# Ndi

Senior Members

188

## Posts posted by Ndi

### How important is calculus for Comptuer Science?

Math in CS/Programming is just like having the right tools for the job.

You can do it both ways, you can fix it with duct tape, or you can use a titanium hinge that will outlast the planet. Both are solutions, except one is better and doesn't require constant maintenance.

A good understanding of the math behind a problem will allow you to understand, solve and optimize your solution, and only code in the minimal, slim, elegant solution. Not having a good grasp on Math will keep you from putting down elegant, simple, fast solutions and making the computer do more work, with complex code.

This applies to all secondary specializations. Having a good grasp of economics means your accounting software will have a nice, simple interface, all the needed tools and will be slim and fast. Having no idea, you rely on instructions from someone who doesn't know how computers work. Generally, the result will be messy and complex; and complex is buggy. KISS isn't just a band.

So, to answer your question, yes, it matters that you can understand and solve the problem BEFORE you write the code. The computer is there to automate the solution, not to find it. You will only be as good as the worst of your grades in a field. You can be 10/10 programmer, but if you only rate as 3/10 in engineering, your engineering software won't ever grade over 3 or 4, even with help. An excellent engineer with mediocre CS skills will write an ugly program everyone will use.

So, if you're not good with math, pick something else and go with that. There are plenty issues that need solving, not all require a deep understanding of calculus. You WILL need the basics.

Here's a basic example:

There are 20 people in the room. They all need to say good bye to each other, and shake hands. How many handshakes?

No math:

cnt := 0;

for i := 1 to X do

for j := 1 to X do

if j>i then inc(cnt); // 1 shakes hands with all, then 2 with 3-20, then 3 with 4-20.

Writeln(cnt);

Math:

// Combination problem; C=X(X-1)/2

Writeln(X*(X-1)/2);

For 20 people, nobody can tell the difference.

Now, if there are one million people in the room, you get to do 10^14 loops, I do 1. You need a 64 bit machine, with a 64 bit compiler, for storing the counter, whereas I don't. And finally, the first code does (at minimum) 2 INC and 2 CMP/JZ per cycle, plus the counter, that's 5 cycles, and that's 5*10^14. At 3 GHz, that takes 166 seconds (way more in real life). Mine is instant.

Better yet, knowing that 2 consecutive numbers have at least one even number, the result of the multiplication is always integer, even, so a SHR (>>) is in order. If your code is designed to run on an (e.g.) ATMEL, that tidbit is the difference between the small and the large CPU, because division will ramp up your code from 400 bytes to 3.7K. That means forever barred from using the small, efficient CPU that sleeps at a microAmp. Mine will run 2 years from a battery, yours will do 2 weeks. Even on a Core i7, optimized to the teeth, division is ~115 cycles, INT division is 26 cycles, SHR is 1. Point is, it does me no good to know SHR is faster unless I know the math behind that tells me that X*(X-1) is always even.

Handling a problem poorly will forever bar you from high end algorithms, like image matching, sound analysis, media, hardware development, any application where every bit counts (like devices/automation) and, not by chance, the best paid positions.

Can you be a surgeon without a scalpel? Yes. Will you be doing any brain surgery? Not likely.

I've celebrated my 20th year as a programmer, and know many successful people in the IT industry. Of those, only one is a pure, dedicated degree, CS graduate. Everyone else has a different specialization, ranging from accountant, to aeronautical engineer, to power distribution. Most if not all can solve differential equations unaided. Most have a good grasp of physics, electronics, logic. Also as a side note, not all of them had these while in school, so don't sweat it.

You wanted advice from a good programmer with mediocre math (hi!). Here it is: get familiar with at least one other industry branch, preferably a lucrative one. It's not all math. You can have a great career with mediocre math. You cannot have one with mediocre everything.

### Using electromagnitism to supply lift for a heavier-than-air airship

1. Is this scientifically plausible?

2. If so, could it be plausible to vary the altitude of the ship by inducing varying amounts of electric charge into the material?

3. Could this effect be somehow manipulated to provide propulsion and maneuvering for such a vessel?

1. On another planet in sci-fi, yes, BUT: You commit to new laws of physics, all materials affected by the field would behave very differently. And that's a lot of materials. Water included. Water is in organisms. Very complicated. A field that powerful will likely make people hover (more likely, slam into the ground).

