Wikipedia:Reference desk/Archives/Science/2018 September 6

= September 6 =

Alpha decay of Be-9?
My inorganic chemistry text book contains this practice problem. The question seems totally nonsensical to me. Firstly, Be-9 is a stable isotope. It doesn't undergo alpha decay. Secondly, even if it did undergo decay, it could not have a daughter product 3 mass units heavier than itself, plus an alpha particle and then neutrons on top of that. The text gives this answer which makes even less sense (many of the other answers to problems are obviously incorrect, so I don't put a lot of stock in what they give). The equation balances, but it involves a step where two Be-9 nuclei first fuse, which is not what I'd call simply "Be-9 undergoes alpha decay"). Any help would be appreciated. 139.194.67.236 (talk) 02:33, 6 September 2018 (UTC)
 * Indeed. Which kind of textbook makes basic errors like this?--Jasper Deng (talk) 02:40, 6 September 2018 (UTC)
 * It's Shriver & Atkins' Inorganic Chemistry, the Atkins being Peter Atkins. His physical chemistry textbooks are the go-to for many courses as they're really probably the best out there, but this inorganic text is full of errors. 139.194.67.236 (talk) 03:00, 6 September 2018 (UTC)
 * The problem is nonsensical, even as a hypothetical sample problem meant to demonstrate simple concepts, like conservation of mass number and atomic number in nuclear decay events. Alpha decay as a process should always produce a smaller atom, not a larger one.  Strictly speaking, if Be-9 were to undergo alpha decay (not that it would, merely if we were to work out the product if it did), the product should be He-5, not C-12.  Something emitting an alpha particle and gaining mass number and atomic number is nonsensical.  -- Jayron 32 15:10, 6 September 2018 (UTC)
 * The fusion reaction given in the answer is allowed as a spontaneous reaction, let's see if anyone here can come up with an estimate of the reaction rate per atom in piece of Be-metal at room temperature. Count Iblis (talk) 13:54, 6 September 2018 (UTC)
 * That reaction is literally how the neutron was discovered. Though it is inaccurate for a textbook to refer to this as decay of 9Be. Someguy1221 (talk) 22:39, 6 September 2018 (UTC)

Providing the current precision limit of our measurements, what is the minimal size of the universe?
I am not referring to the observable universe, but to the entire universe.

As far I understand, current observation is that the universe is flat, and this indicate infinite universe.

However, the measurements have a precision limit, which still allow a very low curvature, beneath our measuring abilities.

Assuming the curvature is as big as possible to exist but still not to be detected, what is the size of the universe? אילן שמעוני (talk) 04:23, 6 September 2018 (UTC)


 * 14 trillion lightyears according to this. 139.194.67.236 (talk) 11:17, 6 September 2018 (UTC)


 * Or it may be that the universe has a high curvature, but the part we can observe just happens to be on a flat spot. Or there may be something we don't understand yet that negates the assumption that flat=infinite. Or perhaps the Simulation hypothesis is true and the owner of the simulator doesn't bother simulating the parts that cannot be observed. --Guy Macon (talk) 16:45, 6 September 2018 (UTC)


 * I think we can eliminate any variation of solipsism from any scientific analysis of the universe. Once we start getting into unfalsifiable ideas, we've moved from the realm of science into something else.  The axiom that reality exists and is knowable (or at least reliably modelable) by humans is necessary for any productive realm of thought to move forward; once we start dealing in fanciful and unfalsifiable possibilities, we're outside the scope of the question.  -- Jayron 32 18:10, 6 September 2018 (UTC)


 * You actually think that the claim that the entire universe (not just the visible universe) has the same curvature throughout is falsifiable? How exactly would you falsify it? --Guy Macon (talk) 23:37, 6 September 2018 (UTC)
 * It could be in principle falsified by information from gravitational waves or neutrino radiation that predate the recombination (and thus give us a larger observable universe, maybe even all of it) or in the event of a big crunch we eventually get to observe all of the universe as it comes hurtling back to us (which ironically, is more likely to happen if we are in some sort of unrepresentative bubble that is unlike the rest of the universe with different cosomological constants and curvature and all that). Could also be primordial wormholes or other exotic phenomena that link us to regions of the "unobservable" universe. We don't have enough certainty to declare that it's definitely unfalsifiable. 202.155.85.18 (talk) 02:08, 7 September 2018 (UTC)
 * Every time you speak you make it clear how unworthwhile it is to listen to you. -- Jayron 32 11:35, 7 September 2018 (UTC)
 * According to The Hitchhiker's Guide to the Galaxy "Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space." I'm not sure we can say all that much more about the size of the entire universe at this point. Dmcq (talk) 13:02, 7 September 2018 (UTC)
 * We can provide lower limits (probably larger than the diameter of the surface of large scattering). The Wikipedia article is Shape of the Universe (note that a flat Universe need not be infinite), but that is weak on observational constraints. Planck results are analysed here, and this should be a readable summary. --Wrongfilter (talk) 17:43, 7 September 2018 (UTC)


 * Its one of the tasks of the James Webb Space Telescope to find that answer. Please have some patience and send some well wishes to NASA.