Wikipedia:Reference desk/Archives/Science/2008 August 19

= August 19 =

light
If light travels through space(a vacuum) at a constant speed does this mean that in theory light can travel on forever 207.118.239.28 (talk) 00:39, 19 August 2008 (UTC)
 * Anything can continue traveling indefinitely in the absence of interfering forces or materials, per Newton's First Law of Motion, and light is no exception. However, space isn't a perfect vacuum, and thus probability dictates that light eventually hits something. &mdash; Lomn 01:41, 19 August 2008 (UTC)
 * If it doesn't hit anything that can absorb it, yes, it will keep on going for ever. The photons we detect as cosmic microwave background radiation have been travelling for over 13 billion years. Because of the expansion of the universe, light travelling for a long time is redshifted, so if it started off as visible light it won't be visible after a while (I'm not sure how long, billions of years probably, longer for blue light, shorter for red). Also, if the light is coming from a point and spreading out in all directions (like from a star, say, as opposed to a laser beam), it's intensity reduces with the square of distance (inverse square law). Neither of those things will ever stop it, though, it gets weaker and weaker and redder and redder, but it never entirely goes away. --Tango (talk) 01:42, 19 August 2008 (UTC)


 * Are other EM wavelengths going around in space in addition to the cosmic microwave spectrum? For example, is ultraviolet light scooting around out there waiting till it is old enough to be visible?


 * If wavelengths get gradually longer with time, is the cosmic microwave distribution shifting toward longer wavelengths, or has it stabilized for some reason? Thanks.  Wanderer57 (talk) 03:46, 19 August 2008 (UTC)
 * Yes all those different wavelengths are radiating through space, see ultraviolet astronomy coming from hot stars, and X ray astronomy from pulsars. Also the cosmological redshift keeps working even today.  What could stop it is the end of time.  Graeme Bartlett (talk) 05:58, 19 August 2008 (UTC)
 * The end of time, or just the end of expansion. If the universe stopped expanding, the redshift would stop (and would turn into blueshift if it started contracting). Current theories do not expect that to happen, however, so redshift should continue indefinitely. --Tango (talk) 18:11, 19 August 2008 (UTC)


 * Here's a point about light intensity. The total intensity does not decrease with the square of the distance. Assuming nothing blocks the light, total intensity remains constant, regardless of distance. What decreases is the intensity per unit of area normal to the direction of radiation. A telescope, at any given distance, magnifies this area. That makes the image dimmer but allows smaller detail to be seen - provided the detail remains sufficiently bright to be perceived. (Hence the value of time-exposure photograpy in astronomy.) Andme2 (talk) 06:03, 19 August 2008 (UTC)
 * According to Intensity (physics), intensity is a measure of power per unit area, so it is correct to say the intensity is reducing, it's the total energy which is staying the same (I'm not sure even that is true once you take relativity into account). --Tango (talk) 18:11, 19 August 2008 (UTC)
 * Also, I should point out that telescopes don't just magnify. The aperture (width of the main lens, roughly speaking) is usually much greater than that of the human eye, so it also increases the total amount of light that reaches the eye. --Tango (talk) 18:14, 19 August 2008 (UTC)


 * I think the definition of intensity depends on your field. As Intensity (physics) says: "In photometry and radiometry, intensity has a different meaning: it is the luminous or radiant power per unit solid angle. This can cause confusion ..." Gandalf61 (talk) 18:23, 19 August 2008 (UTC)

Railguns, rail errosion
If it is possible to coat the rails or the projectile in conductive plasma, could this help mitigate rail erosion at all? ScienceApe (talk) 05:03, 19 August 2008 (UTC)
 * Plasma will increase erosion as the ions and radicals react with the rail. Graeme Bartlett (talk) 05:54, 19 August 2008 (UTC)


 * How about reducing friction by raising the projectile above the rails through "magnetic levitation"? Is this incompatible with the propulsion system?


