Wikipedia:Reference desk/Archives/Science/2017 December 5

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December 5

"Elastic collisions" and time travel

Can the theory of elastic collisions, extended into spacetime, still work when one of the colliding masses' velocities is timelike? IIRC, a character in HG Wells' The Time Machine raises the concern that a time traveller might be killed by arriving inside a solid object. (I'd assume that while traveling at full temporal speed, her density would be effectively reduced, and thus she'd be able to pass through solid matter without inflicting or sustaining significant damage, just as neutrinos can.) I'm wondering whether this could be avoided by colliding elastically with that other object, forcing it to time-travel out of the way in the same temporal direction. NeonMerlin 04:03, 5 December 2017 (UTC)

Whatever about about it being an elastic collision I think something like What If - Relativistic Baseball might happen. Dmcq (talk) 12:41, 5 December 2017 (UTC)

No, because one cannot move backwards in the time dimension. One of the things about Minkowski spacetime is that the time dimension only allows macroscopic objects to move in one direction according to the Arrow of time. Backwards time travel, except under highly restricted conditions (see T-symmetry) does not really happen without violating Causality. All sorts of Temporal paradoxes are introduced, both in terms of basic lay explanations (like the Grandfather paradox), and mathematically rigorous explanations (see Causal structure). Simply put, the timelike dimension does not obey the same rules as the three spacelike dimensions, and cannot be treated as such. It is true that the mathematics is arbitrary as regards to which direction time flows; but basically once you have aligned your arrow of time to designate a future and a past (time orientability), spacetime only moves in one direction if multiple observers all agree on the direction time is moving; i.e. once the universe gets going one way, it all goes that way. --Jayron32 12:55, 5 December 2017 (UTC)

In order to dispute the Arrow of time, space has to be counted as something that in some way can be dissipated. Before it might be yes as well as no. But the OP seem to be assuming some kind of Doppler effect as applied to teleportation. --Askedonty (talk) 16:47, 5 December 2017 (UTC)

Just a side note here — I am not sure that word timelike means what you think it means. A normal (bradyonic) particle's four-momentum is always timelike; the weird case would be spacelike. I can't seem to work out in my head right now just what "four-velocity" ought to be, but I think, if there is such a thing, it would normally be timelike. --Trovatore (talk) 23:32, 5 December 2017 (UTC)

Ah, four-velocity; if it'd been a snake it'd'a bit me. --Trovatore (talk) 23:45, 5 December 2017 (UTC)

Yeah, but after you de-Lorentz-transmogrify it - which you would have to do during any analysis that adds or subtracts vectors - you still get the right answer, and unsurprisingly, the constant "c" drops out exactly as you would expect. One cannot defeat conservation of momentum nor conservation of energy, which are ultimately the only laws of interest when we study collisions under relativistic conditions!

Landau & Lifschitz Mechanics Chapter IV, §16; from whence, those who care to follow, we can derive relativistically-corrected Thomson scattering, and then further, Compton scattering ...

What we have in this question is a fun use of the English language to obfuscate the actual physics - which is much simpler than our OP wants to admit. Even when we consider motion in time- and space- - irrespective of how we write out our vectors - we cannot forget to conserve energy and momentum. These simple facts are what keeps our physics, and our conclusions, grounded in reality. After all, we are all "traveling in time," and always at "velocity" "ct/ct"! We need neither fictional machinery nor new physics to describe it!

At this time, I would remind our new initiates (e.g. our OP, User:NeonMerlin) that relativistically-correct scattering theory is hard physics - it takes several years of gruelling preparation to attain anything close to a complete understanding; but after you put in all the effort and solve a few thousand sample problems, let me assure you that you will never forget your efforts!

Nimur (talk) 19:33, 7 December 2017 (UTC)

Cconceptually, I suspect there are some scenarios within an extremely large gravitational field where the same object can appear arbitrarily close yet separated by long light cones and therefore time. The Einstein Cross is in essence, the same object at different points in time affecting telescopes on earth. It's not time travel per se but an interesting phenomena. Everyones current frame is at the tip of the time arrow and and we are constantly being held by the past. --DHeyward (talk) 00:10, 6 December 2017 (UTC)

Speed of molecular machines

I've seen this video and others of molecular machines. Is the speed shown the speed at which they operate, or are they faster or slower? Bubba73 ^{You talkin' to me?} 04:47, 5 December 2017 (UTC)

The animation is pretty famous - it's an excerpt from a 2003 work called Molecular Movies by Drew Berry, whose various animations have been published, and cited, in many peer-reviewed publications including Science. The original video has detailed narration and scientific commentary.

