Wikipedia:Reference desk/Archives/Science/2007 August 9

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.

August 9

Do freezers use less energy when empty?

Let's say I have two identical freezers set to -10C. One is empty and the other is full of ice at -10C. Over a long time, does the empty freezer use less energy? I can see two competing arguments. The empty freezer only has to keep air frozen, while the full freezer has to keep a lot of water frozen, so it seems like the empty freezer would use less energy. But, since the water ice is already starting out at the temperature of the freezer, all of the energy that would need to go into freezing it has already been accounted for, and as long as the air in the freezer is kept at -10C, the ice will stay at -10C too, never raising, never needing any energy to keep cold. So do empty freezers use less energy than full freezers?

A follow-up question: does it make any difference if the water is replaced with meat? --Cyde Weys 00:21, 9 August 2007 (UTC)

Being a bit removed from thermodynamics (well, not really, I just hated it) I can't answer it, but I can provide some info. The root of the question is, how quickly will thermal energy enter the freezer, for that exactly how fast the freezer must subsequently pump it out. Air is considerably less dense than ice, and so it can't contain much thermal energy to begin with (at a given temperature). It also happens to be an extremely poor conductor of heat, although it can convey heat quite well in some circumstances. In short, ummm, I still don't know ;-) Someguy1221 00:27, 9 August 2007 (UTC)

Empty freezer definitely uses less energy. That is why it is important to keep your freezer defrosted, if you still have an old non frost free freezer. Since the water is a lot more dense then the air, it will take more effort to keep it at a low temperature. Think of this example, say the freezer thermostat is designed to cycle on at -8 and cool something to -12 degrees, purely hypothetical temperatures. If there is ice at -12 it and the air will take longer to get to -8, but once it does, the freezer would quickly cool just the air back to -12, but it will take the frozen ice a lot longer to get back to -12 since it absorbs much more heat getting to -8 then the air alone, so the freezer will have to work a lot longer to get the ice air back to -12 then if there was no water there. As for the second question, i imagine meat and water would have similar heat capacity, from memory meat is around 80% water anyway. Vespine 01:32, 9 August 2007 (UTC)

You guys have it completely backwards. A full freezer uses less energy. [1]--Dacium 03:39, 9 August 2007 (UTC)

The catch, Vespine, is that the ice doesn't warm up to -8°C (in your hypothetical example) very quickly. Ice is a lousy conductor of heat, so it's not going to warm very rapidly, even if you leave the freezer door open. Air, on the other hand, is free to move. Particularly with an upright freezer, cold air will very quickly be replaced with room-temperature air. (Notice how your bare feet get cold when you're standing in front of the fridge?) Every time the door is opened, you have to re-chill all of that air. As for the idea that 'it will take more effort to keep [water] at a low temperature'—well, that's rather misleading. The rate at which heat enters the cold part of the freezer depends solely on the temperature difference between the inside and outside; it doesn't matter what's inside. (Going back to the original question, this means that if you never open the freezer, there won't be a significant difference in energy usage between a full and an empty freezer.)

Now, it is still important to limit frost buildup in a freezer, but not for the reason stated. Rather, the ice tends to build up around the coldest parts of the freezer (the refrigerant coils and such) and – as we just finished saying – ice is a pretty good insulator. Putting a thick layer of ice around the parts of the freezer that remove heat means that they don't remove heat nearly as efficiently. TenOfAllTrades(talk) 04:00, 9 August 2007 (UTC)

(edit conflict)Actually, I think the only questions is if you are opening and closing it. If you start with ice cubes, the energy difference to cool will be fairly small. If you open the doors however, warm air enters the freezer and must be cooled. There is more volume in the empty freezer and more room for warm air. The amount of air is the only thing bringing heat into the freezer in both cases so the freezer with the smaller volume wins (ie filled with ice). I don't think the contents matter unless you take into account the conduction contact inside the freezer which should be small. Water directly on the coils inhibit heat transfer and act like an insulator (see igloo). This reduces thE efficiency of cooling stuff on the other side of the ice (so spread your ice around to let air in). If you don't open it, the temperature difference between the inside of the fridge and the outside is the same so I think that means the heat loss from conduction is the same. Als, if you don't open it, an empty fridge will warm up faster but also cool down faster so the difference is in cycles (air freezer has shorter, more frequent cycles = bad for compressor), not energy. So keep it full. Meat would be my choice. --Tbeatty 04:07, 9 August 2007 (UTC)

