StarCraft® II

I'm a Theoretical Physicist. Ask me things.

Posts: 95
02/06/2012 03:43 AMPosted by Necromaster
so we can't get to zero kelvin because of zero point energy, doesn't really answer many questions (explaining something I don't understand/know with something else I don't understand/know),

Perhaps my answer might expand on what Astrai is saying. That Zero Kelvin and the concept of Zero Point energy are mutually exclusive states. Having ZPE means having a higher than zero K temp.

As to why, well if you change the question slightly and say "How is it we can never *observe* zero K". Then you could explain it with Heisenberg Uncertainty. i.e. We can't observe a system without changing it slightly, the degree of this effect gets worse as we look at smaller and smaller things. Therefore there are some systems which we can never get beyond a certain degree of accuracy on. Something at Zero K has no energy which implies that you could know everything about one of it's particles. However you could never observe this state because in order to observe it something must be acting as an intermediary - transmitting the information to you. This would also have the effect of transferring energy to the object. So it's now no longer at Zero K.
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Posts: 1,522
hmmm ok that makes a bit more sense, Doesn't answer why an object has zero point energy, but it does answer why we could never "see" something at zero kelvin. But what about microwaves . . .? It has something to do with the wavelength and the valence electrons, that much I can understand, but why is it that di-polar substances (like water) vibrate and absorb microwaves, where metals simply reflect them (like a common household microwaves). Also magnets, shouldn't a magnet react by heating up in the presence of microwaves?
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Posts: 142
To Astrai and AbslomDaak, it's basic thermodynamic. You can't cool a system to an arbitrary low temperature. I could review it if you wish (I don't feel like it now). You don't need quantum mechanics to explain that.

For microwaves, it's reflected by metal as they have a small penetration depth (if you think that name is funny, you're a little bit immature). Basically, electrons screen the wave so that it bounces back.

For water and similar molecules, the energy of vibrations is about the energy of the waves so the system can absorb photons. For plastic and similar materials, the band gap (the minimal energy required so that electrons can absorb and conduct) is way larger than the energy microwaves have. In quantum mechanics, you can't absorb a photon if it doesn't have the right energy.

Hope it helps!

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Posts: 95
@MaxPower - If it will help anyone that's cool (no pun intended) but I was just coming up with an explanation that people could picture in their heads.
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Posts: 1,522
according to basic thermodynamics why can you not cool a system to an arbitrary low temperature? What is a band gap? What is penetration depth? How does having small penetration depth prevent microwaves from passing through a material?
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Posts: 142
ok, I was a bit technical...

In a metal, electrons are free to move. When a EM wave (microwave or anything else) enters a metal, electrons will accumulate on the surface and oscillate in the bulk of the material. This is such that the wave is attenuated inside the metal. It's reflected instead. The penetration depth is the distance that the wave can move inside the metal. Small penetration depth means that the wave are 'killed' quickly in the metal, typically a few angstrom (an angstrom is 1 tenth of a billionth of a meter).

In a semi-conductor or insulator (basically not a metal), electrons are bound and cannot move freely. You need to give them a certain energy in order to get them moving. The band gap is that minimal energy. Since microwave energy is much smaller than a typical band gap, most plastic don't react in a microwave oven.
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Posts: 95
02/07/2012 10:39 PMPosted by Astrai
You -really- need to be transferring things to ceramic or glass prior to microwaving, the chemicals which leech from plastics are quite bad for you over long periods of time.

What sort of things? I read an awful lot of the literature on BPA leaching and it wasn't really impressive. You could break the research down into basically two groups:

i) Epidemiology done on mice which showed harm at chronic dosages which were below the understood ingestion level.
ii) Epidemiology done on mice which showed an elevated condition which in humans is correlated with harm at some level.

There was very little data on per dose harm and nothing in terms of human epidemiology.

The BPA stuff originally started with concern about baby bottles which while probably still harmless is at least a reasonable precaution. In adults? Probably not.

IIRC Even the EU - the people who won't let you put "water helps avoid dehydration" on a bottle of water. Thought the US and Canada overreacted.
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Posts: 95

There are some plastics which are purposefully designed to be microwave-safe for cooking food in, and some which are safe for a single use; they will specifically have notification on them that they are, and are designed to be inert with the addition of heat.

I think, it's more accurate to say that these are tested to be with in certain factor - a few orders of magnitude or so from the levels which are shown to cause harm in animals. I rarely find plastics, except perhaps at the dollar store without this marking. Even so, that doesn't actually mean they aren't any less safe than "microwave safe" plastics. Just that they aren't confirmed to be so. In fact given that we don't know exactly how well this is regulated they may all be just as safe or unsafe. ;-)

BPA was a concern in water bottles as well as baby bottles; and it's been so recent that there really hasn't been time for good longitudinal studies to be completed.

