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May 27

Effect of very high redshift on neutrino velocity

If a galaxy were sufficiently far away, and thus with big redshift and big speed away from us, could that cause neutrinos emitted from that galaxy to travel, relative to us, at considerably below lightspeed, say 0.5c, 0.01c, or even less? — Preceding unsigned comment added by Richard L. Peterson (talk • contribs) 07:30, 27 May 2020 (UTC)[reply]

Redshift means that we measure a lower energy and momentum than the correponding values measured in the frame of the galaxy that the neutrino was emitted from, so yes, we would measure a lower velocity. However, neutrinos emitted from stars (like Solar neutrinos) have energies in the MeV range, much larger than their mass (at most eV), so they move effectively at the speed of light. As galaxies have redshifts less than about 10, this remains true for any neutrinos emitted from galactic sources in our Universe. If you want slow neutrinos, you should look at the cosmic neutrino background. This has a temperature of 1.95 K now, which would correspond to typical velocities of 0.02c for a neutrino mass of 1 eV (higher if the neutrino mass is lower). --Wrongfilter (talk) 08:20, 27 May 2020 (UTC)[reply]

Convenience link for readers: MeV and eV refer to (mega)electronvolts. --47.146.63.87 (talk) 22:11, 2 June 2020 (UTC)[reply]

Why do flies make so much noise during sex?

I have often been alerted to the presence of copulating flies (of various species) by the loud sustained buzzing noise they make while doing so. Why do they make such a lot of noise in this way? It surely makes them vulnerable to predation, indeed as reported here. PaleCloudedWhite (talk) 12:27, 27 May 2020 (UTC)[reply]

Reminds me of a true story I saw a while ago (unfortunately I can’t remember the details). A female human was stranded in the wilderness for a while, and had her period during that time; she was worried about the blood attracting predators. I’ve been wondering about that since then. Menstruation (for humans) and Menstruation (mammal) don’t seem to address this directly, although they do suggest that menstruation does not confer an evolutionary advantage. Brianjd (talk) 13:41, 28 May 2020 (UTC)[reply]

Humans and other animals are rather noisy in that situation too. Maybe the buzzing noise excites the flies just as the respective noises excite humans. If it helps to a succesful copulation it is a selective advantage which compensates for the disadvantage of drawing predators' attention. In the long run this 16 percent of flies eaten by bats can be of no relevance if the other 84 percent are not eaten while laiyng more eggs or copulating twice as often or what makes flies more successful in this domain if they are more noisy. Same for menstruation: maybe menstruating women are more vulnerable to predators, but first this is less true since humans live in large communities able to defend them, and importantly the advantage for a freshly fertilized egg to find a soft and fresh mucous membrane to implantate itself in the uterus instead of some dry and fibrous age old tissues must compensate for the disadvantages. In general it seems that 20 or 30 percent losses doesn't matter at all if another 20 or 30 percent ist successful enough. I think the problem comes from the idea that selection has to act only on individuals: this is only true for individuals living alone for themselves. As soon as several individuals live together and act as a group, the group becomes also an object for selection. I know that this view is deprecated today, but the paradigma 'selection can only act onto individuals' seems to me to be simply untenable under several aspects. 2003:F5:6F08:8200:25BC:4DFB:3A9F:92FF (talk) 18:33, 29 May 2020 (UTC) Marco PB[reply]

Considering their short lifespan, they've got to grab all the gusto they can. ←Baseball Bugs ^{What's up, Doc?} carrots→ 19:08, 29 May 2020 (UTC)[reply]

In the case of humans, we tend to not stay quiet and thus not alert predators when we are in groups. Groups of humans are loud. Baboons have the same behavior.[1] I would guess that this is to warn off predators who might be willing to attack one ape but might not want to tangle with a group of them. --Guy Macon (talk) 18:41, 31 May 2020 (UTC)[reply]

Wheelchairs and steps

Can a wheelchair go up a 5cm high step without much effort? 90.192.124.163 (talk) 20:06, 27 May 2020 (UTC)[reply]

What type of wheelchair? What sized wheels? What constitutes "much effort"? --OuroborosCobra (talk) 20:09, 27 May 2020 (UTC)[reply]

Assuming a wheel diameter of 24 inch (almost 61 cm), the initial effort to make the wheel go over the step is about the same as required for going up a 33.3° ramp. In general, for a step of height

s

and wheel radius

r

(half the diameter), when the wheel makes contact with the step, the axle is still at a horizontal distance

d={\sqrt {r^{2}-(r-s)^{2}}}

from the edge of the step. The angle between the vertical and the line from the axle to the point of contact is then

\alpha =\mathrm {arctan} {\frac {d}{r-s}}.

The effort of moving forward is the same as on a slope perpendicular to the line from axle to point of contact, that is, a slope at angle

\alpha

. If

s

is small compared to

r

,

\alpha

can be approximated by

\alpha \approx {\sqrt {\frac {s}{r}}}\times 81^{\circ }.

--Lambiam 21:58, 27 May 2020 (UTC)[reply]

It's important to realise that people use wheelchairs for a wide varieties of reasons. Some paraplegics have better upper body strength and capability than most able bodied people, and could cope with a 5 cm step with no difficulty at all. Others can be closer to being tetraplegic, and while they can use a manual wheelchair successfully on flat, hard surfaces, they would find a 2 cm step an insurmountable barrier. Of course, one needs to consider whether we are talking about manual or motorised wheelchairs, or among the latter, those specifically designed to navigate stairs.

