Listen Now Download as mp3 from the show Nuclear: About to Explode? QuestionBronwyn Higgs asked: Dear Chris,
I have a question about absolute zero.
If we can have a lower limit on temperature absolute zero i.e. so cold that nothing happens, why do we not have an upper limit  i.e. a temperature that is so hot that atoms and particles are so hot that they are ripped into the most elementary units and become so excited that theycannot react with anything around them.
It has always seemed to me that there is a certain asymmetry about this temperature scale.
Thanks for taking questions from interested members of the public.
Regards Bronwyn Higgs AnswerWe posed this question to Sam Gregson, High Energy Particle Physicist at the University of Cambridge... Sam  The temperature of a system is simply related to the amount of energy in that system. Because the system can't have a negative energy, there is only so much heat you can remove from it and so a limit to how cold you can get. This is called absolutely zero. We’ve got very close to it. Scientists in Finland have cooled rhodium atoms to a 10th of a billionth of a degree above absolute zero. On the other hand, an absolute maximum temperature would require there to be a limit to the amount of energy you can give to a particle. As far as we know, there is no such limit. Although the speed of light is the universal speed limit, the reason you can't get there is that this would require an infinite amount of energy. So this speed limit does not limit the amount of energy and therefore, the temperature of an individual particle. The most energetic particle ever observed was a cosmic ray over Utah, travelling at 99.999999999985% of the speed of light. Probably a single proton with about 50 joules of energy. This is equivalent to about 5 trillion trillion degrees Celsius and there is no evidence that this is the hottest you could get to. As far as we know, you are just limited by the amounts of energy you can give to a particle. So you could say that the absolute maximum temperature is a temperature equivalent to all the energy in the universe, concentrated onto one particle. But that limits more accounting than basic physics. Hannah  Thanks, Sam Gregson from Cambridge and CERN. So temperature is related to thermal energy and Einstein’s theory of relativity means that although a particle has a universal speed limit, it doesn’t have an energy limit. If you took all of the energy in the universe and put it into one particle, you'd essentially run out of energy before you run out of capacity for energy which is why we have no absolute maximum temperature. Multimedia
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CommentsI thinik theres 2 aspects to this question: Temperature is a measure of how fast the elements of an object are moving  and the absolute speed limit is the speed of light  so it ought to follow that the absolute max temperature is when the particles are approaching the speed of light  like the protons at CERN in the LHC  they are within a few meters per second of the speed of light. But as things get faster they also get heavier (E=mc^2)  so as you get hotter then the energy needed to raise the temperature further goes up, so it should tend to infinity... chris, Wed, 21st Mar 2012 I think Chris is right. To be more precise I think the second Chris is correct. A maximum finite speed or velocity does not  in relativistic terms  cause a maximum finite energy. As the Kinetic Theory of Gas derived temperature is related to the average Kinetic Energy (rather than the average speed) of particles  at nonrelativistic speeds I look forward to the answer on the next show.
This is incorrect Absolute hot is postulated at something like 1x10^32 C Negative energy exists as well. Alex, Wed, 5th Feb 2014 there does exist the upper limit of temperature.it could be the temperature at the beginning of this universe.it is found to be TEN TO THE POWER OF THIRTY TWO KELVIN. Sudhakaran, Sat, 5th Sep 2015 I realize that this isn't exactly the same concept, but what if a photon was given so much energy that its wavelength approached the Planck length? KB, Mon, 4th Apr 2016 
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