# Thread: We cant achieve absolute zero??

1. So I was watching a documentary on YouTube about helium as a super-fluid...

The fluid actually flowed through the glass container O.o strange stuff!

The one scientist said it is impossible to actually reach ABSOLUTE zero and when asked why, his answer was, "because every time we get close to it, we have to cool it down further..." - Well, that's a lame answer I thought, so I thought I would present the question here and hopefully someone can give me a nice neat and relatively simple answer.

My only guess would be that there is a heat transfer when the object is cooled do to motion? Maybe the motion of the technique used to cool an object causes heat?

PS: here is a clip from the same documentary - http://www.youtube.com/watch?v=9FudzqfpLLs

2.

3. I will request that someone who knows more than me correct me should I misspeak, but...

Absolute zero is equivalent to having no motion whatsoever, and there will always be some small motion with at least some small particles... even if that motion is very slow. With absolute zero, time essentially stops. We can get very very very close to absolute zero, but there will always be some movement of some particles, and therefore (despite being very close to zero) we never truly reach it.

That's from memory, and may be missing something simple, but the idea is that absolute zero is an idealized state which can only be approached, never reached.

There was a brilliant program on PBS Nova sometime back about absolute zero. You can watch it online.

http://www.pbs.org/wgbh/nova/zero/

4. Originally Posted by Rickz2020
So I was watching a documentary on YouTube about helium as a super-fluid...

The fluid actually flowed through the glass container O.o strange stuff!

The one scientist said it is impossible to actually reach ABSOLUTE zero and when asked why, his answer was, "because every time we get close to it, we have to cool it down further..." - Well, that's a lame answer I thought, so I thought I would present the question here and hopefully someone can give me a nice neat and relatively simple answer.

My only guess would be that there is a heat transfer when the object is cooled do to motion? Maybe the motion of the technique used to cool an object causes heat?

PS: here is a clip from the same documentary - http://www.youtube.com/watch?v=9FudzqfpLLs
One of the several equivalent statements of the third law of thermodynamics is that it is impossible to reach absolute zero in a process with a finite number of steps.

5. In order to reduce the temperature of anything you need a heat sink that is colder than that thing. If you had a heat sink at absolute zero your thing needs to be above absolute zero to be able to transfer heat and thus to cool down further.

6. Originally Posted by Bunbury
In order to reduce the temperature of anything you need a heat sink that is colder than that thing. If you had a heat sink at absolute zero your thing needs to be above absolute zero to be able to transfer heat and thus to cool down further.
If that were true refrigerators would not work.

In order to move heat from a low temperature system to a high temperature system you must do work.

Temperatures below 1K have been achieved. There is no natural heat sink of such a low temperature.

7. Originally Posted by DrRocket
Originally Posted by Bunbury
In order to reduce the temperature of anything you need a heat sink that is colder than that thing. If you had a heat sink at absolute zero your thing needs to be above absolute zero to be able to transfer heat and thus to cool down further.
If that were true refrigerators would not work.
A refrigerator cycle creates a heat sink colder than the refrigerator itself by mechanical means. This is not inconsistent with what I wrote.

In order to move heat from a low temperature system to a high temperature system you must do work.
Yes, of course. That's the second law.

Temperatures below 1K have been achieved. There is no natural heat sink of such a low temperature.
Right, but are you saying that mechanical refrigeration can theoretically achieve absolute zero?

8. What reduced the temperature of the heat sink then?

9. Originally Posted by Bunbury
Right, but are you saying that mechanical refrigeration can theoretically achieve absolute zero?
No. That is the content of the third law. You cannot acieve absolute zero in a finite number of theermodynamic cycles.

In (classical) theory a mechanical system could come arbitrarily close.

10. Originally Posted by Bunbury
A refrigerator cycle creates a heat sink colder than the refrigerator itself by mechanical means. This is not inconsistent with what I wrote.
No a refrigerator exhausts heat to the room. It does work to raise the temperature of the working fluid, in the compressor, to do that. The working fluid is then cooled by adiabatic expansion in the evaporator.

