Wikipedia:Reference desk/Archives/Science/2014 July 24

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July 24

Question about Hawking's imaginary time

Please don't hesitate to use advanced math in answering this question:

As I understand it, Stephen Hawking thinks the universe is spherically curved in imaginary time.

Would that mean that imaginary time is the radial dimension of the universe's expansion? Why or why not?

If so, would that mean that the universe is expanding slower than light speed? Why or why not?

If the universe is expanding faster than light speed, then why doesn't that make the FRW metric positive-definite?

166.137.101.167 (talk) 02:32, 24 July 2014 (UTC)Collin237

Hi there. You're asking about the Hartle-Hawking state. Unfortunately, our article about this subject isn't great, and the technical details are rather inaccessible. (I won't claim to be an expert in this area of cosmology.) Given those caveats, there are a few basic points that can provide the answers to your questions:

1. The Hartle-Hawking state is a hypothesis about how to write down a quantum state in quantum gravity around and before the Planck time. It does include a signature change in the metric. However, it doesn't describe the subsequent Friedmann–Lemaître–Robertson–Walker expansion history of the universe and isn't needed to answer questions about the present-day expansion of the universe.
2. Saying that the universe is expanding faster than (or slower than) the speed of light is a vague statement that can mean one of several things when describing cosmology in everyday language. It doesn't have any precise technical meaning. It refers to the recession speed of objects at some distance, so it is not a local property of spacetime.
3. The metric tensor is a local property of spacetime.

So my answer to your first two questions is no, because the Hartle-Hawking hypothesis doesn't describe the modern universe (or anything much after the Planck time); and my answer to your last question is no, because the metric is a local property of spacetime, while "expanding faster than light" (whatever it may mean in some context) is not a local statement. --Amble (talk) 18:13, 24 July 2014 (UTC)

So if there was no time and no space before the big bang, what was around then and where? And in what 'space' did the big bang occur? Also where did all the mass and energy in the present universe come from if there was nothing to start with? Also whats to stop another universe just appearing out of nothing. And anyway, where did God live before the big bang?? This theory must be complete tosh!86.171.5.136 (talk) 16:13, 25 July 2014 (UTC)

The argument from incredulity is a notoriously unreliable guide in matters outside the realm of everyday experience. In addition, you appear to be objecting to certain vague notions about Big Bang Cosmology in general, rather than the Hartle-Hawking state, which is the topic under discussion. --Amble (talk) 16:21, 25 July 2014 (UTC)

Well is it credible that something that violates basic laws of the universe is, in fact, true? Or is it more likely to be false. If false, why?86.171.5.136 (talk) 17:42, 25 July 2014 (UTC)

A hypothesis that violates basic laws of the universe is probably false. A commonsense notion that is violated by nature is probably not a basic law of the universe. Do you have a specific question about the Hartle-Hawking state? --Amble (talk) 17:58, 25 July 2014 (UTC)

It was the beginning of everything, including the basic laws. So there was nothing to violate. 198.228.228.166 (talk) 13:59, 28 July 2014 (UTC)Collin237

Is this a good image of a pediplain?

I'm determined to find an image for this article, and after researching a bit I discovered that the western Atacama Desert, particularly the Tarapaca region, is home to one of the largest and oldest pediplains on Earth.[1] And so I found [this image], and I'm wondering if anyone with expertise or familiarity with the region can confirm that this is in fact (part of) a pediplain. And furthermore, is it a good enough representation to include in the page? If so, I can modify the image with some labels to make it clearer. = NV1982 ^(talk) 06:16, 24 July 2014 (UTC)

Interesting! It looks like you've done your homework. I've read through your links, and it seems right to me... but I'm no geographer or geologist. I suggest (barring any credible, referenced, objections here in the next few days) that you be WP:BOLD and add the image to the article :) SemanticMantis (talk) 14:50, 24 July 2014 (UTC)

I think I'll end up doing that. I'm pretty confident in this case. Although an aerial shot might be more ideal, this at least will point readers to an example of what the article's describing. And it'd be a bonus to make more good use of that photo. Thanks for your input! = NV1982 ^(talk) 17:38, 24 July 2014 (UTC)

Origin of body heat when cycling

When I cycle, which are the main contributors to (over) heating my body - muscles in my legs, heart, diaphragm and intercostal muscles? --129.215.47.59 (talk) 10:20, 24 July 2014 (UTC)

That is waste heat generated when muscles convert chemical energy into kinetic energy. StuRat (talk) 15:16, 24 July 2014 (UTC)

In addition to the excellent links StuRat has provided for you, Thermoregulation may also be an interesting read. --Jayron32 00:58, 25 July 2014 (UTC)

