Science desk
< January 8	<< Dec \| January \| Feb >>	January 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.

January 9

Neutrinos vs Neutron decay

How often on average does a Neutrino pass within one radii of a free Neutron and how does this compare with the halflife to beta decay of a free Neutron? Hcobb (talk) 00:32, 9 January 2012 (UTC)[reply]

Total neutrino flux through any area in the vicinity of the Earth is on the order of 10^15 neutrinos per m^2 per second. "Radius of a free neutron" is not a well-defined concept, but, as long as we're doing order-of-magnitude estimates, let's say 10^-15 m, which makes the answer to your question "once every hundred million years".--Itinerant1 (talk) 00:58, 9 January 2012 (UTC)[reply]

To answer the other part, Neutron#Stability_and_beta_decay says the half-life of a free neutron is about 611 seconds. Which is quite different. --Colapeninsula (talk) 10:34, 9 January 2012 (UTC)[reply]

One more thing. What's the point of that question? Dauto (talk) 15:50, 9 January 2012 (UTC)[reply]

It's to consider the possibility that the Neutron is a stable particle in isolation. If we include the Cosmic neutrino background we "gain" six orders of magnitude, but that's still not enough, unless we switch from the Neutron radius to the Neutrino Compton wavelength, which is of course huge (mass around 0.04 eV, therefore around a few 10^-5 meters), when it isn't zipping around at almost the speed of light. (Like say as part of the CNB.) So every free Neutron is touched by a background neutrino before it goes. Hcobb (talk) 20:53, 9 January 2012 (UTC)[reply]

Neutron is not a stable particle in isolation simply because the mass of neutron is greater than masses of proton and electron, combined. That is sufficient to result in decay. Also, my flux estimate already included cosmic neutrino background. --Itinerant1 (talk) 21:14, 10 January 2012 (UTC)[reply]

Ellipsoidal Dyson shell

Would it not be possible to build a megastructure, essentially an ellipsoidal Dyson shell, with the Sun at one focus and a solar-energy collection device at the other, to collect the Sun's entire output of light? Whoop whoop pull up ^{Bitching Betty | Averted crashes} 02:54, 9 January 2012 (UTC)[reply]

Would it be possible for whom? TenOfAllTrades(talk) 03:00, 9 January 2012 (UTC)[reply]

The idea being that it would be a giant reflector ? StuRat (talk) 03:00, 9 January 2012 (UTC)[reply]

Yes. Whoop whoop pull up ^{Bitching Betty | Averted crashes} 03:05, 9 January 2012 (UTC)[reply]

Would it be theoretically possible, I don't see why not. Would it be feasible or practical? Almost certainly not. Vespine (talk) 03:03, 9 January 2012 (UTC)[reply]

One problem is that a Dyson shell would require far more mass than all the planets and other items in our solar system, outside the Sun, with current technology. StuRat (talk) 03:07, 9 January 2012 (UTC)[reply]

The article you just linked says that if we somehow disassembled the rocky inner planets in the solar system we could make a spherical shell of 42kg/m², more than enough for a mylar envelope, if we could get it to stay open. APL (talk) 09:05, 9 January 2012 (UTC)[reply]

Gravity might pull the sun away from the focus. With a spherical shell, there is no net gravitational force on the sun due to the shell. I'm not sure that this is true for an ellipsoidal shell.--Srleffler (talk) 17:49, 9 January 2012 (UTC)[reply]

Shouldn't a helicopter be able to fly even without a tail rotor?

Since the mass of the helicopter itself is much greater than that of the main rotor, shouldn't the main rotor spin much faster than the helicopter, thus allowing the helicopter to fly even with no tail rotor at all? Whoop whoop pull up ^{Bitching Betty | Averted crashes} 03:34, 9 January 2012 (UTC)[reply]

Yes, the rotor turns much faster, but the helicopter spins in the opposite direction too fast to handle, and who wants a constantly spinning chopper, anyway ? Note that dual top rotor systems can eliminate the need for a tail rotor. StuRat (talk) 03:38, 9 January 2012 (UTC)[reply]