Giant philosophical tree-falling-in-the-forest caveat. An empty freezer never needs to be opened. A freezer full of meat however is a constant attraction. So on science desk, the full freezer uses less energy, but in the philosphy desk, the empty freezer uses less energy. Turn off the empty fridge. --Tbeatty 04:10, 9 August 2007 (UTC)

Hmm….this seemingly simple question has turned surprisingly tough, hasn’t it? What with all the ones answered re: how to set down on neutron stars and so on. Let’s forget about opening and closing the freezer door for the mo, and just look at the fundamentals. If the freezer was totally perfect, then, if you didn’t open the door, you could keep it full of frozen air, water or meat, indefinitely. Once the freezer froze its contents, then no more energy need be expended keeping it cold. But that’s because we are positing that no thermal energy can enter the freezer cavity. But that’s impossible. No matter how well insulated the cavity is, it is made of matter, and all matter must conduct heat to some extent. Therefore, heat will come in from the outside at some rate, however slow. Air is less dense than water, which is less dense than meat, so air cannot keep a lot of latent heat. Because air will not hold a lot of heat at normal pressure, then it will not take much heat from outside to get the freezer air to ambient room temperature again. Water holds a lot more heat, so it will take much more energy to get the water to room temperature, and of course meat is much denser again, so it will take even more. So yes, you will pay more to keep a freezer full of meat cold, than you would a freezer of air. Myles325a 06:26, 9 August 2007 (UTC)

So let's remember how a freezer works. if the temperature rises to the set point it turns on and freezes again. In you scenario, the air freezer should cycle more (it cools and heats faster). the full freezer cycles slower. Imagine the water filled freezer cycles an hour off followed by an hour on. The air freezer might cycle for 15 minutes off and 15 minutes on and the temperature swings are the same. The energy (the integral of the cycling) will be the same becase the energy transfer depends only on the barrier. --Tbeatty 07:41, 9 August 2007 (UTC)

I think, to a first approximation, you've called it correctly: To a first approximation, a closed freezer's energy consumption depends entirely on 1) how much heat leaks in through the walls and 2) how efficient the refrigeration mechanism is at removing that heat. Both of these parameters are independent of the load inside a closed freezer that has been closed long enough to reach nominal thermal equilibrium.

The one second-order nit that I'll pick is that electric motors draw a "starting surge" of current so each start of the refrigeration mechanism draws a certain impulse of energy. The emptier freezer will experience more starts and so more impulses of starting energy will be consumed. This is definitely a second-order effect, though.

Atlant 13:03, 9 August 2007 (UTC)

That sounds about right - but I have a nagging doubt which I'll come to in a moment.

The rate of energy loss is dependent on the square of the temperature difference between the inside of the freezer and the outside (Newton's law of cooling) - so from a purely theoretical point of view, we don't care what's inside the freezer - only what temperature it's at. But - because of the nature of the thermostat, if the thermal inertia of the contents of the freezer is larger (ie if it's full) then the on-off-on-off cycle time will be much longer than for something with less thermal inertia. So if your freezer is full, the motors will turn on and off less often than if it's empty. The total amount of time that the motor will be running will (theoretically) be exactly the same no matter what's inside - but because it takes extra energy to turn the motor on (which is lost when you turn it off again), if the freezer turns on and off 100 times a day then that extra energy is wasted at ten times the rate of a freezer that turns on and off only 10 times a day. That's a strange thing though - the book that came with my freezer said that it runs more efficiently when it's full (which fits this explanation) - and to keep it defrosted in order to save energy (which is contrary to our expectations - the ice just makes the freezer yet more full - so it should actually save energy!) What gives?

We can certainly say that keeping the freezer full will reduce the number of times the motor starts and stops - and that ought to improve the life of the motor and save energy - which is certainly a benefit.