 * Is friction a significant issue with railguns? Wanderer57 (talk) 19:51, 19 August 2008 (UTC)


 * The projectile needs to have current flowing through it, see railgun. The coilgun has no wear on the barrel, but who wants to ONLY fire ferromagnetic projectiles. With a Railgun, you can launch depleted uranium slugs, yay! Coolotter88 (talk) 20:25, 19 August 2008 (UTC)


 * Yea, the railgun can launch anything that is conductive. I also think it can launch projectiles are higher velocities than a coilgun. But the rail erosion is a problem that coilguns don't have. Friction is primarily the problem. Plasma is conductive so I was thinking that maybe plasma could help "lubricate" the rails so to speak. Could a salt solution work instead? ScienceApe (talk) 00:16, 20 August 2008 (UTC)


 * What about silver conductive lubricant? 96.242.14.160 (talk) 15:30, 20 August 2008 (UTC)

Laser
Thank all friends, I have get all about my question. This discussion can be ended now. I will start to write a new title.--Jiachun Zheng (talk) 07:53, 3 September 2008 (UTC)

How to measure the distance using laser? —Preceding unsigned comment added by Jiachun Zheng (talk • contribs) 09:45, 19 August 2008 (UTC)
 * Are you talking about a Laser rangefinder? Zain Ebrahim (talk) 09:48, 19 August 2008 (UTC)

Thank you.

Yes. I think. Part of it is "Laser transducor". I interested about it is used in the Olympic Games, specially about it is used in the field game.

I want to know more morden Tech. of the Laser rangefinder.Thank you.

--Jiachun Zheng (talk) 11:58, 19 August 2008 (UTC)


 * Using interferometry combined with time-domain reflectometry, you can use the laser to determine lengths with accuracy to sub-wavelength levels. I am not aware of commercial applications of such an apparatus outside of the research community.  Nimur (talk) 16:59, 19 August 2008 (UTC)
 * A laser rangefinder is a time-of-flight device, not an interferometer. It sends a laser pulse and counts the time until the pulse returns. the accuracy depend on the accuracy of the clock that measures the time: light travels 300 millimeters per nanosecond. -Arch dude (talk) 02:14, 20 August 2008 (UTC)

Thanks. All friends I interested it specially about how to be used at the field game in the Olympic Games,

I want to know more morden Tech. of the Laser rangefinder.

--Jiachun Zheng (talk) 03:27, 21 August 2008 (UTC)


 * The most basic device contains a laser and some kind of sensitive light-sensor that only responds to light at roughly the frequency of the laser. There is also a very high speed clock - one that "ticks" several times in every nanosecond probably.


 * Something (a computer probably) fires off a brief burst of laser light in the direction of whatever it is that has to be measured and simultaneously starts the clock. The light heads off towards the target (at the speed of light) - bounces off of it and then comes all the way back again and some of the reflected light enters the light detector.  As soon as the detector senses that the laser light has returned, it stops the clock.  By measuring the amount of time taken for the light to go from the rangefinder to the object and back again - and knowing the speed of light - you can calculate the distance the laser pulse travelled.  Divide that by two to find the actual distance.


 * The difficulty is that light moves very fast - and making a clock that ticks fast enough is difficult.  A good rule of thumb is that light travels about 1 foot (or 30cm) in a nanosecond.   Making a clock that ticks once per nanosecond is possible - but it would only be able to measure the distance the light travelled accurately to 1 foot - and so the error in measuring the distance would be about six inches.  In some situations, that kind of precision probably plenty good enough - in others, it's not.


 * The "Interferometry" approach is more difficult - but vastly more accurate. Light is a wave - and if you mix two lightbeams, you get "interference".  When the peaks and troughs of the two waves line up exactly, you get a wave that's twice as big.  When the peak of one wave lines up with the trough of the next, they cancel out.  Between those two extremes, you get light that's somewhere between zero and the sum of the two beam brightnesses.   This effect is called "interference".