Define "Fly". If you mean "Be in the air for some time" then yeah, sure. If you mean "and be controlable well enough not to dash itself into the ground and kill everyone", then probably not. Helicopter#Antitorque_configurations explains the problem: The spinning rotor generates a torque, and because of Newton's third law, this torque results in the body of the helicopter torquing in the opposite direction. Some means of control must be used to counteract this torque. You can use any number of methods, including multiple horizontal rotors (which can be installed coaxially or on seperate axles), or having a perpendicular rotor which deflects the reaction torque. But a single rotor would cause the helicoptor to corkscrew through its flight, making it impossible to control. --Jayron32 03:42, 9 January 2012 (UTC)[reply]

Addendum: there is a helicopter stabilization system known as NOTAR, which does not have an exposed tail rotor, but still uses a powered internal "fan" to direct a stream of air countract the main rotor torque. It still provides an active force to solve the torque problem, but does so through a means different from a tail rotor. --Jayron32 03:47, 9 January 2012 (UTC)[reply]

Slower than the rotor is still too fast. One way to deal with tail rotor failure is to maintain significant forward speed and allow the tail to aerodynamically stabilize the helicopter. This is more easily said than done, particularly when you have to set it down; a runway comes in handy at that point. Tail rotor failure in a hover or at slow speed results in a hard landing at least, and usually something much worse. Acroterion (talk) 03:45, 9 January 2012 (UTC)[reply]

It might be an interesting exercise to make a single rotor helicopter. If you put wheels on it, so it could rotate as it takes off and lands, had a single occupant at the center of gravity so the spinning wouldn't kill him, and used an electronic display that compensated for the spinning to provide views in every direction, you just might be able to get it to work. It would be good for a laugh, if nothing else. An unmanned surveillance helicopter with a single rotor might be of some use, in that the cameras would rotate to view 360°. StuRat (talk) 03:47, 9 January 2012 (UTC)[reply]

Even if the center of rotation was inside of the pilot, that point is still just a point, and his arms, legs, and head would still be outside of it, and still be experiencing rather profound forces which would make operation of such a vehicle difficult to the point of imposibility; unless you could encase him in a pod of somesort which was stable; such as a pod which rotated exactly counter to the spinning helicopter. Which would require a motor of some sort, which could just be used to power a tail rotor instead of some complex stable pod within a rotating helicopter body. There are any number of rather inelegant ways to design a stable helicopter; however not many of them are necessarily better than current designs. Designing a working, but more cumbersome, means of solving a problem which has already been solved doesn't seem like a good use of engineering. --Jayron32 04:00, 9 January 2012 (UTC)[reply]

Well, there's the Spruce Goose, which was also a rather impractical method of solving a problem which had already been solved, by using materials better than wood to build airplanes. Also, the Red Bull Flugtag is full of silly flying machines, and people enjoy that. StuRat (talk) 04:11, 9 January 2012 (UTC)[reply]

Except the Spuce Goose was made with wood specifically because they were looking for alternate materials other than metals to build a plane out of. There were not yet reasonable plastics or composite materials to build a plane out of, and if metals could be freed up for other purposes than building planes, why not use wood, given the stresses put on the U.S. during World War II. The Spruce Goose was wood for a reason, not just "lets see if we can do it". Oh, and guys getting drunk, dressing like birds, and throwing themselves into a lake is hardly a major engineering effort. Designing a working production helicopter as described would be. --Jayron32 04:36, 9 January 2012 (UTC)[reply]

But a wooden plane that size was completely impractical, and I'd have to think they knew that before they started. The weight to strength ratio is just plain wrong. StuRat (talk) 05:33, 13 January 2012 (UTC)[reply]

The pilot could in theory (if not in practice) sit in a seat with controls and gauges in front of it which was rotated by a motor so that it remained stable with respect to the ground while the rest of the ship was allowed to rotate in a direction contrary to the spinning blade. Maybe it would look like a flying saucer. The landing gear could also rotate so as to be stationary with respect to the ground. Not saying this design would be practical or efficient, just cool. Edison (talk) 04:05, 9 January 2012 (UTC)[reply]

The helicopter could also use a cylindrical crew compartment which is completely inertially isolated from the rest of the helicopter, using roller bearings maybe, so that much of the helicopter rotates but the crew compartment doesn't. You could also drive it through a reduction gear train from the main rotor mast ;-) Whoop whoop pull up ^{Bitching Betty | Averted crashes} 11:43, 9 January 2012 (UTC)[reply]