2. Yes, BUT: Magnetic forces vary a LOT with distance. For every meter in length you'll probably have to add another zero to the power and, frankly, by 5 meters you'll probably be better off just putting an engine on it.

3. Not likely. Assuming a model where these rocks are part of the outer crust (otherwise it'd just be silly zeroes everywhere), then the crust would need to be smooth and aligned for this to work (otherwise you'd wobble around like a car on a bad road). If it's smooth then you will be aligned to it or unstable (depending on field alignment) unless aided by aerodynamics.

You could lose altitude with wings attached, and thus move forward, then regain it. The power requirements, however, would be enormous, as lost altitude does not put any more gas in your tank - you'd have to bench-press the ship.

So, no, basically by the time you put all this together you kill so much of physics you'll have trouble explaining how plants don't jump from the ground.

If you want to work with magnetism, go small distances, it will make it workable.

If you want giant ships, I suggest you go with magic density atmosphere. A reasonable engine that takes advantage of this soup will be able to hover relatively easily, and copy the aerodynamics from blimps and subs.

If you just want cool-sounding sci-fi stuff, make the atmosphere magnetic-responsive. You could use a powerful field inside a static engine to compress the atmosphere to a tiny fraction of its size, lure it into another, lower part of the engine, and let it rip. It would look like a static fan, and would make a killer cover - the engines would look like huge wedding rings, with plasma inside from the heat generated by the compression. Add some lightning for the power surging from the superconducting coils and you're set.

### Frontal car collision

Well, matter has since been tested by the Mythbusters (Season 8 ep 07 - Mythssion Control, aired May 5). Small scale test and full crash, nicely done. Turns out physics still works, the crash was equivalent to single speed, not double.

Sorry for popping the question and disappearing, the forum was supposed to notify by mail? Huh. I must've messed it up.

### Frontal car collision

Conservation of energy does not apply to changing reference frames.

Why would I change reference frames? Ground seems like a good idea. Also, how would that change anything? If my reference would be a satellite in orbit, one car would go 10100 and the other 9900. Resulting acceleration during impact would still be the same?

Also, how does crumple zones change anything (in the context of the original question)? Both cars crumple the same, so the net effect on the passengers regarding peak acceleration is the same in both cases?

@swansont: Me or them?

### [forum glitch] Stuck PM from the seventies

Yeah those are mine. Just stuff them with whatever it is you have in your glass and send them over.

### [forum glitch] Stuck PM from the seventies

*public claps*

Works now, thank you very much.

### Frontal car collision

If it's a conservation of energy, then it should be difference squared.

I assume energy is divided directly by masses, so the difference is speed should be relatively simple?

### [forum glitch] Stuck PM from the seventies

Still, Private Messages: Unread 0, Total 1. I'm using 1 one message.

Notification's gone, however, it's much better now

### Frontal car collision

Hi.

I have this nagging issue with people's physics in all kinds of movies and shows and even on the street in which they claim that if you have a frontal collision between two cars, the speeds add up.

Basically, there's car A, going 100m/s that way --> and car B, going 100m/s that way <--. Frontal ideal collision. I hear people say that it's worse than hitting a wall (ideal, concrete, immovable wall), because the speeds add up, essentially people actually smash at 200m/s.

I think they're nuts.

My problem with this is: Car a is in a crash. Essentially, all its energy is converted into molten crash juice (deformation, mostly), equal to speed difference (100 to 0, 100), divided by stopping time (let's assume 10 ms), basically this means that one gets a negative deceleration of 100m/s in 0.01s, which is -what- some 1000G? Math is unimportant.

However, the same thing happens to car B, assuming identical cars. It also goes from 100 to 0 in the same space and time, basically, there is no difference between an impact of 100m/s to a wall or to another car coming at you at the same speed the wrong way, the acceleration is the same (assuming that's what kills you).

Now, if car A throws a rubber ball out the window, the ball goes from 100m/s to -100m/s, effectively adding the speeds and thus the impact (divided by mass ratio).

Am I wrong? Is some energy transferred to the wall I'm not adding up?

### [forum glitch] Stuck PM from the seventies

Fixed now? I think this is a leftover from deleting some spam PMs.

Sorta

Inbox contains 0 messages.

You have 1 messages stored, of a total of ...

I now have Inbox (0), Sentbox(0), but usage is 1. Can't access it though. Oh, and, I also have one notification, which is kind of bugging TBH.

### [forum glitch] Stuck PM from the seventies

Hi.

