2926: Doppler Effect

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Doppler Effect

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

Explanation

This explanation may be incomplete or incorrect: Created by an ALARMED BOT THAT GOES NYOOOOM - Please change this comment when editing this page. Do NOT delete this tag too soon. If you can address this issue, please edit the page! Thanks.

Miss Lenhart is teaching a class about the concept of redshift and using the Doppler effect as a demonstration. The comparison begins as relevant, but then quickly becomes extraneous when she brings up the "BUTTON THAT MAKES IT GO PYEEW! PYEEW!". In fact, it seems she cares more about her special interest in various emergency vehicle sirens than about astronomy, the field in which she teaches, similar to 1519: Venus.

In the second and third panels, Miss Lenhart talks about the strange noise sirens (and cars) make when the pass you. The usual explanation of Doppler effect is that the source of the sound waves is moving. Consider a sound being generated at a frequency of 1000 cycles per second (Hz); each wave will propagate at a fixed speed in the air, the speed of sound. The car generates peak A, which begins moving away at the speed of sound, and 1/1000th of a second later, generates peak B, which also begins moving away at the speed of sound. In that 1/1000th of a second between peaks being generated, the car has traveled slightly forward, so peak B is produced slightly farther along the car's direction of travel than the previous peak. For an observer in front of the car, because peak B was generated a bit closer to the observer, they would measure (hear) a shorter wavelength (higher pitch) than if the peaks were generated from a stationary car. Similarly, for an observer behind the car, because peak B was generated a bit further away, they would hear a lower pitch. However, Miss Lenhart doesn't make this usual explanation and instead starts talking about how cool emergency sirens are.

Redshift is the same concept applied to wavelength of light and stellar objects. Red has longer wavelength than blue, and stars gets red when they move away from us and blue when they move towards 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, and in fact are moving quicker the farther away they are due to universe expansion. Note that unlike the usual explanation of redshift for sirens, a major component of the redshift of light from distant galaxies is due to the expansion of space. This effect is not an important component of the Doppler shift for sirens. Redshift has been mentioned multiple times before, such as in 2764: Cosmological Nostalgia Content and 2853: Redshift.

Still unexplained: What is the "button" that sometimes makes the noise go "pew pew"? Gun triggers?

The title text explains that the Doppler effect particularly affects sirens. This isn't actually true,^{[citation needed]} but it may seem like it because sirens are almost always the analogy given, as in Miss Lenhart's lecture. Then it 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.

Transcript

This transcript is incomplete. Please help editing it! Thanks.

[Miss Lenhart is pointing with a stick to a whiteboard with various scientific drawings and words, including but not only a graph.]

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...

[Zoom in on Miss Lenhart with her arms raised.]

Miss Lenhart: ...but then it zooms past and goes Nyeeeeooooowww?

Miss Lenhart: And sometimes they hit a button that makes it go Pyeew! Pyeew! really loud?

[Miss Lenhart with her finger raised is standing in front of the whiteboard and holding the stick down.]

Miss Lenhart: And in Europe they go Oooo^eeeeoooo^eeee...

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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Discussion

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. 172.68.22.151 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. 172.68.22.151 19:58, 29 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. 172.70.42.213 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? 172.69.246.148 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 ... --172.70.247.172 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. 172.70.90.172 21:06, 29 April 2024 (UTC)