But I wonder whether it's ONLY to do with the motor turning on and off? Because Newton's law of cooling says that energy loss is proportional to the SQUARE of the temperature difference, when we have to cool the freezer down from the upper limit of the thermostat to the lower limit, it's going to be easy to reduce the temperature to start with - and get progressively harder as the temperature drops - so we don't have a linear system here. I can't quite get my head around the effect this has - I think we need some differential equations here. Hmmm - intuition says it shouldn't matter - but I'm not so sure. There may be more to this than just the motor switching cost. SteveBaker 13:54, 9 August 2007 (UTC)

Freezer energy consumption depends only on "cold" loss (imperfectness of heat insulation). If freezer is kept closed, it should not matter, if it is full (of anything) or empty. Covered cooling coil, might affect energy consumption, though. -Yyy 17:18, 9 August 2007 (UTC)

So how do you explain away the effect that Atlant and I are suggesting? Motors do use more energy when starting and stopping - and I'm pretty sure it has to start and stop more often when the freezer is empty than full. SteveBaker 19:38, 9 August 2007 (UTC)

yeah, thats right! -Yyy 11:38, 10 August 2007 (UTC)

The problem with ice that forms on the inside surface of the freezer compartment (in a non "frost-free" freezer, and assuming that the evaporator coils are hidden in the walls) is that it forms insulation between the mechanism's evaporator coil and the interior of the freezer. As a result, the evaporator coils run cooler than they otherwise would but more of that "cool" escapes through the thermal insulation to the outside and proportionally less "cool" gets into the freezer compartment proper. In other words, unless the freezer is filled with one solid block of ice, you probably want air to be able to circulate freely in the freezer compartment and in full contact with the freezer's cold walls.

(And yes, I know it's heat and not "cool" that flows, but the explanation reads more clearly without the extra levels of "inversion".)

Atlant 23:03, 10 August 2007 (UTC)

Insulation, insulation, insulation! Seriously folks! The only efficient freezer, is a full one. Whether it's water, meat or veggies, they'll hold on to that precious frost *much* better than a cubic metre of air.

so what does all this mean in terms of pound notes??? if an empty freezer is less efficient than a full one i should definately try and fill it but if it only means i'll save £3 a year i might not bother with all the hassle!!! (Jon)

centrifugal force and gravity

Why can a child spinning a ball on a string create an absolutely identical force to that created by the presence of trillions of tons of earth? (1G)

Gravity weakens with distance, and is very weak by the time you're at macroscopic distances (and you are many many miles away from nearly all of the earth's mass). Try putting two bowling balls an inch apart and see how long it takes for them to roll together. DMacks 05:01, 9 August 2007 (UTC)

It's the same as asking how can you toss a ball to yourself and defy gravity for the amount of time it takes to return. The answer is the mass of the ball is small and the energy required to move it is small. If you want to compare the earth to ability of a small child, the earth can accelerate a meteor to thousands of miles an hour still at only 1G. --Tbeatty 05:12, 9 August 2007 (UTC)

The force is far from identical, the magnitude of that force is the same in that case but that doesn't make it identical. The same thing can be said about you standing on the earth, how can your feet exert as much force onto the earth as the earth exerts onto your feet?? Imagine what would happen when the child tried doing the same with a car. Vespine 06:04, 9 August 2007 (UTC)

The force is the same; what differs is force divided by mass. —Tamfang 22:56, 14 August 2007 (UTC)

Of the four fundamental forces, gravity is by far the weakest. Our muscles make force by means of chemical reactions which are ultimatively mediated by the electromagnetic force which is very many times stronger. Furthermore, in the case of the ball on a string as well as in the person standing up, the question might also be: why do the forces not tear apart the string or make the person crumble under his own weight, i.e., what causes matter to hold together. This, too, is the action of the electromagnetic force, together with the Pauli exclusion principle. You may turn around your statement: The fact that you need the gravitation of the whole earth just to keep you on the ground shows how incredibly weak gravitation is. (Compare with a small magnet, that can easily counter gravitation when holding a piece of iron.) Simon A. 08:26, 9 August 2007 (UTC)

Yes, exactly. Gravity is such a pathetically small force that if we didn't live near this outrageously large chunk of rock - we'd never really have noticed it. The gravitational force between two 'normal' sized objects (like that pair of bowling balls) is ridiculously tiny - far too small to measure in fact. But the force from a teeny-tiny muscle or a magnet is HUGE by comparison. SteveBaker 13:31, 9 August 2007 (UTC)

Not necessarily too small to measure. Henry Cavendish managed to measure the gravity exerted by 350 lb spheres; you could probably measure the gravity exerted by a truck or something else which is "normal" but heavy. --24.147.86.187 14:30, 9 August 2007 (UTC)

Yup...it's a standard physics lab experiment in high school or introductory college courses. You can buy a kit or even use stuff in your basement. DMacks 18:03, 9 August 2007 (UTC)

Wow! That's impressive. I'd never have guessed you could get that to work. I'm going to try it over the weekend. Thanks for the correction! SteveBaker 17:12, 10 August 2007 (UTC)

centrifugal force and gravity (gravitons)

why are they the same? Are gravitons created by acceleration?

