Talk:2926: Doppler Effect

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)

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)

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)