Science desk
< September 8	<< Aug \| September \| Oct >>	September 10 >

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

September 9[edit]

How does an MRI machine not destroy its own electronics?[edit]

So there's this social media post trending about various large metal things stuck to the MRI machine (some apparently quite violently). This makes me wonder .... since it's impossible to build an electronic computer out of plastic (currently), how does the MRI machine not destroy its electronic components? How did MRI machines work in the days before solid state drives? Yanping Nora Soong (talk) 03:13, 9 September 2019 (UTC)[reply]

One obvious solution is to put the delicate electronics further away, with nothing more than wires and motors and sensors needed close to the magnets. All the image processing could be performed remotely, and the hard drives could be there to store the images. For those components which are close, some form of magnetic shielding would be in order. Would a Faraday cage work ? SinisterLefty (talk) 03:30, 9 September 2019 (UTC)[reply]

A Faraday cage blocks oscillating fields (such as the RF of the MRI experiment itself) but not a static field (such as the one of the main magnet that sucks in large metallic objects). Instead, you would want Mu-metal to shield from the static field (MRI equipment is specifically listed in that article). DMacks (talk) 04:16, 9 September 2019 (UTC)[reply]

Did you check Physics of magnetic resonance imaging and links? Gem fr (talk) 06:53, 9 September 2019 (UTC)[reply]

Our article Magnetic resonance imaging says

"The major components of an MRI scanner are the main magnet, which polarizes the sample, the shim coils for correcting shifts in the homogeneity of the main magnetic field, the gradient system which is used to localize the MR signal and the RF system, which excites the sample and detects the resulting NMR signal. The whole system is controlled by one or more computers."

The main magnet creates a static field, not a time-varying field, so it would not create a large enough EMF to disrupt the electronically active components of the scanner. What electromagnetic fields arise from the presence of the main magnet are desired radiofrequency fields when ~~electrons~~ nuclei of hydrogen in the body's water and fat, and of MRI contrast media (usually compounds of gadolinium or similar metals) are excited by the magnet in the MRI scanner, then revert to a lower-energy state. This signal is of low amplitude compared to the massive magnetic field of the MRI device's main magnet, so it does not disrupt computer or other electronics in the MRI suite.--loupgarous (talk) 23:32, 12 September 2019 (UTC)[reply]

I think you were right the first time. It's NMR-like not EPR. DMacks (talk) 11:40, 13 September 2019 (UTC)[reply]

You're right, good catch. I confused "electron spin" with "nuclear spin". Thanks! --loupgarous (talk) 19:28, 13 September 2019 (UTC)[reply]

Tan colour from indoor tanning compared to natural sun tanning[edit]

Why does the skin colour resulting from indoor tanning differ from natural sun tanning? At least for me, it looks like a kind of orange. My hypothesis was that maybe natural sun tanning would reach the same colour if done thoroughly, but since artificial tanning UV lamps are stronger, it's easier to reach an intense tan with them. Another option is that tanning fast produces different results, in the same way that a greenhouse tomato ripens faster but differently than a natural grown tomato.--C est moi anton (talk) 16:28, 9 September 2019 (UTC)[reply]

Does it, really? Methink orange would result from sunless tanning I suspect would be used at the same time. Tomato ripening only involve the sun as a heat source, light has no part of it; greenhouse tomato ripens just like a natural grown tomato (it is actually a natural grown tomato, just done in a more controlled environment...). Gem fr (talk) 19:45, 9 September 2019 (UTC)[reply]

One appears to be confusing spray tanning with sun bed tanning. Spray tanning is what turns some people orange. Sun bed tanning takes much longer and has a heightened risk of skin cancer. See Spray Tan and sun bed Thanks Anton — Preceding unsigned comment added by 81.131.40.58 (talk) 08:01, 10 September 2019 (UTC)[reply]

I suggest that amongst people who desire to tan artificially and quickly (so as to appear tanned from day one of their beach holiday, for example), many are likely to employ both methods simultaneously. {The poster formerly known as 87.81.230.195} 90.202.210.107 (talk) 11:38, 10 September 2019 (UTC)[reply]

snails[edit]

how long do snails sleep Poydoo (talk) 22:25, 9 September 2019 (UTC)[reply]