Gravity and centrifugal force are not at all "the same"—gravity works even for non-rotating things and centrifugal (or centripetal) force behaves the same regardless of gravity. They are both forces whose size depends on many things, and one can carefully choose "those things" such that the sizes of those forces are the same. DMacks 06:07, 9 August 2007 (UTC)

Your example of a child with a ball on a string above shows that they are not the same. In that case the centripetal force is provided by tension in the string, not by gravity. Capuchin 08:47, 9 August 2007 (UTC)

Centrifugal force doesn't exist - although it's often very convenient to pretend it does for the sake of doing calculations. Some academic physicists go absolutely ballistic (joke!) when their students talk about "centrifugal force" - because strictly speaking, there is no such thing. Others are more pragmatic and point out that using the concept of a centrifugal force makes calculations of circular motion a lot easier to get your head aroud - so from a practical standpoint, we'll very carefully pretend that the force exists.

It comes about like this: An object that's moving in a circular arc must be being pulled into that motion by the application of some external force - because if no other force is being applied then Newton's first law says it'll move in a straight line. Whatever it is that is forcing the object to move in a circle is what is imposing that force. So for a satellite in orbit, gravity is the force that is bending the natural straight-line motion of the satellite into a curve. For a ball swinging around on the end of a string - the tension in the string is providing that force. In both cases, it's convenient to say that the "centrifugal force" is balancing the gravity/string-tension and so the system is in equilibrium. This makes it easy to figure out the size of the orbit, from the speed of the orbital motion and the size of the planet (or whatever). But the truth is that the object that's moving in a circle isn't in equilibrium - that's why it's not travelling in a straight line! There is no force counteracting gravity/string-tension and that's why the object is moving the way it is - it's continually being accellerated towards the center.

That's why we talk about astronauts on the space station as being in "free fall" - it's not really a "zero g" environment - it's that the astronauts are falling towards the center of the earth at exactly the right rate to stop them from shooting off into space in a dead straight line. SteveBaker 13:22, 9 August 2007 (UTC)

I think perhaps the questioner is trying to get at something like this: if gravity has a force particle (graviton), how can that be consistent with general relativity, where Einstein's theory clearly states that a gravitational field is equivalent to acceleration towards the center of a massive body (and Einstein himself chided those who wanted to distriguish between "real" gravity and acceleration within gravitational fields)? Are the idea of gravitons actually consistent with general relativity, whereby gravity is a warping of spacetime by mass? --24.147.86.187 14:35, 9 August 2007 (UTC)

(To which the answer is, I think, that no, gravitons are not very consistent with GR; in fact, the standard model is not consistent with GR, as it stands. The standard model, as our graviton page explains, does not treat gravity as anything special as far as forces go, whereas in GR gravity is a warpage in spacetime itself. But someone with more knowledge of this please correct me.) --24.147.86.187 14:37, 9 August 2007 (UTC)

"Centrifugal force doesn't exist".M My understanding was that it doesn't exist in inertial frames, but does in rotating ones, so I've always thought that it's perfectly ok to talk about centrifugal force in this sort of situation. Is that correct? 80.169.64.22 21:26, 9 August 2007 (UTC)

Short answer: Yes. For most practical purposes, anyway. Whether it's correct in a deeper sense is more a philosophy question than a physics one -- it brings up questions such as realism and Mach's principle. --Trovatore 22:22, 9 August 2007 (UTC)

80 brings up a good point. I dislike it when people say that centrifugal force doesnt exist. Sure, it doesn't exist in the sense that it's just an apparent force due to an accelerating reference frame, but sit in a centrifuge at a couple of gs and then tell me it doesnt. It certainly does exist for the person observing it. Capuchin 08:27, 10 August 2007 (UTC)

