Redshift, Blueshift, & the Doppler Effect

It’s no stretch to say that light can be stretched (…or compressed).

TLDR;

If you haven’t already, please consider reading through The Cosmic Brief’s post on the Electromagnetic Spectrum wherein we spend a little more time fleshing out what light really is. It may help provide some context for this post:

[embed]The Electromagnetic [EM] Spectrum We don’t need our eyes where we’re going (well, maybe a little).medium.com

Light behaves as a ‘wave’ and these ‘waves’ can be both stretched & compressed; the effect becomes more apparent the greater the distance between the observer & the observed object(s). This phenomena of the stretching/compressing of light is what is known as the **Doppler Effect. When the light emitted by an object moves away relative to our position, the light is stretched & we observe a redshift. On the other hand, if the object is moving towards our position, the light is compressed & we observe a blueshift**. As it so happens, distance is one of the foremost variables astronomers must contend with during their research, especially in vast-scale fields like cosmology, galactic, & extragalactic astronomy (be sure to check out TCB’s write-ups on those). Astrophysicists subsequently utilize these principles of physics to characterize cosmological objects by defining things like their distance to us & their speed (velocity) as they travel through the cosmos.

[embed]Astronomy Sub-Fields: Cosmology The origin story of the fabric of our reality. (No joke)medium.com

[embed]Astronomy Sub-Fields: Galactic Astronomy The speed of light is quick ’til you learn crossing the Milky Way takes 200k years.medium.com

[embed]Astronomy Sub-Fields: Extra-Galactic Astronomy Night-time starlight is old. Distant galactic light is an ancient time-traveller.medium.com

Photo by Max Kleinen on Unsplash

What Is The Doppler Effect?

The Doppler Effect is a phenomena that virtually everyone has experienced: Hearing planes landing/taking off, listening to EMS/police sirens rush by, racecars screaming through a track. Simply put, sound is ‘vibrations’ traveling through the air. These sound waves have different frequencies which manifest as different kinds of sounds. In the EMS example: an ambulance siren wails at a certain frequency, but this frequency can be made to sound off-pitch depending on how the vehicle is moving. If the ambulance is moving toward you, the frequency of the sound waves coming from the siren become compressed (i.e. more frequent), resulting in that characteristic ‘higher-and-higher’ pitch as it continues to approach you. Once the vehicle passes you, the vehicle is now moving away & the opposite occurs: the sound waves decompress and become stretched (i.e. less frequent) until you can no longer hear the siren. The result in this case is that odd distortion of sound, followed by the continuously lower pitch as the ambulance drives away.

This stretching & compression of sound waves is what we call The Doppler Effect. In astronomy, we utilize this principle of physics as it applies to light, since light can be measured by its frequencies in a manner similar to sound.

This YouTube Short shows a content creator hammer-throwing a live bluetooth speaker, displaying a really dramatic example of the Doppler Effect. Nonetheless, it’s pretty entertaining to hear the rapid compression/stretching of the sound waves as he swings before finally launching the speaker & hearing the sound waves get stretched.

Visual representation of the compression of incoming vs stretching of outgoing sound waves. Courtesy: Byju’s Doppler Effect Lesson

What Are Redshift & Blueshift?

But wait a second, space is silent isn’t it? That’s correct. There’s no medium (e.g. atmosphere) in the void that allows sound waves to propagate… so why even bring up the Doppler Effect in the first place?

The principles of the Doppler Effect apply to light as well because, like sound, light propagates through space as a wave. The difference between the two is that light doesn’t rely on any sort of medium to propagate through space like sound does and so can freely beam across the cosmos as I’m sure we’re all aware (our night sky is pitch black otherwise).

So, when considering light & thinking back to how the Doppler Effect alters sound, you can imagine that a similar logic follows. In the case of light, the frequency of the waves are still compressed upon approach and still stretch as it moves away. In this case there’s no changing pitch to be able to detect — this is where we introduce redshift and blueshift:

• Redshift: light emitted by object in focus is being stretched, signaling the object is moving away
• Blueshift: light emitted is being compressed, signaling the object is approaching the observer

Astronomers observe light that is redshifted or blueshifted by studying the color spectrum of the object in focus (this is known as spectroscopy; a TCB write-up on this topic may be in order, stay tuned). Instead of manifesting as changes in pitch, the spectrum of light is slightly altered depending on a redshift or blueshift, & to what degree the light is ‘shifted’. Relevancy here: in academic papers readers may come across the term “high redshift” when the author(s) are discussing an object. An example might sound like:

“The central objects of this research were 5 high-redshift galaxies…”

This means that the research group are likely a handful of extragalactic astronomers and/or cosmologists, focusing on a specific group of very distant galaxies moving away from our position on Earth. The other thing that can be assumed with high-redshift objects is that they are likely much older than anything we can observe in a more immediate proximity to us, but here is where we start discussing relativity… a whole different rabbit hole to go down some other time.

Very helpful animation visualizing these concepts as they pertain to light. Notice the redshift vs the blueshift. Courtesy: NASA — Electromagnetics

How Are These Used?

As you might have gathered from the example I came up with, these concepts are used extensively in deep-space astronomy like galactic astronomy & cosmology. That’s not it, however. Redshift & blueshift can be used in stellar and exoplanetary astronomy, where astronomers might focus in on a binary star system or a singular star with exoplanets to determine the orbit patterns of these objects (this does of course depend on how these things are oriented relative to the observer). In stellar astronomy, astronomers will observe supernovae remnants (stars eject matter every which way in their death cycles) & can determine how the ejecta is ‘moving’ by using redshift & blueshift.

Why It Matters

It’s my hope that the reader can reason at this point that these principles of physics are important for a few different reasons. In cosmology, astrophysicists have used redshift along with other complicated physics to suggest that the universe is indeed expanding, even going so far as to attempt to put a number on it via the Hubble Constant. Observing & characterizing the deaths of stars is important in studying the evolution of cosmic nebulae, and studying systems like binary stars and stars hosting exoplanets are important in their own rights since it’s important to have details that help astronomers can compare against similar systems in order to draw certain correlations. What I mean in this case is that if we can define & characterize orbital patterns of something less common like a binary star system, we can compare this to other instances to gain a better idea of what sorts of conditions allow these systems to exist in a stable state.

…am I allowed to just say “because it’s cool”?

Thanks for reading! Catch you on the next one.

A, TCB

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