Wikipedia:Reference desk/Archives/Science/2019 May 12

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May 12

Laser or radar directed at white dwarf

If a spacecraft were 1 million kilometers from a white dwarf and tried to bounce an extremely powerful radar or laser beam off the white dwarf, what would happen? Are white dwarfs reflective? Also, how much blueshift quantitatively would occur by the time the signal hit the surface. If the signal did bounce back to the spacecraft, would the redshift on the return outweigh the original blue shift or what?Rich (talk) 02:38, 12 May 2019 (UTC)

As a practical matter, any signal returned from a reflected laser signal would be totally swamped by the light coming from the star itself. As for radar, I'm not sure how much radio frequency noise is created by a white dwarf. SinisterLefty (talk) 11:25, 12 May 2019 (UTC)

In science sometimes you need to argue that black is white, and this is one of those times. A white dwarf is a black body and so, at roughest approximation, might absorb light very well. That said, this is an approximation and the details of the atmosphere might make all the difference. [1] says something about the colors, but I may not necessarily understand it. White dwarf pairs can also act as masers, apparently. [2] I don't know if a white dwarf can have an ionosphere. None of this approaches being an actual answer to your question, I'm afraid, so I'll be interested to see who we can get to shed light on this. Wnt (talk) 11:29, 12 May 2019 (UTC)

Electromagnetic radiation with the wavelength exceeding the scale height of the WD atmosphere (tens meters) will be efficiently reflected. So, the radiolocation of white dwarfs is possible. Ruslik_Zero 20:05, 12 May 2019 (UTC)

Neutrino cooling by maser or laser

Could a high intensity lasers, acting over distances of several light years, measurably slow down neutrinos? What I have just heard is photons interact with the very small magnetic moment of the neutrino, so I’m wondering if that should make one want to use higher energy photons, that is, lasers, for this, or given the really low mass ( and low momentum?) of neutrinos should one use the lower energy photons in masers to reduce the recoil? Since the photon would interact with a magnetic field on the neutrino, would there be a problem with causing a perpendicular Biot-Savartlike push making neutrinos swerve instead of slow down? Concerning, I’ve read that various types of laser cooling (probably for different reasons)use laser beams in more than one direction, like 180 degrees and 90 degrees, so could that help to bring about a genuine slowing rather than just swerving? I realizie that might be a project for the far future, seeing has how that would involve lasers or masers several light years apart. ThanksRich (talk) 22:56, 12 May 2019 (UTC)

How much slower is a neutrino leaving a neutron star?

If a neutrino is traveling at x% of the speed of light when exiting the surface of a neutron star or white dwarf, what would be the new speed in the limit as the neutrino traveled arbitrarily far away?Rich (talk) 23:02, 12 May 2019 (UTC)

The same as any other particle like that, being a neutrino makes no difference. I think though Measurements of neutrino speed is what you want, nobody has detected a difference from the velocity of light yet and ones from decays may be going at within 10^-19 or less of the speed of light. Good luck at slowing that down to anything readily detectable! Dmcq (talk) 23:24, 12 May 2019 (UTC)

Im not sure how to do the problem for any particle...Doesn’t relativity make computing the change in kinetic energy more complicated than integrating GMm/r^2?Rich (talk) 02:08, 14 May 2019 (UTC)

Reduction of MnO2 in alkaline battery.

The Alkaline battery article states that MnO₂ is reduced to Mn₂O₃, and several sources agree with that. But others say it is reduced to Mn(OH)₂ ( http://chemconnections.org/general/chem121/Electrochem/Electrochem-III-2011wo.pdf ), or to MnO(OH) (http://data.energizer.com/pdfs/alkaline_appman.pdf ). Which one is correct, or do all three reactions take place, perhaps depending on rate or depth of discharge, or on the structural form of the oxide? Prevalence 23:21, 12 May 2019 (UTC)

Oxidation state will help: just calculate the OS of Mn in

MnO₂ +4

MnO(OH) +3

Mn₂O₃ +3

Mn(OH)₂ +2

meaning, as far as oxidation is concerned, MnO(OH) and Mn₂O₃ are the same; the balance between them will depend, I guess, on availibility of OH^-, that is, pH.

Mn(OH)₂ is next step of reduction. Would occur in deep discharge.

Gem fr (talk) 08:23, 13 May 2019 (UTC)

That makes sense, thank you! Prevalence 01:00, 14 May 2019 (UTC)

• Not pH -- 2 MnO(OH) = "Mn₂O₂(OH)₂". 2 OH^- = O^2- + H₂O, so the difference between this and Mn₂O₃ is like the difference between unslaked lime and slaked lime, but with one water molecule per two manganese(III) ions. (An even more direct analogy to this is Manganese(II) oxide versus the Manganese(II) hydroxide listed above) In general, the (OH) tends to signify there may be water available. Wnt (talk) 23:03, 16 May 2019 (UTC)