In the event of a helicopter tail rotor failure, the safest procedure is likely to be to disengage the engine (!). This isn't as nuts as it sounds, as it can then (assuming it has enough height and/or forward speed) autorotate to a safe landing. AndyTheGrump (talk) 04:23, 9 January 2012 (UTC)[reply]

Autorotation is something extensively practiced by helicopter pilots, but would work best if the tail rotor failure is gradual. Sudden tail rotor failure can whip the helicopter around faster than the pilot can kill the throttle, although with enough height there is a chance to regain control. See [1] for a discussion of autorotation and running landings with a failed tail rotor. See also loss of tail-rotor effectiveness which happen more often than actual failure. Acroterion (talk) 04:31, 9 January 2012 (UTC)[reply]

It doesn't matter where the centre of rotation is, the results is the same. The craft has a much higher moment of inertia than the main rotor, and for every action there is an equal and opposite reaction; the the craft and rotor will turn in opposite directions, but the angular acceleration of the craft will be low, and will take some time to reach a terminal angular velocity. Even initially, the craft will experience a torque and will begin to slowly turn. Plasmic Physics (talk) 05:29, 9 January 2012 (UTC)[reply]

Because the angular momentum is distributed over both craft and rotor, the magnitude of lift generated is allot less than with a tail rotor. For this reason, I'd be surprised if the craft could stay airborne once it has attained terminal angular velocity. Plasmic Physics (talk) 06:30, 9 January 2012 (UTC)[reply]

Of course, some helicopters don't need tail rotors.--Shantavira|^{feed me} 08:33, 9 January 2012 (UTC)[reply]

Of course they don't, but the question was concerned with single rotor craft. Plasmic Physics (talk) 09:08, 9 January 2012 (UTC)[reply]

Seriously though, would it be possible to develop an emergency software autopilot that would keep a helicopter with a non-working tail rotor relatively stable in the air and allow it to make a gradual though awkward descent? Wnt (talk) 14:31, 11 January 2012 (UTC)[reply]

Not really. The forces involved in the torque pretty much make "control" a moot point. Cutting the throttle (or at least reducing it substantially) is the only way to keep it controllable. I suppose you could set up a sensor that automatically cuts the throttle back when a tail rotor failure is detected... but then you're taking control away from the pilot. Kind of a Catch-22 in that situation, where neither solution is really viable. — The Hand That Feeds You:^Bite 18:18, 11 January 2012 (UTC)[reply]

If I recall correctly, the Vietcong tactic was to shoot at the tail rotor during landing or taking off at the moment when the helicopter was at a height that made it impossible for the pilot to switch to autorotation or crash-land safely. Was described in a chapter of Ronald J. Glasser's "365 days". Read it 33 years ago, so I don't remember much details... 84.197.185.211 (talk) 10:10, 13 January 2012 (UTC)[reply]

Human chloroplasts

What would happen if someone implanted both chloroplasts and the genes coding for their internal constituents into a human zygote and then placed the newly modified zygote into a woman's body and allowed it to grow? Whoop whoop pull up ^{Bitching Betty | Averted crashes} 03:41, 9 January 2012 (UTC)[reply]

If the idea is to have the humans be able to photosynthesize their own energy while in the sunlight, note that the surface area of our skin is way too small to provide for all of our energy needs. StuRat (talk) 03:54, 9 January 2012 (UTC)[reply]

Yeah, but it could at least provide for some small fraction of them... Whoop whoop pull up ^{Bitching Betty | Averted crashes} 19:43, 9 January 2012 (UTC)[reply]

The reverse would be interisting. To be able to convert fat reserves into light during the daytime. Plasmic Physics (talk) 06:33, 9 January 2012 (UTC)[reply]

-The sendentary art of doing nothing, and still losing weight, perfected. Plasmic Physics (talk) 09:11, 9 January 2012 (UTC)[reply]

I would totally sign up for calorie-burning flashlight skin. APL (talk) 09:19, 9 January 2012 (UTC)[reply]

I imagine that the baby wouldn't be born green, without light, only etioplasts will develop which in turn develop into chloroplasts. Once the baby was exposed to light they would presumably turn green, but as StuRat points out this wouldn't be much use to them. As this has come up though, I have to mention Elysia chlorotica - the sea slug that does photosynthesise. SmartSE (talk) 10:41, 9 January 2012 (UTC)[reply]