 * So, you fire off your laser to the target, catch the reflected light as it comes back and mix the outgoing and incoming light together with some fancy mirrors and stuff. Then you can see whether the two beams are lined up, or out of alignment, or somewhere in between.  If the distance to the object is an exact multiple of the wavelengths of the laser light - then the two beams line up peak-to-peak and trough-to-trough.  If the distance is a little less, they won't add up to quite as much - and so on down to where they cancel out and the distance is half a wavelength more than an exact multiple.


 * This means that providing you know the distance to the object to the nearest half wavelength (which you can do with a really fast clock and the first method I described) - then you can use the interference result to figure out whether this is an exact number of wavelengths or half a wavelength off - or anywhere in between. This complicated method gives accuracies down to a few millimeters or better...but the machines are a LOT more expensive.


 * The other trick you can do with some of these machines is to use them like a police laser speed gun which measures the amount of doppler shift between the incoming and outgoing laser light - which tells you the speed something is moving.


 * SteveBaker (talk) 01:17, 22 August 2008 (UTC)

Thanks for this answer. Thank you for your writing so carefully. I get so much from it. I was a retired Physics teacher. So I think I am able to understand all of your writing. Thank you once again.

Could you give me an answer about what Laser rangefinder is used at the field game in the Olympic Games, specially at the 2008 Beijing Olymic Games. I guess probably,it is more morden Tech. of the Laser rangefinder.

You are so kind that I don't know how to express my mind.

good luck

--Jiachun Zheng (talk) 08:29, 22 August 2008 (UTC)

Who can help me to detect the imformation about which Laser Rangefinder is used at the field game of the Olympic Games, specially in the 2008 Beijing Olymic Games.

I hope to know the lastest Tech. the newest Tech. that is used at 2008 Olympic games about Laser Rangefinder.

--Jiachun Zheng (talk) 08:51, 24 August 2008 (UTC)

I had getten all of the imformation about which Laser Rangefinder is used at the field game of the Olympic Games, specially in the 2008 Beijing Olymic Games.But I don't have been able to write it in English Wikipedia. very sorry. Thank all friends, you helped me so much. I will see you on next question.--Jiachun Zheng (talk) 04:06, 11 September 2008 (UTC)

natural c onception after menopause
Can a woman conceive naturally after menopause? Are there natural foods or treatments to reverse infertility, after many years past menopause? —Preceding unsigned comment added by 64.21.220.29 (talk) 16:49, 19 August 2008 (UTC)
 * I think, by definition, a woman cannot conceive after the menopause, however I suppose it's possible to be mistaken about the menopause having taken place (the cycle may become irregular, but not have actually stopped completely). Being mistaken like that for many years seems unlikely. --Tango (talk) 18:05, 19 August 2008 (UTC)

Is a little lead deadly?
I bought a flat iron and it has a tag with a warning that it contains lead and I should wash my hands after handling it. And it says "The state of California recognizes that lead causes cancer and birth defects" or something. Should I freak out? Is it REALLY necessary to wash my hands every time I straighten my hair? Will having the product near me send lead particles into the air that will harm me?

And since this thing will be in contact with my hair, will the lead damage my hair?

Thanks so much in advance. 68.15.144.36 (talk) 18:18, 19 August 2008 (UTC)HarkUponTheGale
 * Just don't lick it. see lead poisoning. I wouldn't imagine that lead dust would fly out of your iron (they wouldn't sell it) but the time it takes to wash your hands afterwards isn't that much so it's not that inconvenient to be on the safe side and wash your hand with soap and water. Coolotter88 (talk) 18:32, 19 August 2008 (UTC)


 * Regarding lead and hair, note that (at least until recently) the hair coloring product Grecian Formula contained lead. In fact, the active ingredient was lead, which colored the hair by forming black particles of lead sulfide. -- 128.104.112.147 (talk) 19:07, 19 August 2008 (UTC)