I have a PM stuck in my Inbox (don't remember if I had one and got corrupted or it's new). It has no subject, no sender, no nothing. Date is 01-01-1970 (Unix 0 IIRC) With no subject it's unclickable and attempting to get the message via txt or csv gives me an error page.

I haven't tried deleting it in case anyone wants to investigate. Error page contents follow:

-------------------

Warning: Invalid argument supplied for foreach() in [path]/private.php on line 849

Warning: Cannot modify header information - headers already sent by (output started at [path]/includes/class_core.php:3254) in [path]/includes/functions_file.php on line 147

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Date,Folder,Title,From,To,Message "1970-01-01 03:00","Inbox","","","Ndi",""

### Help get my head around basic air pressure /volume transference

Hum.

Well, since nobody tackled this, I'll give it a try.

I was always taught to take the easy path, so every time I see a pressure issue i simplify (to volume) - it can be added, unlike pressure.

Oh, and, also, I'll ignore imperial units since they make math too odd. I didn't even know physics was taught in Imperial. Your equations must be mighty odd. Anyway.

a) If one tank, A, has 100 m3 and one bar, that's 100 m3 of air at normal pressure (1). You can multiply by density to get actual air in grams but we'll skip that. Now, should we cram twice the air in there, we get twice the pressure. So at 2 bar, we have 200 m3 of air in an 100 m3 enclosure.

When you open the valve, you make the two tanks a single tank. Thus, sum of volumes over sum of contents. A is 100 m3 at 2 bar (200), the second is 10 m3 at 20 bar (200). So now you have 400 m3 of air on 110 m3, for a nice 3.63 bar. You see it's a lot closer to the large tank, which makes sense.

b) Again, volume. To push a 100 m3 from 1 to 2 bar, you need 100 m3 of air at standard pressure. That means that the first tube will lose that, meaning you now have 100 m3 at 10 m3 tank (pressure = 10), the new one will have 200 on 100 (pressure 2).

c) well, you get the point. Get actual quantity, do the math, re-apply into original.

Note, however, that none of this will work in real life as it should. Even in the classical interpretation of _ideal_ volumes, PV=k, and your tanks have different "k". Also, upon compression collisions go up and heat starts to really mess things up growing a tanks of air into a branch of physics. The above is an approximation of 1880's approximation of gases. Actual graphs are in 3d and quite curved, and that's ideals only.

But since you said you are now discovering Boyle's, I assume you just want a feel at the basics. Don't quote me on anything.

### Retrocausality

Some of the interpretations of QM may well not be falsifiable, but some of the data cannot be explained by current science.

My analogy is valid. In the year 1012, some experiments were outside of what current physics could explain. So a possible interpretation would be "God did it". While fine at the time, we have now learned that "God did it" is unfalsifiable, meaning you can throw anything to God and nobody can tell you it's false. (since then, people redefine God's intervention to fit this).

You seem to give interpretations and models some power. They have NONE. NONE. I see a tree and my interpretation is it was beamed by Scotty. It's mine, I believe in it and I may or may not be a crackpot (*). If I find more people to believe this load then what I have is religion not truth.

*) This one of the straw men you throw along with the other fallacies. What you do here is make an unjustified parallel between being wrong and being a crackpot. I can be sane AND wrong. E.g. I ask if you took my lighter and you reply "I'm not a thief! You think I'm a thief?". You might have taken it by mistake, that means that you are not a thief AND you have my lighter.

Also, I screamed it once and I'll scream it again. Appeal to authority. Just because X believes something _has_zero_bearing_on_reality_ or truth. The summed up opinion of all the humanity equals zero. Unless there's proof, a theory, falsifiable text, it means nothing. If Michael Jordan thinks I can win against O'Neill in 1-on-1 does NOT make it so.

You forget that 90% of the 7 billion believe they were put here by deity? (or did at some point). Did that make a difference? If Einstein said he believed in the flying spaghetti monster, would that make it real? And don't fall in the trap of "more" true. Making it believable doesn't make it more true.

Some interpretations of QM do provide a possible solution.

See, there's the problem. You confer this interpretation thing power. It has none. Someone sees some light in the window. Their interpretation is "OMG it's a ghost". The complete sum of all interpretations precisely null.

However, if the proposals are not falsifiable directly, there may be experiments that could be devised that might imply them to be true.