Alternate Fuel and Car Technology

Hi wikipedia, I dont have a great article to publish but certainly a new idea which i would like to share it with you. Recently i have been thinking about powering cars with wind energy ,can i do so? i have a basic idea not exactly a technical one , so need help hope so ull reply . — Preceding unsigned comment added by Manojb95 (talk • contribs) 12:05, 24 July 2014 (UTC)

Lots of obvious problems with that. Your best bet is to charge batteries using a turbine, which is already done. Any ideas about sails and turbines mounted on cars are, sadly, not feasible. Zzubnik (talk) 12:25, 24 July 2014 (UTC)

This is a much underappreciated aspect of history - see land sailing, ice boat, ice yachting. We've seen a thousand film images of Conestogas but anyone ever see a "wind wagon" in film (??!) I remember reading something about ice boats having held the land speed record on Lake Michigan for a time but can't find it now. (maybe a logician objected?) Wnt (talk) 12:50, 24 July 2014 (UTC)

These old Chinese devices are pretty close to a wind-propelled wagon Wheelbarrow#Chinese_sailing_carriage. Google /Chinese wheelbarrow sail/ for lots of cool pics and articles. SemanticMantis (talk) 13:17, 24 July 2014 (UTC)

Wind-powered vehicle has examples of the turbine type. Katie R (talk) 13:25, 24 July 2014 (UTC)

We should probably mention some of the problems that make it not feasible:

1) You can't go faster than the wind in a vehicle powered solely by the wind. In most places, the wind doesn't go very fast, most of the time, so your car would be slow. When there is no wind, your car wouldn't move at all. (Addition: While it may be possible to make a stripped down vehicle which can go faster than the wind, I still don't believe this can be done with a practical passenger vehicle.)

2) The wind often goes in the wrong direction. While sailboats can go in the opposite direction as the wind, via tacking, this involves sailing in a zigzag pattern that's not practical for land vehicles following roads. Out on an open desert, it's a bit more practical.

And, in case you are thinking you could power a car by conventional means, and put up a windmill to capture wind energy too, that won't work, since the drag from the windmill will slow the car down more than the electricity generated would speed it up.

So, I agree that the best way to power a car by wind energy is to use a windmill to charge batteries for an electric car. However, for a car that gets much use, if the windmill is only set to charge the batteries and nothing else, the best option is to charge the batteries outside the car, and just swap those in for the discharged batteries in the car whenever you leave home. Alternatively, if wind power is used to generate electricity on the grid, then you can just use a standard plug-in electric vehicle. (In many places you can sell your unused wind energy back to the power company using the grid.)

One other possible use of wind energy for powering a car is when parked away from home. You could conceivable have a windmill that deploys above the car to trickle charge it while parked. However, this would provide rather minimal power, so would only be practical if parked for long periods in windy areas, with only short drives. Also, high winds might tip the car over, necessitating the use of outriggers (I see our article is just on boats, don't we have one for outriggers on cranes, etc. ?), which would add to the weight of the vehicle. StuRat (talk) 14:04, 24 July 2014 (UTC)

I agree that these vehicles aren't practical for everyday use. However, your first claim is wrong. Blackbird (land yacht) can go upwind and downwind faster than the wind speed and doesn't need to tack. Katie R (talk) 14:21, 24 July 2014 (UTC)

(EC) Your comment 1 seems very confusing or inadequately explained as our article sailing faster than the wind (and perhaps vehicle) will indicate. Nil Einne (talk) 14:23, 24 July 2014 (UTC)

The key to understanding the concept is that by using a turbine coupled to the wheels, it can use the speed between the air and the ground, not just the relative speed between the air and the craft. Katie R (talk) 14:26, 24 July 2014 (UTC)

Note that I'm not just thinking of Blackbird concept but the general concept of sailing faster than the wind. StuRat's comment 2 mentioned the issues tacking etc poses for road based vehicles. But comment 1 didn't say anything about that nor did it refer solely to sailing dead downwind. So comment 1 seems to ignore the more general idea where you can use tacking or other methods to achieve a VMG higher than the windspeed particularly on ice boats (which Wnt already mentioned) and unlike Blackbird, I don't think is something only recently demonstrated (if you didn't believe the theoretical calculations), even though StuRat's comment itself could be taken to imply this isn't possible. These may not be practical for most land based vehicles but again, the comment was phrased very generally. So I'm not sure making the claim as StuRat did without additional explaination of what they were referring to, or at least a link to our article (which I found in about 3 seconds), helps the OP much. Nil Einne (talk) 14:44, 24 July 2014 (UTC)