The working fluid is not a "heat sink". The heat sink is the ambient air. The net result is that work is done to move thermal energy from the contents of the refrigerator to the ambient air.

11. A heat sink can be defined as any device or medium that absorbs heat, and that is the sense in which I used it. The cold refrigerant is a heat sink for the thing that it is cooling.

A heat sink is an object that transfers thermal energy from a higher temperature to a lower temperature fluid medium. The fluid medium is frequently air, but can also be water or in the case of heat exchangers, refrigerants and oil.
http://en.wikipedia.org/wiki/Heat_sink

12. Originally Posted by Bunbury
A heat sink can be defined as any device or medium that absorbs heat, and that is the sense in which I used it. The cold refrigerant is a heat sink for the thing that it is cooling.
In that case just about everything is a heat sink, and the concept is useless. I think I will stick to conventional thermodynamics.

13. Bunbury, the point is that pressure and temperature are interchangeable values. You don't need a cold place to transfer the heat to. A low pressure place will do just fine, and low pressure places can be artificially created by mechanical means. ( At the extreme, like to reach zero kelvin I think you might have to do something more exotic than that, but I'm not familiar with how they do that. )

Consider CO2 for example. If you've ever gone paint ball fighting with some friends and you put your hand on your paintball gun's CO2 tank after you've fired off a number of shots, you'll notice that it's cold. That's because the CO2 gets cool when it expands.

http://en.wikipedia.org/wiki/Carbon_dioxide#Refrigerant

14. Kojax, thanks, but I’m quite familiar with refrigeration cycles, the Joule-Thomson effect and the laws of thermodynamics, having designed several systems in which CO2 is compressed, expanded, heated, cooled, liquefied, solidified, vaporized and injected into the ground as a supercritical fluid. Unfortunately my resume doesn’t include a paint ball fight so I suppose I still have much to learn about thermodynamics.

This statement is quite correct when the common engineering usage of the term heat sink is understood:

In order to reduce the temperature of anything you need a heat sink that is colder than that thing. If you had a heat sink at absolute zero your thing needs to be above absolute zero to be able to transfer heat and thus to cool down further.
That is the answer to the original question.

Temperature and pressure are related; they are NOT interchangeable.

15. Originally Posted by Bunbury
A heat sink can be defined as any device or medium that absorbs heat, and that is the sense in which I used it. The cold refrigerant is a heat sink for the thing that it is cooling.

A heat sink is an object that transfers thermal energy from a higher temperature to a lower temperature fluid medium. The fluid medium is frequently air, but can also be water or in the case of heat exchangers, refrigerants and oil.
http://en.wikipedia.org/wiki/Heat_sink
When you first buy a fridge and first plug it in, everything's at room temperature. How then does the inside get cold if you have to have something colder?

16. You make something - the refrigerant fluid- colder by mechanical means. You compress it which makes it hot. Being hot means it can give up heat to the atmosphere. As it gives up heat it condenses to liquid. Then you expand it through a flow restriction which vaporizes some of it and makes it cold. Then you pass it inside the refrigerator where it absorbs heat from the warmer air and cools it down. Then you recompress it and it goes round again.

http://www.chemistry.wustl.edu/~cour...rigeration.htm

17. Originally Posted by Bunbury
You make something - the refrigerant fluid- colder by mechanical means. You compress it which makes it hot. Being hot means it can give up heat to the atmosphere. As it gives up heat it condenses to liquid. Then you expand it through a flow restriction which vaporizes some of it and makes it cold. Then you pass it inside the refrigerator where it absorbs heat from the warmer air and cools it down. Then you recompress it and it goes round again.

http://www.chemistry.wustl.edu/~cour...rigeration.htm
And that atmosphere being a large body can be treated as isothermal which is what one usually means by a "heat sink". This in contrast to the working fluid which is not reasonably described as either isothermal or a "heat sink".

Your description of the thermodynamics of a refrigerator is correct. Your earlier post was, at best, misleading.