BPA has been studied for adverse effects since 1980 in far more than water bottles and baby bottles. You should read the wikipedia article it's a huge list of stuff - a lot of it is kind of fearmongering. For example people often forget that the old adage "correlation does not imply causation" has a statistical implication. When you read on things like the Wiki page that BPA at a certain dosage causes "breast cells to be predisposed to cancer". What they are actually saying is that mice were found to have an increased density in breast tissue. In humans this is correlated with some RR of an increase in breast cancer. The problem is, not only do we not know that breast cancer is caused by this (instead of simply being a co-morbidity with something like difficulty in diagnosis) and we likewise we don't know if BPA really causes this effect in humans BUT even if we knew both those things you CAN'T necessarily use one to claim the other (well wikipedia seems to say so but...). An all-causes correlation is significantly different than a single-cause correlation.

I'm not interested in being a guinea pig for it, myself, however

That's kind of exaggerating don't you think?

Besides how about all the other things that you don't know about? It's kind of interesting, Sackett the bio-statistician said that we must hold preventative medicine to a higher standard of evidence. The reason for this is that unlike "reactive" medicine we often have a good understanding of both the treatment and the treated and untreated prognosis. However with preventive medicine we are simply eschewing one thing for another without knowing much about the other.

So I'm still unclear exactly what chemicals are you concerned about and are you saying that microwavable plastics are safe or not?
Edited by AbslomDaak on 2/9/2012 12:37 PM PST
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Posts: 1,522
phantom started this thread . . . I wonder where he went? Also very interesting tangent, what about styrofoam? (the stuff that a doggy bag (box) from a resteruant is made out of Im not sure its technically syrofoam) Does styrofoam bleed chemicals in the microwave (when heated)?
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Posts: 95
I tend to think of it as: If you put energy into some kind of reasonably inert system. There is always going to be a probability that you "knock a piece out". The question is more of how much energy, how much gets released and at what level (both chronic and instantaneously) it is harmful (since everything - even water - is harmful at some level).
Edited by AbslomDaak on 2/9/2012 12:35 PM PST
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Posts: 95
The question easily gets complicated:

For example "avoid" usually also means we are embracing some alternative. i.e. people who think aspartame is harmful will happily drink non-diet beverages.

Is the relative risk (RR) of some negative outcome for drinking X ml of Diet Coke equal to or less than the RR of some other negative outcome of drinking X ml of Regular Coke?

How do we as people weigh a 1% increase in cancer risk vs. say a 10% increase in obesity risk?
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Posts: 95
Yeah, it has moved quite a bit on...

I do see what you're saying but what I'm talking about isn't so much what can be controlled vs. what can't but rather known vs. unknown (as well as the fact that people value different outcomes differently). Even when we think we are avoiding some risk we can be actually embracing another risk. Drinking water for example increases your intake of whatever pollutants are in it. IIRC there was a study looking at correlation between some developmental disorders in children which had water reservoirs near some sources of industrial waste.

It goes on and on...people who use groundwater instead of city water. Their children are sometimes at significantly higher risk of fluorosis. It's counter-intuitive but the places you are most likely find children who have too much fluoride are in rural communities not in intentionally fluoridated water. Likewise there is some research showing bottled waters being of lower qualities of some contaminates than tap water...

As I said, it's not so much that people can "avoid" risks so much as pick which ones they are the most comfortable with.
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Posts: 1,342
02/10/2012 05:54 PMPosted by AbslomDaak
Drinking water for example increases your intake of whatever pollutants are in it


nah, i filter my water thru an Reverse Osmosis System...although now i cant stand the taste of water with heavy metals and all that other junk in it
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Posts: 95
RO systems are nice but there are definitely classes of contaminants which get through like a number of organics and gasses. Not to mention the device itself probably adds some small amount of contaminants.

Overall you might be better off but as I keep saying it's not as simple as it seems.
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Posts: 1,342
RO systems are nice but there are definitely classes of contaminants which get through like a number of organics and gasses. Not to mention the device itself probably adds some small amount of contaminants.

Overall you might be better off but as I keep saying it's not as simple as it seems.


true true
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Posts: 1,522
Why did you choose to become a theoretical physicist?
on the off topic, I can taste the calcium in the water I drink (i think its calcium based on the color of the mineral deposits on the faucet) and its pretty bad. + side I don't have to drink milk.
Edited by Necromaster on 2/15/2012 2:30 AM PST
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