Aside from the physics of the thing, consider the economics. A typical car engine puts out something of the order of 100kWatts. According to The American Wind Energy Association a 1kWatt turbine costs between $4000 and $9000 (already way too expensive to put onto a car!) and a 100kWatt turbine costs $350,000. So right there, you know that you can't bolt a wind turbine to a car and get free energy. However, since you aren't driving your car 24 hours a day - and the wind might maybe blow 24 hours a day (in a good location) - if you restricted yourself to driving for at most one hour per day, then you could charge the batteries on an electric car with a mere 4kWatt generator. Sadly, that's still going to cost you more than your car...but why bolt the thing to your car? Why not use it to charge the batteries and leave it behind when you drive off? What this leads you to is that electric cars are a good idea...and that using wind energy to make electricity is a good idea. These are completely separate concepts though...linking them together and saying "Let's run a car on wind" is an unnecessary (and exceedingly difficult) linkage! Why have your expensive windmill only work while you're driving your car? Why not use it to power your refrigerator instead?

Taking it one step further - the larger a windmill is, the more efficient it becomes. That's why the wind energy folks use windmills with blades the size of a 747's wing. So having each person who drives an electric car spend all that money on a 4kW windmill is silly. You need to fund your share of a megawatt windmill and share it with everyone else...or, in other words, buy your electricity from a wind-energy company and run an electric car.

What most concerns me is that you're probably thinking "Wow! When I drive my car, and stick my hand out of the window, there's a heck of a lot of wind! Why can't I capture some of that to drive the car?" - and that's a fatally flawed argument. The problem is that windmills cause drag...in fact, that's what their function is - to cause as much drag as possible, slowing down the airflow and stealing energy from it. So if you could bolt a small windmill onto your regular car, it would increase the drag on the car such as to increase fuel consumption by an amount of energy that would most certainly be considerably more than the windmill would generate. That's 100% certain - and it doesn't depend on how clever your design for the windmill is - or how you use it's energy. The laws of thermodynamics guarantee that this approach won't work...no matter how clever you are!

The only thing that might work would be to have a windmill that popped up out of the roof of your car when you stepped on the brakes! The windmill would slow the car down AND generate energy. This might, somehow be a seemingly good idea (I doubt it!) - but cars that recover energy from the braking systems ("regenerative braking") already exist - the Prius does that exact thing - except that it turns the electric motors that drive the wheels into generators when you step on the brakes - and that requires no new mechanical systems - just $10 worth of electronics. So if you already have an electric car, that's a vastly cheaper, easier, more efficient approach than trying to capture the energy from the air flowing over the car.

So, sadly, this idea is a non-starter. Not going to work!

SteveBaker (talk) 14:32, 24 July 2014 (UTC)

No. A moving Prius regenerates from the moment the accelerator pedal is released i.e. during coasting. Let Toyota explain. 84.209.89.214 (talk) 19:26, 24 July 2014 (UTC)

Yeah - but it's simulating the effect of engine braking. When I take my foot off the gas pedal of my conventional car, the engine speed drops to the point where it's actively slowing the car down - the Prius wouldn't do that unless they had something like regenerative braking. The Prius can't harvest energy without slowing the car down more than would otherwise be the case...that would be impossible. SteveBaker (talk) 19:00, 25 July 2014 (UTC)

I agree that it's totally impractical, but it isn't ruled out by the laws of thermodynamics. You can extract power from the air-ground speed difference while moving relative to both. As Katie R mentioned above, people have actually built wind-powered vehicles that move faster than the wind. Also, a wind turbine behind the front grille of a car seems likely to produce more power than it would cost in drag. There's no law of physics saying that it can't. -- BenRG (talk) 17:41, 24 July 2014 (UTC)

"a wind turbine behind the front grille of a car seems likely to produce more power than it would cost in drag"? Citation most definitely needed... AndyTheGrump (talk) 17:46, 24 July 2014 (UTC)

There absolutely IS such a law! The First law of thermodynamics in fact! A machine that produced more power than drag would allow you to build a perpetual motion machine...and thermodynamics has rather strong opinions on such things!

Building a machine that moves faster than the wind (and even against the wind and faster than it!) is perfectly possible if your machine is in contact with the ground (or water or whatever). You can use the relative speed of the wind and ground/water to extract energy - which (if you're careful) does allow you to move faster (relative to the ground) than the wind travels (relative to the ground) - but that in no way frees you from the laws of thermodynamics.