"The dynamics and molecular shapes were based on X-ray crystallographic models and other published scientific data sets. Leading scientists, including many Nobel Laureates, critiqued the animations during their development. Particular effort was made to ensure the relative shapes, sizes and 'real-time' dynamics were as accurate as possible."

And, in this 2012 TED talk, Berry again discusses the accuracy of the animations.

Of course, the real molecules "look" nothing like this - the molecule-scale features are too small to resolve with visible light!

After all, it's just a cartoon - but it is meant to be a realistic model, and effort was put toward that end.

Nimur (talk) 05:30, 5 December 2017 (UTC)

Thanks, that is fascinating stuff. Bubba73 ^{You talkin' to me?} 18:51, 5 December 2017 (UTC)

Resolved

If you look at 1:20 into that video [1], it describes a helicase spinning at the speed of a jet engine. Now a jet engine spins at tens of thousands of RPMs [2] so at least in this case a molecular machine is depicted as moving much slower than it actually does. --Bob K31416 (talk) 19:03, 5 December 2017 (UTC)

Why do people use that "resolved" tag? Don't they know it's like waving a red flag in front of a bull would be, if bulls could see red? More about the helicase here; looks like it has useful links to further reading. The max speed is 10,000 rpm.

I should point out more generally though that molecular machines work differently than the ones we know. ATP doesn't magically jump into a binding site for a helicase to turn, for example; it will jam in this way and that, fly away, another ATP comes, that flies away, between, countless water molecules, the occasional ADP or chloride ion tries ... who knows how long until a fit is made? Which brings us to the point that these are small distances, yet according to the Boltzmann distribution the molecules are moving faster than macroscopic objects. So they are doing an unfathomable amount of stuff that isn't shown in the video between every two frames that have a result you expect. That's why the ATP requirement at all - without it, that helicase would spin 10,000 rpm backward just as often as forward. That's another thing to notice -- in the microscopic world, there's practically no momentum built up by a spinning helicase compared to, say, a jet engine. The distances are small, after all - if something is say 100 angstroms in circumference, 10,000 rpm means it moves a tenth of a millimeter every minute. By contrast, the electrons in atoms that make up that engine may be moving around their nuclei at something like 1/137 the speed of light (this should be discussed at fine structure constant). Wnt (talk) 21:51, 5 December 2017 (UTC)

How are these videos made - do they calculate what the molecules will do or are they an animation of observations? Bubba73 ^{You talkin' to me?} 03:30, 6 December 2017 (UTC)

These specific animations are “cartoons” or pictures drawn by an artist, guided by actual scientific data like x-ray crystallography (which produces graphical results that require scientific training and discipline to interpret). In specific, the digital artist uses Maya and other specialized software. Molecule movements and trajectories can be programmed, like any other computer-aided animation - in a sense, this is Digital puppetry with atom-shaped puppets. The shapes and movements are meant to illustrate molecular motion, but they are not in themselves a “simulation” in the sense of computational chemistry or molecular dynamics scientific software. The author says in his presentation that it is “expressing science through art.”

Here’s another promotional video from the Linear Coherent Light Source team: using X-ray lasers to image nucleic acid reaction kinetics. That equipment, and the data it produces, are much newer but are in the same category as the type of pictures that Drew Berry would use to inform his animations. In the SLAC video, you can see a sort of transition from raw data to “science product” and finally to cartoon-for-layperson-consumption, as the narrator explains the science.

Nimur (talk) 18:43, 6 December 2017 (UTC)

Thank you. Bubba73 ^{You talkin' to me?} 00:34, 7 December 2017 (UTC)

Just here to note that Autodesk Maya has an article, so we can learn it's "specialized" as general purpose high quality animation rather than science-specialized. DMacks (talk) 06:32, 7 December 2017 (UTC)

Anthropology and VO2 max

I am looking for information regarding how VO2 max relates to ethnicity and how it has changed since early humans (possible relating to Endurance running hypothesis). On average do certain ethnicity have better VO2 max L/min? Trick on (talk) 18:01, 5 December 2017 (UTC)

Venlafaxine

One of the reported side effects of Venlafaxine is loss of appetite. By what mechanism does Venlafaxine exert this appetite suppressing effect? — Preceding unsigned comment added by 182.253.176.176 (talk) 19:51, 5 December 2017 (UTC)

Where did you see that reported side effect? ←Baseball Bugs ^{What's up, Doc?} carrots→ 20:29, 5 December 2017 (UTC)