Sleep_in_non-human_animals contain nothing that would hint the matter was investigated. Sad, this would be worth an igNobel Gem fr (talk) 23:48, 9 September 2019 (UTC)[reply]

I did find one paper on sea slugs (snails are, phylogenetically speaking, basically just slugs with shells). It does not completely document the necessary conditions for proving that the animals sleep (notably it does not contain any experiments involving sleep deprivation, which would be required to reveal rebound behavior). They also did not make any observations at night, which is unfortunate. As for the daytime measurements, they generally found that 90% of the slugs are awake while the tank is being cleaned, 100% are awake when they are being fed, and at all other times about 50% appear to be sleeping. From that you might conclude that about half the slugs are asleep when nothing is happening, but that only covers daytime observations, and this is not a natural environment for them. Someguy1221 (talk) 00:11, 10 September 2019 (UTC)[reply]

First we would need to ask, "Do snails sleep ?". It's not obvious that they do. For an animal that moves slowly enough, it could conceivably effect repairs "on the go", rather than shutting down to do repairs periodically. Another purpose of sleep in higher animals seems to be to consolidate memories and mentally try out various scenarios to find better ways to do things. But a snail's brain may not benefit from that. Of course, one universal benefit to all animals is to conserve energy when nothing requires activity. But being inactive isn't really the same as sleep. SinisterLefty (talk) 01:21, 10 September 2019 (UTC)[reply]

Fortunately, biologists have already provided the framework for answering that question. The most general definition of sleep is that it is a period of relative inactivity that is accompanied by an increased arousal threshold, and deprivation of which results in rebound behavior (sleeping to excess at the next opportunity, or sleeping at inappropriate times). Common features also include a circadian rhythm linked to a day-night cycle, and reduced ability to function following deprivation. The available research on slugs satisfies everything but rebound behavior, which was not tested for. It was quite controversial when flies were reported to sleep, and even more controversial when the discovery was extended to nematodes. It was recently and rather dramatically pushed all the way to jellyfish, which don't even have brains. It seems likely that anything with neurons will have a behavioral state matching the basic 3-part definition of sleep. Someguy1221 (talk) 09:55, 10 September 2019 (UTC)[reply]

Sounds like they defined sleep too broadly. That's more like what most of us would call "rest". I would have added the brain wave patterns we associate with sleep, which would obviously preclude anything without a brain. SinisterLefty (talk) 10:21, 10 September 2019 (UTC)[reply]

Snails have brains of a sort. The tricky part would be to get the electrodes attached. ←Baseball Bugs ^{What's up, Doc?} carrots→ 14:17, 10 September 2019 (UTC)[reply]

Snails like other invertebrates do not have a spinal cord nor one single brain. Instead they have a set of ganglia (groupings of neurons) that distribute the control of the various parts of the snail.[1]. DroneB (talk) 16:04, 10 September 2019 (UTC)[reply]

Snails are particularly easy to study brain [sic] function within. The giant marine snails in particular, as the structures are bigger and more accessible. Aplysia (a sea slug) is a particular favourite, with the 2000 Nobel prize awarded. Andy Dingley (talk) 16:21, 10 September 2019 (UTC)[reply]

"rest" would not qualify, as a resting but conscious human, for instance, would not satisfy all three requirements. In any event, the definition of sleep was deliberately broadened because classical definitions based on brain states, while obviously useful for medical diagnoses, wind up automatically excluding anything without a sufficiently comparable central nervous system. Someguy1221 (talk) 19:50, 10 September 2019 (UTC)[reply]

This is an odd exception to how animal behavior is normally approached, where scientists go out of their way to avoid making animals seem like humans. For example, even mating pairs that stay together for life are not said to "love" each other, just to have a pair-bond. But here they seem to go out of their way to describe very different situations using the same term as in humans.