For example (you can actually do this): Take a spool of thread. Rest the spool on its side on the ground and unspool a length of thread so that it runs under the spool. Now, if you pull gently on the loose end of the thread - the spool will move in the opposite direction from what you're pulling and at a speed that's higher than you're moving your hand. Now imagine attaching a large, very lightweight parachute to the loose end of the thread so that it moves away from the spool at more or less the same speed as the wind - and now our little vehicle will move against the wind and at higher speed than the wind...(at least until it runs out of thread...but this is a thought-experiment so the thread and parachute are massless and can be collapsed and reeled back in once in a while for zero energy outlay).

SteveBaker (talk) 18:09, 24 July 2014 (UTC)

Cars already lose energy to air friction, and there's no law that you can't get some of that as useful work instead. Not all of it, or more than all of it, but more than a typical car does. You gave an example yourself with frictional vs. regenerative braking. If the air that enters through the front grille is decelerated to the speed of the car anyway, decelerating it with a turbine doesn't make the drag any worse and generates some power in the bargain.

The best citation I can offer is a car that does have turbines behind the grille, though it could be a gimmick. -- BenRG (talk) 20:37, 24 July 2014 (UTC)

But, if the air in the grill was formerly used for engine cooling, then if you use a turbine to slow that air, you've effectively reduced the efficacy of the air cooling system... and it's also possible that a spinning turbine can increase drag: if the induced drag of the turbine blades is greater than the static drag of the stagnation point (i.e., the moving turbine can be worse for aerodynamics than the stationary material that it replaced, even if the exit air velocity is identical). Anyway, I don't think it's useful or instructive to nitpick at this hypothetical situation too much: it depends on a zillion unspecified details. As a whole, I think we're all very familiar with conservation of energy and we understand the limitations of pushing this analogy. I only wanted to highlight some of the many engineering-details that would make an actual implementation more difficult. Nimur (talk) 23:21, 24 July 2014 (UTC)

Wind powered cars seem impractical now, but that could change back toward how it was in the past. We assume that we have abundant energy storage available, either as fossil fuels or batteries. But land sailing was proposed for some situations on Venus where those may not be so easy and wind more so;^[2] really, any formerly uninhabited planet will probably lack fossil fuels, and some may also lack metals or other usable battery components. Even Earth could end up in that boat in a few millennia, though I suppose (knock on wood) we'll always have water to split and store away as bags of hydrogen at least. Wnt (talk) 00:22, 28 July 2014 (UTC)

Space suit color

Why were the first space suits like those that Shepard wore shiny silver? 20.137.2.50 (talk) 16:20, 24 July 2014 (UTC)

To reflect heat away from the body. The interior of those early spacecraft weren't well insulated against the heat of launch and re-entry - or from the suns' rays while in orbit. SteveBaker (talk) 16:35, 24 July 2014 (UTC)

Thanks. Wasn't Yuri Gagarin's suit on his first space flight orange? 20.137.2.50 (talk) 16:37, 24 July 2014 (UTC)

The SK-1 spacesuit was indeed orange - most likely to aid in spotting the cosmonaut after s/he ejected from the capsule during reentry. WegianWarrior (talk) 16:44, 24 July 2014 (UTC)

Found a citation for it:"The orange colour of the overalls was selected to facilitate the search for the cosmonaut...". Russian Spacesuits, by Isaak P. Abramov and Å. Ingemar Skoog, ISBN 1-85233-732-X. WegianWarrior (talk) 17:45, 24 July 2014 (UTC)

Hmmm - so how does a wind-farm work?

All this talk of wind-powered cars got me thinking.

I'm imagining an invisible cuboid surrounding a wind farm containing hundreds of windmills. Air flows into the cube at some velocity. The windmills steal kinetic energy from it - so the air must leave my imaginary volume at slower speeds than it enters. We know that the cube doesn't fill up with increasingly high pressure air - and what comes in has to go out again! I suppose a larger fraction of the surface area of my imaginary cube has a net low-speed outflow of air than has a high-speed net inflow...but this has to result in some crazy wind directions when several hundred windmills are set up close together over several miles of hilltop (as we see out in West Texas, for example).

What exactly happens there?