There's a comically long list at List of adverse effects of venlafaxine (are all these proved to a level of statistical significance?), which include weight loss, but not "loss of appetite" per se (and theoretically the two can be unlinked, as by uncoupling agents, though I doubt it here). It also gives nausea as a side effect; [3] adds constipation; so maybe that gives an explanation. But I don't know that. Since they are side effects I would be surprised to see detailed information about the mechanism but I haven't looked. Wnt (talk) 21:28, 5 December 2017 (UTC) ... Hmmm, having looked, at least the first four PubMed screens, not much obvious to me; [4] gives nausea as a side effect in meta-analysis but doesn't try (as I expected) to break down cause and effect. Found general support for antidepressants in binge eating disorder. [5] I could look harder if someone got me interested. Wnt (talk) 21:34, 5 December 2017 (UTC)

I just noticed the mixing of "adverse event" and "side effect" here to create "adverse effect." In our pharmacy school, students must know the difference between a Type A adverse event, Type B adverse event, side effect, and toxic effect. I understand that the general public doesn't need to know it, but mixing and matching words between "adverse event" and "side effect" in what claims to be an encyclopedia seems a bit clumsy to me. 209.149.113.5 (talk) 14:20, 6 December 2017 (UTC)

Since you don't have a talk page, I'll encourage here that you raise this issue at the appropriate article talk pages. I'll note that we have separate articles for adverse event and adverse effect, and at a glace they seem to be about (somewhat) distinct things. SemanticMantis (talk) 02:09, 7 December 2017 (UTC)

• At least part of the cause it that venlafaxine is a serotonin reuptake inhibitor. This means it increases serotonin levels in the brain, and one of the effects of serotonin is to reduce appetite. The well-known anti-obesity drug fen-phen relies heavily on that mechanism. Looie496 (talk) 19:36, 6 December 2017 (UTC)

Yes, venlafaxine is an "upper," that is it acts as a psychostimulant for many people, improves attention span, reduces need for sleep, may cause insomnia. It is actually a dual serotonin and norepinephrine reuptake inhibitor. - AboutFace 22 (talk) 02:18, 7 December 2017 (UTC)

Ice-free northern Greenland

The Greenland article says The extreme north of Greenland, Peary Land, is not covered by an ice sheet, because the air there is too dry to produce snow, which is essential in the production and maintenance of an ice sheet. Why doesn't the main ice sheet spread over Peary Land? Is it unable to get past the fjord south of Peary Land (i.e. the ice that would go north just goes into the ocean instead), or is there some other reason? Nyttend (talk) 23:54, 5 December 2017 (UTC)

Didn't you just quote the answer to your own question? --Jayron32 13:14, 6 December 2017 (UTC)

No. Nyttend backup (talk) 14:06, 6 December 2017 (UTC)

Question: "Why doesn't the main ice sheet spread over Peary Land?" Answer: "because the air there is too dry to produce snow, which is essential in the production and maintenance of an ice sheet". I am confused as to what you are asking if that is not the answer. If you can clarify your question in some meaningful way, then people can help you find the answer. --Jayron32 14:47, 6 December 2017 (UTC)

I believe the question meant is "Why is the air so dry there that it cannot produce snow?" I do not know the answer to that, but there are, similarly, small areas of Antarctica without snow. Our article is at Antarctic oasis and it similarly fails to account for why these areas have the low humidity that causes lack of snowfall. Matt Deres (talk) 14:21, 6 December 2017 (UTC)

The question is: "Why doesn't the ice sheet to the south of Peary Land slide North and, eventually, cover Peary Land?" For background, ice sheets move. There are ice sheets moving into Peary Land. It is easy to find many photo collections of the ice sheet edge. They don't cover Peary Land because during the summer it warms too much. The ground is permafrost, which absorbs solar light. Also, ice in dry air will melt in direct sunlight even if the air is below freezing. (Just as you can see ice melting off a roof when it is well below freezing.) So, it is a flow and ebb system. The ice sheet moves in during the winter, but melts away during the summer. Overall, Peary Land is speckled with snow in the winter, but dry and clear in the summer. You can get information on this type of system easier by looking for information on arctic deserts in Alaska and Canada. 209.149.113.5 (talk) 17:47, 6 December 2017 (UTC)

Thank you for that explanation. It didn't occur to me that summer melting would be an issue; I figured that the sheer size of the sheet was great enough that melting wouldn't be able to prevent its expansion. And on sublimation, again I'd figured that only a small percentage would be lost, so thank you for correcting me. Nyttend (talk) 00:28, 7 December 2017 (UTC)

I thought the question clearly was about the ice moving, which you have also answered. It may also be mentioned that ice in dry air and sunlight may not only melt but may also sublime, i.e. evaporate directly from the solid state. --69.159.60.147 (talk) 19:46, 6 December 2017 (UTC)

It looks like the sheet did get in to Peary Land in the past. Here [6] is a scholarly article discussing the historical and current limits of that ice sheet, here [7] is another that includes more maps. SemanticMantis (talk) 19:42, 6 December 2017 (UTC)