Also, all 3 conditions seem to apply to rest in humans, as well. It's a relative period of inactivity. If you didn't let somebody rest all day long, they would perform at a reduced level, due to stress, etc., and would take every opportunity to sit down and rest. Amazon.com warehouse workers have run into this very problem, where automated systems keep them on their toes all day, and it really wears on them. The link to circadian rhythms applies to the afternoon siesta, whether they sleep or just rest. And certainly it would take longer for somebody to get going from that state than if they were up and about. SinisterLefty (talk) 01:13, 11 September 2019 (UTC)[reply]

Love is a complex emotional state, however, and uniquely defined by human experience because animals are unable to describe their own. In the case of sleep, actually, the developing theory, and where the field has been moving for decades, is that these are not very different situations. Rather, it is very seriously proposed that sleep represents an extremely ancient behavioral state that evolved in early animals with simple neuronal networks, if not even earlier. This is due to strikingly common behavioral and biochemical features of the observed sleep-like states in all animals more complex than a sponge. The suspicion is that there is something very fundamental about either neurons or networks of neurons (if not cells generally) that mandates a sleep-like state and explains some or all of the common features. But at the same time, sleep is also necessarily very different between organisms based on the structure of their central nervous system and the lifestyle to which they are adapted.

It is absolutely a fair viewpoint, and one held by most journal editors about sixty years ago, to say that since the word "sleep" is almost always a reference to human behavior, it implicitly refers only to the human experience and things provably very similar. I believe that the push to expand the meaning of the word is motivated by a desire to ~~publish papers~~ change how researchers view the phenomenon of sleep itself. Where it was once commonly assumed that what we experience as sleep is a consequence of the human brain's vast complexity, and that no significant aspect of this behavior could be reduced for study in simple models; it is now considered plausible that sleep may be "figured out" by working first with the simplest models, and then building up through more complex organisms to learn how complexity of the brain adds complexity to the state. So, like most other things, run before you can walk, and if you're interested in learning a complex system works, maybe study the the simple versions before trying to tackle one of the most complex machines every built.

And as for your analysis of the three conditions, 'arousal threshold' is not quite what you think it is. It is both the magnitude of the stimulus required to elicit a reaction and the time until reaction for a stimulus of a given magnitude. "Reacting slower" is too general. It's not referring to people taking longer to get up and moving. It's referring to things like sounds and other stimuli below a certain threshold having literally no effect on the sleeper, or many times as long for a reaction to even initiate. It's not yelling at someone laying on a couch to get up and them rolling their eyes; it's yelling at someone laying on a couch to get up and they don't even move because they can't hear you because they're sleeping. If you want them to hear you, you have to either be way louder, or yell again and again and again. In the case of nematodes, you can actually monitor neuronal activity to ask why they are so difficult to rouse when they appear to be sleeping, and you will find that their sensory neurons literally aren't even sending signals anymore unless you pass a certain threshold. That is, they cannot sense what is going on around them. However, reversibility, if you startle the worm with a great big poke, those neurons immediately go back to having normal thresholds. Someguy1221 (talk) 02:06, 11 September 2019 (UTC)[reply]

That sounds a lot like daydreaming while resting, say in class, where the teacher calling the student's name no longer is sufficient to get their attention, they must yell their name, slam a book on their desk, etc. As for love, understanding pair-bonding behavior in lower animals would be a good start in understanding how love works in humans. SinisterLefty (talk) 03:17, 11 September 2019 (UTC)[reply]

Daydreaming would not consistently require reduction of activity (I can daydream just fine while walking), nor would it exhibit rebound behavior where a person prevented from daydreaming for a day begins uncontrollably daydreaming the next day. The definitions are quite robust, and were actually crafted with input from human sleep researchers (sleep researchers who study humans - though they also happen to be human themselves) to include everything they considered to be human sleep, and simultaneously exclude any human behavior they did not consider sleep. I would highly recommend reading some of the literature on non-human sleep states from what our article cites, as they both demonstrate how the definitions are satisfied, and provide much more detailed explanations of what they mean. Someguy1221 (talk) 04:42, 11 September 2019 (UTC)[reply]

Looking into the sleep literature, it is clear that many types of organism exhibit sleep-like behaviour as Someguy1221 has already said. Specific examples from the gastropods (which snails are part of) include the california sea hare (a sea slug) [2] and lymnaea stagnalis a pond snail [3]. Mikenorton (talk) 15:06, 10 September 2019 (UTC)[reply]

This page has a bit of info about snail dormancy and 'sleep'. PaleCloudedWhite (talk) 22:55, 10 September 2019 (UTC)[reply]