wave of higher frequency than the frequency it was actually emitted at. (The higher the frequency of a sinusoidal wave you emit, the closer the peaks, which means a lower wavelength) It is called the Doppler effect. Hence why the pitch of an incoming siren sounds higher than how it would sound if the siren was stationary.

You can find short gifs illustrating that effect on the corresponding wikipedia page: [https://en.wikipedia.org/wiki/Doppler_effect](https://en.wikipedia.org/wiki/Doppler_effect)

Conversely, if the source is moving away from a given direction, the peaks of the wave are going to be more spaced from each other and thus, in the example of the siren, it will sound with a lower pitch than when it was approaching.

Similarly to the siren example, stars move while emitting light. Hence, to an observer towards whom the star is moving, the wavelength of the waves that observer receives, is going to be slightly decreased (shifted). Conversely, if the star is moving away from said observer, the wavelength of the waves the observer receives, is going to be slightly increased. I’m guessing that second case refers to redshift, given that among the range of wavelengths that correspond to light visible by the human eye (from 400 nanometers to 800 nanometers), red belong to the high wavelengths (around 800 nanometers).

Please note that, as much as what i’m saying regarding the siren example is correct, my analogy with the light example might be incorrect because that light example seem to be influenced not only by the simple Doppler effect but also by relativistic physics: [https://en.wikipedia.org/wiki/Redshift](https://en.wikipedia.org/wiki/Redshift) (which means, there are others phenomenons at play, that, from my impression, an analogy with the siren doppler effect might not suffice to explain)

You are viewing 1 out of 8 answers, click here to view all answers.