Nothing. Some zygotes would die from the implantation but most would still be viable. What you mean, what would happen if the genes for chloroplast were genetically engineered into the zygote?
Sleigh (talk) 12:07, 9 January 2012 (UTC)[reply]

Has someone been reading "by light alone"?Equisetum (talk | contributions) 12:38, 9 January 2012 (UTC)[reply]

Well, if you look up Elysia, you'll see it actually has taken a gene by horizontal gene transfer to help maintain the chloroplast. Even so, it is still dependent on the algae to make them in the first place. Transferring the genetic program for a complete morphological structure into a radically different type of cell is no easy task. I won't say it couldn't be done, though. Wnt (talk) 14:26, 11 January 2012 (UTC)[reply]

Trying to recall some chemistry

I remember seeing a graph which showed the amount of energy required to remove an atom's electrons one by one i.e. the first point showed the amount of energy required to remove the first electron, the second point showed the amount of energy required to remove the second electron etc. Can someone remind what it is which dictates the amount of energy required to remove each particular electron? Widener (talk) 09:35, 9 January 2012 (UTC)[reply]

Do you mean this graph? I found it here. Maybe this article is useful. Von Restorff (talk) 09:47, 9 January 2012 (UTC)[reply]

Galvanic metals

This link gives "galvanic metals" as a name for the group 12 elements. Has anyone heard of this term? Double sharp (talk) 11:33, 9 January 2012 (UTC)[reply]

I have also posted this at Wikipedia talk:WikiProject Elements. Double sharp (talk) 11:33, 9 January 2012 (UTC)[reply]

1200 results on Google (125 in Google Books) so I think the answer is yes. Von Restorff (talk) 11:36, 9 January 2012 (UTC)[reply]

Careful with that search; it shows that the phrase "galvanic metals" is used, but the search doesn't confirm that it is used in the same way as in the reference Double sharp asks about. From the context of the first few hits, it looks like the phrase mostly shows up in descriptions of galvanism, Galvanic corrosion, galvanic cells, or the galvanic series. I haven't yet come across another instance where it is used as a synonym for Group 12 elements, but I may not have come across the right reference yet—and it's rather difficult to prove a negative. TenOfAllTrades(talk) 15:09, 9 January 2012 (UTC)[reply]

Well, I did a bit of Googling and I agree. Many people have heard of the term "galvanic metals", but I was unable to find a source that said it was using this definition. Some of the sources I saw did include non-group 12 elements. Von Restorff (talk) 15:59, 9 January 2012 (UTC)[reply]

Breathing air requirement?

How much air does a slim adult doing reading etc.. require on average, a ballpark figure?, ie m³/h. It ought to be the basis for ventilation, room volume etc. Electron9 (talk) 12:34, 9 January 2012 (UTC)[reply]

The average adult at rest inhales and exhales something like 7 or 8 liters (about one-fourth of a cubic foot) of air per minute. But for safety reasons you want to provide much more, in case someone is exercising and smoking and burning incense etc. The amount of air breathed by an average adult varies from 6 Litre/minute (when at rest) to 50 Litre/minute after hard exercise. But you should read this, and think about oxygen too, not just air. Von Restorff (talk) 13:08, 9 January 2012 (UTC)[reply]

Our article on the per minute inhalation/exhalation rate is respiratory minute volume. -- ToE 13:22, 9 January 2012 (UTC)[reply]

The "respiratory minute volume" article only defines various parameters. No practical data. The "how much oxygen" page is way more on to real life characteristics. I know it's about oxygen. But in most situation thats 21% unless special equipment is used. How much CO₂ buildup that is acceptable is however unclear. The lowest life sustainable percent is 17.5% according to psu.edu, but at what level does it become "unpleasant" low oxygen? Electron9 (talk) 13:51, 9 January 2012 (UTC)[reply]

If life cannot be sustained at less than 0.175 atm partial pressure of O₂, we had best lower the population figures in our Mexico City article. -- ToE 14:45, 9 January 2012 (UTC)[reply]

The articles partial pressure and breathing gas says the same thing: the minimum safe partial pressure of oxygen in a breathing gas is commonly held to be 16 kPa (0.16 bar). Von Restorff (talk) 16:08, 9 January 2012 (UTC)[reply]

Lack of oxygen starts interfering with your normal levels of physical performance below 75% of normal (which is 0.16 kPa). Survival below 75% is possible because your body acclimatizes to low oxygen by producing extra blood cells. It takes time to acclimate, though. 50% of normal is the lowest level where people are known to live for extended periods of time. 35% of normal is the lowest level that your body can tolerate for more than a couple of days with acclimatization.