 * My best guess is that the lead is in the solder for the electrical connections inside the iron. If so, the hazard really does not occur until it is disposed of and tossed in a landfill to leach into the soil. --——  Gadget850 (Ed)  talk  -  19:27, 19 August 2008 (UTC)


 * I really doubt you have anything to worry about. I remember buying a chair once that had the same warning (though it didn't tell me to constantly wash my hands). That message is required by the state of California for any product that contains lead somewhere, I guess. A product that could cause lead poisoning by its standard usage wouldn't get on the market, and the lead concentration is probably inside, like Gadget850 said.--El aprendelenguas (talk) 19:33, 19 August 2008 (UTC)


 * This appears to be a Proposition 65 warning required when you will be ingesting more than 0.5 micrograms of lead per day. That's a very small amount, but the tag doesn't have to show the actual amount. The lead could be in the solder, in could also be an alloy in the iron body itself (for heat conduction). I wouldn't personally advise you to ignore any warning label. Your other option is to take the iron back to the store and look for one that doesn't have a warning tag. Also, as noted in one of the links, pregant women should minimize their exposure to lead from all sources. Franamax (talk) 21:02, 19 August 2008 (UTC)


 * Lead is nasty because it attaches to your brain and makes you stupid. The problem is much worse for small children than for adults for some reason...hence the warning for pregnant women.  As an adult, I honestly wouldn't worry.  So long as you aren't trying to eat the lead in the iron, it's not going to affect you.  The biggest problem with lead poisoning is dust that you can inhale (from crumbling lead-based paint) or lead that's dissolved into something (such as happened in old buildings with lead water pipes).  An actual solid lump of lead isn't going to hurt you measurably. SteveBaker (talk) 21:25, 19 August 2008 (UTC)
 * Well, not usually... - EronTalk 21:28, 19 August 2008 (UTC)


 * While we're on the subject of lead... I have two lead pigs that were given to me as an odd gift. They're not coated in anything—just heavy lead cylinders. At the moment they're just sitting on my radiation cover. I don't touch them. Is there any health risk? What about in the winter, when the radiation is turned on? I'm not asking for medical advice, I just want to know if there's some obvious danger I'm not thinking of. --98.217.8.46 (talk) 22:05, 19 August 2008 (UTC)


 * You mean "radiator" - not "radiation" - right? If you have radiation sources that turn on and off - the lead may be the only thing that's saving you!


 * But - no, just having a couple of lumps of lead in the room with you won't hurt you at all...doubly so if you never touch them. You might want to put them somewhere where kids can't reach them though. SteveBaker (talk) 22:12, 19 August 2008 (UTC)


 * I seem to remember a similar question being asked a few months back. I can't seem to find it in the archives now, but IIRC the consensus that time was that the owner of the cans should get them checked out (local university lab?) before even thinking of reusing them for any purpose, or even opening them up. --Kurt Shaped Box (talk) 00:24, 20 August 2008 (UTC)


 * Haha, yes, Freudian slip on radiatior. Anyway, thanks. I figured such was the case. No worries about kids (they are both out of the way, incredibly boring, and I have no kids around, ever). And yes, KSB, I was the original lead pig dude. I'm not reusing them. They just sit there. And are heavy. And are pretty boring. --98.217.8.46 (talk) 00:26, 20 August 2008 (UTC)


 * As I think I probably mentioned when you asked previously, whatever was previously stored in them may have left a dusty residue on the inside. These things are supposed to be decontaminated before disposal but, as is the way of the world and the people in it, that doesn't always happen. Really, the last thing you need is a faceful (followed by a lungful) of an alpha emitter compound if you accidentally jostle the pigs and the lid comes off one of them... --Kurt Shaped Box (talk) 02:07, 20 August 2008 (UTC)

Wall voltage through your body vs resistance
If the resistance between my hands is 3.9 million Ohms (measured with a multimeter), how come running 230 volts between them is so dangerous? Am I missing something huge, or are there simply points with so much lower resistance that it becomes dangerous?