You seem to round off corners of concepts. If a sentence is unfalsifiable then it's unfalsifiable. If someone comes up with something that determines/falsifies it then we were wrong in calling it unfalsifiable.

_Unbreakable_ means unbreakable. Actually unbreakable. Don't confuse it with "hard to break" "I can't break it" or "nobody has ever broken one". The former are theoretical concepts, they have sharp corners.

Implying something is true means nothing. Tomorrow someone proves if untrue and all your implications are zero. Tomorrow someone proves if true and all your implications are still zero, because it's trube sinceproven, not implied.

I think we may well be talking about people who are pushing the borders of science. Who do you mean?

I mean that the person in a white coat that did the experiment never said that the world blinks when you close your eyes or that the particle knows it's being watched. If he/she did, s/he should have the license removed.

I think that someone twisted "it works as if the particle knew" oversimplification for the masses and turned into "there is a bond between our consciousnesses".

That's an inaccurate analogy. Data is being produced that cannot be explained by current 'mainstream' physics, but could be explained using some interpretations of QM.

Sigh.

Interpretations explain nothing. Interpretations are imaginary tales based on the facts. We have them because it helps us conceive the future experiments.

I see a tree. My interpretation is it's Gods' hands who planted it. Thus I make an interpretation model in which a huge hand plants it. This is falsifiable. How? I wait and see. Later, a new tree grows with zero huge hands. Into the bin it goes. Total value if the interpretation? Zero.

The only thing it helped me with is hint at an experiment that will ruin it. I do it. I ruin it. The more I try and don't ruin it, the closer it's assumed to be to the truth.

You see, if it's unfalsifiable I can't do an experiment. It's an axe with no blade.

### Retrocausality

I would venture that it is you that is ignoring the evidence. You can explain away the evidence if you want, and choose to BELIEVE that its all an error, a hoax, or whatever. You can BELIEVE what you like.

Yes, well, the key difference here is that my ... belief as you put it is a falsifiable theory whereas yours is not. My theory that the equipment/application of experiment was faulty is verifiable, and at some point in the future it WILL be proven right or wrong. This is the difference between tackling a problem and explaining it away. Our theories are nor equivalent.

Do you really think the issues you mention have not been considered by the experimenters? They are not cranks you know!

The experimenters? Probably not, I doubt people pushing the borders of science are crackpots, but we are not talking about them, now are we? We discuss several interpretations of the observed experiments.

If Newton decided that gravity is mass multiplied by gravitational acceleration, conducts an experiment and makes a (reasonable for classical thinking) theory, then good for him, up in the hall of fame he goes.

We should NEVER confuse his experiment with an outside person (the crackpot) who sees gravity as not the apple falling down attracted by earth, but pushed by the tree. Or that claims that if Newton weren't there, the apple would have never fallen, or that apples don't fall unless there's a human there to see it.

I scream appeal to authority.

--

On a lighter note, maybe I missed it but I don't remember anyone arguing towards the following:

In the first experiment, without the observer, the particles form a wave pattern. Is this experiment not observed, even if indirectly? Are the physicists now aware of the particles' behavior and is their consciousness not there in the mix?

Sure they don't check EACH particle, but hey, human glory is human glory, I mean, the tree might not fall in the woods if nobody sees it, but we SEE IT in the first experiment - just not directly, but by the effect. When the tree falls, it makes a sound because we hear the air waves it produces. We see it because the light it reflects. These are indirect observations, we don't put a bell under the tree to directly measure the tree.

A particle behaving like a wave is a particle observed (that's why we know it's there), it's not the observance that changed, it's the measuring mode. I blame the measuring mode.

### Retrocausality

In general terms you are correct, but experiments have been undertaken with pre-recorded data where 'observers' have been asked to try to influence the data e.g make data recorded by a random number generator produce numbers above 50.

The effect seems to work with real-time random number generators but also works with pre-recorded data (the results cannot be accounted for by chance)!

I find this astonishing, unbelievable, and though I'm willing to remain skeptical, am also willing to believe it could be possible.

And this is where you make your mistake - you ignore evidence and choose to BELIEVE. Look, I wasn't kidding, someone call Occam.

If you make a random number generator that yields over-50, then it's busted. Then you do it again, with pre-recorded data that HAS been observed and fixed (some guys in white coats somewhere KNOW what the numbers will be). You again get over-50.

My first guess is you blew it. Bug in siftware, bug in generator, you name it.

