Wikipedia:Reference desk/Archives/Science/2023 June 8

= June 8 =

Expanding Universe and Relativity.
Did a little research on this and the main point I found is that when Einstein assumed the Lorentz transformation, he was totally oblivious to the expanding universe. So in other words, the concept of every point in space moving away from all others was not known to him. It was assumed that space was "still" and not moving relative to itself.

Had he known this, would he have thought that the idea of Relativity (the speed of light is the same to all observers) was a non workable idea?

Because an expanding universe seems to lead to the idea that any point generally in front of a photon will see that photon's speed as c - v and any point behind it will see its speed as c + v (where v is simply the relative velocity of a given point in space to the point the photon is at).

So the question is, how can Relativity possibly not be self violating with the notion of expanding space? Byron Forbes (talk) 06:35, 8 June 2023 (UTC)


 * See the top of this page 'We don't answer requests for opinions, predictions or debate.' This is a reference desk for an encyclopaedia. NadVolum (talk) 07:12, 8 June 2023 (UTC)
 * So where is my opinion, prediction or debate?
 * I have asked a simple question - an answer would be appreciated.
 * Censorship in the name of serving agendas is not what I'm looking for at all. If you don't like the implication of the question then that's your bad luck! Please take your combative attitude elsewhere! Byron Forbes (talk) 07:29, 8 June 2023 (UTC)
 * Please be civil. A question that begins "Had he known this, would he have thought" is clearly unanswerable. Shantavira|feed me 09:05, 8 June 2023 (UTC)
 * You are on a mission to resurrect the concept of a luminiferous aether. I wish you the best of luck with that; only, do not use Wikipedia for the purpose. Wikipedia is not a forum for discussing or advocating any particular point of view, scientific or otherwise. This also applies to user pages and to the Reference desk. --Lambiam 09:58, 8 June 2023 (UTC)
 * The question is simple - everything else is context.
 * And the question is, how does expanding space not violate the 2nd postulate. Who can direct me to info on that? Byron Forbes (talk) 16:25, 8 June 2023 (UTC)
 * For your photon scenario try Velocity-addition_formula (use wisely). For everything else, read a book. --Wrongfilter (talk) 16:49, 8 June 2023 (UTC)
 * a velocity-addition formula is an equation that specifies how to combine the velocities of objects in a way that is consistent with the requirement that no object's speed can exceed the speed of light
 * So assume the theory to prove the theory?
 * I'm sorry, but I'm yet to be convinced that a photon at point B, that was just at point A a moment earlier, doesn't have a velocity of c + v where v is the relative velocity between A and B.
 * We know that the photon's velocity is c in B, and we know A travels away from B at v - the conclusion seems unavoidable to anyone but the religious followers of Relativity. Byron Forbes (talk) 17:21, 8 June 2023 (UTC)
 * I take "religious follower" as a personal insult. --Wrongfilter (talk) 17:27, 8 June 2023 (UTC)
 * The universe doesn't care whether you can be convinced about how it works or not. AndyTheGrump (talk)
 * Maybe hold a seance and ask Einstein yourself. ←Baseball Bugs What's up, Doc? carrots→ 11:21, 8 June 2023 (UTC)



Here’s a thought experiment. Suppose one sends a satellite in lower Earth orbit, with a high-precision clock, and a radio transmitter that regularly broadcasts the clock time to Earth. Suppose one stands on Earth, next to the same kind of high-precision clock, and monitors the difference between the Earth clock time and the satellite broadcast time.

Of course, that difference will increase with the distance between the satellite and the reception station, because light speed is not infinite, but that can be accounted for.

Relativity proponents predict that the satellite clock will run faster than the clock on Earth (i.e. "time is relative" and all that jazz); according to them, the time difference will increase over time by about 38μs/day. Of course, standard Galilean physics say the clock will run exactly as fast, and therefore no such trend will be observed.

Now, what would you say if this (purely hypothetical) experiment was done, and turned out to show a shifting trend exactly as relativity proponents suggest? Tigraan Click here for my talk page ("private" contact) 15:59, 9 June 2023 (UTC)
 * Here's a hint: In 1955, Friedwardt Winterberg proposed a test of general relativity—detecting time slowing in a strong gravitational field using accurate atomic clocks placed in orbit inside artificial satellites. Special and general relativity predicted that the clocks on GPS satellites, as observed by those on Earth, run 38 microseconds faster per day than those on the Earth. The design of GPS corrects for this difference; because without doing so, GPS calculated positions would accumulate errors of up to 10 km/day. Philvoids (talk) 16:33, 9 June 2023 (UTC)