By my (possibly faulted) calculations, the current passing between the hands when resistance is 3.9M ohms is 230/(3.9 * 10^6) = 5.9 * 10^-5 = .000059 A = 0.059 mA, where 10mA is quoted as being deadly (because of the heart). That's more than an order of magnitude more than 0.059 mA. -- Aeluwas (talk) 19:15, 19 August 2008 (UTC)


 * Did you see electric shock? Coolotter88 (talk) 19:43, 19 August 2008 (UTC)


 * It's not so much a case of different points on your body as the condition of your body causing resistance to change. Is your skin dry?  Is it wet?  Is it wet enough?  Grabbing hold of 230V is far from uniformly fatal, but it can be under the proper conditions. &mdash; Lomn 20:06, 19 August 2008 (UTC)


 * I'd take that measurement with a very large grain of salt. High resistances can be very difficult to measure accurately, and as others have already noted, the condition of the skin (especially the presence of moisture) can dramatically alter the conductivity of the body.  Under moist conditions (electrodes or skin are damp), resistance between major extremities can drop at least a hundred-fold  to one thousand ohms or less.  Inadvertently piercing the skin (as, for example, with the sharp cut end of an electrical wire...) will also sharply reduce the resistance of the body by eliminating the resistance of skin at the point of contact.  230 volts across a thousand ohms resistance is 230 milliamps&mdash;more than sufficient to be very fatal.  TenOfAllTrades(talk) 20:52, 19 August 2008 (UTC)


 * OK - PLEASE LISTEN VERY CAREFULLY : People have been killed by multimeters...doing more or less exactly what you did. Sure, the resistance is high - and even 240volts won't directly deliver enough current to kill you.  But that's a VERY simplified view of what's going on.  The crucial piece of information is that the majority of that huge resistance is in the first few millimeters of the skin.  Once you get past the skin, the human body is mostly just salty water. Salty water (and therefore blood) conducts electricity very well and veins and arteries handily "wire" everything directly to your heart and brain.  So if you were to grip the meter probe where you have a cut your finger (DO NOT TRY THIS!) and measure the resistance, the resistance will be MUCH lower and the current supplied by a humble 9 volt battery is quite enough to kill you.


 * I have gotten many 240v shocks several times over the years - and I'm still here to tell the tale - so indeed, 240 volts THROUGH THE SKIN is not usually lethal (it's far from pleasant though!). The key is that the current flow through the muscles of your arm causes them to contract super-quickly.  So the current is only there very briefly.  People who die from electric shocks tend to be doing something like gripping the wire so that their fingers contract around the wire and they can't let go because of the very muscle contraction that saves you in more ordinary situations.  What happens then is that the energy from the current flow turns into heat which is concentrated where the resistance is highest...in the first millimeter of your skin.  This heat causes the cell walls to break down - within a second or two you get burns and blistering (and you still can't let go - no matter how hard you try).  If you haven't let go of the wire before that happens, blood gets in contact with the metal and then the resistance falls spectacularly...then you die.


 * Please treat electricity with respect - even a 9 volt battery can kill you.


 * SteveBaker (talk) 21:04, 19 August 2008 (UTC)
 * Hmm, interesting! I'll certainly think twice before doing that again. Still: does anyone have two multimeters around, to measure the voltage they send out when probing resistance? I doubt it's the full 9 volts; another site measured it to 0.3V. -- Aeluwas (talk) 21:27, 19 August 2008 (UTC)


 * Steve, I believe you on general principle, but do you have a reference for the assertion that multimeters have killed people in that way? It would be a valuable addition to multimeter.  --Sean 23:24, 19 August 2008 (UTC)


 * This is a fairly detailed discussion of an apparent death by multimeter, though that page lists it as "unconfirmed" and hence possibly erroneous. Dragons flight (talk) 23:45, 19 August 2008 (UTC)