Well, I think you're agreeing with me, more or less. But in the context of the original question, maybe the thing to ask is, "can centrifugal force be described in terms of the exchange of virtual field quanta, such as gravitons?". I don't know the answer to that question. I don't know if anyone knows it. If quantum gravity is to be "Machian", then the answer ought to be yes. But whether there's any viable theory of quantum gravity that satisfies Mach's principle -- or for that matter, any viable theory of quantum gravity, period -- is a question way outside my personal expertise. --Trovatore 08:34, 10 August 2007 (UTC)

The experience of sitting in a centrifuge is that the chair is pushing against you - preventing you from travelling in a straight line - that force is simply the tension in the 'arm' to which the chair is attached. There is no doubt that the concept of centrifugal force is useful as a mathematical abstract (especially in rotating frames of reference) - I use it all the time to simplify calculations and casually as a shorthand for a much more long-winded explanation. But to translate that mathematical convenience into an actual force (in the same sense as electroweak/gravitation/strong) is a mistake - and is what leads to the OP. SteveBaker 11:59, 10 August 2007 (UTC)

Isn't one of the postulates of general relativity that a gravitational force is indistinguishable from a pseudoforce due to acceleration (ie centrifugal)? I'm guessing introducting the idea of force carrying particles for "real" gravity, but not for this, is one of the disagreements between GR and quantum theory. 147.197.230.174 14:59, 10 August 2007 (UTC)

You're talking about Mach's principle, not GR per se. It's not clear to me that quantum theory has to deny Mach's principle in this context. After all, the "force-carrying particles" are virtual quanta, awfully ephemeral things. Maybe there's a useful description that has virtual quanta existing in one (accelerated) frame of reference, and not in another (unaccelerated) one. As I say this is out of my competence; I'd like to hear from someone who actually specializes in quantum gravity. --Trovatore 18:11, 10 August 2007 (UTC)

bunsen burner

What does the black soot consists of?

The soot is produced by incomplete combustion of the natural gas; the most sooty flame is the "safety flame"; the bright yellow one produced by a closed airhole. The soot produced is known as "Carbon black", and consists almost entirely of amorphous carbon, mixed in with a few impurities from the gas. Laïka 12:23, 9 August 2007 (UTC)

Marble and Mobile Phones!!

Do marble walls prevent the radiation of base stations from cellular phones?.. mean, if we put a mobile phone in a room with marble walls.. will it receive any signals?.. and why doesn't it receive signals while being in an elevator? Ahmad510 12:51, 9 August 2007 (UTC)

Elevator cars, no matter what decorative interior panels are fitted to them, tend to be steel-walled. This Faraday cage effectively blocks radio signals. Marble shouldn't do this.

Atlant 12:56, 9 August 2007 (UTC)

(edit conflict) An elevator (being essentially a metal box) makes a fine Faraday cage - and a Faraday cage blocks radio waves. Marble (and other kinds of stone, concrete, brick, etc) will block radio signals to some degree - but not as efficiently as a steel cage. Whether a marble lined room would simply lose a bar or two of your reception - or shut it down entirely is one of those "it depends" questions: How good was the reception in the first place? Are you right next to a cell tower - or right on the edge of the cell? How thick is the marble? What is the structure of the building behind the marble? In short - I don't think you're going to get a good answer on that one. Note also, with cell phones it's not just a matter of whether it can receive signals from that big powerful transmitter out there - but also of whether the feeble signals from the teeny-tiny transmitter in the phone can make it back out to the tower. SteveBaker 13:05, 9 August 2007 (UTC)

I can't think of any reason why marble would be any more blocking than plasterboard/brick/concrete/wood of the same thickness (excluding density effects)..87.102.79.111 18:37, 9 August 2007 (UTC)

Marble might well be a better electrical conductor than any of those. Note that granite is a fairly good conductor, which is one reason it's not good to be on top of Half Dome during an electrical storm (other reasons are left as an exercise for the interested reader). Conductors reflect electromagnetic radiation. --Trovatore 22:48, 9 August 2007 (UTC)

I thought my answer was rhetorical but apparently not. Are you sure granite is a good conductor - I'd always thought of it as a total non conductor - the components - quartz, mica, and feldspar are all used as electrical insulators? see http://www.chinamarble.com/E-granite.htm87.102.35.197 or here for data http://www.egmrs.org/EJS/PDF/vo261/25.pdf 10:26, 10 August 2007 (UTC) Still be careful on those mountain tops - high places tend to get struck more often. (especially if wet?)87.102.35.197 11:01, 10 August 2007 (UTC)

I'm not sure, no. Just one of those things that stuck in my head, could be totally wrong. --Trovatore 19:45, 10 August 2007 (UTC)

I found marble resistivity as "Marble 5 x 10^7-10^9 Ohm-m" - which is better than glass - but nothing like a conductor of semiconductor87.102.35.197 11:07, 10 August 2007 (UTC) see http://www.kayelaby.npl.co.uk/general_physics/2_6/2_6_3.html

Thank you for your answers..commendable Ahmad510 10:16, 11 August 2007 (UTC)

Relativity's successor

Is there any serious modern theory, which criticizes Einstein's relativity and assumes the faster-than-light speed?

