absolute zero, the zero point of the ideal gas temperature scale, denoted by 0 degrees on the Kelvin and Rankine temperature scales, which is equivalent to -273.15°C and -459.67°F. For most gases there is a linear relationship between temperature and pressure (see gas laws), i.e., gases contract indefinitely as the temperature is decreased. Theoretically, at absolute zero the volume of an ideal gas would be zero and all molecular motion would cease. In actuality, all gases condense to solids or liquids well above this point. Although absolute zero cannot be reached, temperatures within a few billionths of a degree above absolute zero have been achieved in the laboratory. At such low temperatures, gases assume nontraditional states, the Bose-Einstein and fermionic condensates. http://www.infoplease.com/ce6/sci/A0802223.html

i think it is because at absolute zero, all energy is lost

Here's one answer I found at http://www.newton.dep.anl.gov/askasci/phy99/phy99194.htm : "There is no strict theoretical barrier to T=0 like there is to attaining the speed of light for massive objects, but practically your insulation could never be good enough, and some energy would always leak in from the surroundings, so that T = 0 K(elvin) could only be achieved for an ordinary system like a gas in an entirely empty universe." I could see at least with our present technology that we haven't developed a material or a seal that can stop all particulate leakage.

1) "The third law of thermodynamics is a statistical law of nature regarding entropy and the impossibility of reaching absolute zero of temperature. The most common enunciation of third law of thermodynamics is: “ As a system approaches absolute zero, all processes cease and the entropy of the system approaches a minimum value. ” It can be concluded as 'If T=0K, then S=0' where T is the temperature of a closed system and S is the entropy of the system. The essence of the postulate is that the entropy of the given system near absolute zero depends only on the temperature (i.e. tends to a constant independently of the other parameters)." "The third law was developed by Walther Nernst, during the years 1906-1912, and is thus sometimes referred to as Nernst's theorem or Nernst's postulate. The third law of thermodynamics states that the entropy of a system at zero is a well-defined constant. This is because a system at zero temperature exists in its ground state, so that its entropy is determined only by the degeneracy of the ground state; or, it states that "it is impossible by any procedure, no matter how idealised, to reduce any system to the absolute zero of temperature in a finite number of operations". An alternative version of the third law of thermodynamics as stated by Gilbert N. Lewis and Merle Randall in 1923: “ If the entropy of each element in some (perfect) crystalline state be taken as zero at the absolute zero of temperature, every substance has a finite positive entropy; but at the absolute zero of temperature the entropy may become zero, and does so become in the case of perfect crystalline substances. ” This version states not only ΔS will reach zero at D = 0 K, but S itself will also reach zero." Source and further information: http://en.wikipedia.org/wiki/Third_law_of_thermodynamics 2) "In physics, certain systems can achieve negative temperatures; that is, their thermodynamic temperature can be of a negative quantity. Negative temperatures can be expressed as negative numbers on the kelvin scale. Temperatures that are expressed as negative numbers on the familiar Celsius or Fahrenheit scales are simply colder than the zero points of those scales. By contrast, a system with a truly negative temperature is not colder than absolute zero; in fact, temperatures colder than absolute zero are impossible. Rather, a system with a negative temperature is hotter than the same system with an infinite temperature." "As Kittel and Kroemer (p.462) put it, "The temperature scale from cold to hot runs +0 K, . . . , +300 K, . . . , +∞ K, −∞ K, . . . , −300 K, . . . , −0 K."" Source and further information: http://en.wikipedia.org/wiki/Negative_temperature

The equipment we use to try and get to 0 kelvin emits heat... So, at the moment, we can reach it.