SteveBaker (talk) 16:41, 24 July 2014 (UTC)

There is of course a net reduction in energy in the air when some of it is slowed down to do work on the turbine blade. But it doesn't necessarily result in crazy wind directions beyond scales of a few meters. Turbulence (at all scales) takes care of mixing and averaging of wind speeds, but usually engineers want to diminish wake vortices and vortex shedding to improve efficiency of conversion of kinetic energy in the air into mechanical power. For some relatively long-distance effects, check out this image search for /wake vortex wind turbine/ [3] My understanding is that a turbine array would be most efficient if we could magically eliminate all wake vortices, and then the simple effect of the wind farm would be to reduce the velocity of the laminar flow. Here's a power point presentation from the Sandia national lab [4] that talks about how turbulence effects arrays. Here's a more technical book chapter about mathematical modeling of how turbine blades affect wind [5] These both relate to your question, but do not address your specific phrasing. SemanticMantis (talk) 17:04, 24 July 2014 (UTC)

I think you mean to say that turbulence affects arrays. 84.209.89.214 (talk) 19:02, 24 July 2014 (UTC)

It is true that you can't extract too much energy (aka speed) from the wind at each site, or the air piles up and causes stalls. IIRC, the exit wind speed must be no less that 75% of the source speed, but I don't have a cite for that. CS Miller (talk) 18:04, 24 July 2014 (UTC)

The physical limit is known as Betz's law and says that no wind turbine can extract more than 16/27 (~59%) of the energy in an airflow. Typical real windmills for power generation operate around ~75% of this theoretical maximum efficiency. It also implies that the air behind the windmill will optimally move at about 1/3 the speed of the air approaching the windmill (the exiting air is assumed to move across a larger area). Dragons flight (talk) 19:06, 24 July 2014 (UTC)

Byron Airport (C83) is in very close proximity to a large number of windmills. I can feel their effect on the wind when I fly my Citabria there. Their effect is still smaller in magnitude than the coal power plant, whose "smoke-stack" produces thousands-of-feet-tall vertical updrafts of very significant magnitude. Both the wind farm and the steam plume are documented in the official Airport/Facility Directory entry, "Remarks" section, for C83, which is officially published by the FAA[6] to make sure that aviators are aware of all relevant information. (It's up to us to use our own judgement about the magnitude of the effects)! And it's not only their effect on local wind: they're also large, tall, weirdly-shaped moving obstacles; one generally prefers to avoid flying into such objects. Windmills also have serious effects on modern Doppler RADAR (to air traffic control, the windmills have a radio return that looks exactly like a small aircraft with an inoperative transponder - i.e. an out-of-the-ordinary radar blip that blinks into- and out-of existence, a little bit too close to the rugged terrain). Windmills are also tall and difficult to avoid.

Here's a video of a C182 landing at C83 on Runway 30. (The really exciting footage is the take-off from 23, directly upwind into the rising terrain full of the windmills, but I can't find any online. Maybe next week I'll post a video...) Nimur (talk) 21:18, 24 July 2014 (UTC)

And before anyone nitpicks, I always comply with 14 CFR 91.119 and all other applicable regulations; in particular, I do not fly closer than 500 feet from the structure of a windmill. Even still, I can feel the wind change. Nimur (talk) 21:42, 24 July 2014 (UTC)

I think Steve is asking about the mass balance. If I had a (wind driven) turbine in a duct, I would expect to pressure (and thus density) to be lower downstream of the turbine than upstream. If the downstream velocity of the air is also lower, then there would appear to be a greater rate of air (by mass) flowing into the turbine than out. How can that be? -- ToE 11:06, 25 July 2014 (UTC)

For efficient turbine drive, the cross sectional area of your duct should vary inversely with speed, as in the diagram at Betz's law. 84.209.89.214 (talk) 11:44, 25 July 2014 (UTC)

Thanks. I missed the larger area mention that Dragons flight made earlier. -- ToE 12:30, 25 July 2014 (UTC)

Also see Disk actuator theory. Dolphin (t) 13:12, 25 July 2014 (UTC)

Does Betz's law also apply to water-turbines, and free-flow tidal generators, that look like underwater windmills? The article indicates that it is for air only. As water is non-compressible, a different law may apply. CS Miller (talk) 13:16, 25 July 2014 (UTC)

I too would have assumed that compressibility would be a factor, but the article does say, "Betz' Law applies to all Newtonian fluids, but this article will use wind as an example." -- ToE 14:44, 25 July 2014 (UTC)

I really need to improve my skim-reading. The lead only mentions air/wind, but the first section says all Newtonian fluids. CS Miller (talk) 15:23, 25 July 2014 (UTC)

[7] AndyTheGrump (talk) 06:50, 26 July 2014 (UTC)

Sadness and heart problems

Is it possible that sadness, whether from a sad movie or a real situation such as a breakup, which causes the physical sadness feeling in the chest area, is bad for the physical health of the heart? Could it increase risks of heart disease? — Preceding unsigned comment added by 90.192.100.190 (talk) 23:22, 24 July 2014 (UTC)

Yes. See Takotsubo cardiomyopathy, also known as broken heart syndrome. --Srleffler (talk) 04:59, 25 July 2014 (UTC)