You might not be an experienced person with random numbers but as a 15yr+ programming experience, I'll let you in on a few secrets:

a) random number are never random. They are extracted from the decimals of a function with a very long period. Better systems have a recorded set and work with that

b) random numbers are repeated if you take the same seed. In PCs and similar implementations, Randomize() skips in the decimal string at current time(). If you set the clock at 1:00:00, 19-06-2009 and call randomize() then Random(3), you get 1. Reset time and call again, it again 1. Not only that, but the sequence restarts. I can write you a program that calls Random() 50.000 times, draws a graph and tomorrow morning when you re-run, it will reproduce exactly. Once little bug in code and you have a shot experiment.

c) Random numbers all sum up to zero (or whatever they collapse to). There is no obligation for them to be UNIFORM, though. Perfect random numbers are: 1,1,1,1 ...., 1, -300000. This sequence is random (not determined by a function), averages 0 and it's 300.001 numbers long. Perfect (statistical) string but only works if you take 300.001 samples. Any other subsample and you get skewed results.

c.2) As a result of c), if you draw the graph of random sum function RS, with Rs begin RS(x) := RS(x) + Random(1,-1) (or Signum(-100 + Random(200)) if you will), then the function will hover around zero at some point or other. While it WILL go through zero several times and the sum of the values will tend to be zero, any subsection of the function will look full-positive or full-negative because in that section numbers will tend to be negative, but the sum is positive.

What I'm trying to say if I didn't put you all to sleep is that (ESPECIALLY WITH HUMAN-PC EXPERIMENTS) these things mean squat unless you do many, many, many permutations. PC Random() will settle statistically after 2^31 extractions(signed 32 bit because Randomize() takes such a number. Anything less and you get positive-exceeded or negative-exceeded results.

The problem with humans is that that can only do a few. A thousand is a good guess before they go nuts, and focusing hard wears them out in an hour or so, with a number every 3 seconds that's roughly one thousand.

2^31 is 2 billion. Divided by 1000, it's (we'll say 1024), that 2^21, or 2 million (roughly). Brought into real time, this is the equivalent of someone getting an idea of what you do in a day by observing you for 41.1 milliseconds. This is roughly the time it takes a flash from a photo to light, expose and die off. From a statistical point of view, you are hanging in mid air, frozen.

You think people who play lottery, slots, etc don't WANT to win? There's a reason they never made the news, don't win lottery, don't play blackjack, etc. There are games that lean 51-49 for the house. They'd be rich, now wouldn't they?

### Retrocausality

(cartoon)

Agha. Someone call Occam.

OK, now we have a debate. We have the particle, the results, we all agree on the patterns. So we have:

a) The phenomenon is unknown to us. Cooool, unknown phenomena. We eat tonight! Let's see, bad equipment, magnetic fields, unsuitable equipment, unknown phenomena, someone sneezed, etc. Logic dictates that if it does THIS left alone and THAT with a sensor in, then the sensor is changing the experiment.

b) The ELECTRON has DECIDED to stop in mid-air and WENT through ONE SLIT (emphasis theirs). Favorite explanations feature (depending on century) the sun god, gods, god and, in this enlightened age, the ability of a subatomic particle to choose a path. Alternately, the particle is a tool of higher beings, capable of such feats contrary to all evidence.

Choose one.

--

Seriously though, if you stick a wrench in a sprocket and it stops, it's probably the wrench that stopped it. Saying that it's as if the sprocket KNEW we were putting a wrench in and stopped is not OK.

So what, adding a sensor changed the experiment. All sensors ever added to anything change the experiment. We have known that for millennia. Sometimes it's so small we don't care. Sometimes it's so big that measuring destroys the sample/properties (uncertainty).

Even looking at a blue object changes its color. To see color you need light. You have light, you have energy and the paint slowly ages. In 5 years, it will be light blue. You look at it, the paint changes color. It's not like the paint knows it's being watched. (I am aware that it's the lighting not the watching that does that. Work with me here).

Why would anyone assume the particle has a steering wheel and brakes?

--

evil3penguin: Throughout the ages, *some* people wondered why those things were there. Other wondered why God would put it there. It's no different today, it will be no different for a long time, regardless of that god's name. Human Consciousness is also a type of god, since it can do stuff nobody else can. Will, strength, abstract thinking, awareness. Same deal.

### Retrocausality

Em, sorry to hop in, but bombus, did you skip over my post about data storage?