But it is the maximum tolerable level of CO2 that is more pertinent to this question. I seem to recall that the air exhaled by a human contains 5% of CO2. Our article on CO2 reports that levels of 1% will cause discomfort, levels of 4% are "dangerous", and levels of 8% will cause loss of consciousness within minutes. So, if my recollection is correct, we can say that the human can breathe 1/5'th of the volume of his enclosed space before he starts experiencing discomfort.--Itinerant1 (talk) 20:15, 9 January 2012 (UTC)[reply]

Your recollection is corroborated by the (unreferenced) section Breathing#Components: "The permanent gases in gas we exhale are roughly 4% to 5% more carbon dioxide and 4% to 5% less oxygen than was inhaled.". -- ToE 08:19, 10 January 2012 (UTC)[reply]

The time frame I wonder about is minutes, not days. Thus no acclimatisation ;) Electron9 (talk) 00:11, 10 January 2012 (UTC)[reply]

OK, leave Mexico City (7,350 ft, 2,240 m, 772 mbar, 15.9% atm pO₂) aside and consider the cabin pressurization of a flight departing a coastal airport, thus allowing no acclimatisation. The FAA restricts cabin pressurization to an equivalent altitude of below 8,000 ft. The "Need for cabin pressurization" section addresses hypoxia concerns, saying that "symptoms may begin as low as 5,000 feet (1,500 m) [846 mbar, 17.4% atm pO₂], although most passengers can tolerate altitudes of 8,000 feet (2,400 m) [756 mbar, 15.6% atm pO₂] without ill effect. At this altitude, there is about 25% less oxygen than there is at sea level." (mbar and pO₂ mine.) I'm not saying that I want you to fill my SCBA with O₂ deficient air, but only that I take exception to the "lowest life sustainable percent is 17.5%", and agree that Itinerant1 is addressing the more important concern. Ventilate your enclosure well enough to keep the pCO₂ well down, and pO₂ won't be a problem. -- ToE 02:24, 10 January 2012 (UTC)[reply]

I made a typo above: it should have been 16 kPa, not 0.16 kPa. Personally, I've been on several camping trips where I'd leave the sea level and drive directly to a campground at the altitude of 10,000 feet (pO₂ at 70% of normal), it is noticeably harder to move around and you quickly run out of breath when you get there, but you get used to it after 12-24 hours.

It is also interesting to note that, based on numbers above, a single human adult sleeping inside a car (let's even say a minivan) will reach uncomfortable levels of CO2 in about two hours if the car is sealed tightly enough. Most cars will not be sufficiently airtight for this to happen, but, just in case, if you decide to sleep in a car, crack a window or, better yet, leave some kind of a leak near the floor (CO2 is heavier than the air).--Itinerant1 (talk) 09:15, 10 January 2012 (UTC)[reply]

Time of useful consciousness is what you're looking for, then. Pick your time, find the corresponding altitude, and compute the resulting oxygen partial pressure. --Carnildo (talk) 02:14, 13 January 2012 (UTC)[reply]

Why Ritalin against ADHD?

Why do we treat ADHD, which is a form of hyperactivity, with Ritalin, which is a stimulant? Shouldn't we go the other way round, and try to treat ADHD sufferers with some sedative drug? (if we treat them at all with pills) — Preceding unsigned comment added by 80.39.202.227 (talk) 13:35, 9 January 2012 (UTC)[reply]

It does not have the same effect on people with ADHD. It has the opposite effect. Von Restorff (talk) 13:41, 9 January 2012 (UTC)[reply]

That's the question, Restorff. Why does it work like that? — Preceding unsigned comment added by 80.39.202.227 (talk) 13:44, 9 January 2012 (UTC)[reply]