Bottom line: In order to move heat from a colder body to a warrmer body work must be done. This is one of several equivalent statements of the second law of thermodynamics.

18. Can we achieve absolute zero ?

Shortly said : NO. We cannot.

19. Yes, I know my description is correct, but thanks for mentioning it anyway. And I’m sorry you find my use of “heat sink” so objectionable, but it is widely used the way I used it in the engineering industry. I’ll offer you another example, this one from the engineering procedure known as Pinch Technology in which process streams in (for example) a refinery are analyzed to optimize energy usage. The process streams are identified as heat sources and heat sinks and can be hydrocarbons, cooling water, air and so on. They are usually not isothermal. They can include ambient air but that is just one of many. If you can be bothered, do a word search in this document for “sink”. The first (of many) occurrence is this:

Table 1 shows the thermal data for Pinch Analysis. “Hot steams{sic}” are the streams that need
cooling (i.e. heat sources) while “cold streams” are the streams that need heating (i.e. heat
sinks).
http://www.ou.edu/class/che-design/a...nhoffMarch.pdf

20. Originally Posted by Bunbury
Yes, I know my description is correct, but thanks for mentioning it anyway. And I’m sorry you find my use of “heat sink” so objectionable, but it is widely used the way I used it in the engineering industry. I’ll offer you another example, this one from the engineering procedure known as Pinch Technology in which process streams in (for example) a refinery are analyzed to optimize energy usage. The process streams are identified as heat sources and heat sinks and can be hydrocarbons, cooling water, air and so on. They are usually not isothermal. They can include ambient air but that is just one of many. If you can be bothered, do a word search in this document for “sink”. The first (of many) occurrence is this:

Table 1 shows the thermal data for Pinch Analysis. “Hot steams{sic}” are the streams that need
cooling (i.e. heat sources) while “cold streams” are the streams that need heating (i.e. heat
sinks).
http://www.ou.edu/class/che-design/a...nhoffMarch.pdf
I have at least as much engineering education and experience as do you. I know very well what terminology is used in industry. This discussion is over.

21. Note to self: Don't EVER bring up the concept of a heat sink on thescienceforum. Just to be safe, I also won't bring up kitchen sinks, bathroom sinks, other types of water basin or even sink holes. Additionally, I will not bring up any downward movements in altitude, descents, dips, or drops. Hell, I should probably avoid the entire concept of synchronization too, for that matter, because it abbreviates as sync. Yowza...

22. Originally Posted by Bunbury
You make something - the refrigerant fluid- colder by mechanical means. You compress it which makes it hot. Being hot means it can give up heat to the atmosphere. As it gives up heat it condenses to liquid. Then you expand it through a flow restriction which vaporizes some of it and makes it cold. Then you pass it inside the refrigerator where it absorbs heat from the warmer air and cools it down. Then you recompress it and it goes round again.

http://www.chemistry.wustl.edu/~cour...rigeration.htm
I didn't know you were an expert. Just to be clear then: by your definition of "heat sink", open space can be considered a heat sink, if an object is cooling itself by radiating blackbody radiation out into the darkness in greater amounts than it absorbs?

Also a question on heat/pressure relationship: are there some substances that don't cool down when pressure is relieved?

23. I'm not an expert. I happen to have experience in some areas.

It's not "my"definition. By anyone's definition open space can be called a heat sink, but don't forget that first, space is not at absolute zero, and second, the point I have tried to make clear previously: in order for heat transfer to occur there must be a temperature difference. The radiating object can cool to within infinitesimal difference above the temperature of space, but cannot quite get all the way. If the object is an individual particle then ask a a physicist. I am referring to objects consisting of large numbers of particles.

Hydrogen, helium and argon don't cool down on Joule-Thomson expansion (unless the starting temperature is extremely low). I assume J-T expansion is what you have in mind.

24. Originally Posted by DrRocket
I have at least as much engineering education and experience as do you. I know very well what terminology is used in industry. This discussion is over.
You are not a gracious loser, are you Dr. Rocket. (That's rhetorical, by the way. As you say, the discussion is over.)