Wien displacement law and frequency
Since Wien's displacement law is a linear function "l=b/T" and a wavelength is linked to a frequency "l=C/f", by definition, how can we accept that the Wien's displacement law in frequency is different from "f=CT/b"? Malypaet (talk) 11:43, 8 June 2023 (UTC)
 * Let's start with Planck's_law, which shows how the spectral radiance in terms of wavelength, $$B_\lambda(\lambda, T)$$, is related to the spectral radiance in terms for frequency, $$B_\nu(\nu, T)$$ (the relation between the energy densities $$u_\lambda$$ and $$u_\nu$$ is the same). Note that it is not just a matter of replacing $$\lambda$$ and $$\nu$$ via $$\lambda = c/\nu$$; the wavelength interval has also to be transformed. Your calculation gives the maximum of $$B_\lambda$$, expressed as a frequency. Wien's law in frequency, however, gives the maximum of $$B_\nu$$, a different function with its maximum at a different place. --Wrongfilter (talk) 14:45, 8 June 2023 (UTC)
 * It's just a question of Cartesian logic. If there is a maximum for a certain wavelength, by the relation v=c/l then should also have the frequency for this same maximum. In the same place! Knowing that the only possible experimental verification measurement is on the wavelength, then translated into frequency, right? Malypaet (talk) 16:54, 8 June 2023 (UTC)
 * The difference is in measuring the amount of radiance in a frequency range versus a wavelength range. For the long wavelength radiation, it is concentrated into less frequency and spread over more wavelength. For short wavelengths, radiance is packed into less wavelength range, but more frequency range. SO this distorts the graphs so they are not the same, and maximums re different. Graeme Bartlett (talk) 23:36, 9 June 2023 (UTC)

Planck dλ/dv
Where Planck find this surprising equation :

dλ/dv=-c/dv²

?

When we speak about dv, is it with v+dv, or some other thing ?

Malypaet (talk) 17:06, 8 June 2023 (UTC)


 * Where did you find this surprising equation? It makes no sense, mathematically. --Lambiam 22:13, 8 June 2023 (UTC)
 * I think you meant
 * $$\frac{d\lambda}{d\nu}=-\frac{c}{\nu^2}.$$
 * That is a Greek letter ν|$\nu$, not a Latin letter v. Starting from $$\lambda\nu=c,$$ we have $$d(\lambda\nu)=dc=0,$$ since $$c$$ is constant. But also, by the product rule,
 * $$d(\lambda\nu)=\lambda d\nu+\nu d\lambda,$$
 * so $$c d\nu+\nu^2 d\lambda=\lambda\nu d\nu+\nu^2 d\lambda=\nu(\lambda d\nu+\nu d\lambda)=\nu d(\lambda\nu)=0,$$ which means that $$\nu^2 d\lambda=-c d\nu.$$ Divide both sides by $$\nu^2d\nu$$ and you are done. --Lambiam 22:40, 8 June 2023 (UTC)
 * Thank's, that's great !
 * You helped me. Malypaet (talk) 06:41, 9 June 2023 (UTC)

Earth Circumnavigation Vertically
If money were no object, could modern technology build an aircraft that could circumnavigate the earth longitudinally (crossing both poles) without landing or mid-air refuelling? 24.72.82.173 (talk) 19:33, 8 June 2023 (UTC)
 * Why not? Non-stop non-refueled circumnavigations (non-polar) have happened after all. Ruslik_ Zero 20:37, 8 June 2023 (UTC)
 * Circumnavigation along a meridian and its antipodal partner is a bit more challenging, since the aircraft cannot use the jet streams for assistance. Also, crossing both poles implies the length of the trajectory is at least 40007.863 km. Most aerial non-stop circumnavigations have fallen short of that. The length record for any non-stop non-refueled flight is held by Steve Fossetts 2006 circumnavigation in the GlobalFlyer, a flight of 41466 km. --Lambiam 23:15, 8 June 2023 (UTC)
 * The previous recordholder, of course, was the Rutan Voyager. According to Wikipedia, it flew 26,366 statute miles (42,432 km) on its non-stop, non-refueled circumnavigation but the FAI-certified record is only 40,212 km (which is 24,987 miles). What happened to the other 2,220 km? --142.112.221.43 (talk) 02:44, 9 June 2023 (UTC)
 * I'm not sure, but I remember having read something about it flying in a holding pattern for several hours before landing in order to burn off unused fuel -- might that be the reason for the discrepancy? 2601:646:9882:46E0:1CFE:39D7:62CF:ECBA (talk) 04:02, 9 June 2023 (UTC)
 * For an FAI circumnavigation diploma you do not have to cross the poles, but fly north of 75N and south of 75S and for a total distance of 34,000 km. fiveby(zero) 13:00, 9 June 2023 (UTC)
 * Could Fossett's plane operate under the temperatures it would encounter over the poles? 24.72.82.173 (talk) 03:54, 15 June 2023 (UTC)