Unlike unknown physics at *some* particle level we don't really understand, data storage is something we invented, we know how it works. Implying that we observe when the consciousness observes means that the storage medium is in a state of flux until we look at the screen and that the experiment never happened at that time, but at the time we observed it.

First, media doesn't do that. Remember, WE know how that works. Trust us.

This reminds me of those TV people that believe in ghosts (I don't mean this as an offense). At some point or other, you hear the same magical words: "I ... I don't understand it. No ... I've never seen ANYTHING like it. It can't be explained". And I always wonder - what on Earth pushes these people to jump from "I don't understand" to "it's unexplainable"? Or from "I've never seen anything like it" to "it's not from this world".

Just because you can't taste the world being round doesn't mean it isn't. "Prove it to me" is not equal to "I have to touch it" - if it is for you then quantum mechanics is a bad career choice

Implying that fixed storage is fluxed means that everything is fluxed and changes as we observe it - not a particle, but the whole shamoogle. Everything.

Secondly, you make an unfalsifiable claim - thus arguing outside of science. You asked where the line is drawn between science and speculation/philosophy. The answer is, in your first sentence. When you first asked if it really happens if you don't see it.

The answer is yes, because all evidence points to yes, no evidence points to no, and the proposition that the world really blinks into nonexistence every time you close you eyes is unfalsifiable. As a result, anything linked to that concept is speculation. Looking for a real explanation on an abstract and false proposition is futile and will never yields a result, no matter who you argue with and for how long. It's like the color of Santa. I claim he's green. You can guess where THAT debate ends.

I guess that whole angels on the head of a pin thousand-year debate missed you.

And lastly, as swansont points out, how is this a debate? A debate is between interpretations of the same fact - you argue fact versus interpretation of (nonexistent) different facts. The key difference here is that both parties in a debate agree on the facts but disagree on the implications.

To even get into a debate, we need uniform facts. And this is where the non-falsifiability really wrecks stuff.

### Thermoelectric Modules

In that page I see

(Model Number)

I/V - Maximum voltage and the maximum intensity (amps) when fully loaded (W=I*V here)

ΔT- Maximum temperature difference _in ideal conditions_

Qc Max - Maximum heat transfer at peak efficiency (when cold=hot)

The way I read it, they all get 72C cold-to-hot (IDEAL, please take note. You never get this because the hot side is rarely efficiently cooled and the cold side WILL go up the second you try to move heat, PLUS the hot side radiates in real life, seal is not perfect and the whole shamoogie is metal).

The more power it eats, the more heat it can transfer. You need to give it less heat that the rated transfer _at the rated efficiency_, meaning that a 100W pump will move 100W when the cold equals hot (max efficiency), and to move 100 you need 150. As the delta increases, efficiency drops. At 72, it's zero (thus the maximum temp).

That's why most PC shops give it a rated moving power, meaning a 200W 30C TEC will keep 30C below ambient (22C does -8C) if you feed it less that 200W in heat (quad core at some 4.8 GHz?). Not very scientific, but mighty clear. It will also suck some 300W from the PSU, forging an upgrade.

Also, if you look for a temperature in degrees, you need to know the exact quantity of heat you give it. If you use the cold side to chill an airtight, sealed and insulated fridge, then given enough time and proper ventilation you get almost 72 degrees below hot side (NOT ambient unless you are in a hurricane).

In the heat outside, there will be 30 C, and the hot side will probably stay hot to the touch but not too much so if vented properly. If you can touch it without recoiling in pain it's under 60C (golden rule of an IT specialist ). That would give you -15 theoretical, but most likely below zero, or even a plus 3-4. Thus the fridge applications. (the closer it gets to -15 the worse it performs and each drop of heat that oozes through the insulation takes longer and longer to pump so it stabilizes).

If not insulated, well, then it moves the ambient heat, meaning the ventilated part will lose to the air as the cold will gain. If you don't fan the 50W-ish, you cook it. The more you fan, the more it will go.

Oh, and then there's stacking.

### The strngth of a memory wire coil

We don't really like temperature-controlled stuff unless we want it temperature-controlled because the environment on Earth has such bad temper. Cars are tuned and sometimes remade (parts) for hot/cold areas, and high-low altitude because the same car could work in as low as -30C to as high as +80C in the _temperate_ area, higher in the extremes (I know that's not how hot it gets in the summer but a black car in the sun goes mighty high).