It stimulates certain areas of the brain that help the person to contain impulsive behavior. You can compare this to being drunk. If you drink alcohol, the sedative effect will actually make you behave in a more uncontrolled, impulsive way. I'm not an expert in this area, but I tend to think of this as follows. A very complex system like the brain can be faced with having to cut back on its activity for many reasons. A natural reason could be that the person is exhausted and has a lack of energy. But in Nature, you would still need a working brain to survive, so the brain will tend to cut down on certain higher level sophisticated control mechanisms, rather than on lower level functions. And, that will lead to the person becoming more aggrssive, more impulsive, not less, because that will help you to survive. Count Iblis (talk) 13:46, 9 January 2012 (UTC)[reply]

"Stimulants" are named that way because they are used to stimulate certain parts of the brain that are not acting at normal levels; not because they make you more active. The brains of children with ADHD have too little dopamine because the children's brains have too many molecules that suck up dopamine before it does its thing. Ritalin exerts its attention-increasing effects by increasing the amount of dopamine in brain cell synapses, the space between cells. It does this by blocking dopamine transporters, proteins that normally transport dopamine from the synapse back into dopamine cells, recycling it for future use. More dopamine in the synapse yields a stronger reward signal, and more enjoyment of and motivation to perform certain tasks. When a person concentrates on a task, the part of the brain that is working becomes highly active. But other parts of the brain are active, too. This "white noise" helps a person without ADHD make new, creative associations. But for someone with ADHD, the white noise drowns out the main signal. (Von Restorff (talk) 13:56, 9 January 2012 (UTC)[reply]

(edit conflict)An ADHD brain need more stimulus than other people in order to feel "normal", according to the low arousal theory of ADHD. ADHDers fidget, talk and walk around, etc., because there isn't enough stimulation coming in from the environment. Stimulant medications work on ADHD because they reduce the need for the person to seek stimulation from the environment, thus helping them focus and sit still when necessary. :From the low arousal theory article:

"The low arousal theory is a psychological theory explaining that people with attention-deficit hyperactivity disorder (ADHD) seek self-stimulation or excessive activity in order to transcend their state of abnormally low arousal. The theory states that one with ADHD cannot self-moderate, and his or her attention can only be sustained by means of sustained external/environmental stimuli. This results in an inability to sustain attention on any task of waning stimulation or novelty, as well as explaining compulsive hyperactive behavior."

A sedative might also calm the hyperactive behaviours of ADHDers, but not in such a way that would help the ADHDer to focus and get things done.

I have ADHD myself, and for me and my doctor, the low arousal theory best explains the symptoms and the reason that stimulants work, but please note that there are other theories of ADHD. They can be found at the main ADHD article. Feel free to ask if you have any questions; I don't mind talking about my ADHD. Cheers, Dawn Bard (talk) 14:09, 9 January 2012 (UTC)[reply]

As an aside, I believe that sedatives are sometime also prescribed, to counter-act the effects of the stimulant. I think this is only done with patients that need unusually large dosages that disrupt the patient's sleep schedule. APL (talk) 19:59, 9 January 2012 (UTC)[reply]

E.T.

What are the probabilities of discovering an alien species, capable of replying to a message, in the near future? If we did so what message would we send them? Kittybrewster ☎ 16:08, 9 January 2012 (UTC)[reply]

Us discovering them, in the near future? 0% chance. Von Restorff (talk) 16:21, 9 January 2012 (UTC)[reply]

The SETI article would be a good place to start reading, with lots of links off to other relevant places. The Drake equation is a commonly used (and commonly criticised) equation to estimate the number of likely intelligent alien civilisations. The Pioneer plaque and Voyager Golden Record are two messages we chose to dispatch with spacecraft in the 70s, so may indicate what we would send now. (Realistically though they'll probably pick up signals of Two and a Half Men first and decide that we're all total morons and simply come and destroy us before we know anything about them). --jjron (talk) 16:31, 9 January 2012 (UTC)[reply]

Also, between the pioneer plaque and the voyager record, we sent the Arecibo message. SemanticMantis (talk) 16:47, 9 January 2012 (UTC)[reply]

It can't really be said that there's precisely 0% chance of that happening, but the outlook for that happening certainly doesn't appear too promising. At the very least, any communication with aliens would be extremely slow. Round-trip radio or light communication with the closest known star with a planet around it, Epsilon Eridani, would take about 21 years. And the failure so far of SETI to find any sign of intelligent life within the Milky Way would suggest that if intelligent life ever is discovered, the closest such life would be much further away than Epsilon Eridani. A round-trip radio or light communication to a typical star within the Milky Way would take tens of thousands of years. Red Act (talk) 17:26, 9 January 2012 (UTC)[reply]