 * Yep - that's where I first read about it too. There was some chat about it here on the WP:RD a year or two ago.  It's really "death by a surprisingly little amount of electricity" - the multimeter just provides a plausible reason why someone should be connecting their hands to the + and - ends of a battery. SteveBaker (talk) 03:42, 20 August 2008 (UTC)

Radio wave strength
Is the strength of radio waves broadcast by humans into outer space inversely proportional to the square of the distance from Earth or inversely proportional to the cube of the distance from Earth (like magnets)? Coolotter88 (talk) 20:47, 19 August 2008 (UTC)


 * The square. Electromagnetic radiation follows an inverse square law. --Tango (talk) 20:58, 19 August 2008 (UTC)


 * So it doesn't matter that the radio waves are broadcast from dipoles? Coolotter88 (talk) 21:07, 19 August 2008 (UTC)


 * Yep it's the square. It's easy to remember why.  When you set off a brief radio pulse, the waves spread out at the speed of light in a spherical wave-front - the three dimensional analog of "ripples in a pond".  When you double the radius of a sphere, you quadruple its area.  So however much energy was in the original pulse ends up smeared out over the surface of the sphere...since the area of the sphere increases of the square of the range - the energy on each square meter of the sphere follows an inverse square law.  It doesn't matter what kind of antenna you use - even if you put out a fairly narrow beam of energy - you're broadcasting over a small segment of the surface of the sphere - which still doubles in area as the distance doubles.  The only way to dodge the inverse square thing would be if you could broadcast like an absolutely perfect laser beam - which might never diverge at all.  However, even the 'tightest' laser beams do diverge slowly - so the inverse square law still applies to them. SteveBaker (talk) 21:14, 19 August 2008 (UTC)
 * Yes. For those who's interested, it might be helpful to look at the pyramid and check that the area of the base quadruples in size when you double the height or the length of its sides, regardless of the angle at the apex. So the law holds even when the rays spread only a little. (Since the base we're interested in here is really curved, the rigorous treatment would be found in Spherical cap, but the law holds here too, naturally.)EverGreg (talk) 12:04, 20 August 2008 (UTC)


 * The magnetic field strength from one pole actually decreases as the square of the distance. The reason dipole field strength obeys the inverse cube law at large distances is that the ratio between the poles' separation and the distance decreases as the observer moves away.
 * For a radio transmitter, the changing current causes an electromagnetic wave that radiates out in all directions. It is not the electric charge comprising the current that is detected as radio waves; it's actually the changing magnetic and electric fields produced as a result.  The wave's intensity does not drop as the cube of the distance because it reflects the energy transmitted in a certain direction, not the force exerted by two poles.  --Bowlhover (talk) 06:42, 22 August 2008 (UTC)

Radio astronomy would usually be in terms of a cone rather than a pyramid; the receptor would be in the form of a parabolic reflector having a round periphery. But the same principle of distance and area would apply. Square-periphery radio horns have also been used, resulting in a pyramidical form of the received-energy path. In light astronomy the thought would also be in terms of a cone rather than a pyramid; the light receptor would be a round lens or a mirror. Andme2 (talk) 07:38, 22 August 2008 (UTC)

healing effect of color on the elderly
How does color in the environment help heal the elderly patient76.196.253.118 (talk) 22:25, 19 August 2008 (UTC)


 * Fascinating question. Has it been shown that color in their environment helps "heal" elderly patients?


 * Has any particular color been found to be especially effective?