We usually prefer controllable stuff. Most of our locks are electrical, magnetic or a combination thereof (including mechanical). Shop tags that prevent shop-lifting have a latch inside that is unlocked by a string magnetic field and twisting, heat-tags would have come undone in China or lock up in Norway. Quick-enter and lock systems are also electromagnetic because one can lock/unlock immediately and because they fail-open with disasters, fire and emergencies. That's why they are mandatory in most emergency exits that normally lock.

A thermal lock would be slow, lack power (a magnetic lock with a 10W rating can hold 300Kg, dual lock high and low on a door makes it effectively locked) and could fail left in the sun (or when cold). Electromagnets have no such flaws. Correctly sealed they have 100% reliability and virtual endless lifetime (it's the case that fails). Also, it can't be damaged-shut, like with mechanical and electro-mechanical latches.

Also, for transport, sealing, etc we have magnetic latches - power to lock- power to unlock. This has the huge advantage that it can have a RFID smart chip in it or other auth so it only works with a known and dedicated opener. Sensitive shippings already have smart temperature sensors embedded in the product that actively log and report temperature every 5 minutes so the receiver can know if cooling failed anywhere on the way. It's used for meat, medical supplies, bio-sludge, organs, etc.

The thermal stuff in the article has its advantages when you need slow-moving but no-motors, like implants, especially since Ti is so well tolerated (it's virtually immune to organic juices). I still wonder how they plan on heating it to 100 or so Celsius to make it open a rib cage. Or what happens when you take a really hot shower or heat up during exercise and ... Well, no matter.

For one-way memory (buy it stretched, shrinks-at-home) we have long polymer (shrink-wrap, shrink-tube) that are cheap and don't revert, but nobody cares because it's so cheap you just replace it. Plus, most stuff you shrink-wrap you don't really fiddle with. Also, they plan on doing this on clothes that auto-iron with heat, as they are sold ironed. The technology exists, it's revolutionary, cool, and failed. Long polymers are uncomfortable. Plus, that's why clothes do when ironed, the heat breaks the bonds that are make in the cold (washing machine) leaning only the strong, built-in bonds. Your clothes basically have memory, it's just not that impressive. That's why in-rack steam ironing works.

For both way we have bi-metal which is used extensively since 300 or so years (balance pendulum clocks length) to the newer stuff like thermo-regulated stuff. All that clicks when turning on and off is most likely bi-metal: home plug-in heaters, boilers, irons, most of the old turn-dial thermostats, trip protections in electronics. It's very nice because it never fails (mechanical), it's reusable and it has hysteresis of sorts (once tripped, it takes a little while to re-do instead of locking in a click-click cycle).

I'm not sure what replacement you speak of. If it's opening barrels, a plain spring-lock is nice and reliable, dirt cheap, the dock workers can work it with no additional tools, no power required, works in the dark and blackouts, *and* can be overpowered with a metal rod that lives 100 years in the rain. Why touch it?

### Thermoelectric Modules

What you have there is called a TEC.

It is based (sorta) on the Peltier effect, and it moves heat at a certain rate. They are quite inefficient, and have several drawbacks, such as weld point failing with temperature and failure if the hot side is not maintained cool.

These are used in portable cooling systems because they are compact and solid state, but are woefully inefficient versus the traditional heat pumps. A compressor heat pump can beat 100% "effective" efficiency (measured as heat brought in versus consumption) under favorable conditions, whereas TECs wallow in mediocrity.

Most of the juicy stuff on these you'll find on computer sites, not physics sites, because with computers you have little heat to move (100-200W), and plenty power (maybe a few hundred watts to spare) but the cold side is critical, as is MTBF. Solid state is way, way better. Just look for "TEC" cooling and you get specs, ratings, consumption, everything.

Just try to avoid cases where the hot side is not cool enough. Even the web sites smell of smoke.