There are a large number of theories of everything that explain things that could be explained in General Relativity but not quantum mechanics or vice versa. Many of these involve tachyons, which move faster than light. There are also a few ways that General Relativity allows. I'd recommend reading Faster-than-light. — Daniel 16:46, 9 August 2007 (UTC)

serious modern theory. I would simply say NO. Interesting theories yes, disusssed theories yes, but to get a faster than light theory into a peer reviewed journal NO.--Stone 17:11, 9 August 2007 (UTC)

The trouble is that the predictions of Einstein's theory have played out extremely well in practical experiments. So any future theory that changes things has to do so in such a way that all of the predictions that have already been carefully tested still work out right. The limitations on FTL travel, for example, are well demonstrated and aren't likely to change even if Einstein made a little boo-boo. So maybe some new theory might come along to say that relativity breaks down at the quantum scale of things - or maybe it might somewhat modify the equations in close proximity to an intense gravitational field...but it's not going to really change the core message about not going faster than light under reasonably sane conditions because if it did that, it would be wrong! It's instructive to think about what we might have said about Newton's laws before Einstein came along. Newton predicted all sorts of things that (at the time) tested out perfectly - there were no known problems with it UNTIL we started measuring the speed of light from distant stars and found this peculiar effect where the speed of light didn't change no matter how fast you were moving relative to the source of the light. That provoked Einstein to come up with a new law that modified Newton's laws at extreme speeds - yet very carefully changed nothing at all at low speeds. At 'normal' speeds, Einstein and Newton agree to far more precision than we'll ever be able to measure. It's only in this very special case that Einstein disagrees with Newton. So without an experiment that shows up a problem with relativity, there is really no reason to assume we have a problem...and right now, there really aren't any experiments that do that. If there ever IS an experiment that disproves relativity - it's going to have to be done in some situation that's so bizarre that the change to relativity will be even less important to day-to-day events than Einsteins change was to Newton. SteveBaker 19:32, 9 August 2007 (UTC)

That's one possible scenario but I disagree that it's something we can currently know. Steve, in general you have way too much faith in the idea that nothing radically outside our currently understood conceptions of physics will ever be found. Yes, relativity works just fine to describe the kinds of interactions we now observe between the sorts of matter we know about (except maybe for private observations such as qualia which you as a materialist presumably find some way to dismiss). But that doesn't imply that we'll never observe some other sort of matter or generate other sorts of interactions.

I think this is the reason you came up with such a blatantly wrong answer in this discussion. --Trovatore 20:28, 9 August 2007 (UTC)

Well, in Steve's defense, we are presumably talking about accelerating the kind of matter we know about to FTL speeds. Now you are presuming that 1. there will be kinds of matter we don't know about, and 2. that somehow this matter will interact with the kinds of matter we do know about in ways that will somehow invalidate key aspects of SR and GR. I find the former quite likely, but the latter quite unlikely — the stuff of science fiction more than a "serious modern theory" as the poster asked for. At the moment, it takes a lot less blind faith to say that it is unlikely that FTL will be possible than it does to suggest that it will be, as far as I can see. But I am not a practicing scientist of any sort. --24.147.86.187 23:03, 9 August 2007 (UTC)

I'm not saying that it will be -- just that Steve's arguments that it won't are not so strong as he thinks they are.

If (1) FTL travel is possible and (2) does not have a preferred reference frame, then it is easy to see that you can get causal loops. Causal loops are a serious problem for most people (grandfather paradox and all that). In the earlier discussion referenced above, Steve claimed that you couldn't even get to that point because we already knew there couldn't be FTL travel because the Lorentz formulas would involve taking the square root of a negative number. That argument was just wrong -- perhaps the Lorentz formulas simply don't apply to objects travelling faster than light.