That is just the rate of two-way communication. We could send the entire works of Shakespeare before we found out that that they didn't like Hamlet. Of course we might want to withhold broadcasting the military articles of Wikipedia until we found out if they were non-hostile. Rmhermen (talk) 17:40, 9 January 2012 (UTC)[reply]

More likely they wouldn't understand Hamlet. Of course we don't censor Wikipedia from known hostiles on earth. Kittybrewster ☎ 17:57, 9 January 2012 (UTC)[reply]

Yes, the OP said "...capable of replying to a message...", so I was addressing the time it would take to send a message and receive a reply. When I said "slow", I was just referring to communication latency, not bandwidth. Red Act (talk) 18:01, 9 January 2012 (UTC)[reply]

Maybe an alien species is broadcasting a signal to space, like us BTW, which could reach the earth tomorrow. That falls within your definition of "capable of replying.", but I don't know any way of calculating a probability of something that didn't happen at least a few times. 88.9.214.197 (talk) 17:34, 9 January 2012 (UTC)[reply]

Completely unknown. This could be computed with the Drake Equation, except that virtually every number you need to plug into that equation is also unknown. APL (talk) 19:54, 9 January 2012 (UTC)[reply]

Semantically I would disagree with "completely unknown". That suggests you have no opinion either way; we clearly have quite a lot to suggest the chances are far less then 100%. I don't know if I'd bet my life on it, but I'd probably bet my house that we won't be communicating with intelligent space aliens in the near future, I doubt many scientists in the field would think that was a frivolous wager because the odds are "completely unknown". Vespine (talk) 21:32, 9 January 2012 (UTC)[reply]

All fair points, of course. However, if we're nitpicking semantics, I'd like to point out that "communicating" is not the same as "discovering". If you made that wager and even if the discovering happened right now, odds are that your house would be safe for many years to come. APL (talk) 22:33, 9 January 2012 (UTC)[reply]

All of the responses above assume that communicating with aliens means communicating with their home planet. There's no reason why the first human detection of an alien can't be of a sentient Bracewell probe in our own solar system, in which case two-way communication could be nearly instantaneous. Alternatively, the alien planet could send an alien ambassador upon receiving Earth's radio signals, and there's no reason why this ambassador can't arrive tomorrow. --140.180.15.97 (talk) 04:37, 10 January 2012 (UTC)[reply]

If we assume they can't send their ambassador faster than the speed of light, then they would have to be quite close to have gotten our radio transmissions and sent him/her/it already. StuRat (talk) 21:34, 10 January 2012 (UTC)[reply]

Vacuum metastability event warning?

Could a Gravitational lens give us a few scant million years warning of our inevitable doom from a False vacuum collapse? Hcobb (talk) 20:38, 9 January 2012 (UTC)[reply]

It depends how fast the lower vacuum bubble was expanding, and where in space it nucleated. The farther away, and the slower it expands, the more warning we would get. Goodbye Galaxy (talk) 21:46, 9 January 2012 (UTC)[reply]

I'm assuming a speed of light spread, because otherwise the boundary conditions get really odd. But that is of course speed of light through space and space is bent. Hcobb (talk) 01:12, 10 January 2012 (UTC)[reply]

If it expands at the speed of light then there isn't any warning. If it expands slower then there is some kind of warning (if nothing else, the stars disappear!). Gravitational lensing doesn't make a difference either way. Gravity affects everything equally, even vacuum decay. -- BenRG (talk) 19:55, 10 January 2012 (UTC)[reply]

Health adjusted life table

Is there, in the actuarial science a health-adjusted life table? (note: this is not a disability-adjusted life years or healthy Life Years or quality-adjusted life year). I mean, if you are x years old and healthy, you probably will live longer than the average.88.9.214.197 (talk) 23:19, 9 January 2012 (UTC)[reply]

The problem with that approach is most people don't actually know if they're healthy. Do you know how much plaque has accumulated in your arteries ? Do you know if a small tumor is currently growing inside you ? So, they typically ask questions like your BMI, whether you smoke, how stressful your job is, etc., and use those to determine how healthy you are likely to be. StuRat (talk) 21:28, 10 January 2012 (UTC)[reply]