1. ## Absolute Zero

I was trying to explain why -273.15 C or 0K was the lowest temperature ever to people at church.

My assumption is that 0K comes from the Kinetic Theory of matter and that by the third law of Thermodynamics we can never reach it anyway.

Was I correct on this.

2. The reason we cannot reach -273.15 is Heisenbergs uncertainty principle AFAIK.

3. Originally Posted by Laguna2
The reason we cannot reach -273.15 is Heisenbergs uncertainty principle AFAIK.
Can you elucidate?

4. Originally Posted by Sticks
Can you elucidate?
IIRC the argumentation was as follows:
A particle at absolute zero would have a known impulse AND position, which is impossible in accordance with heisenberg.

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Sticks. Laguna is correct. So even at "zero" there is still a minimum of motion. The radiation associated with this is the zero point radiation, but in order to capture it and use it for something useful....that is as a heat source...you would have to be at an even lower temperature for it to "flow" to....which by definition is impossible. That's why nutcases post "Free energy" schemes....temperatures lower than absolute zero are only found by nutcases, and nobody is getting rich harvesting the zero point radiation.
There may be a higher temperature in the neutrino sea, but to date, nobody is getting rich there either. Invest in methanol fuel cells....and No I don't own any stock there...... pete

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Search the forums. There have been a couple of good threads about absolute zero.

7. The basic idea is that when we first chose the temperature scale of Celsius, it was fit to two points, the freezing and boiling points of water. Then it was discovered that temperature was related to the average energy of the particles-- but only if measured on an absolute scale (i.e., a scale where 0 meant 0 energy). The Celsius scale did not have that property unless you shift it. An analogy with age might be, you could define 0 age to be when you are old enough to drive a car by yourself and 100 when you are old enough to buy alcohol. But then you discover that age can be thought of as proportional to the time you've been alive, so you make a conversion of adding 320. Now you have something proportional to years (just divide by 20), and anything proportional to years will be an "absolute age". That's just like going from Celsius to Kelvin. (The quantum mechanics at absolute zero is very much a detail in this general story. Even without quantum mechanics, absolute zero would still be unattainable in practice, sort of like how it is impossible to get all the toothpaste out of the tube.)

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We cannot reach zero-Kelvin, simply bvecause there is still far too much energy existing at this level. In fact, if you could reach -273.15 C, there would still be a lot of movement, and a massive amountof energy. This i due to the uncetainty principle.

9. Originally Posted by Laguna2
IIRC the argumentation was as follows:
A particle at absolute zero would have a known impulse AND position, which is impossible in accordance with heisenberg.
OK That makes sense, I was thinking along the lines of the third law of thermodynamics

10. Originally Posted by Occams Ghost
We cannot reach zero-Kelvin, simply bvecause there is still far too much energy existing at this level. In fact, if you could reach -273.15 C, there would still be a lot of movement, and a massive amountof energy.
The thing about energy is that the amount of energy never matters, only the changes in energy of various types. So for example there would always be a lot of energy even in the absence of movement: there would be the rest energy, but it is not accessible so plays no role. Similarly, the fact that the kinetic energy has a quantum mechanical minimum is not terribly important, since you cannot access the "zero-point energy". Absolute zero means there is no accessible energy present, so the quantum mechanical minimum is already included in what absolute zero means and does not tell you why you can't get there. You still can't get there though-- for perfectly classical reasons. There is a tendency to overstess the importance of quantum mechanics, I think because it is so cool. Nevertheless, as it happens, quantum effects inevitably do become important at low temperatures in practice.

11. Is there any reason to mention it was at church? (i.e. is there a new and exciting woo-woo theory to debunk?)

12. Originally Posted by Ara Pacis
Is there any reason to mention it was at church? (i.e. is there a new and exciting woo-woo theory to debunk?)
IIRC they were talking about how cold it had been and someone may have suggested a temperature below absolute zero and I said you could not go below absolute zero and some did not seem to accept a lower limit.

Someone even said, they are now proving Einstein wrong

13. It is actually possible to go below absolute zero, in some sense, but it's really just a technicality. Below absolute zero would mean a negative temperature. What temperature really is, by the way, is the amount of heat (in whatever units are appropriate for your definition of temperature) you need to add to a system to get a certain increase in its entropy, which means to fractionally increase the number of states that system has access to. At low temperature, only a little heat is needed to fractionally increase the number of states the system has access to by, say, 1%, but at high temperature, that same 1% fractional increase will require a lot of heat. So negative temperatures just mean that you can actually give some sort of specialized hybrid "system" a fractional increase in the number of states it has access to by removing heat from it, in some prescribed way.

Interestingly, one way to do that, though it is certainly a kind of cheat compared to what the OP was talking about, is to have reservoirs of heat at two different temperatures, a warm reservoir W and a cold reservoir C, and draw heat from W and put it into C, but siphon off a little of that heat and put it into a third system H at an even higher temperature than W. You can choose the numbers such that the net result of that is to create entropy, even though if you only look at W and H, you see heat going from something warm to something hot. Since from the point of view of H, the rest of the universe loses heat and gives it to H, yet the rest of the universe gains entropy, it is as though the rest of the universe is effectively at negative temperature (even though in fact neither W nor C is at negative temperature). This, by the way, is the principle involved in lasers, and is also the principle involved in spontaneous generation of ordered systems like living creatures. You are I were forged in an environment that functions in broad brush like it had a negative temperature, even though in more detail it was effectively two different positive ones (the temperatures of the Sun and the Earth). The interaction of photosynthesis with that temperature difference functions as though the photosynthetic reaction was immersed in a negative temperature environment, which is essentially why a lot of people confuse that with a violation of the second law of thermodynamics.

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