 * For what sort of conditions is the healing process improved by color? Wanderer57 (talk) 23:16, 19 August 2008 (UTC)


 * See colour therapy (or color therapy to you).--Shantavira|feed me 08:15, 20 August 2008 (UTC)


 * I always cheer up when I notice the color of red wine in my glass --- always looking for a sponsor of a long term research project. 93.132.147.52 (talk) 18:36, 20 August 2008 (UTC)

Black holes
I read through the black hole article and found it difficult to comprehend as I'm not so erudite with Science. So I have some questions: 1) Where does the matter go when it is pulled into the black hole? 2) Is there a limit to how much they can "absorb"? 3) If black holes are created via imploding stars, what is underneath/above/around the black hole? Is it just space? I can't imagine anything flat in the middle of space pulling objects towards it. 4) I read through the Hawking radiation article and didn't understand it either. Do black holes ever disappear (or "evaporate")? Does it depend on their size (i.e. Do "smaller" black holes disappear, rather than "big" ones?)? If anyone could answer me these questions in simple laymans terms I would really appreciate it. Thank you in advance :-) Utan Vax (talk) 23:54, 19 August 2008 (UTC)

1) Matter is merely added to the black hole's total mass. It gets bigger the more it "eats".

2) As far as I know, no there isn't. OJ 287 is the most massive black hole we know of with a mass of about 18 billion solar masses.

3) Around a black hole is something called an accretion disk which is basically a giant disk of extremely hot matter surrounding the black hole. There are also polar jet at the poles of the black hole which spew matter and radiation.

4) Yes, they will all evaporate into photons eventually. The bigger the black hole, the slower the rate of evaporation. ScienceApe (talk) 00:22, 20 August 2008 (UTC)


 * Thank you very much for your answers :-) Utan Vax (talk) 00:26, 20 August 2008 (UTC)


 * We have to be very careful when saying "It gets bigger the more it eats". The idea is that the forces between the fundamental particles that made up the atoms of the original star are no longer enough to withstand the crushing force of gravity.  All of the material in the star collapses to an infinitesimal dot.  An object with zero size - no matter what the black hole has eaten, it's still a zero-sized object.  (Actually, if it's spinning it can be a small, flat disk - but it would have zero thickness).  When we talk about the "size" of the black hole - we're talking about the size of the "event horizon".  That's more of a mathematical concept than an actual tangiable object.  It's the distance from the infinitesimal point at which nothing (not even light) can go fast enough to escape the gravity field of that infinitesimal point.  So as it swallows more stuff (gas, stars, planets, moons, light, little green men), it gets heavier - that means it's gravity field grows stronger - so the distance at which light cannot escape increases.  So the size of the event horizon grows - but the infinitesimal dot in the middle is still zero sized - it doesn't grow.  But the event horizon itself is nothing special - if you could fall though it, you wouldn't notice anything special happening.


 * Having said all that - it's really much more messy than that. As things fall towards the black hole, they go faster and faster - until (at the instant they hit the event horizon) they are travelling at the speed of light.  But because of relativity, time slows down - and from the point of view of someone outside the black hole, they never actually reach the event horizon - they just get dimmer and dimmer until they are completely dark and splattered flat onto the event horizon with their time stopped dead.  This is messy to understand and explain!


 * SteveBaker (talk) 01:59, 20 August 2008 (UTC)


 * I should correct a few things. You would notice something at the event horizon. Notably, you would probably be dead because the radiation from accretion disk and polar jets would kill you. Also, depending on the size of the black hole, the tidal forces would kill you too. ScienceApe (talk) 17:55, 20 August 2008 (UTC)


 * I don't think infalling objects ever reach the speed of light. From an outside observer's reference frame, time slows down and the falling object becomes stationary at the event horizon.  For the object crossing the event horizon, a finite gravitational force is applied for a finite amount of time, because the object eventually hits the singularity.  It doesn't reach the speed of light because the energy added is finite.  --Bowlhover (talk) 06:40, 20 August 2008 (UTC)


 * Thank you. Intriguing stuff. But there seems to me to be a contradiction here.  Black holes "will all evaporate into photons eventually." But "light cannot escape". If the photons cannot escape, they remain part of the black hole don't they?  How is the black hole diminished? Wanderer57 (talk) 02:19, 20 August 2008 (UTC)
 * The way Hawking radiation works, the particles which constitute the radiation actually come into being just outside the event horizon, so are never inside it. It's really weird stuff! --Tango (talk) 02:52, 20 August 2008 (UTC)