25. [quote="Ophiolite"]
Originally Posted by DrRocket
You are not a gracious loser, are you Dr. Rocket. (That's rhetorical, by the way. As you say, the discussion is over.)
??????????????

26. Originally Posted by Bunbury
I'm not an expert. I happen to have experience in some areas.

It's not "my"definition. By anyone's definition open space can be called a heat sink, but don't forget that first, space is not at absolute zero, and second, the point I have tried to make clear previously: in order for heat transfer to occur there must be a temperature difference. The radiating object can cool to within infinitesimal difference above the temperature of space, but cannot quite get all the way. If the object is an individual particle then ask a a physicist. I am referring to objects consisting of large numbers of particles.
How would you describe the behavior of a thermocouple, then? Electricity appears to be moving heat to one end of the device and cold to the other in excess of any natural temperature gradient.

http://en.wikipedia.org/wiki/Thermocouple

27. Originally Posted by kojax
How would you describe the behavior of a thermocouple, then? Electricity appears to be moving heat to one end of the device and cold to the other in excess of any natural temperature gradient.

http://en.wikipedia.org/wiki/Thermocouple
I don't see how you come to that conclusion. In a thermocouple, a temperature differential generates an electric current. The reverse is also true.

28. Originally Posted by Bunbury
Originally Posted by kojax
How would you describe the behavior of a thermocouple, then? Electricity appears to be moving heat to one end of the device and cold to the other in excess of any natural temperature gradient.

http://en.wikipedia.org/wiki/Thermocouple
I don't see how you come to that conclusion. In a thermocouple, a temperature differential generates an electric current. The reverse is also true.
How do you mean by "the reverse is also true"? Thermocouples are reversible. You can generate electricity by exposing the metal to a temperature gradient, or you can expend electricity in order to create a temperature gradient.

How does expending electricity to create a temperature gradient involve a "heat sink"? The heat from the end being cooled is transported to the end that's getting hot.

29. The heat from the end being cooled is transported to the end that's getting hot.
Where does the heat from the end being cooled come from? Where the heat from the end getting hot go to?

30. Good point.... the hot end needs somewhere to dissipate heat into or the thermocouple will overheat and probably melt or something.

So, as long as some portion of the system dissipates heat into an area that is cooler than that portion (not cooler than the object being cooled, however) we're still considered to be using a "heat sink"? I find that a good half of my difficulties understanding physics arise from trying to learn the terminology, so hopefully you'll understand why I want to be so clear about this.

31. The inverse thermocouple effect that you speak of is commonly used in Peltier coolers, sometimes used to cool computer CPUs below ambient temps.

I don't recall the exact cause for this effect, and would have to look it up which I'm sure you can do if interested enough, but I remember studying the effect in 4th yr solid state physics and it probably has to do with junction potential differences, just as in semiconductors, with electron flow possible only in one direction. But that is not what my post is about...

Sink, as in heat sink, can have different meanings to an engineer, a physicist and a mathematician.
At one extreme a mathematician would see it as a negative temperature gradient and, at the other, an engineer would see it as a slope of temp. per watt. A refrigeration system involving a heat pump ( expending work to achieve the required cooling ) will still increases total entropy. In their respective capacities both DrR and Bunbury are right. I on the other hand, am neither so don't blame me for trying to make peace betwen the two, but...

As a physicist, absolute zero is defined as the absence of atomic motion, and while one would think that zero motion is not acheivable for a system of atoms because of their statistical nature, it should be possible for a single atom. However Heisenberg's principle rears its ugly head when dealing with a single atom, so that we cannot be sure that we have zero motion. In effect quantum mechanics ultimately keeps us from acheiving absolute zero.

32. Originally Posted by MigL
don't blame me for trying to make peace betwen the two
To make peace there has to be a war. There is no war.

The Peltier effect is also used in wine coolers. I was given one once. It made noise, sucked electricity, took up space and did little cooling so I took it back to the store.