But still, the problem with the causal loops is a serious obstacle, and is a reasonable ground for people to be skeptical about FTL travel (or more generally FTL transmission of information). It's not by itself a refutation, though. Maybe there really is a preferred reference frame, and the laws of (normal) physics conspire to keep us from figuring out which one it is (I think this is called the neo-Lorentzian interpretation of relativity). Or maybe one of the imaginative ways that sci-fi writers incorporate causal loops into their plots will actually turn out to be correct. --Trovatore 23:22, 9 August 2007 (UTC)

You didn't read what I said. I said that IF some new experiment comes along that shows that there is a problem with relativity THEN there has to be a new law BUT the new law can't be very different over the range of current experimentation. I'm not saying that won't happen - it obviously could. However, whatever experiment does blow a hole in it cannot be one that operates within the general boundaries of mass/energy/speed/particle-types that our present experiments have explored. It has to be something outside of our present 'envelope' of experiments. If that happened then the new law would have to produce the almost the exact same answers as our present law over the range of our present experiments. So this hypothetical new law has to describe behaviors under some really strange situations. This is (as I said before) no different from what happened when Einstein overturned Newton...Relativity only changes Newton's laws in situations far beyond the range of experiments that could be done to test Newton's laws over the preceding centuries - and produces almost identical results within that range. Any replacement for relativity has to be a LOT like it. So indeed - it might be that dark matter can go faster than light because (somehow) relativity doesn't apply to it - but there is absolutely no evidence of that - and even if there were, it wouldn't help us to get normal matter going faster than light because we already have pretty good measurements showing that accellerating normal particle towards lightspeed follows what Einstein predicted exactly. Right now, there are NO experiments that even hint that relativity might be wrong. So why assume that it's going to be? It's pointless going around saying "FTL travel can't be proven to be impossible because one of our fundamental laws could be wrong". That's an unfalsifiable proposition - and we don't explore those because they are useless to us. SteveBaker 00:06, 10 August 2007 (UTC)

Well, dark matter going faster than light because relativity doesn't apply to it doesn't sound "a lot like relativity" to me, Steve. But I suppose that's a question of interpretation. However your remarks the last time we discussed causal loops weren't a matter of interpretation; you were just blatantly wrong. Have you seen the light on that discussion? --Trovatore 05:10, 10 August 2007 (UTC)

I still don't believe I was wrong. Your explanations were completely incomprehensible - so I just gave up on you. Now you are being agumentative for the sake of it - so I'm giving up on you again. SteveBaker 11:46, 10 August 2007 (UTC)

My explanations could be followed by a bright high schooler if he hadn't already made up his mind. But I do admit to being a bit argumentative here. You pissed me off with an edit summary that said "Bogus, bogus, bogus" -- that really wasn't appropriate, and if you'd bother to work through the argument and notice that I am, in fact, right (or if you'd just look up the "tachyonic antitelephone" paper by Tolman, who made the argument many decades before I did) you'd see just how inappropriate it was. Still, it's true that I've displayed a less than beautiful aspect of my own character in the exchange. --Trovatore 17:57, 10 August 2007 (UTC)

Just an anecdote here. One of my friend's relatives is working on revealing problems with Einstein's theory of relativity. However, he said in order to demonstrate this, he would need something approximately the mass of The Statue of Liberty, but about the size of a basketball. I inquired on how he reached that necessity, and he explained in a very detailed manner. Sadly, I did not follow. He has been nominated for a Nobel Prize since. Mrdeath5493 07:03, 10 August 2007 (UTC)

One of my relative's friends is working on revealing problems in plate tectonics. They said that all they needed was a continent that was wider than the Earth. I didn't understand why, so asked them to write an explanation for me to study. Unfortunately, this was later burnt in an experiment, so I can't explain it to anyone else. However, they have now been appointed head of Nasa. (Sorry for the sarcasm, I just find the use of this kind of anecdote worse than useless. Probably a failing in myself.) Skittle 22:15, 10 August 2007 (UTC)

Some people like to broadcast and if you don't believe them they get upset! Look what they did to triplebatterylife? A simple question they couldn't answer but drove them berserk to the point of erasing his text ! Gladlast 17:05, 10 August 2007 (UTC)

Firstly, this is not the place to discuss it. Secondly, the correct place is on the Talk: page - where in fact it's already been/being discussed. SteveBaker 19:50, 10 August 2007 (UTC)

Instruments of Measurements

Name one instrument or device that may be used to measure each of the following quantities. Give the units of measurement that are commonly used.

