Wikipedia:Reference desk/Archives/Science/2023 December 1

= December 1 =

AC vs refrigerator
Hi. I was told by my HVAC installers that AC units need regular re-fills of refrigerants. I have followed their advice and have refilled my AC units regularly.

But then I realized that my refrigerator has never had a refill of refrigerants. Nor have I ever heard of any non-commercial user refill their refrigerator with new refrigerants. These three sources seem to confirm my understanding.

How come ACs need refills when refrigerators don't?

Are they not both closed systems? (Assuming no system damage or leaks) Liberté2 (talk) 03:46, 1 December 2023 (UTC)
 * According to this, refrigerators can leak, and then you have to deal with adding more freon. And according to this, air conditioners shouldn't need new freon unless - guess what - they leak! ←Baseball Bugs What's up, Doc? carrots→ 07:36, 1 December 2023 (UTC)


 * Both are closed systems, but the piping in refrigerators is created in the factory under controlled conditions. The piping for AC is installed on site. Combine that with the larger size and length of piping and it becomes probable that the AC systems will leak more. Rmvandijk (talk) 09:18, 1 December 2023 (UTC)
 * Thanks. Is there some sort of comparison between the pipe lengths of the average home refrigerator, vs the pipe lengths of the average home AC unit?
 * Or maybe the total pipe surface area would be a better comparison? Liberté2 (talk) 09:58, 1 December 2023 (UTC)
 * More the number of joints made in the field and the circumference of those joints. Poor joints are the most likely place to get leaks. PiusImpavidus (talk) 11:10, 3 December 2023 (UTC)
 * Thank you.Liberté2 (talk) 21:08, 3 December 2023 (UTC)

Blueshift
Is there any theoretical limit, on the blueshifting effect (whether caused by gravitation or by the Doppler effect)?

i.e. a limit on how much a given object's color can be blueshifted.

I guess there is no such limit, but I want to be sure.

( I ignore the issue, of limits caused - if at all - by the Planck units, which is a controversial issue brought up by another user some days ago. As far as my question is concerned, I'm asking about a process, i.e. about changing the color). HOTmag (talk) 09:58, 1 December 2023 (UTC)
 * The wavelength of a spectral line can be blueshifted to arbitrarily small (but always positive) values — or equivalently, the frequency to arbitrarily large values. This is not different to the redshifting effect(s). However, the usual quantification in terms of $$z = \frac{\lambda_\mathrm{obs} - \lambda_\mathrm{emit}}{\lambda_\mathrm{emit}}$$ is limited to values $$z>-1$$, but that's a numerical limit, not a physical one. --Wrongfilter (talk) 10:22, 1 December 2023 (UTC)
 * Thanks. So, I understand there's no theoretical limit on the very process of blueshifting, when it's caused by the Doppler effect. I guess the same is true for a blueshifting effect caused by gravitation, right? HOTmag (talk) 10:35, 1 December 2023 (UTC)
 * The largest increase in energy should be observed in a photon approaching a massive black hole. While gravitational time dilation will have the effect of decreasing its frequency, the blueshift can only be observed by an observer near the black hole. (Sending the photon back with a mirror makes it travel out of the gravitational well and get redshifted back to normal.) The observer undergoes the same time dilation, so (I think) this effect can be ignored. A limit on the mass of black holes (such as the mass of the observable universe) should then give a limit on the gravitational blueshift. --Lambiam 12:11, 1 December 2023 (UTC)
 * Here's a thought on this type of question and the types of answers that we are giving. What is meant by a "theoretical limit"? Purely within a theory, such as special or general relativity, there may not be a limit on, say gravitation or Doppler redshifts. In special relativity, there is no limit on the energy that a particle can have as we can always transform to an inertial system where the particle's energy is Lorentz boosted to any value you'd like. This is the type of answer that I tend to give, because I don't feel particularly at ease in the real world. There are however limits that are tied to the particular realisation of our universe, these are contingent limits. For instance, we do not expect to observe cosmic rays of arbitrarily high energy, even ignoring the GZK limit, because in our universe there are no processes that create particles of arbitrarily high energy and no relative velocities that would Lorentz boost to arbitrarily high energies. Lambiam's limit would be of the second type. Does that make sense? I'm not sure what you mean by "The observer undergoes the same time dilation" — observers always carry their proper time with them, which I would understand as "observers do not undergo time dilation". But maybe you meant something else. --Wrongfilter (talk) 12:46, 1 December 2023 (UTC)
 * From the point of view of an outside observer, clocks slow down near a black hole, and the frequency of incoming photons decreases accordingly. However, an outside observer cannot measure this frequency; this has to be an inside job. But the clocks of "inside" observers also slow down – still from the point of view of an outside observer. --Lambiam 07:58, 2 December 2023 (UTC)
 * There's no theoretical limit that I know of but there are practical limits as explained in Greisen–Zatsepin–Kuzmin limit and the higher the energy the less far they can go. I guess in the limit the source has to be enormous and the distance they can go gets very small . But that's in astronomical terms ;-) NadVolum (talk) 13:51, 2 December 2023 (UTC)

Longest earthquake?
I keep thinking of a question of the longest earthquake ever recorded, by duration. Can anyone answer what it is? Thank you. Brennan1234567890 (talk) 13:27, 1 December 2023 (UTC)
 * Multiple sources claim that the longest earthquake was 32 years. It was a "slow slip event" that occurred in Sumatra. The event was undetected by the people who lived through it. It ended with an 8.5 magnitude earthquake in 1861. 97.82.165.112 (talk) 15:36, 1 December 2023 (UTC)
 * Source:National Geographic (provided by 1861 Sumatra earthquake)- gadfium 19:54, 1 December 2023 (UTC)
 * The 2011 Tōhoku earthquake and tsunami is also thought to have been at the end of long slow slip event, in that case it lasted 9 years (see here, which also discusses such events in general). Mikenorton (talk) 17:15, 4 December 2023 (UTC)
 * If we ignore slow earthquakes, the longest quake was probably the one with the largest magnitude, the 1960 Valdivia earthquake, which lasted for about 10 minutes or the 2004 Indian Ocean earthquake, which was of similar duration. Mikenorton (talk) 17:15, 4 December 2023 (UTC)