 * Yes, infalling objects don't reach the speed of light at the event horizon. Nothing special happens at the event horizon. But I disagree with the part beginning "From an outside observer's reference frame". The slowdown you're referring to here is a Doppler shift, as I mentioned in the thread below this one. It doesn't have anything to do with reference frames; in fact nothing has anything to do with reference frames. The point of reference frames is that they're all equivalent: you can pick any one to do your calculations in and the result comes out the same. To the extent that you're making physical predictions, such as what's seen by the outside ship, reference frames don't matter. I'm of the opinion that one cannot achieve general relativistic enlightenment until one liberates one's mind from the concept of reference frames. -- BenRG (talk) 14:37, 20 August 2008 (UTC)
 * I don't understand you... reference frames are extremely important. An external observer sees something very different to an observer that's falling in. It's not just a matter of Doppler shift, it's gravitational time dilation as well (the two are related, however). When we say that all reference frames are equivalent we mean that the laws of physics apply equally to all of them, that doesn't mean they are all the same - different frames observe events differently. --Tango (talk) 17:14, 20 August 2008 (UTC)


 * (after ec's)
 * (1) It goes to the gravitational singularity, an mathematical point that contains all of a black hole's mass.
 * (2) There's no limit; a more massive black hole absorbs more cosmic microwave radiation and evaporates less quickly than a less massive one. There is no feedback mechanism that ejects mass/energy more quickly for more massive holes.
 * (3) Black holes are singularities with surrounded by event horizons. When an object travels closer to a singularity than the event horizon, it cannot escape.  Hence, the space inside an event horizon is closed; there is no path which leads outside.
 * (4) The cosmic microwave radiation is constantly feeding energy into black holes, and this energy adds mass (remember Einstein's famous equation, E=mc^2, where E is the energy, m is the corresponding mass, and c is the speed of light in vacuum). In order for a black hole to lose mass over time, it must be evaporating faster than the CMB is adding energy.  Because the rate of evaporation increases with decreasing mass, only black holes below a certain mass can be gradually wasting away.  This mass turns out to be 0.8% of Earth's--must lower than the smallest holes (~3 solar masses) formed by gravitational collapse.  All black holes are currently gaining mass instead of losing it.
 * Note that some black holes may formed shortly after the Big Bang and these can be of any mass. High-energy collisions may also create holes with extremely low mass.  However, there is no evidence for the existence of these objects.
 * Also note that Hawking radiation contains more than photons. Any particle is possible, although according to http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/hawking.html photons and neutrinos are the main components.  --Bowlhover (talk) 02:36, 20 August 2008 (UTC)


 * The cosmic microwave radiation is constantly feeding energy into black holes, and this energy adds mass OK but Because the rate of evaporation increases with decreasing mass, only black holes below a certain mass can be gradually wasting away. This doesn't follow since at some time in the future the CMB radiation could be lower than the evaporation rate. But we're talking about 10100 years here. --Ayacop (talk) 08:17, 20 August 2008 (UTC)


 * Sure, but my intention was to say that all black holes are gaining mass at this moment. To nitpick, the mass limit will exist even far into the future; it just won't be 0.8% of Earth's mass.
 * Edit: Sorry, I see my post implied black holes cannot evaporate away.   --Bowlhover (talk) 11:31, 20 August 2008 (UTC)


 * Assuming the Lambda-CDM model is right, the CMBR temperature will soon start falling by a factor of 2 every 11 billion years or so, so even a billion-solar-mass black hole will be warmer than the CMBR in less than a trillion years. It will take on the order-order of 10100 years to evaporate after that, though. -- BenRG (talk) 14:37, 20 August 2008 (UTC)