P.S. Oh, you wondered about how cold/hot you get from a battery. Quick rules to help you:

a) Any decent TEC will eat 12, 18 or 24 volts. It works by potential difference. Batteries suck.

b) Rule of thumb is - To move some 100W of heat at a constant rate (how hot the sides get is another story), you need 50W. As a result, the hot side needs to lose 150W.

c) Under hold-in-hand conditions, a 50W TEC has a cold side of about -20C, hot side too hot to touch, needs fan. If you put your hand on the cold side, it drops to -7 or so. Temperature depends on the heat load, and when closing the rated heat transfer rate the power the TEC eats starts to add up, the cold side becomes hot and it self-destructs quite rapidly. If shorted, that's good for the batteries.

d) A D battery is 1.5V, maybe 7Ah. That's 11 Watts, with a 50 required of a small -puny-TEC. That would hive you 13 minutes. NOTE that a TEC needs 12V, so it will probably bust, you need a DC-to-DC SMPSU, that takes its toll in efficiency.

e) A high-perf Li-Po has 3.7V nominal, 4Ah, so you'll need 3 of them in-series to get voltage. That gives you 12V, 4A, a total of 48Wh, or 48 minutes of coldness. However, note Li-Po can't be safely charged in-series, so you need a multi-branch charger.

f) On failure, the TEC shorts sometimes. Li-Po cascades on heating, you need trip sensors.

g) Car batteries with fuses work. A SLA has 70-100 AH at 12V, almost a KWh. That could work (and does in portable car fridges and ice boxes)

I wouldn't. Whatever it is you want.

### The strngth of a memory wire coil

Memory stuff works by deforming objects before their (English limitation here) permanent destruction point. As a result, heating does nothing more than reverting to whatever shape it was in before.

Think of it as a frozen spring. You tug, you freeze. It remains like that. Let it warm up and it slowly reverts to its initial state. Sum of all available energy is same as the one put into it to deform it - no energy is gained or lost (ideally) - just stored.

As for coiling into a barrel to make it blow, we have better ways, hydraulic being one - especially with mechanical advantage.

Article also suggests sprinkler systems (we already have bi-metal ones for that) and bi-metals work almost as well if ones needs a simpler open-close action.

While interesting as a material because Titanium has cool properties like being implantable, we have several other "memory" materials that are widespread and pretty cool. Shrink wrap for one.

I wonder how they heat mounted braces to 90 Celsius.

### Light frequencies and the human eye

But as this seems to get into biology

It doesn't have to, the color is still purple when captured by a camera sensor, since it still has R, G and B filtered light ADCs. Advantage being that with a recorded image one can objectively measure resultant color, and latency, persistence, etc are fully known and understood. We have the specs.

Some LEDs are - Laser diodes (Laser LEDs) are monochromatic and coherent - thus the beautiful patter.... ooooohhhh...

### Quick Forum Questions

I couldn't find it in the UCP, can I change date format display? This whole m-d-y is constantly throwing me off.

### Retrocausality

[...] I've heard many people describe these quantum mechanics as though its not the machine but the person the conciousness that determines the outcome of interference

I don't really pretend to understand this stuff about particles and their various effects, but logic seems to dictate that interference is interference regardless of the nature of the observer.

If it isn't, then several scenarios lead to absurdity. For example, you record a pattern, but not look at the recording. Later, when you look at it, it is determined. This implies that the recording medium shifts in anticipation of your play button which is, basically, a showstopper. Unlike particles, I understand storage mediums.

Interference exists always because of the nature of the observed medium. It doesn't become a "pattern" until it is observed since the pattern is a "snapshot" of the phenomenon. This implies a fixed time and place (or a multiple of). Note I'm thinking of interference in terms of a light interference pattern here.

### Retrocausality

Well, I'm not very good at this, but since there are no takers, here's what other people think.

Retrocausality is any of several hypothetical phenomena or processes that reverse causality, allowing an effect to occur before its cause. (emphasis mine).

Apparently, nobody actually thinks effects happen before the cause since, you know, that sort of violates the whole concept of cause and effect. As I see, this is a theoretical model, that is to say a way of looking at something, not a way things actually happen.

As for your example I can't tell if it's bad, badly explained or I can't understand it right (It feels like you were trying to say something else). But in your example,

"If you shoot particles through a double slit at a screen, whether u view them or not changes whether it has an interference pattern or not".

I fail to see the time travel. Observing or not (likely) changes the outcome (uncertainty), but still, you have the shooting, the observing and the results that point to what you have done. It's quite linear.

Also, if any retro stuff happens it will be between emission and reception, storing data does little for time travel, as we all know. You could emit, wait, then in the future run after the beam an observe. But I feel like pointing out that the beam has no interference until observed, then it has when observed. There is little info there and no timetravel.

In addition, to store data you need to have data. To have data you need to observe. If you observe, then it has an interference pattern, no?

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