科學家創造出比絕對零度還低的溫度？！

在絕對零度時，物質就靜止不動了（只剩自旋性），具有最低的能量。而你是沒法叫一個已經靜止不動的粒子，更靜止的，對不對？而一個已經處於最低能量狀態的粒子，又如何產生比最低能量還低的能量狀態？這新聞到底說的是什麼意思呢？

初看起來，這個比克氏絕對溫度還低的溫度，實在很不科學。問題是出在，克氏溫度是量測「亂度」（Entropy），而不是我們一般人熟知的「溫度」（Temperature）。

物理定義下的溫度（Temperature）是指亂度（Entropy）相對於能量（Energy）的斜率（Slope）。

Temperature = the change in the energy of a system relative to the change in entropy。

各位可以想像某系統裡的基本粒子，只可以處於兩個狀態，一個是最低能階狀態，一個是最高能階狀態，若所有基本粒子都處於最高能階狀態，則此系統，不但有最高能量，也具有最低亂度（所有粒子都處於最高能量狀態）。當此系統釋出能量，則此系統的能量降低，某些粒子由最高能階狀態，落入最低能階狀態，則此系統的亂度反而增加，即並非所有粒子全部處於同一能階，就可以產生負的亂度與能量的變化率，即所謂「負溫度」的現象。這樣的實驗，在靜態的粒子上，很久以前就有許多人做過。而這個報導裡的實驗，比較特別的是，它加入動量（Momentum）的考量因素。

克氏(Kelvin)絕對零度是物質粒子＂完全不動＂的狀態，是物質的亂度(Entropy)為零的狀態（嚴格來說，物質即使處在絕對溫度零度，還是有非零的亂度，也就是粒子的自旋性，不過我們不需在此深究）。而溫度（Temperature）是亂度以及能量的變化率（The change rate of entropy）。

在溫度為克氏(Kelvin)絕對零度以上的世界，一般來說，物質加熱（加入能量），他的亂度就增加，就越無序化。而這群科學家是利用多筆高能雷射束抓住粒子，一邊供給它極高的能量，但又極力束縛它，硬是不讓它動，讓它產生一個負溫度的暫態，也就是越加入能量，粒子的運動越有秩序，或越釋出能量，粒子的運動越無秩序，只有在溫度低於克氏絕對零度才會有的怪異現象。

所以，客氏絕對零度仍然是能量增加、亂度增加（溫度為正度數，也就是「正」的亂度與能量變化率），以及能量增加、亂度降低（溫度為負度數，也就是「負」的亂度與能量變化率），兩種亂度／能量變化率系統的中間點。

因此，雖然初聽起來，比絕對零度還低的溫度，蠻令人不可思議的，但科學家還是有可能讓物質產生溫度低於克氏絕對零度的狀態的啦。

Temperature is actually just a measure of the rate of change of energy with respect to entropy. For positive temperature, you gain energy by increasing entropy (e.g. boiling water is more chaotic than cold water). For negative temperature, it's the reverse, where you gain energy by decreasing entropy. The wikipedia article on "negative temperature" has a nice explanation

So absolute zero is still the point which marks the turning point from energy increasing with higher entropy to energy increasing with lower entropy.

The precise statistical mechanics based definition on temperature is not simply that "atoms stop moving" as we learn in high school physics.

It has to do with a rate of change of entropy to energy. At absolute zero, that rate of change is zero, above it the rate is positive, below it the rate is negative.

In the case of this experiment. The atoms at "negative temperature" are still moving about at a faster speed than they would at absolute zero. So in the layman's sense of the word, negative temperatures are "warmer" than absolute zero. Either way, the zero still stands.

• 

  Entropy drop: Scientists create “negative temperature” system

  arstechnica.com

  Bizarre setup may help researchers model dark energy.

 

At absolute zero particles were thought to have zero energy. Now, how can you have a particle with "negative" energy? What does this "negative" energy means? It's so odd...

 

 

Quantum Gas Temperature Drops Below Absolute Zero | Wired Science | Wired.com

www.wired.com

Physicists have created a quantum gas capable of reaching temperatures below absolute zero, paving the way for future quantum inventions.

近代物理學因為已經超過日常生活的經驗範圍，由我當年唸物理系時已經由理論(數學推理)大幅領先，而後再由實驗證明或推翻，如果這篇雜誌報導為真，就先需理論推翻絕對0度。我雖然離開物理界已35年，但還依然關注物理界的理論發展，這種沒理論依據的胡亂實驗沒一個活下來的。

陳律師(vchen123) 於 2013-01-07 08:44 回覆： 再說一次，早在1950年代，「負」的溫度（亂度與能量的梯度），也就是文中所述的能量增加、亂度降低現象，就已經在理論上被推導出來，大家所理解的溫度（Temperature），與克氏絕對溫度（Kelvin）所量測的亂度（Entropy）有所不同。這屬於熱力學（Thermodynamics），屬於量子力學，有能階、自旋、測不準原理，玻色、愛因斯坦膠合子等，與普通物理所述有很大不同，讓大家難以理解。 arstechnica.com的敘述比較對（而Wired的敘述不太正確），你有空的話，請抽空讀一下。他是利用極高能雷射，製造了一個負溫度的暫態，維持一個非常短暫但有意義的時間。 這篇發表在Science期刊的原文在這裡： http://www.sciencemag.org/content/339/6115/52 摘要： Absolute temperature is usually bound to be positive. Under special conditions, however, negative temperatures—in which high-energy states are more occupied than low-energy states—are also possible. Such states have been demonstrated in localized systems with finite, discrete spectra. Here, we prepared a negative temperature state for motional degrees of freedom. By tailoring the Bose-Hubbard Hamiltonian, we created an attractively interacting ensemble of ultracold bosons at negative temperature that is stable against collapse for arbitrary atom numbers. The quasimomentum distribution develops sharp peaks at the upper band edge, revealing thermal equilibrium and bosonic coherence over several lattice sites. Negative temperatures imply negative pressures and open up new parameter regimes for cold atoms, enabling fundamentally new many-body states. Kaglan2Wise, Aged Ars Veteran Temperature in physics is not a discrete property but it is a slope -- a derivative, the change in the energy of a system relative to the change in entropy. Usually, as energy goes up, entropy goes up (and vice versa) so temperature is almost always positive. Now imagine you have a situation where particles can be in one of two states: low energy or high energy. If every particle in the system is the 'high energy' state, that gives the maximum total system energy. However, this system has the lowest possible entropy (every particle the same = no disorder, no entropy). Strangely, if you decrease the system energy -- by allowing some of those particles to randomly move to their 'low energy' states, so you end up with a mix -- entropy increases (more randomness). Because the slope here is backwards of what it should be (more energy -> less chaos, less energy --> more chaos) we say the temperature is negative. So the strangeness is partly because we humans have chosen to define temperature. As people have pointed out, for this to work you need a system that has a 'maximum energy', and that concept doesn't usually appear in our daily lives. I haven't been in the field for a while, but past experiments probably used atomic spin systems where because of quantum mechanics atoms could be limited, as in my example, of being in only one of a limited number of states, with clear maximum energy. What this team did differently is they demonstrated the phenomenon with momentum. From what I understand, you can imagine they filled an energy 'bowl' with marbles (or maybe an ice cube tray, because they used a lattice) and then inverted the bowl in such a way that it did not disturb the marbles. They ended up with a bunch of marbles perched on top of this upside down bowl, this hill, in the highest possible position (energetically), but with low entropy. Last edited by Kaglan2 on Fri Jan 04, 2013 2:09 pm