An alternative and possibly more important question to me is, why on earth does a 2 cm step exist? Surely a sloping ramp of some sort would be more appropriate for all of us there. Architects and builders seem to have an obsession with putting stairs everywhere, even when a sloping ramp would be easier and cheaper to install. I call it the "Gone With The Wind" syndrome, a subconscious desire to create some sort of dramatic statement with a staircase. HiLo48 (talk) 23:59, 27 May 2020 (UTC)[reply]

Whether you meant 2 cm or 5 cm, possible reasons include: incorrect matching of outdoor paving ~~with~~ such as sidewalks with building floor levels; settling of the substrate below the paving materials; incorrect adjustment of the train floor height according to its current load; reduced horizontal clearance between the train and the platform (by placing the bottom of the train floor is slightly above the platform). It's not all about an architectural obsession with stairs. --76.71.5.208 (talk) 00:51, 28 May 2020 (UTC)[reply]

Whoops. Sorry. I meant 5 cm. I'm a mature aged, metricated Australian who tends to do instant cm<->in conversions to keep up with what Americans usually write here, but this time I didn't need to. My mistake. But yes, I agree, small steps can occur "by accident", but even a lot of them could be smoothed out if society thought and cared more about wheelies. HiLo48 (talk) 01:33, 28 May 2020 (UTC)[reply]

2cm could keep water outside a threshold of a building even for nominal ground-floor entrance, or in the gutter rather than spreading up a driveway (roadway design standards seem to be 1–2 inches for that latter context). DMacks (talk) 02:23, 28 May 2020 (UTC)[reply]

But it could be a sloping ramp, rather than a step. HiLo48 (talk) 03:19, 28 May 2020 (UTC)[reply]

The step could be that of the kerb separating a roadway from the pedestrian pavement. --Lambiam 07:59, 28 May 2020 (UTC)[reply]

The equivalent ramp slope only indicates the initial force required to start moving over the step, not a sustained effort. Another measure is the speed of approach for which your momentum will carry you over the step. If I'm not mistaken, it is given by

v={\frac {r}{r-s}}{\sqrt {2gs}},

in which

g

is the gravitational acceleration. For a step of 5 cm, this comes out at 4.3 km/h, a walking pace. The jolt will be considerable but not insufferable. A small ramp, even if as steep as 45°, will significantly reduce the jolt. --Lambiam 07:50, 28 May 2020 (UTC)[reply]

I'm in awe of your calculations Lambiam, but you seem to have forgotten that on a standard manual wheelchair, the wheels making first contact with the step will not be the big 24" ones but the much smaller caster wheels at the front (unless you are going backwards). So it's necessary to tip the wheelchair back to lift these caster wheels over the step before the big wheels make contact. There are users who can perform this manoeuvre unaided, but there are also those who cannot. Just ramming the step at high speed tends to result in the forward ejection of the user, as the small wheels come to a full stop. Wheelchairs (except sports models) have a projecting piece at the back which an assistant can push down on with their foot to lift the front wheels when negotiating steps or kerbs.

The obvious solution to this conundrum is a rubber threshold ramp. The linked site says: "4mm may not seem much to someone not in a chair, but it presents a physical barrier that stops you entering if you are in a chair". They start at GBP 15 (USD 18). [2] Alansplodge (talk) 11:09, 28 May 2020 (UTC)[reply]

For my first calculation I had not forgotten about them, having observed that wheelchair users typically have become adroit at popping the caster wheels by giving the chair a quick backward jolt, a skill acquired mainly by practice.^[3] Doing that at just the right moment while approaching a threshold at 5 km/h is indeed more difficult – although, I expect, not out of reach for most wheelchair users. --Lambiam 14:49, 28 May 2020 (UTC)[reply]

BTW, I doubt the claim that a 4 mm threshold constitutes a problematic barrier. --Lambiam 16:43, 28 May 2020 (UTC)[reply]

If there's a reader here who uses a wheelchair, maybe they could comment. 4 mm is .15 inches, which isn't much. Just going from a bare floor to a carpeted area could be more than that. ←Baseball Bugs ^{What's up, Doc?} carrots→ 16:55, 28 May 2020 (UTC)[reply]

4 mm would not be a barrier to most, but it would be to some. HiLo48 (talk) 23:41, 28 May 2020 (UTC)[reply]

Assuming a 4-inch diameter for the caster wheels, the momentum will carry you over a 4 mm barrier even at a moderate speed of 1.1 km/h (0.68 mph). Section 303 (Changes in Level) of the Architectural Barriers Act Accessibility Standard (ABAAS) of 2006 allows vertical changes in level up to 6.8 mm to remain unbevelled.^[4] --Lambiam 11:10, 29 May 2020 (UTC)[reply]

Thanks, it seems the 4 mm claim is hyperbole. However, that must mean that a step of 50 mm is not permitted in US federally funded buildings. Alansplodge (talk) 11:48, 29 May 2020 (UTC)[reply]

The threshold ramp link mentions doors with thresholds. If it's high enough, you could trip on it just from normal walking, so that could certainly be a problem for a wheelchair. 4 mm is small, as is 6.8 mm - but 50 mm or 5.0 cm would be close to 2 inches, and that could definitely be a struggle in a wheelchair. ←Baseball Bugs ^{What's up, Doc?} carrots→ 13:36, 29 May 2020 (UTC)[reply]