1. Tyre pressure 2. Force 3. Wind speed Briandlazar 15:38, 9 August 2007 (UTC)

Sounds like a homework question. Have you looked up pressure, force, wind speed yet? If you cant find the answers there or by following the links, come back here.--SpectrumAnalyser 15:43, 9 August 2007 (UTC)

Ok heres one Tire-pressure_gauge--SpectrumAnalyser 15:46, 9 August 2007 (UTC)

You can also use a search engine to find your answers to these (and many other) questions. Putting the required terms in quotes returns more directly related results. For example, "wind speed measurement". Flyguy649 ^talk _contribs 15:53, 9 August 2007 (UTC)

This probably puts me in the realm of being an idiot but for some reason I read this and was thinking of musical instruments that would do the measuring. Obviously that would be a lot tougher! I mean you could maybe use say an accordion in some way to measure force through the bellows, I guess you could perhaps use a whistle to measure wind-speed (loudness of the whistle?) and then for tyre-presure, well, I wouldn't have a clueny156uk 22:46, 9 August 2007 (UTC)

• Assuming the tiretyre is inflated at least enough to be semi-rigid, the resiliancy ("squishiness") would correlate with its inflation pressure. Measure the deformation when subjected to a known mass, for example, one bass drum.
• Use a rubber hose to attach the blow-holes two piccolos. Obtain two piccoloists, each with one (1) piccolo. Have each finger the lowest C, but position their instruments vertically, head-joint down. Using a rubber hose, attach the two blow-holes together. Fill with mercury. You now have a manometer! Use a hose to connect air inlet to the end of one pic's foot-joint and collect the mercury as it spills out. Weigh the mercury spill, and from the piccolo's inner diameter you can figure out the height of the column of mercury supported by the tyre pressure. As a bonus, this will render the two pic's unplayable, and thus your orchestra has two less things that are never ever in tune and that nobody can hear anyway.
• I'm still trying to figure out a solution that involves setting violins on fire, or any of the other standard instrument jokes. DMacks 04:44, 10 August 2007 (UTC)

You could measure force by having a range of violins of different strength that will be crushed or withstand the different forces. Or for wind speed, different instruments will be able to withstand different speeds before blowing away. The triangle and flute could withstand high windspeed, but the guitar will blow away easily. And tire pressure could be measured by driving over the flute and seeing if it is crushed or not. Graeme Bartlett 11:47, 10 August 2007 (UTC)

Conv. of units for cholesterol

How do you convert mmol/L to mg/dl for cholesterol? Jack Daw 17:12, 9 August 2007 (UTC)

From Cholesterol: 100 mg/dL is the same as 2.6 mmol/L. Flyguy649 ^talk _contribs 17:23, 9 August 2007 (UTC)

ice cream

Why is eating too much ice cream unhealthy and how much is too much?86.141.240.149 21:47, 9 August 2007 (UTC)

Ice cream is unhealthy because it is high in saturated fat. Eating "too much" ice cream occurs when you push your intake of saturated fat above your recommended level, which depends on your frame, weight, and height. I believe 1 serving of ice cream is one half cup, and eating more than 1/2 cup at a time could also be considered "too much". HYENASTE 22:04, 9 August 2007 (UTC)

Ice creams also tend to contain a lot of sugar. Also Hyenaste forgot to mention it also depends on the specific ice cream since the fat content tends to vary (although there will often be a minimum for it to be called ice cream as set by your countries government) Nil Einne 23:10, 9 August 2007 (UTC)

This is true. Mayfield makes a "snow cream" that contains only 1.5g of saturated fat per serving, whereas other ice creams can contain 5 or more grams of saturated fat per serving. HYENASTE 00:30, 10 August 2007 (UTC)

It makes your head hurt from the cold (Assuming rapidity of consumption)! 68.39.174.238 02:16, 10 August 2007 (UTC)

It might please you to know that we have an article on that: Brain freeze --Mdwyer 03:50, 10 August 2007 (UTC)

"Nurse, this is the worst case of brain freeze I've ever seen, get me 20 ml of hot fudge, stat !" - Julius Hibbert -- StuRat 05:53, 12 August 2007 (UTC)