2926: Doppler Effect

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Doppler Effect
The Doppler effect is a mysterious wavelength-shifting phenomenon which seems to primarily affect sirens, which is why the 🚨 emoji is red.
Title text: The Doppler effect is a mysterious wavelength-shifting phenomenon which seems to primarily affect sirens, which is why the 🚨 emoji is red.


Miss Lenhart is teaching an astronomy class about the concept of redshift in the light from distant galaxies. She states that why this occurs is an interesting question, then follows this by talking about the Doppler effect of sirens. While sirens are commonly used as an example of the concept of the Doppler shift, and is hence relevant to the preceding topic, Miss Lenhart appears to have raised it for a completely unrelated purpose - she simply has a special interest in sirens. This becomes apparent as her explanation quickly veers away from the preceding topic, similarly to 1519: Venus, or due to a form of topical monomania similar to that which Hairbun exhibited in 1610: Fire Ants.

Different emergency vehicles may have different siren tones, and many have different tones on the same vehicle, which they can switch between for different circumstances, such as long NYEEEOOOWWW to alert people at a distance and short PYEEW PYEEW when they are closer to drivers, as for example when crossing an intersection.

In the second and third panels, Miss Lenhart talks about the strange change in perceived noise sirens (and cars) make when they pass you. The usual explanation of Doppler effect is that the source of the sound waves is moving and the wave can sound different depending on whether the source is coming towards you or away from you (for details/explanation see the Doppler effect in Wikipedia).

Redshift is the same concept applied to wavelengths of light. Red has a longer wavelength than blue, so light-emitting objects get redder when they move away from us and bluer when they move toward us. We usually talk about redshift and not blueshift because while stars in our galaxy can move in any direction relative to us, most other galaxies are moving away from us. The fact that more distant galaxies are moving away quicker the farther away they are shows that the universe is expanding.

Unlike the usual explanation of redshift as equivalent to the Doppler effect for sirens, a major component of the redshift of light from distant galaxies is due to the expansion of space in between us and the light source. This effect is not an important component of the Doppler shift for sirens.[citation needed] Redshift has been mentioned multiple times before, including in 2764: Cosmological Nostalgia Content and 2853: Redshift.

The title text claims that the Doppler effect particularly affects sirens. This isn't actually true, but it may seem like it because people hear Doppler shifts for sirens more than for other sounds. Sirens tend to employ predictable tone(s), which people who aren't totally tone deaf would have experienced as a shift in pitch from a passing vehicle's siren, whereas something equally subject to Doppler shift like engine noise could also change pitch according to differences of speed and gearing. Then the text claims that the emoji for sirens is red because they're associated with redshift. Actually, the emoji is a picture of the rotating light on top of emergency vehicles; these tend to be used in conjunction with sirens, and they're red because this color typically signifies danger or warning (though, in fact, blue lights used with sirens are also common).


[Miss Lenhart is pointing with a stick to a whiteboard. There is an unreadable heading and two lines of unreadable text above a drawing of a spiral galaxy, this is what she points at. Below that there is a graph with a curve that looks like it is increasing exponentially. The line is going through a cloud of points, scattered on either side of the curve. Beneath the graph there is another unreadable line of text.]
Miss Lenhart: The more distant a galaxy is, the redder its light.
Miss Lenhart: Why? Well, that's an interesting question.
[Zoom in on Miss Lenhart.]
Miss Lenhart: Ever notice how, when a siren is approaching, it sounds like Bweeeeeeeeee...
[Same setting but Miss Lenhart has raised her arms.]
Miss Lenhart: ...but then it zooms past you and goes Nyeeeeooooowww?
Miss Lenhart: And sometimes they hit a button that makes it go Pyeew! Pyeew! really loud?
[Back to the original view with Miss Lenhart in front of the whiteboard. She is now raising a finger in the air while holding the stick down with her other hand. A student ask a question from off-panel.]
Miss Lenhart: And in Europe they go Ooooeeeeooooeeee...
Off-panel voice: So why are galaxies red?
Miss Lenhart: Oh, no idea.
Miss Lenhart: Anyway, another siren I like is...

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Honestly, this is one of my favorite ones yet, Apollo11 (talk) 18:21, 29 April 2024 (UTC)

I'm not always a big fan of Miss Lenhart comics, but I agree this one is good. Barmar (talk) 18:39, 29 April 2024 (UTC)

I think there might be some confusion in the explanation. It suggests that red shift occurs because of space expansion, not because of relative motion between the light source and observer. My understanding is that there IS relative motion between the light source and observer BECAUSE of expanding space. 19:54, 29 April 2024 (UTC)

Edit - There appears to be a "just" in there that I missed, changing the meaning of the sentence somewhat. Never mind. 19:58, 29 April 2024 (UTC)
My understanding is that there is relative motion between the galaxies, but there is also redshift caused by the expansion of space while the light was traveling, which would occur even if the galaxies were at rest. And IIUC for most galaxies this is the dominant effect -- the Doppler shift caused by the motion of galaxies when the light was emitted is small, but the cosmological redshift caused by the light traveling for a long time is large. Vyzen (talk) 21:54, 29 April 2024 (UTC)
As far as I have understood you could use both methods and get the same result. The speed with which distant galaxies are receding is directly proportional to the expansion of the space between them. Thus whether you look at the speed and say then they must be thus redshifted, or the space they have traveled through that have expanded, the calculated red shift it the same. For objects closer the relative velocities towards or away from each other would have a significant effect but not for very distant objects. --Kynde (talk) 11:39, 30 April 2024 (UTC)

The bit about the Doppler effect being similar to a bullet fired from a moving car is simply incorrect. That's vector addition of velocities. Sound traveling from a source is going to travel at the speed of sound in the medium, and the only addition of velocities would be to the extent that the car is moving the air around it. Also, the Doppler effect doesn't make sounds louder, that's simply a function of the distance between you and the source changing, independent of velocity. Edited the text accordingly. 20:00, 29 April 2024 (UTC)

Although firing an automatic firearm from a moving car can make a pretty decent analogy, as the bullets will pass a person the car is moving toward more frequently or a person the car is moving away from less frequently. Though I think drive-by shootings are probably not the ideal metaphor to use in classrooms. Perhaps a nerf gun? 20:38, 29 April 2024 (UTC)

This comic seems to be poor nerd sniping for explainxkcd to get into a long explanation why galaxies are red ... -- 20:08, 29 April 2024 (UTC)

In the UK, the primary emergency-vehicle (police, ambulance/paramedic, fire, coastguard, anything else similarly official; for road/off-road/air/water vehicles of all kinds) flashing light tends to be blue. There may be alternating reds too, according to vintage, but currently blue lights are the main feature (and 'battenburgs', on marked vehicles, according to the nature of the service involved). Non-emergency vehicles' 'beacons' would be amber, on anything underspeed/stopped/extraordinary on the carriageway (road-sweepers, flatbed car-recovery, exceptional load carriers/escorts) and I think green and red flashers are common for construction site traffic. Interestingly, the other day I saw a police car and an unmarked response car (going to the same incident, both flashing their blues), three ambulances (none obviously going to same incident, and only two with blues) and a fire-engine (not flashing, probably going back to base). Only one of them (an ambulance) was blaring its respective siren, though. I believe emergency drivers are required to use them sparingly/judiciously, rather than just put the blues'n'twos on and barge through. 21:06, 29 April 2024 (UTC)

Blue lights are actually common in most of Europe: https://en.wikipedia.org/wiki/Emergency_vehicle_lighting#Usage_by_country. The light of emergency vehicles is technically also effected by the Doppler Effect, though this is barely measurable at typical driving speeds. -- 09:36, 30 April 2024 (UTC)
Yes also in Denmark they use blue lights. Red light are all around already, so not so good using that to make a difference. Typical US :-p --Kynde (talk) 11:43, 30 April 2024 (UTC)

It appears to be sheer coincidence that sirens were relevant to the discussion, as Miss Lenhart does not actually seem to know that the same phenomenon is at work. -- 22:04, 29 April 2024 (UTC)

I’m an EMT and can explain the button that makes it go PEW PEW! The siren has quite a few different settings with different noise patterns. The noise patterns are chosen based on the surroundings. The default is a T1, the least obtrusive. You use that one on long straight roads. Coming up to an intersection, you would switch to T2, which is more noticeable and lets them know you’re close to the intersection before they can see you. Once you’re crossing the intersection, you switch to T3, the loudest and most irritating patterns, so the chance of someone not noticing you and causing an accident is lowest. There are a couple different patterns for each tier. The sirens are controlled by either dials or buttons (and some touchscreens that I hate) and there’s also a button to make the siren go off at a set tone or pattern for as long as you hold it down. Lathgaertha (talk) 22:50, 29 April 2024 (UTC)

The three tiers are described in this video: https://www.youtube.com/watch?v=salqpgFuOZA. And the sounds can be heard on this video: https://www.youtube.com/watch?v=QFj0q37uvxo. Orion205 (talk) 01:20, 30 April 2024 (UTC)

re: "Pew pew" still unexplained. Note that Miss Lenhart actually says "pyeew" not "pew". It's most likely not a reference to shooting, but to a siren signal that (to my knowledge) is particular to US ambulances. Sound effect in question at ~0:18 here. 07:52, 30 April 2024 (UTC)

This comic has no colors... But the title text makes note of using a colored emoji... Are we agreeing that the comic should still not be listed as one using colors? (I think that should be restricted to the main comic). --Kynde (talk) 11:44, 30 April 2024 (UTC)

With recent new discoveries by James Webb Space Telescope of very old galaxies that shouldn't have time to form, the theory of tired light is making a comeback. It's still far from mainstream, but Miss Lenhart can actually be on to something when she states we don't know why the redshift. (talk) 12:11, 30 April 2024 (please sign your comments with ~~~~)